JPH041265B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a glow plug used for preheating a wide combustion chamber or the inside of a combustion chamber of a diesel engine, and in particular, it is capable of exhibiting a fast heating type function and its heat generation. The present invention relates to a self-temperature-controlled glow plug that has improved characteristics and can extend afterglow for a longer period of time.

[Prior art]

In general, diesel engines have poor starting performance at low temperatures, so a glow plug is installed in the wide combustion chamber or combustion chamber, and by passing an electric current through it and generating heat, it is possible to raise the intake air temperature or to use it as an ignition source. A method has been adopted to improve starting performance. Conventionally, this type of glow plug has a so-called sheath type structure, in which a heat-resistant metal sheath is filled with heat-resistant insulating powder and a coiled heating wire made of iron chrome or nickel is embedded. is commonly used,
The structure was designed to ensure durability under adverse conditions such as the above-mentioned wide combustion chamber or exposure to high temperature gas within the combustion chamber.

However, with such a sheath type glow plug, there is a problem in terms of heat transfer efficiency because indirect heating is performed via heat-resistant insulating powder, sheath, etc., and it takes too much time to raise the temperature of the glow plug.
It could not function as a rapid heating type. With such a glow plug, it takes several tens of seconds to raise the heat generation temperature to, for example, 900° C., making it impossible to start the engine quickly. Furthermore, in the above-mentioned sheath type, the sheath part is not only exposed to adverse conditions such as the engine combustion chamber, but also because there is a large difference in temperature inside and outside when performing rapid heating. The burden on the heat-generating wires was large, and there were problems such as wire breakage due to deterioration of the material.

Therefore, instead of the above-mentioned sheath, a rod-shaped ceramic heater with a heating wire embedded in a ceramic material is used to improve heat transfer efficiency, and a glow plug is disclosed, for example, in Japanese Patent Laid-Open No. 57-41523. It has been proposed by , and has superior heat generation characteristics compared to conventional sheathed types, and it also generates red heat in a short time when heated, greatly improving temperature rise characteristics, and can demonstrate performance as a fast heating type.

However, even with glow plugs that use ceramic heaters, which have these advantages, just one type of heating wire is buried inside, just like the conventional sheathed type, so it is difficult to control the energizing power. There are some problems with this. In other words, in order to significantly improve the temperature rise characteristics during heating in this type of glow plug, it is conceivable to cause the heating wire to rapidly generate heat by passing a large current at the initial stage of energization, but it is possible to prevent the heating wire from fusing. Otherwise, the high heat may adversely affect the ceramic heater, and it may also cause battery damage.
It can also have a negative effect on the electrical circuit, causing problems such as fuses being blown.To prevent this, it is necessary to newly install a temperature control means on the circuit to the heating wire, and as a result, the glow The drawback was that the cost of the entire preheating device including the plug increased.

In this type of glow plug, the heating wire (heating element) has a structure that can self-control the power applied to the heating wire to improve its heating characteristics and prevent overheating at the heater part. This is a so-called self-temperature control type dual-material type, in which a resistor made of a material with a larger positive temperature coefficient of resistance (PTC) is connected in series with the heating wire inside the glow plug as an energizing power control element. For example, Japanese Patent Publication No. 45-11648 and Japanese Unexamined Patent Application Publication No. 1973-
These conventional glow plugs have been proposed in the past, such as in Publication No. 109538, but these conventional glow plugs not only have problems in terms of reliability of their control functions, but also have problems with rapid heating, etc., as mentioned above. This also caused problems in some respects.

That is, in the former case, in order to stably hold the power control resistor disposed inside the holder in an insulated state, the inner wall of the holder is filled with an insulating material such as water glass. Although it has been adopted,
Not only is its structure cumbersome and complicated,
It is virtually impossible to pack insulating material densely, and as a result, the heat dissipation from the resistor varies widely, making it impossible to stabilize the heat capacity in that area, which prevents current flow to the heating wire side. It was not possible to stabilize power control performance.

On the other hand, in the latter case, the heating wire and the resistor are connected in series within the sheath, so the filling efficiency of the insulating material in the resistor part is high, which is effective in improving the heat generation characteristics. On the other hand, the connection between the heating wire and the resistor is troublesome and complicated, which not only poses a problem in terms of assembly, but also ensures that the heat effect from heat generated on the heating wire side does not affect the resistor. It was actually difficult to do so. In such a configuration, although it is possible to increase the filling efficiency of the insulating material in the resistor part to a certain extent and increase the heating rate to some extent, it is not possible to shorten the temperature rise time to within 10 seconds, for example. Furthermore, since the saturation temperature cannot be kept below a certain value (for example, below 1000°C), there is a problem in that it is not possible to prolong the energization so-called afterglow time after the engine is started.

