Efficiency
Efficiency is a measure of how much you get out versus how much you put in. For heat engines, then:
Efficiency = ?= Work done / Heat Added
By the first law, then, the work done is simply the difference in the heat going into the engine minus the heat coming out of the engine. The total heat added is the heat going into the engine. ?= (Qhot - Qcold) /Qhot