Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2000-12-22
2003-07-22
Wong, Don (Department: 2821)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C445S007000
Reexamination Certificate
active
06597089
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine. More particularly, the invention relates to a spark plug having an electrode tip welded onto a center electrode, improved in preventing or suppressing abnormal lateral consumption or rather spark erosion that occurs at a lateral periphery of the electrode tip and/or at the joined potion that joins the electrode tip and the center electrode.
2. Description of the Related Art
The electrode tip which constitutes part of a center electrode of a spark plug is laser welded to the end face of the body of the center electrode. The body is formed from nickel or a nickel alloy. When an electrode tip formed from a noble metal, such as iridium, is used, a joined portion is formed from an alloy which contains the noble metal, such as iridium, and a large amount of nickel. This alloy exhibits good release of thermions as compared with a noble metal, such as iridium. Thus, when a discharge portion and the joined portion are located in the same vicinity, sparks tend to jump toward the joined portion. As a result, the electrode tip tends to suffer abnormal lateral consumption arising in the vicinity of the joined portion and/or at the joined portion, potentially resulting in dropout of the electrode tip.
According to Japanese Patent Application Laid-Open (kokai) No. 11-3765, when the thickness of a fused portion (joined portion) of an electrode tip as measured at a position located half the radius of the electrode tip away in a radial direction from the axis of the electrode tip is not less than 0.2 mm, a spark plug of sufficient durability is obtained. However, when an electrode tip having a diameter of not greater than 1 mm, particularly not greater than 0.6 mm, is laser welded, sufficient spacing, particularly in the radial direction, cannot be established between a discharge portion and the fused portion, unavoidably resulting in abnormal lateral consumption of the electrode tip arising in the vicinity of the fused portion. When the spark plug is actually used in an engine, the electrode tip may come off, and the life may become shorter than an initially set value.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to solve the above-mentioned problems involved in a conventional spark plug, and to provide a spark plug for an internal combustion engine in which a discharge portion and a joined portion are located as far away from each other as possible in the diametral and axial directions of an electrode tip which constitutes part of a center electrode thereof, thereby, in particular, suppressing abnormal lateral consumption or lateral spark erosion of the electrode tip arising in the vicinity of the joined portion and/or at the joined portion, and thus maintaining excellent performance over a long period of time.
A spark plug for an internal combustion engine according to the invention comprises a center electrode which extends in the axial direction and comprises a body and an electrode whose root end is joined to the end face of the body. The spark plug is characterized in that the distance between a joined portion formed from an alloy which contains a component of the body and a component of the electrode tip and the face of a free end of the electrode tip is at least 0.15 mm, and that the diametral dimension of the electrode tip as measured at a position located on the interface between the electrode tip and the joined portion and nearest to the free end of the electrode tip is at least 0.2 mm greater than the diametral dimension of the electrode tip as measured at the free end. In general, this invention provides a structure of an electrode tip that is welded onto a top of the center electrode, improving spark-durability performance and suppressing a lateral-park erosion that becomes more critical to a smaller diametral tip welded on a larger-diameter center electrode.
The above-mentioned “body” is usually formed from nickel or a nickel alloy. The material of the above-mentioned “electrode tip” is not particularly limited, and the electrode tip may be formed from platinum or a platinum alloy, such as a Pt—Ir alloy or a Pt—Ni alloy, or from iridium or an iridium alloy, such as an Ir—Rh alloy, an Ir—Pt alloy, or an Ir—Ni alloy.
No particular limitations are imposed on the material and structure of other component members of the spark plug; i.e., an insulator disposed in contact with the circumferential surface of the body; a metallic shell disposed in contact with the insulator from outside; an outer electrode disposed such that a root end thereof is connected to the metallic shell and that a free end thereof faces the center electrode; and a metallic terminal member connected to the center electrode and disposed at the rear end side of the insulator.
The electrode tip is usually joined to the body by laser welding. The circumferential edge of an end face of the body and the circumferential edge of a root end of the electrode tip are fused together, thereby forming the above-mentioned “joined portion” which contains a component of the body and a component of the electrode tip. The end face of the body to which the electrode tip is joined is not limited to a flat face, but may assume the form of a recess, into which the electrode tip is fitted, followed by joining.
The distance between the joined portion and the face of the free end of the electrode tip (represented by the symbol “L” in
FIG. 2
) is “at least 0.15 mm,” more preferably not less than 0.20 mm, particularly preferably not less than 0.25 mm. By employing a specified distance range, spacing can be established between the discharge portion and the joined portion in the axial direction of the electrode tip. When the distance is less than 0.15 mm, sparks tend to jump from the discharge portion to the joined portion, causing a lateral spark erosion or abnormal lateral consumption of the electrode tip arising in the vicinity of the joined portion including the joined portion. When the distance is 0.5 mm, particularly 0.7 mm, a spark plug is substantially free from this abnormal lateral consumption. The distance does not need to assume a larger value.
It is important that the diametral dimension of the electrode tip measured at a position located at the interface between the electrode tip and the joined portion and nearest to the free end of the electrode tip (the dimension represented by the symbol “D” in
FIG. 2
) is “at least 0.2 mm” greater, or at least 0.25 mm greater (particularly important), than the diametral dimension of the electrode tip measured at the free end (the dimension represented by the symbol “d” in FIG.
2
). By employing an electrode tip in which a diametral dimension as measured at the root end to be joined to the body is greater than that measured at the free end, spacing is established between the discharge portion and the joined portion in the radial direction of the electrode tip, thereby reliably suppressing abnormal lateral consumption of the electrode which would otherwise arise in the vicinity of the joined portion due to sparks jumping from the discharge portion to the joined portion.
An electrode tip in which the diametral dimension as measured at the root end to be joined to the body is greater than that measured at the free end can be formed such that the diameter is increased stepwise from the free end toward the root end. For example, the electrode tip may have a shape which is obtained by coaxially integrating a cylindrical body of large diameter and a cylindrical body of small diameter. In this case, as shown in
FIG. 2
, the upper surface of a stepped portion may be sloped radially outward and downward. Alternatively, an electrode tip may be formed such that the circumferential surface thereof is tapered from the free end toward the root end. In this case, the diameter may be varied or increased continuously from the free-end face to the root-end face, or the inclination angle may be varied in the middle.
Preferably, in the spark plug
A Minh D
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
Wong Don
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