GB2060773A

GB2060773A - Spark igniter

Info

Publication number: GB2060773A
Application number: GB8032707A
Authority: GB
Original assignee: Champion Spark Plug Co
Current assignee: Federal Mogul Ignition Co
Priority date: 1979-10-22
Filing date: 1980-10-10
Publication date: 1981-05-07
Also published as: JPS5667187A; NZ195331A; IT1127892B; FR2468234B1; DE3036223A1; AU6285780A; GB2060773B; JPH0410195B2; BR8006759A; AU537242B2; ZA805008B; IT8049941A0; CA1168531A; FR2468234A1; DE3036223C2; MX148143A

Abstract

The igniter (21) comprises a metal shell (22) having a firing end (23) which terminates at its lower end in an annular ground electrode, an insulator (27), a centre electrode (25) and a plurality of oxidation and erosion resisting inserts embedded within and bonded to the metal shell (22). The insulator (27) is sealed within the metal shell (22) and has a central bore within which the centre electrode is sealed and a lower surface extending inwardly toward the bore from the ground electrode. The centre electrode (25) is so positioned that a spark discharge between it and the ground electrode occurs along the inwardly extending surface of the insulator (27). The inserts are composed of iridium, platinum, rhodium, ruthenium, osmium, tungsten or an alloy or ductile alloy of one of the foregoing metals. <IMAGE>

Description

SPECIFICATION Spark igniter The present invention relates to a spark igniter of the type used in turbine engines, including aircraft jet engines. Such igniters are frequently surface gap spark plugs in which a high energy spark discharge occurs between a centre electrode and a ground electrode, travelling 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 sparking, 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 engines 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 condition with respect to igniter life. Problem erosion of both the centre electrode and the ground electrode occurs in igniters used with turbine engines. A solution to the problem of electrode erosion in such igniters is disclosed in U.S. Patent No. 3,691,419, Van Uum et al; this patent discloses an igniter of the type in question having a centre 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 this igniter, 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 iridium ring of the type suggested by the above U.S. 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 expansion characteristics between iridium and the nickel alloys commonly used as ground electrode materials therein can cause catastrophic failure of igniters of this type.

Various suggestion (see, for example, U.S.

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 al patent discloses a ground electrode made of a molybdenum platinum alloy wire which constitutes an insert in the metal shell of a conventional spark plug) have also been made for reducing electrode erosion in conventional spark plugs where the spark discharge occurs through a gas-filled gap between centre and ground electrodes. What it calls a spark plug with "a multiplicity of semi-surface spark gaps" is also suggested in U.S. Patent No. 2,591,718 which discloses a structure wherein a centre 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 present invention is based upon a spark igniter which does not require the expensive and difficult to obtain iridium ring of the spark plug disclosed in U.S. Patent 3,691,419, but which has substantially equivalent resistance to electrode erosion. In a preferred embodiment the configuration of the spark igniter of the present 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.

The igniter comprises a metal shell having a firing end which terminates at its lower end in an annular ground electrode, an insulator, a centre electrode and a plurality of inserts embedded within, and bonded to, the metal shell. The insulator is sealed within the metal shell and has a central bore within which the centre electrode is sealed and a surface extending inwardly toward the bore from the ground electrode. The centre electrode has a firing end which is in spark gap relation with the ground electrode of the metal shell and is so positioned that a spark discharge between the firing end and the ground electrode occurs along the inwardly extending surface of the insulator.The inserts are composed of an oxidation and erosion resistant material, preferably iridium, platinum, rhodium, ruthenium, osmium, or an alloy or ductile alloy of one of the foregoing metals and, for service where it is not heated to temperatures higher than about 10000F., tungsten and its alloys and ductile alloys.

In the accompanying drawings:- Fig. 1 is an elevational view, partially in section, of an igniter according to the present invention.

Fig. 2 is an end view of the igniter of Fig. 1.

Fig. 3 is a plan view of an assembly that is used in producing the igniter of Figs. 1 and 2.

Fig. 4 is a vertical sectional view of the assembly of Fig. 3, taken along the line 4-4 thereof.

Fig. 5 is a vertical section of a shell assembly which is a part of the igniter of Figs. 1 and 2.

Fig. 6 is a side view of an insert which is a part of the igniter of Figs. 1 and 2 and of the assemblies of Figs. 3 to 5.

Fig. 7 is an end view of the insert of Fig. 6.

Fig. 8 is a view in vertical section of the metal shell of an igniter similar to that of Figs. 1 and 2, but constituting another embodiment of an igniter according to the invention.

Fig. 9 is an end view of the metal shell of Fig. 8.

Fig. 10 is a plan view of an insert which is a part of the metal shell of Figs. 8 and 9.

Fig. 11 is an end view of the insert of Fig. 10.

Fig. 12 is a view in vertical section of the shell of still another embodiment of an igniter according to the present invention.

Fig. 13 is an end view of the shell of Fig. 12.

An igniter 21 according to the present invention is shown in Figs. 1 and 2 and comprises a metal shell 22 having a firing end 23 which terminates at its lower end in an annular ground electrode having a surface 24 (Fig. 2) which is in spark gap relation with a centre electrode 25.

There are four iridium inserts 26 in the annular ground electrode at the firing end 23 of the igniter 21 and the inserts 26 extend radially inwardly beyond the surface 24 of the annular ground electrode toward the centre electrode 25.

The iridium inserts 26 are rectangular in cross section (Fig. 6) and are embedded within and bonded to, for example by a brazing operation, the firing end 23 (Figs. 1 and 5) of the metal shell 22.

The igniter 21 also includes a lower insulator 27 (Fig. 1) and an upper insulator 28. The lower insulator 27 is sealed within the metal shell 22, while the upper insulator 28 is sealed within a composite upper 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 bya 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 mounting in a turbine engine while the inner shell 33 is threaded at 38 for mounting in an ignition harness of the turbine engine.The igniter 21 also includes a terminal 39 which is threaded into the upper insulator 28 and is in electrical contact with the centre electrode 25.

As best seen in Fig. 3, the metal shell 22 with the iridium inserts 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 rectangular slots in a surface 43 thereof. When the assembly 40 is brazed or otherwise bonded to the co-operating 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, Figs. 8 and 9 for the identically shaped shell 22 in the igniter 21 of Fig. 1. Referring again to Figs. 8 and 9, the shell 54 is made of Inconel (Registered Trade Mark) 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 Figs. 10 and 11, the inserts 55 are cylindrical in shape, matching the bores 56.

An igniter according to the present invention can also be produced from a shell 57, (Figs. 12 and 13) having iridium inserts 58 brazed or otherwise 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 Figs.6 and 7.

The shell 22 of the igniter 21, Fig. 1, and the shells 54 and 57 of Figs. 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 present invention.

It will be appreciated that, while the invention has been shown and described in connection with an igniter having iridium inserts adjacent its firing end, inserts made of any other oxidation and erosion resistant material can also be used. The most common materials 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 10000F., 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 20000 F. 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 Figs. 1 to 7 constituting the best mode presently known to us.

Claims

1. An igniter comprising a metal shell having a firing end which terminates at its lower end in an 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 centre electrode sealed within the bore of said insulator 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 centre electrode.

2. An igniter as claimed in claim 1 wherein each of said oxidation and erosion resistant inserts is in the annular ground electrode of said shell and extends generally radially thereof.

3. An igniter as claimed in claim 2 wherein each of said inserts is composed of iridium, platinum, rhodium, ruthenium, osmium, tungsten or an alloy or a ductile alloy of one of the foregoing metals.

4. An igniter as claimed in claim 3 wherein each of said inserts is composed of iridium.

5. An igniter substantially as described with reference to, and as illustrated in, Figs. 1 to 7, or Figs.8 to 11, or Figs.12 12 and 13, of the accompanying drawings.