Patent ID: 7554253
Filing Date: 2009-06-30
Classification: H01T

Abstract:
1. A spark plug for an internal combustion engine comprising: a metal shell having a base end and a top end opposed to the base end in a longitudinal direction of the spark plug; a hollow porcelain insulator having a length which includes a body and an insulator nose, the body being retained within said metal shell, the insulator nose projecting from the top end of said metal shell; a center electrode retained within said porcelain insulator to have a top end protruding from the insulator nose; a main ground electrode which defines a main spark gap between itself and said center electrode; and an auxiliary ground electrode having a base end, a top end opposed to the base end, and a body which has a length between the base end and the top end and is made up of a base end portion continuing to the base end and a top end portion continuing to the top end, the base end portion extending parallel to a longitudinal center line of the spark plug, the top end portion being bent from the base end portion toward the longitudinal center line of the spark plug, the base end being joined to said metal shell to extend from the top end of the metal shell and to define an auxiliary spark gap between the top end of the auxiliary ground electrode and a peripheral side surface of the insulator nose of said porcelain insulator, the top end of said auxiliary ground electrode having an end face facing the longitudinal center line of the spark plug, wherein the end face of said auxiliary ground electrode includes an increasing-radial distance surface which has a base edge closer to the base end of the auxiliary ground electrode and a top edge far from the base end of the auxiliary ground electrode and is located at a distance from the longitudinal center line of the spark plug in a radial direction of the spark plug which increases as approaching to the base edge from the top edge, and wherein the insulator nose of said porcelain insulator has a wall thickness T meeting a relation of 0.3 mm≦T≦0.7 mm, and wherein the increasing-radial distance surface tapers toward the top edge thereof to have widths of 0.3 mm or more in a radius direction and the longitudinal direction of the spark plug.