CA1198331A - Igniter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An igniter for Jet and other internal combustion engines is disclosed. A ceramic insulator containing a substantial proportion of silicon nitride is disposed annularly within the shell of the igniter so that spark discharge occurs along a surface of the silicon nitride insulator adjacent the spark gap between a center electrode and a ground electrode. An igniter having such a silicon nitride insulator is particularly adapted for service under conditions of severe thermal, mechanical, and electrical stress, in an engine having a high voltage, (e.g., 10,000 volts or more), high energy (e.g., up to 20 joules) ignition system. Use of silicon nitride as an insulating material in such igniters eliminates health hazards associated with toxic beryllium oxide insulators previously used, while producing an insulating surface far more durable than one made of alumina.

Description

3 1. ~leld Or the Invention 4This invention relates to an igniter of the high energy type for an internal combustion engine. In service, 6 typically in a ~et engine, such an igniter is fired by a 7 capacitor discharge ignition system of either the high ~ voltage type or the lo~ voltage type. The spark discharge g of high voltage igniters usually occurs along a surface of a dielectric ceramic situated so that it is adjacent a 11 spark gap between a center electrode and a ground 12 electrode. A "high" applied voltage, usually in the range 13of 10,000 to 30,000 volts, is required for ionization of 14 the spark gap to enable discharge of the lgniter.
15Igniters ~or use in low voltage ignition systems, 1~usually in the range of 200 to 5~000 volts, have an 17 electrically semi-conducting surface adjacent a spark gap 18 between a center electrode and a ground electrode. It has 19 been found that, in an igniter with a semi-conducting surrace so positioned, the voltage required to cause a 21 spark discharge is reduced, as compared to an igniter where 22 there is an insulator in this position.
23In the case of either a high voltage igniter or a 24 low voltage igniter, discharge of a previously-charged capacitor occurs when there is a spark between the ground 26 and center electrodes. The discharge of the capacitor 27 causes the spark to be of the high energy type, i.e. up to 28 about 20 ~oules.

1 2. Description o~ the Prior Art

2 Various electrically semi-conducting and

3 insulating materials have heretofore been suggested and

4 used ad~acent the spark gap of igniters designed for low voltage and high voltage ignition systems, respectively.
6 For example, U.S. Pat. No. 3,558,959 discloses a low 7 voltage igniter utilizin~ semi-conductor bodies composed of 8 hot-pressed mixtures of alumina and silicon carbide. It has g also been suggested that semi-conductors suitable for low voltage applications can be produced by pressing a body ~1 from a mixture of si~icon carbide and aluminum silicate, 12 embedding the body in silicon carbide particles, and firing 13 (see, for example, U.S. PatO NosO 3,376,367 and 3,573,231).
14 Also, U.S. Pat. No. 3,968,057 discloses a method for 1~ producing an alumina-bonded silicon carbide semi-conductor 16 7 ror use in a high energy, low voltage ignition system, and 17 U.S. Pat. No. 4,1209829 describes an improved silicon 18 carbide semi-conductor having an electrically 19 non-conductlng glass bonding phase.
Silicon nitride-bonded silicon carbide 21 semi-conductor bodies for use in low voltage igniters are 22 disclosed in U.S. Pat. No. 3,052,814. According to this 3 patent, igniters utilizing a silicon carbide semi-conductor 24 body bonded with silicon nitride are better able to withstand the compressive forces, temper~ture extremes, ~6 vibration, and spark erosion effects encountered durlng ~7 operation in a combustion chamber than are igniters using 28 the materials of the prior art. The use of semi-conductor 29 bodies formed from a nitrided batch of Si and SiC was found to minimize the problems caused by excessive porosity 3 3~

