To see that every reversible engine operating between reservoirs at temperatures T 1 and T 2 must have the same efficiency, assume that two reversible heat engines have different efficiencies, and let the relatively more efficient engine M drive the relatively less efficient engine L as a heat pump. As the right figure shows, this will cause heat to flow from the cold to the hot reservoir without external work, which violates the second law of thermodynamics. Therefore, both (reversible) heat engines have the same efficiency, and we conclude that: