GB2058216A

GB2058216A - Glow plug for internal combustion engines

Info

Publication number: GB2058216A
Application number: GB8028039A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-09-01
Filing date: 1980-08-29
Publication date: 1981-04-08
Also published as: DE2935424A1; IT1132600B; IT8024390A0; DE2935424C2; JPS5637424A; GB2058216B

Abstract

A glow plug for internal combustion engines includes a heating filament 12 embedded in insulating material 13 and enclosed by a heating tube 10 whose wall thickness is substantially uniform throughout the length of the plug, and whose outside diameter in the ignition region 10/1 is reduced. The tube is made of nickel alloy, the filament has a P.T.C. characteristic and the insulating material is magnesium oxide. <IMAGE>

Description

SPECIFICATION Glow plug for internal combustion engines The invention relates to a glow plug for internal combustion engines. Glow plugs are already known from German Patent (Offenlegungsschrift) No. 2744624; the glow plug described in the German Patent has, however, the disadvantage that, in order to achieve a short preheat time, a heating tube is required which is relatively expensive to manufacture and which exhibits variations in wall thickness.

According to the present invention there is provided a glow plug for internal combustion engines, having in its ignition region a heating filament which is located in a single-piece metallic heating tube and is embedded in compacted material having good heat conducting and electrical insulating properties, said filament being attached at its terminal end to a terminal pin, and, at its ignition end, to the base of the heating tube, the heating tube being of reduced diameter at the ignition end thereof but the wall thickness in said reduced diameter region being of the same thickness as in the non-reduced diameter portion.

In contrast to the known art, shaping of the glow plug of the present invention may be performed when the heating rod is finishassembled. No separate machine or forming operation is thus necessary for shaping the heating tube according to the present invention, whereby a substantial reduction in the cost of the heating rod may be achieved. Surprisingly, it has been shown that a heating rod according to the present invention has practically the same heating characteristic as a heating rod according to the German Offenlegungsschrift No. 2744624 which has a heating tube which is relatively expensive to manufacture and which exhibits variations in wall thickness.

Preferably in the glow plug of the present invention the reduced diameter portion is of a length of from 5mm to 1 5mm.

The heating filament preferably is in the form of a coil in which the coils are more closely spaced in the ignition region than in the adjacent region nearer to the terminal end. Preferably at least the region of the filament in the ignition region has a PTC characteristic.

The invention will now be further described by way of example, with reference to the accompanying drawing the single figure of which is an enlarged sectional view of a part of a heater plug in accordance with the invention. Glow plugs for internal combustion engines are well known (see German Patent Nos. 1119598 and 1190254) and a detailed description of such heater plugs has consequently been omitted.

The term "glow plug" is intended to include socalled flame igniter plugs having additionally a fuel feed system, as described, for example, in German Patent No.1301631.

Referring to the drawing, the part of the glow plug shown is that part adjacent to the ignition end, and comprises a single-piece heating tube 10. A terminal pin 11 is disposed inside the heating tube 10, and an electric heating filament 12, in the form of a wire coil is attached at one end to the terminal pin 11 and at its other end is seal-welded to closed end 14 of the heating tube 10. The tube 10 is filled with electrically insulating material 13, which material also is a good conductor of heat. The portion of the heating filament 1 2 adjacent to the ignition end of the glow plug is designated as 12/1 and has coils more closely spaced than those of the region 12/2 of the heating filament 12 adjacent to the terminal pin 11. The purpose of this feature is to impart a high energy density to the ignition region 10/1.In addition, in order to reduce the preheat time it is advantageous if at least the region 12/1 of the heating filament 12 consists of material having a PTC characteristic since this results in rapid heating-up of this region when the current is switched on; however, burning-out of the heating filament owing to the increased electrical resistance is precluded.

The heating tube 10 is made of a hightemperature-resistant, corrosion-resistant material such as a nickel alloy having the trade mark 'Inconel' and has a wall thickness substantially throughout of approximately 0.6 mm. The ignition region of the heater plug is 10 mm long, but may vary from type to type between 5 mm and 1 5 mm.

While the outside diameter of the heating tube 10 in the ignition region 10/1 is 4.5 mm, the diameter of the region 10/2 of the heating tube 10 extending as far as the heater plug housing (not shown) is 6 mm. Owing to the lower heat capacity in the ignition region 1 0/1, and also because of the high degree of compaction (due to circumferential hammering or a similar process) of the insulating material 1 3 consisting, for example, of magnesium oxide, such heater plugs heat up particularly rapidly.The reduction, by circumferential hammering, of the diameter of heating rods having built-in heating filaments embedded in insulating material 13 is known in principle; in the case of the glow plug according to the present invention, however, the ignition region 10/1 of the heating rod is subjected to an additional reduction process whereby its thermal capacity is further reduced and also heat transfer from the region 12/1 of the heating filament 12 to the heating tube 10 is improved. A further possible method of manufacturing a heating rod of this configuration consists in first of all reducing a heating tube 10 of constant diameter to a smaller diameter for a distance of 10 mm from the base 14 of the heating tube by means of circumferential hammering machines, next welding in the heating filament 1 2 and filling with the insulating material 13, and then hammering the hitherto unreduced portion of the initially mounted heating rod to approximately the same diameter as the already reduced region, and, lastly, reducing the ignition region 10/1 to its final diameter by a third circumferential hammering process.

Claims

1. A glow plug for internal combustion engines, having in its ignition region a heating filament which is located in a single-piece metallic heating tube and is embedded in compacted material having good heat conducting and electrical insulating properties, said filament being attached at its terminal end to a terminal pin, and, at its ignition end,~to the base of the heating tube, the heating tube being of reduced diameter at the ignition end thereof but the wall thickness in said reduced diameter region being of the same thickness as in the non-reduced diameter portion.

2. A glow plug as claimed in claim 1, in which said reduced diameter portion is of a length of from 5 mm to 15 mm.

3. A glow plug as claimed in claim 1 or 2, in which the heating filament is in the form of a coil in which coils are more closely spaced in the ignition region than in the adjacent region nearer to the terminal end.

4. A glow plug as claimed in any one of claims 1 to 3, in which at least the region of the filament in the ignition region has a PTC characteristic.

5. A glow plug constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.