Patent Publication Number: US-8970098-B1

Title: Ignition plug

Description:
This application claims the benefit of Japanese Patent Applications No. 2013-202872 filed on Sep. 30, 2013, which is incorporated by reference in its entirety herein. 
     FIELD OF THE INVENTION 
     The present invention relates to an ignition plug for use in an internal combustion engine or the like. 
     BACKGROUND OF THE INVENTION 
     An ignition plug is attached to an internal combustion engine (engine) etc. and used to ignite, for example, an air-fuel mixture in a combustion chamber. Generally, an ignition plug includes an insulator having an axial hole extending in an axial direction, a center electrode inserted into a forward end portion of the axial hole, a metallic shell disposed externally of the insulator, and a ground electrode fixed to a forward end portion of the metallic shell. The insulator is fixed to the metallic shell in a state in which a step portion provided on the outer circumference of the insulator is engaged with an inner circumferential portion of the metallic shell directly or through a metal-made sheet packing. A spark discharge gap is formed between a distal end portion of the ground electrode and a forward end portion of the center electrode. A high voltage is applied across the spark discharge gap to generate spark discharge, whereby an air-fuel mixture, for example, is ignited. 
     In engines proposed in recent years, to improve fuel economy and to cope with environmental regulations, the degree of supercharging and the dree of compression, for example, are increased. In these engines, since the pressure inside each combustion chamber during operation is relatively high, the voltage necessary to generate spark discharge (discharge voltage) is also high. When the discharge voltage is high, spark discharge passing through the insulator (penetration discharge) may occur in a leg portion of the insulator which is located forward of the step portion and whose wall thickness is relatively small, and this may hinder normal spark discharge (may cause a misfire). Particularly, in recent years, to achieve a reduction in ignition plug size, the insulator is further reduced in wall thickness. The possibility of the occurrence of penetration discharge is particularly high in such a thin-walled insulator. 
     A possible measure for suppressing the occurrence of penetration discharge is increasing the denseness of the insulator, i.e., reducing the porosity of the insulator, to thereby increase the dielectric strength of the insulator. In one previously proposed technique (see, for example, Japanese Patent Application Laid-Open (kokai) No. H11-43368), the porosity of the insulator is reduced to 0.5% or less. 
     Problem to be Solved by the Invention 
     When the porosity of the insulator is significantly reduced in order to increase the dielectric strength, the hardness of the insulator increases, so that the Young&#39;s modulus of the insulator becomes relatively high. In the insulator with high Young&#39;s modulus, heating and cooling cause large thermal stress between outer and inner circumferential portions of the leg portion. Therefore, when a thermal cycle is repeated, breakage (cracking) easily occurs in the leg portion. When the porosity of the insulator is increased, the thermal shock resistance of the insulator can be enhanced, but its dielectric strength decreases. Specifically, the dielectric strength and the thermal shock resistance are in a trade-off relation, and therefore it is very difficult to obtain both high dielectric strength and high thermal shock resistance simultaneously. 
     The present invention has been made in view of the above circumstances, and an object of the invention is to provide an ignition plug whose insulator has sufficiently increased dielectric strength and thermal shock resistance. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problem 
     Configurations suitable for achieving the above object will next be described in itemized form. When needed, actions and effects peculiar to the configurations will be described additionally. 
     Configuration 1. An ignition plug of the present configuration comprises: 
     an insulator having an axial hole extending therethrough in the direction of an axial line (axis); 
     a center electrode inserted into a forward end portion of the axial hole; and 
     a metallic shell disposed externally of the insulator, wherein 
     the insulator includes a step portion engaged with an inner circumferential portion of the metallic shell and a leg portion extending forward from a forward end of the step portion, 
     the porosity of the leg portion of the insulator is 3.0% or less, and 
     the porosity of a first region is equal to or more than 1.20 times the porosity of a second region, where the first region is an outermost region of three regions of the leg portion that are radially trisected in a cross section perpendicular to the axial line, and the second region is an innermost region of the three regions. 
     In configuration 1 described above, the porosity of the leg portion is 3.0% or less, and the leg portion is sufficiently dense. Therefore, sufficient dielectric strength can be achieved. 
