Abstract: A spark plug includes: a center electrode; an insulator having a through hole around a part of the center electrode; and a metal shell holding the insulator from an outer peripheral side thereof. The metal shell includes a shelf portion that projects radially inward. The insulator includes an engagement portion engaged with the shelf portion from the front side, and a front end portion at the front side with respect to a front end of the metal shell. The front end portion has an outer diameter larger than an inner diameter of the metal shell at the front side with respect to the shelf portion. The front end portion of the insulator has a diameter-enlarged portion at which a diameter of the through hole increases and which is spaced apart from an outer peripheral surface of the center electrode.

Abstract: An ignition plug includes a center electrode; a cylindrical insulator that surrounds at least the circumference of a front end portion of the center electrode and that includes a bottom portion at the front side; and a cylindrical metal shell that holds the insulator from the outer circumference side. The center electrode includes a shaft portion that extends along an axial line and a head portion disposed at a front end of the shaft portion. The head portion has a width greater than that of the shaft portion in the radial direction. The insulator includes a first and second insulator. The first insulator has an axial hole and a diameter smaller than the maximum diameter of the head portion. The second insulator is joined to the first insulator. The shaft portion is disposed in the axial hole of the first insulator. The second insulator encloses the head portion.

Abstract: In an ignition plug, the volume V1 of a portion of an insulator, which projects from a metallic shell to a front side, is equal to or greater than 45 mm3; and an expression 0.18?V2/V1?0.37 is satisfied, where H is a length along which the insulator projects from the metallic shell to the front side in an axial direction, and V2 is a volume of a portion of the insulator, which projects from a front end of the insulator along a length H/2 in the axial direction.

Abstract: An ignition plug includes a center electrode; a cylindrical insulator that surrounds at least the circumference of a front end portion of the center electrode and that includes a bottom portion at the front side; and a cylindrical metal shell that holds the insulator from the outer circumference side. The center electrode includes a shaft portion that extends along an axial line and a head portion disposed at a front end of the shaft portion. The head portion has a width greater than that of the shaft portion in the radial direction. The insulator includes a first and second insulator. The first insulator has an axial hole and a diameter smaller than the maximum diameter of the head portion. The second insulator is joined to the first insulator. The shaft portion is disposed in the axial hole of the first insulator. The second insulator encloses the head portion.

Abstract: A spark plug includes: a center electrode; an insulator having a through hole around a part of the center electrode; and a metal shell holding the insulator from an outer peripheral side thereof. The metal shell includes a shelf portion that projects radially inward. The insulator includes an engagement portion engaged with the shelf portion from the front side, and a front end portion at the front side with respect to a front end of the metal shell. The front end portion has an outer diameter larger than an inner diameter of the metal shell at the front side with respect to the shelf portion. The front end portion of the insulator has a diameter-enlarged portion at which a diameter of the through hole increases and which is spaced apart from an outer peripheral surface of the center electrode.

Abstract: An ignition plug capable of improving ignition performance while preventing pre-ignition includes a center electrode, a bottomed cylindrical insulator enclosing a front end portion of the center electrode and a cylindrical metal shell holding therein the insulator. The insulator has, on a front end side thereof, a protruding portion protruding from and located frontward of a front end of the metal shell. In the ignition plug, a difference between an outer diameter of the center electrode and an inner diameter of the insulator is 0.01 mm or more; an axial length of the protruding portion is 6.7 mm to 12.7 mm; and the following condition is satisfied: 1.07?S/V1?1.35 where S is an outer surface area of the protruding portion; and V1 is a volume of the protruding portion.

Abstract: An ignition plug capable of improving ignition performance while preventing pre-ignition includes a center electrode, a bottomed cylindrical insulator enclosing a front end portion of the center electrode and a cylindrical metal shell holding therein the insulator. The insulator has, on a front end side thereof, a protruding portion protruding from and located frontward of a front end of the metal shell. In the ignition plug, a difference between an outer diameter of the center electrode and an inner diameter of the insulator is 0.01 mm or more; an axial length of the protruding portion is 6.7 mm to 12.7 mm; and the following condition is satisfied: 1.07?S/V1?1.35 where S is an outer surface area of the protruding portion; and V1 is a volume of the protruding portion.

Abstract: A spark plug having a ground electrode, a noble metal tip having a front end that projects forward of a front end portion of the ground electrode, and a center electrode. When S1 represents an area of an overlap region in which a region onto which the center electrode is projected overlaps a region onto which the noble metal tip is projected when the center electrode and the noble metal tip are projected onto an imaginary plane perpendicular to a center line of the center electrode, and S2 represents an area of an overlap region in which a region onto which the noble metal tip is projected overlaps a region onto which the ground electrode is projected when the noble metal tip and the ground electrode are projected onto the imaginary plane, 0.22?S1/S2?0.68 is satisfied.

Abstract: A non-thermal equilibrium plasma ignition plug including a tubular metallic shell having an axial hole extending along an axial line, an insulator disposed in such a manner as to form a gap in cooperation with a wall surface of the axial hole at a forward end portion of the metallic shell, and a center electrode held at the center of the insulator, and generates nonequilibrium plasma in response to voltage applied thereto from a power supply. The insulator has a plurality of depressions or protrusions formed on a surface thereof which faces a discharge space therearound.

Abstract: A spark plug having a ground electrode, a noble metal tip having a front end that projects forward of a front end portion of the ground electrode, and a center electrode. When S1 represents an area of an overlap region in which a region onto which the center electrode is projected overlaps a region onto which the noble metal tip is projected when the center electrode and the noble metal tip are projected onto an imaginary plane perpendicular to a center line of the center electrode, and S2 represents an area of an overlap region in which a region onto which the noble metal tip is projected overlaps a region onto which the ground electrode is projected when the noble metal tip and the ground electrode are projected onto the imaginary plane, 0.22?S1/S2?0.68 is satisfied.

Abstract: In an ignition plug, the volume V1 of a portion of an insulator, which projects from a metallic shell to a front side, is equal to or greater than 45 mm3; and an expression 0.18?V2/V1?0.37 is satisfied, where H is a length along which the insulator projects from the metallic shell to the front side in an axial direction, and V2 is a volume of a portion of the insulator, which projects from a front end of the insulator along a length H/2 in the axial direction.

Abstract: A non-thermal equilibrium plasma ignition plug including a tubular metallic shell having an axial hole extending along an axial line, an insulator disposed in such a manner as to form a gap in cooperation with a wall surface of the axial hole at a forward end portion of the metallic shell, and a center electrode held at the center of the insulator, and generates nonequilibrium plasma in response to voltage applied thereto from a power supply. The insulator has a plurality of depressions or protrusions formed on a surface thereof which faces a discharge space therearound.