Nuova ricerca

FABIO BERNI

Ricercatore t.d. art. 24 c. 3 lett. B
Dipartimento di Ingegneria "Enzo Ferrari"

Home | Curriculum(pdf) | Didattica |


Pubblicazioni

2024 - A methodology to reduce the computational effort in 3D-CFD simulations of plate-fin heat exchangers [Articolo su rivista]
Torri, Federico; Berni, Fabio; Giacalone, Mauro; Mantovani, Sara; Defanti, Silvio; Colombini, Giulia; Bassoli, Elena; Merulla, Andrea; Fontanesi, Stefano
abstract

The analysis of a plate-fin heat exchanger performance requires the evaluation of key parameters such as heat transfer and pressure drop. In this regard, computational Fluid Dynamics (CFD) can be proficiently adopted, at the design stage, to predict the performance of plate-fin heat exchangers. However, these last are often characterized by a complex geometry, such as in the case of plate exchangers with turbulators, leading to a huge computational effort, which often exceeds the available resources. In this study, a numerical methodology for the simulation of plate heat exchangers is proposed, to bypass the limits imposed by the computational cost. The methodology relies on the simulation of a minimal portion of the exchanger (two plates, one per fluid) characterized by periodic boundary conditions (that mimic the presence of several layers). The total heat exchanged is obtained simply multiplying the calculated heat transfer by the number of plate couples composing the device. Moreover, the two plates allow to calibrate porous media which are adopted to rebuild (in a simplified version) the two fluid circuits of the whole exchanger and obtain the overall pressure drop across the device for both the hot and cold fluids. The proposed approach is validated against experimental data of an oil cooler for automotive application, that is a plate-fin heat exchanger characterized by the presence of turbulators. The numerical outcomes are compared to the experiments in terms of pressure drop and heat transfer for a wide range of volumetric flow rates. Particular attention is devoted to the mesh sensitivity and the adopted computational grid minimizes the number of cells (and, thus, the computational cost), without compromising the accuracy. Moreover, the Reynolds-Stress-Transport turbulence model is accurately selected among the most diffused ones, in order to properly match the test bench data. The proposed methodology allows to reduce of nearly one order of magnitude the total number of cells required for the simulation of the heat exchanger performance. The heat transfer is predicted with high accuracy, i.e. error is always lower than 4%. As for the pressure loss, the deviation compared to the experiments increases up to nearly 15% (for one of the simulated conditions) but it is considered still acceptable.

2024 - Proposal and validation of a numerical framework for 3D-CFD in-cylinder simulations of hydrogen spark-ignition internal combustion engines [Articolo su rivista]
Sfriso, S.; Berni, F.; Fontanesi, S.; D'Adamo, A.; Frigo, S.; Antonelli, M.; Borghi, M.
abstract

2023 - A 3D-CFD Numerical Approach for Combustion Simulations of Spark Ignition Engines Fuelled with Hydrogen: A Preliminary Analysis [Relazione in Atti di Convegno]
Sfriso, S.; Berni, F.; Fontanesi, S.; D'Adamo, A.; Antonelli, M.; Frigo, S.
abstract

With growing concern about global warming, alternatives to fossil fuels in internal combustion engines are searched. In this context, hydrogen is one of the most interesting fuels as it shows excellent combustion properties such as laminar flame speed and energy density. In this work a CFD methodology for 3D-CFD in-cylinder simulations of engine combustion is proposed and its predictive capabilities are validated against test-bench data from a direct injection spark-ignition (DISI) prototype. The original engine is a naturally aspirated, single cylinder compression ignition (Diesel fueled) unit. It is modified substituting the Diesel injector with a spark plug, adding two direct gas injectors, and lowering the compression ratio to run with hydrogen fuel. A 3D-CFD model is built, embedding in-house developed ignition and heat transfer models besides G-equation one for combustion. Three different lean-burn conditions are selected among the tested ones for the validation of the numerical framework. The investigated conditions are characterized by the same revving speed (3000 rpm) but different equivalence ratios (0.4, 0.6 and 0.8, respectively). A good agreement with the experimental dataset is observed, confirming the validity of the proposed CFD approach, and opening the possibility of further virtual optimizations of the engine.