     In configuration 1 described above, the porosity of the first region located on the outer circumferential side is equal to or more than 1.20 times the porosity of the second region on the inner circumferential side, so the denseness of the first region located on the outer circumferential side is relatively low. Therefore, the Young&#39;s modulus in the first region can be reduced, and thermal stress generated between the outer and inner circumferential portions of the leg portion during heating and cooling can be reduced. 
     In configuration 1 described above, since the porosity of the second region is relatively small, inward compressive stress remains in the inner circumferential portion. When the outer circumferential portion shrinks rapidly during rapid cooling and tensile stress occurs on the surface, the remaining compressive stress can further reduce the thermal stress generated between the outer and inner circumferential portions. Therefore, sufficient thermal shock resistance can be obtained. 
     Configuration 2. An ignition plug of the present configuration is characterized in that, in the above-described configuration 1, the porosity of a third region is equal to or less than 1.05 times the porosity of the second region, where the third region is located between the first region and the second region in the cross section. 
     In configuration 2 described above, the porosity of the third region located in the radially central portion of the leg portion is equal to or less than 1.05 times the porosity of the second region formed so as to be relatively dense. Specifically, the third region is as dense as the second region. Therefore, the leg portion can be dense over a wide area in the radial direction, and the dielectric strength can be further increased. 
     Configuration 3. An ignition plug of the present configuration is characterized in that, in the above-described configuration 1 or 2, the maximum wall thickness of the leg portion in a direction perpendicular to the axial line is 0.50 mm or more and 2.00 mm or less. 
     In configuration 3 described above, since the maximum wall thickness of the leg portion is set to 2.00 mm or less, it is very difficult to ensure sufficient dielectric strength. However, when, for example, configuration 1 is employed, sufficient di strength can be obtained even when the leg portion is thin-walled. In other words, configuration 1 etc. are particularly effective for an ignition plug in which the maximum wall thickness of the leg portion is 2.00 mm or less and sufficient dielectric strength is difficult to ensure. 
     In configuration 3 described above, the maximum wall thickness of the leg portion is set to 0.50 mm or more. Therefore, a combination of configuration 3 and configuration 1 etc. allows sufficiently high dielectric strength to be obtained. 
     Configuration 4. An ignition plug of the present configuration is characterized in that, in any of the above-described 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 portion. 
     In configuration 4 described above, the mullite crystal phase and the aluminate crystal phase can reduce the amount of thermal expansion of the leg portion. Therefore, the thermal stress generated between the outer and inner circumferential portions of the leg portion can be further reduced. As a result, the thermal shock resistance can be further enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a partially cutaway front view showing the configuration of an ignition plug. 
         FIG. 2  is an enlarged sectional view of a leg portion etc. taken in a direction orthogonal to an axial line. 
         FIG. 3  is an enlarged sectional view showing the maximum wall thickness of the leg portion. 
         FIG. 4  is a schematic sectional view showing a step in a process of producing an insulator. 
         FIG. 5  is a schematic sectional view showing another step in the process of producing the insulator. 
         FIG. 6  is a schematic sectional view showing another step in the process of producing the insulator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Modes for Carrying out the Invention 
     An embodiment of the present invention will next be described with reference to the drawings.  FIG. 1  is a partially cutaway front view showing an ignition plug  1 . In the following description with reference to  FIG. 1 , the direction of an axial line CL 1  of the ignition plug  1  is referred to as the vertical direction; the lower side is referred to as the forward side of the ignition plug  1 ; and the upper side as the rear side. 
     The ignition plug  1  includes a tubular insulator  2 , a tubular metallic shell  3  that holds the insulator  2 , and other components. 
     The insulator  2  is formed from alumina or the like by firing, as well known in the art. The insulator  2 , as viewed externally, includes a rear trunk portion  10  formed on the rear side; a large-diameter portion  11  that is located forward of the rear trunk portion  10  and protrudes radially outward; an intermediate trunk portion  12  that is located forward of the large-diameter portion  11  and is smaller in diameter than the large-diameter portion  11 ; and a leg portion  13  that is located forward of the intermediate trunk portion  12  and is smaller in diameter than the intermediate trunk portion  12 . The large-diameter portion  11 , the intermediate trunk portion  12 , and most of the leg portion  13  of the insulator  2  are accommodated in the metallic shell  3 . A tapered, stepped portion  14  is formed between the intermediate trunk portion  12  and the leg portion  13 . The insulator  2  is engaged with the metallic shell  3  at the stepped portion  14 . 
