  The concept of thermal machines has evolved from the canonical steam engine
to the recently proposed nanoscopic quantum systems as working fluids. The
latter obey quantum open system dynamics and frequently operate in
non-equilibrium conditions. However, the role of this dynamics in the overall
performance of quantum heat engines remains an open problem. Here, we analyse
and optimize the efficiency and power output of two-stage quantum heat engines
fuelled by non-equilibrium steady states. In a charging first stage, the
quantum working fluid consisting of a qutrit or two coupled qubits is connected
to two reservoirs at different temperatures, which establish a heat current
that stores ergotropy in the system; the second stage comprises a coherent
driving force that extracts work from the machine in finite a amount of time;
finally, the external drive is switched off and the machine enters a new cycle.