Especially with regard to this type of glow plug in recent years,
A request for the adoption of the so-called afterglow method, which maintains the glow plug energized for a certain period of time after the engine starts to ensure smooth and appropriate combustion within the engine. It is required that the afterglow time is large and that the afterglow time is as long as possible. That is, even after the engine has been started, for example in a cold region, the engine is too cold and it takes time for the engine to warm up. Furthermore, in this non-warmed-up state, there were problems such as exhaust noise and noise such as high noise during idling, generation of white smoke due to incomplete combustion, and stalling of the engine.

By the way, the diesel engine mentioned above has
There are direct injection type and wide combustion chamber type, and in the case of the former, the afterglow time is sufficient within about 30 seconds, so even with the conventional glow plug structure described above, there is no adverse effect on its performance or durability. The problems in its use are minor.

However, in the latter case, the afterglow time of approximately 30 seconds is insufficient, and the requirement is 30 seconds.
In such cases, the durability of each part of the glow plug may be adversely affected. In other words, in this type of glow plug, the applied voltage during normal preheating (about 5 seconds) is
It is about 11V, but when the engine is started, the regulator setting voltage is generally about 14V, and this is applied to the glow plug, so
If the afterglow time is prolonged, the temperature will rise too much due to the high voltage, and there is a risk that the durability will be affected, such as deterioration or melting, especially in the internal heating element and resistor part.

In particular, in recent years, diesel engines have been increasingly used in general passenger cars, and in order to compete with gasoline engines, it is desired that they have good startability, and there is a demand for fast-heating glow plugs. Although it is large, it is also desired to achieve a long afterglow as a countermeasure against engine exhaust and noise, and these requirements are contradictory to each other. In other words, in order to achieve the above-mentioned rapid heating, it is necessary to quickly supply a large amount of power to the heating wire at the initial stage of energization. Supply, on the other hand, is a hindrance. Therefore, there is a demand for a simple glow plug that takes some measures to make it possible to satisfy these conflicting demands for heat generation characteristics within the respective necessary frameworks with a simpler structure. ing.

[Summary of the invention]

The present invention has been made in view of the above-mentioned circumstances, and includes a first resistor whose one end is connected in series with a first resistor serving as a heating element in a ceramic heater held at the tip of a holder. A sheathed heater has a second resistor for controlling energized power made of a material with a larger positive temperature coefficient of resistance than the body, and the second resistor is sealed with heat-resistant insulating powder filled in a metal sheath. An air layer is formed between the inner wall of the holder and the outer periphery of the sheath by embedding the sheathed heater in a holder with a heat-resistant insulating tube fitted around its outer periphery. With a simple configuration in which the sheathed heater has a second resistor and is loosely fitted and held, the energization power control function of the sheathed heater having the second resistor can be efficiently exerted.
This allows the tip of the ceramic heater to become red hot quickly and appropriately, enhances its function as a fast heating type, greatly improves engine startability, and enables long-term afterglow as a measure against engine exhaust and noise. To provide an inexpensive self-temperature-controlled glow plug which has a simple overall structure and is easy to assemble.

〔Example〕

Hereinafter, the present invention will be explained in detail using embodiments shown in the drawings.

FIG. 1 shows an embodiment of a self-temperature-controlled glow plug according to the present invention, and in the same figure, the glow plug, whose entire body is designated by the reference numeral 10, has a first resistor whose tip end functions as a heating element inside the glow plug. A rod-shaped ceramic heater 12 for generating heat is formed entirely of heat-resistant and insulating ceramic material with the heater 11 buried therein.
and a substantially tubular metal holder 13 that holds the ceramic heater 12 at its tip, and an external connection terminal 15 is concentrically connected to the rear end of the holder 13 via an insulating bushing 14 made of synthetic material or the like. This external connection terminal 1
Reference numeral 5 denotes a sheathed heater 21 having a second resistor 20 having a power control function, which will be described later.
The resistor 11 is connected to the resistor 11 of FIG.