l weakness in compressive strength, low resistance to spark 2 erosion, and chemical change in the combustion chamber (with a resulting change in electrical characteristics), 4 which had been exhibited by the semiconductors of lgniters ~ suggested by the prior art.
6 Methods for producing shaped silicon nitride 7 bGdies from silicon metal are disclosed in British Pat. No.
8 717,555. The reaction-bonded silicon nitride (~BSN) bodies g so produced are said to be resistant to heat shock, to have high mechanical strength, and to be highly resistant to ll oxidation and chemical attack. Also, the dielectric 12 properties of silicon nitride bodies manufactured according l~ to the methods of this patent are descrlbed as being 14 similar to those of silica. The ~ritish speciflcation l~ further suggests that silicon nitride bodies might be 16 utilized in many high temperature ~pplications; among these 17 are Jet engine combustion cnambers, exhaust nozzle linings, l~ rocket combustion cha~bers and exhaust nozzles, and spark 19 plug bodies.
Although many materials have been suggested for 21 assembly into igniters designed to be operated under 22 conditions of high stress (see, for example~ UOS. Pat. Nos.
23 2,684,665; 2,786,158; 3,344,304; 3,558,95g), the prior art, 2~ so far as is known, has not suggested the use of silicon ~5 nitride, or silicon nitride mixed with other materials, as 26 surfaces of insulators ad~acent the spark gap of igniters 27 fired by high voltage, high energy ignition systems. Such 2~ igniters are sub~ected to exceptionally severe thermal, mechanical and electrical stresses in service in ~et and other internal combustion engines. Previousl~ known 3~

1 insulators utilized in such igniters have been produced 2 from materials such as alumina and beryllium oxide, both 3 kno~n to produce ceramic bodies which perform 4 satis~actorily as electrical insulators when approprlately situated adjacent the ~iring end of a high energy ignlter.
6 However, aluJnina insulators are sub~ect to severe spark 7 erosion and thermal shock degradation in service. Beryllium 8 oxide, although insulators produced there~rom are g considerably more durable and resistant to thermal shock than are ones produced ~rom alumina, is considered toxic to 11 humans and is therefore less desirable for extensive use in 12 the manufacture of igniters.
~ SUMMARY 0~ THE I~VENTION
14 The instant invention is based upon the discovery that an improved igniter of the high energy type can be 16 produced by properly assembling into such an igniter an 17 insulator body composed Or substantially pure silicon 18 nitride or a silicon nltride-based material, and having a 19 surface along which a spark, when discharged, travels between a center and a ground electrode.
21 It has been found that high energy igniters 22 having appropriately shaped bodies containing a substantial ~3 proportion o~ silicon nitride situated properly within 2~ thelr shells have improved durability under service 2~ conditions in such applications as ~et engines, where 2~ igniters are subJected to severe thermal, mechanical, and 27 electrlcal stresses. It is believed that the apparent 28 thermal, mechanical -and electrical propertles of insulator 29 bodies composed o~ silicon nitride materials not only enable these materials to exhibit greater resistance to 33~

spark erosion and thermal shock degradation than insul.ators made of alumina, but also, unexpectedly, enable such bodies to perform almost as well as insulators composed of beryllium oxide, without the toxicity problems associated with the latter material. Therefore, the invention provides an improved igniter utilizing silicon nitride, which can be produced wi-thout the dangers to human health associated with the manufac-ture oE igniters containing beryllium oxide.
Accordingly, it is an object of this invention to provide an improved high energy igniter for jet and other internal combustion engines~
In particular, according to the present invention there is provided an igniter having a firing end and a terminal end, and comprising a metal shell releasably engagable wi-th an internal combustion engine, said shell having at the firing end of said igniter a surface which is an annular ground electrode, a ceramic insulator seated within said metal shell and having a central bore, a center electrode seated within the central bore of sai.d ceramic insulator and having a firing end in spaced, spark-gap relationship with the annular ground eleGtrode of said shell, and an insulator containing at least 50 percent by weight of silicon nitride seated within said shell and having a surface adjacent the spark gap along which a spark travels when discharged between the firing end of said center electrode and the ground electrode of said shell, Other objects and advantages of the invention will be apparent from the description which follows, reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a view in longitudinal cross section o~
a high voltage igniter assembly according to the instant invention.

,. ~ 6 -Figure 2 is an enlarged, part.ially schematic, vertical sectional view of a silicon ni-tride insulator which is disposed within the shell of the igniter assembly of Figure 1.

