describing in words a primitive heated water vapor lift
To carry out the B transformation, you have to heat a lot. If you don't heat, the volume will of course increase, but the pressure will decrease. B is a process difficult to carry out, because it is not easy to  heat enough a gas who is inflated to as to maintain the inner pressure. Then, at the end of B, the temperature is very high. You block the position of the piston, and you cool the system : the pressure will decrease from P2 to P1, during the process C. The last process D is also difficult to carry out. Because in D you must compress the gaz so as to decrease its volume, but the pressure must not increase. It means that the system should be cooled during D. At the end of D, the temperature will be extremely low.