JP2003303661A - Spark plug for internal combustion engine and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure both thermal shock resistance property and voltage- withstanding property of a spark plug for an internal combustion engine. <P>SOLUTION: With the spark plug using a high-pressure-molded insulator 20, surface roughness of the insulator 20 at a part contacting with a packing 60 is made 10 μm or less in ten-point average, so that a contact area with the packing 60 is increased and heat transmitted from combustion gas to the insulator can be more easily released to a housing 10 through the packing 60. In other words, an excellent heat dissipation is realized. On this account, temperature rise of the insulator 20 at the part contacting the packing 60 is made small to improve the thermal shock resistance property of the insulator 20. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spark plug for an internal combustion engine that ignites an air-fuel mixture by spark discharge and a method for manufacturing the spark plug.

[0002]

2. Description of the Related Art This type of spark plug for an internal combustion engine has various required characteristics such as thermal shock resistance and withstand voltage, and these required characteristics must be satisfied at the same time. Japanese Patent Laid-Open No. 11-43368 proposes a method for improving the thermal shock resistance of the insulator by lowering the porosity of the insulator.

[0003]

However, when the insulator is densified by high-pressure molding to improve the dielectric strength of the insulator, the hardness of the insulator is increased (Young's modulus is also increased). Therefore, the thermal shock resistance of the insulator is lowered, and it has been found that the thermal shock resistance of a level required for practical use cannot be obtained only by reducing the porosity of the insulator as described in the above publication.

Further, in the spark plug, the packing disposed between the insulator and the housing prevents the leakage of combustion gas from between the insulator and the housing, but it contacts the packing in the insulator. It was found that the thermal stress was the largest at the part, so that cracking was likely to occur at that part due to thermal shock.

The present invention has been made in view of the above points, and an object thereof is to secure both thermal shock resistance and voltage resistance of an insulator in a spark plug for an internal combustion engine.

[0006]

In order to achieve the above-mentioned object, in the invention described in claim 1, a center electrode (30),
Ceramic insulator (2 with built-in center electrode (30)
0) and the insulator (20) are built in and are arranged between the housing (10) mounted on the cylinder head of the internal combustion engine, the insulator (20) and the housing (10), and the insulator (20) and In a spark plug for an internal combustion engine, which comprises a packing (60) in contact with the housing (10), the insulator (20) has a Vickers hardness of 900 Hv or more, and a surface roughness of a portion of the insulator (20) in contact with the packing (60). Has a ten-point average roughness of 10 μm or less.

According to this, by reducing the surface roughness of the portion of the insulator which is in contact with the packing with the ten-point average roughness of 10 μm or less, the contact area with the packing is increased,
The heat transferred from the combustion gas to the insulator is easily released to the housing via the packing. That is, heat transfer becomes good. For this reason, when heat is applied to the insulator, the temperature rise of the portion of the insulator in contact with the packing is reduced, and the thermal shock resistance of the insulator is improved.

Therefore, even in a spark plug using an insulator having a high hardness formed by high-pressure molding for improving the withstand voltage, both the thermal shock resistance and the withstand voltage property of the insulator should be kept at a practically sufficient level. You can

According to the second aspect of the invention, the porosity of the insulator can be set to 4.5% or less.

According to the invention of claim 3, 50MP
By forming a porcelain by pressurizing after a and forming the porcelain, the Vickers hardness of the porcelain becomes 900 Hv or more, and the structure of the porcelain becomes finer and the withstand voltage is improved.

According to the invention of claim 4, the center electrode (3
0), a ceramic insulator (20) having a center electrode (30) incorporated therein, and a housing (1) incorporating the insulator (20) and attached to a cylinder head of an internal combustion engine.
0) and a packing (60) disposed between the insulator (20) and the housing (10) and in contact with the insulator (20) and the housing (10). An insulator (20) is formed through a molding step of press-molding a ceramic material to form a molded product, a grinding step of grinding the molded product to form a ground product, and a firing step of firing the ground product. Then, the ceramic material is selected so that the Vickers hardness of the insulator (20) after the firing step is 900 Hv or more, the pressurizing pressure is set in the forming step, and the packing (6
The surface roughness of the part in contact with 0) at the completion of the grinding process is
It is characterized in that grinding is performed so that the ten-point average roughness is 15 μm or less.

According to this, the surface roughness of the portion of the ground product contacting the packing at the time of completion of the grinding step is set to 15 μm or less in terms of ten-point average roughness, so that the surface of the portion contacting the packing of the insulator after firing. The roughness can be 10 μm or less in terms of ten-point average roughness. Therefore, the same effect as the invention of claim 1 can be obtained.

According to the fifth aspect of the present invention, by setting the pressure applied in the molding step to 50 MPa or more, the Vickers hardness of the insulator becomes 900 Hv or more, and the structure of the insulator becomes dense and the resistance is increased. The voltage property is improved.

The reference numerals in parentheses of the above means indicate the correspondence with the specific means described in the embodiments described later.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention shown in the drawings will be described.

FIG. 1 is a cross-sectional view of a main part showing a spark plug of this embodiment. This spark plug has a cylindrical housing 10 made of a steel material such as conductive low carbon steel.
The housing 10 is formed with a male screw portion 11.

Then, a male screw portion 11 of the housing 10 is screwed to a female screw portion (not shown) formed in the cylinder head of the internal combustion engine, so that both electrodes 3 to be described later are formed.
The spark plug is fixed to the cylinder head so that 0 and 40 are exposed to the combustion chamber of the internal combustion engine.

Inside the housing 10, an insulator 20 made of ceramic such as alumina (Al ₂ O ₃ ) having a high insulating property is used.
Is fixed, and one end portion 21 of the insulator 20 is provided so as to be exposed from the one end portion 12 of the housing 10. A method of manufacturing the insulator 20 will be described later.

A center electrode 30 is fixed in the shaft hole 22 of the insulator 20, and the center electrode 30 is held in an insulating manner with respect to the housing 10. The center electrode 30 is made of, for example, a metal material having excellent thermal conductivity such as Cu as an inner material and a metal material having excellent heat resistance and corrosion resistance such as a Ni-based alloy, a Fe-based alloy, or a Co-based alloy as an outer material. The columnar body is provided so that one end portion 31 thereof is exposed and extends from one end portion 21 of the insulator 20.

On the other hand, the ground electrode 40 is made of a Ni-based alloy, and is fixed at one end portion 41 to the one end portion 12 of the housing 10 by welding, bent in the middle thereof, and the other end portion 4
The second side faces one end 31 of the center electrode 30 with a discharge gap 50 in between.

A ring-shaped packing 60 made of cold rolled steel plate (SPCD) is arranged between the inner peripheral surface of the housing 10 and the outer peripheral surface of the insulator 20. The outer peripheral surface of the insulator 20 contacts. The portion of the insulator 20 that comes into contact with the packing 60 will be referred to as a jitz portion hereinafter.

Next, a method of manufacturing the insulator 20 will be described.

First, a ceramic material is placed in a rubber mold and pressure-molded to form a molded product (molding step). In this forming step, the pressure applied is set to 50 MPa or higher so that the insulator 20 after firing has a Vickers hardness of 900 Hv or higher. Incidentally, as shown in FIG. 2, when the pressure applied in the molding step is 30 MPa, the porosity is 4.5%,
The porosity decreases with increasing pressurizing pressure.

Next, the outer peripheral surface of the molded product is ground by a grinding wheel to form a ground product (grinding step). In this grinding step, the grinding conditions are set so that the surface roughness of at least the part of the ground product in contact with the packing 60 at the time of completion of the grinding step is 15 μm or less in ten-point average roughness (Rz). There is. Incidentally, the surface roughness is a value measured by a non-contact type three-dimensional measuring device (NH-3 manufactured by Mitaka Koki Co., Ltd.).

Next, the ground product is fired (firing step). Here, at the site where the surface roughness at the time of completion of the grinding step is 10 μm or less in 10-point average roughness, the surface roughness becomes 10 μm or less in 10-point average roughness after firing. Therefore, the insulator 20 of this example
The surface roughness of at least the jitz portion in the insulator 20 after firing is 10 μm or less in ten-point average roughness.

FIG. 3 shows the results of evaluating the withstand voltage of the insulator 20 produced by setting various pressures in the above-mentioned molding process. The insulator 20 is increased as the pressure is increased. Withstand voltage is improved. By the way, by setting the pressure applied in the molding step to 50 MPa or more, a practically sufficient level of withstand voltage can be secured.

Next, the thermal shock resistance of the insulator 20 was evaluated. FIG. 4 shows the evaluation result, and FIG. 5 shows the pattern of the thermal shock resistance evaluation test.

As an evaluation product, the pressurizing pressure in the above molding step was set to 70 MPa, and the surface roughness of the zitz portion after firing was 10 μm in terms of ten-point average roughness, 1 μm, 5 μm, 10 μm, 1
Insulators 20 having a size of 5 μm were manufactured, and spark plugs equipped with these insulators 20 were prepared.

Further, as an evaluation reference product corresponding to the conventional product, the pressure applied in the molding step was set to 30 MPa, and the surface roughness of the jitz portion after firing was set to 15 μm in terms of ten-point average roughness. 20 was manufactured, and a spark plug equipped with the insulator 20 was prepared.

In the evaluation test, the evaluation product and the evaluation reference product were mounted on a 4-cycle single-cylinder engine, and as shown in FIG. 5, first, during a period indicated by a symbol A, the throttle valve was fully opened and the engine speed was 9000 rpm. Then, after the lapse of the period A, the throttle valve is quickly closed.

