Patent Publication Number: US-7906893-B2

Title: Spark plug of internal combustion engine having glaze layers on the spark plug

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a spark plug of an internal combustion engine. Hereinafter, the term “front” refers to a spark gap side with respect to the direction of an axis of the spark plug, and the term “rear” refers to a side opposite the front side. 
     A spark plug is generally provided with a cylindrical metal shell, a center electrode and an insulator and mounted on an internal combustion engine for reliable ignition of air-fuel mixture. The insulator has an axial through hole to hold therein the center electrode and includes a front leg portion, a middle body portion, a rear body portion and a large-diameter portion located between and protruded radially outwardly from the rear and middle body portions. The large-diameter portion of the insulator is often formed at a rear end thereof with a rear shoulder section in a curved surface form etc. 
     The spark plug is exposed to various external stresses such as vibrations due to engine operations and high combustion pressure due to air-fuel mixture combustion. In particular, the rear shoulder section of the insulator is intensively exposed to external stresses. It is thus desired that the insulator has a sufficient level of strength to be protected from cracking under external stresses. 
     On the other hand, there has been a recent demand for size/diameter reduction of the spark plug. In order to satisfy the spark plug miniaturization demand, it is conceivable to decrease the thickness of the insulator. The insulator however decreases in strength with thickness. As a measure for insulator strength improvement, Japanese Laid-Open Patent Publication No. 2003-7424 proposes to form a glaze layer on a surface of the insulator from the rear body portion to the rear shoulder section so as to reinforce the insulator surface under compressive stress and smoothen fine cracks and holes in the insulator surface. If the glaze layer is too small in thickness, the strength of the insulator cannot be improved to a sufficient level so that there arises a possibility of a crack notably in the rear shoulder section of the insulator. If the glaze layer is too large in thickness, it is likely that a crack will occur on the glaze layer and develop in the insulator. It is thus important to control the thickness of the glaze layer appropriately. 
     SUMMARY OF THE INVENTION 
     Even if the rear shoulder section of the insulator attains sufficient strength to withstand relatively high external stress and be protected from cracking under external stress by the formation of the glaze layer with appropriate thickness, the middle body portion of the insulator does not attain sufficient strength to withstand external stress so that the occurrence of cracking in the middle insulator body portion may become pronounced. 
     It is therefore an object of the present invention to provide a spark plug of an internal combustion engine, having an insulator reinforced by glazing to obtain an improvement in overall strength and prevent a failure such as cracking more assuredly. 
     According to an object of the present invention, there is provided a spark plug of an internal combustion engine, comprising: a cylindrical metal shell; an insulator retained in the metal shell and having a through hole in an axial direction of the spark plug, the insulator including a rear body portion, a middle body portion and a large-diameter portion located between and protruded radially outwardly from the rear and middle body portions, the large-diameter portion having a rear shoulder section connected at a rear end thereof to a front end of the rear body portion and a front shoulder section connected at a front end thereof to a rear end of the middle body portion; a center electrode fitted in the through hole of the insulator; and a ground electrode joined to the metal shell and having a given portion facing a front end of the center electrode to define a spark gap between the front end of the center electrode and the given portion of the ground electrode, wherein the insulator has first and second glaze layers formed on an outer surface thereof; the first glaze layer extends over the rear body portion and the rear shoulder section of the large-diameter portion; and the second glaze layer extends over at least part of the middle body portion from some point on the front shoulder section of the large-diameter portion. 
     The other objects and features of the present invention will also become understood from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially cutaway elevation view of a spark plug having a metal shell, a center electrode and an insulator according to one exemplary embodiment of the present invention. 
         FIG. 2  is a partially cutaway elevation view showing a positional relationship between the metal shell and first and second insulator glaze layers of the insulator according to one exemplary embodiment of the present invention. 
         FIG. 3  is an elevation view of positions of the first and second glaze layers on the insulator according to one exemplary embodiment of the present invention. 
         FIG. 4  is a graph of bending test results regarding a relationship of insulator crack position, crack load and first body glaze thickness. 
         FIG. 5  is a graph of bending test results regarding a relationship of insulator crack position, crack load and first corner glaze thickness. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A spark plug  1  for an internal combustion engine according to one exemplary embodiment of the present invention will be described below with reference to the drawings. 
