Patent Publication Number: US-9419415-B2

Title: Spark plug

Description:
RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2013/002619 filed Apr. 18, 2013, which claims the benefit of Japanese Patent Application No. 2012-213321, filed Sep. 27, 2012. 
     FIELD OF THE INVENTION 
     The present invention relates to a spark plug that includes a resistor inside of a through hole of an insulator. 
     BACKGROUND OF THE INVENTION 
     In order to reduce radio wave noise generated by ignition, a spark plug that includes a resistor inside of a through hole of an insulator is known (for example, see JP-A-2009-245716). In this spark plug, a conductive seal is disposed between the resistor and a center electrode. A contact portion between the resistor and the conductive seal is formed in a bowl shape that projects toward a tip end side around the central axis of the through hole. Consequently, this expands the contact portion between the conductive seal and the resistor compared with the case where the contact portion lies in a horizontal plane. This reduces sealing failure (such as peeling) between the conductive seal and the resistor. 
     However, in the above-described technique, the resistor has a shorter effective length compared with the case where the contact portion lies in a horizontal plane. This may decrease radio-wave noise reduction performance. 
     The main advantage of the present invention is a technique that reduces sealing failure between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve at least a part of the above-described problem, and can be realized as the following application examples. 
     Application Example 1 
     In accordance with a first aspect of the present invention, there is provided a spark plug that includes an insulator, a center electrode, a metal terminal nut, a resistor, and a conductive seal. The insulator extends along a central axis, and includes a through hole that passes through the insulator along the central axis. The center electrode extends along the central axis, and includes a rear end positioned inside of the through hole. The metal terminal nut extends along the central axis, and includes a tip end positioned at the rear end side with respect to the rear end of the center electrode inside of the through hole. The resistor is disposed in a position between the center electrode and the metal terminal nut inside of the through hole and apart from the center electrode. The conductive seal is disposed between the resistor and the center electrode inside of the through hole, and contacts both the center electrode and the resistor. The resistor has a contact surface in contact with the conductive seal. The contact surface includes: a portion where a distance in the central axis direction between the contact surface and a virtual plane changes according to a position on the contact surface where the virtual plane includes a rear end of the resistor and is vertical to the central axis; and at least one point where the distance is local maximum and at least one point where the distance is local minimum, in at least one cross section including the central axis. 
     The above-described configuration increases the area of the contact surface between the resistor and the conductive seal while suppressing shortening of the effective length of the resistor. As a result, this reduces sealing failure between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance. 
     Application Example 2 
     In accordance with a second aspect of the present invention, there is provided a spark plug according to application example 1, wherein at least a part of the resistor is positioned at the tip end side with respect to the rear end of the center electrode. 
     According to the above-described configuration, at least a part of the resistor is positioned at the tip end side with respect to the rear end of the center electrode. This expands the area of the contact portion between the resistor and the conductive seal without shortening the effective length of the resistor. As a result, this reduces sealing failure between the conductive seal and the resistor without shortening the radio-wave noise reduction performance. 
     Application Example 3 
     In accordance with a third aspect of the present invention, there is provided a spark plug that includes an insulator, a center electrode, a metal terminal nut, a resistor, and a conductive seal. The insulator extends along a central axis, and includes a through hole that passes through the insulator along the central axis. The center electrode extends along the central axis, and includes a rear end positioned inside of the through hole. The metal terminal nut extends along the central axis, and includes a tip end positioned at the rear end side with respect to the rear end of the center electrode inside of the through hole. The resistor is disposed in a position between the center electrode and the metal terminal nut inside of the through hole and apart from the center electrode. The conductive seal is disposed between the resistor and the center electrode inside of the through hole, and contacts both the center electrode and the resistor. At least a part of the resistor is positioned at the tip end side with respect to the rear end of the center electrode. 
     According to the above-described configuration, at least a part of the resistor is positioned at the tip end side with respect to the rear end of the center electrode. This expands the area of the contact portion between the resistor and the conductive seal without shortening the effective length of the resistor. As a result, this reduces sealing failure between the conductive seal and the resistor without shortening the radio-wave noise reduction performance. 
     Application Example 4 
     In accordance with a fourth aspect of the present invention, there is provided a spark plug according to application example 2 or 3, wherein the resistor includes a portion positioned at the tip end side with respect to the rear end of the center electrode over a whole circumference of a side surface of a rear end portion including the rear end of the center electrode. 
     According to the above-described configuration, a part of the resistor is positioned at the tip end side with respect to the rear end of the center electrode over the whole circumference of the side surface of the rear end portion at the center electrode. This further expands the area of the contact portion between the resistor and the conductive seal without shortening the effective length of the resistor. As a result, this further reduces sealing failure between the conductive seal and the resistor without shortening the radio-wave noise reduction performance. 
     Application Example 5 
     In accordance with a fifth aspect of the present invention, there is provided a spark plug according to application example 2 or 3, wherein a distance in the central axis direction between a tip end of the resistor and the rear end of the center electrode is equal to or less than 1.2 mm (millimeter). 
     The above-described configuration suppresses excessive reduction of the amount of the conductive seal. As a result, this suppresses decrease in load life performance of the spark plug. 
     Application Example 6 
     In accordance with a sixth aspect of the present invention, there is provided a spark plug according to any one of the application examples 1 to 5, wherein a distance in the central axis direction between the rear end of the center electrode and a tip end of the metal terminal nut is equal to or less than 13 mm (millimeter). 
     The above-described configuration reduces sealing failure between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance in a relatively compact spark plug where the distance in the center of axial direction between the rear end of the center electrode and the tip end of the metal terminal nut is equal to or less than 13 mm. 
     Application Example 7 
     In accordance with a seventh aspect of the present invention, there is provided a spark plug according to any one of the application examples 1 to 6 that further includes a metal shell that covers at least a partial range of an outer peripheral surface of the insulator in the central axis direction. The rear end of the resistor is at the tip end side with respect to a rear end of the metal shell. 
     The rear end of the resistor is disposed at the tip end side with respect to the rear end of the metal shell so as to reduce outward leakage of the radio wave noise. In this case, the length of the resistor is limited by the position of the rear end of the metal shell. Thus, it is difficult to ensure the effective length of the resistor. Even in this case, the above-described configuration facilitates ensuring the effective length of the resistor so as to reduce sealing failure between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance. 
     Application Example 8 
     In accordance to an eighth aspect of the present invention, there is provided a spark plug according to application example 7, wherein the insulator includes a large inner diameter portion, a small inner diameter portion, and an insulator shoulder portion. The small inner diameter portion is positioned at the tip end side with respect to the large inner diameter portion, and has a smaller inner diameter of the through hole than an inner diameter of the large inner diameter portion. The insulator shoulder portion is a shoulder portion disposed between the large inner diameter portion and the small inner diameter portion. The center electrode includes an electrode shoulder portion that is a shoulder portion with an outer diameter expanding from the tip end side toward the rear end side. The electrode shoulder portion is a shoulder portion disposed at the tip end side with respect to the rear end of the center electrode and is supported by the insulator shoulder portion. A portion of the center electrode at the rear end side with respect to the electrode shoulder portion, the conductive seal, and the resistor are disposed inside of the through hole in the large inner diameter portion of the insulator. A distance in the central axis direction between a tip end of the electrode shoulder portion and the rear end of the center electrode is equal to or more than 3.8 mm (millimeter). 
