The second law of thermodynamics states that no heat engine can be more efficient
than a reversible heat engine working between two fixed temperature limits
(Carnot cycle) i.e. the maximum thermal efficiency
is equal to the thermal efficiency of the Carnot cycle:
or in other words If the heat input to a heat engine is Q, then the work output of the engine, W will be restricted to an upper limit Wmax i.e.
It should be noted that real cycles are far less efficient than the Carnot cycle due to mechanical friction and other irreversibility. Related topic: