Simulation Techniques to Predict Combustion Variability Validated with Engine Tests: EGR and increased valve overlap have been applied to recent gasoline engines for the purpose of reducing pumping loss and improve fuel economy under part-load operating conditions. Combustion of fuel mixture with inert gas decreases combustion temperature and increases combustion variability from cycle to cycle. Therefore, a method to predict combustion variability particularly under part-load operating conditions is necessary to estimate the fraction of burned gas during the early stage of the engine design process. The information is extremely useful to engine designers who need to know the limit of stable flame propagation so that the optimum settings for either EGR rate or valve overlap can be determined, and the level of fuel economy improvement with reduction in pumping loss can be predicted prior to actual engine design. Such prediction method was developed at Nissan to increase the accuracy of predicting cyclic variability of IMEP. They used the results of cycle-averaged gas motion obtained with Reynolds Averaged Navier-Stokes and computational fluid dynamics to predict the cyclic variability of combustion considering several factors including gas motion patterns, pumping loss, wall heat transfer, and heat release, for example.