Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1999-03-17
2001-10-30
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S142000, C123S1690EL, C123S1690EB
Reexamination Certificate
active
06310430
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug used in an internal combustion engine, such as an automobile engine.
2. Description of the Related Art
As shown in
FIG. 7
, a spark plug
200
is used to ignite an air-fuel mixture in an internal combustion engine, such as an automobile gasoline engine. The spark plug
200
is attached to a cylinder head SH of the engine by an attachment screw portion
201
a
formed on the peripheral surface of a metallic shell
201
. In the installed condition, a spark discharge gap g defined by a ground electrode
204
and a center electrode
203
is located within a combustion chamber K and is used to ignite an air-fuel mixture. The electrodes which define the spark discharge gap g are exposed to a combustion gas while the engine is operating, and thus rises to a considerably high temperature.
In recent years, as a power output of an internal combustion engine used in, for example, an automobile increases, an area occupied by an inlet valve and an exhaust valve within the combustion chamber has been increasing. Accordingly, the spark plug must become more compact.
Additionally, due to employment of a supercharger, a turbocharger, or the like, the temperature within the combustion chamber tends to increase more and more.
In order to attain a sufficiently long life of a spark plug under such severe working conditions, heat must be sufficiently radiated from the electrode portions. The heat is radiated from the spark plug in various passages. Particularly, a heat-radiation passage from an insulator
202
to the cylinder head SH via the attachment screw portion
201
a
exhibits a large heat flow and plays an important role in sufficient heat radiation. In popular spark plugs, the length of the attachment screw portion
201
a
(screw reach) is as high as 19 to 20 mm even in those of the so-called long reach type. Recently, the screw reach of a spark plug has been lengthened to improve the heat radiation performance of the spark plug.
As is evident from
FIG. 7
, when the screw reach of the spark plug
200
increases, the overall length of the insulator
202
must increase accordingly. The insulator
202
is generally manufactured by the steps of: compacting a powder of alumina or a similar material by a rubber press or a similar apparatus; lathing or machining the peripheral surface of the compacted body; and sintering the lathed compacted body. However, if the insulator
202
is excessively long, the compacted body may incur eccentric rotation during the lathing process, or the lathed compacted body may bend during the sintering process. These may potentially result in manufacturing deterioration because of unsatisfactory dimensional accuracy. The increase in the length of the insulator
202
leads to a weight increase of the spark plug
200
, potentially loosening a caulked portion
201
b
of the metallic shell
201
because of excessive inertia of the spark plug
200
upon exposure to vibration or impact.
To prevent excessive increase in the overall length of the insulator
202
, the length of a projecting portion
202
a
of the insulator
202
projecting from the tail end of the metallic shell
201
may conceivably be decreased. However, if the length of the projecting portion
202
a
becomes too short, a discharge may occur between a tail end portion of the center electrode
203
(terminal electrode
205
) and the metallic shell
201
along the surface of the projecting portion
202
a
. This discharge is called a flashover phenomenon. Accordingly, to avoid the flashover phenomenon, the projecting portion
202
a
cannot be shortened beyond a certain length, which is at least 25 mm according to the common standard in the art.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spark plug whose screw reach is relatively long to attain an excellent heat radiation, whose overall insulator length is not excessively long to achieve a good productivity, and which is less susceptible to flashover.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To achieve the above object, the present invention provides a spark plug comprising a center electrode, a metallic shell, a ground electrode, an insulator, and a metallic terminal.
The spark plug has a metallic shell including a tip-end side having an open tip end and a tail-end side having an open tail end. The metallic shell also has an attachment screw portion peripherally formed at the tip-end side thereof. A ground electrode attaches onto the tip-end side of the metallic shell. An insulator includes an axial through-hole formed therein and a tail end portion having a tail end and is disposed within the metallic shell such that the tail end portion of the insulator substantially projects from the tail end of the metallic shell. A center electrode has a tip end and is disposed within the through-hole and facing the ground electrode to define a spark discharge gap therebetween. A metallic terminal has a tail end and is disposed within the through-hole of the insulator such that the tail end of the metallic terminal is set back from the tail end of the insulator into the through-hole, the metallic terminal being electrically connected with the center electrode. The metallic terminal and the tail end of the metallic shell are separated with a distance for preventing an electrical discharge therebetween.
In the spark plug of the present invention, the attachment screw portion has a long screw reach of not less than 25 mm. Thus, the spark plug exhibits excellent heat radiation characteristics and a prolonged life even under severe operating conditions, such as high load and power output. Since the tail end of the metallic terminal is set back from the tail end of the insulator into the insulator through-hole, a surface discharge passage extending from the tail end of the metallic shell to the tail end of the metallic terminal is relatively long. Accordingly, the length of the tail end portion of the insulator projecting from the tail end of the metallic shell can be shortened, i.e., the overall length of the insulator can be shortened, while the flashover phenomenon is suppressed. As a result, the insulator becomes less likely to bend during the sintering process and improves productivity.
Also, the weight of the spark plug can be decreased accordingly. Thus, the metallic shell becomes less susceptible to loosening, which might otherwise result from vibration or impact.
When A represents an axial distance between the tail end of the insulator and the tail end of the metallic terminal located within the insulator through-hole (i.e., the depth of the setback of the tail end of the metallic terminal), and B represents an axial of the tail end portion of the insulator projecting from the tail end of the metallic shell, a length (A+B) is preferably not less than 20 mm. If the length (A+B) becomes less than 20 mm, the aforementioned flashover phenomenon may occur. Preferably, the length (A+B) is not less than 25 mm.
The above-mentioned depth A of the setback of the tail end of the metallic terminal can be appropriately adjusted according to the length B of the projecting tail end portion of the insulator so that the length (A+B) becomes not less than 20, preferably not less than 25 mm. When the length B of the projecting tail end portion of the insulator is shortened to shorten the overall length of the insulator, the depth A of the setback of the tail end of the metallic terminal is increased accordingly to establish a required length (A+B) of the insulation passage.
In the case of a spark plug used in an automobile gasoline engine, a diameter of the attachment screw portion is often set to a range between
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
NGK Spark Plug Co. Ltd.
Patel Nimeshkumar D.
Santiago Mariceli
LandOfFree
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