Universality of efficiency of an unsteady-state heat engine at maximum instantaneous power

In 1975, Curzon and Ahlborn [2] showed that the efficiency of the finite-time Carnot cycle at maximum power is universal. This raises the issue of universality of the efficiency at maximum power. In 2005, Van den Broeck [3] demonstrated that in the linear regime, the efficiency of a steady-state heat engine at maximum power is universal. Further studies indicated that the values of the linear and quadratic coefficients of the efficiency at maximum power are universal for several strong coupling steady-state models and for an overdamped Brownian particle in a time-dependent harmonic potential [M. Esposito, K. Lindenberg, C. Van den Broeck, Phys. Rev. Lett. 102 (2009) 130602]. In this paper, we demonstrate that in the linear regime, the efficiency of an unsteady-state heat engine at maximum instantaneous power is universal.