Spark plug for internal combustion engine

A spark plug for an internal combustion engine with both sides of a surface, on a center electrode side, of an outer electrode and/or both side portions, on the top surface of the center electrode, form wedge-shaped projections. At least one surface having the wedge-shaped projection has no plane surface but is arcuate.

BACKGROUND OF THE INVENTION 
This invention relates to a spark plug for internal combustion engines for 
preventing quench operation of a flame nuclei due to a center electrode 
and an outer or ground electrode, thereby improving ignitability thereof. 
There have been provided many methods for producing the quench operation of 
the flame nuclei due to a center electrode and an outer electrode, 
particularly for preventing misfires under an engine running condition in 
which the ignitability is low, such as during idle speed. 
Within the prior art many such spark plug designs having various 
projections and surfaces are known. Typical are the annular surfaces shown 
in the inner and outer electrodes surfaces of U.S. Pat. No. 4,015,160, the 
annular ring and channel arrangement of U.S. Pat. No. 4,023,058 and the 
V-type ground electrode with a channel shown in U.S. Pat. No. 2,226,415. 
Additional prior art is shown in U.S. Pat. No. 3,970,885 which includes, 
in addition to various groove embodiments a projection provided on the 
ground electrode (element 36, FIG. 25). As set forth in that patent, the 
projection is disposed in the spark discharge area confronting the tip 
surface of the center electrode. The flame nuclei produced by the spark 
are rapidly spread out over the projection to facilitate growth of flame 
nuclei and easy propagation of flame. The hallmark of all these prior art 
devices is the modification of the ground electrode to limit the area of 
the grounded surface. However, it has been found that many deficiencies 
remain, in actual use, so these spark plugs do not satisfactorily perform, 
especially in an idle speed engine condition. 
One method shown in my co-pending application is that an end portion of the 
center electrode is tapered or that a groove is formed in the end surface 
of the center electrode or the outer electrode surface confronting the 
center electrode. However, according to the above-described methods it is 
impossible to obtain a sufficient improvement of ignitability. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a spark plug for 
effectively reducing the quench operation to the flame nuclei and for 
remarkably enhancing the ignitability thereof. 
The spark plug according to the present invention is constructed so that 
both end surfaces of the outer electrode on the center electrode and/or 
both side portions of the end surface of the center electrode which are 
directed to the outer electrode are projected to have wedge-shaped 
projections, and at least one surface having wedge-shaped projections has 
no plane surface. The present invention will be hereinafter described 
referring to the accompanied drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 a first embodiment according to the present 
invention is shown. Reference numeral 1 designates a metal body of a spark 
plug; 2, an insulative material; 3, a center electrode; and 4, an outer or 
ground electrode which is mounted on an end surface 11 of the metal body. 
A surface 41, on the center electrode side (inner side), of the outer 
electrode is concave and formed as a cylindrical surface and both sides 
thereof have wedge-shaped projections 42 and 43 to thereby form spark gaps 
with the center electrode. 
In the thus constructed spark plug, because of the electric concentration 
phenomenon of the end of the wedge-shaped projections 42 and 43 and the 
spark discharge distance thereof is short, a non-uniform electric field is 
generated and the strength of the electric field becomes large. Therefore, 
most of spark discharge occurs between the projections 42 and 43 and edge 
portions 31 and 32 confronting the projections, respectively. Flame nuclei 
are produced in regions a and b positioned on both sides of the spark gap. 
The flame nuclei thus produced regions a and b of both sides of the spark 
gap are not susceptible to the quench operation of the walls of the center 
and outer electrodes applied to the flame nuclei during the growing period 
of the flame nuclei. Accordingly, ignition is positively achieved to 
prevent misfires in the internal combustion engine. 
In contrast, in case where a spark discharge occurs in a central region d 
of the spark gap, a large amount of the quench effect of the end surface 
30 of the center electrode and the surface of the outer electrode on the 
center electrode is applied to the flame nuclei so that misfires are 
liable to result, especially under the engine running conditions such as 
idle speed where difficulty of ignition is most noticed due to low engine 
RPM. 
