An endoreversible Carnot-type heat engine is studied under the usual restrictions: no friction, working substance in internal equilibrium (endoreversibility), no mechanical inertial effects, and under Newton’s cooling law for heat transfer between working fluid and heat reservoirs. A monoparametric family of straight lines which is isoefficient is found; i.e., all points (engine configurations) that belong to same line have the same efficiency. Along each line the power output divided by entropy production is a constant. From these properties and by using some dissipated quantities, relationships are obtained between reversible work and finite-time work and between reversible efficiency and finite-time efficiency. An ‘‘ecological’’ criterion is proposed for the best mode of operation of this heat engine. It consists in maximizing a function representing the best compromise between power and the product of entropy production and the cold reservoir temperature. The corresponding efficiency results almost equal to the average of the Carnot and the Curzon and Ahlborn [Am. J. Phys. 43, 22 (1975)] efficiencies.

We propose an irreversible simplified model for the air standard Otto thermal cycle. This model takes into account the finite-time evolution of the cycle's compression and power strokes and it considers global losses lumped in a friction like term. The proposed model permits the maximization of quantities such as the power output and the efficiency in terms of the compression ratio r. The optimum r values obtained compare well with standard r values for real Otto engines. Our model leads to loop-shaped power-versus-efficiency curves as is common to almost all real heat engines.

In this work we propose that endoreversible Carnot–type heat engines have a general property independent of the heat transfer law used to describe heat exchanges between the working fluid and its thermal reservoirs. This property has to do with the so-called ecological function [F. Angulo–Brown, J. Appl. Phys. 69, 7465 (1991)]. According to this property, the efficiency at the maximum of the ecological function is the semisum of the Carnot and the maximum power efficiencies for any heat transfer law. This result is obtained by using the quasiparabolic behavior of power versus efficiency. From this property, we obtain a corollary over a general quantitative relation between the power (and also the entropy production) of both maximum power and maximum ecological regimes. We also discuss a criterion to find the best ecological function.

In this work, we present the generalization of some thermodynamic properties of the black body radiation (BBR) towards an n-dimensional Euclidean space. For this case, the Planck function and the Stefan-Boltzmann law have already been given by Landsberg and de Vos and some adjustments by Menon and Agrawal. However, since then, not much more has been done on this subject, and we believe there are some relevant aspects yet to explore. In addition to the results previously found, we calculate the thermodynamic potentials, the efficiency of the Carnot engine, the law for adiabatic processes and the heat capacity at constant volume. There is a region at which an interesting behavior of the thermodynamic potentials arises: maxima and minima appear for the n-dimensional BBR system at very high temperatures and low dimensionality, suggesting a possible application to cosmology. Finally, we propose that an optimality criterion in a thermodynamic framework could be related to the 3-dimensional nature of the universe.

A local stability analysis of an endoreversible Curzon-Ahborn-Novikov (CAN) engine, working in a maximum-power-like regime, is presented. The CAN engine in the present work consists of a Carnot engine that exchanges heat with the heat reservoirs T 1 and T 2 (T 1 > T 2 ) through a couple of thermal conductors, both having the same conductance (α). In addition, the working fluid has the same heat capacity (C) in the two isothermal branches of the cycle. From the local stability analysis we conclude that the CAN engine is stable for every value of α, C and τ = T 2 /T 1 ; that after a perturbation the system state exponentially decays to the steady state with either of two different relaxation times; that both relaxation times are proportional to C/α; and that only one of them depends on τ , being a monotonically decreasing function of τ . Finally, when comparing with the system steady-state energetic properties, we find that as τ increases, the system stability is improved, while the system power and efficiency decrease; this suggests a compromise between the stability and energetic properties, driven by τ .