- 6a -1 Figure 3 is a partially schematlc view in 2 longltudinal cross section showing the firing end of 3 another embodiment of a high voltage lgniter assembly 4 according to the invention~

~ Referring in more detail to Flgure 1, a high 7 voltage, high energy igniter, indicated generally at 10, 8 has a firing end 11 and a terminal end 12. The igniter 10 g comprises a metal shell 13, a center electrode 14 and lnsulators 15, 16 and 17 mounted in an annular space 11 between the shell 13 and the electrode 14. The shell 13 has 12 an inwardly-directed annular portion 18, at the firing end 13 11 Or the igniter 10, constituting a ground electrodeO In 14 service, the igniter 10 is releasably engaged so that the annular portion 18 extends into the ~irin~ chamber of an 1~ associated engine (not shown), and is grounded to the 17 engine through contact therewith of the shell 13. The 1~ insulator 15, which is an alumina body, is seated adjacent 19 the terminal end 12 of the igniter 10 so as to form a cylindrical cavity 19 in which a contact of an associated 21 ignition system (not shown3 is electrically engagable with 22 a portion Or the center electrode 14 which extends into the X3 cavity 19. The insulator 16, which is an alumlna body, has 24 a central bore containing the center electrode 14 and extends axially ~rom the base of the cavity 19 to a tubular 26 portion 20 thereof terminating at a point short of the ~7 firing end 11 Or the igniter 10, and is there dlsposed 2~ between the insulator 17 and the center electrode 14 in an 29 annular space 21.

3~

2The insulator 17, which is a substantially pure 3 silicon nitride body, is seated within the annular space 21 4 and has a tubular portion 22 which is situated between a portion of the insulator 16 and the shell 13 and extends 6 axially toward the terminal end 12 of the igniter 10. The 7 center electrode 14 has a radially-enlarged firing end 23 8 which is mounted thereon and is disposed within the annular g space 21 so that a portion thereof is adjacent the silicon nitride insulator 17 at the firing end 11. The silicon 11 nitride insulator 17 extends axially from the tubular 12portion 22 thereof to the firing end 11 of the igniter 10, 13 having a surface 24 adjacent a spark gap between the firing 14 end 23 of the center electrode 14 and the annular ground electrode 18 of the shell 13.
16The silicon nitride insulator 17 is shown in 17~ vertical section to an enlarged scale in ~igure 2.
18Referring now in more detail to Figure 3, the firing end of a high voltage, high energy igniter according ~O to the invention is indicated generally at 25. The igniter 21 25 comprises a lower metal shell 26 connected to an upper 22 metal shell 27 by silver solder as indicated at 2~, a 23 center electrode 29 having a firing tlp 30, and annular 24 insulators 31 and 32. Only a tubular portion of the insulator 31 is shown. The lower shell 26 has an 2~ inwardl~-directed annular portion 33 constituting a ground electrode. In service, the igniter 25 is releasably engaged ~8 so that the annular portion 33 extends into the firing 29 chamber of an associated engine (not shown) and is grounded to the engine through contact therewith of the upper ~hell 27. The insulator 31, which is an alumina body, is seated ?. in an annular space 33 between the upper shell 27 and the 3 center electrode 29 and has a central bore in whlch a a portion Or the center electrode 29 and a tubular portion 34 of the insulator 32 are disposed. A talc seal 35, to 6 prevent gas leakage~ is positioned annularly between the 7 lower shell 26 and the tubular portion 34 of the insulator 8 32 and fills an area between an outwardly-dlrected annular g flange 36 of the insulator 32 and an inwardly-directed annular flange 37 of the lower shell 26.
11 The insulal;or 32, which is a substantially pure 12 silicon nitride body, extends axially from the tubular 13 portion 34 thereof to a point short of the annular ground 14 electrode 33 and is there disposed in an annular space 38 1~ between the lower shell 26 and the center electrode 29. A
16 ~ surface 39 Or the silicon nitride insulator 32 is ad~acent 17 a portion of a radially-decreased segment 40 of the center 13 electrode 29, while a surface 41 thereo:E is ad~acent a 19 spark gap between the firing tip 30 of the center electrode 29 and the annular ground electrode 33 Or the lower shell 2L 2~. .
22 It will be appreciated that an lnsulator having, 23 for example, the overall shape Or one of the in~ulators 17 24 and 32 of Figures 1-3, can be fabricated of any suitable sllicon nltride-based material rather than of substanti.ally 2e pure si:Licon nitride, and then used adJacent the firing end 27 of an igniter according to the lnvention so that spark ~8 discharge travels along a surface thereof. For example, 29 such an insulat.or can be fabricated from silicon-aluminum-oxynitrldes (SIALONs3~ or from compositions ~ r~ m~k 3~

