| 主权项 |
1. 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, P0 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 S1 between the weld portion and the noble-metal chip, P1 designates an intersection point at which a phantom axis line designated by Q1 which is radially distant from the central axis line Q by D/4 intersects with the boundary line S1, P2 designates an intersection point at which a phantom axis line designated by Q2 which is radially distant from the central axis line Q by 3D/8 intersects with the boundary line S1, P3 designates an intersection point at which a phantom axis line designated by Q3 which is radially distant from the central axis line Q by D/2 intersects with the boundary line S1, an angle which a straight line joining the intersection points P0 and P1 makes with the central axis line is θ1, an angle which a straight line joining the intersection points P1 and P2 makes with the central axis line Q is θ2, and an angle which a straight line joining the intersection points P2 and P3 makes with the central axis line Q is θ3, the angle θ1, θ2 and θ3 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 S2 between the weld portion and the base material electrode, and an axial distance along the central axis line Q between the intersection points P3 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 C1 mass % and a content of the chemical composition constituting the noble-metal chip in the second region is C2 mass %, a relational expression of |C1-C2|≦20 mass % is satisfied. |
