JP5870074B2 - Spark plug - Google Patents

Description

  The present invention relates to a spark plug used for an internal combustion engine or the like.

  The spark plug is attached to an internal combustion engine (engine) or the like, and is used to ignite an air-fuel mixture in the combustion chamber. In general, a spark plug is fixed to an insulator having an axial hole extending in the axial direction, a center electrode inserted through the tip end of the shaft hole, a metal shell provided on the outer periphery of the insulator, and a tip of the metal shell. And a ground electrode. Further, the insulator is fixed to the metal shell in a state in which the step provided on the outer periphery thereof is locked to the inner peripheral portion of the metal shell either directly or via a metal plate packing. . Furthermore, a spark discharge gap is formed between the tip of the ground electrode and the tip of the center electrode, and a high voltage is applied to the spark discharge gap to generate spark discharge, thereby igniting an air-fuel mixture or the like. Has been made.

  In recent years, in order to improve fuel efficiency and comply with environmental regulations, engines with high supercharging and high compression have been proposed. In such an engine, the pressure in the combustion chamber becomes relatively large during operation, and thus a voltage (discharge voltage) necessary for generating a spark discharge becomes large. When the discharge voltage becomes large, a spark discharge (penetration discharge) penetrating the insulator is generated in a relatively thin leg portion located on the tip side of the step portion of the insulator, and normal spark discharge May cause trouble (cause misfire). Particularly in recent years, in order to reduce the size of the spark plug, the insulator is made thinner. In such a thin insulator, the occurrence of through discharge is particularly a concern.

  Therefore, it is conceivable to improve the withstand voltage performance of the insulator by densifying the insulator, that is, by reducing the porosity of the insulator in order to suppress the occurrence of through discharge. In the prior art, a technique for reducing the porosity of the insulator to 0.5% or less has been proposed (see, for example, Patent Document 1).

Japanese Patent Laid-Open No. 11-43368

  However, when the porosity of the insulator is made extremely small in order to improve the withstand voltage performance, the hardness of the insulator becomes high and the Young's modulus of the insulator becomes relatively large. In such an insulator having a large Young's modulus, thermal stress generated by heating and cooling between the outer peripheral side portion of the leg length portion and the inner peripheral side portion of the leg length portion increases. For this reason, breakage (cracking) is likely to occur in the leg length portion due to repeated cooling and heating cycles. On the other hand, if the porosity of the insulator is increased, the thermal shock resistance can be improved, but the withstand voltage performance is lowered. That is, the withstand voltage performance and the thermal shock resistance are in a trade-off relationship, and it is very difficult to obtain good performance in both.

  This invention is made | formed in view of the said situation, The objective is to provide the ignition plug which can fully improve both the withstand voltage performance and thermal shock resistance in an insulator.

  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 spark plug of this configuration includes an insulator having an axial hole penetrating in the axial direction;
A center electrode inserted on the tip side of the shaft hole;
A metal shell provided on the outer periphery of the insulator;
The insulator is a spark plug having a stepped portion that is locked to an inner peripheral portion of the metal shell, and a leg long portion that extends from the tip of the stepped portion to the tip side,
While the porosity in the leg length is 3.0% or less,
In the cross section orthogonal to the axis, the leg length portion is equally divided along the radial direction, the region located on the outermost periphery is defined as the first region, and the region located on the innermost periphery is defined as the second region, The porosity of the first region is not less than 1.20 times the porosity of the second region.

  According to the configuration 1, the porosity of the leg length portion is 3.0% or more, and the leg length portion is in a sufficiently dense state. Therefore, good withstand voltage performance can be realized.

  On the other hand, according to the configuration 1, the porosity of the first region located on the outer peripheral side is 1.20 times or more the porosity of the second region located on the inner peripheral side, The first region located is relatively sparse. Therefore, the Young's modulus in the first region can be reduced, and the thermal stress generated by heating / cooling between the outer peripheral side portion of the leg length portion and the inner peripheral side portion of the leg length portion can be reduced.

  Moreover, according to the said structure 1, since the porosity of a 2nd area | region is made comparatively small, the compressive stress toward the inner side will remain in the said inner peripheral part. When the outer peripheral portion contracts suddenly during rapid cooling and a tensile stress is generated on the surface, the thermal stress generated between the inner peripheral portion and the outer peripheral portion is further reduced due to the remaining compressive stress. can do. As a result, good thermal shock resistance can be obtained.