The above-described projections 42 and 43 must be shapened at their tips as 
wedges and at the same time, the tips must be positioned at outermost 
positions of a surface 41, confronting center electrode, of the outer 
electrode. A distance A in the axial direction of the center electrode 
from the center of the surface, confronting the center electrode, of the 
outer electrode to the tip end of the projection is less than about 0.3 
mm. An angle B formed by a contact or tangent surface contacting the inner 
surface of the outer electrode at the tip of the wedge-shaped projection 
and the end surface of the center electrode is in a range from 5.degree. 
to 15.degree.. An angle C of the tip portion of the projection is in a 
range from 80.degree. to 90.degree.. The surface 44, on the center 
electrode side, of the outer electrode is not plane between the 
wedge-shaped projections 42 and 43. The above described numerical 
limitations are determined or desired in view of the durability of the 
spark plug. For enhancement of the ignitability, desirably, a diameter of 
the center electrode is the same as a width of the outer electrode or a 
difference therebetween is less than 1.0 mm. The reason is that if the 
difference deviates, increasing from the range, a spark is generated in 
the central portion, on the electrode side, where the spark discharge is 
large so that a strong quench effect occurs. 
Experimental data showing differences between a spark plug according to the 
present invention as shown in FIG. 1 and that of the prior art in the 
ignitability will be hereinafter described. 
Referring now to FIG. 2, curve (I) is a spark plug in which a spark gap 
between a center electrode and an outer electrode is formed by plane 
surfaces. (II) is a spark plug in which a longitudinal groove is formed in 
a surface, on a center electrode side, of an outer electrode. (III) is a 
spark plug in which wedge-shaped projections are formed on both edge 
portions an outer electrode surface confronting a center electrode 
according to the present invention. The engine used is a four-cycle and 
four-cylinder type, having a total piston displacement volume of 1600 ml 
and the engine is already improved for exhaust emissions. FIG. 2 is a 
graph showing a relationship between the number of misfires incurred and 
CO concentration, of the exhaust emissions, air/fuel ratio when the engine 
is operated in idling. As obvious from a graph shown in FIG. 2, according 
to the present invention, it is possible to operate the engine with the 
lean air/fuel ratio (low CO concentrations) and at the same time, the 
number of the misfires is small. 
FIG. 3 shows a second embodiment of the present invention wherein 
wedge-shaped portions 51 and 52 of the top surface 50 of the center 
electrode 5 are projected. These portions are in alignment with side 
portions 63 and 64 of the outer electrode 6. Also, in the spark plug of 
this embodiment, the spark discharge is generated in regions a and b of 
both sides of the spark gap. As in the first embodiment, an enhancement of 
the ignitability is achieved. 
FIG. 4 shows a third embodiment of the present invention, wherein 
wedge-shaped projections 51, 52, 42 and 43 are formed on both the top end 
surface 50 of the center electrode 5, of outer electrode 4. Also, in this 
embodiment, a spark discharge occurs in region a and b on both sides of 
the spark gap. 
FIG. 5 shows a fourth embodiment in which a surface, on the center 
electrode side, of the outer electrode 7 is V-shaped in a concave manner 
and as a result, both sides 71 and 72 become wedge-shaped projections. 
FIGS. 6 and 7 show combinations between an outer electrode 4 having a 
wedge-shaped projection in both sides of a surface and on the center 
electrode a rectangular concave 8 and a groove 9, respectively. These 
modifications have the same spark discharge generated position and 
ignitability enhancement as the first embodiment. 
FIG. 8 shows an embodiment of a spark plug which is practical, according to 
the present invention. The spark plug according to the present invention 
will be practically used in this form. That is, the apexes of the 
wedge-shape are rounded by machining and the maximum depth of concave 
portion of the outer electrode 4 is very small. A V-shaped groove 9 is 
formed in the center electrode. Dimensions in mm used in FIG. 8 are only 
for reference. 
As mentioned above, a spark plug of the present invention is constructed so 
that at least wedge-shaped projections are formed on both side surfaces, 
on the center electrode, of the outer electrode or both side portions, on 
both sides of the outer electrode, of the end surface of the center 
electrode, as a result of which the spark discharge is positively 
generated in the both side regions of the spark gap. Therefore, the quench 
effect of the center electrode and the outer electrode is very small to 
thereby enhance the ignitability. 
It is apparent that modifications can be made without departing from the 
scope of this invention.