Now we are ready to generalize this conclusion. The Second Law of thermodynamics
implies that all reversible engines have the same efficiency regardless of their construction.
To see the truth of this statement, suppose two reversible engines are coupled
together and run between the same two reservoirs (Fig. 3.8). The working substances
and details of construction of the two engines are entirely arbitrary. Initially, suppose
that engine A is more efficient than engine B, and that we choose a setting of the controls
that causes engine B to acquire energy as heat qc from the cold reservoir and to
release a certain quantity of energy as heat into the hot reservoir.