Reciprocating piston internal combustion engine

A reciprocating piston internal combustion engine comprises at least one primary cylinder and at least one primary piston. The primary piston is mounted for reciprocating movement in the primary cylinder. A cylinder head overlies the primary cylinder and, with the primary cylinder and the face of the primary piston, defines a primary combustion chamber. At least one subsidiary cylinder is formed in the cylinder head and receives a subsidiary piston mounted for reciprocating movement in the subsidiary cylinder. Defined within the subsidiary cylinder is a subsidiary combustion chamber that communicates with the primary combustion chamber. The subsidiary cylinder moves upwardly in the subsidiary cylinder during upward movement of the primary piston in the primary cylinder and at least intermittently interrupts communication between the primary and subsidiary combustion chambers. The subsidiary piston compresses air-fuel mixture in the subsidiary combustion chamber to a higher pressure than air-fuel mixture is compressed in the primary combustion chamber by the primary piston.