Patent Publication Number: US-8115371-B2

Title: Spark plug

Description:
TECHNICAL FIELD 
     The present invention relates to a spark plug built in an internal combustion engine for igniting a fuel mixture. 
     BACKGROUND ART 
     Conventionally, in an internal combustion engine, spark plugs are used to ignite fuel mixtures. As is shown in  FIG. 21 , a general spark plug includes a center electrode, an insulating member which holds the center electrode in an axial hole, a metal shell which surrounds the periphery of the insulating member to hold the insulating member, and a ground electrode which is joined to the metal shell at a proximal end portion and which forms a spark gap at a distal end portion with the center electrode. The fuel mixture is ignited by a spark discharge occurring in the spark gap. Although the form of the spark plug shown in  FIG. 21  is of a so-called projecting type, in addition to this, there are spark plugs of a slant type and a semi-creeping type (refer to JP-A-6-176849). 
     In recent years, valve diameters of intake valves and exhaust valves are required to be extended to increase the output of an internal combustion engine. A larger water jacket is also required to be equipped on such an internal combustion engine whose output is increased in that way in order to cool the engine with good efficiency. However, with these required countermeasures implemented, since a space where to install spark plugs, which are to be install in the internal combustion engine, becomes small, spark plugs with smaller diameters are now required. 
     However, in the event that the diameter of a spark plug is reduced simply, an insulating distance between an insulating member and a metal shell is narrowed. Because of this, depending upon how carbon deposits are accumulated on the insulating member, a lateral spark in which a spark occurs from the center electrode to the metal shell via an insulator or an inside spark in which a spark occurs from the center electrode to the metal shell through a gap between the insulator and the metal shell is generated (refer to  FIG. 21 ). When lateral sparks and inside sparks occur frequently, the frequency of discharge at the normal spark gap decreases, thus, there is a problem of ignition with fuel mixture. 
     In relation to these problems, for example, JP-A-2006-49207 discloses a spark plug for suppression of the lateral spark in which an outside diameter of a front end of an insulating member is formed so as to be increased gradually from a front end side to a rear end side and a volume from the front end of the insulating member to a position lying 0.1 mm rearwards from the front end is 0.38 mm 3  or lower. JP-A-2000-243535 discloses a spark plug including a center electrode having a high melting point metal tip, wherein a thickness of a portion of an insulating member which is positioned to face a front end face of a metal shell is 1.1 mm or larger and further, an outside diameter of a portion of the center electrode which is positioned to face a front end of the insulating member is 1.4 mm or larger and 2.0 mm or smaller. 
     DISCLOSURE OF THE INVENTION 
     A problem that the invention is to solve in view of the problems described above is how to provide a spark plug which can suppress effectively a lateral spark and an inside spark even with a small diameter configuration from a different viewpoint from that of the related art. 
     In order to solve at least a part of the problems described above, a spark plug of one aspect of the invention is configured as follows. That is, the spark plug comprises: a rod-shaped center electrode; a substantially cylindrical insulating member which has an axial hole extending along a direction of an axis of the center electrode and holds the center electrode within the axial hole while allowing a front end portion of the center electrode to be exposed; a substantially cylindrical metal shell which is provided on an outer circumference of the insulating member; and a ground electrode which is joined to a front end face of the metal shell and which forms a spark gap with a front end portion of the center electrode, wherein a front end portion of the insulating member projects 2 mm or larger from the front end face of the metal shell, and a volume of a portion of the insulating member which lies within a range from a front end to a position lying 1 mm towards a rear end of the insulating member from the front end is 11 mm 3  or smaller, and wherein when assuming in a section of the spark plug which passes through the axis that: a corner portion where a front end face of the insulating member and a side surface of the axial hole intersect is referred to as a position PA, a position on the center electrode where a straight-line distance from the position PA to the center electrode within the axial hole becomes shortest is referred to as a position PB, a position where the insulating element first contacts the metal shell from the front end face of the insulating member along a surface of the insulating member is referred to as a position PC, and a position on the insulating member where when a straight line BC which connects the position PB with the position PC is displaced parallel towards an outside of the axis, the straight line BC contacts the surface of the insulating member is referred to as a position PD, a parallel displacement amount E by which the straight line BC is displaced parallel so as to contact the position PD is 0.75 mm or more. 
     In the spark plug formed as described above, the front end portion of the insulating member projects 2 mm or larger from the front end face of the metal shell, and the volume of the portion of the insulating member which lies within the range from the front end to the position lying 1 mm towards the rear end from the front end of the insulating member is specified as being 11 mm 3  or smaller. According to the spark plug formed in this way, since the temperature of the front end of the insulating member can be increased quickly, carbons, which constitutes a cause for a lateral spark, can burned off quickly. As a result of this, even with a spark plug with a smaller diameter than the standard one, the generation of a lateral spark can be suppressed effectively. Further, in the spark plug formed as described above, by the parallel displacement amount E being referred to as 0.75 mm or larger, an outer circumferential projecting amount of the spark plug can be ensured. As a result of this, the generation of an inside spark from the center electrode to the gap between the insulating member and the metal shell can be suppressed. Although the position PC is the position where the insulating member first contacts the metal shell from the front end face of the insulating member along the surface of the insulating member, the concept of the metal shell is understood to include metallic members such as a packing which communicates electrically with the metal shell. The reference characters PA, PB, PC, PD and the like are only given as a matter of convenience to distinguish the positions to which the reference characters are so given from other positions, and hence, the positions can be expressed in another way. 
