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Alessandro D'ADAMO

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

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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

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.

2020 - Application of a zonal hybrid URANS/LES turbulence model to high and low-resolution grids for engine simulation [Articolo su rivista]
Iacovano, C.; d'Adamo, A.; Fontanesi, S.; Di Ilio, G.; Krastev, V. K.

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

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

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

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

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

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

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

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

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

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

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

2020 - MOTORI E SISTEMI PROPULSIVI PER AUTOVEICOLI [Monografia/Trattato scientifico]
D'Adamo, A; Cantore, G

La conoscenza dei motori a combustione interna costituisce una delle principali basi formative che devono caratterizzare la figura dell’Ingegnere Meccanico, in particolar modo dell’Ingegnere del Veicolo. Questo testo si propone di presentarne le principali tipologie e di descriverne i principi di funzionamento, ed è indirizzato a tutti coloro che affrontano per la prima volta, a livello universitario, lo studio di queste macchine complesse. In questa nuova edizione sono state aggiunte alcune sezioni che riflettono l’evoluzione dei motori a combustione interna nell’ultimo decennio, con particolare riferimento alle emissioni inquinanti e al loro trattamento e controllo, così come alle più promettenti frontiere di ricerca sul tema delle combustioni innovative. Inoltre, l’integrazione dei motori a combustione interna in più complessi “sistemi propulsivi” ha motivato la stesura di capitoli riguardanti la descrizione delle più comuni architetture di veicoli ibridi a batteria, corredati da una presentazione dei principi di funzionamento delle pile a combustibile (o “fuel cells”) per uso automobilistico. Si auspica che questo testo possa costituire una solida base di conoscenze, a partire dalle quali lo studente interessato ed appassionato potrà sviluppare i numerosi approfondimenti richiesti dallo studio di questa complessa ed affascinante materia.

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

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

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

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

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

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

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 - Comparison of library-based and detailed chemistry models for knock prediction in spark-ignition engines [Relazione in Atti di Convegno]

The present engine development pathway for increased specific power and efficiency is moving Spark-Ignition engines towards unprecedented levels of mean thermo-mechanical loading. This in turn promotes undesired abnormal combustion events in the unburnt mixture (also called “engine knock”), leading to solid parts failure and constituting a severe upper constraint to engine efficiency. In this context, CFD simulations are regularly used to investigate the fluid-dynamic reasons for engine knock and to address knock suppression strategies, using dedicated models to simulate the chemical reaction rate of the fuel/air/residual mixture at the same thermodynamics states as those encountered in engines. In this paper three different approaches are coherently compared to simulate knock occurrence on a turbocharged GDI engine, representing some of the most popular choices for modelers in the RANS framework. The first one considers the on-the-fly solution of chemical reactions, which represents the state-of-the-art knock modelling approach albeit its problematic computational cost for industrial turnaround times. The other two methods consider pre-calculated libraries of ignition delay times (calculated at constant pressure and volume, respectively) for the same fuel model, and knock timing is predicted using a classical Livengood-Wu approach coupled to the same main combustion model. All the analyzed models for the end-gas reaction rate are coupled with a dedicated combustion model for propagating flame (G-equation). A comprehensive analysis of computational cost and of knock prediction accuracy is carried out for library-based methods against the detailed chemistry model. Finally, results are critically discussed and explained using combined ignition delay time maps and traces for thermodynamic in-cylinder states, and guidelines for the a priori choice for constant pressure- or volume-generated libraries are provided. In this context, the use of a synthetic knock model combined with libraries of ignition delays calculated at constant volume emerges as an accurate and efficient modelling strategy. The study outlines a method for the well-supported use of simplified CPU-efficient models, with a promoted confidence in simulation results from the comparison with detailed chemistry.

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

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

2019 - 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.

