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. These submodels are calibrated from experimental data.In this paper a combustion system featuring high and low swirl generating ports, a flat head and various piston designs is investigated. A parameter study is conducted to determine the effects of in-cylinder geometry on burn duration, heat transfer and fuel consumption. These results indicate that swirl, squish and turbulence intensity levels can be varied to produce a minimum in fuel consumption for the conditions examined. In addition, the isolated effect of the turbulence intensity on engine efficiency is presented for various EGR levels.