Ceramic glow plug having a tungsten-rhenium alloy heating wire

A ceramic glow plug in which a large resistance suitable for adapting the plug for use with a 24 V battery without having to use a thin or long heating wire. The glow plug includes a ceramic heater having a sintered ceramic body and a heating wire made of an alloy of a high melting point metal such as tungsten embedded in the ceramic body. The ceramic heater is secured to one end of a metal sheath with an outer end portion of the ceramic heater protruding from the end of the metal sheath by as small a distance as possible. A mounting shell receives the other end of the metal sheath. The heating wire is preferably made of a tungsten alloy containing 5 to 39 wt % rhenium to provide the desired resistance.

BACKGROUND OF THE INVENTION 
The present invention relates to a ceramic glow plug for starting a diesel 
engine, and more particularly to a ceramic heater of such a glow plug. 
It is well known generally that a diesel engine is hard to start at low 
temperatures. In order to resolve this problem, it has been the practice 
to provide a glow plug in the engine's cylinders or in auxiliary 
combustion chambers thereof for increasing the temperature of the 
cylinders or the auxiliary combustion chambers. In order to provide quick 
starting, a glow plug has to have a rapid heating characteristic. Further, 
due to the recent tendency to use glow plugs not only for starting but 
also during the engine operation to stabilize the fuel combustion in the 
cylinder after the engine has started, improvements in the electrical and 
chemical durabilities of glow plugs have been demanded. 
In order to meet this demand, a ceramic glow plug of a rapid-heating type 
has been developed which includes a ceramic heater composed of a sintered 
ceramic body and a tungsten heating wire embedded therein. Tungsten is 
used because it has a high melting point and is heat durable. 
FIG. 1 shows, in partial cross section, an example of such a conventional 
ceramic glow plug. In FIG. 1, a ceramic glow plug 1 includes a ceramic 
heater 3 constituted by a sintered ceramic body of Si.sub.3 N.sub.4 and a 
heating wire 2 in the form of a coil embedded therein. The ceramic heater 
3 is fixedly secured to one end of a metal sheath 4 to which one end of 
the heating wire 2 is connected. The metal sheath 4, which is connected to 
a mounting shell 5, functions as a negative electrode. The other end of 
the heating wire 2 is connected to a center conductor 6 supported in the 
shell 5 and electrically insulated therefrom. The center conductor 6 
functions as a negative electrode. 
A portion 3' of the ceramic heater 3 of the ceramic glow plug 1 in which 
the heating coil 2 is embedded protrudes from the end of the metal sheath 
4 by a distance l. The distance l is typically 12 to 15 mm for a glow plug 
operated by a 12 V battery. This distance should be as short as possible 
for reasons of mechanical strength against mechanical shock applied to the 
ceramic glow plug. In the conventional ceramic glow plug, however, it is 
impossible to shorten the distance l because, if the ceramic heater 3 is 
merely pushed into the metal sheath 4 as shown in FIG. 2 to reduce the 
distance l of the exposed portion 3' thereof to l.sub.1, the portion of 
the heating wire of the ceramic heater 3 corresponding to a distance 
l.sub.2 is covered by the metal sheath 4, resulting in a reduced heating 
efficiency of the heating coil. In addition, the heating wire in the metal 
sheath may overheat, causing a soldering material used to connect the 
ceramic heater 3 to the metal sheath 4 to be melted. 
For a ceramic glow plug used with a 24 V battery, in order to obtain the 
same temperaure of the ceramic heater as that of the 12 V ceramic heater, 
the resistance of the heating coil must be about four times that of the 
heating coil for the 12 V battery. If such a large resistance is obtained 
by reducing the diameter of the heating wire, which is usually about 0.2 
mm for the 12 V battery, the diameter must be about 0.1 to 0.13 mm for the 
24 V battery. When such a thin heating wire is embedded in a ceramic body, 
it cannot withstand the unavoidable torsional stresses applied thereto due 
to the difference in thermal expansion coefficients between the ceramic 
and the heating wire. On the other hand, if the increase of the resistance 
is obtained by using a longer wire, the distance l of the ceramic heater 
portion protruding from the end of the metal sheath is necessarily 
increased. Thus, these approaches are not usable in view of the mechanical 
strength of the ceramic glow plug. 
SUMMARY OF THE INVENTION 
The present invention was made in view of the above mentioned state of art, 
and an object of the present invention is to provide a ceramic glow plug 
having a ceramic heater the mechanical strength of which is remarkably 
improved. 
According to the present invention, the above object is achieved by 
employing, as a heating wire material of the ceramic heater, a tungsten 
alloy containing at least one element selected from the group consisting 
of cobalt, rhenium, thorium, zirconium and thorium oxide. Such a tungsten 
alloy has a specific resistance larger than that of pure tungsten, which 
is used conventionally. Preferably, the tungsten alloy contains 5 to 30 
wt% rhenium. 
By employing heating wire of such a tungsten alloy, the heating coil can be 
made smaller compared with the conventional coil, and thus the length l of 
the ceramic heater which protrudes from the metal sheath can be shortened, 
resulting in an improvement of the mechanical strength of the ceramic glow 
plug.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a graph showing the relation between the amount of rhenium 
contained in a tungsten alloy and the specific resistance of the alloy, 
with the amount of rhenium being varied from 5 wt% to 30 wt%. 
When a heating coil prepared by using a tungsten alloy containing 15 wt% 
rhenium is used in a 12 V ceramic heater, the effective length of the 
tungsten alloy wire is about half that of the conventional wire of pure 
tungsten, and therefore the length of the ceramic heater which protrudes 
from the metal sheath is about half that of the conventional heater. 
The following table shows the results of tests conducted on three 
conventional ceramic glow plugs, the length l of the protrusion 3' of 
which is 15 mm, and three ceramic glow plugs according to the present 
invention for comparing the durability thereof against mechanical shocks. 
The tests were performed by dropping each ceramic glow plug 1 held 
horizontally onto a floor 7 as shown in FIG. 4 and measuring the height H 
at which the exposed portion 3' of the ceramic heat 1 was broken. 
TABLE 
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Sample 
No. L (mm) l (mm) H (m) 
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Conventional 
1 25 15 1.3 
glow plug 2 25 15 1.1 
3 25 15 1.5 
Invention 1 25 8 4.2 
2 25 8 3.8 
3 25 8 4.5 
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As can be seen from the Table, the height in the case of the ceramic glow 
plug according to the present invention is about three times that in the 
case of the conventional glow plug, showing superior anti-shock strength. 
In another embodiment of a ceramic glow plug designed for a 24 V battery 
system, the heating wire is made of a tungsten alloy containing 25 wt% 
rhenium. In this case, the effective length of the heating wire in the 
shape of coil can be made substantially equal to that of the 12 V 
embodiment described previously. 
The ceramic glow plugs according to the present invention were subjected to 
a heat cycle test where it was found that their ceramic heaters showed a 
superior durability. 
As described hereinbefore, according to the present invention, the ceramic 
heater of a ceramic glow plug can be minimized in size by employing a 
tungsten alloy having the described composition and having a specific 
resistance larger than that of pure tungsten wire, which has been used in 
the conventional ceramic heater. Thus, the size of the exposed portion of 
the ceramic heater can be minimized, resulting in a superior anti-shock 
performance. Furthermore, the specific resistance of the tungsten alloy 
heating wire can be arbitrarily selected by appropriately selecting the 
materials contained in the alloy. Therefore, there is no need of making 
the diameter thereof small or increasing the length thereof to increase 
the resistance of the wire.