     In the spark plug of the above aspect, a small diameter portion where a diameter of the front end portion of the center electrode is reduced may be formed at the front end portion of the center electrode, and the diameter R 1  of the front end portion of the center electrode and the diameter R 2  of the small diameter portion may have the following relationship, 0.75≦R 2 /R 1 ≦0.95. Further, in the spark plug of the above aspect, a depth of a gap defined between the small diameter portion and the insulating member from the front end face of the insulating member may be 0.5 mm or larger and 2.0 mm or smaller. 
     According to the spark plug formed in this way, since the temperature at the front end portion of the insulating member can be increased quickly, the lateral spark can be suppressed effectively. 
     In the spark plug of the above aspect, the front end portion of the insulating member and the front end portion of the metal shell may be disposed to provide a predetermined gap in a position corresponding to the front end face of the metal shell, and a dimension of the gap may be 0.8 to 1.3 times a dimension of a spark gap defined between the ground electrode and the center electrode. 
     According to this form, since the gap between the insulating member and the metal shell and the dimension of the spark gap can be set to an optimal ratio, even with the spark plug with the small diameter, an igniting performance equal to or better than that of a spark plug with the standard diameter can be ensured. Further, according to the ratio, even in the event that the diameter of the spark plug is reduced, the thicknesses of the metal shell and the ground electrode do not have to be thinned more than required. Because of this, even in the event that the diameter of the spark plug is decreased, the strength thereof can be ensured. 
     In the spark plug of the above aspect, a dimension of the spark gap may be 0.6 mm or larger and 1.2 mm or smaller. According to this form, a sufficient gap can be ensured between the front end portion of the insulating member and the front end portion of the metal shell while ensuring the ignition performance. 
     In the spark plug of the above aspect, a thickness of a portion of the insulating member which lies in a position situated 1 mm towards the rear end from the front end of the insulating member may be 0.7 mm or larger. According to this form, an inside spark, which tends to take place with no carbon deposit, can be suppressed effectively. 
     In the spark plug of the above aspect, an outside diameter of the center electrode in a position corresponding to the front end face of the metal shell may be 1.2 mm or larger and 2.1 mm or smaller. According to the center electrode formed in that way, the realization of a spark plug having a smaller diameter than the standard one can be facilitated. 
     In the spark plug of the above aspect, a noble metal tip may be provided on at least either of the front end portion of the center electrode and a distal end portion of the ground electrode. According to the form, the ignition performance of the spark plug can be improved. 
     In the spark plug of the above aspect, the front end portion of the center electrode and the distal end portion of the ground electrode may face each other on the axis of the center electrode. Further, in the spark plug of the above aspect, the front end portion of the center electrode and the distal end portion of the ground electrode may face each other outside the axis of the center electrode. 
     In the spark plug formed as described above, the metal shell may include a mounting portion having threads provided for tightening the spark plug to an internal combustion engine in part of the metal shell, a thread portion of the mounting portion being M 10  or M 12 . According to this form, a spark plug whose diameter is smaller than that of a spark plug with a standard size M 14  can be provided by selecting from existing specified sizes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional view of a spark plug  100 . 
         FIG. 2  is an enlarged view of the vicinity of a front end portion  22  of a center electrode  20 . 
         FIG. 3  is a view showing respective dimensions of portions in the vicinity of the front end portion  22  of the center electrode  20 . 
         FIG. 4  is a view showing respective dimensions of portions in the vicinity of the front end portion  22  of the center electrode  20 . 
         FIG. 5  is a graph showing results of an evaluation test in a first example. 
         FIG. 6  is a table showing dimensions of samples prepared in a second example. 
         FIG. 7  is a graph showing an occurrence rate of lateral spark that occurred when a smoldering fouling test was carried out. 
         FIG. 8  is a graph showing results of an evaluation test in a third example. 
         FIG. 9  is a graph showing results of an evaluation test in a fourth example. 
         FIG. 10  is a graph showing results of an evaluation test in a fifth example. 
         FIG. 11  is a graph showing results of the evaluation test in the fifth example. 
         FIG. 12  is a graph showing results of an evaluation test a sixth example. 
         FIG. 13  is a graph showing results of the evaluation test in the sixth example. 
         FIG. 14  is an explanatory view showing another form of a mounting position of an electrode tip. 
         FIG. 15  is an explanatory view showing a further form of a mounting position of an electrode tip. 
         FIG. 16  is an explanatory view showing another form of a mounting position of an electrode tip. 
         FIG. 17  is an explanatory view showing a cross-sectional shape of a ground electrode  30 . 
         FIG. 18  is an explanatory view showing another cross-sectional shape of a ground electrode  30 . 
         FIG. 19  is an explanatory view showing a further cross-sectional shape of a ground electrode  30 . 
