Spark plug electrode for use in internal combustion engine

In a spark plug electrode in which an alloyed tip is secured to a firing end of one of opposed electrodes forming a spark gap therebetween, the tip has a spark-erosion resistant noble metal component, and the noble metal component varies as a weight percentage of the alloyed tip in an axial direction of the electrode depending on a magnitude of spark discharges to which a specified portion of the tip is subjected between the opposed electrodes, wherein a largest weight percentage of the noble metal component is provided at areas on the electrode subjected to relatively high magnitude spark discharges, and wherein a smallest weight percentage of the noble metal component is provided at areas on the electrode subjected to relatively low magnitude spark discharges.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a spark plug electrode in which a spark-erosion 
resistant noble metal containing tip is secured to a firing end of a 
center or ground electrode, and particularly to an improved spark plug 
electrode in which a noble metal component of the tip is altered depending 
on a total magnitude of the spark discharges to which a specified portion 
of the tip is subjected. 
2. Description of Prior Art 
With the recent demand of enhancing a spark-erosion resistant property of 
an electrode for use in an internal combustion engine, Japanese Patent 
Publication No. 62-31797 introduces a technique in which annular noble 
metal tips 103, 104 in turn cover firing ends of center electrode metals 
101, 102 of multi-polarity type and semi-creeping discharge type spark 
plugs 100, 100a as shown in FIGS. 11 and 
Due to the positional relationship between the center electrode metals 101, 
102 and ground electrode 105, 106, the tips 103, 104 tend to be locally 
eroded in a manner as shown at a left half in FIGS. 11 and 12. The spark 
erosion makes spark gaps 107, 108 unacceptably greater to exhaust service 
life of the spark plug, and an expensive noble metal component of the tip 
partially remains uneroded so as to make the technique uneconomical. 
In order to avoid the uneconomical disadvantage, it is considered to 
previously leave out an uneroded part of the noble metal tip as shown at 
the left half 109 in FIG. 13a. 
However, the center electrode metal 101 is spark eroded significantly as 
spark discharges repeatedly occur between the electrodes as shown in FIGS. 
13b through 13d. This makes the service life of the spark plug 110 
increasingly shorter than that of the spark plugs 100, 100a. 
This is because the spark discharges presumably occur to the portion 109 of 
the electrode 101 in which the noble metal tip 103 is left out although a 
total magnitude of spark discharges is substantially limited when compared 
to the portion to which the noble metal tip 103 is subjected. 
Therefore, it is an object of the invention to provide a spark plug 
electrode for use in an internal combustion engine which is capable of 
controlling a rapid spark erosion of an electrode to which a limited 
magnitude of spark discharges is subjected while maintaining an economical 
advantage by substantially reducing a quantity of noble metal component 
used for a portion of the electrode to which the limited magnitude of 
spark discharges is subjected. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a spark plug electrode in 
which an alloyed tip is secured to a firing end of one of opposed 
electrodes which forms a spark gap therebetween. The alloyed tip has a 
noble metal component varied in an axial direction of the electrode 
depending on a magnitude of spark discharges to which a specified portion 
of the tip is subjected between the opposed electrodes. 
In this instance, it is desirable to predetermine that a difference between 
an upper limit and lower limit of the noble metal component of the alloyed 
tip is 10% by weight or more. 
According to the invention, there is provided a spark plug electrode in 
which an alloyed tip is secured to a firing end of one of opposed 
electrodes which forms a spark gap therebetween. The alloyed tip has a 
noble metal component varied in an axial direction of the electrode 
depending on a quantity of spark erosion to which a specified portion of 
the tip is subjected between the opposed electrodes. 
In this instance, it is also desirable to predetermine that a difference 
between an upper limit and lower limit of the noble metal component of the 
alloyed tip is 10% by weight or more. Further, the electrode has a clad 
metal and a heat-conductive core concentrically embedded in the clad 
metal. The clad metal is made of a corrosion and erosion resistant nickel 
alloy containing 15.0 wt % Cr and 8.0 wt % Fe, and the heat-conductive 
core is made of silver or copper. 
