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In this paper, we investigate the effects of autoignition on the heat release characteristics of a spark-ignition (SI) engine, under knocking conditions. In a normal, flame-propagation combustion, the heat release rate increases smoothly to a maximum, and then progressively decreases as the entire mixture is consumed. When autoignition occurs, the heat release rate profile shows a departure from its normal profile: since autoignition results in an explosive combustion, an abnormal rapid increase in heat release rate is generated, with significantly higher peak heat release rates and faster fuel consumption. Three distinct heat-release-rate profiles for autoignition can be identified at different engine speeds, which differ in the phasing of the sudden increase in release rate due to autoignition, relative to the peak release rate due to normal combustion.