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Stefano FONTANESI

Professore Associato
Dipartimento di Ingegneria "Enzo Ferrari"


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Pubblicazioni

- Application of a zonal hybrid URANS/LES turbulence model to high and low-resolution grids for engine simulation [Articolo su rivista]
Iacovano, C.; D'Adamo, A.; Fontanesi, S.; Di Ilio, G.; Krastev, V. K.
abstract

A zonal hybrid unsteady Reynolds-averaged Navier–Stokes/large eddy simulation (URANS-LES) Zonal detached-eddy simulation (ZDES) model is applied to internal combustion engine (ICE) simulation and comparisons of predicted flow morphology and variability are carried out against on the transparent combustion chamber (TCC-III) particle image velocimetry (PIV) data set for motored conditions. To this aim, a previously developed model derived from a standard seamless-detached eddy simulation (DES) formulation is adopted for two different grid resolutions. In particular, two zonalization choices are evaluated based on previous single-grid results, in order to assess the model outcomes based on the joint turbulence treatment/grid density: the seamless-DES mode is applied (1) only to the cylinder (TCC-Z1) and (2) to the cylinder and intake port (TCC-Z2). Multi-cycle simulations (50 samples) are carried out and the results are compared to experimental data in terms of PIV images using multiple quality indices on multiple planes (Y = 0 and X = 0). Finally, comparison of predicted mean flow fields is extended to standard URANS mode. Results show that the use of a cylinder-only seamless-DES treatment on a relatively coarse grid results in a quantitative agreement between simulated and measured (PIV) flow fields, both in terms of average morphology and flow variability, whereas the extension of the DES mode to the intake port does not introduce relevant variations. Quality indicators seem to be moderately sensitive to the grid resolution, thus confirming the adaptive potential of a ZDES–like model and promoting the use of DES–type turbulence modelling even on relatively low-resolution grids. The analysis of average fields compared to URANS simulations highlights the benefit for both grids of a scale-resolving ZDES modelling when the same underlying turbulence model (k-ε RNG) is used. This study reinforces the recommendation in the use of hybrid URANS-LES models to simulate ICE flows. The adopted ZDES formulation based on the two-equation k-ε RNG model shows that high-quality results can be obtained even on engineering-grade grids, both in terms of average and cycle-to-cycle variation. The numerical results obtained using the two grids with variable resolution are consistent, and this further promotes a wider adoption of this class of models to simulate engine flows in industrial applications.


2022 - A zonal secondary break-up model for 3D-CFD simulations of GDI sprays [Articolo su rivista]
Berni, F.; Sparacino, S.; Riccardi, M.; Cavicchi, A.; Postrioti, L.; Borghi, M.; Fontanesi, S.
abstract

In Gasoline Direct Injection (GDI) engines, the secondary break-up plays a significant role in air–fuel mixing. In fact, spray granulometry affects evaporation rate, liquid penetration and plume morphology. Operating pressures and temperatures of both liquid and gaseous phases strongly influence the droplet disruption mechanism in the combustion chamber. In 3D Computational Fluid Dynamics (CFD), several models can be adopted to simulate the secondary break-up process, among which the Reitz-Diwakar and the Kelvin-Helmholtz Rayleigh-Taylor (KHRT) are the most diffused ones in the engine community. However, application of such models in their original versions is limited to a reduced range of injection parameters and ambient conditions. As a matter of fact, large variations of them usually require ad-hoc calibrations of the model constants. To improve the predictive capabilities and reduce the need of case-by-case tuning, an alternative secondary break-up model is proposed in the present paper. It is based on the Reitz-Diwakar one but, compared to the latter, a zonalization of the break-up regimes is proposed. Specifically, it is assumed that only Stripping break-up can occur near the nozzle, while Bag break-up only takes place sufficiently far from it. Moreover, model parameters are now treated as functions of the operating conditions. In particular, the impact of the ambient density on the model parameters is analysed in the present work. The proposed model is calibrated via constant volume vessel simulations on a single-hole GDI research injector at vacuum-to-pressurized conditions (namely at 0.4, 1.0, and 3.0 bar(a) of back pressure), on equal temperature. Model parameters are found to be linear functions of the ambient density. Thereafter, model validation is carried out on two different GDI injectors. The first is again a single-hole (remarkably different compared to the previous one), while the second is a 5-hole prototype. Numerical results provided by the proposed model show a satisfactory agreement compared to the experiments in terms of liquid penetration, Phase Doppler Anemometry (PDA) data and imaging, without any dedicated tuning. Conversely, the Reitz-Diwakar and KHRT models, applied to simulate (with default calibration constants) the same injectors, provide results which remarkably deviate from the experiments.


2022 - Laminar flame speed correlations of ammonia/hydrogen mixtures at high pressure and temperature for combustion modeling applications [Articolo su rivista]
Pessina, V.; Berni, F.; Fontanesi, S.; Stagni, A.; Mehl, M.
abstract

Ammonia/hydrogen mixtures are among the most promising solutions to decarbonize the transportation and energy sector. The implementation of these alternative energy carriers in practical systems requires developing suitable numerical tools, able to estimate their burning velocities as a function of both thermodynamic conditions and mixture quality. In this study, laminar flame speed correlations for ammonia/hydrogen/air mixtures are provided for high pressures (40 bar–130 bar) and elevated temperatures (720 K–1200 K), and equivalence ratios ranging from 0.4 to 1.5. Based on an extensive dataset of chemical kinetics simulations for ammonia/hydrogen blends (0-20-40-60-80-90-100 mol% of hydrogen), dedicated correlations are derived using a regression fitting. Besides these blend-specific correlations, a generalized (i.e., hydrogen-content adaptive) formulation, with hydrogen content used as additional parameter, is proposed and compared to the dedicated correlations.


2022 - Methodology for PEMFC CFD Simulation Including the Effect of Porous Parts Compression [Articolo su rivista]
Corda, G.; Fontanesi, S.; D'Adamo, A.
abstract

In this paper, a three-dimensional, multi-physics and multi-phase CFD model is presented and validated on straight single-channel configurations to analyse the influence of the channel/rib width ratio. In the first part, two cases with wide/narrow channel/rib spacing are reproduced from a literature campaign including neutron radiography (NRG) measured water distribution, which is well reproduced in simulations thanks to the novel implementation of an in-house developed macro-homogeneous catalyst layer sub-model. In the second part, the inclusion of an adapted compression model from literature allows to investigate in detail the effect that the deformation of the porous parts (diffusion media and catalyst layers) has on the cell performance, considering two levels of compression (i.e. clamping pressure). All transport properties (flow/energy/charge) are locally modified as a function of the inhomogeneous compression acted by the BPPs, e.g. influencing flow permeability and thermo/electrical conductivity. The obtained numerical results are compared against those from the undeformed geometry, highlighting a relevant operational difference and explaining it as a compression-related oxygen starvation. The study presents a comprehensive model for PEMFC simulation, including an efficient catalyst layer model and demonstrating the relevance of including the often-neglected compression effect on full-scale cell (or stack) models.


2022 - Modeling of gaseous emissions and soot in 3D-CFD in-cylinder simulations of spark-ignition engines: A methodology to correlate numerical results and experimental data [Articolo su rivista]
Berni, F.; Mortellaro, F.; Pessina, V.; Paltrinieri, S.; Pulvirenti, F.; Rossi, V.; Borghi, M.; Fontanesi, S.
abstract


2022 - Numerical Comparison of the Performance of Four Cooling Circuit Designs for Proton Exchange Membrane Fuel Cells (PEMFCs) [Relazione in Atti di Convegno]
Corda, G.; Fontanesi, S.; D'Adamo, A.
abstract

Polymer Electrolyte Membrane Fuel Cell (PEMFC) are among the most promising technologies as energy conversion devices for the transportation sector due to their potential to eliminate, or greatly reduce, the produc- tion of greenhouse gases. One of the current issues with this type of technology is thermal management, which is a key aspect in the design and optimization of PEMFC, whose main aim is an effective and balanced heat removal, thus avoiding thermal gradients leading to a cell lifetime reduction as well as a decrease in the output performance. In addition, a uniform temperature distribution contributes to the achieve- ment of a uniform current density, as it affects the rate of the electrochemical reaction. This is made even more challenging due to the low operating temperature (80°C), reducing the temperature difference for heat dissipation, and leaving a critical role to the design and optimization of the cooling circuit design. In this paper, a three-dimensional and multi-physics CFD approach is used to compare four different liquid cooling flow fields within the bipolar plates, using a conventional cooling fluid. Several typical cooling flow rates will be tested and equalized among all the considered cases, in order to carry out consistent comparisons. Numerical analyses will include Index of Uniform Temperature (IUT), minimum and maximum temperature gradient, coolant circuit pressure drop, thermal power absorbed by the coolant and performance of the cell, thus providing a detailed and comprehensive overview of a PEMFC thermal survey. The study paves the way for a conjugate fluid-dynamic/thermal characterization of a full PEMFC stack, thus constituting a fundamental step towards a CAE-based engineering of fuel cells.


2021 - A critical review of flow field analysis methods involving proper orthogonal decomposition and quadruple proper orthogonal decomposition for internal combustion engines [Articolo su rivista]
Rulli, Federico; Fontanesi, Stefano; D’Adamo, Alessandro; Berni, Fabio
abstract

Experimental techniques like particle image velocimetry provide a powerful technical support for the analysis of the spatial and temporal evolution of the flow field in internal combustion engines. Such techniques can be used to investigate both ensemble-averaged flow structures and their cyclic variations. These last are among the major causes of cycle-to-cycle variability of the engine processes (mixture formation, combustion, heat transfer, emission formation), the reduction of which has become a paradigm recently in engine development. Proper orthogonal decomposition has been largely used in conjunction with particle image velocimetry to analyze flow field characteristics. Several methods involving proper orthogonal decomposition have been proposed in the recent years to analyze engine cycle-to-cycle variability. In this work, phase-invariant proper orthogonal decomposition analysis, conditional averaging and triple and quadruple proper orthogonal decomposition methods are first introduced and applied to a large database of particle image velocimetry data from a well-known research engine. Results are discussed with particular emphasis on the capability of the methods to perform both quantitative and qualitative evaluations on cycle-to-cycle variability. Second, a new quadruple proper orthogonal decomposition methodology is proposed and compared to those available in the literature. All the methods are found to be helpful to identify the turbulent structures responsible for cycle-to-cycle variability. They can be equally applied to both experimental and numerical datasets to analyze turbulent fields in detail and to make comparisons.


2021 - A Data-Driven Methodology for the Simulation of Turbulent Flame Speed across Engine-Relevant Combustion Regimes [Articolo su rivista]
D’Adamo, Alessandro; Iacovano, Clara; Fontanesi, Stefano
abstract


2021 - A Simple CFD Model for Knocking Cylinder Pressure Data Interpretation: Part 1 [Relazione in Atti di Convegno]
Corrigan, D. J.; Breda, S.; Fontanesi, S.
abstract

Knock is one of the main limitations on Spark-Ignited (SI) Internal Combustion Engine (ICE) performance and efficiency and so has been the object of study for over one hundred years. Great strides have been made in terms of understanding in that time, but certain rather elementary practical problems remain. One of these is how to interpret if a running engine is knocking and how likely this is to result in damage. Knocking in a development environment is typically quantified based on numerical descriptions of the high frequency content of a cylinder pressure signal. Certain key frequencies are observed, which Draper [1] explained with fundamental acoustic theory back in 1935. Since then, a number of approaches of varying complexity have been employed to correlate what is happening within the chamber with what is measured by a pressure transducer. Whilst such phenomena can be well described by 3D Computational Fluid Dynamics (CFD) with moving meshes, small time-steps and chemical kinetics, such an approach is computationally intensive. Analytical calculations or Finite Element Methods (FEM) on the other hand, can estimate modal frequencies but not their likelihood of occurrence. In the present work, a simple stationary 3D CFD model, taking inspiration from an experiment by Draper [1] in 1934, is implemented in STAR CCM+ software. One or more autoignition events are simulated, and the corresponding frequency spectra and modal pressure distributions are described. It is shown that the model can reproduce the expected knocking frequencies from numerical analysis and experimental data. Sensitivity to autoignition and pressure transducer location is commented upon. Time Frequency Analysis (TFA) is applied to moving mesh data and demonstrates that little accuracy is lost in considering the stationary case. The current model is considered to be an appropriate means for analysis of knocking cycles with trace and moderate intensity, and can be used to bridge the gap between what is measured by a pressure transducer and what is occurring in the combustion chamber.


2021 - A wall-adapted zonal URANS/LES methodology for the scale-resolving simulation of engine flows [Articolo su rivista]
Iacovano, C.; d'Adamo, A.; Fontanesi, S.; Di Ilio, G.; Krastev, V. K.
abstract

In the present paper, a comprehensive, wall-adapted zonal URANS/LES methodology is shown for the multidimensional simulation of modern direct-injection engines. This work is the latest update of a zonal hybrid turbulence modeling approach, specifically developed by the authors for a flexible description of in-cylinder turbulent flow features with an optimal balance between computational costs and accuracy. Compared to the previous developments, a specific near-wall treatment is added, in order to preserve full-URANS behavior in the first near-wall cells, having in mind typically available mesh resolution in this part of the fluid domain. The updated methodology is applied to the multi-cycle simulation of a reference single-cylinder optical engine, which features a twin-cam, overhead-valve pent-roof cylinder head, and is representative of the current generation of spark-ignited direct-injection thermal power units. Results based on phase-specific flow field statistics and synthetic quality indices demonstrate the consistency and effectiveness of the proposed methodology, which is then qualified as a suitable candidate for affordable scale-resolving analyses of cycle to cycle variability (CCV) phenomena in direct-injection engines.


2021 - CFD modelling of a hydrogen/air PEM fuel cell with a serpentine gas distributor [Articolo su rivista]
D'Adamo, A.; Riccardi, M.; Borghi, M.; Fontanesi, S.
abstract

Hydrogen-fueled fuel cells are considered one of the key strategies to tackle the achievement of fully-sustainable mobility. The transportation sector is paying significant attention to the development and industrialization of proton exchange membrane fuel cells (PEMFC) to be introduced alongside batteries, reaching the goal of complete de-carbonization. In this paper a multi-phase, multi-component, and non-isothermal 3D-CFD model is presented to simulate the fluid, heat, and charge transport processes developing inside a hydrogen/air PEMFC with a serpentine-type gas distributor. Model results are compared against experimental data in terms of polarization and power density curves, including an improved formulation of exchange current density at the cathode catalyst layer, improving the simulation results’ accuracy in the activation-dominated region. Then, 3D-CFD fields of reactants’ delivery to the active electrochemical surface, reaction rates, temperature distributions, and liquid water formation are analyzed, and critical aspects of the current design are commented, i.e., the inhomogeneous use of the active surface for reactions, limiting the produced current and inducing gradients in thermal and reaction rate distribution. The study shows how a complete multi-dimensional framework for physical and chemical processes of PEMFC can be used to understand limiting processes and to guide future development.


2021 - Combustion modelling of turbulent jet ignition in a divided combustion chamber [Articolo su rivista]
Olcuire, M.; Iacovano, C.; D'Adamo, A.; Breda, S.; Lucchini, T.; Fontanesi, S.
abstract

Turbulent jet ignition is seen as one of the most promising strategies to achieve stable lean-burn operation in modern spark-ignition engines thanks to its ability to promote rapid combustion. A nearly stoichiometric mixture is ignited in a small-volume pre-chamber, following which multiple hot turbulent jets are discharged in the main chamber to initiate combustion. In the present work, a detailed computational investigation on the turbulent combustion regime of premixed rich propane/air mixture in a quiescent divided chamber vessel is carried out, to study the characteristics of the jet flame without the uncertainties in mixing and turbulent conditions typical of real-engine operations. In particular, the paper investigates the dependency of flame propagation on nozzle diameter (4, 6, 8, 12 and 14 mm) and pre-chamber/main-chamber volume ratio (10% and 20%); CFD results are compared to the experimental outcomes. Results show that the combustion regime in the quiescent pre-chamber follows a well-stirred reaction mode, rendering the limitation in using conventional flamelet combustion models. Furthermore, due to the very high turbulence levels generated by the outflowing reacting jets, also the main chamber combustion develops in a well-stirred reactor type, confirming the need for a kinetics-based approach to combustion modelling. However, the picture is complicated by thickened flamelet conditions possibly being verified for some geometrical variations (nozzle diameter and pre-chamber volume). The results show a general good alignment with the experimental data in terms of both jet phasing and combustion duration, offering a renewed guideline for combustion simulations under quiescent and low Damköhler number conditions.


2021 - Computational Fluid Dynamics (CFD) Analysis of Lubricant Oil Tank Sloshing of a High-Performance Car under Racetrack Maneuvers [Articolo su rivista]
Fontanesi, S.; Olcuire, M.; Cicalese, G.; Lamberti, L.; Pulvirenti, F.; Berni, F.
abstract

The paper proposes a methodology to perform sloshing analyses through multidimensional Computational Fluid Dynamics (CFD), with particular focus on a lubricant tank of a high-performance sports car. Lubricant tanks are usually fed by a mixture of oil and air, which makes Volume of Fluid (VoF) models unsuitable for this kind of simulation. Hence, a different approach based on a Eulerian MultiPhase (EMP) model is investigated and adopted. In contrast to the VoF approach, which is the most consolidated technique to handle the numerical analysis of sloshing problems, the EMP accounts for interactions between liquid and gaseous phases, such as mixing and separation. It also reduces numerical constraints on time-step and mesh size. EMP is therefore applied to the analysis of a sports car lubricant tank where mist and foam formation and subsequent phase separation are of primary importance. Comparison between the EMP and VoF approach is performed on cases of increasing complexity. Firstly, a rectangular tank with internal baffles and under pitch oscillations, for which experimental measurements are available, is analyzed. The EMP approach shows improved responsiveness in representing both phase mixing and separation. Secondly, a current production lubricant oil tank, for which experimental test-rig measurements of foam percentage shortly upstream the feeding pump are available, confirms the ability of the EMP approach to quantitatively estimate foam formation. Thirdly, the analysis of a current production lubricant oil tank subject to typical racetrack maneuvers is performed. Such final step confirms the ability of the EMP approach to simulate complex interactions between the phases, which largely affect tank and lubricating circuit performance in high-performance sports car applications. Moreover, the EMP approach allows a massive reduction of computational time compared to VoF.


2021 - Impact of Grid Density and Turbulence Model on the Simulation of In-Cylinder Turbulent Flow Structures - Application to the Darmstadt Engine [Relazione in Atti di Convegno]
Barbato, A.; Fontanesi, S.; D'Adamo, A.
abstract

The paper reports a wide numerical analysis of the well-known "Darmstadt engine"operated under motored condition. The engine, which features multiple optical accesses and is representative of currently made four-valve pentroof GDI production engines, is simulated using computational grids of increasing density and two widely adopted approaches to model turbulence, Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES). In the first part of the paper, attention is focused on the increase of grid density within the RANS modelling framework: both bulk-flow grid density and near-wall grid density are varied in order to analyse potentials and limitations of the different grid strategies and evaluate the trade-off between accuracy and computational cost. In the second part of the paper, an analysis is made, on equal grid density, to compare RANS and LES modelling frameworks and to highlight the improved capability of the latter in representing the spatial and temporal evolution of the in-cylinder turbulent flow structures. Results from the CFD analyses are compared with detailed PIV measurements on different planes and at different crank angle positions; furthermore, quantitative comparisons are performed using well-established quality indices.


2021 - Knock: A Century of Research [Articolo su rivista]
Corrigan, D. J.; Fontanesi, S.
abstract

Knock is one of the main limitations on increasing spark-ignition (SI) engine efficiency. This has been known for at least 100 years, and it is still the case today. Knock occurs when conditions ahead of the flame front in an SI engine result in one or more autoignition events in the end gas. The autoignition reaction rate is typically much higher than that of the flame-front propagation. This may lead to the creation of pressure waves in the combustion chamber and, hence, an undesirable noise that gives knock its name. The resulting increased mechanical and thermal loading on engine components may eventually lead to engine failure. Reducing the compression ratio lowers end-gas temperatures and pressures, reducing end-gas reactivity and, hence, mitigating knock. However, this has a detrimental effect on engine efficiency. Automotive companies must significantly reduce their fleet carbon dioxide (CO2) values in the coming years to meet targets resulting from the 2015 Paris Agreement. One path towards meeting these is through partial or full electrification of the powertrain. However, the vast majority of automobiles in the near future will still feature a gasoline-fueled SI engine; hence, improvements in combustion engine efficiency remain fundamental. As knock has been a key limitation for so long, there is a huge amount of literature on the subject. A number of reviews on knock have already been published, including in recent years. These generally concentrate on current understanding and status. The present work, in contrast, aims to track the progress of research on knock from the 1920s right through to the present day. It is hoped that this can be a useful reference for new and existing researchers of the subject and give further weight to occasionally neglected historical activity, which can still provide important insights today.


2021 - Modelling methods and validation techniques for CFD simulations of PEM fuel cells [Articolo su rivista]
D'Adamo, A.; Haslinger, M.; Corda, G.; Hoflinger, J.; Fontanesi, S.; Lauer, T.
abstract

The large-scale adoption of fuel cells system for sustainable power generation will require the combined use of both multidimensional models and of dedicated testing techniques, in order to evolve the current technology beyond its present status. This requires an unprecedented understanding of concurrent and interacting fluid dynamics, material and electrochemical processes. In this review article, Polymer Electrolyte Membrane Fuel Cells (PEMFC) are analysed. In the first part, the most common approaches for multi-phase/multi-physics modelling are presented in their governing equations, inherent limitations and accurate materials characterisation for diffusion layers, membrane and catalyst layers. This provides a thorough overview of key aspects to be included in multidimensional CFD models. In the second part, advanced diagnostic techniques are surveyed, indicating testing practices to accurately characterise the cell operation. These can be used to validate models, complementing the conventional observation of the current-voltage curve with key operating parameters, thus defining a joint modelling/testing environment. The two sections complement each other in portraying a unified framework of interrelated physical/chemical processes, laying the foundation of a robust and complete understanding of PEMFC. This is needed to advance the current technology and to consciously use the ever-growing availability of computational resources in the next future.


2021 - Potentials of the Oversizing and H2-Supported Lean Combustion of a VVA SI Gasoline Engine towards Efficiency Improvement [Relazione in Atti di Convegno]
Bozza, F.; Berni, F.; Cicci, F.; D'Adamo, A.; De Bellis, V.; Fontanesi, S.; Malfi, E.; Pessina, V.; Teodosio, L.
abstract

In recent years, internal combustion engine (ICE) downsizing coupled with turbocharging was considered the most effective path to improve engine efficiency at low load, without penalizing rated power/torque performance at full load. On the other side, issues related to knocking combustion and excessive exhaust gas temperatures obliged adopting countermeasures that highly affect the efficiency, such as fuel enrichment and delayed combustion. Powertrain electrification allows operating the ICE mostly at medium/high loads, shifting design needs and constraints towards targeting high efficiency under those operating conditions. Conversely, engine efficiency at low loads becomes a less important issue. In this track, the aim of this work is the investigation of the potential of the oversizing of a small Variable Valve ActuationSpark Ignition gasoline engine towards efficiency increase and tailpipe emission reduction. To enhance the potential improvements of such an approach, a lean combustion concept is adopted, where the flame speed propagation is supported by doping gasoline with the addition of a percentage of hydrogen (10% by mass). The analysis is carried out by a 1D simulation tool, widely validated for the base engine supplied with pure gasoline and under stoichiometric/rich combustion. The combustion and knock models are here extended to handle the flame speed and auto-ignition characteristics of gasoline/H2 blends. The comparison between the base gasoline engine and the oversized gasoline/H2 variant highlights significant efficiency advantages at full load operations, which are due to the possibility to remove fuel enrichment and combustion delays. Exceptfor unburned hydrocarbons, pollutants and CO2 emissionsare reducedthanks to the synergic effects of H2addition and ultra-lean mixtures. A certain penalization of efficiency arises at very low loads, where engine oversizingdegrades the combustion process.


2021 - Quantitative investigation on the impact of injection timing on soot formation in a GDI engine with a customized sectional method [Articolo su rivista]
Fontanesi, S.; Del Pecchia, M.; Pessina, V.; Sparacino, S.; Di Iorio, S.
abstract

Soot engine-out emissions are no longer a prerogative of Diesel engines. Emission regulations related to Gasoline units aim to curb the soot emissions along with other pollutants. In this scenario, Computational Fluid Dynamics (CFD) is a very promising research and development tool to explore the influence of engine design and operational parameters, as well as of the fuel chemical nature, on the particulate matter formation. Among the soot models, the Sectional Method is an advanced resource to provide information on Particle Number, Particulate Mass and Particle Size Distribution. In this study, the Sectional Method is applied in conjunction with a customized soot library, where the source terms governing the soot sections transport equations are stored. The library is computed via chemical kinetics simulation of a 0D constant pressure reactor, which provides fuel-related coefficients for each individual source term over the entire range of conditions experienced by the 3D-CFD model. 3D-CFD simulations are then carried out for three different injection timings without case-by-case tuning. Numerical results are then compared to the experimental dataset by using a consistent methodology. A satisfactory agreement between 3D-CFD results and experimental measurements is reached for soot mass and particle numbers, while the particle size distribution function is only partially reproduced. Soot-related quantities are thoroughly analyzed for each of the examined injection strategies to understand the mechanisms leading to soot formation and emissions.


