Using simulating models for internal combustion engine cycles is appreciable method for predicting the engines performance for saving the time and the effort. Fuel air ratio and gas variable specific heats are taken into account in the present work. Irreversibilities resulted from nonisentropic compression and expansion processes and heat loss through the cylinder wall are also taken into account in the present model. Finite difference method is applied for estimating the states through the heat addition process and compression and expansion strokes. Computer program is designed for the model includes all the above conditions and the cycle parameters. Experimental test was carried out on a single cylinder constant speed diesel engine to verify the obtained results using the present model. The obtained results show a good agreement with the corresponding data recorded from the experimental tests. Other Comparisons are done with the corresponding results of an actual engine model results which published for gasoline and diesel engines. The obtained results from the model show a good agreement with the corresponding data in researches. The effect of the cycle parameters (inlet air temperature, inlet air pressure, air fuel ratio, compression ratio, and compression and expansion efficiencies) on the power output and thermal efficiency are studied. It is shown that the power and thermal efficiency increase with the increase of compression and expansion efficiencies, inlet air pressure and compression ratio. For gasoline engine cycle the optimum value of compression ratio is around10 to be prevented from detonation, and for diesel the optimum value is around 20. With increasing air fuel ratio the power output increase then decrease and the thermal efficiency increases, so the optimum value of air fuel ratio for gasoline engine cycle is around 13 and for diesel around 15. With increasing the inlet air temperature the power output and thermal efficiency are decreased. The Specific Fuel Consumption decreases with increasing power for the two cycles. The benefit from the research is that optimum parameters for operating are predicted by the model. The obtained results would be more realistic and implemented on the performance evaluation of the internal combustion engine