PURPOSE: To warp a disk spring as the combustion pressure is increased, increase the capacity of a combustion chamber, and keep the combustion pressure at the maximum allowable combustion pressure by fitting the disk spring to the upper section of an inner piston, and fitting an outer piston outside them.

CONSTITUTION: A disk spring 1 is fitted to the upper section of an inner piston 2, and an outer piston 3 is fitted outside them. The constant-load spring characteristic is applied to the disk spring 1. When the combustion pressure is increased to the maximum allowable combustion pressure, the disk spring 1 is warped to increase the capacity of a combustion chamber, and the combustion pressure is kept at the maximum allowable combustion pressure. Even when combustion is continued and the combustion pressure is increased, the disk spring 1 is warped to keep the maximum allowable combustion pressure. When the combustion pressure is lowered, the deflection of the disk spring 1 is returned to decrease the capacity of the combustion chamber, and the combustion pressure is kept at the maximum allowable pressure.