Patent Publication Number: US-8975809-B2

Title: Spark plug

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ignition plug for use in an internal combustion engine or the like. 
     BACKGROUND OF THE INVENTION 
     An ignition plug is used in an internal combustion engine or the like, and includes an insulator formed of an insulating ceramic such as alumina and having an axial bore extending in the axial direction thereof. A center electrode is provided in a forward end portion of the axial bore. A terminal electrode is provided in a rear end portion of the axial bore, and a ground electrode which forms a spark discharge gap in cooperation with the center electrode. Spark discharge is generated at the spark discharge gap by applying a predetermined voltage to the spark discharge gap through the terminal electrode. 
     The terminal electrode includes a head portion which projects from the rear end of the insulator and to which a high tension cable (plug cap) for power supply is attached, and a rod-shaped leg portion which is inserted into the axial bore and whose forward end portion is fixed to the insulator by means of glass seal or the like. The head portion is provided in a state in which a flat-shaped portion of the head portion is in contact with a flat-shaped portion (flat portion) of a rear end surface of the insulator (in some cases, at least a portion of the forward end surface of the head portion may separate from the rear end surface of the insulator). 
     In recent years, the fuel consumption of an internal combustion engine has been strictly regulated from the viewpoint of environmental protection, etc. In order to prevent a decrease in the output of the internal combustion engine while complying with the regulation on fuel consumption, the displacement of the internal combustion engine has been decreased. A decrease in the output of the engine has been prevented by increasing the degree of compression and the degree of supercharging of the engine. 
     In an internal combustion engine having an increased degree of compression and an increased degree of supercharging, a higher voltage is needed so as to generate spark discharge. However, when the applied voltage is increased, current may leak from the terminal electrode to the metallic shell while creeping along the surface of the insulator, and misfire may occur due to discharge anomaly. In view of this, there has been proposed increasing the length of a portion (rear trunk portion) of the insulator located between the rear end of the metallic shell and the head portion of the terminal electrode to thereby prevent leakage of current (so-called flashover) (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-155839). 
     Moreover, in recent years, an ignition plug has been demanded to have a reduced size; in particular, a reduced diameter, and the insulator thereof may have a reduced diameter in order to meet such demand. In the case of the insulator having a reduced diameter, the wall thickness of the rear end portion is decreased, and the area of the above-mentioned flat portion of the insulator is decreased. 
     Incidentally, when a vibration generated as a result of operation of an internal combustion engine or the like acts on an ignition plug used therein, the head portion of the terminal electrode shakes, with a forward end portion of the leg potion fixed to the insulator serving as the center of the shaking motion, whereby the leg portion may come into contact with the inner circumference surface of a rear end portion of the insulator. In the case where the wall thickness of the rear end portion of the insulator is small, breakage such as cracking may occur on the inner circumferential surface of the rear end portion of the insulator as a result of the contact with the leg portion. 
     Also, the vibration applies a load onto the head portion of the terminal electrode in a direction intersecting with the axis, and due to this load, the forward end surface of the head portion may be pressed against the flat portion of the insulator. When the forward end surface of the head portion is pressed against (comes into pressure contact with) the insulator, a compressive load acts on a portion of the insulator against which the head portion is pressed, and a tensile load acts on a portion of the insulator adjacent to the portion on which the compressive load acts. Since the insulator is strong against compressive load but relatively weak against tensile load due to its nature, the above-mentioned tensile load may cause breakage at the rear end surface (flat portion) of the insulator. 
     In particular, in the case of an insulator in which the length of its rear trunk portion is increased and/or the area of the above-mentioned flat portion of the insulator is decreased, breakage is more likely to occur. 
     The present invention has been conceived in view of the above circumstances. An advantage of the invention is an ignition plug which can prevent breakage at a rear end portion of an insulator even in the case where a rear trunk portion of the insulator is long and/or the area of a flat portion of the insulator is small. 
     SUMMARY OF THE INVENTION 
     Configurations suitable for achieving the above advantage will next be described in itemized form. When needed, actions and effects peculiar to the configurations will be described additionally. 
     Configuration 1. In accordance with a first aspect of the present invention, there is provided an ignition plug of the present configuration comprised of: 
     an insulator having an axial bore extending in a direction of an axis; 
     a center electrode inserted into a forward end portion of the axial bore; 
     a terminal electrode having a rod-shaped leg portion inserted into a rear end portion of the axial bore, and a head portion projecting from a rear end of the insulator; and 
     a tubular metallic shell disposed around the insulator such that a portion of the insulator projects from a rear end of the metallic shell, a length L of the portion of the insulator projecting from the rear end of the metallic shell measured along the axis being not less than 28 mm but not greater than 34 mm, 
     wherein, a thickness t of a thinnest portion of the insulator is 1.0 mm or greater within a region which extends 5.5 mm from the rear end of the insulator toward the forward end side with respect to the direction of the axis; 
     a forward end surface of the head portion and a rear end surface of the insulator have respective flat portions; and 
     when the flat portion of the rear end surface of the insulator and the flat portion of the forward end surface of the head portion are projected along the axis onto a plane orthogonal to the axis, an area D of a region where a projected area of the flat portion of the rear end surface overlaps with a projected area of the flat portion of the forward end surface is set to 8 mm 2  or greater. 
     Notably, the “region which extends 5.5 mm from the rear end of the insulator toward the forward end side with respect to the direction of the axis” means a region in which the leg portion may come into contact with the insulator when the head portion shakes. Accordingly, in the case where the axial bore has a larger diameter portion which is located within the above-mentioned region and whose diameter is greater than that of the remaining portion of the axial bore, and the leg portion does not come into contact with a portion of the insulator corresponding to the larger diameter portion, that portion of the insulator is excluded from the portion whose thickness is set to 1.0 mm or greater. Namely, even the portion of the insulator located within the above-mentioned region can have a thickness less than 1.0 mm in a region where the leg portion does not come into contact with that portion of the insulator due to shaking of the head portion (this applies to the following configurations). 
     Configuration 2. In accordance with a second aspect of the present invention, there is provided an ignition plug as described above in Configuration 1, wherein the flat portion of the rear end surface of the insulator has an area of 25 mm 2  or less. 
     Configuration 3. In accordance with a third aspect of the present invention, there is provided an ignition plug of the present configuration comprised of: 
     an insulator having an axial bore extending in a direction of an axis; 
     a center electrode inserted into a forward end portion of the axial bore; 
     a terminal electrode having a rod-shaped leg portion inserted into a rear end portion of the axial bore, and a head portion projecting from a rear end of the insulator; and 
     a tubular metallic shell disposed around the insulator, 
     wherein, a thickness t of a thinnest portion of the insulator is 1.0 mm or greater within a region which extends 5.5 mm from the rear end of the insulator toward the forward end side with respect to the direction of the axis; 
     a forward end surface of the head portion and a rear end surface of the insulator have respective flat portions; 
     the flat portion of the rear end surface of the insulator has an area of 25 mm 2  or less; and 
     when the flat portion of the rear end surface of the insulator and the flat portion of the forward end surface of the head portion are projected along the axis onto a plane orthogonal to the axis, an area D of a region where a projected area of the flat portion of the rear end surface overlaps with a projected area of the flat portion of the forward end surface is set to 8 mm 2  or greater. 
     Configuration 4. In accordance with a fourth aspect of the present invention, there is provided an ignition plug as described in any of Configurations 1 to 3, wherein a part of the head portion located within a region which extends at least 2 mm from the forward end of the head portion toward the rear end side with respect to the direction of the axis has an outer diameter equal to or greater than that of the forward end surface of the head portion. 
     According to the ignition plug of Configuration 1, the length L is set to 28 mm or greater. Therefore, a high degree of insulation can be obtained between the head portion of the terminal electrode and the metallic shell, whereby leakage of current between the head portion and the metallic shell can be prevented more reliably. However, when the length L is set to 28 mm or greater, the rear end portion of the insulator becomes likely to break due to vibration. 
     According to the ignition plug of Configuration 1, the thickness t of the thinnest portion of the insulator is 1.0 mm or greater within the region which extends 5.5 mm from the rear end of the insulator toward the forward end side with respect to the axial direction. Namely, the thickness of a portion of the insulator with which the leg portion may come into contact due to shaking of the head portion is set to 1.0 mm or greater. Accordingly, the insulator has sufficient strength against the contact with the leg portion, whereby it becomes possible to more reliably prevent breakage at the rear end portion of the insulator, which breakage would otherwise occur due to contact with the leg portion. 
     Further, according to the above-described Configuration 1, when the flat portion of the insulator and the flat portion of the head portion are projected along the axis onto a plane orthogonal to the axis, the area D of the region where the projected area of the flat portion of the insulator and the projected area of the flat portion of the head portion overlap with each other is set to 8 mm 2  Accordingly, the compressive force which is applied from the head portion to the rear end surface of the ceramic insulator due to vibration can be dispersed effectively, whereby the tensile load acting on the insulator can be decreased more reliably. As a result, it becomes possible to more reliably prevent breakage at the rear end portion of the insulator, which breakage would otherwise occur due to pressure contact with the head portion. 
     As described above, according to the above-described Configuration 1, the thickness t is set to 1.0 mm or greater and the area D is set to 8 mm 2  or greater, whereby both of the breakage attributable to contact with the leg portion and the breakage attributable to pressure contact with the head portion can be prevented more reliably even in the case where the length L is 28 mm or greater and the rear end portion of the insulator is likely to break. As a result, breakage at the rear end portion of the insulator can be prevented effectively. 
     Moreover, in the case where the length L is greater than 34 mm, when a load is applied to the head portion, breakage may occur in a front-end-side part (part located near the metallic shell) of the portion (rear trunk portion) of the insulator which projects from the metallic shell. However, according the above-described Configuration 1, since the length L is set to 34 mm or less, breakage at the front-end-side part of the rear trunk portion can be prevented effectively. As a result, combined with the effect of preventing breakage at the rear end portion of the insulator which is achieved by setting the thickness t to 1.0 mm and setting the area D to 8 mm 2  or greater, setting the length L to 34 mm or less makes it possible to more reliably prevent occurrence of breakage at various portions of the insulator. 
     According to the ignition plug of Configuration 2, the area of the flat portion of the insulator is set to 25 mm 2  or less. Therefore, the rear end portion of the insulator is likely to have a relatively low strength. Accordingly, the rear end portion of the insulator is more likely to break. However, through employment of the above-described Configuration 1, breakage at the rear end portion of the insulator can be prevented more reliably. In other words, above-described Configuration 1 is particularly effective in an ignition plug in which the length L is set to 28 mm or greater, the area of the flat portion of the insulator is set to 25 mm 2  or less, and therefore, the rear end portion of the insulator is more likely to break. 
     In the case where the area of the flat portion of the insulator is small, the rear end portion of the insulator is likely to have a relatively low strength. Therefore, in the case where the area of the flat portion of the insulator is set to 25 mm 2  or less as in the case of the ignition plug of Configuration 3, breakage at the rear end portion of the insulator due to vibration is more likely to occur. 
     According to the ignition plug of Configuration 3, the above-mentioned thickness t is set to 1.0 mm or greater. Therefore, the insulator has sufficient strength against contact with the leg portion, whereby breakage at the rear end portion of the insulator due to contact with the leg portion can be prevented more reliably. 
     Also, since the area D is set to 8 mm 2  or greater, the tensile load acting on the insulator can be decreased more reliably, whereby breakage at the rear end portion of the insulator due to pressure contact with the head portion can be prevented more reliably. 
     As described above, according to the above-described Configuration 3, breakage at the rear end portion of the insulator can be prevented effectively even in the case where the area of the flat portion of the insulator is set to 25 mm 2  or less, and the rear end portion of the insulator is likely to break. 
