U.S. patent application number 12/335682 was filed with the patent office on 2009-07-23 for sheathed glow plug.
Invention is credited to Marcello Cino, Sandro Goretti, John Hoffman, Camillo Rena, William Walker, JR..
Application Number | 20090184101 12/335682 |
Document ID | / |
Family ID | 40796124 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090184101 |
Kind Code |
A1 |
Hoffman; John ; et
al. |
July 23, 2009 |
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/.degree. 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.
Inventors: |
Hoffman; John; (Perrysburg,
OH) ; Goretti; Sandro; (Rubiera, IT) ; Rena;
Camillo; (Modovi, IT) ; Walker, JR.; William;
(Toledo, OH) ; Cino; Marcello; (Rubiera,
IT) |
Correspondence
Address: |
ROBERT L. STEARNS;Dickinson Wright PLLC
38525 Woodward Avenue, Ste. 2000
Bloomfield Hills
MI
48304-2970
US
|
Family ID: |
40796124 |
Appl. No.: |
12/335682 |
Filed: |
December 16, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61014122 |
Dec 17, 2007 |
|
|
|
61061387 |
Jun 13, 2008 |
|
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Current U.S.
Class: |
219/270 |
Current CPC
Class: |
F23Q 7/001 20130101 |
Class at
Publication: |
219/270 |
International
Class: |
F23Q 7/22 20060101
F23Q007/22 |
Claims
1. A glow plug, comprising: an annular metal shell having an
axially extending bore; an electrically and thermally conductive
tubular sheath having an open end disposed within said bore in
electrical contact with said shell and a closed end projecting from
said bore; an electrode extending into said open end of said
sheath; a resistance heating element disposed in said sheath having
a proximal end which is electrically connected to said electrode
and a distal end which is electrically connected to said closed end
of said sheath; an electrically insulating, thermally conductive
powder disposed within said sheath and surrounding said resistance
heating element; and a glass seal disposed in said open end and in
sealing engagement with said sheath and said electrode.
2. The glow plug of claim 1, wherein said glass seal comprises a
glass selected from a group consisting of a silicate glass, a
borate glass and a borosilicate glass.
3. The glow plug of claim 2, further comprising an oxide of a
transition metal as a constituent of said glass.
4. The glow plug of claim 3, wherein said transition metal is
selected from a group consisting of chromium, cobalt, nickel, iron
and copper.
5. The glow plug of claim 3, wherein said oxide comprises 10 mole
percent or less of said glass.
6. The glow plug of claim 2, wherein said glass comprises a
recrystallized microstructure.
7. The glow plug of claim 6, wherein said recrystallized
microstructure comprises greater than 90 volume percent of said
glass.
8. The glow plug of claim 2, wherein said glass is substantially
lead free.
9. The glow plug of claim 2, further comprising a filler as a
constituent of said glass.
10. The glow plug of claim 9, wherein said filler is a ceramic
oxide.
11. The glow plug of claim 10, wherein said ceramic oxide is
selected from a group consisting of quartz, eucryptites, leucites,
cordierites, beta-spodumene, glass-ceramics, low-expansion
glass(CTE<5 ppm/.degree. C.) , mullite, zircon, zirconia and
alumina.
12. The glow plug of claim 1, wherein said sheath has an outer
diameter that varies along its length such that said outer diameter
has a reduced diameter portion proximate said open end.
13. The glow plug of claim 12, wherein said glass seal has a
length, and said reduced diameter portion has a length, and said
length of said reduced diameter portion is greater than said length
of said glass seal.
14. The glow plug of claim 1, further comprising a protective gas
disposed in said cavity.
15. The glow plug of claim 14, wherein said protective gas is
selected from a group consisting of nitrogen, helium, neon, argon,
krypton and xenon.
16. The glow plug of claim 1, wherein said resistance heating
element comprises a metal wire spiral.
