Patent Publication Number: US-10777975-B2

Title: Spark plug

Description:
FIELD OF THE INVENTION 
     The present invention relates to a spark plug. Herein, the term “front” refers to a spark discharge side with respect to the direction of a center axis of a spark plug; and the term “rear” refers to a side opposite the front side. 
     BACKGROUND OF THE INVENTION 
     Japanese Laid-Open Patent Publication No. H09-266055 discloses one example of spark plug, which includes: a cylindrical insulator; a center electrode held in the insulator; a ground electrode defining a spark gap with the center electrode; and a resistor disposed on a rear end side of the center electrode within the insulator so as to suppress radio noise generated from the center electrode. In this spark plug, a conductive seal material such as glass seal material is arranged between the resistor and the center electrode in contact with an outer circumferential surface of the resistor so as to ensure electrical conduction to the center electrode while providing an improved seal on the inside of the insulator. 
     In the above-disclosed type of spark plug, a difference in thermal expansion coefficient between the insulator and the center electrode tends to be large so that a clearance may occur between the center electrode and the conductive seal material due to such a difference in thermal expansion coefficient during the manufacturing of the spark plug. 
     In the case where the conductive glass seal material is fixed between the center electrode and the resistor by hot pressing, for example, the conductive glass seal material (in the form of a raw material powder) is placed between the center electrode and the resistor in a thorough hole of the insulator, melted under heating and then solidified under cooling. However, the amount of thermal shrinkage of the center electrode during the cooling becomes larger than that of the insulator as the difference in thermal expansion coefficient between the center electrode and the insulator increases. This makes it likely that a clearance will occur at a location adjacent to an interfacial surface of the center electrode. The higher the occupation rate of the center electrode in the through hole of the insulator, the more likely the influence of the difference in thermal expansion coefficient will occur. The influence of the difference in thermal expansion coefficient is of particular concern in the vicinity of a rear-end-side collar portion of the center electrode. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances. An advantage of the present invention is a spark plug which has a center electrode and a conductive seal material arranged on a rear end side of the center electrode so as to suppress the occurrence of a clearance between the center electrode and the conductive seal material and improve fixing of the conductive seal material to the center electrode. 
     In accordance with a first aspect of the present invention, there is provided a spark plug, comprising: a cylindrical metal shell having a front end portion to which a ground electrode is joined; an insulator having an through hole formed therethrough in a direction of an axis of the spark plug, the through hole including a first hole region and a second hole region having an inner diameter larger than that of the first hole region and being connected to the first hole region via a step region; a center electrode including a collar portion disposed in the second hole region and supported on the step region and a shaft portion extending from the collar portion toward the first hole region, the center electrode having a thermal expansion coefficient higher than that of the insulator; a resistor disposed in the second hole region, with a front end of the resistor being located apart from a rear end of the center electrode; and a conductive seal material arranged at least between the center electrode and the resistor within the second hole region, the conductive seal material having a thermal expansion coefficient lower than that of the center electrode, wherein the center electrode has formed therein a recess recessed from the rear end thereof toward the front, wherein the recess is provided at least at a location of a maximum outer diameter section of the collar portion in the direction of the axis, and wherein the conductive seal material is filled into the recess from the rear end of the center electrode. 
     In the first aspect, the recess is formed in the part of the center electrode from the rear end of the center electrode through to the maximum outer diameter section of the collar portion, without the entire collar portion being formed of the material of the center electrode; and the conductive seal material, which is lower in thermal expansion coefficient than the center electrode, is filled into the recess. With this configuration, it is possible to decrease the thickness of the center electrode at the location of the maximum outer diameter section of the collar portion and suppress thermal expansion or shrinkage of the maximum outer diameter section during heating or cooling in the manufacturing of the spark plug. Thus, the spark plug effectively prevents the occurrence of a clearance in the vicinity of the maximum outer diameter section due to a difference in thermal expansion coefficient between the insulator and the center electrode. Furthermore, it is possible to ensure a large area of contact between the conductive seal material and the rear end part of the center electrode as the conductive seal material is filled into the recess. The spark plug thus achieves effectively improved fixing of the conductive seal material to the center electrode. 
     In accordance with a second aspect of the present invention, there is provided a spark plug as described above, wherein, in a cross section of the spark plug taken along any plane passing through the axis, the center electrode has a ratio α/β of 40% or higher where α and β are an inner diameter of the recess and an outer diameter of the collar portion, respectively, at the location of the maximum outer diameter section of the collar portion in the direction of the axis. 
     In second aspect, the volume rate of the recess is set high at the location of the maximum outer diameter section of the collar portion so that it is possible to further decrease the thickness of the center electrode at the location of the maximum outer diameter section and further decrease the amount of thermal expansion or shrinkage of the maximum outer diameter section during heating or cooling. As the relationship of α/β≥40% is satisfied in each cross section of the spark plug taken along any plane passing through the axis, the thickness of the center electrode is decreased throughout its entire circumference. Thus, the spark plug more reliably suppresses the occurrence of a clearance in the vicinity of the maximum outer diameter section due to the difference in thermal expansion coefficient. 
     In accordance with a third aspect of the present invention, there is provided a spark plug as described above, wherein the step region includes a tapered area having an inner diameter gradually decreasing toward the first hole region, wherein a front-side surface of the collar portion is brought into contact with a surface of the tapered area, and wherein a front end of the recess is located frontward of a front end of the tapered area. 
     In the third aspect, it is possible to decrease the thickness of the center electrode at least in the range from the rear end of the center electrode to the front end of the tapered area in the direction of the axis, whereby the spark plug reliably suppresses the occurrence of a clearance in this axis direction range. It is also possible to ensure a larger area of contact between the conductive seal material and the center electrode by increasing the depth of the recess (i.e. the length of the recess in the direction of the axis), whereby the spark plug achieves further improved fixing of the conductive seal material to the center electrode. 
