JP6248601B2 - Spark plug for internal combustion engine - Google Patents

Description

  The present invention relates to a spark plug for an internal combustion engine.

In an internal combustion engine such as an automobile, a spark plug is used as an ignition means. The spark plug includes an insulator that inserts and holds the center electrode inward, a housing that inserts and holds the tip side of the insulator inward, and a ground electrode that is joined to the housing and forms a spark discharge gap. have. In the spark plug, by applying a high voltage from the ignition coil, the spark discharge gap is broken between the center electrode and the ground electrode to generate a discharge spark.
In recent years, due to the downsizing of internal combustion engines and the like, there has been a demand for miniaturization of spark plugs, which requires a reduction in the diameter of the insulator, and an improvement in withstand voltage between the center electrode and the housing is desired.

  An example of such a spark plug is disclosed in Patent Document 1. The spark plug of Patent Document 1 attempts to improve the withstand voltage of the insulator by atomizing alumina forming the insulator.

JP 2010-208901 A

However, the spark plug disclosed in Patent Document 1 has the following problems.
In the spark plug of Patent Document 1, the smaller the alumina particle size, the better the voltage resistance of the insulator, but there is a limit to atomization of alumina. For this reason, there is a limit to the effect of improving the voltage resistance of the insulator obtained by atomizing alumina, and a structure that can further improve the voltage resistance is desired.

  The present invention has been made in view of such a background, and an object of the present invention is to provide a spark plug for an internal combustion engine that can prevent a decrease in voltage resistance of an insulator and obtain excellent insulation.

One embodiment of the present invention includes a center electrode;
An insulator for inserting and holding the center electrode inside;
A housing that inserts and holds the insulator inside while exposing a part of the base end side of the insulator;
A grounding electrode joined to the housing and forming a spark discharge gap with the central electrode;
At least on the surface of the insertion part disposed inside the insulator in the central electrode, a coating layer made of a material having a work function larger than the work function in the surface of the insertion part is formed ,
In the spark plug for an internal combustion engine , the surface roughness of the coating layer is smaller than the surface roughness of the insertion portion .

  The spark plug for an internal combustion engine (hereinafter referred to as a spark plug as appropriate) has a high temperature in the vicinity of its tip due to the combustion of fuel and the flow of discharge current. At this time, in the center electrode heated to a high temperature, thermal electron emission is generated in which electrons excited by heat are emitted from the surface of the center electrode. It is considered that electrons emitted from the center electrode enter the inside of the insulator, thereby promoting dielectric breakdown in the insulator.

  In the spark plug, the coating layer made of a material having a work function larger than the work function of the surface of the center electrode is formed on the surface of the center electrode. Since thermionic emission occurs on the surface of the member, thermionic emission can be prevented by forming the coating layer on the surface of the central electrode. Thereby, it is possible to prevent electrons from entering the insulator and to prevent a decrease in the voltage resistance of the insulator. Therefore, the occurrence of dielectric breakdown in the spark plug can be suppressed.

  As described above, according to the spark plug for an internal combustion engine, it is possible to prevent a decrease in the voltage resistance of the insulator and to obtain an excellent insulating property.

1 is a cross-sectional view showing a spark plug for an internal combustion engine in Embodiment 1. FIG. 1 is a partially enlarged cross-sectional view showing a spark plug for an internal combustion engine in Embodiment 1. FIG. 2 is a cross-sectional view of a center electrode in Example 1. FIG. 6 is a partial enlarged cross-sectional view showing a spark plug for an internal combustion engine in the second embodiment. Sectional drawing of the center electrode in Example 2. FIG. FIG. 7 is a partial enlarged cross-sectional view showing a spark plug for an internal combustion engine in the third embodiment. FIG. 10 is a partial enlarged cross-sectional view showing a spark plug for an internal combustion engine in the fourth embodiment.

  In the spark plug for the internal combustion engine, an engaging portion that is engaged with an engaged portion that protrudes outward from the outer peripheral surface of the insulator protrudes inwardly on the inner peripheral surface of the housing. It is preferable that the coating layer is formed at least on a portion of the center electrode disposed inside the engaging portion. Since the distance between the engagement portion and the center electrode is small, the withstand voltage is likely to be lowered at this portion. Therefore, by forming the covering layer inside the engaging portion and preventing thermionic emission from the center electrode, it is possible to suppress a decrease in voltage resistance of the insulator.

  Further, in the insulator, an insulator side coating layer made of a material having a work function larger than a work function on the surface of the insertion portion is formed on at least a part of the inner peripheral surface facing the insertion portion. It is preferable. In this case, the effect of preventing thermionic emission from the center electrode can be obtained by the insulator-side coating layer in addition to the coating layer. Thereby, the fall of the withstand voltage property in the said insulator can be suppressed more effectively.

