Patent ID: 7360531

Claim:
A combustion chamber structure for a spark-ignition engine, comprising: a combustion chamber defined between a bottom surface of a cylinder head and a top surface of a piston in such a manner that the bottom surface of said cylinder head serves as a ceiling wall thereof; and a spark plug having a sparking end protruding from the ceiling wall into the combustion chamber, wherein: when said piston is at a top dead center, a principal space of the combustion chamber is parted into a first combustion space around the sparking end of said spark plug and a second combustion space around a circumference of a cylinder bore, said ceiling wall and the top surface of said piston are configured such that a narrowed space is formed therebetween so that said narrowed space functions as a small interspace zone which communicates between said first combustion space and said second combustion space said spark plug is located around a radial center of said cylinder bore; and said small interspace zone includes a small interspace sub-zone and a minimum interspace sub-zone said small interspace sub-zone being defined in an intermediate position between said spark plug and at last an intake-side region of the circumference of said cylinder and said minimum interspace sub-zone being defined in an intermediate position between said spark plug and at least an exhaust-side region of the circumference of said cylinder bore with said minimum interspace sub-zone being narrower than the small interspace sub-zone.