Experimental Investigation of Combustion and Emission Characteristics of Stoichiometric Stratified Flame Ignited (SFI) Hybrid Combustion in a 4-Stroke PFI/DI Gasoline Engine

Controlled Auto-Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), can improve the fuel economy of gasoline engines and simultaneously achieve ultra-low NOx emissions. However, the difficulty in combustion phasing control and violent combustion at high loads limit the commercial application of CAI combustion. To overcome these problems, stratified mixture, which is rich around the central spark plug and lean around the cylinder wall, is formed through port fuel injection and direct injection of gasoline. In this condition, rich mixture is consumed by flame propagation after spark ignition, while the unburned lean mixture auto-ignites due to the increased in-cylinder temperature during flame propagation, i.e., stratified flame ignited (SFI) hybrid combustion. The combustion and emissions characteristics in the SFI combustion were experimentally investigated in a naturally aspirated single-cylinder 4-stroke gasoline engine at medium-high loads when direct injection timing was kept at -60 °CA after top dead center and direct injection ratio was less than or equal to 0.4 at stoichiometry. The results show that advanced spark timing or decreased direct injection ratio alters the SFI events from flame-dominated combustion to auto-ignition-dominated one, resulting in shorter combustion duration. The start of SFI combustion advances with advanced spark timing. But it is slightly affected by direct injection ratio. Thermal efficiency increases around 10% with advanced spark timing and its maximum value appears at high direct injection ratio. Unburned hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx) emissions in SFI combustion increase with advanced spark timing at different direct injection ratios.