     In the case where the distance in the central axis direction between the tip end of the electrode shoulder portion and the rear end of the center electrode is equal to or more than 3.8 mm, the adhesion between the center electrode and the conductive seal improves. In this case, ensuring the effective length of the resistor becomes more difficult when the distance in the central axis direction between the tip end of the electrode shoulder portion and the rear end of the center electrode is equal to or more than 3.8 mm. In this case, the above-described configuration facilitates ensuring the effective length of the resistor so as to reduce sealing failure between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance. 
     Application Example 9 
     In accordance with a ninth aspect of the present invention, there is provided a spark plug according to any one of the application examples 1 to 8, wherein a minimum inner diameter of a portion where the resistor is disposed in the through hole of the insulator is equal to or less than 2.9 mm (millimeter). 
     In this relatively compact spark plug, the contact area between the resistor and the conductive seal are prone to be small. In this case, the above-described configuration expands this contact area while suppressing decrease in radio-wave noise reduction performance, thus reducing sealing failure between the conductive seal and the resistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a spark plug, illustrating an embodiment of the invention. 
         FIG. 2  is a diagram showing a structure in the proximity of a head of an electrode base material and a tip end face of a resistor. 
         FIG. 3  is a flowchart of a manufacturing process of an insulator assembly. 
         FIGS. 4(A) to 4(D)  are diagrams for explaining the manufacture of the insulator assembly. 
         FIGS. 5(A) to 5(C)  are diagrams exemplarily showing comparative embodiments. 
         FIGS. 6(A) and 6(B)  are examples showing the measurement result of the samples and the evaluation result of the samples. 
         FIG. 7  is an example showing the measurement result of the samples and the evaluation result of the samples. 
         FIG. 8  is a diagram showing a compression rod member used in manufacture of the insulator assembly in a modification. 
         FIGS. 9(A) to 9(C)  are diagrams showing an exemplary shape of a tip end face of the resistor in the modification. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. Embodiment 
     A-1. Configuration of Spark Plug: 
     Hereinafter, an aspect of present invention will be described with reference to embodiments.  FIG. 1  is a sectional view of a spark plug  100 . The one-dot chain line in  FIG. 1  indicates the central axis CO of the spark plug  100 . A direction (the vertical direction in  FIG. 1 ) parallel to the central axis CO is referred to as a central axis direction or an axial direction. The lower side in  FIG. 1  is referred to as a tip end side of the spark plug  100 . The upper side in  FIG. 1  is referred to as a rear end side of the spark plug  100 . The spark plug  100  includes a ceramic insulator  10  as an insulator, a center electrode  20 , a ground electrode  30 , a metal terminal nut  40 , and a metal shell  50 . 
     The ceramic insulator  10  is formed by sintering alumina and similar material. The ceramic insulator  10  is an approximately cylindrical shape member that extends along the central axis and has a through hole  12  (an axial hole) passing through the ceramic insulator  10 . The ceramic insulator  10  includes a flange portion  19 , a rear-end-side trunk portion  18 , a tip-end-side trunk portion  17 , a shoulder portion  15 , and an insulator leg portion  13 . The flange portion  19  is a portion positioned at approximately the center of the ceramic insulator  10  in the axial direction. The rear-end-side trunk portion  18  is positioned at the rear end side with respect to the flange portion  19 , and has a smaller outer diameter than the flange portion  19 . The tip-end-side trunk portion  17  is positioned at the tip end side with respect to the flange portion  19 , and has a smaller outer diameter than the rear-end-side trunk portion  18 . The insulator leg portion  13  is positioned at the tip end side with respect to the tip-end-side trunk portion  17 , and has a smaller outer diameter than the tip-end-side trunk portion  17 . The insulator leg portion  13  has a reduced diameter (i.e., is tapered) toward the tip end side, and is exposed to a combustion chamber of an internal combustion engine (not shown) when the spark plug  100  is installed on the internal combustion engine. The shoulder portion  15  is formed between the insulator leg portion  13  and the tip-end-side trunk portion  17 . 
     The metal shell  50  is formed of conductive metallic material (for example, low-carbon steel material), and is a cylindrically-shaped metal shell to secure the spark plug  100  to an engine head (not shown) of the internal combustion engine. In the metal shell  50 , an insertion hole  59  passes through the metal shell  50  along the central axis CO. The ceramic insulator  10  is inserted and held in the insertion hole  59  of the metal shell  50 . The metal shell  50  covers a portion from a part of the rear-end-side trunk portion  18  of the ceramic insulator  10  to the insulator leg portion  13 . The tip end of the ceramic insulator  10  is exposed from the tip end of the metal shell  50 . The rear end of the ceramic insulator  10  is exposed from the rear end of the metal shell  50 . 
     The metal shell  50  includes a hexagonal prism-shaped tool engagement portion  51  to engage a spark plug wrench, a mounting screw portion  52  for installation to the internal combustion engine, and a flanged seal portion  54  formed between the tool engagement portion  51  and the mounting screw portion  52 . A length between mutually parallel side surfaces of the tool engagement portion  51 , that is, a length between opposite sides is, for example, 9 mm to 14 mm. An outer diameter M (nominal diameter) of the mounting screw portion  52  is, for example, 8 mm to 12 mm. 
     An annular gasket  5  is fitted by insertion between the mounting screw portion  52  and the seal portion  54  in the metal shell  50 . The gasket  5  is formed by folding a metal plate. The gasket  5  seals the clearance between the spark plug  100  and the internal combustion engine (the engine head) when the spark plug  100  is installed on the internal combustion engine. 
     The metal shell  50  further includes a thin walled caulking portion  53  and a thin walled compression deformation portion  58 . The caulking portion  53  is disposed at the rear end side of the tool engagement portion  51 . The compression deformation portion  58  is disposed between the seal portion  54  and the tool engagement portion  51 . An annular region is formed between an inner peripheral surface in an area of the metal shell  50  from the tool engagement portion  51  to the caulking portion  53  and an outer peripheral surface of the rear-end-side trunk portion  18  of the ceramic insulator  10 . In the annular region, annular ring members  6  and  7  are disposed. Powders of talc  9  are filled up between the two ring members  6  and  7  in this region. The rear end of the caulking portion  53  is folded radially inward, and secured to the outer peripheral surface of the ceramic insulator  10 . Regarding the compression deformation portion  58  of the metal shell  50 , during manufacturing, the caulking portion  53  secured to the outer peripheral surface of the ceramic insulator  10  is pushed toward the tip end side so that the compression deformation portion  58  is compressively deformed. The compression deformation of the compression deformation portion  58  pushes the ceramic insulator  10  toward the tip end side within the metal shell  50  via the ring members  6  and  7  and the talc  9 . The shoulder portion  15  (an insulating-insulator-side shoulder portion) of the ceramic insulator  10  is pushed by the shoulder portion  56  (a metal-shell-side shoulder portion) formed in a position of the mounting screw portion  52  at the inner periphery of the metal shell  50 , via an annular plate packing  8 . As a result, the plate packing  8  prevents outward leakage of gas in the combustion chamber of the internal combustion engine from the clearance between the metal shell  50  and the ceramic insulator  10 . On the tip end side with respect to the metal-shell-side shoulder portion  56 , a clearance C with a predetermined dimension is disposed between the metal shell  50  and the insulator leg portion  13  of the ceramic insulator  10 . 
