Bipolar diesel engine glow plug having a U-shaped ceramic heater

A glow plug for a diesel engine includes an electrically conductive ceramic (e.g., SIALON) heater having a U-shaped heating portion with a pair of integral parallel leg portions extending into the end of an elongated metallic holder. The leg portions have a thickness larger than the U-shaped heating portion and are electrically insulated from the holder by an electrically insulative coating formed therebetween. An electrically insulating sheet having essentially the same thermal expansion coefficient as the ceramic heater fills the gap between the leg portions and is bonded thereto to form a seal preventing combustion products from reaching the interior of the holder. A pair of coaxially arranged external connecting terminals extend into the other end of the holder and are electrically connected to the leg portions.

BACKGROUND OF THE INVENTION 
The present invention relates to a glow plug for preheating a subcombustion 
or combustion chamber of a diesel engine and, more particularly, to an 
improvement of a diesel engine glow plug of a bipolar two-line system 
having a ceramic heater which has high-speed and self temperature 
saturation properties and which allows "after glow" operation for a long 
period of time. 
Conventional glow plugs having various types of structures have been 
proposed. Among these glow plugs, a plug having a ceramic heater has 
received a great deal of attention as a fast heating plug. 
A glow plug of a ceramic heater type is described in Japanese Patent 
Prepublication No. 60-14784. This glow plug has a structure wherein a 
heating element is exposed on the outer surface of a heater by using a 
conductive ceramic material having substantially the same thermal 
expansion coefficient as that of an insulating ceramic material with a 
heater insulating element, so that the heating element is integrally 
formed with the heater insulating element. With this structure, the distal 
end of the heater can be immediately heated to obtain a fast heating glow 
plug. At the same time, bonding between the heating element and the heater 
insulating element can be optimally and appropriately maintained to 
improve reliability for heat resistance and the like to some extent. 
In a conventional glow plug of a ceramic heater type having the structure 
as described above, however, many problems are left unsolved from 
structural and functional points of view when such a plug is used as a 
glow plug in an operating engine. 
With the above structure, the heating element is exposed on the outer 
surface of the heater to achieve fast heating. However, the heating 
element is formed by a simple U-shaped laminated structure, and both ends 
thereof are guided to the rear end portion of the heater. In order to 
produce a practical glow plug, an electrode extraction portion and a 
holding portion to be coupled to a holder must be specifically designed. 
For example, when external connection electrodes are led out from the 
ceramic heater, the lead portion must be separated from the heating 
element at the distal end of the heater by as great a distance as possible 
to minimize a thermal influence, thereby improving reliability for bonding 
strength or the like. The thermal influence must also be considered at a 
bonding portion held by the holder by silver brazing. The heat resistance 
of the bonding portion must be assured. It is very difficult to satisfy 
these requirements. 
In the conventional ceramic heater as described above, the heating element 
has an integral structure of conductive and insulating ceramic materials. 
Although the thermal expansion coefficients of these ceramic materials are 
substantially the same, reliability of the bonding portion of a ceramic 
heater in a glow plug heated to 1,100.degree. C. or more is undesirably 
low. 
In a glow plug of this type, smooth and efficient combustion inside the 
engine can be achieved due to improvements in the starting characteristics 
of the diesel engine; durability for high-temperature operating conditions 
as a result of the widespread use of turbo mechanisms and; maintenance of 
an energized state for the glow plug for a predetermined period of time 
after starting the engine. This better combustion results in a reduction 
of exhaust gas and noise. Market demand has arisen for such an after glow 
system, and maximum prolongation of the after glow time (e.g., 10 minutes) 
is required. In order to prolong the after glow time, energization power 
to the heating element must be self-controlled to greatly improve the 
heating characteristics. Overheating of the heater portion must be 
prevented, and a self temperature saturation function is required to keep 
the saturation temperature below an appropriate temperature. During after 
glow operation, a voltage applied to the glow plug is kept lower than that 
applied at the time of energization of the plug so as to assure durability 
of the heating wire. In consideration of these points, a demand has arisen 
for development of a low-cost glow plug having a ceramic heater having 
fast heating and self temperature saturation properties as well as high 
reliability for heat resistance and the like. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide a diesel 
engine glow plug wherein heat resistance can be greatly improved. 
It is another object of the present invention to provide a diesel engine 
glow plug wherein fast heating can be achieved. 
It is still another object of the present invention to provide a diesel 
engine glow plug wherein voltage control during after glow operation can 
be easily achieved, and durability of the heating element can be improved. 
It is still another object of the present invention to provide a diesel 
engine glow plug wherein the plug can be easily mounted to a holder by 
simple mechanical coupling. 
In order to achieve the above objects of the present invention, a glow plug 
for a diesel engine, is disclosed. The invented glow plug comprises a 
hollow holder, a ceramic heater having a U-shape as a whole and supported 
by the hollow holder with the U-shaped heating element projecting 
outwardly from the holder, a pair of external connecting terminals 
arranged coaxially, means for connecting both the ends of the ceramic 
heater and the external connecting terminals, respectively, and insulating 
member means for supporting the external connecting terminals in an 
insulated condition with respect to the hollow holder. The ceramic heater 
includes an outwardly projecting U-shaped heating portion, and a pair of 
lead portions of a resistive ceramic material extending backward from both 
the ends of the U-shaped heating portion and parallel to each other. The 
U-shaped heating portion is integral with said pair of lead portions, and 
the lead portions are insulated from the hollow holder and connected to 
the external connecting terminals through the interior of the hollow 
holder and via the connecting means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described in detail with reference to the 
preferred embodiments in conjunction with the accompanying drawings. 
FIGS. 1 to 4B show a diesel engine glow plug according to an embodiment of 
the present invention. The schematic structure of a glow plug 10 in FIG. 2 
will be briefly described. The glow plug 10 comprises a rod-like ceramic 
heater 11 whose distal end portion serves as a heating element and a 
tubular metal holder 12 for holding the heater 11 at its distal end. A 
terminal assembly 15 is fitted and held in the rear end portion of the 
holder 12. The terminal assembly 15 is prepared such that first and second 
external connecting terminals 13 and 14 are embedded and extended through 
an insulating material such as a synthetic resin material. The terminals 
13 and 14 are respectively connected through metal wires 16 and 17 to lead 
portions (to be described later) of a conductive ceramic material 
constituting the heater 11. The metal wires 16 and 17 serve to 
mechanically protect the heater 11 from an external mechanical force such 
as various kinds of vibrations and a fastening torque, all of which act on 
the external connecting terminals 13 and 14. In this sense, the metal 
wires 16 and 17 must be flexible to a given extent. Therefore, in this 
embodiment, each of the metal wires 16 and 17 is constituted by a 
plurality of fine conductive wires. A single wire may be used if it is 
flexible. A threaded portion 12a is formed on the outer surface of the 
holder 12 and can be threadably engaged with a screw hole in an engine 
cylinder head (not shown). The distal end of the heater 11 extends into a 
subcombustion chamber (or a combustion chamber). 
The terminal assembly 15 has the first external connecting terminal 13, the 
second external connecting terminal 14, and an electrically insulating 
assembly body 15a. The first connecting terminal 13 is located on the axis 
of the assembly 15 and has a rod 13a at an inner end side thereof. The rod 
13a is connected to the metal wire 16. The second external connecting 
terminal 14 comprises a cylindrical member disposed around the first 
external connecting terminal 13 spaced by a predetermined distance 
therefrom. A lead piece 14a extending from part of the inner end of the 
second external connecting terminal 14 is connected to the metal wire 17. 
The assembly body 15a electrically insulates the terminals 13 and 14 from 
each other and comprises an insulating layer on the outer surface of the 
second terminal 14. In this manner, the assembly body 15a integrally 
supports and holds the first and second terminals 13 and 14. A connection 
reinforcing metal pipe 15b is fitted on the outer surface of the body 15a. 