2023 - An integrated 0D/1D/3D numerical framework to predict performance, emissions, knock and heat transfer in ICEs fueled with NH3–H2 mixtures: The conversion of a marine Diesel engine as case study [Articolo su rivista]
Berni, F.; Pessina, V.; Teodosio, L.; D'Adamo, A.; Borghi, M.; Fontanesi, S.
abstract

In the maritime transportation, e-fuels represent a valid alternative to fossil energy sour- ces, in order to accomplish the European Union goals in terms of climate neutrality. Among the e-fuels, the ammonia-hydrogen mixtures can play a leading role, as the combination of the two allows to exploit the advantages of each one, simultaneously compensating their gaps. The main goal of the present publication is the proposal of a robust numerical frame- work based on 0D, 1D and 3D tools for CFD analyses of internal combustion engines fueled with ammonia-hydrogen mixtures. The 1D engine model provides boundary conditions for the multi-dimensional in- vestigations and estimates the overall engine performance. 3D in-cylinder detailed ana- lyses are proficiently used to predict combustion efficiency (via the well-established G-equation model supported by laminar flame speed correlations for both ammonia and hydrogen) and emissions (with a detailed chemistry based approach). Heat transfer and knock tendency are evaluated as well, by in-house developed models. As for the 0D/1D chemical kinetics calculations, firstly they support 3D analyses (for example via the gen- eration of ignition delay time tables). Moreover, they allow insights on aspects such as NOx formation, to individuate mixture qualities able to strongly reduce the emissions.

2023 - CFD Simulations and Potential of Nanofluids for PEM Fuel Cells Cooling [Relazione in Atti di Convegno]
D'Adamo, A.; Corda, G.; Berni, F.; Diana, M.; Fontanesi, S.
abstract

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are undergoing a rapid development, due to the ever-growing interest towards their use to decarbonize power generation applications. In the transportation sector, a key technological challenge is their thermal management, i.e. the ability to preserve the membrane at the optimal thermal state to maximize the generated power. This corresponds to a narrow temperature range of 75-80°C, possibly uniformly distributed over the entire active surface. The achievement of such a requirement is complicated by the generation of thermal power, the limited exchange area for radiators, and the poor heat transfer performance of conventional coolants (e.g., ethylene glycol). The interconnection of thermal/fluid/electrochemical processes in PEMFCs renders heat rejection as a potential performance limiter, suggesting its maximization for power density increase. To this aim, suspensions of coolants and nanoparticles (nanofluids) have been proposed for PEMFCs cooling, although their characterization has often been limited to the superior thermal conductivity, overlooking a comprehensive understanding, and leaving a relevant research gap. In this paper, nanofluids cooling is simulated using 3D-CFD in a small laboratory scale (25 cm2) model of a hydrogen-air PEMFC with a liquid cooling circuit. The variation of the coolant fluid is studied considering flow uniformity, heat rejection, pressure losses, and power generation, ultimately leading to a high-level analysis on the trade-off between heat transfer/storage, relevant for coolant channels in PEMFCs. The study elucidates the membrane conditions and the compositional requirements for ethylene glycol and water based nanofluids to lead to a net gain in the generated power density, modelled in the range of +5/10% for high particle loading (10%) and envisaged to reach +15% for hypothesized ideal compositions. The study clarifies the role of nanofluids for PEMFC cooling and redefines their enabler contribution in the development of high power density PEMFCs, indicating guidelines for their application-designed formulation.

2023 - Evaluation of TPMS Structures for the Design of High Performance Heat Exchangers [Relazione in Atti di Convegno]
Torri, F.; Berni, F.; Fontanesi, S.; Mantovani, S.; Giacalone, M.; Defanti, S.; Bassoli, E.; Colombini, G.
abstract

The development of the additive manufacturing tech nology has enabled the design of components with complex structures that were previously unfeasible with conventional techniques. Among them, the Triply Periodic Minimal Surface (TPMS) structures are gaining scientific interest in several applications. Thanks to their high surface-to-volume ratio, lightweight construction, and excep tional mechanical properties, TPMS structures are being investigated for the production of high-performance heat exchangers to be adopted in different industrial fields, such as automotive and aerospace. Another significant advantage of the TPMS structures is their high degree of design flexibility. Each structure is created by replicating a characteristic unit cell in the three spatial dimensions. The three key parameters, namely cell type, cell dimension and wall thickness can be adjusted to provide considerable versatility in the design process. As for the heat exchangers, the variation of these parameters results in different values of heat transfer and pressure drop. If, on the one and, this flexibility leads to a wide range of design possibilities, on the other hand it generates uncertainty when the most suitable cell (with the best set of parameters) has to be selected. Therefore, the aim of the paper is to address the initial challenge in the design process of an innovative heat exchanger that incorporates a TPMS structure, which is the selection of the unit cell. Based on a literature review, four TPMS structures are selected as the most promising ones for the purpose, namely Gyroid, I-WP, Primitive and Diamond. Small prototypes of the selected structures are numerically tested at laminar and turbulent flow conditions to compare their performances in terms of heat transfer and pressure drop against a more traditional solution. In order to ensure an unbiased comparison between the structures, they are compared on equal volume of the specimen, wall thickness and unit cell dimension. Finally, a compact plate heat exchanger based on turbulators is added to the comparison, to investigate the capabilities of the TPMS structures compared to a more conventional solution.

2023 - Impact of fuel surrogate formulation on the prediction of knock statistics in a single cylinder GDI engine [Articolo su rivista]
Fontanesi, S.; Shamsudheen, F. A.; Gonzalez, E. G.; Sarathy, S. M.; Berni, F.; D'Adamo, A.; Borghi, M.; Breda, S.
abstract

The statistical tendency of an optically accessible single-cylinder direct-injection spark-ignition engine to undergo borderline/medium knocking combustion is investigated using 3D-CFD. Focus is made on the role of fuel surrogate formulation for the characterization of anti-knock quality and flame speed of the actual fuel. An in-house methodology is used to design surrogates able to emulate laminar flame speed and autoignition delay times of the injected fuel. Two different surrogates, characterized by increasing level of complexity, are compared. The most complex one (six components) improves the representation of the real fuel, highlighting the crucial role of accurate fuel kinetics to predict flame propagation and unburnt mixture reactivity. A devoted chemical mechanism including the oxidation pathways for all the species in the surrogate is also purposely developed for the current analysis. Knock is investigated using a proprietary statistical knock model (GruMo-UNIMORE Statistical Knock Model, GK-PDF), which can infer the probability of knocking events within a RANS formalism. Predicted statistical distributions are compared to measured counterparts. The proposed numerical/experimental comparison demonstrates the possibility to efficiently integrate complex-chemistry driven information in 3D-CFD combustion simulations without online solving chemical reactions: a combination of laminar flame speed correlations, ignition delay look-up tables, and a statistics-based knock model is adopted to estimate the percentage of knocking cycles in a GDI engine while limiting the computational cost of the simulations.

2023 - 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

2023 - Optimization via genetic algorithm of a variable-valve-actuation spark-ignition engine based on the integration between 1D/3D simulation codes and optimizer [Articolo su rivista]
Teodosio, L.; Berni, F.
abstract