     Referring to  FIGS. 1 and 2 , the spark plug  1  includes a ceramic insulator  2 , a metal shell  3 , a center electrode  5 , a terminal electrode  6 , a ground electrode  27 , a resistive element  7  and conductive glass seal materials  8  and  9 . 
     The ceramic insulator  2  is made of ceramic material such as sintered alumina and formed into a substantially cylindrical shape. More specifically, the ceramic insulator  2  has a through hole  4  in an axial direction C 1  of the spark plug  1  and includes a rear body portion  10 , a middle body portion  12  and a large-diameter portion  11  continuously located between and protruded radially outwardly from the rear and middle body portions  10  and  12 . The large-diameter portion  11  has a rear shoulder section  23  connected at a rear end thereof to a front end of the rear body portion  10  to define a rear corner  41  between the front end of the rear body portion  10  and the rear end of the rear shoulder section  23  and a front shoulder section  24  connected at a front end thereof to a rear end of the middle body portion  12  to define a front corner  42  between the front end of the front shoulder section  24  and the rear end of the middle body portion  12 . The rear corner  41  is herein defined as an area including at least a corner boundary across the front end of the rear body portion  10  and the rear end of the rear shoulder section  23 . Similarly, the front corner  42  is defined as an area including at least a corner boundary across the front end of the front shoulder section  24  and the rear end of the middle body portion  12 . The rear shoulder section  23  can be in a curved surface form or in a continuous or stepwise inclined surface form. The front shoulder section  24  can be in a curved surface form or in a continuous inclined surface form. In the present embodiment, the rear shoulder section  23  and the front shoulder section  24  have a curved surface form and a gradually, continuously inclined surface form, respectively. Further, both of the corners  41  and  42  are L-like shaped in cross section in the present embodiment. The ceramic insulator  2  also includes a front leg portion  13  located on a front side of the middle body portion  12  and having a smaller outer diameter than that of the middle body portion  12  to define a step  14  between the middle body portion  12  and the front leg portion  13 . 
     The metal shell  3  is made of metal material such as iron-based or low-carbon steel material such as S17C or S25C and formed into a substantially cylindrical shape to retain therein the ceramic insulator  2  in such a manner that the front leg portion  13  of the ceramic insulator  2  is partially protruded from a front end of the metal shell  3  and exposed to a combustion chamber of the engine. In the present embodiment, the metal shell  3  includes an external thread portion  15 , a plug seat portion  16  and a tool engagement portion  19 . The thread portion  15  is made on an outer surface of the metal shell  3  and screwed into a cylinder head (plug hole) of the engine so as to mount the spark plug  1  on the engine cylinder head. A ring-shaped gasket  18  is fitted around a rear thread neck end  17  of the thread portion  15 . The plug seat portion  16  is located on a rear side of the thread portion  15  and seated on the engine cylinder head via the gasket  18 . The tool engagement portion  19  is located on a rear side of the plug seat portion  16  so as to engage with a tool such as a wrench for mounting the spark plug  1  on the engine cylinder head. Although the tool engagement portion  19  has a hexagonal cross section in the present embodiment, the cross sectional profile of the tool engagement portion  19  is not limited to the hexagonal shape. The tool engagement portion  19  may alternatively be of any other cross sectional profile such as Bi-HEX (bi-hexagonal) according to ISO 22977: 2005(E). Further, the metal shell  3  has an inner diameter reduced stepwisely to define a step  21  on an inner surface thereof. A rear end  20  of the metal shell  3  is crimped onto the ceramic insulator  2  in such a manner that the inner surface of the crimped rear end  20  conforms to the rear shoulder section  23  of the ceramic insulator  2 , thereby retaining the ceramic insulator  2  in the metal shell  3  with the step  14  of the ceramic insulator  2  engaged on the step  21  of the metal shell  3 . Although the rear end  20  of the metal shell  3  is crimped directly onto the rear shoulder section  23  of the ceramic insulator  2  in the present embodiment, it is alternatively feasible to dispose a pair of ring members filled with talc, or an annular metal plate packing, in a gap between the crimped rear end  20  of the metal shell  3  and the rear shoulder section  23  of the ceramic insulator  2  (i.e. between the outer surface of the ceramic insulator  2  and the inner surface of the metal shell  3 ). An annular plate packing  22  is arranged between the step  14  of the ceramic insulator  2  and the step  21  of the metal shell  3  to provide a seal between the outer surface of the ceramic insulator  2  and the inner surface of the metal shell  3  and prevent leakage of combustion gas from the engine combustion chamber to the outside through between the ceramic insulator  2  and the metal shell  3 . 
