A zero-dimensional (or, quasi-dimensional) model for combustion in S.I. engines has been developed using conservation equations for mass and energy, thermodynamic state equations and correlations for mass burning rate and wall heat transfer. The model was applied to a single cylinder, four-stroke gasoline engine. The computed pressure crank angle results were compared with the measured pressure histories obtained for nine engine operating conditions representing various engine speeds, air-fuel ratios, and loads. The extensive comparison study, which hitherto had not been conducted to validate any zero-dimensional model, ended with surprising success in view of the fact that the zero-dimensional model is very simple and an engine is a very complex system. Satisfactory comparisons were obtained simultaneously for all the operating conditions with only one set of model constants. The spark timings used in the computations were same as the actual ones.