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 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

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

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

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

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 - Analysis and Simulation of Non-Flamelet Turbulent Combustion in a Research Optical Engine [Relazione in Atti di Convegno]
Iacovano, Clara; D'Adamo, Alessandro; Cantore, Giuseppe

In recent years, the research community devoted many resources to define accurate methodologies to model the real physics behind turbulent combustion. Such effort aims at reducing the need for case-by-case calibration in internal combustion engine simulations. In the present work two of the most widespread combustion models in the engine modelling community are compared, namely ECFM-3Z and G-equation. The interaction of turbulent flows with combustion chemistry is investigated and understood. In particular, the heat release rate characterizing combustion, and therefore the identification of a flame front, is analysed based on flame surface density concept rather than algebraic correlations for turbulent burn rate. In the first part, spark-ignition (S.I.) combustion is simulated in an optically accessible GDI single-cylinder research engine in firing conditions. The turbulent combustion regime is mapped on the Borghi-Peters diagram for all the conditions experienced by the engine flame, and the consistency of the two combustion models is critically analysed. In the second part, a simple test case is defined to test the two combustion models in an ideally turbulence-controlled environment: this allows to fully understand the main differences between the two combustion models under well-monitored conditions. and results are compared against experimental databases of turbulent burn rate for wide ranges of Damkohler (Da) and Karlovitz (Ka) numbers. The joint experimental and numerical study presented in this paper evaluates different approaches within the unified flamelet/non-flamelet framework for modelling turbulent combustion in SI engines. It also indicates guidelines for reduced calibration effort in widespread combustion models.

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

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

2018 - 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2015 - Effects of fuel composition on charge preparation, combustion and knock tendency in a high performance GDI engine. Part I: RANS analysis [Relazione in Atti di Convegno]
Giovannoni, Nicola; D'Adamo, Alessandro; Nardi, Luca; Cantore, Giuseppe

The paper analyses the effects of fuel composition modelling in a turbocharged GDI engine for sport car applications. Particularly, a traditional single-component gasoline-surrogate fuel is compared to a seven-component fuel model available in the open literature. The multi-component fuel is represented using the Discrete-Continuous-Multi-Component modelling approach, and it is specifically designed in order to match the volatility of an actual RON95 European gasoline. The comparison is carried out following a detailed calibration with available experimental measurements for a full load maximum power engine speed operation of the engine, and differences are analyzed and critically discussed for each of the spray evolution, mixture stratification and combustion. In the present paper (Part I), a RANS approach is used to preliminarily investigate the behaviour of the fuel model on the average engine cycle. In the subsequent Part II of the same paper, the numerical framework is evolved into a more refined LES approach, in order to take into account cycle-to-cycle variations in mixture formation and knock tendency.

2015 - Effects of fuel composition on charge preparation, combustion and knock tendency in a high performance GDI engine. Part II: Les analysis [Relazione in Atti di Convegno]
D'Adamo, Alessandro; Giovannoni, Nicola; Nardi, Luca; Cantore, Giuseppe; D'Angelis, Angelo

As discussed in the Part I of this paper, a numerical activity is carried out in order to analyse the effects of fuel composition modelling in a turbocharged GDI engine for sport car applications. While Part I analyses the "ensemble averaged" macroscopic effects on spray evolution, mixture stratification, combustion and knock tendency, in Part II of this paper cycle-to-cycle variations are analysed and discussed using a multi-cycle LES numerical framework, again comparing results from a more traditional single-component fuel surrogate model to those of a multi-component one. A purposely developed numerical approach is applied to properly account for the effects of the Discrete-Continuous-Multi-Component fuel formulation on the charge preparation: just before the spark timing, each vaporized fuel fraction is lumped back into a single-component surrogate fuel to allow the combustion model (ECFM-3Z, in its LES formulation) to take place. At the beginning of a new injection process, the numerical framework for the injected spray is switched back to Multi-Component, thus allowing each fuel fraction to independently spread, vaporize and diffuse in the combustion chamber according to the cycle-specific characteristics. A detailed comparison between the two fuel formulations is carried out on both average and rms values of the most influencing fields just before the spark discharge.

2015 - Effects of Fuel-Induced Piston-Cooling and Fuel Formulation on the Formation of Fuel Deposits and Mixture Stratification in a GDI Engine [Relazione in Atti di Convegno]
Giovannoni, Nicola; D'Adamo, Alessandro; Cicalese, Giuseppe; Cantore, Giuseppe

