SHEATHED GLOW PLUG

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 61/014,122, filed December 17, 2007, and U.S. Provisional Application Serial No. 61/061,387, filed June 13, 2008, both of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

[0002] This invention relates generally to glow plugs and, more particularly, to sheathed glow plugs.

2. Related Art

[0003] 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 may have a positive temperature coefficient characteristic (PTC characteristic), and in those using two series connected electrical resistances, the resistance which is connected to the electrode of the glow plug has a higher PTC characteristic than the resistance which is connected to the free closed end of the tubular sheath.

[0004] Glow plugs of the type described, whether having one or more resistance elements, have their resistance elements totally embedded in the insulating powder, and the insulating powder is sealed in the tubular sheath using an elastomeric o-ring seal or other seal shape. These o-ring seals have been made using numerous elastomers or plastics, including various fiuoropolymers such as those sold by DuPont under the Viton ^® brand. 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 ⁰ C, Recently, glow plug

applications are emerging where a higher operating temperature range is needed and o- ring seals are unsuitable.

[0005] 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. Other materials, including gases, that are adsorbed onto the surface of the magnesium oxide powder may also contribute to the degradation of the heating element wire. This failure mechanism can serve to reduce or otherwise limit the operational life of the glow plug. [0006] In view of the above, there remains a need for glow plugs that can be used at operating temperatures above 200 ⁰ C, that have resistance elements that can withstand elevated temperatures, and further, that can provide an improved seal between the electrode and the sheath.

SUMMARY OF THE INVENTION

[0007] 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 ⁰ C.

[0008] In one aspect, the glass of the seal is selected from a group consisting of a silicate glass, a borate glass and a borosilicate glass. The glass may preferably be substantially lead free.

[0009] In another aspect, the glass may include an oxide of a transition metal as a constituent of the glass. The transition metal may be selected from a group consisting of chromium, cobalt, nickel, iron and copper. The oxide may be 10 mole percent or less of the glass.

[00010] In another aspect, the glass may include a recrystallized microstructure.

The recrystallized microstructure may include more than 90 volume percent of the glass. [00011] In another aspect, the glass may include a filler as a constituent of the glass. The filler may include a ceramic oxide. The ceramic oxide may be selected from a group consisting of quartz, eucryptites, cordierites, glass-ceramics, mullite, alumina, zircon, zirconia, beta-spodumene, low-expansion glass (CTE<5ppm/°C) and leucite. [00012] In another aspect, the sheath has an outer diameter that varies along its length such that the outer diameter has a reduced diameter portion proximate the open end. The outer diameter and the reduced diameter portion may have a diametral difference of about 0.4 mm. The glass seal has a length, and the reduced diameter portion has a length, and the length of the reduced diameter portion is greater than the length of the glass seal. In an exemplary embodiment, the length of the reduced diameter portion is about 8 mm. The sheath may include a metal. The sheath may have a deformed microstructure.

[00013] In another aspect, the glass seal includes a hermetic seal enclosing a cavity provided by the sheath, and further includes a protective gas disposed in the cavity. The protective gas may be selected from a group consisting of nitrogen, helium, neon, argon, krypton and xenon.

[00014J In another aspect, the resistance heating element may include a metal wire spiral. The metal wire spiral may include a metal selected from a group consisting of pure nickel, a nickel alloy, a nickel-iron-chromium alloy and an iron-cobalt alloy. [00015] In another aspect, the resistance heating element may include a metal wire spiral. The metal wire spiral may include a metal selected from a group consisting of tungsten, molybdenum, alloys containing tungsten, molybdenum, nickel, iron, tantalum, niobium, titanium, vanadium, osmium and chromium.

[00016] In another aspect, the thermally conductive, electrically insulating powder may include magnesium oxide.

[00017] In accordance with another aspect, a method of making a heater assembly for a glow plug is provided. The method includes providing an electrically and thermally conductive tubular sheath having an open end and a closed end, and further providing an electrode having a resistance heating element electrically connected to one end of the electrode. Further, extending the resistance heating element into the open end of the sheath and connecting an end of the resistance heating element to a distal end of the sheath to form the closed end of the sheath. Then, disposing an electrically insulating, thermally conductive powder within a cavity between the sheath and the resistance heating element. Then, forming a hermetic seal in the open end of the sheath between and in sealing engagement with the sheath and the electrode to close off the cavity from potential permeation of water vapor and/or oxygen into the cavity.

[00018] In another aspect, the method further includes evacuating any oxygen within the cavity by disposing inert gas into the cavity prior to forming the hermetic seal.

The inert gas can be selected from a group consisting of nitrogen, helium, neon, argon, krypton and xenon.

[00019] hi another aspect, selecting the resistance heating element from a group consisting of tungsten, molybdenum, alloys containing tungsten, molybdenum, nickel, iron, tantalum, niobium, titanium, vanadium, osmium and chromium.

[00020] In another aspect, the method further includes forming the hermetic seal as a glass seal.

[00021] In another aspect, selecting the glass of the seal from a group consisting of a silicate glass, a borate glass and a borosilicate glass.

[00022] In another aspect, providing the glass to include an oxide of a transition metal as a constituent of the glass. The transition metal may be selected from a group consisting of chromium, cobalt, nickel, iron and copper.

[00023] In another aspect, providing the glass to include a filler as a constituent of the glass. The filler may include a ceramic oxide. The ceramic oxide ma\ be selected from a group consisting of quartz, eucryptites, cordierites, glass-ceramics, mulhte, alumina, zircon, zircom ^' a, beta-spodumene, low-expansion glass (CTE<5ppm _' ^/o C) and leucite.

[00024] In another aspect, forming the glass seal by inserting a glass preform into the open end of the sheath and heating the glass preform for a time and temperature sufficient to melt the glass and form the glass seal.

BRIEF DESCRIP TION OF THE DRAWINGS

[00025] These and other aspects, features and advantages of the invention will become more readily appreciated when considered in connection with the following detailed description of presently preferred embodiments and best mode, appended claims and accompanying drawings, in which:

[00026] Figure 1 is a partial cross-sectional view of a sheathed heater assembly and glow plug of the invention;

[00027] Figure 2 is a cross-sectional view of a tubular sheath preform of the invention;

[00028] Figure 3 is a front view of a resistance heater element of the invention;

[00029] Figure 4A is a schematic sectional view of a glass preform inserted in the annular gap between the electrode and tubular sheath;

[00030] Figure 4B is a schematic sectional view of a glass seal in the annular gap between the electrode and tubular sheath formed by melting the glass preform of FIG.

4A; and

[00031] Figure 5 is a flow chart of a method of making a glow plug of the invention.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS [00032] This invention provides a glow plug with an improved seal and heating element assembly which reduces the exposure of the thermally conductive and electrically insulating powder and spiral wire heating element that is embedded in the powder to oxygen and water vapor, thereby eliminating or substantially reducing the degradation processes described above. In glow plugs of the invention the components are processed in such a way that oxygen and water vapor are removed or substantially reduced within the powder bed during the installation of the seal. Once installed, the seal eliminates or greatly reduces the ability of the ambient atmosphere, which includes oxygen and water vapor, to permeate the insulating powder and reach the wire heating

element, thereby inhibiting the potential for degradation of the wire heating element as described above.

[00033] Glow plugs and glow plug heater assemblies of the invention utilize a glass or glass-ceramic seal in lieu of an elastomeric or plastic seal, such as an o-ring seal. The glass or glass-ceramic seal provides electrical resistance between the shell and electrode and produces a hermetic seal between the powder bed containing the resistance heating element and the ambient atmosphere. A glass sealing material is positioned in the glow plug assembly either as a preform or from a loose powder that is formed in place by tamping or compaction. The preform may comprise a compressed powder or green powder compact or a portion of a substantially fully dense glass tube. The glass preform is heated to melt the glass and cause it to bond to the electrode and the sheath. The material may also be heat-treated to transform the glass and form a recrystallized glass-ceramic. Heating of the glow plug heater assembly to form the seal causes the magnesium oxide powder to out-gas, which removes potential reactant species such as oxygen and water that are known to contribute to degradation of the heating element wire. A preferred method is to heat the heater assembly in a vacuum and/or inert gas atmosphere in order to more completely remove the aforementioned reactant species. Accordingly, upon constructing a glow plug in accordance with the invention, the glow plug is able to exhibit a long and enhanced useful life that is substantially free from the potentially negative affects discussed above with regard to elastomeric or plastic seals, while also being able to operate in extremely high temperature engine application environments.

[00034] Referring in more detail to the drawings, Figure 1 illustrates a glow plug

10 constructed in accordance with one presently preferred embodiment of the invention. The glow plug 10 has an annular metal shell 12 with a bore 14 which extends along a longitudinal axis 15 of the shell 12. The metal shell 12 may be formed from any suitable metal, such as various grades of steel, and may also incorporate a plating or coating layer, such as a nickel or nickel alloy coating, on the surfaces thereof, including an exterior surface 16 and the bore 14, to improve the resistance of shell 12 to high temperature oxidation and corrosion. The glow plug 10 also includes a heater assembly 18. The heater assembly 18 has a tubular sheath 20, an electrode 22, a resistance heating element 24, an insulating powder packing material 26, an inert gas 27 occupying any space that is not occupied by solid matter, and a hermetic seal, referred to hereafter as a

glass seal 28 unless otherwise specified, to prevent atmospheric species, e.g. oxygen and water vapor, from entering the sealed area within the tubular sheath 20. [00035] The tubular sheath 20 is electrically and thermally conductive, and is preferably formed from a metal. Any suitable metal may be used to form the tubular sheath 20, but the metal will preferably be resistant to high temperature oxidation and corrosion, particularly with respect to combustion gases and reactant species associated with the operation of an internal combustion engine. An example of a suitable metal alloy is a nickel-chrome-iron-aluminum alloy. The tubular sheath 20 has an open end 30, which is disposed within the bore 14 of the metal shell 12 and in electrical contact with the shell 12, and a closed end 32 which projects from the bore 14. The tubular sheath 20 can be provided with an outer diameter (Dl) that varies along its length such that the outer diameter has a reduced diameter portion 34 proximate open end 30. The outer diameter (Dl) maybe any suitable diameter, with a typical outer diameter for many glow plug applications being about 4mm, for example. The reduced diameter portion 34 generally has a length greater than the length of the glass seal 28. In an exemplary embodiment, without limitation, the length of the reduced diameter portion 34 was about 8mm. The diametral difference between outer diameter (Dl) and reduced diameter portion 34 maybe any desired amount, depending on the application requirements, but in an exemplary embodiment, the differential was about 0.4mm. The tubular sheath 20 may have a deformed microstructure, such as a cold-worked microstructure, where a sheath preform 36 (Figure 2) is formed by swaging or otherwise to reduce the diameter and increase the density of the insulating powder 26 in the sheath 20. In an exemplary embodiment, deformation may amount to about a 20% reduction in the wall thickness of sheath preform 36, as shown schematically in phantom in Figure 2. [00036] The electrode 22 extends into the open end 30 of the sheath 20. The electrode 22 may be made from any suitable electrically conductive material, but is preferably a metal. In an exemplary embodiment, the electrode 22 is made from steel. Examples of suitable grades of steel include AISI 1040, AISI 300/400 family, EN 10277- 3 family; Kovar *UNS K94610 and ASTM F15, 29-17 alloy. The outer surface 38 of the electrode 22 will generally be cleaned thoroughly prior to incorporation into the heater assembly 18 to remove volatile contaminants, such as oils, from the surface of the electrode in order to enhance the ability of the glass seal 28 to bond to the electrode 22. The outer surface 38 of electrode 22 proximate the glass seal 28 may also be oxidized.