17. The glow plug of claim 16, wherein said metal wire spiral
comprises a metal selected from a group consisting of pure nickel,
a nickel alloy, a nickel-iron-chromium alloy and an iron-cobalt
alloy.
18. The glow plug of claim 16, wherein said metal wire spiral
comprises a metal selected from a group consisting of tungsten,
molybdenum, or alloys containing tungsten, molybdenum, nickel,
iron, tantalum, niobium, titanium, vanadium, osmium and
chromium.
19. A heater assembly for a glow plug, comprising: an electrically
and thermally conductive tubular sheath having an open end and a
closed end; an electrode extending into said open end of said
sheath; a resistance heating element disposed in said sheath and
having a proximal end electrically connected to said electrode and
a distal end electrically connected to said closed end of said
sheath; an electrically insulating, thermally conductive powder
disposed within said sheath and surrounding said resistance heating
element; and a glass seal disposed in said open end and in sealing
engagement with said sheath and said electrode.
20. The heater assembly of claim 19, wherein said glass seal
comprises a glass selected from a group consisting of a silicate
glass, a borate glass and a borosilicate glass.
21. The heater assembly of claim 20, further comprising an oxide of
a transition metal as a constituent of said glass.
22. The heater assembly of claim 21, wherein said transition metal
is selected from a group consisting of chromium, cobalt, nickel,
iron and copper.
23. The heater assembly of claim 20, further comprising a filler as
a constituent of said glass.
24. The heater assembly of claim 23, wherein said filler is a
ceramic oxide.
25. The heater assembly of claim 24, wherein said ceramic oxide is
selected from a group consisting of quartz, eucryptites, leucites,
cordierites, beta-spodumene, glass-ceramics, low-expansion
glass(CTE<5 ppm/.degree. C.) , mullite, zircon, zirconia and
alumina.
26. The heater assembly of claim 19, wherein said sheath has an
outer diameter that varies along its length such that said outer
diameter has a reduced diameter portion proximate said open
end.
27. The heater assembly of claim 26, wherein said glass seal has a
length, and said reduced diameter portion has a length, and said
length of said reduced diameter portion is greater than said length
of said glass seal.
28. The heater assembly of claim 19, further comprising a
protective gas disposed in said cavity.
29. The heater assembly of claim 28, wherein said protective gas is
selected from a group consisting of nitrogen, helium, neon, argon,
krypton and xenon.
30. The heater assembly of claim 19, wherein said resistance
heating element comprises a metal wire spiral selected from a group
consisting of pure nickel, a nickel alloy, a nickel-iron-chromium
alloy and an iron-cobalt alloy.
31. The heater assembly of claim 19, wherein said resistance
heating element comprises a metal wire spiral selected from a group
consisting of tungsten, molybdenum, or alloys containing tungsten,
molybdenum, nickel, iron, tantalum, niobium, titanium, vanadium,
osmium and chromium.
32. A method of making a heater assembly for a glow plug comprising
the steps of: forming a tubular sheath preform, electrode and
resistance heating element; attaching a distal end of the electrode
to a proximal end of the resistance heating element; inserting the
resistance heating element and electrode into the tubular sheath
preform; attaching the distal end of the resistance heating element
to the distal end of the tubular sheath preform to form the closed
end of the sheath; disposing electrically insulating, thermally
conductive powder into the sheath preform to surround the
resistance heating element; inserting a glass preform into the open
end; and heating the glass preform for a time and temperature
sufficient to melt the glass and form the glass seal.
33. The method of claim 32, further comprising a step of reducing
an outer diameter of the sheath preform to form the tubular
sheath.
34. The method of claim 32, further comprising performing the
heating in vacuum or under a blanket of a protective gas.
35. The method of claim 34, further comprising selecting the
protective gas from a group consisting of nitrogen, helium, neon,
argon, krypton and xenon.
36. The method of claim 32, further comprising a step of forming an
oxide layer on one of the electrode or the sheath proximate the
location of the glass seal.
37. The method of claim 32, further comprising forming the
resistance heating element 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 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.
