Benchmarking Black Hole Heat Engines
  We present the results of initiating a benchmarking scheme that allows for cross-comparison of the efficiencies of black holes used as working substances in heat engines. We use a circular cycle in the p-V plane as the benchmark engine. We test it on Einstein-Maxwell, Gauss-Bonnet, and Born-Infeld black holes. Also, we derive a new and surprising exact result for the efficiency of a special `ideal gas' system to which all the black holes asymptote. 
Avik Chakraborty and Clifford V. Johnson
hep-th