In the period A, the temperature of the combustion gas rises with the lapse of time, and the preignition reaches 100 from a certain point.
% Will occur. Reference numeral B in FIG. 5 represents an operating time (hereinafter referred to as pre-ignition keep time) in a situation where 100% pre-ignition occurs, and the pre-ignition keep time B is variously changed, and
Insulator 20 after the test is repeated 4 times.
The presence or absence of cracks was evaluated.

In each test, 10 spark plugs of the evaluation product and the evaluation reference product were tested. In FIG. 4, a spark plug in which the insulator 20 did not crack was marked with ○, and a spark plug in which the insulator 20 cracked. Is indicated by x.

As is clear from the results shown in FIG. 4, the spark plugs having a surface roughness of the zitz portion of 10 μm or less among the evaluation products obtained the same or higher results than the spark plug of the evaluation reference product. That is, it was confirmed that even in the high-pressure molded spark plug, the thermal shock resistance of the insulator 20 can be improved by setting the surface roughness of the jitz portion to 10 μm or less.

Incidentally, when the surface roughness of the jitz portion of the insulator 20 is reduced to 10 μm or less in terms of ten-point average roughness,
The contact area between the jitz portion and the packing 60 increases, and the heat transferred from the combustion gas to the insulator 20 easily escapes to the housing 10 via the packing 60. That is, heat transfer becomes good. Therefore, when heat is applied to the insulator 20, the temperature rise of the jitz portion is reduced, and the thermal shock resistance of the insulator 20 is improved.

According to the present embodiment, by setting the pressurizing pressure in the molding process to 50 MPa or more, the withstand voltage is sufficiently high to meet the needs of downsizing of the plug and high output engine. Can be secured. Moreover, the thermal shock resistance of the insulator 20 is improved by reducing the surface roughness of the jitz portion of the insulator 20 to 10 μm or less in terms of ten-point average roughness. Therefore, insulator 2
With respect to the thermal shock resistance and the withstand voltage of 0, it is possible to secure a level that can sufficiently meet the needs such as downsizing of the plug and compatibility with a high-power engine.

(Other Embodiments) The packing 60 of the above embodiment may be made of copper or stainless steel, or may be a disc spring shape in which a disk having a hole in the center is made into a conical shape.

[Brief description of drawings]

FIG. 1 is a sectional view of essential parts showing an embodiment of a spark plug according to the present invention.

FIG. 2 is a diagram showing a relationship between a pressurizing pressure and a porosity in a molding process.

FIG. 3 is a diagram showing a relationship between a pressure applied and a withstand voltage in a molding process.

FIG. 4 is a diagram showing evaluation results of thermal shock resistance of the insulator 20.

FIG. 5 is a diagram showing a pattern of a thermal shock resistance evaluation test.

[Explanation of symbols]

10 ... Housing, 20 ... Insulator, 30 ... Center electrode, 60
... packing.

Claims (5)

[Claims] 1. A center electrode (30) and the center electrode (3)
0) built-in ceramic insulator (20), the insulator (20) built-in, the housing (10) mounted on the cylinder head of the internal combustion engine, the insulator (20) and the housing ( 10) is placed between
A spark plug for an internal combustion engine comprising the insulator (20) and a packing (60) in contact with the housing (10), wherein the insulator (20) has a Vickers hardness of 900 Hv or more, and the insulator (20) includes: The surface roughness of the part in contact with the packing (60) is 10-point average roughness.
A spark plug for an internal combustion engine, which has a thickness of less than μm. 2. The spark plug for an internal combustion engine according to claim 1, wherein the insulator (20) has a porosity of 4.5% or less. 3. The spark plug for an internal combustion engine according to claim 1, wherein the insulator (20) is pressure-molded at 50 MPa or more and then fired. 4. A center electrode (30) and the center electrode (3)
0) built-in ceramic insulator (20), the insulator (20) built-in, the housing (10) mounted on the cylinder head of the internal combustion engine, the insulator (20) and the housing ( 10) is placed between
A method of manufacturing a spark plug for an internal combustion engine, comprising: the insulator (20); and a packing (60) in contact with the housing (10), comprising: a molding step of pressure-molding a ceramic material to form a molded article; The insulator (20) is formed through a grinding process of grinding the molded product to form a ground product and a firing process of firing the grinding product, and the Vickers hardness of the insulator (20) after the firing process is formed. The ceramic material is selected so that the pressure is 900 Hv or more, and the pressurizing pressure is set in the forming step. Further, at the time when the grinding step is completed at a portion of the ground product in contact with the packing (60). The method for producing a spark plug for an internal combustion engine, wherein the grinding process is performed so that the surface roughness of 10-point average roughness is 15 μm or less. 5. The pressure applied in the molding step is 50 MPa.
The method for manufacturing a spark plug for an internal combustion engine according to claim 4, wherein the setting is made as described above.