Spark-ignition engine

A spark-ignition engine comprising:

• a cylinder head having a combustion chamber roof and a cylinder head-side squish surface provided on an outer periphery of the combustion chamber roof;
• a piston head having an outer periphery and a piston head-side squish surface provided on the outer periphery of a piston head opposed to the cylinder head-side squish surface;
• a combustion chamber section recessed at a center of the combustion chamber roof;
• an intake valve port provided on one opposed section of two opposed sections of the combustion chamber recessed section;
• an exhaust valve port provided on another of the two opposed section;
• and
• a spark-plug attachment hole facing the combustion chamber recessed section,
• wherein the cylinder head-side squish surface and the piston head-side squish surface are upwardly inclined toward the combustion chamber recessed section, and in a vicinity of a compression top dead center a squish flow is ejected from a squish area formed between the cylinder head-side squish surface and the piston head-side squish surface toward a center of the combustion chamber recessed section,
• when viewed from a direction parallel to a cylinder center axis, given a central virtual line passing a squish area maximum-width section and the center of the combustion chamber recessed section, and one area a and another area b divided by the central virtual line, and
• one of the intake valve port and the exhaust valve port is provided in one area of any opposed section of the two opposed sections of the combustion chamber recessed section, and the spark-plug attachment hole is provided in the other area of the opposed section,
• such that a spark discharge section at a tip of a spark plug is provided in a position not to overlap the central virtual line when viewed from the direction parallel to the cylinder center axis.