JP6440582B2 - Spark plug and ignition system - Google Patents

Description

  The present specification relates to an ignition plug and an ignition system for igniting an air-fuel mixture in an internal combustion engine or the like.

  As an ignition plug for igniting a fuel gas in an internal combustion engine, there is an ignition plug that generates plasma by supplying electric power to a spark generated in an ignition part using a high-frequency power source or the like (for example, a patent) Reference 1). Thereby, since ignition to fuel gas can be performed using high energy plasma, ignition performance improves compared with the case where AC power supply is not used. For example, it is possible to reliably ignite a lean air-fuel mixture with a higher air-fuel ratio.

International Publication No. 2012/032846

  However, in the above technique, it cannot be said that sufficient contrivance has been made for the structure of the ground electrode. For this reason, for example, the growth of plasma is hindered by the ground electrode, and the ignition performance may be lowered.

  The present specification discloses a technique for improving the ignition performance of a spark plug that generates plasma.

The technology disclosed in the present specification can be realized as the following application examples or forms .
[Form 1]
A rod-shaped center electrode extending along the axial direction;
A cylindrical insulator that holds the center electrode so that the tip of the center electrode protrudes from the tip of the center electrode to the tip side;
A cylindrical metal shell for holding the insulator;
A ground electrode joined to the metal shell and opposed to the front end surface of the center electrode via a gap;
A spark plug to which a plurality of peak voltages are supplied after the trigger discharge voltage is applied to the center electrode,
The ground electrode is formed with a through-hole penetrating in the axial direction through a position facing the tip surface of the center electrode.
The through hole includes a rear end side hole, a front end side hole located at a front end side with respect to the rear end side hole, and a front end side hole communicating with the front end of the rear end side hole;
The length of the hole on the rear end side in a direction perpendicular to the axis is longer from the front end side toward the rear end side.
[Form 2]
A rod-shaped center electrode extending along the axial direction;
A cylindrical insulator for holding the center electrode;
A cylindrical metal shell for holding the insulator;
A ground electrode joined to the metal shell and opposed to the front end surface of the center electrode via a gap;
A spark plug to which a plurality of peak voltages are supplied after the trigger discharge voltage is applied to the center electrode,
The ground electrode is formed with a through-hole penetrating in the axial direction through a position facing the tip surface of the center electrode.
The through hole includes a rear end side hole, a front end side hole located at a front end side with respect to the rear end side hole, and a front end side hole communicating with the front end of the rear end side hole;
The length in the direction perpendicular to the axis of the hole on the rear end side becomes longer from the front end side toward the rear end side,
The center electrode is disposed inside the shaft hole of the insulator,
The ground electrode is a plate-like member in which the through-hole is formed and disposed on the tip side of the insulator,
A cavity is formed by the surface on the front end side of the center electrode, the inner surface of the insulator, and the surface on the rear end side of the ground electrode,
The diameter R1 of the front end surface of the center electrode, the length R2 of the rearmost end in the direction perpendicular to the axis, and the length of the front end in the direction perpendicular to the axis R3 satisfies R3 <R1 <R2,
The distance D in the axial direction between the tip of the center electrode and the surface of the ground electrode on the rear end side where the through hole is formed satisfies 0.5 mm ≦ D ≦ 2.0. Features a spark plug.
[Form 3]
A rod-shaped center electrode extending along the axial direction;
A cylindrical insulator for holding the center electrode;
A cylindrical metal shell for holding the insulator;
A ground electrode joined to the metal shell and opposed to the front end surface of the center electrode via a gap;
A spark plug to which a plurality of peak voltages are supplied after the trigger discharge voltage is applied to the center electrode,
The ground electrode is formed with a through-hole penetrating in the axial direction through a position facing the tip surface of the center electrode.
The through hole includes a rear end side hole, a front end side hole located at a front end side with respect to the rear end side hole, and a front end side hole communicating with the front end of the rear end side hole;
The length in the direction perpendicular to the axis of the hole on the rear end side becomes longer from the front end side toward the rear end side,
The diameter W1 of the front end surface of the center electrode, the length W2 in the direction perpendicular to the axis of the rear end of the holes on the rear end side, and the length in the direction perpendicular to the axis of the hole on the front end side W3 is a spark plug characterized by satisfying W3 <W1 <W2.

Application Example 1 A rod-shaped center electrode extending along the axial direction;
A cylindrical insulator for holding the center electrode;
A cylindrical metal shell for holding the insulator;
A ground electrode joined to the metal shell and opposed to the front end surface of the center electrode via a gap;
A spark plug to which a plurality of peak voltages are supplied after the trigger discharge voltage is applied to the center electrode,
The ground electrode is formed with a through-hole penetrating in the axial direction through a position facing the tip surface of the center electrode.
The through hole includes a rear end side hole, a front end side hole located at a front end side with respect to the rear end side hole, and a front end side hole communicating with the front end of the rear end side hole;
The length of the hole on the rear end side in a direction perpendicular to the axis is longer from the front end side toward the rear end side.

  According to the above configuration, the length of the through hole in the direction perpendicular to the axis of the hole on the rear end side becomes longer from the front end side toward the rear end side, so that the plasma generated in the gap passes through the through hole. It tends to be emitted from the ground electrode to the tip side through the hole. As a result, the ignition performance of the spark plug can be improved.

[Application Example 2] The spark plug according to Application Example 1,
The diameter W1 of the front end surface of the center electrode, the length W2 in the direction perpendicular to the axis of the rear end of the holes on the rear end side, and the length in the direction perpendicular to the axis of the hole on the front end side W3 is a spark plug characterized by satisfying W3 <W1 <W2.

  According to the above configuration, the plasma generated in the gap can be more efficiently guided to the through hole. As a result, the ignition performance of the spark plug can be further improved.

[Application Example 3] The spark plug according to Application Example 1 or 2,
The length L1 of the tip side hole in the axial direction and the length L2 of the through hole in the axial direction are 1.2 mm ≦ L2 ≦ 1.7 mm and 0.4 ≦ (L1 / L2) A spark plug characterized by satisfying ≦ 0.7.

  According to the above configuration, the directionality of the plasma passing through the through hole is improved, and the amount of heat drawn by the ground electrode can be suppressed. As a result, the ignition performance of the spark plug can be further improved.

[Application Example 4] The spark plug according to any one of Application Examples 1 to 3,
The spark plug according to claim 1, wherein the hole on the front end side includes a portion between the front end and the rear end that is shorter than the front end and the rear end in a direction perpendicular to the axial direction.

  According to the said structure, the discharge | release rate of the plasma which passes along a through-hole can be made faster. As a result, the ignition performance of the spark plug can be further improved.

Application Example 5 The spark plug according to any one of Application Examples 1 to 4,
An ignition system comprising: a voltage supply unit that supplies the trigger discharge voltage and the plurality of peak voltages to the center electrode.

[Application Example 6] The spark plug according to Application Example 1,
The center electrode is disposed inside the shaft hole of the insulator,
The ground electrode is a plate-like member in which the through-hole is formed and disposed on the tip side of the insulator,
A cavity is formed by the surface on the front end side of the center electrode, the inner surface of the insulator, and the surface on the rear end side of the ground electrode,
The diameter R1 of the front end surface of the center electrode, the length R2 of the rearmost end in the direction perpendicular to the axis, and the length of the front end in the direction perpendicular to the axis R3 satisfies R3 <R1 <R2,
The distance D in the axial direction between the tip of the center electrode and the surface of the ground electrode on the rear end side where the through hole is formed satisfies 0.5 mm ≦ D ≦ 2.0. Features a spark plug.

  According to the above configuration, it is possible to increase the length of the discharge during the occurrence of the discharge while suppressing an increase in the voltage necessary for the occurrence of the discharge, and thus it is possible to further improve the ignition performance.

[Application Example 7] The spark plug according to Application Example 6,
A spark plug characterized in that the length E of the shortest path from the surface of the center electrode to the surface of the rear end side of the ground electrode through the inner surface of the insulator satisfies E ≧ 5D.

  According to the above configuration, it is possible to increase the probability that the discharge generation path becomes an air path that passes through the gas in the cavity, and to reduce the probability that the discharge generation path becomes a creeping path that passes through the inner surface of the insulator. As a result, the ignition performance can be further improved.

  Note that the technology disclosed in the present specification can be realized in various modes. For example, it can be realized in a mode of an internal combustion engine equipped with an ignition plug, an internal combustion engine equipped with an ignition system, or the like. it can.