In this work, a turbocharged spark ignition engine equipped with a variable valve actuation device is investigated to numerically optimize the Brake specific fuel consumption (BSFC) at different loads and speeds by employing a genetic algorithm. The engine is preliminary analyzed at the test bench under both part and full load operations and different valve strategies. A system schematization is realized in a 1D code. The developed model is integrated with user-defined sub-models for the description of the in-cylinder processes, and then is validated over the measurements. A 3D CFD model of a single cylinder is developed in a commercial code and validated against experimental mean in-cylinder pressure and combustion indicators. The validated 1D engine model is coupled to an external optimizer, to identify the optimal calibration, performing multi-variable and multi-objective optimizations with the adoption of the MOGA genetic algorithm. The latter aims at minimizing the BSFC in a BMEP sweep, at fixed speed, while controlling the load through the Inlet Valve Closure (IVC) at fully opened throttle valve. The optimization results show that an advanced control of the intake valve strategy allows a maximum BSFC advantage of 26% at medium/high loads and medium speeds, if compared to the manufacturer-advised engine calibration. The outcomes of the optimization process are also confirmed by the 3D CFD tool. The latter not only contributes to the tuning of the 1D model, but it also provides an in-depth on detailed 3D aspects, such as turbulence and knock, that could not be assessed via a simplified 1D approach. The presented methodology represents a valuable tool to refine the virtual calibration of VVA engines and to support the design phase, thus remarkably reducing the experimental efforts. Moreover, it is a promising example of integration between 1D and 3D CFD tools.

2023 - Refined Structural Design and Thermal Analyses of a High-Speed Wound-Field Generator for the More Electrical Aircraft [Relazione in Atti di Convegno]
Guiducci, A.; Barbieri, S. G.; Nuzzo, S.; Barater, D.; Berni, F.; Cicalese, G.; Fontanesi, S.; Franceschini, G.
abstract

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.

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 - 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 - 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 - 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.

2020 - 3D-CFD Simulation of a GDI Injector under Standard and Flashing Conditions [Relazione in Atti di Convegno]
Sparacino, S.; Berni, F.; Riccardi, M.; Cavicchi, A.; Postrioti, L.
abstract

In the optimization of GDI engines, fuel injection plays a crucial role since it can affect the combustion process and, thus, fuel efficiency and pollutant emissions. The challenging task is to obtain the required fuel distribution and atomization inside the combustion chamber over a wide range of engine operating conditions. To achieve such goals, flash-boiling can be exploited. Flash-boiling is a phenomenon occurring when fuel temperature exceeds saturation temperature or, similarly, when ambient pressure is lower than saturation one. Under these conditions, which can occur inside the injector or directly in the combustion chamber, the fuel undergoes extremely accelerated breakup and quickly evaporates. The proposed manuscript shows the application of an alternative flashboiling model, recently implemented by Siemens-PLM in STAR-CD V.2019.1, to be applied in 3D-CFD Lagrangian simulations of GDI sprays. Results are validated against experimental data, provided by the SprayLAB of the University of Perugia, on a single-hole research injector. The new flash-boiling model consists of three main parts: an atomization model able to compute droplet initial conditions and the overall spray cone angle; an evaporation model and, finally, a droplet break-up model; the last two models are designed to simulate all the physical events occurring when droplets are injected into the combustion chamber. As for the investigated operating condition, vessel pressure and temperature are 40 kPa and 293K, respectively; as for the fuel (n-Heptane) temperature, it ranges from 303.15 K to 393.15 K, on equal injection pressure (10 MPa). The numerical-experimental comparison is carried out in terms of liquid penetration, imaging, and droplet sizing.

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 - 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 - 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 - 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 - 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 - 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 - Design of a Hybrid Power Unit for Formula SAE Application: Packaging Optimization and Thermomechanical Design of the Electric Motor Case [Articolo su rivista]
Mangeruga, V.; Giacopini, M.; Barbieri, S. G.; Berni, F.; Mattarelli, E.; Rinaldini, C.
abstract

