Patent ID: 7944134

Claim:
A spark plug comprising: a center electrode extending from a leading end thereof to a base end thereof in an axial direction and comprising an electrode base member and a core member disposed inside the electrode base member, the core member having a thermal conductance higher than that of the electrode base member; an electrode tip containing a noble metal as a main component, the electrode tip being joined to a leading end portion of the center electrode via a molten bond in which the electrode tip and the center electrode are fused, the molten bond extending from a leading end thereof to a base end thereof in the axial direction; an insulator having an axial hole extending in the axial direction, the insulator holding the center electrode in the axial hole on a leading end side portion of the axial hole; a metal shell extending from a leading end thereof to a base end thereof in the axial direction and surrounding and holding a radial periphery of the insulator; and a ground electrode having one end joined to a leading end surface of the metal shell and another end defining a spark discharge gap between the electrode tip and the ground electrode; wherein following relationships (1) are (2) are satisfied: d≦2.1 [mm] (1); and −0.09 ×d+ 0.33 <Vb /( Va+Vb )<−0.2 ×d+ 0.75 (2) where: d is an outer diameter of the center electrode at a position 4 mm closer to the base end of the center electrode from the base end of the molten bond in the axial direction; Va is a volume of the electrode base member in a region from the base end of the molten bond to a position of the center electrode 4 mm closer to the base end of the center electrode from the base end of the molten bond in the axial direction; and Vb is a volume of the core member in a region from the base end of the molten bond to a position 4 mm closer to the base end of the center electrode from the base end of the molten bond in the axial direction.