[0006] 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 fluoropolymers such as
those sold by DuPont under the Viton.RTM. 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.degree. C. Recently, glow plug applications
are emerging where a higher operating temperature range is needed
and o-ring seals are unsuitable.
[0007] 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.
[0008] In view of the above, there remains a need for glow plugs
that can be used at operating temperatures above 200.degree. 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
[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.degree. C.
[0010] 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.
[0011] 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.
[0012] In another aspect, the glass may include a recrystallized
microstructure. The recrystallized microstructure may include more
than 90 volume percent of the glass.
[0013] 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<5
ppm/.degree. C.) and leucite.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] In another aspect, the thermally conductive, electrically
insulating powder may include magnesium oxide.
[0019] 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.
[0020] 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.
[0021] In 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.
[0022] In another aspect, the method further includes forming the
hermetic seal as a glass seal.
[0023] In another aspect, selecting the glass of the seal from a
group consisting of a silicate glass, a borate glass and a
borosilicate glass.
[0024] 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.
[0025] 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 may be selected from a group consisting of
quartz, eucryptites, cordierites, glass-ceramics, mullite, alumina,
zircon, zirconia, beta-spodumene, low-expansion glass (CTE<5
ppm/.degree. C.) and leucite.
[0026] 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 DESCRIPTION OF THE DRAWINGS
[0027] 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:
[0028] FIG. 1 is a partial cross-sectional view of a sheathed
heater assembly and glow plug of the invention;
[0029] FIG. 2 is a cross-sectional view of a tubular sheath preform
of the invention;
[0030] FIG. 3 is a front view of a resistance heater element of the
invention;
[0031] FIG. 4A is a schematic sectional view of a glass preform
inserted in the annular gap between the electrode and tubular
sheath;
[0032] FIG. 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
[0033] FIG. 5 is a flow chart of a method of making a glow plug of
the invention.
DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS
[0034] 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.
[0035] 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.
[0036] Referring in more detail to the drawings, FIG. 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.
[0037] 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 (D1) that varies along its
length such that the outer diameter has a reduced diameter portion
34 proximate open end 30. The outer diameter (D1) maybe any
suitable diameter, with a typical outer diameter for many glow plug
applications being about 4 mm, 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 8 mm. The
diametral difference between outer diameter (D1) and reduced
diameter portion 34 maybe any desired amount, depending on the
application requirements, but in an exemplary embodiment, the
differential was about 0.4 mm. The tubular sheath 20 may have a
deformed microstructure, such as a cold-worked microstructure,
where a sheath preform 36 (FIG. 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 FIG. 2.
[0038] 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.
[0039] 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.degree. 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 (FIG. 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 FIGS. 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 (FIG. 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.
[0040] 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.degree. C.
[0041] 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.
[0042] 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 FIG.
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 FIG. 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.
[0043] 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 standing preform, the glass preform 56 will
generally be compacted using a pressure in the range of 1-50 kpsi
(6.9-340 MPa).
[0044] 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.degree. 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.degree. 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.degree. C., the glass transition temperature will generally
be about 200.degree. C. or more. For a heater assembly intended for
use at higher operating temperatures, for example from
600-800.degree. C., the glass transition temperature will generally
be about 700.degree. 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<5 ppm/.degree. 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.
[0045] 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 1 k.OMEGA. for applied
voltages up to 24V DC over the operating temperature range of the
heater assembly 18 which is about -40 to 3422.degree. 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.
[0046] 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.
[0047] 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.sub.electrode<CTE.sub.glass<CTE.sub.sheath over the
operating temperature range of the heater assembly.
[0048] Referring to FIG. 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 27 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.
[0049] 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 FIG. 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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 (D1) of sheath
20.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.degree. C. More
particularly, it is adapted for operation at temperatures greater
than 600.degree. C., and even more particularly is adapted for
operation up to about 3422.degree. C.
[0062] 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.
* * * * *