Numerical Study of the Effect of Piston Shapes and Fuel Injection Strategies on In-Cylinder Conditions in a PFI/GDI Gasoline Engine

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In order to stabilize the hybrid combustion process, the port fuel injection (PFI) combined with gasoline direct injection (GDI) strategy is proposed in this study to form the in-cylinder fuel stratification to enhance the early flame propagation process and control the auto-ignition combustion process. The effect of bowl piston shapes and fuel injection strategies on the fuel stratification characteristics is investigated in detail using three-dimensional computational fluid dynamics (3-D CFD) simulations. Three bowl piston shapes with different bowl diameters and depths were designed and analyzed as well as the original flat piston in a single cylinder PFI/GDI gasoline engine. An engine operating load of IMEP=3.6 bar was selected to evaluate the effect of piston shapes on the in-cylinder conditions, including flow conditions, fuel stratification patterns, thermal conditions and fuel evaporation ratios. The GDI ratios and direct injection timings were also varied to find the most appropriate injection strategy to achieve optimal in-cylinder condition. The simulation results presented in this study clarified the effect of bowl piston shapes, GDI ratios and direct injection timings on the in-cylinder fuel stratification in a PFI/GDI gasoline engine. They would provide the fundamental knowledge and effective guidance on selecting the optimal bowl piston shape and fuel injection strategy to stabilize the SI-CAI hybrid combustion in the future experimental investigations.