Combustion chamber structure for engine

A combustion chamber structure for an engine, comprising:

• a combustion chamber defined by a crown surface of a piston, an inner wall surface of a cylinder in which the piston is slidably housed, and a pent roof-shaped ceiling surface;
• the combustion chamber having a geometrical compression ratio of 15 or more, and in the combustion chamber, flame propagation combustion and partial compression ignition combustion being performed in combination, the flame propagation combustion being a mode in which mixture gas combusts by flame propagation, the partial compression ignition combustion being a mode in which mixture gas combusts by flame propagation and then combusts by compression ignition;
• wherein
• the crown surface includes
• a cavity recessed to have a bowl-shape;
• and
• a pair of raised portions having a mound-shape along the pent roof shape of the ceiling surface and provided in an outer circumferential region between the cavity and an outer edge portion of the piston to protrude in a cylinder axis direction;
• wherein
• the cavity includes a bottom portion which is a lower region of the recessed part, the bottom portion having an outer circumferential edge which is circular in a top view, and
• with a height of the raised portion relative to a height position of a deepest portion of the cavity being represented as H1 and a diameter of an outer circumferential edge of the bottom portion of the cavity being represented as D, H1/D as a ratio of the height H1 of the raised portion to the diameter D of the cavity is set to be in a range of 0.050 or more and 0.235 or less.