The efficiency of a heat engine is defined as the work output divided by the heat input. The Carnot cycle is the most efficient cycle possible for a heat engine.
Efficiency of Heat Engine Equation (Carnot cycle):
  Efficiency_max = 1 - T_c/T
where:
T_c = Temperature of the Cold Reservior, in Kelvin
T = Temperature of the Hot Reservior, in Kelvin