JP6006658B2 - Plasma jet ignition plug and ignition system - Google Patents

Description

  The present invention relates to a plasma jet ignition plug that ignites an air-fuel mixture or the like by generating plasma, and an ignition system having a plasma jet ignition plug.

  Conventionally, in an internal combustion engine or the like, an ignition plug that ignites an air-fuel mixture or the like by spark discharge is used. In recent years, in order to meet the demand for higher output and lower fuel consumption of internal combustion engines, as an ignition plug that can be ignited more reliably even with a lean mixture with a fast combustion spread and a higher ignition limit air-fuel ratio. Plasma jet spark plugs have been proposed.

  Generally, a plasma jet ignition plug is disposed on a cylindrical insulator having a shaft hole, a center electrode inserted into the shaft hole with the tip surface being more immersed than the tip surface of the insulator, and an outer periphery of the insulator. And a ring-shaped ground electrode joined to the tip of the metal shell. Further, the plasma jet ignition plug has a space (cavity part) formed by the front end side surface of the center electrode and the inner peripheral surface of the insulator, and the cavity part passes through a through hole formed in the ground electrode. To communicate with the outside.

  In such a plasma jet ignition plug, the air-fuel mixture or the like is ignited as follows. First, a voltage is applied between the center electrode and the ground electrode to cause a spark discharge between the two electrodes, thereby causing a dielectric breakdown between the two electrodes. In addition, a discharge state is transitioned by applying power between both electrodes, and plasma is generated inside the cavity portion. Then, the generated plasma is ejected from the cavity through the through hole, so that the air-fuel mixture is ignited.

  By the way, as a method for realizing even better ignitability, it is conceivable to generate a larger plasma by increasing the electric power input after the spark discharge. However, when a large amount of electric power is applied, the center electrode tends to be consumed, and the voltage (discharge voltage) necessary for spark discharge may increase rapidly.

  Therefore, by providing a step on the inner peripheral surface of the insulator, or by providing a reduced diameter part that reduces the diameter toward the tip side, an aperture is provided in the cavity part, which is excellent even with relatively small input power. A method for realizing the ignitability has been proposed (see, for example, Patent Document 1).

WO2008 / 156035A1

  However, with the spark discharge, a phenomenon (so-called channeling) occurs in which the insulator located on the spark discharge generation path is scraped. In the above method, the spark discharge is generated in a state in which the spark discharge is pressed against the inner peripheral surface of the insulator, so that the insulator is easily cut. Furthermore, among the paths connecting the center electrode and the ground electrode, the path that passes through the part where the insulator has been cut is shorter than the other paths, so that a spark discharge is concentrated on that path, resulting in local channeling. Concentrating. As a result, the insulator is deeply cut in a streak shape, and a groove connecting the rear electrode side surface (the surface facing the tip of the insulator) and the center electrode is formed on the inner periphery of the insulator. It may be formed on the surface. Even if a spark discharge is generated along the groove to generate plasma, the presence of the ground electrode makes it difficult for the plasma to be ejected to the outside of the cavity portion. That is, according to the technique described in Patent Document 1, although excellent ignitability can be realized in the initial stage, there is a risk that the ignitability may rapidly decrease with use.

  The present invention has been made in view of the above circumstances, and its purpose is to maintain excellent ignitability over a long period of time by suppressing rapid progress of channeling while improving ignitability. It is an object of the present invention to provide a plasma jet spark plug and an ignition system that can be used.

  Hereinafter, each configuration suitable for solving the above-described object will be described in terms of items. In addition, the effect specific to the corresponding structure is added as needed.

Configuration 1. The plasma jet ignition plug of this configuration includes an insulator having an axial hole extending in the axial direction;
A center electrode inserted into the shaft hole so that a tip surface is located on the rear end side in the axial direction from the tip of the insulator;
A metal shell disposed on the outer periphery of the insulator;
A ground electrode fixed to the tip of the metal shell,
A cavity portion formed by the inner peripheral surface of the insulator and the tip-side surface of the center electrode;
A plasma jet ignition plug comprising a through hole formed in the ground electrode and communicating with the cavity and the outside,
While causing a spark discharge between the center electrode to which a positive voltage is applied and the ground electrode,
The shaft hole has a large-diameter portion in which the tip portion of the center electrode is disposed, and a small-diameter portion that is located on the tip side of the large-diameter portion and whose inner diameter is smaller than the inner diameter of the large-diameter portion. And
The tip of the center electrode is spaced from the inner peripheral surface of the insulator, and is located on the rear end side in the axial direction from the rear end of the small diameter portion,
The distal end portion of the center electrode has a convex edge portion toward the cavity portion side, and a minimum distance from the central axis of the small diameter portion along the direction orthogonal to the axis to the edge portion is Smaller than the diameter of the small diameter part,
The inner diameter of the through hole is larger than the inner diameter of the small diameter portion,
A gap is provided between a portion of the ground electrode where the through hole is formed and the tip of the insulator.

  According to the configuration 1, the shaft hole includes the large-diameter portion where the tip portion of the center electrode is disposed, and the small-diameter portion that is located on the tip side of the large-diameter portion and whose inner diameter is smaller than the inner diameter of the large-diameter portion. And have. Accordingly, it is possible to increase the plasma ejection pressure toward the opening side (the front end side in the axial direction) of the cavity portion.

  Moreover, according to the said structure 1, the internal diameter of the through-hole is made larger than the internal diameter of a small diameter part. Accordingly, the plasma can be smoothly ejected from the through hole, and, as described above, the plasma ejection length from the cavity opening is extremely large in combination with the increase in the plasma ejection pressure. It can be. As a result, excellent ignitability can be realized.

  Furthermore, according to the said structure 1, while being configured so that a spark discharge is generated by applying a positive voltage to the center electrode, the tip of the center electrode has a minimum distance from the center axis of the small diameter portion. Is smaller than the diameter (radius) of the small diameter portion (that is, located on the inner peripheral side of the inner peripheral surface of the small diameter portion). Therefore, at the time of spark discharge, electrons fly (move) from the ground electrode toward the edge portion located on the inner peripheral side of the small diameter portion. In addition, according to the configuration 1, the gap is provided between the portion of the ground electrode where the through hole is formed and the tip of the insulator. Therefore, in the vicinity of the ground electrode, it is difficult for the electrons flying from the ground electrode to crawl on the tip end surface of the insulator or the inner peripheral surface on the tip end side of the insulator. Moreover, according to the said structure 1, the front-end | tip of a center electrode is spaced apart from the internal peripheral surface of an insulator, and is located in the rear end side rather than the rear end of a small diameter part. Therefore, electrons flying toward the edge portion are less likely to crawl the inner peripheral surface of the insulator.

  And since the above-mentioned operation effect acts synergistically, spark discharge is likely to occur in a route passing through the air in a wide range along the axial direction. Furthermore, even if a spark discharge occurs in a path including the path that crawls the inner peripheral surface of the insulator, electrons fly from the ground electrode toward the edge located on the inner peripheral side, so the spark discharge is It is unlikely to be pressed against the inner peripheral surface of the insulator. Therefore, it is possible to effectively prevent a situation in which the insulator is sharply scraped (the channeling progresses rapidly) due to the spark discharge. In addition, since the spark discharge occurs mainly in the air, the spark discharge path can be further dispersed. As a result, a situation in which only a portion of the insulator is deeply cut off is significantly less likely to occur. Therefore, even after the spark discharge is repeatedly performed, the spark discharge can be more reliably generated between the portion of the ground electrode where the through hole is formed and the center electrode. As a result, the above-described excellent ignitability can be maintained over a long period of time.

  Configuration 2. The plasma jet ignition plug of the present configuration is configured so that, in the above-described configuration 1, when the minimum distance from the axis to the edge along the direction orthogonal to the axis is a and the diameter of the small diameter is r, a ≦ It is 0.7r.

  According to the configuration 2, it is configured to satisfy a ≦ 0.7r, and the edge portion is provided at a position sufficiently separated from the inner peripheral surface of the small diameter portion along the direction orthogonal to the axis. . Therefore, the spark discharge that crawls the inner peripheral surface of the insulator is further less likely to occur, and the spark discharge is caused to pass through the air in a wider range along the axial direction. As a result, the progress rate of channeling can be further reduced, and excellent ignitability can be maintained over a longer period.

  Configuration 3. The plasma jet ignition plug of this configuration is characterized in that, in the above configuration 1 or 2, a distance along the axis line between the edge portion and the rear end of the small diameter portion is 0.2 mm or more and 0.8 mm or less. And

  Usually, the electric field strength is relatively high at the rear end of the edge portion and the small diameter portion. For this reason, when the two are excessively close to each other, spark discharge is likely to occur through a path connecting the two. If a spark discharge occurs along the path connecting the two, the spark discharge occurs along a path over the entire inner peripheral surface of the small-diameter portion along the axial direction (that is, channeling tends to progress). There is a risk of becoming.

  In this regard, according to the configuration 3, the distance between the edge portion and the rear end of the small diameter portion is 0.2 mm or more, and the edge portion and the rear end of the small diameter portion are sufficiently separated from each other. Has been. Therefore, it is possible to more reliably suppress the occurrence of spark discharge that passes through the path connecting the two. As a result, rapid progress of channeling can be prevented more reliably.

  Furthermore, when a voltage (discharge voltage) required to cause a spark discharge between the center electrode and the ground electrode is large, a path with a relatively small insulation resistance (that is, a path over the inner peripheral surface of the insulator) is included. In the path 3, spark discharge is likely to occur. According to the configuration 3, the distance between the edge portion and the rear end of the small diameter portion is set to 0.8 mm or less. Therefore, the discharge voltage can be made sufficiently small, and the occurrence of spark discharge in the shape of the inner peripheral surface of the insulator can be more effectively suppressed. As a result, the progress speed of channeling can be further reduced.

  Configuration 4. In the plasma jet ignition plug of this configuration, in any of the above configurations 1 to 3, the distance along the axis between the portion of the ground electrode where the through hole is formed and the tip of the insulator is 0. .2 mm or more and 0.5 mm or less.

  According to the configuration 4, the distance between the portion of the ground electrode where the through hole is formed and the tip of the insulator is 0.2 mm or more. Therefore, when electrons move from the ground electrode to the center electrode, the electrons easily move along a path passing through the air. As a result, spark discharge is more likely to occur in the path through the air, and the progress of channeling can be more effectively suppressed.

  Furthermore, according to the said structure 4, the distance between the site | part which forms a through-hole among ground electrodes and the front-end | tip part of an insulator shall be 0.5 mm or less. Therefore, the discharge voltage can be further reduced, and the occurrence of spark discharge that crawls the inner peripheral surface of the insulator can be more reliably suppressed.

  Configuration 5. In the plasma jet ignition plug of this configuration, 1/2 of the value obtained by subtracting the inner diameter of the small diameter portion from the inner diameter of the through hole is 0.2 mm or more and 0.5 mm or less in any of the above configurations 1 to 4. It is characterized by.

  According to the configuration 5, ½ of the value obtained by subtracting the inner diameter of the small diameter portion from the inner diameter of the through hole (that is, the radius difference between the through hole and the small diameter portion) is 0.2 mm or more. Therefore, plasma can be ejected more smoothly from the through hole, and the ignitability can be further improved.

  Moreover, according to the said structure 5, the said radius difference shall be 0.5 mm or less. Therefore, it is difficult to generate a spark discharge that hits the front end surface of the insulator, and as a result, the occurrence of a spark discharge that hits the inner peripheral surface of the insulator can be more reliably suppressed. As a result, the progress of channeling can be more effectively suppressed.

Configuration 6. The ignition system of this configuration includes an insulator having an axial hole extending in the axial direction;
A center electrode inserted into the shaft hole so that a tip surface is located on the rear end side in the axial direction from the tip of the insulator;
A metal shell disposed on the outer periphery of the insulator;
A ground electrode fixed to the tip of the metal shell,
A cavity portion formed by the inner peripheral surface of the insulator and the tip-side surface of the center electrode;
A plasma jet ignition plug provided with a through hole formed in the ground electrode and communicating with the cavity portion and the outside; and
An ignition system comprising a voltage application device for applying a voltage to the center electrode,
By applying a positive voltage from the voltage application device to the center electrode, a spark discharge is generated between the center electrode and the ground electrode,
The shaft hole has a large-diameter portion in which the tip portion of the center electrode is disposed, and a small-diameter portion that is located on the tip side of the large-diameter portion and whose inner diameter is smaller than the inner diameter of the large-diameter portion. And
The tip of the center electrode is spaced from the inner peripheral surface of the insulator, and is located on the rear end side in the axial direction from the rear end of the small diameter portion,
The distal end portion of the center electrode has a convex edge portion toward the cavity portion side, and a minimum distance from the central axis of the small diameter portion along the direction orthogonal to the axis to the edge portion is Smaller than the diameter of the small diameter part,
The inner diameter of the through hole is larger than the inner diameter of the small diameter portion,
A gap is provided between a portion of the ground electrode where the through hole is formed and the tip of the insulator.