     In accordance with a fourth aspect of the present invention, there is provided a spark plug as described above, wherein the conductive seal material is filled in between an outer circumferential surface of the collar portion and an inner circumferential surface of the through hole, and wherein a front end of the recess is located frontward of a front end of a part of the conductive seal material between the outer circumferential surface of the collar portion and the inner circumferential surface of the through hole. 
     In the fourth aspect, it is possible to improve the seal between the center electrode and the insulator by filling the conductive seal material in between the outer circumferential surface of the collar portion and the inner circumferential surface of the through hole. In the case where the thickness of the collar portion is large, the collar portion tends to show a large amount of expansion or shrinkage during heating or cooling so that a clearance becomes likely to occur between the outer circumferential surface of the collar portion and the conductive seal material. It is however possible to suppress thermal expansion or shrinkage of the collar portion as a whole by filling the conductive seal material into the recess. The spark plug thus reliably suppresses the occurrence of a clearance in the vicinity of the collar portion. It is also possible to ensure a larger area of contact between the conductive seal material and the center electrode by increasing the depth of the recess (i.e. the length of the recess in the direction of the axis). The spark plug thus achieves further improved fixing of the conductive seal material to the center electrode. 
     In accordance with a fifth aspect of the present invention, there is provided a spark plug as described above, wherein the collar portion includes a diameter decreasing section located frontward of the maximum outer diameter section and having an outer diameter gradually decreasing toward the shaft portion, and wherein the recess includes a small diameter region located frontward of a rear end of the diameter decreasing section and having an inner diameter smaller than a maximum inner diameter of the maximum outer diameter section. 
     In fifth aspect, it is possible by increasing the depth of the recess to suppress the difference in thermal expansion efficient between the insulator and the center electrode over a wider range and improve fixing of the conductive seal material to the center electrode. As the diameter decreasing portion (whose outer diameter gradually decreases toward the shaft portion) is provided in the front end side of the collar portion, the thickness of the diameter decreasing section may become too small to attain sufficient strength by the formation of the deep recess from the rear end of the center electrode through to the diameter decreasing section. However, the small diameter region (whose inner diameter is smaller than the maximum inner diameter of the maximum outer diameter section) is provided in the recess at a position frontward of the rear end of the diameter decreasing section. It is thus possible to form the recess with a greater depth while ensuring a larger thickness of the diameter decreasing section. 
     In accordance with a sixth aspect of the present invention, there is provided a spark plug as described above, wherein the recess includes a diameter increasing region located in a rear end side thereof and having an inner diameter gradually increasing toward the rear end of the center electrode. 
     In the sixth aspect, it is easier by the diameter increasing region to charge and fill the conductive seal material (in the form of a raw material powder) into the recess during manufacturing of the spark plug and thereby possible to easily increase the density of the conductive seal material in the recess. 
     As described above, the spark plug according to the present invention suppresses the occurrence of a clearance between the conductive seal material and the center electrode and achieves improved fixing of the conductive seal material to the center electrode. 
     The other objects and features of the present invention will also become understood from the following description. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a spark plug according to a first embodiment of the present invention. 
         FIG. 2  is an enlarged cross-sectional view of a part of the spark plug in the vicinity of a rear end of a center electrode according to the first embodiment of the present invention. 
         FIG. 3  is an enlarged cross-sectional view of a part of a spark plug in the vicinity of a rear end of a center electrode according to a second embodiment of the present invention. 
         FIG. 4  is an enlarged cross-sectional view of a part of a spark plug in the vicinity of a rear end of a center electrode according to a first modification example of the present invention. 
         FIG. 5  is an enlarged cross-sectional view of a part of a spark plug in the vicinity of a rear end of a center electrode according to a second modification example of the present invention. 
         FIG. 6  is an enlarged cross-sectional view of a part of a spark plug in the vicinity of a rear end of a center electrode according to a third modification example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. First Embodiment 
     A-1. Basic Structure of Spark Plug 
       FIG. 1  is a cross-sectional view of a spark plug  1  according to the first embodiment of the present invention. In  FIG. 1 , a cross section of the spark plug  1  is taken along a center axis CL of the spark plug  1 . In the present description, the center axis CL is also simply referred to as “axis CL”; and the direction parallel to the center axis CL is also referred to as “direction of the axis CL”. 
     As shown in  FIG. 1 , the spark plug  1  includes an insulator  3 , a center electrode  4 , a metal terminal  5 , a metal shell  7 , an electrical connection part  60  and a ground electrode  8 . 
     The metal shell  7  has a cylindrical shape (more specifically, substantially circular cylindrical shape) to hold therein the insulator  3  while accommodating a part of the insulator  3 . The ground electrode  8  is joined to a front end portion of the metal shell  7 . A thread portion  9  is formed an outer circumferential surface of a front end part of the metal shell  7  such that the spark plug  1  is mounted to a cylinder head of an internal combustion engine by means of the thread portion  9 . 
     The insulator  3  is held and fixed to an inner circumference of the metal shell  7  via a talc  10  and a packing  14 , with a front end portion of the insulator  3  protruding from a front end surface of the metal shell  7 . A through hole  3 A is formed through the insulator  3  so as to extend between front and rear ends of the insulator  3  in the direction of the axis CL. 
     In the first embodiment, the through hole  3 A includes: a first hole region  11  located on a front end side of the insulator  3  and holding therein the center electrode  4 ; and a second hole region  12  located rearward of the first hole region  11  and accommodating therein the electrical connection part  60 . Both of inner circumferential surfaces of the first and second hole regions  11  and  12  are circular cylindrical in shape centering on the axis CL. A diameter of the inner circumferential surface of the second hole region  12  (hereinafter simply referred to as “inner diameter of the second hole region  12 ”) is larger than a diameter of the inner circumferential surface of the first hole region  11  (hereinafter simply referred to as “inner diameter of the first hole region  11 ”). There is a tapered step region  13  formed between the first and second hole regions  11  and  12 , as a part of the through hole  3 A, such that an inner circumferential surface of the step region  13  has a diameter gradually increasing toward the rear. The inner diameter of the first hole region  11  is constant within a range from a front end of the step region  13  to a front end of the insulator  3 , whereas the inner diameter of the second hole region  12  is constant within a range from a rear end of the step region  13  to a predetermined point rearward of a front end of the metal terminal  5 . Namely, the through hole  3 A is in the form of an axial hole where the first hole region  11  and the second hole region  12  of larger inner diameter than the first hole region  11  are connected to each other via the step region  13 . 
