Combustion chamber structure for engines

A spark ignited engine, comprising:

• a piston;
• a combustion chamber ceiling surface formed on a cylinder head;
• an injector provided on the combustion chamber ceiling surface;
• an ignition plug provided on the combustion chamber ceiling surface, and including an ignition portion provided in such a way as to face the combustion chamber;
• and
• an intake opening and an exhaust opening opened in the combustion chamber ceiling surface, wherein
• a side where the intake opening is opened is defined as an intake port side of the combustion chamber, and a side where the exhaust opening is opened is defined as an exhaust port side of the combustion chamber, with respect to a center portion of the combustion chamber as a reference, in a plan view from one side in a cylinder axis direction,
• the ignition plug is configured in such a way that the ignition portion is disposed on the intake port side, and that the ignition plug is ignited at a timing after the piston passes a compression top dead center,
• the injector is disposed on the center portion, and is configured to inject fuel at least toward the exhaust port side,
• the piston has a cavity which is recessed in the cylinder axis direction in a region at least on the exhaust port side with respect to a position below the ignition portion in a crown surface of the piston, an intake-side flat portion at an end on the intake port side of the crown surface, and an exhaust-side flat portion at an end on the exhaust port side of the crown surface, the exhaust-side flat portion being apart from the intake-side flat portion,
• the intake-side flat portion and the exhaust-side flat portion define a smaller gap against the combustion chamber ceiling surface than the center position when the piston is at the compression top dead center,
• the intake-side flat portion constitutes a reverse squish flow generation portion, which draws an air-fuel mixture toward the intake port side as the piston moves in an expansion stroke, in combination with the combustion chamber ceiling surface,
• an area of the intake-side flat portion is larger than an area of the exhaust-side flat portion, and
• a maximum width of the exhaust-side flat portion is set to be narrower than a maximum width of the intake-side flat portion in an engine output axis direction.