When oxidation is employed, the oxide layer will be developed to a thickness suitable to provide the needed adhesion, which for most oxide layers will be in the range of about 0.2-5.0 microns. Also, the distal end 39 of the electrode 22 may be formed, such as by reducing the diameter, to facilitate fitting the resistance heating element 24 onto the electrode 22 and to provide a shoulder 41 to seat the element, if desired, in conjunction with its attachment to electrode 22.

[00037] The resistance heating element 24 may be provided of any suitable heating device and have any suitable resistance characteristics so long as it is operable to provide the necessary time/temperature heating response characteristics needed for the glow plug 10, and can be provided to withstand extremely high operating temperatures between about 2000-3422 ⁰ C. This may include an element comprising a single electrical resistance element with a positive temperature coefficient characteristic (PTC characteristic), or two electrical resistance elements connected in series (Figure 3), where the first resistance element 40, which is connected to the electrode 22 of the glow plug 10, has a higher PTC characteristic than the second resistance element 42, which is connected to the closed end 32 of the tubular sheath 20. Thus, the first resistance element 40 acts as a current limiter or regulator element, and the second resistance element 42 acts a the heating element. The spiral wire resistance heating elements can be formed from any suitable material, including various metals, such as pure nickel, and various nickel, nickel-iron-chromium and iron-cobalt alloys, for example. However, if an extremely high temperature application is present, the resistance heating element or elements are preferably formed from a high-temperature resistant material, such as, for example, tungsten, molybdenum, or alloys containing tungsten, molybdenum, nickel, iron, tantalum, niobium, titanium, vanadium, osmium and chromium. Referring again to Figures 1 and 3, a spiral wire, two resistance element heating element 24 is disposed in the tubular sheath 20 with a proximal end 44 which is electrically connected and mechanically fixed by a metallurgical bond, such as a weld, to the electrode 22 and a distal end 46 which is electrically connected and mechanically fixed by a metallurgical bond to the closed end 32 of the sheath 20. This mechanical attachment and metallurgical bond is formed when the distal end 46 of the resistance heating element 24 is welded to the distal end 48 of the sheath preform 36 (Figure 2). This weld also forms the closed end 32 of the tubular sheath 20 by sealing an opening 50 in the distal end of the sheath preform 36.

[00038] The electrically insulating, thermally conductive packing powder 26 is disposed within the sheath 20 to surround the electrode 22 and to completely fill all the space between the resistance heating element 24 and the inner volume of a cavity 52 of the sheath 20. The powder 26 may include any suitable electrically insulating and thermally conductive powder, such as magnesium oxide, aluminum dioxide, or mullite. Loose powder is inserted into the cavity 52 of the sheath preform 36, which is the space between the inner surface 58 of the sheath 20 and the combination of the outer surface 38 of the electrode 22 and the outer surface of resistance heating element 24, through an annular gap 54 after the attachment of resistance heating element 24 to the sheath preform 36 and the closure of the opening 50 by the associated weld which attaches these elements to one another. The thickness of the annular gap 54 may be any suitable thickness; however, it is believed that a width of the annular gap 54 between about 0.2- 1.0 mm will be suitable for many applications of the resistance heater assembly 18. The width of the annular gap 54 is determined by the diametral difference of the inside diameter of the sheath preform 36 and the outside diameter of the electrode 22 at the proximal end 30 of the sheath preform 36. The cavity 52 generally does not include the space in the annular gap 54 which is the necked-down portion at the proximal end of sheath preform 36, as this is the space that is operative to receive the glass preform 56; however, powder may extend slightly into the distal end of the annular gap 54 so long as sufficient space remains to form the desired configuration of the glass seal 28. Oxidation of both the sheath 20 and electrode 22 may be done in conjunction with heating the powder 26 in oxygen following its placement into the cavity of sheath 20 in order to reduce or eliminate adsorbed water from the powder 26. In an exemplary embodiment of the invention, the electrically insulating, thermally conductive powder 26 is magnesium oxide which can be compacted around the resistance heating element 24 in conjunction with reducing the diameter of the sheath preform 36 to form sheath 20. The compacted powder 26 provides the desired thermal conductivity while also electrically isolating the resistance heating element 24 from the sheath 20. The powder 26 must also be operative for use over the extended operating temperature range of glow plug 10, and in extremely high temperature engine environments, can operate up to about 2000-3422 ⁰ C. [00039] The inert gas 27 can be provided, by way of example and without limitation, as helium, neon, argon, krypton, or xenon. The inert gas 27 is disposed to fill all the space within the cavity previously occupied by oxygen, and thus, the oxygen is

