Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1998-12-09
2001-04-10
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S144000, C123S1690EL, C445S007000
Reexamination Certificate
active
06215234
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of Japanese Patent Applications No. 9-369059, filed on Dec. 27, 1997, No. 9-369155 filed on Dec. 26, 1997, No. 10-320863 filed on Nov. 11, 1998, and No. 10-320864, filed on Nov. 11, 1998, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a spark plug for an internal combustion engine suitable for a vehicle, a cogenerator, a gas transporting pump, or the like.
2. Description of the Related Art
A large engine, which is applied to a large generator using gaseous fuel (cogenerator), a gas transporting pump, or the like, usually includes a large spark plug having, for example, a housing outer diameter of 18 mm, an insulator head portion diameter of 14 mm, a central electrode diameter of 2.5 mm, and an entire length of 115 mm. This kind of large engine is always operated without intervals. Therefore, the large spark plug for the engine is especially required to have long life in order to improve a maintenance property and in order to reduce plug exchanging cost.
It is conceivable, to secure the long life of the spark plug, that a spark gap is prevented from being enlarged during spark discharge so as not to increase a required voltage necessary for the spark discharge. Therefore, as shown in
FIG. 1
, discharge members
25
made of a noble metal material having favorable expendability resistance are generally disposed on respective front ends of a central electrode
2
and an earth electrode
3
, thereby preventing the spark gap from being enlarged.
The discharge members
25
having large areas, however, can lowers a spark discharge ignition property. The reason is considered as follows. That is, when the discharge members
25
have large areas to have specific calorific capacities, flame cores
7
produced by the spark discharge contact the discharge members
25
and are immediately cooled down. That is, the flame cores
7
are quenched without growing, resulting in deterioration of the spark discharge ignition property.
To solve this problem, JP-A-4-242090 proposes a noble metal chip as a discharge member that has a groove on a discharge face. However, in the spark plug, because the electrical insulating propery of the spark plug is so high that sparks are not liable to be produced, it is difficult to sufficiently comply with the above-described requirements of a large engine. In addition, it is easy for cracks to develop in the the noble metal chip due to thermal stress produced by a thermal expansion difference between the noble metal chip and the central electrode, so that it is easily separated from the central electrode, shortening the spark plug life.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems. An object of the present invention is to provide a long-life spark plug suitable for an internal combustion engine and capable of providing a low required voltage for generating sparks and a high ignition property. Another object of the present invention is to provide a long-life spark plug including a noble metal chip with high strength and toughness.
According to a first aspect of the present invention, a spark plug has, an insulator, a central electrode disposed in a through hole of the insulator, a housing, and an earth electrode fixed to the housing and facing the central electrode to define a spark gap G therebetween. The spark gap G is within a range of0.2 mm≦G<0.4 mm, and a space which is larger than a sphere having a diameter equal to or larger than 0.4 mm is provided in the spark gap G.
Therefore, the space can hold a flame core having a diameter equal to or larger than 0.4 mm not to quench the flame core, so that spark discharge is effectively performed, resulting in high ignition property. This is because the flame core which grows to have a diameter equal to or larger than 0.4 mm is not easily quenched by contacting the electrodes. Because the spark gap G is in the range of 0.2 mm≦G<0.4 mm, a required voltage for producing sparks is relatively low.
Preferably, a noble metal chip is disposed on one of the central electrode and the earth electrode to face another one of the central electrode and the earth electrode with the spark gap G therebetween. More preferably, the noble metal chip has a groove for forming the space. When the spark plug dispenses with the noble metal chip, one of the central electrode and the earth electrode can have a groove for forming the space.
According to another aspect of the present invention, a noble metal chip is bonded to one of the central electrode and the earth electrode to face another one of the central electrode and the earth electrode, and the noble metal chip includes a plurality of crystal grains having an average grain diameter N &mgr;m in an axial direction thereof and an average grain diameter K &mgr;m in a radial direction thereof. The average grain diameters N, K fulfill relationships of N>K and (N+K)2≦50 &mgr;m.
Therefore, the noble metal chip can have high strength and toughness with respect to thermal stress in the radial direction thereof which is produced by a thermal expansion difference between the noble metal chip and the electrode. As a result, cracks capable of causing separation of the noble metal chip are prevented from being produced in the noble metal chip, resulting in spark plug long-life. Even when the noble metal chip has a diameter equal to or larger than 1.5 mm not to be wasted by spark discharge, cracks and separation can be prevented from occurring to the noble metal chip due to the crystal grain structure described above.
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patent: 5202601 (1993-04-01), Takamura et al.
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patent: 5514929 (1996-05-01), Kawamura
patent: 5998912 (1999-12-01), Schwab
patent: 6046532 (2000-04-01), Matsutani et al.
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Abe Nobuo
Kawashima Yasushi
Day Michael H.
Denso Corporation
Haynes Mack
Nixon & Vanderhye PC
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