In 1975 Curzon and Ahlborn (Am. J. Phys; vol. 43, p. 22) studied a heat engine with thermal losses and derived an expression for its efficiency that described the performance of actual heat machines better than the traditional result due to Carnot. In their original derivation, time appears explicitly and this is disappointing in the framework of classical thermodynamics. In this note a derivation of the maximum efficiency of a heat engine with thermal losses is given without any explicit reference to time.
