A spark plug having a center electrode that includes a columnar noble metal tip at one end thereof, and a ground electrode that forms a spark gap between the ground electrode and a circular discharge surface of the tip. In the tip, a mass % of Pt is largest and a content percentage of Ni is more than or equal to 0 mass % and less than or equal to 40 mass %. In each of both a cross-section of the noble metal tip parallel to the discharge surface and a cross-section of the tip perpendicular to the discharge surface, particles each having an aspect ratio of more than or equal to 1 and less than or equal to 10 occupy more than or equal to 70% of observed particles in an area extending from an outline of the cross-section by a distance of 10% of a diameter of the discharge surface.

A spark plug having a center electrode that includes a columnar noble metal tip at one end thereof, and a ground electrode that forms a spark gap between the ground electrode and a circular discharge surface of the tip. In the tip, a mass % of Pt is largest and a content percentage of Ni is more than or equal to 0 mass % and less than or equal to 40 mass %. In each of both a cross-section of the noble metal tip parallel to the discharge surface and a cross-section of the tip perpendicular to the discharge surface, particles each having an aspect ratio of more than or equal to 1 and less than or equal to 10 occupy more than or equal to 70% of observed particles in an area extending from an outline of the cross-section by a distance of 10% of a diameter of the discharge surface.

A spark plug includes a first discharge chip and a second discharge chip facing the first discharge chip to define a spark gap therebetween. The spark gap has a size or distance between the first and second discharge chips which is 0.23 mm or more and 0.6 mm or less. At least one of the first and second discharge chips is made from material mainly containing iridium or platinum and an additive of tantalum of 0.5 Wt % to 5.0 Wt %. This minimizes a risk that a portion of the first or second discharge chip may be separated or such a separated portion may be redeposited on the first or second discharge portion.

A spark plug includes a first discharge chip and a second discharge chip facing the first discharge chip to define a spark gap therebetween. The spark gap has a size or distance between the first and second discharge chips which is 0.23 mm or more and 0.6 mm or less. At least one of the first and second discharge chips is made from material mainly containing iridium or platinum and an additive of tantalum of 0.5 Wt % to 5.0 Wt %. This minimizes a risk that a portion of the first or second discharge chip may be separated or such a separated portion may be redeposited on the first or second discharge portion.

A pre-chamber ignition system for a spark-ignited reciprocating piston internal combustion engine includes a housing where the housing is installable in a cylinder head of the spark-ignited reciprocating piston internal combustion engine and where the housing has a cavity. An ignition device is disposed in the housing and the ignition device projects into the cavity of the housing. A cover covers the cavity with respect to a combustion chamber of the spark-ignited reciprocating piston internal combustion engine and the cavity and the combustion chamber are connectable together in a gas-guiding manner by an aperture in the cover. The housing consists of a material having a thermal conductivity of more than 170 W/(m*K) and the cover consists of a material having a thermal conductivity of less than 170 W/(m*K).

A spark-ignited reciprocating piston internal combustion engine may include a pre-chamber ignition system which is arranged in the cylinder head of the reciprocating piston internal combustion engine and in which an ignition device is arranged, comprising an ignition electrode in a housing, said ignition electrode protruding into a cavity of the pre-chamber ignition system. The cavity is covered from the combustion chamber of the reciprocating piston internal combustion engine by a cover, and the cover has at least one first opening which connects the cavity and the combustion chamber together so as to conduct gas. A metal transverse brace with at least one second opening, which connects opposite sides of the housing together, is arranged in the cavity in the region of the ignition electrode, wherein the metal transverse brace is made of a first metal material in a central region and a second metal material in a region contacting the housing.

A spark plug has an insulator having a through hole formed along an axial direction, a center electrode which is partially inserted into a portion of the through hole on a forward end side in the axial direction, and a glass seal portion which is in contact with the insulator and the center electrode within the through hole, in which the glass seal portion contains glass and an electrically conductive substance. The glass contains an Si component and a B component in a total amount of 50 mass % or more, as reduced to SiO.sub.2 and B.sub.2O.sub.3, a Zn component in an amount of 20 mass % to 35 mass % as reduced to ZnO, and an alkali metal component. The glass contains, as the alkali metal component, an Na component in an amount less than 1 mass % as reduced to Na.sub.2O.

A spark plug has an insulator having a through hole formed along an axial direction, a center electrode which is partially inserted into a portion of the through hole on a forward end side in the axial direction, and a glass seal portion which is in contact with the insulator and the center electrode within the through hole, in which the glass seal portion contains glass and an electrically conductive substance. The glass contains an Si component and a B component in a total amount of 50 mass % or more, as reduced to SiO.sub.2 and B.sub.2O.sub.3, a Zn component in an amount of 20 mass % to 35 mass % as reduced to ZnO, and an alkali metal component. The glass contains, as the alkali metal component, an Na component in an amount less than 1 mass % as reduced to Na.sub.2O.

A ground electrode for a spark plug includes a ground electrode body arranged to face a center electrode of the spark plug. The ground electrode includes a discharge portion mounted on the ground electrode body to face the center electrode with a spark gap between the discharge portion and the center electrode. The discharge portion is made of a platinum-based alloy of platinum, rhodium, and nickel. A first mass percent of the rhodium contained in the platinum-based alloy of the platinum, the rhodium, and the nickel is within a first range from 2 wt % to 20 wt % inclusive. A second mass percent of the nickel contained in the platinum-based alloy of the platinum, the rhodium, and the nickel being within a second range from 2.5 wt % to 12 wt % inclusive.

A ground electrode for a spark plug includes a ground electrode body arranged to face a center electrode of the spark plug. The ground electrode includes a discharge portion mounted on the ground electrode body to face the center electrode with a spark gap between the discharge portion and the center electrode. The discharge portion is made of a platinum-based alloy of platinum, rhodium, and nickel. A first mass percent of the rhodium contained in the platinum-based alloy of the platinum, the rhodium, and the nickel is within a first range from 2 wt % to 20 wt % inclusive. A second mass percent of the nickel contained in the platinum-based alloy of the platinum, the rhodium, and the nickel being within a second range from 2.5 wt % to 12 wt % inclusive.