Michela Costa

CFD Modeling of a Mixed Mode Boosted GDI Engine and Performance Optimization for the Avoidance of Knocking

The paper applies simulation techniques for the prediction and optimization of the thermo-fluid-dynamic phenomena characterizing the energy conversion process in a GDI engine. The 3D CFD model validation is realized on the ground of experimental measurements of in-cylinder pressure cycles and optical images collected within the combustion chamber. The model comprehends properly developed submodels for the spray dynamics and its impingement over walls. This last is particularly important due to the nature of the mixture formation mode, being wall-guided. Both homogeneous stoichiometric and lean stratified charge operations are considered. In the case of stoichiometric mixture, the possible occurrence of knocking is also accounted for by means of a submodel able to reproduce the preflame chemical activity. The CFD tool is finally included in a properly formulated optimization problem aimed at minimizing the engine-specific fuel consumption with the avoidance of knocking through a non-evolutionary algorithm.

Modeling and performance optimization of a direct injection spark ignition engine for the avoidance of knocking

The paper applies simulation techniques for the prediction and optimization of the thermo-fluid-dynamic phenomena characterising the energy conversion process in an internal combustion engine. It presents the development and validation of a 3D CFD model for a GDI optically accessible engine operating either under stoichiometric homogeneous charges or under overall lean mixtures. The model validation is realized on the ground of experimental measurements of the in-cylinder pressure cycle and of the available optical images. The model comprehends properly developed sub-models for the spray dynamics and the spray-wall interaction. This last is particularly important due to the nature of the mixture formation mode, being of the wall-guided type. In the stoichiometric mixture case, the possible occurrence of knocking is also considered by means of a sub-model able to reproduce the pre-flame chemical activity. The CFD tool is finally included in a properly formulated optimization problem aimed at minimizing the engine specific fuel consumption with the avoidance of knocking. The optimization, performed through a non-evolutionary algorithm, allows determining the best engine control parameters (spark advance and start of injection).

Optimization of a GDI engine operation in the absence of knocking through numerical 1D and 3D modeling

Various solutions are being proposed and adopted by manufactures and researchers to improve the energetic and environmental performance of internal combustion engines within the transportation sector. For automotive spark ignition engines, gasoline direct injection is one of the presently preferred technologies, in conjunction with turbocharging and downsizing. One of the limiting phenomena of this kind of engines, however, still remains the occurrence of knocking, namely the self-ignition of the so-called end-gas zones of the mixture, not yet reached by the flame front. This phenomenon causes strong in-cylinder pressure oscillations, high stress levels and even damage to engine components.Present work focuses on a numerical and experimental study of a turbocharged GDI engine and is aimed at assessing CFD-O (computational fluid dynamics optimization) procedures to be used in the phase of design as a decision making tool for the development of control strategies for a smooth and efficient operation. A preliminary experimental analysis is performed in order to characterize the considered engine and to investigate the phenomenon of knocking that occurs under some circumstances as the spark advance is increased. The collected data are employed to elaborate a predictive criterion for the appearance of this kind of abnormal combustion, as well as to validate both a 1D and a 3D model for the simulation of the engine working cycle. Various numerical optimization procedures are then realized to increase the engine power output and simultaneously avoid conditions leading to undesired self-ignitions. These are either based on the use of a non-evolutionary algorithm or employ a genetic algorithm in the case multiple contrasting objectives are set. The response surface methodology is also explored as a way to reduce the computational effort.

Image Processing for Early Flame Characterization and Initialization of Flamelet Models of Combustion in a GDI Engine