Fuel deposits in DISI engines promote unburnt hydrocarbon and soot formation: due to the increasingly stringent emission regulations (EU6 and forthcoming), it is necessary to deeply analyze and well-understand the complex physical mechanisms promoting fuel deposit formation. The task is not trivial, due to the coexistence of mutually interacting factors, such as complex moving geometries, influencing both impact angle and velocity, and time-dependent wall temperatures. The experimental characterization of actual engine conditions on transparent combustion chambers is limited to highly specialized research laboratories; therefore, 3D-CFD simulations can be a fundamental tool to investigate and understand the complex interplay of all the mentioned factors. The aim is pursued in this study by means of full-cycle simulations accounting for instantaneous fuel/piston thermal interaction and actual fuel characteristics. To overcome the standard practice, based on the adoption of time-independent wall temperatures, solid cell layers are added onto the piston crown. In particular, thermal boundary conditions on the lower face of the piston portion are derived from a complete CHT simulation, thus considering both the actual piston shape and the point-wise cooling effect by the oil jets, the friction contribution and the heat transfer to the cylinder liner and the connecting rod. Furthermore, the use of a simplified fuel model based on a single-component formulation is compared to a more realistic hydrocarbon blend. The methodology is applied to a currently produced turbocharged DISI engine operating at full load peak power and maximum torque regimes; the piston thermal field is completely resolved in space and time during the engine cycle, and its effects on spray guidance, fuel impingement and liquid film formation are carefully analyzed.

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

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

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

2015 - Large-eddy simulation of cycle-resolved knock in a turbocharged SI engine [Relazione in Atti di Convegno]
D'Adamo, Alessandro; Breda, Sebastiano; Cantore, Giuseppe

The paper presents a numerical study of cycle-to-cycle variability in a turbocharged GDI engine. The Large-Eddy Simulation technique is adopted in this study in conjunction with the recent ISSIM-LES model for spark-ignition, allowing a dedicated treatment of both the flame kernel formation and flame development phases. Numerical results are compared with an extended dataset of experimental test-bed acquisitions, where the engine is operated at knock-limited spark advance. The agreement of both ensemble averaged combustion pressure history and of its standard deviation confirm the validity of the adopted numerical framework able to correctly quantify the degree of CCV measured by the experiments. Knock tendency is evaluated by means of an in-house developed knock model, based on a tabulation technique for AI delays of the same RON98 gasoline as the one used in experiments. The results confirm the knock-free condition of the experimental KLSA, for which the cycle-resolved knock signature is extremely weak just for the cycles in the highest band of the CCV-affected combustion. The visualization of the pressure wave allows to identify the exhaust side as the most knock-prone region. Finally, spark-advance is increased by 3 CA with respect to the experimental edge-of knock limit, in order to simulate an experimentally prevented operating condition. Local pressure measurements mimicking flush-mounted transducers confirm the severe knock damage related to this condition. The predictive capability of the combustion CCV and of the adopted knock model confirm the heavy and recurrent cycle-resolved knock damage.

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

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

2015 - Numerical investigation on the effects of 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

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.

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

2012 - Experimental and Numerical Investigation of the Idle Operating Engine Condition for a GDI Engine [Relazione in Atti di Convegno]
Malaguti, Simone; d'ADAMO, Alessandro; Cantore, Giuseppe; P., Sementa; B. M., Vaglieco

The increased limitations to both NOx and soot emissions have pushed engine researchers to rediscover gasoline engines. Among the many technologies and strategies, gasoline direct injection plays a key-role for improving fuel economy and engine performance. The paper aims to investigate an extremely complex task such as the idle operating engine condition when the engine runs at very low engine speeds and low engine loads and during the warm-up. Due to the low injection pressure and to the null contribution of the turbocharger, the engine condition is far from the standard points of investigation. Taking into account the warm-up engine condition, the analyses are performed with a temperature of the coolant of 50°C. The paper reports part of a combined numerical and experimental synergic activity aiming at the understanding of the physics of spray/wall interaction within the combustion chamber and particular care is used for air/fuel mixing and the combustion process analyses. In order to properly describe the engine condition, different injection strategies are investigated. Late and early injection strategies are deeply analyzed and compared in terms of combustion stability and pollutant emissions. UV-visible imaging and spectral measurements are carried out in real engine with wide optical accesses… Measurements are performed in the optically accessible combustion chamber realized by modifying a real engine. The cylinder head was modified in order to allow in the fourth cylinder the visualization of the fuel injection and the combustion process with high spatial and temporal resolution. The 3D-CFD engine simulations are reproduced by means the commercial code Star-CD. Due to the warm-up condition and the many physical sub-models a numerical methodology is implemented and particular care is used to boundaries conditions analyses. CFD analysis is used to find a possible explanation of the high cycle to cycle variability. The experimental and numerical comparisons, in terms fuel mixing and front flame propagation, give an explanation of the idle condition.

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

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