Effects of reactivity inhomogeneities on knock combustion in a downsized spark-ignition engine

Abstract

The reactivity inhomogeneities are inevitably generated in the cylinder with the implementation of knock suppression strategies in downsized engines with direct-injection spark ignition. However, the mechanism of its influence on combustion has not yet been fully understood. A fundamental study was conducted by large eddy simulation to comprehensively study the effects of reactivity inhomogeneity formed by an independent variable of fuel stratification or temperature stratification on the in-depth mechanism of end-gas autoignition in a downsized spark ignition engine. The turbulent flame propagation is determined by an improved G-equation turbulent combustion model, and the detailed chemistry mechanism of a primary reference fuel is employed to observe the detailed reaction process in the end-gas autoignition process. Knock suppression results are observed in the temperature stratification case. In a large temperature gradient case, the premature autoignition timing caused by higher local temperature provides sufficient fresh gas for spontaneous combustion, leading to a better power performance compared to slight temperature gradient cases. Results of fuel stratification show that the knock is suppressed and gradually disappears with increasing ΔΦ. However, the subsequent reduction of peak pressure and the lengthening of combustion duration results in a terrible drop in power performance. The impact of flame propagation velocity dominates the pressure evolution compared to the effect of a highly local rich mixture reducing the ignition delay time under present conditions. This work will give an underlying understanding of the knock mechanism through different strategies.