Sheathed glow plug

Abstract

A glow plug which includes an annular metal shell, thermally conductive tubular sheath, central electrode; resistance heating element, and electrically insulating, thermally conductive powder includes a glass seal in sealing engagement with the sheath and the electrode to form a sealed cavity within the sheath. The glass seal may include silicate, borate and borosilicate glasses, and may include one or more transition metal oxides, such as oxides of chromium, cobalt, nickel, iron and copper. The glass may also include a filler, including a ceramic oxide, such as one selected from a group consisting of quartz, eucryptites, leucites, cordierites, beta-spodumene, glass-ceramics, low-expansion glass(CTE<5 ppm / ° C.), mullite, zircon, zirconia and alumina. The sealed cavity may house a protective inert gas. The resistance heating element may be formed from a metal selected from a group consisting of tungsten, molybdenum, or alloys containing tungsten, molybdenum, nickel, iron, tantalum, niobium, titanium, vanadium, osmium and chromium.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application claims the benefit of U.S. Provisional Application Ser. No. 61 / 014,122, filed Dec. 17, 2007, and U.S. Provisional Application Ser. No. 61 / 061,387, filed Jun. 13, 2008, both of which are incorporated herein by reference in their entirety.BACKGROUND OF THE INVENTION[0002]1. Technical Field[0003]This invention relates generally to glow plugs and, more particularly, to sheathed glow plugs.[0004]2. Related Art[0005]Sheathed glow plugs, such as those used in diesel engine applications, generally have an electrical resistance heater which includes one or more resistance elements, such as a spiral wound resistive wire, which is embedded in an electrically insulating, thermally conductive powder, e.g., magnesium oxide, so as to be electrically insulated from the tubular sheath they are located in, except for electrical connection with a free closed end of the tubular sheath. Glow plugs using a single electrical resistance element m...

AI Technical Summary

Benefits of technology

[0009]In general terms, one aspect of this invention provides a sheathed heater for a glow plug which includes an annular metal shell having an axially extending bore; an electrically and thermally conductive tubular sheath having an open end disposed within the bore in electrical contact with the shell and a closed end projecting from the bore; an electrode extending into the open end of the sheath; a resistance heating element disposed in the sheath having a proximal end which is electrically connected to the electrode and a distal end which is electrically connected to the closed end of the sheath; an electrically insulating, thermally conductive powder disposed within the sheath and surrounding the resistance heating element; and a glass seal disposed in the open end and in sealing engagement with the sheath and the electrode. The heater assembly may be inserted into a shell to form a glow plug. The glass seal used provides improved hermeticity and thus improved resistance to environmental degradation of the resistance heater element and extends the operating range of the glow plug up to 600-800° C.

Problems solved by technology

Upon creating the seal, oxygen is commonly present within the interstices of the powder, and thus, the resistance element is potentially prone to oxidize in the presence of the oxygen.

While o-ring seals have been used in glow plug applications, their useful operating temperature range is about 150-200° C. Recently, glow plug applications are emerging where a higher operating temperature range is needed and o-ring seals are unsuitable.

During thermal cycling which occurs during operation of the glow plug, the surface of the wire oxidizes, reducing the effective cross-section of the wire, eventually leading to higher current density in this portion of the wire leading to overheating of the wire and failure of the heating element.

A factor affecting this mode of failure is the imperfect seal provided by the rubber or plastic gaskets or o-rings, which allows oxygen and water vapor to permeate into the packed powder bed and react with the heating element wire, resulting in oxidation and the reduction in effective cross-section described above.

Reaction of the magnesium oxide with the water vapor may form magnesium hydroxide, which can corrode or oxidize the metal resistance wire, thus, resulting in failure of the part even when the glow plug is not in service.

This failure mechanism can serve to reduce or otherwise limit the operational life of the glow plug.

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