 However, if the cycle is performed faster than the relaxation time of the working medium, the fluctuations of work are inevitable.Nevertheless, when work and heat fluctuations are counted, there is exact equality that relates the exponential average of work performed by any heat engine and the heat transfer from the hotter heat bath.Carnot realized that in reality it is not possible to build a thermodynamically reversible engine, so real heat engines are even less efficient than indicated by Equation 3.