Characterization of Knock in a Spark-Ignition Engine

Spark-ignition engine knock was characterized in terms of when during the engine cycle and combustion process knock occurred and its magnitude or intensity. Cylinder pressure data from a large number of successive individual cycles were generated from a single-cylinder engine of hemispherical chamber design over a range of operating conditions where knock occurred in some or all of these cycles. Mean values and distributions of following parameters were quantified: knock occurrence crank angle, knock intensity, combustion rate and the end-gas thermodynamic state. These parameters were determined from the cylinder pressure data on an individual cycle basis using a mass-burn-rate analysis. The effects of engine operating variables on these parameters were studied, and correlations between these parameters were examined.

The individual cycle knock occurrence crank angle correlates well with the combustion rate or a thermodynamic parameter representing the end-gas temperature history on an individual cycle basis. But the individual cycle knock intensity does not correlate with any of the relevant variables measured although the mean knock intensity did correlate well with the mean unburned fuel fraction at knock. This lack of correlation on an individual cycle basis is thought to be due to differences in the end-gas temperature and species concentration distributions, and differences in the geometry and location of the end-gas region, cycle-by-cycle.