WO2016136149A1 - Plasma jet plug - Google Patents

Abstract

This plasma jet plug comprises: a tubular insulating body having an axial hole that extends along the direction of an axial line; a center electrode placed inside the axial hole; a main metallic fitting placed in the outer periphery of the insulating body; and an orifice electrode that is electrically connected to the main metallic fitting and that is placed at the leading end side of the insulating body. A cavity for generating plasma is formed by the surface of the center electrode, the inner surface of the insulating body, and the inner surface of the orifice electrode. In this plasma jet plug, a minimum route length D1 of a creepage route from the surface of the center electrode to the inner surface of the orifice electrode via the inner surface of the insulating body in the cavity, is greater than or equal to five times an air gap G which is a minimum distance between the center electrode and the orifice electrode.

Description

Plasma jet plug Cross-reference of related applications

The present application is filed with an application number 2015-036010 filed on February 26, 2015, an application number 2015-0755551 filed on April 2, 2015, and an application number 2015 filed on May 25, 2015. Claims priority based on Japanese Patent Application No. 105326, the entire disclosure of which is incorporated herein by reference.

The present invention relates to a plasma jet plug that ignites a fuel mixture by injecting plasma.

The plasma jet plug is a spark plug having a space for generating plasma called a cavity (Patent Document 1). An orifice electrode (also referred to as a “ground electrode”) having an opening is provided at the exit of the cavity, and a center electrode is provided inside the cavity via the orifice electrode and a gap. The wall surface in the cavity is made of an insulator except for the orifice electrode and the center electrode. A large current is supplied to this cavity, and the fuel mixture is ignited by filling the cavity space and ejecting it with a large amount of plasma. When applying a large current to the cavity, first, a high voltage is applied between the orifice electrode and the center electrode to cause a dielectric breakdown to form a discharge path in the cavity, and then a large voltage is applied at a low voltage. Superimpose current.

JP 2008-045449 A

As the discharge path in the cavity, an air path that is a path in a space away from the wall surface of the cavity and a creeping path along the wall surface of the cavity (particularly the surface of the insulator) can be formed. Usually, a creeping route is more easily formed than an air route. When the creeping path is formed, a phenomenon called “channeling” occurs in which the surface of the insulator in contact with the creeping path melts in a groove shape due to the current at the time of dielectric breakdown. When channeling occurs, the cavity shape changes greatly, and the plasma ejection performance deteriorates. Furthermore, there arises a problem that a deeper groove is formed by concentration of discharge in the groove formed by channeling. Thus, a technique is desired that can prevent the occurrence of creeping discharge and can stably perform air discharge and suppress the occurrence of channeling.

The inventor of the present application also shows that when the length of the exposed portion of the central electrode in the cavity is large, the area where the central electrode comes into contact with the plasma becomes large, and the consumption of the central electrode due to the heat of the plasma becomes excessively large. I found out that there is a problem that. The inventors of the present application have further found that when the inner surface of the orifice electrode is exposed in the cavity, there is a problem that the inner surface of the orifice electrode is excessively consumed by the heat of the plasma.

The present invention has been made to solve the above-described problems, and can be realized as the following modes.

(1) According to the first aspect of the present invention, a plasma jet plug is provided. The plasma jet plug includes a cylindrical insulator having an axial hole extending along an axial direction, a center electrode disposed inside the axial hole, a metal shell disposed on an outer periphery of the insulator, An orifice electrode electrically connected to the metal shell and disposed on the distal end side of the insulator, and a plasma generating cavity is formed by the surface of the center electrode, the inner surface of the insulator, and the inner surface of the orifice electrode. Is formed. In the plasma jet plug of the first form, the shortest path length D1 of the creeping path from the surface of the center electrode to the inner surface of the orifice electrode through the inner surface of the insulator in the cavity is such that the center electrode and the It is characterized by being 5 times or more the air gap G which is the shortest distance between the orifice electrodes.
According to this plasma jet plug, since the shortest path length D1 of the creeping path is sufficiently larger than the air gap G, creeping discharge hardly occurs and air discharge can be stably performed. Ring generation can be suppressed.

(2) In the plasma jet plug, the inner surface of the insulator may have one or more grooves that form a concave path in the creeping path, and the groove width may be 0.1 mm or more. .
According to this configuration, by providing the groove on the inner surface of the insulator, the shortest path length D1 of the creeping path can be increased while suppressing the volume of the cavity and facilitating the ejection of plasma. By setting the groove width to 0.1 mm or more, the effective length of the shortest path length D1 of the creeping path can be set to the length along the groove portion, so that air discharge can be performed more stably. Can do.

(3) The said plasma jet plug WHEREIN: The depth of the said groove part is good also as what is 3 times or less of the said groove width.
According to this configuration, by setting the depth of the groove portion to three times or less of the groove width, it is possible to increase the shortest path length D1 of the creeping path and to suppress the volume of the cavity and facilitate the ejection of plasma. Is possible.

(4) In the plasma jet plug, a surface area of a side surface of the center electrode facing the cavity may be 20 mm ² or less.
According to this configuration, by setting the surface area of the side surface of the center electrode facing the cavity to 20 mm ² or less, the phenomenon that the plasma is cooled by the center electrode can be suppressed, and the plasma can be easily ejected.

(5) In the plasma jet plug, the insulator facing the cavity may be composed of a plurality of members.
According to this configuration, if the insulator facing the cavity is composed of a plurality of members, it is easy to form the inner surface shape of the insulator facing the cavity so as to increase the path length D1 of the creeping path.

(6) In the plasma jet plug, the plurality of members of the insulator include a first member provided on an outer peripheral side of the center electrode and a second member provided on an outer peripheral side of the first member. The first member is made of a first insulating material having a higher thermal conductivity than the second member, and the second member has a second insulation having a higher withstand voltage than the first member. It may be formed of a material.
According to this configuration, since the thermal conductivity of the first member is higher than the thermal conductivity of the second member, the heat extraction from the center electrode by the first member can be increased, and the durability of the center electrode is improved. Can be made. Moreover, since the withstand voltage of the second member is higher than that of the first member, the withstand voltage of the entire insulator can be improved.

(7) In the plasma jet plug, the side surface of the center electrode in the cavity is covered with an insulating material, and the distance from the tip of the insulating material provided on the side surface of the center electrode to the tip of the center electrode L may be 0.4 mm or less.
According to this configuration, since the length L of the tip portion of the center electrode exposed from the insulating material is as short as 0.4 mm or less, the consumption of the center electrode due to the heat of plasma can be suppressed.

(8) In the plasma jet plug, a distance H between the side surface of the center electrode and the inner wall surface of the cavity when measured along a direction perpendicular to the axial direction is larger than the air gap G. It may be a thing.
According to this configuration, creeping discharge hardly occurs along the path from the side surface of the center electrode to the inner wall surface of the cavity along the direction perpendicular to the axial direction, so that stable air discharge can be performed. it can.

(9) In the plasma jet plug, an inner surface of the orifice electrode around the through hole of the orifice electrode is covered with an insulating material except an exposed surface adjacent to the through hole, and is perpendicular to the axial direction. A distance J between the outermost peripheral position of the exposed surface and the side surface of the center electrode when measured along the direction may be smaller than the distance H.
According to this configuration, since the inner surface of the orifice electrode is covered with the insulating material leaving the exposed surface adjacent to the through hole, it is possible to suppress the consumption of the inner surface of the orifice electrode due to plasma.

(10) In the plasma jet plug, a distance K between the outermost peripheral position of the exposed surface and the tip of the center electrode may be larger than the air gap G.
According to this configuration, creeping discharge hardly occurs along the path from the tip of the center electrode to the insulating material covering the inner surface around the through hole of the orifice electrode, so that stable air discharge can be performed. Can do.

(11) According to the second aspect of the present invention, a plasma jet plug is provided. The plasma jet plug includes a cylindrical insulator having an axial hole extending along an axial direction, a center electrode disposed inside the axial hole, a metal shell disposed on an outer periphery of the insulator, An orifice electrode electrically connected to the metal shell and disposed on the distal end side of the insulator, and a plasma generating cavity is formed by the surface of the center electrode, the inner surface of the insulator, and the inner surface of the orifice electrode. Is formed. The plasma jet plug according to the second aspect includes an air gap G that is the shortest distance between the center electrode and the orifice electrode, and a shortest distance Dr between the tip edge of the center electrode and the inner surface of the insulator. The relationship is characterized by satisfying 1.5 × G ≦ Dr. The characteristic part of the plasma jet plug of the second form can be adopted in combination with the plasma jet plug of the first form described above, and is independent of the presence or absence of the characteristic part of the plasma jet plug of the first form. It is also possible to adopt.
According to the plasma jet plug of the second embodiment, since the shortest distance Dr between the tip edge of the center electrode and the inner surface of the insulator is sufficiently larger than the air gap G, creeping discharge hardly occurs. Since air discharge can be performed stably, the occurrence of channeling can be suppressed.

(12) In the plasma jet plug, the inner surface of the insulator facing the cavity has a reduced diameter portion provided so that the inner surface of the insulator is reduced in diameter toward the rear end side of the insulator. The cavity has a first cavity portion on the front end side of the rear end of the reduced diameter portion of the insulator and a second cavity portion on the rear end side of the rear end of the reduced diameter portion. Also good.
According to this configuration, since the shortest distance Dr between the tip edge of the center electrode and the inner surface of the insulator can be increased by the second cavity portion having a small volume, the volume of the entire cavity can be suppressed while suppressing the occurrence of creeping discharge. It is possible to make the plasma easy to be ejected while keeping the value small.

(13) In the plasma jet plug, a radial spatial distance Dp that is a distance measured in a radial direction perpendicular to the axial direction between the surface of the center electrode and the inner surface of the insulator in the second cavity portion. It is good also as what is 0.1 mm or more.
According to this configuration, the occurrence of creeping discharge in the second cavity portion can be suppressed and air discharge can be stably performed, so that the occurrence of channeling can be suppressed.