Here, as is well known, the above-mentioned ceramic heater 12 has a substantially elliptical cross section, and is used for pressure firing of ceramic powder filled with the first resistor 11 held in a mold. In this case, the cross section is made into a substantially elliptical shape in order to improve the density of the ceramic material compared to a circular shape, and to make its heat resistance strength, thermal conductivity, etc. more effective. The ceramic heater 12 is made of silicon, a so-called fine ceramic material that is stable in performance even at high temperatures (up to about 1700 degrees Celsius) and has excellent insulation and thermal shock resistance. Non-oxides such as silicon-based nitrides are preferred. In particular, the silicon-based nitride mentioned above has high-temperature strength that is much superior to metal materials, alumina, etc., and also has thermal shock resistance and electrical insulation properties at high temperatures.
It is also excellent in terms of wear resistance and chemical resistance, and can almost satisfy the characteristics required for this type of glow plug.

Note that 17a and 17b in the figure are metal pipes and terminal caps that are fitted into the longitudinal center and rear ends of the ceramic heater 12 and fixed by silver brazing, etc., and the first resistor buried therein. Both ends of the lead portion of the body 11 are electrically connected via metal coating layers 16a and 16b. The first resistor 11 in the heater is connected to the holder 13 side through a pipe 17a that holds the central portion of the heater 12, and the conductive wire 18 extends rearward through a cap 17b. It is connected to the external connection terminal 15 via the control sheathed heater 21 .

Further, an insulating ring 19a, a fixing nut 19b thereof, a nut 19c for tightening an external lead, etc. are screwed into the threaded portion formed on the rear end side of the external connection terminal 15. Connect the lead wires from the battery to nuts 19b, 1.
By tightening the pinch nut 19c between the terminals 9c, the external connection terminal 15 is electrically connected to the battery terminal. By screwing the threaded portion 13a formed on the outer circumference of the holder 13 into a threaded hole formed in the cylinder head of the engine, the first ceramic heater 12 on the heat generating side is connected via the holder 13. The other end of the resistor 11 is electrically connected to ground, and at the same time, the tip of the ceramic heater 12 is disposed to protrude into the combustion chamber or into the combustion chamber.

Furthermore, 14a in the figure is a holder 13 that is fitted onto the outer circumference of an insulating bushing 14 that holds an external connection terminal 15 at the rear end of the holder 13 and is caulked during assembly.
It is a metal pipe that buckles in its axial direction due to the high pressure applied by the rear end, and integrates the insulating bushing 14 with the holder 13 side with the required mechanical strength to create a structure that is less susceptible to temperature effects. This solves the problem that conventional resin insulating bushings 14 alone would expand and contract due to external temperature changes, causing them to loosen with respect to the holder 13. This is to improve the mechanical strength and prevent this. Of course, the above-mentioned insulating bushing 14 is formed integrally with the external connection terminal 15 to ensure joint strength against twisting in the rotational direction.

In addition, the heater 12 made of the ceramic material described above is
On the other hand, the reason why the sheathed heater 21 connected to the external connection terminal 15 is connected by the metal conductor wire 18 is to strongly protect the heater 12 from mechanical external forces such as various vibrations and tightening torques applied to the external connection terminal 15. It is preferable to use a material that has a certain degree of flexibility as the material for this conductive wire 18. In this case, if the conducting wire 18 is formed in a wavy manner as shown in the figure, its protective effect can be further enhanced, but depending on its flexibility, it may be formed in a straight line as shown in FIG. You can connect it as is.

According to the above-described rod-shaped ceramic heater 12, the first resistor 11, which serves as a heating element, is directly embedded in the main body made of ceramic material, so it is more effective as a faster heating type than the conventional sheath type. It is possible to make the most of this. Since the tip of the heater 12 can quickly become red hot, the startability of the engine can be greatly improved, and the output can be made appropriate and good.

The first resistor 11 embedded in the ceramic heater 12 is made of a conductive material with a high melting point, such as Pt (platinum), W (tungsten), etc., taking into account the firing temperature of ceramic material such as hot pressing. It is recommended to form the

Now, according to the present invention, in the glow plug 10 having the above-described configuration, as shown in FIG.
Connected in series to one end of the first resistor 11 to a rod-shaped ceramic heater 12 which is held at the tip of the holder 13 and is formed by embedding the first resistor 11 serving as a heating element in an insulating ceramic material. A sheathed heater 21 having a second resistor 20 for controlling the energized power is connected in series through the conductor 18, and the sheathed heater 21 is connected to a heat-resistant insulating powder 23 filled in a metal sheath 22. The holder 1 is formed with a second resistor 20 buried therein in a sealed state, and a heat-resistant insulating tube 24 fitted on the outer periphery of the sheath 22.
3, the sheath 22 outer circumferential portion and the holder 13 inner wall portion are held in a loosely fitted state such that an air layer is formed between the outer peripheral portion of the sheath 22 and the inner wall portion of the holder 13.