The metal pipe 15b is caulked at the edge of the rear opening of the 
holder 12 at a high pressure. The inner side of the metal pipe 15b is bent 
to engage the side of the assembly body 15a. In this manner, the outer 
side of the metal pipe 15a firmly engages the inner wall surface of the 
holder 12, thereby solving problems associated with an external force and 
thermal shrinkage. 
Reference numerals 18a and 18b respectively denote an insulating ring and a 
washer, both of which are mounted on the second terminal 14 extending in 
the rear portion of the holder 12; 18c, an insulating member mounted on 
the side of the first terminal 13 at the outer end of the washer 18b; and 
18d and 18e, a spring washer and a fastening nut, respectively, both of 
which are threadably engaged with the threaded portion formed on the outer 
end of the first terminal 13. Lead wires (not shown) connected to a 
battery are clamped between the washer 18b and the insulating member 18c 
and between the insulating member 18c and the spring washer 18d so that 
the terminals 13 and 14 are electrically connected to the battery 
terminals. Reference numerals 16a and 17a denote insulating members such 
as tubes coated on the metal wires 16 and 17, respectively. 
With the glow plug 10 having the structure as described above, the rod-like 
ceramic heater 11 held at the distal end of the holder 12 is designed to 
be a substantially U-shaped structure wherein a U-shaped heating element 
20 and a pair of parallel lead portions 21 and 22 extending backward from 
the both ends of the U-shaped heating element are integrally made of a 
conductive ceramic material. Insulating coating layers 23 and 24 are 
formed on the outer surfaces of the lead portions 21 and 22 and are bonded 
to the distal end portion of the holder 12. At the same time, the rear end 
portions of the lead portions 21 and 22 are connected to the first and 
second external connecting terminals 13 and 14 insulatively held at the 
rear end portion of the holder 12 through the metal wires 16 and 17. 
The above characteristic feature of the present invention will be described 
in more detail. The ceramic heater 11 comprises a conductive ceramic 
heating element 20 of an outer diameter and a cross section, both of which 
are smaller than the lead portions 21 and 22. A slit 25 is formed to 
extend between the lead portions 21 and 22 and the heating element 20 at 
the central portion of the heater 11 along its longitudinal direction. 
Insulating coating layers 23 and 24 (the outside layers thereof are 
nickel-plated layers) are formed on the outer surfaces at the central 
portions along the longitudinal direction of the lead portions 21 and 22 
integrally formed of a conductive ceramic material together with the 
heating element 20. The layers 23 and 24 and the nickel-plated layers 
formed thereon allow sliver brazing of the ceramic heater 11 to the distal 
portion of the holder 12. In this case, the bonding surface portions of 
the holder 12 may have nickel-plated layers, as needed. However, the 
surface portions need not be coated with the nickel-plated layers. 
The lead portions 21 and 22 have electrode extraction ends 26 and 27 
extending backward therefrom, respectively. The electrode extraction ends 
26 and 27 are electrically connected to the distal ends of the metal wires 
16 and 17 extending from the first and second external connecting 
terminals 13 and 14 through terminal caps 28 and 29, respectively. A 
current is thus supplied through the ceramic heater 11, as indicated by 
the arrows in FIG. 1. Reference numerals 26a and 27a denote metallized 
layers formed on the electrode extraction ends 26 and 27 to connect 
terminal caps 28 and 29 thereto, respectively. In this case, nickel-plated 
layers are formed on the metallized layers 26a and 27a, respectively, and 
the metal material can be appropriately and firmly bonded to the ceramic 
material. The terminal caps 28 and 29 have a shape matching with the 
electrode extraction ends 26 and 27 at the rear end portions of the heater 
11. The terminal caps 28 and 29 are brazed by silver with the electrode 
extraction ends 26 and 27 in a furnace while the caps 28 and 29 are 
engaged with the ends 26 and 27, respectively. The metal wires 16 and 17 
are bonded to the caps 28 and 29 such that the flange portions at the 
distal ends thereof are attached to the end faces of the caps 28 and 29 at 
the time of sliver brazing as described above, thereby constituting the 
heater assembly. The other end of each of the metal wires 16 and 17 is 
spot-welded to the rod 13a and the lead piece 14a of the first and second 
external connecting terminals 13 and 14 in the terminal assembly 15. 
