Experimental Study on Spark Assisted Compression Ignition (SACI) Combustion with Positive Valve Overlap in a HCCI Gasoline Engine

The spark-assisted compression ignition (SACI) is widely used to expend the high load limit of homogeneous charge compression ignition (HCCI), as it can reduce the high heat release rate effectively while partially maintain the advantage of high thermal efficiency and low NOx emission. But as engine load increases, the SACI combustion traditionally using negative valve overlap strategy (NVO) faces the drawback of higher pumping loss and limited intake charge availability, which lead to a restricted load expansion and a finite improvement of fuel economy.

In this paper, research is focused on the SACI combustion using positive valve overlap (PVO) strategy. The characteristics of SACI combustion employing PVO strategy with external exhaust gas recirculation (eEGR) are investigated. Two types of PVO strategies are analyzed and compared to explore their advantages and defects, and the rules of adjusting SACI combustion with positive valve overlap are concluded. Based on that, the fuel efficient strategy for SACI combustion at mid-high load is identified. Using the optimized PVO strategy, stable SACI combustion is achieved in a wider load range, with significant improvements on fuel economy and pumping loss.