completed evacuated from the cavity. Accordingly, the inert gas 27 fills all the space between the sheath 20, the electrode 22, the resistance heating element 24 and the space between the individual grains of the powder 26.

[00040] The glass seal 28 is located in the open end 30 of the sheath 20 and is in sealing engagement with the sheath 20 and the electrode 22 in the annular gap 54. Following insertion of powder 26 into cavity 52, and upon evacuating all the oxygen within the cavity 52 by introducing the inert gas 27 in its place, a glass preform 56 is inserted into the annular gap 54, as shown in Figure 4A. Then, the glass preform 56 is heated sufficiently to fully densify and bond the preform 56 to the outer surface of the electrode 22 and the inner surface 58 of the sheath 20, preferably by melting the glass, followed by cooling, to form the glass seal 28, as shown in Figure 4B. The glass seal 28 forms a hermetic seal between the cavity 52 and the powder 26 and the bore 14 of the shell 12, thus preventing any contaminants, including combustion gases and ambient oxygen or water vapor, which penetrate the bore 14, from entering the cavity 52. [00041] Formation of the glass preform 56 may include providing a glass powder which is poured into the annular gap 54 and tamped to compact the powder and form the glass preform 56, or a compacted green powder preform which is formed separately and simply inserted into the annular gap 54, or a portion of a fully dense glass tube which is cut to the appropriate length and inserted into the annular gap 54. Where pre-compacted preforms are used, further compaction may be performed after the preform is placed into the heater assembly and prior to melting the glass to form glass seal 28. All manner of preforms are contemplated within the scope of the invention, including those that are pressed, uniaxially, isostatically or otherwise, and then sintered at a temperature less than the softening point of the glass to consolidate and sinter the material, for example, up to about 95 % of theoretical to facilitate handling the preform, such as placement into the heater assembly. The glass powder includes powdered, granulated and spray-dried glass materials. Whether compacted in place in the heater assembly or a free-standing preform, the glass preform 56 will generally be compacted using a pressure in the range of 1-50 kpsi (6.9-340 MPa).

[00042] The glass seal 28 may be formed from any suitable glass, including a silicate glass, a borate glass or a borosilicate glass in any combination. Where more than one type of glass or glass composition is used, it is preferred that the glass transition temperature of the more abundant glass be greater than the glass transition temperature

