SPARK PLUG

Abstract

A plurality of insulator samples of different dimensions are subjected to multiple regression analysis to thereby induce an expression for estimating the amount of unevenness in wall thickness of an insulator; specifically, 0.01 (17.527+0.141A−0.285B−6.124C−0.14D+1.105E), where A is an overall length of the insulator, B is the outside diameter of a rear trunk portion of the insulator, C is the diameter of a large diameter portion of the axial bore of the insulator, D is the length of the large diameter portion of the bore, and E is the diameter of a small diameter portion of the axial bore. The insulator is designed such that the amount of unevenness in wall thickness thereof estimated through calculation by this expression is less than 0.07 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view in half section of a spark plug according to the present invention;

FIG. 2 is a full sectional view of an insulator of the spark plug of FIG. 1;

FIG. 3 is a schematic view showing a step of manufacturing the insulator of FIG. 2;

FIG. 4 is a schematic view showing a step of manufacturing the insulator of FIG. 2;

FIG. 5 is a graph of the amount of unevenness in wall thickness of an insulator vs. the ratio of the length D of a large diameter portion of an axial bore to the overall length A of the insulator, plotting measured amounts for insulators in groups; and

FIG. 6 is a graph of the amount of unevenness in wall thickness of an insulator vs. the ratio of the length D of a large diameter portion of an axial bore to the overall length A of the insulator, plotting measured amounts of FIG. 5 and estimated (calculated) amounts in a superposed representation for the insulators in groups.

Claims

1. A spark plug comprising an insulator having a bore extending in an axial direction and holding a center electrode for generating spark discharge in a front end portion of the axial bore and a terminal electrically connected to the center electrode, at a rear end portion of the axial bore, wherein the axial bore of the insulator comprises a large diameter portion continuous with an opening located at a rear end of the axial bore and a small diameter portion continuous with a front end of the large diameter portion and smaller in diameter than the large diameter portion, andwherein an axial length A in mm of the insulator, an outside diameter B in mm of a rear trunk portion located rearward of a portion of the insulator having a maximum outside diameter, a diameter C in mm of the large diameter portion, an axial length D in mm of the large diameter portion, and a diameter E in mm of the small diameter portion satisfy the following inequalities: A≧65E≦3.40.01(17.527+0.141A−0.285B−6.124C−0.14D+1.105E)<0.07

2. A spark plug comprising an insulator having a bore extending in an axial direction and holding a center electrode for generating spark discharge in a front end portion of the axial bore and a terminal electrically connected to the center electrode in a rear end portion of the axial bore, wherein the axial bore of the insulator comprises a large diameter portion continuous with an opening located at a rear end of the axial bore and a small diameter portion continuous with a front end of the large diameter portion and smaller in diameter than the large diameter portion,wherein an axial length A in mm of the insulator, an outside diameter B in mm of a rear trunk portion located rearward of a portion of the insulator having a maximum outside diameter, a diameter C in mm of the large diameter portion, an axial length D in mm of the large diameter portion, and a diameter E in mm of the small diameter portion satisfy the following inequalities and ratios: A≧65F≦3.4C/E≧1.16C/B≦0.47D/A≧0.09, andwherein as viewed on a plane which is perpendicular to the axial direction and on which a front end face of the insulator is projected, a distance between the center of a projected outer circumference of the front end face of the insulator and the center of a projected circumference of the opening of the axial bore is less than 0.07 mm.

3. A spark plug comprising, in combination, an insulator having an axial bore which receives a center electrode, a terminal electrically connected to one end of the center electrode and a spark discharge gap at another end of the center electrode, the insulator defining an axial length A in millimeters, a rear trunk portion having an outside diameter B in millimeters located rearwardly of a portion of the insulator having a maximum outside diameter,the axial bore of the insulator defining a large diameter portion having a diameter C in millimeters, an axial length D in millimeters and a small diameter portion having a diameter E in millimeters, andwherein the following inequalities are satisfied: A≧65E≦3.40.01(17.527+0.141A−0.285B−6.124C−0.14D+1.105E)<0.07.