Patent ID: 7321187
Filing Date: 2008-01-22
Classification: H01T

Abstract:
1. A spark plug comprising: a ground-electrode spark portion fixedly attached to a side surface of a ground electrode, which ground-electrode spark portion is disposed to face a center-electrode spark portion made from a noble metal and fixedly attached to a distal end of a center electrode, thereby forming a spark discharge gap between the center-electrode spark portion and the ground-electrode spark portion; the ground-electrode spark portion is formed from a noble metal which contains Pt as a main component, and is joined to the ground electrode via an alloy layer in which the noble metal that constitutes the ground-electrode spark portion and a metal that constitutes the ground electrode are alloyed with each other; and the ground-electrode spark portion is configured such that the distal end surface facing the spark discharge gap is smaller in diameter than a bottom surface fixedly attached to the ground electrode; the distal end surface is protrusively located closer to the distal end of the center electrode than is the side surface of the ground electrode; and when the ground-electrode spark portion is viewed in plane from the distal end surface, a portion of a surface of the ground-electrode spark portion is viewed as a peripheral exposed-region surface which is exposed on the side surface of the ground electrode so as to surround the distal end surface, wherein the entire peripheral exposed-region surface is located closer to the center electrode than is the side surface of the ground electrode, wherein the peripheral exposed-region surface of the ground electrode-spark portion is formed in parallel with the distal end surface of the ground-electrode spark portion, wherein, when G represents a shortest distance along an axial direction of the center electrode between a distal end surface of the center-electrode spark portion and the distal end surface of the ground-electrode spark portion, and L represents a length of a line segment connecting, by a shortest distance, a peripheral edge of the distal end surface of the center-electrode spark portion and a peripheral edge of the peripheral exposed-region surface, the following relational expression is satisfied: when, in orthogonal projection on a plane perpendicularly intersecting an axis of the center electrode, A represents a width of the peripheral exposed-region surface, W represents a width of the ground electrode, and d represents a diameter of the distal end surface of the ground-electrode spark portion, the following relational expression is satisfied: