Carnot heat engine

Carnot heat engine

A Carnot heat engine [In French, Carnot uses "machine à feu", which Thurston translates as "heat-engine" or "steam-engine". In a footnote, Carnot distinguishes the steam-engine ("machine à vapeur") from the heat-engine in general. (Carnot, 1824, p. 5 and Carnot, 1890, p. 43)] is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded upon by Benoit Paul Émile Clapeyron in 1834 and mathematically elaborated upon by Rudolf Clausius in the 1850s and 60s from which the concept of entropy emerged.



In the adjacent diagram, from Carnot's 1824 work, "Reflections on the Motive Power of Heat", [Sometimes translated as "Reflections on the Motive Power of Fire". [] ] there are "two bodies "A" and "B", kept each at a constant temperature, that of "A" being higher than that of "B". These two bodies, to which we can give or from which we can remove the heat without causing their temperatures to vary, exercise the functions of two unlimited reservoirs of caloric. We will call the first the furnace and the second the refrigerator.” [English translation by Thurston (Carnot, 1890, p. 51-52).] Carnot then explains how we can obtain motive power, i.e. “work”, by carrying a certain quantity of heat from body "A" to body "B".

Modern diagram

The previous image shows the original piston-and-cylinder diagram used by Carnot in discussing his ideal engine. The figure at right shows a block diagram of a generic heat engine, such as the Carnot engine. In the diagram, the “working body” (system), a term introduced by Clausius in 1850, can be any fluid or vapor body through which heat "Q" can be introduced or transmitted through to produce work. Carnot had postulated that the fluid body could be any substance capable of expansion, such as vapor of water, vapor of alcohol, vapor of mercury, a permanent gas, or air, etc. Although, in these early years, engines came in a number of configurations, typically "QH" was supplied by a boiler, wherein water was boiled over a furnace; "QC" was typically a stream of cold flowing water in the form of a condenser located on a separate part of the engine. The output work "W" here is the movement of the piston as it is used to turn a crank-arm, which was then typically used to turn a pulley so to lift water out of flooded salt mines. Carnot defined work as “weight lifted through a height”.

Carnot's theorem

Carnot's theorem is a formal statement of this fact: "No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs."

This maximum efficiency eta is defined to be::eta=frac{W}{Q_H}=1-frac{T_C}{T_H}quadquadquadquadquadquadquadquadquad(1)where: W is the work done by the system (energy exiting the system as work),: Q_H is the heat put into the system (heat energy entering the system),: T_C is the absolute temperature of the cold reservoir, and : T_H is the absolute temperature of the hot reservoir.

A corollary to Carnot's theorem states that: "All reversible engines operating between the same heat reservoirs are equally efficient."


Efficiency of real heat engines



: langle T_H angle = frac{1}{Delta S} int_{Q_{in TdS

: langle T_C angle = frac{1}{Delta S} int_{Q_{out TdS

at which heat is input and output, respectively. Replace "TH" and "TC" in Equation (1) by <"TH"> and <"TC"> respectively.

For the Carnot cycle, or an equivalent, <"TH"> is the highest temperature available and <"TC"> the lowest. For other less efficient cycles, <"TH"> will be lower than "TH" , and <"TC"> will be higher than "TC". This can help illustrate, for example, why a reheater or a regenerator can improve thermal efficiency.

:"See also: Heat Engine (efficiency and other performance criteria)"

ee also

*Carnot cycle
*Thermal efficiency
*History of the internal combustion engine



*cite book |title=Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance |last=Carnot |first=Sadi |authorlink=Nicolas Léonard Sadi Carnot |coauthors= |year=1824 |publisher=Bachelier |location=Paris |isbn= |pages= |url= fr icon
*cite book |title=Reflections on the Motive Power of Heat and on Machines Fitted to Develop That Power |last=Carnot |first=Sadi |authorlink= |coauthors= Thurston, Robert Henry (editor and translator) |year=1890 |publisher=J. Wiley & Sons |location=New York |isbn= |pages= ( [ full text of 1897 ed.)] ) ( [ html] )
*cite book
author = Feynman, Richard P.; Leighton, Robert B.; Sands, Matthew
title = The Feynman Lectures on Physics
publisher = Addison-Wesley Publishing Company
year = 1963
isbn = 0201021161

*cite book |title=Physics |last=Halliday |first=David |authorlink= |coauthors=Resnick, Robert |year=1978 |edition=3rd ed. |publisher=John Wiley & Sons |location= |isbn=0471024562 |pages=541-548
*cite book |title=Thermal Physics |last=Kittel |first=Charles |authorlink=Charles Kittel |coauthors=Kroemer, Herbert |year=1980 |edition=2nd ed. |publisher=W. H. Freeman Company |location= |isbn=0-7167-1088-9 |pages=

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