US3890518A - Spark ignition plug - Google Patents

Abstract

A spark ignition plug comprises an inner electrode 13 a portion 15 of which is formed from tungsten and an outer electrode 12, there being an air gap disposed between the electrodes. The tungsten is provided with a diffused surface layer 16 of oxidation resistant material.

Description

United States Paten Tombs 1' June 17, 1975 SPARK IGNITION PLUG {56] References Cited [75] Inventor: Terence Leslie Tombs, Birmingham, UNITED STATES PATENTS England 2.391.458 12/1945 Hense] 313/141 2,926,275 2/1960 Peras [73] Assgnee' 9 Lucas undustr'es) 3,488,556 111970 Burley I. 313/131 R Birmlngham, England [22] Filed: June 1, I973 Primary Examiner-R. V. Rolinec Assistant Examiner-Darwin R. Hostetter [2]] Appl' 7 Attorney, Agent, or Firm-Holman and Stern [30] Foreign Application Priority Data {57] ABSTRACT June I, I972 UIIIICd Kingdom 7. 72-25628 A Spark ignition comprises an inner electrode a portion 15 of which is formed from tungsten and an [52] Cl 313/130 313/131 33,131 outer electrode 12, there being an air gap disposed be- I t Cl 26:: tween the electrodes. The tungsten is provided with a n I I t d l t- -t t t [58] Field of Search 313/130, 131 R, 137, 13s, ri'aL Se ace 0 ma 8 4 Claims, I Drawing Figure PATENTEDJUN 17 ms 1 SPARK IGNITION PLUG This invention relates to a spark ignition plug of the kind including an outer, annular electrode, an inner electrode formed at least in part of tungsten and an air gap between the electrodes.

According to the invention, in a spark ignition plug of the kind specified the tungsten part of the inner electrode is provided with a diffused surface layer of an oxidation resistant material.

Preferably, the oxidation resistant material is silicon or alternatively is aluminium.

Preferably a member formed from a material having a high dielectric constant is interposed between the air gap and one of the electrodes.

Conveniently said material having a high dielectric constant is calcium titanate.

The accompanying drawing is a sectional view of part of a spark ignition plug according to one example of the invention.

Referring to the drawing, the spark ignition plug shown is a surface discharge igniter plug and includes a hollow, cylindrical electrode body 11 formed of 18/8 stainless steel. Extending within the bore in the body 11, but spaced from the inside surface thereof, is an inner electrode 13. The electrode 13 is formed in two parts, that is a main part 14 formed from nickel supplied by the lnternational Nickel Company Limited as NILO 36, and an electrode tip 15 formed from commercially pure tungsten obtained from the Tungsten Manufacturing Company Limited of Brighton, Sussex. The electrode tip 15, which is of substantially cylindrical configuration, is resistance welded at one end to the main electrode part 14 and is so dimensioned that the opposite, free end surface of the tip lies flush with one annular, end surface 12 of the body 11.

The electrode tip 15 is provided with an oxidation resistant layer 16 of diffused silicon in its surface presented to the body 11. Conveniently, the siliconised layer 16 is formed by surrounding the tip 15, prior to its connection to the main electrode part 14, with a powder pack containing fine silicon powder, such as of 3 micron particle size, and ammonium chloride and heating the assembly at 1060C for 4 hours in a flow of argon. In this way, a siliconized layer 16 is produced over the tip 15, although alternatively the layer 16 may be produced by siliciding. A typical value for the thickness of the layer 16 is microns. When production of the layer 16 is complete, the layer is removed from the axial end surface of the tip 15 to be connected to the main electrode part 14 so that the tip can be resistance welded to the part 14.

lnterposed between the electrode 13 and the body ll is an insulating member 17, an end portion 17a of which engages the body 11 and lies flush with the annular, free end surface 12 of the body ll. The insulating member 17 is formed ofa material having a high dielectric constant and, since the igniter plug of the present example is intended for use in gas turbine engines for igniting an air-fuel mixture, the material of the member 17 is also arranged to be resistant to corrosion by the hot gases resulting from combustion of such a mixture. A convenient material for the insulating member [7 is therefore found to be calcium titanate. and, for example, that supplied by Steatite and Porcelain Products Limited as TEMPRADEX NlSOO. This material has a dielectric constant of at l MC/s and 20C.

The arrangement of the electrode 13, the electrode 12 and the insulating member 17 is such that there exists a small, annular air gap 18 between the electrode tip 15 and the end portion 17a of the insulating member 17. In operation, when the electrodes l2, 13 are connected to a voltage surce sufficient to break down the air gap 18 a spark occurs between the tip 15 and the insulating member 17, which then quickly transfers to the outer electrode 12. Any tendency for the surface of the tungsten tip 15 presented to the insulating member 17 to be oxidised during sparking is substantially prevented by the surface layer 16. In conventional igniter plugs of this kind, however, the surface layer 16 is not present and it is found that an oxide film tends to grow over the surface of the tungsten tip which can result cracking of the member 17 and hence failure of the plug.

[t is to be appreicated that other oxidation resistant materials than silicon, such as aluminium, can be used to provide the layer 16, although in general silicon is preferred.

I claim:

1. A spark ignition plug including an outer, annular electrode, an inner electrode formed at least in part of tungsten and an air gap between the electrodes, the tungsten part of the inner electrode being provided with a diffused, oxidation resistant surface layer of silicon.

2. A plug as claimed in claim 1 including a member formed from a material having a high dielectric constant interposed between the air gap and one of the electrodes.

3. A plug as claimed in claim 2 in which said member is positioned adjacent the outer annular electrode.

4. A plug as claimed in claim 2 in which the material is calcium titanate.

Claims (4)

1. A SPARK IGNITION PLUG INCLUDING AN OUTER, ANNULAR ELECTRODE, AN INNER ELECTRODE FORMED AT LEAST IN PART OF TUNGSTEN AND AN AIR GAP BETWEEN THE ELECTRODES, THE TUNGSTEN PART OF THE INNER ELECTRODE BEING PROVIDED WITH A DIFFUSED, OXIDATION RESISTANT SURFACE LAYER OF SILICON.

2. A plug as claimed in claim 1 including a member formed from a material having a high dielectric constant interposed between the air gap and one of the electrodes.

3. A plug as claimed in claim 2 in which said member is positioned adjacent the outer annular electrode.

4. A plug as claimed in claim 2 in which the material is calcium titanate.

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