Effect of piston shapes and fuel injection strategies on stoichiometric stratified flame ignition (SFI) hybrid combustion in a PFI/DI gasoline engine by numerical simulations

• SFI was proposed to enhance the control of combustion and moderate PRRmax.
• Effect of pistons, SOI timings and DI ratios on SFI was studied using 3-D CFD.
• Shallow bowl pistons can optimize mixture stratification and reduce PRRmax.
• Later SOI timing and larger DI ratio can significantly reduce the PRRmax.
• Spark timing is effective to control CA50, IMEP and PRRmax of SFI.

In this research, the stratified flame ignition (SFI) hybrid combustion process was proposed to enhance the control of SI–CAI hybrid combustion and moderate the maximum pressure rise rate (PRRmax) by the combination of port fuel injection (PFI) and direct injection (DI). The effect of the stratified flame formed by different piston shapes, start of direct injection (SOI) timings and direct injection ratios (rDI) on the stoichiometric SFI hybrid combustion and heat release process was studied using the three-dimensional computational fluid dynamics (3-D CFD) simulations. The spark ignited flame propagation near the spark plug and the auto-ignition heat release process of the diluted mixture were modelled in the framework of 3-Zones Extended Coherent Flame Model (ECFM3Z) by the extended coherent flame model and tabulated auto-ignition chemistry of a 4-component gasoline surrogate, respectively. The operating load of indicated mean effective pressure (IMEP) 3.6 bar was selected to represent a typical part-load operation. The sweep of the spark timing (ST) was performed for different pistons, SOI timings and direct injection ratios. The SFI hybrid combustion process with the same combustion phasing was investigated in details. The optimal stratified mixture pattern, characterized with the central rich mixture around spark plug and stratified lean mixture at the peripheral region, formed by the newly designed Piston A and B effectively lowers the PRRmax with a slight deterioration of IMEP. The later SOI timing advances the crank angle of 50% total heat release (CA50) and significantly reduces the PRRmax with a little deterioration of IMEP. As the direct injection ratio is increased, both the PRRmax and IMEP decrease. During the SFI hybrid heat release process, spark timing is effective to control CA50, IMEP and PRRmax regardless the piston shapes, SOI timings and direct injection ratios. However, the sensitivity of SFI hybrid combustion to the stratified mixture varies with the spark timing. The reduction of the PRRmax caused by the stratified flame enables the advance of spark timing to achieve maximum IMEP.