  According to the configuration 6, the same effect as the configuration 1 is achieved.

It is a block diagram which shows schematic structure of an ignition system. It is a partially broken front view which shows the structure of a spark plug. It is an expanded sectional view which shows the structure of the front-end | tip part of a spark plug. It is an enlarged bottom view which shows the structure of the front-end | tip part of a spark plug. (A) is an expanded sectional view showing a path of a spark discharge when a negative voltage is applied to the center electrode, and (b) is a spark discharge when a negative voltage is applied to the center electrode. It is an expanded sectional view which shows the path | route of the spark discharge, the shape of an insulator, etc. after producing repeatedly. (A) is an expanded sectional view showing a path of a spark discharge when a positive voltage is applied to the center electrode, and (b) is a spark discharge when a positive voltage is applied to the center electrode. It is an expanded sectional view which shows the path | route of the spark discharge, the shape of an insulator, etc. after producing repeatedly. It is a graph which shows the result of the durability evaluation test in the negative polarity sample which applied the negative polarity voltage to the center electrode, and the positive polarity sample which applied the positive polarity voltage to the center electrode. It is a graph which shows the result of the durability evaluation test in the sample which changed a / r variously. It is a graph which shows the result of the durability evaluation test in the sample which changed various distance b. It is a graph which shows the result of the durability evaluation test in the sample which changed distance c variously. It is a graph which shows the result of the durability evaluation test in the sample which changed Rr variously. (A), (b) is an expanded sectional view which shows the structure of the edge part in another embodiment. It is an expanded sectional view showing composition of a tip part of a spark plug in another embodiment. It is an expanded sectional view showing composition of a convex part in another embodiment. It is a figure which shows the structure of the front-end | tip part of the ignition plug in another embodiment, (a) is an expanded sectional view, (b) is an enlarged bottom view.

  Hereinafter, an embodiment will be described with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of an ignition system 101 including a plasma jet ignition plug (hereinafter referred to as “ignition plug”) 1, a voltage application device 61, and a power input device 71. In FIG. 1, only one spark plug 1 is shown, but the internal combustion engine EN is provided with a plurality of cylinders, and the spark plugs 1 are provided corresponding to the respective cylinders. A voltage application device 61 and a power input device 71 are provided for each spark plug 1.

  As shown in FIG. 2, the spark plug 1 includes an insulator 2 as a cylindrical insulator, a cylindrical metal shell 3 that holds the insulator 2, and the like. In FIG. 2, the direction of the axis CL1 of the spark plug 1 is the vertical direction in the drawing, the lower side is the front end side of the spark plug 1, and the upper side is the rear end side.

  As is well known, the insulator 2 is formed by firing alumina or the like, and in its outer portion, a rear end side body portion 10 formed on the rear end side, and a front end than the rear end side body portion 10. A projecting portion 11 projecting radially outward on the side, a middle body portion 12 having a smaller diameter on the tip side than the projecting portion 11, and a tip side more than the middle body portion 12. The leg length part 13 formed more narrowly than this is provided. In addition, of the insulator 2, the protruding portion 11, the middle body portion 12, and the leg length portion 13 are accommodated inside the metal shell 3. A tapered step portion 14 is formed at the connecting portion between the middle body portion 12 and the long leg portion 13, and the insulator 2 is locked to the metal shell 3 at the step portion 14.

  Further, a shaft hole 4 extending in the direction of the axis CL <b> 1 is formed through the insulator 2, and a rod-shaped center electrode 5 is inserted on the tip side of the shaft hole 4. The center electrode 5 includes an inner layer 5A made of a metal having excellent thermal conductivity (for example, copper, copper alloy, pure nickel (Ni), etc.), and an alloy mainly composed of Ni (for example, Inconel (trade name) 600, 601 etc.]. Further, the center electrode 5 includes a cylindrical convex portion 5C formed of a metal having excellent wear resistance (for example, an alloy containing iridium as a main component) at the center of the tip portion thereof. In addition, the center electrode 5 has a rod shape (cylindrical shape) as a whole, and the front end surface thereof is located on the rear end side in the axis line CL1 direction with respect to the front end of the insulator 2.

  A terminal electrode 6 is inserted and fixed on the rear end side of the shaft hole 4 in a state of protruding from the rear end of the insulator 2.

  Further, a columnar glass seal portion 9 containing a conductive metal and glass is disposed between the center electrode 5 of the shaft hole 4 and the terminal electrode 6. The center electrode 5 and the terminal electrode 6 are electrically connected to each other by the glass seal portion 9 and are fixed to the insulator 2.

  In addition, the metal shell 3 is formed in a cylindrical shape from a metal such as low carbon steel, and a spark plug 1 is attached to the outer peripheral surface of the metal shell 3 (for example, an internal combustion engine or a fuel cell reformer). A threaded portion (male threaded portion) 15 for attachment to the hole is formed. A flange-shaped seat 16 is formed on the rear end side of the screw portion 15, and a ring-shaped gasket 18 is fitted on the screw neck 17 at the rear end of the screw portion 15. Further, on the rear end side of the metal shell 3, a tool engaging portion 19 having a hexagonal cross section for engaging a tool such as a wrench when the metal shell 3 is attached to the combustion device is provided. 1 is provided with a caulking portion 20 for holding the insulator 2. In addition, an annular engagement portion 21 that protrudes toward the distal end side in the axis CL1 direction is formed on the outer periphery of the distal end portion of the metal shell 3, and a ground electrode that will be described later with respect to the inner periphery of the engagement portion 21 27 is joined.