This paper presents the development of a parallel hybrid power unit for Formula SAE application. In particular, the system is made up of a brand new, single-cylinder 480 cc internal combustion engine developed on the basis of the Ducati "959 Superquadro" V90 2-cylinders engine. The thermal engine is assisted by a custom electric motor (30 kW), powered by a Li-Ion battery pack. The performance of the ICE has been optimized through CFD-1D simulation (a review of this activity is reported in a parallel paper). The main design goal is to get the maximum amount of mechanical energy from the fuel, considering the car typical usage: racing on a windy track. The Ducati "959 Superquadro" engine is chosen because of its high power-to-weight ratio, as well as for its V90 2-cylinder layout. In fact, the vertical engine head is removed and it is subsequently replaced by the electric motor directly engaged to the crankshaft using the original valvetrain transmission chain, thus achieving a very compact package. The mechanical behaviour of the original chain is investigated for this purpose. A specific electric motor case is then designed and manufactured via Additive Manufacturing technology, in order to include the chain housing, the electric motor cooling system and the lubrication system. Furthermore, the case flange is designed to perfectly fit to the original engine deck in order to allow the engine cooling circuit to match with the electric motor cooling circuit. Several types of circuit layout - around the stator - are analysed via CFD simulations comparing pressure drop and heat transfer coefficients. Finally, a thermo-structural analysis is performed in order to assess the mechanical strength of the electric motor case.

2019 - Effects of nanofluid contaminated coolant on the performance of a spark ignition engine [Relazione in Atti di Convegno]
Teodosio, L.; Bozza, F.; Berni, F.
abstract

In this work, the effects of alumina nanoparticle contaminated coolant on the performance of a small Spark Ignition engine are investigated by 1D and 3D models. An analysis regarding the alumina nanofluid properties is carried out, and, in particular, a reliable correlation for the thermal conductivity ratio between nanofluid and base coolants is selected. Firstly, a single-cylinder 3D-CFD model of a similar engine cooling system is developed in Star-CCM+ and it is employed to derive the relative increase of the convective in-cylinder heat transfer coefficient. The latter takes into account the geometrical effects of the cooling system, the flow conditions and the nanofluid characteristics. Secondly, a 1D engine model is developed and validated against the available experimental findings with standard coolant fluid. The model is then employed, in a predictive way, to perform full and part load analyses, where the 3D-predicted heat transfer coefficient of the nanofluid contaminated coolant is imposed as an input. The outcomes reveal the potential of the considered nanofluid to achieve fuel consumption improvements (up to about 5.4%) at full load, mainly due to decreased knock tendency and reduced mixture over-fuelling, while minor fuel consumption penalizations (about 1.0 %) are observed at low loads.

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 - Impact of different droplets size distribution on the morphology of GDI sprays: Application to multi-hole injectors [Relazione in Atti di Convegno]
Sparacino, S.; Berni, F.; Cavicchi, A.; Postrioti, L.
abstract

The scientific literature focusing on the numerical simulation of fuel sprays is plenty of atomization and secondary break-up models, all aiming at simulating GDI sprays under possibly any engine condition in terms of injection pressure and cylinder back pressure. However, it is well known that the predictive capabilities of even the most diffused break-up models are affected by injection parameters, especially backpressure. As a consequence, model constants require usually substantial tuning based on the specific operating conditions. In this manuscript, an alternative atomization methodology is proposed for the 3D-CFD simulation of GDI sprays, aiming at reducing case-to-case tuning of the model constants for variations of the operating conditions. In particular, attention is focused on the effects of back pressure, which has a huge impact on both the morphology and the sizing of GDI sprays. 3D-CFD Lagrangian simulations of two different multi-hole GDI injectors are presented. The first injector is a 5-hole GDI prototype unit operated at ambient conditions. The second one is the well-known Spray G, characterized by a higher back pressure (up to 0.6 MPa). Results are compared against experimental data in terms of liquid penetration, PDA (Phase Doppler Anemometry) data of droplet sizing / velocity and imaging. CFD results are demonstrated to be highly sensitive to the spray vessel pressure, mainly because of the atomization strategy. The proposed alternative approach proves to strongly reduce such dependency. To confirm the improvements, such approach is combined to two different well-known secondary break-up models, namely Reitz's model and the KHRT one.