(14) In the plasma jet plug, a depth Dq of the second cavity portion measured along the axial direction may satisfy 0 <Dq ≦ 3 × Dp.
According to this configuration, by setting the depth Dq of the second cavity portion in this range, it is possible to increase the tendency that air discharge is more likely to occur than creeping discharge, and the volume of the second cavity portion. Can be prevented from becoming excessively large, and plasma can be easily ejected.

(15) In the plasma jet plug, the relationship between the radial space distance Dp of the second cavity portion and the shortest distance Dr between the tip edge of the center electrode and the inner surface of the insulator is Dp / It is good also as satisfy | filling Dr <= 0.5.
According to this configuration, plasma can be more easily ejected by setting Dp / Dr within this range.

The present invention can be realized in various modes. For example, an ignition device using a plasma jet plug or a plasma jet plug, an internal combustion engine equipped with the plasma jet plug, or the plasma jet plug is used. This can be realized in the form of an internal combustion engine or the like equipped with the conventional ignition device.

The fragmentary sectional view of the plasma jet plug as one embodiment. Sectional drawing which expanded the front-end | tip part of the plasma jet plug. The block diagram of an ignition device. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 1st Embodiment and a modification. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 2nd Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 3rd Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 4th Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 5th Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 6th Embodiment. Explanatory drawing which shows the test result regarding D1 / G. Explanatory drawing which shows the test result regarding a groove width. Explanatory drawing which shows the test result regarding the relationship between groove depth and groove width. Explanatory drawing which shows the test result regarding the surface area of the side surface of the center electrode which faces a cavity. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 7th Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 8th Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 9th Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 10th Embodiment. Explanatory drawing which shows the test result regarding the exposure length of a center electrode. Explanatory drawing which shows the test result regarding the insulator coating | cover of an orifice electrode inner surface. Sectional drawing which expanded the front-end | tip part of the plasma jet plug of 11th Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 11th Embodiment. The enlarged view of the cross section of the front-end | tip part of the plasma jet plug of 12th Embodiment. Explanatory drawing which shows the test result regarding Dr / G. Explanatory drawing which shows the test result regarding the radial direction spatial distance Dp of a 2nd cavity part. Explanatory drawing which shows the test result (the 1) regarding Dq / Dp. Explanatory drawing which shows the test result (the 2) regarding Dq / Dp. Explanatory drawing which shows the test result regarding Dp / Dr.

A. overall structure:
FIG. 1 is a partial cross-sectional view of a plasma jet plug 100 as an embodiment of the present invention. FIG. 2 is an enlarged cross-sectional view of the tip portion of the plasma jet plug 100. 1 and 2, the lower side along the direction of the axis O of the plasma jet plug 100 is referred to as the front end side of the plasma jet plug 100, and the upper side is referred to as the rear end side. A direction that intersects the axis O and is perpendicular to the axis O is referred to as a “radial direction”.

In FIG. 1, the right side of the axis O shows the appearance of the plasma jet plug 100, and the left side of the axis O shows a cross-sectional view. The plasma jet plug 100 includes an insulator 10, a metal shell 50 that holds the insulator 10, a center electrode 20 that is held inside the insulator 10, and an orifice electrode 30 that is disposed at a distal end portion 57 of the metal shell 50. And a terminal fitting 40 disposed at the rear end of the insulator 10.

The insulator 10 is formed by firing a ceramic material such as alumina, and is a cylindrical insulating member having an axial hole 12 extending in the direction of the axis O. A flange portion 19 having the largest outer diameter is formed substantially at the center in the direction of the axis O, and a rear end side body portion 18 is formed on the rear end side. Further, a distal end side body portion 17 having an outer diameter smaller than that of the rear end side body portion 18 on the front end side from the flange portion 19, and a further outer diameter than the front end side body portion 17 on the front end side of the front end side body portion 17. A small leg length 13 is formed. Between the leg long part 13 and the front end side body part 17, it is formed in a step shape. A portion of the shaft hole 12 corresponding to the inner periphery of the long leg portion 13 is formed as an electrode housing portion 15. The electrode housing portion 15 is smaller in diameter than any inner peripheral portion of the front end side body portion 17, the flange portion 19, and the rear end side body portion 18. A center electrode 20 is held inside the electrode housing portion 15. An enlarged inner diameter portion 16 having a larger inner diameter than the leg length portion 13 is formed on the distal end side of the leg length portion 13 of the insulator 10.

The center electrode 20 is a rod-shaped conductive member extending along the axis O, and is disposed inside the shaft hole 12 of the insulator 10. In the present embodiment, the center electrode 20 is an integrally molded product formed of a high melting point material such as tungsten. However, various other configurations can be adopted as the configuration of the center electrode 20. For example, a configuration having a double structure of a base material and a core material embedded in the base material may be adopted.

As shown in FIG. 2, the center electrode 20 has a head 21 on the most rear end side and a leg portion 22 that is located on the tip side of the head 21 and has an outer diameter smaller than that of the head 21. The leg portion 22 of the center electrode 20 is housed in the electrode housing portion 15, and the head portion 21 of the center electrode 20 is housed in a portion on the rear end side from the reduced inner diameter portion 10 z of the shaft hole 12. The surface on the front end side of the head 21 and the surface on the rear end side of the reduced inner diameter portion 10z are in close contact with each other, and are sealed over the entire circumference in the circumferential direction.

As shown in FIG. 1, the center electrode 20 is electrically connected to the terminal fitting 40 on the rear end side through a conductive seal body 4 made of a mixture of metal and glass provided in the shaft hole 12. It is connected. With this seal body 4, the center electrode 20 and the terminal fitting 40 are fixed in the shaft hole 12 and are electrically connected to each other. A high voltage cable (not shown) is connected to the terminal fitting 40 via a plug cap (not shown).

The main metal fitting 50 is a cylindrical metal fitting for fixing the plasma jet plug 100 to the engine head of the internal combustion engine, and holds the insulator 10 so as to surround it. The metal shell 50 includes a tool engaging portion 51 into which a plug wrench is fitted and a screw portion 52 to be screwed into the engine head. A caulking portion 53 is provided on the rear end side of the metal fitting 50 from the tool engaging portion 51. Annular ring members 6, 7 are interposed between the metal shell 50 from the tool engaging portion 51 to the caulking portion 53 and the rear end side body portion 18 of the insulator 10, and two ring members Between 6 and 7, talc 9 (talc) powder is filled. Then, by crimping the crimping portion 53, the insulator 10 is pressed toward the distal end side in the metal shell 50 through the ring members 6, 7 and the talc 9. As a result, the stepped portion between the leg length portion 13 of the insulator 10 and the front end side body portion 17 is provided with an annular packing 80 on the locking portion 56 formed in a step shape on the inner peripheral surface of the metal shell 50. The metal shell 50 and the insulator 10 are integrated with each other. By this packing 80, airtightness between the metal shell 50 and the insulator 10 is maintained, and combustion gas is prevented from flowing out. Further, a flange 54 is formed between the tool engaging portion 51 and the screw portion 52, and the gasket 5 is inserted into the vicinity of the rear end side of the screw portion 52, that is, the seat surface 55 of the flange 54. ing.

The orifice electrode 30 is provided at the front end portion 57 of the metal shell 50. As shown in FIG. 2, a concave portion 57A is formed on the inner peripheral side of the distal end portion 57 of the metal shell 50, and the orifice electrode 30 is fitted in the concave portion 57A. The orifice electrode 30 is an annular plate member having a through hole 31 at the center. The through hole 31 functions as an ejection hole for ejecting plasma. The peripheral edge of the orifice electrode 30 is joined to the metal shell 50 by laser welding or the like over the entire circumference. The metal shell 50 and the orifice electrode 30 are electrically connected. Since the metal shell 50 is screwed to the engine head and grounded, the orifice electrode 30 is also grounded. The orifice electrode 30 covers the opening in the distal direction of the metal shell 50.

As shown in FIG. 2, the cavity CV for generating plasma is formed between the inner surface of the tip portion of the insulator 10, the surface of the tip portion of the center electrode 20, and the inner surface of the orifice electrode 30. . Plasma is generated by applying a voltage between the center electrode 20 and the orifice electrode 30.

FIG. 3 is a block diagram showing the configuration of the ignition device 120 that ignites the plasma jet plug 100. The ignition device 120 includes a spark discharge circuit unit 140, a plasma discharge circuit unit 160, and two control circuit units 130 and 150 for controlling them. Control circuit units 130 and 150 are connected to the ECU of the automobile.

The spark discharge circuit section 140 is for performing a so-called trigger discharge in which a high voltage is applied to the gap between the center electrode 20 and the orifice electrode 30 of the plasma jet plug 100 to cause a dielectric breakdown to start a spark discharge. It is a power supply circuit. The plasma discharge circuit unit 160 is a power supply circuit for supplying a large current to a gap where dielectric breakdown has occurred due to trigger discharge. The plasma discharge circuit unit 160 includes a capacitor 162 for storing electric energy and a high voltage generation circuit 161 for charging the capacitor 162. One end of the capacitor 162 is grounded, and the other end is connected to the center electrode 20. When a discharge occurs in the gap between the center electrode 20 and the orifice electrode 30, the gas in the cavity CV is excited by a large current supplied from the ignition device 120 to form plasma. When the plasma formed in the cavity CV expands and the pressure in the cavity CV increases, the plasma in the cavity CV is ejected from the through hole 31 of the orifice electrode 30. The air-fuel mixture in the combustion chamber of the internal combustion engine is ignited by the ejected plasma.

B. Various embodiments of the tip portion of the plasma jet plug:
FIG. 4A is an enlarged view of the cross section of the tip portion of the first embodiment of the plasma jet plug, and FIG. 4B is an enlarged view of the cross section of the tip portion of the modification. 4 is upside down with respect to FIGS. 1 and 2, the upper side of FIG. 4 is the front end side of the plasma jet plug, and the lower side of FIG. 4 is the rear end side of the plasma jet plug.