     The center electrode  20  is a rod-shaped member that extends along the central axis CO. The center electrode  20  has a construction including an electrode base material  21  and a core material  22  buried inside of the electrode base material  21 . The electrode base material  21  is formed of Nickel, or alloy (inconel (registered trademark)  600  or similar alloy) that contains Nickel, as a main constituent. The core material  22  is formed of copper, or alloy that contains copper, as a main constituent with excellent thermal conductivity compared with the alloy forming the electrode base material  21 . In the center electrode  20 , the greater portion including the rear end is positioned inside of the through hole  12  of the ceramic insulator  10 . The tip end of the center electrode  20  is exposed at the tip end side of the ceramic insulator  10 . 
     The center electrode  20  includes a flange portion  24  (referred to also as an electrode flange portion or a flanged portion), a head  23  (an electrode head), and a leg portion  25  (an electrode leg). The flange portion  24  is disposed in a predetermined position in the central axis direction. The head  23  is a portion at the rear end side with respect to the flange portion  24 . The leg portion  25  is a portion at the tip end side with respect to the flange portion  24 . The tip end portion of the leg portion  25  of the center electrode  20  has a tapered shape with a smaller diameter toward the tip end. An electrode tip  28  is sealed (i.e., attached) to this tip end portion, for example, by laser welding. The electrode tip  28  is formed of material that contains noble metal with high melting point as a main constituent. This material of the electrode tip  28  employs, for example, iridium (Ir) or an alloy containing Ir as a main constituent. Specifically, Ir-5Pt alloy (iridium alloy containing five mass % platinum) or similar alloy is frequently used. 
     The ground electrode  30  is sealed (i.e., attached) to the tip end of the metal shell  50 . The electrode base material of the ground electrode  30  is formed of metal with a high corrosion resistance, for example, nickel alloy such as inconel (registered trademark)  600 . A base-material base end portion  32  of this ground electrode  30  is sealed to the tip end face of the metal shell  50  by welding. A base-material tip end portion  31  of the ground electrode  30  is bent. One side surface of the base-material tip end portion  31  faces the electrode tip  28  of the center electrode  20  on the central axis CO in the axial direction. On the one side surface of the base-material tip end portion  31 , an electrode tip  38  is welded by resistance welding in a position facing the electrode tip  28  of the center electrode  20 . The electrode tip  38  employs, for example, Pt (platinum) or alloy containing Pt as a main constituent, specifically, Pt-20Ir alloy (platinum alloy containing 20 mass % of iridium) or similar alloy. A spark gap is formed between a pair of these electrode tips  28  and  38 . 
     The metal terminal nut  40  is a rod-shaped member that extends along the central axis CO. The metal terminal nut  40  is formed of conductive metallic material (for example, low-carbon steel), and has a surface where an anticorrosion metal layer (for example, a Ni layer) is formed by plating or similar method. The metal terminal nut  40  includes a flange portion  42  (a terminal nut jaw portion), a plug cap installation portion  41 , and a leg portion  43  (a terminal nut leg portion). The flange portion  42  is formed at a predetermined position in the central axis direction. The plug cap installation portion  41  is positioned at the rear end side with respect to the flange portion  42 . The leg portion  43  is positioned at the tip end side with respect to the flange portion  42 . The plug cap installation portion  41  including the rear end of the metal terminal nut  40  is exposed at the rear end side of the ceramic insulator  10 . The leg portion  43  including the tip end of the metal terminal nut  40  is inserted (press-fitted) into the through hole  12  of the ceramic insulator  10 . That is, the tip end of the metal terminal nut  40  is positioned inside of the through hole  12 . A plug cap (not shown), connected to a high-voltage cable (not shown), is installed on the plug cap installation portion  41 , and receives a high voltage for generating a spark. 
     Inside of the through hole  12  of the ceramic insulator  10 , the tip end of the metal terminal nut  40  (the tip end of the leg portion  43 ) is positioned at the rear end side with respect to the rear end of the above-described center electrode  20 . Inside of the through hole  12  of the ceramic insulator  10 , a resistor  70  is disposed in a region between the tip end of the metal terminal nut  40  and the rear end of the center electrode  20  to reduce radio wave noise during sparking. The resistor is formed of compositions including glass particles as a main constituent, ceramic particles other than glass, and a conductive material. The conductive material includes, for example, a non-metal conductive material such as carbon particles (such as carbon black), TiC particles, and TiN particles and a metal such as Al, Mg, Ti, Zr, and Zn. The material of the glass particles can employ, for example, B2O3-SiO2 system, BaO—B2O3 system, and SiO2-B2O3-CaO—BaO system. The material of the ceramic particles can employ, for example, TiO2 and ZrO2. The resistance value of the resistor  70  is preferred to be, for example, 0.1 kΩ to 30 kΩ, and further preferred to be 1 kΩ to 20 kΩ. 
     The clearance between the resistor  70  and the center electrode  20  inside of the through hole  12  is filled up with a conductive seal  60 . The clearance between the resistor  70  and the metal terminal nut  40  is filled up with the conductive seal  80 . That is, the conductive seal  60  contacts both the resistor  70  and the center electrode  20 , while the conductive seal  80  contacts both the resistor  70  and the metal terminal nut  40 . As a result, the center electrode  20  and the metal terminal nut  40  are electrically connected to each other via the resistor  70  and the conductive seals  60  and  80 . The conductive seal includes, for example, the above-described various glass particles and metal particles (such as Cu and Fe) in a ratio of about 1 to 1. The conductive seal has properties intermediate between: the material property of the center electrode  20  and the metal terminal nut  40  as metals, and the material property of the resistor  70  that includes glass as a main constituent. As a result, interposing the conductive seals  60  and  80  stabilizes the contact resistance between the laminated members, thus stabilizing the resistance value between the center electrode  20  and the metal terminal nut  40 . 
     Here, a rear end MB of the resistor  70  is positioned at the tip end side with respect to a rear end UK of the metal shell  50 . That is, the outer peripheral surface of the ceramic insulator  10  is covered with the metal shell  50  over the whole range where the resistor  70  is disposed in the central axis direction. As a result, the radio wave noise emitted from the spark plug  100  to the outside is blocked by the metal shell  50 . This reduces the radio wave noise emitted from the spark plug  100 . 
     From the aspect of ensuring the compact spark plug  100 , a distance UL in the center of axial direction between the rear end of the ceramic insulator  10  and the rear end of the center electrode  20  (the rear end of the head  23 ) is preferred to be equal to or less than 25 mm. Also, from the aspect of productivity, an insulator nose length BL (a distance in the central axis direction between the tip end of the flange portion  42  and the tip end of the leg portion  43  of the metal terminal nut  40 ) in the central axis direction of the leg portion  43  of the metal terminal nut  40  is preferred to be equal to or more than 12 mm. Accordingly, in the case where these conditions are satisfied, a distance SL (this distance is also referred to as seal length SL) in the central axis direction between the tip end of the metal terminal nut  40  and the rear end of the center electrode  20  is equal to or less than 13 mm. 
     Here, the radio-wave noise reduction performance by the resistor  70  depends on the effective length EL of the resistor  70 . The effective length EL is a distance between the tip end of a rear end face  72  (a contact surface between the resistor  70  and the conductive seal  80 ) of the resistor  70  and the rear end of a tip end face  71  (a contact surface between the resistor  70  and the conductive seal  60 ) of the resistor  70 . In the compact spark plug  100  where the above-described conditions of the distance UL and the insulator nose length BL are satisfied, it is especially desired to improve the radio-wave noise reduction performance by ensuring the longest possible effective length EL in a range that the above-described seal length SL equal to or less than 13 mm. 