         FIG. 20  is an explanatory view showing a modified example of a positional relationship between a distal end portion of a ground electrode  30  and the front end portion of the center electrode  20 . 
         FIG. 21  is an explanatory view showing concepts of a lateral spark and an inside spark. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment of a spark plug, which is a mode of the invention, will be described by reference to the drawings. The embodiment of the spark plug will be described in the following order. 
     A. Construction of Spark Plug: 
     B. Respective Dimensions of Portions: 
     C. Examples: 
     D. Modified Examples: 
     A. CONSTRUCTION OF SPARK PLUG 
       FIG. 1  is a partial sectional view of a spark plug  100 , and  FIG. 2  is an enlarged view of the vicinity of a front end portion  22  of a center electrode  20  of the spark plug  100 . In  FIG. 1 , the description will be made with a direction of an axis O of the spark plug  100  referred to as a vertical direction, a lower side of the figure referred to as a front end side of the spark plug  100  and an upper side of the figure referred to as a rear end side thereof. 
     As is shown in  FIG. 1 , the spark plug  100  includes an insulator  10  as an insulating member, a metal shell  50  which holds the insulator  10 , a center electrode  20  which is held within the insulator  10  in a direction of an axis O of the spark plug  100 , a ground electrode  30  which is welded to a front end face  57  of the metal shell  50  at a base portion  32  thereof and which faces a front end portion  22  of the center electrode  20  on one side surface of a distal end portion  31  thereof, and a terminal metal fitting  40  which is provided at a rear end portion of the insulator  10 . 
     As is known, the insulator  10  is formed by calcining alumina or the like and has a cylindrical shape in which an axial hole  12  is formed in a center of thereof so as to extend in the direction of the axis O. A flange portion  19  having a largest outside diameter is formed substantially at a center of the insulator  10  in the direction of the axis O, and a rear end side body portion  18  is formed to extend rearwards from the flange portion  19  towards a rear end side (an upper side in  FIG. 1 ). A front end side body portion  17 , whose outside diameter is smaller than that of the rear end side body portion  18 , is formed to extend forwards from the flange portion  19  towards a front end side (a lower side in  FIG. 1 ), and a long leg portion  13 , whose outside diameter is smaller than that of the front end side body portion  17 , is formed to extend forwards from the front end side body portion  17 . The long leg portion  13  is reduced in diameter as it extends towards the front end side so as to be exposed in a combustion chamber when the spark plug  100  is mounted in a cylinder head  200  of an engine. A riser portion  15  is formed between the long leg portion  13  and the front end side body portion  17 . 
     As is shown in  FIG. 2 , the center electrode  20  is a rod-shaped electrode and has a construction in which a core material  25 , which is made of copper or an alloy which contains as a primary ingredient copper having better heat transfer properties than those of an electrode base material  21  which is formed of a nickel such as Inconel (trade name) 600 or 601 or an alloy which contains nickel as a primary ingredient, is embedded in an interior of the electrode base material  21 . Normally, a center electrode  20  is prepared by filling a core material  25  in an interior of an electrode base material  21  which is formed into a bottomed cylindrical shape and stretching the electrode base material  21  by extruding it from a bottom side. Although the core material  25  has a constant outside diameter in a body portion, the core material  25  is formed into a tapered shape on a front end side thereof. 
     The front end portion  22  of the center electrode  20  projects further forwards than a front end portion  11  of the insulator  10  and is formed so as to be reduced in diameter as it extends towards a front end side thereof. With a view to improving the spark wear resistance thereof, an electrode tip  90  made of a noble metal having a high melting point is joined to a front end face of the front end portion  22  of the center electrode  20 . The electrode tip  90  can be formed of, for example, iridium (Ir) or an Ir alloy which contains Ir as a primary ingredient and one or two or more of platinum (Pt), rhodium (Rh), ruthenium (Ru), palladium (Pd) and rhenium (Re). 
     The joining of the center electrode  20  and the electrode tip  90  is implemented by laser welding which goes round a full outer circumference of mating surfaces of the electrode tip  90  and the center electrode  20  while being aimed at the mating surfaces. In the laser welding, since both the materials are melted to be mixed together by being irradiated by laser, the electrode tip  90  and the center electrode  20  are joined together strongly. The center electrode  20  is extended towards the rear end side within the axial hole  12  and is electrically connected to the terminal metal fitting  40  at the rear (upper in  FIG. 1 ) by way of a seal member  4  and a ceramic resistance  3  (refer to  FIG. 1 ). A high-tension cable (not shown) is connected to the terminal metal fitting  40  via a plug cap (not shown) so that a high voltage is applied to the terminal metal fitting  40 . 
     The ground electrode  30  is made of a metal having high corrosion resistance, and as an example, a nickel alloy such as Inconel (trade name) 600 or 601 is used. This ground electrode  30  has a substantially rectangular cross-sectional shape when taken along a plane at right angles to a longitudinal direction thereof, and the base portion  32  is joined to the front end face  57  of the metal shell  50  by welding. The distal end portion  31  of the ground electrode  30  is bent so as to face the front end portion  22  of the center electrode  20  on the one side thereof on the axis O. 
