  The triumph of heat engines is their ability to convert the disordered energy
of thermal sources into useful mechanical motion. In recent years, much effort
has been devoted to generalizing thermodynamic notions to the quantum regime,
partly motivated by the promise of surpassing classical heat engines. Here, we
instead adopt a bottom-up approach: we propose a realistic autonomous heat
engine that can serve as a testbed for quantum effects in the context of
thermodynamics. Our model draws inspiration from actual piston engines and is
built from closed-system Hamiltonians and weak bath coupling terms. We
analytically derive the performance of the engine in the classical regime via a
set of nonlinear Langevin equations. In the quantum case, we perform numerical
simulations of the master equation. Finally, we perform a dynamic and
thermodynamic analysis of the engine's behaviour for several parameter regimes
in both the classical and quantum case, and find that the latter exhibits a
consistently lower efficiency due to additional noise.
