JP3589684B2 - Ceramic glow plug - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a ceramic glow plug used as a starting assist device in an internal combustion engine such as a diesel engine.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, two types of ceramic glow plugs used as a start-up assist device in a diesel engine or the like have been roughly classified.
One is a type in which a coil-shaped tungsten wire is used as a heating element, an electrode is electrically coupled to the heating element, and a ceramic heater embedded in an insulating ceramic body is provided. The other is to use a conductive ceramic or a mixture of powder of metal and insulator as the heating element, electrically connect the electrodes to the heating element, and embed the ceramic in the insulating ceramic insulator. It is equipped with a heater.
[0003]
Of the above two types of ceramic heaters, a ceramic heater using a tungsten wire as a heating element has a thermal stress caused by a difference in linear expansion coefficient between the tungsten wire and the insulating ceramic. In some cases, the coil heater may be damaged and the ratio of the diameter of the circumscribed circle of the coiled tungsten wire to the outer diameter of the ceramic heater is specified as disclosed in Japanese Patent Application Laid-Open No. 59-84025. However, for the heating element using conductive ceramic or a mixture of powder of metal and insulating material, the linear expansion coefficient of the insulating ceramic can be adjusted by selecting the material to be used. There is no need to take great care for stress.
[0004]
The insulating ceramic insulator of the above-described ceramic heater is mainly composed of a non-oxide ceramic such as silicon nitride. However, with the recent trend of extending the life of ceramic glow plugs, there is a problem that the insulating ceramics are corroded and gradually disappear in a corrosive atmosphere caused by gas in the engine and with repeated use of high and low temperatures for a long time. It is becoming possible. For example, in a ceramic heater using a conductive ceramic or a mixture of a metal and an insulating powder as a conductive member, a problem that the conductive member is exposed after prolonged use in an engine may be observed. is there. Since the conductive member is inferior in heat resistance and corrosion resistance as compared with the insulating ceramic insulator, if it is exposed on the surface, it is subject to corrosion (oxidation) by a combustion gas, and in the worst case, the resistance value increases and finally stops functioning. There is fear. As a countermeasure, a ceramic heater having an alumina coating layer formed on the surface thereof has been proposed as disclosed in Japanese Patent Application Laid-Open No. 63-297925. However, as disclosed in Japanese Patent Application Laid-Open No. 63-297925, the insulating ceramic insulator of the ceramic heater is generally made of silicon nitride, has a large difference in linear expansion coefficient from alumina, and has a high engine output. In recent years, the combustion temperature has risen, and cracks may occur in the alumina coating layer due to the further increase in thermal stress due to the difference in linear expansion coefficient, and the alumina coating layer may eventually fall off. Therefore, there is a need for a technique for quickly reducing the corrosion of the insulating ceramic insulator of the ceramic heater.
[0005]
[Problems to be solved by the invention]
The present invention has been made in view of the above problems, and provides a ceramic glow plug including a highly durable ceramic heater in which a heating element is not exposed due to corrosion disappearance of an insulating ceramic insulator even when used for a long time.
[0006]
[Means for Solving the Problems]
Therefore, in the present invention, the holding means, the ceramic heater protruding from one end of the holding means, and the ceramic heater held by the holding means, and the ceramic heater and the metal protruding from the other end of the holding means A ceramic glow plug comprising a center shaft held by the holding means so as to be electrically conducted through a wire;
The ceramic heater includes an insulating ceramic insulator containing silicon nitride as a main component, a heating element embedded in the insulating ceramic insulator and made of conductive ceramic, and at least a part embedded in the insulating ceramic insulator. A ceramic glow comprising an electrode directly connected to the heating element, wherein the surface roughness of the entire surface of the ceramic heater protruding from one end of the holding means has a ten-point average surface roughness of 10 μm or less. It provides a plug.
[0007]
[Action]
With the above configuration, a deep surface flaw can be removed. As a result, it is possible to eliminate the cause of the situation where the corrosion is more likely to progress due to the higher stress concentration, so that the progress of the corrosion of the insulating ceramic insulator can be suppressed.
[0008]
【The invention's effect】
By the above-described action, the progress of corrosion of the insulating ceramic insulator is suppressed, and thus the disappearance of corrosion of the insulating ceramic insulator is suppressed. This makes it possible to provide a ceramic plug having a highly durable ceramic heater in which the heating element is not exposed.
