Pressure assist piston for internal combustion engine

A controlled pressure combustion chamber for a reciprocating engine, the engine comprising a main cylinder including means permitting admission of a fuel mixture and escape of exhaust material therefrom, means for initiating combustion of the fuel mixture in the main cylinder, a piston supported for reciprocating movement through the main cylinder, the main cylinder having a head, toward and away from which head the piston reciprocates in the cylinder;

• a second cylinder in the piston head, the second cylinder opening toward the main cylinder;
• a free piston disposed for displacement along the second cylinder and communicating with the main cylinder for prolonging the compression and combustion stages and absorbing energy upon displacement of the first mentioned piston by the compression and combustion;
• the second cylinder being closed on the side of the free piston away from the main cylinder, for defining a pneumatic cushion in the second cylinder which generates a force resisting movement of the second piston away from the main cylinder, when said pneumatic cushion is compressed by movement of the second piston away from the main cylinder, said pneumatic cushion being substantially the only source of such a resisting force, whereby when the pressure in the pneumatic cushion and the pressure in the first cylinder have become equalized due to displacement of the first and second pistons, the free piston commences to return energy to the first piston in the main cylinder on the stroke thereof while the gases of combustion expand.