Lord Kelvin's choice of words misleads text 'skimmers'.
An unintended red herring for the casual reader.
Reversibility is irrelevant.

   Previous Next

Reversibility gives evidence to support Conservation of Energy, but is not required for Conservation of Energy to remain true.

Conservation of Energy is an Axiom

The general law of Conservation of Energy, as well as Conservation of Momentum in heat engines are both "Axiom"s. They are not provable, but based on observation.

All our theories support both Axiom's as do all our observations. But no one is keeping track of every photon or atom in the universe, to see that they have precisely the right wavelength or momentum after bumping into something.

An impossibility to prove there can be nothing in the universe we don't yet know about that creates or destroys energy. Because we don't know about it.

If energy was not conserved in some as yet unkown transformation, it does not affect the conclusions regarding heat engines. Heat engines exchange energy in the form of momentum of ordinary matter. All that is required for reversibility is the narrower condition of conservation of momentum. Converting heat to motion, is converting tiny motion to larger motion. "Consuming" heat is literally slowing down molecules, which is reducing moleculular momentum.

 

Lord Kelvin uses the Law of Conservation of Energy in his famous proof regarding efficiency of reversible vs irreversible engines.

In the early 1800's an irreversible engine was a purely hypothetical device.

Today, irreversible engines are still a purely hypothetical device.

All our engines are vapor expansion engines, and that expansion is reversible. A careful reading of shows both Sadi Carnot and Lord Kelvin considered all steam and air driven heat engines had the property of reversibility. "Reflections", page 161, Lord Kelvin, footnote, writes the math of the Carnot Cycle

"it is applicable to any conceivable kind of thermodynamic engine."

Pretty clear intent there. Also, "Reflections" page 163

"we conclude, generally, that
any two engines, constructed on the principles laid
down above, whether steam-engines with different
liquids, an air-engine and a steam-engine, or two
air-engines with different gases, must derive the
same amount of mechanical effect from the same
thermal agency."

Also pretty clear. All engines derive exactly the same mechanical effect as any other engine, when consuming a given amount of heat. In the 1800's they knew it. Today "we" don't.

He writes that should their be an engine producing more work than a reversible engine for a given amount of heat, that amounts to a way to create energy from nothing, by coupling it with a reversible engine. He cites that as an impossibility.

He does not write about the possibility of some other reversible engine type, which perhaps gives 2 units of motion for 1 unit of heat, both forward and in reverse. Perhaps he thought that was an impossibility too obvious to justify writing down, or perhaps it just did not occur to him.

Either way, exactly the same argument says their can be no engine that is LESS efficient than a reversible engine. Coupled with a reversible engine, one could make energy disappear, equally impossible. A thorough treatment of the theorem would conclude all heat engines must exchange heat and motion in exactly equal quantity.

So, whether or not the reader believes all engines are reversible, Lord Kelvin's proof and its consequences say that all engines must behave the same, regarding the exchange of heat and motion. Anything else would be a counter-example, a disproof, of the Law of Conservation of Momentum and of Energy. Reversibility cannot affect the amount of any type of energy obtained for a given amount of heat energy.