The effects of residual gases on the combustion process of engines are examined through analysing the cyclic variations of autoignition in a motored engine fuelled with homogeneous gaseous fuel-air mixtures. The changes in composition and temperature of residual gases as well as the associated rates of the preignition reactions are followed over a number of consecutive working cycles at a constant engine speed to establish whether autoignition will take place and how many cycles are need for its occurrence. It is in that the residual gases associated with partial oxidation reactions tend to have strong kinetic but hardly any thermal or diluting effects, while residual gases produced from the more complete combustion following autoignition tend to possess significant thermal, kinetic and diluting effects. The build up in the concentrations of active species in the residual gases over a number of cycles and associated thermal effects contribute to the observed cyclic variations in the onset of autoignition during the operation of homogeneous charge gas fuelled engines. Analytical results showed good agreement with the corresponding experimental trends.