     Also, since the length L is set to 34 mm or less, it is possible to effectively prevent the breakage at the front-end-side part of the rear trunk portion, which breakage would otherwise occur when a load acts on the head portion. 
     According to the ignition plug of Configuration 4, the part of the head portion located within a region which extends at least 2 mm from the forward end of the head portion toward the rear end side with respect to the axial direction has an outer diameter equal to or greater than that of the forward end surface of the head portion. Namely, a portion of the head portion which comes into contact with the rear end surface of the insulator has a thickness of 2 mm or greater in the axial direction. Therefore, the load applied from the head portion to the rear end surface of the insulator due to vibration tends to become large, and the rear end portion of the ceramic insulator is highly likely to break. However, through employment of the above-described Configuration 1, etc., breakage at the rear end portion of the insulator can be prevented more reliably. In other words, above-described Configuration 1, etc. are particularly effective in the case where the part of the head portion located within the region which extends at least 2 mm from the forward end of the head portion toward the rear end side with respect to the axial direction has an outer diameter equal to or greater than that of the forward end surface of the head portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially cutaway front view showing the structure of an ignition plug. 
         FIG. 2  is an enlarged sectional view showing the structures of a terminal electrode and a ceramic insulator. 
         FIG. 3  is a projection view showing a projected area of a flat portion of the terminal electrode and a projected area of a flat portion of the ceramic insulator. 
         FIG. 4  is a graph showing the results of a cracking load evaluation test performed on samples in which a length L and an area D were changed within respective ranges. 
         FIG. 5  is a graph showing the results of a cracking load evaluation test performed on samples in which the length L and the area D were changed within respective ranges. 
         FIG. 6  is a graph showing the results of a cracking load evaluation test performed on samples in which a thickness t and the area D were changed within respective ranges. 
         FIG. 7  is a graph showing the results of a cracking load evaluation test performed on samples in which the length L was set to 30 mm, and an area A and the area D were changed within respective ranges. 
         FIG. 8  is a graph showing the results of a cracking load evaluation test performed on samples in which the length L was set to 25 mm, and the area A and the area D were changed within respective ranges. 
         FIG. 9  is a graph showing the results of a cracking load evaluation test performed on samples in which the length L was set to 30 mm, and a thickness X and the area D were changed within respective ranges. 
         FIG. 10  is a graph showing the results of a cracking load evaluation test performed on samples in which the length L was set to 25 mm, and the thickness X and the area D were changed within respective ranges. 
         FIG. 11  is an enlarged sectional view showing the structure of the terminal electrode in another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     One embodiment will now be described with reference to the drawings.  FIG. 1  is a partially cutaway front view showing an ignition plug  1 . In  FIG. 1 , the direction of an axis CL 1  of the ignition plug  1  is referred to as the vertical direction. In the following description, the lower side of  FIG. 1  is referred to as the forward end side of the spark plug  1 , and the upper side of  FIG. 1  is referred to as the rear end side of the spark plug  1 . 
     The ignition plug  1  includes a tubular ceramic insulator  2  which corresponds to the insulator in the claims, a tubular metallic shell  3  which holds the ceramic insulator  2  therein, etc. 
     The ceramic insulator  2  is formed from alumina or the like by firing, as well known in the art. The ceramic insulator  2 , as viewed externally, includes a rear trunk portion  10  formed at its rear end side; a large-diameter portion  11  located forward of the rear trunk portion  10  and projecting radially outward; an intermediate trunk portion  12  located forward of the large-diameter portion  11  and being smaller in diameter than the large-diameter portion  11 ; and a leg portion  13  located forward of the intermediate trunk portion  12  and being smaller in diameter than the intermediate trunk portion  12 . The large-diameter portion  11 , the intermediate trunk portion  12 , and the greater part of the leg portion  13  of the ceramic insulator  2  are accommodated within the metallic shell  3 . The rear trunk portion  10  of the ceramic insulator  2  projects from the rear end of the metallic shell  3 . A tapered, stepped portion  14  is formed at a connection portion between the intermediate trunk portion  12  and the leg portion  13 . The ceramic insulator  2  is seated on the metallic shell  3  at the stepped portion  14 . 
     The rear trunk portion  10  has a plurality of annular grooves  31  extending along its circumferential direction and formed at predetermined intervals along the direction of the axis CL 1 . The grooves  31  increase a creeping distance, measured along the surface of the ceramic insulator  2 , between a head portion  6 B of a terminal electrode  6 , which will be described later, and the rear end of the metallic shell  3 . 
     The ceramic insulator  2  has an axial bore  4  extending therethrough along the axis CL 1 . A center electrode  5  is fixedly inserted into a forward end portion of the axial bore  4 . The center electrode  5  includes an inner layer  5 A formed of a metal having excellent thermal conductivity [e.g., copper, a copper alloy, or pure nickel (Ni)], and an outer layer  5 B formed of an alloy which contains Ni as a main component. The center electrode  5  assumes a rodlike (circular columnar) shape as a whole, and its forward end portion protrudes from the forward end of the ceramic insulator  2 . 
     A rod-shaped terminal electrode  6  formed of a metal such as low-carbon steel is provided in a rear end portion of the axial bore  4 . The terminal electrode  6  has a rod-shaped leg portion  6 A which is inserted into the rear end portion of the axial bore  4 , and a head portion  6 B which is greater in diameter than the leg portion  6 A and which projects from the rear end of the ceramic insulator  2 . 
     In the present embodiment, in order to facilitate insertion of the leg portion  6 A into the axial bore  4 , an annular gap is formed between the outer circumferential surface of the leg portion  6 A and the wall surface of the axial bore  4 . The axial bore  4  has a diameter-increased portion  4 A at the rearmost end of thereof. The diameter of the diameter-increased portion  4 A is greater than the diameter of a portion of the axial bore  4  located adjacent to and forward of the diameter-increased portion  4 A. A relatively large gap is formed between the leg portion  6 A and the diameter increased portion  4 A. A high tension cable or the like for power supply (not shown) is attached to the head portion  6 B. 
     As shown in  FIG. 2 , a flat portion  6 F having a planar shape is provided on a forward end surface of the head portion  6 B. In the present embodiment, the flat portion  6 F is in contact with a flat portion  2 F provided on a rear end surface of the ceramic insulator  2  and has a planar shape. In the present embodiment, in order to reduce the size of the ignition plug  1 , the diameter of the ceramic insulator  2  is reduced. Therefore, the area A of the flat portion  2 F of the ceramic insulator  2  is set to a predetermined value (e.g., not less than 10 mm 2  but not greater than 25 mm 2 ). A layer of glaze is provided on the outer surface of at least a portion (e.g., the rear trunk portion  10 ) of the ceramic insulator  2 . The layer of glaze may be provided on the flat portion  2 F. 