In the plasma jet plug 100 of the first embodiment shown in FIG. 4A, the tip portion of the center electrode 20 is formed as a columnar leg portion 22. An enlarged inner diameter portion 16 having an inner diameter larger than that of the leg length portion 13 is formed in the leg length portion 13 in the vicinity of the tip of the insulator 10. The long leg portion 13 is also referred to as a “small inner diameter portion 13”. A reduced diameter portion 14 is formed between the leg length portion 13 and the enlarged inner diameter portion 16. In this example, the reduced diameter portion 14 is formed as a surface perpendicular to the axis O, but may be formed in a tapered shape. On the outer edge of the reduced diameter portion 14 of the insulator 10, an annular groove Gr <b> 1 that is recessed toward the rear end side from the surface of the reduced diameter portion 14 is formed. This groove part Gr1 forms a concave path in the creeping path. The size of the groove part Gr1 is defined by the width Wa1 and the depth Wd1 of the groove part Gr1. The effect of forming the groove part Gr1 will be described later.

The cavity CV is a space surrounded by the surface 20s of the center electrode 20, the inner surface 10in of the insulator 10, and the inner surface 30in of the orifice electrode 30. However, the cavity CV does not include the portion of the through hole 31 of the orifice electrode 30, and means a space inside the inner surface 30in of the orifice electrode 30 when it is assumed that there is no through hole 31. Note that a minute gap (less than 0.06 mm) is formed between the outer peripheral surface of the leg portion 22 of the center electrode 20 and the inner surface of the insulator 10 in order to assemble them. A space with a gap of less than 0.06 mm is very small and plasma is not generated, so it does not function as a part of the cavity CV. In this specification, the “cavity” means a space where plasma is generated, and means a space having a gap of 0.06 mm or more. More specifically, the “cavity” in the first embodiment of FIG. 4A includes the inner surface 10 in of the tip portion of the insulator 10, the surface of the tip portion of the center electrode 20, and the inner surface 30 in of the orifice electrode 30. Means a space having a gap of 0.06 mm or more, and does not include a space having a gap of less than 0.06 mm.

In FIG. 4A, the following dimensions are further shown.
(1) D1: The shortest length of the creeping path from the surface 20s of the center electrode 20 through the inner surface of the insulator 10 to the inner surface 30in of the orifice electrode 30 (hereinafter referred to as “the creepage shortest path length”). In FIG. 4A, the creeping shortest path length D1 includes the length of the concave path along the groove part Gr1.
(2) E: Inner diameter of the through hole 31 of the orifice electrode 30.
(3) G: A distance G in the axial direction between the inner surface 30 in of the orifice electrode 30 and the tip surface 20 t of the center electrode 20. This distance G is also referred to as “air gap G”. A typical value range of the air gap G is, for example, 0.3 mm to 1.5 mm.

Note that the inner diameter E of the through hole 31 of the orifice electrode 30 is preferably smaller than the outer diameter of the leg portion 22 at the tip of the center electrode 20. This is to facilitate air discharge in the air gap G.

FIG. 4B is an enlarged view of a cross section of a tip portion of a plasma jet plug 100r as a modification. This plasma jet plug 100r is obtained by omitting the groove Gr1 of the insulator 10 from the plasma jet plug 100 of the first embodiment, and the other configuration is the same as that of the first embodiment. The shortest creepage path length D1r in this modification is shorter than the shortest creepage path length D1 in the first embodiment by the length of the groove Gr1 (= Wa1 + 2 × Wd1).

In the first embodiment shown in FIG. 4A, since the groove portion Gr1 is provided in a part of the inner surface 10in of the insulator 10, the creeping shortest path length D1 can be increased as compared with the modified example. As a result, creeping discharge can be made difficult to occur, and air discharge can be stably performed. From this point of view, it is particularly preferable that the creepage shortest path length D1 is 5 times or more the air gap G. However, also in the modification shown in FIG. 4B, if the creepage shortest path length D1r is 5 times or more of the air gap G, it is possible to make it difficult to generate the creeping discharge, and to stabilize the air discharge. Therefore, it can be adopted as an embodiment of the present invention. However, as in the first embodiment shown in FIG. 4A, if the groove portion Gr1 that forms the concave path is provided in the creeping path, the creeping shortest path length D1 without excessively increasing the volume of the cavity CV. Is preferable in that it can be lengthened.

The groove width Wa1 of the groove part Gr1 may be 0.06 mm or more, but is preferably 0.1 mm or more. This is because if the groove width Wa1 is excessively small, the groove part Gr1 may not have a function of extending the creeping path (that is, a discharge is generated by jumping over the groove part Gr1). Providing the groove part Gr1 having a groove width Wa1 of 0.1 mm or more is preferable in that the creepage shortest path length D1 can be increased while keeping the volume of the cavity small. The maximum value of the groove width Wa1 is not particularly limited, but for example, the groove width Wa1 is preferably 0.5 mm or less, and more preferably 0.3 mm or less.

Further, it is preferable that the depth Wd1 of the groove part Gr1 is not more than three times the groove width Wa1. In this way, it is possible to make it easier to eject plasma by reducing the volume of the cavity CV while increasing the shortest creepage path length D1.

FIG. 5 is an enlarged view of a cross section of the tip portion of the plasma jet plug 100a in the second embodiment. This plasma jet plug 100a is obtained by adding an annular second groove Gr2 to the inner surface 10in of the insulator 10 of the plasma jet plug 100 (FIG. 4A) of the first embodiment. The same as in the first embodiment. That is, in the plasma jet plug 100a of the second embodiment, two grooves Gr1 and Gr2 are provided on the inner surface 10in of the insulator 10. The groove depth Wd1 of the second groove portion Gr2 is the same as the groove depth of the first groove portion Gr1 in the example of FIG. 5, but the depths of both may be changed. Further, the groove width Wa2 of the second groove part Gr2 may be the same as or different from the groove width Wa1 of the first groove part Gr1. Further, three or more grooves may be provided. In the example of FIG. 5, the groove portions Gr <b> 1 and Gr <b> 2 are provided in the reduced diameter portion 14 of the insulator 10, but the groove portion may be formed on the cylindrical inner surface of the insulator 10 along the axis O.

FIG. 6 is an enlarged view of the cross section of the tip portion of the plasma jet plug 100b in the third embodiment. This plasma jet plug 100b is obtained by extending the cavity CV portion of the plasma jet plug 100 (FIG. 4A) of the first embodiment along the direction of the axis O, and the other configuration is the first embodiment. Is the same. That is, in the plasma jet plug 100b of the second embodiment, the side surface 20f of the center electrode 20 facing the cavity CV is longer than that of the first embodiment. The surface area S _20f of the side surface 20f of the center electrode 20 is expressed as follows.
S _20f = 2πR · L (1)
Here, R is the radius of the exposed portion of the center electrode 20, and L is the length of the exposed portion of the center electrode 20 in the axial direction. A typical value range for the radius R is, for example, 0.25 mm to 1 mm. A typical value range of the length L is, for example, 0 mm to 5 mm.

If the surface area S _20f of the side surface _20f of the center electrode 20 becomes excessively large, the plasma is cooled by the center electrode 20 and the plasma ejection performance may be lowered. Considering this point, it is preferable that the surface area S _20f of the side surface 20f of the center electrode 20 is 20 mm ² or less. In this way, the phenomenon that the plasma is cooled by the center electrode 20 can be suppressed, and the plasma can be easily ejected.

FIG. 7 is an enlarged view of a cross section of the tip portion of the plasma jet plug 100c in the fourth embodiment. In the plasma jet plug 100c, the insulator 10 facing the cavity CV is configured by a plurality of members 13c and 16c, and other configurations are the same as those of the first embodiment. More specifically, the leg length portion 13 of the insulator 10 and the portion of the enlarged inner diameter portion 16 following the distal end side thereof are provided on the outer peripheral side of the first member 13 c provided on the outer peripheral side of the center electrode 20. The second member 16c was separated into two members. The first member 13 c corresponds to the leg long part 13 of FIG. 4A and is formed to have a smaller outer diameter than the leg long part 13. The second member 16c is a substantially annular member, and is fitted and fixed to the outer peripheral side of the first member 13c.

Further, in FIG. 7, a groove part Gr1 that forms a concave path in the creeping path is formed at a position where the first member 13c and the second member 16c are in contact with each other. This groove part Gr1 is formed in the boundary part of the two members 13c and 16c. If the insulator 10 facing the cavity CV is constituted by a plurality of members 13c and 16c, there is an advantage that the groove Gr1 can be formed more easily. However, the groove portion Gr1 may be omitted and the shape similar to that shown in FIG.

Further, the insulator 10 facing the cavity CV is constituted by a plurality of members 13c and 16c, and further advantages can be obtained by changing their materials. For example, the first member 13c on the inner peripheral side is formed of a first insulating material (for example, aluminum nitride (AlN)) having a higher thermal conductivity than the second member 16c on the outer peripheral side, and the second member on the outer peripheral side. the 16c, may be formed in the second insulating material withstand voltage is higher than the first member 13c on the inner circumferential side (for example, alumina (Al ₂ O _3)). If such a structure is employ | adopted, the heat sink from the center electrode 20 by the 1st member 13c can be increased, and durability of the center electrode 20 can be improved. Moreover, since the withstand voltage of the second member 16c is higher than that of the first member 13c, the withstand voltage of the entire insulator 10 can be improved.

FIG. 8 is an enlarged view of a cross section of the tip portion of the plasma jet plug 100d in the fifth embodiment. In the plasma jet plug 100d, as in the fourth embodiment (FIG. 7), the insulator 10 facing the cavity CV is composed of a plurality of members 13d and 16d. In FIG. 8, the first groove 13r is provided in the first member 13d of the insulator 10, and the second groove Gr2 is provided at the boundary position between the first member 13d and the second member 16d. In other words, a part of the wall surface of the second groove part Gr2 is configured by the surface of the first member 13d, and the other part is configured by the surface of the second member 16d. As a result, it is easy to sufficiently lengthen the creepage shortest path length D1 by the plurality of grooves Gr1 and Gr2.

