New polytropic model to predict the performance of beta and gamma type Stirling engine

During the compression and expansion processes, the polytropic number is obtained near the adiabatic number. It was shown that there exists an optimum rotation speed for maximum output power. The output power evolution among different average pressures becomes more important with the rotation speed increasing for both helium and hydrogen as working gas. Increasing engine's average pressure can increase the thermal efficiency, but the increased amplitude decreases with pressure's increasing. Hydrogen is more suitable for high rotation speed engine comparing with helium. An optimum displacer clearance size was determined for a maximum output power. This optimum value increases with the engine pressure increasing.