  Configuration 2. In the spark plug of this configuration, the porosity of the third region located between the first region and the second region in the cross section in the above configuration 1 is 1.05 times the porosity of the second region. It is characterized by the following.

  According to Configuration 2, the porosity of the third region located in the radial center of the long leg portion is 1.05 times or less than the porosity of the second region formed relatively densely. That is, the third region is also as dense as the second region. Therefore, the leg length can be made dense in a wide range along the radial direction, and the withstand voltage performance can be further enhanced.

  Configuration 3. The spark plug of this configuration is characterized in that, in the above configuration 1 or 2, the maximum thickness along the direction perpendicular to the axis of the leg long portion is 0.50 mm or more and 2.00 mm or less.

  According to the above configuration 3, the maximum thickness of the leg length portion is 2.00 mm or less, and it is very difficult to secure a good withstand voltage performance. Even when the leg length is thin, good withstand voltage performance can be obtained. In other words, the above configuration 1 or the like is particularly effective for a spark plug in which the maximum thickness of the leg length portion is 2.00 mm or less and it is difficult to ensure sufficient withstand voltage performance.

  Moreover, according to the said structure 3, since the maximum thickness of the leg length part shall be 0.50 mm or more, combined with the said structure 1 grade | etc., Can fully obtain the withstand voltage performance.

  Configuration 4. The spark plug of this configuration is characterized in that in any one of the above configurations 1 to 3, at least one of a mullite crystal phase and an aluminate crystal phase is present on the outer surface of the leg length portion.

  According to the said structure 4, the amount of thermal expansion of a leg long part can be reduced with a mullite crystal phase or an aluminate crystal phase. Therefore, the thermal stress generated between the outer peripheral side part of the leg long part and the inner peripheral side part of the leg long part can be further reduced. As a result, the thermal shock resistance can be further improved.

It is a partially broken front view which shows the structure of a spark plug. It is an expanded sectional view of a leg long part etc. along the direction orthogonal to an axis. It is an expanded sectional view showing the maximum thickness of a leg long part. It is a cross-sectional schematic diagram which shows one process of the manufacturing process of an insulator. It is a cross-sectional schematic diagram which shows one process of the manufacturing process of an insulator. It is a cross-sectional schematic diagram which shows one process of the manufacturing process of an insulator.

  Hereinafter, an embodiment will be described with reference to the drawings. FIG. 1 is a partially cutaway front view showing a spark plug 1. In FIG. 1, the direction of the axis CL <b> 1 of the spark plug 1 is the vertical direction in the drawing, the lower side is the front end side, and the upper side is the rear end side.

  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.

  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 large-diameter portion 11 that protrudes radially outward on the side, a middle body portion 12 that is smaller in diameter than the large-diameter portion 11, and a tip portion that is more distal than the middle body portion 12. The leg length part 13 formed in diameter smaller than this on the side is provided. In addition, of the insulator 2, the large diameter portion 11, the middle trunk portion 12, and most of the leg long portions 13 are accommodated inside the metal shell 3. A tapered step portion 14 is formed between the middle trunk portion 12 and the leg long portion 13, and the insulator 2 is locked to the metal shell 3 at the step portion 14.

  Further, a shaft hole 4 extending along the axis CL <b> 1 is formed through the insulator 2, and a center electrode 5 is inserted through the tip end 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 outer layer 5B made of an alloy containing Ni as a main component. Further, the center electrode 5 has a rod shape (cylindrical shape) as a whole, and a tip portion thereof protrudes from the tip of the insulator 2.

  In addition, 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 cylindrical resistor 7 is disposed between the center electrode 5 and the terminal electrode 6 of the shaft hole 4. Both ends of the resistor 7 are electrically connected to the center electrode 5 and the terminal electrode 6 through conductive glass seal layers 8 and 9, respectively.

  In addition, the metal shell 3 is formed in a cylindrical shape from a metal such as low carbon steel, and a screw portion (male screw) for attaching the spark plug 1 to an internal combustion engine, a fuel cell reformer, or the like on the outer peripheral surface thereof. Part) 15 is formed. Further, a seat portion 16 is formed on the rear end side of the screw portion 15 so as to protrude toward the outer peripheral side, and a ring-shaped gasket 18 is fitted into 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 hexagonal cross-section tool engaging portion 19 for engaging a tool such as a wrench when the spark plug 1 is attached to an internal combustion engine or the like, and radially inward. A caulking portion 20 that bends is provided. In this embodiment, in order to reduce the diameter of the spark plug 1, the metal shell 3 is reduced in diameter (for example, the screw diameter of the screw portion 15 is equal to or less than M12).