     The metal shell  50  is a cylindrical metallic shell which fixes the spark plug  100  in the cylinder head  200  of the internal combustion engine. The metal shell  50  holds the insulator  10  in an interior thereof so as to surround a portion of the insulator  10  which extends from part of the rear end side body portion  18  to the long leg portion  13 . The metal shell  50  is formed of a low carbon steel material and includes a tool engagement portion  51  with which a spark plug wrench, not shown, is brought into engagement and a mounting screw portion  52  where screw threads are formed which screw into a tapped mounting hole  201  in the cylinder head  200  provided in an upper portion of the internal combustion engine. 
     A flange-like seal portion  54  is formed between the tool engagement portion  51  and the mounting screw portion  52 . A ring-like gasket  5 , which is formed by bending a plate member, is fittingly inserted into a screw neck  59  between the mounting screw portion and the seal portion  54 . The gasket  5  is pressed and collapsed to be deformed between a seating surface of the seal portion  54  and an opening circumferential portion  205  of the tapped mounting hole  201 . A gap between the spark plug  100  and the cylinder head  200  is sealed by the deformation of the gasket  5 , whereby the interruption of gastightness within the engine via the tapped mounting hole  201  is prevented. 
     A thin crimping portion  53  is provided further rearwards towards the rear end than the tool engagement portion  51  of the metal shell  50 . A thin buckling portion  58 , which is as thin as the crimping portion  53 , is provided between the seal portion  54  and the tool engagement portion  51 . Annular ring members  6 ,  7  are interposed between an inner circumferential surface of a portion of the metal shell  50  which extends from the tool engagement portion  51  to the crimping portion  53  and an outer circumferential surface of the rear end side body portion  18  of the insulator  10 , and powder of talc  9  is filled between both the ring members  6 ,  7 . The insulator  10  is pressed towards the front end side within the metal shell  50  via the ring members  6 ,  7  and the talc  9  by crimping the crimping portion  53  while bending it inwards. By this, the riser portion  15  of the insulator  10  is supported on a riser portion  56  formed in a position where the mounting screw portion  52  resides on an inner circumference of the metal shell  50  via a ring-like plate packing  8  made of iron, whereby the metal shell  50  and the insulator  10  are made integral. As this occurs, gastightness between the metal shell  50  and the insulator  10  is held by the plate packing  8 , whereby combustion gases are prevented from flowing out. Since the buckling portion  58  is designed to be deflected and deformed outwards as a compression force is applied thereto when the crimping occurs, a compression stroke of the talc  9  in the direction of the axis O is increased. As a result of this, the gastightness within the metal shell  50  is increased. A clearance C of a predetermined dimension is provided between the metal shell  50  and the insulator  10  in an area situated further forwards towards the front end side than the riser portion  56 . 
     B. RESPECTIVE DIMENSIONS OF PORTIONS 
     Next, referring to  FIGS. 2 to 4 , respective dimensions of portions of the spark plug  100  will be described. As is shown in  FIG. 2 , in the spark plug  100  of this embodiment, an outside diameter M (a nominal diameter) of the mounting screw portion  52  is referred to as M 10  which is smaller than M 14  which is a standard outside diameter. An outside diameter R 1  of a portion of the center electrode  20  which lies in the vicinity of the front end face  57  of the metal shell  50  is referred to as 1.2 mm or larger and 2.1 mm or smaller. In this embodiment, although the outside diameter M of the mounting screw portion  52  is described as being referred to as M 10 , the outside diameter M may be referred to as M 12 . 
     In this embodiment, a projecting amount H (mm) of the insulator  10  by which it projects from the front end face  57  of the metal shell  50  towards the front end side in the direction of the axis O is specified to 2 mm or larger. The reason that the projecting amount is specified to that dimension will be described in a first example, which will be described later. 
     In this embodiment, a volume Vc (mm 3 ) of a hatched portion of the insulator  10  shown in  FIG. 2  is specified to 11 mm 3  or smaller. The volume Vc of the hatched portion in  FIG. 2  represents a volume of a front end side portion of the insulator  10  when the center electrode  20  is cut at a plane P (whose section is indicated by a chain double-dashed line P-P) which passes through a position which is situated 1 mm away towards the rear end side from a front end of the insulator  10  in the direction of the axis O and intersects the axis O at right angles. The reason that the volume Vc is referred to as 11 mm 3  or smaller will be described in a second example, which will be described later. 
     In this embodiment, the clearance C defined between the front end portion of the metal shell  50  and the front end portion of the insulator  10  is specified so as to satisfy the following relation (1) with a spark gap G (mm) in the position corresponding to the front end face  57  of the metal shell  50 . Note that the spark gap G is a distance between the distal end portion  31  of the ground electrode  30  and the electrode tip  90  which is provided at the front end of the center electrode  20 . The reason that the relation (1) is established will be described in the second example, which will be described later.
 
0.8≦( C/G )≦1.3  (1)
 
     In this embodiment, the spark gap G is referred to as 0.6 mm or larger and 1.2 mm or smaller. Because of this, it is inevitable that the clearance C becomes a dimension of 0.48 mm or larger and 1.56 mm or smaller based on the relation (1) above in accordance with the dimension of the spark gap G. 