     The center electrode  5  is generally formed into a rod (cylindrical column) shape and fitted in a front side of the insulator axial hole  4  in such a manner that the front portion of the center electrode  5  is partially protruded from a front end of the ceramic insulator  2 . In the present embodiment, the front portion of the center electrode  5  tapers down toward the front. Alternatively, the center electrode  5  may be of rod (cylindrical column) shape throughout its length. Further, the center electrode  5  is provided with a body and a tip  31 . The body of the center electrode  5  has a two-layer structure consisting of an inner layer  5 A of copper or copper alloy and an outer layer  5 B of nickel alloy for efficient thermal radiation in the present embodiment. Alternatively, the center electrode body may have a single-layer structure. 
     The electrode tip  31  is made of known precious metal such as Pt—Ir alloy, formed into a cylindrical column shape and joined to a flattened front end face of the center electrode body by welding (e.g. laser welding, electron-beam welding or resistance welding) of mating surface edges of the center electrode body and the electrode tip  31 . 
     The ground electrode  27  is provided with a body and a tip  32 . The body of the ground electrode  27  is made of e.g. nickel alloy (Inconel alloy etc.), joined at a rear end thereof to a front end face  26  of the metal shell  26  and bent into a L-shape to have a given portion e.g. a front end facing the front end of the center electrode  5  (the electrode tip  31 ). Alternatively, the ground electrode body may be formed by cutting the front end of the metal shell  3  (or a part of a metal attachment to the front end of the metal shell  3 ) as disclosed in Japanese Laid-Open Patent Publication No. 2006-236906. 
     The electrode tip  32  is made of known precious metal such as Pt—Ir alloy, formed into a cylindrical column shape and joined to the front end of the ground electrode body by welding (e.g. laser welding, electron-beam welding or resistance welding) of mating surface edges of the ground electrode body and the electrode tip  32 . 
     There is a spark gap  33  defined between the electrode tip  31  and the electrode tip  32  as shown in  FIGS. 1 and 2  in the present embodiment. Alternatively, the spark plug  1  may omit either or both of the electrode tips  31  and  32  so that the spark gap  33  is defined between the electrode tip  31  of the center electrode  5  and the body of the ground electrode  27  or between the body of the center electrode  5  and the electrode tip  32  of the ground electrode. 
     The terminal electrode  6  is fitted in a rear side of the insulator axial hole  4  in such a manner that a rear end of the terminal electrode  6  is protruded from a rear end of the ceramic insulator  2 . 
     The resistive element  7  is disposed between the center electrode  5  and the terminal electrode  6  within the insulator axial hole  4  and electrically connected at front and rear ends thereof to the electrodes  5  and  6  via the glass seal materials  8  and  9 , respectively. 
     Additionally, two glaze layers  28  and  29  are formed on the outer cylindrical surface of the ceramic insulator  2  as indicated by dotted hatching in  FIGS. 2 and 3 . There is no particular restriction on the compositions of the glaze layers  28  and  29 . Each of the glaze layers  28  and  29  can be of any appropriate composition such as those disclosed in Japanese Laid-Open Patent Publication No. 2001-319755. 
     The rear glaze layer  28  (as a first glaze layer) extends over the rear body portion  10  and the rear shoulder section  23 . In general, the rear shoulder section  23  is intensively subjected to external mechanical stress. The rear shoulder section  23  is reinforced by the glaze layer  28  so as to withstand such external stress and be protected from cracking. 
     The front glaze layer  29  (as a second glaze layer) extends over at least part of the middle body portion  12  from some point on the front shoulder section  24 . The middle body portion  12  is reinforced by the glaze layer  29  so as to withstand external stress and be protected from cracking. The reinforcement effect of the glaze layer  29  becomes more pronounced when the insulator  2  has a small thickness of 1.3 to 1.65 mm at the front corner  42 . 
     It is accordingly possible to improve both of the strength of the rear shoulder section  23  and the strength of the middle body portion  12  and, by extension, the overall strength of the ceramic insulator  2  and thereby prevent a cracking failure of the ceramic insulator  2  effectively and assuredly by the formation of the glaze layers  28  and  29 . 