  A tapered portion 21 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 with respect to the metal shell 3, and the step 14 of the metal shell 3 is locked to the taper portion 21 of the metal shell 3. It is fixed to the metal shell 3 by caulking the rear end side opening portion radially inward, that is, by forming the caulking portion 20. An annular plate packing 22 is interposed between the stepped portion 14 and the tapered portion 21. Thereby, the airtightness in the combustion chamber is maintained, and the fuel gas entering the gap between the leg long portion 13 of the insulator 2 exposed to the combustion chamber 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 23 and 24 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 23 , 24 is filled with talc 25 powder. That is, the metal shell 3 holds the insulator 2 via the plate packing 22, the ring members 23 and 24, and the talc 25.

  Further, a rod-shaped ground electrode 27 which is bent back at an intermediate portion of the metal shell 3 at its middle portion and whose side surface on the tip side faces the tip of the center electrode 5 is joined to the tip portion 26 of the metal shell 3. A spark discharge gap 28 is formed between the tip of the center electrode 5 and the tip of the ground electrode 27, and spark discharge is performed in the spark discharge gap 28 in a direction substantially along the axis CL1. It has become.

  Furthermore, in this embodiment, the porosity in the leg length part 13 is 3.0% or less. The porosity can be obtained by the following method. That is, the long leg portion 13 is cut in the direction of the axis CL1, and the cut surface is mirror-polished. After that, the polished surface is observed with an SEM (for example, acceleration voltage 20 kV, spot size 50, COMPO image, composition image), and an image in which the pores of the entire polished surface can be distinguished is divided into one or several sheets. Get. And a porosity can be calculated | required by measuring the area ratio of a pore part from the acquired image. The area ratio of the pore portion can be measured using predetermined image analysis software (for example, Analysis Five manufactured by Soft Imaging System GmbH). In the case of using the illustrated image analysis software, an appropriate threshold value is set so that the pore portion is selected in the entire image of the polished surface.

  Further, as shown in FIG. 2, in the cross section orthogonal to the axis CL1, the leg length portion 13 is divided into three equal parts along the radial direction, and the region located on the outermost periphery is defined as the first region AR1, and is located on the innermost periphery. Let the region be a second region AR2. At this time, the porosity PO1 of the first area AR1 is 1.20 times or more the porosity PO2 of the second area AR2, and the first area AR1 is configured to be sparser than the second area AR2. ing.

  In addition, a region located between the first region AR1 and the second region AR2 in the cross section is referred to as a third region AR3. At this time, the porosity PO3 of the third area AR3 is set to 1.05 times or less of the porosity PO2 of the second area AR2. That is, the third area AR3 is configured to be as dense as the second area AR2 in a relatively dense state.

  Further, as the diameter of the metal shell 3 is reduced, the insulator 2 is made thinner. As shown in FIG. 3, the maximum thickness T along the direction perpendicular to the axis CL1 of the leg long portion 13 is 2.00 mm or less. It is said that. In the present embodiment, the maximum thickness T is set to 0.50 mm or more in the leg length portion 13 in order to prevent the withstand voltage performance and the mechanical strength from excessively decreasing.

Further, in the present embodiment, at least one of a mullite crystal phase and an aluminate crystal phase exists on the outer surface of the leg length portion 13. The mullite crystal phase can be generated from alumina (Al ₂ O ₃ ) and silica (SiO ₂ ) contained in the raw material powder, for example, in a firing step for forming the insulator 2 described later. Further, for example, by mixing mullite powder in the raw material powder, the mullite crystal phase can be configured to exist on the outer surface of the leg length portion 13. Further, the aluminate crystal phase is obtained by, for example, alumina and rare earth elements (for example, Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho contained in the raw material powder in the firing step. , Er, Tm, Yb, and Lu), or alumina and a Group 2 element (for example, Mg, Ca, Sr, and Ba). Further, for example, by mixing aluminate powder in the raw material powder, the aluminate crystal phase can be configured to exist on the outer surface of the leg portion 13.

  Next, the manufacturing method of the spark plug 1 comprised as mentioned above is demonstrated.