     In the present embodiment, a part of the head portion  6 B located in a region which extends at least 2 mm from the forward end of the head portion  6 B toward the rear end side with respect to the direction of the axis CL 1  has an outer diameter equal to or greater than the outer diameter of the forward end surface of the head portion  6 B. Namely, a portion of the head portion  6 B which comes into contact with the rear end surface of the ceramic insulator  2  has a thickness of at least 2 mm or more. 
     Referring back to  FIG. 1 , a circular columnar resistor  7  is disposed within the axial bore  4  to be located between the center electrode  5  and the terminal electrode  6 . Glass seal layers  8  and  9  are provided on opposite sides, respectively, of the resistor  7 . The glass seal layers  8  and  9  are formed by firing a glass powder mixture which contains an electrically conductive material (e.g., carbon black) and glass powder. In the present embodiment, a forward end portion of the terminal electrode  6  (the leg portion  6 A) is fixed to the ceramic insulator  2  by the glass seal layer  9 . 
     The metallic shell  3  is formed into a tubular shape from low-carbon steel or a like metal. The metallic shell  3  has a threaded portion (externally threaded portion)  15  which is formed on the outer circumferential surface of a forward end portion thereof. The threaded portion  15  is used to mount the ignition plug  1  onto a combustion apparatus such as an internal combustion engine or a fuel cell reformer. The metallic shell  3  also has a seat portion  16  located rearward of the threaded portion  15  and protruding radially outward. A ring-shaped gasket  18  is fitted to a screw neck  17  at the rear end of the threaded portion  15 . Furthermore, the metallic shell  3  has, near the rear end thereof, a tool engagement portion  19  having a hexagonal cross section and allowing a tool, such as a wrench, to be engaged therewith when the metallic shell  3  is to be mounted to the combustion apparatus. Also, the metallic shell  3  has a crimped portion  20  which is provided at the rear end thereof and which curves toward the radially inner side. 
     Also, the metallic shell  3  has, on its inner circumferential surface, a tapered, stepped portion  21  adapted to allow the ceramic insulator  2  to be seated thereon. The ceramic insulator  2  is inserted forward into the metallic shell  3  from the rear end of the metallic shell  3 . In a state in which the stepped portion  14  of the ceramic insulator  2  butts against the stepped portion  21  of the metallic shell  3 , a rear-end opening portion of the metallic shell  3  is crimped radially inward; i.e., the crimped portion  20  is formed, whereby the ceramic insulator  2  is fixed to the metallic shell  3 . An annular sheet packing  22  intervenes between the above-mentioned stepped portions  14  and  21 . This retains airtightness of a combustion chamber and prevents outward leakage of fuel gas entering a clearance between the leg portion  13  of the ceramic insulator  2  and the inner circumferential surface of the metallic shell  3 , the clearance being exposed to the combustion chamber. 
     Furthermore, in order to ensure airtightness which is established by crimping, annular ring members  23  and  24  intervene between the metallic shell  3  and the ceramic insulator  2  in a region near the rear end of the metallic shell  3 , and a space between the ring members  23  and  24  is filled with powder of talc  25 . That is, the metallic shell  3  holds the ceramic insulator  2  via the sheet packing  22 , the ring members  23  and  24 , and the talc  25 . 
     A ground electrode  27  is joined to a forward end portion  26  of the metallic shell  3  and is bent toward the center electrode  5  such that a side surface of a distal end portion of the ground electrode  27  faces a forward end surface of the center electrode  5 . A spark discharge gap  28  is formed between the forward end surface of the center electrode  5  and the side surface of the distal end portion of the ground electrode  27 . Spark discharges are performed across the spark discharge gap  28  substantially along the axis CL 1 . 
     In the present embodiment, the length L of a portion of the ceramic insulator  2 , which portion projects from the rear end of the metallic shell  3 , measured along the axis CL 1  is set to fall within a range of 28 mm to 34 mm. Making the length L relatively large, coupled with provision of the grooves  31  to increase the creeping distance as described above, greatly enhances the insulation between the head portion  6 B and the metallic shell  3 . As a result, occurrence of leakage of current between the head portion  6 B and the metallic shell  3  (flashover) can be restrained effectively. 
     Incidentally, as described above, a gap is present between the outer circumferential surface of the leg portion  6 A and the wall surface of the axial bore  4 . A forward end portion of the terminal electrode  6  is fixed to the ceramic insulator  2  by the glass seal layer  9 . Therefore, when a vibration generated as a result of operation of an internal combustion engine or the like acts on the ignition plug  1 , the head portion  6 B (on which a large acceleration acts because a high tension cable or the like is attached to the head portion  6 B) may shake, with a forward end portion of the terminal electrode  6  (the leg potion  6 A) serving as the center of the shaking motion. When the head portion  6 B shakes, a rear end portion of the leg portion  6 A comes into contact with the inner circumference surface of a rear end portion of the ceramic insulator  2 , and breakage (cracking or the like) may occur on the inner circumferential surface of the rear end portion of the ceramic insulator  2 . 
     In view of this, in the present embodiment, as shown in  FIG. 2 , the thickness t of a thinnest portion  2 A of the ceramic insulator  2  is set to 1.0 mm or greater in a region which extends 5.5 mm from the rear end of the ceramic insulator  2  toward the forward end side with respect to the direction of the axis CL 1 . This setting is performed for preventing breakage of the ceramic insulator  2 , which breakage would otherwise occur when the leg portion  6 A comes into contact with the inner circumference surface of the rear end portion of the ceramic insulator  2 . 
     Notably, the “region which extends 5.5 mm from the rear end of the ceramic insulator  2  toward the forward end side with respect to the direction of the axis CL 1 ” means a region in which the leg portion  6 A may come into contact with the ceramic insulator  2  when the head portion  6 B shakes. Accordingly, in the case where the diameter increased portion  4 A is provided at the rearmost end of the axial bore  4  as in the present embodiment and the leg portion  6 A does not come into contact with the wall surface of the diameter increased portion  4 A even when the head portion  6 B shakes, a portion of the ceramic insulator  2  where the diameter increased portion  4 A is located is excluded from the thinnest portion  2 A whose thickness is set to 1.0 mm or greater. 