Ignition and flame inception are well recognised as affecting performance and stable operation of spark ignition engines. The very early stage of combustion is indeed the main source of cycle-to-cycle variability, in particular in gasoline direct injection (GDI) engines, where mixture formation may lead to non-homogenous air-to-fuel distributions, especially under some speed and load conditions. From a numerical perspective, 3D modelling of combustion within Reynolds Averaged Navier Stokes (RANS) approaches is not sufficient to provide reliable information about cyclic variability, unless proper changes in the initial conditions of the flow transport equations are considered. Combustion models based on the flamelet concept prove being particularly suitable for the simulation of the energy conversion process in internal combustion engines, due to their low computational cost. These models include a transport equation for the flame surface density, which needs proper initialization. A flame collocation is indeed to be properly made when starting the calculations, often just based on the users skill and without resorting to any quantitative data derived from experiments. However, the way to define initial conditions for cyclic variability prediction is often based on just statistical considerations. This work aims at exploiting information derived from images collected in a single cylinder 4-stroke GDI engine to properly collocate the flame at the start of the combustion calculation. The considered engine is optically accessible through a wide fused-silica window fixed on the piston crown having a Bowditch design. Image processing methodologies are applied to evaluate local and integral luminous intensity, and flame morphology parameters. The collected data allows improving the numerical simulation and gaining hints about the main parameters defining the engine cyclic variability.

Multidimensional modelling of diesel combustion by a detailed kinetic scheme and comparison with in-cylinder optical measurements

The present work aims at a comprehensive study of diesel combustion based on detailed kinetic modelling and experimental investigations performed on an optically accessible engine. A diesel fuel surrogate model of combustion is included in a three-dimensional CFD code properly designed for application to internal combustion engines. The fuel spray is treated by discrete droplet models tuned on the ground of measurements of penetration lengths effected in a vessel at controlled conditions. The detailed chemistry of the turbulent reacting flow is computed by means of the partially stirred reactor approximation, which leads to an appreciable computational economy. Test-bench experimental data allow the code validation and the assessment of its predictability as operating conditions are modified. Optical measurements clarify major physical and chemical aspects related to fuel autoignition and combustion evolution. The proposed approach is believed to be valuable in optimising performances of diesel engines equipped with Common Rail (CR) injection systems.

Schlieren and Mie scattering techniques for the ECN “spray G” characterization and 3D CFD model validation

Purpose This paper aims to study the heat transfer phenomenon occurring between heated walls and impinging fuel, showing the strict relationship between cooling effect after impingement and enhancing of wallfilm formation. The study focuses on a fundamental task in terms of pollutant emissions in internal combustion engines, aiming at giving a major contribution to the optimization of energy conversion systems in terms of environmental impact. Design/methodology/approach The paper is based on experimental campaigns relevant at taking measurements of an impinging spray over a heated wall in a confined vessel. The results, in both qualitative and quantitative terms (measurements of liquid and vapour radial penetration and thickness), are numerically reproduced by a computational model based on a Reynolds Averaged Navier Stokes approach, properly validated through customized sub-models. Findings The paper provides quantitative results about the agreement between radial penetration and vapour thickness between measurements and simulation, achieved by taking into account the cooling effect determined by the fuel impingement. This validation of the numerical model allows the author to give more considerations about the link between wall temperature and wallfilm formation. Originality/value This paper presents an original approach for the simulation of wall heat transfer, by imposing a boundary condition at the wall that may consider the heat conduction and temperature cooling given by fuel impingement in both lateral and normal directions. The classical Dirichlet boundary condition, characterized by imposing a fixed temperature value, is, instead, replaced by an approach based on calculating the unsteady process that couples the heat fluxes between the fluid and the solid material and within the solid itself.

Biofuel Powering of Internal Combustion Engines: Production Routes, Effect on Performance and CFD Modeling of Combustion