  First, the metal shell 3 is processed in advance. That is, a through hole is formed by subjecting a cylindrical metal material (for example, an iron-based material such as S17C or S25C or a stainless steel material) to a cold forging process, and a rough shape is manufactured. Thereafter, the outer shape is adjusted by cutting to obtain a metal shell intermediate.

  Subsequently, a ground electrode 27 made of Ni alloy is resistance-welded to the front end surface of the metal shell intermediate. During the welding, so-called “sag” occurs, and after removing the “sag”, the threaded portion 15 is formed by rolling at a predetermined portion of the metal shell intermediate body. Thereby, the metal shell 3 to which the ground electrode 27 is welded is obtained. The metal shell 3 to which the ground electrode 27 is welded is galvanized or nickel plated. In order to improve the corrosion resistance, the surface may be further subjected to chromate treatment.

On the other hand, the insulator 2 is prepared separately from the metal shell 3. More specifically, first, a raw material composed mainly of alumina (Al ₂ O ₃ ) powder and containing at least one kind of sintering aid containing silica (SiO ₂ ), rare earth elements, Group 2 elements and the like. A slurry is prepared by wet-mixing the powder with water as a solvent after containing a predetermined binder. Then, the adjusted slurry is spray-dried to obtain a granular body.

  Next, the obtained granule is subjected to rubber press molding with a rubber press molding machine 41 to produce a molded body. As shown in FIG. 4, the rubber press molding machine 41 includes a cylindrical inner rubber mold 43 having a cavity 42 extending along the direction of the central axis CL <b> 2, and a cylindrical shape provided on the outer periphery of the inner rubber mold 43. An outer rubber mold 44, a molding machine main body 45 provided on the outer periphery of the outer rubber mold 44, and a bottom lid 46 and a lower holder 47 for closing the lower opening of the cavity 42 are provided. Further, the molding machine main body 45 is provided with a liquid channel 45A, and by applying a hydraulic pressure to the outer peripheral surface of the outer rubber mold 44 in the radial direction via the liquid channel 45A, the cavity 42 can be reduced in the radial direction.

  Returning to the description of the manufacturing method, first, the particulate matter PM is filled in the cavity 42 of the inner rubber mold 43. Next, as shown in FIG. 5, a press pin 51 for forming the shaft hole 4 made of a high hardness material such as metal or ceramic is disposed in the cavity 42.

  Next, by applying a hydraulic pressure through the liquid channel 45A, pressure is applied from the outer peripheral sides of the inner rubber mold 43 and the outer rubber mold 44, and the cavity 42 is reduced. Then, after releasing the hydraulic pressure after a predetermined time has elapsed, as shown in FIG. 6, the granular material PM is compressed together with the press pins 51 by pulling the press pins 51 from the rubber press molding machine 41 in the direction of the axis CL2. Then, the formed body CP formed is removed from the cavity 42. Thereafter, by rotating the press pin 51 relative to the molded body CP, the press pin 51 is extracted from the molded body CP, and the molded body CP is obtained. In this embodiment, the hydraulic pressure is adjusted so that the pressure increase speed of the inner rubber mold 43 (the contraction speed of the inner rubber mold 43) is relatively high, or the inner rubber mold 43 and the outer rubber mold 44 are relatively thin. It is comprised so that the site | part located in the vicinity of the press pin 51 among the powdery bodies PM may be compressed more. Thereby, the molded object CP is comprised so that it may become sparse from the radial direction inner side toward the outer side. In the present embodiment, the pressure increasing speed is suppressed to a certain level so that the central portion in the radial direction of the molded body CP is also sufficiently dense. Furthermore, by adjusting the pressure applied to the powder body PM and the application time of the pressure, the molded body CP is in a state where the pores are sufficiently small.

  Next, the outer periphery of the obtained molded body CP is ground to obtain an insulator intermediate that has substantially the same outer shape as the insulator 2. And in the baking process, the insulator 2 is obtained by baking the said insulator intermediate body with a baking furnace. Note that, as described above, the molded body CP is configured so as to become denser and sparser from the inside in the radial direction toward the outside. Therefore, in the obtained insulator 2, the porosity PO1 of the first region AR1 is obtained. Is not less than 1.20 times the porosity PO2 of the second region AR2. Further, since the central portion in the radial direction of the molded body CP is sufficiently dense, in the obtained insulator 2, the porosity PO3 of the third region AR3 is 1.05 times or less than the porosity PO2 of the second region AR2. It has become. Furthermore, since the molded body CP has a sufficient number of pores, the porosity of at least the leg length portion 13 is 3.0% or less.

