A
free piston engine is configured with a pair of opposed engine cylinders located on opposite sides of a fluid pumping
assembly. An inner
piston assembly includes a pair of inner pistons, one each operatively located in a respective one of the engine cylinders, with a push rod connected between the inner pistons. The push rod extends through an inner pumping chamber in the fluid pumping
assembly and forms a fluid
plunger within this chamber. An outer
piston assembly includes a pair of outer pistons, one each operatively located in a respective one of the engine cylinders, with at least one pull rod connected between the outer pistons. The pull rod extends through an outer pumping chamber in the fluid pumping assembly and forms a fluid
plunger within this chamber. The movement of the inner and outer
piston assemblies during engine operation will cause the fluid plungers to pump fluid from a low pressure container into a
high pressure chamber as a means of storing the energy output from the engine. Alternatively, the piston assemblies may drive a
linear alternator. At least one of the pistons includes one or more generally axially extending bores partially filled with a
sodium compound. As the piston reciprocates, the
sodium moves back and forth in each cooling bore, thereby better distributing heat in the piston.