     The insulator  2  has an axial hole  4  extending therethrough along the axial line CL 1 , and a center electrode  5  is inserted into a forward end portion of the axial hole  4 . The center electrode  5  includes an inner layer  5 A formed of a metal having good thermal conductivity (e.g., copper, a copper alloy, or pure nickel (Ni)) and an outer layer  5 B formed of an alloy containing nickel (Ni) as a main component. The center electrode  5  has a rod-like (circular columnar) shape as a whole and has a forward end portion protruding from the forward end of the insulator  2 . 
     A terminal electrode  6  is fixedly inserted into a rear end portion of the axial hole  4  and protrudes from the rear end of the insulator  2 . 
     A circular columnar resistor  7  is disposed within the axial hole  4  between the center electrode  5  and the terminal electrode  6 . Opposite end portions of the resistor  7  are electrically connected to the center electrode  5  and the terminal electrode  6  via electrically conductive glass seal layers  8  and  9 , respectively. 
     The metallic shell  3  is formed into a tubular shape from a metal such as low-carbon steel and has, on its outer circumferential surface, a threaded portion (externally threaded portion)  15  for mounting the ignition plug  1  on an internal combustion engine, a fuel cell reformer, etc. A seat portion  16  protruding outward is formed rearward of the threaded portion  15 . A ring-like gasket  18  is fitted to a screw neck  17  located at the rear end of the threaded portion  15 . The metallic shell  3  further has, at its rear end portion, a tool engagement portion  19  and a crimped portion  20  bent radially inward. The tool engagement portion  19  has a hexagonal cross section and allows a tool such as a wrench to be engaged therewith when the ignition plug  1  is attached to, for example, an internal combustion engine. In the present embodiment, to reduce the diameter of the ignition plug  1 , the metallic shell  3  is reduced in diameter (e.g., the thread diameter of the threaded portion  15  is M12 or less). 
     A tapered portion  21  with which the insulator  2  is engaged is provided on the inner circumferential surface of the metallic shell  3 . The insulator  2  is inserted into the metallic shell  3  from the rear end side thereof toward the forward end side, and the step portion  14  is engaged with the tapered portion  21  of the metallic shell  3 . In this state, a rear opening portion of the metallic shell  3  is crimped radially inward, i.e., the crimp portion  20  is formed. As a result, the insulator  2  is fixed to the metallic shell  3 . An annular sheet packing  22  is interposed between the step portion  14  and the tapered portion  21 . This maintains airtightness inside a combustion chamber, so that fuel gas entering the gap between the inner circumferential surface of the metallic shell  3  and the leg portion  13  of the insulator  2 , exposed to the combustion chamber, is prevented from leaking to the outside. 
     To make the seal achieved by crimping more complete, annular ring members  23  and  24  are interposed between the metallic shell  3  and the insulator  2  at the rear end of the metallic shell  3 , and the gap between the ring members  23  and  24  is filled with powder of talc  25 . More specifically, the metallic shell  3  holds the insulator  2  through the sheet packing  22 , the ring members  23  and  24 , and the talc  25 . 
     A rod-shaped ground electrode  27  is joined to a forward end portion  26  of the metallic shell  3 . The ground electrode  27  is bent at its intermediate portion, and a side surface of a distal end portion of the ground electrode  27  faces the forward end portion of the center electrode  5 . A spark discharge gap  28  is formed between the forward end portion of the center electrode  5  and the distal end portion of the ground electrode  27 , and spark discharge is generated in the spark discharge gap  28  in a direction substantially along the axial line CL 1 . 
     In the present embodiment, the porosity of the leg portion  13  is 3.0% or less. The porosity can be determined by the following method. The leg portion  13  is cut in a direction orthogonal to the axial line CL 1 , and the cut section is mirror-polished. Then the polished surface is observed under an SEM (e.g., acceleration voltage: 20 kV, spot size: 50, COMPO image (composition image)) to acquire one image or a plurality of split images so that pores over the entire polished surface can be identified. The area ratio of the pore portions is measured in the acquired image(s), and the porosity can thereby be determined. The area ratio of the pore portions can be measured using prescribed image analysis software (e.g., Analysis Five of Soft Imaging System GmbH). When the exemplified image analysis software is used, an appropriate threshold value is set such that the pore portions can be selected over the entire image of the polished surface. 
     Next, as shown in  FIG. 2 , the leg portion  13  is trisected radially in a cross section perpendicular to the axial line CL 1 . An outermost region is referred to as a first region AR 1 , and an innermost region is referred to as a second region AR 2 . In this case, the porosity PO1 of the first region AR 1  is equal to or more than 1.20 times the porosity PO2 of the second region AR 2 , and therefore the leg portion  13  is formed such that the first region AR 1  is less dense than the second region AR 2 . 