It is a block diagram of the ignition system of this embodiment. It is sectional drawing of an example of the ignition plug of embodiment. 2 is an enlarged view of the vicinity of the tip of a spark plug 100. It is explanatory drawing of the sample of a comparison form. It is explanatory drawing of the ground electrode 30b of the ignition plug of 2nd Embodiment. It is sectional drawing which cut | disconnected the front-end | tip part vicinity of the ignition plug of 3rd Embodiment by the surface where axis line CO is included. It is an enlarged view of the vicinity of the cavity formation part 135 among the cross sections of FIG. It is explanatory drawing of the ground electrode 30f of the spark plug of a modification. It is explanatory drawing of the ground electrode 30g of the spark plug of a modification. It is explanatory drawing of the ground electrodes 70B and 70C of the spark plug of a modification.

A. First embodiment:
A-1. Ignition system configuration:
FIG. 1 is a block diagram of the ignition system of the present embodiment. The ignition system 600 includes a spark plug 100, a discharge power source 640, a high frequency power source 650, a mixing circuit 660, an impedance matching circuit 670, a control device 680, and a plug cord 690 connected to a terminal fitting of the spark plug 100. And.

  The discharge power source 640 is connected to a battery (not shown) and the mixing circuit 660. The discharge power source 640 includes, for example, an ignition coil, and generates a relatively high voltage, for example, a trigger voltage of 5 kV to 30 kV, using battery power. The generated trigger voltage is supplied to the spark plug 100 via the mixing circuit 660 and the plug cord 690.

  The high frequency power source 650 is connected to a battery (not shown) and an impedance matching circuit 670. The high-frequency power source 650 includes, for example, a DC / AC converter, and generates a voltage (an AC voltage in the present embodiment) of a relatively high frequency (for example, 50 kHz to 100 MHz) using battery power. The generated AC voltage is supplied to the spark plug 100 via the impedance matching circuit 670, the mixing circuit 660, and the plug cord 690.

  The impedance matching circuit 670 is connected to the mixing circuit 660 and the high frequency power source 650. The impedance matching circuit 670 matches the output impedance on the high frequency power source 650 side with the input impedance on the mixing circuit 660 side. Thereby, the attenuation of the AC voltage supplied to the spark plug 100 is suppressed.

  The mixing circuit 660 is connected to the discharging power source 640, the impedance matching circuit 670, and the plug cord 690. The mixing circuit 660 includes a coil 662 connected between the discharge power source 640 and the plug cord 690, and a capacitor 663 connected between the impedance matching circuit 670 and the plug cord 690. The mixing circuit 660 suppresses the flow of current from one of the discharge power supply 640 and the high-frequency power supply 650 to the other, and converts both the trigger voltage from the discharge power supply 640 and the AC voltage from the high-frequency power supply 650 to the plug cord. 690 is supplied to the spark plug 100. The coil 662 allows a relatively low frequency current from the discharge power source 640 to flow, and suppresses a relatively high frequency current from the high frequency power source 650 from flowing. Capacitor 663 allows a relatively high-frequency current from high-frequency power supply 650 to flow, and suppresses a relatively low-frequency current from discharging power supply 640 from flowing. The coil 662 may be substituted by an ignition coil included in the discharge power source 640.

  The control device 680 is connected to the discharge power source 640 and the high frequency power source 650. The control device 680 is a computer including a processor and a memory, for example. The control device 680 controls the timing at which the trigger voltage is supplied from the discharge power source 640 to the spark plug 100 and the timing at which the AC voltage is supplied from the high frequency power source 650 to the spark plug 100.

  The operation of the ignition system 600 will be briefly described. The control device 680 controls the discharge power source 640 to supply a trigger voltage to the spark plug 100. As a result, a trigger voltage is supplied between the center electrode and the ground electrode of the spark plug 100, and spark discharge due to dielectric breakdown occurs in the gap between these electrodes. Spark discharge due to dielectric breakdown is also called trigger discharge. The control device 680 controls the high-frequency power source 650 immediately after the trigger voltage is supplied, and supplies an AC voltage to the spark plug 100. As a result, plasma is generated by supplying the AC voltage energy to the trigger discharge generated by the trigger voltage. The air-fuel mixture in the combustion chamber of the internal combustion engine is ignited by the generated plasma energy. The spark plug 100 used in such an ignition system 600 is also called a high-frequency plasma plug.

  In addition, although the high frequency power supply 650 of this embodiment produces | generates alternating voltage, it may replace with alternating voltage and may generate the voltage containing several rectangular pulse voltage. It can be said that an AC voltage or a voltage including a plurality of rectangular pulse voltages is a voltage including a plurality of peak voltages. That is, the high frequency power source 650 may generate a voltage (for example, an AC voltage or a pulse voltage) including a plurality of high frequency peak voltages. The whole of the discharge power source 640 and the high-frequency power source 650 can be called a voltage supply unit that supplies a trigger voltage for trigger discharge and a plurality of peak voltages for plasma generation to the spark plug 100.

A-2. Spark plug configuration:
FIG. 2 is a cross-sectional view of an example of the ignition plug according to the embodiment. The illustrated line CO indicates the axis of the spark plug 100. The illustrated cross section is a cross section including the axis CO. A direction parallel to the axis CO is also referred to as an “axis direction”. A radial direction of a circle on a plane that is centered on the axis CO and is perpendicular to the axis CO is simply referred to as a “radial direction”, and a circumferential direction of the circle is also referred to as a “circumferential direction”. Of the directions parallel to the axis CO, the lower direction in FIG. 1 is referred to as a front end direction FD, and the upper direction is also referred to as a rear end direction BD. The distal direction FD is a direction from the terminal fitting 40 described later toward the electrodes 20 and 30. The front end direction FD side is referred to as the front end side of the spark plug 100, and the rear end direction BD side is referred to as the rear end side of the spark plug 100.

  The spark plug 100 includes an insulator 10 as an insulator, a center electrode 20, a ground electrode 30, a terminal fitting 40, and a metal shell 50.

  The insulator 10 is formed by firing alumina or the like. The insulator 10 is a substantially cylindrical member having an axial hole 12 extending along the axial direction and penetrating the insulator 10. The insulator 10 includes a flange part 19, a rear end side body part 18, a front end side body part 17, a step part 15, and a leg length part 13. The rear end side body portion 18 is located on the rear end side of the flange portion 19 and has an outer diameter smaller than the outer diameter of the flange portion 19. The distal end side body portion 17 is located on the distal end side from the flange portion 19 and has an outer diameter smaller than the outer diameter of the flange portion 19. The long leg portion 13 is positioned on the distal end side from the distal end side body portion 17 and has an outer diameter smaller than the outer diameter of the distal end side body portion 17. The leg portion 13 is exposed to the combustion chamber when the spark plug 100 is attached to an internal combustion engine (not shown). The step portion 15 is formed between the leg long portion 13 and the distal end side body portion 17.

  The metal shell 50 is formed of a conductive metal material (for example, a low carbon steel material), and is a substantially cylindrical member for fixing the spark plug 100 to an engine head (not shown) of the internal combustion engine. The metal shell 50 is formed with an insertion hole 59 penetrating along the axis CO. The metal shell 50 is disposed on the outer periphery of the insulator 10. That is, the insulator 10 is inserted and held in the insertion hole 59 of the metal shell 50. The tip of the insulator 10 protrudes from the tip of the metal shell 50 toward the tip side. The rear end of the insulator 10 protrudes toward the rear end side from the rear end of the metal shell 50.

  The metal shell 50 is formed between a hexagonal column-shaped tool engagement portion 51 with which a plug wrench engages, an attachment screw portion 52 for attachment to an internal combustion engine, and the tool engagement portion 51 and the attachment screw portion 52. And a bowl-shaped seat portion 54. The nominal diameter of the mounting screw portion 52 is, for example, one of M8 (8 mm (millimeters)), M10, M12, M14, and M18.

  An annular gasket 5 formed by bending a metal plate is fitted between the mounting screw portion 52 and the seat portion 54 of the metal shell 50. The gasket 5 seals a gap between the spark plug 100 and the internal combustion engine (engine head) when the spark plug 100 is attached to the internal combustion engine.

  The metal shell 50 further includes a thin caulking portion 53 provided on the rear end side of the tool engaging portion 51, and a thin compression deformation portion 58 provided between the seat portion 54 and the tool engaging portion 51. And. An annular region formed between the inner peripheral surface of the portion of the metal shell 50 from the tool engaging portion 51 to the crimping portion 53 and the outer peripheral surface of the rear end side body portion 18 of the insulator 10 has an annular shape. Ring members 6 and 7 are arranged. Between the two ring members 6 and 7 in the region, talc (talc) 9 powder is filled. The rear end of the crimped portion 53 is bent radially inward and fixed to the outer peripheral surface of the insulator 10. The compression deformation portion 58 of the metal shell 50 is compressed and deformed when the crimping portion 53 fixed to the outer peripheral surface of the insulator 10 is pressed toward the distal end during manufacture. The insulator 10 is pressed toward the front end side in the metal shell 50 through the ring members 6 and 7 and the talc 9 by the compression deformation of the compression deformation portion 58. A step portion 15 (insulator side step) of the insulator 10 is formed by a step portion 56 (metal side step portion) formed on the inner periphery of the mounting screw portion 52 of the metal shell 50 through the metal annular plate packing 8. Part) is pressed. As a result, the gas in the combustion chamber of the internal combustion engine is prevented by the plate packing 8 from leaking outside through the gap between the metal shell 50 and the insulator 10.

