Effect of temperature disturbance on end-gas autoignition and detonation development

Abstract

Knocking is one of the main constrains in improving the thermal efficiency of spark ignition engines. It is generally accepted that normal knock and super-knock are respectively caused by autoignition and detonation development in end-gas. In this study, the effect of temperature disturbance on end-gas autoignition and detonation development in a closed circular domain is examined through 2D simulations considering detailed chemistry. In simulations we find typical end-gas combustion modes including triple-detonation, double-detonation and double-tongue structures, which were also observed in previous rapid compression machine (RCM) experiments. It is shown that the detonation development in end-gas is very sensitive to the temperature disturbance, Δ. As Δ increases, the first autoignition in end-gas induced by temperature disturbance occurs earlier while the corresponding pressure wave is weaker, which subsequently results in different combustion modes in end-gas. Specifically, for small Δ, a supersonic autoignition is initiated and then it triggers a triple-detonation structure consisting of a radial detonation induced by shock-flame coupling and two circumferential detonations that are caused by the near-wall shock compression induced detonation (NWSCD) mechanism. For moderate Δ, the radial detonation is suppressed due to the earlier first autoignition and weaker pressure waves, and thereby the double-detonation structure consisting of two circumferential detonations appears. These two detonations are formed through near wall autoignition induced detonation (NWAID) mechanism. For relatively large Δ, there is no detonation development since the end-gas is quickly consumed by autoignition, which results in a double autoignition front structure, referred to as the double-tongue structure. In this study, the formation of complicated autoignition and detonation structures is interpreted. The results provide insight in understanding the development of normal knock and super-knock in spark ignition engines.