What did the Thermodynamic forefathers say about reversibility?
Carnot and Lord Kelvin pointed out that all volume changes of all vapors is reversible, and that volume change is how heat engines are made. So all vapor volume change heat engines are reversible engines.
A reversible engine, R,
gives an equal amount of heat for an equal amount of work. They asserted that one could not have a MORE efficient
machine, M, or one could use M to make work, and R to turn the work back in to MORE HEAT, and create energy from nothing.
They theorized there may be some as yest unknown heat engines besides vapor expansion engines. (None have been found/invented yet.) They don't call them irreversible, nor imply irreversibility is possible.
Lord Kelvin has a famous theorem he believed proved a better engine could not be found. (See theorems section). It is stated as a negative, so it is easy to assume incorrectly that means other engines must be worse. (kelvin does not say so.)
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Were they wrong?
A less efficient engine LE, reversible or not, brings about the same impossibility as a more efficient engine. One could use LE to convert heat to work, R to convert it back to LESS heat, and it would make energy disappear to nowhere. Run it in a closed insulated room, the room will freeze to absolute zero, exporting no work or heat.
The property that governs efficiency is Conservation of Energy (or Momentum).
Energy = Heat + Work. If it does not add up, it is wrong. Its possible to convert only part of the heat, but not to lose the energy. No engine can produce less work from a given amount of heat than any other engine. Engines that APPEAR to produce less work, are in fact leaking heat.
To put it another way, if machine A and B both produce exactly W amount of work, how much heat did they convert? The answer is exactly the same heat energy as work H=W.
They were correct that there cannot be a more efficient engine, but the implied opposite is incorrect. There also cannot be a less efficient engine, if the Law of Conservation of Momentum is true. Otherwise, energy is being destroyed.
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