  A tapered step portion 22 for locking the insulator 2 is provided on the inner peripheral surface of the metal shell 3. The insulator 2 is inserted from the rear end side to the front end side of the metal shell 3, and the step 14 of the metal shell 3 is locked to the step 22 of the metal shell 3. It is fixed by caulking the rear end side opening portion radially inward, that is, by forming the caulking portion 20. An annular plate packing 23 is interposed between the step portions 14 and 22. Thereby, the airtightness in the combustion chamber is maintained, and the fuel gas that enters the gap between the leg long portion 13 of the insulator 2 and the inner peripheral surface of the metal shell 3 is prevented from leaking outside.

  Further, in order to make the sealing by caulking more complete, annular ring members 24 and 25 are interposed between the metal shell 3 and the insulator 2 on the rear end side of the metal shell 3, and the ring member 24. , 25 is filled with powder of talc (talc) 26. That is, the metal shell 3 holds the insulator 2 via the plate packing 23, the ring members 24 and 25, and the talc 26.

  In addition, a ground electrode 27 that is formed of an alloy containing Ni as a main component and has a disk shape is fixed to the tip of the metal shell 3. The ground electrode 27 is joined by welding its outer peripheral portion to the engaging portion 21 while being engaged with the engaging portion 21 of the metal shell 3. In addition, in the present embodiment, the entire ground electrode 27 is in a state slightly spaced from the tip surface of the insulator 2.

  Furthermore, a cavity 31 is formed at the tip of the insulator 2 by the inner peripheral surface of the insulator 2 and the tip-side surface of the center electrode 5 and is a space that opens toward the tip. The ground electrode 27 is formed with a cylindrical through hole 28 that penetrates the ground electrode 27 in the plate thickness direction, and the cavity portion 31 communicates with the outside through the through hole 28. In the present embodiment, the center axis of the through hole 28 is configured to coincide with the axis line CL1.

  In the spark plug 1, a voltage is applied to the center electrode 5 to cause a spark discharge between the center electrode 5 and the ground electrode 27, and then power is applied to both the electrodes 5 and 27. By changing the discharge state, plasma is generated in the cavity portion 31 and the plasma is ejected through the through hole 28. Then, next, the structure of the said voltage application apparatus 61 which applies a voltage to the center electrode 5, and the power input apparatus 71 which inputs electric power between both the electrodes 5 and 27 is demonstrated.

  As shown in FIG. 1, the voltage application device 61 is connected to the spark plug 1 (terminal electrode 6) via a backflow prevention diode 81, and includes a primary coil 62, a secondary coil 63, a core 64, and An igniter 65 is provided.

  The primary coil 62 is wound around the core 64, and one end of the primary coil 62 is connected to the power supply battery 66 and the other end is connected to the igniter 65. The secondary coil 63 is wound around the core 64, and one end thereof is connected between the primary coil 62 and the battery 66, and the other end is connected to the terminal electrode 6 of the spark plug 1. Yes.

  In addition, the igniter 65 is formed by a predetermined transistor, and switches between supply and stop of power supply from the battery 66 to the primary coil 62 in accordance with an energization signal input from a predetermined ECU (electronic control unit) 91. . When applying a voltage to the spark plug 1, a current is passed from the battery 66 to the primary coil 62, a magnetic field is formed around the core 64, and then the energization signal from the ECU 91 is switched from on to off, The current from the battery 66 to the primary coil 62 is stopped. When the current is stopped, the magnetic field of the core 64 changes, and a primary voltage is generated in the primary coil 62 by self-dielectric action, and a positive high voltage (for example, 5 kV to 30 kV) is generated in the secondary coil 63. By applying this positive high voltage to the center electrode 5 of the spark plug 1, a spark discharge can be generated between the electrodes 5 and 27. In addition, the timing which generate | occur | produces a spark discharge can be changed by adjusting the timing which turns off the said electricity supply signal. In the present embodiment, the voltage application device 61 is a full transistor method, but the voltage application device 61 is not limited to this, and may be, for example, a CDI ignition method.

  In addition, the power input device 71 includes a power source 72 and a capacitor 73.

  The power supply 72 is a power supply circuit capable of generating a positive high voltage (for example, 500 V to 1000 V), and is electrically connected to the spark plug 1 through a backflow preventing diode 82 and a current adjusting coil 83. Connected. In addition, one end of the capacitor 73 is grounded and the other end is connected to a power source 72, and the capacitor 73 is configured to be charged by the power source 72. Then, when a spark discharge occurs between the center electrode 5 and the ground electrode 27, and the dielectric breakdown occurs between the electrodes 5 and 27, the electric energy stored in the capacitor 73 is input to the spark plug 1 to generate plasma. It has come to be.

  Next, the configuration of the tip of the spark plug 1 will be described in detail with reference to FIG.

  As shown in FIG. 3, the shaft hole 4 has a large-diameter portion 4A in which the distal end portion of the center electrode 5 is disposed, and is positioned on the distal end side with respect to the large-diameter portion 4A. It has a small diameter portion 4B smaller than the inner diameter. In the present embodiment, the large diameter portion 4A and the small diameter portion 4B are configured such that the inner diameter is constant along the direction of the axis CL1. The small-diameter portion 4B is configured such that the central axis CL2 coincides with the axis CL1, and the small-diameter portion 4B and the through hole 28 are each configured with the axis CL1 as the central axis. Note that the inner diameters of the large diameter portion 4A and the small diameter portion 4B are not necessarily constant along the direction of the axis CL1. Therefore, for example, the large-diameter portion 4A or the like has a tapered shape toward the distal end side or a tapered shape toward the distal end side, and the inner peripheral surface of the large-diameter portion 4A or the like is slightly (for example, It may be inclined (about ± 5 °).

  In addition, the tip of the center electrode 5 is separated from the inner peripheral surface of the insulator 2 and is located on the rear end side in the axis line CL1 direction with respect to the rear end of the small diameter portion 4B.

