On the maximum efficiency of realistic heat engines
  In 1975, Courzon and Ahlborn studied a Carnot engine with thermal losses and got an expression for its efficiency that described better the performance of actual heat machines than the traditional result due to Carnot. In their original derivation, time appears explicitly and this is disappointing in the framework of classical thermodynamics. In this note a derivation is given without any explicit reference to time. 
E. N. Miranda
physics.gen-ph