     In this embodiment, a thickness T of a portion of the insulator  10  which lies in the position corresponding to the front end face  57  of the metal shell  50  is specified to 0.7 mm or larger. The reason that the thickness is specified to that dimension will be described in a third example, which will be described later. 
     In this embodiment, as is shown in  FIG. 3 , the positions PA to PD in the section which passes through the axis O of the spark plug  100  are defined as below, and a projecting amount E which is calculated based on these positions is referred to as 0.75 mm or larger. The reason for this dimension will be described in a fourth example, which will be described later. The projecting amount E is a dimension which represents an extent by which the insulator  10  projects outwards of the axis O. 
     Position PA: A corner portion where a front end face of the insulator intersects a side surface of the axial hole  12 . 
     Position PB: A position on the center electrode  20  where a straight line from the position PA to the center electrode  20  within the axial hole  12  becomes shortest. In other words, the PB is the position of a contact point between the center electrode  20  and an imaginary circle contacts the center electrode  20  when the imaginary circle is drawn from the position PA. 
     Position PC: A position where the insulator  10  first contacts the metal member (the metal shell  50  or the plate packing  8  which electrically communicates with the metal shell  50 ) in an area extending from the front end face of the insulator  10  along a surface of the insulator  10 . 
     Position PD: A position on the insulator  10  where when a straight line BC which connects the position PB with the position PC is displaced parallel towards an outside of the axis O, this straight line BC tangentially contacts the surface of the insulator  10 . In other words, in  FIG. 3 , the position PD is the position of a contact point between a straight line B′C′ which results when the straight line BC is displaced parallel and the surface of the insulator  10 . 
     Projecting Amount E: A parallel displacement amount by which the straight line BC is displaced parallel so as to contacts the position PD. 
     In this embodiment, as is shown in  FIG. 4 , a diameter R 1  of a portion of the front end portion  22  of the center electrode  20  where the axial hole  12  contacts the center electrode  20  and a diameter R 2  of a small diameter portion  23  where the diameter of the front end portion  22  of the center electrode  20  is relatively reduced by one size via a tapered portion  24  are specified so as to satisfy the following relation (2). The reason that this relation (2) is established will be described in a fifth example, which will be described later.
 
0.75≦ R 2/ R 1≦0.95  (1)
 
     In this embodiment, a depth F of a gap (hereinafter, referred to as a “pocket portion  26 ”) defined between the small diameter portion  23  and the axial hole  12  in the insulator  10  which is measured from the front end face of the insulator  10  is referred to as 0.5 mm or larger and 2.0 mm or smaller. The reason for this range will be described in a sixth example, which will be described later. 
     Thus, as has been described heretofore, in the spark plug  100  which has the relatively small diameter as is represented by its outside diameter of M 10 , the occurrence of lateral spark and inside spark can be suppressed effectively by controlling the respective dimensions of the portions of the spark plug  100  of the embodiment. 
     The spark plug  100  can be fabricated by the following fabricating method, for example. Namely, it is a fabricating method comprising the steps of preparing a center electrode  20 , an insulator  10 , a metal shell  50  and a ground electrode  30  which adopt the constructions and dimensions that have been described above, assembling the insulator  10  so as to cover an outer circumference of the center electrode  20  with a front end portion of the center electrode  20  exposed, assembling the metal shell  50  on to an outer circumference of the insulator  10  so that a front end portion of the insulator  10  projects 2 mm or larger from a front end face of the metal shell, and joining a base portion of the ground electrode  30  to the front end face of the metal shell  50  with a distal end portion of the ground electrode  30  caused to face the front end portion of the center electrode  20 . 
     C. EXAMPLES 
     Hereinafter, the reasons that the respective dimensions of the individual portions are specified as described above will described based on various examples. 
     C-1 First Example 
     In a first example, the reason that the projecting amount H is referred to as 2 mm or larger will be described. Firstly, in this first example, a plurality of samples of spark plugs  100  were prepared which had different projecting amounts H by which the front end of the insulator  10  projects and volumes Vc. Specifically, samples were prepared whose volumes were 5, 8, 11, 12 and 13 mm3, and projecting amounts H of their insulators  10  were adjusted from −0.5 mm to 3.0 mm in 0.5 mm increments, whereby a total of 40 different types of samples was prepared. 
     In this example, front ends of the insulators  10  of these samples were heated by a burner, and time was measured which was spent until the temperature of the front ends of the insulator  10  had reached 500° C. since the start of the heating. The temperature of 500° C. is a temperature at which carbon sticking to the vicinity of the front ends of the insulators  10  start to be burned off. 
       FIG. 5  is a graph showing results of an evaluation test. As shown in the figure, according to this example, it could be verified that times spent by the samples whose projecting amounts H were 2 mm or larger in reaching 500° C. were intentionally shorter than those of the other samples. Because of this, the projecting amount H of the spark plug  100  of the embodiment is referred to as 2 mm or larger. With the projecting amount H referred to as that amount, even in the event that carbon sticks to the insulator  10 , the carbon so sticking can be burned off quickly, thereby making it possible to suppress the occurrence of lateral spark, which tends to occur when carbon is deposited. 