  The center electrode 20 includes a rod-shaped center electrode main body 21 extending in the axial direction, and a columnar center electrode tip 29 joined to the tip of the center electrode main body 21. The center electrode main body 21 is disposed at a tip side portion inside the shaft hole 12 of the insulator 10. The center electrode main body 21 has a structure including an electrode base material 21A and a core portion 21B embedded in the electrode base material 21A. The electrode base material 21A is made of, for example, nickel or an alloy containing nickel as a main component, in this embodiment, Inconel 600 ("INCONEL" is a registered trademark). The core 21B is made of copper, which is superior in thermal conductivity to the alloy forming the electrode base material 21A, or an alloy containing copper as a main component, in this embodiment, copper.

  The center electrode main body 21 includes a flange portion 24 (also referred to as a flange portion) provided at a predetermined position in the axial direction, a head portion 23 (electrode head portion) that is a portion on the rear end side of the flange portion 24. And a leg portion 25 (electrode leg portion) which is a portion on the tip side of the collar portion 24. The flange 24 is supported by the step 16 of the insulator 10. The distal end portion of the leg portion 25, that is, the distal end of the center electrode main body 21 protrudes toward the distal end side from the distal end of the insulator 10.

  The terminal fitting 40 is a rod-shaped member extending in the axial direction. The terminal fitting 40 is formed of a conductive metal material (for example, low carbon steel), and a metal layer (for example, Ni layer) for corrosion protection is formed on the surface of the terminal fitting 40 by plating or the like. The terminal fitting 40 includes a collar part 42 (terminal jaw part) formed at a predetermined position in the axial direction, a cap mounting part 41 located on the rear end side of the collar part 42, and a leg part 43 on the distal side of the collar part 42. (Terminal leg). The cap mounting portion 41 of the terminal fitting 40 is exposed to the rear end side from the insulator 10. The leg 43 of the terminal fitting 40 is inserted into the shaft hole 12 of the insulator 10. A plug cap to which the plug cord 690 is connected is mounted on the cap mounting portion 41, and the trigger voltage and the alternating voltage described above are supplied.

In the shaft hole 12 of the insulator 10, a gap between the tip of the terminal fitting 40 (tip of the leg 43) and the rear end of the center electrode 20 (back end of the head 23) is buried with a conductive seal 60. Yes. The conductive seal 60 is formed of, for example, a composition containing glass particles such as B ₂ O ₃ —SiO ₂ and metal particles (Cu, Fe, etc.).

A-3. Configuration of the tip portion of the spark plug 100:
The configuration near the tip of the spark plug 100 will be described in more detail. FIG. 3 is an enlarged view of the vicinity of the tip of the spark plug 100. FIG. 3A shows a cross-sectional view in which the vicinity of the tip of the spark plug 100 is cut by a plane including the axis CO. The cross section of FIG. 3A further passes through the center of the rear end portion of the ground electrode 30 in the circumferential direction. For this reason, the cross section of FIG. 3A includes the cross section of the ground electrode 30.

  The center electrode tip 29 is joined to the tip of the center electrode main body 21 (tip of the leg portion 25) using, for example, laser welding. A portion indicated by reference numeral 27 in FIG. 3A is a melted portion formed by laser welding when the center electrode tip 29 is joined. The center electrode tip 29 is formed of a material mainly composed of a high melting point noble metal. For example, iridium (Ir) or an alloy containing Ir as a main component is used as the material of the center electrode tip 29. In this embodiment, Ru is ruthenium (Ru), rhodium (Rh), nickel (Ni). An alloy to which is added is used.

  The ground electrode 30 is a curved rod having a square cross section. The ground electrode 30 is formed using a metal having conductivity and high corrosion resistance, for example, a nickel alloy, and Inconel 601 in this embodiment. The rear end 31 of the ground electrode 30 is joined to the front end surface 50 </ b> A of the metal shell 50. Thereby, the metal shell 50 and the ground electrode 30 are electrically connected. The tip 32 of the ground electrode 30 is a free end.

  The ground electrode 30 includes a curved portion 33 including a rear end 31 and a facing portion 34 including a distal end 32. The facing portion 34 faces the tip surface 29S of the center electrode tip 29 in the axial direction through the gap g. FIG. 3B is a view of the facing portion 34 viewed from the rear end side along the axis CO toward the front end direction FD.

  A through hole 35 is formed in the facing portion 34 so as to penetrate the position facing the tip surface 29S of the center electrode 20, that is, the position where the axis CO and the facing portion 34 intersect in the axial direction in the present embodiment. The through-hole 35 includes a rear end side hole 36 and a front end side hole 37 that is located on the front end side with respect to the rear end side hole 36 and communicates with the front end of the rear end side hole 36 and the rear end. Yes. The rear end side hole 36 increases in diameter from the front end side toward the rear end side. That is, the length of the rear end side hole 36 in the direction perpendicular to the axis CO is longer from the front end side toward the rear end side. The tip-side hole 37 is a cylindrical hole having a constant diameter regardless of the position in the axial direction. The diameter of the hole 37 on the front end side is the same as the most advanced diameter of the hole 36 on the rear end side.

  When the trigger voltage described above is supplied to the electrodes (the center electrode 20 and the ground electrode 30) of the spark plug 100, the tip end surface 29S of the tip 29 and the diameter-expanded surface 36S that forms the hole 36 on the rear end side of the ground electrode 30. , Trigger discharge TE occurs (FIG. 3A). In FIG. 3B, the diameter-expanded surface 36S is hatched.

  The diameter of the front end surface 29S of the center electrode 20, that is, the diameter of the front end surface 29S of the center electrode tip 29 in this embodiment is W1. Further, the diameter (the length in the direction perpendicular to the axis) of the rear end of the hole 36 on the rear end side of the ground electrode 30 is W2. The diameter (the length in the direction perpendicular to the axis) of the hole 37 on the tip side of the ground electrode 30 is W3. The length in the axial direction of the hole 37 on the tip end side of the through hole 35 is L1. The length of the opposing portion 34 of the ground electrode 30 in the axial direction, that is, the length of the through hole 35 in the axial direction is L2. The gap distance between the rear end surface 34A of the facing portion 34 and the front end surface 29S of the center electrode 20 is L3.

  The diameter W3 of the hole 37 on the distal end side of the ground electrode 30 is made smaller than the diameter W1 of the distal end surface 29S of the center electrode 20 (W3 <W1). If the diameter W3 of the hole 37 on the front end side of the ground electrode 30 is larger than the diameter W1 of the front end face 29S of the center electrode 20, the entire portion facing the front end face 29S in the axial direction becomes the through hole 35. This is because the trigger discharge TE is difficult to occur.

A-4: First Evaluation Test In the first evaluation test, as shown in Table 1, 18 types of spark plug samples A1 to A18 were prepared, and an ignition performance test and a wear resistance test were performed. It was. The dimensions common to each sample are as follows.
Length L1 in the axial direction of the hole 37 on the tip side: 0.4 mm
Length L2 in the axial direction of the through hole 35: 1.5 mm
Distance L3 between the rear end surface 34A of the facing portion 34 and the front end surface 29S of the center electrode 20: 0.8 mm

  In the 18 kinds of samples, at least one of the diameter W1 of the front end surface 29S, the diameter W2 of the rearmost end of the rear end side hole 36, and the diameter W3 of the front end side hole 37 is different from each other. . The diameter W1 of the distal end surface 29S is set to any value of 1.6 mm, 1.2 mm, and 2 mm. The diameter W2 of the rearmost end of the hole 36 on the rear end side is set to any value of 0.8 mm, 1 mm, 1.5 mm, 2 mm, and 2.5 mm. The diameter W3 of the hole 37 on the tip side is set to any value of 0.8 mm, 1 mm, and 1.2 mm. For the reasons described above, the diameter W3 of the tip-side hole 37 is smaller than the diameter W1 of the tip face 29S in all samples.

