A thermodynamic cycle simulation was used to examine the effects of oxygen enriched combustion air on engine performance for a range of operating conditions and for different sized engines. The use of oxygen enriched combustion air will have a direct effect on the combustion process and on the overall engine thermodynamics. For example, for cases with higher inlet oxygen concentration (and hence less nitrogen dilution), for the same operating conditions, the combustion gas temperatures and engine cylinder heat losses will be higher. In addition, the engine using oxygen enriched combustion air will be smaller than an engine using normal air for the same power output. The major objective of this study was to quantify these expectations for a range of operating conditions. One special feature of a portion of the current study is the constant engine power output by decreasing engine size as the oxygen concentration increased in the combustion air. Results include detail thermodynamic results of temperatures, pressures and properties as functions of the oxygen concentration of the combustion air. Results also include engine performance parameters such as power, torque, fuel consumption, thermal efficiency, and exhaust temperatures. For one comparison, engine performance and fuel consumption were obtained for an equivalence ratio of 1.0, MBT spark timing, and 2500 rpm. For oxygen enriched combustion air with 32% oxygen, equal power output was obtained with 73% of the displaced volume (all else the same). For the higher oxygen case, the brake fuel consumption increased about 11% primarily due to higher heat losses and higher exhaust gas energy which were a consequence of the higher gas temperatures. For the MBT spark timing case, the nitric oxide emissions increased by about 11% as the oxygen concentration increases from 21% to 25%.