     The reason that the position for defining the volume Vc is specified to the position lying 1 mm rearwards from the front end of the insulator  10  is that it could be verified that the temperature of the portion ranging from the front end to the position lying 1 mm rearwards therefrom was extremely higher than that of a portion lying further rearwards towards the rear end side. 
     C-2 Second Example 
     In a second example, the reason that the volume of the front end portion of the insulator  10  is referred to as 11 mm 3  or smaller and the reason that the clearance C and the spark gap G are specified so as to satisfy the relation (1) will be described. In this second example, firstly, samples of spark plugs  100  were prepared in which diameters D 1  (refer to  FIG. 2 ) of holes in front ends of metal shells  50 , outside diameters D 2  (refer to  FIG. 2 ) of front ends of insulators  10 , clearances C (refer to  FIG. 2 ) and spark gaps G (refer to  FIG. 2 ) were varied variously. 
       FIG. 6  is a table showing part of dimensions of the samples prepared in this example. As is shown in the table, in the samples prepared in this example, although the hole diameters D 1  of the metal shells  50  were all 6 mm, the outside diameters D 2  of the insulators  10  were caused to vary from 3.3 mm to 5.2 mm, the clearances C from 0.4 mm to 1.35 mm, and the gaps from 0.6 mm to 1.1 mm. Ratios of clearances C to spark gaps G (hereinafter, referred to as a “clearance ratio”) of the individual samples are shown on a most right-hand column of the table. The clearance C is a value resulting from subtracting the outside diameter D 2  of the insulator  10  from the hole diameter D 1  of the metal shell and dividing the resulting value by 2. As to volume Vc, a plurality of volumes were prepared which ranged from 5 mm 3  to 13 mm 3  by changing the diameters of the center electrodes  20  of the samples. 
       FIG. 7  is a graph showing occurrence rates of lateral spark which occurred when a smoldering fouling test was carried out. The smoldering fouling test is a test specified under “D 1606” of JIS (Japanese Industrial Standards). Specifically, the smoldering fouling test is a test for studying the degree at which spark plugs are fouled through smoldering when a motor vehicle is driven to a predetermined driving pattern which is close to an actual driving condition by placing a motor vehicle on a chassis dynamometer in a low temperature test room and installing spark plugs in an engine of the vehicle. 
     In the graph shown in  FIG. 7 , an X axis indicates clearance ratio (C/G), a Y axis volume Vc (mm 3 ) of the front end portion of the insulator, and a Z axis occurrence rates of lateral spark (%). In this graph, a thick solid line is given to a position where the occurrence rate of lateral spark is 24%. This thick solid line indicates an occurrence rate of lateral spark for a general spark plug of M 14 . Namely, it means that spark plugs having occurrence rates of lateral spark which lie at or below the thick solid line have ignition performances equal to or better than that of the spark plug of M 14 . 
     As is shown in  FIG. 7 , with samples whose clearance ratios were generally 0.8 or larger and volumes Vc were 11 mm 3  or smaller, the occurrence rate of lateral spark became 24% or smaller. It has been able to be verified when reference is made to  FIG. 5  showing the results of the evaluation test in the first example that when the volume Vc exceeded 11 mm 3 , it became difficult to burn off carbon quickly. Further, when the Vc exceeds 11 mm 3  or the clearance ratio exceeds 1.3, the clearance C (refer to  FIG. 2 ) has to be secured largely. Then, the thicknesses of the metal shell  50  and the ground electrode  30  need to be reduced, which causes the occurrence of bending or melting of the ground electrode  30 . From the observation of these facts, in the embodiment, the clearance ratio is specified to 0.8 or larger, and the volume Vc to 11 mm 3  or smaller. With the spark plug  100  configured in this way, the spark plug with a smaller diameter can be provided which has the ignition performance and strength which are equal to or better than those of the spark plug of M 14 . 
     C-3 Third Example 
     In a third example, the reason that the thickness T of the insulator  10  is specified to 0.7 mm or larger will be described. According to various experiments carried out by the applicant, it has been able to be verified that when the insulator was fouled with carbon, many lateral sparks occurred, whereas when the insulator was not so fouled, many inside sparks occurred. Then, in this third example, the following experiment was carried out to mainly suppress the occurrence of inside spark. 
     Namely, an experiment was carried out to study about a spark gap which triggers a inside spark by preparing samples in which thicknesses T of front end portions of insulators  10  were caused to vary in many ways, and adjusting dimensions of spark gaps of the samples so prepared. In this example, spark discharge was made 100 times for each spark gap, and when an inside spark occurred even once, it was judged that an inside spark was triggered with the spark gap. Namely, it means that with spark gaps larger than the spark gap, more inside sparks would occur. 
       FIG. 8  is a graph showing results of the evaluation test in this example. An axis of abscissa indicates thicknesses T of insulators  10 , and an axis of ordinate indicates dimensions of spark gaps G where inside spark is triggered. In this graph, inside spark triggered gaps are plotted in association with thicknesses. A horizontal thick line shown in the graph indicates a dimension of a spark gap G of a general spark plug  100  of M 14 . In general, the ignitability is increased by such an extent that the spark gap is increased. Because of this, when a spark plug has an inside spark triggered gap which takes a value equal to or larger than the value expressed by the thick line in the figure, it means that the spark plug has an ignition performance equal to or better than that of the spark plug of M 14 . 
