Effects of Direct Injection Timing and Air Dilution on the Combustion and Emissions Characteristics of Stratified Flame Ignited (SFI) Hybrid Combustion in a 4-Stroke PFI/DI Gasoline Engine

Controlled Auto-Ignition (CAI) combustion can effectively improve the thermal efficiency of conventional spark ignition (SI) gasoline engines, due to shortened combustion processes caused by multi-point auto-ignition. However, its commercial application is limited by the difficulties in controlling ignition timing and violent heat release process at high loads. Stratified flame ignited (SFI) hybrid combustion, a concept in which rich mixture around spark plug is consumed by flame propagation after spark ignition and the unburned lean mixture closing to cylinder wall auto-ignites in the increasing in-cylinder temperature during flame propagation, was proposed to overcome these challenges. The combustion and emissions characteristics in the SFI hybrid combustion were experimentally investigated in a single-cylinder 4-stroke gasoline engine operating at medium to high loads when direct injection timing was retarded from -100 °CA to -40 °CA after top dead center (ATDC) and excess air coefficient was increased from 1.0 to 1.2 at the direct injection ratio of 30%. The experimental results show that direct injection timing and excess air coefficient control the ignition timing and combustion duration. Ignition timing advances with increased excess air coefficient at the same direct injection timing. Long combustion duration occurs at earlier direct injection timing close to -100 °CA ATDC or very late direct injection timing near -40 °CA ATDC at different excess air coefficients. In the meantime, combustion duration reduces with increased excess air coefficient when auto-ignition occurs during the combustion processes, while it increases with excess air coefficient without auto-ignition. Nitrogen oxides emissions increase first and then decrease with retarded direct injection timing at different excess air coefficients, and their maximum values occur at the direct injection timing of -60°CA ATDC.