Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2001-11-27
2004-04-13
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
Reexamination Certificate
active
06720716
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a spark plug having noble metallic or comparable tips fixed to opposed center and ground electrodes so as to cause spark discharge between them. Furthermore, this invention relates to a method for manufacturing this spark plug.
This kind of spark plug is for example disclosed in the unexamined Japanese patent publication No. 52-36237, as schematically shown in
FIGS. 19A and 19B
. First, an arrangement shown in
FIG. 19A
comprises a noble metallic tip (i.e., center electrode tip) J
2
fixed to an apical end of a center electrode J
1
and a noble metallic tip (i.e., ground electrode tip) J
4
fixed to an apical end of a ground electrode J
3
. The center electrode tip J
2
and the ground electrode tip J
4
are opposed to each other along an axis of the center electrode J
1
.
Second, an arrangement shown in
FIG. 19B
comprises the center electrode tip J
2
fixed to an apical end of the center electrode J
1
and the ground electrode tip J
4
fixed to an apical end of the ground electrode J
3
. The center electrode tip J
2
and the ground electrode tip J
4
are opposed to each other along a line normal to the axis of the center electrode J
1
.
However, according to the arrangement shown in
FIG. 19A
, the ground electrode J
3
is long and therefore the heat releasing ability of ground electrode J
3
is insufficient. When subjected to the heat of combustion, the ground electrode J
3
will have a high temperature. This worsens the heat resistance and lowers the mechanical strength of the ground electrode J
3
. Furthermore, the noble metallic tip of the ground electrode will be worn hardly. Accordingly, it is difficult to assure a proper life of the ground electrode.
In general, the flow of gas mixture in a combustion chamber is normal to the axis of a plug (i.e., the axis of center electrode J
1
) as shown by an arrow Y in FIG.
19
B. Such a flow of gas mixture tends to forcibly shift a flame kernel toward the ground electrode J
3
when the flame kernel is produced in the discharge gap between the opposed tips J
2
and L
4
in response to an ignition.
In this case, according to the arrangement of
FIG. 19B
, the center electrode J
1
and the ground electrode J
3
are positioned close and parallel to each other. Hence, due to the flow of gas mixture, the flame kernel collides with the ground electrode J
3
and is cooled by the ground electrode J
3
. This worsens the ignitability of a spark plug.
Furthermore, this kind of spark plug is disclosed in the unexamined Japanese patent publication No. 61-45583, as schematically shown in FIG.
20
. An arrangement shown in
FIG. 20
comprises a ground electrode J
6
having a proximal portion fixed to a metallic housing J
5
and a distal portion extending toward an apex of center electrode J
7
so that an acute angle is formed between an axis of the distal portion of ground electrode J
6
and the axis of center electrode J
7
.
The arrangement shown in
FIG. 20
is advantageous in that the ground electrode is short in length and excellent in heat resistance and mechanical strength when compared with an ordinary ground electrode having a distal portion perpendicular to an axis of the center electrode and overhanging an apex of the center electrode (refer to FIG.
19
A).
However, according to the arrangement shown in
FIG. 20
, a ground electrode tip J
8
is provided within an area of an end surface J
61
of the distal portion of ground electrode J
6
. A distance J
10
between the distal portion of ground electrode J
6
and the center electrode tip J
9
must be short to form an appropriate discharge gap between the ground electrode tip J
8
and the center electrode tip J
9
.
Accordingly, when the flame kernel is shifted toward the ground electrode J
6
due to the flow of gas mixture, the flame kernel is cooled by the ground electrode J
6
. The ignitability of a spark plug becomes dissatisfactory. Furthermore, as a fixing portion of the ground electrode tip J
8
to the ground electrode J
6
is positioned close to the center electrode tip J
9
, a discharge may occur at the fixing portion of the ground electrode tip J
8
. Thus, the reliability of the tip fixing portion cannot be assured.
SUMMARY OF THE INVENTION
In view of the foregoing problems of the prior art, the present invention has an object to provide a spark plug having noble metallic or comparable tips fixed to opposed center and ground electrodes so as to cause spark discharge between them which is capable of shortening the length of the ground electrode and improving the heat resistance and mechanical strength, and also capable of preventing the discharge from occurring from a tip fixing portion of the ground electrode, thereby assuring the fixing reliability of the tip and realizing excellent ignitability.
To accomplish the above and other related objects, the present invention provides a first spark plug comprising a cylindrical metallic housing, a center electrode accommodated in the metallic housing with one end protruding and extending from one end of the metallic housing, a center electrode tip fixed to the one end of the center electrode and extending in the same direction as an axis of the center electrode, a ground electrode having a proximal portion fixed to the one end of the metallic housing and a distal portion extending toward the one end of the center electrode, and a columnar ground electrode tip fixed to an end surface of the distal portion of the ground electrode with an apical surface of the ground electrode tip opposed to an apical surface of the center electrode tip via a discharge gap. The first spark plug is characterized in that an acute angle is formed between an axis of the distal portion of the ground electrode and the axis of the center electrode when the ground electrode is projected on a virtual plane including the axis of the center electrode and a cross-sectional centroid of a proximal end of the ground electrode where the ground electrode is fixed to the metallic housing. The ground electrode tip extends along an axis crossing with the axis of the distal portion of the ground electrode, so that the ground electrode tip protrudes from the end surface of the distal portion of the ground electrode and extends toward the center electrode. And, an axis of the center electrode tip is in a cross or skew relationship with an axis of the ground electrode tip.
According to the first spark plug, the ground electrode having the proximal portion fixed to one end of the metallic housing has the distal portion extending toward one end of the center electrode so that an acute angle is formed between the axis of the distal portion of the ground electrode and the axis of the center electrode. Thus, it becomes possible to shorten the length of the ground electrode and improve the heat resistance and mechanical strength when compared with an ordinary ground electrode having a distal portion perpendicular to the axis of the center electrode and overhanging an apex of the center electrode.
Furthermore, according to the first spark plug, the columnar ground electrode tip protrudes from the end surface of the distal portion of the ground electrode and extends toward the center electrode in the direction crossing with the axis of the distal portion of the ground electrode. And, the axis of the center electrode tip is in a cross or skew relationship with the axis of the ground electrode tip. Thus, it becomes possible to provide an appropriate distance between the fixing portion of the ground electrode tip and the apical surface of the center electrode tip which is sufficiently longer than a distance between the apical surface of the ground electrode tip and the apical surface of the center electrode tip.
Namely, the distance from the apical surface of the ground electrode tip to the apical surface of the center electrode tip is sufficiently shorter than the distance from the fixing portion of the ground electrode tip to the apical surface of the center electrode tip. This ensures that the discharge only occurs between the apic
Hiramatsu Hiromi
Ishinada Teiji
Shibata Masamichi
Denso Corporation
Krishnan Sumati
Nixon & Peabody PC
O'Shea Sandra
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