According to the bipolar two-line glow plug 10 having the above structure, 
the electrode extraction portions from the ceramic heater 11 are located 
within the holder 12 spaced away from the heating element 20. The 
electrode extraction portions can be maintained at a relatively low 
temperature. Therefore, reliability for heat resistance and the like can 
be greatly improved as compared with the conventional structure. As 
described above, the bonding portion between the ceramic heater 11 and the 
holder 12 does not require electrical ground but mechanical bonding. 
Therefore, reliability for bonding strength and the like can be improved. 
The ceramic heater 11 is prepared such that a conductive ceramic paste is 
injected into a mold, and that a molded body is sintered. Alternatively, a 
ceramic heater having a rod-like shape is formed into a predetermined 
shape. After molding or forming, the insulating coating layers 23 and 24 
(flame spraying with alumina) and the metallized layers 26a and 27a are 
respectively formed on the lead portions 21 and 22 and the outer surfaces 
of the electrode extraction ends 26 and 27. Nickel-plated layers are 
formed on the surface portions to be bonded to the metal holder 12. 
The ceramic heater 11 prepared as described above is incorporated into the 
holder 12 in a known manner such that the outer surfaces of the lead 
portions 21 and 22 are brazed through the insulating layers (23 and 24), 
and that the rear end portions of the metal wires 16 and 17 are connected 
to the first and second external connecting terminals 13 and 14 held at 
the rear end portion of the holder 12, thereby assembling the glow plug 
10. 
A suitable conductive ceramic material for making a substantially U-shaped 
ceramic heater 11 is SIALON obtained by mixing titanium nitride (TiN) into 
a SIALON (40% of Si.sub.3 N.sub.4, 30% of Al.sub.2 O.sub.3, and 30% of 
Y.sub.2 O.sub.3) containing .beta.-phase SIALON or .alpha.+.beta.-phase 
SIALON. It is found that an electrical conductivity of positive 
resistance-temperature characteristics can be obtained when about 30% or 
more of TiN is added to the SIALON (i.e., a conductive SIALON). When the 
content of TiN is increased, the resistivity of the resultant SIALON is 
known to be continuously changed. Therefore, a SIALON compound containing 
a predetermined content of TiN can be used as needed. 
However, the conductive ceramic material serving as the resistor material 
for the ceramic heater 11 is not limited to the above-mentioned SIALON. It 
is therefore essential to use a ceramic material whose performance is 
stable at high temperatures (e.g., up to 1,200.degree. C.) and has good 
heat impact resistance. At least one nonoxide conductive material selected 
from the group consisting of SiC, and a carbonate, a borate, a nitride, or 
a carbon nitride of Group IVa, Va, and VIa of the Periodic Table is mixed 
with Al or an Al compound as a sintering binder to prepare a SIALON 
sintered body. 
The structure according to the present invention solves the conventional 
problem wherein a metal heating wire is embedded in a sheath or insulating 
ceramic material and fails to obtain fast heating because of internal 
heating. The above solution is given by the structure wherein the heating 
element 20 made of a conductive ceramic material is exposed on the outer 
surface of the heater 11 to improve heating characteristics, thereby 
heating the element. In particular, according to the present invention, 
since the heating element 20 is made of only a conductive ceramic material 
which does not contain foreign materials, reliability for heat resistance 
is high even if thermal stress repeatedly acts on the heating element 20. 
The heating element 20 also has high durability and good workability. 
Therefore, the fabrication cost becomes low. 