     Preferably, the insulator  3  is made of a material having mechanical strength, thermal strength and electrical strength. For example, the insulator  3  can be a sintered ceramic body predominantly composed of alumina. The insulator  3  has a thermal expansion coefficient lower than that of the center electrode  4  and lower than those of the after-mentioned conductive seal materials  61  and  62 . 
     The center electrode  4  is held in a front end side of the through hole  3 A, with a front end portion of the center electrode  4  protruding frontward from a front end surface of the insulator  3 , and is kept insulated from the metal shell  7 . The center electrode  4  includes: a collar portion  44  located on a rear end side thereof; a shaft portion  42  located adjacent to and frontward of the collar portion  44  and having an outer diameter smaller than that of the collar portion  44 ; and a cylindrical rear end portion  45  located adjacent to and rearward of the collar portion  44  and having an outer diameter smaller than that of the collar portion  44 . The collar portion  44  is disposed in the second hole region  12  and is retained and supported on the step region  13 . The cylindrical rear end portion  45  extends rearward from a rear end of the collar portion  44 , and is disposed together with the collar portion  44  in the second hole region  12 . The shaft portion  42  extends frontward from the collar portion  44  toward the first hole region  11 , and is inserted in the first hole region  11 . 
     Preferably, the center electrode  4  is made of a material having thermal conductivity and mechanical strength. For example, the center electrode  4  can be made of a Ni-based alloy such as Inconel (tradename). A core of high thermal conductivity material such as Cu or Ag may be embedded in an axial center part of the center electrode  4 . The center electrode  4  has a thermal expansion coefficient higher than that of the insulator  3  and higher than those of the after-mentioned conductive seal materials  61  and  62 . 
     The ground electrode  8  is bent at a middle portion thereof and has a base end portion joined to the front end surface of the metal shell  7  and a distal end portion facing the front end portion of the center electrode  4  via a gap. The ground electrode  8  can be made of the same material as the center electrode  4 . 
     Noble metal tips  29  and  30 , each of which is made of a platinum alloy, iridium alloy or the like, are respectively joined to opposed surfaces of the center and ground electrodes  4  and  8 . There is a spark discharge gap g defined between these noble metal tips  29  and  30 . Either one or both of the noble metal tips  29  and  30  may be omitted. 
     The metal terminal  5  is used to apply a voltage from external equipment to the center electrode  4  for the generation of spark discharge between the center electrode  4  and the ground electrode  8 . The metal terminal  5  is held in a rear end side of the through hole  3 A, with a rear end portion of the metal terminal  5  protruding rearward from a rear end surface of the insulator  3 . Although not specifically shown in the drawings, an outer circumferential surface of a front end part of the metal terminal  5  is formed with fine ridges and grooves by knurling etc. so as to increase adhesion of the metal terminal  5  and the conductive seal material  62  and securely fix the metal terminal  5  and the insulator  3  together. The metal terminal  5  can be made of e.g. low carbon steel with a Ni plating layer. 
     The electrical connection part  60  is arranged between the center electrode  4  and the metal terminal  5  within the through hole  3 A, as an electrical conduction path, such that the center electrode  4  and the metal terminal  5  are electrically connected and conducted to each other by the electrical connection part  60 . The electrical connection part  60  includes a resistor  63  and first and second conductive seal materials  61  and  62 . 
     The resistor  63  exhibits electrical conductivity and serves as electrical resistance between the center electrode  4  and the metal terminal  5  to suppress the generation of radio noise at the time of spark discharge. The resistor  63  is disposed in the second hole region  12 , with a front end of the resistor  63  being located apart from a rear end  4 A of the center electrode  4  (see also  FIG. 2 ). The resistor  63  is formed by sintering a resistive composition that contains e.g. a glass powder and a carbon-containing conductive powder. 
     The first and second conductive seal materials  61  and  62  are provided in layer form to establish a seal on the inside of the insulator  3 . These conductive seal materials  61  and  62  are formed by sintering a raw seal material powder that contains e.g. a powder of metal such as Cu or Fe and a powder of glass such as borosilicate soda glass. As mentioned above, each of the first and second conductive seal materials  61  and  62  has a thermal expansion coefficient lower than that of the center electrode  4  and higher than that of the insulator  3 . 
     The first conductive seal material  61  is arranged between the center electrode  4  and the resistor  63  within the second hole region  12  for sealing and fixing of the insulator  3  and the center electrode  4  in the through hole  3 A. In the first embodiment, the first conductive seal material  61  corresponds to the claimed conductive seal material. The first conductive seal material  61  is in contact with a surface of a rear end part of the center electrode  4  (including the collar portion  44  and the cylindrical rear end portion  45 ) and with a front end surface of the resistor  63  so as to provide electrical connection and conduction between the center electrode  4  and the resistor  63 . 
     The second conductive seal material  62  is arranged between the metal terminal  5  and the resistor  63  within the second hole region  12  for sealing and fixing of the insulator  3  and the metal terminal  5  in the through hole  3 A. The second conductive seal material  62  is in contact with the surface of the front end part of the metal terminal  5  and with a rear end surface of the resistor  63  so as to provide electrical connection and conduction between the metal terminal  5  and the resistor  63 . 
     A-2. Characteristic Features of First Embodiment 
       FIG. 2  is an enlarged cross-sectional view of a part of the spark plug  1  in the vicinity of the rear end  4 A of the center electrode  4 . 
     As shown in  FIG. 2 , the collar portion  44  of the center electrode  44  has a maximum outer diameter section  44 B and a diameter decreasing section  44 A. The maximum outer diameter section  44 B is a part of the collar portion  44  whose outer circumferential surface is maximum in diameter. The outer circumferential surface of the maximum outer diameter section  44 B has a circular cylindrical shape constant in diameter within a predetermined range AR 1  in the direction of the axis CL. The diameter decreasing section  44 A is located adjacent to and frontward of the maximum outer diameter section  44 B. The diameter decreasing section  44 A has a tapered shape whose outer diameter gradually decreases toward the shaft portion  44 . In  FIG. 2 , a rear end  44 Z of the diameter decreasing section  44 A coincides with a front end of the maximum outer diameter section  44 B; and a front end  44 Y of the diameter decreasing section  44  coincides with a rear end of the shaft portion  42 . 
     On the other hand, the step region  13  of the insulator  3  has a tapered area  13 A as shown in  FIG. 2 . In the first embodiment, the entire step region  13  is formed as the tapered area  13 A. A rear end of the tapered area  13 A coincides with a front end of the second hole region  12 ; and a front end of the tapered area  13 A coincides with a rear end of the first hole region  11 . The tapered area  13 A has an inner diameter gradually decreasing toward the first front region  11  such that the inner diameter of the rear end of the tapered area  13  is equal to the inner diameter of the second hole region  12  and such that the inner diameter of the front end of the tapered area  13  is equal to the inner diameter of the first hole region  11 . A front-side surface of the collar portion  44  (more specifically, a surface of the diameter decreasing section  44 A) is hence brought into contact with a surface of the tapered area  13 A. 
