Note: Descriptions are shown in the official language in which they were submitted.
1 168531
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The present invention relates to a spark igniter of the type used
in turbine engines, including aircraft jet engines. Such igniters are fre-
quently surface gap spark plugs in which a high energy spark discharge occurs
between a center electrode and a ground electrode, traveling along the surface
of a ceramic member. The spark discharge in such igniters is of the "high
energy" type because of the nature of the ignition system used to cause spa-rk-
ing, the system including a condenser which is charged as the voltage applied
thereto and across the igniter increases; when the applied voltage becomes
sufficiently large to cause a spark discharge the electrical energy stored by
the capacitor is discharged, flowing across the spark gap. The stored energy
in capacitor discharge ignition systems that are used with jet aircraft en-
gines is usually at least one joule.
Electrode erosion has been a problem with spark igniters used with
turbine engines for jet aircraft, sometimes constituting the limiting condi-
tion with respect to igniter life. Problem erosion of both the center elec-
trode and the ground electrode occurs in igniters used with turbine engines.
A solution to the problem of electrode erosion in such igniters is suggested
in United States Patent No. 3,691,419, Van Uum et al; this patent discloses
an igniter of the type in question having a center electrode with a firing
end made of spark resistant metal such as tungsten and a ground electrode
having a ductile iridium ring welded therein and positioned so that it is
immediately adjacent the spark gap. In the igniter of the Van Uum et al
patent the ground electrode to which the iridium ring is welded is a portion
of the metal shell of the igniter, a common structure.
It has been found that iridium and other precious metal rings, if
they can be obtained at all, are extremely expensive. On the basis of price
quotations that have been received, it has been estimated that the use of an
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11~8531
iridium ring of the type suggested by the Van Uum et al patent in an igniter
that is presently commercially available would approximately double the cost
of that igniter. It has also been found that the differences in thermal ex-
pansion characteristics between iridium and the nickel alloys comrnonly used
as ground electrode materials therein can cause catastrophic failure of ig-
niters of the type suggested by Van Uum et al.
Various suggestions* have also been made for reducing electrode
erosion in conventional spark plugs where the spark discharge occurs through
a gas-filled gap between center and ground electrodes. What it calls a spark
plug with "a multiplici~y of semi-surface spark gaps" is also suggested in
United States Patent No. 2,591,71~ to Paul; this patent discloses a structure
wherein a center electrode terminates flush with an insulator end and is in
spark gap relation along the insulator end with four rod-type electrodes each
of which just touches the insulator surface.
The instant invention is based upon the discovery of a spark igniter
which does not require the expensive and difficult to obtain iridium ring of
the spark plug suggested by the Van Uum et al patent, but which has substan-
tially equivalent resistance to electrode erosion. In a preferred embodiment
the configuration of the spark igniter of the instant invention minimizes the
stresses which occur as a consequence of different coefficients of thermal
expansion between an insert of an oxidation and erosion resistant material
such as iridium and an annular ground electrode containing the insert.
According to the invention there is provided an igniter comprising
a metal shell having a firing end which terminates at its lower end in an
* See, for example, United States Patents 2,391,455; 2,391,456; 2,391,458;
2,470,033 (all to Hensel); and 2,344,597 ~to Chaston et al). The Chaston
et all patent discloses a ground electrode made of a molybdenum platinum
alloy wire which constitutes an insert in the metal shell of a convention-
al spark plug.
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1 1~8~31
annular ground electrode, an insulator sealed within said metal shell and
having a central bore and a surface extending inwardly toward the bore from
the ground electrode, a center electrode sealed within the bore of said in-
sulator and having a firing end which is in spark gap relation with the ground
electrode of said metal shell and so positioned that a spark discharge between
the firing end and the ground electrode occurs along the inwardly extending
surface of said insulator, and a plurality of oxidation and erosion resistant
inserts, each of said inserts comprising a body embedded within and bonded to
said metal shell and having an exposed surface which extends inwardly from the
ground electrode toward the firing end of said center electrode.
In the accompanying drawings:
Figure 1 is a view in elevation, partially in section, of an igniter
according to the instant invention.
Figure 2 is an end view of the igniter of Figure 1.
Figure 3 is a plan view of an assembly that is used in producing the
igniter of Figures 1 and 2.
Figure 4 is a vertical sectional view of the assembly of Figure 3,
taken along the line 4-4.
Figure 5 is a view in vertical section of a shell assembly which is
a part of the igniter of Figures 1 and 2.
Figure 6 is a side view of an insert which is a part of the igniter
of Figures 1 and 2 and of the assemblies of Figures 3-5.
Figure 7 is an end view of the insert of Figure 6.
Figure ~ is a view in vertical section of the metal shell of an ig-
niter similar to that of Figures 1 and 2, but constituting another embodiment
of an igniter according to the invention.
Figure 9 is an end view of the metal shell of Figure 8.
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. Figure 10 is a plan view of an insert which is a part of the metal
shell of Figures 8 and 9.
