Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to spark plugs for
u~e in internal combustion engines, and more particularly
to the cente~ electrode used in such spark plugs, and
especially to a composite electrode structure for such spark
plugs which has high conductivity with respect to heat and
electricity, and is resistant to corrosion and burning.
Various types of internal combustion engine~ used in
automotive power plants, to provide power forvarious types
of appliances, and the like, require different types of
spark plugs. It is difficult to provide a single type of
spark plug for different motors, and even for motors which
are of essentially similar construction, in view of~the
different power loading on the motor, different operatian,
compression ratio, speed, cooling, carburation, and type
of fuel supply. If a single type of spark plug would be
used for various engines, or for an engine subject to
different operating conditions, then such a spark plug
might, under some operating conditions, become too hot
~` while, under other operating conditions, or with different
motors, would have too low an average temperature. If the
spark plug becomes too hot, then the fuel-air mixture can
self-ignite on the elements of the spark plugs extending
into the combustion space of the engine, resulting in
ignition due to glow of the spark plug, and in misfires.
If the spark plug operates too cool, then the insulator tip
~ ourrounding the center electrode, extending into the
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combustion ~pace of the engine, would rapidly become
contaminated by combustion residues. Misfires, in the
sense of failure of sparking, would result due to spurious
electrical paths through combustion residues or deposits.
Thus, the spark plug to be used in the internal combustion
engine ~ust be matched to the engine, so that the plug, in
a specific motor, does not become too hot, nor remain
too cold. This requires spark plugs with different load
characteristics. These different load characteristic~ are
evaluated by designation by a heat number, associated with
each spark plug. The heat number~ or heat value of a spark
plug, expressed as an arbitrary number, characterizes the
resistance of the spark plug with respect to overheating: ;
a higher heat number permits higher loading of the spark
plug without danger of spurious ignition due to glowing of
the spark plug but, on the other hand, it has a higher
tendency to become contaminated, since the self-cleaning
temperature is not reached as rapidly upon starting of a
c~ld engine and is easily passed in downward direction upon
throttling of the motor.
Spark plugs can be designed to have various heat
numbers by suitably shaping the insulator surface which has ''
to accept the heat to which the spark plug is subjected.
Assuming an egual diameter for a specific spark plug, that
25~ is, the same diameter of the spark plug housing, the length
of the inculator tip adjacent tho electrode tip has a
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definite bearing on the heat number. Spark plugs with a
high heat value have short insulator tips; spark plugs with
a low heat ~wmber have long insulator tips. The spark plug
must reach a temperature of about 500C in order to be
self-cleaning, that is, to spontaneously burn off deposits
such as soot, oil, carbonized oil,or the like, which may
deposit on the insulator of the spark plug. The burning
of these deposits should be sufficiently effective to
prevent short circuits for the spark current being supplied
to the electrode. To permit such burn-off, the spark plug
must have an insulator tip with a large surface, that is,
the spark plug must have a long insulator tip. It is
difficult, however, to find a suitable compr~mise since the
use of longer insulating tips can be realized only if the
heat generated upon ignition in the ignition space of the
engine can also be conducted away since, otherwise, the
longer tips may tend to cause self-ignition due to the glow
of the t~p, or deposits thereon. It has already been
proposed to conduct heat away rapidly and effectively over
the center electrode by so c~nstructing the center
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electrode that it is easily heat conductive, that is, I
I
consists of a material having a high heat conductivity.
Composite electrodes have been made in which silver is
` used as a matrix material and as the highly heat conductive
~-~ 25 substance, in which at least one filament or strand of
` nickel is embedded. The fi~mentsor strandæ of nickel
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tend to reduce corrosion, and burn-off of the electrode, 80
that the distance between the center electrode and the counter
electrode remains essentially constant. While the solution is
technically good, the cost of such electrodes is high since
sllver i8 expensive.
It ls an ob~ect of the present invention to provide
a spark plug with a composite electrode which permits use of
a long insulating tip, and which has the advantages of composite
electrodes of high heat conductivity without, however, requiring
the expensive metal of silver as a matrix material.
Sub~ect matter of the present invention: Briefly,
the present invention provides a spark plug for use in internal
combustion engines having a housing shaped for assembly in the
engine, an insulator sleeve extending through the housing, an
essentially cylindrical central electrode extending through the
insulator, and a counter electrode secured to the housing,
wherein the central electrode comprises an electrode tip located
opposite the counter electrode to provide for spark discharge
from an end face of the tip to the counter electrode and formed
as a composite structure having an outer ~acket of corrosion
re8i8tant materlal, at least one strand of corrosion resistant -- -
electrically conductive material, and a matrix filler material
fllling the ~acket and within which the at least one strand is
embeddet; and wherein the matrix filler material comprises a
material selected from the group consisting of copper; a
corrosion resistant copper alloy, and forms between 5% and 50%,
by volume, of the central electrode, and the ~acket is ~
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essentially a cylindrical cover extending around the outer
circumference of the filler material leaving the end face exposed ;~
and facing the counter electrode and which end face comprises
the matrix filler and the at least one strand.