  Moreover, the front-end | tip part of the center electrode 5 is formed between the front end surface and side surface of the convex part 5C, and has the edge part 32 convex toward the cavity part 31 side. Furthermore, the minimum distance a (mm) from the central axis CL2 of the small diameter portion 4B to the edge portion 32 along the direction orthogonal to the axis CL1 is configured to be smaller than the radius r (mm) of the small diameter portion 4B. ing. That is, as shown in FIG. 4, when the spark plug 1 is viewed from the front end side in the direction of the axis CL <b> 1, at least a part of the edge portion 32 (in this embodiment, on the inner peripheral side from the inner peripheral surface of the small diameter portion 4 </ b> B) The entire area of the edge portion 32 is present. In particular, in the present embodiment, it is configured to satisfy a ≦ 0.7r, and the edge portion 32 is provided at a position sufficiently separated from the inner peripheral surface of the small diameter portion 4B along the direction orthogonal to the axis CL1. ing.

  Further, as shown in FIGS. 3 and 4, the radius R (mm) of the through hole 28 is configured to be larger than the radius r of the small diameter portion 4 </ b> B. 1/2 [= R−r] of a value obtained by subtracting the inner diameter of the small diameter portion 4B from the inner diameter is configured to be 0.2 mm or more and 0.5 mm or less.

  Further, as described above, since the ground electrode 27 is disposed at a position slightly separated from the front end surface of the insulator 2, the portion 27 K of the ground electrode 27 that forms the through hole 28 and the front end portion of the insulator 2. There is a gap between them. In this embodiment, the distance c (mm) along the axis CL1 between the portion 27K and the tip of the insulator 2 is configured to be 0.2 mm or more and 0.5 mm or less.

  In addition, the distance b (mm) along the axis CL1 between the edge portion 32 and the rear end of the small diameter portion 4B is configured to be 0.2 mm or more and 0.8 mm or less.

  As described above in detail, according to the present embodiment, the shaft hole 4 has the large-diameter portion 4A and the small-diameter portion 4B whose inner diameter is smaller than the inner diameter of the large-diameter portion 4A. Therefore, it is possible to increase the plasma ejection pressure toward the opening side (the tip end side in the axial direction) of the cavity portion 31. Further, since the inner diameter of the through hole 28 is larger than the inner diameter of the small diameter portion 4B, plasma can be smoothly ejected from the through hole. As a result, as described above, combined with the increase in the plasma ejection pressure, the plasma ejection length from the cavity opening can be made very large, and excellent ignitability can be achieved. can do.

  Furthermore, in the present embodiment, a spark discharge is generated when a positive voltage is applied to the center electrode 5, and the tip of the center electrode 5 is separated from the center axis CL <b> 2 of the small diameter portion 4 </ b> B. An edge portion 32 is formed in which the minimum distance a is smaller than the diameter (radius) r of the small diameter portion 4B. Therefore, at the time of spark discharge, electrons fly (move) from the ground electrode 27 toward the edge portion 32 located on the inner peripheral side of the small diameter portion 4B. In addition, in this embodiment, a gap is provided between a portion 27 </ b> K of the ground electrode 27 where the through hole 28 is formed and the tip of the insulator 2. For this reason, in the vicinity of the ground electrode 27, it is difficult for the electrons flying from the ground electrode 27 to crawl the tip end surface of the insulator 2 and the tip side inner peripheral surface of the insulator. Further, since the tip of the center electrode 5 is separated from the inner peripheral surface of the insulator 2 and is located on the rear end side with respect to the rear end of the small diameter portion 4B, electrons flying toward the edge portion 32 are insulated. It becomes difficult to scratch the inner peripheral surface of the insulator 2.

  And by the above-mentioned action effect acting synergistically, spark discharge is likely to occur in a route passing through the air in a wide range along the direction of the axis CL1. Furthermore, even when spark discharge occurs in a path including a path that crawls the inner peripheral surface of the insulator 2, electrons fly from the ground electrode 27 toward the edge portion 32 located on the inner peripheral side. The spark discharge is unlikely to be pressed against the inner peripheral surface of the insulator 2. Therefore, the situation in which the insulator 2 is sharply scraped (the channeling progresses rapidly) with spark discharge can be extremely effectively prevented. In addition, since the spark discharge occurs mainly in the air, the spark discharge path can be further dispersed. As a result, a situation in which only a part of the insulator 2 is deeply cut is significantly less likely to occur. Therefore, even after the spark discharge is repeatedly performed, the spark discharge can be more reliably generated between the portion 27K of the ground electrode 27 where the through hole 28 is formed and the center electrode 5. As a result, the above-described excellent ignitability can be maintained over a long period of time.

  Further, in the present embodiment, a ≦ 0.7r is satisfied, and the edge portion 32 is provided at a position sufficiently separated from the inner peripheral surface of the small diameter portion 4B along the direction orthogonal to the axis line CL1. Therefore, the spark discharge that crawls the inner peripheral surface of the insulator 2 is further less likely to occur, and the spark discharge is caused to pass through the air in a wider range along the direction of the axis CL1. As a result, the progress rate of channeling can be further reduced, and excellent ignitability can be maintained over a longer period.

  In addition, since the distance b between the edge portion 32 and the rear end of the small diameter portion 4B is 0.2 mm or more, the spark discharge that passes through the path connecting the edge portion 32 and the rear end of the small diameter portion 4B is performed. Generation | occurrence | production can be suppressed more reliably. As a result, rapid progress of channeling can be prevented more reliably.

  Moreover, since the distance b is 0.8 mm or less, the discharge voltage can be made sufficiently small. As a result, it is possible to more effectively suppress the occurrence of spark discharge in the shape of the inner peripheral surface of the insulator 2 and further reduce the channeling progress rate.

  Furthermore, since the distance c is set to 0.2 mm or more, when electrons move from the ground electrode 27 to the center electrode 5, the electrons easily move along a path through the air. As a result, spark discharge is more likely to occur in the path through the air, and the progress of channeling can be more effectively suppressed.

  In addition, since the distance c is set to 0.5 mm or less, the discharge voltage can be further reduced, and the occurrence of spark discharge that crawls the inner peripheral surface of the insulator 2 can be more reliably suppressed.

  In addition, in this embodiment, ½ (= R−r) of a value obtained by subtracting the inner diameter of the small diameter portion 4B from the inner diameter of the through hole 28 is set to 0.2 mm or more. Therefore, plasma can be ejected more smoothly from the through hole 28, and the ignitability can be further improved.