Example 1
Examples of the spark plug for the internal combustion engine will be described with reference to FIGS.
As shown in FIG. 1, the spark plug 1 for an internal combustion engine includes a center electrode 2, an insulator 6 inserted and held inside the center electrode 2, and a part of the base end side of the insulator 6 while exposing the insulator. And a ground electrode 71 which is joined to the housing 7 and forms a spark discharge gap G between the housing 7 and the center electrode 2.
A coating layer 5 made of a material having a work function larger than the work function of the surface of the insertion part 21 is formed on the surface of the insertion part 21 disposed inside the insulator 6 in the center electrode 2.

This will be described in more detail below.
As shown in FIG. 1, the spark plug 1 of this example is for igniting an air-fuel mixture in an internal combustion engine. In the axial direction, one end of the spark plug 1 is connected to an ignition coil (not shown), and the other end on the opposite side is arranged in a combustion chamber (not shown) of the internal combustion engine. In this example, the side connected to the ignition coil in the axial direction is referred to as the base end side, and the side disposed in the combustion chamber is referred to as the front end side.

  As shown in FIGS. 1 and 2, the spark plug 1 includes a cylindrical insulator 6, a cylindrical housing 7 that holds the insulator 6 on the inside, and an insulator 6 inside the insulator 6 so that the tip portion protrudes. The center electrode 2 held, the stem 8 disposed on the base end side of the center electrode 2, and the ground electrode 71 connected to the housing 7 are provided.

The housing 7 is formed in a substantially cylindrical shape, and the insulator 6 is inserted and held therein. The housing 7 exposes the proximal end side of the insulator 6 and covers the distal end side of the insulator 6. On the inner peripheral surface of the housing 7, an engaging portion 72 that protrudes in an annular shape toward the inside on the same circumference is formed, and is configured to be engageable with the engaged portion 65 of the insulator 6. . A screw thread for screwing with a cylinder head (not shown) of the internal combustion engine is formed on the outer peripheral side surface of the housing 7, and a ground electrode 71 is disposed on the front end surface of the housing 7.
The ground electrode 71 includes an extending portion 711 extending from the end surface of the housing 7 toward the distal end side, and an opposing portion 712 bent at a right angle to the axial direction from the distal end of the extending portion 711 toward the inside in the radial direction. have. A spark discharge gap G is formed between the facing portion 712 and the center electrode 2.

  As shown in FIGS. 1 and 2, the insulator 6 is formed of alumina in a substantially cylindrical shape, and is formed so as to hold the center electrode 2 and the stem 8 inside thereof. The insulator 6 has a proximal end cylindrical portion 61 formed in a cylindrical shape on the proximal end side, an insulator large diameter portion 62 formed on the distal end side of the proximal end cylindrical portion 61, and a distal end side of the insulator large diameter portion 62. The insulator has a small-diameter portion 63 formed and a tapered portion 64 formed on the distal end side of the small-diameter portion 63 of the insulator. A step is formed between the small-diameter portion 63 and the tapered portion 64 so as to connect the two, and this step forms an engaged portion 65 that engages with the engaging portion 72 of the housing 7. ing.

Inside the insulator 6, a stem 8 connected to the ignition coil and a center electrode 2 connected to the distal end side of the stem 8 are inserted and arranged.
The stem 8 has a stem body 81 inserted and held inside the insulator 6, and a terminal 82 exposed from the insulator 6 on the proximal end side of the stem body 81 and connected to the ignition coil.
A resistor 9 is disposed between the stem 8 and the center electrode 2. This register 9 is for suppressing ignition noise in the spark plug 1.

As shown in FIGS. 2 and 3, the center electrode 2 connected to the stem 8 through the resistor 9 has an electrode base material 3 and a discharge chip 4 joined to the tip of the electrode base material 3. . The electrode base material 3 has a core material 31 made of a copper alloy and a surface layer 32 made of a nickel alloy and covering the core material 31.
The core material 31 includes a cylindrical core material large-diameter portion 311 formed on the base end side, and a cylindrical core material small-diameter portion 312 having a diameter smaller than the core material large-diameter portion 311 when viewed from the axial direction. have. A core material reduced-diameter portion 313 is formed at the tip of the core material small-diameter portion 312 so as to reduce the diameter in an arc shape toward the tip when viewed from a direction orthogonal to the axial direction.

  As shown in FIGS. 2 and 3, the surface layer 32 is formed so as to cover the surface of the core material 31. The surface layer 32 is formed with a certain thickness on the surfaces of the core material small diameter portion 312 and the core material large diameter portion 311. Further, the core layer reduced diameter portion 313 is formed with a surface layer 32 so as to have a cylindrical shape having the same outer diameter as the outer diameter of the surface layer formed on the outer peripheral surface of the core material small diameter portion 312. . That is, the center electrode 2 is formed by the core small diameter portion 312, the core material small diameter portion 313 and the electrode small diameter portion 22 formed by the surface layer 32, the core material large diameter portion 311 and the surface layer 32, and is formed from the axial direction. It has an electrode large diameter portion 23 whose outer diameter is larger than the electrode small diameter portion 22. Note that the distal end shape of the core material small diameter portion 313 has a tapered shape that decreases in diameter toward the distal end side.