  Separately from the metal shell 3 and the insulator 2, the center electrode 5 is manufactured. That is, the center electrode 5 is produced by forging a Ni alloy in which a copper alloy or the like for improving heat dissipation is arranged at the center.

  Then, the insulator 2 and the center electrode 5, the resistor 7, and the terminal electrode 6 obtained as described above are sealed and fixed by the glass seal layers 8 and 9. The glass seal layers 8 and 9 are generally prepared by mixing borosilicate glass and metal powder, and the prepared material is injected into the shaft hole 4 of the insulator 2 with the resistor 7 interposed therebetween. After being heated, it is baked and hardened by pressing with the terminal electrode 6 from behind while heating in the baking furnace. At this time, the glaze layer may be fired simultaneously on the surface of the rear end body portion 10 of the insulator 2 or the glaze layer may be formed in advance.

  Thereafter, the insulator 2 provided with the center electrode 5 and the terminal electrode 6 and the metal shell 3 provided with the ground electrode 27 are fixed. More specifically, after the insulator 2 is inserted through the metal shell 3, the rear end side opening of the metal shell 3 formed relatively thin is crimped radially inward, that is, the crimp portion 20 is formed. By doing so, the insulator 2 and the metal shell 3 are fixed.

  Finally, the intermediate portion of the ground electrode 27 is bent toward the center electrode 5 side, and the size of the spark discharge gap 28 formed between the center electrode 5 and the ground electrode 27 is adjusted, whereby the spark plug 1 described above is used. Is obtained.

  As described above in detail, according to the present embodiment, the porosity of the leg length portion 13 is 3.0% or more, and the leg length portion 13 is in a sufficiently dense state. Therefore, good withstand voltage performance can be realized. In particular, in this embodiment, since the maximum thickness T of the leg length portion 13 is 2.00 mm or less, it is difficult to ensure sufficient withstand voltage performance, but according to this embodiment, good withstand voltage performance is achieved. Can be obtained.

  In the present embodiment, the porosity PO1 of the first area AR1 is 1.20 times or more the porosity PO2 of the second area AR2, and the first area AR1 is relatively sparse. . Therefore, the Young's modulus in the first region AR1 can be reduced, and the thermal stress generated by heating / cooling between the outer peripheral side portion of the leg length portion 13 and the inner peripheral side portion of the leg length portion 13 can be reduced. it can.

  Furthermore, in this embodiment, since the porosity PO2 of the second region AR2 is relatively small, the compressive stress toward the inside remains in the inner peripheral portion. When the outer peripheral portion contracts suddenly during rapid cooling and a tensile stress is generated on the surface, the thermal stress generated between the inner peripheral portion and the outer peripheral portion is further reduced due to the remaining compressive stress. can do. As a result, good thermal shock resistance can be obtained.

  In addition, the porosity PO3 of the third region AR3 is 1.05 times or less the porosity PO2 of the relatively densely formed second region AR2, and the third region AR3 is also the same as the second region AR2. It is considered as a dense state. Therefore, the leg length portion 13 can be in a dense state in a wide range along the radial direction, and the withstand voltage performance can be further enhanced.

  Moreover, in this embodiment, it is comprised so that at least one of a mullite crystal phase and an aluminate crystal phase may exist in the outer surface of the leg long part 13. FIG. Therefore, the amount of thermal expansion of the leg long portion 13 can be reduced, and the thermal stress generated between the outer peripheral side portion of the leg long portion 13 and the inner peripheral side portion of the leg long portion 13 can be further reduced. As a result, the thermal shock resistance can be further improved.

  Next, in order to confirm the effect achieved by the above embodiment, the porosity PO1 (%) of the first region, the porosity PO2 (%) of the second region, the porosity PO3 (%) of the third region, the leg length Make several spark plug samples with various changes in the porosity PO0 (%) of the whole part and the presence or absence of mullite crystal phase or aluminate crystal phase on the outer surface of the leg length part, and withstand voltage performance evaluation test for each sample And the thermal shock resistance evaluation test was done.