     Furthermore, the center electrode  4  has a recess  46  formed therein from the rear end  4 A toward a front end  4 B of the center electrode  4  through the collar portion  44  as shown in  FIG. 2  (also see  FIG. 1 ). The recess  46  is a space recessed along and centering on the axis CL. As the recess  46  has a depth in the direction of the axis CL, a maximum depth (most recessed) point of the recess  46  corresponds to a front end  46  of the recess  46 . As a consequence, the part of the center electrode  4  in which the recess  46  is formed is hollow, substantially cylindrical in shape. 
     The recess  46  is provided at least at a location of the maximum outer diameter section  44 B of the collar portion  44  in the direction of the axis CL. In the first embodiment, the recess  46  extends through the whole of the axis direction range AR 1  (in which the maximum outer diameter section  44 B is located) so that the front end  46 A of the recess  46  is located frontward of the front end of the maximum outer diameter section  44 B as shown in  FIG. 2 . More specifically, the recess  46  extends over the entire tapered area  13 A and reaches a greater depth so that the front end  46 A of the recess  46  is located frontward of the front end of the maximum outer diameter section  44 B. 
     The recess  46  includes: a circular cylindrical region  48  having an inner circumferential surface circular cylindrical in shape centering on the axis CL; a diameter increasing region  49  located rearward of the circular cylindrical region  48 ; and a small diameter region  47  located frontward of the circular cylindrical region  48 . 
     The circular cylindrical region  48  is provided over a predetermined range in the direction of the axis CL so as to extend astride the cylindrical rear end portion  45  and the collar portion  44 . An inner diameter of the circular cylindrical region  48  is constant throughout the predetermined range. A front end of the circular cylindrical region  48  is located at a position close to the front end of the maximum outer diameter section  44 B within the axis direction range AR 1 . In  FIG. 2 , the inner diameter of the circular cylindrical region  48  (corresponding to a maximum inner diameter of the recess  46 ) is designated as D 1 ; the outer diameter of the maximum outer diameter section  44 B (corresponding to a maximum outer diameter of the collar portion  44 ) is designated as D 2 ; and the inner diameter of the second hole region  12  is designated as D 3 . 
     The diameter increasing region  49  is provided, in a rear end side of the recess  46 , with a tapered inner circumferential surface such that an inner diameter of the diameter increasing region  49  gradually increases toward a rear end of the recess  46 . In  FIG. 2 , the rear end  4 A of the center electrode  4  corresponds to a rear end of the diameter increasing region  49  and also corresponds to the rear end of the recess  46 . A front end of the diameter increasing region  49  coincides with a rear end of the circular cylindrical region  48 . Further, the inner diameter of the front end of the diameter increasing region  49  is equal to the inner diameter D 1  of the circular cylindrical region  48 . 
     The small diameter region  47  is provided in a front end side of the recess  46 . An inner diameter of the small diameter region  47  is smaller than the inner diameter D 1  of the circular cylindrical region  48  and gradually decreases toward the front end  46 A of the recess  46 . As the inner diameter D 1  of the circular cylindrical region  48  corresponds to a maximum inner diameter of the maximum outer diameter section  44 B, the inner diameter of the small diameter region  47  is smaller than the maximum inner diameter of the maximum outer diameter section  44 B. A rear end of the small diameter region  47  is located at the same position as the rear end  44 Z of the diameter decreasing section  44 A in the direction of the axis CL or slightly rearward of the rear end  44 Z of the diameter decreasing section  44 A; and a front end of the small diameter region  47  is located frontward of the front end  44 Y of the diameter decreasing section  44 A. In other words, the small diameter region  47  is at least partially located frontward of the rear end  44 Z of the diameter decreasing section  44 A. In the first embodiment, the small diameter region  47  is provided throughout the whole axis direction range in which the diameter decreasing section  44 A is located, and is gradually decreased in inner diameter toward the front as shown in  FIG. 2 . This makes it easy to ensure the thickness of the center electrode  4  at a location of the diameter decreasing section  44 A. 
     In the first embodiment, the conductive seal material  61  is charged into the recess  46  from the rear end  4 A of the center electrode  4  such that the whole of the recess  46  is filled with the conductive seal material  61 . The conductive seal material  61  is also charged and filled in between the outer circumferential surface of the rear end part of the center electrode  4  and the inner circumferential surface of the insulator  3  so as to entirely circumferentially surround the rear end part of the center electrode  4 . More specifically, the conductive seal material  61  is filled in between the outer circumferential surfaces of the cylindrical rear end portion  45  and the maximum outer diameter section  44 B and the inner circumferential surface of the through hole  3 A so as to entirely circumferentially surround the cylindrical rear end portion  45  and the maximum outer diameter section  44 B. A front end  61 A of the part of the conductive seal material  61  on the outer circumferential side of the center electrode  4  (i.e. between the outer circumferential surface of the collar portion  44  and the inner circumferential surface of the through hole  3 A) is located at the same position as the front end of the maximum outer diameter section  44 B in the direction of the axis CL or frontward of the front end of the maximum outer diameter section  44 B (e.g. at a position between the diameter decreasing section  44 A and the tapered area  13 A). Further, the front end  46 A of the recess  46  is located frontward of the front end  61 A of the part of the conductive seal material  61  on the outer circumferential side of the center electrode  4 . 
     With the above-mentioned configuration, it is possible to allow the entry of the conductive seal material  61  into the recess  46  of the center electrode  4  while limiting the thickness of the center electrode  4  at least at the location of the maximum outer diameter section  44 B. 
     The spark plug  1  is herein configured to, when viewed in cross section along a plane passing through the axis CL, satisfy the relationship of α/β≥40% where α and β are the inner diameter of the recess  46  and the outer diameter of the collar portion  44 , respectively, in the axis direction range AR 1  in which the maximum outer diameter section  44 B is located. It is preferable to, in a cross section of the spark plug  1  taken along any plane perpendicular to the axis CL and passing through the maximum outer diameter section  44 B (for example, in a cross section of the spark plug  1  taken along any arbitrary imaginary plane P 1  perpendicular to the center axis CL within the axis direction range AR 1 ), satisfy the relationship of α/β≥40%. It is more preferable to, in each of cross sections of the spark plug  1  taken along any imaginary planes passing through the maximum outer diameter section  44 B, satisfy the relationship of α/β≥40%. 
     The satisfaction of the above relationship can be judged by specifying the position of the maximum outer diameter section  44 B in the spark plug  1  by a computed tomography (CT) technique, cutting through the spark plug  1  at the specified position, grinding the cross section of the spark plug  1 , observing the cross section of the spark plug with an electron scanning microscope (SEM), measuring the inner diameter α of the recess  46  and the outer diameter β of the collar portion  44  in the cross section along any direction passing through and perpendicular to the axis CL and calculating the percentage ratio of α to β. 
