Abstract: A torch ignition type internal combustion engine has a precombustion chamber formed in a cylinder head in communication with a main combustion chamber through a passage so that a part of a lean air-fuel mixture is forced into the precombustion chamber on compression stroke. A spark plug is mounted on the cylinder head to ignite the lean air-fuel mixture in the precombustion chamber so that the ignited air-fuel mixture is burnt therein to form a torch jet which runs from the precombustion chamber through the passage into the main combustion chamber to ignite the lean air-fuel mixture therein. The spark plug has an insulator extending at one end axially beyond that end face of a metal shell of the spark plug from which grounded electrodes extend. The one end of the insulator supports a center electrode.

Abstract: An ignition plug for internal combustion engines in which a spark gap is established between a central electrode and a ground electrode opposed thereto and a spark discharge is generated in the spark gap so as to ignite a mixture, wherein the ground electrode takes a unique form such that it is provided with a kerf, groove, depression, projection or the like at its spark discharge area confronting the central electrode whereby the ignition ability is improved with the result that a lean mixture is useful to reduce the quantity of harmful components in the exhaust gas.