The efficiency of the engine can also be referred to as Carnot Efficiency, is the fraction of the in-going heat energy that is converted to available work.For example the efficiency of a Carnot engine having a hot reservoir of boiling water and a cold reservoir ice cold water will be 1-(273/373)=0.27, just over a quarter of the heat energy is transformed into useful work.  This is the very same expression Carnot found from his water wheel analogy.

Note we can never reach 100% efficiency, because we cannot have a cold reservoir at T_C=0_K  and, even if we did, after the first cycle the heat dumped into it would warm it up!

epsilon_ideal = 1 −  T_C/T_H

T_H is the temperature at which the high temperature reservoir operates ( T_Hot ).
 T_C the temperature at which the low temperature reservoir operates ( T_Cold ).

Notes

To get maximal efficiency is must be a heat engine where all processes are reversible. An irreversible heat engine operating between two heat reservoirs at constant temperatures cannot have efficiency greater than that of a reversible heat engine operating between the same two temperatures. Reversible engines operating between the same temperatures have the same efficiency.

• there exist no real reversible engine but there is a upper limit
•  Does not matter what kind of substances and states of matter are used
•  It doesn’t matter what kind of machines we use
•  What matters is T_c and T_h