A comprehensive cycle analysis has been developed for four-stroke spark-ignited engines from which the indicated performance of a single cylinder engine was computed with a reasonable degree of accuracy. The step-wise cycle calculations were made using a digital computer. This analysis took into account mixture composition, dissociation, combustion chamber shape (including spark plug location), flame propagation, heat transfer, piston motion, engine speed, spark advance, manifold pressure and temperature, and exhaust pressure.A correlation between the calculated and experimental performance is reported for one engine at a particular operating point. The calculated pressure-time diagram was in good agreement with the experimental one in many respects. The calculated peak pressure was 10 per cent lower and the thermal efficiency 0.8 per cent higher than the measured values. Thus this calculational procedure represents a significant improvement over constant volume cycle approximations.When the procedure was developed it was used to study the influence of varying the rates of flame propagation and heat transfer on the performance of this engine. In addition the availability and irreversibility of the working fluid were determined as a function of crank angle.It appears that this is a valid approach for predicting spark-ignited engine cycle behavior and the effects of changing operating variables. Moreover it offers great potential in the study of the importance to the engine cycle of phenomena such as turbulence, heat transfer, and the influence of combustion chamber shape.