Experimental investigation of combustion and emissions characteristics of stoichiometric stratified flame ignited (SFI) hybrid combustion in a 4-stroke PFI/DI gasoline engine 2019-01-0960

Controlled Auto-Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), has the potential to improve the fuel economy of gasoline engines and simultaneously achieve ultra-low NOx emissions. However, lack control of combustion phasing and violent combustion at high loads limit the commercial application of CAI combustion. To overcome above problems, stratified fuel, which is rich mixture around the central spark plug and lean mixture around the cylinder wall, is formed with port fuel injection and direct injection strategies, and rich mixture consumed by flame propagation triggered by spark ignition increases the temperature of unburned mixture and achieve, i.e. stratified flame ignited (SFI) hybrid combustion. The combustion and emissions characteristics of stoichiometric SFI hybrid combustion were investigated in a single-cylinder 4-stroke gasoline engine at around 7 bar net indicated mean effective pressure when direct injection timing was kept at -60 °CA after top dead center, the ratio of direct injection gasoline was no more than 0.4, and the excess air coefficient was fixed at 1.0. The results show that advanced spark timing or reduced direct injection ratio alters the SFI hybrid combustion dominated by flame propagation to that dominated by auto-ignition, and shortens the combustion duration. Ignition timing advances at earlier spark timing, but is slightly affected by direct injection ratio. Thermal efficiency increases with advanced spark timing, and is increased around 10% at high direct injection ratio of 0.4. Advanced spark timing results in higher HC, CO and NOx emissions. Increased direct injection ratio decreases HC and NOx emissions, but increases CO emissions.