  Archetypal motors produce work when two slowly varying degrees of freedom
(DOF) move around a closed loop of finite area in the parameter space. Here,
instead, we propose a simple autonomous {\it monoparametric} optomechanical
engine that utilizes nonlinearities to turn a constant energy current into a
nonconservative mechanical force. The latter self-sustains the periodic motion
of a mechanical DOF whose frequency is orders of magnitude smaller than the
photonic DOF. We have identified conditions under which the maximum extracted
mechanical power is invariant and show a new type of self-induced robustness of
the power production against imperfections and driving noise.
