Resemblance of the efficiencies between reversible Otto and Joule-Brayton-like cycles and finite-time Curzon-Ahlborn engines

In this work we analyze a connection between reversible thermal cycles and Curzon-Ahlborn finite-time thermal engines. Since the 19th century, with the formalization of the thermal engines study, analyzing their efficiencies has been a most relevant issue. The first landmark was the named Carnot efficiency, a cornerstone in thermodynamics, establishing the maximum possible efficiency for any energy converter working between two extreme heat reservoirs, hardly reachable since it requires infinite time processes. First Novikov and Chambadal and Curzon and Ahlborn after that made a step towards more realistic irreversible engines was made. Thus Finite Time Thermodynamics emerged. Its formulation supposes engines with dissipations or irreversibilities derived from the couplings between an internal reversible Carnot cycle and its surroundings (endorreversible engine). This kind of model proceeds at finite-time. The connection we have pointed out and that now is analyzed in this work arose from a notable proximity between the reversible and irreversible engines efficiencies through the thermal properties of the working substance and the nature of the heat exchanges. This can be explained by rethinking on how to look at the internal reversible cycle, thanks to a not yet exploited flexibility of the endorreversibility hypothesis it is possible to look at it not as an ideal separate core, but as a hybrid mixture of reversible and irreversible processes.