~ Figure 11 is an end view of the insert of Figure 10.
Figure 12 is a view in vertical section of the shell of still another
~:~ embodiment of an igniter according to the present invention.
Figure 13 is an end view of the shell of Figure 12.
An igniter according to the instant invention is indicated generally
` at 21 in Figures 1 and 2. The igniter 21 comprises a metal shell 22 having a
, firing end 23 which terminates at its lower end in an annular ground electrode
~; ~ 10 having a surface 24 (Figure 2~ which is in spark gap relation with a center
electrode 25. There are four iridium inserts 26 in the annular ground elec-
trode at the firing end 23 of the igniter 21. The inserts 26 extend radially
~?; inwardly beyond the surface 24 of the annular ground electrode toward the cen-
ter electrode 25. The iridium inserts 26 are rectangular in cross section
(Figure 6) and are embedded within and bonded to, for example by a brazing
operation, the firing end 23 ~Figures 1 and S) of the metal shell 22.
The igniter 21 also includes a lower insulator 27 (Figure 1) and
an upper insulator 28. The lower insulator 27 is sealed within the metal
.~ shelI 22, while the upper insulator 28 is sealed within a composite upper
~; 20 shell 29. The upper shell 29 comprises an outer shell part 30 which engages
the shell 22 as indicated generally at 31 and is threaded at 32 to an inner
;.
. shell part 33. The lower insulator 27 is sealed to the shell 22 by a body
34 of compacted talc, while the upper insulator 28 is sealed to the outer
shell 30 by a body 35 of talc which is compacted by an end 36 of the inner
~ shell 33. The outer shell 30 is threaded at 37 for engagement with a turbine
.~ engine while the inner shell 33 is threaded at 38 for engagement with an ig-
nition harness of the turbine engine. The igniter 21 also includes a terminal
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39 which is threaded into the upper insulator 28 and is in electrical contact
with the center electrode 25.
As best seen in Figure 3, the metal shell 22 with the iridium in-
serts 26 embedded therein and bonded thereto can readily be produced by
brazing or otherwise bonding a sub-assembly 40 to a cooperating shell part
(not illustrated) to produce the shell 22. The sub-assembly 40 comprises an
annular ring 41 in which the iridium inserts 26 are staked by arms 42 in rec-
tangular slots in a surface 43 thereof. When the assembly 40 is brazed or
otherwise bonded to the cooperating part (not illustrated) to produce the
shell 22, the annular ring 41 becomes an integral part of the shell 22 and,
simultaneously, the iridium inserts 26 are bonded within and to the shell 22.
A spark igniter according to the invention can also be produced by
substituting a shell 54, Figures 8 and 9 for the identically shaped shell 22
in the igniter 21 of Figure 1. Referring again to Figures 8 and 9, the shell
54 is made of inconel or other suitable nickel alloy, and has iridium inserts
55 brazed or otherwise bonded in bores 56 of the shell 54. As best seen in
Figures 10 and 11, the inserts 55 are cylindrical in shape, matching the bores
56.
An igniter according to the instant invention can also be produced
from a shell 571 Figures 12 and 13 having iridium inserts 58 brazed or other-
wise bonded in slots 59 which are adjacent the firing end 60 thereof. The
inserts 58 are rectangular in cross section, having the same configuration
as the inserts 39 of Figures 6 and 7.
The shell 22 of the igniter 21, Figure 1, and the shells 54 and 57
of Figures 8 and 12, have grooves 61 extending longitudinally thereof adjacent
their respective firing ends. These slots 61 are frequently used in igniters
to facilitate cooling thereof and form no part of the instant invention.
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1 168531
It will be apparent that various changes and modifications can be
made from the specific details of the igniter shown in the attached drawings
and described in connection therewith without departing from the spirit and
scope of the invention as defined in the appended claims. For example, while
the invention has been shown and described in connection with an igniter hav-
ing iridium inserts adjacent its firing end, inserts made of any other oxida-
tion and erosion resistant material can also be used. The most common mater-
ials having the requisite degree of oxidation and erosion resistance, in
addition to iridium, are platinum, rhodium, ruthenium, osmium, alloys and
ductile alloys of the named metals and, for service where it is not heated
to temperatures higher than about 1000F., tungsten and its alloys and ductile
alloys. Because of their refractory nature, parts composed of the named
metals are frequently made by powder metallurgical techniques and may be
comparatively brittle immediately after sintering. Such brittleness can
usually be reduced to acceptable limits by working the parts at comparatively
low temperatures, for example in the vicinity of 2000F. It is sometimes
desirable to increase the ductility of such materials; this can be done by
producing so-called ductile alloys: refractory metal powders are blended
with other metal powders, for example nickel and copper or nickel and iron,
which form a comparatively low melting phase which, upon firing, bonds the
refractory metal particles together, forming a matrix which is ductile by
comparison with the pure refractory metal. Iridium is the preferred insert
material, the embodiment of Figures 1-7 constituting the best mode presently
known to the inventor.
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