  The outline of the withstand voltage performance evaluation test is as follows. That is, 30 samples each having the same porosity in the first region are prepared, and the tip of the sample is immersed in a predetermined insulating oil so that no spark discharge occurs between the center electrode and the ground electrode. did. In addition, the voltage is applied to the center electrode, and the applied voltage is gradually increased to apply when a discharge penetrating the insulator (leg leg) occurs between the center electrode and the metal shell. The voltage (through voltage) was measured. Furthermore, the average value (average penetration voltage) of the penetration voltage in 30 samples was calculated. Here, a sample having an average through voltage of 40 kV or more and less than 41 kV is evaluated as “◯” because it has good withstand voltage performance, and a sample having an average through voltage of 41 kV or more is very excellent. It was decided to give an evaluation of “◎” as having a withstand voltage performance. On the other hand, a sample having an average through voltage of less than 40 kV was evaluated as “x” because it had poor withstand voltage performance.

  The outline of the thermal shock resistance evaluation test is as follows. That is, after making the heating temperature different, the sample was heated for 30 minutes, and then the sample was dropped into 20 ° C. water and rapidly cooled. Thereafter, a predetermined test solution was applied to the surface of the leg length part to clarify the presence or absence of cracks in the leg length part, and whether or not the leg length part was cracked was visually confirmed. Here, when the heating temperature was set to 180 ° C. or more and less than 200 ° C., the sample in which the crack of the leg length portion occurred was evaluated as “◯” as having good thermal shock resistance, and the heating temperature was set to 200 ° C. or more. The sample in which cracks occurred in the leg long part was evaluated as “◎” as having excellent thermal shock resistance. On the other hand, when the heating temperature was less than 180 ° C., the sample with cracks in the leg length was evaluated as “x” because it was inferior in thermal shock resistance.

  Table 1 shows the results of both tests. The porosity PO1 and the like in the first region were changed by adjusting the pressure increase speed when forming the molded body. Separately from the test sample, a sample for measuring the porosity was separately prepared under the same conditions as the test sample. And the porosity PO1 etc. of the 1st field were measured with the sample for measuring the porosity. Specifically, the leg long part was cut in a direction perpendicular to the axis, and the cut surface was mirror-polished. Then, the porosity at each of the 10 points in the first to third regions was measured with a predetermined electron microscope, and the average value of the measured porosity was defined as the porosity PO1 to PO3 in the first to third regions. . Moreover, the average value of the porosity of 30 points in total in the first to third regions was defined as the overall porosity PO0 of the leg length portion. Further, when the outer surface of the leg length portion is subjected to X-ray diffraction analysis by a predetermined X-ray diffractometer, the outer surface of the leg length portion is determined depending on whether or not the intrinsic peak value of the mullite crystal phase or the aluminate crystal phase is detected. It was determined whether a mullite crystal phase or an aluminate crystal phase was present.

  In addition, in sample 6, the thickness of the insulator is significantly smaller than the thickness of the insulator in the other samples (specifically, about 2/3), thereby ensuring good withstand voltage performance. It was extremely difficult.

  As shown in Table 1, it was found that the withstand voltage performance of the sample (sample 1) in which the overall porosity PO0 of the leg length portion was over 3.0% was insufficient.

  Furthermore, although the overall porosity PO0 of the leg length portion is 3.0% or less, PO1 / PO2 is less than 1.20 (that is, the porosity PO1 of the first region is 1.20 of the porosity PO2 of the second region). It was revealed that the sample (sample 2), which was less than double), had good withstand voltage performance, but was inferior in thermal shock resistance. This is presumably because the first region is very dense and the Young's modulus of the first region is relatively large, so that the thermal stress generated in the leg length portion during rapid cooling increases.

On the other hand, PO1 / PO2 is 1.20 or more while the overall porosity PO0 of the leg length portion is 3.0% or less (that is, the porosity PO1 of the first region is equal to 1 of the porosity PO2 of the second region). It was confirmed that the samples (samples 3 to 8) which were 20 times or more) had good performance in both withstand voltage performance and thermal shock resistance. This is considered to be due to the following (1) to (3).
(1) By setting the porosity PO0 to 3.0% or less, the denseness of the leg length has been sufficiently increased.
(2) By setting the porosity PO1 to 1.20 times or more of the porosity PO2, the Young's modulus of the first region is sufficiently small, and the thermal stress generated in the leg length portion during rapid cooling is reduced.
(3) Since the porosity PO2 is made relatively small, the compressive stress toward the inner side remains in the inner peripheral portion of the insulator, so that the outer peripheral portion of the insulator contracts rapidly during rapid cooling. When the tensile stress is generated on the surface, the thermal stress generated between the inner peripheral portion and the outer peripheral portion is further reduced.

