Wall Temperature Effect on SI-CAI Hybrid Combustion Progress in a Gasoline Engine

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In order to investigate the effect of the thermal boundary condition on the hybrid combustion, the experiments with different coolant temperatures are performed to adjust the chamber wall temperature in a gasoline engine. The experimental results indicate that increasing wall temperature would advance the combustion phasing, enlarge the peak heat release rate and shorten the combustion duration. While the capacity of the wall temperature effect on the hybrid combustion characteristics are more notable in the auto-ignition dominated hybrid combustion. The flame propagation dominated hybrid combustion shows the apparent inconsistency of the changing degree of CA10 (10% of the total heat released) and CA90 (90% of the total heat released) with the wall temperature changing. The three-dimensional simulations using a hybrid combustion model are carried out to further investigate the influence mechanism of the wall temperature effect on the SI-CAI hybrid combustion. The simulation results reveal that the early flame propagation, compared to the subsequent auto-ignition, is less sensitive to the variation of the in-cylinder temperature distribution caused by the wall temperature change, which in turn accounts for the inconsistency of the changing degree of CA10 and CA90 observed at different loads. The understanding of the combustion characteristics with different wall temperatures provided in this paper would facilitate the combustion control and have a full advantage of the SI-CAI hybrid combustion.