Effect of active ignition on the end-gas autoignition with detonation combustion in confined space

Abstract

In spark-ignition engines, the presence of knock significantly prevents the engines from further improving thermal efficiency. In this work, the effect of active ignition in the end-gas region on the end-gas autoignition with detonation combustion was optically observed in the self-designed constant volume combustion bomb (CVCB-TJU). The different combustion modes together with pressure oscillation were presented. The results indicate that advancing the ignition timing of the end-gas spark can reduce the pressure oscillation intensity and even suppress the occurrence of autoignition. In addition, the present work observed a statistically positive relationship between unburned mass fraction (UMF) and detonation intensity for the knock induced by developing detonation. The underlying influencing mechanism of end-gas ignition on auto-ignition was presented. The end-gas ignition influences the occurrence and intensity of detonation and pressure oscillation by two means: 1) reducing the UMF burned by autoignition, 2) influencing the flame acceleration, thereby reducing the flame tip velocity, and the subsequent shock intensity. Furthermore, the burned mass fraction (BMF) consumed by end-gas flame when jet flames emerge is applied to substitute the ignition timing when measuring its influence at different oxygen concentrations of 29% and 33%. It is found there exists a critical BMF above which the autoignition would be suppressed. And this critical value increases with increased oxygen concentration.