  This chapter presents autonomous quantum engines that generate work in the
form of directed motion for a rotor. We first formulate a prototypical
clock-driven model in a time-dependent framework and demonstrate how it can be
translated into an autonomous engine with the introduction of a planar rotor
degree of freedom. The rotor plays both the roles of internal engine clock and
of work repository. Using the example of a single-qubit piston engine, the
thermodynamic performance is then reviewed. We evaluate the extractable work in
terms of ergotropy, the kinetic energy associated to net directed rotation, as
well as the intrinsic work based on the exerted torque under autonomous
operation; and we compare them with the actual energy output to an external
dissipative load. The chapter closes with a quantum-classical comparison of the
engine's dynamics. For the single-qubit piston example, we propose two
alternative representations of the qubit in an entirely classical framework:
(i) a coin flip model and (ii) a classical magnet moment, showing subtle
differences between the quantum and classical descriptions.
