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The nature of autoignition and knocking is investigated experimentally and theoretically in an optical engine by high speed direct light photography and laser schlieren filming. Special emphasis is devoted experimentally and theoretically to the role of exothermic centres in the end-gas in initiating knocking combustion and subsequent knock damage to the combustion chamber walls. The optical engine is a modified single cylinder ported two stroke engine equipped with a large head window for unlimited access to both the entire combustion chamber and the ring crevice region. In some experiments the formation of exothermic centres was stimulated by microscopic aluminium particles that deposited on the mirrored piston surface. The data are analysed by numerically modelling the transition from normal combustion to autoignition with a simplified 2D-code.