  In addition, since Rr is 0.5 mm or less, it is difficult for spark discharge to crawl the tip surface of the insulator 2 and thus more reliably suppress the occurrence of spark discharge to crawl the inner peripheral surface of the insulator 2. can do. As a result, the progress of channeling can be more effectively suppressed.

  Next, in order to confirm the operational effects achieved by the above embodiment, a sample of an ignition system (negative sample) that generates a spark discharge by applying a negative high voltage to the center electrode, and a positive polarity on the center electrode An ignition system sample (positive polarity sample) that generates a spark discharge by applying a high voltage was prepared, and a durability evaluation test was performed on both samples. The outline of the durability evaluation test is as follows. That is, after attaching a sample spark plug to a predetermined chamber, the pressure in the chamber is set to 0.4 MPa, and the frequency is set to 60 Hz by a predetermined ignition method (for example, CDI method) (that is, 3600 per minute). A positive or negative voltage was applied to each sample (at this time) (At this time, only spark discharge was generated without applying power between the electrodes from the power application device). Then, after 100 hours have elapsed, a voltage is applied to the center electrode and power is supplied from the power input device, thereby ejecting plasma (frame) and imaging the ejected plasma (frame) from the outer peripheral side of the spark plug. . Next, by binarizing the obtained captured image, the plasma (frame) portion was specified, and the number of pixels (frame area) of the portion was measured. In addition, it can be said that the larger the frame area, the better the effect of suppressing channeling, and the better initial ignitability can be maintained over a longer period.

FIG. 7 shows the test results of the test. In both samples, the shaft hole, the center electrode, and the like satisfy the following configurations (1) to (5).
(1) The shaft hole has a large diameter portion and a small diameter portion.
(2) The front end of the center electrode is separated from the inner peripheral surface of the insulator and is positioned on the rear end side with respect to the rear end of the small diameter portion (that is, b> 0 is satisfied).
(3) An edge portion is provided at the tip of the center electrode, and the minimum distance a from the central axis of the small diameter portion to the edge portion is smaller than the radius r of the small diameter portion (that is, a <r is satisfied).
(4) The radius R of the through hole is larger than the radius r of the small diameter portion (that is, R> r is satisfied).
(5) A gap is provided between the portion of the ground electrode where the through hole is formed and the tip of the insulator (that is, c> 0 is satisfied).

  As shown in FIG. 7, it was found that the negative sample in which a negative voltage was applied to the center electrode had a relatively small frame area and channeling was likely to occur. This is considered to be due to the following reason.

  That is, as shown in FIG. 5 (a), when a negative voltage is applied to the center electrode, a spark discharge [in FIG. 5 (a), (b), the main spark discharge path is indicated by a bold line] Electrons move from the center electrode toward the ground electrode. At this time, since electrons move toward the ground electrode located on the outer peripheral side of the tip of the center electrode, spark discharge occurs in a state where it is pressed over the entire inner peripheral surface of the small diameter portion. The inner peripheral surface is more easily consumed. In addition, since the spark discharge is intensively generated along the path around the inner peripheral surface of the small diameter portion, the insulator is more easily consumed. As a result, the inner peripheral surface of the insulator is scraped in a short time. Then, after repeatedly performing the spark discharge, as shown in FIG. 5 (b) (in FIG. 5 (b), only the consumption of the insulator is shown, and the consumption of the center electrode and the ground electrode is not shown). In addition, the inner peripheral surface of the insulator is deeply cut in a part along the circumferential direction, and the spark discharge passes through this deeply cut portion to the rear end side surface of the ground electrode (on the front end surface of the insulator). It occurs between the opposing surface) and the center electrode. In this state, the generated plasma is difficult to be ejected from the through hole, and the frame area becomes extremely small. As a result, it is considered that the above test results were obtained.

  On the other hand, as shown in FIG. 7, the positive polarity sample in which the positive polarity voltage is applied to the center electrode has a remarkably large frame area, and has an excellent channeling suppression effect. It became. This is considered to be due to the following reason.

  That is, as shown in FIG. 6 (a), when a positive voltage is applied to the center electrode, spark discharge [the main spark discharge path is indicated by a bold line in FIGS. 6 (a) and (b)]. Electrons move from the ground electrode toward the center electrode. At this time, since electrons move toward the center electrode (edge part) located on the inner peripheral side of the ground electrode, the spark discharge is not a path that crawls the inner peripheral surface of the small diameter part, but in a wide range along the axial direction. It tends to occur on a route that passes through the air. In addition, even when a spark discharge occurs in a path including a path that crawls the inner peripheral surface of the small diameter portion, the spark discharge is not pressed against the inner peripheral surface of the small diameter portion. The peripheral surface is hard to wear out. In addition, since spark discharge occurs mainly in the air, the spark discharge path is more dispersed, and it is difficult to cause a situation in which only a part of the insulator is deeply shaved. As a result, even after repeated discharge of sparks, FIG. 6 (b) [In FIG. 6 (b), only consumption of the insulator is shown, and consumption of the center electrode and the ground electrode is not shown. As shown in FIG. 4, the consumption of the inner peripheral surface of the insulator is effectively suppressed, and spark discharge continues to occur between the portion of the ground electrode where the through hole is formed and the center electrode. In this state, the generated plasma is smoothly ejected from the through hole, so that the frame area is sufficiently large. As a result, it is considered that the above test results were obtained.

  From the results of the above test, from the viewpoint of improving channeling resistance and maintaining good ignitability over a long period of time, while satisfying the above-described configurations (1) to (5), the center electrode is positive. It can be said that it is preferable that a spark discharge is generated by applying a positive voltage.

  Next, after changing the radius r of the small diameter portion to 0.25 mm or 0.5 mm, the ratio (a / r) of the shortest distance a (mm) from the edge portion to the central axis of the small diameter portion with respect to the radius r was variously changed. Samples of the ignition system were prepared, and the durability evaluation test described above was performed on each sample.

  FIG. 8 shows the results of the test. In FIG. 8, the test results of the sample with the radius r of 0.25 mm are indicated by circles, and the test results of the sample with the radius r of 0.5 mm are indicated by triangles (hereinafter, in FIGS. 9 to 11). The same). In addition, each sample was configured such that a positive voltage was applied to the center electrode, and at least (1), (2), (4), (5) described above was satisfied.

