JPH04149985A - Ignition plug - Google Patents

Abstract

PURPOSE:To achieve a high discharge characteristic, and make a dielectric breakdown difficult to occur by forming a cylindrical member for insulating both of a center electrode of an ignition plug and a circular electrode having a gap of ceramic consisting of a main component of alumina. CONSTITUTION:For a discharge gap 5 between an end part 4a of a center electrode 4 and a gap forming part 6a of a circular electrode 6, a thermal impact by quick heating/quick cooling is applied periodically to a leg part 1a in a combustion chamber. By covering the leg part 1a of a coating layer 20 of a higher heat conductivity than that of alumina, heat applied to it is transmitted through the layer 20 and an O-ring 12 to the electrode 6 to be emitted to the external. A thermal load applied to the part 1a is thus restricted, and the thermal resistance characteristic is improved, thereby the discharge characteristic is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a spark plug used for igniting an air-fuel mixture supplied to a combustion chamber of an internal combustion engine.

(Prior Art) In an internal combustion engine, an air-fuel mixture supplied to a combustion chamber is ignited by a spark plug and combusted. A spark plug has a center electrode, which is usually shaped like a rod, and
A gap forming part is provided to form a discharge gap between a cylindrical insulating member surrounding the center electrode and one end of the center electrode that is fitted into the cylindrical insulating member and projects outward from the cylindrical insulating member. The annular electrode is grounded and a pulse voltage is applied to the center electrode, so that spark discharge occurs in the discharge gap between the center electrode and the gap forming part of the annular electrode. It is said that

For example, the annular electrode has a groove formed on its outer peripheral surface, and is fixed by being screwed into the combustion chamber forming part of the internal combustion engine, and one end of the center electrode forms the discharge gap. and a gear knob forming portion provided on the annular electrode face into the combustion chamber.

In such a spark plug, the portion surrounding one end of the center electrode protruding from the cylindrical insulating member is particularly referred to as a long leg portion, and the long leg portion has excellent dielectric strength characteristics. At the same time, it is required to have excellent thermal shock resistance. Therefore, generally, the entire cylindrical insulating member including the long leg portion is formed of ceramics made of an insulating material containing alumina as a main component. There is also a spark plug whose cylindrical insulating member including the part is made of ceramics made of an insulating material mainly composed of silicon nitride, which has better thermal shock resistance than ceramics made of an insulating material mainly composed of alumina. Are known.

(Problem to be solved by the invention) Ceramics made of an insulating material mainly composed of alumina have inferior thermal shock resistance compared to ceramics made of an insulating material mainly composed of silicon nitride, but on the other hand, they have a high specific resistance. It also has excellent dielectric strength characteristics.

Therefore, in the case of a spark plug in which the cylindrical insulating member including the long leg portion is formed of ceramics made of an insulating material containing alumina as a main component, the thermal shock resistance of the cylindrical insulating member includes the cylindrical insulation member including the long leg portion. Although this is not as good as a spark plug whose members are made of ceramics made of an insulating material mainly composed of silicon nitride, the cylindrical insulating member including the long leg part is made of ceramics made of an insulating material mainly composed of silicon nitride. In addition to providing superior discharge characteristics compared to spark plugs that are said to have
Even if the pulse voltage applied between the annular electrode and the center electrode is relatively high, there is an advantage that the cylindrical insulating member is unlikely to cause dielectric breakdown.

However, with the increase in output and rotation speed in internal combustion engines, the thermal load imposed on the cylindrical insulating member of the spark plug, especially the long leg portion, tends to increase more and more. In spark plugs in which the cylindrical insulating member including the long leg portion is formed of ceramics using an insulating material containing alumina as a main component, the thermal shock resistance of the long leg portion is considered to be unstable.

In view of this, the present invention provides a structure in which the cylindrical insulating member including the long leg portions is formed of ceramics made of an insulating material containing alumina as a main component, so that excellent discharge characteristics can be obtained and high pulse voltage can be applied. In addition, the long legs of the cylindrical insulating member have improved thermal shock resistance and are unlikely to be damaged by the imposed thermal load. The purpose is to provide spark plugs that are

(Means for Solving the Problem) In order to achieve the above-mentioned object, a spark plug according to the present invention has the following features:
A center electrode formed to form a rod-shaped body, a cylindrical insulating member made of ceramic made of an insulating material containing alumina as a main component and surrounding the center electrode with at least one end protruding outside, and a cylindrical insulating member. an annular electrode that is fitted into the member and is provided with a gap forming part that forms a discharge gap between one end of the center electrode that protrudes from the cylindrical insulating member to the outside; The peripheral portion of one end of the center electrode, that is, the long leg portion is covered with a coating layer made of an insulating material having higher thermal conductivity than alumina.

