Internal-combustion engine having a sub-combustion chamber

Claim 1

An internal-combustion engine comprising a cylinder, a cylinder head having a recess opening to the inner surface thereof, a piston for sliding movement within said cylinder, a main combustion chamber being defined between the inner surface of said cylinder head and the top surface of said piston and having an intake valve for admitting the lean air-fuel mixture thereinto and an exhaust valve for discharging the exhaust gas therefrom, a sub-chamber element fitted into said recess of the cylinder head in overall close contact relation with and securely held in position under contact pressure against said recess and formed therein with a sub-chamber and a passage which intercommunicate said sub-chamber and said main combustion chamber for providing the only supply of air-fuel mixture for said sub-chamber in which the coefficient of thermal expansion of said cylinder head is 1.5 to 2.5 times that of said sub-chamber element, thereby preventing said sub-chamber element from dropping into said main combustion chamber, and a spark plug having a sparking gap for exposing said gap to said