     Incidentally, when the vibration of the spark plug  1  applies a load onto the head portion  613  in a direction intersecting the axis CL 1 , the forward end surface (the flat portion  6 F) of the head portion  6 B may be pressed against (come into pressure contact with) the rear end surface (the flat portion  2 F) of the ceramic insulator  2 . When the forward end surface of the head portion  6 B is pressed against the rear end surface of the ceramic insulator  2 , a tensile load acts on the rear end surface of the ceramic insulator  2 , and due to this tensile load, breakage (cracking) may occur at the ear end surface of the ceramic insulator  2 . 
     In view of this, in the present embodiment, breakage of the ceramic insulator  2  due to the pressure contact with the head portion  6 B is prevented as follows. Specifically, when the flat portion  2 F of the ceramic insulator  2  and the flat portion  6 F of the head portion  6 B are projected along the axis CL 1  onto a plane VS orthogonal to the axis CL 1  as shown in  FIG. 3 , the area D of a region AR where a projected area PA 1  (a hatched portion in  FIG. 3 ) of the flat portion  2 F and a projected area PA 2  (a dotted portion in  FIG. 3 ) of the flat portion  6 F overlap with each other is set to 8 mm 2  or greater. 
     As having been described in detail, according to the present embodiment, the thickness t of the thinnest portion  2 A of the ceramic insulator  2  is set to 1.0 mm or greater in the region which extends 5.5 mm from the rear end of the ceramic insulator  2  toward the forward end side with respect to the direction of the axis CL 1 . Namely, the thickness of a portion of the ceramic insulator  2  with which the leg portion  6 A may come into contact as a result of shaking of the head portion is set to 1.0 mm or greater. Accordingly, the ceramic insulator  2  has sufficient strength against the contact with the leg portion  6 A, whereby it becomes possible to more reliably prevent breakage at the rear end portion of the ceramic insulator  2 , which breakage would otherwise occur due to contact with the leg portion  6 A. 
     Also, since the above-mentioned area D is set to 8 mm 2 , the compressive force which is applied from the head portion  6 B to the rear end surface of the ceramic insulator  2  due to vibration can be dispersed effectively, whereby the tensile load acting on the ceramic insulator  2  can be decreased more reliably. As a result, it becomes possible to more reliably prevent breakage at the rear end portion of the ceramic insulator  2 , which breakage would otherwise occur due to pressure contact with the head portion  6 B. 
     Moreover, in the present embodiment, since the length L is set to 34 mm or less, when a load acts on the head portion  6 B, breakage at a forward end portion of the rear trunk portion  10  (on the inner circumferential side of the crimp portion  20 ) can be prevented effectively. As a result, combined with the effect of preventing breakage at the rear end portion of the ceramic insulator  2  which is achieved by setting the thickness t to 1.0 mm and setting the area D to 8 mm 2  or greater, setting the length L to 34 mm or less makes it possible to more reliably prevent occurrence of breakage at various portions of the ceramic insulator  2 . 
     In particular, in the case where the length L is set to 28 mm or greater, the area of the flat portion  2 F is set to 25 mm 2  or less, and the outer diameter of a forward end part of the head portion  6 B, which part has a length of at least 2 mm, is rendered equal to or greater than the outer diameter of the forward end surface of the head portion  6 B as in the present embodiment, breakage is very likely to occur at the rear end portion of the ceramic insulator  2 . However, when the thickness t and the area D are determined in the above-described manner, breakage of the ceramic insulator  2  can be prevented more reliably. 
     Notably, in the present embodiment, the length L is set to 28 mm or greater, the area of the flat portion  2 F is set to 25 mm 2  or less, and, for the ceramic insulator  2  which is very likely to suffer breakage at the rear end portion thereof, the thickness t is set to 1.0 mm or greater, and the area D is set to 8 mm 2  or greater. However, for a ceramic insulator in which the length L is less than 28 mm and the area of the flat portion  2 F is set to 25 mm 2  or less (a ceramic insulator which is likely to suffer breakage at the rear end portion thereof although the likelihood is lower than that in the case of the ceramic insulator  2  of the above-described embodiment), the thickness t may be set to 1.0 mm or greater, and the area D may be set to 8 mm 2  or greater. Also, for a ceramic insulator in which the area of the flat portion  2 F is greater than 25 mm 2  and the length L is set to 28 mm or greater (a ceramic insulator which is likely to suffer breakage at the rear end portion thereof although the likelihood is lower than that in the case of the ceramic insulator  2  of the above-described embodiment), the thickness t may be set to 1.0 mm or greater, and the area D may be set to 8 mm 2  or greater. 
     In order to confirm the action and effects achieved by the above-described embodiment, a cracking load evaluation test was performed on samples of the ignition plug which were manufactured while the above-mentioned length L was changed in a range of 25 mm to 37 mm and the above-mentioned area D (mm 2 ) was changed within a predetermined range. The outline of the cracking load evaluation test is as follows. Namely, a sample was attached to a predetermined test bench, and a load was applied to the head portion of the terminal electrode in a direction orthogonal to the axis by using a predetermined autograph (speed: 1 mm/min) A load (cracking load) at the time when the ceramic insulator cracked was measured, and the ratio (strength ratio) of the measured cracking load to the cracking load of a sample in which the length L was set to 25 mm and the area D was set to 20 mm 2  was calculated.  FIG. 4  shows the test results of samples in which the length L was set in a range of 25 mm to 31 mm, and  FIG. 5  shows the test results of samples in which the length L was set in a range of 32 mm to 37 mm. 
     In  FIG. 4 , the test results of samples in which the length L was set to 25 mm are indicated by black circles; the test results of samples in which the length L was set to 26 mm are indicated by black triangles; the test results of samples in which the length L was set to 27 mm are indicated by black squares; the test results of samples in which the length L was set to 28 mm are indicated by white circles; the test results of samples in which the length L was set to 29 mm are indicated by white triangles; the test results of samples in which the length L was set to 30 mm are indicated by white squares; and the test results of samples in which the length L was set to 31 mm are indicated by white rhombuses. In  FIG. 5 , the test results of samples in which the length L was set to 32 mm are indicated by white circles; the test results of samples in which the length L was set to 33 mm are indicated by white triangles; the test results of samples in which the length L was set to 34 mm are indicated by white squares; the test results of samples in which the length L was set to 35 mm are indicated by black circles; the test results of samples in which the length L was set to 36 mm are indicated by black triangles; and the test results of samples in which the length L was set to 37 mm are indicated by black squares. 
     Notably, for reference, the strength ratio of each sample is shown in Table 1. In each sample, the thickness t of the thinnest portion of the ceramic insulator was set to 1.5 mm. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Area D (mm 2 ) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 5 
                 8 
                 10 
                 13 
                 15 
                 18 
                 20 
               