With the alloyed tip having the noble metal component varied in the axial 
direction of the electrode depending on a magnitude of spark discharges to 
which the specified portion of the tip is subjected, it is possible to 
increase the noble metal component of the tip to the portion to which 
increasing spark discharges are subjected. Further, it is also possible to 
decrease the noble metal component of the tip to the portion to which the 
limited spark discharges are subjected. This ensures that there is no 
substantial difference between a quantity of spark erosion to which a low 
noble metal component of the tip is subjected and a quantity to which a 
high noble metal component of the tip is subjected. This apparently 
insures a uniform quantity of spark erosion all through the alloyed tip 
secured to an entire area of the firing end of the electrode, thus 
enabling an extended service life of the spark plug with a minimum 
quantity of noble metal. 
With the alloyed tip having the noble metal component varied in the axial 
direction of the electrode depending on a quantity of spark erosion to 
which the specified portion of the tip is subjected, it is possible to 
increase the noble metal component of the tip to the portion to which 
increasing spark erosion is subjected. Further, it is also possible to 
decrease the noble metal component of the tip to the portion to which the 
limited spark erosion is subjected. This ensures that there is no 
substantial difference between a quantity of spark erosion to which a low 
noble metal component of the tip is subjected and a quantity to which a 
high noble metal component of the tip is subjected. This apparently 
insures a uniform quantity of spark erosion all through the alloyed tip 
secured to an entire area of the firing end of the electrode, thus 
enabling an extended service life of the spark plug with a minimum 
quantity of noble metal. 
These and other objects and advantages of the invention will be apparent 
upon reference to the following specification, attendant claims and 
drawings.

DETAILED DESCRIPTION OF THE EMBODIMENT OF THE INVENTION 
Referring first to FIGS. 1 and 2 which show a lower portion of a circular 
discharge type spark plug 1 for use in an internal combustion engine 
according to a first embodiment of the invention, the spark plug 1 has a 
tubular insulator 2, and a metallic shell 3 in which the insulator 2 is 
placed. From a lower end of the metallic shell 3, a ring-shaped Ground 
electrode 4 is integrally extended. In a manner to be surrounded by an 
inner wall 4a of the ring-shaped Ground electrode 4, a lower portion of a 
center electrode 5 is arranged which is concentrically placed in the 
insulator 2. The insulator 2 is made of a sintered ceramic body such as, 
for example, alumina (Al.sub.2 O.sub.3) with its inner space as an axial 
bore 6. The insulator 2 is further engaged against a shouldered inner wall 
3a of the metallic shell 3 through a packing 7. 
Meanwhile, the metallic shell 3 is made of an electrically conductive metal 
such as a low carbon steel or the like so as to form a housing of the 
circular discharge type spark plug 1. To an outer surface of the metallic 
shell 3, a male thread portion 8 is provided to secure the metallic shell 
3 to a cylinder head (not shown) of the internal combustion engine. A 
gasket 10 is provided between barrel portion 9 of the metallic shell and 
the cylinder head so as to air-tightly seal a combustion chamber (not 
shown) of the internal combustion engine. 
The lower end of the ground electrode 4 is arranged such as to oppose that 
of the center electrode 5, and being terminated short of the combustion 
chamber. Between the inner wall 4a of the ground electrode 4 and an outer 
surface of the center electrode 5, there is provided a spark gap G (e.g. 
1.0 mm). 
The electrode 5 has an elongated metal column 11 and a noble metal tip 13 
secured to a firing end 12 of the center electrode 5 which establishes a 
spark discharge against the inner wall 4a of the ground electrode 4. The 
metal column 11 of the center electrode 5 has a clad metal 14 and a 
heat-conductive core 15 concentrically embedded in the clad metal 14. The 
clad metal 14 has a corrosion and erosion resistant nickel alloy 
containing 15.0 wt % Cr and 8.0 wt % Fe. The clad metal 14 is solidly 
supported in the insulator 2 with its lower end somewhat extended beyond 
the insulator 2. The core 15 is made of a heat-conductive metal such as 
silver, copper, copper-based alloy or the like. 
As shown in FIG. 2, an increased quantity of the platinum component of the 
tip 13 resides in a portion corresponding to an upper limit 16 of the 
firing end 12 of the metal column 11. A decreased quantity of the platinum 
component of the tip 13 resides in a portion corresponding to a lower 
limit 17 of the firing end 12 of the metal column 11. The platinum 
component of the tip 13 is adapted to gradually change between the 
portions corresponding to the upper and lower limits of the firing end 12 
of the metal column 11. 
