The thermodynamics governing 'endoreversible' engines

TL;DR

This paper clarifies the thermodynamics of endoreversible engines, especially the Curzon-Ahlborn engine, showing that their irreversibility stems from reservoir selection and that optimal conditions are reversible, reaffirming Carnot efficiency limits.

Contribution

It revises the criterion for adiabatic equilibration in endoreversible engines and demonstrates that optimal operation is reversible when reservoirs are in thermal contact.

Findings

Irreversibility arises from cold reservoir selection.

Optimal heat exchange results in reversible operation.

Carnot efficiency remains the ultimate limit.

Abstract

The thermodynamics of the Curzon-Ahlborn engine, which is a prototype of endoreversible engines, is elucidated. In particular, their criterion for adiabatic equilibration is revised. The so-called irreversibility of endoreversible engines arises from the selection of the coldest reservoir for heat rejection. Rather, if the reservoirs are allowed to come into thermal and mechanical contact, a mean value results which optimizes the work output and heat uptake, and is entirely reversible. The Carnot efficiency cannot be beaten because nothing is as cold as the coldest reservoir.