2021 - Towards grid-independent 3D-CFD wall-function-based heat transfer models for complex industrial flows with focus on in-cylinder simulations [Articolo su rivista]
Berni, F.; Cicalese, G.; Borghi, M.; Fontanesi, S.
abstract

Convective heat transfer heavily affects both efficiency and reliability in many industrial problems. For this reason, its proper estimation is mandatory since the early design stage. 3D-CFD simulations represent a powerful tool for the prediction of the heat fluxes. This is even more true considering that typical operating conditions of many applications prevent experimental characterization. As for 3D-CFD computations, the combination of Reynolds Averaged Navier Stokes (RANS) turbulence modeling and high-Reynolds wall treatment is still widely diffused in the industrial practice, to save both computational cost and time. The adoption of a high-Reynolds wall treatment based on wall functions, which permits the use of relatively coarse near-wall grids, implies specific restrictions for the height of the near-wall cell layer. In particular, the first cell-centroid must be placed in the fully turbulent (log-) region of the boundary layer. The main drawback of a cell-centroid falling into the viscous sub-layer consists in a huge overestimation of both wall shear stress and wall heat transfer. The lower the y+ is (i.e. the lower the wall distance is), the higher the predicted values are. As for many other industrial applications, Internal Combustion Engine (ICE) in-cylinder simulations remarkably suffer from the presence of low y+ values in the computational domain, mostly at part-loads and low-revving speeds. At specific operating points, such as idle conditions, it is nearly impossible to maintain y+ in the log-region, even during the compression stroke, when the velocity field should allow the dimensionless distance to reach the highest values in the engine cycle. To avoid such undesired overestimations of shear stress and heat transfer, a modified formulation of the thermal law of the wall (T+) to be used in the viscous sub-layer is proposed in the present paper. To further reduce the grid-dependency of the high-Reynolds wall treatment, a similar modification is applied to the velocity wall function (u+). Resulting wall heat flux and wall shear stress are shown to be grid-independent, at least for y+>3. The proposed alternative modeling for u+ inside the viscous sub-layer is validated in terms of flow field against experimental Laser-Doppler Anemometry (LDA) data and Direct Numerical Simulation (DNS) results. Despite the present analysis focuses on in-cylinder simulations, the alternative u+ and T+ formulations can be applied to any complex flow. Furthermore, the proposed modified laws of the wall can be adopted in conjunction with any wall-function-based heat transfer model.


2021 - Validation of a les Spark-Ignition Model (GLIM) for Highly-Diluted Mixtures in a Closed Volume Combustion Vessel [Relazione in Atti di Convegno]
Iacovano, C.; Zeng, Y.; Anbarasu, M.; Fontanesi, S.; D'Adamo, A.
abstract

The establishment of highly-diluted combustion strategies is one of the major challenges that the next generation of sustainable internal combustion engines must face. The desirable use of high EGR rates and of lean mixtures clashes with the tolerable combustion stability. To this aim, the development of numerical models able to reproduce the degree of combustion variability is crucial to allow the virtual exploration and optimization of a wide number of innovative combustion strategies. In this study ignition experiments using a conventional coil system are carried out in a closed volume combustion vessel with side-oriented flow generated by a speed-controlled fan. Acquisitions for four combinations of premixed propane/air mixture quality (φ=0.9,1.2), dilution rate (20%-30%) and lateral flow velocity (1-5 m/s) are used to assess the modelling capabilities of a newly developed spark-ignition model for large-eddy simulation (GLIM, GruMo-UniMORE LES Ignition Model). The model accounts for all the main physical phenomena governing flame kernel growth, including electric circuit and over-adiabatic thermal expansion, which are included in the LES formalism of ECFM-LES combustion model. In the first part of the study the agreement of the simulation results against measurements is carried out to assess a validated non-reacting condition and converged flow statistics. Then, combustion simulations are carried out, and ignition events are repeated at random timings to replicate flow variability at ignition. Finally, optical comparison is carried out for simulated enflamed volume against high-frequency Schlieren images for all the cases, and measurements of flame radius growth are presented. The agreement of the quantitative flame measurements, as well as the qualitative resemblance of flame development, indicate the use of the presented GLIM ignition model as a valuable model for multi-cycle engine simulations, with particular relevance on unstable and CCV-affected conditions


2020 - A methodology to formulate multicomponent fuel surrogates to model flame propagation and ignition delay [Articolo su rivista]
Del Pecchia, M.; Fontanesi, S.
abstract

Nowadays, the leading driver for the development of internal combustion engines is the search for increased fuel efficiency and reduced emissions. To develop and optimize new combustion systems, a high degree of accuracy is needed in 3D-CFD simulations. In particular, detailed chemical kinetics models and fuel surrogates able to represent the main physical and chemical properties of the real commercial fuels are needed. A series of fuel surrogate formulation methodologies were presented in the recent past with the aim of matching fuel properties and combustion-related characteristics using a set of well-known compounds. While most of the gasoline-targeted available studies mainly focused on the possibility to simultaneously match the gross fuel properties, the evaporating characteristics and the auto-ignition behaviour of the fuel, none of them explicitly targeted the flame propagation characteristics. In this work, a novel methodology is introduced to formulate gasoline fuel surrogates able to match the main chemical and physical properties, the auto-ignition and the flame propagation characteristics of a commercial gasoline. Due to the increasing presence of oxygenated fuels in the market share, an average gasoline fuel named ULG95, representative of a European oxygenated gasoline with Research Octane Number RON = 95, is targeted to validate the presented methodology. Three fuel surrogates of increasing complexity are formulated and validated against laminar flame speed, shock-tube and rapid compression machine experiments available in literature for oxygenated gasolines. The results suggest that a unique gasoline fuel surrogate can be used, together with validated chemical kinetics mechanisms, to model auto-ignition and flame propagation characteristics.


2020 - A Preliminary 1D-3D Analysis of the Darmstadt Research Engine under Motored Condition [Relazione in Atti di Convegno]
Iacovano, C.; Berni, F.; Barbato, A.; Fontanesi, S.
abstract

In the present paper, 1D and 3D CFD models of the Darmstadt research engine undergo a preliminary validation against the available experimental dataset at motored condition. The Darmstadt engine is a single-cylinder optical research unit and the chosen operating point is characterized by a revving speed equal to 800 rpm with intake temperature and pressure of 24 °C and 0.95 bar, respectively. Experimental data are available from the TU Darmstadt engine research group. Several aspects of the engine are analyzed, such as crevice modeling, blow-by, heat transfer and compression ratio, with the aim to minimize numerical uncertainties. On the one hand, a GT-Power model of the engine is used to investigate the impact of blow-by and crevices modeling during compression and expansion strokes. Moreover, it provides boundary conditions for the following 3D CFD simulations. On the other hand, the latter, carried out in a RANS framework with both highand low-Reynolds wall treatments, allow a deeper investigation of the boundary layer phenomena and, thus, of the gas-to-wall heat transfer. A detailed modeling of the crevice, along with an ad hoc tuning of both blow-by and heat fluxes lead to a remarkable improvement of the results. However, in order to adequately match the experimental mean in-cylinder pressure, a slight modification of the compression ratio from the nominal value is accounted for, based on the uncertainty which usually characterizes such geometrical parameter. The present preliminary study aims at providing reliable numerical setups for 1D and 3D models to be adopted in future detailed investigations on the Darmstadt research engine.


2020 - A threshold soot index-based fuel surrogate formulation methodology to mimic sooting tendency of real fuels in 3D-CFD simulations [Articolo su rivista]
Del Pecchia, M.; Fontanesi, S.; Prager, J.; Kralj, C.; Lehtiniemi, H.
abstract

Particulate Matter emission is an increasing concern for engine manufacturers due to the strong limits imposed by worldwide regulations. Fuel composition plays a key role in determining the extent to which soot is formed during the combustion process. The availability of advanced multidimensional computational fluid-dynamics soot models incorporating soot chemistry pushed researchers to formulate fuel surrogates able to represent sooting tendency of real fuels in the numerical framework. Such studies, which provide information to target research grade fuels, are scarcely present in literature. A methodology is proposed to estimate sooting tendency of commercial gasolines, based on Threshold Soot Index and basic composition information, as well as to formulate tailored ethanol-toluene reference fuel surrogates. The technique relies on a purely mathematical approach to estimate Threshold Soot Index of individual compounds and blends using a structural group contribution-based approach, available in literature, which allows to adapt fuel surrogate palette as needed. Firstly, this approach is demonstrated by means of constant pressure reactor simulations using the Method of Moments. Secondly, a methodology is proposed to formulate fuel surrogates simultaneously targeting the main chemical and physical auto-ignition characteristics and estimated Threshold Soot Index. Several oxygenated and non-oxygenated commercial gasolines available on the market are targeted to provide a wide number of validation cases. Finally, surrogates are used to generate a selection of constant pressure-based soot libraries tested in conjunction with the Sectional Method on a 3D-CFD model of a single-cylinder optically accessible gasoline engine. Surrogates exhibit the same qualitative ranking estimated based on Threshold Soot Index and retain a quantitative scaling in terms of particulate matter formation.


2020 - A 3D-CFD methodology to investigate boundary layers and assess the applicability of wall functions in actual industrial problems: A focus on in-cylinder simulations [Articolo su rivista]
Berni, F.; Fontanesi, S.
abstract

In the industrial practice, 3D-CFD in-cylinder simulations still largely rely on RANS turbulence models and high-Reynolds wall treatments, i.e. based on wall functions. However, the use of the latter represents a potential source of error, leading to poor estimations of shear stress and heat flux at the wall. In fact, universal laws of the wall can be claimed only under very restricted conditions, which are hardly (to say never) met in industrial applications. As a result, typical dimensionless profiles of velocity and temperature on the combustion chamber walls are far from standard wall functions. In the present paper, a methodology to investigate the presence of dimensionless profiles comparable to universal wall laws in boundary layers of actual industrial problems is presented. In particular, attention is focused on 3D-CFD in-cylinder simulations. While the existing literature deals with DNS or hybrid URANS/LES approaches applied to simplified geometries and low revving speed conditions (for computational cost reasons), in the present paper a RANS k-ε turbulence model with a low-Reynolds wall treatment is adopted. In addition, an alternative strategy to extract velocity and temperature dimensionless profiles from the computed fields is proposed. The methodology is preliminary tested on a 2D plane channel (where the existence of wall functions is a priori acknowledged), at both quasi-isothermal and highly non-isothermal conditions. Afterwards, it is applied to the well-known “GM Pancake” engine test case, showing that both u+ and T+ calculated on the combustion chamber walls remarkably differ from analytical standard wall functions. Finally, in order to demonstrate the importance of dimensionless profiles to properly predict heat transfer, two different high-Reynolds simulations of the “GM Pancake” engine are proposed, one with standard wall functions and one with u+ and T+ profiles provided by the low-Reynolds analysis. While the former underestimates heat fluxes, the latter provides results in good agreement with the experiments.


2020 - Development of a Sectional Soot Model Based Methodology for the Prediction of Soot Engine-Out Emissions in GDI Units [Relazione in Atti di Convegno]
Del Pecchia, M.; Sparacino, S.; Pessina, V.; Fontanesi, S.; Breda, S.; Irimescu, A.; Di Iorio, S.
abstract

With the aim of identifying technical solutions to lower the particulate matter emissions, the engine research community made a consistent effort to investigate the root causes leading to soot formation. Nowadays, the computational power increase allows the use of advanced soot emissions models in 3D-CFD turbulent reacting flows simulations. However, the adaptation of soot models originally developed for Diesel applications to gasoline direct injection engines is still an ongoing process. A limited number of studies in literature attempted to model soot produced by gasoline direct injection engines, obtaining a qualitative agreement with the experiments. To the authors' best knowledge, none of the previous studies provided a methodology to quantitatively match particulate matter, particulate number and particle size distribution function measured at the exhaust without a case-by-case soot model tuning. In the present study, a Sectional Method-based methodology to quantitatively predict gasoline direct injection soot formation is presented and validated against engine-out emissions measured on a single-cylinder optically accessible gasoline direct injection research engine. While adapting the model to the gasoline direct injection soot framework, attention is devoted to modelling the dependence of the processes involved in soot formation on soot precursors chemistry. A well-validated chemical kinetics mechanism is chosen to accurately predict soot precursors formation pathways retaining an accurate description of the main oxidation pathways for oxygenated fuel surrogates. To account for the prominent premixed combustion mode characterizing modern GDI units, a constant pressure reactor library is generated containing the rates for the chemistry-based processes involved in soot formation and evolution at engine-like conditions. The proposed methodology is successfully applied to a 3D computational fluid dynamics model of the engine to predict soot engine-out emissions at the exhaust.


2020 - Evaluation of hole-specific injection rate based on momentum flux measurement in GDI systems [Articolo su rivista]
Cavicchi, A.; Postrioti, L.; Berni, F.; Fontanesi, S.; Di Gioia, R.
abstract

A novel methodology for the estimation of the mass flow rate delivered by each hole of a multi-hole GDI nozzle is presented and discussed in this paper. The proposed method is based on the measurement of the impact force of each jet and it is able to evaluate the individual hole injection rate and injected mass using the relationship between the spray momentum flux and the mass flow rate. Three different nozzles are tested, one featuring equal hole diameters and two with differentiated hole-to-hole dimensions. Firstly, results are validated in terms of total injection rate, comparing the sum of the individual hole flow rates with the signal from a Zeuch-based flow meter. Secondly, outcomes are compared with the direct measurement of the injected quantity by means of a special device able to collect and weight the fuel delivered from each hole. Results evidence an excellent agreement in terms of mass flow rate dynamic profile as the proposed method is able to detect both opening and closure transients and the static flow rate. The proposed method proved to be able to capture the dynamic mass flow rate in transient conditions, i.e. very short injections, overcoming a limitation of the methodology proposed in a previous study. Moreover, in terms of injected mass, the results show a percentage error lower than 5% for medium and long energizing times and a maximum error of 9.5% for short injections in ballistic operating conditions.


2020 - Experimental Validation of a 3D-CFD Model of a PEM Fuel Cell [Relazione in Atti di Convegno]
Riccardi, M.; D'Adamo, A.; Vaini, A.; Romagnoli, M.; Borghi, M.; Fontanesi, S.
abstract

The growing energy demand is inevitably accompanied by a strong increase in greenhouse gas emissions, primarily carbon dioxide. The adoption of new energy vectors is therefore seen as the most promising countermeasure. In this context, hydrogen is an extremely interesting energy carrier, since it can be used as a fuel in both conventional energy systems (internal combustion engines, turbines) and in Fuel Cells (FC). In particular, PEM (Polymeric Electrolyte Membrane) FC are given growing attention in the transportation sector as a Life-Cycle viable solution to sustainable mobility. The use of 3D CFD analysis of for the development of efficient FC architectures is extremely interesting since it can provide a fast development tool for design exploration and optimization. The designer can therefore take advantage of a robust and accurate modelling in order to define and develop fuel cell systems in a more time-efficient and cost-efficient way, to optimize their performance and to lower their production costs. So far, studies available in the scientific literature lack of quantitative validation of the CFD simulations of complete PEM fuel cells against experimental evidence. The proposed study presents a quantitative validation of a multi-physics model of a Clearpak PEM cell. The chemistry and physics implemented in the methodology allow the authors to obtain both thermal and electrical results, characterizing the performance of each component of the PEM. The results obtained, compared with the experimental polarization curve, show that the model is not only numerically stable and robust in terms of boundary conditions, but also capable to accurately characterize the performance of the PEM cell over almost its entire polarization range.


2020 - Gasoline-ethanol blend formulation to mimic laminar flame speed and auto-ignition quality in automotive engines [Articolo su rivista]
Del Pecchia, M.; Pessina, V.; Berni, F.; D'Adamo, A.; Fontanesi, S.
abstract

Several environment agencies worldwide have identified biofuels as a viable solution to meet the stringent targets imposed by future regulations in terms of on-road transport emissions. In the last decades, petroleum-based gasoline has been increasingly blended with oxygenated fuels, mostly ethanol. Blending ethanol with gasoline has two major effects: an increase of the octane number, thus promoting new scenarios for engine efficiency optimization, and a potential reduction of soot emissions. 3D-CFD simulations represent a powerful tool to optimize the use of ethanol-gasoline blends in internal combustion engines. Since most of the combustion models implemented in 3D-CFD codes are based on the “flamelet assumption”, they require laminar flame speed as an input. Therefore, a thorough understanding of the gasoline-ethanol blend chemical behavior at engine-relevant conditions is crucial. While several laminar flame speed correlations are available in literature for both gasoline and pure ethanol at ambient conditions, none is available, to the extent of authors’ knowledge, to describe laminar flame speed of gasoline-ethanol blends (for different ethanol volume contents) at engine relevant conditions. For this reason, in the present work, laminar flame speed correlations based on 1D detailed chemical kinetics calculations are derived targeting typical full-load engine-like conditions, for different ethanol-gasoline blends. A methodology providing a surrogate able to match crucial properties of a fuel is presented at first and validated against available experimental data. Then, laminar flame speed correlations obtained from 1D chemical kinetics simulations are proposed for each fuel blend surrogate.


2020 - Large eddy simulation analysis of the turbulent flow in an optically accessible internal combustion engine using the overset mesh technique [Articolo su rivista]
Rulli, F.; Barbato, A.; Fontanesi, S.; d'Adamo, A.
abstract

Computational fluid dynamics has become a fundamental tool for the design and development of internal combustion engines. The meshing strategy plays a central role in the computational efficiency, in the management of the moving components of the engine and in the accuracy of results. The overset mesh approach, usually referred to also as chimera grid or composite grid, was rarely applied to the simulation of internal combustion engines, mainly because of the difficulty in adapting the technique to the specific complexities of internal combustion engine flows. The article demonstrates the feasibility and the effectiveness of the overset mesh technique application to internal combustion engines, thanks to a purposely designed meshing approach. In particular, the technique is used to analyze the cycle-to-cycle variability of internal combustion engine flows using large eddy simulation. Fifty large eddy simulation cycles are performed on the well-known TCC-III engine in motored condition. Results are analyzed in terms of tumble center trajectory and using proper orthogonal decomposition to objectively characterize the spatial and temporal evolution of turbulent flow field in internal combustion engines. In particular, an original decomposition method previously applied by the authors to the TCC-III measured flow fields is here extended to computational fluid dynamics results.


2020 - Large-Eddy simulation of lean and ultra-lean combustion using advanced ignition modelling in a transparent combustion chamber engine [Articolo su rivista]
D'Adamo, A.; Iacovano, C.; Fontanesi, S.
abstract

The need for Internal Combustion Engines (ICEs) to face near-future challenges of higher efficiency and reduced emissions is leading to a renewed interest towards lean-combustion. Several operational issues are associated to lean combustion, such as an abrupt increase of combustion cycle-by-cycle variability (CCV), leading to unbearable levels of Indicated Mean Effective Pressure (IMEP) variation and to misfiring cycles. Significant potential in the wide-scale establishment of lean combustion might come from Large Eddy Simulation (LES), which is able to elucidate the relationships between local physical processes (e.g. velocity magnitude, Air to Fuel Ratio (AFR), etc.) and early combustion progress (e.g. 1%) in unprecedented manners. To this aim, an improved ignition model for LES is proposed in the paper. Two premixed propane-air lean strategies are selected from the wide TCC-III database. A lean-stable (λ=1.10, also named lean) and lean-unstable (λ=1.43, also named ultra-lean) conditions are simulated, highlighting the model capability to well reproduce the sudden rise in CCV for increased mixture dilution. Explanations are given for the observed behaviour and a hierarchical quest for CCV dominant factors is presented. Finally, the different role of local flow field is highlighted for the two cases, and the comparison of optical acquisitions of OH* emission against simulated flame iso-surface up to 1% burn duration reinforce the simulation fidelity. The study shows the investigation possibilities of innovative combustion strategies given by advanced LES simulations. The understanding of turbulent combustion dynamics and the knowledge of the related lean-burn instabilities are key enabler for the exploration of new efficient lean-burn operations.


2020 - Methodology for the large-eddy simulation and particle image velocimetry analysis of large-scale flow structures on TCC-III engine under motored condition [Articolo su rivista]
Ko, I.; Rulli, F.; Fontanesi, S.; D'Adamo, A.; Min, K.
abstract

Large-eddy simulation has been increasingly applied to internal combustion engine flows because of their improved potential to capture the spatial and temporal evolution of turbulent flow structures compared with Reynolds-averaged Navier Stokes simulation. Furthermore, large-eddy simulation is universally recognized as capable of simulating highly unsteady and random phenomena, which drive cycle-to-cycle variability and cycle-resolved events such as knocks and misfires. To identify large-scale structure fluctuations, many methods have been proposed in the literature. This article describes the application of several analysis methods for the comparison between different datasets (experimental or numerical) and the identification of large-structure fluctuations. The reference engine is the well-known TCC-III single-cylinder optical unit from the University of Michigan and GM Global R&D center; the analyses were carried out under motored engine conditions. A deep analysis of in-cylinder gas dynamics and flow structure evolution was performed by comparing the experimental results (particle image velocimetry of the velocity fields) with a dataset of consecutive large-eddy simulation cycles on four different cutting planes at engine-relevant crank angle positions. Phase-dependent proper orthogonal decomposition was used to obtain further conclusions regarding the accuracy of the simulation results and to apply conditional averaging methods. A two-point correlation and an analysis of the tumble center are proposed. Finally, conclusions are drawn to be used as guidelines in future large-eddy simulation analyses of internal combustion engines.


2020 - Numerical Simulation of a High Current Density PEM Fuel Cell [Relazione in Atti di Convegno]
D'Adamo, A.; Riccardi, M.; Locci, C.; Romagnoli, M.; Fontanesi, S.
abstract

The ever-increasing quest for sustainable mobility is pushing the automotive sector towards electric-based technologies, allowing the reduction of localized emission sources in highly populated urban areas. Among the many possible solutions, Proton Exchange Membrane Fuel Cells (PEMFC) have the potential to de-carbonise the automotive sector without the range anxiety of present and future batteries. The interaction between physical and chemical processes in PEMFC is crucial to their maximum attainable efficiency, albeit the complexity of such interplay still limits a complete understanding of the governing processes. In this paper a canonical PEMFC from literature is simulated using 3D-CFD, and results are compared against experiments. A Eulerian multi-phase/multi-physics non-isothermal framework is used to account for both fluid (gas channels, porous gas diffusion layers) and solid (bi-polar plates, membrane), as well as for electrochemical and sorption reactions. The model is also able to account for the heat balance and for the liquid water formation at cathode catalyst layer, as well as to predict the water transport and the membrane hydration state fundamental for high-efficiency operation. Simulations are compared with measurements and polarization curves are analysed for two membrane thicknesses and rib/channel spacing. The study presents the investigation possibilities given by 3D-CFD in the field of PEMFC and how this can be used to design high efficiency fuel cells. The presented numeric analysis shows how the virtual design of high-efficiency PEMFC for the automotive sector can be guided by simulations amongst multiple degrees of freedom, thus aiding the development of innovative PEMFC.


2020 - Standard and consistent Detached-Eddy Simulation for turbulent engine flow modeling: An application to the TCC-III engine [Relazione in Atti di Convegno]
Krastev, V. K.; Di Ilio, G.; Iacovano, C.; D'Adamo, A.; Fontanesi, S.
abstract

Multidimensional modeling of Cycle-to-Cycle Variability (CCV) has become a crucial support for the development and optimization of modern direct-injection turbocharged engines. In that sense, the only viable modeling options is represented by scale-resolving approaches such as Large Eddy Simulation (LES) or hybrid URANS/LES methods. Among other hybrid approaches, Detached-Eddy Simulation (DES) has the longest development story and is therefore commonly regarded as the most reliable choice for engineering-grade simulation. As such, in the last decade DESbased methods have found their way through the engine modeling community, showing a good potential in describing turbulence-related CCV in realistic engine configurations and at reasonable computational costs. In the present work we investigate the in-cylinder modeling capabilites of a standard two-equation DES formulation, compared to a more recent one which we call DESx. The DESx form differs from standard DES in the turbulent viscosity switch from URANS to LES-like behavior, which for DESx is fully consistent with Yoshizawa's one-equation sub-grid scale model. The two formulations are part of a more general Zonal-DES (ZDES) methodology, developed and validated by the authors in a series of previous publications. Both variants are applied to the multi-cycle simulation of the TCC-III experimental engine setup, using sub-optimal grid refinement levels in order to stress the model limitations in URANS-like numerical resolution scenarios. Outcomes from this study show that, although both alternatives are able to ouperform URANS even in coarse grid arrangements, DESx emerges as sligthly superior and thus it can be recommended as the default option for in-cylinder flow simulation.


2020 - Validation of a zonal hybrid URANS/LES turbulence modeling method for multi-cycle engine flow simulation [Articolo su rivista]
Krastev, V. K.; D'Adamo, A.; Berni, F.; Fontanesi, S.
abstract

A zonal hybridization of the RNG (Formula presented.) - (Formula presented.) URANS model is proposed for the simulation of turbulent flows in internal combustion engines. The hybrid formulation is able to act as URANS, DES or LES in different zones of the computational domain, which are explicitly set by the user. The resulting model has been implemented in a commercial computational fluid dynamics code and the LES branch of the modified RNG (Formula presented.) - (Formula presented.) closure has been initially calibrated on a standard homogeneous turbulence box case. Subsequently, the full zonal formulation has been tested on a fixed intake valve geometry, including comparisons with third-party experimental data. The core of the work is represented by a multi-cycle analysis of the TCC-III experimental engine configuration, which has been compared with the experiments and with prior full-LES computational studies. The applicability of the hybrid turbulence model to internal combustion engine flows is demonstrated, and PIV-like flow statistics quantitatively validate the model performance. This study shows a pioneering application of zonal hybrid models in engine-relevant simulation campaigns, emphasizing the relevance of hybrid models for turbulent engine flows.