     In addition, a region between the first region AR 1  and the second region AR 2  in the cross section is referred to as a third region AR 3 . In this case, the porosity PO3 of the third region AR 3  is equal to or less than 1.05 times the porosity PO2 of the second region AR 2 . Specifically, the third region AR 3  is formed so as to be as dense as the relatively dense second region AR 2 . 
     Since the metallic shell  3  is reduced in diameter, the insulator  2  is also reduced in wall thickness. Therefore, as shown in  FIG. 3 , the maximum wall thickness T of the leg portion  13  in a direction orthogonal to the axial line CL 1  is set to 2.00 mm or less. In the present embodiment, to prevent an excessive reduction in the dielectric strength and mechanical strength of the leg portion  13 , the maximum wall thickness T is set to 0.50 mm or more. 
     In the present embodiment, the leg portion  13  is configured such that at least one of a mullite crystal phase and an aluminate crystal phase is present on the outer surface of the leg portion  13 . The mullite crystal phase may be produced from alumina (Al 2 O 3 ) and silica (SiO 2 ) contained in a raw material powder in a later-described firing step of forming the insulator  2 . Alternatively, the leg portion  13  may be formed such that the mullite crystal phase is present on the outer surface of the leg portion  13  by mixing mullite powder into a raw material powder in advance. The aluminate crystal phase may be produced in the firing step from alumina and a rare-earth element (such as Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) contained in a raw material powder or from alumina and a group 2 element (such as Mg, Ca, Sr, or Ba) contained in the raw material powder. Alternatively, the leg portion  13  may be formed such that the aluminate crystal phase is present on the outer surface of the leg portion  13  by mixing aluminate powder into a raw material powder in advance. 
     Next, a description will be given of a method of producing the ignition plug  1  configured as described above. 
     First, the metallic shell  3  is formed beforehand. Specifically, a circular columnar metal material (e.g., an iron-based or stainless steel material such as S17C or S25C) is subjected to cold forging to form a through hole, whereby a general shape is formed. Then cutting is performed to adjust the outer shape, and a metallic shell intermediate is thereby obtained. 
     Then the ground electrode  27  formed of a Ni alloy is resistance-welded to a forward end face of the metallic shell intermediate. Since so-called “sags” occur during the resistance welding, the “sags” are removed, and then the threaded portion  15  is formed at a prescribed portion of the metallic shell intermediate by rolling. A metallic shell  3  with the ground electrode  27  welded thereto is thereby obtained. The metallic shell  3  with the ground electrode  27  welded thereto is plated with zinc or nickel. In order to enhance corrosion resistance, the plated surface may be further subjected to chromate treatment. 
     Separately from the metallic shell  3 , the insulator  2  is produced in advance. More specifically, first, a prescribed binder is added to a raw material powder including alumina (Al 2 O 3 ) powder as a main component and a sintering agent containing at least one of silica (SiO 2 ), a rare-earth element, a group 2 element, etc., and these components are wet-mixed using water as a solvent to thereby prepare a slurry. The prepared slurry is sprayed and dried to obtain a particulate material. 
     The obtained particulate material is subjected to rubber press forming using a rubber press machine  41  to produce a green compact. As shown in  FIG. 4 , the rubber press machine  41  includes a cylindrical inner rubber die  43  having a cavity  42  extending in the direction of a center axis CL 2 , a cylindrical outer rubber die  44  disposed externally of the inner rubber die  43 , a rubber press machine main body  45  disposed externally of the outer rubber die  44 , a bottom lid  46  for closing a lower opening of the cavity  42 , and a lower holder  47 . Liquid passages  45 A are provided in the rubber press machine main body  45 . By applying hydraulic pressure to the outer circumferential surface of the outer rubber die  44  in the radial direction through the liquid passages  45 A, the cavity  42  can be shrunk radially. 
     Returning to the description of the production method, first, the particulate material PM is charged into the cavity  42  of the inner rubber die  43 . Next, as shown in  FIG. 5 , a pressing pin  51  made of a high-hardness material such as a metal or ceramic and used to form the axial hole  4  is disposed inside the cavity  42 . 
