Efficiency at maximum power of minimally nonlinear irreversible heat   engines
  We propose the minimally nonlinear irreversible heat engine as a new general theoretical model to study the efficiency at the maximum power $\eta^*$ of heat engines operating between the hot heat reservoir at the temperature $T_h$ and the cold one at $T_c$ ($T_c \le T_h $). Our model is based on the extended Onsager relations with a new nonlinear term meaning the power dissipation. In this model, we show that $\eta^*$ is bounded from the upper side by a function of the Carnot efficiency $\eta_C\equiv 1-T_c/T_h$ as $\eta^*\le \eta_C/(2-\eta_C)$. We demonstrate the validity of our theory by showing that the low-dissipation Carnot engine can easily be described by our theory. 
Yuki Izumida and Koji Okuda
cond-mat.stat-mech