     With reference to  FIG. 2 , a further description will be given.  FIG. 2  is a view showing a structure in the proximity of the head  23  of the electrode base material  21  and the tip end face  71  of the resistor  70 .  FIG. 2  shows a cross section of the spark plug  100  taken along the cross section including the central axis CO. The through hole  12  of the ceramic insulator  10  has inner diameter that differs on the tip end side and the rear end side in the proximity of the location of the flange portion  24  of the center electrode  20 . That is, seen from the aspect of the inner diameter of the through hole  12 , the ceramic insulator  10  includes a large inner diameter portion BRP that has a first diameter R 1  as the inner diameter of the through hole  12  and a small inner diameter portion SRP that has a second diameter R 2  smaller than the first diameter R 1  as the inner diameter of the through hole  12 . The small inner diameter portion SRP is positioned at the tip end side with respect to the large inner diameter portion BRP. A shoulder portion  16  (referred to also as an insulator shoulder portion  16 ) is disposed between the large inner diameter portion BRP and the small inner diameter portion SRP. The shoulder portion  16  is a portion where the inner diameter of the through hole  12  decreases from the first diameter R 1  to the second diameter R 2 , heading from the rear end side toward the tip end side. Here, the first diameter R 1  is, for example, 2.0 mm to 4.0 mm, and equal to or less than 2.9 mm in the compact spark plug  100 . The second diameter R 2  is 1.0 mm to 3.2 mm, and equal to or less than 2.4 mm in the compact spark plug  100 . In the case where, for example, the first diameter R 1  is relatively small (for example, the first diameter R 1  is equal to or less than 2.9 mm), the tip end face  71  of the resistor  70  has a small area. The smaller area of the tip end face  71  more easily causes peeling between the conductive seal  60  and the resistor  70  in the case where an impact (for example, an impact caused by vibration of the internal combustion engine) is applied to the tip end face  71  of the resistor  70  (the contact surface between the conductive seal  60  and the resistor  70 ). Thus, the impact resistance of the spark plug  100  is prone to decrease. Accordingly, it is, especially, desired to improve impact resistance in the compact spark plug  100  with the relatively small first diameter R 1 . 
     The flange portion  24  (the flanged portion) of the center electrode  20  includes a shoulder portion  24   f  at the tip end side (referred to as an electrode shoulder portion  24   f ). The electrode shoulder portion  24   f  is a portion where the outer diameter increases from the tip end side toward the rear end side. The electrode shoulder portion  24   f  is supported by the insulator shoulder portion  16 . Accordingly, the head  23  of the center electrode  20  is disposed inside of the through hole  12  in the large inner diameter portion BRP of the ceramic insulator  10 . The leg portion  25  of the center electrode  20  is disposed inside of the through hole  12  in the small inner diameter portion SRP of the ceramic insulator  10 . The side surface of the head  23 , and the side surface and the rear end face of the flange portion  24  are in contact with conductive seal  60 . Here, in the center electrode  20 , a length TL (a distance TL in the central axis direction between the tip end of the flange portion  24  and the rear end of the head  23 ) from the tip end of the flange portion  24  (that is, the tip end of the electrode shoulder portion  24   f ) to the rear end of the head  23  (that is, the rear end of the center electrode  20 ) is preferred to be equal to or more than 3.8 mm. In this case, the volume of the head  23  becomes relatively large. This reduces temperature rise of the head  23  due to heat generated by the internal combustion engine, thus reducing thermal expansion of the head  23 . As a result, this improves adhesion between the center electrode  20  and the conductive seal  60 , thus prolonging the service life of the spark plug  100 . In the case where the length TL from the tip end of the flange portion  24  to the rear end of the head  23  is relatively long (for example, the length TL is equal to or more than 3.8 mm), it is difficult to ensure the compact spark plug  100  and seal length SL at the same time. Therefore, it is especially desired to improve the radio-wave noise reduction performance by ensuring the longest possible effective length EL with a relatively short seal length SL. 
     Additionally, a head outer diameter R 3  of the head  23  is preferred to be set, for example, within a range of 60% to 70% of the first diameter R 1  to ensure the clearance NT at the head side surface. It is preferred to ensure the clearance NT at the head side surface to an extent of 0.4 mm to 0.6 mm. 
     In the spark plug  100  of this embodiment, the shape of the tip end face  71  of the resistor  70  is devised to ensure the compatibility between ensuring the effective length EL of the resistor  70  and expanding the area of the tip end face  71 . Hereinafter, the shape of the tip end face  71  will be described. 
     The tip end face  71  has a peripheral edge portion  73  that includes a portion projecting further toward the tip end side of a center portion  74  of the tip end face  71  over the whole circumference. A detailed description will be given using a distance in the central axis direction (an axial distance) between the rear end MB of the resistor  70  (a virtual plane MS (in  FIG. 1 ) that includes the rear end MB and is vertical to the central axis CO) and a point on the tip end face  71 , that is, a length from the rear end MB of the resistor  70  to the point on the tip end face  71 . In the cross section including the central axis CO of the resistor  70  (in  FIG. 2 ), the tip end face  71  includes two local maximum points SP 1  and SP 2  at the local maximum axial distance and a local minimum point BP 1  at the local minimum axial distance. That is, the axial distance becomes larger from a first contact position PP 1  with the inner peripheral surface of the ceramic insulator  10  toward the central axis CO in the cross section shown in  FIG. 2 , and has the local maximal value at the first local maximum point SP 1 . Then, the axial distance becomes smaller from the first local maximum point SP 1  toward the central axis CO, and has the local minimal value at the local minimum point BP 1  near the central axis CO. Additionally, the axial distance becomes local maximum at the second local maximal value SP 2  between the central axis CO and a second contact position PP 2  with the inner peripheral surface of the ceramic insulator  10  to have a shape that is approximately line-symmetrical to the shape from the first contact position PP 2  to the central axis CO with respect to the target axis of the central axis CO. 
     Here, the local maximum points SP 1  and SP 2  of the tip end face  71  are positioned at the tip end side with respect to the rear end of the head  23  of the center electrode  20 . That is, the resistor  70  includes a portion positioned at the tip end side with respect to the rear end of the center electrode  20 . Here, the peripheral edge portion  73  including the local maximum points SP 1  and SP 2  in the cross section shown in  FIG. 2  includes a portion positioned at the tip end side with respect to the rear end of the side surface of the head  23  over the whole circumference on the side surface of the head  23  of the center electrode  20  (over the whole circumference of the inner peripheral surface of the ceramic insulator  10 ). That is, the tip end face  71  includes a portion in a bowl shape (an inversed bowl shape in the orientation of the illustration shown in  FIG. 2 ) where the local minimum point BP 1  is on the bottom portion side and the local maximum points SP 1  and SP 2  are on the opening side. The rear end of the center electrode  20  is positioned at the bottom portion side (the rear end side) with respect to the opening of the bowl shape. The outer surface (the side surface and the rear end face) of the head  23  of the center electrode  20  is not in contact with the tip end face  71 , and is separated from the tip end face  71  by the conductive seal  60 . 