"~~ 1 of silicon nitride or SIALONs and one or more additional 2constituents added as sintering aids, such as Y20 3, Ce203, 3La23, SC23- Cr2 03 , MgO, ZnO, ~iO, TiO 2~ SnO2and SrO~ .
4 Such sintering aids are particularly helpful during pressureless-sintering Or SIALONs. However, an lgniter 6 according to the present invention includes an insulator in 7 which the proportion Or silicon nitride is substantial, ~ i.e. at least 50 percent by weight, and most desirably at g least 65 percent by weight.
10Only two embodiments of the invention have been 11 described above with reference to the dra~ings. An ignlter 12 according to the invention can also be produced using a 13 two-piece insulator having, for example, the over-all shape 1~ of the insulator 17 of ~igures 1 and 2 and consisting, for example, of a hot-pressed substantially pure sllicon 16 nitride, or silicon nitride-based, "button" bonded to an 17 alumina insulator. The surface 24 should be of such sillcon 18 nitride materials; the button can be as thin as about 1/8 19 inch, preferably being at least 3/16 inch, and most 2~ desirably at least 1/ll inch thick. Silicon nitride 2L rnaterials are unexpectedly resistant to erosion by high 22 energy sparks of the type which occur in the igniter 10 and 2S in similar igniters, such as that illustrated in Fi~ure 3.
The silicon nitride insulator of an igniter ~5 according to the invention, whether a complete insulator or 26 a button adhered to an alumina insulator~ and, in either case, whether composed substantially of silicon nitride or ~8 of silicon nitride mixed with other materials, can be 29 produced by hot pressing, as indicated above, or by pressureless-sintering or reaction-bonding processes.

~ T~e ~71ark' 33~

1 Hot-pressed silicon nitride insulators are preferred, 2 however, because they have been found generally to exhibit 3 greater resistance to erosion than do such insulators 4 formed by pressureless-sintering or reaction-bonding methods. Accordingly, hot-pressed bodies are partlcularly 6 suitable for use in igniters which are subjected to severe 7 stresses in service, as, for example, in ~et engines.

~ However, where the geometry of an igniter requires g machining of an insulator body after fabrication and 1~ firing, pressureless-sintering or reaction-bonding is 11 preferred, as these methods provide enhanced machineability 12 for the body.

13 Although the preferred embodlments of the 14 invention have been described, it is to be understood that the scope of the invention is not limited thereto or 16 thereby. It will be apparent that various changes and 17 modifications can be made from the specific disclosure 1~ hereof without departing from the spirit and scope of the 19 invention as defined in the following claims.
~0 ~2 ~7

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An igniter having a firing end and a terminal end, and comprising a metal shell releasably engagable with an internal combustion engine, said shell having at the firing end of said igniter a surface which is an annular ground electrode, a ceramic insulator seated within said metal shell and hav-ing a central bore, a center electrode seated within the central bore of said ceramic insulator and having a firing end in spaced, spark-gap relation-ship with the annular ground electrode of said shell, and an insulator con-taining at least 50 per cent by weight of silicon nitride seated within said shell and having a surface adjacent the spark-gap along which a spark travels when discharged between the firing end of said center electrode and the ground electrode of said shell.

2. An igniter as claimed in claim 1 wherein said silicon nitride insulator and said ceramic insulator have tubular portions which are dis-posed within an annular space between said metal shell and at least a por-tion of said center electrode.

3. An igniter as claimed in claim 2 wherein said silicon nitride insulator extends axially from said tubular portion thereof toward the fir-ing end of the igniter to the surface along which the spark discharge occurs.

4. An igniter as claimed in any of claims 1-3 wherein the silicon nitride insulator is a hot-pressed ceramic.

5. An igniter as claimed in any of claims 1-3 wherein the silicon nitride insulator is a pressureless-sintered ceramic.

6. An igniter as claimed in any of claims 1-3 wherein the silicon nitride insulator is a reaction-bonded ceramic.