An Exact Efficiency Formula for Holographic Heat Engines
  Further consideration is given to the efficiency of black hole heat engines that perform mechanical work via the pdV terms present in the First Law of extended gravitational thermodynamics. It is noted that when the engine cycle is a rectangle with sides parallel to the (p,V) axes, the efficiency can be written simply in terms of the mass of the black hole evaluated at the corners. Since an arbitrary cycle can be approximated to any desired accuracy by a tiling of rectangles, a general geometrical algorithm for computing the efficiency follows. A simple generalization of the algorithm renders it applicable to more general classes of heat engine, beyond the black hole context. 
Clifford V. Johnson
hep-th