  Of the 18 types of samples, two types of samples A11 and A18 are samples of the first comparative form, and the other 16 types of samples are samples of the first embodiment described above. FIG. 4 is an explanatory diagram of a sample of a comparative form. FIG. 4A shows a ground electrode 30d of the sample of the first comparative form. Unlike the spark plug 100 of the first embodiment, the through hole 37d formed in the facing portion 34d of the ground electrode 30d of the sample of the first comparative embodiment is a cylindrical hole having the same diameter as a whole. That is, the facing portion 34d of the ground electrode 30d of the sample of the first comparative form does not have the rear end side hole 36 formed by the enlarged diameter surface 36S. For this reason, in Table 1, in the samples A11 and A18 of the first comparative embodiment, the diameter W2 and the diameter W3 are equal.

  In the ignition performance test, the air-fuel ratio (the ratio of air to fuel in the mixture) of the ignition limit of 18 types of samples was examined. Specifically, the test vessel was filled with a mixture of propane gas and air at a pressure of 0.15 MPa, and the sample was ignited once in the test vessel. In one ignition, a trigger voltage and an alternating voltage were supplied using the ignition system 600 of FIG. 1, and energy of 500 mJ was supplied to the sample. Then, the air-fuel mixture in the test container is replaced every time ignition is performed, and ignition is performed 5 times with a combination of one type of sample and one type of air-fuel mixture. The combination of sample and gas mixture was accepted. Then, the air-fuel ratio of the air-fuel mixture was increased step by step, and the maximum acceptable air-fuel ratio was examined. The higher the air-fuel ratio at the ignition limit, the better the ignition performance.

  Further, in order to determine the air-fuel ratio at the ignition limit serving as a reference, a similar test was performed using a sample (also referred to as a comparison plug) of the second comparative form shown in FIG. A through hole is not formed in the facing portion 34e of the ground electrode 30e of the comparison plug. The configuration of other parts of the comparison plug is the same as that of the other samples. In the comparison plug, the diameter W1 of the distal end surface 29S was 1.6 mm, and the other dimensions were the common dimensions described above.

  Then, the air-fuel ratio at the ignition limit was compared between the comparison plug and each sample, and each sample was evaluated. When the difference (AF1-AF2) between the air-fuel ratio AF1 at the ignition limit of the sample and the air-fuel ratio AF2 at the ignition limit of the comparison plug is 0 or less, the evaluation of the sample is “E”. The sample was evaluated as “D”. Evaluation of a sample having a difference (AF1-AF2) of 0.3 or more and less than 0.5 was “C”, and evaluation of a sample having a difference of 0.5 or more and less than 1.0 was “B”. Further, the evaluation (A1) when the difference (AF1−AF2) is 1.0 or more and less than 1.5 is “A”, and the evaluation of the sample whose difference is 1.5 or more is “S”.

  Table 1 shows the evaluation results of the ignition performance test of each sample. None of the samples rated “E”. That is, in all samples, the ignition performance was improved over the comparative plug. Since the through hole is formed in the ground electrode in all the samples, the plasma generated in the gap g is emitted from the ground electrode to the tip side (that is, the mixture side) through the through hole. Can promote growth. On the other hand, since no through hole is formed in the ground electrode 30e of the comparison plug, the discharge of the plasma to the air-fuel mixture side is hindered, and the plasma diverges in a direction perpendicular to the axial direction. As a result, plasma growth is not promoted. For this reason, it is considered that the ignition performance of all the samples is improved as compared with the comparison plug.

  The evaluation of the two types of samples A11 and A18 of the first comparative form was “D”. On the other hand, the evaluation of samples A1 to A10 and A12 to A17 of all the embodiments was “C” or more.

  The reason is estimated as follows. Unlike the two types of samples A11 and A18 in the first comparative embodiment, the samples A1 to A10 and A12 to A17 in the embodiment have a rear end side hole 36 formed by the enlarged diameter surface 36S. As a result, the plasma is guided through the through hole 35 by the hole 36 on the rear end side. For this reason, as compared with the case where there is no hole 36 on the rear end side, the plasma generated in the gap is likely to be emitted to the front end side from the ground electrode through the through hole 35. As a result, the ignition performance of the spark plug is further improved.

  Furthermore, among the samples A1 to A10 and A12 to A17 of the embodiment, the diameter W2 of the rearmost end of the hole 36 on the rear end side is smaller than the diameter W1 of the front end surface 29S of the center electrode tip 29 (W1> W2). The evaluation of A4, A7, A10, A12, and A15 was “C”. Samples A1, A2, A5, A6, A8, A9, A13, A14, and the diameter W2 of the rearmost end of the rear end side hole 36 are larger than the diameter W1 of the front end surface 29S of the center electrode tip 29 (W1 <W2). The evaluation of A16 and A17 was “B”.

  The reason is estimated as follows. The trigger discharge TE is likely to occur at a position close to the outer edge of the tip surface 29S of the center electrode tip 29. For this reason, plasma is also likely to be generated at a position near the outer edge of the tip surface 29S of the center electrode tip 29. For this reason, when the diameter W2 of the rear end of the hole 36 on the rear end side is larger than the diameter W1 of the front end face 29S, the diameter W2 of the rear end of the hole 36 on the rear end side is smaller than the diameter W1 of the front end face 29S. Compared to the case, the generated plasma can be guided to the through hole 35 more efficiently. As a result, the plasma generated in the gap is likely to be emitted from the ground electrode to the tip side through the through hole 35. As a result, the ignition performance of the spark plug can be further improved.

  In the first evaluation test, a test for confirming the wear resistance performance was further performed. In the wear resistance test, for each of the 18 types of samples, ignition was repeated 30 times per second for 20 hours in a test container having a nitrogen atmosphere of 0.15 MPa. In one ignition, a trigger voltage and an alternating voltage were supplied using the ignition system 600 of FIG. 1, and energy of 500 mJ was supplied to the sample. And the increase amount of the gap distance L3 after a test was investigated by comparing with the gap distance L3 before a test. The smaller the increase in the gap distance L3, the better the wear resistance.

  The evaluation of the sample in which the increase amount of the gap distance L3 is within 0.1 mm is “A”, and the evaluation of the sample larger than 0.1 mm is “B”. Table 1 shows the evaluation results of the wear resistance performance test of each sample. All the samples A1 to A18 were evaluated as “A”, and no sample was evaluated as “B”.

  The reason for this is estimated as follows. As described above, the trigger discharge TE is likely to occur at a position near the outer edge of the front end surface 29S of the center electrode tip 29. Even if the rear end side hole 36 is formed in a portion facing the front end surface 29S, there is an enlarged surface 36S at a position facing the outer edge of the front end surface 29S of the center electrode tip 29 in the axial direction. For this reason, even if the hole 36 on the rear end side is provided, the area of the discharge surface on the ground electrode side for generating the trigger discharge TE does not become insufficient. Therefore, for example, the phenomenon that the trigger discharge TE concentrates locally on the ground electrode 30 and the ground electrode 30 is excessively consumed does not occur, and the wear resistance performance is not inferior as compared with the plug of the comparative form.

  As described above, the through-hole 35 including the rear end side hole 36 and the front end side hole 37 formed by the enlarged diameter surface 36S is formed in the ground electrode 30 based on the result of the first evaluation test. As a result, it was confirmed that the ignition performance of the spark plug can be improved and that the wear resistance performance is not impaired.

A-5: Second Evaluation Test In the second evaluation test, six types of sample groups B1, B2, B8, B9, B13, and B17 were prepared. As in the first evaluation test, the ignition performance test and the wear resistance And performance tests. The six types of sample groups B1, B2, B8, B9, B13, and B17 are created based on the samples A1, A2, A8, A9, A13, and A17 of Table 1 with the same number at the end. Each sample group includes eight types of samples. That is, in the second evaluation test, (6 × 8) = 48 types of samples were evaluated. Six types of samples included in one type of sample group are different from each other in the ratio (L1 / L2) of the length L1 in the axial direction of the hole 37 on the distal end side to the length L2 in the axial direction of the through-hole 35. The ratios (L1 / L2) of these eight types of samples are 0.18, 0.33, 0.40, 0.53, 0.58, 0.70, 0.83, and 0.88, respectively. .

For the ratio (L1 / L2), the length L2 in the axial direction of the through hole 35 and the length L1 in the axial direction of the hole 37 on the tip side, which were common values in the first evaluation test, were set to the following values: It has been changed by setting.
(L1 / L2) = 0.18: L1 = 0.3 mm, L2 = 1.7 mm
(L1 / L2) = 0.33: L1 = 0.4 mm, L2 = 1.2 mm
(L1 / L2) = 0.40: L1 = 0.6 mm, L2 = 1.5 mm
(L1 / L2) = 0.53: L1 = 0.9 mm, L2 = 1.7 mm
(L1 / L2) = 0.58: L1 = 0.7 mm, L2 = 1.2 mm
(L1 / L2) = 0.70: L1 = 1.2 mm, L2 = 1.7 mm
(L1 / L2) = 0.83: L1 = 1 mm, L2 = 1.2 mm
(L1 / L2) = 0.88: L1 = 1.5 mm, L2 = 1.7 mm

  Thus, the axial length L2 of the through hole 35 is set in the range of 1.2 mm ≦ L2 ≦ 1.7 mm.