     In the present embodiment, the outer diameter of the front shoulder section  24  and the rear part of the middle body portion  12  of the ceramic insulator  2  is smaller than the inner diameter of the corresponding part of the metal shell  3  so that the outer surface of the front shoulder section  24  and the rear part of the middle body portion  12  is spaced away from the inner surface of the corresponding part of the metal shell  3  to leave a relatively large clearance (of about 0.05 to 0.5 mm) therebetween as shown in  FIGS. 1 and 2 . As shown in  FIGS. 2 and 3 , the glaze layer  29  is formed to cover the front part of the front shoulder section  24  and the rear part of the middle body portion  12  and is kept from contact with the inner surface of the metal shell  3  in the present embodiment. Even if the thickness of the glaze layer  29  varies, the glaze layer  29  does thus not interfere with the metal shell  3  at the time the ceramic insulator  2  and the metal shell  3  are assembled together. This makes it possible to avoid a workability deterioration during assembling of the ceramic insulator  2  and the metal shell  3  and secure a sufficient gas seal between the ceramic insulator  2  and the metal shell  3  in the assembled state of the ceramic insulator  2  and the metal shell  3 . Alternatively, the glaze layer  29  may extend to a front part of the middle body portion  12  to be brought into contact with the metal shell  3 . In this case, it is desirable to adjust the outer diameter of the middle body portion  12  and the thickness of the glaze layer  29  etc. in such a manner that the outer diameter of the middle body portion  12  with the glaze layer  29  becomes equal to the inner diameter of the metal shell  3  to minimize the clearance for sufficient gas seal between the ceramic insulator  2  and the metal shell  3  while avoiding workability deterioration during the assembling of the ceramic insulator  2  and the metal shell  3 . 
     More specifically, the rear glaze layer  28  includes a first corner glaze section  43  to cover the rear corner  41  and a first body glaze section  45  located adjacent to the first corner glaze section  43  to cover the rear body portion  10  (from the rear end to near the front end of the rear body portion  10 ). The front glaze layer  29  also includes a second corner glaze section  44  to cover the front corner  42  and a second body glaze section  46  located adjacent to the second corner glaze section  44  to cover the at least part of the middle body portion  12 . 
     Each of the first and second corner glaze sections  43  and  44  preferably has a thickness of 5 to 150 μm (for example, 10 to 100 μm). In particular, the corners  41  and  42  of the ceramic insulator  2  are intensively subjected to external mechanical stress. When the corner glaze sections  43 ,  44  have a relatively large thickness of 5 to 150 μm, the corners  41  and  42  attain sufficiently improved strength. In addition, the corner glaze section  43 ,  44  provides a smoother bend (e.g. more smoothly curved form) to the corner  41 ,  42  for efficient stress dispersion. The ceramic insulator  2  can thus obtain further improvements in overall strength and be protected from cracking more assuredly. If the thickness of the corner glaze section  43 ,  44  is smaller than 5 μm, the strength of the corner  41 ,  42  may not be increased to a sufficient level. If the thickness of the corner glaze section  43 ,  44  is larger than 150 μm, it is likely that a crack will occur on the corner glaze section  43 ,  44  and develop in the ceramic insulator  2 . For further strength improvements, the thickness of the corner glaze section  43 ,  44  can be controlled to 10 μm or larger. 
     Further, each of the first and second body glaze sections  45  and  46  preferably has a thickness of 5 to 30 μm (for example, 10 to 20 μm). When the body glaze sections  45  and  46  have a thickness of 5 to 30 μm, the rear and middle body portions  10  and  12  attain sufficiently improved strength. The ceramic insulator  2  can thus obtain further improvements in overall strength and be protected from cracking more assuredly. If the thickness of the body glaze section  45 ,  46  is smaller than 5 μm, the strength of the body portion  10 ,  12  may not be increased to a sufficient level. If the thickness of the body glaze section  45 ,  46  is larger than 30 μm, it is likely that a crack will occur on the body glaze section  45 ,  46  and develop in the ceramic insulator  2 . 
     As the rear corner  41  is subjected to external mechanical stress more intensively than the rear body portion  10 , the first corner glaze section  43  is preferably made larger in thickness than the first body glaze section  45  for effective strength improvements. Similarly, the second corner glaze section  44  is preferably made larger in thickness than the second body glaze section  46  for effective strength improvements as the front corner  42  is subjected to external mechanical stress more intensively than the middle body portion  12 . 
