  We study the mechanical performance of quantum rotor heat engines in terms of
common notions of work using two prototypical models: a mill driven by the heat
flow from a hot to a cold mode, and a piston driven by the alternate heating
and cooling of a single working mode. 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 to the energy output for the case of an external
dissipative load and for externally driven engine cycles. Our results connect
work definitions from both physical and information-theoretical perspectives.
In particular, we find that apart from signatures of angular momentum
quantization, the ergotropy is consistent with the intuitive notion of work in
the form of net directed motion. It also agrees with the energy output to an
external load or agent under optimal conditions. This sets forth a consistent
thermodynamical description of rotating quantum motors, flywheels, and clocks.
