Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2001-06-22
2002-04-02
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S118000
Reexamination Certificate
active
06366008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a spark plug and a method of making the same.
2. Description of the Related Art
Conventionally, spark plugs have been known which have a sealing material layer composed mainly of talc filled in a space between the outer face of the insulator and the inner face of the metal shell so as to seal the space for checking gas leakage from a combustion chamber. The spark plug is exposed to high temperatures and high pressure because of influences by combustion gas generated in a combustion process within the combustion chamber, and sometimes served under severe circumstances receiving vibrations, and therefore the spark plug is demanded to meet needs for completing performance under such circumstances, and in particular it is desirable to sufficiently secure a sealing property in a sealing material.
Of late, direct injection of gasoline or lean-burn system has extensively been progressed as instruments for realizing high output and low fuel combustion. Such an engine is apt to enlarge a valve diameter or bring valve position nearly to a plug hole at a center of the cylinder head, and a demand for making the spark plug small size has been arisen to reduce the diameter thereof as possible. Practically, a distance between two parallel opposite faces of a tool engaging portion for attaching the engine for fitting such as a wrench is conventionally 16 mm or more, and it has been required to reduce from 16 mm to less than 16 mm as 14 mm. While satisfying the demand for miniaturization, it has been called for to provide a spark plug considered to have a sealing property (anti-looseness) and impact resistance.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide such a spark plug having a sealing material layer, enabling to secure an excellent sealing property under circumstances at high temperature, using powder of talc as a main component. In particular, it is an object to provide a miniaturized spark plug excellent in the impact resistance and the sealing property and a method of making the same.
For solving the above mentioned problems, the invention is to provide a spark plug having a center electrode, an insulator provided around the center electrode, a metal shell provided around the insulator and a ground electrode disposed in opposition to the center electrode so as to form a spark discharge gap, and having a sealing material composed mainly of talc filled in a space between the inner face of the metal shell and the outer face of the insulator so as to seal the space, characterized in that a packing density of the sealing material is 1.5 g/cm
3
to 3.0 g/cm
3
.
If the sealing material layer is filled in the space between the inner face of the metal shell and the outer face of the insulator such that a packing density of the sealing material is 1.5 g/cm
3
to 3.0 g/cm
3
, compressibility of the sealing material is remarkably improved and the sealing property of the sealing material layer is heightened. Thus, if being exposed to severe using conditions to generate load in the sealing material layer by vibration, pressure and others, air tightness can be well secured between the metal shell and the insulator. Especially in the spark plug, when taking, as a front side, a side where the spark discharge gap is formed, a rear-side circumferential part of the metal shell defines a press-fitting portion facing outside, deterioration is difficult to happen even at high temperature and high pressure depending on the above mentioned sealing material layer, and the press-fitting portion can be usefully controlled from loosening to heighten the sealing property.
In the spark plug, when a distance between two parallel opposite faces of the tool engaging portion (called as “opposite side sizes” hereafter) to be formed 6 in the metal shell for attaching the engine is W, W<16 mm, an inner diameter D
S
of a portion surrounding the sealing material layer in the metal shell satisfies 9.0 mm<D
S
<13.0 mm, and when an outer diameter of a portion surrounded by the sealing material layer in the insulator is D
I
, D
S
−D
I
>1.6 mm and D
I
≧7.0 mm, and it is desirable that the filling density of the sealing material is 1.5 g/cm
3
to 3.0 g/cm
3
.
In the small sized spark plug, the metal shell and the insulator are required to reduce diameter. Specifically, the opposite side sizes are required to be less than 16 mm. On the other hand, from the viewpoint of mechanical strength of the spark plug, the size reduction of the insulator is limited to keep enough strength. It is accordingly assumed that the sealing material layer is not furnished between the metal shell and the insulator, and such a spark plug is structured to have a large diameter of the insulator. However, the spark plug designed not to have a sealing material layer is involved with problems that the impact resistance is weak and the air-tightness is considerably lowered after giving impact. Such problems are remarkable in a spark plug, wherein the opposite side sizes of the tool engaging portion are less than 16 mm, because inevitable lack of thickness of the metal shell decreases strength thereof.
In the small sized spark plug of the opposite side sizes being less than 16 mm, the sizes of the insulator and the metal shell are determined as mentioned above (as in a second invention), and the sealing material layer is furnished between the metal shell and the insulator so as to moderate impact to the metal shell, effecting as a buffer, thereby enabling to realize a structure satisfying the impact resistance and the air-tightness. Especially, if furnished with the sealing material layer where the filling density is adjusted to range 1.5 g/cm
3
to 3.0 g/cm
3
difference in diameter between the inner face of the metal shell and the outer face of the insulator is shortened in comparison with conventional ones, and even in the small sized spark plug limiting the amount of the sealing material layer, the structure having excellent impact resistance and air-tightness can be realized.
A miniaturization of the spark plug reduces the difference in diameter between the inner face of the metal shell and the outer face of the insulator, and by making D
S
−D
I
>1.6 mm in said difference, it is possible to pack the sealing material layer uniformly and at a proper density (the filling density of 1.5 g/cm
3
to 3.0 g/cm
3
) in the gap between the metal shell and the insulator. If the difference of D
S
−D
I
is less than 1.6 mm, when filled with powder, the difference is too small to be filled with the sealing material layer On the other hand, when a shaped body (ring), which is preliminary shaped with powder, is disposed (filled) in the space between the metal shell and the insulator, the thickness of the ring should be less than 0.8 mm. However, the difficulties in forming thin ring brings may result in lower strength. Further, if the outer diameter D
I
of the insulator is less than 7.0 mm, insufficient strength thereof results in inferior function of the spark plug. On the contrary, if being D
I
>7.0 mm, enough strength can be given to the insulator.
By the way, in the spark plug as mentioned above since it is structurally difficult to make thickness of the metal shell (actually, thickness of the tool engaging portion) larger than necessary. Therefore, if the filling density is larger than 3.0 g/cm
3
, high pressing pressure should be applied when the sealing material layer is filled. Such high pressure may causes deformation of the tool engaging portion, which results in deviation from tolerance. Therefore, it is desirable that the filling density of the sealing material layer is 3.0 g/cm
3
in the above size determination (that is, W<16 mm, 9.0 mm<D
S
<13.0 mm, D
S
−D
I
>1.6 mm and D
I
>7.0 mm). Thus, if the filling density of the sealing material layer is 3.0 g/cm
3
or lower, even in the miniaturized spark plug difficult to make thickness of the metal shell large, the filling density can be
Fukushima Osamu
Sugimoto Makoto
Tanabe Hiroyuki
NGK Spark Plug Co. Ltd.
Patel Vip
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