     A-2. Method for Manufacturing the Spark Plug: 
     The above-described spark plug  100  can be manufactured by, for example, the following manufacturing method. First, a ceramic insulator assembly (an assembly where the center electrode  20 , the metal terminal nut  40 , the resistor  70 , and similar member are assembled to the ceramic insulator  10 ) manufactured through a manufacturing process described later, the metal shell  50 , and the ground electrode  30  are prepared. Subsequently, the metal shell  50  is assembled to the outer periphery of the ceramic insulator assembly, and the base-material base end portion  32  of the ground electrode  30  is sealed to the tip end face of the metal shell  50 . The electrode tip  38  is welded to the base-material tip end portion  31  of the sealed ground electrode  30 . Subsequently, the ground electrode  30  is bent so that the base-material tip end portion  31  of the ground electrode  30  faces the tip end portion of the center electrode  20 . Thus, the spark plug  100  is completed. 
     A description will be given of the manufacturing process of the insulator assembly with reference to  FIG. 3 .  FIG. 3  is a flowchart of the manufacturing process of the insulator assembly.  FIGS. 4(A) to 4(D)  are diagrams for explaining the manufacture of the insulator assembly. In step S 50 , necessary members and raw material powders, specifically, the ceramic insulator  10 , the center electrode  20  where the electrode tip  28  is sealed to its tip end, the metal terminal nut  40 , and the respective raw material powders  65 ,  85 , and  75  of the conductive seals  60  and  80  and the resistor  70  are prepared. 
     In step S 100 , the center electrode  20  is inserted from the opening of the rear end inside of the through hole  12  of the prepared ceramic insulator  10 . As described above with reference to  FIG. 2 , the center electrode  20  is supported by the shoulder portion  16  of the ceramic insulator  10  and secured inside of the through hole  12  ( FIG. 4(A) ). 
     In step S 200 , the raw material powder  65  of the conductive seal  60  is filled into the through hole  12  of the ceramic insulator  10  from the opening of the rear end, that is, the upper side of the center electrode  20 . In step S 300 , pre-compression is performed on the raw material powder  65  filled inside of the through hole  12 . The pre-compression is performed by compressing the raw material powder  65  using a compression rod member  200 . The compression rod member  200  is a rod-shaped member that has an outer diameter slightly smaller than the first diameter R 1  of the through hole  12 . The tip end of the compression rod member  200  has a planar surface vertical to the axial direction of the compression rod member  200 . The rear end face of the raw material powder  65  after the pre-compression has a planar shape vertical to the central axis CO. 
     In step S 400 , the raw material powder  75  of the resistor  70  is filled into the through hole  12  of the ceramic insulator  10  from the opening of the rear end, that is, from the upper side of the raw material powder  65 . In step S 500 , similarly to step S 300  described above, the pre-compression is performed on the raw material powder  75  filled inside of the through hole  12  using the compression rod member  200 . The filling of the raw material powder  75  (in S 400 ) and the pre-compression (in  5500 ) can be performed over several cycles For example, filling of a half of the prescribed filling quantity of the raw material powder  75  and the pre-compression after the filling are each performed twice in alternation. 
     In step S 600 , the raw material powder  85  of the conductive seal  80  is filled into the through hole  12  of the ceramic insulator  10  from the opening of the rear end, that is, from the upper side of the raw material powder  75 . In step S 700 , similarly to step S 300  described above, the pre-compression is performed on the raw material powder  85  filled inside of the through hole  12  using the compression rod member  200 . 
       FIG. 4(B)  shows the center electrode  20  and the raw material powders  65 ,  75 , and  85  that are inserted and filled into the ceramic insulator  10  and the through hole  12  of the ceramic insulator  10  at the time the manufacturing process up to step S 700  is completed. Here, the partial expansion figure of  FIG. 4(B)  shows a central portion  65 C where the head  23  of the center electrode  20  is present on tip end side and a peripheral edge portion  65 P where the head  23  of the center electrode  20  is not present on the tip end side in the filled raw material powder  65 . The central portion  65 C includes a region through which the central axis CO passes. The peripheral edge portion  65 P includes a ring-shaped region surrounding the radially outside of the central portion  65 C. 
     In the pre-compression (in S 300 ), the pressure applied to the central portion  65 C is higher than a pressure applied to the peripheral edge portion  65 P. That is, the peripheral edge portion  65 P receives a relatively low pressure to be sandwiched between: the tip end face of the compression rod member  200 ; and the rear end face of the head  23  at a relatively close distance to this tip end face. On the other hand, the central portion  65 C receives a relatively high pressure to be sandwiched between: the tip end face of the compression rod member  200 ; and the rear end faces of the flange portion  24  and the shoulder portion  16  relatively far distance from this tip end face. 
     As a result, the raw material powder  65  has a density in the peripheral edge portion  65 P that is lower than a density of the raw material powder  65  in the central portion  65 C. 
     In this state, in step S 800 , the ceramic insulator  10  is transferred into a tunnel kiln and heated to a predetermined temperature. The predetermined temperature is, for example, a temperature higher than the softening point of glass constituent contained in the raw material powders  65 ,  75 , and  85 , specifically, 800 to 950 degrees Celsius. In a state heated to the predetermined temperature, in step S 900 , the metal terminal nut  40  is press-fitted in the central axis direction from the opening of the rear end of the through hole  12  in the ceramic insulator  10  (in  FIG. 4(C) ). As a result, the respective raw material powders  65 ,  75 , and  85  laminated inside of the through hole  12  of the ceramic insulator  10  are pressed (compressed) in the central axis direction by the tip end of the metal terminal nut  40 . As a result, as shown in  FIG. 4(D) , the respective raw material powders  65 ,  75 , and  85  are compressed and sintered to form the respective conductive seal  60 , resistor  70 , and conductive seal  80  described above. The insulator assembly is completed through the above-described manufacturing process. 
     Here, as described above, the raw material powder  65  before compression and sintering has a difference in density between the central portion  65 C and the peripheral edge portion  65 P. As a result, in the peripheral edge portion  65 P, the tip end portion of the resistor  70  to be molded by compression and sintering is molded to extend to the tip end side with respect to the central portion  65 C. A distance H and a distance K shown in  FIG. 2  depend on the difference in density (referred to also as a difference in raw material powder density) generated between the central portion  65 C and the peripheral edge portion  65 P in the raw material powder  65  before compression and sintering. The distance H is a distance in the central axis direction between the tip end of the resistor  70  (the tip ends SP 1  and SP 2  of the peripheral edge portion  73 ) and the rear end of the center electrode  20  (the head  23 ) (see  FIG. 2 ). The distance H is, in other words, a penetrating length of the tip end of the resistor  70  into the tip end side with respect to the rear end of the center electrode  20 . Thus, the distance H is also referred to as a penetration length H below. The distance K is a distance in the central axis direction between the rear end of the center portion  74  and the tip end of the peripheral edge portion  73  (see  FIG. 2 ). The distance K is, in other words, a projecting length of the tip ends SP 1  and SP 2  of the peripheral edge portion  73  toward the tip end side with respect to the center portion  74  adjacent to the central axis CO in the tip end face  71  of the resistor  70 . Thus, the distance K is also referred to as a projection length K below. 
