  The Curzon-Ahlborn efficiency has long served as the definite upper bound for
the thermal efficiency at maximum output power, and has thus shaped the
development of finite-time thermodynamics. In this paper, we repeal the ruling
consensus according to which it has a genuine universal character that can be
derived from linear irreversible thermodynamics. We demonstrate that the
Curzon-Ahlborn efficiency should instead properly be associated with a
particular case of nonlinear heat engines, and we derive a generalized
expression for the efficiency at maximum power beyond the restrictive case of
linear models.