     Next, hydraulic pressure is applied through the liquid passages  45 A, and pressure is thereby applied externally to the inner rubber die  43  and the outer rubber die  44  to shrink the cavity  42 . After a lapse of a prescribed time, the application of the hydraulic pressure is released. Then the pressing pin  51  is pulled up from the rubber press machine  41  in the direction of the center axis CL 2  as shown in  FIG. 6 . As a result, the compact CP formed by compressing the particulate material PM is removed from the cavity  42  together with the pressing pin  51 . Then the pressing pin  51  is rotated relative to the compact CP to pull the pressing pin  51  out from the compact CP, whereby the compact CP is obtained. In the present embodiment, a portion of the particulate material PM that is located in the vicinity of the pressing pin  51  is compressed to a higher degree. This is achieved by controlling the hydraulic pressure such that the pressure increasing speed of the inner rubber die  43  (the shrinking speed of the inner rubber die  43 ) is relatively fast or by using a relatively thin-walled inner rubber die  43  and/or a relatively thin-walled outer rubber die  44 . In this manner, the compact CP is formed such that its denseness decreases radially from the inner side toward the outer side. In the present embodiment, by controlling the pressure increasing speed to a certain speed, the radially central portion of the compact CP becomes sufficiently dense. By controlling the pressure applied to the particulate material PM and the time of application of the pressure, the amount of pores in the compact CP is sufficiently reduced. 
     Next, the outer circumference of the obtained compact CP is subjected to grinding to obtain an insulator intermediate having an outer shape substantially the same as the outer shape of the insulator  2 . The insulator intermediate is fired in a firing furnace in the firing step to obtain the insulator  2 . As described above, the compact CP is formed such that its denseness decreases radially from the inner side toward the outer side. Therefore, in the obtained insulator  2 , the porosity PO1 of the first region AR 1  is equal to or more than 1.20 times the porosity PO2 of the second region AR 2 . Moreover, the radially central portion of the compact CP is sufficiently dense. Therefore, in the obtained insulator  2 , the porosity PO3 of the third region AR 3  is equal to or less than 1.05 times the porosity PO2 of the second region AR 2 . In addition, since the amount of pores in the compact CP is sufficiently small, the porosity of at least the leg portion  13  is 3.0% or less. 
     Separately from the metallic shell  3  and the insulator  2 , the center electrode  5  is produced in advance. Specifically, a Ni alloy in which, for example, a copper alloy for improving heat dissipation is disposed at the center is subjected to forging to produce the center electrode  5 . 
     The insulator  2  and the center electrode  5  that are obtained as described above, the resistor  7 , and the terminal electrode  6  are fixed in a sealed condition via the glass seal layers  8  and  9 . The glass seal layers  8  and  9  are generally formed as follows. A mixture of borosilicate glass and a metal powder is charged into the axial hole  4  of the insulator  2  so as to sandwich the resistor  7 . Then, while the charged mixture is heated in a firing furnace, pressure is applied to the mixture from its rear side through the terminal electrode  6  to thereby fire the mixture. At the same time, a glaze layer may be formed on the surface of the rear trunk portion  10  of the insulator  2  through firing. Alternatively, the glaze layer may be formed in advance. 
     Next, the insulator  2  provided with the center electrode  5  and the terminal electrode  6  and produced as described above is fixed to the metallic shell  3  including the ground electrode  27  and produced as described above. More specifically, after the insulator  2  is inserted into the metallic shell  3 , the relatively thin-walled rear opening portion of the metallic shell  3  is crimped radially inward, i.e., the crimp portion  20  is formed, whereby the insulator  2  and the metallic shell  3  are fixed to each other. 
     Finally, the intermediate portion of the ground electrode  27  is bent toward the center electrode  5 , and the size of the spark discharge gap  28  formed between the center electrode  5  and the ground electrode  27  is adjusted, whereby the above-described ignition plug  1  is obtained. 
     As described above in detail, in the present embodiment, the porosity of the leg portion  13  is 3.0% or less, and the leg portion  13  is sufficiently dense. Therefore, sufficient dielectric strength can be achieved. Particularly, in the present embodiment, the maximum wall thickness T of the leg portion  13  is set to be 2.00 mm or less. In such a case, it is generally difficult to ensure sufficient dielectric strength. However, according to the present embodiment, sufficient dielectric strength can be obtained. 
     In the present embodiment, the porosity PO1 of the first region AR 1  is equal to or more than 1.20 times the porosity PO2 of the second region AR 2 , and the denseness of the first region AR 1  is relatively low. Therefore, the Young&#39;s modulus of the first region AR 1  can be reduced, and thermal stress generated between the outer and inner circumferential portions of the leg portion  13  during heating and cooling can be reduced. 
     In the present embodiment, since the porosity PO2 of the second region AR 2  is relatively small, inward compressive stress remains in the inner circumferential portion. When the outer circumferential portion shrinks rapidly during rapid cooling and tensile stress occurs on the surface, the remaining compressive stress can further reduce the thermal stress generated between the outer and inner circumferential portions. Therefore, sufficient thermal shock resistance can be obtained. 
