This paper presents confirmation of the accuracy of prediction of an engine simulation model. The experimental data used to compare with the output of the simulation model are from a single cylinder four-stroke cycle engine and from a single-cylinder two-stroke cycle engine; both engines are naturally aspirated and use spark- ignition. In addition, for the two-stroke cycle engine, the experimental data includes two cylinders with different scavenging characteristics which induce variations of performance characteristics of up to 20%. The fundamentals of the theoretical approach have been presented before to SAE (1)* and this paper extends that theory by providing a detailed discussion on the inclusion of measured scavenging characteristics to enable the simulation model to predict the mechanism of the in-cylinder gas exchange process. This permits the designer to use the simulation model to detail completely the internal geometry of an engine, its porting or valving, and its intake or exhaust ducting, and to deduce the overall performance characteristics such as power, torque, airflow or fuel consumption. Further detailed information is presented from the use of the simulation model on scavenging, trapping, and charging efficiencies and delivery ratio and also the pressures, temperatures, particle velocities, charge purities and densities as a function of time in various location throughout the engine. Such detailed insights into the internal gas dynamic and gas exchange characteristics are invaluable in that many are almost impossible to acquire experimentally and so they give the designer a ‘feel’ for the engine behaviour which could not otherwise be obtained.The level of accuracy of correlation between measurement and calculation is such that the model can be used for design purposes with considerable confidence.