2019 - A comparison between different moving grid techniques for the analysis of the TCC engine under motored conditions [Relazione in Atti di Convegno]
Barbato, Alessio; Rulli, Federico; Fontanesi, Stefano; D'Adamo, Alessandro; Berni, Fabio; Cicalese, Giuseppe; Perrone, Antonella
abstract

The accurate representation of Internal Combustion Engine (ICE) flows via CFD is an extremely complex task: it strongly depends on a combination of highly impacting factors, such as grid resolution (both local and global), choice of the turbulence model, numeric schemes and mesh motion technique. A well-founded choice must be made in order to avoid excessive computational cost and numerical difficulties arising from the combination of fine computational grids, high-order numeric schemes and geometrical complexity typical of ICEs. The paper focuses on the comparison between different mesh motion technologies, namely layer addition and removal, morphing/remapping and overset grids. Different grid strategies for a chosen mesh motion technology are also discussed. The performance of each mesh technology and grid strategy is evaluated in terms of accuracy and computational efficiency (stability, scalability, robustness). In particular, a detailed comparison is presented against detailed PIV flow measurements of the well-known "TCC Engine III" (Transparent Combustion Chamber Engine III) available at the University of Michigan. Since many research groups are simultaneously working on the TCC engine using different CFD codes and meshing approaches, such engine constitutes a perfect playground for scientific cooperation between High-Level Institutions. A motored engine condition is chosen and the flow evolution throughout the engine cycle is evaluated on four different section planes. Pros and cons of each grid strategy as well as mesh motion technique are highlighted and motivated.


2019 - A refined OD turbulence model to predict tumble and turbulence in SI engines [Articolo su rivista]
Bozza, F.; Teodosio, L.; De Bellis, V.; Fontanesi, S.; Iorio, A.
abstract

In this work, the refinement of a phenomenological turbulence model developed in recent years by the authors is presented in detail. As known, reliable information about the underlying turbulence intensity is a mandatory prerequisite to predict the burning rate in phenomenological combustion models. The model is embedded under the form of “user routine” in the GT-Power™ software. The main advance of the proposed approach is the potential to describe the effects on the in-cylinder turbulence of some geometrical parameters, such as the intake runner orientation, the compression ratio, the bore-to-stroke ratio, and the valve number. The model is based on three balance equations, referring to the mean flow kinetic energy, the tumble vortex momentum, and the turbulent kinetic energy (3-eq. concept). An extended formulation is also proposed, which includes a fourth equation for the dissipation rate, allowing to forecast also the integral length scale (4-eq. concept). The model consistency is verified against 3D results under motored operations for various operating conditions and engine geometrical architectures. The temporal evolutions of the 0D-derived mean flow velocity, turbulence intensity, and tumble velocity present very good agreement with the 3D outcomes. The model exhibits the capability to accurately predict the tumble trends by varying some engine geometrical parameters. The proposed 0D model proves to correctly estimate the in-cylinder turbulence characteristics, without requiring any tuning adjustment with the engine speed and the valve strategy. In addition, it demonstrates the capability to properly take into account the intake duct orientation and the compression ratio without any case-dependent tuning. Some minor tunings are required to predict the effects of the bore-to-stroke ratio. The model also shows an adequate accuracy for a two-valve per cylinder engine and for two different high-performance engines.


2019 - CFD analysis and knock prediction into crevices of piston to liner fireland of an high performance ICE [Relazione in Atti di Convegno]
Rosetti, A.; Iotti, C.; Bedogni, A.; Cantore, G.; Fontanesi, S.; Berni, F.
abstract

The paper aims at defining a methodology for the prediction and understanding of knock tendency in internal combustion engine piston crevices by means of CFD simulations. The motivation for the analysis comes from a real design requirement which appeared during the development of a new high performance SI unit: It is in fact widely known that, in high performance engines (especially the turbocharged ones), the high values of pressure and temperature inside the combustion chamber during the engine cycle may cause knocking phenomena. "Standard" knock can be easily recognized by direct observation of the in-cylinder measured pressure trace; it is then possible to undertake proper actions and implement design and control improvements to prevent it with relatively standard 3D-CFD analyses. Some unusual types of detonation may occur somewhere else in the combustion chamber: Knocking inside piston/liner crevices belongs to the latter category and damages on the piston top land (very similar to pitting) are one of the evidence of knock onset in this region. The very localized regions of damage onset, the cycle to cycle variability and the very short duration of the phenomena do not allow to obtain fully reliable experimental data concerning the investigated problem. A new methodology is therefore implemented in CFD to drive the root causes identification and understanding the impact of crevice design. A preliminary CFD 3D in-cylinder analysis is performed, in order to understand the criticalities in the piston to liner fireland due to local pressure and temperature temporal evolution. Then a "model reduction" is proposed, which is necessary in order to study the problem with reasonable computational costs and times. A 2D simplified model is developed which is able to maintain the possibility to correctly represent the local thermo fluid dynamic effects, especially the auto-ignition conditions. Finally, new geometries are studied in order to prevent local knocking and retard auto-ignition such to improve the KLSA.


2019 - Effects of the Domain Zonal Decomposition on the Hybrid URANS/LES Modeling of the TCC-III Motored Engine Flow [Relazione in Atti di Convegno]
Krastev, V; D'Adamo, A; Rulli, F; Fontanesi, S
abstract

Hybrid URANS/LES turbulence modeling is rapidly emerging as a valuable complement to standard LES for full-engine multi-cycle simulation. Among the available approaches, zonal hybrids are potentially attractive due to the possibility of clearly identify URANS and LES zones, eventually introducing further zone types with dynamically switching behavior. The present work aims at evaluating the impact of different zonal configurations on the simulated flow statistics using the well-assessed TCC-III experimental engine setup. More specifically, different methods (URANS, LES or seamless DES) are applied inside the cylinder volume, as well as into the intake/exhaust ports and plenums. For each of the five tested configurations, in-cylinder flow features are compared against the reference TCC-III experimental measurements, in terms of velocity field statistics and quality indices. In addition, a detailed analysis using Proper Orthogonal Decomposition (POD) is carried out to quantitatively compare the results from experiments and simulation sets. The study outcomes are used as a starting point for discussing the applicability of zonal hybrid turbulence modeling to realistic engine geometries, critically analyze the model assumptions (e.g. the domain zonal decomposition) and provide guidelines for general application of such method.


2019 - Evaluation of the single jet flow rate for a multi-hole GDI nozzle [Relazione in Atti di Convegno]
Cavicchi, A.; Sparacino, Simone; Berni, F.; Postrioti, L.; Fontanesi, S.
abstract

Fuel injectors featuring differentiated hole-to-hole dimensions improve the fuel distribution in the cylinder ensuring a more efficient and cleaner combustion for GDI (Gasoline Direct Injection) engines. A proper diagnostic system able to detect the actual fuel flow rate exiting each hole of a GDI nozzle is requested in order to optimize the matching between the spray and the combustion chamber. Measuring the spray impact force of a single plume allows the detection of the momentum flux exiting the single hole and, under appropriate hypotheses, the evaluation of the corresponding mass flow rate time-profile. In this paper two methodologies for the hole-specific flow rate evaluation, both based on the spray momentum technique, were applied to two different GDI nozzles, one featuring equal hole dimensions and one with two larger holes. Three different energizing times at 100 bar of fuel pressure were tested in order to cover a wide range of operating conditions. The results were validated in terms of injected mass by means of a proper device able to collect and weigh the fuel injected by each single nozzle hole, and in terms of mass flow rate using a Zeuch-method flow meter as reference. Both the proposed methodologies showed an excellent accuracy in the fuel amount detection with percentage error lower than 5% for standard energizing times and lower than 10% for very short injections working in ballistic conditions. The mass flow rate time-profile proved a good accuracy in the detection of the start and end of injection and the static flow rate level.


2019 - Experimental and numerical study on the adoption of split injection strategies to improve air-butanol mixture formation in a DISI optical engine [Articolo su rivista]
Breda, S.; D'Orrico, F.; Berni, F.; d'Adamo, A.; Fontanesi, S.; Irimescu, A.; Merola, S. S.
abstract

Gasoline replacement with alternative non-fossil fuels compatible with existing units is widely promoted around the world to reduce the dependency on oil-based products by adopting domestic renewable sources. In this context, the possibility to obtain bio-alcohols from non-edible residues of food and plants is particularly attractive for gasoline replacement in SI (Spark Ignition) engines. Such bio-fuels are characterized by higher laminar flame speed (LFS) and octane rating, resulting in improved thermal efficiency and reduced regulated emissions. Low-carbon alcohols (e.g. ethanol) are disadvantageous as gasoline replacement due to poor energy density and high corrosive action on distribution pipelines, whereas high-carbon ones (e.g. n-butanol) are particularly promising candidates thanks to the physical properties and the energy density closer to those of gasoline. High latent heat of vaporization and low saturation pressure are the most relevant weaknesses of n-butanol related to gasoline replacement in DISI (Direct Injection SI) power units. On equal injection pressure and phasing, the slow evaporation rate of n-butanol leads to poor mixture preparation and larger fuel deposits. In particular, this is emphasized by low charge and wall temperatures during part load operation, reducing combustion efficiency and promoting the formation of pollutant particles. Split injection is a promising strategy to improve charge preparation contemporary reducing fuel deposits and improving mixture homogeneity, mostly for low-evaporating fuels. In the present work different split injection strategies are tested in an optically accessible SI engine fueled with n-butanol and simulated through CFD with the aim of identifying trends and understanding the root causes behind measured behaviors. CFD simulations help in understanding changes in charge stratification using different injection strategies, allowing to explain both combustion behavior and soot formation tendency from the analysis of fuel distribution. Mixture quality in the spark region and the presence of very rich mixture pockets in the combustion chamber are identified as the most critical aspects that should be optimized when changing the injection strategy; this in turn contributes to avoid slow burn rates or excessive soot production during operation with low evaporating fuels such as n-butanol. A strong correlation between diffusive flames and rich mixture pockets is found in terms of both location and intensity, proving the first order role of fuel deposits formation and mixture homogenization on both combustion development and soot formation.


2019 - Numerical Simulation of Syngas Blends Combustion in a Research Single-Cylinder Engine [Relazione in Atti di Convegno]
Pessina, V.; D'Adamo, A.; Iacovano, C.; Fontanesi, S.; Martinez, S.; Lacava, P
abstract

Despite syngas is a promising alternative fuel for internal combustion engines (ICEs), its extensive adoption has not been adequately investigated so far. The dedicated literature offers several fundamental studies dealing with H2/CO blends burning at high pressure and room temperature, as well as preheated mixture at low pressure. However, these thermodynamic states are far from the operational conditions typical of ICEs. Therefore, it is essential to investigate the syngas combustion process at engine-like conditions to shed light on this fuel performance, in order to fully benefit from syngas characteristics in ICE application. One of the key properties to characterize a combustion process is laminar flame speed, which is also used by the most widespread turbulent combustion models. In the first part, a database of premixed laminar burning rates at engine-like conditions for different syngas (H2/CO) blends is created based on one-dimensional unstretched flame simulations using two validated chemical mechanisms. Then the resulting laminar flame speed values are fitted using a validated in-house method based on logarithmic correlations. In the second part of the paper, these are implemented in the G-equation combustion model and three-dimensional simulations of a four stroke Spark Ignition (SI) optical access engine fueled by syngas are carried out. The combustion characteristics of two H2/CO blends (50/50 and 75/25 volume fraction, respectively) are investigated and the simulation results are compared to the available experimental data for the same fuels. This joint numerical/experimental study allows to investigate and optimize the syngas combustion for ICEs and it provides general guidelines to further understand the feasibility of this alternative fuel in terms of ICE utilizations.


2019 - The potential of statistical RANS to predict knock tendency: Comparison with LES and experiments on a spark-ignition engine [Articolo su rivista]
D'Adamo, A.; Breda, S.; Berni, F.; Fontanesi, S.
abstract

Pollutant regulations and fuel consumption concerns are the driving guidelines for increased thermal efficiency and specific power in current internal combustion engines. The achievement of such challenging tasks in Spark Ignition units is often limited by the onset of knock, which hinders the possibility to operate the engine with the optimal combustion phasing. The sporadic occurrence of individual knocking events is related to cycle-to-cycle variability of turbulent combustion. This is avoidable by only accepting a safety margin from its earliest onset. On one side, the stochastic nature of knock and turbulence-related combustion variability would indicate Large-Eddy Simulation (LES) as the most appropriate technique for CFD analyses. Nevertheless, Large-Eddy Simulation remains a very time- and CPU-demanding approach, hardly integrated in the industrial timeframe for the design exploration and development of new units. Therefore, Reynolds Averaged Navier Stokes (RANS) models representing the average flow are chosen to limit CPU and development times, though they suffer from the intrinsic inability to account for cycle-dependent phenomena (e.g. knock). This is particularly critical at knock-borderline conditions, where far-from-average knocking events may occur. A previously developed statistical RANS-PDF knock model partly overcomes this limitation using equations for mixture fraction and enthalpy variance, ultimately reconstructing log-normal distributions of knock intensity. This allows RANS simulations to be directly compared to the usual statistical knock analysis at the test-bench. In this paper all the mentioned modelling techniques (LES, RANS and RANS-PDF) are applied to simulate combustion and knock in a currently made turbocharged GDI engine under knock-safe, knock-limited and light-knocking conditions. The study relevance lays in the critical comparison of the results. The full potential of the statistical RANS-PDF model for engine development is highlighted on a coherent basis. The possibility to preserve the RANS formalism while enriching the results with knock statistical description is a relevant advancement in the virtual design of high-efficiency engines.


2018 - Development of Chemistry-Based Laminar Flame Speed Correlation for Part-Load SI Conditions and Validation in a GDI Research Engine [Articolo su rivista]
Del Pecchia, Marco; Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Irimescu, Adrian; Merola, Simona
abstract

The detailed study of part-load conditions is essential to characterize engine-out emissions in key operating conditions. The relevance of part-load operation is further emphasized by the recent regulation such as the new WLTP standard. The combustion development at part-load operations depends on a complex interplay between moderate turbulence levels (low engine speed and tumble ratio), low in-cylinder pressure and temperature and stoichiometric-to-lean mixture quality (to maximize fuel efficiency at partial loads). From a modelling standpoint, the reduced turbulence intensity compared to full-load operations complicates the interaction between different sub-models (e.g. re-consideration of the flamelet hypothesis adopted by common combustion models). In this paper, the authors focus on chemistry-based simulations for laminar flame speed of gasoline surrogates at conditions typical of part-load operations. The analysis is an extension of a previous study focused on full-load operations of a methodology based on detailed chemistry 1D simulations of the flame structure. The comparison with the previous research reveals that flames at partial loads experience analogous temperature levels, despite the generally lower pressure. Therefore, particular attention will be devoted to the temperature scaling of flame speed, as well as to the extension to lean mixtures. The proposed correlation is applied to simulate the combustion development on a single-cylinder research engine operated at a 0.7 bar absolute pressure part-load condition provided with an optical access to the combustion chamber. The experimental data derived by the aforementioned kind of equipment allows a detailed description of the flame development since early flame kernel growth and, therefore, the role of an accurate laminar flame speed modelling is discussed in details. The correlation for laminar flame speed proposed by the authors constitutes a useful reference for similar studies and it can be used in conjunction with the most common CFD combustion models.


2018 - Experimental and Numerical Analysis of Spray Evolution, Hydraulics and Atomization for a 60 MPa Injection Pressure GDI System [Relazione in Atti di Convegno]
Postrioti, Lucio; Cavicchi, Andrea; Brizi, Gabriele; Berni, Fabio; Fontanesi, Stefano
abstract

In recent years, the GDI (Gasoline Direct Injection) technology has significantly spread over the automotive market under the continuous push toward the adoption of combustion systems featuring high thermodynamic conversion efficiency and moderate pollutant emissions. Following this path, the injection pressure level has been progressively increased from the initial 5-15 MPa level nowadays approaching 35 MPa. The main reason behind the progressive injection pressure increase in GDI engines is the improved spray atomization, ensuring a better combustion process control and lower soot emissions. On the other hand, increasing injection pressure implies more power absorbed by the pumping system and hence a penalty in terms of overall efficiency. Therefore, the right trade-off has to be found between soot formation tendency reduction thanks to improved atomization and the energetic cost of a high pressure fuel injection system. In this paper, a 5-hole, side-mounted prototype GDI injector was tested in a wide range of injection pressure conditions - from 5 up to 60 MPa - in terms of injection rate and spray development. The injection rate was detected by means of a Zeuch-method-based Injection Analyzer. The spray global shape was investigated by high speed imaging, while the atomization level and droplet velocity were measured by means of a PDA (Phase Doppler Anemometry) system over several measuring stations from 20 to 50 mm downstream the nozzle. A numerical model of the spray was developed and validated against the experimental data in order to simulate the spray penetration, cone angle and atomization over a wide range of injection pressure levels. The results show that the decreasing trend for the drops SMD (Sauter Mean Diameter) from 5 up to 60 MPa approaches its asymptote, suggesting an adequate cost/benefits analysis in terms of soot reduction for further injection pressure level increases.


2018 - Impact of Grid Density on the les Analysis of Flow CCV: Application to the TCC-III Engine under Motored Conditions [Relazione in Atti di Convegno]
Ko, Insuk; Min, Kyoungdoug; Fontanesi, Stefano; Rulli, Federico; Ha, Taehun; Choi, Hoimyung
abstract

Large-eddy simulation (LES) applications for internal combustion engine (ICE) flows are constantly growing due to the increase of computing resources and the availability of suitable CFD codes, methods and practices. The LES superior capability for modeling spatial and temporal evolution of turbulent flow structures with reference to RANS makes it a promising tool for describing, and possibly motivating, ICE cycle-to-cycle variability (CCV) and cycle-resolved events such as knock and misfire. Despite the growing interest towards LES in the academic community, applications to ICE flows are still limited. One of the reasons for such discrepancy is the uncertainty in the estimation of the LES computational cost. This in turn is mainly dependent on grid density, the CFD domain extent, the time step size and the overall number of cycles to be run. Grid density is directly linked to the possibility of reducing modeling assumptions for sub-grid scales. The extent of the computational domain influences the impact of the boundary conditions on the CFD results. The time-step size needs to be set according to the size of the resolved turbulent eddies. It is therefore closely tied to local grid size with the constraint that the CFL number should be lower than unity everywhere in the domain for the highest accuracy. The overall number of simulated cycles influences the soundness of the statistical analysis of LES outcomes. This paper focuses on the impact of grid density on the LES description of the TCC-III single-cylinder optical engine flow under motored conditions. In particular, attention is focused on the intake stroke of the engine cycle, which governs the induced flow motion. LES results are first evaluated by means of well-established quality indices to find the insufficient grid resolution region to be refined. Second, comparisons with available PIV measurements are carried out. Finally, COV and proper orthogonal decomposition analyses are adopted to further assess the impact of grid density on CCV.


2018 - Refinement of a 0D Turbulence Model to Predict Tumble and Turbulent Intensity in SI Engines. Part II: Model Concept, Validation and Discussion [Relazione in Atti di Convegno]
Bozza, Fabio; Teodosio, Luigi; De Bellis, Vincenzo; Fontanesi, Stefano; Iorio, Agostino
abstract

As known, reliable information about underlying turbulence intensity is a mandatory pre-requisite to predict the burning rate in quasi-dimensional combustion models. Based on 3D results reported in the companion part I paper, a quasi-dimensional turbulence model, embedded under the form of "user routine" in the GT-Power™ software, is here presented in detail. A deep discussion on the model concept is reported, compared to the alternative approaches available in the current literature. The model has the potential to estimate the impact of some geometrical parameters, such as the intake runner orientation, the compression ratio, or the bore-to-stroke ratio, thus opening the possibility to relate the burning rate to the engine architecture. Preliminarily, a well-assessed approach, embedded in GT-Power commercial software v.2016, is utilized to reproduce turbulence characteristics of a VVA engine. This test showed that the model fails to predict tumble intensity for particular valve strategies, such LIVC, thus justifying the need for additional refinements. The model proposed in this work is conceived to solve 3 balance equations, for mean flow kinetic energy, tumble vortex momentum, and turbulent kinetic energy (3-eq. concept). An extended formulation is also proposed, which includes a fourth equation for the dissipation rate, allowing to forecast the integral length scale (4-eq. concept). The impact of the model constants is parametrically analyzed in a first step, and a tuning procedure is advised. Then, a comparison between the 3- and the 4-eq. concepts is performed, highlighting the advantages of the 3-eq. version, in terms of prediction accuracy of turbulence speed-up at the end of the compression stroke. An extensive 3-eq. model validation is then realized according to different valve strategies and engine speeds. The user-model is then utilized to foresee the effects of main geometrical parameters analyzed in part I, namely the intake runner orientation, the compression ratio, and the bore-to-stroke ratio. A two-valve per cylinder engine is also considered. Temporal evolutions of 0D- and 3D-derived mean flow velocity, turbulent intensity, and tumble velocity present very good agreements for each investigated engine geometry and operating condition. The model, particularly, exhibits the capability to accurately predict the tumble trends by varying some geometrical parameter of the engine, which is helpful to estimate the related impact on the burning rate. Summarizing, the developed 0D model well estimates the in-cylinder turbulence characteristics, without requiring any tuning constants adjustment with engine speed and valve strategy. In addition, it demonstrates the capability to properly take into account the intake duct orientation and the compression ratio without tuning adjustments. Some minor tuning variation allows predicting the effects of bore-to-stroke ratio, as well. Finally, the model is verified to furnish good agreements also for a two-valve per cylinder engine, and with reference to two different high-performance engines.


2018 - Understanding the origin of cycle-to-cycle variation using large-eddy simulation: Similarities and differences between a homogeneous low-revving speed research engine and a production DI turbocharged engine [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Berni, Fabio; Fontanesi, Stefano
abstract

A numerical study using large-eddy simulations (LES) to reproduce and understand sources of cycle-to-cycle variation (CCV) in spark-initiated internal combustion engines (ICEs) is presented. Two relevantly different spark-ignition (SI) units, that is, a homogeneous-charge slow-speed singlecylinder research unit (the transparent combustion chamber (TCC)-III, Engine 1) and a stratifiedcharge high-revving speed gasoline direct injection (GDI) (Engine 2) one, are analyzed in fired operations. Multiple-cycle simulations are carried out for both engines and LES results well reproduce the experimentally measured combustion CCV. A correlation study is carried out, emphasizing the decisive influence of the early flame period variability (1% of mass fraction burnt (MFB1)) on the entire combustion event in both ICEs. The focus is moved onto the early flame characteristics, and the crucial task to determine the dominant causes of its variability (if any) is undertaken. A two-level analysis is carried out: the influence of global parameters is assessed at first; second, local details in the ignition region are analyzed. A comparison of conditions at combustion onset is carried out and case-specific leading factors for combustion CCV are identified and ranked. Finally, comparative simulations are presented using a simpler flame deposition ignition model: the simulation flaws are evident due to modeling assumptions in the flame/flow interaction at ignition. The relevance of this study is the knowledge extension of turbulence-driven phenomena in ICEs allowed by advanced CFD (Computational Fluid Dynamics) simulations. The application to different engine types proves the soundness of the used models and it confirms that CCV is based on enginespecific factors. Simulations show how CCV originates from the interplay of small- and large-scale factors in Engine 1, due to the lack of coherent flows, whereas in Engine 2 the dominant CCV promoters are local air-to-fuel ratio (AFR) and flow velocity at ignition. This confirms the absence of a generally valid ranking, and it demonstrates the use of LES as a development and designorienting tool for next-generation engines.


2017 - A Comprehensive CFD-CHT Methodology for the Characterization of a Diesel Engine: From the Heat Transfer Prediction to the Thermal Field Evaluation [Relazione in Atti di Convegno]
Cicalese, Giuseppe; Berni, Fabio; Fontanesi, Stefano; D'Adamo, Alessandro; Andreoli, Enrico
abstract

High power-density Diesel engines are characterized by remarkable thermo-mechanical loads. Therefore, compared to spark ignition engines, designers are forced to increase component strength in order to avoid failures. 3D-CFD simulations represent a powerful tool for the evaluation of the engine thermal field and may be used by designers, along with FE analyses, to ensure thermo-mechanical reliability. The present work aims at providing an integrated in-cylinder/CHT methodology for the estimation of a Diesel engine thermal field. On one hand, in-cylinder simulations are fundamental to evaluate not only the integral amount of heat transfer to the combustion chamber walls, but also its point-wise distribution. To this specific aim, an improved heat transfer model based on a modified thermal wall function is adopted to estimate correctly wall heat fluxes due to combustion. On the other hand, a detailed Conjugate Heat Transfer model including both the solid components and the coolant circuit of the engine is needed, accounting for all the thermo-mechanical effects acting simultaneously during actual operations. Such comprehensive CHT methodology is here presented, with particular emphasis on a dedicated framework for the thermal simulation of the piston, to account for the mutual influence of many interplaying phenomena such as oil jet impingement, frictional losses and conduction with the surrounding components. The predictive capabilities of the methodology are demonstrated both in terms of global thermal balance and local engine temperature distribution. In fact, numerical coolant heat rejection and thermal field are compared with experimental data provided by thermal survey and point-wise temperature measurements for two different mid-to-low revving speed operating conditions.