     In addition, the porosity PO3 of the third region AR 3  is equal to or less than 1.05 times the porosity PO2 of the second region AR 2  formed so as to be relatively dense, and the third region AR 3  is as dense as the second region AR 2 . Therefore, the leg portion  13  can be dense over a wide area in the radial direction, and the dielectric strength can be further increased. 
     The present embodiment is configured such that at least one of the mullite crystal phase and the aluminate crystal phase is present on the outer surface of the leg portion  13 . Therefore, the amount of thermal expansion of the leg portion  13  can be reduced, and the thermal stress generated between the outer and inner circumferential portions of the leg portion  13  can be further reduced. As a result, the thermal shock resistance can be enhanced. 
     To examine the actions and effects obtained by the above embodiment, a plurality of ignition plug samples were produced. These ignition plug samples were different in the porosity PO1(%) of the first region, the porosity PO2(%) of the second region, the porosity PO3(%) of the third region, the overall porosity PO0(%) of the leg portion, and the presence and absence of the mullite crystal phase and the aluminate crystal phase on the outer surface of the leg portion. A dielectric strength evaluation test and a thermal shock resistance evaluation test were performed on each of the samples. 
     The outline of the dielectric strength evaluation test is as follows. Specifically, 30 samples with the same porosity of the first region, etc. were prepared. The forward end portion of each sample was immersed in a prescribed insulating oil so that no spark discharge occurred between the center electrode and the ground electrode. Then voltage was applied to the center electrode, and the applied voltage was gradually increased. The applied voltage (penetration voltage) when discharge passing through the insulator (leg portion) occurred between the center electrode and the metallic shell was measured. Then the average of the penetration voltages of the 30 samples (the average penetration voltage) was computed. Samples with an average penetration voltage of 40 kV or more and 41 kV or less were considered to have sufficient dielectric strength and evaluated as “good.” Samples with an average penetration voltage of 41 kV or more were considered to have very high dielectric strength and evaluated as “excellent.” Samples with an average penetration voltage of less than 40 kV were considered to have poor dielectric strength and evaluated as “poor.” 
     The outline of the thermal shock resistance evaluation test is as follows. Specifically, samples were heated at different heating temperatures for 30 minutes. Then each sample was dropped into water at 20° C. to quench that sample. Then a prescribed test solution was applied to the surface of the leg portion to make clear whether or not cracking was present in the leg portion, and whether or not cracking occurred in the leg portion was checked visually. Samples in which cracking occurred in the leg portion when the heating temperature was 180° C. or higher and lower than 200° C. were considered to have sufficient thermal shock resistance and evaluated as “good.” Sample in which cracking occurred in the leg portion when the heating temperature was 200° C. or higher were considered to have very high thermal shock resistance and evaluated as “excellent.” Samples in which cracking occurred in the leg portion when the heating temperature was lower than 180° C. were considered to have poor thermal shock resistance and evaluated as “poor.” 
     Table 1 shows the results of these two tests. The porosity PO1 of the first region etc. were changed by controlling the pressure increasing speed when the compact was formed. Separately from the samples for the above tests, samples for porosity measurement were separately produced under the same conditions as those for the samples for the above tests. The porosity PO1 of the first region etc. were measured using each of the samples for porosity measurement. Specifically, the leg portion was cut in a direction orthogonal to the axial line, and the cut section was mirror-polished. Then the porosity was measured at ten points for each of the first to third regions under a prescribed electron microscope, and the averages of the measured porosity values were used as the porosities PO1 to PO3 of the first to third regions. The average of the porosities at the 30 points in the first to third regions was used as the overall porosity PO0 of the leg portion. In addition, whether or not the mullite crystal phase or the aluminate crystal phase was present on the outer surface of the leg portion was determined by examining whether or not peaks intrinsic to the mullite crystal phase or the aluminate crystal phase were detected when the outer surface of the leg portion was subjected to X-ray diffraction analysis using a prescribed X-ray diffraction apparatus. 
     In samples 6, the wall thickness of the insulator was significantly smaller than (specifically, about ⅔) the wall thickness of the insulator in any of the other samples. In this case, it is generally very difficult to ensure sufficient dielectric strength. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 First 
                 Second 
                 Third 
                 Leg 
                   