(Function) In the spark plug according to the present invention configured as described above, since the cylindrical insulating member is formed of ceramics made of an insulating material containing alumina as a main component, excellent discharge characteristics can be obtained and , it is said that dielectric breakdown is unlikely to occur even when a high voltage pulse voltage is applied between the annular electrode and the center electrode, and furthermore, the long legs of the cylindrical insulating member are made of ceramic that forms the cylindrical insulating member. By being covered with a coating layer made of an insulating material having higher thermal conductivity than alumina, which is the main component, for example, aluminum nitride, boron nitride, or beryllium oxide, the cylindrical insulating member is The heat applied to the long leg portions is efficiently dissipated to the outside through the coating layer and further through the annular electrode, thereby avoiding damage to the long leg portions of the cylindrical insulating member due to the heat load imposed thereon. Therefore, in the spark plug according to the present invention, the cylindrical insulating member including the long leg portion is formed of ceramics made of an insulating material containing alumina as a main component, so that good discharge characteristics and dielectric strength characteristics are obtained, and thermal shock resistance is achieved. It is believed that the characteristics can be improved.

(Example) FIG. 2 shows an example of a spark plug according to the present invention.

The spark plug shown in Figure 2 is made of alumina (Aj220
A cylindrical insulating member 1 is formed into a cylindrical shape by ceramics made of an insulating material mainly composed of 3), and has a long leg portion 1a on one end side, and a cylindrical insulating member 1 is fitted to the other end of the cylindrical insulating member 1. The electrode terminal part 3 is connected to the pulse voltage generating part, and the cylindrical insulating member 1 is formed to form a rod-shaped body, with one end 4a of the rod-shaped body protruding outward from the long leg part 1a of the cylindrical insulating member 1. The center electrode 4 is surrounded by the member 1 and has its other end 4b engaged with the electrode terminal portion 3, and the center electrode 4 is fitted into the cylindrical insulating member 1 and protrudes outward from the long leg portion 1a of the cylindrical insulating member 1. A gap forming portion 6a forming a discharge gear pump 5 between one end portion 4a of the center electrode 4 and one end portion 4a of the center electrode 4
and a ring-shaped electrode 6 provided at one end. The center electrode 4 is made of a prepared alloy, and has one end 4a projecting outward from the long leg portion 1a of the cylindrical insulating member 1.
The surface of the portion including the is covered with a layer 4C of Inconel, which is a heat-resistant iron/nickel/chromium alloy.

A sealing member 9 is disposed between the inner surface of the portion of the cylindrical insulating member 1 closer to the center than the long leg portion 1a and the outer surface of the center electrode 4 surrounded by the inner surface of the portion of the cylindrical insulating member 1 that is closer to the center than the long leg portion 1a. A sealing member 10 is disposed between the outer surface of the more central portion and the inner surface of the other end of the annular electrode 6 that fits therein.

Further, an O-ring 12 is disposed between the inclined surface portion 1b forming the base of the long leg portion 1a of the cylindrical insulating member I and the inclined inner surface portion 6b of the annular electrode 6 surrounding it. Furthermore, the long leg portion 1a of the cylindrical insulating member 1 in the annular electrode 6
A threaded portion 6c is provided on the outer periphery of the portion surrounding the gasket 14, and a gasket 14 is fitted into the threaded portion 6c.

As shown in an enlarged view in FIG. 1, the leg portions 1a of the cylindrical insulating member 1 have a resistivity comparable to that of alumina, which is the main component of the ceramic forming the cylindrical insulating member 1, and higher than alumina. It is covered with a thin coating layer 20 having a thickness of, for example, 20 μm, which is formed by plasma deposition of an insulating material having thermal conductivity. The insulating material forming the coating layer 20 is, for example, aluminum nitride (A/!N).
.

Either boron nitride (BN) or beryllium oxide (BeO) is selected. Aluminum nitride.

The thermal conductivity and coefficient of thermal expansion for boron nitride and helium oxide are shown in Table A below, along with the thermal conductivity and coefficient of thermal expansion for alumina.

Table-A In addition, alumina, aluminum nitride, boron nitride, and
The specific resistance of beryllium oxide is 10I4
It is Ω・1 or more.

The covering layer 20 provided in this manner has one end formed on the inclined surface portion l forming the base of the long leg portion 1a of the cylindrical insulating member 1.
b is brought into contact with the O-ring 12.