FIG. 9 is an enlarged view of the cross section of the tip portion of the plasma jet plug 100e in the sixth embodiment. In the plasma jet plug 100e, as in the fourth embodiment (FIG. 7) and the fifth embodiment (FIG. 8), the insulator 10 facing the cavity CV is composed of a plurality of members 13e and 16e. . 9 differs from FIG. 7 in that a tip opening 16p having a small opening is provided at the tip of the second member 16d and covers the inner surface 30in of the orifice electrode 30. The tip opening 16p of the second member 16d may cover the entire inner surface 30in of the orifice electrode 30, or may cover only a part thereof. Thus, if the tip opening 16p is provided in the second member 16d so as to cover the inner surface 30in of the orifice electrode 30, the creeping shortest path length D1 can be further increased.

As can be understood from the above embodiments of FIGS. 4 to 9, the insulator in the shortest creepage path from the surface 20s of the center electrode 20 through the inner surface of the insulator 10 to the inner surface 30in of the orifice electrode 30. If one or more grooves are provided on the inner surface of the surface 10, the shortest creepage path length D1 can be made sufficiently long. As a result, creeping discharge can be made difficult to occur, and air discharge can be stably performed. Further, as can be understood from the examples of FIGS. 7 to 9, if the insulator 10 facing the cavity CV is composed of a plurality of members, the insulation facing the cavity is increased so as to increase the shortest creepage path length D1. There is an advantage that it is easy to form the inner surface shape of the body.

C. Test results:
In the following, the test results relating to the preferred dimensions of the plasma jet plug shown in FIGS. 4 to 9 will be described sequentially.

FIG. 10 is an explanatory diagram showing test results regarding the ratio D1 / G between the creepage shortest path length D1 and the air gap G. FIG. FIG. 10A is a schematic plan view of the test apparatus. In this test, the insulator 210 having the groove 212 is installed in the pressure chamber, and the first electrode 220 and the second electrode 230 are installed facing each other with the groove 212 sandwiched on the surface of the insulator 210. did. The insulator 210 was made of alumina. The gap Dg between the two electrodes 220 and 230 was set to a constant value of 0.5 mm. The groove width Da of the groove portion 212 was set to a constant value of 0.2 mm, and the groove portion path length DL was changed by changing the groove depth Dd of the groove portion 212. The “groove path length DL” is the shortest path length that follows the inner surface of the groove 212 and is given by DL = Da + 2Dd.

The two electrodes 220 and 230 simulate the center electrode 20 and the orifice electrode 30. The following two discharge paths may occur between the two electrodes 220 and 230.
(1) First discharge path RT1: A discharge path that jumps over the groove 212 in the vicinity of the upper surface 210s of the insulator 210 (indicated by a black arrow in FIG. 10A).
(2) Second discharge path: a creeping path (not shown) that follows the upper surface 210s of the insulator 210 and the groove path length DL.
Since these two discharge paths have a common path portion along the upper surface 210s of the insulator 210, the difference between the two discharge paths passes through the air path of the groove width Da in the first discharge path RT1, and the second The discharge path is only a point passing through a concave creepage path having a groove path length DL. Therefore, when this structure is applied to the structure of FIG. 4, the groove width Da has a role as a dimension for simulating the air gap G of FIG. 4, and the groove path length DL is the creepage shortest path length D1. It can be understood that it has a role as a simulated dimension.

In the discharge path confirmation test of FIG. 10, discharge was performed 100 times each in a state where the pressure chamber was pressurized to 0.4 MPa, 1.2 MPa, and 2.0 MPa (all in the atmosphere). Then, the discharge path was photographed using a high-speed camera, and the ratio of the number of times the discharge occurred in the second discharge path described above out of 100 discharges was measured, and this was defined as the “creeping discharge ratio”. Here, “creepage discharge” means discharge along the second discharge path described above, and “air discharge” means discharge along the first discharge path RT1.

FIG. 10B shows the relationship between the value of the ratio DL / Da and the creeping discharge rate. According to this test result, the creeping discharge ratio decreased as the value of the ratio DL / Da increased, and when DL / Da was 5 or more, the creeping discharge did not occur, and all were in the air. This result can be understood as follows. That is, when the groove portion path length DL in FIG. 10A is increased, the creeping discharge that passes through the second discharge path described above becomes difficult to occur, and the air discharge that passes through the first discharge path RT1 is likely to occur. Therefore, by setting DL / Da to 5 or more, air discharge can be stably generated. Incidentally, as described above, the groove portion path length DL simulates the creeping shortest path length D1 of FIG. 4, and the groove width Da simulates the air gap G. Therefore, it can be considered that the horizontal axis of FIG. 10B simulates the ratio D1 / G between the creepage shortest path length D1 and the air gap G. Considering this test result, it is preferable to set the value of the ratio D1 / G of the creepage shortest path length D1 and the air gap G to 5 or more in the plasma jet plug. In other words, the creepage shortest path length D1 is preferably 5 times or more the air gap G. If it carries out like this, generation | occurrence | production of the creeping discharge in the cavity CV can be suppressed, and air discharge can be performed stably.

FIG. 11 is an explanatory diagram showing test results regarding the groove width Wa1 of the groove part Gr1. The test apparatus shown in FIG. 11A is the same as that shown in FIG. 10A, but the setting of dimensions is different from the test shown in FIG. That is, in the test of FIG. 11, the groove width Da was changed to several values, and the groove depth Dd was also changed so that the groove depth Dd became equal to the groove width Da. Furthermore, the air gap Dg was set to a value obtained by adding 0.3 mm to the groove width Da. In this test, the groove width Da simulates the groove width Wa1 of the groove part Gr1 in FIG. Further, in this discharge path confirmation test, discharge was performed 100 times in a state where the pressure chamber was pressurized to 0.8 MPa (atmosphere), and discharge occurred in the first discharge path RT1 out of 100 discharges. The ratio of the number of times was measured, and this was defined as the “in-air discharge ratio”.

FIG. 11 (B) shows the relationship between the value of the groove width Da and the air discharge rate. According to this test result, the air discharge ratio decreased as the value of the groove width Da increased. When the groove width Da was 0.1 mm or more, no air discharge was generated, and all the surface discharge occurred. This result can be understood as follows. That is, when the groove width Da is small, air discharge is likely to occur along the first discharge path RT1 without passing through the concave creeping path (second discharge path) along the groove 212. On the other hand, as the groove width Da increases, creeping discharge along the concave creeping path along the groove 212 is likely to occur. In other words, when the groove width Da of the groove part 212 is less than 0.1 mm, the concave path along the groove part 212 hardly functions as a discharge path, whereas the groove width Da is 0.1 mm or more. Then, the concave path along the groove 212 sufficiently functions as a discharge path. Considering this test result, in the plasma jet plug of FIG. 4, it is preferable to set the groove width Wa1 of the groove part Gr1 to 0.1 mm or more. The same applies to the groove widths of the other grooves Gr2 (FIGS. 5 and 8). If the groove width Wa1 is set to 0.1 mm or more, the creeping path can be made substantially longer by the groove part Gr1, so that the occurrence of creeping discharge in the cavity CV is further suppressed, and air discharge is stably performed. Can be done.

FIG. 12 is an explanatory diagram showing test results regarding the groove depth Wd1 and the groove width Wa1 of the groove part Gr1. In this test, a plurality of types of samples having different groove depths Wd1 and groove widths Wa1 were prepared. In these samples, L + G (L is the length of the exposed portion of the center electrode 20 and G is the air gap) is set to 3.5 mm, and the outer diameter 2R of the center electrode 20 is set to 1.5 mm. The inner diameter of each of the ten enlarged inner diameter portions 16 was set to 3.5 mm. The groove width Wa1 is set to three values of 0.2 mm, 0.3 mm, and 0.5 mm, and the groove depth Wd1 is set so that the value of Wd1 / Wa1 ranges from 0.5 to 5.0. Set to. Then, the plasma jet plug sample is discharged in a state where the pressure chamber is pressurized to 0.6 MPa (atmosphere), and the plasma ejected from the through-hole 31 of the orifice electrode 30 is photographed from the side to obtain a schlieren image. did. Then, the Schlieren image was binarized and classified into pixels representing a high-density portion and pixels representing a low-density portion, and the number of pixels representing the high-density portion was calculated as the size of the ejected plasma. In addition, 10 schlieren imaging | photography was performed for every sample, and the average value of the pixel number of the plasma calculated by 10 imaging | photography was made into the ejection area.

FIG. 12B shows the relationship between the value of the ratio Wd1 / Wa1 of the groove depth Wd1 and the groove width Wa1 and the plasma ejection area. According to this test result, regardless of the value of the groove width Wa1, when the value of Wd1 / Wa1 is 3 or more, the plasma ejection area decreases as the value of Wd1 / Wa1 increases. The reason for this is presumed that when the groove depth Wd1 is excessively increased, the volume of the cavity CV is excessively increased, and it becomes difficult to eject plasma. Considering this test result, it is preferable that the depth Wd1 of the groove part Gr1 is not more than three times the groove width Wa1. The same applies to the groove widths of the other groove portions Gr2. In this way, it is possible to make it easier to eject plasma by reducing the volume of the cavity CV while increasing the shortest creepage path length D1.

FIG. 13 shows the results of the plasma ejection test for the surface area of the side surface of the central electrode facing the cavity. In this test, as shown in FIG. 6, by changing the length L of the center electrode 20 exposed in the cavity CV, the surface area S _20f of the side surface 20f of the center electrode 20 facing the cavity CV is different. A variety of samples were created. In these samples, the air gap G is set to 0.5 mm or 1.0 mm, the outer diameter 2R of the center electrode 20 is 1 mm, the groove width Wa1 is 0.2 mm, and the groove depth Wd1 is 0.00. The constant value was 4 mm. Then, schlieren imaging was executed under the same conditions as in FIG. 12, and the average value of the number of plasma pixels calculated by 10 imaging operations was taken as the ejection area.