     That is, a larger difference in raw material powder density ensures larger penetration length H and projection length K. A smaller difference in raw material powder density ensures smaller penetration length H and projection length K. The difference in raw material powder density depends on a filling quantity of the raw material powder  65 . That is, a smaller filling quantity of the raw material powder  65  ensures larger penetration length H and projection length K. This is because the smaller filling quantity of the raw material powder  65  ensures a larger ratio of the volume of the peripheral edge portion  65 P to the volume of the central portion  65 C, and this result in a difference in compression ratio by the pre-compression consequently becomes larger. Here, a larger projection length K and penetration length H ensure a larger area of the tip end face  71  of the resistor  70 . However, in the case where the filling quantity of the raw material powder  65  becomes smaller than a specific value, the amount of the conductive seal  60  at the completion becomes excessively small. Thus, the center electrode  20  and the resistor  70  directly contact each other, and the thickness of the conductive seal  60  over the head  23  becomes excessively thin. As a result, as described later, a resistance value between the center electrode  20  and the resistor  70  is not stabilized, and the load life of the spark plug  100  may become shorter. Accordingly, the filling quantity of the raw material powder  65  is preferred to be designed considering a balance between maintaining the load life and expanding the area of the tip end face  71  of the resistor  70 . The sizes of the penetration length H and the projection length K depend also on a distance NT ( FIG. 2 : referred to also as a clearance NT of the head side surface) between the side surface of the head  23  of the center electrode  20  and the inner peripheral surface of the ceramic insulator  10 . It is preferred that the size of the clearance NT of the head side surface be also considered. 
     According to the spark plug  100  of this embodiment, the above-described configuration and manufacturing method of which have been described above, the contact surface (the tip end face  71 ) between the resistor  70  and the conductive seal  60  has a plurality of points (SP 1 , SP 2 , and BP 1 ) where the distance in the central axis direction from the rear end of the resistor  70  becomes local maximum or local minimum in the cross section including the central axis CO. This increases a contact area between the resistor  70  and the conductive seal  60  while restricting the effective length EL of the resistor  70  to be short. As a result, this reduces sealing failure (peeling) between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance, thus improving impact resistance. 
       FIGS. 5(A) to 5(C)  are diagrams exemplarily showing comparative embodiments. Like first and second comparative embodiments shown in  FIGS. 5(B) and 5(C) , in the case where the tip end face of the resistor has only one local maximum or local minimum point in the cross section including the central axis CO of the resistor, this configuration does not sufficiently achieve the compatibility between ensuring the effective length EL of the resistor and expanding the area of the tip end face of the resistor. For example, a spark plug of the first comparative embodiment shown in  FIG. 5(B)  is an example where a distance SK 1  in the central axis direction between the tip end and the rear end at a tip end face  71 A of a resistor  70 A is relatively short. In this example, the tip end face  71 A of the resistor  70 A has an approximately flat shape. In this case, since the distance SK 1  is relatively short, the proportion of the effective length EL to the overall length of the resistor  70 A (a length from the rear end to the tip end of the resistor  70 ) can be set relatively large. However, the area ratio of the tip end face  71 A to the area of the cross section perpendicular to the central axis CO of the through hole  12  cannot be set large. That is, the area of the tip end face  71 A cannot be set sufficiently large, and this might not sufficiently reduce the sealing failure (peeling) between the conductive seal  60 A and the resistor  70 A. 
     A spark plug of the second comparative embodiment shown in  FIG. 5(C)  is an example where a distance SK 2  in the central axis direction between the tip end and the rear end at a tip end face  71 B of a resistor  70 B is relatively long. In this case, since the distance SK 2  is relatively long, the area ratio of the tip end face  71 B to the area of the cross section perpendicular to the central axis CO of the through hole  12  can be set large to some extent. However, the proportion of the effective length EL to the overall length of the resistor  70 B becomes small. That is, this does not ensure a sufficient effective length EL, and may cause decrease in radio-wave noise reduction performance. 
     On the other hand, in the spark plug  100  of this embodiment (in  FIG. 2  and  FIG. 5(A) ), the tip end face  71  is constituted in a wavelike shape to have the local maximum points SP 1 , SP 2 , and BP 1  in the cross section shown in  FIG. 2  even in the case where the distance SK in the central axis direction between the tip end and the rear end at the tip end face  71  of the resistor  70  is relatively small. This can sufficiently expand the area of the tip end face  71 . Accordingly, as described above, this reduces sealing failure (peeling) between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance, thus improving the impact resistance. 
     Furthermore, the resistor  70  includes the portion positioned at the tip end side with respect to the rear end of the center electrode  20  to expand the area of the tip end face  71  without shortening the effective length EL of the resistor  70 . As a result, this further reduces sealing failure between the conductive seal  60  and the resistor  70  without shortening the radio-wave noise reduction performance. In this embodiment, the resistor  70  includes the portion positioned at the tip end side with respect to the rear end of the side surface of the head  23  over the whole circumference of the side surface of the head  23  in the center electrode  20 . Accordingly, the area of the tip end face  71  can be expanded more efficiently. 
     Here, penetration length H (the distance H (in  FIG. 2 ) in the central axis direction between the tip end of the resistor  70  and the rear end of the center electrode  20  (the head  23 )) is preferred to be equal to or less than 1.2 mm. The penetration length H equal to or less than 1.2 mm suppresses excessive reduction of the amount of the conductive seal  60  arranged between the resistor  70  and the center electrode  20 . If the amount of the conductive seal  60  arranged between the center electrode  20  and the resistor  70  is excessively reduced, the resistance value between the center electrode  20  and the resistor  70  is not stabilized. Therefore, the load life performance of the spark plug  100  may be decreased. In the case where the clearance NT of the head side surface is, for example, in a range of 0.2 mm&lt;NT&lt;0.5 mm, specifically the penetration length H equal to or less than 1.2 mm suppresses excessive reduction of the amount of the conductive seal  60  arranged between the resistor  70  and the center electrode  20 . 
     In the case where the distance (the seal length SL) in the central axis direction between the rear end of the center electrode  20  and the tip end of the metal terminal nut  40  is equal to or less than 13 mm (millimeter), this reduces sealing failure between the conductive seal  60  and the resistor  70  while suppressing decrease in radio-wave noise reduction performance within the limitations of the seal length SL. 
     In this embodiment, the rear end MB of the resistor  70  can be positioned at the tip end side with respect to the rear end UK of the metal shell  50  without shortening the effective length EL of the resistor  70 . As a result, as described above, the radio wave noise emitted from the spark plug  100  to the outside is blocked by the metal shell  50 . This reduces the radio wave noise emitted from the spark plug  100 . 
     Additionally, in the case where the distance in the central axis direction between the tip end of the flange portion  24  and the rear end of the center electrode  20  is set equal to or more than 3.8 mm, it is more difficult to ensure the effective length EL of the resistor  70  restricted by the position of the rear end of the metal shell  50 . In this case, the above-described embodiment facilitates ensuring the effective length EL of the resistor  70  so as to reduce sealing failure between the conductive seal  60  and the resistor  70  while suppressing decrease in radio-wave noise reduction performance. 
     Additionally, in the case where the inner diameter (seal diameter) at the position where the resistor  70  is disposed in the through hole  12  of the ceramic insulator  10  is equal to or less than 2.9 mm, the area of the tip end face  71  is prone to be small. In the case where the inner diameter of the portion where the resistor  70  is disposed in the through hole  12  changes according to the position parallel to the central axis CO, the area of the tip end face  71  is prone to be small similarly to the case where the minimum inner diameter of the portion where the resistor  70  is disposed in the through hole  12  is equal to or less than 2.9 mm. This relatively compact spark plug efficiently expands this contact area while suppressing decrease in radio-wave noise reduction performance, thus reducing sealing failure between the conductive seal  60  and the resistor  70 . 
