Shape of combustion chamber for direct-injection diesel engine

A combustion chamber for a direct-injection diesel engine, in which the combustion chamber is formed at a top of a piston, fuel is injectable into the combustion chamber, and a mixture of the fuel and air is allowed to be burnt, comprising:

• an inside wall of the combustion chamber being substantially formed to shape a rotating body about an axis of the piston
• an opening at an upper end of the combustion chamber having a substantially regular hexagonal shape;
• wherein the inside wall of the combustion chamber comprises:
• a first wall portion formed as an upper circumferential wall extending downward from an upper surface of the piston at an angle a;
• a second wall portion formed as an intermediate circumferential wall extending downward and radially outward from a lower end of the upper circumferential wall at an angle b with respect to the upper surface of the piston;
• a third wall portion formed as a lower circumferential wall extending downward and radially outward from the lower end of the intermediate circumferential wall at an angle c with respect to the upper surface of the piston;
• wherein angles a, b, and c are as follows:
• a=90°;
• 65°<b<75°;
• and 40°<c<55°;
• and
• a center projection projecting upwardly from a central bottom area of the combustion chamber, the center projection having a top wall and a foot end;
• and
• a radially outward expanding portion formed at an outer peripheral portion of the combustion chamber between an outer peripheral edge of the top wall of the center projection and the foot of the center projection.