                   
                   
                   
                   
               
               
                   
                 region 
                 region 
                 region 
                 portion 
                   
                   
                   
                   
                 Thermal 
               
               
                   
                 porosity 
                 porosity 
                 porosity 
                 porosity 
                   
                   
                 Crystal 
                 Dielectric 
                 shock 
               
               
                 No. 
                 PO1 (%) 
                 PO2 (%) 
                 PO3 (%) 
                 PO0 (%) 
                 PO1/PO2 
                 PO3/P02 
                 phase 
                 strength 
                 resistance 
               
               
                   
               
             
            
               
                 1 
                 3.87 
                 3.85 
                 3.85 
                 3.9 
                 1.01 
                 1.00 
                 None 
                 Poor 
                 Excellent 
               
               
                 2 
                 3.03 
                 2.96 
                 2.98 
                 3.0 
                 1.02 
                 1.01 
                 None 
                 Good 
                 Poor 
               
               
                 3 
                 3.26 
                 2.72 
                 2.97 
                 3.0 
                 1.20 
                 1.09 
                 None 
                 Good 
                 Good 
               
               
                 4 
                 3.24 
                 2.64 
                 2.77 
                 2.9 
                 1.23 
                 1.05 
                 None 
                 Excellent 
                 Good 
               
               
                 5 
                 2.14 
                 1.73 
                 1.81 
                 1.9 
                 1.24 
                 1.05 
                 None 
                 Excellent 
                 Good 
               
               
                 6 
                 3.22 
                 2.68 
                 2.78 
                 2.9 
                 1.20 
                 1.04 
                 None 
                 Good 
                 Good 
               
               
                 7 
                 3.24 
                 2.64 
                 2.77 
                 2.9 
                 1.23 
                 1.05 
                 Mullite 
                 Excellent  
                 Excellent 
               
