Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2002-02-26
2003-08-26
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S011500, C313S142000, C123S1690EL
Reexamination Certificate
active
06611084
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine.
2. Description of the Related Art
A conventional spark plug generally includes a center electrode projecting downward from the tip face of an insulator, and a parallel ground electrode disposed in opposition to the center electrode while one end of the ground electrode is joined to a metallic shell. The spark plug is adapted to ignite an air-fuel mixture by means of spark discharge effected across an air gap between the center electrode and the parallel ground electrode. In addition to such a parallel-electrode spark plug, a creeping-discharge spark plug is known which is a spark plug for use in an internal combustion engine and which features improved fouling resistivity. The creeping-discharge spark plug is configured such that sparks produced in a spark discharge gap creep along the surface of an insulator in the form of creeping discharge at all times or under certain conditions.
For example, a so-called semi-creeping-discharge spark plug includes an insulator having a center through-hole formed therein; a center electrode held in the center through-hole and disposed at a tip portion of the insulator; a metallic shell for holding the insulator such that a tip portion of the insulator projects from the tip face thereof; and a semi-creepage ground electrode disposed such that one end thereof is joined to the metallic shell while the other end thereof faces either the side peripheral surface of the center electrode or the side peripheral surface of the insulator. Creeping discharge involves air discharge effected between the spark face of the semi-creepage ground electrode and the surface of the insulator and sparking that creeps along the tip surface of the insulator. In the spark plug of creeping discharge type, spark discharge occurs so as to creep along the surface of the insulator, thereby continuously burning off fouling and thus exhibiting enhanced fouling resistivity as compared with a spark plug of air discharge.
A hybrid spark plug has been proposed which combines functions of the parallel-electrode type spark plug and the semi-creeping-discharge type spark plug. Since dimensions of the hybrid spark plug are determined such that sparking occurs across a semi-creepage gap even when the tip face of an insulator is not fouled, channeling can be effectively suppressed while fouling resistivity is established, and ignition property can be improved.
Among hybrid spark plugs composed of a parallel ground electrode and a semi-creepage ground electrode, a certain hybrid spark plug includes a heat release acceleration metal portion provided in a center electrode in order to accelerate heat release from the center electrode, the heat release acceleration metal portion being made of a material higher in heat conduction than an electrode base material. As shown in
FIG. 10
, the heat release acceleration metal portion
2
m
is provided in the interior of the electrode base material so as to accelerate heat release from the entire center electrode, thereby effecting good heat release from the center electrode. The larger the portion of the electrode base material occupied by the heat release acceleration metal, the greater the heat release effect.
3. Problems Solved by the Invention
However, for structural reasons, increasing a portion of the center electrode occupied by the heat release acceleration metal portion unavoidably involves a reduction in the wall thickness of the electrode base material. This potentially results in impaired durability against surface erosion of the electrode base material stemming from spark discharge across a semi-creepage gap.
The hybrid spark plug potentially involves a variation over the course of time in the frequency of sparking across a certain gap depending on engine conditions, engine characteristics, and the like. Dimensions of the hybrid spark plug are determined such that sparking across the semi-creepage gap occurs, even when carbon fouling does not occur as well as when carbon fouling occurs. In the case of such a spark plug involving highly frequent sparking against the side surface of a center electrode, a problem of spark erosion of the side surface of the center electrode arises.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a hybrid spark plug including a parallel ground electrode and a semi-creepage ground electrode, which spark plug exhibits good heat release from a center electrode and excellent durability against spark erosion by effectively protecting a portion of the side peripheral surface of the center electrode subjected to frequent spark impact.
To achieve the above object, the present invention provides a spark plug comprising:
an insulator having a center through-hole formed therein; a center electrode held in the center through-hole, disposed in a tip portion of the insulator, and having a noble metal chip located at a tip portion thereof, a metallic shell for holding the insulator such that a tip portion of the insulator projects from a tip face thereof, a parallel ground electrode disposed such that one end thereof is joined to the tip face of the metallic shell while the other end thereof faces a tip face of the center electrode so as to form a main air gap; and a plurality of semi-creepage ground electrodes each disposed such that one end thereof is joined to the metallic shell while the other end thereof faces at least either the side peripheral surface of the center electrode or the side peripheral surface of the insulator so as to form a semi-creepage gap.
The spark plug is characterized in that a tip portion of the center electrode as projected orthogonally on a virtual plane in parallel with the axis of the center electrode includes a tapered portion which is tapered such that its diameter reduces axially frontward, where the term frontward refers to an axial direction directed into an internal combustion engine; a convex portion is formed at an axially intermediate position of the tapered portion such that an outline thereof as viewed on the virtual plane projects radially outward with respect to the axis; the axially measured distance between the vertex of the convex portion (hereinafter may be called the convex vertex) and the tip of the insulator is less than 0.5 mm; a heat release acceleration metal portion higher in thermal conductivity and linear expansion coefficient than an electrode base material, which forms a surface layer portion of the center electrode, is present at a position located 1.5 mm axially rearward from the convex vertex while being enclosed by the electrode base material; and the heat release acceleration metal portion is formed such that the electrode base material has a wall thickness of not less than 0.6 mm as measured at a position located 1.5 mm axially rearward from the convex vertex.
As described above, the center electrode has a convex portion formed such that the axially measured distance between the convex vertex and the tip face of the insulator is less than 0.5 mm, thereby yielding the following effect: sparks which creep along the tip surface of the insulator can readily reach the convex vertex, which is angular and on which an electric field concentrates, thereby maintaining good ignition property at a gap between the semi-creepage ground electrode and the center electrode. Since sparks generated between the electrodes creep along the tip face of the insulator, the sparks erode, for example, a portion of the center electrode located rearward of the convex vertex, such as the region C in FIG.
10
.
Thus, by employing the above-described configuration in which the heat release acceleration metal portion is present at a position located 1.5 mm axially rearward from the vertex of the convex portion of the center electrode having the noble metal chip located at the tip portion, the heat release acceleration metal portion suppresses an increase in electrode temperature. Additionally, by imparting to the electr
Kato Tomoaki
Musasa Mamoru
Teramura Hideki
Macchiarolo Peter
NGK Spark Plug Co. Ltd.
O'Shea Sandra
Sughrue & Mion, PLLC
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