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A computer simulation for the performance of a four-stroke spark-ignition engine is used to assess the effects of in-cylinder flow processes on engine efficiency. The engine simulation model is a thermodynamic model coupled to submodels for the various physical processes of in-cylinder swirl, squish and turbulent velocities, heat transfer and flame propagation. The swirl and turbulence models are based on an integral formulation of the angular momentum equation and a K-ε turbulence model, These models account for the effects of changes in geometry of the intake system and the chamber design on in-cylinder flow processes. The combustion model is an entrainment burn-up model applicable to the mixing controlled region of turbulent flame propagation. The flame is assumed to propagate spherically from one or two spark plug locations. A heat transfer model that is dependent upon the turbulence level is used to compute the heat loss from the unburned and burned gases.