Patent ID: 6707237
Filing Date: 2004-03-16
Classification: H01T

Abstract:
A spark plug comprising:an insulator (1) having a center through hole (1D); a center electrode (2) disposed in said center through hole (1D) and extends along the direction of axial line (O); a metal shell (5) having a screw (5B) for assembling an internal combustion engine provided external of said insulator (1); and a ground electrode (11) joined at one end thereof through a joint portion (55) to said metal shell (5), and on the other end of which having a discharge plane (111A) being arranged so as to oppose to an end face (22B) of said center electrode (2) to thereby form a spark discharge gap (g), wherein said insulator (1) is engaged to said metal shell (5) through an engagement portion (15), and said center electrode (2) is protruded out from an end face (1E) of the insulator (1) and has formed therein a divergent portion (G), having a diameter increasing towards the end, on the end side beyond said engagement portion (15) and between the outer peripheral surface of the center electrode (2) and the inner peripheral surface of the insulator (1); and the end of said center electrode (2) forming said spark discharge gap (g) comprises a noble metal member (22) having a straight rod portion (22A) of 1.0 mm or less in diameter and 0.2 mm or more in length; assuming a primary intersectional line (PKL) as being defined as an intersectional line formed between said end face (22B) of said center electrode (2) or a plane (P1) extended therefrom and a lateral plane (22S) of said straight rod portion (22A) or a cylindrical plane (C1) extended therefrom; further assuming a secondary intersectional line (SKL) as being defined as an intersectional line formed between said discharge plane (111A) or a plane (P2) extended therefrom and an end face (112B) of said ground electrode (11) or a plane extended therefrom; further assuming a primary virtual line (PVL) as being defined as a virtual line containing a primary intersectional point (PP) and being in parallel to a virtual center axial line (O) of the spark plug referring to said screw (5B) for assembling the internal combustion engine, said primary intersectional point (PP) being a first point encountered said primary intersectional line (PKL)when a standard line (SL) parallel to said virtual center axial line (O) is moved across said spark discharge gap g to the joint portion 55 of the ground electrode 11 from the side opposite to such joint portion 55 placing the virtual center axial line O in between; and further assuming a secondary virtual line (SVL) as being defined as a virtual line containing a secondary intersectional point (SP) and being in parallel to said virtual center axial line (O), said secondary intersectional point (SP) being a last point where the standard line (SL) similarly moved intersects with said primary intersectional line (PKL); a width of coverage (K) as being defined as a distance between the primary virtual line (PVL) and secondary intersectional line (SKL) is set so as to satisfy a relation of âˆ’d&lE;K&lE;0.5 (in mm: where d represents the diameter of the end face (22B) of said center electrode (2); and sign for K is defined as negative when the secondary intersectional line (SKL) stands closer to the joint portion (55) than the primary virtual line (PVL), and as positive when stands further); and the width (w) of a portion of said discharge plane (111A) which falls within a range (WDS) between the secondary virtual line (SVL) and primary virtual line (PVL) satisfies a relation of w<2.1âˆ’K (in mm) where K is the foregoing width of coverage.