     A-3. Working Example 
     A plurality of samples #1 to #16, different in projection length K and penetration length H, of the spark plug  100  in the above-described embodiment were manufactured, and evaluation tests were performed. The respective samples were manufactured in accordance with the above-described manufacturing process. In order to vary the projection length K and the penetration length H, the filling quantity of the raw material powder  65  is varied among the samples. The manufacturing conditions other than the filling quantity of the raw material powder  65 , for example, the filling quantity of the raw material powder  75  of the resistor  70 , the respective members (the ceramic insulator  10 , the center electrode  20 , the metal shell  50 , and the metal terminal nut  40 ) are not varied between the samples. 
     Various dimensions of the spark plug  100  that are common to the respective samples are as follows. 
     The first diameter R 1  of the large inner diameter portion BRP of the ceramic insulator  10  (in  FIG. 2 ): 3.0 mm 
     The second diameter R 2  of the small inner diameter portion SRP of the ceramic insulator  10  (in  FIG. 2 ): 2.0 mm 
     The outer diameter R 3  of the head  23  of the center electrode  20  (in  FIG. 2 ): 2.1 mm 
     The clearance NT of the head side surface (in  FIG. 2 ): 0.45 mm 
     The length TL from the tip end of the flange portion  24  to the rear end of the head  23 : 3.5 mm 
     The distance UL between the rear end of the ceramic insulator  10  and the rear end and of the center electrode  20 : 47.5 mm 
     The insulator nose length BL of the metal terminal nut  40  (in  FIG. 1 ): 36.5 mm 
     The seal length SL (in  FIG. 1 ): 11.0 mm 
       FIGS. 6(A) and 6(B)  and  FIG. 7  are examples showing the measurement result of the samples and the evaluation result of the samples. In this working example, eight types of Samples #1 to #16 were manufactured in pluralities in which filling quantities of the raw material powder  65  are each different. Subsequently, each of Samples #1 to #8 manufactured in the respective pluralities was individually sectioned along the cross section including the central axis CO. The minimum penetration length HA, the minimum projection length KA, the maximum penetration length HD, and the maximum projection length KD among the penetration lengths H and the projection lengths K in the peripheral edge portion  73  over the whole circumference were each measured (in  FIG. 6(A) ). It may be said that if these values HA, KA, HD, and KD become larger, the area of the tip end face  71  becomes larger. Additionally, each one of the plurality of the respective manufactured Samples #9 to #16 was sectioned along the cross section including the central axis CO. The minimum penetration length HA among the penetration lengths H in the peripheral edge portion  73  over the whole circumference was measured (in  FIG. 6(B) ). 
     A-3-1. Test of Impact Resistance: 
     An impact resistance test was carried out using the Samples #1 to #8. The impact resistance test was carried out based on test conditions compliant with Japanese Industrial Standard B8031: 2006 (internal combustion engine-spark plug) section 7.4. However, a condition (30 minutes) more severe than the stipulation (10 minutes) of Japanese Industrial Standard was adopted as duration for applying the impact. The impact resistance was evaluated using a changing rate of the resistance value between the metal terminal nut  40  and the center electrode  20  before and after the test. The evaluation standard of this test is as follows. 
     Evaluation Result A: the changing rate is equal to or less than ±15%, Evaluation Result B: the changing rate is equal to or less than ±25%, Evaluation Result C: the changing rate is equal to or less than ±30%, and Evaluation Result D: the changing rate is equal to or more than ±30. 
     As shown in  FIG. 6(A) , respective evaluation results of the impact resistance of Samples #1 to #8 were either the evaluation result A or the evaluation result B. As apparent from  FIG. 6(A) , it was confirmed that the impact resistance tended to improve when the minimum penetration length HA, the minimum projection length KA, the maximum penetration length HD, and the maximum projection length KD became larger, that is, the area of the tip end face  71  became larger. 
     A-3-2. Reduction Performance Test for Radio Wave Noise: 
     A reduction performance test for radio wave noise was carried out using Samples #9 to #16. Specifically, the electrical field intensity of the interfering wave emitted from the spark plug as each sample was measured in a range of test frequency of 50 to 900 MHz by measuring procedure specified by International Special Committee on Radio Interference standard (CISPR). The radio-wave noise reduction performance was evaluated using an improvement rate of attenuation with reference to the attenuation (units were decibels: the attenuation compared with the spark plug without the resistor) of the electrical field intensity of the interfering wave in Sample #10 where the minimum penetration length HA was “0”. The evaluation standard of this test is as follows. 
     Evaluation Result A: the improvement rate of the attenuation is equal to or more than 3%, Evaluation Result B: the improvement rate of the attenuation is less than 3%, and Evaluation Result C: Reference level 
     Respective evaluation results of the radio-wave noise reduction performance of Samples #9 to #16 are as shown in  FIG. 6(B)  and  FIG. 7 . That is, as shown in  FIG. 6(B) , it was confirmed that the radio-wave noise reduction performance tended to improve when the minimum penetration length HA became larger. Additionally, as shown in  FIG. 7 , it was confirmed that the radio-wave noise reduction performance tended to improve over the entire range of the test frequency of 50 to 900 MHz when the minimum penetration length HA became larger. This is considered to be because the effective length EL of the resistor  70  is lengthened since the rear-endmost position among the contact points (such as the point PP 1  and the point PP 2  in  FIG. 2 ) between the inner peripheral surface of the through hole  12  and the tip end face  71  is more toward the tip end side as the minimum penetration length HA becomes larger. 
     A-3-3. Load Life Test of Resistor: 
     A load life test of the resistor  70  was carried out using Samples #9 to #16. The load life test was carried out based on test conditions compliant with Japanese Industrial Standard B8031: 2006 (internal combustion engine-spark plug) section 7.14. However, a condition more severe than the stipulation of Japanese Industrial Standard was adopted by heating to 400 degrees Celsius instead of the normal temperature. The load life (durability) was evaluated using a changing rate of the resistance value between the metal terminal nut  40  and the center electrode  20  before and after the test. The evaluation standard of this test is as follows. 
     Evaluation Result A: the changing rate is equal to or less than ±15%, Evaluation Result B: the changing rate is equal to or less than ±25%, Evaluation Result C: the changing rate is equal to or less than ±30%, and Evaluation Result D: the changing rate is equal to or more than ±30. 
     As shown in  FIG. 6(B) , in respective evaluation result of the impact resistance of Samples #9 to #16, it was confirmed that the durability tended to improve when the minimum penetration length HA became smaller. Furthermore, the durability was found to be considerably improved in the case where the minimum penetration length HA is equal to or less than 1.2 mm compared with 1.3 mm (or more). That is, it was found that the penetration length H was preferred to be set equal to or less than 1.2 mm. 
     B. Modification: 
     (1)  FIG. 8  is a diagram showing a compression rod member  200 B used in manufacture of the insulator assembly in a modification. A tip end face  210 B of the compression rod member  200 B shown in  FIG. 8  is molded in a shape approximated by the shape of the tip end face  71  of the resistor  70  in the insulator assembly to be manufactured, unlike the tip end face of the compression rod member  200  (in  FIG. 4(A) ) in the embodiment. The shape of the tip end face  71  changes from the shape before compression and sintering when the raw material powders  65 ,  75 , and  85  are compressed and sintered. Therefore, the shape of the tip end face  71  might not conform to the shape of the tip end face  210 B of the compression rod member  200 B. However, in the case where the shape of the tip end face  210 B is molded in the shape approximated by the shape of the tip end face  71  of the resistor  70  in the insulator assembly to be manufactured, this facilitates molding the shape of the tip end face  71  in any desired shape. 