     Then, an approximate line was drawn based on individual evaluation values in the graph and a point was obtained where the approximate line intersected the thick line. As a result, the thickness T of the point of intersection was generally 0.7 mm. Namely, with the insulator  10  whose thickness T is referred to as 0.7 mm or larger, the spark plug can be provided which has the ignition performance equal to or better than that of the spark plug of M 14  while suppressing inside spark. 
     C-4 Fourth Example 
     In a fourth example, the reason that the projecting amount E is specified to 0.75 mm or larger will be described. In the fourth example, samples were prepared in which projecting amounts were caused to vary in many ways, and a similar experiment to that in the third example was carried out. 
       FIG. 9  is a graph showing results of an evaluation test carried out in this example. An axis of abscissa denotes projecting mounts E of insulators  10 , and an axis of ordinate denotes dimensions of spark gaps G which trigger inside spark. A horizontal thick line shown in the graph indicates a dimension of a spark gap G of a general spark plug of M 14 . As has been described above, the ignition performance is increased by such an extent that the spark gap G is increased. Because of this, when a spark plug has an inside spark triggered gap which takes a value equal to or larger than the value expressed by the thick line in the figure, it means that the spark plug has an ignition performance equal to or better than that of the spark plug of M 14 . 
     Then, an approximate line was drawn based on individual evaluation values in the graph and a point was obtained where the approximate line intersected the thick line. As a result, the projecting amount E of the point of intersection was generally 0.75 mm. Namely, with the insulator  10  whose projecting amount E is referred to as 0.75 mm or larger, since the distance of a path along which an inside spark is likely to occur (a path from the position PB to the position PC in  FIG. 3 ) can be lengthened, the spark plug can be provided which has the ignition performance equal to or better than that of the spark plug of M 14  while suppressing inside spark. 
     C-5 Fifth Example 
     In a fifth example, the reason that the diameter R 1  of the center electrode  20  (hereinafter, referred to as a “center shaft diameter R 1 ”) and the diameter R 2  of the small diameter portion  23  (hereinafter, referred to as a “pocket diameter R 2 ”) are specified so as to satisfy the relation (2) will be described. In this fifth example, spark plugs  100  whose center shaft diameter R 1  was 1.9 mm and spark plugs  100  whose center shaft diameter R 1  was 2.1 mm were prepared, and their pocket diameters R 2  were varied to be 0.55 time, 0.65 time, 0.75 time, 0.85 time, and 1.0 time the center shaft diameters R 1  thereof. Times spent in reaching a temperature of 500° C. were measured on the samples so prepared. 
       FIG. 10  is a graph showing results of an evaluation test carried out in this example. An axis of abscissa represents rations R 2 /R 1  of center shaft diameter R 1  to pocket diameter R 2 . In this example, times spent reaching the temperature of 500° C. were measured on the spark plugs  100  whose center shaft diameter R 1  was 1.9 mm and the spark plugs  100  whose center shaft diameter R 1  was 2.1 mm. The times spent reaching the temperature of 500° C. showed almost similar values in the individual pocket diameters R 2 . Because of this, only one 500° C. reaching time is plotted for the individual rations R 2 /R 1  in  FIG. 10 . 
     According to the experiment whose results are shown in  FIG. 10 , it was found that as the value of the ratio R 2 /R 1  of center shaft diameter R 1  to the pocket diameter R 2  was reduced, that is, the gap of the pocket portion  26  was increased, the 500° C. reaching time was shortened. Namely, as the front end portion of the insulator  10  moves farther away from the center electrode  20 , the temperature tends to be increased easily. Consequently, as the value of the ratio R 2 /R 1  becomes lower, carbon can be burned off more quickly, whereby the occurrence of lateral spark can be suppressed effectively. Although a result of the evaluation test is shown in  FIG. 10  in which the ratio R 2 /R 1  was “1,” that is, there existed no gap between the center electrode  20  and the insulator  10 , this case resulted in the 500° C. reaching time becoming extremely longer than the sample in which even a slight gap exited between the center electrode  20  and the insulator  10 . Namely, it means that providing a gap between the center electrode  20  and the insulator  10  is better than providing no gap therebetween. Then, in the embodiment, an upper limit value of the ratio R 2 /R 1  of center shaft diameter R 1  to pocket diameter R 2  is specified to “0.95.” 
     Incidentally, although as the temperature of the front end portion of the insulator  10  becomes higher, carbon can be burned off more quickly, preignition tends to occur easily. Then, to determine a lower limit value for the ratio R 2 /R 1 , in this example, an advance angle which triggers a preignition was studied by use of a known spark advance method. The spark advance method is a method for studying an angle which triggers a preignition by following steps (a) to (c). 
     (a) A certain spark advance angle is set, and a full load driving is started under a predetermined engine speed. Whether or not a preignition occurs is observed by an ion current detecting method during a continuous driving of two minutes. 
