Effect of dilution strategies and direct injection ratios on stratified flame ignition (SFI) hybrid combustion in a PFI/DI gasoline engine

Three-dimensional (3-D) computational fluid dynamics (CFD) simulations were used to investigate and obtain a fundamental understanding of the effect of dilution strategies and direct injection ratios on the stratified flame ignition (SFI) hybrid combustion. The combination of port fuel injection (PFI) and direct injection (DI) was used to form the homogeneous lean/diluted mixture and stratified charge respectively. Studies were carried out on effects of dilution strategies with different combinations of fuel/air equivalence ratio (ϕair) and fuel/dilution equivalence ratio (ϕdilution) with negative valve overlap (NVO). Compared to the stoichiometric SFI hybrid combustion, the air-diluted SFI hybrid combustion optimizes the early flame propagation process because of the avoidance of over-rich mixture around spark plug. In order to explore the potential of SFI hybrid combustion under a high compression ratio (14:1) operation, the lean boosted dilution strategy with additional intake air and internal residual gas was proposed to address the trade-off between indicated mean effective pressure (IMEP) and maximum pressure rise rate (PRRmax) in air-diluted SFI hybrid combustion. Furthermore, the effect of direct injection ratio (rDI) was investigated as a means to optimize the fuel/air equivalence ratio distribution as well as the air-diluted SFI hybrid combustion performance. It is found that the optimal SFI hybrid combustion with rDI of 0.16 can be used to both achieve higher IMEP for a given amount of fuel and moderate the rate of heat release. Finally, three different combustion regimes, including pure flame propagation zone, hybrid combustion zone and pure auto-ignition zone, are proposed to understand the effect of typical fuel/air equivalence ratio distribution patterns on the air-diluted SFI hybrid combustion characteristics and performances. In order to obtain optimal hybrid combustion with high IMEP and low PRRmax, the in-cylinder stratified mixture should avoid over-rich condition around spark plug and over-lean condition at outer region. In addition, the internal residual gas in the dilution strategy should be carefully controlled to maintain sufficient thermal condition and ensure the stable auto-ignition of the lean mixture at outer region.