  Furthermore, samples (samples 4 to 8) having PO3 / PO2 of 1.05 or less (that is, the porosity PO3 of the third region is 1.05 times or less of the porosity PO2 of the second region) are more excellent in resistance. It was found that good withstand voltage performance can be secured even in sample 6 having voltage performance and having a very small insulator thickness. This is considered to be due to the fact that the leg length portion is dense in a wide range along the radial direction.

  In addition, the sample (sample 7) in which the mullite crystal phase is present on the outer surface of the leg length part and the sample (sample 8) in which the aluminate crystal phase is present on the surface of the leg length part have extremely excellent thermal shock resistance. It became clear to have. This is presumably because the amount of thermal expansion of the leg length portion was reduced by the mullite crystal phase or the aluminate crystal phase, and the thermal stress generated in the leg length portion during quenching was further reduced.

  From the results of the above test, in order to obtain good performance in both the withstand voltage performance and the thermal shock resistance, the porosity at the leg length is set to 3.0% or less, and the porosity PO1 in the first region is set to the second. It can be said that it is preferable to be 1.20 times or more the porosity PO2 of the region.

  Furthermore, from the viewpoint of further improving the withstand voltage performance, it can be said that the porosity PO3 of the third region is more preferably 1.05 times or less of the porosity PO2 of the second region.

  In addition, from the viewpoint of further improving the thermal shock resistance, it can be said that it is more preferable that at least one of a mullite crystal phase and an aluminate crystal phase exist on the outer surface of the leg length portion.

  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 maximum thickness T of the leg length portion 13 is set to 2.00 mm or less, but the technical idea of the present invention is applied to a spark plug having a maximum thickness T exceeding 2.00 mm. May be.

  (B) In the above embodiment, the spark plug 1 ignites an air-fuel mixture or the like by generating a spark discharge in the spark discharge gap 28. The structure of the spark plug to which the technical idea of the present invention can be applied. Is not limited to this. Therefore, for example, with respect to an ignition plug (plasma jet ignition plug) having a cavity (space) at the tip of the insulator and igniting an air-fuel mixture or the like by ejecting plasma generated in the cavity The technical idea of the present invention may be applied. Further, the technical idea of the present invention may be applied to an ignition plug (high frequency plasma ignition plug) that generates plasma between both electrodes by applying high frequency power between the center electrode and the ground electrode.

  (C) In the above embodiment, the case where the ground electrode 27 is joined to the distal end portion 26 of the metal shell 3 is embodied. However, a part of the metal shell (or the tip metal fitting previously welded to the metal shell is used. The present invention is also applicable to the case where the ground electrode is formed by cutting out a part of the ground (for example, Japanese Patent Application Laid-Open No. 2006-236906).

  (D) In the above embodiment, the tool engaging portion 19 has a hexagonal cross section, but the shape of the tool engaging 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)].

1 ... Spark plug 2 ... Insulator (insulator)
DESCRIPTION OF SYMBOLS 3 ... Metal shell 4 ... Shaft hole 5 ... Center electrode 13 ... Leg long part 14 ... Step part AR1 ... 1st area | region AR2 ... 2nd area | region AR3 ... 3rd area | region CL1 ... Axis line

Claims (4)

An insulator having an axial hole penetrating in the axial direction;
A center electrode inserted on the tip side of the shaft hole;
A metal shell provided on the outer periphery of the insulator;
The insulator is a spark plug having a stepped portion that is locked to an inner peripheral portion of the metal shell, and a leg long portion that extends from the tip of the stepped portion to the tip side,
While the porosity in the leg length is 3.0% or less,
In the cross section orthogonal to the axis, the leg length portion is equally divided along the radial direction, the region located on the outermost periphery is defined as the first region, and the region located on the innermost periphery is defined as the second region, A spark plug characterized in that the porosity of the first region is 1.20 times or more the porosity of the second region.   The porosity of the third region located between the first region and the second region in the cross section is 1.05 or less of the porosity of the second region. The spark plug described.   3. The spark plug according to claim 1, wherein a maximum thickness of the leg long portion along a direction orthogonal to the axis is 0.50 mm or more and 2.00 mm or less.   The spark plug according to any one of claims 1 to 3, wherein at least one of a mullite crystal phase and an aluminate crystal phase is present on an outer surface of the leg length portion.

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