An engine you can buy today with efficiency over 90%, and a Carnot Limit of 0%!

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A compressed air engine still runs on heat, but has a 0 difference between "hot" and "cold" reservoirs. So, compressed air engines have a Carnot Limit of 0%. Multiple manufacturers make real world car and utility engines you can go out and buy today.

Compressed Air engines are an example of Heat Engines

It is less obvious, because compressed air engines are powered with the heat already in the air.

However, this is simply a different strategy for creating a heat engine.

MDI is one company making compressed air power cars. Read about it here.

Engine Air is another compressed air engine manufacturer. Here is their web site.

An actual heat engine without any heat (added).

An engine can achieve infinitely better than the "Carnot Limit" efficiency.

Review: The Carnot Limit says an engine efficiency cannot exceed the ratio of the "Cold Reservoir" temperature to the "Hot Reservoir" temperature.

Consider an engine powered by "compressed air". Its not actually powered by air. Like all vapor expansion engines, it is powered by heat. In this case, the heat that is already in the air.

There is no "heat increase", so the conventional wisdom shows this engine has a "Carnot Limit" of 0, as in Zero, Zilch or Nada. There is no heat increase at all, and the only "Cold Reservoir" is the air. So Hot minus Cold is Zero, divided by Hot is still Zero. This engine cannot move, according to the "Limit".

Further, one would expect abysmal efficiency. The energy gets put into compressing air, and the heat is all dissipated into the atmosphere. Gone forever?

If the car's were "insulated" the air tank would reach very low temperatures, and lose pressure, be unable to move.

But the cars are not insulated. So heat used up is replaced by the heat from the Atmosphere. Quite different from "losing" heat, it "stores" the heat in a free heat battery called the Atmosphere.

This is the most striking counter-example to the "Carnot Limit" possible. It illustrates the "Hot/Cold" reservoir models are faulty, and so all the limitations predicted by that model are faulty.

Limitations are those based on the design and construction, and the only real limitation on energy is that you can't create it from nothing.

Interesting note. In general one cannot "multiply" energy. In this case, compressing on a very cold day, 0° C, and driving on a very warm day, 30° C, would result in about 10% more energy coming out than went in to the storage bottle. 110% efficient. Even Conservation of Energy has the odd loophole. Circumstances or design may get more out than was put in. :-)