The recent improvements on spark ignition engines have been the result of improved engine thermodynamic cycles, made possible by advances in technologies such as variable valve timing, variable compression ratio, and turbines of variable geometry. To assess the improvement capability of engine performance under a changing thermodynamic cycle, performance of an irreversible Miller cycle in internal combustion engines is analyzed using finite time thermodynamics. In this model, the nonlinear relation between the specific heats of a working fluid and its temperature, the frictional loss computed according to the mean velocity of the piston, and heat transfer loss through the cylinder wall are considered. The relations between the power output and the compression ratio and between the thermal efficiency and the compression ratio are indicated by detailed numerical examples. Moreover, the effects of variation of combustion chamber volume and variation of piston displacement volume on the cycle performance are analyzed. The results obtained herein have realistic significance and may provide guidelines for the design and evaluation of practical internal combustion engines.