Maybe "cause and effect" is somewhat in question these days, but generally, assuming it exists, seems like a good rule of thumb, which is probably my biggest objection to the "Carnot Limit", as it has no identifiable cause.
Conservation of energy demands the energy put into a system, leaves the system in equal measure in one form or another including "work" output, friction, vibration, or whatever.
The Carnot formula insists, supposedly, that out of all the heat going into the engine only 20% or whatever can come out one way as "work" and the other 80% for some reason must come out some other way into the "cold reservoir" and does not account for friction loses and such at all.
The cause/effect relationship or reason is unclear.
How can the temperatures precisely limit engine efficiency?
Yes, it seems pretty obvious you can't get out more than you put in, but the Carnot limit says you can only get out, under the absolute best conditions, with zero friction loses, 20% of what you put in.
Is it coincidence that 20% is just the temperature difference on the kelvin scale? No, it's just a simple derivation. The high temperature is 20% higher than the low temperature. Of course this varies with temperature, but I'm talking LTD running on hot water type temperature.
With heat as "caloric" a cause can be imagined. A fluid flows down. So the height it can fall makes at least a plausible hypothesis.
With heat as energy, that half baked assumption falls apart.
But, it also does not hold up experimentally. If it did, it might be worth looking for a cause, but in my book, if it doesn't hold up experimentally that's where it ends. You can't assign a cause to a non-phenomenon.