Predicting the Effects of Air and Coolant Temperature, Deposits, Spark Timing and Speed on Knock in Spark Ignition Engines

The prediction of knock onset in spark-ignition engines requires a chemical model for the autoignition of the hydrocarbon fuel-air mixture, and a description of the unburned end-gas thermal state. Previous studies have shown that a reduced chemistry model developed by Keck et al. adequately predicts the initiation of autoignition. However, the combined effects of heat transfer and compression on the state of the end gas have not been thoroughly investigated. The importance of end-gas heat transfer was studied with the objective of improving the ability of our knock model to predict knock onset over a wide range of engine conditions. This was achieved through changing the thermal environment of the end gas by either varying the inlet air temperature or the coolant temperature.

Results show that there is significant heating of the in-cylinder charge during intake and a substantial part of the compression process. The effects of deposits on the combustion chamber walls in promoting knock were also investigated. Their primary effect is a thermal one: since the outer surface temperature of the deposits is hotter than that of the clean engine walls, greater bulk gas heating occurs. However, our results suggest that active species in the end gas carried over from preceding cycles may play a more important role in enhancing knock when deposits are present. Finally, initial work in developing a knock prediction methodology for investigating the audible knock limit of an engine was undertaken. A comparison of the audible knock predictions with the experimental limits as a function of speed is discussed.