The greatest quantity of the platinum component of the tip 13 is 
approximately 85% by weight, while the smallest quantity of the platinum 
component of the tip 13 is approximately 70% by weight. Thus, a difference 
between upper and lower limits of the platinum component is about 15% by 
weight. 
The following is a method for securing the tip 13 to the firing end 12 of 
the column metal 11 of the center electrode 5 (FIGS. 3a.about.3c). 
An annular groove 18 is provided on the firing end 12 of the metal column 
11 (2.5 mm in diameter) by means of milling or the like. The groove 18 is 
0.6 mm in length and 0.15 mm in depth. A distance between the lower limit 
of the metal column 11 and a center of the groove 18 measures 1.5 mm. A 
noble metal wire 19 is made of platinum, and formed into an annular 
configuration, both ends of which meet in a manner to have a slit 20 
therebetween. The noble metal wire 19 is circular in section and 0.3 mm in 
diameter. The noble metal wire 19 is arranged so that its volume is 
substantially the same as that of the groove 18. 
After fitting the wire 19 into the groove 18 of the metal column 11, by 
means of electrical resistance welding, the wire 19 is provisionally 
secured to the side of the Groove 18 which corresponds to the upper limit 
of the firing end 12 subjected to the increased incidence of the spark 
discharges. Then, laser beams (LB) are applied perpendicular to the center 
of the groove 18 with a laser spot e.g. 1.4 mm in diameter as shown in 
FIG. 3b, the wire 19 is thermally fused into the column metal portion in 
which the groove 18 is located. In this instance, the metal column 11 is 
rotated around its axis at the speed of e.g. (5.pi./6) rad/sec, while at 
the same time, applying 48-round of the laser beams (LB) to the length of 
the wire 19 to carry out a seam welding. 
Upon carrying out the seam welding, a laser welding machine is used to 
generate a pulse-type YAG laser with an underfocus 10 mm from an outer 
surface to the center of the metal column 11. The YAG laser is used with 
an output and pulse width 6.5 J and 2.0 milliseconds respectively. Instead 
of the YAG laser, a CO.sub.2 laser may be used. Any type of welding may be 
used including electron beam welding so long as the wire is thermally 
fused into the groove 18 of the metal column 11. 
By revolving the metal column 11 about its axis, the stationary wire 19 is 
automatically wound around the column 11, and the laser beams are applied 
to the wire 19 from its leading end to the successive portions 
continuously. 
After applying the YAG laser welding to the noble metal wire 19, the noble 
metal tip 13 is provided on the firing end 12 in the form of an alloyed 
layer in which the nickel alloy component of the metal column 11 and the 
platinum component of the tip 13 are thermally fused as shown in FIG. 3c. 
In this instance, the platinum component of the alloyed layer gradually 
increases from the lower limit to the upper limit of the metal column 11 
as previously shown in FIG. 2. 
With the structure thus far described, a high voltage is intermittently 
applied across the electrodes 4, 5 with the circular discharge type spark 
plug 1 mounted on the cylinder head of the internal combustion engine. The 
high voltage repeatedly induces the spark discharge between the inner wall 
4a of the Ground electrode 4 and the noble metal tip 13 provided on the 
firing end 12 of the metal column 11. 
As previously shown in FIG. 2, the platinum component of the tip 13 is 
increased in the direction in which the increased incidence of the spark 
discharges occurs, while the platinum component of the tip 13 is decreased 
in the direction in which the decreased incidence of the spark discharges 
occurs. In other words, an increased quantity of the platinum component of 
the tip 13 resides in a portion corresponding to an upper limit of the 
firing end 12 of the metal column 11. Conversely, a decreased quantity of 
the platinum component of the tip 13 resides in a portion corresponding to 
a lower limit of the firing end 12 of the metal column 11. 
Due to the increased number of cycles of spark discharges between the inner 
wall 4a of the ground electrode 4 and the noble metal tip 13 caused by 
extending the use of the circular discharge type spark plug 1, the inner 
wall 4a of the ground electrode 4 and the noble metal tip 13 provided on 
the firing end 12 of the metal column 11. The spark erosion of the tip 13 
depends on the platinum component and the incidence of the spark 
discharges to which the tip 13 is subjected. This means that there is no 
significant difference between the spark erosion of the tip portion to 
which the increased spark discharge incidence and increased platinum 
component are subjected and the spark erosion of the tip portion to which 
the decreased spark discharge incidence and decreased platinum component 
are subjected. The eroded layer of the tip 13 is uniformly retained all 
through the firing end 12 of the metal column 11 when the circular 
discharge type spark plug 1 exhausts its service life. 
