Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2002-10-30
2004-09-28
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S142000, C313S144000, C313S118000, C445S007000
Reexamination Certificate
active
06798125
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
Ordinarily, in spark plugs, many proposals for enhancing resistance to wear due to sparks, in which a noble metal chip mainly made of Pt, Ir or the like is welded on the front end of an electrode to form an wear resistant portion have been made. Particularly, the wear resistant portion on the part of a center electrode which, in many cases, is set to be negative polarity at the time of discharging a spark is liable to be subjected to a strong attack of the spark and be worn down, so that a noble metal has increasingly been used for this portion.
On the other hand, in regard to an internal combustion engine, a lean-burn method has increasingly been adopted as seen in direct fuel injection engines, to comply with strict exhaust gas control regulations. Also, in order to obtain optimum combustion, a configuration in which the spark discharge gap of the spark plug protrudes further inside a combustion chamber than an ordinary spark plug has increasingly been adopted. As a result, the electrodes of the spark plug, in particular the ground electrode which is located further inside the combustion chamber, is subjected to severe high temperature conditions. Therefore, it is an important task to improve heat resistance and spark wear resistance of the ground electrode and, accordingly, not only formation of a noble metal spark emitting portion but also changing the material of the electrode itself to a metal that has a higher heat resistance has been attempted. For example, Inconel 600 (trade name; available from Inco Ltd., in the U.K.), a Ni-based heat resistant alloy, has ordinarily been adopted as the material for the ground electrode, but adoption of Inconel 601 which has higher proportions of Cr and Fe and to which Al is also added to have a further enhanced high temperature strength and high temperature oxidation resistance is under consideration.
Heretofore, there are many cases in which a noble metal wear resistant portion of a ground electrode is formed by joining a noble metal chip to the ground electrode by means of resistance welding. However, the present inventors' studies have found that, when the noble metal chip having a high melting point was resistance welded to a heat resistant alloy of higher grade such as Inconel 601 or the like, it was difficult to sufficiently ensure joining strength by resistance welding under the above-described severe use environment. Specifically, when the noble metal wear resistant portion is subjected to a severe heat cycle while the spark plug is in actual use, the noble metal wear resistant portion is peeled away from the ground electrode so that normal ignition can not be performed.
Heat resistant alloys such as Inconel 601, in which the proportion of corrosion resistance-improving components such as Cr, Al or the like are increased, intrinsically have the tendency to decrease in weldability in direct proportion to an increase in oxidation resistance. Therefore, it may seem at first that the cause of the above-described decrease of joining strength is insufficient melting due to such a decrease of weldability, but the present inventors' research has found that the intrinsic cause is not insufficient melting.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a spark plug in which, even when the ground electrode comprises a heat resistant alloy having an increased content of corrosion resistance improving component such as Cr, Al, the peeling resistance of a noble metal wear resistant portion welded to the ground electrode can sufficiently be ensured, whereupon the spark plug can be used for a long period of time even under severer working conditions.
A spark plug according to the present invention, in which a spark discharge gap formed by securing a noble metal wear resistant portion to the face of a ground electrode facing the front face of the center electrode, is characterized in that a portion containing at least the sides of the ground electrode comprises a Ni alloy containing from 21% by mass to 25% by mass of Cr, from 1% by mass to 2% by mass of Al, from 7% by mass to 20% by mass of Fe and from 58% by mass to 71% by mass of Ni, the noble metal wear resistant portion is joined to the side face of the ground electrode via a welding portion, &Dgr;&agr;≡&agr;2−&agr;1 is adjusted to be 4.55×10
−6
/K or less, wherein &agr;1 represents the linear expansion coefficient at 800 K of the noble metal constituting the noble metal wear resistant portion; and &agr;2 represents the linear expansion coefficient of the Ni alloy constituting a portion comprising the above-described side face of the ground electrode, and, further, the outer diameter of the noble metal wear resistant portion, which is defined as the diameter of the circle having the same surface area as that of an orthogonal projection of the noble metal wear resistant portion on a plane perpendicular to a center axis line of the center electrode, is from 0.6 mm to 1.5 mm.
In the above configuration, a raw material (hereinafter referred to as an electrode base material) composing a side face portion of the ground electrode which is subjected to high temperature particularly while the spark plug is in actual use is made to be a Ni alloy having the above-described composition which is excellent in high-temperature resistance and anti-oxidation property to a further extent than Inconel 600 or the like which has ordinarily been used. As a result, durability of the spark plug according to the present invention in a high temperature environment is enhanced and defects thereof such as corrosion, breakage or the like are less liable to occur.
Further, the present inventors have studied in detail factors affecting peeling resistance in the case where the noble metal wear resistant portion is joined to the electrode base material having the above composition, and it was clearly found that peeling is caused by a difference in the linear expansion coefficients of the noble metal constituting the wear resistant portion and the electrode base material rather than by insufficient melting due to decreased weldability of the electrode base material or the like. Accordingly, as a result of still further study it has been found that the peeling resistance of the noble metal wear resistant portion on the ground electrode can be enhanced to a great extent by adjusting &Dgr;&agr;≡&agr;2−&agr;1 to be 4.55×10
−6
/K or less, wherein &agr;1 represents the linear expansion coefficient at 800 K of the noble metal constituting the noble metal wear resistant portion; and &agr;2 represents the linear expansion coefficient at 800 K of the electrode base material.
However, it has been found that, under a use environment in which attainable temperature of the ground electrode is higher than ordinary, for example, during high speed or high load driving with a lean-burn or direct fuel injection engine, it is difficult to sufficiently ensure peeling resistance of the noble metal wear resistant portion only by adjusting such a difference &Dgr;&agr; in linear expansion coefficients. To deal with this, the outer diameter of the noble metal wear resistant portion on the ground electrode as defined above is made to be from 0.6 mm to 1.5 mm, by which the peeling resistance can further be enhanced and the durability of the noble metal wear resistant portion can sufficiently be ensured even under such a severe use environment as described above.
As for the composition of the Ni alloy constituting the electrode base material, the Cr content can be from 21% by mass to 25% by mass. When the Cr content is less than 21% by mass, it is difficult to ensure desired high temperature oxidation resistance and high temperature strength. On the other hand, when the Cr content is over 25% by mass, ductility of the material is deteriorated whereupon impact resistance is deterio
Sugiura Yoshitaka
Sugiyama Yoshihisa
NGK Spark Plug Co. Ltd.
Patel Ashok
Roy Sikha
Sughrue & Mion, PLLC
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