Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2000-10-20
2003-04-15
Kim, Robert H. (Department: 2882)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S143000, C445S007000
Reexamination Certificate
active
06548945
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application is based on and incorporates herein by reference Japanese Patent Application No. 11-300208 filed Oct. 21, 1999.
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug that is inserted into a combustion chamber of an engine, and suitable for use in an engine that has a thick engine head for a higher engine output.
In conventional spark plugs, as shown in
FIG. 3
, a cylindrical insulator
301
surrounding a center electrode
305
is held inside a metallic fitting
303
in such a manner that one end
302
of the insulator
301
protrudes from one end
304
of the fitting
303
. A ground electrode
307
is fixed to the fitting
303
so that the ground electrode
307
faces the top end of the center electrode
305
protruding from the insulator
301
through a discharge gap
308
. This spark plug is threaded into a thread hole
310
formed in an engine head
309
that defines a combustion chamber
320
therein.
The fitting
303
is formed, on its outer peripheral surface, with a thread part
311
and a taper part
312
from the side of the end
304
. The thread part
311
is engaged with the thread hole
310
by turning the plug. The taper part
312
has a diameter gradually decreasing toward the thread part
311
. The taper part
312
contacts a taper surface
313
formed on the thread hole
310
to restrict leakage of gas from the combustion chamber
320
.
The fitting
303
is produced by a cold-forging and then machine-cut to form the taper part
312
and the thread part
311
in shape. The machine-cutting tends to produce traces of a cutting tool (tool mark) on the taper part surface, resulting in a high surface roughness. Further, the machine-cutting tends to produce deflection of longitudinal axes between the thread part
311
and the taper part
312
. As a result, sealing characteristics of the taper part
312
is lessened.
In high output-type engines, the engine head is made thicker to ensure more coolant flow for higher cooling efficiency. The fitting of the spark plug for such engines generally has a reach length RL of more than 12 mm. As known well in the art, the reach length is defined as a length from the end
304
of the fitting
303
to the point where the diameter of the taper part
312
is 14.8 mm. In the case of a spark plug having a longer reach length for high output type engines, it must have a sufficient sealing ability because the pressure in the combustion chamber
320
increases.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a spark plug that can provide a sufficient sealing ability for use in high output-type engines.
According to the present invention, a spark plug comprises a center electrode, a ground electrode and a tubular fitting for engagement with an engine head. The fitting has a thread part and a taper part from one end thereof toward another end thereof. The fitting is formed by cold-forging a low carbon steel so that the taper part has its cold-forged surface roughness of less than about 10 &mgr;m. The fitting is machine-cut to provide the thread part which deflects less than about 0.15 mm from the taper part with respect to longitudinal axes. This fitting is suitably used for spark plugs for high output engines in which a reach length of the fitting is at least 12 mm.
REFERENCES:
patent: 5088311 (1992-02-01), Inoue
patent: 5581145 (1996-12-01), Kato et al.
patent: 59-130391 (1984-09-01), None
patent: 60-133592 (1985-09-01), None
patent: 2000-48930 (2000-02-01), None
Denso Corporation
Gemmell Elizabeth
Kim Robert H.
Nixon & Vanderhye PC
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