Piston assembly for free piston internal combustion engine

A free piston internal combustion engine (10), particularly suitable for use in a vehicle, is provided with a combustion cylinder (12) and a piston assembly (14). The piston assembly (14) includes a piston (18) and a plunger shaft (20). The piston (18) is reciprocally movable within the combustion cylinder (12). The piston (18) includes a crown (22), a skirt (24) extending from the crown (22) and having a piston ring groove (30) therein, a hub (26) attached to the plunger shaft (20), and at least one support block (42). Each support block (42) is positioned adjacent to the crown (22) and extends from the hub (26) toward the skirt (24). Each support block (42), skirt (24) and piston ring groove (30) defines a cut-out (52) there between. The cut-out (52) provides mechanical support to the piston (18) during acceleration, and also provides effective heat transfer for cooling the piston ring groove (30).