Low- and high-temperature chemistry effects on engine knock prediction

This paper examines the performance of high-temperature and low-temperature chemical kinetic models in 3D engine simulations with respect to knock prediction. The study is motivated by the high computational cost associated with using large detailed chemical kinetic models that contain low-temperature chemical kinetics. Hitherto, it has been largely assumed that reduced chemical kinetic models that only contain high-temperature chemical kinetics cannot predict engine knock with reasonable accuracy. It is necessary to verify this tacit assumption and quantify any differences in the predictive ability of high- and low-temperature models. If it can be established that high-temperature chemical kinetic models predict knock reasonably well, the computational cost reduction for exploratory high-compression ratio engine design can be significant. In this study, the CFD software package, ANSYS Forte, is used to comparatively investigate the knocking behavior of a high-compression ratio engine (compression ratio of 14.28). Two reduced chemical kinetic models are used. The larger version contains 1071 reactions among 203 species and it is able to predict low-temperature kinetic behavior such as the Negative-Temperature Coefficient (NTC) of ignition delay times. The smaller reduced model consisting of 99 species and 669 reactions is only capable of predicting high temperature kinetic behavior but not the NTC ignition behavior. The results show that the smaller high-temperature chemical kinetic model is equally capable of predicting knock onset and intensity, comparable to the larger model that includes low-temperature chemical kinetics. This comparable predictive ability spans a wide range of load, spark timing, and engine speed conditions, with small differences being observed only at low engine speeds, and larger but tolerable differences at low engine speeds and late spark timing. This comparable predictive ability is explained by the fact that in both cases, the onset of knocking is largely dependent on the higher temperature and pressure established after compression heating by the spark-ignited flame. Autoignition, that characterizes engine knock, occurs mostly near the flame front, on account of this dependence on the high temperature gradient arising from the spark-ignited flame.