Abstract: The invention relates to a combustion chamber formed in the upper end of the piston of the diesel engine and, more particularly, to a geometrical shape of a combustion chamber of a piston. The combustion chamber includes a main combustion chamber and a plurality of auxiliary chambers. The main combustion chamber has a diameter d in the range of 0.522-0.552 times the piston diameter D. The auxiliary chamber has a tunnel like shape having a half-circle shaped cross section having a base line with a radius in the range of 0.095D-0.200D or 0.179d-0.370d from the upper end to the lower end of the side wall of the main combustion chamber of the piston cavity. The above half-circle is centered at a point where it is on a circumference whose radius is in the range of 0.105D-0.238D or 0.190d-0.446d from the center of the main combustion chamber circle and is located in between two sprays.