               
                   
               
               
                 L = 25 mm 
                 0.62 
                 0.65 
                 0.79 
                 0.93 
                 0.95 
                 0.96 
                 0.98 
                 1.00 
               
               
                 L = 26 mm 
                 0.60 
                 0.62 
                 0.81 
                 0.93 
                 0.95 
                 0.96 
                 0.98 
                 0.97 
               
               
                 L = 27 mm 
                 0.59 
                 0.61 
                 0.81 
                 0.88 
                 0.91 
                 0.93 
                 0.92 
                 0.25 
               
               
                 L = 28 mm 
                 0.30 
                 0.31 
                 0.81 
                 0.92 
                 0.95 
                 0.95 
                 0.93 
                 0.94 
               
               
                 L = 29 mm 
                 0.29 
                 0.32 
                 0.81 
                 0.92 
                 0.95 
                 0.95 
                 0.93 
                 0.94 
               
               
                 L = 30 mm 
                 0.29 
                 0.32 
                 0.81 
                 0.92 
                 0.95 
                 0.95 
                 0.93 
                 0.94 
               
               
                 L = 31 mm 
                 0.31 
                 0.36 
                 0.85 
                 0.92 
                 0.91 
                 0.92 
                 0.93 
                 0.93 
               
               
                 L = 32 mm 
                 0.35 
                 0.35 
                 0.81 
                 0.88 
                 0.87 
                 0.88 
                 0.89 
                 0.91 
               
               
                 L = 33 mm 
                 0.29 
                 0.33 
                 0.80 
                 0.81 
                 0.84 
                 0.89 
                 0.90 
                 0.88 
               
               
                 L = 34 mm 
                 0.31 
                 0.32 
                 0.81 
                 0.82 
                 0.89 
                 0.86 
                 0.88 
                 0.89 
               
               
                 L = 35 mm 
                 0.30 
                 0.32 
                 0.28 
                 0.27 
                 0.28 
                 0.31 
                 0.30 
                 0.31 
               
               
                 L = 36 mm 
                 0.28 
                 0.30 
                 0.31 
                 0.28 
                 0.31 
                 0.30 
                 0.27 
                 0.27 
               
               
                 L = 37 mm 
                 0.28 
                 0.30 
                 0.28 
                 0.27 
                 0.30 
                 0.29 
                 0.31 
                 0.29 
               
               
                   
               
            
           
         
       
     
