An Experimental Study on a Six-Stroke Gasoline Homogeneous Charge Compression Ignition (HCCI) Engine with Continuously Variable Valve Duration (CVVD) 2021-01-0512

An experimental study was conducted on a multi-cylinder engine to understand the feasibility of a six-stroke homogeneous charge compression ignition (HCCI) operation under stoichiometric conditions. State-of-the-art technologies such as continuously variable valve duration (CVVD) and high-pressure gasoline direct injection (GDI) were experimentally exploited to increase the degree of freedom of engine control. The motivation of six-stroke HCCI combustion is to remedy the load limitation and the cyclic variation in four-stroke HCCI combustion with two additional strokes: compression and expansion strokes. The six-stroke HCCI combustion occurs in the following order. First, hot residual gas is trapped by applying negative valve overlap (NVO). Next, fresh air enters, fuel is injected, and lean HCCI combustion occurs in the 1^st power stroke (PS). Subsequently, additional fuel is injected, and the 2^nd combustion occurs with the remaining oxygen in the two additional strokes. In this study, we discuss the effect of various control variables, i.e., fuel split ratio, and injection timings, on the combustion phases of the two power strokes. In addition, we present the challenge of achieving proper combustion phasing in stoichiometric six-stroke HCCI operation, mainly in high load operation when we intend to split the load. As the load is more distributed to the 1^st PS, it may lead to a significantly high mixture temperature at the beginning of the 2^nd PS, which can result in a premature 2^nd combustion phase. We discuss how the aforementioned control variables can be utilized to mitigate the challenge.