Fuel effects on knock, heat release and ''CARS'' temperatures in a spark ignition engine

Net heat release, knock characteristics and temperature were derived from in-cylinder pressure and end-gas CARS measurements for different fuels in a single-cylinder engine. The maximum net heat release rate resulting from the final phase of autoignition is closely associated with knock intensity. Aromatic fuels have lower maximum heat release rates and lower knock intensities than expected from their octane number when compared to paraffinic fuels; this is observed even when there is significant heating of the end-gas from pre-flame reactions. Leaner mixtures have lower combustion rates so that pressure development is slowed and hence ignition needs to be more advanced to get knock to occur as frequently as in a richer mixture. However, for a given frequency of knock occurrence, there is no significant difference in peak net heat release rates and hence in knock intensities for different mixture strengths.