  We present a quantum heat engine based on a cavity with two oscillating
mirrors that confine a quantum field as the working substance. The engine
performs an Otto cycle during which the walls and a field mode interact via a
nonlinear Hamiltonian. Resonances between the frequencies of the cavity mode
and the walls allow to transfer heat from the hot and the cold bath by
exploiting the conversion between phononic and photonic excitations. We study
the time evolution of the system and show that net work can be extracted after
a full cycle. We evaluate the efficiency of the process.
