Prediction of knock occurrence in a spark-ignition engine

Abstract

It is generally accepted that knock in a spark-ignition engine is caused by end-gas autoignition. Hu and Keck have recently developed a reduced kinetic model which simulates the kinetic processes involved in autoignition of hydrocarbon-air mixtures. The model was used successfully to correlate measurements of explosion limits in a constant volume bomb and ignition delay times in rapid compression machines. In this paper, the ability of Hu and Keck's model to predict the onset of knock in a spark-ignition engine is evaluated Experimental data from a large number of individual cycles were generated from a single-cylinder engine over a range of operating conditions where knock occurred. The unburned gas temperature used in the kinetic model was calculated from the measured cylinder pressure data assuming that knock originates in that part of the end-gas region which is compressed adiabatically. The model indicates that autoignition under knocking conditions is a two stage process. The model predictions of when knock occurs generally match well with measured knock angle on a cycle-by-cycle basis for a range of fuel (isooctane, primary reference, and indolene) and engine operating conditions. Agreement is least satisfactory when knock occurs close to the end of the burning process when the end-gas mass fraction is small and its temperature is not known precisely.