  The discharge chip 4 is joined to the tip of the core material small diameter portion 313 so as to be exposed from the tip of the insulator 6. As the material of the discharge chip 4, a noble metal such as iridium, platinum, rhodium or an alloy thereof is used. The discharge chip 4 does not necessarily need to be a noble metal, and may be a high melting point material made of, for example, tungsten, rhenium, tantalum, niobium, or an alloy thereof.

  A coating layer 5 made of a material having a work function larger than that of the material of the surface layer 32 is formed on the surface of the center electrode 2. In this example, the coating layer 5 is made of a platinum plating film and is formed on the entire surface of the center electrode 2. Further, the surface roughness of the coating layer 5 is set to be smaller than the surface roughness of the surface layer 32. The surface layer 32 can be formed by, for example, thermal spraying, vapor deposition, sputtering, etc. in addition to plating.

Next, the function and effect of this example will be described.
In the spark plug 1, a coating layer 5 made of a material having a work function larger than that of the surface of the center electrode 2 is formed on the surface of the center electrode 2. Since thermionic emission occurs on the surface of the member, the formation of the coating layer 5 on the surface of the central electrode 2 can prevent thermionic emission. Thereby, it is possible to prevent electrons from entering the insulator 6 and to prevent a decrease in the voltage resistance of the insulator 6. Therefore, the occurrence of dielectric breakdown in the spark plug 1 can be suppressed.

  In the spark plug 1 for an internal combustion engine, an engaging portion 72 that engages with an engaged portion 65 that protrudes outward from the outer peripheral surface of the insulator 6 protrudes inward on the inner peripheral surface of the housing 7. The coating layer 5 is formed at least at a portion of the center electrode 2 disposed inside the engaging portion 72. Since the distance between the engaging portion 72 and the center electrode 2 is small, the withstand voltage is likely to be lowered at this portion. Therefore, by forming the coating layer 5 inside the engaging portion 72 and preventing thermionic emission from the center electrode 2, it is possible to suppress a decrease in withstand voltage in the insulator 6.

  Further, an electrode large diameter portion 23 whose outer shape viewed from the axial direction of the center electrode 2 is larger than the distal end side is formed at the base end portion of the center electrode 2, and at least the surface of the electrode large diameter portion 23 has a coating layer 5 is formed. Between the electrode large-diameter portion 23 and the housing 7, the distance between the two becomes small, so that the withstand voltage property tends to decrease at this portion. Therefore, by forming the coating layer 5 on the electrode large-diameter portion 23 and improving the voltage resistance, the occurrence of dielectric breakdown in the insulator 6 can be suppressed.

  Further, the surface roughness of the coating layer 5 is smaller than the surface roughness of the insertion site 21. Therefore, the work function in the coating layer 5 can be increased by reducing the surface roughness. Thereby, generation | occurrence | production of the dielectric breakdown in the insulator 6 can be suppressed.

  As described above, according to the spark plug 1 for an internal combustion engine of the present example, it is possible to prevent a decrease in the voltage resistance of the insulator 6 and obtain an excellent insulating property.

  The material of the coating layer is not limited to platinum, and may be any material having a work function larger than the work function of the surface at the insertion portion of the center electrode, such as Ir, Au, and Rh. Moreover, when the center electrode is formed of a plurality of materials, it is preferable that the coating layer is formed of a material having a work function larger than that of the material disposed on the surface side on which the coating layer is formed.

(Example 2)
In this example, as shown in FIGS. 4 and 5, the configuration of the spark plug 1 for the internal combustion engine of the first embodiment is partially changed.
In the spark plug 1 of this example, the covering layer 5 is formed on the outer peripheral side surface of the center electrode 2 facing the inner side of the engaging portion 72 of the housing 7 and the outer peripheral side surface of the core member large diameter portion 311.

Of the reference numerals used in this example or the drawings relating to this example, the same reference numerals as those used in the first embodiment represent the same components as in the first embodiment unless otherwise specified.
Also in this example, the same effects as those of the first embodiment can be obtained.