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It has been found, surprisingly, that copper or
corrosion resistant, slightly alloyed copper can be used
as the matrix material although, without protection by a
jacket, it could not be used due to the tendency to corrode
when exposed to hot gases. The-copper, when
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jacketed and having the filaments or strands of corrosion
resistant material embedded therein is eminently suitable
to conduct heat away from the electrode tip, and from the
insulator, while being highly electrically conductive, and
permitting construction of an electrode which is
at least half and usually less than half as expensive as
a composite elect~ode including silver as the matrix
material.
The number of filaments embedded in the matrix can
vary widely. Up to 200 filaments or strands can be
embedded in spark plugs of conventional size for automotive-
type engines; as the number of strands increases beyond
about 20 or so strands for the ordinary automotive-type -
electrode, the corrosion tendency within the electrode,
and the price of the electrode increase. In a preferred
form, the strands or filaments are so located within the
copper matrix material that the strands do not touch each
other, and are essentially uniformly distributed over the
cross-sectional area of the electrode. If the strands touch ¦
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` ~ ~ 20 each other, then the danger of corrosion is increased.
It has been found, desirably and surprisingly, that
the ignition voltage necessary for a spark at the electrodes
i8 lower than with spark plug electrodes without the
filament or strand inserts, given a fixed electrode gap.
The invention will be described by way of example with
reference to the accompanying drawings) wherein:
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Fig. l is a side view, partly perspective and partly
in section, of a spark plug, to an enlarged scale;
Fig. ~ is a cross-sectional view along line I-I
of the center electrode of Fig. l, to a still greater scale;
Fig. 3 is a cross-sectional view along line I-I of
Fig. l of a modiication of an electrode to essentially the
same scale as Fig. 2;
and Fig. 4 is a longitudinal cross-sectional view
along line III-III of the electrode of Fig. 3.
The spark plug of Fig. l has an electrical insulator
body 12, made of ceramic material, through which a central
electrode extendQ. The outer connecting portion 10 is -
suitably shaped to receive an electrical connector, as
well known in the art, and connects with a center elsctrode
11, which passes through the insulator 12. The insulator
~ 12 is sealed into a housing 13, made of metal, and formed
; ~ ~ with a screw thread 15 for insertion into the engine block
of an internal combustion engine, not shown. The metal
housing has an inner extension 14 forming the ground or
~ counter electrode. The portion of the insulator 12 which
extends into the combustion area forms a tip 16. The
insulator 12 is a unitary body and, including the insulator
,
tip 16, is made of a ceramic based on aluminum oxide, with
additives of~glassy substance and fired to havo high
~ ` 25 ~ mechanical str~ngth in special furnaces at high -
'`Sr~ ' ', temperatures~
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The center electrode, in accordance with the
present invention, has a diameter of a~out 2.4 mm at the
tip portion. The electrode 11 is essentially cylindrical
has a jacket 17 of abbut 0.35 mm wall thickness. The
jacket 17 is made of an alloy having (by weight) 96% nickel
as corrosion resistant material to which such elements as
chromium, manganese, or silicon have been added to improve
the corrosion resistance. The jacket 17 may also be made of
other corrosion resistant alloys having a nickel, chromium
or cobalt base. This jacket 17, the thickness of which may
vary between 0.2 and 0,5 mm, surrounds and protects the
matrix material 18 but leaves the end face free Matrix
material 18 is subject to attack by corrosion by hot gases
and, in accordance with the present invention, is made of
copper~ or a copper-base alloy. Seven strands l9 of a mat-
erial which essentially may be the same material as the jacket
17 are embedded in the copper or copper-base alloy matrix 18,
The diameter of the strands 19 is approximately 0,3 mm. The
strands are so arranged in the matrix that they provide a
center strand about which the other six strands are located
with uniform spacing, The center strand extends axially in
the electrode 11, The strands 19 are so spaced that they do
not touch each other. This particular arrangement of the
8trands 19 has been found to be advantageous in operation.
Depending on the use to which the spark plug is to be put,
a larger or lesser ~umber of strands can be used~; up to 200
~rands
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can be embedded in the matrix 18. It has been found,
however, that as the number of strands 19 increases,
corrosion ~lso increases on the center electrode 11.