In the ceramic heater 11 according to the present invention, the 
resistivity of the conductive SIALON constituting the heating element 20 
and the pair of lead portions 21 and 22 can be controlled by the content 
of titanium nitride, and the thickness of the members can be arbitrarily 
controlled. In particular, the width (i.e., the sectional area) of the 
heating element 20 can be minimized to achieve fast heating, and its 
saturation temperature can be properly controlled to provide the after 
glow operation for a long period of time. The conductive SIALON has a 
large positive resistance-temperature coefficient and ha an advantage in 
the self temperature saturation characteristics. 
The thickness of the heater 11 can be arbitrarily controlled at the time of 
molding, and the resistance of the heater 11 can therefore be arbitrarily 
controlled. For example, assume that the diameter of the heater 11 is 5 
mm, that the diameter of the heating element 20 is 3 mm, and that its 
length is 50 mm (excluding a length of 5 mm of the electrode extraction 
end 26 or 27). Under these assumptions, the length of the heating element 
20 is set to be 10 mm, and the insulating coating layers and 23 and 24 are 
formed for a length of 20 mm from the position 25 mm from the distal end. 
The heat capacity of the heating element 20 can be smaller than that of 
the lead portions 21 and 22. A desired resistance can be set to obtain the 
required self temperature saturation characteristics. These results were 
confirmed by tests or the like. 
When a substantially U-shaped integral ceramic heater 11 made of the 
conductive ceramic material is used, good characteristics of the glow plug 
10 can be obtained, as shown in FIG. 5. More specifically, according to 
the glow plug 10 of this embodiment, the plug can be kept at about 
1,100.degree. C. when the plug is heated to 800.degree. C. in 3.5 seconds 
and the allowable range of the saturation temperature is set to be 
1,200.degree. C. or less, as indicated by the solid line in FIG. 5. 
The bipolar two-line glow plug 10 can employ an energization circuit as 
shown in FIG. 6. A voltage magnitude is changed between the heating mode 
and the after blow mode to improve durability of the heating element 20 in 
the after glow mode for a long period of time. Reference numeral 40 
denotes a battery power source; and 41 and 42, first and second relays, 
respectively. The two bipolar two-line glow plugs (represented by 11) and 
two unipolar glow plugs 43 are arranged in this circuit. The relays 41 and 
42 are selectively turned on/off for the glow plugs 11 and 43. During 
abrupt preheating, the four glow plugs are connected in parallel with each 
other. During after glow operation, two bipolar plugs are connected in 
series with the two unipolar plugs. 
With the above circuit arrangement, a voltage of, e.g., 12 V is supplied to 
the glow plugs 11 and 43 in the abrupt preheating mode to achieve fast 
heating of the heating elements. In the after glow mode, a voltage of, 
e.g., 6 V is supplied to the glow plugs 11 and 43 to lower the heating 
temperature to improve durability of the glow plugs. 
In the glow plug 10 having the arrangement described above, as is apparent 
from FIGS. 1 to 3, the internal space of the holder 12 communicates with 
the corresponding engine combustion chamber by the slit 25 formed along 
the longitudinal direction of the ceramic heater 11. Leakage of a 
combustion pressure and combustion heat at the time of explosion in the 
combustion chamber outside the engine must be taken into consideration. In 
this embodiment, a sealing sheet 30 made of asbestos or rubber is mounted 
at the outer end of the terminal assembly 15 having the first and second 
external connecting terminals 13 and 14 at the rear end opening of the 
holder 12, as shown in FIG. 2, thereby mechanically sealing the outer end 
of the terminal assembly. However, the position of the sealing sheet and 
the sealing method are not limited to these. For example, an O-ring or the 
like may be mounted at the inner end of the terminal assembly 15 so as to 
seal it from the holder 12. The above-mentioned sealing means may comprise 
a structure shown in FIG. 7. In the ceramic heater 11, an insulating sheet 
50 made of, e.g., an insulating ceramic material is interposed between the 
lead portions 21 and 22 of the rear end side of the ceramic heater 11 at 
least a portion corresponding to the distal end portion of the holder 12. 