2017 - A modified thermal wall function for the estimation of gas-to-wall heat fluxes in CFD in-cylinder simulations of high performance spark-ignition engines [Articolo su rivista]
Berni, Fabio; Cicalese, Giuseppe; Fontanesi, Stefano
abstract

Last generation spark ignition (SI) engines are characterized by a simultaneous reduction of the engine displacement and an increase of the brake power; such conflicting targets are achieved through the adoption of several techniques such as turbocharging, direct fuel injection, variable valve timing and variable port lengths. This design approach, referred to as “engine downsizing”, leads to a remarkable increase in the thermal loads acting on the engine components facing the combustion chamber. Hence, an accurate evaluation of the thermal field is of primary importance in order to avoid thermo-mechanical failures. Moreover, the correct evaluation of the temperature distribution improves the prediction of point-wise abnormal combustion onset. Due to the complexity of the experimental measurement of instantaneous gas-to-wall heat fluxes, 3D-CFD simulations of the in-cylinder processes are a fundamental tool to evaluate not only the global amount of heat transferred to the combustion chamber walls, but also its point-wise distribution. Several heat transfer models and thermal laws of the wall are available in literature, most of which were developed in the past decades and calibrated against experiments carried out in research laboratories at relatively low-load/low-speed engine operations. In the present paper two widely adopted heat transfer models are proved to be effective at such conditions to predict gas-to-wall heat flux, as demonstrated by their application to the well-known GM pancake engine test case. However, despite such comforting results, they manifest evident shortages when used for highly-charged/highly-downsized current production SI engines, since operated at specific thermal loads and engine speeds very different from the above experiments. In particular, overestimations of the wall heat transfer predicted by such thermal laws of the wall are pointed out thanks to experimental engine thermal surveys and temperature measurements on four current production engines. Therefore an alternative heat transfer model is proposed by the authors and tested on such currently made turbocharged SI engines, operated at different conditions. Compared to the existing models differences are pointed out, especially in terms of law of the wall expression. Experimental engine thermal survey and point-wise temperature measurements are used to validate the numerical heat flux. In particular the increased predictive capabilities of the 3D-CFD gas-to-wall heat transfer simulations are revealed both in terms of global thermal balance and temperature distribution of the metal for all the investigated engines. In fact model adoption in a combined in-cylinder/CHT (Conjugate Heat Transfer) simulation loop leads to a correct characterization of the thermal status of all the analyzed engines. Finally, alternative model adoption for the investigated current production high specific power DISI turbocharged engines operated at full load and high revving speed is critically motivated adopting the “isothermicity parameter” ζ which represents an indication of the thermal state of the boundary layer, being a characteristic scale of the ratio between gas and wall temperatures.


2017 - A RANS knock model to predict the statistical occurrence of engine knock [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Fontanesi, Stefano; Irimescu, Adrian; Merola, Simona Silvia; Tornatore, Cinzia
abstract

In the recent past engine knock emerged as one of the main limiting aspects for the achievement of higher efficiency targets in modern spark-ignition (SI) engines. To attain these requirements, engine operating points must be moved as close as possible to the onset of abnormal combustions, although the turbulent nature of flow field and SI combustion leads to possibly ample fluctuations between consecutive engine cycles. This forces engine designers to distance the target condition from its theoretical optimum in order to prevent abnormal combustion, which can potentially damage engine components because of few individual heavy-knocking cycles. A statistically based RANS knock model is presented in this study, whose aim is the prediction not only of the ensemble average knock occurrence, poorly meaningful in such a stochastic event, but also of a knock probability. The model is based on look-up tables of autoignition times from detailed chemistry, coupled with transport equations for the variance of mixture fraction and enthalpy. The transported perturbations around the ensemble average value are based on variable gradients and on a local turbulent time scale. A multi-variate cell-based Gaussian-PDF model is proposed for the unburnt mixture, resulting in a statistical distribution for the in-cell reaction rate. An average knock precursor and its variance are independently calculated and transported; this results in the prediction of an earliest knock probability preceding the ensemble average knock onset, as confirmed by the experimental evidence. The proposed model estimates not only the regions where the average knock is promoted, but also where and when the first knock is more likely to be encountered. The application of the model to a RANS simulation of a modern turbocharged direct injection (DI) SI engine with optical access is presented and the analysis of the knock statistical occurrence obtained by the proposed model adds an innovative contribution to overcome the limitation of consolidated “average knock” analyses typical of a RANS approach.


2017 - An Experimental and Simulation Study of Early Flame Development in a Homogeneous-Charge Spark-Ignition Engine [Articolo su rivista]
Shekhawat, Y; Haworth, Dc; D'Adamo, A; Berni, F; Fontanesi, S; Schiffmann, P; Reuss, Dl; Sick, V.
abstract

An integrated experimental and Large-Eddy Simulation (LES) study is presented for homogeneous premixed combustion in a spark-ignition engine. The engine is a single-cylinder two-valve optical research engine with transparent liner and piston: the Transparent Combustion Chamber (TCC) engine. This is a relatively simple, open engine configuration that can be used for LES model development and validation by other research groups. Pressure-based combustion analysis, optical diagnostics and LES have been combined to generate new physical insight into the early stages of combustion. The emphasis has been on developing strategies for making quantitative comparisons between high-speed/high-resolution optical diagnostics and LES using common metrics for both the experiments and the simulations, and focusing on the important early flame development period. Results from two different LES turbulent combustion models are presented, using the same numerical methods and computational mesh. Both models yield Cycle-to-Cycle Variations (CCV) in combustion that are higher than what is observed in the experiments. The results reveal strengths and limitations of the experimental diagnostics and the LES models, and suggest directions for future diagnostic and simulation efforts. In particular, it has been observed that flame development between the times corresponding to the laminar-to-turbulent transition and 1% mass-burned fraction are especially important in establishing the subsequent combustion event for each cycle. This suggests a range of temporal and spatial scales over which future experimental and simulation efforts should focus.


2017 - CFD Optimization of n-Butanol Mixture Preparation and Combustion in an Research GDI Engine [Relazione in Atti di Convegno]
Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Del Pecchia, Marco; Merola, Simona; Irimescu, Adrian
abstract

The recent interest in alternative non-fossil fuels has led researchers to evaluate several alcohol-based formulations. However, one of the main requirements for innovative fuels is to be compatible with existing units' hardware, so that full replacement or smart flexible-fuel strategies can be smoothly adopted. n-Butanol is considered as a promising candidate to replace commercial gasoline, given its ease of production from bio-mass and its main physical and chemical properties similar to those of Gasoline. The compared behavior of n-butanol and gasoline was analyzed in an optically-accessible DISI engine in a previous paper [1]. CFD simulations explained the main outcomes of the experimental campaign in terms of combustion behavior for two operating conditions. In particular, the first-order role of the slower evaporation rate of n-butanol compared to gasoline was highlighted when the two fuels were operated under the same injection phasing. The poor n-butanol/air mixture homogeneity was found to be a major limiting factor on the potential benefit of the use of n-butanol. This outcome is further deepened in this paper by numerically exploring different mixture preparation strategies for n-butanol. To this aim, variations of the injection phasing and profile are analyzed, including the use of multiple injection strategies. An optimized fuel injection strategy is then numerically identified considering mixture homogeneity, engine torque output and tailpipe emissions of soot and NOx. In order to confirm the validity of the CFD approach, this strategy is experimentally tested to finally draw conclusions on the potential of n-butanol in modern GDI units.


2017 - Chemistry-Based Laminar Flame Speed Correlations for a Wide Range of Engine Conditions for Iso-Octane, n-Heptane, Toluene and Gasoline Surrogate Fuels [Relazione in Atti di Convegno]
D'Adamo, Alessandro; Del Pecchia, Marco; Breda, Sebastiano; Berni, Fabio; Fontanesi, Stefano; Prager, Jens
abstract

CFD simulations of reacting flows are fundamental investigation tools used to predict combustion behaviour and pollutants formation in modern internal combustion engines. Focusing on spark-ignited units, most of the flamelet-based combustion models adopted in current simulations use the fuel/air/residual laminar flame propagation speed as a background to predict the turbulent flame speed. This, in turn, is a fundamental requirement to model the effective burn rate. A consolidated approach in engine combustion simulations relies on the adoption of empirical correlations for laminar flame speed, which are derived from fitting of combustion experiments. However, these last are conducted at pressure and temperature ranges largely different from those encountered in engines: for this reason, correlation extrapolation at engine conditions is inevitably accepted. As a consequence, relevant differences between proposed correlations emerge even for the same fuel and conditions. The lack of predictive chemistry-grounded correlations leads to a wide modelling uncertainty, often requiring an extensive model tuning when validating combustion simulations against engine experiments. In this paper a fitting form based on fifth order logarithmic polynomials is applied to reconstruct correlations for a set of Toluene Reference Fuels (TRFs), namely iso-octane, n-heptane, toluene and for a commercial gasoline fuel surrogate. Experimental data from literature are collected as well as existing computations for laminar flame speed. These last are extended up to full-load engine-relevant conditions, where experiments are not available; they constitute a model-based prediction of flame behaviour at such states. The mentioned literature and calculated data, which are shown to be representative of a wide range of engine-typical operating points, constitute the target values for the fitting polynomials. The model-based correlations of this study constitute a reference to increase the accuracy of flamelet combustion simulations, and to reduce the modelling approximations when dealing with full-load engine operations.


2017 - Critical aspects on the use of thermal wall functions in CFD in-cylinder simulations of spark-ignition engines [Articolo su rivista]
Berni, Fabio; Fontanesi, Stefano; Cicalese, Giuseppe; D'Adamo, Alessandro
abstract

CFD and FE tools are intensively adopted by engine manufacturers in order to prevent thermo-mechanical failures reducing time and cost-to market. The capability to predict correctly the physical factors leading to damages is hence essential for their application in the industrial practice. This is even more important for last generation SI engines, where the more and more stringent need to lower fuel consumption and pollutant emissions is pushing designers to reduce engine displacement in favor of higher specific power, usually obtained by means of turbocharging. This brings to a new generation of SI engines characterized by higher and higher adiabatic efficiency and thermo-mechanical loads. A recent research highlighted the different behavior of the thermal boundary layer of such engines operated at high revving speeds and high loads if compared to the same engines operated at low loads and revving speeds or even engines with a lower specific power. This means that CFD heat transfer models proposed and validated in the past decades on these last may not be predictive when applied to high specific power engines. This is why an alternative formulation was proposed in a previous work, for the estimation of the heat transfer in in-cylinder CFD simulations of high performance turbocharged SI engines. Nevertheless, for both the proposed alternative thermal wall function and the other ones available in literature, there are essential limitations due to the dimensionless distance y+. In fact, even if heat transfer models provide a further formulation for low y+ (viscous sub-layer), industrial practice seldom makes use of turbulence models enabling the integration up to the wall and low Reynolds approaches are even less used because of computational costs. Therefore in the present paper the authors aim to analyze critically the use of thermal wall functions along with high Reynolds turbulence models for the prediction of heat transfer in CFD in-cylinder simulations for different values of y+.


2017 - Development of a RANS-Based Knock Model to Infer the Knock Probability in a Research Spark-Ignition Engine [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Iaccarino, Salvatore; Berni, Fabio; Fontanesi, Stefano; Zardin, Barbara; Borghi, Massimo; Irimescu, Adrian; Merola, Simona
abstract

Engine knock is one of the most limiting factors for modern Spark-Ignition (SI) engines to achieve high efficiency targets. The stochastic nature of knock in SI units hinders the predictive capability of RANS knock models, which are based on ensemble averaged quantities. To this aim, a knock model grounded in statistics was recently developed in the RANS formalism. The model is able to infer a presumed log-normal distribution of knocking cycles from a single RANS simulation by means of transport equations for variances and turbulence-derived probability density functions (PDFs) for physical quantities. As a main advantage, the model is able to estimate the earliest knock severity experienced when moving the operating condition into the knocking regime. In this paper, improvements are introduced in the model, which is then applied to simulate the knock signature of a single-cylinder 400cm3 direct-injection SI unit with optical access; the engine is operated with two spark timings, under knock-safe and knocking conditions respectively. The statistical prediction of knock resulting from the presented knock model is compared to the experimental evidence for both investigated conditions. The agreement between the predicted and the measured knock distributions validates the proposed knock model. Finally, limitations and some unprecedented possibilities given by the model are critically discussed, with particular emphasis on the meaning of RANS knock prediction.


2017 - Investigation of Sub-Grid Model Effect on the Accuracy of In-Cylinder les of the TCC Engine under Motored Conditions [Relazione in Atti di Convegno]
Ko, Insuk; Min, Kyoungdoug; Rulli, Federico; D'Adamo, Alessandro; Berni, Fabio; Fontanesi, Stefano
abstract

The increasing interest in the application of Large Eddy Simulation (LES) to Internal Combustion Engines (hereafter ICEs) flows is motivated by its capability to capture spatial and temporal evolution of turbulent flow structures. Furthermore, LES is universally recognized as capable of simulating highly unsteady and random phenomena driving cycle-to-cycle variability (CCV) and cycle-resolved events such as knock and misfire. Several quality criteria were proposed in the recent past to estimate LES uncertainty: however, definitive conclusions on LES quality criteria for ICEs are still far to be found. This paper describes the application of LES quality criteria to the TCC-III single-cylinder optical engine from University of Michigan and GM Global R&D; the analyses are carried out under motored condition. In particular, attention is focused on sub-grid scale (SGS) model effects, which are evaluated using single grid estimators to compare three different sub-filter models: static Smagorinsky, dynamic Smagorinsky and dynamic structure model. Information on LES quality criteria are cross-linked to the analysis of in-cylinder gas-dynamics and flow structures. These are in turn analyzed by comparing experimental results (Particle Image Velocimetry (PIV) velocity fields) with a dataset of consecutive LES cycles on four different cutting planes at engine-relevant crank angle positions. Finally, phase-dependent Proper Orthogonal Decomposition is used to draw further considerations on the connection between LES quality indices and the accuracy of simulation results and conclusions are drawn to be used as guidelines in future LES analyses of ICEs.


2017 - Numerical and experimental analysis of the spray momentum flux measuring on a GDI injector [Articolo su rivista]
Cavicchi, Andrea; Postrioti, Lucio; Giovannoni, Nicola; Fontanesi, Stefano; Bonandrini, Giovanni; Di Gioia, Rita
abstract

In direct injection combustion systems, the spray momentum flux measurement can provide significant insight in the fuel jet development and the in-cylinder fuel-air mixing potential. The spray momentum flux can be determined by means of the impact force method, which was proved to be completely consistent when the basic measuring hypotheses are fulfilled, i.e. during the steady phase of the injection process. Conversely, in the transients the measurement technique details can significantly affect the results. An appropriate analysis of the possible effects exerted by the experimental setup is thus mandatory. In order to deepen the knowledge about this measurement technique applied to GDI (Gasoline Direct Injection) systems and to support the design of the experiment, in this paper a CFD 3-D model of a single-hole, research injector was developed and assessed with experimental data in terms of spray penetration curve, overall shape and droplet sizing and velocity. The validated numerical tool was then used to simulate the spray momentum measuring procedure in order to investigate the possible effects of the experimental set-up details on the momentum flux results. To this end, numerical results were compared to experimental data for different values of the main measurement parameters: target size, nozzle/target distance, discharge ambient pressure. This analysis confirmed both the experimental procedure sensitivity to some setup details and the model ability to capture the spray-target interaction phenomenon, supporting the need of a combined use of numerical and experimental approaches to obtain an adequate insight in the spray evolution.


2017 - Numerical Simulation and Flame Analysis of Combustion and Knock in a DISI Optically Accessible Research Engine [Articolo su rivista]
Iaccarino, Salvatore; Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Irimescu, Adrian; Merola, Simona
abstract

The increasing limitations in engine emissions and fuel consumption have led researchers to the need to accurately predict combustion and related events in gasoline engines. In particular, knock is one of the most limiting factors for modern SI units, severely hindering thermal efficiency improvements. Modern CFD simulations are becoming an affordable instrument to support experimental practice from the early design to the detailed calibration stage. To this aim, combustion and knock models in RANS formalism provide good time-to-solution trade-off allowing to simulate mean flame front propagation and flame brush geometry, as well as “ensemble average” knock tendency in end-gases. Still, the level of confidence in the use of CFD tools strongly relies on the possibility to validate models and methodologies against experimental measurements.In the paper, two sets of cycle-resolved flame visualizations are available from a single-cylinder 400 cm3 direct-injection spark-ignition (DISI) unit with optical access. The engine is operated at two spark timings, ranging from knock-safe to light-knock conditions.On this basis, a numerical analysis is carried out to reproduce flame kernel growth and propagation using the well-known ECFM-3Z combustion model for all the operating conditions. CFD results are compared in terms of enflamed volume and flame morphology against cycle averaged experimental data. In addition, average knock is simulated by means of the in-house built UniMORE Knock Model [1] in terms of knock onset location and phasing.The agreement between predicted and measured position of the flame front and knock inception location for the two different operating conditions confirms the validity of the adopted models and proves their predictive capability for engine design and optimization


2017 - Numerical simulation of gasoline and n-butanol combustion in an optically accessible research engine [Articolo su rivista]
Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; D’Orrico, Fabrizio; Irimescu, Adrian; Merola, Simona; Giovannoni, Nicola
abstract

Conventional fossil fuels are more and more regulated in terms of both engine-out emissions and fuel consumption. Moreover, oil price and political instabilities in oil-producer countries are pushing towards the use of alternative fuels compatible with the existing units. N-Butanol is an attractive candidate as conventional gasoline replacement, given its ease of production from bio-mass and key physico-chemical properties similar to their gasoline counterpart. A comparison in terms of combustion behavior of gasoline and n-Butanol is here presented by means of experiments and 3D-CFD simulations. The fuels are tested on a single-cylinder direct-injection spark-ignition (DISI) unit with an optically accessible flat piston. The analysis is carried out at stoichiometric undiluted condition and lean-diluted mixture for both pure fuels. Numerical simulations are carried out on the same operating points and a dedicated set of detailed chemistry simulations are used to accurately predict laminar flame speed for both gasoline and n-Butanol at selected engine-relevant conditions. Moreover, a method to accurately fit target results is presented and it is applied to obtain a polynomial form of laminar flame speed for both fuels. Mixture preparation and combustion development are carefully analyzed to explain the experimental evidence and to argument the differences between the two fuels, as well as the fuel-specific tolerance to mixture leaning. Finally, conclusions are drawn to summarize the obtained results and to outline the foreseen advantages of using n-Butanol as gasoline substitute in modern SI engines.


2017 - Pressure Losses in Multiple-Elbow Paths and in V-Bends of Hydraulic Manifolds [Articolo su rivista]
Zardin, Barbara; Cillo, Giovanni; Borghi, Massimo; D'Adamo, Alessandro; Fontanesi, Stefano
abstract

Hydraulic manifolds are used to realize compact circuit layouts, but may introduce high pressure losses in the system because their design is usually oriented to achieving minimum size and weight more than reducing the pressure losses. The purpose of this work is to obtain the pressure losses when the internal connections within the manifold are creating complex paths for the fluid and the total loss cannot be calculated simply as the sum of the single losses. To perform the analysis both Computational Fluid Dynamic (CFD) analysis and experimental tests have been executed. After the comparison between numerical and experimental results, it was possible to assess that the numerical analysis developed in this work is able to depict the correct trends of the pressure losses also when complex fluid path are realized in the manifold. Successively, the numerical analysis was used to calculate the pressure loss for inclined connections of channels (or V-bends), a solution that is sometimes adopted in manifolds to meet the design requirements aimed towards the minimum room-minimum weight objective.


2017 - Study of les Quality Criteria in a Motored Internal Combustion Engine [Relazione in Atti di Convegno]
Ko, Insuk; D'Adamo, Alessandro; Fontanesi, Stefano; Min, Kyoungdoug
abstract

In recent years, Large-Eddy Simulation (LES) is spotlighted as an engineering tool and severe research efforts are carried out on its applicability to Internal Combustion Engines (ICEs). However, there is a general lack of definitive conclusions on LES quality criteria for ICE. This paper focuses on the application of LES quality criteria to ICE and to their correlation, in order to draw a solid background on future LES quality assessments for ICE. In this paper, TCC-III single-cylinder optical engine from University of Michigan is investigated and the analysis is conducted under motored condition. LES quality is mainly affected by grid size and type, sub-grid scale (SGS) model, numeric schemes. In this study, the same grid size and type are used in order to focus on the effect on LES quality of SGS models and blending factors of numeric scheme only. In the first section of the study, single grid estimators are used to compare two sub-filter models which are static Smagorinsky model and dynamic Smagorinsky model. Also, two cases which are simulated with different blending factors for numeric schemes and same SGS model are compared. In the second section, the in-cylinder gas-dynamics and flow structures are analyzed by comparing experimental results (pressure transducers and Particle Image Velocimetry (PIV) velocity fields) with a dataset of consecutive LES cycles. The flow analysis focuses at four different crank angle positions (bottom dead center (BDC), middle of exhaust and intake valve opening timing and mid-compression stroke) on the same section plane as PIV visualizations. Finally, the connection between the LES quality criteria and the accuracy of simulation results with experiments is discussed and conclusions are drawn to outline a best practice in LES quality for ICE.


2016 - A RANS-Based CFD Model to Predict the Statistical Occurrence of Knock in Spark-Ignition Engines [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Fontanesi, Stefano; Cantore, Giuseppe
abstract

Engine knock is emerging as the main limiting factor for modern spark-ignition (SI) engines, facing increasing thermal loads and seeking demanding efficiency targets. To fulfill these requirements, the engine operating point must be moved as close as possible to the onset of abnormal combustion events. The turbulent regime characterizing in-cylinder flows and SI combustion leads to serious fluctuations between consecutive engine cycles. This forces the engine designer to further distance the target condition from its theoretical optimum, in order to prevent abnormal combustion to severely damage the engine components just because of few individual heavy-knocking cycles. A RANS-based model is presented in this study, which is able to predict not only the ensemble average knock occurrence but also a knock probability. This improves the knock tendency characterization, since the mean knock onset alone is a poorly meaningful indication in a stochastic event such as engine knock. The model is based on a look-up table approach from detailed chemistry, coupled with the transport of the variance of both mixture fraction and enthalpy. These perturbations around the ensemble average value are originated by the turbulent time scale. A multivariate cell-based Gaussian-PDF model is proposed for the unburnt mixture, resulting in a statistical distribution for the in-cell reaction rate. An average knock precursor and its variance are independently calculated and transported, and the earliest knock probability is always preceding the ensemble average knock onset, as confirmed by the experimental evidence. This allows to identify not only the regions where the average knock first occurs, but also where the first knock probability is more likely to be encountered. The application of the model to a RANS simulation of a modern turbocharged direct injection (DI) SI engine is presented and a small percentage of knocking cycles is predicted by the model although the average behavior is knock-free, in agreement with the experiments. The estimate of the knocking probability improves the consolidated “average knock” RANS analysis and gives an indication of the statistical knock tendency of the engine


2016 - CFD Analysis of Combustion and Knock in an Optically Accessible GDI Engine [Articolo su rivista]
Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Giovannoni, Nicola; Testa, Francesco; Irimescu, Adrian; Merola, Simona; Tornatore, Cinzia; Valentino, Gerardo
abstract

The occurrence of knock is the most limiting hindrance for modern Spark-Ignition (SI) engines. In order to understand its origin and move the operating condition as close as possible to onset of this potentially harmful phenomenon, a joint experimental and numerical investigation is the most recommended approach. A preliminary experimental activity was carried out at IM-CNR on a 0.4 liter GDI unit, equipped with a flat transparent piston. The analysis of flame front morphology allowed to correlate high levels of flame front wrinkling and negative curvature to knock prone operating conditions, such as increased spark timings or high levels of exhaust back-pressure. In this study a detailed CFD analysis is carried out for the same engine and operating point as the experiments. The aim of this activity is to deeper investigate the reasons behind the main outcomes of the experimental campaign. A tabulated knock model is presented, based on detailed chemical mechanism for the surrogate gasoline. Combustion and knock simulations are carried out in a RANS framework through the use of validated models and the results are compared with cycle-resolved acquisition from the test-bed. The results of the CFD analysis explain the experimentally observed flame behavior and allow to proficiently understand the reasons of the sensitivity to knock of the analyzed unit


2016 - Development of a phenomenological turbulence model through a hierarchical 1D/3D approach applied to a VVA turbocharged engine [Articolo su rivista]
De Bellis, Vincenzo; Bozza, Fabio; Fontanesi, Stefano; Severi, Elena; Berni, Fabio
abstract

It is widely recognized that spatial and temporal evolution of both macro-and micro-turbulent scales inside internal combustion engines affect air-fuel mixing, combustion and pollutants formation. Particularly, in spark ignition engines, tumbling macro-structure induces the generation of a proper turbulence level to sustain the development and propagation of the flame front. As known, 3D-CFD codes are able to describe the evolution of the in-cylinder flow and turbulence fields with good accuracy, although a high computational effort is required. For this reason, only a limited set of operating conditions is usually investigated. On the other hand, thanks to a lower computational burden, 1D codes can be employed to study engine performance in the whole operating domain, despite of a less detailed description of in-cylinder processes. The integration of 1D and 3D approaches appears hence a promising path to combine the advantages of both. In the present paper, a 0D phenomenological mean flow and turbulence model belonging to the K-k model family is presented in detail. The latter is implemented in the GT-Power™ software under the form of “user routine”. The model is tuned against in-cylinder results provided by 3D-CFD analyses carried out by the Star-CD™ code at two engine speeds under motored operation. In particular, a currently produced twin-cylinder turbocharged VVA engine is analyzed. The 0D model is then validated against further 3D results at various engine speeds and intake valve lifts, including early closure strategies, both under motored and fired operation. The proposed 0D mean flow and turbulence model shows the capability to accurately estimate the temporal evolution of the incylinder turbulence for all the considered operating conditions, without requiring any case-dependent tuning, proving its generality and reliability.


2016 - Guidelines for the optimization of a muffler in a small two stroke engine [Articolo su rivista]
Testa, Francesco; Gagliardi, Vincenzo; Ferrari, Marco; Fontanesi, Stefano; Bertani, Andrea
abstract

It is well known that 3D CFD simulations can give detailed information about fluid and flow properties in complex 3D domains while 1D CFD simulation can provide important information at a system level, i.e. about the performance of the entire engine. The drawbacks of the two simulation methods are that the former requires high computational cost while the latter is not able to capture complex local 3D features of the flow. Therefore, the two simulation methods are to be seen as complementary, indeed a coupling of the two approaches can benefit from the pros of the two methods while minimizing the cons. In particular, with a multi-scale modeling approach (1D-3D) it is possible to simulate large and complex domains by modeling the complex part with a 3D approach and the rest of the domain with a 1D approach. This paper describes an optimization cycle analysis of the unsteady flow of a single cylinder, two stroke gasoline engine using advanced numerical tools, which are in turn validated by means of experimental measurements. In particular, a 3D model (based on STAR-CD code) of the entire engine and a 1D-3D integrated fluid dynamics model (based on GT-POWER 1D and Converge-LITE 3D codes) is developed and applied for the representation of the geometrical domain and for the prediction of performance and gasdynamics in the whole intake and exhaust systems. The methodology allows users to accurately predict and deeply understand unsteady phenomena in the whole engine and capture the wave motion, which strongly affects the muffler 3D design in small two stroke engines equipped with resonance pipes.