  As shown in FIG. 8, a sample in which a / r is 0.7 or less, that is, a sample that satisfies a ≦ 0.7r has a very large frame area, and can effectively suppress channeling. confirmed. This is because the edge portion is provided at a position sufficiently separated from the inner peripheral surface of the small-diameter portion along the direction orthogonal to the axis, so that the spark discharge that crawls the inner peripheral surface of the insulator is less likely to occur. This is considered to be because the air passes through the air in a wider range along the axial direction.

  From the results of the above test, it can be said that it is more preferable to satisfy a ≦ 0.7r in order to further improve the channeling resistance.

  Next, after setting the radius r of the small-diameter portion to 0.25 mm or 0.5 mm, samples of the ignition system in which the distance b (mm) between the edge portion and the rear end of the small-diameter portion are variously changed are prepared. The sample was subjected to the durability evaluation test described above.

  FIG. 9 shows the results of the test. Each sample was configured such that a positive voltage was applied to the center electrode, and at least (1), (3) to (5) described above were satisfied.

As shown in FIG. 9, it was found that the sample having the distance b of 0.2 mm or more and 0.8 mm or less has a very large frame area and has further excellent channeling resistance. This is considered to be due to the following (a) and (b).
(A) Since the electric field strength is relatively high at both the edge portion and the rear end of the small-diameter portion, spark discharge is likely to occur in a route including a route connecting the two, but the distance b is set to 0.2 mm or more. By sufficiently separating the edge portion and the rear end of the small-diameter portion, spark discharge in the route including the route connecting the two is more reliably suppressed, and as a result, the spark in the route that crawls the inner peripheral surface of the insulator. Discharge was suppressed more reliably.
(B) When a voltage (discharge voltage) necessary for generating a spark discharge between the center electrode and the ground electrode is large, a path with a relatively low insulation resistance (that is, a path over the inner peripheral surface of the insulator) Spark discharge is likely to occur in the route including), but by setting the distance b to 0.8 mm or less, the discharge voltage becomes sufficiently small, and moreover spark discharge in the form of scooping the inner peripheral surface of the insulator It became difficult to occur.

  From the results of the above test, it can be said that the distance b is more preferably 0.2 mm or more and 0.8 mm or less in order to more reliably suppress channeling.

  Next, the radius r of the small diameter portion was set to 0.25 mm or 0.5 mm, and the distance c (mm) between the portion of the ground electrode where the through hole was formed and the tip portion of the insulator was variously changed. Samples of the ignition system were prepared, and the durability evaluation test described above was performed on each sample.

  FIG. 10 shows the results of the test. Each sample was configured so that a positive voltage was applied to the center electrode, and at least (1) to (4) described above were satisfied.

As shown in FIG. 10, it was revealed that the sample in which the distance c is 0.2 mm or more and 0.5 mm or less has a significantly large frame area and can more effectively suppress channeling. This is considered to be due to the following (c) and (d).
(C) By setting the distance c to 0.2 mm or more, when electrons move from the ground electrode to the center electrode, the electrons easily move along the path through the air, and the path including the air Spark discharge is likely to occur.
(D) By setting the distance d to 0.5 mm or less, the discharge voltage becomes sufficiently small, and spark discharge in the form of scooping the inner peripheral surface of the insulator is less likely to occur.

  From the results of the above test, it can be said that the distance c is more preferably 0.2 mm or more and 0.5 mm or less in order to further improve the channeling resistance.

  Next, by setting the radius r of the small diameter portion to 0.25 mm or 0.5 mm and changing the radius R of the through hole, samples of the ignition system having different Rr (mm) were prepared. Each sample was subjected to the durability evaluation test described above.

  FIG. 11 shows the results of the test. Each sample was configured so that a positive voltage was applied to the center electrode, and at least satisfied (1) to (3) and (5) described above.

As shown in FIG. 11, it was confirmed that the sample having Rr of 0.2 mm or more and 0.5 mm or less has a more excellent channeling suppression effect. This is considered to be due to the following (e) and (f).
(E) Since Rr is 0.2 mm or more, plasma can be ejected more smoothly from the through hole.
(F) By setting Rr to 0.5 mm or less, it becomes difficult to generate spark discharge in the form of scooping the tip surface and inner peripheral surface of the insulator, and spark discharge passing through the air is more likely to occur. .

  From the results of the above test, in order to suppress channeling more effectively, ½ of the value obtained by subtracting the inner diameter of the small diameter portion from the inner diameter of the through hole (that is, R−r) is 0.2 mm or more and 0.5 mm. It can be said that the following is more preferable.

  In addition, it is not limited to the description content of the said embodiment, For example, you may implement as follows. Of course, other application examples and modification examples not illustrated below are also possible.

  (A) In the above embodiment, the edge portion 32 is formed by providing the convex portion 5C at the tip of the center electrode 5, but the method for forming the edge portion is not limited to this. . Therefore, for example, as shown in FIGS. 12 (a) and 12 (b), by providing recesses 5D and 5E at the tip of the center electrode 5, the tip surface of the center electrode 5 and the side surfaces of the recesses 5D and 5E You may comprise so that the edge parts 33 and 34 may be formed in between.

  Further, when the edge portion is formed by providing the concave portion, as shown in FIG. 13, the front end surface 5F of the center electrode 5 is configured to be inclined toward the rear end side in the axis CL1 direction toward the outer peripheral side. May be. In this case, the electric field strength of the edge portion 33 can be further increased, and a spark discharge can be generated more reliably between the edge portion 33 and the ground electrode 27. Further, the outer peripheral side portion located in the vicinity of the inner peripheral surface of the insulator 2 in the front end surface 5 </ b> F of the center electrode 5 is further away from the ground electrode 27. Therefore, it is possible to more reliably prevent the occurrence of a spark discharge that crawls the inner peripheral surface of the insulator 2 between the outer peripheral side portion and the ground electrode 27, thereby further improving the channeling resistance.

  (B) In the above embodiment, the convex portion 5C has a cylindrical shape. However, as shown in FIG. 14, the convex portion 5G has a cylindrical discharge portion 5G1 located on the tip side, and a rear side of the discharge portion 5G1. You may comprise so that it may be located in an end side and may have the base 5G2 whose own outer diameter is larger than the outer diameter of the discharge part 5G1. And it is good also as forming the edge part 35 between the front end surface of the discharge part 5G1, and a side surface. In this case, a spark discharge may be more reliably generated between the edge portion 35 and the ground electrode 27 by making the entire outer peripheral surface of the base portion 5G2 have a curved surface.

