Modeling of DISI Engines

Research Project, 2009 – 2017

The goal of this project is to develop models, methods, and a numerical platform for simulations of stratified turbulent burning in direct injection (DI) spark ignition (SI) engines that use various fuels, e.g. gasoline, ethanol, or gasoline-ethanol blends. Within the framework of the project, semi-detailed chemical mechanisms of combustion of gasoline-air and gasoline-ethanol-air mixtures were developed and validated against a wide set of published experimental data on laminar flame speeds and ignition delay times. Subsequently, laminar flame speeds computed using the former mechanism under a wide range of conditions associated with burning in SI engines were approximated, followed by implementation of this approximation into an open-source CFD library known as OpenFOAM. Moreover, implementations of several spray models (KHRT, LISA, and TAB breakup models, as well as O’Rourke collision model) into OpenFOAM were corrected in order to be fully consistent with the model descriptions in the original papers. Subsequently, experiments with hollow-cone gasoline and ethanol sprays discharged by a pintle injector into Chalmers spray rig, that were earlier performed by our colleagues within the framework of another project entitled “Spray Guided Gasoline Direct Injection”, were numerically simulated by running OpenFOAM and using various spray models. Based on the obtained results, the KHRT model was selected for DI SI engine simulations. Furthermore, the Flame Speed Closure (FSC) model of premixed turbulent combustion, developed and thoroughly validated earlier at Chalmers, was extended to stratified flames by combining it with a presumed Probability Density Function (PDF) approach to allowing for mixture-fraction fluctuations. A new method for converting the mass-weighted PDF, which was commonly presumed in the literature, to the canonical PDF required to average reaction rates in stratifies flames was developed and implemented into OpenFOAM. After implementation of all aforementioned models, the code was applied to unsteady multidimensional RANS simulations of fuel injection, evaporation, turbulent mixing, spark ignition, and combustion in a research gasoline DI engine available at the Combustion Division. In all three highly stratified (late injection) cases, which have yet been studied, computed pressure traces agreed well with data measured earlier by our colleagues within the framework of the aforementioned experimental project “Spray Guided Gasoline Direct Injection”.

In the future, the semi-detailed chemical mechanism will be used to obtain a library that contains profiles and integrated reaction rates for key species relevant to combustion of gasoline-ethanol-air mixtures under DI SI engine conditions. In order to develop a numerical platform for simulating emissions from such engines, the library will be implemented into OpenFOAM and will be used jointly with (i) the aforementioned extended FSC/PDF model of stratified turbulent burning and (ii) a new approach for presuming PDF for the combustion progress variable. The approach is under development now.