A piston for a four valve direct injection diesel engine defines a combustion chamber comprising a recess (10) in the piston crown, which is rotationally symmetrical about the axis of the piston in axial cross-section and is defined by a base and a side wall. A generally conical projection (5) is upstanding from the base. The recess (10) includes a lower toroidal portion (8) and an upper portion (10), whose diameter progressively increases towards the piston crown. The upper and lower portions of the recess are separated by an annular, arcuate lip (12) extending into the recess. The included angle of the conical projection (5) is between 104° and 108°, the maximum depth of the recess (10) is between 13% and 22% of the diameter of the piston, the conical projection (5) merges with the base of the recess along a circular line whose diameter is between 38% and 44% of the diameter of the piston, the diameter of the lip (12) is between 54% and 59% of the diameter of the piston, the line of minimum diameter of the lip (12) is situated at a distance between 5% and 10% of the diameter of the piston from the plane of the piston crown and the diameter of the upper portion (10) of the recess increases from 54% to 59% of the diameter of the piston at the line of minimum diameter of the lip to between 72% and 76% of the diameter of the piston at the line where the side wall of the recess intersects the plane of the piston crown.