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The goal of this comment note is to express my concerns about the recent paper by Tian Zhao et al. (<i>Entropy</i> <b>2018</b>, <i>20</i>, 542). It is foreseen that this comment will stimulate a fruitful discussion of the issues involved. The principle of the least thermodynamic action is applicable for the analysis of the Carnot cycle using the entropy (not heat) generation extrema theorem. The transversality conditions of the variational problem provide the rectangular shape of the <i>ST</i> diagram for the Carnot cycle.