Numerical analysis of knocking characteristics and heat release under different turbulence intensities in a gasoline engine

Abstract

In this work, different knocking characteristics and heat release have been numerically studied in a downsized spark-ignition engine, with addressing turbulence intensity and spark-ignition timing. The results show that knock intensity mainly depends on the heat release of hot-spots, and the heat release is not only associated with unburned mass fraction but also closely related to subsequent autoignition development processes, e.g. end-gas autoignition mode. Under low turbulence intensity, local mixture autoignition is induced by a single hot-spot, accompanied by an autoignition reaction wave propagation; whereas under high turbulence intensity, a nearly homogeneous autoignition event characterizing quasi constant-volume combustion is observed. Meanwhile, compared with the single hot-spot autoignition, the homogeneous one can lead to more intensive local overpressure and thus stronger knock intensity. Further analysis on the underlying reasons shows that the higher heat release rate under homogeneous autoignition conditions is the main reason for the stronger knock intensity. Besides, the dependency of knock intensity and heat release on unburned mass fraction is found effective only for the same autoignition scenarios. Current study can provide insights into knocking suppression with consideration of intake system optimization in modern downsized gasoline engines.