3D modeling of mixing, ignition and combustion phenomena in highly stratified gasoline engines

The present paper describes recent developments realised at IFP on the 3D modeling of combustion in spark ignition engines. They consist in improvements made to the classical coherent flame model (CFM) to yield the extended coherent flame model (ECFM), specifically adapted to simulating the combustion process in direct injection-spark ignition (DI-SI) engines. The principal idea of this extension consists in describing locally the fuel/air (FIA) equivalence ratio in fresh gases, composition (including residual gases) and temperature, allowing to improve the description of large scale stratification. A generalisation of this approach to multi-component fuels is proposed. The effect of small scale stratification is included into the ECFM model via a variance/scalar dissipation model in combination with a presumed probability density function approach for the fuel stratification. The spark ignition model AKTIM, developed to represent the initiation of combustion at the spark plug, has been modified here in order to account for the flame wrinkling by turbulence. It is shown that this effect is essential in the case of high turbulence levels. Finally, a model to predict knock in SI engines is briefly described. These developments are then validated on two engine configurations: an optical access engine, for which LIF (laser induced fluorescence) measurements are available and the gasoline direct injection Mitsubishi engine for global validations.