2016 - Integrated In-Cylinder / CHT Methodology for the Simulation of the Engine Thermal Field: An Application to High Performance Turbocharged DISI Engines [Articolo su rivista]
Cicalese, Giuseppe; Berni, Fabio; Fontanesi, Stefano
abstract

New SI engine generations are characterized by a simultaneous reduction of the engine displacement and an increase of the brake power; such targets are achieved through the adoption of several techniques such as turbocharging, direct fuel injection, variable valve timing and variable port lengths. This design approach, called "downsizing", leads to a marked increase in the thermal loads acting on the engine components, in particular on those facing the combustion chamber. Hence, an accurate evaluation of the thermal field is of primary importance in order to avoid mechanical failures. Moreover, the correct evaluation of the temperature distribution improves the prediction of pointwise abnormal combustion onset. The paper proposes an evolution of the CFD methodology previously developed by the authors for the prediction of the engine thermal field, which is applied to two different high performance turbocharged DISI engines: the methodology employs both in-cylinder 3D-CFD combustion simulations and CHT (Conjugate Heat Transfer) simulations of the whole engine, inclusive of both the solid components and the coolant circuit. In-cylinder analyses are used as thermal boundary conditions for the CHT simulations, which are in turn a fundamental benchmark to evaluate the accuracy of the combustion heat flux estimation by means of a combination of global engine thermal survey and local temperature measurements. A preliminary evaluation of some consolidated heat transfer models is carried out to evaluate the accuracy of the predicted gas-to-wall heat fluxes. Then, a modified heat transfer model is proposed, critically motivated and applied to the specific engine conditions under investigations. The proposed model strongly improves the predictive capability of the combined in-cylinder/CHT methodology in terms of both global thermal balance and pointwise temperature distribution for both the investigated engines


2015 - A methodology for the reduction of numerical diffusion in sloshing analyses through automated mesh adaptation [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; De Pasquale, Gianluca
abstract

The paper proposes a methodology to improve the accuracy of Volume of Fluid (VoF) multiphase problems involving liquid/gas sloshing in fuel/lubricant tanks without penalizing the computational cost of the simulations. In order to correctly track the complex trajectory of the liquid/gas interface and the presence of liquid droplets in the gas phase, the VoF method requires a fine mesh at each interface location to reduce modeling errors. The investigated case is a lubricant tank of a sport car subject to typical race track maneuvers. Due to the geometrical extent and the complexity of the computational domain and to the relevant accelerations, resulting in dispersed liquid structures within the gas phase, the use of a generalized fine mesh would result in computational costs far beyond the industrial practice. A methodology is then proposed to reduce the overall number of computational cells through a combination of local interface tracking and mesh refinement, which is combined with an active control of the time step to comply with Courant-Friedrichs- Lewy number limits. The methodology is at first validated against experimental measurements for a simplified test case, and then applied to the actual oil tank sloshing case, showing a relevant reduction of the numerical diffusion and a consequent higher accuracy.


2015 - A Numerical Investigation on the Potentials of Water Injection as a Fuel Efficiency Enhancer in Highly Downsized GDI Engines [Relazione in Atti di Convegno]
D'Adamo, Alessandro; Berni, Fabio; Breda, Sebastiano; Lugli, Mattia; Fontanesi, Stefano; Cantore, Giuseppe
abstract

Engine downsizing is gaining popularity in the high performance engine market sector, where a new generation of highly downsized engines with specific power outputs around or above 150 HP/litre is emerging. High-boost and downsizing, adopted to increase power density and reduce fuel consumption, have to face the increased risks of pre-ignition, knock or mega-knock. To counterbalance autoignition of fuel/air mixture, such engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter reduces performance and induces an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A relatively simple yet effective solution to increase knock resistance is investigated by means of 3-D CFD analyses in the paper: water is port injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. Full-load engine operations of a currently made turbocharged GDI engine are investigated comparing the adopted fuel-only rich mixture with stoichiometric mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-injected engine allows the Spark Advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption.


2015 - CFD Analysis of the Effects of Fuel Composition and Injection Strategy on Mixture Preparation and Fuel Deposit Formation in a GDI Engine [Relazione in Atti di Convegno]
Giovannoni, Nicola; Breda, Sebastiano; Paltrinieri, Stefano; D'Adamo, Alessandro; Fontanesi, Stefano; Pulvirenti, Francesco
abstract

In spark-ignited direct-injected engines, the formation of fuel pools on the piston is one of the major promoters of unburnt hydrocarbons and soot: in order to comply with the increasingly stringent emission regulations (EU6 and forthcoming), it is therefore necessary to limit fuel deposit formation. The combined use of advanced experimental techniques and detailed 3D-CFD simulations can help to understand the mechanisms driving fuel pool formation. In the paper, a combined experimental and numerical characterization of pool formation in a GDI engine is carried out to investigate and understand the complex interplay of all the mentioned factors. In particular, a low-load low-rpm engine operation is investigated for different ignition phasing, and the impact of both fuel formulation and instantaneous piston temperature variations in the CFD analyses are evaluated. The investigated engine operation shows some interesting features which are suited to deeply investigate the interplay between fuel film formation, mixing and soot. In particular, the relatively low wall temperature and low injection pressure allow the fuel to form deposits and then slowly evaporate, with possible presence of liquid fuel at the time of ignition. The simultaneous presence of slow fuel evaporation, reduced turbulence and presence of liquid fuel leads to the formation of extremely rich mixture pockets (with equivalence ratios well above 5) which are the major promoters for soot inception. Four different start of injection (hereafter SOI) values are analyzed, for which tailpipe Soot concentration measurements are available. For one SOI value, two different injection profiles are also evaluated. In particular, the analyses focus on the formation of fuel pads on the combustion chamber walls and on the mixture stratification, and a correlation between these two factors and the tailpipe soot level is found. The proposed methodology proves to be able to capture the Soot trend for the different SOI values without simulating the combustion process; it is therefore promising since it avoids the need for a dedicated calibration of the combustion model parameters and provides reasonable results (at least in terms of trends) with limited computational resources.


2015 - Employment of an auto-regressive model for knock detection supported by 1D and 3D analyses [Relazione in Atti di Convegno]
Siano, D.; Severi, E.; De Bellis, V.; Fontanesi, S.; Bozza, F.
abstract

In this work, experimental data, carried out on a twin-cylinder turbocharged engine at full load operations and referred to a spark advance of borderline knock, are used to characterize the effects of cyclic dispersion on knock phenomena. 200 consecutive incylinder pressure signals are processed through a refined Auto-Regressive Moving Average (ARMA) mathematical technique, adopted to define the percentage of knocking cycles, through a prefixed threshold level. The heuristic method used for the threshold selection is then verified by 1D and 3D analyses. In particular, a 1D model, properly accounting for cycle-by-cycle variations, and coupled to a reduced kinetic sub-model, is used to reproduce the measured cycles, in terms of statistical distribution of a theoretical knock index. In addition, few individual cycles, representative of the whole dataset, are selected in a single operating condition in order to perform a more detailed knock analysis by means of a 3D CFD approach, coupled to a tabulated chemistry technique for auto-ignition modeling. Outcomes of 1D and 3D models are compared to the ARMA results and a substantial coherence of the numerical and experimental results is demonstrated. The integrated 1D and 3D analyses can hence help in supporting the choice of the experimental threshold level for knock identification, following a more standardized theoretical approach.


2015 - Large-Eddy simulation analysis of spark configuration effect on cycle-to-cycle variability of combustion and knock [Articolo su rivista]
Fontanesi, Stefano; D'Adamo, Alessandro; Rutland, C. J.
abstract

Cycle-to-cycle variability is numerically simulated for high-speed, full-load operation of a turbocharged gasoline direct injection engine. Large-Eddy simulation is adopted to replicate the fluctuations of the flow field affecting the turbulent combustion. Experimental data were provided at knock onset, and large-Eddy simulation was validated for the same condition. In the original engine configuration, the spark plug is displaced toward the exhaust side, while the electrodes orientation is arbitrary. A 90 rotation is imposed to evaluate the effects of the aerodynamic obstruction caused by the electrode with respect to the flow field and the flame kernel growth. A second speculative analysis is performed modifying the position of the spark plug. The electrodes are shifted 2mm toward the intake side since this variation is compatible with the cylinder head layout. For both variations in orientation and position, the effects on the flow field around the spark plug are investigated. Statistical analysis is carried out on early flame kernel formation and knock tendency. The results highlight that the orientation of the electrodes affects the flow field for each cycle but plays a negligible role on the statistical cyclic variability, indirectly justifying the lack of an imposed orientation. As for the spark plug position, the numerical analysis indicate that the shifting of the electrodes toward the intake side slightly improves the knock limit mainly because of a reduction in in-cylinder peak pressure. In general, it is inferred that improvements may be achieved only through a simultaneous modification of the fuel jet orientation and phasing.


2015 - LES Modelling of Spark-Ignition Cycle-to-Cycle Variability on a Highly Downsized DISI Engine [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Fontanesi, Stefano; Cantore, Giuseppe
abstract

The paper reports an activity aiming at characterizing cycle-to-cycle variability (CCV) of the spark-ignition (SI) process in a high performance engine. The numerical simulation of spark-ignition and of early flame kernel evolution are major challenges, mainly due to the time scales of the spark discharge process and to the reduced spatial scales of flame kernel. Typical mesh resolutions are insufficient to resolve the process and a dedicated treatment has to be provided at a subgrid level if the ignition process is to be properly modelled. The focus of this work is on the recent ISSIM-LES (Imposed Stretch Spark-Ignition Model) ignition model, which is based on an extension of the flame surface density (FSD) transport equation for a dedicated flame kernel treatment at subgrid scales. The FSD equation is solved immediately after spark discharge. The interaction of the flame kernel with the flow field is fully accounted for since spark formation and a transition is provided from ignition to propagation phase. The comparison is carried out with the AKTIM-Euler ignition model in terms of flame interaction with the flow field (e.g. arc convection, flame blow-off, flame holder effect). A multiple cycle LES activity provided a set of cycle-resolved conditions for spark-ignition comparisons, and the flame kernel development is carefully analyzed for the two ignition models on a wide range of thermo-physical conditions. Spark-ignition cyclic variability and combustion traces are compared with experiments. Results confirm that the simulated cycle-to-cycle variability increases through the adoption of the ISSIM-LES ignition model.


2015 - Numerical Investigation on the Effects of Water/Methanol Injection as Knock Suppressor to Increase the Fuel Efficiency of a Highly Downsized GDI Engine [Relazione in Atti di Convegno]
Berni, Fabio; Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Cantore, Giuseppe
abstract

A new generation of highly downsized SI engines with specific power output around or above 150 HP/liter is emerging in the sport car market sector. Technologies such as high-boosting, direct injection and downsizing are adopted to increase power density and reduce fuel consumption. To counterbalance the increased risks of pre-ignition, knock or mega-knock, currently made turbocharged SI engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter induce an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A possible solution to increase knock resistance is investigated in the paper by means of 3D-CFD analyses: water/methanol emulsion is port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. The peak power engine operation of a currently made turbocharged GDI engine is investigated comparing the adopted fuel-only rich mixture with stoichiometric-to-lean mixtures, for which water/methanol mixture is added in the intake port under constant charge cooling in the combustion chamber and same air consumption level. In order to find the optimum fuel/emulsion balance analytic considerations are carried out. Different strategies are evaluated in terms of percentage of methanol-water emulsion rate, to assess the effects of different charge dilutions and mixture compositions on knock tendency and combustion efficiency. Thanks to the lower chemical reactivity of the diluted end gases and the faster burn rate allowed by the methanol addition, the water/methanol-injected engine allows the spark advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption.


2015 - Two-stage turbocharging for the downsizing of SI V-engines [Relazione in Atti di Convegno]
Rinaldini, Carlo Alberto; Breda, Sebastiano; Fontanesi, Stefano; Savioli, Tommaso
abstract

One of the most critical challenges for the specific power increase of turbocharged SI engines is the low end torque, limited by two aspects. First, the big size of the compressor necessary to deliver the maximum airflow does not allow high boost pressures at low speed, due to the surge line proximity. Second, the flame front velocity may become slower than the end gas auto-ignition rate, thus increasing the risk of knocking. This study is based on a current SI GDI V8 turbocharged engine, modeled by means of CFD tools, both 1d and 3d. The goal of the activity is to lower by 20% the displacement, without reducing brake torque, all over the engine speed range. It was decided to adopt a smaller bore, keeping stroke constant. Obviously, the combustion chamber, the valves and the intakeexhaust ports have been re-designed, as well as the whole intake and exhaust system. Instead of the two turbochargers, one for each bank of cylinders, a triple-turbocharger layout has been considered. The development of the engine has been carried out by means of 1D engine cycle simulations, using predictive knock models, calibrated with the support of both experiments and CFD-3d simulations. A few operating conditions for the final configuration have been also analyzed by means of a 3-d CFD tool. The paper presents the results of this activity, and describes in details the guidelines followed for the development of the engine.


2014 - A Methodology to Improve Knock Tendency Prediction in High Performance Engines [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; D'Adamo, Alessandro; Cantore, Giuseppe
abstract

The paper presents a comprehensive numerical methodology for the estimation of knock tendency in SI engines, based on the synergic use of different frameworks [1]. 3D-CFD in-cylinder analyses are used to simulate the combustion and to estimate the point-wise heat flux acting on engine components. The resulting heat fluxes are used in a conjugate heat transfer model in order to reconstruct the actual point-wise wall temperature distribution. An iterative loop is established between the two simulation realms. In order to evaluate the effect of temperature on knock, in-cylinder analyses are integrated with an accurate chemical description of the actual fuel.


2014 - Analysis of Knock Tendency in a Small VVA Turbocharged Engine Based on Integrated 1D-3D Simulations and Auto-Regressive Technique [Articolo su rivista]
Fontanesi, Stefano; Severi, Elena; Daniela, Siano; Fabio, Bozza; Vincenzo De, Bellis
abstract

In the present paper, two different methodologies are adopted and critically integrated to analyze the knock behavior of a last generation small size spark ignition (SI) turbocharged VVA engine. Particularly, two full load operating points are selected, exhibiting relevant differences in terms of knock proximity. On one side, a knock investigation is carried out by means of an Auto-Regressive technique (AR model) to process experimental in-cylinder pressure signals. This mathematical procedure is used to estimate the statistical distribution of knocking cycles and provide a validation of the following 1D-3D knock investigations. On the other side, an integrated numerical approach is set up, based on the synergic use of 1D and 3D simulation tools. The 1D engine model is developed within the commercial software GT-Power™. It is used to provide time-varying boundary conditions (BCs) for the 3D code, Star-CD™. Particularly, information between the two simulation tools are at first exchanged under motored conditions to tune an “in-house developed” turbulence sub-model included in the 1D software. 1D results are then validated against the experimental data under fired full load operations, by employing a further “in-house developed” combustion sub-model. BCs are finally passed back to the 3D code to carry out a detailed knock analysis for two full load points, namely 2100 and 4000 rpm. In particular, the knock intensity is predicted, for experimentally actuated and earlier spark advances, and the results are qualitatively compared to the AR model outcomes.


2014 - CFD Analysis of the Acoustic Behavior of a Centrifugal Compressor for High Performance Engine Application [Relazione in Atti di Convegno]
Fontanesi, Stefano; Paltrinieri, Stefano; Cantore, Giuseppe
abstract

The paper reports an activity aiming at the characterization of the acoustic noise of a centrifugal compressor for a currently made high performance engine. All the analyses are carried out through the use of Detached Eddy Simulation. During high-load / low-ngine speed operations of the engine, the compressor exhibits noise peaks above 150 dBA at relatively low frequencies, whose origin is relatively hard to rationalize. The use of three-dimensional CFD simulation appears to be very promising to gain a better understanding of the complex flow structures at the compressor inlet as well as to promote design optimizations aiming at limiting the acoustic emissivity of the component. In view of the dependency of the acoustic phenomena on the instantaneous pressure waves and flow structures, fully transient CFD simulations are highly recommended, together with the use of sophisticated numerical techniques such as Large Eddy and Detached Eddy simulation [1], [2], which are widely recognized to be able to better capture highly unstable features than the common RANS approach [3], [4]. In order to limit the computational cost of the analyses, preliminary steady-state RANS simulations are carried out to both initialize the flow field and to evaluate the grid capability to properly match the desired frequency spectrum.


2014 - Hierarchical 1D/3D Approach for the Development of a Turbulent Combustion Model Applied to a VVA Turbocharged Engine. Part I: Turbulence Model [Relazione in Atti di Convegno]
Vincenzo De, Bellis; Severi, Elena; Fontanesi, Stefano; Fabio, Bozza
abstract

It is widely recognized that air-fuel mixing, combustion and pollutant formation inside internal combustion engines are strongly influenced by the spatial and temporal evolution of both marco- and micro- turbulent scales. Particularly, in spark ignited engines, the generation of a proper level of turbulence intensity for the correct development of the flame front is traditionally based on the onset, during the intake and compression strokes, of a tumbling macro-structure. Recently, in order to both reduce pumping losses due to throttling and develop advanced and flexible engine control strategies, fully variable valve actuation systems have been introduced, capable of simultaneously governing both valve phasing and lift. Despite the relevant advantages in terms of intake system efficiency, this technology introduces uncertainties on the capability of the intake port/valve assembly to generate, at low loads, sufficiently coherent and stable structures, able therefore to promote adequate turbulence levels towards the end of the compression, with relevant effects on the flame front development. It is a common knowledge that 3D-CFD codes are able to describe the evolution of the in-cylinder flow field and of the subsequent combustion process with good accuracy; however, they require too high computational time to analyze the engine performance for the whole operating domain. On the contrary, this task is easily accomplished by 1D codes, where, however, the combustion process is usually derived from experimental measurements of the in-cylinder pressure trace (Wiebe correlation). This approach is poorly predictive for the simulation of operating conditions relevantly different from the experimental ones. To overcome the above described issues, enhanced physical models for the description of in-cylinder turbulence evolution and combustion to be included in a 1D modeling environment are mandatory. In the present paper (part I), a 0D (i.e. homogeneous and isotropic) phenomenological (i.e. sensitive to the variation of operative parameters such as valve phasing, valve lift, intake and exhaust pressure levels, etc.) turbulence model belonging to the K-k model family is presented in detail. The model is validated against in-cylinder results provided by 3D-CFD analyses carried out with the Star-CD code for motored engine operations. In particular, a currently produced small turbocharged VVA engine is analyzed at different speeds, with valve actuations typical of full load and partial load (EIVC) operations, as well. The proposed turbulence model shows the capability, once tuned, to accurately estimate the temporal evolution of the in-cylinder turbulence according to the engine operating conditions. In the subsequent part II of the same paper, the developed turbulence model will be employed within a quasi-dimensional fractal combustion model.


2014 - Hierarchical 1D/3D Approach for the Development of a Turbulent Combustion Model Applied to a VVA Turbocharged Engine. Part II: Combustion Model [Relazione in Atti di Convegno]
Vincenzo De, Bellis; Severi, Elena; Fontanesi, Stefano; Fabio, Bozza
abstract

As discussed in the part I of this paper, 3D models represent a useful tool for a detailed description of the mean and turbulent flow fields inside the engine cylinder. 3D results are utilized to develop and validate a 0D phenomenological turbulence model, sensitive to the variation of operative parameters such as valve phasing, valve lift, engine speed, etc. In part II of this paper, a 0D phenomenological combustion model is presented, as well. It is based on a fractal description of the flame front and is able to sense each of the fuel properties, the operating conditions (air-to-fuel ratio, spark advance, boost level) and the combustion chamber geometry. In addition, it is capable to properly handle different turbulence levels predicted by means of the turbulence model presented in the part I. The turbulence and combustion models are included, through user routines, in the commercial software GT-Power". With reference to a small twin-cylinder VVA turbocharged engine, the turbulence/combustion model, once properly tuned, is finally used to calculate in-cylinder pressure traces, rate of heat release and overall engine performance at full load operations and brake specific fuel consumption at part load, as well. An excellent agreement between numerical forecasts and experimental evidence is obtained.


2014 - Integrated In-Cylinder/CHT Analysis for the Prediction of Abnormal Combustion Occurrence in Gasoline Engines [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; Cantore, Giuseppe; D'Adamo, Alessandro
abstract

In order to improve fuel conversion efficiency, currently made spark-ignited engines are characterized by the adoption of gasoline direct injection, supercharging and/or turbocharging, complex variable valve actuation strategies. The resulting increase in power/size ratios is responsible for substantially higher average thermal loads on the engine components, which in turn result in increased risks of both thermo-mechanical failures and abnormal combustion events such as surface ignition or knock. The paper presents a comprehensive numerical methodology for the accurate estimation of knock tendency of SI engines, based on the integration of different modeling frameworks and tools. Full-cycle in-cylinder analyses are used to estimate the point-wise heat flux acting on the engine components facing the combustion chamber. The resulting cycle-averaged heat fluxes are then used in a conjugate heat transfer model of the whole engine in order to reconstruct the actual point-wise temperature distribution of the combustion chamber walls. The two simulation realms iteratively exchange information until convergence is met. Particularly, the effect of point-wise temperature distribution on the onset of abnormal combustion events is evaluated. In-cylinder analyses account for the actual autoignition behavior of the air/fuel mixture through a look-up table approach: the combustion chamber is treated as a two-zone region (burnt/unburnt), where ignition delay tabulation, generated off-line using a constant pressure reactor, is applied to the unburnt region to estimate cell-wise knock proximity. The methodology is applied to a high performance engine and the importance of an accurate representation of the combustion chamber thermal boundary conditions when aiming at precisely evaluating the surface ignition/knock tendency is highlighted.


2014 - Investigation of boundary condition and field distribution effects on the cycle-to-cycle variability of a turbocharged GDI engine using LES [Articolo su rivista]
Fontanesi, Stefano; Paltrinieri, Stefano; d'ADAMO, Alessandro; S., Duranti
abstract

The paper reports some preliminary results of a numerical activity aiming at characterizing cycle-to-cycle variability of a highly-downsized turbocharged DISI engine for high-performance car applications, using a customized version of the commercial software Star-CD licensed by CD-adapco. During experimental investigations at the engine testbed, high cycle to cycle dispersion was detected even for relatively stable peak-power / full-load operations of the engine, thus limiting the overall engine performance. Despite the complex architecture of the V-8 engine, the origin of such cyclic variability could not be related to cyclic fluctuations of the gas-dynamics within the intake and exhaust pipes. Several subsequent acquisitions of the instantaneous pressure traces were measured at both the intake port entrance and exhaust port junction, showing almost negligible differences in terms of both amplitude and phasing compared to those within the cylinder. In order to explore the potentials of the LES application to the analysis and understanding of the cycle-to-cycle variability, which notoriously strongly limits the overall engine performance, a numerical activity is carried out using full-cycle LES simulations over several subsequent engine cycles. Despite the very early stage of the investigation, two main issues are addressed in the paper: the analysis of the possible causes originating the high cycle to cycle variability and the influence of the boundary conditions on the predicted cyclic dispersion. Concerning the former aspect, a detailed investigation of local and global instantaneous fields is carried out aiming at identifying both a possible hierarchy of responsibilities on one side and limitations and possible improvements of the adopted numerical procedure on the other side. Concerning the latter aspect, a set of simulations, performed applying cycle-independent averaged experimental conditions, is compared to those resulting from the application of cycle-specific variable pressure traces at both intake and exhaust sides, in order to analyze the influence of port fluctuations on the in-cylinder pressure history. Results are also qualitatively compared to those resulting from a multi-cycle RANS simulation using the same grid-size, in order to better highlight the superior LES potentials.


2014 - LES Analysis of Cyclic Variability in a GDI Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Paltrinieri, Stefano; Cantore, Giuseppe
abstract

The paper critically discusses Large-Eddy Simulation (LES) potential to investigate cycle-to-cycle variability (CCV) in internal combustion engines. Particularly, the full load/peak power engine speed operation of a high-performance turbocharged GDI unit, for which ample cycle-to-cycle fluctuations were observed during experimental investigations at the engine test bed, is analyzed through a multi-cycle approach covering 25 subsequent engine cycles. In order to assess the applicability of LES within the research and development industrial practice, a modeling framework with a limited impact on the computational cost of the simulations is set up, with particular reference to the extent of the computational domain, the computational grid size, the choice of boundary conditions and numerical sub-models [1, 2, 3]. In order to evaluate the applicability of the adopted approach to the resolution of an adequate portion of the overall turbulent energy spectrum, different grid metrics are at first introduced, based on criteria available in literature [4, 5]. A qualitative comparison between CFD results and experimental evidence is then carried out in terms of both in-cylinder pressure envelope and coefficients of variation for any of indicated mean effective pressure, 10%, 50% and 90% of fuel burnt distributions among the investigated cycles. Particularly, a detailed analysis of the physical factors influencing the exhibited cycle-to-cycle variability is performed through the use of correlation coefficients, aiming at highlighting possible hierarchies between the many involved phenomena and the observed engine behavior. Finally, a phase-dependent Proper Orthogonal Decomposition (POD). Particularly, while POD applications available in literature mainly cover vector fields and flow structures [6, 7], in the present paper the analysis is extended to scalar fields describing the combustion process evolution and its cyclic variability, and results are critically analyzed and commented.