of the less abundant glass, and more preferred that the difference in glass transition temperatures be about 30 ⁰ C or more. It is further preferred that the less abundant glass be present in an amount of about 45 volume percent or less of the total glass constituents, and more preferred that the less abundant glass be present in an amount of about 5-45 volume percent. It is believed to be preferred that the glass utilized be substantially lead free. The glass material will be selected so to ensure no softening of the glass during operation of the heater assembly 18, generally 100-300 ⁰ C above the specified maximum operating temperature. Thus, for the heater assembly 18 used in an application having a maximum operating temperature similar to that of prior art glow plugs of about 100-150 ⁰ C, the glass transition temperature will generally be about 200 ⁰ C or more. For a heater assembly intended for use at higher operating temperatures, for example from 600- 800 ⁰ C, the glass transition temperature will generally be about 700 ⁰ C or more. The glass seal 28 may also incorporate an oxide of a transition metal as a constituent of the glass. The transition metal oxide may include oxides of chromium, cobalt, nickel, iron or copper, either separately or in any combination. Where used, transition metal oxides will generally be used in the amount of about 10 mole percent or less of the glass and may be added as fine particulate oxide powders to the glass powder prior to forming a glass preform as described herein, or directly to a molten glass prior to forming the molten glass into a glass preform. The glass of glass seal 28 may also incorporate as a constituent a filler, including fillers of one or more ceramic oxides. Ceramic oxides may include quartz, eucryptites, leucites, cordierites, beta-spodumene, glass-ceramics, low- expansion glass(CTE<5ppm/°C) , mullite, zircon, zirconia or alumina, either separately or in any combination. Where present, fillers will preferably be used in an amount of about 45 volume percent of the glass or less. Where fillers are used, it is generally desirable that the coefficient of thermal expansion (CTE) of the filler be less than that of any glasses used. One purpose of the fillers is to enhance the toughness, including fracture toughness of the glass. In some cases however, filler with higher thermal expansion coefficient than the glass, such as leucite, may be desirable in order to adjust the thermal expansion characteristics of the combination.

[00043] In order to provide electrical isolation of the electrode 22 from the sheath

20, it is generally preferred that the glass seal 28 have a resistance of at least about lkω for applied voltages up to 24V DC over the operating temperature range of the heater assembly 18 which is about -40 to 3422 ⁰ C, The glass seal 28 will have mechanical

strength, both tensile and shear, through the thickness and at the interfaces with the sheath 20 and the electrode 22 to resist an external applied pressure of up to 10 bar. [00044] The glass seal 28 may have an amorphous microstructure typical of many glasses, and may be formed using a single-step ramp, soak or hold at temperature, followed by a suitable slow cooling process. Additionally, the glass seal 28 may be heat- treated (i.e., ramped heating followed by a soak at temperature) with suitable constituents to form a recrystallized microstructure. Where a recrystallized microstructure is developed, it will preferably occupy more than 90 volume percent of the glass. The glass preform 56 may be heated using any suitable heat source or heating method, including induction heating of one or both of the electrode 22 and the sheath 20 sufficiently to melt the glass preform 56. Heat treatment may be performed using any suitable heat treatment atmosphere, such as in a pressurized chamber of the inert gas atmosphere. By using an elevated pressure of the inert gas 27, a residual amount of the inert gas 27 is sealed in the cavity 52 during formation of the glass seal 28. This has the benefit of both driving off adsorbed contaminants, such as oxygen and water vapor from the powder 26, as well as sealing a residual amount of the inert gas 27 within the cavity 52 to provide ongoing protection of the resistance heating element 24 during operation of the glow plug 10.

[00045] Materials for the electrode 22, sheath 20 and glass seal 28 should be selected, particularly from the standpoint of their relative CTE's, to avoid or minimize to the extent possible tensile stresses over the entire operating range of the heater assembly at these interfaces, or where present to maintain any such stresses at a level sufficient to avoid the creation and propagation of cracks in the glass. It is preferable that these materials be selected with CTE's which maintain the glass seal 28 in compression, particularly at the respective interfaces with the electrode 22 and the sheath 20. One approach to maintaining the stress states described above is to select these materials such that the CTE of the glass seal 28 is approximately equal to those of the sheath 20 and the electrode 22, or where the CTE of the glass seal 28 material, including all of the glass constituents, is within 10% of the CTE's of both the sheath 20 and the electrode 22 over the operating temperature range of the heater assembly, including the processing necessary to form the glass seal 28. Another approach to maintaining the stress states described above is to select these materials such that the CTE of the glass seal 28 is