  The dimensions of the six sample groups B1, B2, B8, B9, B13, and B17 other than the lengths L1 and L2 are the same as those of the base samples A1, A2, A8, A9, A13, and A17.

  In Table 2, among the evaluations shown for one cell, the left side is the evaluation result of the ignition performance, and the right side is the evaluation result of the wear resistance performance. For example, a sample with an evaluation of “B / A” has an ignition performance evaluation of “B” and a wear resistance performance evaluation of “A”.

  As shown in Table 2, the evaluation of the ignition performance of all samples having a ratio (L1 / L2) of 0.40 or more and 0.70 or less among 48 types of samples was “A”. . Among the 48 types of samples, the evaluation of the ignition performance of all samples having a ratio (L1 / L2) of less than 0.40 or greater than 0.70 was “B”.

  The reason is estimated as follows. When the ratio (L1 / L2) is less than 0.40, the length L1 in the axial direction of the hole 37 on the distal end side becomes excessively short relative to the length L2 in the axial direction of the through hole 35. When the hole 37 on the front end side having the same diameter is excessively short, the direction of plasma emission through the through hole 35 cannot be directed along the axial direction. As a result, the plasma emitted from the through hole 35 toward the tip side does not spread in the tip direction FD, but easily spreads in the direction (radial direction) perpendicular to the axis. As a result, it is difficult for the plasma to expand as compared with the case where the ratio (L1 / L2) is 0.40 or more and 0.70 or less.

  On the other hand, when the ratio (L1 / L2) is greater than 0.70, the axial length L1 of the tip-side hole 37 is excessively longer than the axial length L2 of the through hole 35. The amount of heat drawn by the ground electrode 30 when the plasma passes through the front-side hole 37 having a relatively small diameter is the amount of heat drawn by the ground electrode 30 when the plasma passes through the rear-side hole 36 having a relatively large diameter. Larger than the amount. For this reason, if the axial length L1 of the hole 37 on the tip side is excessively long, the amount of heat drawn by the ground electrode 30 becomes excessively large. As a result, compared with the case where the ratio (L1 / L2) is 0.40 or more and 0.70 or less, the thermal energy of plasma is easily lost.

  Regarding the wear resistance test, the evaluation of all 48 types of samples was “A”, and there was no sample with an evaluation of “B”. That is, wear resistance performance was not impaired for all 48 types of samples.

  As described above, based on the result of the second evaluation test, at least in the range of 1.2 mm ≦ L2 ≦ 1.7 mm, regardless of the values of the diameters W1 to W3, 0.4 ≦ (L1 / L2) ≦ It was found that satisfying 0.7 is preferable from the viewpoint of improving the ignition performance. In this way, the directionality of the plasma that has passed through the through hole 35 is improved, and the amount of heat drawn by the ground electrode 30 can be suppressed. As a result, the ignition performance of the spark plug can be further improved. It was also found that the wear resistance is not impaired.

  Moreover, the value of the ratio (L1 / L2) that the ignition performance was found to be improved by the ignition performance test was 0.40, 0.53, 0.58, and 0.70. Any value among these values can be adopted as the upper limit value and / or the lower limit of the preferred range of the ratio (L1 / L2). For example, as a preferable range of the ratio (L1 / L2), a range of 0.53 to 0.70 can be employed.

B. Second embodiment:
B-1. Configuration of Spark Plug FIG. 5 is an explanatory diagram of the ground electrode 30b of the spark plug according to the second embodiment. FIG. 5A shows a diagram in which the ground electrode 30b is cut along a plane including the axis CO. FIG. 5B shows a view of the facing portion 34b of the ground electrode 30b as viewed from the rear end side toward the front end direction FD along the axis CO.

  A constricted portion 38b is formed at the center in the axial direction of the hole 37b on the tip side of the facing portion 34b. The inner diameter W4 of the throttle portion 38b is smaller than the diameter W3 of the front end and the rear end of the hole 37b on the front end side. The narrowed portion 38b is a portion in which a protruding portion that protrudes radially inward from the inner peripheral surface that forms the tip-side hole 37b is formed over the entire circumference in the circumferential direction of the inner peripheral surface. Since the structure of the other part of the spark plug of 2nd Embodiment is the same as the spark plug 100 of 1st Embodiment, the description is abbreviate | omitted.

B-2. Third Evaluation Test In the third evaluation test, as shown in Table 3, eight types of samples C1-1, C9-1, C13-1, C17-1, C1-2, C9 of the spark plug of the second embodiment. -2, C13-2, and C17-2 were prepared, and the ignition performance test and the wear resistance test were performed in the same manner as the first evaluation test. Eight types of samples C1-1, C9-1, C13-1, C17-1, C1-2, C9-2, C13-2, C17-2 are samples A1 in Table 1 with the same number after C, It is created based on A9, A13, and A17. Each sample is provided with the aforementioned narrowed portion 38b in the hole 37b on the distal end side. As shown in Table 3, the inner diameter W4 of the narrowed portion is either 0.6 mm or 0.9 mm.

  The four types of samples C1-1, C9-1, C13-1, and C17-1 have L1 = 0.6 mm and L2 = 1 so that the ratio (L1 / L2) is 0.40. .5mm is set. Further, another four types of samples C1-2, C9-2, C13-2, and C17-2 have L1 = 1.2 mm and L2 so that the ratio (L1 / L2) is 0.70. = 1.7 mm.

  8 samples C1-1, C9-1, C13-1, C17-1, C1-2, C9-2, C13-2, C17-2 dimensions other than W4, L1, L2 are base samples The same as A1, A9, A13, A17.

  As shown in Table 3, the ignition performance of the eight types of C1-1, C9-1, C13-1, C17-1, C1-2, C9-2, C13-2, and C17-2 of the second embodiment. The evaluation was “S”. That is, the eight types of C1-1, C9-1, C13-1, C17-1, C1-2, C9-2, C13-2, and C17-2 of the second embodiment are the sample A1 that does not have the diaphragm 38b. The ignition performance was higher than A9, A13, and A17.

  The reason is estimated as follows. By providing the throttle portion 38b in the tip-side hole 37, the speed of the plasma passing through the tip-side hole 37 is faster than in the case where there is no throttle portion 38b. As a result, it is possible to increase the discharge rate of the plasma toward the tip. As a result, by increasing the plasma growth rate, the plasma expansion range can be expanded, and the ignition performance of the spark plug can be further improved.

  Further, regarding the wear resistance test, the evaluation of all eight types of samples was “A”, and there was no sample with an evaluation of “B”. That is, the wear resistance performance was not impaired for all eight types of samples.

  As described above, based on the results of the third evaluation test, it has been found that it is preferable to provide the narrowed portion 38b in the hole 37 on the tip side from the viewpoint of improving the ignition performance. By so doing, the ignition performance of the spark plug can be further improved by increasing the discharge rate of the plasma passing through the through hole.

C. Third Embodiment C-1. Configuration of Spark Plug FIG. 6 is a cross-sectional view of the vicinity of the tip of the spark plug according to the third embodiment cut along a plane including the axis CO. The configuration of the rear end side of the plate packing 8 in the spark plug of the third embodiment is the same as that of the spark plugs of the first embodiment and the second embodiment.

  Unlike the leg length portion 13 of the first embodiment shown in FIG. 2, the outer diameter of the leg length portion 13B on the tip side of the step portion 15 of the insulator 10B of the third embodiment is constant. In the mounting screw portion 52B of the metal shell 50B of the third embodiment, the inner diameter of the portion 52F on the front end side from the step portion 56 is smaller than the inner diameter of the portion 52E on the rear end side from the step portion 56. It is slightly smaller than the outer diameter of the portion 13B. And the front end surface 52BT (that is, the front end surface of the metal shell 50) of the mounting screw portion 52B and the front end surface 13BT of the leg long portion 13B (that is, the front end surface of the insulator 10) are substantially equal in the axial direction.

  The ground electrode 70 of the third embodiment is a substantially disk-shaped plate member. The ground electrode 70 is made of, for example, a refractory metal such as tungsten or nickel alloy. The rear end side of the outer edge portion of the ground electrode 70 is joined to the front end surface 52BT of the metal shell 50B by, for example, resistance welding. As a result, the metal shell 50B and the ground electrode 70 are electrically coupled.

  Furthermore, a cylindrical cavity forming portion 135 is formed on the distal end surface 13BT of the leg long portion 13B of the insulator 10B.

  The configuration in the vicinity of the cavity forming portion 135 will be described in more detail. FIG. 7 is an enlarged view of the vicinity of the cavity forming portion 135 in the cross section of FIG.