2019 - Impact of the Primary Break-Up Strategy on the Morphology of GDI Sprays in 3D-CFD Simulations of Multi-Hole Injectors [Articolo su rivista]
Sparacino, Simone; Berni, Fabio; D'Adamo, Alessandro; Krassimirov Krastev, Vesselin; Cavicchi, Andrea; Postrioti, Lucio
abstract

The scientific literature focusing on the numerical simulation of fuel sprays is rich in atomization and secondary break-up models. However, it is well known that the predictive capability of even the most diused models is aected by the combination of injection parameters and operating conditions, especially backpressure. In this paper, an alternative atomization strategy is proposed for the 3D-Computational Fluid Dynamics (CFD) simulation of Gasoline Direct Injection (GDI) sprays, aiming at extending simulation predictive capabilities over a wider range of operating conditions. In particular, attention is focused on the eects of back pressure, which has a remarkable impact on both the morphology and the sizing of GDI sprays. 3D-CFD Lagrangian simulations of two dierent multi-hole injectors are presented. The first injector is a 5-hole GDI prototype unit operated at ambient conditions. The second one is the well-known Spray G, characterized by a higher back pressure (up to 0.6 MPa). Numerical results are compared against experiments in terms of liquid penetration and Phase Doppler Anemometry (PDA) data of droplet sizing/velocity and imaging. CFD results are demonstrated to be highly sensitive to spray vessel pressure, mainly because of the atomization strategy. The proposed alternative approach proves to strongly reduce such dependency. Moreover, in order to further validate the alternative primary break-up strategy adopted for the initialization of the droplets, an internal nozzle flow simulation is carried out on the Spray G injector, able to provide information on the characteristic diameter of the liquid column exiting from the nozzle.

2019 - On the existence of universal wall functions in in-cylinder simulations using a low-Reynolds RANS turbulence model [Relazione in Atti di Convegno]
Berni, F.; Cicalese, G.; Sparacino, S.; Cantore, G.
abstract

Heat Transfer plays a fundamental role in internal combustion engines, as able to affect several aspects, such as efficiency, emissions and reliability. As for this last, a proper heat transfer prediction is mandatory for the estimation of the engine temperatures at peak power condition, it being the most critical one from a thermal point of view. At part-load/low revving speed operations, heat transfer is detrimental for the engine efficiency, deeply reducing indicated work of the burnt gases on the piston. Focusing on the in-cylinder domain, 3D-CFD simulations represent an irreplaceable tool for the estimation of gas-to-wall heat fluxes. Several models have been developed in the past, aiming at providing a reliable estimation of the heat transfer at any condition in terms of load and revving speed. To save computational cost and time, the most diffused wall approach for the numerical simulation of confined reacting flows is the high-Reynolds one, which means that heat transfer model is based on a thermal wall function. Unfortunately, wall functions (logarithmic profiles of the inertial layer) can be claimed only at restricted conditions, such as isothermal steady-state flow, velocity parallel to the wall and negligible pressure gradient. In practice, none of these assumptions is valid for industrial applications such as an in-cylinder simulation. Therefore in these cases, as demonstrated by different works in the past, wall functions do not exist and their adoption leads to a non-negligible error in the estimation of the heat transfer. The main goal of this work is to build up a methodology able to investigate the presence of wall functions in actual industrial applications, in particular in 3D-CFD in-cylinder analyses. Compared to previous works available in literature, where DNS or LES are carried out on simplified geometries and/or at low revving speed conditions because of the computational cost, in the present paper a RANS approach to turbulence and a low-Reynolds wall treatment are adopted. Moreover, a new strategy to obtain dimensionless profiles of velocity and temperature from computed fields is introduced. At first, the proposed methodology is validated on a 2D plane channel. Then, a preliminary application on a research engine, namely the GM Pancake engine, is proposed, showing that dimensionless profiles of velocity and temperature calculated on the combustion chamber walls are remarkably different from standard analytical wall functions.

