Patent ID: 8196561

Claim:
A spark ignition engine comprising: a cylinder block; a piston sliding inside the cylinder block; a cylinder head disposed at an upper part of the cylinder block; and ignition points for igniting mixture in a combustion chamber defined by the cylinder block, the piston, and the cylinder head, wherein a bottom face of the cylinder head defining the combustion chamber is formed substantially in a spherical shell; the ignition points are arranged in three or more on a concentric circle of 55 to 70% of a diameter of the cylinder block or the vicinity thereof; and combustion is performed with an air excess ratio in the combustion chamber of 1.5 or more; further comprising a connecting rod and a crank connected to the connecting rod for converting a motion of the piston to a rotational motion, wherein ignition timing is set so that an angle of the crank when a pressure in the combustion chamber in an expansion stroke becomes the maximum is 9 to 12° after the piston comes to top dead center.