  A tip made of noble metal is not attached to the tip of the leg portion 25B of the center electrode 20B of the third embodiment. The center electrode 20B is made of a refractory metal such as tungsten or nickel alloy. Instead of this, a configuration having a double structure of a base material and a core material embedded in the base material may be adopted as in the first embodiment. The front end surface 25BT of the leg portion 25B (that is, the front end surface of the center electrode 20B) is located on the rear end side from the front end surface 13BT of the leg long portion 13B.

  As shown in FIG. 7, the inner surface of the leg long portion 13B (specifically, the bottom surface 135Sb of the cavity forming portion 135 and the side surface 135Sa of the cavity forming portion 135), the surface 73 on the rear end side of the ground electrode 70, the leg A cavity CV is formed by the surface on the front end side of the portion 25B (specifically, the front end surface 25BT and the side surface 25BS of the leg portion 25B). As can be seen from FIG. 7, the gap (gap) in which a spark is formed between the center electrode 20B and the ground electrode 70 is located in the cavity CV.

  In the ground electrode 70, a through hole 75 penetrating in the axial direction is formed at a portion facing the distal end surface 25BT of the leg portion 25B. In the present embodiment, the through hole 75 is formed at a position where the ground electrode 70 and the axis CO intersect. As can be seen from FIG. 7, the cavity CV communicates with the outside of the cavity CV (in use, the combustion chamber of the internal combustion engine) through the through hole 75.

  It should be noted that the gap between the front end surface 13BT of the leg long portion 13B and the surface 73 on the rear end side of the ground electrode 70, the side surface 25BS of the leg portion 25B, and the inner periphery that forms the shaft hole 12B of the insulator 10B. The gaps between the surfaces are extremely small (for example, 0.05 mm or less), and plasma generated in the cavity CV (described later) is configured to be ejected from the through hole 75 to the outside of the cavity CV.

  The through hole 75 has the same configuration as the through hole 35 of the first embodiment. That is, the through-hole 75 includes a rear end side hole 752, a front end side hole 751 that is located on the front end side of the rear end side hole 752, and the front end of the rear end side hole 752 communicates with the rear end. Contains. The hole 752 on the rear end side increases in diameter from the front end side toward the rear end side. In other words, the length of the rear end side hole 752 in the direction perpendicular to the axis CO is linearly longer from the front end side toward the rear end side. The surface on which the rear end side hole 752 is formed is also referred to as an enlarged surface 752S. The hole 751 on the distal end side is a cylindrical hole having a constant diameter regardless of the position in the axial direction. The diameter of the hole 751 on the front end side is the same as the most advanced diameter of the hole 752 on the rear end side.

  Here, the diameter of the front end surface 25BT of the leg portion 25B is R1. The diameter (the length in the direction perpendicular to the axis) of the rear end of the hole 752 on the rear end side of the ground electrode 70 is R2. The diameter (length in the direction perpendicular to the axis) of the hole 751 on the tip side of the ground electrode 30 is R3. In addition, a distance in the axial direction from the front end surface 25BT of the leg portion 25B to the surface 73 on the rear end side of the ground electrode 70 (hereinafter also referred to as an interelectrode distance) is D. The length of the shortest route SR (hereinafter also referred to as the shortest creepage route) among the routes from the surface of the center electrode 20 to the surface of the ground electrode 70 along the inner surface of the insulator 10 is defined as E. In the present embodiment, as shown in FIG. 7, the shortest creepage path SR passes from the side surface 25BS of the leg portion 25B through the bottom surface 135Sb and the side surface 135Sa of the cavity forming portion 135 to the rear end side of the ground electrode 70. This is a route to the surface 73.

  Here, the diameter of the cavity forming portion 135 (also referred to as the cavity diameter) is R4, and the axial distance from the bottom surface 135Sb of the cavity forming portion 135 to the surface 73 on the rear end side of the ground electrode 70 (in the axial direction of the cavity). The length E of the shortest creepage path is E = [{(R4-R1) / 2} + H1] where H1 is also referred to as the length).

  Further, the thickness of the ground electrode 70 in the axial direction, that is, the length of the through hole 75 in the axial direction is H2, and the length of the through hole 75 in the axial direction of the rear end side hole 752 is H3. . Among the through holes 75, the length in the axial direction of the hole 751 on the distal end side is (H2-H3).

  This spark plug operates as follows. When the trigger voltage described above is supplied to the electrodes of the spark plug (the center electrode 20B and the ground electrode 70), the diameter-expanded surface 752S that forms the front end surface 25BT of the leg 25B and the hole 752 on the rear end side of the ground electrode 70 is formed. , A trigger discharge occurs at a position SP1 connecting them with the shortest distance (FIG. 7A). Then, AC voltage energy is supplied to the trigger discharge, and plasma is generated in the cavity CV. The generated plasma passes through the through hole 75 and is released into the combustion chamber of the internal combustion engine. The air-fuel mixture in the internal combustion engine is ignited by the plasma energy.

  According to the spark plug of the third embodiment described above, similarly to the spark plugs of the first embodiment and the second embodiment, the rear end side hole 36 formed by the enlarged diameter surface 752S is provided. ing. As a result, the plasma in the cavity CV is induced to pass through the through hole 75 by the hole 752 on the rear end side, so that the plasma is efficiently discharged into the combustion chamber of the internal combustion engine. As a result, the ignition performance of the spark plug is improved.

C-2. Fourth Evaluation Test In the fourth evaluation test, as shown in Table 4, 18 types of spark plug samples D1 to D18 of the third embodiment were prepared and the ignition performance was tested. The dimensions common to each sample are as follows.
Thickness H2 of the ground electrode 70: 1.5 mm
Length H3 in the axial direction of the hole 752 on the rear end side: 0.5 mm
Rear end diameter R2 of the hole 752 on the rear end side: 2.5 mm
Diameter R3 of hole 751 on the tip side: 0.5 mm

  In the 18 types of samples D1 to D18, at least one value among the diameter R1 of the distal end surface 25BT, the inter-electrode distance D, and the length E of the shortest creepage path is different from each other. The diameter R1 of the front end surface 25BT is set to any value of 0.4 mm, 1 mm, 1.5 mm, and 2.6 mm. The inter-electrode distance D is any one of 0.3 mm, 0.5 mm, 1 mm, 1.5 mm, 2 mm, and 2.3 mm. The inter-electrode distance D was changed by adjusting the length of the leg portion 25 in the axial direction.

  The length E of the shortest creepage path is 1 mm, 4 mm, or 8 mm. The length E of the shortest creepage path was changed by fixing the cavity diameter R4 to 4.0 mm and adjusting the axial length H1 of the cavity CV.

  In the fourth evaluation test, the ignition limit air-fuel ratio (ratio of air to fuel in the mixture) of 18 types of samples D1 to D18 was examined. Specifically, the sample was assembled in an in-line 4-cylinder engine with a displacement of 1600 cc, and 1000 cycles of operation were performed at a rotational speed of 1600 rpm. During operation, a trigger voltage and an alternating voltage were supplied using the ignition system 600 of FIG. 1, and energy of 500 mJ was supplied to the sample. For each sample, the misfire rate (rate of change in the indicated mean effective pressure) was measured three times for each air-fuel ratio while gradually increasing the air-fuel ratio (A / F) of the air-fuel mixture. Then, from the result of plotting the air-fuel ratio and the misfire rate, the air-fuel ratio at the time of the misfire rate of 2% of each sample was calculated by approximate calculation and adopted as the air-fuel ratio AFR at the ignition limit. It means that the larger the air-fuel ratio AFR at the ignition limit, the better the ignition performance.

  The evaluation of the sample with the ignition limit air-fuel ratio AFR of 23 or more is “A”, the evaluation of the sample with the ignition limit air-fuel ratio AFR of 22 or more and less than 23 is “B”, and the ignition limit air-fuel ratio AFR is 22 Evaluation of less than samples was set to “C”.

  The evaluation of eight samples D3, D4, D7, D8, D11, D12, D15, and D16 satisfying R3 <R1 <R2 and satisfying 0.5 mm ≦ D ≦ 2.0 mm was B.

  On the other hand, the evaluations of the samples that did not satisfy R3 <R1 <R2 were all “C”. For example, the evaluation of four samples D2, D6, D10, and D14 with R3> R1 was “C” even though 0.5 mm ≦ D ≦ 2.0. Further, the evaluation of the four samples D5, D9, D13, and D17 where R1> R2 was “C” even though 0.5 mm ≦ D ≦ 2.0 was satisfied.

  Moreover, the evaluation of the sample which does not satisfy | fill 0.5mm <= D <= 2.0mm was all "C". For example, the evaluation of the samples D1 and D18 with D = 0.3 mm and D = 2.3 mm was “C” even though R3 <R1 <R2 was satisfied.