     It is also preferable to, in a cross section of the spark plug  1  taken along any imaginary plane perpendicular to the axis CL and passing through a part of the center electrode  4  rearward of the maximum outer diameter section  44 B (e.g. passing through the cylindrical rear end portion  45 ), satisfy the relationship of α/β≥40%. 
     A-3. Characteristic Features of Second Embodiment 
       FIG. 3  is an enlarged cross-sectional view of a part of a spark plug  201  according to the second embodiment of the present invention. The spark plug  201  according to the second embodiment is structurally similar to the spark plug  1  according to the first embodiment, except for the arrangement configuration of a center electrode  204  and a conductive seal material  61  in an axis direction range Z of the through hole  3 A from the front end of the resistor  63  to the vicinity of the front end of the step region  13  as shown in  FIG. 3 . In the second embodiment, parts and portions other than the center electrode  204  and the conductive seal material  61  are designated by the same reference numerals as in the first embodiment; and detailed explanations of those other parts and portions are omitted herefrom. 
     In the second embodiment, the center electrode  204  has a recess  246  formed from the rear end of the center electrode  204  toward the front. The recess  246  is provided at least at a location of the maximum outer diameter section  44 B of the collar portion  44  in the direction of the axis CL as in the case of the first embodiment. More specifically, the recess  246  extends through the whole or substantially the whole of the axis direction range AR 1  (in which the maximum outer diameter section  44 B is located) as shown in  FIG. 3 . A front end  246 A of the recess  246  can be located at the same position or, frontward or rearward of the front end of the maximum outer diameter section  44 B, in the direction of the axis CL. The recess  246  as a whole consists of a circular cylindrical region  248  having an inner circumferential surface circular cylindrical in shape centering on the axis CL. An inner diameter D 1  of the cylindrical surface section  248  is constant throughout the whole range from the front end  246 A to the rear end  246 B of the recess  246  in the direction of the axis CL. 
     The conductive seal material  61  is filled into the recess  246  from the rear end of the center electrode  204 . 
     Even with the above configuration, it is possible to allow the entry of the conductive seal material  61  in the recess  246  of the center electrode  204  while limiting the thickness of the center electrode  204  at least at the location of the maximum outer diameter section  44 B. 
     As in the case of the first embodiment, the spark plug  201  is configured to configured to satisfy the relationship of α/β≥40% in the second embodiment. 
     A-4. Effects 
     The first and second embodiments provides the following effects. 
     (1) In the first and second embodiments of  FIGS. 2 and 3 , the recess  46 ,  246  is formed in the part of the center electrode  4 ,  204  from the rear end of the center electrode  4 ,  204  through to the maximum outer diameter section  44 B of the collar portion  44 , without the entire collar portion  44  being formed of the material of the center electrode  4 ,  204 ; and the conductive seal material  61 , which is lower in thermal expansion coefficient than the center electrode  4 ,  204 , is filled into the recess  46 ,  246  from the rear end of the center electrode  4 ,  204 . It is consequently possible to decrease the thickness of the center electrode  4 ,  204  at the location of the maximum outer diameter section  44 B of the collar portion  44  and suppress thermal expansion or shrinkage of the maximum outer diameter section  44 B during heating or cooling in the manufacturing of the spark plug  1 ,  201 . Thus, the spark plug  1 ,  201  effectively prevents the occurrence of a clearance in the vicinity of the maximum outer diameter section  44 B due to a difference in thermal expansion coefficient between the insulator  3  and the center electrode  4 ,  204 . If a clearance occurs in the vicinity of the maximum outer diameter section  44 B, it is likely that a crack will develop starting from the clearance. The development of such a crack results in a deterioration of sealing and fixing between the conductive seal material  61  and the center electrode  4 ,  204 . This deterioration problem is however avoided in the spark plug  1 ,  201 . Furthermore, it is possible to ensure a large area of contact between the conductive seal material  61  and the rear end part of the center electrode  4 ,  204  as the conductive seal material  61  is filled into the recess  46 ,  246 . The spark plug  1 ,  201  thus achieves effectively improved fixing of the conductive seal material  61  to the center electrode  4 ,  204 . 
     (2) In the first and second embodiments of  FIGS. 2 and 3 , the spark plug  1 ,  201  is configured to, when viewed in cross section along any plane passing through the axis CL, satisfy the relationship of α/β≥40%. By satisfaction of this relationship, the volume rate of the recess  46 ,  246  is set high at the location of the maximum outer diameter section  44 B so that it is possible to further decrease the thickness of the center electrode  4 ,  204  at the location of the maximum outer diameter section  44  and further decrease the amount of thermal expansion or shrinkage of the maximum outer diameter section  44 B during heating or cooling. As the relationship of α/β≥40% is satisfied in every cross section of the spark plug  1 ,  201  taken along any plane passing through the axis CL, the thickness of the center electrode  204  is decreased throughout its entire circumference. Thus, the spark plug  1 ,  201  more reliably suppresses the occurrence of a clearance in the vicinity of the maximum outer diameter section  44 B due to the difference in thermal expansion coefficient. 
     (3) In the first embodiment of  FIG. 2 , the step region  13  (as the part of the through hole  3 A of the insulator  3 ) is provided with the tapered area  13 A such that the inner diameter of the tapered area  13 A gradually decreases toward the first hole region  11 ; the front-side surface of the collar portion  14  is brought into contact with the surface of the tapered area  13 A; and the front end  46 A of the recess  46  is located frontward of the front end of the tapered area  13 A. It is consequently possible to decrease the thickness of the center electrode  4  at least in the axis direction range from the rear end  4 A of the center electrode  4  to the front end of the tapered area  13 A, whereby the spark plug  1  reliably suppresses the occurrence of a clearance in this axis direction range. It is also possible to ensure a larger area of contact between the conductive seal material  61  and the center electrode  4  by increasing the depth of the recess  46  (i.e. the length of the recess  46  in the direction of the axis CL), whereby the spark plug  1  achieves further improved fixing of the conductive seal material  61  to the center electrode  4 . 
