A field emission device is configured as a heat engine. In one embodiment, an apparatus comprises: a cathode, an anode, wherein the anode and cathode are receptive to a first power source to produce an anode electric potential higher than a cathode electric potential; a gate positioned between the anode and the cathode, the gate being receptive to a second power source to produce a gate electric potential selected to induce electron emission from the cathode for a first set of electrons having energies above a first threshold energy; a suppressor positioned between the gate and the anode; at least one region including gas located between the cathode and the anode; and at least one path traversable for a first portion of the first set of electrons, extending from the cathode to the anode.