I have some trouble understanding the carnot cycle and why you have to make it so complicated, i.e. this process involves 2 isothermal and 2 adiabatic processes. If you want an engine with maximum efficiency why can't you just have a system whose temperature is just a tiny inch above your hot resevoir and then absorbs heat at a rate such that it continually expands isothermally? - by this I mean that all the heat goes to producing work and keeping the proces isothermal. Edit: I also have a second question. The efficiency of an engine is given by: e = 1- Tc/Th , where Tc and Th are the temperatures of the hot and cold resevoir. You can see that the smaller temperature for the cold resevoir the higher efficiency - why is that? Then the entropy expelled to the environment is bigger but how does that make you lose less energy?