That is, according to the present invention, as described above, the rod-shaped ceramic heater 1 can exhibit its performance as a fast heating type and is excellent in terms of heat resistance and the like.
2, a sheathed heater 21 having a configuration similar to that of a conventional sheathed glow plug is used as a means for controlling the energized power, and by fitting and disposing this in the holder 13 as a unit, the glow plug This makes it possible to realize the rapid heating type and low saturation temperature required for No. 10. According to such a sheathed heater 21, the second
The resistor 20 can be embedded in a sealed state with the heat-resistant insulating powder 23 filled in the sheath 22, and a heat-resistant insulating tube 24 is interposed between the sheath 22 and the inner wall of the holder 13. As a result, an air layer is formed, which improves the conventional problems such as heat dissipation, stabilizes heat conduction from the second resistor 20 to the outside, and improves the ceramic heater 12.
The heat capacity of this part is made to have a larger heat capacity than that of the ceramic heater 12 serving as the heating element, and the heat generation characteristics are controlled by the second resistor 20 inside the ceramic heater 12. It is capable of properly demonstrating its functions.

To explain this in detail, the sheathed heater 21 is
With the second resistor 20 disposed inside the sheath 22, a heat-resistant insulating powder 23 such as magnesia (MgO) or zirconia (ZrO ₂ ), for example, is filled, and the heat-resistant insulating powder 23 is further packed at a high density. The sheath 22 is formed by reducing the diameter of the sheath 22, so-called swaging processing, etc., so that the sheath 22 becomes in a state of This can dramatically improve density.

That is, what is required in the above-mentioned sheathed heater 21 for power control is to suppress the temperature rise at the second resistor 20 portion to some extent in order to obtain an appropriate temperature rise at the heat generating ceramic heater 12. While it is desirable to make the heat capacity larger than that of the heat generating heater 12, it is necessary to have a certain degree of power control function to appropriately control the peak temperature and saturation temperature on the heat generating heater 12 side. An increase in temperature is necessary;
It can be said that the present invention has been made to meet these demands by providing a configuration that can satisfy both of these contradictory demands.

According to the second resistor 20 embedded in the heat-resistant insulating powder 23 filled with high density, in the initial stage of energization to the glow plug 10, the first resistor 20 on the heat-generating ceramic heater 12 side
A high voltage is applied to the first resistor 11 side depending on the resistance value ratio with that of the resistor 11, so that it quickly becomes red hot, and after a certain period of time, the second resistor 20 itself generates heat. It has a control function to reduce the voltage applied to the first resistor 11 side by the resistance value which increases.

Further, in the sheathed heater 21 using the sheath 22 having the same structure as a conventional sheath type glow plug and having an open rear end as in this embodiment, the second
Since the resistor 20 portion of the resistor 20 can be integrated with the external connection terminal 15 into a unit structure, not only can that portion be easily and reliably molded, but also these can be simply used as electrode rods to form the heat-resistant insulating tube 24. The metal conductor 18 on the ceramic heater 12 side is inserted into the holder 13 with the
It has the advantage of being easy to assemble, as it can be assembled simply by connecting it to the base by spot welding or the like.

Here, as the material of the second resistor 20 mentioned above, it is desirable that the material has a larger positive temperature coefficient of resistance (PTC) than the material of the first resistor 11 on the heat generating heater 12 side, such as tungsten ( It is preferable to use W), nickel (Ni), iron (Fe), or the like. In the glow plug 10 configured as described above, the first and second resistors 1
It has been confirmed through experiments that the resistance ratio of 1:20 may be set to, for example, about 2:1 in its initial state. According to the present invention, since the heat generation temperature in the sheathed heater 21 portion is low, there is a large degree of freedom in selecting the material of the second resistor 20 buried inside the sheathed heater 21. There are also some advantages.

