Modeling of Flame Propagation in Spark Ignition Engines
Developers and manufacturers of gasoline engines are struggling to improve fuel economy and minimize negative environmental effects. Hence, lean burn and direct injection (DI), in particular stratified charge (SC), spark ignition (SI) engines are of special interest. These engines have the potential to reduce the fuel consumption by 20-25% compared with conventional SI engines. However, the combustion process, is governed by complex interactions between chemistry and fluid dynamics, some of which are incompletely understood. Improved knowledge of combustion is, therefore, of vital importance for both direct use in the design of engines, and for the development of reliable simulation tools for engine development.
In this thesis, a brief review is presented of the theory of flame propagation under conditions found in engines, to provide a basis for the formulation of a combustion model. This model, called the turbulent Flame Speed Closure (FSC) model, predicts the instantaneous flame speed, in both laminar and turbulent cases. It is also able to predict the flame development characteristics such as burning velocity and flame brush thickness during propagation. The model is based on time-dependent turbulent diffusivity found in the early phase of flame kernel growth and it includes a single universal model constant that needs to be tuned. To consider local mixture properties, a complex chemistry mechanism consisting of 100 species and 475 reactions is used to determine chemical time scales, required by the model. Validation against multiple databases for spherical flames shows that the model is capable of predicting the flame propagation in a variety of conditions for different fuels and mixture ratios.
The combustion processes in two different SI engines are simulated using the commercial CFD-code FIRE? connected with 1-D gas exchange simulations. The FSC model was implemented in the code for the simulations of combustion processes Four speed/load combinations were simulated of which one featured SC combustion. The computations show good agreement with experimental data over a range of speeds and loads using the same value of the single model constant as in all the validation computations. The computations also show that the developed engine combustion simulation system is time effective, robust and able to reproduce the main features of homogeneous and stratified combustion, in a particular triple flame formation.
Current DI SC engines suffer from too high unburned hydrocarbon (HC) emissions. Approximately 60-70% of the total HC emissions from the SC engine case is found to be due to overmixing, i.e. the fuel has mixed beyond flamability limits. To improve this, further development of the SC engine should be focussed on reducing mixing time available between start of injection and ignition.