Benoit Paul Émile Clapeyron (February 26, 1799 - January 28, 1864) was an French engineer and physicist, considered as one of the founders of thermodynamics.

Born in Paris, Émile Clapeyron makes his studies at the École Polytechnique and the École des Mines, before leaving to Saint Petersburg in 1820 to teach at the École des Travaux Publics. It returns to Paris only after the Revolution of July 1830, undertaking the construction of the first railway line connecting Paris to Versailles and Saint-Germain.

In 1834, he contributes its first share to the creation of modern thermodynamics by publishing a report entitled the Driving force of the heat, in which it develops the work of the physicist Nicolas Léonard Sadi Carnot, deceased two years before. In this memory, it gives in particular a graphic translation of the Carnot theorem , which stipulates that the output of a heat engine with two heat reservoirs is maximum if the machine functions in a reversible way. This diagram of Clapeyron constitutes a representation of the states of a fluid by means of curves giving the pressure of the fluid according to its volume to a given temperature.

In 1843, Clapeyron defines the concept of reversible transformation. This concept enables him to write the principle of Carnot (corresponding to the second principle of thermodynamics) in the form of an equality, which makes it more usable in practice. Within this framework, it establishes the Clapeyron formula, relation which gives the latent heat of change of state of a pure substance . Clapeyron is also illustrated by many other work relating to the formulation of the equation of perfect gases, the equilibrium of homogeneous solids, or calculations of the constraints applying to beams. He was elected member of the Académie des Sciences in 1858.

See also: Stefan problem