Further, the heat-resistant insulating tube 24 may be formed of a material having high heat resistance and insulating properties, such as glass, ceramic, asbestos, or the like. According to this embodiment, this heat-resistant insulating tube 24 is connected to the sheathed heater 21.
The reason why the sheathed heater 21 is fitted only near the approximate center in the axial direction on the outer circumference is to hold the sheathed heater 21 in the holder 13 in an electrically and thermally insulated state, and to This is to make the outer circumference face the inner wall of the holder 13 with an air layer in between, so that the heat dissipation from the heater 21 can be properly controlled. It is desirable to provide one. In this way, the sheathed heater 21 is held in the holder 13 in a loosely fitted state, and an air layer is formed around it to appropriately suppress heat dissipation from the heater 21 to the holder 13 side, and the ceramic heater 12 It has a great effect in maintaining a large heat capacity to appropriately control the temperature on the side, and also has the advantage of ensuring its insulation properties. In particular, the existence of the above-mentioned air layer is effective in controlling the heat dissipation from the sheathed heater 21 to the holder 13 and further to the mounting parts such as the cylinder head of the engine to which this holder 13 is attached, under the desired conditions. It is possible. That is, when such a sheathed heater 21 is held in a fitted state in direct contact with the inner wall of the holder 23, the second resistor 2 inside it
The heat generated at 0 is generated from the sheathed heater 21 to the holder 13.
Since the heat is directly transferred to the cylinder head side through the sheathed heater 21, it is difficult to ensure the desired temperature rise characteristics in the sheathed heater 21 portion. Furthermore, depending on the mounting condition of the glow plug 10 on the cylinder head mentioned above, a problem arises in that the degree of heat dissipation varies, and as a result, the heat dissipation characteristics of the sheathed heater 21 vary depending on the engine model. There is a risk that variations may occur, leading to problems such as not being able to maintain a constant energizing power characteristic by the sheathed heater 21.According to the present invention, the presence of an air layer suppresses heat dissipation. However, it is possible to obtain a temperature rise in the sheathed heater 21 portion in a desired state.

Note that the heat-resistant insulating tube 24 described above may be attached to the sheathed heater 21 in any suitable modification, for example, as shown in FIG. Of course, it may be provided on the end side so as to extend to the rear end side of the holder 13.

In the glow plug 10 configured as described above, when a voltage is applied to the sheathed heater 21 via the external connection terminal 15 and further to the ceramic heater 12 side via the metal conductor wire 18, in the initial stage of energization, the applied voltage is The voltage is both heaters 12, 2
The voltage is divided by the resistance ratio of each resistor 11 and 20 in one part, and a voltage larger than that of the sheathed heater 21 is applied to the ceramic heater 12 side whose heat capacity is small, resulting in a relatively large power density with respect to the sheathed heater 21. Its tip rapidly generates heat. At this time, as described above, it is clear that the ceramic heater 12 is capable of exhibiting a faster heating function than the conventional sheath type.

Moreover, if the sheathed heater 21 side gradually generates heat during a predetermined period of time after the start of energization, and its resistance value increases accordingly, both these heaters 12, 2
The voltage applied between the ceramic heaters 12 and 1 gradually changes, and the ceramic heater 12 reaches its peak temperature at about 1100 to 1250°C, and then saturates at about 1000°C, thereby preventing overheating. That is, at this point, the resistance value of the second resistor 20 on the sheathed heater 21 side approaches that of the first resistor 11 serving as the heating element on the ceramic heater 12 side. Since the control function of the second resistor 20 limits the voltage applied to the first resistor 11 to a predetermined value or less, its durability can be improved when performing afterglow for a long time. This can be fully guaranteed.

Therefore, according to the glow plug 10 having such a configuration, the control function of the voltage applied to the ceramic heater 12 by the sheathed heater 21, which is easily incorporated into the holder 13 as a unit, eliminates the need for a conventional complicated control circuit. This is a great advantage because it can achieve the function of a rapid heating type and the afterglow for a long time by itself without providing it separately. In particular, according to the above-mentioned configuration, it is possible to reduce the time it takes for the heat generation temperature to reach 900°C within about 4 seconds, and also to reduce the saturation temperature to 1000°C.
In addition, it is possible to suppress the peak temperature to 1250°C or less and enable afterglow for more than 3 minutes. This will be clear from the temperature characteristics a and b of the body 20. Here, c in the figure is the temperature characteristic when only the above-mentioned ceramic heater 12 is used alone.