The use of liquid or gaseous biofuels in reciprocating internal combustion engines (ICEs) is today a relevant issue as these systems are largely diffused for both steady power generation and transportation due to their flexibility and easiness of use. The improvement and perfect control of the combustion process under non-conventional fuelling is mandatory to achieve high-energy efficiency without substantial changes to the architecture or the fuel supply system. In this perspective, the detailed characterization of multiphase reacting systems achievable though computational fluid dynamics (CFD) may give a decisive contribution. However, the assessed combustion models used for fossil fuels (diesel oil, gasoline, methane), tuned on the ground of a massive amount of experimental data, often results poor in predicting the actual behaviour of renewable fuels whose composition and properties may change also according to technology for their production. Present work aims at filling some existing gaps in biofuel combustion modeling by performing investigations on two representative engine cases, for their characterization and performance enhancement. Two approaches are followed, namely through reduced chemical kinetics coupled with turbulence within a coherent flame schematization, and through a turbulent species transport approach with detailed kinetics. Simulations are first carried out on a compression ignition (CI) ICE. The formulation of a 3D CFD model is described to reproduce the performance of this engine in a dual-fuel mode with premixed syngas from biomass gasification and a biodiesel pilot injection leading to self-ignition. Pollutants formation and energy efficiency are calculated as syngas amount and the biodiesel start of injection (SOI) are varied. Attention is then focused on the implementation of renewable alcohol fuels (ethanol and butanol), as these lasts are receiving large interest due to low production costs. A validated reduced kinetic mechanism for PRF-ethanol-butanol combustion performs well in multi-component oxidation conditions, as well as in neat fuel oxidation conditions, in terms of ignition delay time, laminar flame speed and HCCI combustion conditions. The paper shows that CFD, even at different level of approximation, may describe into detail the combustion process and provide important guidelines for the design of new generation ICEs fuelled by biofuels.

Analysis of the GDI Spray Dynamics Through Multidimensional Modeling and Flow Visualization in an Optically Accessible SI Engine

Present work is aimed at studying into detail mixture formation and combustion in a gasoline direct injection (GDI) engine working under stoichiometric mixture conditions. The study is performed both numerically and experimentally. From the experimental side, the engine, optically accessible, is characterized by collecting, for various injection strategies, in-cylinder pressure cycles and digital images. From the numerical side, a 3D engine model is developed, that includes proper sub-models for the spray dynamics and the spray-wall interaction. This last phenomenon is studied into detail by resorting to a preliminary 3D simulation of the spray impingement realized in a proper experiment, where the engine injector is mounted at a certain distance from a cold or hot wall.An interesting comparison between numerical and experimental images of the in-cylinder spray dynamics is presented, that also allows individuating the difference in the wallfilm deposition under various injection strategies. This opens the way to understand the difference in the combustion development arising as injection is anticipated or retarded in the engine working cycle.© 2014 ASME

Numerical Modelling and Optimization of the Mixture Formation Processby Multi-Hole Injectors in a GDI Engine

The problem of the environmental impact of energy conversion systems is particularly felt in the automotive field as a consequence of the wide diffusion of internal combustion engines within the transportation systems, and of the very high concentration of vehicles in the urban areas. Several actions, therefore, are today being taken by car manufacturers and researchers towards the development of more and more efficient propulsion systems, characterised by lower and lower pollutants emissions. In fact, despite the recent efforts aimed at developing alternative technologies, it is likely that the internal combustion engine will remain dominant for the next 30 years and beyond. This implies that the study and the optimisation of the thermo-fluid-dynamic processes characterising its operation will undoubtedly continue to play a determining role in the forthcoming scenario. The major difficulties today encountered in the experimental characterization of combustion and pollutants formation in both spark ignition (SI) and compression ignition (CI) engines rely in the low spatial and temporal resolution achievable from measurements, as well as in the possible influence of instruments on the same phenomena to be investigated. The diagnostics capability surely benefits of the development of non-intrusive optical techniques, although constructive and economic problems still hinder their broad use. On the other hand, the introduction of increasingly accurate physical and chemical models and the simultaneous growth of the processors speed have led to a diffuse use of computational fluid dynamics (CFD) techniques, especially in the phase of engine design. A wide variety of geometrical configurations or sets of engine parameters, indeed, are today suitable of being analysed into detail through models of various complexities at relatively low costs, or optimised according to predefined objectives. As regards SI engines, in particular, the most pursued solution for the improvement of fuel economy relies on engine downsizing, coupled with turbo-charging and direct injection (DI): the engine displacement is reduced, whereas an increase of the low end torque is realised by air boosting, compression ratio rising and gasoline injection directly in the combustion chamber. These measures allow overwhelming the main shortcoming of engines mounting port fuel injection (PFI) systems, with mixture formation occurring within the intake ducts, namely the significant engine pumping losses at part-load operation (where the engine works during most of an urban driving cycle), caused by the throttle load