As described above, the tip 13 is uniformly eroded all through the firing 
end 12 of the metal column 11 when repeated spark discharge is induced 
between the inner wall 4a of the ground electrode 4 and the noble metal 
tip 13 provided on the firing end 12 of the metal column 11. This enables 
a reduction in the quantity of the expensive platinum component used at 
the tip portion to which the decreased incidence of the spark discharges 
is subjected without disadvantageously losing the spark erosion resistant 
property. This results in a long service life of the spark plug with a 
minimum use of the expensive platinum, thus significantly reducing a 
manufacturing cost for mass production in industrial applications. 
Further, as previously shown in FIG. 2, the greatest quantity of the 
platinum component of the tip 13 is approximately 85% by weight, while the 
least quantity of the platinum component of the tip 13 is approximately 
70% by weight. Thus, a difference between upper and lower limits of the 
platinum component is about 15% by weight. This maintains a good spark 
erosion resistant property of the tip portion to which the decreased 
incidence of the spark discharges is subjected. It is possible to 
satisfactorily retain the above advantages by insuring the difference of 
15% by weight between the upper and lower limits of the platinum component 
of the tip 13. 
In addition, the platinum component of the tip 13 changes in the axial 
direction of the metal column 11, thereby dispersing thermal stress which 
would otherwise work locally on the tip 13 due to repeated heat-and-cool 
cycles while the circular discharge type spark plug 1 is in service. This 
arrangement also decreases the thermal expansion difference between the 
tip 13 and the metal column 11, which also reduces the thermal stress. 
This also prevents cracks from developing on an interface between the tip 
13 and the metal column 11, and prevents the tip 13 from inadvertently 
falling off the metal column 11. 
FIG. 5 is a graph showing how the platinum component changes depending on 
the position of the tip 13 according to a second embodiment of the 
invention. 
In this embodiment of the invention, an increased quantity of the platinum 
component of the tip 13 resides in a portion corresponding to an upper 
limit of the firing end 12 of the metal column 11. A decreased quantity of 
the platinum component of the tip 13 resides in a portion corresponding to 
a lower limit of the firing end 12 of the metal column 11. The platinum 
component of the tip 13 is adapted to abruptly change between the portions 
corresponding to the upper and lower limits of the firing end 12 of the 
column metal 11. 
The greatest quantity of the platinum component of the tip 13 is 
approximately 83% by weight, while the least quantity of the platinum 
component of the tip 13 is approximately 71% by weight. This concludes 
that the difference between the upper and lower limits of the platinum 
component is about 12% by weight. This arrangement makes it possible to 
insure the same advantages as obtained by the first embodiment of the 
invention since the difference between the upper and lower limits of the 
platinum component is 10% by weight or more. 
It is noted that the noble metal tip 13 may be used in a full creeping 
discharge type spark plug. 
FIG. 6 shows a third embodiment of the invention in which a multi-polarity 
type spark plug 22 is used with paired ground electrodes 21 extended into 
the combustion chamber of the internal combustion engine. The noble metal 
tip 13 is welded to the metal column 11 of the center electrode 5 in the 
same manner as described with respect to the first and second embodiments 
of the invention. 
FIGS. 7 through 9 show a fourth embodiment of the invention which is 
applied to a semi-creeping discharge type spark plug 24. In this 
embodiment of the invention, a discharge gap (Ga) is provided which creeps 
between a front end surface 5a of the center electrode 5 and a discharge 
end 23a of a ground electrode 23 along a front end surface 2a of the 
insulator 2. An air gap (Gb) is provided between the discharge end 23a of 
the ground electrode 23 and an outer surface 3b of the insulator 2. 
The noble metal alloy tip 13 is made of a corrosion and erosion resistant 
platinum (Pt) or platinum alloy containing Ni and Ir, and formed into an 
annular configuration as shown in FIGS. 3a.about.3c. With an experiment 
carried out to previously measure the incidence of spark discharges 
subjected to the tip 13, the noble metal component of the tip 13 is 
designed to change according to an axial position on the elongated metal 
column 11 on the basis of the incidence of the spark discharges subjected 
to a specified portion of the tip 13. Consequently, the platinum component 
of the tip 13 increases in which the incidence of the spark discharges 
increases as shown at numeral 16 (the upper limit) in FIG. 2. Conversely, 
the platinum component of the tip 13 decreases in which the incidence of 
the spark discharges decreases as shown at numeral 17 (the lower limit) in 
FIG. 2. 