     (4) In the first embodiment of  FIG. 2 , the conductive seal material  61  is filled in between the outer circumferential surface of the collar portion  44  of the center electrode  4  and the inner circumferential surface of the through hole  3 A of the insulator  3 ; and the front end  46 A of the recess  46  is located frontward of the front end  61 A of the part of the conductive seal material  61  on the outer circumferential side of the center electrode  4 . By filling the conductive seal material  61  in between the outer circumferential surface of the collar portion  44  and the inner circumferential surface of the through hole  3 A, it is possible to improve the seal between the center electrode  4  (collar portion  44 ) and the insulator  3  (through hole  3 A). In the case where the thickness of the collar portion  44  is large, the collar portion  44  tends to show a large amount of expansion or shrinkage during heating or cooling so that a clearance becomes likely to occur between the outer circumferential surface of the collar portion  44  and the conductive seal material  61 . It is however possible to suppress thermal expansion or shrinkage of the collar portion  44  as a whole by filling the conductive seal material  61  into the recess  46 . The spark plug  1  thus reliably suppresses the occurrence of a clearance in the vicinity of the collar portion  44 . It is also possible to ensure a larger area of contact between the conductive seal material  61  and the center electrode  4  by increasing the depth of the recess  46  to a level where the front end  46 A of the recess  46  is located frontward of the front end  61 A of the part of the conductive seal material  61  on the outer circumferential side of the center electrode  4 . The spark plug  1  thus achieves further improved fixing of the conductive seal material  61  to the center electrode  4 . 
     (5) In the first embodiment of  FIG. 2 , the diameter decreasing section  44 A is provided in the collar portion  44  at a position frontward of the maximum outer diameter section  44 B such that the outer diameter of the diameter decreasing section  44 A gradually decreases toward the shaft portion  42 ; and the small diameter region  47  is provided in the recess  46  at a position frontward of the rear end of the diameter decreasing section  44 B such that the inner diameter of the small diameter region  47  is smaller than the maximum inner diameter D 1  of the maximum outer diameter section  44 B. In this embodiment, it is possible by increasing the depth of the recess  46  to suppress the difference in thermal expansion efficient between the insulator  3  and the center electrode  4  over a wider range and improve fixing of the conductive seal material  61  to the center electrode  4 . As the diameter decreasing section  44 A (whose outer diameter gradually decreases toward the shaft portion  42 ) is provided in the front end side of the collar portion  44 , the thickness of the diameter decreasing section  44 A may become too small to attain sufficient strength by the formation of the deep recess  46  from the rear end of the center electrode  4  through to the diameter decreasing section  44 A. It is however possible to form the recess with a greater depth and, at the same time, ensure a larger thickness of the diameter decreasing section  44 A as the small diameter region  47  (whose inner diameter is smaller than the maximum inner diameter of the maximum outer diameter section  44 B) is provided in the recess  46 . 
     (6) In the first embodiment of  FIG. 2 , the diameter increasing region  49  is provided in the rear end side of the recess  46  such that the inner diameter of the diameter increasing region  49  gradually increases toward the rear end  4 A of the center electrode  4 . In the case where the conductive seal material  61  is fixed by hot pressing during manufacturing of the spark plug  1 , for example, the conductive seal material  61  (in the form of a raw material powder) is placed between the center electrode  4  and the resistor  63  in the thorough hole  3 A of the insulator  3 , melted under heating and then solidified under cooling. When it is difficult to charge and fill the conductive seal material  61  (raw material powder) into the recess  46 , however, the density of the conductive seal material  61  in the recess  46  becomes low in the final product of the spark plug  1 . This can lead to the occurrence of a crack etc. in the vicinity of the recess  46  during use of the spark plug  1 . By the formation of the diameter increasing region  49 , however, it is easier to charge and fill the conductive seal material  61  (raw material powder) into the recess  46  during manufacturing of the spark plug  1  and thereby possible to easily increase the density of the conductive seal material  61  in the recess  46 . 
     A-5. Evaluation Test 
     The following evaluation test was conducted to verify the effects of the present invention. 
     Eighteen kinds of samples of the spark plug  201  according to the second embodiment of  FIG. 3  were prepared as test samples of Examples 1 to 18. In Examples 1 to 18, the inner circumferential surface of the recess  246  (circular cylindrical region  248 ) was formed into a circular cylindrical shape, with an inner diameter D 1 , centering on the axis CL; the outer circumferential surface of the maximum outer diameter section  44 B of the collar portion  44  was formed into a circular cylindrical shape, with an outer diameter D 2 , centering on the axis CL; and the inner circumferential surface of the second hole region  12  was formed into a circular cylindrical shape, with an inner diameter D 3 , centering on the axis CL. 
     The inner diameter D 1  of the recess  246 , the outer diameter D 2  of the maximum outer diameter section  44 B and the inner diameter D 3  of the second hole region  12  were set to different values. More specifically, the spark plug samples of Examples 1 to 6 were the same except that the inner diameter D 1  of the recess  246  was set to different values. The outer diameter D 2  of the maximum outer diameter section  44 B in the spark plug samples of Examples 7 to 12 were set to different values from those in the spark plug samples of Examples 1 to 6. The spark plug samples of Examples 7 to 12 were the same except that the inner diameter D 1  of the recess  246  was set to different values. The inner diameter D 3  of the second hole region  12  in the spark plug samples of Examples 13 to 18 were set to different values from those in the spark plug samples of Examples 1 to 12. The spark plug samples of Examples 13 to 18 were the same except that the inner diameter D 1  of the recess  246  was set to different values. 
     Further, each of the spark plug samples of Examples 1 to 18 was so configured that the ratio α/β (as determined as D 1 /D 2 ) had a constant value in respective cross sections of the spark plug sample taken along any imaginary planes perpendicular to the axis CL and passing through the maximum outer diameter section  44 B. 
     As test samples of Comparative Examples 1 and 2, spark plugs were prepared in the same manner as in Examples 1 to 18, but each using the center electrode  204  with no recess  246 . 
     The following seal performance evaluation test was performed on the thus-prepared spark plug samples. 
     The front end part of the spark plug sample (in the vicinity of the front end portion of the insulator  3 ) was put into a fluid resin within a given container. The fluid resin used was a cold-mounting epoxy resin (manufactured under the trade name of “Specifix 200” from Struers). In this state, the space in which the spark plug sample was placed (i.e. the space outside the fluid resin) was evacuated to a predetermined vacuum level. 
     In each of Examples 1 to 18 and Comparative Examples 1 and 2, three test samples were prepared and tested under different vacuum levels of 10000 Pa, 5000 Pa and 1000 Pa. 
     After the above test operation, the spark plug sample was subjected to cutting and grinding whereby a half section of the spark plug sample was taken through the front end of the maximum outer diameter section  44 B (see  FIG. 3 ) along a plane perpendicular to the axis CL. The cross section of the spark plug sample was observed by an energy dispersive spectrometer (EDS) with a scanning electron microscope (SEM) to examine the presence of the resin in the cross section of the spark plug sample. 
     The seal performance of the spark plug sample was evaluated as: “A” when the resin was observed in the cross section of the spark plug sample after the test operation under the vacuum level of 10000 Pa; “◯” when the resin was observed in the cross section of the spark plug sample after the test operation under the vacuum level of 5000 Pa; “⊚” when the resin was observed in the cross section of the spark plug sample after the test operation under the vacuum level of 1000 Pa; and “⋆” when the resin was not observed in the cross section of the spark plug sample even after the test operation under the vacuum level of 1000 Pa. The evaluation results are shown in TABLE 1. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Outer 
                   