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 - 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 intake valve strategies on fuel consumption and knock tendency of a spark ignition engine [Articolo su rivista]
Teodosio, L.; Pirrello, D.; Berni, F; De Bellis, V; Lanzafame, R; D'Adamo, A
abstract

Nowadays various technical solutions have been proposed in order to improve the performance of spark-ignition internal combustion engines both at part and full load operations, especially in terms of Brake Specific Fuel Consumption (BSFC). Among the most advanced technical solutions, a fully flexible valve control system (VVA – Variable Valve Actuation) appears a very robust and reliable approach to attain the above aim. In fact advanced valve strategies, such as Early Intake Valve Closure (EIVC) and Late Intake Valve Closure (LIVC), proved to be an effective way to decrease the fuel consumption: at part load through a reduction of the pumping work and, at high load, through a knock mitigation and an over-fueling reduction. In this paper, a comparative numerical study is realized to evaluate the influence of the intake valve strategy on the performance of a small-size turbocharged spark-ignition engine. The analyzed engine is equipped with a fully flexible VVA on the intake side, based on the “lost motion” principle and able to realize both EIVC and Full Lift strategies, while the virtual modification of the intake cam profile allows for the actuation of LIVC profiles. First, a 1D model of the tested engine is developed in GT-Power™ framework. It is integrated with in-house developed sub-models for the description of in-cylinder phenomena, including turbulence, combustion, knock and heat transfer. The adopted approach is validated against 3D turbulence results, measured global performance parameters and in-cylinder pressure cycles. The consistency of the proposed approach, without requiring any case-dependent tuning, is demonstrated at various speeds, loads and intake valve strategies. The validated engine model is used to perform a parametric analysis for different intake valve closure angles in two representative operating points at full and part load. The results point out that both EIVC and LIVC induce an improved fuel consumption with respect to a conventional Full Lift valve strategy. EIVC proves to be more effective at part load than LIVC, while similar BSFC advantages are obtained at high load. The proposed approach, based on refined sub-models for in-cylinder phenomena description, shows the capability to predict the effects of advanced valve strategies, making the implementation of a “virtual” calibration of a VVA engine possible.

2018 - Refinement of a 0D Turbulence Model to Predict Tumble and Turbulent Intensity in SI Engines. Part I: 3D Analyses [Relazione in Atti di Convegno]
Bozza, Fabio; De Bellis, Vincenzo; Berni, Fabio; D'Adamo, Alessandro; Maresca, Luigi
abstract

Recently, a growing interest in the development of more accurate phenomenological turbulence models is observed, since this is a key pre-requisite to properly describe the burn rate in quasi-dimensional combustion models. The latter are increasingly utilized to predict engine performance in very different operating conditions, also including unconventional valve control strategies, such as EIVC or LIVC. Therefore, a reliable phenomenological turbulence model should be able to physically relate the actuated valve strategy to turbulence level during the engine cycle, with particular care in the angular phase when the combustion takes place. Similarly, the capability to sense the effects of engine architecture and intake geometry would improve the turbulence model reliability. 3D-CFD codes are recognized to be able to accurately forecast the evolution of the in-cylinder turbulence field, taking into account both geometrical features (compression ratio, bore-to-stroke ratio, intake runner orientation, valve, piston and head shapes, etc.) and operating conditions (engine speed, boost level, valve strategy). Instead, more common 0D turbulence models usually synthesize geometrical effects in a number of tuning constants and "try" to be sensitive to the operating conditions as much as possible. In this two-part paper, the final goal is the refinement of a previously developed 0D turbulence model, here extended to directly predict the tumble vortex intensity and its close-to-TDC collapse into turbulence. In addition, the model is enhanced to become sensitive to engine geometrical characteristics, such as intake runner orientation, compression ratio, bore-to-stroke ratio and valve number, without requiring any preliminary estimation of the tumble coefficient on a flow bench. Part I describes a background study, where 3D analyses are performed to highlight the effects of operating conditions and main engine geometrical parameters on tumble and turbulence evolution during the engine cycle. In a preliminary stage, the averaging process influence to define representative quantities of mean flow and turbulence is discussed, in order to take into account not-uniformities inside the combustion chamber. 3D simulations are carried out under motored conditions on a VVA engine, at various engine speeds. The VVA device is controlled to simulate both standard, early and late valve closures. The results highlight substantial differences in the mean flow velocity, turbulence intensity and tumble speed among the above cases. To focus the engine geometry impact on the turbulence evolution, further analyses are performed on a different engine, by changing the angle between the intake runners and the cylinder axis. The geometrical compression ratio and the bore-to-stroke ratio are modified, as well. Finally, a two-valve version of this engine is also considered. Results of 3D analyses are discussed to widely assess the effects of valve strategy and main engine geometrical parameters on mean flow, tumble and turbulence evolution inside the combustion chamber. The presented information constitute an extended database for the development and validation of a refined quasi-dimensional model, discussed in the companion part II of the paper.