Note that the present invention is not limited to the structure of the embodiment described above, and the shape, structure, etc. of each part may be modified and changed as appropriate. For example, instead of the sheathed heater 21 in the above embodiment, a sheathed heater 31 having a sheath 30 with both ends open may be used, as shown in FIG. Here, this sheathed heater 31 has a second resistor 20 disposed therein, and a heat-resistant insulating powder 2
3, and as shown in FIG. 5, after applying rocking vibration, pressure is applied from both ends to increase the filling efficiency of the heat-resistant insulating powder 23 inside. It will be easily understood that the same effects as those described above can be obtained. Here, 24a and 24b in the figure are heat-resistant insulating tubes that are fitted into both ends of the sheathed heater 31 and supported in a loosely fitted state with an air layer formed between the inner wall and the holder 13. , 32a, 32b
33a and 33b are jigs for filling the heat-resistant insulating powder 23, and 33a and 33b are jigs for filling the heat-resistant insulating powder 23. Further, in this case, an example is shown in which both ends of the second resistor 20 embedded therein are extended and joined to the ceramic heater 12 and external connection terminal 15 side, but the present invention is not limited to this. It should be obvious.

As explained above, according to the self-temperature-controlled glow plug according to the present invention, a rod-shaped ceramic heater in which a first resistor serving as a heating element is embedded is held in the tip of the holder, and the ceramic A sheathed heater has a second resistor for controlling energized power that is connected in series to one end of the first resistor in the heater. By embedding the sheathed heater in a sealed state, and incorporating the sheathed heater into the holder with a heat-resistant insulating tube fitted to the outer periphery of the sheath, an air layer is created between the inner wall of the holder and the sheathed heater. Although the configuration is simple and inexpensive, the sheathed heater in which the second resistor is sealed in a high-density heat-resistant insulating powder can be used. Furthermore, by making the outer periphery face the inner wall of the holder with an air layer in between, it is possible to set the heat dissipation in the sheathed heater part to the desired state, thereby controlling the energization power control function. The ceramic heater achieves rapid and appropriate red heat at the tip of the ceramic heater, greatly improving the startability of the engine that enhances its function as a fast-heating type, and also provides long-term after-sales service as a measure against engine exhaust and noise. It has various excellent effects, such as enabling glow, and having a simple overall structure and excellent ease of assembly.

[Brief explanation of drawings]

Fig. 1 is a longitudinal cross-sectional side view showing one embodiment of a self-temperature control type glow plug according to the present invention, Fig. 2 is a characteristic diagram showing the temperature characteristics of the ceramic heater and sheathed heater, which are the main parts thereof, and Fig. 3 4 is a longitudinal sectional side view showing another embodiment of the present invention, and FIG. 5 is a schematic explanatory diagram showing the manufacturing process of a sheathed heater as the main part. . DESCRIPTION OF SYMBOLS 10... Self-temperature control glow plug, 11... First resistor, 12... Ceramic heater, 13... Hollow holder, 15... External connection terminal, 18... Metal conductor wire, 20... Second resistor, 21, 31 ... Sheathed heater, 22, 30... Sheath, 23... Heat-resistant insulating powder, 24 (24a, 24b)... Heat-resistant insulating tube.

Claims (1)

[Scope of Claims] 1. A rod-shaped ceramic heater in which a first resistor serving as a heating element is embedded in an insulating ceramic material and is held at the tip of a hollow holder with one end protruding to the outside. and a second resistor for controlling energized power made of a material having a larger positive temperature coefficient of resistance than the first resistor and connected in series to one end of the first resistor buried in the ceramic heater. This sheathed heater is formed by filling a metal sheath with heat-resistant insulating powder and embedding a second resistor therein in a sealed manner. A heat-resistant insulating tube fitted to the outer circumference of the sheath is assembled into the holder so that it is loosely fitted and held in the holder, and between the outer circumference of the sheath and the inner wall of the holder. A self-temperature-controlled glow plug characterized in that it is configured to form an air layer. 2. The sheathed heater is assembled into a holder with a heat-resistant insulating tube fitted to a part of the outer periphery of the sheath that avoids the heat-generating part caused by the second resistor buried inside the sheathed heater. The self-temperature control type glow plug according to claim 1, wherein the glow plug is held in a loosely fitted state with an air layer interposed between the glow plug and the inner wall thereof.