     The example shown in  FIG. 8  is an example of the compression rod member  200 B to realize the shape (in  FIG. 2 ) of the tip end face  71  described in the embodiment. That is, in the shape of the tip end face  210 B of the compression rod member  200 B, a peripheral edge portion  212 B positioned at the radially outside of a center portion  213 B is positioned at the tip end side compared with the center portion  213 B close to the central axis CO, similarly to the shape of the tip end face  71  (in  FIG. 2 ). 
     (2)  FIGS. 9(A) to 9(C)  are diagrams showing an exemplary shape of the tip end face of the resistor in the modification. As shown in  FIG. 9(A) , a tip end face  71 C of a resistor  70 C does not necessarily have a plurality of local maximum points or local minimum points in the cross section including the central axis CO, and may have a configuration with only one of the local maximum point and the local minimum point (the total number of the local maximum point and the local minimum point indicates the total number of the local maximum point and the local minimum point formed in positions apart from the inner surface of the through hole of the insulator (the through hole  12  of the ceramic insulator  10 ). In the example shown in  FIG. 9(A) , the peripheral edge portion of the resistor  70 C does not project toward the tip end side with respect to the center portion of the resistor  70 C over the whole circumference, but only a part of the peripheral edge portion of the resistor  70 C projects toward the tip end side with respect to the center portion of the resistor  70 C. 
     However, in the case where the configuration has only one local maximum point or local minimum point, the tip end of the resistor  70 C is preferred to be positioned at the tip end side with respect to the rear end of the head  23 . In this case, the resistor  70 C includes the portion positioned at the tip end side with respect to the rear end of the head  23 . This expands the area of the tip end face  71 C of the resistor  70 C without shortening the effective length EL in the given portion. As a result, this reduces sealing failure between the conductive seal and the resistor without shortening the radio-wave noise reduction performance. 
     (3) As shown in  FIGS. 9(B) and 9(C) , tip end faces  71 D and  71 E of resistors  70 D and  70 E do not necessarily include a portion positioned at the tip end side with respect to the rear end of the center electrode  20 . However, in the case where the tip end faces  71 D and  71 E do not include a portion positioned at the tip end side with respect to the rear end of the center electrode  20 , the contact surface between the resistor and the conductive seal includes a portion where the distance in the central axis direction between the contact surface and the virtual plane (the virtual plane vertical to the central axis) including the rear end of the resistor changes according to the position along the contact surface. Furthermore, at least one cross section among a plurality of cross sections including the central axis CO (a plurality of cross sections with mutually different directions perpendicular to the cross sections) is preferred to have a plurality of points (referred to also as extremal points) where the distance from the rear end of the resistor in the central axis direction becomes local maximum or local minimum (especially, preferred to include one or more points (referred to also as the local maximum point) where the distance becomes local maximum and include one or more points (referred to also as the local minimum point) where the distance becomes local minimum). Here, the number of extremal points (the number of local maximum points and the number of local minimum points) indicates the number of extremal points (the number of local maximum points and the number of local minimum points) formed in positions apart from the inner surface of the through hole of the insulator (the through hole  12  of the ceramic insulator  10 ). The tip end face  71 D of the resistor  70 D in  FIG. 9(B)  is an example including three extremal points (two local maximum points SP 5  and SP 7  and one local minimum point SP 6 ). The tip end face  71 E of the resistor  70 E in  FIG. 9(C)  is an example including two extremal points (one local maximum point SP 8  and one local minimum point SP 9 ). In this case, the tip end faces  71 D and  71 E of the resistors  70 D and  70 E do not include the portion positioned at the tip end side with respect to the rear end of the center electrode  20 , but include the plurality of extremal points. This expands the respective areas of the tip end faces  71 D and  71 E without excessively shortening the effective length EL. 
     (4) The configuration of the spark plug is not limited to the configuration shown in the above-described embodiments and modifications. Various configurations may be adopted. For example, the shape of the rear end portion of the center electrode  20  (in  FIG. 2 ) is not limited to the shape including the flange portion  24  and the head  23 . Various shapes may be adopted. For example, the outer diameter of the head  23  may be the same as the outer diameter of the flange portion  24  (that is, the outer diameter may be uniform without change on the rear end side with respect to the shoulder portion  24   f ). In either case, the resistor  70  is preferred to include a portion positioned at the tip end side with respect to the rear end of the center electrode over the whole circumference of the side surface in the rear end portion including the rear end of the center electrode. This further expands the area of the contact portion between the resistor and the conductive seal without shortening the effective length of the resistor. 
     The inner diameter of the large inner diameter portion BRP in the through hole  12  of the ceramic insulator  10  (in  FIG. 1 ) may be changed in accordance with the position along the direction parallel to the central axis CO (for example, a portion with an inner diameter expanding from the tip end side toward the rear end side may be disposed). Similarly, the inner diameter of the small inner diameter portion SRP may be changed in accordance with the position along the direction parallel to the central axis CO (for example, a portion with an inner diameter expanding from the tip end side toward the rear end side may be disposed). In either case, the large inner diameter portion BRP and the small inner diameter portion SRP are preferred to be constituted so that the inner diameter of the large inner diameter portion BRP becomes larger than the inner diameter of the small inner diameter portion SRP. Thus, the insulator shoulder portion  16  disposed between the large inner diameter portion BRP and the small inner diameter portion SRP is preferred to support the shoulder portion  24   f  of the center electrode. 
     (5) The sizes of the respective areas in the spark plug  100  described in the above-described embodiment are examples. This should not be construed in a limiting sense. As described above, the present invention is more ideally suited to the compact spark plug, but may be applied to a spark plug with a typical diameter or a large diameter. For example, the present invention may be applied to a spark plug where a diameter of the mounting screw portion  52  is 13 mm to 18 mm and a distance between opposite sides of the tool engagement portion  51  is 15 mm to 20 mm. 
     The embodiment and the modifications of the present invention are described above. However, the present invention is not limited to these embodiment and modifications. The present invention may be practiced in various forms without departing from its spirit and scope. 
     REFERENCE LIST 
     
         
           10  ceramic insulator 
           12  through hole 
           13  insulator leg portion 
           15  shoulder portion 
           16  shoulder portion 
           17  tip-end-side trunk portion 
           18  rear-end-side trunk portion 
           19  flange portion 
           20  center electrode 
           21  electrode base material 
           22  core material 
           23  head 
           24  flange portion 
           25  leg 
           28  electrode tip 
           30  ground electrode 
           31  base-material tip end portion 
           32  base-material base end portion 
           38  electrode tip 
           40  metal terminal nut 
           41  plug cap installation portion 
           42  flange portion 
           43  leg 
           50  metal shell 
           51  tool engagement portion 
           52  mounting screw portion 
           53  caulking portion 
           54  seal portion 
           56  shoulder portion 
           56  metal-shell-side shoulder portion 
           58  compression deformation portion 
           59  insertion hole 
           60  conductive seal 
           70  resistor 
           80  conductive seal 
           100  spark plug 
           200  compression rod member