FIG. 13 (B) shows the relationship between the ejection area of the surface area _{S 20f} and the plasma side 20f of the center electrode 20 facing the cavity CV. As can be understood from the test results, the plasma jet area of the larger the value of the surface area S _20f side 20f is increased of the center electrode 20 tends to be smaller. In view of this test result, the value of the surface area S _20f side 20f of the center electrode 20 is small, it is preferable. However, even if the value of the surface area S _20f is smaller than 20 mm ^{2, the} plasma ejection area does not increase so much, so it is sufficient that the value of the surface area S _20f is 20 mm ² or less. A shape in which the length L of the center electrode 20 facing the cavity CV is negative (a shape in which the leg portion 22 at the front end of the center electrode 20 is retracted to the rear end side from the reduced diameter portion 14 of the insulator 10). It is also possible to do. However, such a shape may easily cause creeping discharge. Considering this point, the length L of the center electrode 20 facing the cavity CV is set to 0 mm or more, that is, the surface area S _20f of the side surface 20f of the center electrode 20 facing the cavity CV is set to 0 mm ² or more. Is preferred.

D. Other embodiments:
FIG. 14 is an enlarged view of the cross section of the tip portion of the plasma jet plug 100f in the seventh embodiment. This plasma jet plug 100f is common to the fourth embodiment (FIG. 7) in that the insulator 10 facing the cavity CV is composed of a plurality of members 13f and 16f, and the fourth embodiment is the following two points. It is different from the form. The first difference is that the reduced diameter portion 14f of the insulator 10 extends so as to cover the side surface of the distal end portion (leg portion 22) of the center electrode 20 with a part of the distal end portion of the center electrode 20 exposed. It is a point. At this time, the distance L from the tip 14t of the reduced diameter portion 14f (insulating material) provided on the side surface of the center electrode 20 to the tip of the center electrode 20 is preferably set to 0.4 mm or less. By doing so, the distance L (referred to as “exposed length L of the center electrode 20”) becomes sufficiently short, so that the consumption of the center electrode due to the heat of plasma can be suppressed. The second difference is that the distance H between the side surface of the center electrode 20 and the inner wall surface of the cavity CV when measured along the direction perpendicular to the direction of the axis O is smaller than that in the fourth embodiment (FIG. 7). Is a point. In this case, however, the distance H is preferably larger than the air gap G. This makes it difficult for creeping discharge to occur along the path from the side surface of the center electrode 20 to the inner wall surface of the cavity CV along the direction perpendicular to the direction of the axis O, so that air discharge can be performed stably. Can do. The condition that G <H is preferably satisfied in other various embodiments.

FIG. 15 is an enlarged view of a cross section of the tip portion of the plasma jet plug 100g in the eighth embodiment. The plasma jet plug 100g is different from the seventh embodiment (FIG. 14) in that the distal end portion (leg portion) of the center electrode 20 is replaced by an insulating member 14g different from the insulator 10 instead of the reduced diameter portion 14f of the insulator 10. 22) and the other configuration is the same as that of the seventh embodiment. The insulating member 14g can be formed of any insulating material such as alumina, for example. The insulating member 14g can be formed so as to cover the periphery of the center electrode 20 using any method such as plating.

FIG. 16 is an enlarged view of the cross section of the tip portion of the plasma jet plug 100h in the ninth embodiment. This plasma jet plug 100h differs from the seventh embodiment (FIG. 14) in the following two points. The first difference is that the reduced diameter portion 14h of the insulator 10 has a tip portion 14e covering the tip portion of the center electrode 20, but a gap GP is formed on the lower side (rear end side) of the tip portion 14e. It is a point that is formed. However, this gap GP may not be present. The second difference is that the distance H between the side surface of the center electrode 20 and the inner wall surface of the cavity CV when measured along the direction perpendicular to the direction of the axis O is larger than that in the seventh embodiment (FIG. 14). Is a point. However, since the importance of the second difference is low, this difference may not be provided.

As can be understood from the seventh to ninth embodiments described above, a part of the insulator 10 can be used as the insulating material covering the side surface of the center electrode 20 in the cavity CV. It is also possible to use different insulating materials (for example, the insulating member 14g in FIG. 15). In these embodiments, since the exposed length L of the center electrode 20 is sufficiently short, consumption of the center electrode due to the heat of plasma can be suppressed.

FIG. 17 is an enlarged view of a cross section of the tip portion of the plasma jet plug 100j in the tenth embodiment. The plasma jet plug 100j is different from the seventh embodiment (FIG. 14) in that a tip opening having a small opening at the tip of the second member 16j of the insulator 10 is the same as the sixth embodiment shown in FIG. The point 16 p is provided to cover the inner surface of the orifice electrode 30. However, the opening of the tip opening 16p is larger than the through hole 31 of the orifice electrode 30, and the exposed surface 32 that is not covered by the tip opening 16p remains on the inner surface of the orifice electrode 30. The exposed surface 32 exists at a position adjacent to the through hole 31 of the orifice electrode 30. In addition, the outermost peripheral position 32 e of the exposed surface 32 is preferably located on the radially outer side than the edge portion at the tip of the center electrode 20. Here, the “radial direction” means a direction perpendicular to the axis O direction. At this time, the distance J between the outermost peripheral position 32e of the exposed surface 32 and the side surface of the center electrode 20 when measured along the radial direction is the distance between the side surface of the center electrode 20 and the inner wall surface of the cavity CV. It is preferably smaller than H. By doing so, the inner surface of the orifice electrode 30 is covered with the insulating material leaving the exposed surface 32 adjacent to the through hole 31, so that the inner surface of the orifice electrode 30 due to plasma can be suppressed.

The tenth embodiment further has a feature that the linear distance K between the outermost peripheral position 32e of the exposed surface 32 and the tip of the center electrode 20 is larger than the air gap G. If the condition of G <K is satisfied, creeping discharge occurs along the path from the tip of the center electrode 20 to the insulating material (tip opening 16p) covering the inner surface around the through hole 31 of the orifice electrode 30. Since it becomes difficult, air discharge can be performed stably. In the tenth embodiment, as the insulating material covering the inner surface around the through hole 31 of the orifice electrode 30, the tip opening 16p of the second member 16j that is a part of the insulator 10 is used. Instead, an insulating material different from the insulator 10 may be used.

In the seventh to tenth embodiments, the insulator 10 is composed of a plurality of members (for example, two members 13f and 16f in FIG. 14). Instead, the insulator 10 is composed of one member. May be.

FIG. 18 is an explanatory diagram showing a test result regarding the exposed length L of the center electrode 20. FIG. 18A shows the shape of the sample, which is in conformity with the seventh embodiment shown in FIG. The following parameters were used in this test.
-Creeping shortest path length D1: 3.5mm
・ Inner diameter E of through-hole 31 of orifice electrode 30: 0.5 mm
・ Air gap G: 0.5mm
・ Outer diameter 2R of the center electrode 20: 1.5 mm
・ Inner diameter Dcv of cavity CV (inner diameter of enlarged inner diameter portion 16f): 3.5 mm
The distance H between the side surface of the center electrode 20 and the inner wall surface of the cavity CV: 1.0 mm
The exposed length L of the center electrode 20 (when the insulating member 14f is shielded): 0 to 0.6 mm
-The exposed length L of the center electrode 20 (in the case of no shielding by the insulating member 14f): 2.0 mm

FIG. 18B is a graph showing test results regarding the relationship between the exposed length L of the center electrode 20 and the amount of wear at the tip of the center electrode 20. The vertical axis represents a ratio obtained by dividing the amount of wear at the tip of the center electrode 20 when the side surface of the center electrode 20 is shielded by the amount of wear when there is no shield. Here, “the side of the center electrode 20 is shielded” means that the side of the tip of the center electrode 20 is covered with the reduced diameter portion 14f of the insulator 10 (L = 0 to 0.6 mm). ing. Further, “no shielding of the side surface of the center electrode 20” means that the side surface of the tip portion of the center electrode 20 is not covered with the reduced diameter portion 14f of the insulator 10 (L = 2.0 mm). The “consumed amount” is a value obtained by measuring the volume lost from the tip portion of the center electrode 20 after performing a spark discharge endurance test at 30 Hz for 30 hours.

As can be understood from the result of FIG. 18B, when the side surface of the center electrode 20 is shielded with an insulating material, the amount of wear at the tip of the center electrode 20 is reduced as compared to the case without shielding. In particular, if the exposed length L of the center electrode 20 is set to 0.4 mm or less, the effect of suppressing the consumption of the center electrode due to the heat of plasma is remarkable.

FIG. 19 is an explanatory diagram showing test results regarding the insulator coating on the inner surface of the orifice electrode 30. FIG. 19A shows the shape of the sample, which is in conformity with the tenth embodiment shown in FIG. The following parameters were used in this test.
-Creeping shortest path length D1: 4.0 mm
・ Inner diameter E of through-hole 31 of orifice electrode 30: 0.5 mm
・ Air gap G: 0.5mm
・ Outer diameter 2R of the center electrode 20: 1.5 mm
・ Inner diameter Dcv of cavity CV (inner diameter of enlarged inner diameter portion 16f): 3.5 mm
The distance H between the side surface of the center electrode 20 and the inner wall surface of the cavity CV: 1.0 mm
The outer diameter D32 of the exposed surface 32 on the inner surface of the orifice electrode 30: 1.4 to 1.7 mm

The outer diameter D32 of the exposed surface 32 is the same as the inner diameter of the tip opening 16p that covers the inner surface around the through hole 31 of the orifice electrode 30. Further, the distance J between the outermost peripheral position 32e of the exposed surface 32 and the side surface of the center electrode 20 when measured along the radial direction is equal to J = (D32-2R) / 2.

