Free piston stirling engine

A free piston Stirling engine, comprising a power piston fitted into a cylinder further includes: a support structure carrying moving magnets for a linear alternator; and a passive structure that at normal operating power and frequency produces a restoring force on the piston in the absence of contact with the cylinder. In one variation, the passive structure further comprises a mass suspended within the piston from at least one spring, such that the mass oscillates under influence of movement of the piston at normal operating power and frequency so as to produce the restoring force. In another variation, the passive structure further comprises: a magnet disposed outside the cylinder at a position and in an orientation to produce a field that opposes a field of a moving magnet carried by the support structure when the piston moves toward the magnet.