  That is, 8 samples satisfying R3 <R1 <R2 and satisfying 0.5 mm ≦ D ≦ 2.0 mm do not satisfy either R3 <R1 <R2 or 0.5 mm ≦ D ≦ 2.0 mm. Ignition performance was better than the sample. The reason for this is estimated as follows. First, for the same reason that the spark plug of the first embodiment satisfies W3 <W1 <W2, when R3 <R1 <R2 is satisfied, the plasma is effectively induced, and the combustion chamber May be released. Further, when R3 <R1 <R2 is satisfied, the trigger discharge is generated at the position SP1. As shown in FIG. 7, the position SP1 is a position that connects the end of the front end surface 25BT of the center electrode 20B and the diameter-enlarged surface 752S with the shortest distance. Thereafter, the discharge is considered to move to a position where the length of the discharge becomes longer, that is, a position SP2 closer to the axis CO.

  When the trigger discharge TE is generated, a shorter discharge length is preferable in terms of ignition performance since a discharge failure can be prevented by lowering the voltage required for dielectric breakdown, that is, the trigger voltage. Further, by lowering the trigger voltage, it is difficult to generate a discharge along the creepage path SR, which is preferable in terms of ignition performance. On the other hand, once the discharge has occurred, while the AC voltage energy is being supplied, the longer the discharge length, the greater the thermal energy, which is preferable in terms of ignition performance. When R3 <R1 <R2 is satisfied, as described above, the discharge occurs at the position SP1 where the discharge becomes relatively short, and then the discharge moves to the position SP2 where the discharge becomes relatively long. It is considered that the ignition performance is improved because the thermal energy of the discharge can be increased while the voltage is kept low. However, if the inter-electrode distance D is excessively short, for example, 0.5 mm> D, for example, even if R3 <R1 <R2 is satisfied, the length of the discharge is not sufficiently long even at the position SP2. It is considered that the ignition performance is not improved so much. In addition, if the interelectrode distance D is excessively long, for example, 2.0 mm <D, the discharge length is not sufficiently shortened even at the position SP1, and the trigger voltage becomes too high, resulting in a discharge failure. It is considered that the ignition performance is not improved so much. From the above, it is considered that when R3 <R1 <R2 is satisfied and 0.5 mm ≦ D ≦ 2.0 mm is satisfied, the ignition performance is improved as compared with the case where this is not satisfied.

C-3. Fifth Evaluation Test In the fifth evaluation test, as shown in Table 5, 18 types of spark plug samples E1 to E18 of the third embodiment were prepared and the ignition performance test was performed. The dimensions of the samples E1 to E18 excluding the length E of the shortest creepage path are the same as the samples D1 to D18 in Table 4. In samples D1 to D18 in Table 4, the length E of the shortest creepage path is set to 1 mm, 4 mm, or 8 mm. As a result, the ratio of the length E of the shortest creepage path to the distance D between the electrodes (E / D) is a value within a range of less than 5, specifically, any one of 2, 2.67, 3.33, 3.48, and 4. On the other hand, in samples E1 to E18 in Table 5, the length E of the shortest creepage path is set to any of 3 mm, 8 mm, and 12 mm. The ratio (E / D) of the length E is set to a value within a range of 5 or more, specifically 5.21, 5.33, 6, 8, or 10.

  In the fifth evaluation test, using the same evaluation standard as the fourth evaluation test, the ignition performance of 18 types of samples was evaluated in the three stages of “A”, “B”, and “C” described above. did.

  Evaluation of 8 samples E3, E4, E7, E8, E11, E12, E15, E16 satisfying R3 <R1 <R2 and satisfying 0.5 mm ≦ D ≦ 2.0 mm was “A”. .

  On the other hand, the evaluation of the samples that did not satisfy any of R3 <R1 <R2 and 0.5 mm ≦ D ≦ 2.0 mm was “C”.

  From the above, eight samples (evaluation “B”) in Table 4 that satisfy R3 <R1 <R2 and 0.5 mm ≦ D ≦ 2.0 mm and eight samples in Table 5 (evaluation “A” )), The sample of Table 5 in which (E / D) is 5 or more, that is, the sample satisfying E ≧ 5D was superior in ignition performance to the sample of Table 4 not satisfying E ≧ 5D. .

  The reason for this is estimated as follows. First, the creeping path has a lower electrical resistance per unit length than the air path (for example, the path at position SP1 in FIG. 7) that passes through the air without being along the surface of the insulator 10. For this reason, when the length E of the shortest creepage path is not sufficiently long, for example, when E ≧ 5D is not satisfied, the discharge along the creepage path occurs instead of the discharge along the air path. It becomes easy. The discharge along the creeping path is generated at a position away from the through hole 75 in the cavity CV. For this reason, even if the plasma based on the said discharge generate | occur | produces in the cavity CV, since it is hard to discharge | release from the through-hole 75, ignition performance falls. On the other hand, when E ≧ 5D is satisfied, the discharge along the creeping path can be sufficiently suppressed, so that it is considered that the ignition performance is further improved.

  As described above, the following has been found by the fourth evaluation test and the fifth evaluation test. In the spark plug of the third embodiment, the diameter R1 of the front end surface 35BT of the center electrode 20B, the diameter R2 of the rear end of the holes 752 on the rear end side of the ground electrode 70, and the diameter R3 of the hole 751 on the front end side Is such that R3 <R1 <R2 and the distance D in the axial direction between the tip of the center electrode 20B and the surface on the rear end side of the portion where the through hole 75 of the ground electrode 70 is formed is 0.5 mm. It is preferable to satisfy ≦ D ≦ 2.0. By so doing, it is possible to increase the length of the discharge during the occurrence of the discharge while suppressing an increase in the voltage required for the occurrence of the discharge, and thus the ignition performance can be further improved.

  Furthermore, it is further preferable that the length E of the shortest path from the surface of the center electrode 20B to the surface of the rear end side of the ground electrode 70 via the inner surface of the insulator 10B satisfies E ≧ 5D. In this way, it is possible to increase the probability that the discharge generation path becomes an air path passing through the gas in the cavity CV, and to reduce the probability that the discharge generation path becomes a creeping path passing through the inner surface of the insulator 10B. As a result, the ignition performance can be further improved.

D. Variation:
(1) In each of the above embodiments, the number of the holes 37 on the tip side is one, but a plurality (for example, two, three, and four) may be used. Further, in each of the above embodiments, the cross-sectional shape obtained by cutting the tip-side hole 37 in a cross section perpendicular to the axis CO is a circle, but is not limited to a circle. FIG. 8 is an explanatory diagram of a ground electrode 30f of a spark plug according to a modification. FIG. 8A shows a diagram in which the ground electrode 30f is cut along a plane including the axis CO. FIG. 8B shows a view of the facing portion 34f of the ground electrode 30f as viewed from the rear end side toward the front end direction FD along the axis CO.

  The rear end side hole 36 formed in the facing portion 34f of the ground electrode 30f is the same as the rear end side hole 36 (FIG. 3) of the first embodiment. In the facing portion 34f of the modification, a bottom surface 36BT is formed at the tip of the rear end side hole 36, and a plurality of tip side holes 37f1, 37f2 penetrating the bottom surface 36BT in the tip direction FD are formed. Has been. And the cross-sectional shape which cut | disconnected the hole 37f1 and 37f2 of the front end side by the cross section perpendicular | vertical to the axis line CO is not a circle but a quadrangle. The cross-sectional shape of the holes 37f1 and 37f2 on the front end side may be a star shape, an ellipse, or a polygon having three or more sides.

(2) In each of the above embodiments, the cross-sectional shape of the rear end side hole 36 cut along a cross section perpendicular to the axis CO is a circle, but is not limited to a circle. FIG. 9 is an explanatory diagram of a ground electrode 30g of a spark plug according to a modification. FIG. 9 shows a view of the facing portion 34g of the ground electrode 30g as viewed from the rear end side toward the front end direction FD along the axis CO.

  The cross-sectional shape obtained by cutting the rear end side hole 36g formed in the facing portion 34f of the ground electrode 30f along a cross section perpendicular to the axis CO is a quadrangle. In the direction perpendicular to the axis CO of the hole 36g on the rear end side, the length in the vertical direction in FIG. 9B increases from the front end side toward the rear end side. Similarly, in the direction perpendicular to the axis CO of the hole 36g on the rear end side, the length in the left-right direction in FIG. 9B increases from the front end side toward the rear end side. And the cross-sectional shape which cut | disconnected the hole 37g of the front end side formed in the opposing part 34f by the cross section perpendicular | vertical to the axis line CO is also a rectangle. The cross-sectional shape of the rear end side hole 36g may be a star shape, an ellipse, or a polygon having three or more sides.

