Thermodynamics and Propulsion  

The upper limit of work that can be done occurs during a reversible cycle, for which the total entropy change ( ) is zero. In this situation:
Also, for a reversible cycle of the engine,
These constraints apply to all reversible heat engines operating between fixed temperatures. The thermal efficiency of the engine is
We can approach this last point in another way. The engine work is given by
or,
The total entropy change can be written in terms of the Carnot cycle efficiency and the ratio of the work done to the heat absorbed by the engine. The latter is the efficiency of any cycle we can devise:
The second law says that the total entropy change is equal to or greater than zero. This means that the Carnot cycle efficiency is equal to or greater than the efficiency for any other cycle, with the total equality only occurring if .
Muddy Points
So, do we lose the capability to do work when we have an irreversible process and entropy increases? (MP 6.1)
Why do we study cycles starting with the Carnot cycle? Is it because it is easier to work with? (MP 6.2)
UnifiedTP