Effects of flame propagation speed on knocking and knock-limited combustion in a downsized spark ignition engine

Abstract

Engine knock remains the key factor that ultimately restricts the peak thermal efficiency in modern downsized spark ignition (SI) engines. The flame propagation speed can affect knock tendency by the residence time and thermodynamic state of end-gas, which are competing with each other. The investigations in this study focus on flame propagation speed impacts on engine knock characteristics and thermal efficiency under a high load/low speed operating condition using three-dimensional numerical simulations. The results show that knock intensity (KI) increases first and thereafter decreases with the increase of SI flame speed under knocking condition. Behind the nonmonotonic relationship, the control mechanisms of end-gas auto-ignition are different. The low to moderate speed SI combustions are linked to local hot spots. Higher KI is caused by the interaction between the pressure waves induced by these hot spots. Then with the acceleration of SI flame, a homogeneous autoignition event occurs along the reaction front, leading to more intense pressure fluctuations. At the high-speed SI combustion, local hot spots disappear gradually, and the end-gas autoignition is dominated by flame-induced sequential auto-ignition. The corresponding KI decreases due to the reduction of available fresh gases. A posteriori analysis is also performed applying theories proposed by Zeldovich and Bradley. It’s shown that an increase in the SI flame speed results in lower first and thereafter higher temperature gradient in end-gas, leading to a drastic change in the auto-ignition behaviors. In addition, the relative effects of knock suppression and combustion duration reduction on thermal efficiency improvement are clarified.