Controlled Auto-Ignition (CAI), also known as Homogeneous charge compression ignition (HCCI), has been the subject of extensive research because of their ability to providing simultaneous reduction in fuel consumption and NOx emissions in a gasoline engine. However, due to its limited operation range, combustion mode switching between CAI and spark ignition (SI) combustion is essential to cover the overall operational range of a gasoline engine for passenger car applications. Previous research has shown that the SI-CAI hybrid combustion has the potential to control the ignition timing and heat release process during both steady state and transient operations. However, it was found that the SI-CAI hybrid combustion process is often characterized with large cycle-to-cycle variations, due to the flame instability at high dilution conditions.
In order to control the heat release process stably and expand the operating range of SI-CAI hybrid combustion, based on the idea that increased flame speed would improve the combustion stability at diluted conditions, the stratified flame ignition (SFI) and micro flame ignition strategy (MFI) involved in the SI-CAI hybrid operation were analyzed respectively based on the engine experiments carried out on a single cylinder research engine equipped with both intake and exhaust mechanical variable valve actuation systems. The distribution of fuel concentration and activity were named as SFI and MFI respectively due to their characteristics. The premixed homogenous dilution charge as main fuel was injected by intake port injector. Meanwhile, the stratified gasoline fuel (in SFI) or DME (in MFI) injected by direct injector in the cylinder was used as an enhanced flame kernel. The results show that both SFI and MFI strategy are useful to expand the dilution combustion range and adjust the combustion phase of SI-CAI hybrid combustion. But SFI strategy faced the high cyclic variation of heat release process in continuous cycles, while the key problem of MFI is knock and the worse caused by fast heat release. Both of these two strategies require optimization to be feasible to combustion control at different conditions.