Glow Plug for Diesel Engines

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

FIG. 1 is a vertical sectional view of a conventional glow plug for diesel engines;

FIG. 2a is a vertical sectional view of a glow plug for diesel engines, constructed in accordance with an embodiment of the invention;

FIG. 2b is an enlarged sectional view of a portion of the plug shown in FIG. 2b; and

FIG. 3 is a vertical sectional view of a glow plug in accordance with another embodiment of the invention.

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.

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.