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A Stirling engine

A Stirling engine is a classical heat engine. The engine is heated from outside (any type work, a fire, electrical heating, heating by concentrated solar radiation), and cold water is passed through it to cool the engine (the water leaving the engine is warmer). When operated like this, the engine can drive mechanical processes (Movie 1). The “working fluid” is air or some other simple gas. When the flow of cooling water is reduced, say, by a factor of two, the temperature difference of the fluid changes by a factor of two as well.

Interpretation

The following interpretation is by Sadi Carnot from his famous book The Motive Power of Heat (1824). “Every one knows that heat can produce motion. That it possesses vast motive-power no one can doubt, in these days when the steam-engine is everywhere so well known. To heat also are due the vast movements which take place on the earth …

“According to established principles at the present time, we can compare with sufficient accuracy the motive power of heat to that of a fall of water … . The motive power of a fall of water depends on its height and on the quantity of the liquid; the motive power of heat depends also on the quantity of caloric used, and on what may be termed, on what in fact we will call, the height of its fall, that is to say, the difference of temperature of the bodies between which the exchange of caloric is made. In the fall of water the motive power is exactly proportional to the difference of level between the higher and lower reservoirs. In the fall of caloric the motive power undoubtedly increases with the difference of temperature between the warm and the cold bodies.”

Heat (or caloric) falls from the high temperature of the furnace to the low temperature of the cooler. (*) Doing so it releases energy used for driving the mechanical process of the engine. All the heat supplied by the furnace has to leave through the cooler into the environment. That explains why the cooling water gets warmer if its flow through the engine is smaller.

(*) Note: The technical term for quantities of heat is entropy. What we call heat, and what is officially called entropy by engineers and physicsist is closely related to what scientisits before 1850 called caloric. When presenting phenomena in terms of standard language, it makes sense to keep using the word heat for what we normally call entropy.