What is the reason behind choosing adiabatic and isothermal process for carnot engine?
My book states the following reason. I am not able to understand it. It would be great if you could put it in simpler words.
A process is not quasi-static if it involves finite temperature difference between the system and reservoir. Hence, in a reversible
heat engine operating between two temperatures, heat should be
absorbed and released isothermally.Employing any other processes other than adiabatic, say isochoric, to take the system from one temperature to another, we shall need
series of reservoirs in the temperature range $T_2$ and $T_1$ to
ensure that the process is quasi-static.
Best Answer
I feel there are much better ways to explain the need for reversible isothermal and adiabatic processes in the Carnot cycle than the way it is explained in your book. But since you have to deal with the material you have, I will attempt to clarify what they are saying.
Regarding the first bullet, you start with the requirement that a Carnot cycle only exchanges heat with two fixed temperature thermal reservoirs. The second requirement is that the heat transfers must be reversible. That means the temperature of the system has to be essentially the same as the reservoir during the process, i.e., the system temperature has to be constant. The only process where heat transfer occurs reversibly with the system temperature constant is a reversible isothermal process.
Regarding the second bullet, to complete the cycle you need processes that link together the high and low temperature isothermal processes. In order to do this you need to lower and raise the temperature of the system. The only way to lower and raise the system temperature without transferring heat with additional heat reservoirs are with processes that don't require heat transfer. Those are reversible adiabatic processes.
Hope this helps.