JPH07103482A - Ceramic glow plug - Google Patents

Abstract

PURPOSE:To prevent an exposure of a heat generating body caused by corrosion wear of an insulated body by embedding a heat generating body and an electrode in an insulating ceramic insulation body and by specifying a coarseness of an edge side surface of a ceramic heater of a projecting section from a holding means. CONSTITUTION:A ceramic heater 2 has a composition wherein in a top edge inner section of an insulating ceramic insulation body 12, a heat generating body 11 and a pair of electrodes 13, 14 which are electrically connected to this body are embedded. At a side face 2a of a ceramic heater 2 wherein an edge section 14a of an electrode 14 is exposed a Ni-plating is executed and a fixture by brazing is so executed that a ceramic hollow pipe 3 encloses a ceramic heater 2. Besides, on an edge section 2b of a ceramic heater 2 wherein an edge section 13a of the electrode 13 is exposed, a Niplating is executed. And, a mean coarseness at a middle point of a section that is exposed from a hollow pipe 3 of the ceramic heater 2 is made to be 10mum and less. By this, a corrosion progress of the insulation body of the insulating ceramic can be suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic glow plug used as a starting aid device in an internal combustion engine such as a diesel engine.

[0002]

2. Description of the Related Art Conventionally, two types of ceramic glow plugs, which have been used as starting aids for diesel engines and the like, have been roughly classified. One is
A coil heater 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. Another one
One is to use a conductive ceramic or a mixture of metal and powder such as an insulator as the heating element, and to electrically connect the electrode to the heating element and to embed a ceramic heater embedded in the insulating ceramic insulator. It is equipped with.

Among the above-mentioned two types of ceramic heaters, the ceramic heater using a tungsten wire as a heating element has a thermal stress due to a difference in linear expansion coefficient between the tungsten wire and the insulative ceramic, and is therefore mounted in an internal combustion engine when used. In some cases, the ceramic heater may be damaged.
As disclosed in Japanese Patent No. 4025, there is one that defines the ratio of the diameter of the circumscribed circle of the coiled tungsten wire to the outer diameter of the ceramic heater. However, when a conductive ceramic or a mixture of powder of metal and insulator is used as the heating element, it is possible to match the linear expansion coefficient with the insulating ceramic by selecting the material used. There is no need to pay much attention to stress.

The insulating ceramic insulator of the above-mentioned ceramic heater is mainly composed of a non-oxide ceramic such as silicon nitride. However, due to the trend toward longer life of ceramic glow plugs in recent years, there is a problem that insulating ceramics are corroded and gradually disappear due to repeated use of high temperature and cold heat in a corrosive atmosphere due to gas in the engine. Is becoming available. For example, in the case of a ceramic heater using a conductive ceramic or a mixture of metal and an insulating powder as the conductive member, there is a problem that the conductive member is exposed after long-time use in the engine. is there. Compared to insulating ceramic insulators, conductive members are inferior in heat and corrosion resistance, so
If it is exposed to the surface, it may be corroded (oxidized) by the combustion gas, and in the worst case, the resistance value may increase and eventually it may not function. As a countermeasure, JP-A-6
As disclosed in Japanese Patent Laid-Open No. 3-297925, there is proposed a ceramic heater having an alumina coating layer formed on the surface thereof. However, JP-A-63-29792
As also disclosed in Japanese Patent Publication No. 5, the insulating ceramic insulator of the ceramic heater is generally made of silicon nitride and has a large difference in coefficient of linear expansion from that of alumina, so that the combustion temperature increases as the engine output increases. In recent years, due to a further increase in thermal stress due to the difference in linear expansion coefficient, cracks may occur in the alumina coating layer and eventually fall off. Therefore, there is an urgent need for a technique for reducing the corrosion of the insulating ceramic insulator of the ceramic heater.

[0005]

In view of the above problems, the present invention provides a ceramic having a highly durable ceramic heater in which the heating element is not exposed even when used for a long time due to corrosion disappearance of the insulating ceramic insulator. It provides a glow plug.

[0006]

A ceramic glow having a holding means, a ceramic heater partly protruding from the holding means and a means for supplying electric power from an external power source to the ceramic heater is provided. In the plug, the ceramic heater includes a heating element, an electrode coupled to the heating element, and an insulating ceramic insulator made of a non-oxide, and the heating element and the electrode are the insulating ceramic. And the surface roughness of a part of the part protruding from the holding means on the tip side of the ceramic heater or the whole part of the protruding part is 10 μm or less in ten-point average surface roughness. The above-mentioned problem is solved by using the ceramic glove plug including the ceramic heater.

