Heat engine

Papin engine 2Hot body cold body (diagram)
The 1690 Papin engine (left), the model engine on which the theoretical 1824 Carnot engine (right) was conceived, both of which depict the basic "heat engine": composed of a hot body, a working body, and a cold body.
In engines, heat engine is a generic name for a machine, configuration, or instrument whose heat flow operations can produce "impelling power" (work per unit time) through a difference in temperature by alternating contraction and expansion a working substance, which may be any substance, e.g. air, vapor, liquid, a solid body, etc. When the working substance is water, the heat engine is called a "steam engine". These distinctions were outlined in 1824 by French physicist Sadi Carnot in his On the Motive Power of Fire. [1]

History
See also: Engine development timeline
The first functional steam engine, i.e. that which could produce useful work on a cyclical basis, was built by English engineer Thomas Savery in 1697 and called the the "Miner's friend", by virtue of its ability to help miners pump water out of mines and carry coal out of its shafts. [2] Soon more variations of such engines were made.

In 1824, French physicist Sadi Carnot set out to generalize the principles of operation for all types of heat engines. In particular, Carnot stated that “in order to consider in the most general way the principle of the production of motion by heat, it must be considered independently of any mechanism or any particular agent.” Specifically:

“It is necessary to establish principles applicable not only to steam-engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated.”

In an endnote to this comment, Carnot details that “we distinguish here the steam-engine from the heat engine in general … the latter may make use of any agent whatever, of the vapor of water or of any other, to develop the motive power of heat.” In more detail, regarding this any-type-of-substance proposal, Carnot points out that:

“Wherever there exists a difference of temperature, wherever it has been possible for the equilibrium of the caloric to be re-established, it is possible to have also the production of impelling power. Steam is a means of realizing this power, but it is not the only one. All substances in nature can be employed for this purpose, all are susceptible of change of volume, of successive contradictions and dilations, through the alternation of heat and cold.”

References
1. Carnot, Sadi. (1824). Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power. Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
2. Savery, Thomas. (1702). The Miner’s Friend – or an Engine to Raise Water by Fire. London.

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