Electric lamp and discharge devices – Spark plugs
Reexamination Certificate
2002-02-26
2003-08-05
Wong, Don (Department: 2821)
Electric lamp and discharge devices
Spark plugs
C445S007000
Reexamination Certificate
active
06603244
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug used for an internal combustion engine. Moreover, the present invention relates to a method of producing the spark plug.
A spark plug is used for igniting an internal combustion engine of a motor vehicle and the like. For increasing engine output and reducing fuel consumption, temperature in a combustion chamber of the engine is likely to increase. For improving ignitability, a discharge portion of the spark plug is likely to protrude into the combustion chamber of the engine. Such type of engine is more and more increased in number. Under the above circumstance, the discharge portion of the spark plug is exposed to high temperature, thus causing failures (attributable to spark) such as wear, breakage and the like of the ground electrode.
Furthermore, as part of maintenance-free measures of the automotive engine, recently, durability of the spark plug is required with no replacement, for consecutive vehicle drive not less than 160,000 km or not less than 240,000 km (cumulative). To meet this requirement, the spark plug has the following metal: The central electrode and/or the ground electrode is made of a material having high heat conductivity such as Cu, Cu alloy, and the like (having heat conductivity equivalent to that of the former two). The material (hereinafter referred to as “Cu core and the like”) is coated with Ni alloy. The Cu core and the like and the Ni alloy coating contribute to reduction in temperature, to thereby secure durability of the central electrode and/or the ground electrode.
Forming the Cu core and the like in the ground electrode for improving durability, however, reduces the temperature of the ground electrode attributable to thermal conduction. Although durability is secured, the ground electrode will cause reduction in temperature at high engine speed. Moreover, such reduction in temperature is seen even at intermediate engine speed and at low engine speed. Contacting the ground electrode that is reduced in temperature, flame kernel (generated during spark plug discharge) is likely to be extinguished. In other words, ignitability is deteriorated.
Moreover, another method for improving the durability of the ground electrode is taken into account. More specifically, use of another material for the ground electrode, which material is higher in heat resistance (strength). Included in the another material is, for example, a super heat resisting alloy and the like. Use of such another material, however, involves increase in ordinary temperature resistance (strength), and thereby involves deterioration in plastic machinability (bendability). Therefore, when the ground electrode (made of the another material) is bent, for example, in such a manner that a side face of the ground electrode faces the central electrode, plastic machinability (bendability) of the ground electrode is of difficulty. The difficulty in plastic machinability (bendability) is responsible for reduction in productivity.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a spark plug that is used for an internal combustion engine at high engine speed, and that is excellent in durability and ignitability.
It is another object of the present invention to provide a method of producing the above mentioned spark plug.
According to a first aspect of the present invention, there is provided a spark plug comprising: a central electrode; an insulator surrounding the central electrode radially; a metallic shell surrounding the insulator radially; and a ground electrode having a first end connected to the metallic shell and a second end defining a side face. The ground electrode is so bent that the side face of the second end faces the central electrode. The ground electrode contains: a nickel in a range from 58% to 71% by weight, a chromium in a range from 21% to 25% by weight, an iron in a range from 7% to 20% by weight, and an aluminum in a range from 1% to 2% by weight. The ground electrode has a Vickers hardness in a range from Hv 140 to Hv 220 measured through a Vickers hardness test specified in Japanese Industrial Standard Z2244. A load 9.8 N is applied to the ground electrode in the Vickers hardness test.
According to a second aspect of the present invention, there is provided a method of producing a spark plug having a central electrode; an insulator surrounding the central electrode radially; a metallic shell surrounding the insulator radially; and a ground electrode having a first end connected to the metallic shell and a second end defining a side face. The ground electrode is so bent that the side face of the second end faces the central electrode. The method comprises the following sequential operations of: preparing the ground electrode composed of an alloy material, annealing the alloy material of the ground electrode at an annealing temperature not lower than 800° C., so as to allow the alloy material of the ground electrode to have a Vickers hardness in a range from Hv 140 to Hv 220 measured through a Vickers hardness test specified in Japanese Industrial Standard Z2244; welding the ground electrode to the metallic shell; and bending the ground electrode in such a manner as to allow the side face of the second end of the ground electrode to face the central electrode. The alloy material composing the ground electrode at the preparation contains: a nickel in a range from 58% to 71% by weight, a chromium in a range from 21% to 25% by weight, an iron in a range from 7% to 20% by weight, and an aluminum in a range from 1% to 2% by weight. A load 9.8 N is applied to the ground electrode in the Vickers hardness test.
A spark plug under the present invention has a ground electrode composed of an alloy containing Ni 58% to 71% by weight, Cr 21% to 25% by weight, Fe 7% to 20% by weight, and Al 1% to 2% by weight. Thereby, the ground electrode secures sufficient durability at high temperature. The thus obtained ground electrode is preferably used for a combustion chamber at high temperature caused by high engine speed of the internal combustion engine.
Moreover, conventionally, improvement in high temperature durability (namely, heat resistance, oxidation resistance, and the like) occasionally deteriorates plastic machinability (bendability) of the alloy. Vickers hardness (Hv 140 to Hv 220) of the ground electrode under the present invention, however, features a good plastic machinability (bendability). Therefore, even when the ground electrode is bent in such a manner that a side face of the ground electrode faces a central electrode, plastic machining (bending) of the ground electrode is easy. The easy plastic machining (bending) is expected to contribute to improvement in productivity.
Vickers hardness higher than Hv 220 makes the alloy (composing the ground electrode) too hard, to thereby make it unfavorably difficult to bend the ground electrode. Moreover, annealing is carried out for improving bendability. In this case, however, annealing the ground electrode to such an extent as higher than Hv 220 in hardness requires annealing condition for about 800° C. This temperature causes deposition of carbide on the grain boundary, to thereby deteriorate toughness. As a result, the ground electrode may cause minor cracks and the like during bending operation. With the cracks, the electrode may cause an unfavorable breakage attributable to vibrations and the like caused when the spark plug is used.
Contrary to the above, obtaining Vickers hardness lower than Hv 140 requires the annealing temperature as high as 1150° C. This temperature is responsible for remarkable grain growth, to thereby cause grain corrosion attributable to S, Pb and the like. As a result, the ground electrode is likely to be broken. Moreover, some of the after-mentioned methods of producing the spark plug are not capable of producing the ground electrode with ease.
The ground electrode is more preferably has Vickers hardness in a range from Hv 160 to Hv 200.
Obtaining the above Vickers hardness (Hv 140 to
Kumagai Kenichi
Sugiyama Yoshihisa
Suzuki Tomoharu
A Minh D
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
Wong Don
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