  Thus, when the rear end side hole 36g is not circular, the maximum and minimum lengths in the direction perpendicular to the axis CO at the rearmost end of the rear end side hole 36g are W2max and W2min. . When the tip-side hole 37g is not circular, the maximum and minimum lengths in the direction perpendicular to the axis CO of the tip-side hole 37g are set to W2max and W2min. In this case, the relationship of the diameter W1 of the tip surface 29S of the center electrode tip 29 of the center electrode 20 preferably satisfies W3min <W1 <W2max, and more preferably satisfies W3max <W1 <W2min. If it carries out like this, plasma can be efficiently guide | induced to a through-hole and the ignition performance of a spark plug can be improved.

(3) In each of the above embodiments, the diameter of the hole on the rear end side increases linearly from the front end side toward the rear end side, but is not limited thereto. For example, the diameter of the hole on the rear end side may be increased in a curved line from the front end side toward the rear end side, or the diameter may be partially constant on the way from the front end side to the rear end side. May be included. FIG. 10 is an explanatory diagram of a ground electrode of a spark plug according to a modification. FIG. 10A shows a diagram in which the ground electrode 70B is cut along a plane including the axis CO. FIG. 10B shows a diagram in which the ground electrode 70C is cut along a plane including the axis CO. The diameter of the hole 752B on the rear end side of the ground electrode 70B in FIG. 10A becomes smaller as it goes from the front end side toward the rear end side. As a result, the cross section of the hole 752C on the rear end side is lower. Has a convex shape. Further, the diameter of the hole 752C on the rear end side of the ground electrode 70C in FIG. 10B becomes larger as it goes from the front end side toward the rear end side. As a result, the cross section of the hole 752C on the rear end side is as follows. , Has a convex shape. Other configurations of the ground electrodes 70B and 70C are the same as those of the ground electrode 70 of FIG.

(4) As described above, the ignition performance of the ignition plug of each of the above embodiments is improved by providing the rear end side hole 36 and setting the various dimensions W1 to W4, L1, and L2 within a preferable range. It is thought that it is achieved by doing. Therefore, elements other than these parameters, for example, the material and details of the metal shell 50 and the material and details of the insulator 10 can be variously changed. For example, the material of the metal shell 50 may be low-carbon steel plated with zinc or nickel, or low-carbon steel that is not plated. The insulator 10 may be made of various insulating ceramics other than alumina.

  As mentioned above, although this invention was demonstrated based on embodiment and a modification, embodiment mentioned above is for making an understanding of this invention easy, and does not limit this invention. The present invention can be changed and improved without departing from the spirit and scope of the claims, and equivalents thereof are included in the present invention.

  5 ... Gasket, 6 ... Ring member, 8 ... Plate packing, 9 ... Talc, 10 ... Insulator, 12 ... Shaft hole, 13 ... Leg length, 15 .. Step part, 16 ... Step part, 17 ... Front end side body part, 18 ... Rear end side body part, 19 ... Gutter part, 20 ... Electrode, 20 ... Center electrode, 21 ... center electrode body, 21A ... electrode base material, 21B ... core, 23 ... head, 24 ... buttock, 25 ... leg, 29 ... center electrode Tip: 30 ... Ground electrode, 34 ... Counter part, 35 ... Through hole, 36 ... Rear end side hole, 36S ... Expansion surface, 37 ... Front end side hole, 37 g ... hole, 38 b ... throttle part, 40 ... terminal fitting, 41 ... cap mounting part, 42 ... collar part, 43 ... leg part, 50 ... metal shell, 51 ... tool engaging part, 52 ... mounting screw part, 53 ... caulking part, 54 ... seat part, 56 ... step part, 58 ... compression deformation part, 59 ... Insertion hole, 60 ... conductive seal, 100 ... spark plug, 600. ..Ignition system, 640 ... Power supply for discharge, 650 ... High frequency power supply, 660 ... Mixed circuit, 662 ... Coil, 663 ... Capacitor, 670 ... Impedance matching circuit, 680 .. .Control device, 690 ... plug cord

Claims (9)

A rod-shaped center electrode extending along the axial direction;
A cylindrical insulator that holds the center electrode so that the tip of the center electrode protrudes from the tip of the center electrode to the tip side;
A cylindrical metal shell for holding the insulator;
A ground electrode joined to the metal shell and opposed to the front end surface of the center electrode via a gap;
A spark plug to which a plurality of peak voltages are supplied after the trigger discharge voltage is applied to the center electrode,
The ground electrode is formed with a through-hole penetrating in the axial direction through a position facing the tip surface of the center electrode.
The through hole includes a rear end side hole, a front end side hole located at a front end side with respect to the rear end side hole, and a front end side hole communicating with the front end of the rear end side hole;
The length of the hole on the rear end side in a direction perpendicular to the axis is longer from the front end side toward the rear end side.   The spark plug according to claim 1,
  The ground electrode includes a bar-like facing portion having a side surface facing the tip surface side of the center electrode,
  The spark plug, wherein the hole on the rear end side of the through hole is a hole that is recessed from the side surface to the front end side of the facing portion. The spark plug according to claim 1 or 2 ,
The diameter W1 of the front end surface of the center electrode, the length W2 in the direction perpendicular to the axis of the rear end of the holes on the rear end side, and the length in the direction perpendicular to the axis of the hole on the front end side W3 is a spark plug characterized by satisfying W3 <W1 <W2. The spark plug according to any one of claims 1 to 3 ,
The length L1 of the tip side hole in the axial direction and the length L2 of the through hole in the axial direction are 1.2 mm ≦ L2 ≦ 1.7 mm and 0.4 ≦ (L1 / L2) A spark plug characterized by satisfying ≦ 0.7. The spark plug according to any one of claims 1 to 4 ,
The spark plug according to claim 1, wherein the hole on the front end side includes a portion between the front end and the rear end that is shorter than the front end and the rear end in a direction perpendicular to the axial direction. The spark plug according to any one of claims 1 to 5 ,
An ignition system comprising: a voltage supply unit that supplies the trigger discharge voltage and the plurality of peak voltages to the center electrode. A rod-shaped center electrode extending along the axial direction;
A cylindrical insulator for holding the center electrode;
A cylindrical metal shell for holding the insulator;
A ground electrode joined to the metal shell and opposed to the front end surface of the center electrode via a gap;
A spark plug to which a plurality of peak voltages are supplied after the trigger discharge voltage is applied to the center electrode,
The ground electrode is formed with a through-hole penetrating in the axial direction through a position facing the tip surface of the center electrode.
The through hole includes a rear end side hole, a front end side hole located at a front end side with respect to the rear end side hole, and a front end side hole communicating with the front end of the rear end side hole;
The length in the direction perpendicular to the axis of the hole on the rear end side becomes longer from the front end side toward the rear end side,
The center electrode is disposed inside the shaft hole of the insulator,
The ground electrode is a plate-like member in which the through-hole is formed and disposed on the tip side of the insulator,
A cavity is formed by the surface on the front end side of the center electrode, the inner surface of the insulator, and the surface on the rear end side of the ground electrode,
The diameter R1 of the front end surface of the center electrode, the length R2 of the rearmost end in the direction perpendicular to the axis, and the length of the front end in the direction perpendicular to the axis R3 satisfies R3 <R1 <R2,
The distance D in the axial direction between the tip of the center electrode and the surface of the ground electrode on the rear end side where the through hole is formed satisfies 0.5 mm ≦ D ≦ 2.0. Features a spark plug. The spark plug according to claim 7 , wherein
A spark plug characterized in that the length E of the shortest path from the surface of the center electrode to the surface of the rear end side of the ground electrode through the inner surface of the insulator satisfies E ≧ 5D. A rod-shaped center electrode extending along the axial direction;
A cylindrical insulator for holding the center electrode;
A cylindrical metal shell for holding the insulator;
A ground electrode joined to the metal shell and opposed to the front end surface of the center electrode via a gap;
A spark plug to which a plurality of peak voltages are supplied after the trigger discharge voltage is applied to the center electrode,
The ground electrode is formed with a through-hole penetrating in the axial direction through a position facing the tip surface of the center electrode.
The through hole includes a rear end side hole, a front end side hole located at a front end side with respect to the rear end side hole, and a front end side hole communicating with the front end of the rear end side hole;
The length in the direction perpendicular to the axis of the hole on the rear end side becomes longer from the front end side toward the rear end side,
The diameter W1 of the front end surface of the center electrode, the length W2 in the direction perpendicular to the axis of the rear end of the holes on the rear end side, and the length in the direction perpendicular to the axis of the hole on the front end side W3 is a spark plug characterized by satisfying W3 <W1 <W2.

Images