In the noble metal tip 13 welded to the firing end 12 of the metal column 
11 in the semi-creeping discharge type spark plug 24, the platinum 
component of the tip 13 is increased in the direction in which the 
increased incidence of the spark discharges occurs, while the platinum 
component of the tip 13 is decreased in the direction in which the 
decreased incidence of the spark discharges occurs. Namely, as shown in 
FIG. 9, an increased quantity of the platinum component of the tip 13 
resides in a portion 25 corresponding to a central area of the firing end 
12 of the metal column 11. Conversely, a decreased quantity of the 
platinum component of the tip 13 resides in portions 26, 27 corresponding 
to the upper and lower limits, respectively, of the firing end 12 of the 
column metal 11. 
In this instance, the greatest quantity of the platinum component of the 
tip 13 is approximately 86% by weight, while the smallest quantity of the 
platinum component of the tip 13 is approximately 72% by weight. A 
difference between the central portion 25 and upper/lower limits of the 
platinum component is therefore about 14% by weight. 
FIG. 10 shows a fifth embodiment of the invention in which an iridium (Ir) 
or iridium-alloyed (Ir--Y.sub.2 O.sub.3, Ir--La.sub.2 O.sub.3, 
Ir--ZrO.sub.2) layer 30 is secured to a front end surface 29a of a center 
electrode 29 by means of laser welding, electrical resistance welding or 
the like. 
In a spark plug 28 according to the fifth embodiment of the invention, a 
corrosive and erosion resistant platinum-alloyed tip 33 is secured to a 
firing end 32 of a ground electrode 31 by means of the laser welding. The 
center electrode 29 has a heat-conductive core 34 cladded by a metal 
column 36, while the ground electrode 31 has a heat-conductive core 35 
cladded by a column metal 37. 
As apparently from the foregoing description, the noble metal component of 
the tip is changed depending on the magnitude of spark discharges to which 
the tip portion is subjected. Thus, the eroded layer of the tip is 
uniformly retained all through the firing end 12 of the metal column 11 
when the spark plug exhausts its service life. This enables a reduction in 
the quantity of the expensive platinum component used at the tip portion 
to which the decreased incidence of the spark discharges is subjected 
without disadvantageously losing the spark erosion resistant property. 
This results in a long service life of the spark plug with a minimum use 
of the expensive platinum, thus significantly reducing a manufacturing 
cost for mass production in industrical applications. 
It is noted that the tip may be made of gold, palladium, iridium, rhodium 
or the like instead of the platinum metal used for the noble metal tip 13 
according to each of the embodiments of the invention. 
It is also noted that the ground electrode 4 may be discretely prepared 
with its spark erosion taken into consideration instead of making it 
integral with the metallic shell 3 in the first embodiment of the 
invention. 
It is observed that the noble metal tip may be used on a firing end of the 
ground electrode in the first through fourth embodiment of the invention. 
It is also observed that instead of the noble metal layer 30, the noble 
metal tip 33 may be used on the front end surface 29a of the center 
electrode 29. 
It is appreciated that the platinum component of the tip 13 may be altered 
in its axial direction according to a spark erosion pattern predetermined 
on an experimental test result in which the noble metal tip is actually 
eroded in the first, second and fourth embodiments of the invention. 
It is also appreciated that the platinum component of the tip 13 welded to 
the ground electrode may be altered in its radial direction according to 
the spark erosion of a firing portion of the ground electrode. 
Further, it is observed that a noble tip may be previously made in which 
the platinum component is altered in its axial direction depending on the 
magnitude of the spark discharges to which the tip portion is subjected, 
and thereafter the tip may be secured to the firing end 12 of the metal 
column 11 of the center electrode 5 by means of the electrical resistance 
welding or the like. 
While the invention has been described with reference to the specific 
embodiments, it is understood that this description is not to be construed 
in a limiting sense in as much as various modifications and additions to 
the specific embodiments may be made by skilled artisan without departing 
from the spirit and scope of the invention.