2014 - Numerical investigation of the cavitation damage in the wet cylinder liner of a high performance motorbike engine [Articolo su rivista]
Fontanesi, Stefano; Giacopini, Matteo; Cicalese, Giuseppe; Sissa, Simone; S., Fantoni
abstract

In this paper a numerical methodology is proposed which aims at understanding the origin of a particular failure occurred in a two-cylinder high performance spark ignition engine for motorbike applications. A relevant cavitation damage/erosion has been detected at the water side of the engine cylinder liner during severe reliability bench tests, performed at the early stage of the engine design process. On the contrary, no damages have been registered during parallel high-load long runs of the motorbike. This contribution investigates in detail the differences between the bench test cooling circuit layout and the actual motorbike cooling circuit layout in order to find an explanation of the engine critical behaviour. In particular, CFD-CHT analyses of the water cooling jacket are performed, the computational domain covering both the coolant galleries and the surrounding metal components (head, block, gasket, valves, valve seats, valve guides, cylinder liner, spark plug). The contribution of a two-phase approach which takes into account the effect of a phase transition within the engine coolant is considered. Different engine operating conditions are investigated and a detailed analysis of different thermo-mechanical parameters influencing the engine behaviour is carried out. Results of the CFD simulations asses the methodology capability to correctly capture and understand the origin of the engine failure, thus providing a useful design tool for a faster and more effective design modification.


2013 - Analysis of Turbulence Model Effect on the Characterization of the In-Cylinder Flow Field in a HSDI Diesel Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; Severi, Elena
abstract

In-cylinder large scale and small scale structures are widely recognized to strongly influence the mixing process in HSDI Diesel engines, and therefore combustion and pollutant emissions. In particular, swirl motion intensity and temporal evolution during the intake and compression strokes must be correctly estimated to properly target the spray jets. The experimental characterization of the attitude of a valve/port assembly to promote swirl is traditionally limited to the steady flow bench, in which the analysis is carried out for fixed valve positions / fixed pressure drops and with no piston. Since flow bench analyses cannot reproduce the highly complex instantaneous flow conditions typical of actual engine operations, the use of fully-transient in-cylinder numerical simulations can become extremely useful to correctly address the engine ability to promote adequate flow structures and patterns. CFD analyses of in-cylinder flow motion development and decay are usually carried out using relatively simple yet stable turbulence models, among which k-epsilon and RNG k-epsilon, in conjunction with near-wall algebraic functions, are probably the most popular ones. Nevertheless, it is widely recognized that such models show deficiencies in correctly capturing complex rotating flow structures such as those dominating in-cylinder flows of compression ignition engines. While waiting for the diffusion of more refined approaches such as Large Eddy Simulation (LES) or Detached Eddy Simulation (DES), the use of better performing RANS models such as Reynolds Stress and k-omega SST, in conjunction with adequate resolution of the near wall flow, would be recommendable. The paper reports a numerical activity aiming at evaluating the influence of the turbulence model choice on the prediction of in-cylinder flow patterns in a HSDI Diesel engine for automotive applications. Three different engine operations are investigated, i.e. two part-load / low engine speeds ones and a full-load / peak power engine speed one. For each engine condition, both wall-function k-epsilon and low-Reynolds k-omega SST models are used, and for the last one two different near-wall grid refinements are adopted. In order to better assess the predictive capabilities of the models, analyses are preliminarily compared to experimental measurements for both a simplified engine-like geometry and flow structure available in literature and steady flow-bench operations of the actual engine head.


2013 - Assessment of the Potential of Proper Orthogonal Decomposition for the Analysis of Combustion CCV and Knock Tendency in a High Performance Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; D'Adamo, Alessandro; Paltrinieri, Stefano; Cantore, Giuseppe; C. J., Rutland
abstract

The paper reports the application of Proper Orthogonal Decomposition (POD) to LES calculations for the analysis of combustion and knock tendency in a highly downsized turbocharged GDI engine that is currently under production. In order to qualitatively match the cyclic variability of the combustion process, Large-Eddy Simulation (LES) of the closed-valve portion of the cycle is used with cycle-dependent initial conditions from a previous multi-cycle analysis [1, 2, 3]. Detailed chemical modelling of fuel’s auto-ignition quality is considered through an ad-hoc implemented look-up table approach, as a trade-off between the need for a reasonable representation of the chemistry and that of limiting the computational cost of the LES simulations. Experimental tests were conducted operating the engine at knock-limited spark advance (KLSA) and the proposed knock model was previously validated for such engine setup [3]. All the presented calculations are carried out for an increased spark advance (SA) to both promote knock onset over a large set of cases and to assess the modelling framework sensitivity to small variations in engine operations. The analysis of combustion development and knock onset is carried out analysing 20 subsequent engine cycles through POD of both flame front evolution and local autoignition locations. Particularly, phase-dependent three-dimensional POD is implemented over the scalar distributions of progress variable of the chemical reactions and auto-ignition location, estimated based on the work by Lafossas et al. [4]. The method of snapshots introduced by Sirovich is used for POD [5]. The proposed POD analysis is critically discussed in terms of physical soundness, capability to investigate the engine knock-characteristics and applicability to the optimization of the combustion chamber.


2013 - Combined In-cylinder / CHT Analyses for the Accurate Estimation of the Thermal Flow Field of a High Performance Engine for Sport Car Applications [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; A., Tiberi
abstract

The paper describes an integrated methodology for the accurate characterization of the thermal behavior of internal combustion engines, with particular reference to a high performance direct injected SI engine for sport car applications. The engine is operated at full load and maximum power revving speed, which is known to be critical from the point of view of thermal stresses on the engine components. In particular, two different sets of 3D-CFD calculations are adopted: on one side, full-cycle in-cylinder analyses are carried out to estimate the point wise thermal heat flux due to combustion on the engine components facing the combustion chamber. On the other side, full-engine multi-region CHT calculations covering the engine coolant jacket and the surrounding metal components are used to compute the point wise temperature distribution within the engine head, liner and block. An iterative procedure is then implemented in order to exchange relevant thermal data between the two modeling frameworks: at first, local gas temperatures and heat transfer coefficients from the in-cylinder simulations are applied as thermal loads to the CHT calculations; then, the resulting surface temperature maps are re-applied to the combustion chamber walls for the subsequent in-cylinder CFD analysis. The procedure is iteratively repeated until convergence is met in terms of thermal characterization of the engine. The procedure takes into account the non-uniform spatial distribution of both thermal loads from the combustion process and resulting temperature distribution on the combustion chamber walls, which are known to be particularly evident when considering direct injected stratified charged engines. The resulting thermal fields are compared to those deriving from a traditional approach for both the in-cylinder and CHT calculations, i.e. the application of uniform heat fluxes on the CHT side and the use of uniform wall temperatures on the in-cylinder side. The resulting differences are highlighted and critically discussed, with particular emphasis on those which could have a relevant impact on both the fatigue strength evaluation of the engine and the arising of local undesired phenomena such as surface ignition and knocking.


2013 - Combustion Optimization of a Marine DI Diesel Engine [Relazione in Atti di Convegno]
Mattarelli, Enrico; Fontanesi, Stefano; Rinaldini, Carlo Alberto; G., Valentino; S., Iannuzzi; Severi, Elena; V., Golovitchev
abstract

Enhanced calibration strategies and innovative engine combustion technologies are required to meet the new limits on exhaust gas emissions enforced in the field of marine propulsion and on-board energy production. The goal of the paper is to optimize the control parameters of a 4.2 dm3 unit displacement marine DI Diesel engine, in order to enhance the efficiency of the combustion system and reduce engine out emissions. The investigation is carried out by means of experimental tests and CFD simulations. For a better control of the testing conditions, the experimental activity is performed on a single cylinder prototype, while the engine test bench is specifically designed to simulate different levels of boosting. The numerical investigations are carried out using a set of different CFD tools: GT-Power for the engine cycle analysis, STAR-CD for the study of the in-cylinder flow, and a customized version of the KIVA-3V code for combustion. All the models are calibrated through the above mentioned experimental campaign. Then, CFD simulations are applied to optimize the injection parameters and to explore the potential of the Miller combustion concept. It is found that the reduction of the charge temperature, ensuing the adoption of an early intake valve closing strategy, strongly affects combustion. With a proper valve actuation strategy, an increase of boost pressure and an optimized injection advance, a 40% reduction of NOx emissions can be obtained, along with a significant reduction of in-cylinder peak pressure, without penalizing fuel efficiency.


2013 - Knock Tendency Prediction in a High Performance Engine Using LES and Tabulated Chemistry [Articolo su rivista]
Fontanesi, Stefano; Paltrinieri, Stefano; D'Adamo, Alessandro; Cantore, Giuseppe; C. J., Rutland
abstract

The paper reports the application of a look-up table approach within a LES combustion modelling framework for the prediction of knock limit in a highly downsized turbocharged DISI engine. During experimental investigations at the engine test bed, high cycle-to-cycle variability was detected even for relatively stable peak power / full load operations of the engine, where knock onset severely limited the overall engine performance. In order to overcome the excessive computational cost of a direct chemical solution within a LES framework, the use of look-up tables for auto-ignition modelling perfectly fits with the strict mesh requirements of a LES simulation, with an acceptable approximation of the actual chemical kinetics. The model here presented is a totally stand-alone tool for autoignition analysis integrated with look-up table reading from detailed chemical kinetic schemes for gasoline. The look-up table access is provided by a multi-linear interpolating routine internally developed at the “Gruppo Motori (GruMo)” of the University of Modena and Reggio Emilia. As the experimental tests were conducted operating the engine at knock-limited spark advance, the tool is at first validated for three different LES cycles in terms of knock tolerance, i.e. the safety margin to knock occurrence. As a second stage, the validation of the methodology is performed for discrete spark advance increases in order to assess the sensitivity of the modelling strategy to variations in engine operations. A detailed analysis of the unburnt gas physical state is performed which confirms the knock-limited condition suggested by the experimental tests.


2013 - LES Multi-cycle Analysis of a High Performance GDI Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Paltrinieri, Stefano; D'Adamo, Alessandro; A., Tiberi
abstract

The paper reports the application of LES multi-cycle analysis for the characterization of cycle to cycle variability (hereafter CCV) of a highly downsized DISI engine for sport car applications. The analysis covers several subsequent engine cycles operating the engine at full load, peak power engine speed. Despite the chosen engine operation is usually considered relatively stable, relevant fluctuations were experimentally measured in terms of in-cylinder pressure evolution and combustion phasing. On one hand, despite the complex architecture of the V-8 engine, the origin of such CCV is considered to be poorly related to cyclic fluctuations of the gas-dynamics within the intake and exhaust pipes, since acquisitions of the instantaneous pressure traces at both the intake port entrance and exhaust port junction by fast-response pressure measurements over 250 subsequent engine cycles showed almost negligible differences in both amplitude and phasing compared to those within the cylinder. On the other hand, being the combustion affected by a complex chain of preceding factors (air admission during the intake stroke, variations in the residual gas fraction, generation of complex turbulent flow structures, fuel injection and dispersion in the combustion chamber and subsequent mixing, interaction between the spark discharge and the surrounding local flow pattern, etc.) a clear understanding of the actual origin of cyclic variability is far from being trivial. LES CFD simulations can therefore become a very powerful tool to help investigating the possible causes of such cyclic variations, since detailed analyses of both global and local parameters can be carried out on an almost unlimited set of available virtual measurements. In the first part of the paper, the modeling framework is presented and considerations on the adopted numerical strategy are presented, with particular emphasis on grid size, grid distribution and numerical parameters. Subsequently, LES results are analyzed and discussed in order to understand the cycle-to-cycle variations through the use of correlation coefficients between global/local flow variables in order to highlight the major causes of CCV and establish a possible hierarchy among the analyzed quantities. Finally, criticalities of the currently adopted approach and possible enhancements are briefly discussed at the end of the paper. The results presented in the paper clearly highlight the potential of the modeling methodology to help understanding the origin of CCV as well as to address possible engine optimizations to limit the cyclic dispersion.


2013 - Modelling of primary breakup process of a multi-hole spray for Gasoline Direct Engine applications [Articolo su rivista]
Malaguti, Simone; Fontanesi, Stefano; Cantore, Giuseppe; A., Montanaro; L., Allocca
abstract

The paper proposes a numerical methodology for the simulation of a gasoline spray generated by a multi-hole injector of a current production wall-guided Gasoline Direct Injection engine. Particular care is dedicated to the accurate representation of the spray primary breakup by means of an atomization model. The model is purposely implemented to take into account cavitation phenomena and turbulent effects induced by the nozzle geometry through a simplified approach. Since a high primary breakup rate is expected, an initial distribution of atomized droplets is predicted at the nozzle hole exit by the numerical approach. The spray is at first experimentally investigated in a test vessel at non-evaporative ambient conditions and under quiescent conditions, injecting commercial gasoline at two different injection pressures (10.0 and 20.0 MPa). The spray is characterised in terms of both instantaneous mass flow rate and morphology. Numerical simulations are performed and then compared against experiments in order to evaluate their capability to correctly predict liquid spray penetration, droplet size distribution and spray morphology. The new approach is a fairly simple yet reliable solution able to predict the influence of the nozzle hole (in terms of discharge coefficient, diameter and length) and neglecting geometrical details usually far from being easily accessed by engine developers.


2013 - Multiphase CFD–CHT optimization of the cooling jacket and FEM analysis of the engine head of a V6 diesel engine [Articolo su rivista]
Fontanesi, Stefano; Giacopini, Matteo
abstract

The present paper proposes a numerical methodology aiming at analyzing and optimizing an internal combustion engine water cooling jacket, with particular emphasis on the assessment of the fatigue strength of the engine head. Full three-dimensional CFD and FEM analyses of the conjugate heat transfer and of the thermo-mechanical loading cycles are presented for a single bank of a currently made V6 turbocharged Diesel engine under actual operating conditions. A detailed model of the engine, consisting of both the coolant galleries and the surrounding metal components is employed in both fluid-dynamic and structural analyses to accurately mimic the influence of the cooling system layout on the thermo-mechanical behavior of the engine. In order to assess a proper CFD setup useful for the optimization of the thermal behavior of the engine, the experimentally measured temperature distribution within the engine head is compared to the CFD forecasts. Particular attention is paid to the modeling of the phase transition and of the vapor nuclei formation within the coolant galleries. Thermo-mechanical analyses are then carried out aiming at addressing the design optimization of the engine in terms of fatigue strength. Because of the wide range of thermal and mechanical loadings acting on the engine head, both high-cycle and low-cycle fatigue are considered. An energy-based multi-axial criterion specifically suited for thermal fatigue is employed in the low-cycle fatigue region (i.e. the combustion dome) while well-established multi-axial stress/strain-based criteria are adopted to investigate the high-cycle fatigue regions of the engine head (i.e. the coolant galleries). The proposed methodology shows very promising results in terms of point-wise detection of possible engine failures and proves to be an effective tool for the accurate thermo-mechanical characterization of internal combustion engines under actual life-cycle operating conditions.


2012 - Investigation of Scavenging, Combustion and Knock in a Two-Stroke SI Engine Operated with Gasoline and CNG [Articolo su rivista]
Fontanesi, Stefano; Severi, Elena; F., Bozza; A., Gimelli
abstract

The paper reports a combined experimental and numerical investigation of a small unit displacement two-stroke SI engine operated with either Gasoline and Natural Gas (CNG). It is widely recognized that for two-stroke, crankcase scavenged, carbureted engines the scavenging patterns (fuel short-circuiting, residual gas distribution, point wise lambda field, etc.) plays a fundamental role on both engine performance and tailpipe emissions. To properly characterize the engine behavior in terms of scavenging patterns and combustion, a detailed multi-cycle 3D-CFD analysis of the scavenging process is at first performed starting from preliminary 1D computed boundary conditions provided by a in-house developed 1D model of the whole engine. In order to assess the accuracy of the adopted numerical approach, comparisons between numerical forecasts and experimental measurements of the instantaneous in-cylinder pressure history for steady-state operations of the engine are at first performed and shown in the paper. Subsequently, the activity is focused on the investigation of knock occurrence. In order to limit the computational cost of the simulations, calculations are at first carried out within the 1D modeling framework, where customized quasi-dimensional combustion and knock models are used. In particular, the 1D model is used to compute a numerical knock index which can be useful to address the tuning of the spark advance, given a prescribed and controlled percentage of knock released heat. At the end of the simulation process, the 1D knock index is qualitatively compared to results obtained from full 3D knocking analyses for different in-cylinder compositions and spark timings. The intrinsic knock-resistance of the CNG fuel is finally numerically exploited, through variations of both compression ratio and spark advance.


2012 - Numerical and Experimental Investigation of Fuel, Effects on Knock Occurrence and Combustion Noise in a 2-Stroke Engine [Articolo su rivista]
F., Bozza; Fontanesi, Stefano; A., Gimelli; Severi, Elena; D., Siano
abstract

Knock occurrence is a widely recognized phenomenon to be controlled during the development and optimization of S.I. engines, since it bounds both compression ratio and spark advance, hence reducing the potential in gaining a lower fuel consumption. As a consequence, a clear understanding of the engine parameters affecting the onset of auto-ignition is mandatory for the engine setup. In view of the complexity of the phenomena, the use of combined experimental and numerical investigations is very promising. The paper reports such a combined activity, targeted at characterizing the combustion behavior of a small unit displacement two-stroke SI engine operated with either Gasoline or Natural Gas (CNG). In the paper, detailed multi-cycle 3D-CFD analyses, starting for preliminary 1D computed boundary conditions, are performed to accurately characterize the engine behavior in terms of scavenging efficiency and combustion. In order to assess the accuracy of the adopted numerical approach, comparisons between numerical forecasts and experimental measurements of instantaneous in-cylinder pressure histories are carried out for both gasoline- and CNG-fueled engine operations. 3D analyses are also used to investigate the knock sensitivity of the engine to variations of spark timings in a limited set of operating conditions. The activity is simultaneously developed within a 1D modeling framework, where a detailed quasi-dimensional combustion and knock model is applied to perform a wider investigation of engine performance and knock occurrence for both Gasoline and Natural Gas fuelling. Results from 3D simulations are here used to improve the 1D simulations through a better description of scavenging and combustion processes. Once validated, 1D analyses are in particular finalized to find the knock-limited spark advance by changing both compression ratio and spark timing in order to reduce the fuel consumption. In this phase, a dedicated routine is also developed to have information on combustion related noise, which may limit fuel consumption improvements. Further confirmations on the validity of the 1D approach to the modeling of the knock onset are derived from full-3D knocking analyses over a limited set of engine operating conditions. Advantages and limitations of CNG operations of the engine are briefly pointed out at the end of the paper.


2011 - CFD Investigation of the Thermo-Mechanical Behavior of a High Performance Bike Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; S., Fantoni; M., Rosso
abstract

The paper presents a combined experimental and numerical activity carried out to improve the accuracy of conjugate heat transfer CFD simulations of a high-performance S.I. motorbike engine water cooling jacket. The computational domain covers both the coolant jacket and the surrounding metal components (head, block, gasket, valves, valve seats, valve guides, cylinder liner, spark plug).In view of the complexity of the modeled geometry, particular care is required in order to find a tradeoff between the accuracy and the cost-effectiveness of the numerical procedure. The CFD-CHT simulation of water cooling jackets involves many complex physical phenomena: in order to setup a robust numerical procedure, the contribution of some relevant CFD parameters and sub-models was discussed by the authors in previous publications and is referred to [1-4].Among the formers, the effects of a proper set of boundary conditions and a detailed representation of the physical properties of the involved materials were evaluated. Among the latter, the contribution of a two-phase approach taking into account the effects of phase transition within the engine coolant was considered.The CFD-CHT setup is now applied to investigate and understand the origin of a critical engine behavior occurring at the engine test bench under a severe reliability test. Additional sub-models are introduced and their impact on the results is discussed. Different engine operations are modeled and a detailed analysis of the many thermo-mechanical factors influencing the engine fatigue strength is carried out.At the end of the process, CFD simulations are able to correctly capture and understand the origin of the engine failure, thus leading to a faster and more effective design modification.


2011 - Experimental and Numerical Investigation of the Idle Operating Engine Condition for a GDI Engine [Relazione in Atti di Convegno]
Malaguti, Simone; Fontanesi, Stefano; B. M., Vaglieco; P., Sementa; F., Catapano
abstract

The paper investigates the idle operating condition of a current production turbocharged Gasoline Direct Injected (GDI) high performance engine both from an experimental and a numerical perspective. Due to the low engine speed, to the low injection pressure and to the null contribution of the turbocharger, the engine condition is far from the standard points of investigation. According to the low heat flux due to combustion, temperature levels are low and reduced fuel evaporation is expected. Consequently, fuel spray evolution within the combustion chamber and spray/wall interaction are key points for the understanding of the combustion process.In order to properly investigate and understand the many complex phenomena, a wide set of engine speeds was experimentally investigated and, as far as the understanding of the physics of spray/wall interaction is concerned, many different injection strategies are tested. Among the wide set of experiments, the present paper focuses on a restricted portion which is then numerically reproduced and further investigated.UV-visible imaging and spectral measurements are carried out in the engine to investigate the spray characteristics and flame propagation. Measurements are performed in the optically accessible combustion chamber realized by modifying the actual engine. The cylinder head is modified in order to allow the visualization of the fuel injection and the combustion process in the fourth cylinder using a high spatial and temporal resolution ICCD detector.The complete engine cycle is reproduced by means of 3D-CFD simulations using a commercial code; due to the many physical submodels an ad hoc numerical methodology is validated and implemented. The CFD models are validated against experiments and particular care is devoted to the spray and wall film simulations. A lagrangian approach is implemented in order to simulate the GDI multihole spray. The experimental and numerical comparisons, in terms fuel mixing and flame front propagation, give a good understanding of the idle condition.CFD analyses prove to be a very useful tool to investigate and understand the effects generated by the direct injection into the combustion chamber and they integrate the information provided by the optical investigations.


2011 - Rilevanza delle proprietà termofisiche di materiali e interfacce di accoppiamento nell’analisi termomeccanica di componenti motore [Relazione in Atti di Convegno]
Giacopini, Matteo; Fontanesi, Stefano; C., Forte; A., Morri
abstract

All’interno del presente contributo verrà descritta una metodologia di calcolo integrata CFD/FEM che consente il calcolo del campo di temperatura in diversi componenti motore. Al fine di impostare correttamente le singole simulazioni sarà necessario considerare diversi parametri termofisici in termini di conducibilità termica dei materiali e resistenze di contatto tra i vari componenti, alcuni dei quali sono di incerta reperibilità.Verranno poi presentate opportune analisi di sensibilità a questi parametri al fine di evidenziare l’influenza che ognuno di questi ha sul campo di temperatura calcolato all’interno del componente. Infine, saranno presentate opportune tecniche di validazione sperimentale della metodologia proposta, necessarie a causa delle inevitabili incertezze che permangono all’atto del settaggio delle diverse analisi.


2011 - Validation of a CFD Methodology for the Analysis of Conjugate Heat Transfer in a High Performance SI Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; D'Adamo, Alessandro; G., Pivetti
abstract

The paper presents a combined experimental and numerical activity carried out to improve the accuracy of conjugate heat transfer CFD simulations of a high-performance S.I. engine water cooling jacket.Due to the complexity of the computational domain, which covers both the coolant jacket and the surrounding metal cast (both head and block), particular care is required in order to find a tradeoff between the accuracy and the cost-effectiveness of the numerical procedure. In view of the presence of many complex physical phenomena, the contribution of some relevant CFD parameters and sub-models is separately evaluated and discussed.Among the formers, the extent of the computational domain, the choice of a proper set of boundary conditions and the detailed representation of the physical properties of the involved materials are separately considered. Among the latters, the choice between a simplified single-phase approach and a more complex two-phase approach taking into account the effects of phase transition within the engine coolant is discussed.The predictive capability of the CFD-CHT methodology is assessed by means of the comparison between CFD results and experimental measurements provided by the engine manufacturer for different engine operating conditions.At the end of the validation process, a methodology for the correct and cost-effective characterization of conjugate heat transfer is proposed, showing a reasonable trade-off between the predictive capability and the computational effort of the simulations.


2011 - 1D and 3D CFD Investigation of Burning Process and Knock Occurrence in a Gasoline or CNG fuelled Two-Stroke SI Engine [Relazione in Atti di Convegno]
F., Bozza; A., Gimelli; Fontanesi, Stefano; Severi, Elena
abstract

The paper presents a combined experimental and numerical investigation of a small unit displacement two-stroke SI engine operated with gasoline and Natural Gas (CNG). A detailed multi-cycle 3D-CFD analysis of the scavenging process is at first performed in order to accurately characterize the engine behavior in terms of scavenging patterns and efficiency. Detailed CFD analyses are used to accurately model the complex set of physical and chemical processes and to properly estimate the fluid-dynamic behavior of the engine, where boundary conditions are provided by a in-house developed 1D model of the whole engine. It is in fact widely recognized that for two-stroke crankcase scavenged, carbureted engines the scavenging patterns (fuel short-circuiting, residual gas distribution, pointwise lambda field, etc.) plays a fundamental role on both of engine performance and tailpipe emissions.In order to assess the accuracy of the adopted numerical approach, comparisons between numerical forecasts and experimental measurements of instantaneous in-cylinder pressure history for steady-state operations of the engine are at first performed and shown in the paper.Subsequently, results from 3D simulations are used to improve the scavenging characterization within the 1D model, where particular emphasis is now devoted to the investigation of the knock occurrence. In order to limit the computational cost of the simulations, the activity is at first carried out within the experimental and 1D modeling frameworks, where a quasi-dimensional combustion and knock model is used.The 1D model is used to compute a numerical knock index which can be useful to address the tuning of the spark advance, given a prescribed and controlled percentage of knock released heat. At the end of the simulation process, the 1D knock index is qualitatively compared to results from full 3D knocking analyses for different in-cylinder compositions and spark timings.