[0007]

By virtue of the above-mentioned constitution, deep surface scratches can be removed. As a result, it is possible to eliminate the cause of the situation in which corrosion is more likely to proceed due to a larger stress concentration, and thus it is possible to suppress the progress of corrosion of the insulating ceramic insulator.

[0008]

As described above, the progress of corrosion of the insulative ceramic insulator is suppressed by the above-described action, and thus corrosion disappearance of the insulative ceramic insulator is suppressed. Thereby,
It is possible to provide a ceramic plug including a highly durable ceramic heater that does not expose the heating element.

[0009]

【Example】

(First Example) The present inventor conducted extensive studies while finding out the importance of the relationship between the surface roughness of a ceramic heater and the progress of corrosion while making various studies on the progress of corrosion. As a result of the research, the corrosion loss on the surface of the insulating ceramic insulator is particularly large in the part exposed to the fuel spray, but the insulating ceramic insulator is not uniformly corroded and disappeared in the part exposed to the fuel spray, It was possible to clarify the feature that corrosion disappeared frequently in very small recesses on the surface. Further, as a result of further studying the influence of the depth of the recesses, it has been found that the deeper the recesses are, the more corrosion disappears. To consider the relationship between the depth of the recess and the disappearance of corrosion, for example, a conductive member that uses a mixture of conductive ceramics or a powder of metal and an insulating material, the linear expansion coefficient of Although matching is relatively easy, it is difficult to match the complete linear expansion coefficient in the entire operating temperature range from room temperature to high temperature. Therefore, under repeated environments of high temperature and cold heat, thermal stress occurs due to the difference in linear expansion coefficient, although the value is small.
Further, it is considered that the concave portion corresponds to the cutout portion, stress concentration occurs at the bottom of the concave portion, and corrosion is likely to proceed. Therefore, deep recesses become large notches,
It is considered that the stress concentration is larger and the corrosion is more likely to proceed.

However, since it is impossible to make the surface of the ceramic heater have a surface roughness of 0 μm without unevenness, in view of the above consideration, the surface where corrosion due to stress concentration in the recesses does not easily progress in use in an engine. Various prototypes were prepared in order to study the influence of the size of the minute unevenness, that is, the surface roughness. Hereinafter, the present invention will be described based on the embodiments shown in the drawings.

FIG. 1 is a sectional view showing an embodiment of a ceramic heater 2 held by a hollow pipe 3 which constitutes a holding means of the present invention. The ceramic heater 2 has a rod-shaped insulating ceramic insulator 12 having a circular cross section as a main component, and a heating element 11 made of a U-shaped conductive member inside the tip of the insulating ceramic insulator 12. The heating element 11 is electrically connected to the heating element 11. And a pair of electrodes 13 and 14 made of tungsten and bonded to each other. The side surface 2a of the ceramic heater 2 from which the end portion 14a of the electrode 14 is exposed is nickel-plated, and the metal hollow pipe 3 holds the ceramic heater 2 to hold the ceramic heater 2. It is fixed by brazing so as to include it. Further, the ceramic heater 2 in which the end portion 13a of the electrode 13 is exposed
The end portion 2b of the is plated with nickel, and the coiled metal wire 5 is fixed by brazing.

FIG. 2 is a sectional view showing an example in which the ceramic heater 2 of the present invention is assembled to the ceramic glow plug 1. On the outer periphery of the hollow pipe 3, a metal housing 4 forming a cylindrical holding means having a mounting screw 4a for an engine.
One end of each is joined by brazing. One end of the coil-shaped metal wire 5 is welded to the center shaft 6 and is electrically continuous with a power source (not shown) via the terminal screw portion 6a of the center shaft 6. A glass seal 7 and an insulating bush 8 insulate the space between the center shaft 6 and the record housing 4, and a nut 9 is tightened and fixed. With such a configuration, a power source (not shown), the metal wire 5, the electrode 13, the heating element 11, the electrode 14, the hollow pipe 3, and the housing 4 are used for the illustration. The engine block is electrically grounded to form a power supply means.