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 - 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 - 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.

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 - 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 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 - A numerical investigation on the potentials of water injection to increase knock resistance and reduce fuel consumption in highly downsized GDI engines [Relazione in Atti di Convegno]
Berni, Fabio; Breda, Sebastiano; Lugli, Mattia; Cantore, Giuseppe
abstract

3D CFD analyses are used to analyse the effects of port-injection of water in a high performance turbocharged GDI engine. Particularly, water injection is adopted to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock resistance. A full-load / maximum power engine operation of a currently made turbocharged GDI engine is investigated comparing the actual adopted fuel-only rich mixture to stoichiometric-to-lean 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 - Effects on knock intensity and specific fuel consumption of port water/methanol injection in a turbocharged GDI engine: Comparative analysis [Relazione in Atti di Convegno]
Breda, Sebastiano; Berni, Fabio; D'Adamo, Alessandro; Testa, Francesco; Severi, Elena; Cantore, Giuseppe
abstract

The recent rise in fuel prices, the need both to reduce ground transport-generated emissions (increasingly constrained by legislation) and to improve urban air quality have brought fuel-efficient, low-emissions powertrain technologies at the top of vehicle manufacturers' and policy makers' agenda. To these aims, engine design is now oriented towards the adoption of the so-called downsizing and down-speeding techniques, while preserving the performance target. Therefore, brake mean effective pressure is markedly increasing, leading to increased risks of knock onset and abnormal combustions in last-generation SI engines. 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. Possible solutions to increase knock resistance are investigated in the paper by means of 3D-CFD analyses: water, water/methanol emulsion and methanol are port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. The aim of the work is therefore the replacement of the gasoline-only rich mixture with a global stoichiometric one while avoiding power loss and improving fuel consumption. In order to maintain the same knock tendency, water, methanol or a mixture of the two is then added in the intake port to keep the same charge cooling of the original rich mixture. Different strategies in terms of methanol/water ratios of the port injected mixture are compared in order to find the best trade-off between fuel consumption, performance and knock tendency.

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 - Numerical investigation on the effects of bore reduction in a high performance turbocharged GDI engine. 3D investigation of knock tendency [Relazione in Atti di Convegno]
Severi, Elena; D'Adamo, Alessandro; Berni, Fabio; Breda, Sebastiano; Lugli, Mattia; Mattarelli, Enrico
abstract

Downsizing is a must for current high performance turbocharged SI engines. This is often achieved through the reduction of cylinder number, while keeping constant unit displacement and increasing boost pressure. However, the ensuing higher loads strongly increases the risk of abnormal combustion and thermo-mechanical failures. An alternative path to downsizing is the reduction of cylinder bore: this approach is more expensive, requiring a brand new design of the combustion system, but it also provides some advantages. The goal of the present paper is to explore the potential of bore reduction for achieving a challenging downsizing target, while preserving the engine knock safety margins. A current V8 GDI turbocharged sporting engine is taken as a reference, and a preliminary CFD-3D analysis is carried out in order to define the most suitable bore-to-stroke ratio. On this basis, bore is reduced by 11% at constant stroke, thus obtaining a reduction of about 20% on the engine displacement. In order to achieve the same peak power target, both engine boost and spark advance are adjusted until the knock safety margin of the original engine is met. 3D CFD tools, accurately calibrated on the reference engine, are used to address engine design and the calibration of the operating parameters.