2010 - A Numerical Characterization of New High-Pressure Multi-Hole GDI Injector [Relazione in Atti di Convegno]
Malaguti, Simone; Fontanesi, Stefano; Cantore, Giuseppe
abstract

The paper reports a numerical activity aiming at investigating the spray structure originated by a new-generation GDI injector. The spray is analyzed under quiescent conditions, injecting the fuel in a test vessel at non-evaporative ambient conditions. Results from 3D-CFD simulations are compared to experimental measurements available in literature: commer-cial gasoline at two different injection pressures (10 and 20 MPa) was injected and the spray evolution was ana-lyzed throughout the injection duration.The spray was investigated along the jet axis by the phase Doppler anemometry in order to provide droplet size and velocity, in terms of both axial and radial components. Data were analyzed using the ensemble averaging technique in order to provide mean values.Experimental measurements briefly described above are used to test and validate some lagrangian spray numerical sub-models and numerical parameters such as grid density, numerical setup, primary and secondary fuel breakup and droplet to droplet interaction. Particular care is devoted to the accurate representation of the spray primary breakup, in view of the lack of ad-hoc developed models available in literature. A wide CFD activity is then performed in order to investigate grid effects on the prediction of liquid spray penetration and droplet velocity.Results from the CFD analyses show a relevant dependency of the spray structure on both the computational cell size and the adopted CFD model ensemble.


2010 - Analisi Termo-Meccanica a Fatica di un Motore Diesel Automobilistico [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Cicalese, Giuseppe; Strozzi, Antonio; Giacopini, Matteo
abstract

L’articolo presenta alcuni risultati relativi all’analisi termo-meccanica di un motore Diesel automobilistico. Lo studio è condotto utilizzando simulazioni disaccoppiate CFD e FEM allo scopo di valutare la resistenza a fatica del motore. Una metodologia semplificata per stimare la caratteristica termo-meccanica di testate motore soggette alle reali condizioni operative è stata proposta dagli autori in precedenti pubblicazioni [1,2], ed è ora affinata apportando rilevanti miglioramenti su entrambi i fronti di simulazione.Dal lato CFD, l’analisi CHT (conjugate heat transfer) include nel dominio di calcolo anche il metallo del basamento, dei componenti forzati della testa, nonché della guarnizione. Particolare cura è rivolta alla rappresentazione dello strato limite e all’applicazione delle condizioni al contorno termiche, in particolare alla distribuzione dei flussi termici tra i vari componenti motore. Al fine di massimizzare l’accuratezza delle previsioni CFD, sono valutati criticamente gli effetti dell’ebollizione del refrigerante sulla previsione dello scambio termico.L’accuratezza della previsione numerica viene valutata mediante confronto con misure sperimentali di temperatura in alcuni punti della testa per condizioni stazionarie di funzionamento del motore. I risultati delle simulazioni CFD, ed in particolare la distribuzione puntuale del flusso di calore all’interfaccia fluido/solido, sono trasferiti come condizione al contorno all’analisi termo-strutturale tramite una routine appositamente realizzata. Dal lato termo-meccanico, la principale novità introdotta è l’implementazione di un criterio di tipo energetico per la stima della resistenza a fatica a basso numero di cicli; tale criterio, utilizzato congiuntamente ai più classici criteri tensionali o deformativi, consente di disporre di uno strumento di progettazione capace di predire la resistenza delle singole parti del motore soggette ai differenti carichi agenti. Sono infatti analizzati carichi affaticanti sia ad alto sia a basso numero di cicli, e la metodologia proposta è applicata con successo per predire i possibili punti di innesco di fratture sulla testa e migliorare le caratteristiche del circuito di raffreddamento.


2010 - Multiphase CFD-CHT Analysis and Optimization of the Cooling Jacket in a V6 Diesel Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cicalese, Giuseppe; Giacopini, Matteo
abstract

The paper presents a numerical activity directed at the analysis and optimization of internal combustion engine water cooling jackets, with particular emphasis on the fatigue-strength assessment and improvement.In the paper, full 3D-CFD and FEM analyses of conjugate heat transfer and load cycle under actual engine operation of a single bank of a current production V6 turbocharged Diesel engine are reported.A highly detailed model of the engine, made up of both the coolant galleries and the surrounding metal components, i.e. the engine head, the engine block, the gasket, the valve guides and valve seats, is used on both sides of the simulation process to accurately capture the influence of the cooling system layout under thermal and load conditions as close as possible to actual engine operations.Concerning the CFD side, a 50-50 binary mixture of ethylene-glycol and water is used in order to correctly reproduce the coolant behavior, while boundary conditions are derived from a combination of experimental measurements and a CFD-1D model of the whole engine.In order to find a proper CFD setup for the optimization of the thermal behavior of the engine, a preliminary comparison between experimental temperature distribution within the engine head and CFD forecasts is carried out. Eight thermocouples are used to measure the engine head local temperature at some critical locations.Among the many competing numerical sub-models involved in the CFD simulations, particular attention is devoted to the modeling of phase transition and vapor nuclei formation within the coolant galleries.Concerning the FEM side, thermo-mechanical analyses are carried out aiming at addressing the design optimization of the engine in terms of fatigue strength. In view of the wide range of thermal and load conditions, both high-cycle and low-cycle fatigue must be properly characterized by means of ad-hoc criteria. An energy-based criterion specifically suited for low-cycle fatigue regions is therefore superimposed to well-established S-N o ε-N criteria for the high cycle fatigue regions.The proposed methodology shows very promising results in terms of point-wise detection of possible engine failures ans proves to be an effective tool for the accurate thermo-mechanical characterization of internal combustion engines under actual life-cycle operations.


2010 - Numerical Analysis of GDI Engine Cold-Start at Low Ambient Temperatures [Relazione in Atti di Convegno]
Malaguti, Simone; Fontanesi, Stefano; Severi, Elena
abstract

The paper investigates the low-temperature cranking operation of a current production automotive Gasoline Direct Injected (GDI) by means of 3D-CFD simulations. Particular care is devoted to the analysis of the hollow cone spray evolution within the combustion chamber and to the formation of fuel film deposits on the combustion chamber walls. Due to the high injected fuel amount and the strongly reduced fuel vaporization, wall wetting is a critical issue and plays a fundamental role on both the early combustion stages and the amount of unburnt hydrocarbons formation. In fact, it is commonly recognized that most of the unburnt hydrocarbon emissions from 4-stroke gasoline engines occur during cold start operations, when fuel film in the cylinder vaporize slowly and fuel can persist until the exhaust stroke.In view of the non-conventional engine operating conditions (in terms of injected fuel amount, engine speed, ambient and wall temperature and almost null fuel atomization and breakup), an understanding of the many involved phenomena by means of an optically accessible engine would be of crucial importance. Nevertheless, the application of such technique appears to be almost unfeasible even in research laboratories, mainly because of the relevant wall wetting.CFD analyses prove then to be a very useful tool to gain a full insight of the overall process as well as to correlate fuel deposits to both the combustion chamber design and the injection strategy. In order to better understand where, and how thick, these wall films are formed during the intake and compression, a detailed description of the spray interaction with both the piston wall and the intake valves was performed by the authors in a previous paper [ 1 ]. Subsequently, a wide set of injection strategies was simulated in order to better understand the physics of spray/wall interaction and to minimize the formation of deposits in the combustion chamber most critical locations [ 2 ].In order to limit the overall number of modeling uncertainties (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) the spray model was at first validated against experimental data under low injection pressure, and results from the comparison were reported in [ 1 ].In the present paper, cold start operations at decreasing ambient temperatures are modeled and results are analyzed in terms of both fuel film distribution on the combustion chamber walls and resulting fuel/air mixture distribution within the combustion chamber. The use of CFD simulations prove to be useful to investigate and understand the influence of both combustion chamber design and injection profile on the amount and distribution of fuel deposits, showing a high potential to address future engine optimization


2010 - Sviluppo di un Motore Diesel Due Tempi Veloce per Propulsione Aeronautica [Relazione in Atti di Convegno]
Cantore, Giuseppe; Mattarelli, Enrico; Fontanesi, Stefano; Paltrinieri, Fabrizio; Rinaldini, Carlo Alberto; Perini, Federico; Malaguti, Simone; Severi, Elena; Cicalese, Giuseppe
abstract

Nel campo della propulsione aeronautica per velivoli leggeri, si è recentemente sollevato un forte interesse verso i motori Diesel a due tempi, allo scopo di sostituire i tradizionali motori ad accensione comandata, per i quali risulterà sempre più difficile il reperimento del carburante “avio” negli aeroporti. L’obbiettivo di questo studio è dunque quello di individuare e confrontare tra loro possibili configurazioni adatte all’applicazione aereonautica. Il propulsore scelto come riferimento è prodotto dall’azienda australiana WAM, ha una potenza di 100/120 HP, ed è dotato di sovralimentazione a due stadi, iniezione indiretta, lavaggio unidirezionale (con valvole di scarico in testa).Il primo “step” evolutivo che si è studiato è la trasformazione ad iniezione diretta, con camera a tazza ricavata nel pistone ed iniettore di tipo Common Rail: questa modifica offre il vantaggio di un notevole incremento di potenza ed efficienza, abbinata ad una riduzione delle masse radianti. Oltre a ciò, si è anche analizzato a calcolo un sistema di combustione innovativo, basato su un lavaggio ad anello, senza ausilio di valvole. A fronte della notevole compattazione del motore, aspetto assai apprezzabile in campo aeronautico, con questa soluzione risulta però più difficile ottimizzare lavaggio e combustione, mancando completamente il riferimento a motori moderni.Partendo dal motore base, è stato anzitutto costruito e calibrato sperimentalmente un modello di simulazione termo-fluidodinamico monodimensionale. In parallelo sono state svolte anche simulazioni CFD-3D utilizzando STAR-CD per l’analisi del lavaggio, e KIVA-3V per lo studio della combustione. Queste analisi di dettaglio hanno consentito di caratterizzare i principali processi termo-fluidodinamici che avvengono nelle diverse configurazioni alternative, che sono poi state poste a confronto tramite analisi di ciclo.


2010 - Sviluppo di una metodologia CFD e FEM per l’analisi a fatica di componenti motoristici [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Cicalese, Giuseppe; Strozzi, Antonio; Giacopini, Matteo
abstract

L’articolo presenta alcuni risultati relativi all’analisi termo-meccanica di un motore Diesel automobilistico 6 cilindri a V di cilindrata complessiva 2900cc. Lo studio è condotto utilizzando simulazioni disaccoppiate CFD e FEM allo scopo di valutare la resistenza a fatica dei componenti. La distribuzione di fluido nel circuito di raffreddamento è stata in precedenza oggetto di approfondite analisi e ottimizzazioni al fine di migliorare le caratteristiche dei passaggi del refrigerante. Una metodologia semplificata al fine di stimare la caratteristica termo-meccanica di testate motore soggette alle reali condizioni operative è stata proposta dagli autori in precedenti pubblicazioni. Come conseguenza dell’elevata complessità dei vari fenomeni coinvolti, in questo articolo si introducono alcune importanti migliorie, che consentono un’analisi più accurata della resistenza a fatica del motore, soggetto a carichi affaticanti ad alta frequenza e a bassa frequenza. La metodologia oggetto del presente articolo si basa ancora una volta sull’analisi disaccoppiata CFD e FEM, con rilevanti miglioramenti apportati su entrambi i fronti di simulazione. Dal lato CFD, si utilizza una nuova tipologia di griglia poliedrica, che riesce a combinare l’elevata risoluzione spaziale della mesh con una richiesta computazionale accettabile e un’elevata stabilità numerica della simulazione; particolare attenzione viene dedicata alla rappresentazione del flusso in parete. Mediante l’analisi CFD – CHT (conjugate heat transfer) è valutata la distribuzione puntuale del flusso di calore al refrigerante, includendo nel dominio di calcolo anche il metallo del basamento, della testa completa dei componenti forzati, nonché della guarnizione. Al fine di valutare e incrementare l’accuratezza della previsione numerica, sono stati effettuati e vengono mostrati alcuni confronti con misure sperimentali di temperatura in alcuni punti della testa per condizioni stazionarie di funzionamento del motore. Particolare cura è rivolta alla rappresentazione dello strato limite, fluidodinamico e termico. Allo stesso tempo, grande attenzione è data all’applicazione delle condizioni al contorno termiche, in particolare alla distribuzione dei flussi termici tra i vari componenti affacciati alla camera di combustione. Al fine di massimizzare l’accuratezza delle previsioni CFD, sono valutati criticamente gli effetti dell’ebollizione del refrigerante sulla previsione dello scambio termico tra refrigerante e metallo. I risultati delle simulazioni CFD, ed in particolare la distribuzione puntuale del flusso di calore sulla superficie di contatto fluido/solido, sono successivamente trasferiti come condizione al contorno all’analisi termo-strutturale per la valutazione della resistenza a fatica del componente. A tal fine, si utilizza una routine appositamente realizzata, in grado di mappare la distribuzione puntuale dei flussi termici calcolata tramite le simulazioni CFD su una griglia di calcolo ottimizzata per le analisi FEM. Dal lato termo-meccanico, la principale novità introdotta è l’implementazione di un criterio di tipo energetico per la stima della resistenza a fatica a basso numero di cicli; tale criterio, utilizzato in congiunzione con i più classici criteri tensionali o deformativi, consente di disporre di uno strumento di progettazione capace di predire la resistenza delle singole parti del motore soggette ai differenti carichi agenti. Sono infatti analizzati carichi affaticanti sia ad alto sia a basso numero di cicli, e la metodologia proposta è applicata con successo al fine di predire i possibili punti di innesco di fratture sulla testa e di migliorare le caratteristiche del circuito di raffreddamento.


2009 - CFD Investigation of Fuel Film Formation within a GDI Engine under Cold Start Cranking Operation [Relazione in Atti di Convegno]
Fontanesi, Stefano; Malaguti, Simone
abstract

numerical study on the investigation of spray evolution and liquid film formation within the combustion chamber of a current production automotive Gasoline Direct Injected (GDI) engine characterised by a swirl-type side mounted injector is presented. Particularly, the paper focuses on low-temperature cranking operation of the engine, when, in view of the high injected fuel amount and the strongly reduced fuel vaporisation, wall wetting becomes a critical issue and plays a fundamental role on the early combustion stages. In fact, under such conditions, fuel deposits around the spark plug region can affect the ignition process, and even prevent engine start-up. In order to properly investigate and understand the many involved phenomena, experimental visualisation of the full injection process by means of an optically accessible engine would be a very useful tool. Nevertheless, the application of such technique, far from being feasible from an industrial point of view, appears to be very difficult even in research laboratories, due to the relevant wall wetting at cranking conditions. A numerical program was therefore carried out in order to analyze in depth and investigate the wall/spray interaction and the subsequent fuel deposit distribution on the combustion chamber walls. The CFD model describing the spray conditions at the injector nozzle was previously implemented and validated against experimental evidence. Many different injection strategies were tested and results compared in terms of both fuel film characteristics and fuel/air mixture distribution within the combustion chamber. Low-temperature cranking conditions proved to be an open challenge for the in-cylinder numerical simulations, due to the simultaneous presence of many physical sub-models (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) and the very low motored engine speed. Nevertheless, the use of a properly customized and validated numerical setup led to a good understanding of the overall injection process as well as of the effects of both injection strategy and spray orientation modifications on both the air/fuel and fuel/wall interaction.


2009 - CFD investigation of wall wetting in a GDI engine under low temperature cranking operations [Relazione in Atti di Convegno]
Malaguti, Simone; Fontanesi, Stefano; Cantore, Giuseppe; A., Rosetti; R., Lupi
abstract

The paper reports a numerical activity on the investigation of the spray evolution within the combustion chamber of an automotive DISI engine under low-temperature cranking operations. In view of the high injected fuel amount and the strongly reduced fuel vaporisation at cold cranking, wall wetting becomes a critical issue. Under such conditions, fuel deposits around the spark plug region can affect the ignition process, and even prevent engine start-up. In fact, due to the low injection pressure at engine start-up, the fuel shows almost negligible atomisation and breakup, and the spray structure at the swirl-type injector nozzle is characterized by a single column of liquid fuel, strongly limiting the subsequent vaporisation and enhancing the fuel-wall interaction.In order to properly investigate and understand the many involved phenomena, experimental visualisation of the full injection process by means of an optically accessible engine would be a very useful tool. Nevertheless, the application of such technique, far from being feasible from an industrial point of view, appears to be very difficult even in research laboratories, due to the relevant wall wetting at cranking conditions.CFD analyses prove therefore to be the sole chance to gain a full insight of the overall process, to correlate spark plug wetting to both the combustion chamber design and the injection profile and eventually address either design modifications or changes in the injection strategies. In order to limit the overall number of modelling uncertainties, and to validate the spray model under actual cranking conditions, comparisons with available experimental data at low temperature and low injection pressure were performed and are reported in the paper.


2009 - CFD Methodology Assessment for the Investigation of Convective Heat Transfer Properties of Engine Coolants under Boling Conditions [Relazione in Atti di Convegno]
Fontanesi, Stefano; Malaguti, Simone; E., Mcassey
abstract

The paper presents a combined experimental and numerical program directed at defining a cost/effective methodology for conjugate heat transfer CFD simulations of engine water cooling jackets. As a first step in the process, deficiencies in current numerical strategies for the analysis of conjugate heat transfer problems under typical engine operating conditions are exposed and commented. Results are shown form a wide validation program based on the comparison between experimental measurements from a test facility at Villanova University and CFD predictions at the University of Modena. On the experimental side, the test apparatus consists of a test section, pump, accumulator tank, rejection heat exchanger and required pumping. The test section is provided with a constant volumetric flow rate, and consists of a cylindrical aluminum body with a drilled horizontal flow channel. The section is heated by ten cartridge heaters located at a constant radial distance from the cylinder axis. The test section is connected to the flow loop by means of two calming sections, respectively at the cylinder inlet and exit. Twenty thermocouples are used to measure the test section local temperature along a radial plane cutting the cylinder. Water / ethylene-glycol binary mixture and pure water are tested and compared during the experimental program, in order to reproduce a set of thermal situations as close as possible to actual engine cooling system operation. On the CFD side, an extensive program reproducing the experiments is carried out in order to assess the predictive capabilities of some of the most commonly used eddy viscosity models available in literature. Both non-evaporating and evaporating conditions are tested, showing severe limitations to the use of simplified boiling models to correctly capture the complex interaction between turbulent boundary layer and vapor bubble dynamics. In order to overcome the above stated deficiencies under boiling conditions, a methodology is then proposed to both improve the accuracy of the CFD forecasts and reduce the computational costs of the simulations. A few preliminary results from the validation process are shown and briefly discussed at the end of the paper.


2009 - CFD-3D Multi-Cycle Analysis on a New 2-Stroke HSDI Diesel Engine [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Malaguti, Simone; Mattarelli, Enrico
abstract

The paper describes a CFD multidimensional and multi-cycle engine analysis applied to a novel 2-Stroke HSDI Diesel engine, under development since a few years at the University of Modena and Reggio Emilia. In particular, six operating conditions are considered, two of them at full load and four at partial. The simulation tool is STAR-CD, a commercial software extensively applied by the authors to HSDI Diesel engines. Furthermore, an experimental calibration of the combustion model has been performed and reported in this paper, carrying out CFD simulations on a reference Four Stroke HSDI Diesel engine. As expected, in the multi-cycle analysis a wide dependence of pollutants on trapped charge composition has been found. Much less relevant is the cycle-by-cycle variation in terms of performance parameters, such as trapped mass, IMEP, combustion efficiency, etc. For these parameters, 2 cycle are sufficient to reach a reasonable convergence, while for pollutants 3 or more cycles are required. Information from multi-cycle simulation has been used to refine a 1D engine cycle model, providing a prediction on engine performance at several operating conditions. In particular, the influence of the combustion sub-model has been investigated, comparing the results obtained by entering empirical heat release laws to the results provided by a combustion model refined through multi-cycle 3D simulations.


2009 - Experimental and numerical investigation of conjugate heat transfer in a HSDI Diesel engine water cooling jacket [Relazione in Atti di Convegno]
Fontanesi, Stefano; E., Mcassey
abstract

The paper presents a combined experimental and numerical program directed at improving the accuracy of conjugate heat transfer CFD simulations of engine water cooling jackets.As a first step in the process, a comparison between experimental measurements from a test facility at Villanova University and CFD numerical predictions by at the University of Modena is reported. The experimental test section consists of a horizontal aluminium channel heated electrically and supplied with a constant volumetric flow rate. The operating fluid is a binary 50/50 mixture by volume of ethylene-glycol and water, in order to reproduce a situation as close as possible to actual engine cooling system operations. Temperatures are measured along the channel at several axial locations.On the CFD side, an extensive program reproducing the experiments is carried out in order to assess the predictive capabilities of some of the most commonly used eddy viscosity models available in literature. Both non-evaporating and evaporating conditions are evaluated, showing severe limitations to the use of simplified boiling models to correctly capture the complex interaction between turbulent boundary layer and vapor bubble dynamics.At the end of the validation process, v2-f model proves to yield the best trade off between numerical accuracy and computational costs at least when reproducing non evaporating or slightly evaporating thermal conditions, and the model is therefore applied to predict the temperature distribution within the engine head and block of a 3.0 L HSDI Diesel engine for automotive applications.


2008 - A new decoupled CFD and FEM methodology for the fatigue strength assessment of an engine head [Relazione in Atti di Convegno]
Fontanesi, Stefano; Carpentiero, Davide; Malaguti, Simone; Giacopini, Matteo; Margini, Stefano; L., Arnone
abstract

A 2200 cc engine head for marine applications has been analysed and optimized by means of decoupled CFD and FEM simulations in order to assess the fatigue strength of the component. The fluid distribution within the cooling jacket was extensively analysed and improved in previous works, in order to enhance the performance of the coolant galleries.As a consequence of the many complex phenomena involved, an improved approach is presented in this paper, capable of a better characterization of the fatigue strength of the engine head under both high-cycle and low-cycle fatigue loadings. The improved methodology is once again based on a decoupled CFD and FEM analysis, with relevant improvements added to both simulation realms.From the CFD side, a new generation polyhedral grid is employed to combine high resolution surface spacing, computational demand, and numerical stability of the CFD simulations, with particular emphasis on the boundary layer representation. The local heat flux distribution is calculated by means of CFD analyses of the coolant galleries, now including the engine block portion, plus the engine head metal cast. In order to tune and improve the accuracy of the numerical forecasts, comparisons are carried out with experiments in terms of local metal cast temperature distribution for steady operation of the cooling circuit. Once again, particular care is devoted to the CFD representation of the boundary layer, both fluid and thermal. At the same time, great attention is paid to the thermal boundary conditions, i.e. the distribution of the heat fluxes among the many components facing the combustion process. In order to improve the accuracy of the CFD forecasts, effects of coolant boiling on the heat transfer forecast are investigated and included in the procedure.As a result, a pointwise heat transfer distribution on the fluid/solid interface is transferred as a boundary condition to a thermo-structural analysis for the evaluation of the fatigue strength of the component. An ad-hoc routine is used to map the CFD computed pointwise distribution of the heat flux on a FEM-optimized grid.From the FEM side, an energy based fatigue strength criterion is now implemented in order to create a design tool capable of predicting the fatigue strength of automotive parts subjected to different thermo-mechanical loadings. Both high-cycle fatigue and low-cycle fatigue regions are analysed, and the proposed methodology is successfully applied to predict the site of crack nucleation on an actual engine head and to improve the cooling jacket behaviour.


2008 - Analisi Termofluidodinamica Multidimensionale del Ciclo di un Innovativo Motore Diesel, 2 Tempi Veloce [Relazione in Atti di Convegno]
Cantore, Giuseppe; Carpentiero, Davide; Fontanesi, Stefano; Malaguti, Simone; Mattarelli, Enrico
abstract

L’analisi termofluidodinamica multidimensionale del ciclo di un motore a combustione interna presenta notevoli difficoltà, sia concettuali che pratiche. Da un punto di vista teorico occorre infatti definire una metodologia in grado di simulare in maniera sufficientemente accurata tutti i complessi fenomeni che avvengono all’interno del cilindro e dei condotti (in particolare la miscelazione aria-combustibile, la combustione, l’efflusso attraverso le valvole e/o le luci). Tutt’altro che banale risulta anche la generazione della griglia di calcolo e la gestione del movimento del pistone e degli eventi di apertura e chiusura delle valvole. L’affinamento dei modelli e l’esigenza di reiterare i calcoli per più cicli consecutivi del motore si scontra poi sempre con la limitatezza delle risorse di calcolo, per quanto la potenza dei sistemi di elaborazione sia in costante espansione.Il presente articolo descrive un’analisi multidimensionale e multiciclo applicata ad un innovativo motore Diesel 2 Tempi veloce, in corso di sviluppo presso il Dipartimento di Ingegneria Meccanica e Civile dell’Università di Modena e Reggio Emilia. In particolare, sono state considerate quattro condizioni operative, due a pieno carico e due a carico parziale.Lo strumento di simulazione CFD è un software commerciale, in cui i modelli di spray e combustione sono stati calibrati sulla base di dati sperimentali, ottenuti su di un motore Diesel veloce a 4 Tempi, avente medesimo alesaggio.La metodologia utilizzata si è dimostrata in grado di fornire risultati stabili mediamente dopo tre cicli, ciascuno dei quali ha richiesto circa 200 ore di calcolo su di una macchina quadri-processore. Molto significativa è risultata essere la variazione ciclica per quello che riguarda le emissioni inquinanti, a causa delle differenze di composizione iniziale della carica.E’ stata infine valutata l’influenza delle leggi di rilascio del calore, ottenute mediante queste simulazioni multidimensionali e multiciclo, sui parametri prestazionali del motore a due tempi, calcolati con l’ausilio di un codice gasdinamico mono-dimensionale.


2007 - Multidimensional Cycle Analysis on a Novel 2-Stroke HSDI Diesel Engine [Relazione in Atti di Convegno]
Mattarelli, Enrico; Fontanesi, Stefano; Gagliardi, Vincenzo; Malaguti, Simone
abstract

The Department of Mechanical and Civil Engineering (DIMeC) of the University of Modena and Reggio Emilia is developing a new type of small capacity HSDI 2-Stroke Diesel engine, featuring a specifically designed combustion system. The present paper is focused on the analysis of the scavenging process, carried out by means of 3D-CFD simulations, supported by 1D engine cycle calculations.First, a characterization of the flow through the ports and within the cylinder is performed under conventional operating conditions. Then, a complete 3D cycle simulation, including combustion, is carried out at four actual operating conditions, at full load.The CFD results provide fundamental information to address the development of the scavenging system, as well as to calibrate a comprehensive 1D engine model.