  (C) As shown in FIGS. 15 (a) and 15 (b), for example, by performing an unevenness forming process such as knurling, an uneven part 5H is provided on the tip surface of the center electrode 5, and the protrusions of the uneven part 5H are provided. The edge portion 36 may be configured by a portion.

  (D) In the above embodiment, the distance from the central axis CL2 of the small diameter portion 4B along the direction orthogonal to the axis CL1 to the edge portion 32 is configured to be constant over the entire circumferential direction. You may comprise so that the distance from an edge part to the central axis CL2 may differ in the circumferential direction by making the convex part 5C into a prismatic shape. In this case, the minimum distance a between the edge portion and the center axis CL2 only needs to be smaller than the radius of the small diameter portion 4B. In other words, when the ignition plug 1 is viewed from the front end side in the direction of the axis CL1, it is only necessary that at least a part of the edge portion exists on the inner peripheral side of the inner peripheral surface of the small diameter portion 4B.

  (E) In the above embodiment, the entire area of the ground electrode 27 is configured to be separated from the front end surface of the insulator 2. However, by providing a protrusion or the like on the rear end side surface of the ground electrode 27, You may comprise so that a part may contact the front end surface of the insulator 2. In this case, the heat of the ground electrode 27 can be efficiently conducted to the insulator 2 and the wear resistance of the ground electrode 27 can be improved. The protrusion may be integrated with the ground electrode 27 or may be a separate body.

  (F) In the above embodiment, the convex portion 5C is formed of a material different from that of the outer layer 5B. However, the convex portion 5C and the outer layer 5B may be formed of the same material.

  (G) In the above embodiment, the tool engagement portion 19 has a hexagonal cross section, but the shape of the tool engagement portion 19 is not limited to such a shape. For example, it may be a Bi-HEX (deformed 12-angle) shape [ISO 22777: 2005 (E)].

  DESCRIPTION OF SYMBOLS 1 ... Spark plug (plasma jet spark plug), 2 ... Insulator (insulator), 3 ... Main metal fitting, 4 ... Shaft hole, 4A ... Large diameter part, 4B ... Small diameter part, 5 ... Center electrode, 27 ... Ground electrode , 28 ... through-hole, 31 ... cavity part, 32 ... edge part, 61 ... voltage applying device, 101 ... ignition system, CL1 ... axis, CL2 ... center axis (small diameter part).

Claims (6)

An insulator having an axial hole extending in the axial direction;
A center electrode inserted into the shaft hole so that a tip surface is located on the rear end side in the axial direction from the tip of the insulator;
A metal shell disposed on the outer periphery of the insulator;
A ground electrode fixed to the tip of the metal shell,
A cavity portion formed by the inner peripheral surface of the insulator and the tip-side surface of the center electrode;
A plasma jet ignition plug comprising a through hole formed in the ground electrode and communicating with the cavity and the outside,
While causing a spark discharge between the center electrode to which a positive voltage is applied and the ground electrode,
The shaft hole has a large-diameter portion in which the tip portion of the center electrode is disposed, and a small-diameter portion that is located on the tip side of the large-diameter portion and whose inner diameter is smaller than the inner diameter of the large-diameter portion. And
The tip of the center electrode is spaced from the inner peripheral surface of the insulator, and is located on the rear end side in the axial direction from the rear end of the small diameter portion,
The distal end portion of the center electrode has a convex edge portion toward the cavity portion side, and a minimum distance from the central axis of the small diameter portion along the direction orthogonal to the axis to the edge portion is Smaller than the diameter of the small diameter part,
The inner diameter of the through hole is larger than the inner diameter of the small diameter portion,
A plasma jet ignition plug, wherein a gap is provided between a portion of the ground electrode where the through hole is formed and a tip portion of the insulator.   2. A ≦ 0.7r, wherein a is a minimum distance from the axis along the direction orthogonal to the axis to the edge portion, and a is a diameter of the small diameter portion. The described plasma jet ignition plug.   The plasma jet ignition plug according to claim 1 or 2, wherein a distance along the axis line between the edge portion and a rear end of the small diameter portion is 0.2 mm or more and 0.8 mm or less.   The distance along the axis line between the portion of the ground electrode where the through hole is formed and the tip of the insulator is 0.2 mm or more and 0.5 mm or less. The plasma jet ignition plug according to any one of the above.   5. The plasma jet according to claim 1, wherein ½ of a value obtained by subtracting the inner diameter of the small-diameter portion from the inner diameter of the through-hole is 0.2 mm or more and 0.5 mm or less. Spark plug. An insulator having an axial hole extending in the axial direction;
A center electrode inserted into the shaft hole so that a tip surface is located on the rear end side in the axial direction from the tip of the insulator;
A metal shell disposed on the outer periphery of the insulator;
A ground electrode fixed to the tip of the metal shell,
A cavity portion formed by the inner peripheral surface of the insulator and the tip-side surface of the center electrode;
A plasma jet ignition plug provided with a through hole formed in the ground electrode and communicating with the cavity portion and the outside; and
An ignition system comprising a voltage application device for applying a voltage to the center electrode,
By applying a positive voltage from the voltage application device to the center electrode, a spark discharge is generated between the center electrode and the ground electrode,
The shaft hole has a large-diameter portion in which the tip portion of the center electrode is disposed, and a small-diameter portion that is located on the tip side of the large-diameter portion and whose inner diameter is smaller than the inner diameter of the large-diameter portion. And
The tip of the center electrode is spaced from the inner peripheral surface of the insulator, and is located on the rear end side in the axial direction from the rear end of the small diameter portion,
The distal end portion of the center electrode has a convex edge portion toward the cavity portion side, and a minimum distance from the central axis of the small diameter portion along the direction orthogonal to the axis to the edge portion is Smaller than the diameter of the small diameter part,
The inner diameter of the through hole is larger than the inner diameter of the small diameter portion,
An ignition system, wherein a gap is provided between a portion of the ground electrode where the through hole is formed and a tip of the insulator.

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