In the Carnot cycle, then working substance (the one which undergoes thermodynamic processes) undergoes entirely reversible processes. These processes are isothermal and adiabatic. This can be shown on a p-V graph:
For the isothermal processes:
For the adiabatic processes:
These can be combined to find:
Using both the adiabatic and isothermal equations, we now know:
This gives the efficiency:
This sets an upper limit for the efficiency of real engines.