                   
                   
               
               
                   
                 Inner 
                 diameter 
                 Inner 
               
               
                   
                 diameter 
                 D2 (mm) 
                 diameter 
                   
                 Entry of 
               
               
                   
                 D3 (mm) 
                 of 
                 D1 (mm) 
                   
                 resin into 
               
               
                   
                 of second 
                 collar 
                 of 
                 D1/D2 
                 interfacial 
               
               
                   
                 hole region 
                 portion 
                 recess 
                 (%) 
                 clearance 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 3.9 
                 3.6 
                 1.0 
                 27.8 
                 ◯ 
               
               
                 Example 2 
                 3.9 
                 3.6 
                 1.3 
                 36.1 
                 ◯ 
               
               
                 Example 3 
                 3.9 
                 3.6 
                 1.4 
                 38.9 
                 ◯ 
               
               
                 Example 4 
                 3.9 
                 3.6 
                 1.5 
                 41.7 
                 ⊚ 
               
               
                 Example 5 
                 3.9 
                 3.6 
                 2.0 
                 55.6 
                 ⊚ 
               
               
                 Example 6 
                 3.9 
                 3.6 
                 2.5 
                 69.4 
                 ⋆ 
               
               
                 Example 7 
                 3.9 
                 2.7 
                 0.9 
                 33.3 
                 ◯ 
               
               
                 Example 8 
                 3.9 
                 2.7 
                 1.0 
                 37.0 
                 ◯ 
               
               
                 Example 9 
                 3.9 
                 2.7 
                 1.1 
                 40.7 
                 ⊚ 
               
               
                 Example 10 
                 3.9 
                 2.7 
                 1.2 
                 44.4 
                 ⊚ 
               
               
                 Example 11 
                 3.9 
                 2.7 
                 1.5 
                 55.6 
                 ⊚ 
               
               
                 Example 12 
                 3.9 
                 2.7 
                 1.7 
                 63.0 
                 ⋆ 
               
               
                 Example 13 
                 3.0 
                 2.7 
                 0.9 
                 33.3 
                 ◯ 
               
               
                 Example 14 
                 3.0 
                 2.7 
                 1.0 
                 37.0 
                 ◯ 
               
               
                 Example 15 
                 3.0 
                 2.7 
                 1.1 
                 40.7 
                 ⊚ 
               
               
                 Example 16 
                 3.0 
                 2.7 
                 1.2 
                 44.4 
                 ⊚ 
               
               
                 Example 17 
                 3.0 
                 2.7 
                 1.5 
                 55.6 
                 ⊚ 
               
               
                 Example 18 
                 3.0 
                 2.7 
                 2.0 
                 74.1 
                 ⋆ 
               
               
                 Comparative 
                 3.9 
                 3.6 
                 0.0 
                 0.0 
                 Δ 
               
               
                 Example 1 
               
               
                 Comparative 
                 3.9 
                 2.7 
                 0.0 
                 0.0 
                 Δ 
               
