Patent ID: 7464683

Claim:
A reciprocating engine comprising: a first piston ring adjacent to a top surface of a piston defining a combustion chamber; a second piston ring which defines an annular gas chamber in cooperation with said first piston ring and which is adjacent to said first piston ring such that a pressure-receiving area of a side surface of said piston in said annular gas chamber becomes greater on a thrust side than on a counter-thrust side; and a plurality of gas passages which are disposed in an inner surface of a cylinder in such a manner as to be juxtaposed in a circumferential direction of the inner surface of said cylinder and which allow said annular gas chamber to communicate with said combustion chamber on the thrust side, wherein said plurality of gas passages comprise a first recessed portion which is disposed in such a manner as to oppose a center portion of said piston concerning a direction which is perpendicular to both a reciprocating direction of said piston and an axial direction of a piston pin when said piston is at a top dead center or during a staring period of the fall from the top dead center, and a pair of second recessed portions, said first recessed portion being disposed between said pair of second recessed portions in said circumferential direction.