Patent ID: 7122948
Filing Date: 2006-10-17
Classification: H01T

Abstract:
1. A spark plug comprising: a hollow metal shell having a first end and a second end opposed to the first end, said metal shell also having a threaded portion on an outer periphery thereof and an inner chamber opening at the first end, the threaded portion having an outer diameter equal to or less than 10 mm; an insulator having a length with a first end and a second end opposed to the first end of said insulator, said insulator also having a bore formed therein, said insulator being fixed in the inner chamber of said metal shell such that the first end of said insulator protrudes from the first end of said metal shell; a center electrode secured in the bore of said insulator, said center electrode having an end protruding from the first end of said insulator; and a ground electrode having a side surface, said ground electrode being joined to the first end of said metal shell such that the side surface of said ground electrode is opposed to the end of said center electrode through a spark gap, wherein a distance between an inner surface of said metal shell defining the inner chamber and an outer surface of said insulator has a maximum value on a first reference plane defined to extend perpendicular to the length of said insulator through an inner edge of the first end of said metal shell, and a minimum value on a reference plane defined to extend parallel to and spaced a given distance from the first reference plane, and wherein the following dimensional relationships are defined: X is a distance between an inner surface of said insulator defining the bore and an outer surface of said center electrode on a second reference plane defined to extend parallel to the first reference plane through an inner edge of the first end of said insulator, Y is a minimum distance from the inner edge of the first end of said insulator to the first reference plane along the first end and the outer surface of said insulator, Y Z is a distance between the inner surface of said metal shell and the outer surface of said insulator on the first reference plane, G is a space of the spark gap between the end of said center electrode and the side surface of said ground electrode, and W is a minimum distance on the outer surface of said insulator between the first reference plane and a third reference plane on which a distance between the inner surface of said metal shell and the outer surface of said insulator has the same value as the space G of the spark gap, the third reference plane being parallel to the first reference plane.