     It was confirmed that, as shown in  FIG. 4 ,  FIG. 5 , and Table 1, in the case of the samples in which the length L was set to 28 mm or greater, the strength ratio decreases greatly in some cases, and the ceramic insulator is likely to crack. Notably, in the case of the samples in which the length L was set to 35 mm or greater, the ceramic insulator cracked at a portion located on the inner side of the crimp portion, and, in the case of the samples in which the length L was set to be not less than 28 mm but not greater than 34 mm, the ceramic insulator cracked at a rear end portion thereof even though the ceramic insulator did not crack at the portion located on the inner side of the crimp portion. 
     As described above, it was found that, of the samples in which cracking occurs at the rear end portion of the ceramic insulator (namely, the samples in which the length L was set to be not less than 28 mm but not greater than 34 mm), the samples in which the area D was set to 8 mm 2  or greater (samples whose test results are indicated by white marks in  FIGS. 4 and 5 ) have very high strength ratios and are excellent in the effect of restricting cracking of the ceramic insulator. Conceivably, this advantageous effect was attained for the following reason. Since the area D was set to 8 mm 2  or greater, when a load is applied to the head portion of the terminal electrode, the load applied from the head portion of the terminal electrode to the rear end surface of the ceramic insulator was dispersed, and the tensile load acting on the rear end portion of the ceramic insulator decreased. 
     Next, there were manufactured samples of the ignition plug in which the length L was set to 30 mm and the above-mentioned thickness t (mm) and the above-mentioned area D (mm 2 ) were changed within respective ranges, and the above-described cracking load evaluation test was performed on the samples.  FIG. 6  shows the results of the test. 
     In  FIG. 6 , the test results of samples in which the thickness t was set to 0.2 mm are indicated by black circles; the test results of samples in which the thickness t was set to 0.5 mm are indicated by black triangles; and the test results of samples in which the thickness t was set to 0.7 mm are indicated by black squares. Also, the test results of samples in which the thickness t was set to 1.0 mm are indicated by white circles; the test results of samples in which the thickness t was set to 1.2 mm are indicated by white triangles; the test results of samples in which the thickness t was set to 1.5 mm are indicated by white squares; the test results of samples in which the thickness t was set to 1.7 mm are indicated by white rhombuses; and the test results of samples in which the thickness t was set to 2.0 mm are indicated by X. Notably, for reference, the strength ratio of each sample is shown in Table 2. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Area D (mm 2 ) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 5 
                 8 
                 10 
                 13 
                 15 
                 18 
                 20 
               
               
                   
               
               
                 t = 0.2 mm 
                 0.29 
                 0.32 
                 0.27 
                 0.21 
                 0.22 
                 0.23 
                 0.28 
                 0.21 
               
               
                 t = 0.5 mm 
                 0.31 
                 0.29 
                 0.25 
                 0.21 
                 0.23 
                 0.25 
                 0.21 
                 0.22 
               
               
                 t = 0.7 mm 
                 0.28 
                 0.25 
                 0.31 
                 0.29 
                 0.30 
                 0.21 
                 0.22 
                 0.23 
               
               
                 t = 1.0 mm 
                 0.24 
                 0.35 
                 0.79 
                 0.81 
                 0.82 
                 0.88 
                 0.89 
                 0.87 
               
               
                 t = 1.2 mm 
                 0.30 
                 0.31 
                 0.80 
                 0.83 
                 0.90 
                 0.88 
                 0.87 
                 0.89 
               
               
                 t = 1.5 mm 
                 0.29 
                 0.32 
                 0.81 
                 0.92 
                 0.95 
                 0.95 
                 0.93 
                 0.94 
               
               
                 t = 1.7 mm 
                 0.28 
                 0.30 
                 0.79 
                 0.83 
                 0.92 
                 0.95 
                 0.92 
                 0.98 
               
               
                 t = 2.0 mm 
                 0.28 
                 0.30 
                 0.81 
                 0.83 
                 0.85 
                 0.88 
                 0.90 
                 0.92 
               
               
                   
               
            
           
         
       
     
