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Two classes of heat engines are studied to examine whether there is any consistent relation between the entropy production and the thermal efficiency or the work output of an engine. The first group of heat engines examined includes the Curzon-Ahlborn engine, the Novikov’s and modified Novikov’s engines, and the Carnot vapor cycle, which are all endoreversible. The second class of engines studied includes irreversible Otto, Diesel and Brayton cycles. The results and the discussions presented in this article support the following main conclusions.


  • In an endoreversible heat engine, a reduction in the entropy production corresponds to an increase in the thermal efficiency.

  • In an irreversible heat engine, a reduction in entropy production is neither equivalent to an increase in the thermal efficiency; nor to an increase in the work output.

  • The minimum entropy production, the maximum thermal efficiency, and the maximum work output criteria may become equivalent at the condition of fixed heat input. The first two criteria are also equivalent when the power (work output) of engine is constant.




Endo-reversible engines

Irreversible heat engines





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