[0009]
【Example】
(First embodiment)
The inventor has conducted various studies on the progress of corrosion and found out the importance of the relationship between the surface roughness of the ceramic heater and the progress of corrosion. As a result of research, the corrosion disappearance of the surface of the insulating ceramic insulator is particularly large in the part exposed to the fuel spray, but the part exposed to the fuel spray is not uniform but also the corrosion of the insulating ceramic insulator. The feature that corrosion disappears very much in the very small concave part of the surface was clarified. Further, as a result of further studying the influence of the depth of the concave portion, it was found that the deeper the concave portion, the more corrosion disappearance. Considering the relationship between the depth of the recess and the disappearance of corrosion, for example, a conductive member made of a mixture of conductive ceramic or powder of a metal and an insulator is used as a conductive member. Although the adjustment is relatively easy, it is difficult to completely adjust the coefficient of linear expansion in the entire operating temperature range from room temperature to high temperature. Therefore, in a repetitive environment of high and low temperatures, a small value, but a thermal stress due to the difference in linear expansion coefficient is generated. Further, it is considered that the recesses corresponded to the notches, and stress concentration occurred at the bottom of the recesses, so that corrosion was likely to progress. Therefore, it is considered that a deep recess has a large notch, and the stress concentration is greater, and the corrosion is more likely to proceed.
[0010]
However, since it is impossible to make the surface of the ceramic heater to have a surface roughness of 0 μm without unevenness, in consideration of the above considerations, a minute surface on which corrosion due to stress concentration in a concave portion does not easily progress in use in an engine is considered. Various prototypes were produced in order to examine the size of the irregularities, that is, the influence of the surface roughness.
Hereinafter, the present invention will be described based on an embodiment shown in the drawings.
[0011]
FIG. 1 is a sectional view showing an embodiment of a ceramic heater 2 held by a hollow pipe 3 constituting a holding means of the present invention. The ceramic heater 2 has a heating element 11 made of a U-shaped conductive member inside a tip of an insulating ceramic insulator 12 mainly composed of rod-shaped silicon nitride having a circular cross section, and an electric connection to the heating element 11. And a pair of electrodes 13 and 14 made of tungsten, which are connected to each other. The side surface 2a of the ceramic heater 2 from which the end 14a of the electrode 14 is exposed is plated with nickel, and the metal hollow pipe 3 holds the ceramic heater 2 in order to hold the ceramic heater 2. It is brazed and fixed so as to cover it. Further, the end 2b of the ceramic heater 2 where the end 13a of the electrode 13 is exposed is plated with nickel, and the coil-shaped metal wire 5 is brazed and fixed.
[0012]
FIG. 2 is a cross-sectional view showing an example in which the ceramic heater 2 of the present invention is assembled to the ceramic glow plug 1. One end of a metal housing 4 serving as a cylindrical holding means having a screw 4a for mounting to an engine is joined to the outer periphery of the hollow pipe 3 by brazing. One end of the coil-shaped metal wire 5 is welded to the center shaft 6. The center shaft 6 is electrically connected to a power source (not shown) via a terminal screw portion 6a of the center shaft 6. The center shaft 6 and the other end of the metal housing 4 are insulated by a glass seal 7 and an insulating bush 8, and are fixed by tightening a nut 9. With such a configuration, power is supplied from a power source (not shown) through the center shaft 6, the metal wire 5, the electrode 13, the heating element 11, the electrode 14, the hollow pipe 3, and the housing 4, The electric power supply means is electrically grounded to the engine block which is not used.
[0013]
Here, the ceramic heater 2 was manufactured to have various ten-point average surface roughnesses from 2.5 μm to 25 μm in a portion of the ceramic heater 2 exposed from the hollow pipe 3 (the surface roughness complies with JIS B0601).
Hereinafter, test results showing the effects of the present invention will be described.
First, the depth of the concave portion and the progress of corrosion were tested on the insulating ceramic insulator 12 with the ceramic glow plug 1 of the ceramic heater 2 containing silicon nitride as a main component. In the test, a 2000 cc diesel engine was subjected to a cooling test so that high-temperature corrosion and thermal stress in the concave portion were easily generated. The conditions are as follows: the energization of the ceramic glow plug 1 is OFF; O. The operation was performed for 300 cycles (20 hours), with 2 cycles at T4000 rpm and 2 minutes at idling as low temperature conditions as one cycle. Thereafter, the ceramic heater 2 was taken out, embedded in a resin, and a cross section was observed. An example is shown in FIG. From FIG. 3, it can be seen that the entire surface is corroded, but particularly the deep part of the concave portion is significantly corroded. Although the tip of the projection is slightly corroded, it is considered that this is because it tends to become a hot spot due to the thermal influence of combustion. However, the projection is corroded regardless of the surface roughness.