               
                 8 
                 3.16 
                 2.57 
                 2.68 
                 2.8 
                 1.23 
                 1.04 
                 Aluminate 
                 Excellent 
                 Excellent 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 1, in samples in which the overall porosity PO0 of the leg portion was more than 3.0% (samples 1), dielectric strength was insufficient. 
     As is clear from Table 1, in samples in which although the overall porosity PO0 of the leg portion was 3.0% or less, PO1/PO2 was less than 1.20 (i.e., the porosity PO1 of the first region was less than 1.20 times the porosity PO2 of the second region) (samples 2), dielectric strength was sufficient, but thermal shock resistance was poor. This may be because since the first region was very dense and the Young&#39;s modulus of the first region was relatively high, thermal stress generated in the leg portion during rapid cooling was large. 
     However, in samples in which the overall porosity PO0 of the leg portion was 3.0% or less and PO1/PO2 was 1.20 or more (i.e., the porosity PO1 of the first region was equal to or more than 1.2 times the porosity PO2 of the second region) (samples 3 to 8), both the dielectric strength and the thermal shock resistance were found to be sufficient. This may be because of the following (1) to (3). 
     (1) Since the porosity PO0 was 3.0% or less, the denseness of the leg portion was sufficiently increased. 
     (2) Since the porosity PO1 was equal to or more than 1.20 times the porosity PO2, the Young&#39;s modulus of the first region was sufficiently small, and the thermal stress generated in the leg portion during rapid cooling was small. 
     (3) Since the porosity PO2 was relatively small, inward compression stress remained in the inner circumferential portion of the insulator. Therefore, when the outer circumferential portion of the insulator shrank rapidly during rapid cooling and tensile stress occurred in the surface, the thermal stress generated between the inner circumferential portion and the outer circumferential portion was further reduced. 
     It was found that samples in which PO3/PO2 was 1.05 or less (i.e., the porosity PO3 of the third region was equal to or less than 1.05 times the porosity PO2 of the second region) (samples 4 to 8) had higher dielectric strength. It was also found that, even in samples 6 in which the wall thickness of the insulator was significantly reduced, sufficient dielectric strength could be ensured. This may be because the leg portion was dense over a wide area in the radial direction. 
     It was found that samples in which the mullite crystal phase was present on the outer surface of the leg portion (samples 7) and samples in which the aluminate crystal phase was present on the surface of the leg portion (samples 8) had very high thermal shock resistance. This may be because the mullite crystal phase and the aluminate crystal phase reduced the amount of thermal expansion of the leg portion and therefore the thermal stress generated in the leg portion during rapid cooling was further reduced. 
     As can be seen from the results of the above tests, to obtain sufficient dielectric strength and sufficient thermal shock resistance simultaneously, it is preferable that the porosity of the leg portion is set to 3.0% or less and the porosity PO1 of the first region is set to be equal to or more than 1.20 times the porosity PO2 of the second region. 
     From the viewpoint of further increasing the dielectric strength, it is more preferable that the porosity PO3 of the third region is set to be equal to or less than 1.05 times the porosity PO2 of the second region. 
     In addition, from the viewpoint of further enhancing the thermal shock resistance, it is preferable to configure the leg portion such that at least one of the mullite crystal phase and the aluminate crystal phase is present on the outer surface of the leg portion. 
     The present invention is not limited to the contents of the description of the above embodiment and may be embodied, for example, as follows. It will be appreciated that application examples and modifications other than those exemplified below are also possible. 
     (a) In the above embodiment, the maximum wall thickness T of the leg portion  13  is set to 2.00 mm or less. However, the technological ideal of the present invention may be applied to an ignition plug in which the maximum wall thickness T is more than 2.00 mm. 
     (b) In the above embodiment, the ignition plug  1  is used to ignite, for example, an air-fuel mixture by generating spark discharge in the spark discharge gap  28 . However, the configuration of the ignition plug to which the technological ideal of the present invention is applicable is not limited thereto. Therefore, the technological ideal of the present invention may be applied to, for example, an ignition plug that has a cavity (space) at the forward end portion of an insulator and jets plasma generated in the cavity to ignite, for example, an air-fuel mixture (a plasma jet ignition plug). The technological ideal of the present invention may be applied to an ignition plug that generates plasma between a center electrode and a ground electrode by applying high-frequency power between these electrodes (a high-frequency plasma ignition plug). 
     (c) In the above embodiment, the ground electrode  27  is joined to the forward end portion  26  of the metallic shell  3 . However, the present invention is applicable to the case where a portion of a metallic shell (or a portion of an end metal piece welded beforehand to the metallic shell) is machined to form a ground electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-236906). 
     (d) In the above embodiment, the tool engagement portion  19  has a hexagonal cross section. However, the shape of the tool engagement portion  19  is not limited thereto. For example, the tool engagement portion  19  may have a Bi-HEX (modified dodecagonal) shape [ISO22977:2005(E)]. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1 : ignition plug 
           2 : insulator 
           3 : metallic shell 
           4 : axial hole 
           5 : center electrode 
           13 : leg portion 
           14 : step portion 
         AR 1 : first region 
         AR 2 : second region 
         AR 3 : third region 
         CL 1 : axial line