PURPOSE: To improve a rate of use of air in a combustion chamber of a direct injection type Diesel engine by a method wherein an outer form of a top surface of the combustion chamber is made to be a polygonal shape corresponding to the number of injection holes in the fuel valve and further a bottom surface shows similarly a polygonal shape having its top part circumferentially displaced.
CONSTITUTION: For example, when there are four injection holes in the combustion valve arranged at the central position of the combustion chamber 20 in the cylinder head and an injecting direction of each of the injection holes is in a direction indicated by arrow 7, the upper edge of the combustion chamber 20 formed at a top part of the piston 1 is formed as a rectangular shape, and correspondingly the bottom surface is also formed in a rectangular shape. The upper and lower rectangular shapes are formed in such a way as they may be displaced with each other by, for example, 45° in a circumferential direction of the piston 1 at an projected plane of the piston axis. The injection 7 is performed in such a way as it strikes obliquely against a wall surface of the combustion chamber viewed in a top plan thereof while the wall surface of the combustion chamber is inclined by an angle of α with respect to the upper surface of the piston. In order to disperse the injected fuel after its strking against the wall toward the upper surface of the piston, a wall surface of the combustion chamber between the injection points is inclined at an angle of β.