We propose a finite-time thermodynamics model for an Otto thermal cycle. Our model considers global losses in a simplified way lumped into a friction-like term, and takes into account the departure from an endoreversible regime through a parameter (R) arising from the Clausius inequality. Our numerical results suggest that the cycle's power output and efficiency are very sensitive to that parameter. We find that R is the ratio of the constant-volume heat capacities of the reactants and products in the combustion reaction occurring inside the working fluid. Our results have implications in the search for new fuels for internal combustion engines.

SYNOPSISIn this work we present a simplified irreversible Otto engine model which includes power losses and a chemical combustion reaction at the end of the adiabatic compression. The heat released in the combustion reaction is taken as a fluctuant quantity around its mean value. The fluctuations are simulated by means of a Gaussian stochastic process for a rich-air intake mixture and by two non-linear deterministic noisy maps in the case of poor-air mixtures. The fluctuant combustion heat drives the Otto engine model producing in turn fluctuant power output and fluctuant cycle efficiency. We analyse the size of the fluctuant outputs in terms of the fluctuant input.

We study a photothermal engine resulting from a combination of a Müser engine with an endoreversible Curzon - Ahlborn cycle. This model was proposed by De Vos as an approach to a certain class of photothermal conversion devices. We analyse that engine with and without solar concentrators. We find that typical performance quantities such as the power output, entropy production, engine efficiency and solar conversion efficiency are functions both of the collector's temperature and of the concentration factor. For the case of an engine with concentrators, we find that there is an optimum value for the concentration factor if one wishes to maximize both the solar energy efficiency and a parameter measuring a compromise between the engine's efficiency and the solar conversion efficiency.

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In this brief paper we present an addendum to a recently published analysis of a photothermal engine model. Here, we numerically demonstrate that the design parameters, the sunlight concentration factor and the thermal conductances of materials employed as thermal conductors are linked by a simple relationship, if one wishes to obtain the maximization of the power output of the photothermal engine.