     The above spark plug  1  can be manufactured by the following procedure. 
     The metal shell  3  is first produced in a semifinished form by cold forging a cylindrical column metal piece of iron-based or stainless steel material to form an axial through hole in the metal piece, and then, cutting the outside shape of the metal piece. 
     The body of the ground electrode  27  is joined by resistance welding to the front end face  26  of the metal shell  3 . After removing weld shear drops from the joint between the metal shell  3  and the ground electrode  27 , the thread portion  15  is formed at a predetermined position on the metal shell  3  by component rolling. The thus-obtained metal shell subassembly unit is given zinc plating or nickel plating. The metal shell subassembly unit may be further treated by chromating for corrosion resistance improvement. 
     The electrode tip  32  is then joined to the front end of the ground electrode body by resistance welding or laser welding. For reliable welding, it is feasible to remove the plating of the front end surface of the ground electrode body prior to the welding, or to mask the front end surface of the ground electrode body at the welding. Alternatively, the electrode tip  32  may be welded to the front end of the ground electrode body subsequent to the after-mentioned assembling process. 
     On the other hand, the ceramic insulator  2  is produced by preparing a powder mixture of alumina and binder etc. with a granulation material, molding the ceramic power mixture into a cylindrical shape with a rubber press, shaping the ceramic mold by grinding, sintering the ceramic mold in a furnace, and then, finishing the sintered ceramic mold by various grinding operations. 
     The center electrode  5  is also produced by forging the electrode layer  5 B of nickel alloy and forming the electrode layer  5 A of copper or copper alloy in the center of the electrode layer  5 B. 
     The electrode tip  32  is joined to the front end of the center electrode  5  by resistance welding or laser welding. 
     The ceramic insulator  2 , the center electrode  5 , the resistive element  7  and the terminal electrode  6  are assembled and fixed together as follows. The glass seal materials  8  and  9  are generally prepared from borosilicate glass and metal powder. The resistive element  7  is inserted into the axial through hole  4  of the ceramic insulator  2 , followed by filling the glass seal materials  8  and  9  into the insulator through hole  4  to sandwich the resistive element  7  between the glass seal materials  8  and  9 . The center electrode  5  and the terminal electrode  6  are fitted in the front and rear sides of the insulator though hole  4  by baking the glass seal materials  8  and  9  with these electrodes  5  and  6  placed under pressure. At this time, the glaze layers  28  and  29  are formed concurrently. Either or both of the glaze layers  28  and  29  may alternatively be formed in advance. 
     The metal shell and insulator subassembly units are assembled and fixed together by hot crimping. In this hot crimping process, the metal shell  3  is crimped at the rear end  20  onto the ceramic insulator  2  under the condition that a thin portion  25  of the metal shell  3  between the plug seat portion  16  and the tool engagement portion  19  is heated for reduction in deformation resistance. This allows crimping of the metal shell  3  due to not only plastic deformation but also difference in thermal expansion between the metal shell  3  and the ceramic insulator  2 . When the thin portion  25  gets cooled from the thermally expanded state and contracts in the plug axial direction C 1 , the crimped metal shell end  20  forces the rear shoulder section  23  toward the front. With this, the ceramic insulator  2  is fixed securely in the metal shell  3  by engagement between the step  14  of the ceramic insulator  2  and the step  21  of the metal shell  3 . In the assembled state, the ceramic insulator  2  is held under stress in the plug axial direction C. 
     Finally, the ground electrode  27  is bent in such a manner as to define the spark gap  33  between the electrode tips  31  and  32 . 
     The present invention will be described in more detail by reference to the following examples. It should be however noted that the following examples are only illustrative and not intended to limit the invention thereto. 
     Experiment 1 
     Fifty-two test samples of the spark plug  1  were manufactured by the above-mentioned procedure, except that the front glaze layer  29  was not formed (i.e., on the rear glaze layer  28  was formed). The thickness of the first body glaze section  45  of the glaze layer  28  varied from sample to sample. 
