In 1834, he made his first contribution to the creation of modern thermodynamics by publishing a report entitled Mémoire sur la puissance motrice de la chaleur (Memoir on the Motive Power of Heat), in which he developed the work of the physicist Nicolas Léonard Sadi Carnot, deceased two years before.

Though Carnot had developed a compelling analysis of a generalised heat engine, he had employed the clumsy and already unfashionable caloric theory.

Clapeyron, in his memoire, presented Carnot's work in a more accessible and analytic graphical form, showing the Carnot cycle as a closed curve on an indicator diagram, a chart of pressure against volume (named in his honor Clapeyron's graph).

Clapeyron's analysis of Carnot was more broadly disseminated in 1843 when Johann Poggendorff translated it into German.

[3] In 1842 Clapeyron published his findings on the "optimal position for the piston at which the various valves should be opened or closed.