A computer simulation of the four-stroke spark-ignition engine cycle has been developed for studies of the effects of variations in engine design and operating parameters on engine performance, efficiency and NO emissions. The simulation computes the flows into and out of the engine, calculates the changes in thermodynamic properties and composition of the unburned and burned gas mixtures within the cylinder through the engine cycle due to work, heat and mass transfers, and follows the kinetics of NO formation and decomposition in the burned gas. The combustion process is specified as an input to the program through use of a normalized rate of mass burning profile. From this information, the simulation computes engine power, fuel consumption and NO emissions.Predictions made with the simulation have been compared with data from a single-cylinder CFR engine over a range of equivalence ratios, spark-timings and compression ratios. Predicted fuel consumption and NO emissions showed acceptable quantitative agreement with the data.A parametric study of the effect of variations in load, speed, combustion duration, timing, equivalence ratio, exhaust gas recycle and compression ratio was then carried out for a 5.7 ℓ displacement engine. Pairs of parameters were selected for analysis, and variations in brake specific fuel consumption, brake specific NO emissions and mean exhaust temperature were determined.