Patent ID: 8344605
Filing Date: 2013-01-01
Classification: H01T

Abstract:
1. A spark plug comprising a center electrode extending in a direction of an axis, an insulator which holds the center electrode, a metallic shell which holds the insulator, a ground electrode whose proximal end portion is joined to a front end portion of the metallic shell and which is bent and fixed such that an inside surface of a distal end portion thereof faces a front end portion of the center electrode, and a noble metal tip joined to the inside surface of the ground electrode, a spark discharge gap being formed between the center electrode and the noble metal tip of the ground electrode, the spark plug being characterized in that: the inside surface of the ground electrode has a columnar protrusion projecting in the direction of the axis and formed of an electrode base metal of the ground electrode which contains nickel as a main component; the noble metal tip whose cross-sectional area is smaller than an area of a distal end surface of the protrusion is joined to the distal end surface of the protrusion, and a discharge allowance surface is a part of the distal end surface of the protrusion and is formed around at least a portion of a periphery of the noble metal tip, the discharge allowance surface is formed of the electrode base metal of the ground electrode; a dimension of the spark discharge gap, which is a distance between a discharge surface of the center electrode and a discharge surface of the noble metal tip of the ground electrode as measured along the direction of the axis, is 0.8 mm or greater; a projecting dimension of the noble metal tip of the ground electrode, which is a distance between the inside surface of the ground electrode and the discharge surface of the noble metal tip of the ground electrode as measured along the direction of the axis, is 0.5 mm or greater; and when the discharge surface of the center electrode and the discharge surface of the noble metal tip of the ground electrode are projected onto a plane orthogonal to the direction of the axis, a projected image of the discharge surface of the center electrode does not protrude from a projected image of the discharge surface of the noble metal tip of the ground electrode.