a little about heat engine efficiency here. efficiency = W/Q_H = (Q_H-Q_C)/W which must always be less than 1 by conservation of energy. But the 2nd law of thermodynamics dictates a more stringent limit: efficiencty = < eff_max = 1-T_C/T_H where T_C is usually the temperature of ambient medium (e.g., air temperature) and T_H