(Example 3)
In this example, as shown in FIG. 6, the configuration of the spark plug 1 for the internal combustion engine of the first embodiment is partially changed.
In the spark plug 1 of this example, an insulator-side coating layer 67 made of a material having a work function larger than the work function on the surface of the insertion part 21 is formed on the inner peripheral surface facing the insertion part 21 of the insulator 6. ing. In this example, the insulator-side coating layer 67 made of platinum was formed on the entire surface of the inner peripheral surface of the insulator 6 in the range facing the insertion portion 21 of the center electrode 2. In addition, the material of the insulator side coating layer 67 should just have a work function larger than the work function of the surface in the insertion part of a center electrode other than platinum. As a material of the insulator side coating layer 67, for example, Ir, Au, Rh, or the like can be used.
Of the reference numerals used in this example or the drawings relating to this example, the same reference numerals as those used in the first embodiment represent the same components as in the first embodiment unless otherwise specified.

In the spark plug 1 of this example, the effect of preventing thermionic emission from the center electrode 2 can be obtained by the insulator side coating layer 67 in addition to the coating layer 5. Thereby, the fall of the withstand voltage property in the insulator 6 can be suppressed more effectively.
Also in this example, the same effects as those of the first embodiment can be obtained.

Example 4
As shown in FIG. 7, this example is an example in which the configuration of the spark plug 1 for the internal combustion engine of the first embodiment is partially changed.
In the spark plug 1 of this example, the insulator-side coating layer 67 formed on the inner peripheral surface of the insulator 6 has an inner peripheral surface located inside the engaging portion 72 included in the housing 77 and a core included in the center electrode 2. The large diameter portion 311 and the inner peripheral surface facing each other are formed. Each insulator-side coating layer 67 has an entire length of the engaging portion 72 and the core material large-diameter portion 311 in the axial direction of the center electrode 2 so as to include portions facing the engaging portion 72 and the core material large-diameter portion 311, respectively. Each is formed in a wider range.
Of the reference numerals used in this example or the drawings relating to this example, the same reference numerals as those used in the first embodiment represent the same components as in the first embodiment unless otherwise specified.

In the spark plug 1 for the internal combustion engine of the present example, the amount of material used for the insulator side covering layer 67 is reduced by forming the insulator side covering layer 67 at a portion where the distance between the center electrode 2 and the housing 77 is small. However, the effect of preventing thermionic emission from the center electrode 2 can be obtained.
Also in this example, the same effects as those of the first embodiment can be obtained.

DESCRIPTION OF SYMBOLS 1 Spark plug for internal combustion engines 2 Center electrode 21 Insertion site 23 Large diameter part 5 Covering layer 6 Insulator 65 Engagement part 7 Housing 71 Ground electrode 72 Engagement part

Claims (6)

A center electrode (2);
An insulator (6) for inserting and holding the center electrode (2) inside;
A housing (7) for inserting and holding the insulator (6) inward while exposing a part of the base end side of the insulator (6);
A ground electrode (71) joined to the housing (7) to form a spark discharge gap with the center electrode (2);
At least the surface of the insertion part (21 ) disposed inside the insulator (6) in the central electrode (2) is made of a material having a work function larger than the work function of the surface of the insertion part (21 ). A coating layer (5) is formed ,
The surface roughness of the coating layer (5) is a spark plug for an internal combustion engine, characterized in that less than the surface roughness of the insertion site (21) (1).   On the inner peripheral surface of the housing (7), an engaging portion (72) engaged with an engaged portion (65) protruding outward from the outer peripheral surface of the insulator (6) faces inward. The covering layer (5) is formed in at least a portion of the center electrode (2) disposed inside the engaging portion (72). The spark plug (1) for internal combustion engines as described.   A large-diameter portion (23) whose outer shape viewed from the axial direction of the central electrode (2) is larger than the distal end side is formed at the base end portion of the central electrode (2), and at least the large-diameter portion ( The spark plug (1) for an internal combustion engine according to claim 1 or 2, characterized in that the coating layer (5) is formed on the surface of 23). In the insulator (6), the insulator side made of a material having a work function larger than the work function of the surface of the insertion part (21) is provided on at least a part of the inner peripheral surface facing the insertion part (21). The spark plug (1) for an internal combustion engine according to any one of claims 1 to 3, wherein a covering layer (67) is formed. On the inner peripheral surface of the housing (7), an engaging portion (72) engaged with an engaged portion (65) protruding outward from the outer peripheral surface of the insulator (6) faces inward. protrudes Te, on the inner peripheral surface of the insulator (6) on the inside of at least the engaging section (72), according to claim 4, characterized in that the insulator-side covering layer (67) is formed A spark plug (1) for an internal combustion engine according to claim 1. A large-diameter portion (23) whose outer shape viewed from the axial direction of the center electrode (2) is larger than the distal end side is formed at the base end portion of the center electrode (2), and the insulator (6) 6. The internal combustion engine for an internal combustion engine according to claim 4 , wherein the insulator-side coating layer (67) is formed at least on a surface of the inner peripheral surface of the portion facing the large-diameter portion (23). Spark plug (1).

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