[0014]
Based on the above test results, the relationship between the surface roughness and the durability was tested. The test was performed using the ceramic heater 2 having a surface roughness of 4 to 25 μm and having a diameter of 3.5 mm as the insulating ceramic insulator 12 and containing silicon nitride as a main component. Further, in order to apply a severe heat load equivalent to the vehicle life of 200,000 km required in recent years, the vehicle was operated for 7,500 cycles (500 hours) under the same conditions as in FIG. After the test, the diameter of the portion of the ceramic heater 2 where corrosion was lost most was measured, and the difference between the radii was determined as the amount of corrosion lost. The portion of the ceramic heater 2 where the most corrosion disappeared was concentrated in the half of the tip side where the heat load from the engine was large. The temperature of the metal pipe side is lower than that of the tip side due to heat conduction to the metal pipe, and corrosion is lighter than that of the tip side. The result is shown in FIG. From FIG. 4, it can be seen that the corrosion loss of the surface roughness of about 10 μm or less is about 0.4 mm at the maximum, but the corrosion loss increases at about 10 μm or more. Considering this test result, even if the surface roughness is small and the stress concentration of the thermal stress due to the difference in linear expansion coefficient is small at the concave part, the maximum corrosion of about 0.4 mm under repeated high and low temperatures in the engine Disappearance was observed, which is considered to be the heat resistance and durability of the ceramic heater containing silicon nitride as a main component regardless of the surface state of the ceramic heater 2. The amount of corrosion disappearance when the surface roughness is about 10 μm or more is equal to or more than the amount of corrosion disappearance due to the heat resistance and durability of the ceramic heater 12. Under repeated high and low temperatures in the engine, stress concentration occurs due to the notch effect of the concave part of thermal stress due to the difference in linear expansion coefficient, the corrosion progresses rapidly in the deep part of the concave part, and corrosion and disappearance are repeated, It is considered that the actual surface roughness excluding the corroded layer does not significantly change even after long-time use.
[0015]
As described above, according to the results of FIG. 4, the corrosion of the insulating ceramic insulator 12 is reduced by setting the ten-point average surface roughness of the ceramic heater 2 to 10 μm or less and the burying depth of the conductive member to 0.4 mm or more. Exposure of the heating element due to disappearance can be prevented. Thereby, a long-life ceramic glow plug can be provided.
Although the insulating ceramic insulator 12 is made of a ceramic containing silicon nitride as a main component, a non-oxide ceramic which is oxidized and corroded is similarly effective.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an embodiment of a ceramic heater according to the present invention.
FIG. 2 is a sectional view showing an embodiment of a ceramic glow plug using the ceramic heater of the present invention.
FIG. 3 is an enlarged schematic view showing a corroded state of the ceramic heater.
FIG. 4 is a diagram showing the amount of corrosion disappearance after an engine test.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 ceramic glow plug 2 ceramic heater 2 a ceramic heater side surface 2 b ceramic heater end 2 c ceramic heater protrusion 3 hollow pipe 4 housing 4 a mounting screw portion 5 metal wire 6 center shaft 6 a terminal screw portion 7 glass seal 8 insulating bush 9 nut 11 heating element 12 Insulating ceramic insulator 13 Electrode 13a Electrode end 14 Electrode 14a Electrode end 3,4 Holding means 3,4,5,6,13,13a, 14,14a Power supply means

Claims (3)

A holding means, a ceramic heater protruding from one end side of the holding means, and being held by the holding means; and a ceramic heater projecting from the other end side of the holding means, and electrically connected to the ceramic heater via a metal wire. A ceramic glow plug consisting of a central shaft held by the holding means so as to conduct,
The ceramic heater includes an insulating ceramic insulator containing silicon nitride as a main component, a heating element embedded in the insulating ceramic insulator and made of conductive ceramic, and at least a part embedded in the insulating ceramic insulator. And the electrode is directly connected to the heating element, and the surface roughness of the entire surface of the ceramic heater protruding from one end of the holding means is 10 μm or less in ten-point average surface roughness. Features a ceramic glow plug. The ceramic glow plug according to claim 1, wherein the metal wire has a coil shape. The ceramic glow plug according to claim 1, wherein the heating element and the electrode embedded in the insulating ceramic insulator are embedded at a depth of 0.4 mm or more.

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