Spark plug

A spark plug comprising:

• an insulator having an axial hole;
• a center electrode provided in the axial hole;
• a ground electrode provided so as to form a gap between the ground electrode and the center electrode;
• and
• a noble metal tip provided on at least one of the center electrode and the ground electrode, wherein,
• the noble metal tip contains Mp (Mp is an element group consisting of Pt or Pt and Pd, and the amount of Pd is 20 mass % or less with respect to the mass of the noble metal tip), Cu, and M (M is at least one element selected from the element group consisting of Rh, Ir, Ru, Re, and W) in a total amount of 95 mass % or more;
• the proportions by mass of Mp, Cu, and M (Mp, Cu, M) in a Mp-Cu-M ternary composition diagram fall within a region defined by a line connecting point A (97, 3, 0), point B (80, 3, 17), point C (75, 25, 0), and point A (97, 3, 0) in this order (the region including the line);
• and
• welding area S (mm2), tip protrusion height H (mm), covering length L (mm), and tip-welded portion distance h (mm) satisfy the following relations:
• (a) H≦0.13S+1.18, (b) S≦5, and (c) 0.1≦h or 0.03≦L, wherein
• welding area S is defined as being the area of a region in which, when the noble metal tip is provided on an end surface or a peripheral side surface of the center electrode and/or the ground electrode, as viewed in a direction X—which is a direction perpendicular to a bonding surface of a mounting metallic body (the body corresponding to the center electrode, the ground electrode, or a base provided between each of these electrodes and the noble metal tip) to which the noble metal tip is bonded via a welded portion formed through fusion between the noble metal tip and the mounting metallic body, a projection region formed by projection of the mounting metallic body on a surface perpendicular to the direction X overlaps a projection region formed by projection of the noble metal tip on a surface perpendicular to the direction X (when the noble metal tip is bonded to the mounting metallic body via a plurality of surfaces of the mounting metallic body, welding area S is defined as being the total area of regions in which, as viewed in directions Y—which are perpendicular to the surfaces, the surfaces overlap one another);
• tip protrusion height H is defined as being the distance between the bonding surface of the mounting metallic body and the end surface of the noble metal tip most distal from the bonding surface, the distance being determined in a direction in which the noble metal tip faces a facing metallic protrusion (the protrusion corresponding to the noble metal tip, a portion protruded from the front end of the center electrode, or portion protruded from the distal end of the ground electrode) (when the welded portion is provided between the noble metal tip and the mounting metallic body so as to cover the entire top surface of the mounting metallic body, tip protrusion height H is defined as being the distance between a point corresponding to ½ the thickness of the thinnest portion of the welded portion in a direction of an axis PX of the noble metal tip, and the surface of the noble metal tip most distal from the point in the direction of the axis PX);
• and
• covering length L and tip-welded portion distance h are defined as follows:
• in the case where the axial hole extends in a direction of an axis AX of the center electrode,
• (1) when the noble metal tip and the facing metallic protrusion are arranged so as to face each other in the direction of the axis AX, and the noble metal tip does not project from the mounting metallic body in a direction perpendicular to the axis AX,
• covering length L is defined as being the minimum distance, as viewed in the direction of the axis AX, between a straight line group which includes a point k1 on a peripheral side surface corresponding to the maximum diameter of the noble metal tip and which is parallel to the axis AX, and a straight line group which includes a point k2 on a peripheral side surface corresponding to the maximum diameter of the facing metallic protrusion, facing the noble metal tip, and which is parallel to the axis AX;
• and
• tip-welded portion distance h is defined as being the distance in the direction of the axis AX as measured, on a surface of the noble metal tip which includes the point k1 and is parallel to the axis AX, from the end of the noble metal tip to the boundary between the tip and the welded portion;
• or
• (2) when the noble metal tip projects from the ground electrode in a direction perpendicular to the axis AX, and the front end surface of the facing metallic protrusion faces the noble metal tip in the direction of the axis AX,
• covering length L is defined as being the minimum distance, as viewed in the direction of the axis AX, between a point k3 on a projection region formed by projection of the front end surface of the facing metallic protrusion on a virtual surface perpendicular to the direction of the axis AX, and an intersection point k4 provided by intersection of the contour of a projection region formed by projection of the ground electrode on the virtual surface with the contour of a projection region formed by projection of the noble metal tip on the virtual surface;
• and
• (i) in the noble metal tip, tip-welded portion distance h is defined as being the distance as measured, on a surface of the noble metal tip which includes the point k4 and is parallel to the axis AX, from the end of the noble metal tip to the boundary between the tip and the welded portion;
• or
• (ii) when the facing metallic protrusion is the noble metal tip provided on the center electrode, tip-welded portion distance h is defined as being the distance in the direction of the axis AX as measured, on a surface of the noble metal tip which includes the point k3 and is parallel to the axis AX, from the end of the noble metal tip to the boundary between the tip and the welded portion.