The spark plug configured as described above has its annular electrode 6
A threaded portion 6C provided on the outer periphery of a portion surrounding the long leg portion 1a of the cylindrical insulating member 1 is screwed into and fixed to the combustion chamber forming portion of the internal combustion engine, and is connected to one end portion 4a of the center electrode 4 in an annular shape. Gap forming part 6a provided in electrode 6
The discharge gap 5 formed between the two faces into the combustion chamber. Then, while the annular electrode 6 is electrically grounded through the combustion chamber forming part of the internal combustion engine, high voltage from the pulse voltage generator is applied through the electrode terminal part 3 whose other end 4b engages with the center electrode 4. A pulse voltage is applied, thereby creating a state in which a spark discharge occurs in the discharge gap 5.

In this way, the discharge gap 5 formed between the one end 4a of the center electrode 4 and the gap forming part 6a provided on the annular electrode 6 faces into the combustion chamber, so that the cylindrical insulating member The long leg portion 1a of the l is assumed to be the part to which thermal shock is applied due to rapid heating/quick cooling during the periodic operation stroke of an internal combustion engine, but the long leg portion 1a is made of an insulating material having higher thermal conductivity than alumina. By being covered with the covering layer 20 made of material, the heat applied thereto is efficiently transmitted from the covering layer 20 to the annular electrode 6 through the O-ring 12 to which one end is connected, and the annular electrode 6 is emitted to the outside. Thereby,
The long leg portions 1a of the cylindrical insulating member 1 are designed to suppress the thermal load imposed on them, thereby improving thermal shock resistance.

Such an improvement in the thermal shock resistance of the spark plug according to the present invention has been confirmed through experiments conducted by the present applicant, and the experiments conducted by the present applicant and their results will be described below.

In such an experiment, as the spark plug according to the present invention, the long leg portion of the cylindrical insulating member made of ceramics made of an insulating material containing alumina as a main component was
Three types of spark plugs PAI, PA2, and PA3 were prepared with lengths of 3 mm, 5 mm, and 9 aon, and were covered with an aluminum nitride coating layer with a thickness of 20 μm. The lengths of the long legs of the cylindrical insulating members made of ceramics made of insulating material mainly composed of alumina are 3mm, 5ma+, and 9IIIfi, and the coating layer is not provided on the surface. Spark plug Ps
i.

PS2 and PS3 were prepared. And spark plug P
A thermal shock test was conducted on the long leg portion of the cylindrical insulating member of each of AL-PA3 and Psi-PS3 by repeating 5 consecutive cycles of heating with a propane gas burner for 30 seconds followed by cooling with air for <60 seconds. The testing was continued until a crack appeared in the long leg portion of the cylindrical insulating member of each spark plug. And the amount of heating in the 20 thermal shock test (
The value calculated by converting the amount of propane gas supplied into heat value is 3°5K in the first cycle for 5 consecutive cycles.
cal/egin, and was to be increased by 0°5 Kcal/sin in the subsequent four cycles.

As a result, the results shown in Figure 3 (vertical axis: amount of heat CA applied until cracks appeared in the long leg portion of the cylindrical insulating member, horizontal axis: length of the long leg portion of the horizontal axis of the cylindrical insulating member) The experimental results were obtained as shown below.

In FIG. 3, points a, b and C show the results for spark plugs PAL, PA2 and PA3, respectively;
In addition, points a', b'' and C'' are respectively the spark plug Ps
i, PS2 and PS3. From such FIG. 3, spark plugs PAI, PA2 and P
The amount of heat applied to A3 until a crack occurs in the long leg portion of the cylindrical insulating member is the amount of heat applied until the long leg portion of the cylindrical insulating member cracks for spark plugs Psi, PS2, and PS3. Therefore, the spark plugs PAL, FA2, and PA3 prepared as spark plugs according to the present invention are the spark plugs prepared as comparison samples. It can be seen that the thermal shock resistance was significantly improved compared to Psi, PS2 and SA3.

In addition, in the case of each of the spark plugs PAI, PA2, and PA3, a coating layer of aluminum nitride, which has extremely high thermal conductivity compared to alumina, covers the long leg portion of the cylindrical insulating member, as shown in Table A. In the above-mentioned thermal shock test, the heat applied to the long leg portion of the cylindrical insulating member was effectively transmitted to the annular electrode side to suppress the temperature rise of the long leg portion of the cylindrical insulating member. Spark plug PAL, PA2. PA3. Psl, PS2 and PS
This was confirmed by measuring the temperature of the surface portion of the long leg portion of each of the cylindrical insulating members of No. 3.

Furthermore, through the thermal shock test described above, the spark plug PA
For each of I, PA2, and PA3, no peeling of the aluminum nitride coating layer covering the long leg portion of the cylindrical insulating member was observed. This is because, as shown in Table A above, there is not much difference between the coefficient of thermal expansion of aluminum nitride and the coefficient of thermal expansion of alumina, and the coating layer of aluminum nitride has high thermal conductivity. This is because it is difficult for this to occur.

