GB2216952A - Glow plugs - Google Patents

Abstract

A sheathed element self-limiting glow plug incorporates a unitary element 26 formed from a single length of iron-cobalt alloy wire. In order to concentrate the heating effect at a tip portion 22 of the plug metal sheath 20 the wire may be more closely wound into a fine pitched portion 30 in that portion compared with a more coarsely pitched winding 28 in the remaining part of the sheath. The wire may be 0.3mm gauge. <IMAGE>

Description

GLOW PLUGS This invention relates to sheathed element selflimiting glow plugs as used in vehicle diesel and other compression ignition engines to facilitate speedy engine start-up.

The object of the invention is to provide a glow plug of particularly simple construction, which is durable and reliable in use, and which gives a quick response time without risk of damage due to overheating.

According to the invention there is provided a sheathed element self-limiting glow plug characterised by a unitary element of uniform composition throughout its length being a single length of resistance wire formed from an alloy of iron and cobalt, which element provides both operative heating of the plug and operative heating current self-limiting.

Iron/cobalt alloys which have been found to be particularly suitable are those manufactured or supplied by Vacuumschmelze GmbH of Hanau, German Federal Republic under their Trade Marks or designations "UACOFE" and "Co SPa11.

These alloys, formed as resistance wire, have a very high ratio of increase in electrical resistance in relation to rise in temperature, said ratio being of the order of 11 to 1.

An example of the invention is now more particularly described with reference to the accompanying drawing being a diagrammatic longitudinal section of a glow plug for a vehicle engine.

The body and sheath of the plug are of generally conventional construction comprising a housing 10 threaded at 12 for mounting in the engine and having a hexagon head 14 at its outer end. An insulated electrode 16 provided with a connecting terminal 18 at its outer end is sealed into and extends through housing 10 in known manner.

A hollow cylindrical metal sheath 20 projects from the inner end of body 10. It has a closed distal tip 22 and its proximal end part 24 is sealed within a bore of the inner end of housing 10 in known manner. Sheath 20 may be formed from the metal alloy known has "Inconel".

A single unitary coiled resistance wire element 26 is connected within sheath 20 between the inner end of electrode 16 and the interior of sheath 20 at the extremity of the tip 22 e.g. by welding. Element 26 is of uniform composition throughout its length and it is formed of an alloy of iron and cobalt, for example one of the alloys supplied by Vacuumschmelze GmbH referenced above.

In order to concentrate the heating effect within the tip 22 of the sheath the coiled element 26 is differently wound along its length, approximately 50% of the total wire length is formed into a coarse pitched coiled portion 28 extending along approximately two thirds of the length of sheath 20 being the proximal part extending from the inner end of electrode 16, and the remaining 50% of the length is wound into a fine pitch coil portion 30 occupying the remaining and distal one third length of sheath 20 so concentrating the heating effect at the tip.

The interior of sheath 20 is packed with a filler, typically magnesium oxide, to surround and electrically insulate element 26 in known manner.

Various lengths and gauges of the iron/cobalt resistance wire may be used and the coils may be disposed uniformly along its length or with varying pitches differing from those indicated above. However, particularly successful results have been obtained using 0.3mm gauge wire having a total length of about 170 185mm wound as described above giving rapid heating of the -tip 22 of sheath 20 to a temperature around 8500 centigrade over about 6mm back from the distal extremity in around 7 seconds or less so providing rapid start-up of the engine, yet the high ratio of approximately 11 to 1 change in electrical resistance of the element 26 in relation to its rise in temperature prevents overheating and ensures safe and reliable operation without the need to incorporate separate current-limiting resistance coils or elements.

It has not hitherto been practical to manufacture a glow plug of such simple and reliable construction and operation using a single element.

The construction using the single element avoids the need to join together heating and resistance elements or coils formed from dissimilar materials within the sheath thus reducing manufacture costs and increasing reliability and durability.

Claims (8)

1. A sheathed element self-limiting glow plug characterised by a single unitary resistance wire element, the material forming said element dyeing an alloy of iron and cobalt and being of uniform composition thoroughout the element length, said element providing both operative heating of the plug and operative resistive self-limiting of the heating current.

2. A plug as in Claim 1 wherein said alloy wire has a high ratio of increase in electrical resistance in relation to rise in temperature.

3. A plug as in Clai 2 wherein said ratio is of the order of 11 to 1.

4. A plug as in any preceding claim wherein the said alloy is selected from those manufactured or supplied under the Trade Rarks or designations VAC5-E or CO 8re .

5. h plug as in any preceding claim wherein the element is coiled a first portion of the total element length being formed into a coarse pitched coiled portion positioned in a proximal part of the sheath and the remainder of said length being wound into a fine pitch portion in a distal part of the sheath for concentratinc, the heating effect at a tip of the sheath.

6. A plug as in Claim 5 wherein each said portion of the element length is approximately 50 of its total length and said coarse pitched portion occupies approximately two thirds of the length of the sheath with the fine pitched portion occupying the remaining distal one third length.

7. A plug as in Claim 6 wherein the element is formed of 0.3mm gauge wire, said wire having a total length of about 170-185mm.

8. A sheathed element self-limiting glow plug substantially as hereinbefore described with reference to and as shown in the accompanying drawing.