Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2002-04-11
2004-03-09
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S118000, C313S122000, C313S143000, C123S1690EB, C123S1690EC, C123S1690EL, C123S1690PH, C123S1690EB
Reexamination Certificate
active
06703770
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug for an internal combustion engine and particularly a spark plug adapted to be fixed in a plug hole of a cylinder head without the necessity of being screwed into the plug hole. The present invention further relates to a structure for attaching such a spark plug to a cylinder head of an internal combustion engine. Hereinafter, the terms “front” and “rear” are used to indicate opposite sides of the spark plug that are nearer to and further from a spark gap thereof, respectively.
A spark plug is used in an internal combustion engine for a vehicle so as to ignite an air-fuel mixture in a combustion chamber of the engine.
An example of the spark plug is shown in
FIG. 8. A
spark plug
300
comprises a central electrode
303
extending axially of the spark plug
300
, an insulator
302
surrounding the central electrode
303
and a tubular metallic shell
301
accommodating therein the insulator
302
. The metallic shell
301
includes a front end portion
312
provided with a male thread
312
a
on an outer surface thereof and a protrusion
312
b
on an inner surface thereof, and a hexagon head portion
313
. The insulator
302
has a step
302
a
engaged with the protrusion
312
b
via a packing
351
. The metallic shell
301
thus retains therein the insulator
302
so as to prevent the insulator
302
from slipping out of the metallic shell
301
from the front side, and at the same time, to prevent the passage of air. The spark plug
300
further comprises a ground electrode
304
having one end joined to a front end of the front end portion
312
and the other end opposed to the central electrode
303
with a spark gap g therebetween. Herein, the section at which the ground electrode
304
is joined to the metallic shell
1
is referred to as a joint section
317
.
The spark plug
300
is attached to a cylinder head of the engine. More specifically, the spark plug
300
is screwed into a plug hole of the cylinder head by a predetermined torque applied to the hexagon head portion
313
with the use of a wrench (not shown). Then, the spark plug
300
is fixed in the plug hole through engagement of the male thread
312
a
and a female thread of the plug hole.
In the above arrangement, when heat is applied to the central electrode
303
and the insulator
302
, the heat is transmitted from the step
302
a
to the protrusion
312
b
via the packing
351
, and then, transmitted from the metallic shell
301
to the cylinder head through the male thread
312
a
and the female thread.
However, if the spark plug
300
is screwed into the plug hole in such a way that the spark plug
300
forms an angle with respect to an axis of the plug hole, the male thread
312
a
and the female thread do not engage appropriately and causes so-called “galling”. As a result, the male thread
312
a
and the female thread are damaged. In the worst case, a passage leaking combustion gas from the combustion chamber is formed, and the front end portion
312
of the metallic shell
301
and the central electrode
303
are heated excessively to high temperature. Various problems, such as preignition and electrode fusing, are thus caused.
In addition, if the ground electrode
304
is positioned so as to prevent swirl from flowing into the spark gap g in the combustion chamber at compression stroke, the spark plug
300
only ignites a relatively rich mixture. For this reason, the spark plug
300
is generally attached to the cylinder head in consideration of a positional relationship between the joint section
317
and the flow direction of swirl so that the ground electrode
304
is placed in a proper position and orientation. However, with the spark plug
300
screwed into the plug hole, it is difficult to determine the position of the joint section
317
(i.e., to determine the position and orientation of the ground electrode
304
) in the combustion chamber.
SUMMARY OF THE INVENTION
In view of the foregoing, it is proposed to fix the spark plug in the plug hole by means of a plug fixing member without providing the spark plug and the plug hole with threads. This makes it possible to attach the spark plug to the cylinder head without any problems resulting from the engagement of the threads. In this case, some clearance needs to be created between the spark plug and the plug hole so that the spark plug is smoothly inserted into and detached from the plug hole. However, there arises a problem that heat cannot be easily transmitted from the spark plug to the cylinder head. More specifically, air remains in the clearance between the spark plug and the plug hole and functions as a thermal barrier. When the spark plug (especially, its central electrode and insulator) is subjected to high temperature owing to, e.g., combustion gas, the air in the clearance prevents the heat from being transmitted to the cylinder head. The spark plug is therefore excessively heated, and faces the problems of preignition, electrode fusing and the like.
It is therefore an object of the present invention to provide a spark plug adapted to be easily fixed in a plug hole of a cylinder head without the necessity of being screwed into the plug hole, and at the same time, having a good heat resistance so as to transmit heat rapidly to the cylinder head.
It is also an object of the present invention to provide a structure for attaching a spark plug to a cylinder head.
According to one aspect of the present invention, there is provided a spark plug attachment structure, comprising: a spark plug comprising a central electrode extending axially of the spark plug, an insulator surrounding the central electrode, and a metallic shell retaining therein the insulator and provided with a plug bearing surface and a front end portion on a front side of the plug bearing surface, the front end portion having a smooth cylindrical outer surface; a cylinder head defining therein a plug hole provided with a shoulder and a front end section having a smooth cylindrical inner surface, the spark plug having a clearance fit with the plug hole, with the plug bearing surface supported on the shoulder and the front end portion accommodated in the front end section, so as to provide a clearance between the cylindrical outer surface of the front end portion of the metallic shell and the cylindrical inner surface of the front end section of the plug hole; and a sealant provided in the clearance.
According to another aspect of the present invention, there is provided a spark plug for attachment to a cylinder head, the cylinder head defining therein a plug hole provided with a shoulder and a front end section having a smooth cylindrical inner surface, the spark plug comprising: a central electrode extending axially of the spark plug; an insulator surrounding the central electrode; and a metallic shell retaining therein the insulator and provided with a plug bearing surface and a front end portion on a front side of the plug bearing surface, the front end portion having a smooth cylindrical outer surface, the spark plug being adapted to have a clearance fit with the plug hole, with the plug bearing surface supported on the shoulder and the front end portion accommodated in the front end section, so as to provide a clearance between the cylindrical outer surface of the front end portion of the metallic shell and the cylindrical inner surface of the front end section of the plug hole to be provided with a sealant.
REFERENCES:
patent: 424027 (1890-03-01), Roberts
patent: 2693082 (1954-11-01), Arthur
patent: 5091672 (1992-02-01), Below
patent: 5101783 (1992-04-01), Car et al.
patent: 5186132 (1993-02-01), Runge
Iwata Kazuya
Kawamura Kazumi
Suzuki Akira
Suzuki Takahiro
Harper Holly
Patel Nimeshkumar D.
Sughrue & Mion, PLLC
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