GB2198786A - Glow plug - Google Patents

Abstract

Electrical glow plug, e.g. for starting diesel engines, of the sheathed element self-limiting type incorporates a low resistance body 30, typically of nickel or other metal, connected between heater and resistance coils 26, 28 within the sheath 20. <IMAGE>

Description

GLOW PLUG This invention relates to electrically operated glow plugs used as a aid to speedy startup of internal combustion engines, in particular diesel and other compression ignition engines as used in vehicles.

More specifically the invention relates to glow plugs of the kind including a housing adapted to mount the plug in the wall of a cylinder or other combustion chamber of the engine in use, a tubular sheath carried by the housing to project into the combustion chamber in use, the distal extremity or tip of the sheath containing a totally enclosed heating element whereby the tip of the sheath speedily attains a high temperature when electric current is applied thereto by way of an electrode carried in the housing, and a resistance element also totally enclosed in the sheath electrically interconnecting the heating element with the electrode and formed of a material having a high positive temperature coefficient of resistance. This kind of glow plug is hereinafter referred to as sheathed element self-limiting glow plug".

Examples of this kind of plug are described in U.S. Patents 4592134 and 4477717 (Wellman Thermal Systems Corporation); and in U.S. Patent 4549071 (Jidosha Kiki Co Ltd), British Patent 1376B17 (Robert Bosch GmbH), German Patent 2744624 (Robert Bosch GmbH) and British Patent 2013277 (Beru-Werk Albert Ruprecht GmbH 1 Co KG).

The object of the present invention is to provide a glow plug of said kind which is economical to manufacture, reliable and durable in use, and which will attain its working temperature very quickly for fast starting of an engine from cold yet without any risk of burning out or damaging its heating element even if current is applied for a long period.

According to the invention there is provided a sheathed element self-limiting glow plug characterised in that the heater and resistance elements are wire coils and are electrically interconnected by way of a low resistance body which is within an intermediate zone of the portion of the sheath which extends longitudinally beyond the housing so that the coils are spaced from each other longitudinally of said portion of the sheath.

Preferably said body is a solid body, e.g. a cylindrical metal rod, having the ends of the coils anchored to its respective end portions, e.g. by welding.

Preferably said intermediate zone along which the body extends forms a substantial proportion of the length of the sheath between the tip occupied by the heating element and the housing, e.g. about one quarter or possibly one third or more of said length.

It is also preferred that a substantial portion of the resistance coil is located within a proximal end part of the sheath which is located within the housing, preferably with the housing in close fitting contact with the exterior of said part of the sheath.

The presence of the body between the two coils provides a heat sink effect which restricts any tendency for heat flow between the coils particularly in the initial period of operation in which the heating coil attains its working temperature. Moreover use of the body greatly facilitates the assembly of the coils and the forming of a good and reliablel electrical interconnection between them, reducing manufacturing costs and facilitating volume production to consistently high quality standards.

If a substantial part of the resistance coil extends back into the part of the sheath intimately enclosed by the housing, as is preferred, this provides a further heat sink effect so that the selflimiting effect of the resistance coil is slowed during initial operation to give full current feed to the heating coil for fast response yet the resistance coil can be dimensioned to provide full and adequate limiting protecting the heat coil once working temperature has been reached so that, even if the plug is left inadvertently switched on it will not burn out or be damaged.

One example of the invention will now be described with reference to the accompanying drawing which is a diagrammatic side elevation of a glow plug with its sheath portion shown in section.

The plug comprises a tubular metal housing 10 of conventional shape including an externally screwthreaded portion 12 which will be received in a threaded bore through the wall of an engine combustion chamber in use, its outer end being provided with a hexagon head 14 for engagement by a spanner. An insulated electrode 16 having 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. The distal extremity or tip 22 remote from the body is totally closed and the proximal end part 24 of the sheath is closely fitted with a bore of the inner end of housing 10 and is closed in sealing engagement with electrode 16 in known manner, for example as described in said U.S.

Patents 4477717 and 4592134.

A wound wire heating coil 26 is located within tip 22, its outer end being electrically connected, e.g. by welding, to the tip (sheath 20 and housing 10 constituting the earth return side of the electrical circuit).

A wound wire resistance coil 28 within sheath 20 has one end electrically connected to electrode 16, e.g. by welding. This coil is substantially longer than coil 26 and extends from the part of sheath 20 within housing 10 along the exposed portion of the sheath towards coil 26.

Between the two coils within said exposed portion is located a body of low resistance material, in this example a metal, e.g. nickel, solid cylindrical rod 30, the other ends of coils 26 and 28 being electrically connected to the two end parts of rod 30 to complete the circuit while spacing apart the two coils.

The rod 30 occupies, in length, a substantial intermediate zone of the exposed portion of sheath 20 outside housing 10.

While various dimensions and proportions may be employed depending, for example, on the available space within the combustion chamber, the operational conditions within the chamber, and the desired working temperature of the plug, it is contemplated that the rod 30 could have an axial length of around 6mm and could occupy one fifth or more of the exposed length of the sheath extending from housing 11 to the tip 22 which is occupied by heating coil 26, and that about one third or more of the length of resistance coil 28 would be located within that part of sheath 20 enclosed by the housing 10.

Heating coil 26 may be made of "KanthalR/alloy and the resistance coil 28 may be made of nickel or nickel alloy.

Sheath 20 may be filled with electrically insulating material which would also keep the coils and rod in place, for example an inert granular material, in known manner. Sheath 20 is typically formed from a stainless steel.

Claims (10)

1. A sheathed element self-limiting glow plug characterised in that the heater and resistance elements are wire coils and are electrically interconnected by way of a low resistance body which is within an intermediate zone of the portion of the sheath which extends longitudinally beyond the housing so that the coils are spaced from each other longitudinally of said portion of the sheath.

2. A glow plug as in Claim 1 wherein said body is a solid body.

3. A glow plug as in Claim. 2 wherein said body is a cylindrical metal rod having the ends of the coils anchored to its respective end portions.

4. A glow plug as in any preceding Claim wherein the body is formed of nickel.

5. A glow plug as in any preceding Claim wherein said intermediate zone along which the body extends form a substantial proportion of the sheath exposed between the tip occupied by the heating element and the housing.

6. A glow plug as in Claim 5 wherein said intermediate zone extends for at least one-fifth of the length of the sheath so exposed.

7. A glow plug as in any preceding Claim wherein a substantial portion of the resistance element is located within a proximal end part of the sheath which is located within the housing.

8. A glow plug as. in Claim 7 wherein said proximal end part is intimately enclosed by the housing in close fitting contact therewith.

9. A glow plug as in Claim 7 or Claim 8 wherein about one third or more of the length of the resistance element is located within said proximal end part.

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