  We discuss an autonomous motor based on a Brownian particle driven from
thermal equilibrium by periodic in time variation of the internal potential
through which the particle interacts with molecules of the surrounding thermal
bath. We demonstrate for such a motor the absence of a linear response regime:
The average driving force and drift velocity are shown to be quadratic in both
the frequency and amplitude of the variation. The adiabatic approximation (of
an infinitely slow variation) and the leading correction to it (linear in the
variation's frequency) both lead to zero drift and are insufficient to describe
the motor's operation.
