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Effects of direct injection timing and air dilution on the combustion and emission characteristics of stratified flame ignited (SFI) hybrid combustion in a 4-stroke PFI/DI gasoline engine
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
Controlled Auto-Ignition (CAI) combustion can effectively improve the thermal efficiency of traditional spark ignition (SI) flame propagation combustion, due to the obviously shortened combustion process resulted by the occurrence of auto-ignition. However, its commercial application is limited by the difficult control in the occurrence of combustion process and violent heat release process at high loads. Stratified flame ignited (SFI) hybrid combustion has been proposed to overcome these challenges, 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 due to the increased in-cylinder temperature during the flame propagation. The combustion and emission characteristics in the SFI hybrid combustion were experimentally investigated in a single-cylinder 4-stroke gasoline engine at medium-high loads when direct injection (DI) timing was retarded from -100 °CA after top dead center (ATDC) to -40 °CA ATDC and excess air coefficient was increased from 1.0 to 1.2 at the DI ratio of 0.3. The results show that DI timing and excess air coefficient control the ignition timing and combustion duration. Ignition timing advances with increased excess air coefficient at the same DI timing. Elongated combustion duration occurs at earlier DI timing closing to -100 °CA ATDC or very late DI timing near -40 °CA ATDC at different excess air coefficients. In the meantime, combustion duration reduces with increased excess air coefficient in the case of the occurrence of auto-ignition during the combustion process, while it becomes longer at higher excess air coefficient in the combustion process without auto-ignition. Nitrogen oxides emissions increase firstly and then decrease with retarded DI timing at different excess air coefficients, and its maximum value occurs at the DI timing of -60°CA ATDC.