A variational approach to some properties of endoreversible heat engines

Endoreversible heat engine models are irreversible thermal cycles where the whole entropy production is only ascribed to the couplings between the working fluid and its heat reservoirs. This kind of thermodinamic systems have been extensively studied under diverse criteria of merit. In this work we recover the so-called semisum property for an endoreversible engine working in a regime that represents a good compromise between high power output and low entropy production (the ecological regime) by means of a variational approach. We apply variational calculus over functionals corresponding to thermodynamic processes.