In this study, a quasi-realistic thermodynamic analysis was performed to investigate the effects of design and operating parameters on the performance of a single-cylinder modern Atkinson cycle engine. Fortran was used for all calculations. It was assumed that the fuel-air mixture was used as the working fluid, and iso-octane was used as the fuel. The Wiebe function was used for the combustion process and it was assumed that the specific heat of the working fluid varies with temperature. In the calculations, heat transfer loss, combustion efficiency, mechanical friction, and pumping losses were taken into account. In the analysis, the closing of the intake valve, equivalence ratio, geometric compression ratio, and the initial conditions of the intake proses were used as independent variables. The effects of these variables on brake mean effective pressure, effective power, specific fuel consumption, and thermal efficiency were investigated. Increasing the inlet pressure, increasing the geometric compression ratio, and delaying the closing of the intake valve increased the mean effective pressure, thermal efficiency, engine output power, and torque. The increase in the inlet temperature adversely affected the engine performance and the specific fuel consumption increased. Engine performance parameters worsened when the equivalence ratio fell below 0.8 and rose above 0.9.