Why can we say that a Carnot's machine efficiency does not depend on the fluid that you are heating? Is it because Carnot's machine is very ideal, a mere thought experiment? I have another question. I know that the efficiency of a heat engine depends on the quantity of heat that enter and leaves. I also understand that in a reversible machine the efficiency only depends on the temperature of the reservoirs, but if I had a reservoir that was at 20 C and another one at 10 C and my fluid was at 30 C, the fluid would contract, nonetheless, right? Now if we were talking about the number of cycles produced in a certain time, then the 10 C would be more "efficient" since you would transfer heat faster. So where am I wrong? Is it because the efficiency is the same whether you divide energy or power?( after all the times will cancel out.) Thank you!