True nature of the Curzon-Ahlborn efficiency

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 letter, we repeal the ruling consensus according to which it has a genuine universal character and pertains to linear irreversible thermodynamics. We demonstrate that the Curzon-Ahlborn efficiency should instead properly be associated with a particular case of nonlinear heat engines. We thus derive a generalized expression for the efficiency at maximum power beyond the restrictive case of linear models.