Homogeneous Charge Compression Ignition (HCCI) and Spark Ignition (SI) dual-mode operation provides a practical solution to apply HCCI combustion in gasoline engines. However, the different requirements of air-fuel ratio and EGR ratio between HCCI combustion and SI combustion results in enormous control challenges in HCCI/SI mode switch.
In this paper, HCCI combustion was achieved in a four-cylinder gasoline direct injection engine without knock and misfire using close-loop control by knock index. Assisted Spark Stratified Compression Ignition (ASSCI) combustion was obtained stably at medium-high load. ASSCI combustion exhibits two-stage heat release with initial flame propagation and controlled auto-ignition. The knock index of ASSCI combustion is less than HCCI combustion due to the lower pressure rise rate. The stable operation range of stoichiometric ASSCI combustion is from 0.35MPa to 0.65MPa Indicated Mean Effective Pressure (IMEP), which is higher than normal gasoline HCCI combustion. Therefore, ASSCI mode naturally fills the gap between HCCI at low load and SI combustion at high load in gasoline HCCI engine. Applying ASSCI strategy can effectively reduce the frequency of engine combustion mode switch.
HCCI/SI and ASSCI/SI combustion mode switch were investigated and compared in a four-cylinder GDI engine which was equipped with dual VVT (intake and exhaust), two-stage cam-profile system (CPS) and electronic throttle control (ETC). Experimental results indicated that, fuel injection in compression stroke in ASSCI mode can avoid the misfire and improve stability of switching process without the requirement of changing control variables such as throttle, CPS and injection strategy simultaneously. Meanwhile, ASSCI combustion mode can extend the load range of the compression ignition (CI). Stable HCCI/ASSCI switch was achieved by adjusting s negative valve overlap (NVO) duration only. It is therefore concluded that the switch control strategy of ASSCI/SI combustion was optimized by stoichiometric operation before/after switch and the robust combustion switch was achieved.