     It was found that, as shown in  FIG. 6  and Table 2, the samples in which the area D is set to 8 mm 2  or greater and the thickness t is set to 1.0 mm or greater (their test results are indicated by white marks or X in  FIG. 6 ) have very high strength ratios and are excellent in durability. Conceivably, this advantageous effect was attained because the rear end portion of the ceramic insulator had a sufficient strength against the force applied from the leg portion. 
     The above-described test results show that the area D is preferably set to 8 mm 2  or greater and the thickness t is preferably set to 1.0 mm or greater in order to more reliably prevent breakage of the ceramic insulator in ignition plugs in which the length L is set to be not less than 28 mm but not greater than 34 mm and the rear end portion of the ceramic insulator is more likely to break. 
     Next, there were manufactured samples of the ignition plug in which the length L was set to 30 mm or 25 mm and the area D (mm 2 ) and the area A (mm 2 ) of the flat portion of the insulator were changed within respective ranges, and the above-described cracking load evaluation test was performed on the samples.  FIG. 7  shows the test results of the samples in which the length L was set to 30 mm.  FIG. 8  shows the test results of the samples in which the length L was set to 25 mm. 
     In  FIGS. 7 and 8 , the test results of samples in which the area A was set to 10 mm 2  are indicated by white circles; the test results of samples in which the area A was set to 15 mm 2  are indicated by white triangles; the test results of samples in which the area A was set to 20 mm 2  are indicated by white squares; and the test results of samples in which the area A was set to 25 mm 2  are indicated by white rhombuses. Also, the test results of samples in which the area A was set to 30 mm 2  are indicated by black circles; and the test results of samples in which the area A was set to 35 mm 2  are indicated by black triangles. In each sample, the thickness t was set to 1.5 mm. 
     It was found that, as shown in  FIGS. 7 and 8 , in the case of the samples in which the area A is set to 25 mm 2  or less (whose test results are indicated by white marks in  FIGS. 7 and 8 ), when the area D is set to be less than 8 mm 2 , the strength ratio becomes low and the rear end portion of the ceramic insulator becomes more likely to occur, but when the area D is set to 8 mm 2  or greater, the strength ratio increases greatly, and excellent durability which is comparable to or more excellent than those of the samples in which the area A is set to be greater than 25 mm 2  can be realized. 
     In particular, it was found that, as shown in  FIG. 7 , in the case of the samples in which the length L is set to 28 mm or greater and the area A is set to 25 mm 2  or less, when the area D is set to be less than 8 mm 2 , the strength ratio becomes very low and the rear end portion of the ceramic insulator becomes highly likely to occur, but when the area D is set to 8 mm 2  or greater, the strength ratio increases dramatically, and excellent durability can be realized. 
     The above-described test results show that the area D is preferably set to 8 mm 2  or greater and the thickness t is preferably set to 1.0 mm or greater in order to more reliably prevent breakage of the ceramic insulator in ignition plugs in which the area A is set to 25 mm 2  or less and the rear end portion of the ceramic insulator is more likely to break. 
     Also, in the case of ceramic insulators in which the length L is set to 28 mm or greater and the area A is set to 25 mm 2  or less, and the rear end portion thereof is highly likely to break, setting the thickness t to 1.0 mm or greater and setting the area D to 8 mm 2  or greater is considerably effective for preventing the breakage of the ceramic insulator. 
     Next, there were manufactured samples of the ignition plug in which the length L was set to 30 mm or 25 mm, a brim portion having an outer diameter equal to or greater than the outer diameter of the forward end surface of the head portion was provided at the forward end of the head portion, and the thickness X (mm) of the brim portion along the axis was changed within a predetermined range. The above-described cracking load evaluation test was performed on the samples.  FIG. 9  shows the test results of the samples in which the length L was set to 30 mm.  FIG. 10  shows the test results of the samples in which the length L was set to 25 mm. Notably, in the test, the strength ratio of each sample was calculated while the cracking load of an ignition plug in which the length L was set to 27 mm, the area D was set to 20 mm 2 , the thickness t was set to 1.5 mm was used as a reference. 
     In  FIGS. 9 and 10 , the test results of samples in which the thickness X was set to 0.5 mm are indicated by black circles; the test results of samples in which the thickness X was set to 1.0 mm are indicated by black triangles; the test results of samples in which the thickness X was set to 1.5 mm are indicated by black squares; the test results of samples in which the thickness X was set to 2.0 mm are indicated by white circles; the test results of samples in which the thickness X was set to 2.5 mm are indicated by white triangles; and the test results of samples in which the thickness X was set to 3.0 mm are indicated by white squares. In each sample, the thickness t was set to 1.5 mm. In addition, the area A was set to 30 mm 2  in the samples in which the length L was set to 30 mm, and the area A was set to 25 mm 2  in the samples in which the length L was set to 25 mm. 
     It was revealed that, as shown in  FIGS. 9 and 10 , in the case of the samples in which the thickness X is set to 2.0 mm or greater (whose test results are indicated by white marks in  FIGS. 9 and 10 ), when the area D is set to be less than 8 mm 2 , the strength ratio becomes very low and the rear end portion of the ceramic insulator becomes more likely to occur, but when the area D is set to 8 mm 2  or greater, the strength ratio increases greatly, and excellent durability can be realized. 
     From the above-described test results, it can be said that setting the thickness t to 1.0 mm or greater and setting the area D to 8 mm 2  or greater are particularly meaningful in ignition plugs in which the distance X is 2.0 mm or less (namely, a part of the head portion located within a region which extends at least 2 mm from the forward end thereof toward the rear end side with respect to the axial direction has an outer diameter greater than the outer diameter of the forward end surface of the head portion), and the rear end portion of the ceramic insulator is more likely break. 
     The present invention is not limited to the above-described embodiment, but may be embodied, for example, as follows. Of course, applications and modifications other than those exemplified below are also possible. 
     (a) In the above-described embodiment, the terminal electrode  6  is configured such that a part of the head portion  6 B located within a region which extends at least 2 mm from the forward end thereof toward the rear end side with respect to the direction of the axial CL 1  has an outer diameter greater than the outer diameter of the forward end surface of the head portion  6 B. However, a terminal electrode  36  as shown in  FIG. 11  may be employed. The terminal electrode  36  is configured such that a brim portion  36 D projecting radially outward is provided at the forward most end of a head portion  36 B, and at least a portion of the part of the head portion  36 B, which part is located within a region which extends at least 2 mm from the forward end thereof toward the rear end side with respect to the direction of the axial CL 1 , has an outer diameter greater than the outer diameter of the forward end surface of the head portion  36 B. In this case, when a load acts on the head portion  36 B, the load applied from the head portion  36 B to the ceramic insulator  2  can be decreased, whereby breakage at the rear end portion of the ceramic insulator  2  can be prevented more reliably. 
     (b) In the above-described embodiment, the grooves  31  are provided on the rear trunk portion  10 ; however, the grooves  31  may be omitted. 
     (c) In the above-described embodiment, the ignition plug  1  is configured to ignite a fuel gas (air-fuel mixture) by generating spark discharges at the spark discharge gap  28 . However, the structure of the ignition plug to which the technical idea of the present invention is applicable is not limited thereto. Accordingly, the technical idea of the present invention may be applied to an ignition plug (plasma jet ignition plug) which has a cavity (space) in a forward end portion of the ceramic insulator and which jets the plasma generated in the cavity so as to ignite the fuel gas. 
     (d) In the above-described embodiment, the tool engagement portion  19  has a hexagonal cross section. However, the shape of the tool engagement portion  19  is not limited thereto. For example, the tool engagement portion may have a Bi-HEX (modified dodecagonal) shape [ISO22977:2005(E)] or the like. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1 : ignition plug 
           2 : ceramic insulator (insulator) 
           2 A: thinnest portion 
           2 F: flat portion (of the ceramic insulator) 
           3 : metallic shell 
           4 : axial bore 
           5 : center electrode 
           6 : terminal electrode 
           6 A: leg portion 
           6 B: head portion 
           6 F: flat portion (of the terminal electrode) 
         CL 1 : axis 
         PA 1 : projected area (of the flat portion of the ceramic insulator) 
         PA 2 : projected area (of the flat portion of the terminal electrode)