Trade-off on fuel economy, knock, and combustion stability for a stratified flame-ignited gasoline engine

A combination of port fuel injection (PFI) and direct injection (DI), called P-DI strategy, was used in a four-cylinder gasoline engine. The aim was to achieve a stratified flame ignition (SFI) hybrid combustion and manage the trade-off among fuel economy, knock, and combustion stability (EKS) in a gasoline engine. In the proposed P-DI strategy, DI was used to enrich the local mixture around the spark plug to enhance the early spark-ignition combustion. On the other hand, PFI was used to form a largely lean homogeneous mixture to achieve a moderately controlled auto-ignition combustion in the outer region of the cylinder. The effects of DI fraction (RDI) and start of injection (SOI) timing of DI on the SFI hybrid combustion were investigated experimentally. It was found that an increased RDI resulted in a parabolic-like effect on the combustion phasing and combustion stability. The earliest combustion phasing was achieved with an RDI of approximately 35%. The fuel economy deteriorated monotonously with increasing RDI. In comparison, the effect of SOI on the SFI hybrid combustion was more complicated. It was found that an SOI between 50°CA before top-dead-center (BTDC) and 90°CA BTDC showed a potential to achieve a satisfactory trade-off among EKS. Based on the above findings, a cost function (J) was proposed to represent the EKS trade-off and reduce the calibration burden for optimal SOI at different engine operating conditions. An extremum-seeking algorithm was adopted to search for the maximum value of J and obtain the optimal SOI timing at each operating point. The proposed algorithm was then validated by experimental results.