Original diagram of the Carnot engine (fig. 1 of Carnot's Reflections).

In thermodynamics, the Carnot engine is a heat engine, depicted adjacent, within which a working substance (generally steam), contained within the piston-and-cylinder, is taken through a Carnot cycle of operations to produce mechanical work from heat. In the 1899 view of American physicist John Shedd: [1]

“There is, perhaps, no proposition in the range of physics that is more difficult of comprehension by the average student than that embodied in the so-called Carnot Engine and Carnot Cycle.”

History
The Carnot engine is a theoretical engine conceived by French physicist Sadi Carnot in 1824, in his Reflections on the Motive Power of Fire, based generally on the design of the typical steam engine of the era, i.e. on the Watt steam engine (c.1775), but in more exact detail on the design of the Papin engine (1690), according to which caloric heat particles were visualized to pass from the hot body (fire) to the cold body (stream of cool water) via what was called re-establishment of the equilibrium in the caloric. [2]

In 1871, the term “Carnot’s engine” was already in use by Scottish physicist James Maxwell. [3] The shortened term "Carnot engine" soon became a standard term in the decades to follow.

References
1. Shedd, John C. (1899). “A Mechanical Model of the Carnot Engine”, Physical Review, (pgs. 174-180). No. 3, Jan.
2. 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.
3. Maxwell, James C. (1871). Theory of Heat (pg. 148). Dover.

External links
● Carnot heat engine – Wikipedia.
● Carnot engine (in operation) – Java Script animation by Xing Min Wang.