FIG. 19B is a graph showing test results regarding the relationship between the outer diameter D32 of the exposed surface 32 on the inner surface of the orifice electrode 30 and the amount of wear on the inner surface of the orifice electrode 30. The vertical axis represents the ratio obtained by dividing the amount of wear on the inner surface of the orifice electrode 30 when the inner surface of the orifice electrode 30 is shielded by the amount of wear when there is no shield. Here, “the inner surface of the orifice electrode 30 is shielded” means that the inner surface of the orifice electrode 30 is covered with the tip opening 16p of the insulator 10. Further, “no shielding of the inner surface of the orifice electrode 30” means that the inner surface of the orifice electrode 30 is not covered with the tip opening 16p of the insulator 10. The “consumed amount” is a value obtained by measuring the volume lost from the inner surface of the orifice electrode 30 after performing a spark discharge durability test at 30 Hz for 30 hours.

As shown in the lower end of FIG. 19B, when D32 = 1.4 mm and 1.5 mm, the distance K between the outermost peripheral position 32e of the exposed surface 32 and the tip of the center electrode 20 is the air gap. Equal to G. In these cases, some channeling occurred in the tip opening 16p of the insulator 10. On the other hand, when D32 = 1.6 mm and 1.7 mm, the distance K between the outermost peripheral position 32e of the exposed surface 32 and the tip of the center electrode 20 is larger than the air gap G. In these cases, channeling did not occur in the tip opening 16p of the insulator 10. The reason for this is that when G <K is satisfied, creeping discharge occurs along a path from the tip of the center electrode 20 to an insulating material (tip opening 16p) covering the inner surface around the through hole 31 of the orifice electrode 30. It is presumed that this is difficult to occur.

As can be understood from the result of FIG. 19 (B), it is preferable to shield the inner surface of the orifice electrode 30 with an insulating material in that the amount of wear on the inner surface of the orifice electrode 30 is reduced as compared with the case without shielding. Further, it can be understood that it is preferable to cover the inner surface around the through hole 31 of the orifice electrode 30 with an insulating material so as to satisfy the condition of G <K in order to make the creeping discharge difficult to occur.

E. Still other embodiments:
FIG. 20 is an enlarged cross-sectional view of the tip portion of the plasma jet plug 100k according to the eleventh embodiment. In this plasma jet plug 100k, the center electrode 20k has a head 21 on the most rear end side, a leg 22 having a smaller outer diameter than the head 21 and a position on the most tip side. However, it has a tip small diameter portion 27 having the smallest outer diameter. Since the other structure of the plasma jet plug 100k is almost the same as that shown in FIG. 2, the description thereof is omitted here.

FIG. 21 is an enlarged view of the cross section of the tip portion of the plasma jet plug 100k of the eleventh embodiment. 21 is upside down with respect to FIGS. 1 and 20, the upper side of FIG. 21 is the front end side of the plasma jet plug 100k, and the lower side of FIG. 21 is the rear end side of the plasma jet plug 100k.

As described above, the leg portion 22 and the tip small diameter portion 27 are formed in the vicinity of the tip of the center electrode 20k. Each of the leg portion 22 and the tip small diameter portion 27 has a cylindrical shape. A reduced diameter portion 28 is provided between the leg portion 22 and the distal end small diameter portion 27. Although the reduced diameter portion 28 is formed in a tapered shape in the example of FIG. 21, the reduced diameter portion 28 may be formed in a plane perpendicular to the axis 0 instead of the tapered shape.

An enlarged inner diameter portion 16 having an inner diameter larger than that of the leg length portion 13 is formed in the leg length portion 13 near the tip of the insulator 10. The long leg portion 13 is also referred to as a “small inner diameter portion 13”. A reduced diameter portion 14 is formed between the leg length portion 13 and the enlarged inner diameter portion 16. In this example, the reduced diameter portion 14 is formed in a tapered shape, but it may be formed in a surface perpendicular to the axis 0 instead of the tapered shape. The reduced diameter portion 14 of the insulator 10 is provided on the distal end side with respect to the reduced diameter portion 28 of the center electrode 20k. The outer periphery of the tip small-diameter portion 27 of the center electrode 20k and the inner surface of the leg long portion 13 of the insulator 10 are separated by a distance Dp. The annular groove having a width of the distance Dp corresponds to a second cavity CV2 described below.

The cavity CV is a space surrounded by the surface 20s of the center electrode 20k, the inner surface 10in of the insulator 10, and the inner surface 30in of the orifice electrode 30. However, the cavity CV does not include the portion of the through hole 31 of the orifice electrode 30, and means a space inside the inner surface 30in of the orifice electrode 30 when it is assumed that there is no through hole 31. Note that a minute gap (less than 0.06 mm) is formed between the outer peripheral surface of the leg portion 22 of the center electrode 20k and the inner surface of the insulator 10 for assembly of both. A space with a gap of less than 0.06 mm is very small and plasma is not generated, so it does not function as a part of the cavity CV. In this specification, the “cavity” means a space where plasma is generated, and means a space having a gap of 0.06 mm or more. More specifically, the “cavity” in the eleventh embodiment of FIG. 21 is defined between the inner surface 10 in of the tip portion of the insulator 10, the surface of the tip portion of the center electrode 20 k, and the inner surface 30 in of the orifice electrode 30. Means a space with a gap of 0.06 mm or more, and does not include a space with a gap of less than 0.06 mm. The cavity CV can be classified into the following two.
(A) 1st cavity part CV1: The cavity part which exists in the front end side rather than the rear end 14e of the diameter reducing part 14 of the insulator 10.
(B) 2nd cavity part CV2: The cavity part which exists in the rear end side rather than the rear end 14e of the diameter reducing part 14 of the insulator 10.

FIG. 21 further shows the following dimensions.
(1) Dp: A distance between the outer periphery of the tip small-diameter portion 27 of the center electrode 20k and the leg long portion 13 of the insulator 10 (referred to as “radial direction spatial distance Dp”). The radial space distance Dp corresponds to the width of the second cavity portion CV2.
(2) Dq: distance between the rear end 28e of the reduced diameter portion 28 of the center electrode 20k and the rear end 14e of the reduced diameter portion 14 of the insulator 10. This distance Dq corresponds to the axial depth of the second cavity CV2.
(3) Dr: the shortest distance between the tip edge 20c of the center electrode 20k and the inner surface 10in of the insulator 10. The “shortest distance” means the minimum value when the distance from the tip edge 20c of the center electrode 20k to the inner surface 10in of the insulator 10 is measured in an arbitrary direction.
(4) Ds: difference between the inner radius of the enlarged inner diameter portion 16 of the insulator 10 and the inner radius of the leg length portion 13. This difference Ds corresponds to the difference between the inner radius of the enlarged inner diameter portion 16 of the insulator 10 and the outer radius of the leg portion 22 of the center electrode 20k.
(5) D27: The outer diameter of the tip small diameter portion 27 of the center electrode 20k.
(6) D22: The outer diameter of the leg portion 22 of the center electrode 20k.
(7) E: Inner diameter of the through hole 31 of the orifice electrode 30.
(8) G: A distance in the axial direction between the inner surface 30in of the orifice electrode 30 and the tip surface 20t of the center electrode 20k. This distance G is also referred to as “air gap G”.
(9) Z: Distance between the inner surface 30 in of the orifice electrode 30 and the rear end 14 e of the reduced diameter portion 14 of the insulator 10. This distance Z corresponds to the axial depth of the first cavity CV1.

It should be noted that the inner diameter E of the through hole 31 of the orifice electrode 30 is preferably smaller than the outer diameter D27 of the tip small diameter portion 27 of the center electrode 20k. This is to facilitate air discharge in the air gap G.

FIG. 22 is an enlarged view of a cross section of the tip portion of the plasma jet plug 100m in the twelfth embodiment. The center electrode 20m of the plasma jet plug 100m does not have the tip small-diameter portion 27 that the center electrode 20k of the plasma jet plug 100k in FIG. 21 has, and the leg portion 22 is extended to the tip as it is. Have. Therefore, the second cavity CV2 existing in the plasma jet plug 100k of FIG. 21 does not exist in the plasma jet plug 100m of FIG.

Even in the plasma jet plug 100m that does not have the second cavity portion CV2 as shown in FIG. 22, the shortest distance Dr between the tip edge 20c of the center electrode 20m and the inner surface 10in of the insulator 10 is sufficiently large. Creeping discharge hardly occurs and air discharge can be stably performed. However, if the second cavity portion CV2 is provided as shown in FIG. 21, the shortest distance Dr between the tip edge 20c of the center electrode 20m and the inner surface 10in of the insulator 10 can be increased, so that creeping discharge occurs. In addition, the volume of the entire cavity CV can be suppressed to be small, and plasma can be easily ejected.

Hereinafter, several test results performed using the dimensions related to the plasma jet plug shown in FIGS. 21 and 22 as parameters will be sequentially described.

FIG. 23 shows the result of the discharge path confirmation test regarding the relationship between the shortest distance Dr between the tip edge 20c of the center electrode 20n and the inner surface 10in of the insulator 10 and the air gap G. Here, FIG. 23A shows a longitudinal sectional view of a plasma jet plug 100n for a discharge path confirmation test, and FIG. 23B shows a plan view thereof. The plasma jet plug 100n has a configuration in which the orifice electrode 30 of the plasma jet plug 100m without the second cavity portion CV2 shown in FIG. 22 is replaced with a rod-shaped electrode 30bar. This is because it is difficult to photograph the inside of the cavity CV from the through hole 31 (FIG. 22) of the orifice electrode 30. In the discharge path confirmation test, the plasma jet plug 100n was attached in the pressure chamber, and discharge was performed 100 times in a state where the pressure chamber was pressurized to 1.0 MPa (atmosphere). At this time, the discharge path in the cavity CV was photographed using a high-speed camera, and the ratio of the number of occurrences of creeping discharge out of 100 discharges was measured.

