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I'm reading Feynman's discussion of Reversible engines (Vol I 44.3) in The Feynman Lectures on Physics. His discussion is very abstract, and leaves a lot of practical questions unanswered. Rather than attempting to ask several questions which come to mind, it seems reasonable to consider a real mechanism which comes "as close as possible" to an ideal heat engine.

Has anybody created such a device? That is, one which requires very little additional energy in order to get the machine to return to its initial state after "doing work one the Universe" by transferring the work from the Universe back to the engine?

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  • $\begingroup$ The meaning of "as close as possible" is generally going to depend on what question yo are asking. For most pedagogical purpose, what is wrong with a (good approximation to a) Carnot engine? $\endgroup$ – By Symmetry Apr 23 '18 at 16:22
  • $\begingroup$ Can you show me a picture of such a machine which has actually been constructed? The Carnot engine is the example Feynman uses. But it raises questions such as: how does one isothermally expand gas so that the work done by the gas is stored in some kind of recoverable potential, without the use of some kind of frictionless resistance? I "understand" the Carnot cycle, in so much as I can talk through it, and construct the pressure-volume graph. I can show how a Carnot machine can lift a certain weight as stored energy, and then recover that energy while reversing the engine. $\endgroup$ – Steven Thomas Hatton Apr 23 '18 at 16:47
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The Rankine cycle is a very important model of steam turbine operation in which the isothermal compression segment is via steam to liquid phase change and also while heat is absorbed there is an isothermal expansion segment during which liquid to steam phase change takes place. There are two near adiabatic compression/expansion segments of the cycle and to the extent these are reversible are also isentropic. This is not quite a Carnot cycle but most of the "action" is like that, see details in https://en.wikipedia.org/wiki/Rankine_cycle .

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In the low power limit real Stirling engines approach ideal efficiency.

The actual cycle of these machines is not ideal over finite temperature differences, but they can run over very small temperature differences (albeit at low power) minimizing their deviation from ideal behavior.

There is also an idealized cycle, but it is not terribly representative of real machines.

You can buy low power demo version for a few hundred dollars that are designed to run on the temperature difference between a hot or cold beverage and the ambient temperature of the room (they reverse direction when you move them from your coffee to your ice tea!).

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Well known examples of (almost) reversible heat engines are the Stirling engines developed by Philips NV, The Netherlands, since the 1950. These Stirling engines use the Stirling cycle which has the same efficiency as the Carnot cycle. As motors (generators) they work with different external heat supplies (combustion, solar, etc.). With the reverse cycle and mechanical work input they work as compact and small heat pumps that are used for cooling and liquid air production. Thus the Stirling engines are prime examples of commercially implemented heat engines that use a reversible thermodynamic process in both directions.

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