               
                 Example 2 
               
               
                   
               
            
           
         
       
     
     In Comparative Examples 1 and 2 where the recess  246  was not formed in the center electrode  204 , the resin was observed even after the test operation under the vacuum level of 10000 Pa as shown in TABLE 1. In Examples 1 to 18 where the recess  246  was formed in the center electrode  204 , by contrast, the resin was not observed after the test operation under the vacuum level of 10000 Pa as shown in TABLE 1. The reason for these results is assumed as follows. In Examples 1 to 18, the recess  236  was filled with the conductive seal material  61  so that the influence of the difference in thermal expansion coefficient between the center electrode  204  and the insulator  3  was relieved in the vicinity of the maximum outer diameter section  44 B. Thus, a clearance (for entry of the fluid resin) was difficult to occur at an interfacial surface of the maximum outer diameter section  44 B. 
     As shown in TABLE 1, the resin was not observed even after the test operation under the vacuum level of 5000 Pa in Examples 4 to 6, 9 to 12 and 15 to 18 where the ratio D 1 /D 2  (α/β) was set to 40% or higher. The reason for these results is assumed as follows. The influence of the difference in thermal expansion coefficient between the center electrode  204  and the insulator  3  was more relieved due to the high occupation rate of the conductive seal material  61  at the location of the maximum outer diameter section  44 B. A clearance was thus more difficult to occur at the interfacial surface of the maximum outer diameter section  44 B. 
     2. Modification Examples 
     Although the present invention has been described with reference to the above embodiments, the above embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention thereto. Various changes and modifications can be made to the above embodiments without departing from the scope of the present invention. It is feasible to appropriately replace or combine any of the technical features mentioned above in “Summary of the Invention” and “Description of the Embodiments” in order to solve part or all of the above-mentioned problems and/or to achieve part or all of the above-mentioned effects. Any of these technical features, if not explained as essential in the present specification, may be eliminated as appropriate. For example, the following modifications can be made to the above embodiments. 
     In the above embodiments, the front end of the recess  46 ,  246  is located frontward of the center point of the axis direction range AR 1  (in which the maximum outer diameter section  44 B is located). Alternatively, the front end of the recess  46 ,  246  may be located rearward of the center point of the axis direction range AR 1 . 
     Although the recess  46 ,  246  was formed throughout the whole of the axis direction range AR 1  (in which the maximum outer diameter section  44 B is located) in the above embodiments, the recess  46 ,  246  may alternatively be formed in at least a part of the axis direction range AR 1 . 
     The inner diameter of the circular cylindrical region  48 ,  248  of the recess  46 ,  246  is not limited to those of the above embodiments and can be set larger or smaller than those of the above embodiments. 
     In the above first embodiment of  FIG. 2 , the diameter increasing region  49  may be omitted such that the circular cylindrical  48  extends to the rear end of the center electrode  4 . Further, the small diameter region  47  may be omitted such that that the front end of the circular cylindrical region  48  corresponds to the front end of the recess  46  in the above first embodiment of  FIG. 2 . 
     In the above embodiments, the front end of the recess  46 ,  246  can be located frontward or rearward of the front end of the maximum outer diameter section  44 B and can be located frontward or rearward of the front end of the diameter decreasing section  44 A as long as the front end of the recess  46 ,  246  is situated at least frontward of the rear end of the maximum outer diameter section  44 B in the direction of the axis CL. 
     The above first embodiment may be modified as shown in  FIGS. 4 to 6 .  FIGS. 4, 5 and 6  are enlarged cross-sectional views of parts of spark plugs  301 ,  401  and  501  as first, second and third modification examples of the spark plug  1 . The spark plugs  301 ,  401  and  501  are structurally similar to the spark plug  1 , except for the arrangement configuration of a center electrode  304 ,  404 ,  504  and a conductive seal material  61  in the axis direction range Z of the through hole  3 A. In the first to third modification examples, parts and portions other than the center electrode  304 ,  404 ,  504  and the conductive seal material  61  are designated by the same reference numerals as in the first embodiment; and detailed explanations of those other parts and portions are omitted herefrom. 
     In the first modification example of  FIG. 4 , the center electrode  304  includes no cylindrical rear end portion located rearward of the collar portion  44 . The rear end of the collar portion  44  thus corresponds to the rear end of the center electrode  304 . This center electrode  304  has a recess  346  formed from the rear end of the collar portion  44  toward the front. The recess  346  includes: a circular cylindrical region  348  constant in inner diameter over a predetermined range from the rear end of the collar portion  44 ; and a small diameter region  347  located frontward of the circular cylindrical region  348 . The conductive seal material  61  is filled into the recess  346 . 
     In the second modification example of  FIG. 5 , the center electrode  404  includes a non-cylindrical rear end portion located rearward of the collar portion  44 , with one side of the rear end portion protruding rearward from the rear end of the collar portion  44 . This center electrode  404  has a recess  446  formed from a point slightly frontward of the rear end of the center electrode  404  toward the front. The recess  446  includes: a circular cylindrical region  448  constant in inner diameter over a predetermined range from the rear end of the collar portion  44 ; and a small diameter region  447  located frontward of the circular cylindrical region  448 . The conductive seal material  61  is filled into the recess  446 . 
     In the third modification example of  FIG. 6 , the center electrode  504  includes no cylindrical rear end portion located rearward of the collar portion  44 . The rear end of the collar portion  44  thus corresponds to the rear end of the center electrode  504 . This center electrode  504  has a recess  546  formed from the rear end of the collar portion  44  toward the front. The recess  546  includes: a diameter increasing region  549  extending over a predetermined range from the rear end of the collar portion  44  and increased in diameter toward the rear; and a circular cylindrical region  548  located frontward of the diameter increasing region  549 . The conductive seal material  61  is filled into the recess  546 . 
     Even in these first to third modification examples, it is possible to obtain the same effects as mentioned above. 
     The entire contents of Japanese Patent Application No. 2018-071288 (filed on Apr. 3, 2017) are herein incorporated by reference. The scope of the present invention is defined with reference to the following claims.