Entropy and the Stirling engine

Abstract

The thermodynamic approach developed in the Romanelli (2017 Am. J. Phys. 85 926–31). achieves, among other things, an adequate description of the experimental pressure-volumes diagrams of Stirling engines. However, an unusual and relevant result of such an approach is that, during part of its cycle, the working gas reaches temperatures above and below the temperatures of the two thermal reservoirs that keep the engine running. Within this framework, the present paper essentially shows that the predictions of the above model are not in conflict with the second law of thermodynamics. Analytical expressions for the entropies of the working gas and the environment are developed, and the definition of efficiency is discussed. The theoretical results are applied numerically to a specific Stirling engine.