Abstract
Instructional and complete versions of thermodynamics engine cycle simulations for spark-ignition engines were compared. The instructional version of the cycle simulation used constant specific heats as compared to using variable properties and composition for the complete simulation. For the proper selection of constant properties, the global engine performance parameters obtained from the instructional version of the cycle simulation were in close agreement to the values obtained from using the complete version of the simulation. The specific values of items such as maximum pressure and temperature, however, were not exactly duplicated. Examples are given based on a commercial, spark-ignition engine. For the cases studied here, the brake power and thermal efficiency as obtained from the constant property version of the simulation (for a ratio of specific heats of 1.30 and a gas constant of 0.287 kJ/kg K) were in excellent agreement with the same parameters from the complete simulation for a range of operating conditions.