Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2001-06-28
2003-09-09
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S143000, C123S1690EB
Reexamination Certificate
active
06617769
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug which can be mounted into a plug hole formed in a cylinder head without need of screwing into the plug hole, and a structure for mounting the spark plug.
A spark plug
10
used as an ignition source to a fuel-air mixture in the internal combustion engine comprises a shaft-shaped central electrode
3
, an insulator
2
for surrounding the central electrode
3
, and a main metal shell
1
for holding the insulator
2
, as shown in
FIG. 1
, for example. The main metal shell
1
has a flange portion
11
with a plug bearing surface
13
formed on the tip side (where a spark discharge gap g is formed) and a top end portion
12
extending from the plug bearing surface
13
to the top end in an axial direction O. On a top end face of this top end portion
12
, an ground electrode
4
for forming a spark discharge gap g in cooperation with the central electrode
3
is connected to form a connecting portion (ground electrode connecting portion)
17
. Further, an external thread
12
a
is formed around an outer circumference of this top end portion
12
. Also, a caulking portion
16
, a hexagonal portion
15
and a caulking groove portion
14
are formed on the rear end side of the flange portion
11
, with the external thread
12
a
formed around the outer circumference of the top end portion
12
.
More particularly, the insulator
2
comprises a head portion
2
b
formed with a corrugation
2
c
, a diameter portion
2
e
having the greatest diameter, a middle trunk portion
2
g
having a smaller diameter than the diameter portion
2
e
, and a leg length portion
2
i
located at the top-most end. Generally, an engagement face
2
h
, which is a connecting portion between the leg length portion
2
i
and the middle trunk portion
2
g
, is engaged with a main metal shell side engaging portion
18
(more particularly a face for engaging the insulator) protruded inwardly into an inner circumferential face of the main metal shell
1
via a packing
51
. The engagement face
2
h
of this insulator
2
is intimately contacted with the main metal shell side engaging portion
18
via the packing
51
to retain air-tightness between the insulator
2
and the main metal shell
1
, and makes a part of a main passage for diffusing a heat flowing into the central electrode
3
and the leg length portion
2
i
of the insulator
2
, through the insulator
2
, the packing
51
, the metal shell side engaging portion
18
, and the top end portion
12
(external thread
12
a
) to a cylinder head as will be described later.
In addition, this spark plug
10
is mounted on the cylinder head of the internal combustion engine, when in use. When the spark plug
10
is mounted on the cylinder head, the spark plug
10
is inserted into the plug hole formed in the cylinder head, and the external thread
12
a
formed at the top end portion
12
of the main metal shell
1
is screwed into an internal thread formed in the plug hole at a predetermined torque, with a plug wrench set on the hexagonal portion
15
of the main metal shell
1
.
When the spark plug
10
is correctly rotated around a central axis of the plug hole having the internal thread therein, the spark plug
10
can be fastened into the plug hole without problem. However, if the spark plug
10
is rotated in a state of being slanted from the central axis of the plug hole, the external thread
12
a
is not correctly mated with the internal thread, resulting in a so-called gall state. Therefore, the external thread
12
a
or the internal thread may be damaged, and in extreme cases, a blow-by of a combustion gas from the combustion chamber is incurred so that the top end portion
12
of the main metal shell
1
or the central electrode
3
is heated to high temperatures, with the risk of possibly giving rise to a preignition or melting away the electrode.
SUMMARY OF THE INVENTION
In order to prevent the possible risk from generating, it can be conceived to construct a spark plug mounting structure as follow. The spark plug
10
(top end portion
12
) may be inserted with a fine clearance into the plug hole, while the external thread
12
a
at the top end portion
12
of the main metal shell
1
and the internal thread in the plug hole are not formed. The spark plug
10
is then secured to the cylinder head by a plug fixture separately.
However, since there is a clearance between the top end portion
12
and an inner wall face of the plug hole with a constitution having no external thread
12
at the top end portion
12
, the diffusion of the heat becomes difficult. Therefore, the temperature of the central electrode
3
is prone to rise, leading to the risk of giving rise to preignition or melting away the electrode.
This invention has been achieved in the light of the aforementioned problems, and it is an object of the invention to provide a spark plug and a spark plug mounting structure for mounting the spark plug into a plug hole formed in a cylinder head, in which the spark plug is easily mounted in the plug hole and can diffuse the heat excellently to have an improved heat resistance.
The above object can be achieved by, according to the present invention, a spark plug mounting structure comprising:
a spark plug including,
a central electrode extending in an axial direction thereof,
an insulator having a tip end portion which surrounds the outer circumferential face of the central electrode, and
a main metal shell holding the insulator, the main metal shell having a flange portion with a plug bearing surface at a tip end of the flange portion, and the main metal shell having a top end portion which extends in an axial direction thereof from the plug bearing surface to a tip end side of the main metal shell; and
a cylinder head having plug hole with a top end couterpart which receives the top end portion of the main metal shell,
wherein the top end portion of the main metal shell is formed into a substantially cylindrical shape having an outer circumferential face on which no external thread is provided, and the top end portion of the main metal shell and the top end counter part of the plug hole satisfies a following relation,
&phgr;
D−&phgr;d≦
0.15
where &phgr;d (unit: mm) defines the outer diameter of the top end portion, and &phgr;D (unit: mm) denotes the inner diameter of the top end counterpart.
Also, according to the invention, there is provided a spark plug adapted to be mounted in a plug hole formed in a cylinder head, the spark plug comprising;
a central electrode extending in an axial direction thereof;
an insulator having a tip end portion which surrounds the outer circumferential face of the central electrode; and
a main metal shell holding the insulator, the main metal shell having a flange portion with a plug bearing surface at a tip end of the flange portion, and the main metal shell having a top end portion which extends in an axial direction thereof from the plug bearing surface to a tip end side of the main metal shell,
wherein the top end portion of the main metal shell is formed into a substantially cylindrical shape having an outer circumferential face on which no external thread is provided, and the outer diameter of the top end portion satisfying a following relation,
&phgr;D−&phgr;d≦
0.15
where &phgr;d (unit: mm) defines the outer diameter of the top end portion of the main metal shell, and &phgr;D (unit: mm) denotes the inner diameter of a top end counterpart of the plug hole where is in confront with the top end portion.
With such a constitution of the invention, the top end portion of the main metal shell constituting the spark plug is formed substantially cylindrical without having a plug attaching external thread on the outer circumference. Thereby, when mounted, the spark plug is only inserted with a fine clearance into the plug hole, without needs of the screwing operation employing a plug wrench. In particular, the DOHC engines lately developed have mostly the suction and exhaust valves of large area, and the plug h
Guharay Karabi
Morgan & Lewis & Bockius, LLP
NGK Spark Plug Co. Ltd.
Patel Nimeshkumar D.
LandOfFree
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