FIG. 4 shows parts of another example of the spark plug according to the present invention. The example shown in FIG. 4 has a three-layered covering layer 20 instead of the covering layer 20 in the example shown in FIGS. 1 and 2.
5 is provided.

The three-layer coating layer 2 provided on the long leg portion 1a of the cylindrical insulating member l of the example of the spark plug according to the present invention shown in FIG.
5 consists of, for example, an aluminum nitride layer 25A provided on the surface of the long leg portion 1a, an alumina layer 25B laminated on the aluminum nitride layer 25A, and an aluminum nitride layer 25C laminated on the alumina layer 25B. There is. The aluminum nitride layer 25A, the alumina layer 25B, and the aluminum nitride layer 25C as a whole have higher thermal conductivity than alumina, which is the main component of the ceramic forming the cylindrical insulating member 1. be taken as a thing.

In such a three-layer coating layer 25, the cylindrical insulating member 1
The heat applied from the outside to the long leg portion 1a is reduced due to the heat transfer effect of the aluminum nitride layer 25C which takes the outermost position 1, and the alumina layer 2 which takes the middle position
5B, and the heat of the alumina layer 25B, whose temperature has increased due to such insulation, is reduced by the heat transfer effect of the aluminum nitride layer 25A, which takes the innermost position, and the long leg portion 1
This means that almost no temperature rise occurs at point a. Therefore, the thermal shock resistance of the spark plug in such a case is said to be further improved.

In the example shown in FIG. 4, the three-layer coating layer 25 includes aluminum nitride layers 2 at the outermost and innermost positions, respectively.
5C and 25A, but instead of the aluminum nitride layer 25C at the outermost position and the aluminum nitride layer 25A at the innermost position, a layer made of boron nitride or a layer made of beryllium oxide is used. may be arranged.

(Effects of the Invention) As is clear from the above description, the spark plug according to the present invention includes a long leg portion surrounding the center electrode to insulate the center electrode from the annular electrode provided with the gap forming portion. Since the cylindrical insulating member is made of ceramic made of an insulating material containing alumina as its main component, excellent discharge characteristics can be obtained, and a high voltage pulse voltage can be applied between the annular electrode and the center electrode. The long legs of the cylindrical insulating member are made of an insulating material that is said to be less likely to cause dielectric breakdown, and has a higher thermal conductivity than alumina, which is the main component of the ceramic that forms the cylindrical insulating member, for example, Aluminum nitride.

By being covered with a coating layer made of either boron nitride or beryllium oxide, the heat applied to the long leg portion of the cylindrical insulating member is efficiently transferred to the outside through the coating layer and further through the annular electrode. This prevents the long legs of the cylindrical insulating member from being damaged due to the thermal load imposed thereon. Therefore, in the spark plug according to the present invention, the cylindrical insulating member including the long leg portion is formed of ceramics made of an insulating material containing alumina as a main component, so that in addition to obtaining good discharge characteristics and dielectric strength characteristics, , the thermal shock resistance is expected to be improved.

[Brief explanation of the drawing]

FIG. 1 is a partial cross-sectional view showing the main parts of an example of the spark plug according to the present invention, FIG. 2 is a cross-sectional view showing an example of the spark plug according to the present invention, and FIG. 3 is an example of the spark plug according to the present invention. FIG. 4 is a sectional view showing the main parts of another example of the spark plug according to the present invention. In the figure, 1 is a cylindrical insulating member, 1a is a long leg part, 4 is a center electrode, 5 is a discharge gap, 6 is an annular electrode, 6a is a gap forming part, 12 is an 0 ring, 20 is a covering layer, and 25 is a 3-layer It is a covering layer. Figure 1 Figure 4 (rryn)

Claims (1)

[Scope of Claims] 1. A center electrode formed to form a rod-shaped body, and a cylinder made of ceramic made of an insulating material containing alumina as a main component, surrounding the center electrode with at least one end thereof protruding outside. an annular electrode provided with a gap forming part that forms a discharge gap between a cylindrical insulating member and one end of the center electrode that fits into the cylindrical insulating member and projects outside from the cylindrical insulating member; A surrounding portion of one end of the center electrode protruding from the cylindrical insulating member in the cylindrical insulating member is covered with a coating layer made of an insulating material having higher thermal conductivity than alumina. A spark plug characterized by: 2. The cylindrical insulating member is characterized in that the coating layer covering the peripheral portion of one end of the center electrode protruding from the cylindrical insulating member is formed of any one of aluminum nitride, boron nitride, and beryllium oxide. The spark plug according to claim 1.