The maths looks right Intuitively an isothermal system wastes a great deal of the expansion work because you lose the energy that you put into warming the gas when you compress it A 100 efficient expansion would be adiabatic that is there would be no heat transfer between the gas and the environment at any stage The problem with that is that you have to insulate the reservoir and if you don t then the expansion process freezes up This is the fundamental stupidity with the aircar proposal Realistic systems that were not just investor scams spent a lot of effort using heat exchangers to prevent this I suggest that a graph of the thermodynamics would improve this article