FIG. 23C shows samples S101 to S104 in which the value of the ratio Dr / G between the shortest distance Dr between the tip edge 20c of the center electrode 20n and the inner surface 10in of the insulator 10 and the air gap G is used as a parameter. Various dimensions are shown. Since these samples S101 to S104 do not have the second cavity portion CV2, the dimensions Dp and Dq related to the second cavity portion CV2 are zero, and Dr = Ds. In this test, four samples S101 to S104 were used in which the air gap G was constant at 0.5 mm and the value of the shortest distance Dr was changed in the range of 0.25 mm to 1.00 mm.

FIG. 23 (D) shows the creeping discharge ratio obtained in the discharge path confirmation test. According to this test result, the creeping discharge rate decreased as the value of Dr / G increased, and when Dr / G was 1.5 or more, the creeping discharge did not occur and all became air discharge. Considering this result, the value Dr / G of the ratio of the shortest distance Dr to the air gap G is preferably as large as possible, and it is particularly preferable that the following relationship is satisfied.
1.5 × G ≦ Dr (1)
If this equation (1) is satisfied, the shortest distance Dr between the tip edge 20c of the center electrode 20n and the inner surface 10in of the insulator 10 is sufficiently larger than the air gap G, so that creeping discharge occurs. It becomes difficult, and air discharge can be performed stably. As a result, the occurrence of channeling can be suppressed.

The relationship of the above formula (1) is not limited to the plasma jet plug 100m without the second cavity portion CV2 as shown in FIG. 22, but the plasma jet plug with the second cavity portion CV2 as shown in FIG. It is estimated that the same applies to 100k. The reason for this is that even when the second cavity portion CV2 is present, if the above formula (1) is satisfied, the shortest distance Dr becomes sufficiently larger than the air gap G, and therefore it is difficult for creeping discharges to occur and is stable. This is because air discharge is expected to occur.

By the way, it is preferable to set the value of Dr so as to satisfy the above formula (1) from the viewpoint of facilitating the generation of air discharge rather than creeping discharge. On the other hand, the value of Dr is the value of the cavity CV. It is preferable to keep the volume within a range that does not become excessively large. This is because if the volume of the cavity CV becomes excessively large, the plasma ejection performance may deteriorate. In this sense, the value of Dr is, for example, preferably 2 mm or less, more preferably 1.5 mm or less, and most preferably 1 mm or less.

FIG. 24 is an explanatory diagram showing a discharge test result with respect to the radial space distance Dp of the second cavity portion CV2. FIG. 24A is a schematic plan view of the test apparatus, and FIG. 24B is a cross-sectional view taken along the line BB. In this test, the first electrode 210 is installed in the pressure chamber 300, the rectangular insulator 220 is fitted into the recess on the upper surface of the first electrode 210, and the cylindrical second electrode 230 is placed on the insulator 220. installed. At one end of the first electrode 210, a wall portion 212 rising vertically upward was formed, and a spatial distance Dp was set between the wall portion 212 and the insulator 220. In addition, the creepage distance on the insulator 220 in the creeping path from the side surface of the second electrode 230 toward the wall portion 212 of the first electrode 210 was set to 0.5 mm. Furthermore, the distance Dq between the upper surface of the first electrode 210 and the upper surface of the insulator 220 was changed by changing the thickness of the insulator 220 to several values. Further, the spatial distance Dp was adjusted so that the distance Dq and the spatial distance Dp were equal. The wall portion 212 of the first electrode 210 simulates the center electrode 20k in FIG. 21, and the groove portion GV between the wall portion 212 of the first electrode 210 and the insulator 220 forms the second cavity portion CV2 in FIG. Mock up. That is, the spatial distance Dp in FIG. 24 (B) simulates the radial spatial distance Dp of the second cavity portion CV2 (FIG. 21), and the distance Dq in FIG. 24 (B) is equal to the second cavity portion CV2. Depth Dq is simulated.

In this discharge test, discharge was performed 100 times in a state where the pressure chamber was pressurized to 0.2 MPa, 0.6 MPa, and 1.0 MPa (all in the atmosphere). And the discharge path | route was image | photographed using the high speed camera, and the ratio of the frequency | count that the air discharge generate | occur | produced among 100 discharges was measured. Here, “air discharge” means discharge that does not pass along the creeping path along the surface of the insulator 220, and “creeping discharge” means discharge that passes along the creeping path along the surface of the insulator 220. .

FIG. 24 (C) shows the relationship between the spatial distance Dp and the air discharge rate. According to this test result, the air discharge rate decreased as the value of the space distance Dp increased, and when the space distance Dp was 0.1 mm or more, no air discharge was generated, and all were creeping discharges. This result can be understood as follows. That is, when the spatial distance Dp in FIG. 24B increases, an air discharge that reaches the second electrode 230 in the lateral direction from the surface of the wall portion 212 of the first electrode 210 through the air becomes difficult to occur. When this is applied to the plasma jet plug 100k of FIG. 21, when the radial space distance Dp of the second cavity portion CV2 increases, the groove portion of the second cavity portion CV2 jumps from the side surface of the center electrode 20k through the air. After that, it can be presumed that the discharge hardly occurs along the creeping path on the inner surface of the insulator 10. Therefore, in order to suppress creeping discharge in the plasma jet plug 100k having the second cavity portion CV2 as shown in FIG. 21, it is preferable to increase the radial space distance Dp of the second cavity portion CV2, particularly 0.1 mm. The above is preferable. If it carries out like this, generation | occurrence | production of creeping discharge can be suppressed and air discharge can be performed stably.

By the way, in terms of facilitating the generation of air discharge rather than creeping discharge, the radial space distance Dp of the second cavity portion CV2 is preferably set to 0.1 mm or more. On the other hand, the value of Dp is It is preferable that the volume of the second cavity portion CV2 is within a range that does not become excessively large. In this sense, the value of Dp is preferably, for example, 1 mm or less, more preferably 0.7 mm or less, and most preferably 0.5 mm or less.

FIG. 25 shows the test results for the ratio Dq / Dp between the depth Dq of the second cavity CV2 and the radial space distance Dp. In this test, a plasma jet plug 100k was attached in the pressure chamber, and discharge was performed 100 times in a state where the pressure chamber was pressurized to 1.0 MPa (atmosphere), and the discharge voltage was measured. Here, the “discharge voltage” means a voltage when a dielectric breakdown occurs by applying a high voltage.

FIG. 25 (A) shows the dimensions of samples S201 to S216. Sample S201 is a plug having the shape of FIG. 22 without the second cavity CV2. Samples S202 to S206 are samples in which the radial space distance Dp of the second cavity part CV2 is set to a constant value of 0.1 mm and the depth Dq of the second cavity part CV2 is changed. Samples S207 to S211 are samples in which the radial space distance Dp of the second cavity part CV2 is set to a constant value of 0.3 mm and the depth Dq of the second cavity part CV2 is changed. Samples S212 to S215 are samples in which the radial space distance Dp of the second cavity part CV2 is set to a constant value of 0.5 mm, and the depth Dq of the second cavity part CV2 is changed. The difference in the radial space distance Dp among the three sample groups S202 to S206, S207 to S211, and S212 to S216 was adjusted by changing the outer diameter D27 of the tip small diameter portion 27 of the center electrode 20k. In this test, in all the samples S201 to S216, the inner diameter E of the through hole 31 of the orifice electrode 30 was set to 2.5 mm, which was an excessive value compared to a normal value (about 1.0 mm). The reason for this is to ensure that a creeping discharge occurs without causing an air discharge from the center electrode 20k to the orifice electrode 30.

FIG. 25B shows the relationship between the value of Dq / Dp and the discharge voltage. As can be understood from this result, the discharge voltage tends to increase as the value of Dq / Dp increases. Note that, as described above, the test sample has a shape in which creeping discharge is always generated without generating an air discharge from the center electrode 20k to the orifice electrode 30, and therefore, the discharge voltage in FIG. 25B is high. In fact, in the actual plasma jet plug 100k, air discharge from the center electrode 20k to the orifice electrode 30 is likely to occur. Therefore, a higher discharge voltage in this test is preferable because air discharge is likely to occur and creeping discharge is less likely to occur. Specifically, the value of the ratio Dq / Dp between the depth Dq of the second cavity part CV2 and the radial space distance Dp exceeds 0 (that is, the second cavity part CV2 exists). In addition, even if the value of Dq / Dp becomes 3 or more, the discharge voltage does not increase any more, so that the value of Dq / Dp is 3 or less.

FIG. 26 shows the result of the plasma ejection test for the ratio Dq / Dp of the depth Dq of the second cavity CV2 and the radial space distance Dp. In this test, the plasma jet plug 100k was discharged in a state where the pressure chamber was pressurized to 0.6 MPa (atmosphere), and the plasma ejected from the through hole 31 of the orifice electrode 30 was photographed from the side to obtain a Schlieren image. I got it. Then, the Schlieren image was binarized and classified into pixels representing a high-density portion and pixels representing a low-density portion, and the number of pixels representing the high-density portion was calculated as the size of the ejected plasma. In addition, 10 schlieren imaging | photography was performed for every sample, and the average value of the pixel number of the plasma calculated by 10 imaging | photography was made into the ejection area.

FIG. 26 (A) shows the dimensions of samples S302 to S316. The dimensions of the samples S302 to S316 are the same as the samples S202 to S216 shown in FIG. 25A except that the inner diameter E of the through hole 31 of the orifice electrode 30 is 1.0 mm (normal value). The reason why the inner diameter E of the through hole 31 of the orifice electrode 30 is 1.0 mm in the samples S302 to S316 in FIG. 26A is that the air discharge is caused by the air gap G between the center electrode 20k and the orifice electrode 30. To generate.