Here, various ten-point average surface roughness of the portion of the ceramic heater 2 which is exposed from the hollow pipe 3 from 2.5 μm to 25 μm was prepared (the surface roughness is JIS B 0601). According to). Less than,
The test results showing the effect of the present invention will be shown. First, the insulating ceramic insulator 12 was tested with the ceramic glow plug 1 of the ceramic heater 2 containing silicon nitride as a main component for the depth of the recess and the progress of corrosion.
The test was a 2000 cc diesel engine, and a cold heat test was performed so that high temperature corrosion and thermal stress in the recesses were likely to occur. The conditions are as follows: a high temperature condition of W. O. T4000r
Operation was performed for 300 cycles (20 hours), with 2 minutes at pm and 2 minutes at low temperature as idling as one cycle.
After that, the ceramic heater 2 was taken out and embedded in a resin to observe a cross section. An example thereof is shown in FIG. From Figure 3,
Although the entire surface is corroded, it can be seen that particularly the corrosion of the deep part of the recess is remarkable. Although the tip of the convex portion is also slightly corroded, it is considered that this is because hot spots are likely to be formed due to the thermal effect of combustion, but the convex portion is corroded regardless of the surface roughness.

Based on the above test results, the relationship between surface roughness and durability was tested. The test was carried out with the ceramic heater 2 having a surface roughness of 4 to 25 μm and having φ3.5 mm containing silicon nitride as a main component as the insulating ceramic insulator 12. In addition, in order to apply a severe heat load equivalent to 200,000 km of the vehicle life required in recent years, 7500 cycles (500 hours) of operation were performed under the same conditions as in FIG. After the test, the diameter of the portion of the ceramic heater 2 where corrosion was most lost was measured, and the difference in radius was determined as the amount of corrosion lost. The most corrosive part of the ceramic heater 2 is half of the tip side where the heat load from the engine is large.
Was concentrated on the part. The temperature of the metal pipe side is lower than that of the tip side because of heat conduction to the metal pipe, and the corrosion is less than that of the tip side. The result is shown in FIG. It can be seen from FIG. 4 that when the surface roughness is about 10 μm or less, the maximum corrosion loss is about 0.4 mm, but when it is about 10 μm or more, the corrosion loss increases. Considering the results of this test, even if the surface roughness is small and the thermal stress due to the difference in linear expansion coefficient is small in the stress concentration in the recesses, corrosion of up to about 0.4 mm under repeated high temperature and cold heat in the engine Disappearance was recognized, which is considered to be the heat resistance durability of the ceramic heater containing silicon nitride as a main component, regardless of the surface state of the ceramic heater 2. Surface roughness is about 10
The amount of corrosion disappearance of .mu.m or more is equal to or more than the amount of corrosion disappearance due to the heat resistance durability of the ceramic heater 12.
In addition to 4 mm, stress concentration occurs due to the notch effect of the thermal stress due to the difference in linear expansion coefficient due to the difference in linear expansion coefficient when the engine is repeatedly subjected to high temperature and cold heat, and the corrosion progresses rapidly in the deep part of the recess, causing corrosion and disappearance. Therefore, it is considered that the substantial surface roughness excluding the corrosion layer does not change significantly even after long-term use.

From the results shown in FIG. 4, the ten-point average surface roughness of the ceramic heater 2 is 10 μm or less, and the burying depth of the conductive member is 0.4 mm or more. It is possible to prevent the heating element from being exposed due to the disappearance of the corrosion of 12. This makes it possible to provide a long-life ceramic glow plug. Although the insulating ceramic insulator 12 is a ceramic containing silicon nitride as a main component, the same effect can be obtained even if it is a non-oxide ceramic that oxidizes and corrodes.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a ceramic heater of 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 corrosion state of a ceramic heater.

FIG. 4 is a diagram showing a corrosion disappearance amount after an engine test.

[Explanation of symbols]

1 Ceramic Glow Plug 2 Ceramic Heater 2a Ceramic Heater Side 2b Ceramic Heater End 2c Ceramic Heater Projection 3 Hollow Pipe 4 Housing 4a Mounting Screw 5 Metal Wire 6 Center Shaft 6a Terminal Screw 7 Glass Seal 8 Insulation Bushing 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 (1)

[Claims] 1. A ceramic glow plug comprising a holding means, a ceramic heater part of which is projected and held from the holding means, and a means for supplying electric power from an external power source to the ceramic heater, wherein: The ceramic heater is composed of a heating element, an electrode coupled to the heating element, and an insulating ceramic insulator made of a non-oxide, and the heating element and the electrode are embedded in the insulating ceramic insulator. Having a structure,
The ceramic heater is characterized in that the surface roughness of a part of the part protruding from the holding means on the tip side of the ceramic heater or the whole part of the protruding part is 10 μm or less in ten-point average surface roughness. The above-mentioned ceramic glow plug.