End-Gas Autoignition Mechanism in a Downsized Spark-Ignition Engine: Effect of Inhomogeneity

It is generally accepted that knock in spark-ignition engines is caused by end-gas autoignition. However, limited studies regarding to the effect of the inhomogeneity of temperature and composition on end-gas autoignition are presented. The major objective of this paper is to quantitatively understand the flame propagation velocity and the end-gas autoignition mechanism affected by the reactivity gradient formed by thermal stratification and composition stratification in the combustion chamber of a downsized SI engine. The conclusions were obtained based on a knock combustion case under an initial condition of high temperature and high pressure. In the present work, fuel stratification has a significant influence on the propagation velocity of the main flame. The flame propagation velocity declined with an increase in the level of fuel stratification. However, the temperature stratification had little effect on the flame propagation velocity. It was found that the onset of autoignition advanced with increasing the level of temperature stratification, which demonstrated an approximately linear relationship. It can be found that a high level of fuel stratification induced deflagration because of multi-spot spontaneous combustion. On the contrary, a very intense knock with detonation wave was observed under a slight level of temperature stratification. Under this condition, the temperature gradient near the nascent AI kernel was small and contributed to sequential spontaneous combustion. In fact, both the fuel stratification and temperature stratification are essentially the reactivity stratification. Therefore, similar results also can be conducted from fuel stratification. This work will give a new insight into suppressing knock mechanisms by different strategies.