Additionally, the manufacturing cost of making the
center electrode increases. It is, therefore, desirable to
keep the number of strands 19 at or below twenty-one. The
electrode 11 can have a copper content, in accordance with
the invention, which is between ~and 50% by volume;
preferably, the copper content is about 30% by volume.
Corrosion resistant, low alloyed copper can be used as the
matrix material 18, for example by the addition~of o.1%to
1.5% chromium,or zirconium (by weight). Such additions to
the copper improve the effect of the strands 19 in the
matrix material. The end surface 20 of the center electrode
does not require any additional protection again~t
corro~ion, thus permitting inexpensive mass production of
the center electrodes 11, starting from a wire base.
Various modifications may be made; Figs. 3 and 4
illustrate a center electrode 11 ' which, instead of a
~1 20 central core and surrounding strands is formed with an!-
enlarged single central wire 19' of nickel, embedded in a
matrix 18' of copper or copper alloy. The center wire 19'
has a diameter of about 0.9 mm and extends axially centrally
within the electrode 11'. In all other respects, that is,
.
- 25 dimensions of the electrode as such, the jacket,and the
entire spark plug arrangement, reference is made to the
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embodiments of Figs. 1 and 2.
The jacket 17 (Fig. 2), 17~ (Figs. 3, 4) as well as
the str~nds 19, or the wire 19', may be made of nickel or
other corrosionrasistant alloys based on nicke~ chromiwm,
or cobalt. Examples: "Inconel 600" (trademark) which
includes about 75% nickel, 15% chromium and 10% iron, a
chromium-base corrosion resistant material may have 20%
chromium, 5% aluminum, and 75% iron; a cobalt-base material
can be "Vakumelt ATS 115" (trademark) which has 70% cobalt,
20% chromium and 10% nickel.
In a preferred form, the center electrode has 20% to
40YO by volume copper, or copper-base alloy, although the
range is not critical. ~he jacket and strands, or wire
embedded in the matrix 18, for manufacturing reasons,
preferably are of the same material, but this i5 not a
requirement; in a suitable form, they contain at least
95%, by weight, nickel. ~he copper matrix material must be~
protected at the outer circumference due to the tendency
to corrode when used as electrodes in spark plugs; without
the insert of additional corrosion resistant material,
however, the electrode is not suitable, and the combination
of the jacket, and tha embedded strands (Fig. 2) or
central wire (Figs. ~, 4) results in an electrode which is
eminently suitable as a spark plug electrode, without
corroding, but still providing the necessary heat and
electrical conductivity, permitting rapid heating of the
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insulator tip 16 but not heating of the tip 16 to a
temperature which might cause undesired ignition due to
deposit of carbonized or glowing substances. Electrodes
of the di~ensions given and having either seven strands
(Fig. 2) or a central- strand, or wire (Figs. 3, 4) have
been found to have excellent properties and long operating
time, using jackets 17, 17' of about 0.3 to 0.4 mm,thick-
ness, preferably about 0.35 mm thickness.
Various chan~es and modifications may be made within
the scope of the inventive concept. In the specifiacation
and claims hereof, the term "copper" is deemed to include
copper and minute quantities of alloying constituents,
for example chromium or zirconium.
A preferred material for the jacket 17, or the
filament or strands 19 contains 95 % nickel, 1,8 %
chromium, 2 % mangsnese snd 0,5 1 silioon (7y ~eight).
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The center electrode 11 can be made as a separate
tip to be inserted in a central conductor joined to the
terminal 10 and extending through the length of the
electrode, or the electrode, as such, can extend up to and
inwardly Or a terminal element 10. If the electrode 11 is
inserted into a central conductor, then the central
conductor may consist of any suitable material, well-known
in the art, and which can readily be sealed into the
ceramic material 12 forming the insulator. The tip o~ the
electr~de is then preferably inserted into the central
conductor sealed through the insulator 12 before embedding
the central conductor and the terminal 10 in the ceramic
12 and assembling the ceramic into the housing or metal
holder 13.
However, in the pre~erred embodiment, the center
electrode 11 is made as a separate tip to be connected
by an electrically conducting gla~s seal twell-known in
the art) to a central conductor which is joined to the
terminal 10; also in this case the central conductor - .
may consist of any suitable material well-known in the
art. The tip of the electrode is then inserted and : I
;i mechanically fixed in the insulator 12, the glass brought
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; as a powder upon the electrode tip part inside the ,-
insulator 12, the insulator 12 with the electrode tip
and the glas~ powder is`then heated to melt the glass
powder and after that the central conductor is fed into
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the molten glass; after cooling the electrode ti~
will be electricall~ connected with the central
conductor and will be embedded gastight in the
insulator 12 (see e.g. US-Patent 3 360 676).
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