If we consider a heat engine which is less effective than Carnot cycle (i.e., the work W produced by this engine is less than the maximum predicted by Carnot's theorem), its work output is capped by Carnot efficiency as: W < ( 1 − − T C T H ) Q H Substitution of the work W as the net heat into the inequality above gives us: Q H T H + Q C T C < 0 or in terms of the entropy change Δ Δ S r , i : Δ Δ S r , H + Δ Δ S r , C > 0 A Carnot cycle and an entropy as shown above prove to be useful in the study of any classical thermodynamic heat engine: other cycles, such as an Otto, Diesel or Brayton cycle, could be analyzed from the same standpoint. Notably, any machine or cyclic process converting heat into work (i.e., heat engine) what is claimed to produce an efficiency greater than the one of Carnot is not viable — due to violation of the second law of thermodynamics.