The present invention pertains to a spark plug for an internal combustion engine, comprising a metal outer shell extending in a longitudinal direction from a proximal end to a distal tip end configured to be oriented towards a combustion chamber, said outer shell comprising a fixation portion for attachment of the spark plug to a metal sleeve of the internal combustion engine and arranged at a region proximal of the tip end, wherein the outer shell furthermore comprises a first contact surface arranged at a region distal of the fixation portion and configured to contact a distal end of the sleeve, when the spark plug is attached to the sleeve.

A spark plug includes a tubular metallic shell having an attachment screw portion having an external thread, an insulator disposed inside the metallic shell and having an axial hole, and a center electrode disposed in the axial hole. The attachment screw portion has a pitch diameter local maximum portion at which the external thread has a locally maximum pitch diameter. The pitch diameter local maximum portion is located on the forward end side of a rear end of the center electrode in a direction along an axial line of the metallic shell.

A spark plug has a housing of a cylindrical shape, an insulator of a cylindrical shape fitted into the inside of the housing, a center electrode and a ground electrode. No male screw part is formed on an outer peripheral surface of the housing. A housing support surface is formed on an inner peripheral surface of the housing. An outer peripheral surface of the insulator has an insulator support surface which is supported by the housing support surface in an axial direction of the spark plug. The housing has a pressure fitting part to be presses to a cylinder head of an internal combustion engine. A part of the pressure fitting part and a part of at least one of the housing support surface and the insulator support surface are arranged to be overlapped with each other in a radial direction of the spark plug.

A spark plug has a housing of a cylindrical shape, an insulator of a cylindrical shape fitted into the inside of the housing, a center electrode and a ground electrode. No male screw part is formed on an outer peripheral surface of the housing. A housing support surface is formed on an inner peripheral surface of the housing. An outer peripheral surface of the insulator has an insulator support surface which is supported by the housing support surface in an axial direction of the spark plug. The housing has a pressure fitting part to be presses to a cylinder head of an internal combustion engine. A part of the pressure fitting part and a part of at least one of the housing support surface and the insulator support surface are arranged to be overlapped with each other in a radial direction of the spark plug.

A cylinder head (3) is formed with a pre-chamber (12) surrounded by a thin pre-chamber wall (11) sticking out from the inside wall surface of the cylinder head (3) to the inside of a main combustion chamber (5). Inside the pre-chamber (12), the electrode of a spark plug (15) is arranged. When the spark plug (15) is used to burn the air-fuel mixture inside the pre-chamber (12), jet flames are ejected from the communication holes (13) to the main combustion chamber (5). The thin pre-chamber wall (11) is formed into a two-layer structure of an outside wall (11a) facing the main combustion chamber (5) and an inside wall (11b) facing the pre-chamber (12). The inside wall (11b) is formed by a material with a higher heat conductivity than the outside wall (11a).

A cylinder head (3) is formed with a pre-chamber (12) surrounded by a thin pre-chamber wall (11) sticking out from the inside wall surface of the cylinder head (3) to the inside of a main combustion chamber (5). Inside the pre-chamber (12), the electrode of a spark plug (15) is arranged. When the spark plug (15) is used to burn the air-fuel mixture inside the pre-chamber (12), jet flames are ejected from the communication holes (13) to the main combustion chamber (5). The thin pre-chamber wall (11) is formed into a two-layer structure of an outside wall (11a) facing the main combustion chamber (5) and an inside wall (11b) facing the pre-chamber (12). The inside wall (11b) is formed by a material with a higher heat conductivity than the outside wall (11a).

A spark plug that improves the heat resistance performance of an ignition plug.

An ignition plug includes a tubular insulator, a metallic shell disposed around the outer circumference of the insulator, a center electrode disposed in an axial hole of the insulator, and a ground electrode connected to the forward end of the metallic shell and facing the center electrode. The metallic shell has a threaded portion to be engaged with an internal combustion engine. The relational expression Ss/(Sa+Sb)2.6 is satisfied, where Ss is the surface area of an outer circumferential surface of the metallic shell extending from the rear end of the threaded portion to the forward end of the threaded portion, Sa is the surface area of that portion of the metallic shell which is to be exposed to combustion gas of the internal combustion engine, and Sb is the surface area of that portion of the insulator which is to be exposed to the combustion gas.

An ignition plug includes a tubular insulator, a metallic shell disposed around the outer circumference of the insulator, a center electrode disposed in an axial hole of the insulator, and a ground electrode connected to the forward end of the metallic shell and facing the center electrode. The metallic shell has a threaded portion to be engaged with an internal combustion engine. The relational expression Ss/(Sa+Sb)2.6 is satisfied, where Ss is the surface area of an outer circumferential surface of the metallic shell extending from the rear end of the threaded portion to the forward end of the threaded portion, Sa is the surface area of that portion of the metallic shell which is to be exposed to combustion gas of the internal combustion engine, and Sb is the surface area of that portion of the insulator which is to be exposed to the combustion gas.

A spark plug includes a plug cap having a plurality of through holes. In a cross section including an axial line, the plug cap includes an enlarging portion in a portion of a region between a first imaginary straight line and a second imaginary straight line, the first imaginary straight line extending through a front end of the end portion of a ground electrode and being perpendicular to the axial line, the second imaginary straight line extending through a back end of one of inner open ends of the through holes that is closest to a front end of the spark plug and being perpendicular to the axial line. The enlarging portion being a portion in which a minimum distance from an outer surface to an inner surface of the plug cap increases with increasing distance in a direction from back to front.