     The test samples were subjected to bending strength test. The bending strength test was conducted as follows. The spark plug  1  was mounted on a test stand, with the axis of the spark plug  1  horizontally oriented, by screwing the thread portion  15  into a screw hole of the test stand with a tightening torque of 25 N·m. Using an autograph, a vertical load was applied from above to the terminal electrode  6 . The load was gradually increased. The load under which a crack occurred in the ceramic insulator  2  was measured as a crack load. Further, the position of the crack in the ceramic insulator  2  was identified. The test results are indicated in  FIG. 4 . In  FIG. 4 , the triangular plots indicate the occurrence of cracking in the rear shoulder section  23  of the ceramic insulator  2  and the quadrangular plots indicate the occurrence of cracking in the middle body portion  12  of the ceramic insulator  2 . Further, the first body glaze thickness of 0 mm means the absence of the rear glaze layer  28 . 
     As shown in  FIG. 4 , the rear shoulder section  23  of the ceramic insulator  23  was cracked even under relatively small loads when the thickness of the first body glaze section  45  was less than 5 μm. When the thickness of the first body glaze section  45  exceeded 30 μm, the rear shoulder section  23  of the ceramic insulator  23  was also cracked even under relatively small loads. In addition, the crack load (i.e. the maximum load which the ceramic insulator  2  could withstand without cracking) decreased with increase in the thickness of the first body glaze section  45 . On the other hand, the crack was prevented from occurring in the rear shoulder section  23 , but did occur in the middle body portion  12  of the ceramic insulator  2  under relatively large loads, when the thickness of the first body glaze section  45  was in the range of 5 to 30 μm. 
     Experiment 2 
     Test samples of the spark plug  1  were manufactured by the above-mentioned procedure, except that the front glaze layer  29  was not formed (i.e., on the rear glaze layer  28  was formed). The thickness of the first corner glaze section  43  of the glaze layer  28  varied from sample to sample. 
     The test samples were subjected to bending strength test in the same manner as above. The test results are indicated in  FIG. 5 . In  FIG. 5 , the triangular plots indicate the occurrence of cracking in the rear shoulder section  23  of the ceramic insulator  2  and the quadrangular plots indicate the occurrence of cracking in the middle body portion  12  of the ceramic insulator  2 . Further, the first corner glaze thickness of 0 mm means the absence of the rear glaze layer  28 . 
     As shown in  FIG. 5 , the rear shoulder section  23  of the ceramic insulator  23  was cracked even under relatively small loads when the thickness of the first corner glaze section  43  was less than 5 μm. When the thickness of the first corner glaze section  43  exceeded 150 μm, the rear shoulder section  23  of the ceramic insulator  23  was also cracked even under relatively small loads. On the other hand, the crack was prevented from occurring in the rear shoulder section  23 , but did occur in the middle body portion  12  of the ceramic insulator  2  under relatively large loads, when the thickness of the first corner glaze section  43  was in the range of 5 to 150 μm (preferably 10 μm or larger). 
     As a result of Experiments 1 and 2, it has been concluded that the ceramic insulator  2  can obtain sufficient strength improvement in the rear shoulder section  23  to withstand relatively large loads without cracking by the formation of the glaze layer  28  of appropriate thickness. It has been concluded from further studies that the strength improvement of the rear shoulder section  23  can be achieved effectively and efficiently by making the corner glaze section  43  thicker than the body glaze section  45 . It has further been concluded that, by the formation of the glaze layer  29  in addition to the glaze layer  28 , the ceramic insulator  2  can obtain sufficient strength improvements not only in the rear shoulder section  23  but also in the middle body portion  12  to withstand relatively large loads without cracking; and that the same thickness control of the second corner and body glaze sections  44  and  46  can provide the same strength improvement effects as of the first corner and body glaze section  43  and  45 . 
     As described above, it is possible according to the present invention to impart a sufficient level of strength to the ceramic insulator  2  and prevent a cracking failure of the ceramic insulator  2  effectively and assuredly by forming the first and second glaze layers  28  and  29  on the ceramic insulator  2  and controlling the thickness of these first and second glaze layers  28  and  29  to within the above specific ranges. 
     The entire contents of Japanese Patent Application No. 2007-090183 (filed on Mar. 30, 2007) and No. 2008-006392 (filed on Jan. 16, 2008) are herein incorporated by reference. 
     Although the present invention has been described with reference to the above-specific embodiment of the invention, the invention is not limited to this exemplary embodiment. Various modification and variation of the embodiment described above will occur to those skilled in the art in light of the above teachings. For example, the spark plug  1  is not limited to the above single ground electrode configuration and may alternatively be provided with a multiple ground electrode configuration of e.g. two to four ground electrodes. The scope of the invention is defined with reference to the following claims.