intermediate those of the sheath 20 and electrode 22, and particularly so that CTE _e i _ectrode < CTEgi _ass < CTE _she ath over the operating temperature range of the heater assembly. [00046] Referring to Figure 5, in accordance with the invention, in accordance with another aspect of the invention, a method 100 of making a heater assembly 18 for a glow plug 10 which includes an electrically and thermally conductive tubular sheath 20 having an open end 30 and a closed end 32; an electrode 22 extending into the open end of the sheath; a resistance heating element 24 disposed in the sheath 20 having a proximal end 44 which is electrically connected to the electrode 22 and a distal end 46 which is electrically connected to the closed end 32 of the sheath 20; an electrically insulating, thermally conductive powder 26 disposed within the sheath 20 and surrounding the resistance heating element 24; an inert gas 21 completely occupying the space between the sheath 20, electrode 22, the resistance heating element 24 and between the individual grains of the powder 26; and a glass seal 28 disposed in the open end 30 and in sealing engagement with the sheath 20 and the electrode 22; includes the steps of: forming 110 a tubular sheath preform 36, electrode 22 and resistance heating element 24; attaching 120 a distal end 39 of the electrode 22 to a proximal end 44 of the resistance heating element 24; inserting 130 the resistance heating element 24 and electrode 22 into the tubular sheath preform 36; attaching 140 the distal end 46 of the resistance heating element 24 to the distal end 32 of the tubular sheath preform 20 to form the closed end 32 of the sheath 20; inserting 150 the powder 26 into the sheath preform 36 to surround the resistance heating element 24; inserting 155 a glass preform 56 into the open end 30; inserting 160 a temporary seal to maintain the glass preform and powder in the sheath preform 20; optionally reducing 165 an outer diameter of the sheath preform 36 to form the tubular sheath 20; heating 170 the glass preform 56 in a vacuum for a time and temperature sufficient to evaporate the temporary seal; further heating 175 the glass preform in the inert gas 27 under positive relative pressure for a time and temperature sufficient to melt the glass and form the glass seal 28; cooling 180 the glow plug under positive relative pressure in the inert gas 27; and joining 190 the heater assembly 18 into an axially extending bore 14 of an annular metal shell 12.

[00047] The step of forming 110 a tubular sheath preform 36, electrode 22 and resistance heating element 24 may utilize any suitable methods of forming these components, such as drawing a wire or rod of a suitable electrode material, cutting the wire or rod to length, and coining, machining or otherwise forming the necessary relief

features shown in Figure 1. The resistance heating element 24 may be made by drawing a wire (or wires) of the appropriate resistance heating material or materials, winding the desired forms around a mandrel and, in the case of a two-piece heating element, joining the first resistance element to the second resistance element. The tubular sheath preform 36 may be made by forming a cylindrical tube of the suitable sheath material and forming the ends to reduce the diameter to form the desired tapers or curvature using known processes for forming these features.

[00048] The step of attaching 120 a distal end 39 of the electrode 22 to a proximal end 44 of the resistance heating element 24 may be performed by forming the outer diameter of the distal end of the electrode end of the proximal end of the resistance heating element so as to create an interference, and then press-fitting the distal end of the electrode into the proximal end of the heating element. This may in turn be followed by welding the proximal end of resistance heating element to the distal end of the electrode using any welding process suitable for joining the respective materials. [00049] The step of inserting 130 the resistance heating element 24 and electrode

22 into the tubular sheath preform 36 will generally be combined with the step of attaching 140 the distal end 46 of the resistance heating element 24 to the distal end 48 of the tubular sheath preform 20 to form the closed end 32 of the sheath 20 as the former is a necessary precursor to the latter. The step of inserting 130 may include use of dwelling fixture or jig which allows these components to be oriented as described with respect to one another. The step of attaching 140 may include any suitable method of joining the resistance heating element 24 and the sheath preform 36 while also closing the opening 50 in the sheath preform 36. Preferably, attaching 140 will include welding these components together and closing the opening 50 in the sheath preform 36 with the resulting weldment.

