Combustion chamber structure for engine

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 provided with an intake port and an exhaust port, wherein
• the engine is configured to perform, in the combustion chamber, flame propagation combustion and partial compression ignition combustion 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,
• the crown surface includes:
• a cavity recessed to have a bowl-shape;
• a pair of first 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, the pair of first raised portions each including a ridge having a greatest dimension along a ridge extending direction;
• and
• a second raised portion provided to protrude in a convex shape at an intake side opposed to the intake port, in the outer circumferential region, at a position orthogonal to the ridge extending direction of the pair of first raised portions, and
• with a height of an uppermost portion of the first raised portion relative to a height position of a deepest portion of the cavity being represented as H1 and a height of an uppermost portion of the second raised portion in the cylinder axis direction relative to the height position of the deepest portion being represented as H2, H1/H2 as a ratio of the height H1 of the first raised portion to the height H2 of the second raised portion is set to be in a range of 1.92 or more and 2.75 or less.