Spark plug internal combustion engine

Details Spark plug internal combustion engine Spark plug internal combustion engine

2001-12-28

2004-03-23

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Spark plugs

Particular electrode structure or spacing

C313S144000

Reexamination Certificate

active

06710523

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a spark plug for an internal combustion engine. More specifically, the present invention relates to a spark plug for an internal combustion engine that is capable of preventing an electrode tip from being anomalously corroded by a Pb component included in gasoline, that is excellent in resistance to spark consumption, and that is low in electrode erosion caused by oxidation at high temperatures.
BACKGROUND ART
A spark plug that employs an electrode tip largely composed of Pt is used in practical applications in order to improve resistance to spark consumption. A spark plug that employs an electrode tip of a Pt—Ir alloy largely composed of Pt has also been developed as one that has better resistance to spark consumption. However, there is a case in which, if these electrode tips are joined to the basic body of a center electrode and the base of a ground electrode that are each largely composed of Ni or a Ni alloy according to electric resistance welding, the tips will peel and fall off therefrom because of thermal stress as a result of a thermal expansion difference under a high temperature when they are used.
Therefore, a spark plug has been proposed in which an interlayer including Pt and Ni is disposed between the basic body or the like and the electrode tip so as to ease thermal stress. In addition, practical use is made of a spark plug in which the basic body and the electrode tip are welded to each other according to laser beam welding especially in the center electrode, and a junction layer made of an alloy including Pt, Ni, etc., is formed so as to ease thermal stress.
With the spread of motorization in recent years, a long life spark plug, such as the above-mentioned one, that is excellent in resistance to spark consumption and that is capable of preventing peeling and fall-off of an electrode tip caused by thermal stress has come to be used in many parts of the world. However, in a spark plug provided with an electrode tip chiefly composed of Pt, a compound whose melting point is low is formed between Pt and Pb if gasoline includes Pb. Accordingly, the electrode tip causes anomalous corrosion because of Pb, and, disadvantageously, the life cycle of the spark plug, which has been designed to have high performance and a long life, is on the contrary shortened.
Conventionally, a spark plug that excels in resistance to spark consumption and has a long life, or a spark plug that prevents peeling and fall-off of an electrode tip caused by the difference in a thermal expansion coefficient, as described above in detail, is disclosed by Publication of Unexamined Patent Application No. Hei 1-319284, Publication of Unexamined Patent Application No. Hei 6-45050, Publication of Unexamined Patent Application No. Hei 9-7733, and Publication of Unexamined Patent Application No. Hei 11-3765. However, a full study of the anomalous corrosion of the electrode tip caused by Pb has not been made, and, additionally, there is no description of a spark plug in which an electrode tip is prevented from peeling/falling off because of thermal stress especially when the electrode tip has been joined according to electric resistance welding in a ground electrode.
It is therefore an object of the present invention to provide a spark plug for an internal combustion engine in which a tip is prevented from falling off even if the tip for a ground electrode that is made of Ir or is chiefly composed of Ir is joined to an interlayer that has a specific thermal expansion coefficient and that is formed at a predetermined position of the ground electrode according to electric resistance welding, and, even if gasoline includes Pb, anomalous corrosion is not caused while maintaining great durability.
DISCLOSURE OF INVENTION
A spark plug for an internal combustion engine according to the present invention is characterized in that the spark plug comprises a center electrode comprising a basic body and a tip for the center electrode joined to the basic body and a ground electrode comprising a base, an interlayer formed at a predetermined position of the base, and a tip for the ground electrode joined to the surface of the interlayer, wherein the tip for the center electrode and the tip for the ground electrode are disposed to face each other, the tip for the center electrode and the tip for the ground electrode are each made of Ir or made of an alloy chiefly composed of Ir, a thermal expansion coefficient of the interlayer is between a thermal expansion coefficient of the base and a thermal expansion coefficient of the tip for the ground electrode, the basic body and the tip for the center electrode are joined to each other by laser welding, and the interlayer and the tip for the ground electrode are joined to each other by electric resistance welding.
The “spark plug for an internal combustion engine” may be a parallel electrode type of spark plug in which a tip for a center electrode joined to an end surface of a basic body and a tip for a ground electrode joined to an inner side surface of an end edge of a base are caused to face each other through an interlayer. Alternatively, the “spark plug for an internal combustion engine” may be a multi-electrode type of spark plug provided with a plurality of ground electrodes in which a tip for a center electrode joined to a side surface of an end edge of a basic body and a tip for a ground electrode joined to an end surface of a base through an interlayer are caused to face each other.
Herein, let the end surface or the side surface of the basic body and that of the base signify not only a flat surface but also a bottom surface of a concave part if the concave part is provided to fit and join the tip for the center electrode or the tip for the ground electrode. The material and structure of the members that constitute other parts of this spark plug, i.e., an insulator disposed to be in contact with the circumferential surface of the basic body, main metal fittings disposed to circumscribe the insulator, and terminal metal fittings consecutively connected to the center electrode and disposed on the other side of the insulator are not limited to specific ones, and can be general ones.
The “basic body” that constitutes the center electrode and the “base” that constitutes the ground electrode are each normally made of a Ni simplex or a Ni alloy such as Inconel. The “tip for a center electrode” joined to the basic body and the “tip for a ground electrode” joined to the base through the interlayer are each made of an Ir simplex or an Ir alloy, such as an Ir—Rh alloy and an Ir—Pt alloy, that is chiefly composed of Ir. The “interlayer” is made of an alloy constituting Ir, Ni, Rh, Pt, etc., which are included in the base and in the tip for the ground electrode, and the “thermal expansion coefficient” thereof is between that of the base and that of the tip for the ground electrode.
The tip for the center electrode is joined to the basic body by “laser welding”, so as to form a junction that is made of an alloy including the component of the basic body and the component of the tip for the center electrode, i.e., Ni, Ir, etc. The tip for the center electrode has satisfactory resistance to lead corrosion and has excellent durability and is enforced by a material largely composed of Ir, but, from the view point of mass production, it is preferable to make the basic body thereof of a material largely composed of Ni. If the tip and the basic body are made of such materials as above, a difference in the melting point between the tip and the basic body will become much greater than a case in which the tip mainly composed of Pt is welded to the basic body, which has been conventionally carried out.
Further, the center electrode is required to have a thin end in order to improve ignitability and reduce a discharge voltage, and, if a member made of a material that has a very great difference in the melting point to such a thinned end part is welded by electric resistance welding, a large protuberance will be generated, or buckling will occ

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