2007 - Thermo-mechanical analysis of an engine head by means of integrated CFD and FEM [Relazione in Atti di Convegno]
Fontanesi, Stefano; Carpentiero, Davide; Gagliardi, Vincenzo; Malaguti, Simone; Margini, Stefano; Giacopini, Matteo; Strozzi, Antonio; L., Arnone; M., Bonanni; D., Franceschini
abstract

A 2200 cc engine head for marine applications has been analysed and optimized by means of both fluid-dynamic and thermo-structural simulations. First, the fluid distribution within the cooling jacket has been deeply investigated, in order to point out critical aspects of the current jacket layout and propose modified gaskets aiming at modifying the coolant path and increasing the cooling performance. A new generation polyhedral grid has been employed to combine high resolution surface spacing, computational demand, and numerical stability of the CFD simulations. Different turbulence models and near-wall approaches have been tested in order to accurately predict the boundary layer behaviour, which is fundamental for the subsequent thermal analysis. Comparisons have been carried out between the different gasket layouts in terms of both cylinder to cylinder flow balancing and cooling effectiveness in the critical regions of the engine head.At a second stage, the CFD model has been extended to the whole engine head, i.e. covering both the cooling jacket and the metal cast, and heat flux distribution on the fluid/solid interface has been computed and transferred as a boundary condition to a structural finite elements code for the analysis of the fatigue strength of the component. To this aim, an ad-hoc developed routine has been created to map the computed punctual distribution of the heat transfer coefficient on a FEM-optimized grid. Particular attention has been paid to the thermal boundary conditions, i.e. the distribution of the heat losses among the combustion chamber and pre-chamber components.Along with this coherent approach of thermo-mechanical loading, the mechanical constitutive law of the material, the damage parameters and an energy based fatigue strength criterion have been considered in order to create a design strategy capable of performing predictive calculations of automotive parts subjected to thermo-mechanical loading. The methodology favoured in this study has been successfully applied to predict the site of incipient crack on an actual engine head.


2006 - CFD optimisation of the in-cylinder flow patterns in a small unit displacement HSDI Diesel Engine for off-highway applications [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Gagliardi, Vincenzo; Malaguti, Simone
abstract

The aim of the paper is to provide information about the in-cylinder flow field optimisation in a high speed, direct injection (HSDI) four valve per cylinder diesel engine for off-highway applications.Fully transient CFD analyses of different valve profile strategies for the intake and compression strokes are at first performed, in order to evaluate the effects on both engine permeability and in-cylinder flow field evolution. Modifications are applied to each intake valve separately: gradually stretched cam profiles are imposed so that strategies range from the standard operation, i.e. the adoption of a unique cam profile for the two intake valves, up to the limit case characterized by a 40 % difference between the intake valves maximum valve lifts for three different engine conditions. Organized flow structures (i.e., swirl) and turbulent flow patterns are investigated, in order to address rules for ad-hoc strategies aiming at finding the best trade off between engine performance and pollutant emission.The effectiveness of the valve strategies is validated by means of full injection and combustion simulations using state of the art models. At first, results for the base case are validated against experiments; subsequently, both full-load / peak-torque and mid-load / low-speed operations for the most promising cases are performed.Relative valve profile strategies prove to strongly influence the flow field within the combustion chamber, and therefore the subsequent spray evolution and fuel combustion, confirming the importance of an ad-hoc optimization in order to meet the best trade-off between performance and pollutant emissions.


2006 - Detailed Experimental and Numerical Investigation of the Spray Structure in a GDI High-Pressure Swirling Injector [Relazione in Atti di Convegno]
Fontanesi, Stefano; Gagliardi, Vincenzo; Malaguti, Simone; G., Valentino
abstract

The paper reports a detailed experimental and numerical investigation of the spray evolution, in terms of flow pattern and droplets size, of a gasoline hollow-cone spray generated by a high-pressure swirl injector for Gasoline Direct Injected (GDI) engine applications. Experiments were carried out, injecting the fuel in a chamber at ambient temperature and atmospheric pressure, in the range of injection pressures between 6 and 10 MPa by means of a common rail injection system, a commercial swirled type injector with a nozzle diameter of 0.50 mm and a cone angle of 67°. A 2-D imaging technique was used to follow the global evolution of the spray as function of the injection time in order to estimate the jet development, the morphology of the spray, and the instantaneous velocity field of fuel droplets by Particle Image Velocimetry (PIV). Images of the spray and PIV shots were captured, firstly, aligning the light sheet to the vertical axis of the spray; then, experiments were also taken with the light sheet placed through the cross section of the spray in order to explore the structure and velocity field at different distances from the nozzle.A PDA system was used to acquire, simultaneously, the droplets velocity as well the droplets size (D10). The system, equipped with an argon-ion laser, was set in forward scattering mode at an off-axis of 30°. Measurements of the axial velocity component and size of droplets were performed, at the same operative conditions as for the PIV ones, close to the nozzle exit (distances of Z=7.5 and 10 mm) and at different radii over 100 injection cycles. As a result, for each measurement, a data set with a minimum of about 40,000 valid data were collected and analyzed, off-line, using the ensemble averaging technique.CFD computations are simultaneously carried out by means of the STAR-CD software. Fuel jet atomization and break-up are evaluated by means of a user-implemented set of models. A preliminary evaluation of the fuel droplet velocity at the injector exit is performed in order to define an instantaneous mass flow rate, aiming at capturing the spray temporal evolution throughout the injection process.


2005 - CFD parametric analysis of the combustion chamber shape in a small HSDI Diesel engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Gagliardi, Vincenzo; Malaguti, Simone; Mattarelli, Enrico
abstract

The paper aims at providing information about the influence of the combustion chamber shape on the combustion process evolution in a high speed direct injection (HSDI) small unit displacement engine for off-highway applications. Small HSDI Diesel engines require a deep optimisation process in order tomaximize specific power output, while limiting pollutant emissions without additional expensive pollutant aftertreatment equipments. Making reference to a current production engine, the purpose of this paper is to investigate the influence of combustion chamber design on both engine performances and combustion efficiency. The actual piston omega-shape is progressively distorted in order to assess the influence of some of the main bowl-features on both mean-flow evolution, mixture formation and pollutants.Using previously validated intake stroke CFD results as initial conditions, compression, injection and combustion simulations are performed in an attempt to trend the influence of bowl depth, bowl entrance angle, omega curvature radius and relative spray-bowl orientation on the engine performances and pollutants. Spray mixing effectiveness, combustion efficiency and NOx and Soot formation are used to compare the different geometries, operating the engine at high-speed full load.


2005 - Effects of Relative Port Orientation on the In-Cylinder Flow Patterns in a Small Unit Displacement HSDI Diesel Engine [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Gagliardi, Vincenzo; Malaguti, Simone
abstract

The paper aims at providing information about the in-cylinder flow structure and its evolution of a high speeddirect injection (HSDI) four valve per cylinder engine for off-highway applications.Fully transient CFD analyses by means of state-of-the-art tools and methodologies are carried out for the wholeintake and compression strokes, in order to evaluate port effects on both engine permability and in-cylinder flowfield evolution. Organized mean motions (i.e., swirl, tumble and squish) are investigated, trying to establishgeneral rules in the port design optimization process, addressing relationships between the relative portorientation and the in-cylinder flow structure. Different port configurations are compared, each deriving from therotation of the BASE port configuration on two different planes, the former being perpendicular to the cylinderaxis, while the latter being parallel to the cylinder axis.Relative intake port orientation proves to strongly influence the flow field evolution within the combustionchamber, and is therefore expected to play a non negligible role on the subsequent spray evolution and fuelcombustion.


2005 - Experimental And Numerical Investigation On Mixture Formation In A HDDI Diesel Engine With Different Combustion Chamber Geometries [Relazione in Atti di Convegno]
Fontanesi, Stefano; Gagliardi, Vincenzo; Malaguti, Simone; S., Alfuso; M., Auriemma; A., Montanaro
abstract

One of the most important phases in thedevelopment of direct-injected Diesel engines is theoptimisation of the fuel spray evolution within thecombustion chamber, since it strongly influences boththe engine performance and the pollutant emissions.Aim of the present paper is to provide information aboutmixture formation within the combustion chamber of aheavy duty direct injection (HDDI) diesel engine formarine applications. Spray evolution, in terms of tippenetration, is at first investigated under quiescentconditions, both experimentally and numerically,injecting the fuel in a vessel under ambient temperatureand controlled gas back-pressure. Results of penetrationand images of the spray from the optically accessiblehigh pressure vessel are used to investigate thecapabilities of some state of the art spray models withinthe STAR-CD software in correctly capturing sprayshape and propagation.The experimental investigation is carried outusing a mechanical injection pump equipping a heavyduty eight cylinder engine. Only one of its plungers isactivated, and the fuel is discharged through a sevenhole nozzle, 0.40 mm in diameter, mounted on amechanical injector. Tests are carried out at two differentload fuel amounts, representing 50%, and 100%respectively, and results are used as data base for theCFD setup. CFD analyses of the intake andcompression strokes are at first performed in order tocompare two different combustion chambers anddifferent jet orientations with respect to the combustionchamber cavities, running the engine under motoredconditions and injecting 50% load fuel amount. Both thetwo tested pistons show two-stage deep valve pocketshollowed under the valve seats projections, but somerelevant differences exist in the piston outer region andin the squish area.Subsequently, full CFD analyses of the intake,compression and combustion processes are performedfor the two different combustion chambers and thepreviously optimised jet orientation, operating the engineat full load, maximum revving speed. Numericalpredictions are used to assess the influence of bothcombustion chamber shapes on the mixture formationeffectiveness and the engine-out emissions.


2005 - Investigation of Mixture Formation Process in a HDDI Diesel Engine by CFD and Imaging Technique [Relazione in Atti di Convegno]
Fontanesi, Stefano; Gagliardi, Vincenzo; Malaguti, Simone; L., Allocca; M., Auriemma; F. E., Corcione; G., Valentino; G., Riganti
abstract

The paper aims at providing information about the spray structure and its evolution within the combustion chamber of a heavy duty direct injection (HDDI) diesel engine. The spray penetration is investigated, firstly under quiescent conditions, injecting the fuel in a vessel under ambient temperature and controlled back pressure by both numerical and experimental analyses using the STAR-CD code and the imaging technique, respectively. Experimental results of fuel injection rate, fuel penetration, and spray cone angle are used as initial conditions to the code and for the comparison of predictions.The experimental investigation is carried out using a mechanical injection pump equipped by the heavy duty eight cylinder engine. Only one of its plungers has been activated and the fuel is discharged through a seven holes mechanical injector, 0.40 mm in diameter. Measurements of fuel injection rate have been performed at 900 rpm pump speed by the AVL Bosch tube at engine loads ranging between 10 to 100% that correspond to the injected fuel from 85 to 600 mg/str. Spray tip penetrations have been measured by an imaging technique in an optically accessible high pressure vessel at different instant from the start of injection and different load conditions.CFD analysis is first focused on in-cylinder flow structure during the intake and compressions strokes to evaluate the swirl and turbulence intensity, as well the tangential profile of the air velocity within two combustion chambers having a different geometry. The prediction of liquid fuel and vapour mass fraction is carried out at 50 and 100% spray load rates considering different jet orientation with respect to the combustion chamber cavities. The predictions are carried to estimate the influence of both shape and jet orientation on the spray behaviour.


2005 - Optimization of a Cooling Circuit in an Internal Combustion Engine for Marine Applications [Relazione in Atti di Convegno]
Cantore, G.; Fontanesi, S.; Gagliardi, V.; Malaguti, S.; Baldini, A.; Giacopini, M.; Strozzi, A.; Rosi, R.
abstract

An optimization study involving both fluid-dynamic and thermostructural aspects has been carried out for a 2200 cc turbocharged engine head for marine applications. In this cross-disciplinary problem, the structural and thermodynamic aspects have been decoupled. A preliminary set of CFD numerical analyses of the cooling jacket layout has been performed, in order to investigate critical aspects of the present configuration and improve the cooling performance, by means of local flow patterns and flow distribution analysis. At a second stage, temperature distributions within the metal cast parts have been derived from CFD in order to assess the fatigue strength of the component with structural finite elements. A proper choice of both CFD methodology and boundary conditions is carried out in order to determine a trade-off between computational effort and actual engine behavior representation. The proposed modelling procedure allows a sensitivity study to be carried out of the engine head to variations of the leading geometric parameters, thus devising an optimized component. The methodology favored in this study is finally applied to carry out a comparison between the basic configuration and a fluid-dynamic improved solution, in order to estimate the effectiveness of the design optimization on the fatigue strength of the component.


2004 - Comparison Between Steady and Unsteady CFD Simulations of Two Different Port Designs in a Four Valve HSDI Diesel Engine: Swirl Intensity and Engine Permeability [Relazione in Atti di Convegno]
Fontanesi, Stefano; Cantore, Giuseppe; Montorsi, Luca; P., Ortolani
abstract

Swirl control strategies are useful methods for controllingmixture formation in HSDI Diesel engines. Test rigs allowsonly steady state measurements of the Swirl number, and giveonly a rough estimation of the charge motion during the actualcompression stroke within the engine. On the contrary, CFDsimulations are powerful tools to characterize the air flowdrawn into the cylinder, since they allow not only steady stateoperations, but also full dynamic modeling of the intake andcompression strokes. This paper studies an application ofcomputational fluid dynamics for predicting intake swirlintensity in an automotive 4 valve per cylinder D.I. Dieselengine. Two different intake ports are compared and the besttrade off between engine permeability and swirl intensity isassessed. Both steady state and dynamic simulations of theinduction process are carried out, and results demonstrate thatsteady state analysis is a reliable tool for predicting the portpermeability, while the same capability is not proved ininvestigating the organized charge motion within the chamber.


2004 - Development and validation of a boundary layer control system to increase intake port steady permeability [Relazione in Atti di Convegno]
Fontanesi, Stefano
abstract

Engine permeability, which is commonly knownto exert a strong influence on engine performances, isusually experimentally addressed by means of thedefinition of a global parameter, the steady dischargecoefficient. Nevertheless, the use of such a parameter todescribe valve-port assembly behaviour appearssometimes to be insufficient to determine portfluidynamic behaviour, due to the simultaneousconcurrency of complex mechanisms, such as meanflow distortions and boundary layer detachments. CFDsimulation appears therefore to be a fundamental tool tofully understand port fluidynamic behaviour.In the present paper, two engine intake portassemblies are investigated by using the STAR-CD CFDcode, showing a strongly different behaviour from thepoint of view of secondary detached flows generationacross the valve. Flow separation in the valve seatregion reveals to be detrimental on engine steadybreathing performances, since the subsequentrecirculation region strongly limits the valve curtainusage and forces the mean flow to crash against thevalve. In order to reduce the growth of secondarydetached flows upstream of the valve seat, the detachfavourableport is equipped with a boundary layer controlpneumatic device, which proves to be capable of nearlyeliminating flow separation in the valve region. Thissolution is finally compared to the non-detaching design,showing a non negligible benefit in terms of dischargecoefficient, and therefore engine permeability. Since theevaluation of the steady-flow discharge coefficient andflow patterns of ICE port assembly is strongly sensitiveto the capability of the turbulence sub-models incapturing the boundary layer dynamics, cubic low-Reynolds k-ε model is used for simulations.


2004 - Numerical Analysis of Swirl Control Strategies in a Four Valve HSDI Diesel Engine [Relazione in Atti di Convegno]
Fontanesi, Stefano; Mattarelli, Enrico; Montorsi, Luca
abstract

Recent four valve HSDI Diesel engines are able to controlthe swirl intensity, in order to enhance the in-cylinder flowfield at partial load without decreasing breathing capabilities atfull load.Making reference to a current production engine, thepurpose of this paper is to investigate the influence of portdesign and flow-control strategies on both engine permeabilityand in-cylinder flow field.Using previously validated models, 3-D CFD simulationsof the intake and compression strokes are performed in orderto predict the in-cylinder flow patterns originated by thedifferent configurations.The comparison between the two configurations in termsof airflow at full load indicates that Geometry 2 can trap3.03% more air than Geometry 1, while the swirl intensity atIVC is reduced (-30%).The closure of one intake valve (the left one) is veryeffective to enhance the swirl intensity at partial load: theSwirl Ratio at IVC passes from 0.7 to 2.6 for Geometry 1,while for Geometry 2 it varies from 0.4 to 2.9.


2004 - Optimization of the Intake System on Diesel Engines Featuring a High Pressure Injection System [Capitolo/Saggio]
Borghi, Massimo; Cantore, Giuseppe; Mattarelli, Enrico; Milani, Massimo; Fontanesi, Stefano; Montorsi, Luca; D., Balestrazzi
abstract

Combustion in Diesel engines is controlled by the interaction between fuel jet and mean and turbulent flow field. Therefore, the use of novel fuel injection strategies should be always integrated with the optimisation of the mean and turbulent flow field within the cylinder. While in the last years injection system technology has evolved at an impressive rate, establishing new standards, the development of design criteria for intake ports, ducts and plenums is not keeping the same pace. The authors believe that a substantial work should be carried out in this field. For this purpose numerical simulation should play a fundamental role to cut development time, as well as to gain a better understanding of the complex thermo-fluid-dynamics phenomena occurring within the engines.This paper reviews the fundamentals of the experimental and theoretical work carried out at DIMEC-University of Modena and Reggio for supporting the development of more efficient HSDI Diesel engines.


2004 - The influence of Swirl Control Strategies on the Intake Flow in Four Valve HSDI Diesel Engines [Relazione in Atti di Convegno]
Mattarelli, Enrico; Borghi, Massimo; D., Balestrazzi; Fontanesi, Stefano
abstract

Standard design practice usually adopts steady flow tests for addressing optimisation of the intake valve-port assembly. Recently, with more user-friendly CFD tools and with increased computer power, intake stroke simulations, handling both piston and valves motion, have become practical. The purpose of this paper is to compare the design guidelines provided by the standard steady flow tests (both experimental and numerical) and the information coming from a CFD-3D intake stroke analysis. Reference is made to a four valve HSDI Diesel Engine.Three swirl control strategies are investigated. It is supposed that one intake valve is kept closed, while the other one operates normally (fist strategy). The second strategy consists in a 50% reduction of the lift of both valves. Finally, the third possibility is the blockage of one intake port by means of a simple butterfly valve.While the steady flow tests (both numerical and experimental) indicate that the first and the third strategy massively enhance the swirl intensity in comparison to the baseline (+75 %, according to experiments), intake stroke calculations show an almost null advantage. Furthermore, the actual in-cylinder flow field presents complex patterns that cannot be described in terms of Swirl Ratio.


2003 - On the applications of low-reynolds cubic k-ε turbulence Models in 3D simulations of ICE intake flows [Relazione in Atti di Convegno]
Bianchi, G. M.; Fontanesi, Stefano
abstract

The evaluation of the steady-flow dischargecoefficient of ICE port assemble is known to be verysensitive to the capability of the turbulence sub-modelsin capturing the boundary layer dynamics. Despite thefact that the intrinsically unsteady phenomena related toflow separation claim for LES approach, the presentpaper aims to demonstrate that RANS simulation canprovide reliable design-oriented results by using low-Reynolds cubic k-ε turbulence models.Different engine intake port assemblies andpressure drops have been simulated by using the CFDSTAR-CD code and numerical results have beencompared versus experiments in terms of both globalparameters, i.e. the discharge coefficient, and localparameters, by means of static pressure measurementsalong the intake port just upstream of the valve seat.Computations have been performed by comparing twoturbulence models: Low-Reynolds cubic k-ε and High-Reynolds cubic k-ε.The analysis leaded to remarkable assessmentsin the definition of a correct and reliable methodology forthe evaluation of engine port breathing capabilities.Comparison between numerical results and experimentsshowed that the low-Reynolds cubic k-ε model isunavoidable to correctly capture the influence of portfeature variations on engine permeability. In particular,the deficiencies demonstrated by High-Reynolds cubick-ε turbulence model in resolving the influence of nearwallshear and adverse pressure gradient effect onboundary layer dynamics are completely overcome bythe use of the Low-Reynolds formulation.


2002 - A methodology for the in-cylinder flow field evaluation in a low stroke-to-bore SI engine [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Mattarelli, Enrico; Bianchi, G. M.
abstract

This paper presents a methodology for the 3D CFD simulation of the intake and compression processes of four stroke internal combustion engines.The main feature of this approach is to provide very accurate initial conditions by means of a cost-effective initialization step. Calculations are applied to a low stroke-to-bore SI engine, operated at full load and maximum engine speed. It is demonstrated that initial conditions for this kind of engines have an important influence on flow field development, particularly in terms of mean velocities close to the firing TDC


2002 - CFD Analysis of the In-Cylinder Flow in DI Diesel Engines [Capitolo/Saggio]
Borghi, Massimo; Cantore, Giuseppe; Mattarelli, Enrico; Milani, Massimo; Fontanesi, Stefano; Montorsi, Luca
abstract

Non disponibile


2002 - Turbulence Modelling in CFD Simulations of ICE Intake Flows: The Discharge Coefficient Prediction [Relazione in Atti di Convegno]
G. M., Bianchi; Cantore, Giuseppe; Fontanesi, Stefano
abstract

The paper is focused on the influence of theeddy viscosity turbulence models (EVM) in CFD threedimensionalsimulations of steady turbulent engineintake flows in order to assess their reliability inpredicting the discharge coefficient. Results have beenanalyzed by means of the comparison with experimentalmeasurements at the steady flow bench. High Reynoldslinear and non-linear and RNG k-ε models have beenused for simulation, revealing the strong influence ofboth the constitutive relation and the ε-equationformulation on the obtained results, while limits in theapplicability of more sophisticated near-wall approachesare briefly discussed in the paper.Due to the extreme complexity of typical ICEflows and geometries, the analysis of the behavior ofEVM turbulence models has been subsequently appliedto a test-case available in literature, i.e. a high-Reynoldscompressible flow over a inclined backward facing step(BFS). Different high-Reynolds and low-Reynolds linearand quadratic k-ε models, linear and quadratic RNG k-εmodels and linear and quadratic two-layer models havebeen used for simulation. The predicted mean velocityprofiles at different locations along the duct have beencompared versus experiments available in literature.


2001 - A new concept of supercharging applied to high speed DI Diesel engines [Relazione in Atti di Convegno]
Cantore, Giuseppe; Mattarelli, Enrico; Fontanesi, Stefano
abstract

The supercharging system investigated in this study is made up of a traditional turbocharger, coupled with a Roots-type positive displacement compressor. An electrically actuated clutch allows the compressor to be disengaged from the engine at high speed and under partial load steady operations (such as the ones occurring in a driving cycle).This concept of supercharging has been applied to the downsizing of a reference engine (a 2.5 litre, turbocharged, four cylinder, high speed DI Diesel engine), without penalization on the maximum brake power (110 kW) and transient response.For such a purpose, a “paper” engine has been theoretically characterized. The gross engine parameters have been optimised by means of 1-D numerical simulations, using a computational model previously validated against experiments.Performances of the reference and the downsized engine have been compared, considering both steady and transient operating conditions, full and partial load. The two-stage supercharging system allows the “paper” engine to provide higher values of torque at low engine speed and full load, and to perform slightly better in terms of transient response. Furthermore, when considering operating conditions occurring in the European Driving cycle (Roots compressor disengaged), the downsized engine shows lower fuel consumption (from 6 to 24%), and lower pollutant emissions.


2001 - La sovralimentazione volumetrica per motori ad accensione comandata ad alte prestazioni [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Mattarelli, Enrico; Montorsi, Luca
abstract

Analisi numerica monodimensionale dei vantaggi derivanti dalla sovralimentazione applicata ai motori ad accensione comandata, nell'ottica dell'engine downsizing.


2001 - Turbulence modelling in CFD simulation of ICE intake flow [Relazione in Atti di Convegno]
Bianchi, G. M.; Cantore, Giuseppe; Fontanesi, Stefano
abstract

The paper is focused on the influence of the eddy viscosity turbulence models (EVM) in CFD three-dimensional simulations of steady turbulent engine intake flows in order to assess their reliability in predicting the discharge coefficient. Results have been analyzed by means of the comparison with experimental measurements at the steady flow bench. High Reynolds linear and nonlinear and RNG k-ge models have been used for simulation, revealing the strong influence of both the constitutive relation and the -equation formulation on the obtained results, while limits in the applicability of more sophisticated near-wall approaches are briefly discussed in the paper. Due to the extreme complexity of typical ICE flows and geometries, the analysis of the behavior of EVM turbulence models has been subsequently applied to a test-case available in literature, i.e., a high-Reynolds compressible flow over an inclined backward-facing step (BFS). Different high-Reynolds and low-Reynolds linear and quadratic k-ge models and linear and quadratic two-layer models have been used for simulation. The predicted velocity profiles at different locations along the duct have been compared versus experiments available in literature. The EVM model constitutive relation as well as near-wall treatment was found to be fundamental for accurately predicting the flow characteristics. In the recirculation regions the nonlinear EVM models behave much better than the standard linear EVM thanks to their more accurate physical ground, thus determining the best agreement with experimental data. Simulations revealed also limits of the RANS approach and related EVM when faced with typically unsteady and complex phenomena like flow separation as those occurring in engine intake ducts.


2000 - Analisi del “Matching” motore - turbocompressore in un motore Diesel automobilistico ad iniezione diretta [Relazione in Atti di Convegno]
Cantore, Giuseppe; Fontanesi, Stefano; Mattarelli, Enrico; Montorsi, Luca; Luppino Bertoni, F.
abstract

n.d.