Patent ID: 8878427

Claim:
A spark plug for an internal combustion engine comprising: a center electrode; an insulator disposed around an outer periphery of the center electrode; a mounting bracket disposed around an outer periphery of the insulator; a ground electrode disposed so as to extend from the mounting bracket and form a spark discharge gap with the center electrode; and a columnar noble-metal chip having a diameter of D and joined, through a weld portion, to a distal end of at least one of the center electrode and the ground electrode as a base material electrode, wherein, when Q designates a central axis line of the noble-metal chip, P 0 designates an intersection point in a cross-section of the noble-metal chip passing through the central axis line Q at which the central axis line Q intersects with a boundary line designated by S 1 between the weld portion and the noble-metal chip, P 1 designates an intersection point at which a phantom axis line designated by Q 1 which is radially distant from the central axis line Q by D/4 intersects with the boundary line S 1 , P 2 designates an intersection point at which a phantom axis line designated by Q 2 which is radially distant from the central axis line Q by 3D/8 intersects with the boundary line S 1 , P 3 designates an intersection point at which a phantom axis line designated by Q 3 which is radially distant from the central axis line Q by D/2 intersects with the boundary line S 1 , an angle which a straight line joining the intersection points P 0 and P 1 makes with the central axis line is θ 1 , an angle which a straight line joining the intersection points P 1 and P 2 makes with the central axis line Q is θ 2 , and an angle which a straight line joining the intersection points P 2 and P 3 makes with the central axis line Q is θ 3 , the angle θ 1 , θ 2 and θ 3 are all larger than or equal to 70 degrees, and wherein, when an axial thickness along the central axis line Q of the weld portion is B, X designates an intersection point at which an extension of a contour line of the base material electrode in the vicinity of the weld portion intersects with a boundary line designated by S 2 between the weld portion and the base material electrode, and an axial distance along the central axis line Q between the intersection points P 3 and X is A, relational expressions of B≧0.7A and 0.3 mm≦A≦0.6 mm are satisfied, and wherein when the weld portion is separated into a first region and a second region by a cross-section which passes through a midpoint of the central axis line Q and is perpendicular to the central axis line Q, the first region being closer to the noble-metal chip than to the base material electrode, the second region being closer to the base material electrode than to the noble-metal chip, and when a content of a chemical composition constituting the noble-metal chip in the first region is C 1 mass % and a content of the chemical composition constituting the noble-metal chip in the second region is C 2 mass %, a relational expression of |C 1 -C 2 |≦20 mass % is satisfied.