FIG. 26B shows the relationship between the value of Dq / Dp and the plasma ejection area. As can be understood from the test results, the plasma ejection area tends to decrease as the value of Dq / Dp increases. Therefore, from the result of FIG. 26B, it is preferable that the value of the ratio Dq / Dp between the depth Dq of the second cavity portion CV2 and the radial space distance Dp is small. However, even if the value of Dq / Dp is smaller than 3, the plasma ejection area does not increase so much, so it is sufficient that the value of Dq / Dp is 3 or less.

Considering the results of FIGS. 25 and 26 described above, it is preferable that the radial space distance Dp and the depth Dq of the second cavity portion CV2 satisfy the following relationship.
0 <Dq ≦ 3 × Dp (2)
If the depth Dq of the second cavity portion CV2 is set within the range of the expression (2), the tendency of generating air discharge more easily than creeping discharge can be increased (FIG. 25B). In addition, it is possible to prevent the volume of the second cavity portion from becoming excessively large, and to facilitate the ejection of plasma (FIG. 26B).

FIG. 27 shows the plasma ejection test for the ratio Dp / Dr between the radial space distance Dp of the second cavity CV2 and the shortest distance Dr between the tip edge 20c of the center electrode 20k and the inner surface 10in of the insulator 10. Results are shown. This plasma ejection test was performed under the same conditions as in FIG. 26 except for the shape of the sample.

FIG. 27 (A) shows the dimensions of samples S401 to S405. In these samples S401 to S405, the radial distance Dp of the second cavity portion CV2 was changed by changing the outer diameter D22 of the leg portion 22 of the center electrode 20k. However, the inner radius and leg length of the enlarged inner diameter portion 16 of the insulator 10 are set so that the shortest distance Dr between the tip edge 20c of the center electrode 20k and the inner surface 10in of the insulator 10 becomes a constant value (1.0 mm). The difference Ds from the inner radius of 13 was adjusted.

FIG. 27 (B) shows the relationship between the Dp / Dr value and the plasma ejection area. As can be understood from this result, the plasma ejection area tends to decrease as the value of Dp / Dr increases. Therefore, from the result of FIG. 27B, it is preferable that the value of Dp / Dr is small. However, even if the value of Dq / Dp is smaller than 0.5, the plasma ejection area does not increase any more, so it is sufficient that the value of Dq / Dp is 0.5 or less. In the structure of FIG. 21, Dp is the distance from the side surface (outer peripheral surface) of the center electrode 20k to the wall surface of the insulator 10 constituting the outer periphery of the second cavity portion CV2. Dr is the shortest distance from the tip edge 20c of the center electrode 20k to the inner surface 10in of the insulator 10 constituting the outer periphery of the first cavity portion CV1. The result of FIG. 27B shows that when the value of the ratio Dp / Dr of these distances exceeds 0.5, the plasma easily spreads to the back of the second cavity portion CV2, so that the through hole 31 of the orifice electrode 30 It can be understood that the jet power at which plasma is ejected to the outside decreases.

Considering the test result of FIG. 27B, the relationship between the radial space distance Dp of the second cavity portion CV2 and the shortest distance Dr between the tip edge 20c of the center electrode 20k and the inner surface 10in of the insulator 10 is as follows. The following relationship is preferably satisfied.
(Dp / Dr) ≦ 0.5 (3)
If Dp / Dr is set so as to satisfy this equation (3), it is possible to make it easier to eject plasma.

Modification Examples The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the scope of the invention.

・ Modification 1:
The structural features described in FIGS. 4 to 19 and the structural features described in FIGS. 21 to 27 can be employed at the same time, or can be employed individually. .

Modification 2
As the configuration of the plasma jet plug, various configurations other than the configurations shown in FIGS. 4 to 9, 14 to 17, and 21 to 22 can be adopted. For example, the shape near the tip of the center electrode 20 may not be a simple cylindrical shape, but may be provided with irregularities on the surface.

Further, the tip of the center electrode 20 is not an acute edge, and may be chamfered such as R chamfering or C chamfering. In this way, since electric field concentration is unlikely to occur, it is possible to further suppress the consumption of the center electrode 20 due to the heat of the plasma.

DESCRIPTION OF SYMBOLS 4 ... Sealing body 5 ... Gasket 6 ... Ring member 9 ... Talc 10 ... Insulator 10z ... Reduced inner diameter part of the insulator 10in ... Inner surface of the insulator 12 ... Shaft hole of the insulator 13 ... Leg long part of the insulator (small inner diameter part) )
13c, 13d, 13e ... first member 14 ... reduced diameter portion of insulator 15 ... electrode housing portion of insulator 16 ... enlarged inner diameter portion of insulator 16c-16j ... second member of insulator 16p ... opening of tip of insulator Part 17: Insulator front end side body part 18 ... Insulator rear end side body part 19 ... Insulator flange 20, 20k to 20n ... Center electrode 20f ... Side face of center electrode 20s ... Surface of center electrode 20t ... Center Electrode tip surface 21 ... Head of center electrode 22 ... Leg portion of center electrode 30 ... Orifice electrode 30in ... Inner surface of orifice electrode 31 ... Through hole of orifice electrode 32 ... Exposed surface of orifice electrode 32e ... Exposed surface of orifice electrode Outermost peripheral position 40 ... Terminal fitting 50 ... Metal fitting 51 ... Tool engagement portion 52 of metal fitting 52 ... Threaded portion of metal fitting 53 ... Clamping portion 54 of metal fitting 54 ... Hard part of metal fitting 55 ... Seat of metal shell 56 ... Locking portion of metal shell 57 ... Tip portion of metal shell 57A ... Concavity at tip of metal shell 80 ... Packing 100, 100a to 100n, 100r ... Plasma jet plug 120 ... Ignition device 130 ... Control circuit section 140 ... Spark discharge circuit section 160 ... Plasma discharge circuit section 161 ... High voltage generation circuit 162 ... Capacitor 210 ... Insulator 210s ... Upper surface of the insulator 212 ... Insulator groove 220 ... First electrode 230 ... Second electrode

Claims (15)

1. A cylindrical insulator having an axial hole extending along the axial direction, a central electrode disposed inside the axial hole, a metal shell disposed on the outer periphery of the insulator, and the metal shell electrically A plasma jet plug having a plasma generating cavity formed by the surface of the central electrode, the inner surface of the insulator, and the inner surface of the orifice electrode. In
   In the cavity, the shortest path length D1 of the creeping path from the surface of the center electrode through the inner surface of the insulator to the inner surface of the orifice electrode is the shortest distance between the center electrode and the orifice electrode. A plasma jet plug characterized by being 5 times or more the air gap G.
2. The plasma jet plug according to claim 1,
   The inner surface of the insulator has one or more grooves forming a concave path in the creeping path;
   A plasma jet plug, wherein the groove has a groove width of 0.1 mm or more.
3. The plasma jet plug according to claim 2,
   The plasma jet plug according to claim 1, wherein a depth of the groove portion is three times or less of the groove width.
4. A plasma jet plug according to any one of claims 1 to 3,
   The plasma jet plug according to claim 1, wherein a surface area of a side surface of the center electrode facing the cavity is 20 mm ² or less.
5. The plasma jet plug according to any one of claims 2 to 4,
   The plasma jet plug, wherein the insulator facing the cavity is composed of a plurality of members.
6. The plasma jet plug according to claim 5,
   The plurality of members of the insulator include a first member provided on the outer peripheral side of the center electrode and a second member provided on the outer peripheral side of the first member;
   The first member is made of a first insulating material having a higher thermal conductivity than the second member, and the second member is made of a second insulating material having a higher withstand voltage than the first member. A plasma jet plug characterized by being formed.
7. The plasma jet plug according to any one of claims 1 to 6,
   A side surface of the central electrode in the cavity is covered with an insulating material;
   The plasma jet plug according to claim 1, wherein a distance L from a tip of the insulating material provided on a side surface of the center electrode to a tip of the center electrode is 0.4 mm or less.
8. The plasma jet plug according to claim 7,
   A plasma jet plug, wherein a distance H between a side surface of the center electrode and an inner wall surface of the cavity when measured along a direction perpendicular to the axial direction is larger than the air gap G.
9. The plasma jet plug according to claim 7 or 8,
   The inner surface of the orifice electrode around the through hole of the orifice electrode is covered with an insulating material leaving an exposed surface adjacent to the through hole;
   A plasma jet plug characterized in that a distance J between an outermost peripheral position of the exposed surface and a side surface of the central electrode when measured along a direction perpendicular to the axial direction is smaller than the distance H.
10. The plasma jet plug according to claim 9, wherein
    A plasma jet plug characterized in that a distance K between the outermost peripheral position of the exposed surface and the tip of the center electrode is larger than the air gap G.
11. A plasma jet plug according to any one of claims 1 to 10,
    The plasma jet plug characterized in that the relationship between the air gap G and the shortest distance Dr between the leading edge of the center electrode and the inner surface of the insulator satisfies 1.5 × G ≦ Dr.
12. The plasma jet plug according to claim 11,
    The inner surface of the insulator facing the cavity has a reduced diameter portion provided such that the inner surface of the insulator is reduced in diameter toward the rear end side of the insulator,
    The cavity has a first cavity portion on the front end side of the rear end of the reduced diameter portion of the insulator, and a second cavity portion on the rear end side of the rear end of the reduced diameter portion. Plasma jet plug to be used.
13. A plasma jet plug according to claim 12,
    A radial space distance Dp, which is a distance measured in a radial direction perpendicular to the axial direction between the surface of the center electrode and the inner surface of the insulator in the second cavity portion, is 0.1 mm or more. Characteristic plasma jet plug.
14. The plasma jet plug according to claim 13,
    A plasma jet plug characterized in that a depth Dq of the second cavity portion measured along the axial direction satisfies 0 <Dq ≦ 3 × Dp.
15. The plasma jet plug according to claim 14,
    The relationship between the radial space distance Dp of the second cavity portion and the shortest distance Dr between the tip edge of the center electrode and the inner surface of the insulator satisfies Dp / Dr ≦ 0.5. Characteristic plasma jet plug.

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