[00050] The step of inserting 150 the powder 26 into the sheath preform 36 to surround the resistance heating element 24 may be performed using any suitable method of getting powder 26 into the sheath preform, including pouring a loose powder into the cavity 52 of the sheath preform 36. Typically, it is desirable to also employ methods to consolidate the loose powder in the cavity and fill the spaces around and within the resistance heating element 24, such as by vibrating the sheath electrode assembly after the powder has been inserted.

[00051] The step of inserting 155 a glass preform 56 into the open end 30 may employ any suitable glass preform 56, including a loose powders, green powder preforms or fully dense glass tubes or other preform shapes, and may employ any suitable method of placing the preform 56 into annular gap 54, including pouring a free- flowing powder, or injecting a powder under pressure, or placement of a preform having a fixed shaped into annular gap 54,

[00052] The step of inserting 160 a temporary seal to maintain the glass preform

56 and powder in the preform may be performed by inserting a PTFE seal ring to abut the glass preform 56 in the open end 30 of the sheath 20.

[00053] The method 100 may also optionally include a step of reducing 165 an outer diameter of the sheath preform 36 to form the tubular sheath 20. This serves to further compact powder 26 and thereby increase the thermal conductivity of the powder. It also serves to more securely capture resistance heating element 24 and reduce any opportunity for movement or vibration of the resistance heating element 24 within the cavity 52. This may be performed by any suitable mechanism and method of reducing a tubular metal preform such as sheath preform 36, including swaging, coining, roll forming and the other known methods for reducing the diameter of a metal tube. It is desirable that any reduction of the outer surface of the sheath preform 36 avoid that portion of the outer surface proximate glass seal 28, so as to avoid cracking or otherwise disturbing the seal. In order to accomplish this, it is desirable that the diameter of the portion of the sheath preform 36 proximate the glass seal 28 be less than the outer diameter (Dl) of sheath 20.

[00054] The step of heating 170 the glass preform 56 in a vacuum for a time sufficient to evaporate the temporary seal includes vaporizing the temporary seal while also removing any oxygen that may exist from the cavity 52.

[00055] The step of heating 175 the glass preform 56 in the inert gas 27 under positive relative pressure at a temperature sufficient to melt the glass and form the glass seal 28 includes filling any space voided from oxygen within the cavity 52 with the inert gas 27, wherein the heating may use any suitable heating mechanism and method, including those described above.

[00056] The step of cooling 180 the glow plug 10 under positive relative pressure in the inert gas 27 includes finishing formation of the hermetic seal that keeps the inert

gas 27 within the cavity 52 and keeps oxygen outside the cavity from entering the cavity, even during use.

[00057] In order to form a glow plug 10, the method 100 may also include an additional step of joining 190 the heater assembly 18 into an axially extending bore 14 of an annular metal shell 12. This may include sizing the outer diameter of the sheath 20 and the axially extending the bore 14 of shell 12 so as to create an interference, and then press fitting the heater assembly 18 into the bore 14 to create the desired exposure length of the distal end of the sheath 20.

[00058] In order to form a glow plug 10, method 100 may also include an additional step of joining the heater assembly into an axially extending bore 14 of an annular metal shell 12. This may include sizing the outer diameter of sheath 20 and the axially extending bore 14 of shell 12 so as to create an interference, and then press fitting heater assembly 18 into the bore 14 to create the desired exposure length of the distal end of sheath 20.

[00059] By eliminating the use of an elastomeric seal and the use of glass seal 28 which has a much higher melting point and superior high temperature mechanical and electrical properties, and the selection and use of compatible high temperature materials for electrode 22, sheath, 20, electrical resistance element 24 and thermally conductive, electrically insulating powder 26, the heater assembly 18 of a glow plug 10 of the invention is adapted for operation at temperatures greater than 200 ⁰ C. More particularly, it is adapted for operation at temperatures greater than 600 ⁰ C, and even more particularly is adapted for operation up to about 3422°C.

[00060] The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by construing the claims issuing herefrom.