According to Carnot's theorem, Carnot engine is the most efficient engine. I wanted to know its proof. I looked up and I found a proof on Khan Academy in which he used a reverse carnot engine to prove that an engine more efficient than Carnot engine cannot exist.
But my question is can there be an engine not having 2 adiabatic and 2 isothermal processes, and more efficient or equally efficient to Carnot Engine? If not, can anyone please give me an idea why having 2 isothermal and 2 adiabtic processes makes it the most efficient and why not any other series of processes?