Glow plug for diesel engines

A glow plug for preheating the combustion chamber of a diesel engine. The glow plug has a heat generating portion and a mounting portion. The heat generating portion is provided with a ceramic central electrode made of an electrically insulating material and provided with at least one spiral screw-thread groove formed in the outer peripheral surface thereof. A ceramic electric resistor serving as a heat generating member is disposed in the groove. The electric resistor is made of TiC or SiC with or without addition of Al.sub.2 O.sub.3.

BACKGROUND OF THE INVENTION 
The present invention relates to a pre-heating plug, i.e. glow plug, for 
diesel engines.

DESCRIPTION OF THE PRIOR ART 
A typical conventional glow plug has a heat generating portion including a 
heat generating body embedded in a sheath body through the medium of an 
insulator. A typical example of such a glow plug is shown in FIG. 1. This 
glow plug has a housing 3 consisting of a mounting portion 1 and a heat 
generating portion 2 and adapted to be mounted at the mounting portion 1 
on the cylinder head of an engine. The glow plug further has a central 
electrode 9 disposed in the housing 3 through the medium of an insulator 
4. The heat generating portion 2 includes a metallic electric resistor 7 
wound round a central rod 6 of a ceramic material and fixed in a sheath 9 
through the medium of an insulator 8. The electric resistor 7 is connected 
at its both ends to the central electrode 5 and the housing 3. As electric 
voltage is applied between the central electrode 5 and the housing 3, 
electric current flows through the electric resistor 7 to generate heat 
which in turn is transmitted, through the heat insulator 8, to the sheath 
9 to red-heat the sheath 9. Since the transfer of the heat to the sheath 9 
is made through the insulator 8, the efficiency of the heat transfer is 
impractically low and a comparatively long time is required until the 
sheath is heated up to the desired high temperature. Furthermore, since 
the resistance member has to be maintained at a sufficiently high 
temperature, there is a fear that the electric resistor 7 is burnt out. 
Also, there is a tendency that the metallic sheath member is liable to be 
corroded and deteriorated. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to provide a glow plug for 
diesel engines improved to overcome the above-described problems of the 
prior art. 
To this end, according to the invention, there is provided a glow plug for 
diesel engines, having a heat generating portion and a mounting portion, 
wherein the heat generating portion includes a central rod made of an 
electrically insulating material and provided in the outer peripheral 
surface thereof with a spiral groove, and a heat generating body made of a 
semiconductive material and disposed in the spiral groove. 
The above and other objects, features and advantages of the invention will 
become clear from the following description of the preferred embodiments 
taken in conjunction with the accompanying drawings. 
A glow plug in accordance with a first embodiment of the invention will be 
described hereinunder with reference to FIGS. 2a and 2b. The glow plug of 
the first embodiment has a mounting portion 101 and a heat generating 
portion 102. The mounting portion 101 includes a housing 103 by means of 
which the glow plug is mounted on a head cover of a cylinder of the 
engine, and a central electrode 105 fixed in the housing 103 through an 
insulator 104. 
The heat generating portion 102 has a central rod 106 which extends 
downwardly from the lower end of the housing 103 coaxially with the 
latter. A single screw-thread groove 109 of a suitable pitch is formed in 
the peripheral surface of the central rod 106. An electric resistor 107 
made of a semiconductive material such as a ceramic material is disposed 
in the groove 109. The ceramic central rod 106 is made of alumina which is 
chemically stable enough to avoid any oxidation or deterioration even in 
oxidizing atmosphere of high temperature. On the other hand, the ceramic 
electric resistor 107 is made of TiC or SiC. As an additional material, it 
is possible to add Al.sub.2 O.sub.3 to the material mentioned above. This 
ceramic electric resistor 107 can stably generate heat at the surface 
thereof. The electric resistor 107 is connected at its upper end 108 to 
the housing 103. The resistor 107 is extended from its lower end 110 
upwardly through the ceramic central rod 106 along the axis of the latter, 
and is connected at its upper end 111 to the central electrode 105 by 
means of a silver paste. 
A preferred method of forming the heat generating portion 102 will be 
explained, although the same can be formed by other suitable methods. A 
granular material is formed by adding polyvinyl alcohol as a binder to the 
powder of Al.sub.2 O.sub.3 (alumina). A rod-shaped member is formed by a 
press with this granular material. This rod-shaped material has a size 
about 20% greater than that of the final size of the ceramic central rod 
106. This rod-shaped member is temporarily fired in an electric furnace 
and the spiral screw-thread groove 109 is formed in the fired peripheral 
surface by means of a lathe. Then, a granular material is prepared from 
powdered SiC or TiC. In order to adjust the electric resistance and the 
thermal expansion coefficient, Al.sub.2 O.sub.3 may be added to this 
material. A press work is conducted with the granular material charged in 
the space between the rod-shaped member and a mould and the portions of 
the granular material other than the portion thereof in the spiral 
screw-thread groove are removed. Then, the rod-shaped member together with 
the pressed granular material remaining in the groove is fired 
temporarily. Then, the end of the rod-shaped member is mechanically 
processed into the form shown by full-line in FIG. 2b. The rod-shaped 
member is then finally fired to extinguish the binder while contracting 
the size, thereby to form the heat generating portion 102 in which the 
ceramic resistor 107 is disposed in the spiral screw-thread groove 109. 
In operation, the glow plug is mounted on the cylinder cover of an engine 
at its mounting portion 101. Then, as a battery is connected between the 
central electrode 105 and the housing 103, the electric current flows 
through the central electrode 105 to the ceramic resistor 107, so that the 
ceramic resistor 107 produces heat to heat up the whole part of the heat 
generating portion 102 up to the desired high temperature. 
According to the arrangement stated above, the ceramic resistor is disposed 
directly into the atmosphere, so that the heat generating portion can be 
heated to the desired high temperature in a shorter period of time and a 
high thermal efficiency is obtained. In addition, the undesirable 
deterioration and breakdown of the material due to heating to high 
temperature are avoided advantageously. 
Referring now to FIG. 3, a glow plug in accordance with another embodiment 
of the invention has a pair of screw-thread grooves formed in the 
peripheral surface of a ceramic central rod 206 in the heat genrating 
portion 202. Ceramic resistors 207 and 208 are embedded in these grooves 
just under the peripheral surface of the heat generating portion 202. The 
resistors 207 and 208 are shaped substantially identically to each other. 
The ceramic central rod 206 and the resistors 207, 208 are made from the 
same materials as the materials of the central rod 106 and the resistor 
107 of the first embodiment. The resistor 207 is connected at its upper 
end 214 to the central electrode 205, while the resistor 208 is connected 
at its upper end 211 to the housing 203. The resistor 207 and the resistor 
208 are connected directly to each other at their lower ends 209 and 210. 
Other portions of the glow plug of this second embodiment are materially 
identical to those of the first embodiment. In operation, a battery is 
connected between the central electrode 205 and the housing 203 to supply 
the resistors 207 and 208 with electric power to generate heat for heating 
up the heat generating portion 202 as a whole up to the desired high 
temperature. In the glow plug of the second embodiment, the heating time 
is further shortened to further enhance the thermal efficiency because the 
resistors 207 and 208 are laid just beneath the peripheral surface of the 
heat generating portion 202.