The holder 12 is bonded integrally with the lead portions 21 and 22, 
thereby closing the slit 25 by the portion of the holder 12 and firmly 
preventing leakage of the combustion pressure and the combustion heat. 
With this structure, the mechanical strength of the rear end portion held 
in the holder 12 in the ceramic heater 11 can be improved, and the sealing 
sheet 30 used in the above embodiment can be omitted, resulting in a great 
advantage. 
The insulating ceramic material may comprise a SIALON or the like obtained 
such that the content of titanium nitride (TiN) is controlled to select 
desired insulating or conductive properties in the same manner as in the 
conductive ceramic material constituting the ceramic heater 11. When such 
a material is selected, the insulating sheet 50 can be made of a material 
having substantially the same thermal expansion coefficient as that of the 
resistor. Therefore, the bonding strength can be increased, and 
reliability for heat resistance or the like can be assured. In order to 
bond the insulating and conductive ceramic material members made of 
SIALON, an oxide sintering assistant agent such as Y.sub.2 O.sub.3 may be 
inserted between these two members, and the resultant structure is 
sintered while a diffusion layer is formed firmly between the bonded 
surfaces of the members. However, conventional ceramic bonding techniques 
such as a halogen compound method, a brazing method, or a solid-phase 
bonding method may be used. An insulating ceramic material constituting 
the insulating sheet 50 may be a material containing, e.g., SiC, Si.sub.3 
N.sub.4, AlN or Al.sub.2 O.sub.3 as a major constituent and having a good 
heat resistance and a good adhesion property with the conductive ceramic 
material. In addition, the insulating material may be glass or the like. 
The present invention is not limited to the particular embodiment described 
above. The shapes, structures, and the like of the respective components 
may be changed and modified within the spirit and scope of the invention. 
The electrode extraction ends 26 and 27 from the ceramic heater 11 may be 
staggered along the longitudinal direction, as shown in FIG. 7. 
Alternatively, one of the terminal caps may comprise an annular member 60 
partially having a lead piece 60a. The annular member 60 may be fitted on 
and fixed directly to one of the lead portions 21 and 22. In this case, an 
insulating layer 61 may be formed at a portion corresponding to the other 
lead portion. With this structure, the insulating state between the lead 
portions 21 and 22 can be guaranteed and at the same time, the electrodes 
can be easily terminated, thus providing many practical advantages. 
According to the present invention as described above, the rod-like ceramic 
heater comprises a U-shaped heating element and a pair of parallel lead 
portions extending from the both ends of the heating element. The heating 
element and the lead portions are made of a conductive ceramic material. 
The insulating layers are formed on the outer surfaces of the lead 
portions, and the lead portions are bonded to the distal end of the holder 
through the insulating layers. The rear end portions of the lead portions 
are connected through the metal wires to the first and second external 
connecting terminals insulatively held by the rear end portions of the 
holder. Although the resultant glow plug has a simple low cost structure, 
the thermal stress does not repeatedly act on the heating element since 
the heating element as the characteristic feature of the present invention 
is made of a conductive ceramic material which does not contain foreign 
materials. Reliability for heat resistance or the like can be improved. In 
addition, the heating element is exposed on the surface of the heater, and 
the distal end of the heater can be immediately heated, thus obtaining 
fast heating performance. Furthermore, the heat capacity of the heating 
element can be reduced by the conductive ceramic material, and the self 
temperature saturation property can be obtained to appropriately control 
the saturation temperature. The after glow operation for reducing the 
engine exhaust gas and noise can be maintained for a long period of time. 
The electrodes can be extracted from the rear end portions of the heater 
with a bipolar two-line structure. The thermal influence of the electrode 
extraction portions can be reduced to improve reliability. In addition, 
the voltage can be easily controlled in the preheating and after glow 
modes to improve durability of the heating element. The glow plug is 
mechanically bonded to the holder. In this manner, many practical 
advantages can be obtained.