     (b) In case there is observed no preignition during the continuous driving of two minutes, the ignition timing is advanced repeatedly step by step in increments of an appropriate amount until a preignition is observed. 
     (c) In case a preignition occurs during a driving with a certain advance angle, the advance angle is recorded. 
       FIG. 11  shows results of a measurement carried out by use of the spark advance method. In the figure, an axis of abscissa represents ratio R 2 /R 1  of center shaft diameter R 1  to pocket diameter R 2 , and an axis of ordinate represents advance angle which triggers a preignition. According to the results of the measurement shown in  FIG. 11 , it was found that a preignition occurred somewhere the ratio R 2 /R 1  was 0.75. The delay in advance angle which triggers a preignition represents that the heat resistance of the spark plug  100  is low by such an extent, resulting in a lateral spark becoming easy to occur. Consequently, in the embodiment, the lower limit value for the ratio R 2 /R 1  of center shaft diameter D 1  to pocket diameter D 2  is specified to “0.75” from the results of the measurement carried out. 
     C-6 Sixth Example 
     In a sixth example, the reason that the depth F of the pocket portion  26  is specified to 0.5 mm or larger and 2.0 mm or smaller. In this example, the depth F of the pocket portions  26  of spark plugs  100  whose ratio R 2 /R 1  of center shaft diameter R 1  to pocket diameter R 2  was “0.75” was caused to vary in many ways, and an experiment was carried out to study about 500° C. reaching time and preignition triggered advance angle. 
       FIG. 12  is a graph showing 500° C. reaching times of the individual samples in which the depth F of the pocket portions  26  was changed from 0.25 mm to 2.0 mm. According to results of the experiment shown in this figure, it was found that with the depths of the pocket portions  26  being 0.5 mm or larger, the 500° C. reaching times were reduced intentionally to be shorter than those of the samples in which the depths of the pocket portions  26  were less than 0.5 mm. Because of this, in the embodiment, the lower value for the depth F of the pocket portion  26  is specified to 0.5 mm. 
       FIG. 13  is a graph showing preignition triggered advance angles of the individual samples in which the depths F of the pocket portions  26  were changed from 0.25 mm to 2.0 mm. According to results of the experiment shown in this figure, it was found that with the depths F of the pocket portions  26  being not more than 2.0 mm, the advance angle at which the preignition was triggered was not delayed that much. Because of this, in the embodiment, the upper limit value for the depth F of the pocket portion  26  is specified to 2.0 mm. 
     D. MODIFIED EXAMPLES 
     While the embodiment and various examples of the invention have been described heretofore, the invention is not limited to the embodiment and examples that have been described above, and hence, needless to say, the invention can take various configurations without departing from the spirit and scope thereof. For example, the following modifications can be made. 
     D-1 Modified Example 1 
     In the embodiment, as is shown in  FIG. 2 , the electrode tip  90  is described as being provided at the front end of the center electrode  20 . However, the mounting position of the electrode tip  90  is not limited thereto, and hence, the electrode tip  90  can be mounted in various positions. 
       FIGS. 14 to 16  are explanatory views showing other forms of electrode tip mounting positions.  FIG. 14  shows an example in which an electrode tip  91  is provided at a distal end portion of a ground electrode  30 . In this case, a spark gap G becomes a distance between the electrode tip  91  provided at a front end of the ground electrode  30  and a front end portion of a center electrode  20 .  FIG. 15  shows an example in which electrode tips  90 ,  91  are provided both on a front end portion of a center electrode  20  and a distal end portion of a ground electrode  30 , respectively. In this case, a spark gap G becomes a distance between the electrode tip  90  and the electrode tip  91 . As is shown in these examples, by the electrode tip/s being mounted on the front end portion of the center electrode  20  or/and the distal end portion of the ground electrode  30 , the ignitability of the spark plug  100  can be improved. Of course, as is shown in  FIG. 16 , a form can also be acceptable in which an electrode tip is provided on neither a center electrode  20  nor a ground electrode  30 . In this case, a spark gap G becomes a distance between a front end portion of the center electrode  20  and a distal end portion of the ground electrode  30 . Namely, whether or not the electrode tip is provided, the spark gap G denotes a dimension of a portion where a spark discharge is normally generated. 
     D-2 Modified Example 2 
     In the embodiment, as is shown in  FIG. 17 , the ground electrode  30  is described as having the substantially rectangular shape as the cross section (a section taken along the line a-a in the figure) which is taken along the plane at right angles to the longitudinal direction thereof. Dimensions of the cross section can be 1.1 mm wide and 2.2 mm long. However, the shape of the cross section of the ground electrode  30  is not limited thereto, and hence, the ground electrode  30  can have various cross-sectional shapes. 
       FIGS. 18 and 19  are explanatory views showing other forms of cross-sectional shapes of ground electrodes  30 .  FIG. 18  shows an example in which a cross section of a ground electrode  30  is a substantially circle.  FIG. 19  shows an example in which a cross section of a ground electrode  30  is a substantially semi-circle whose flat plane portion is oriented towards a center electrode  20 . In these forms, the sectional areas of the ground electrodes  30  can be referred to as a cross-sectional area (=1.1 mm×2.2 mm) which is approximately the same as that of the rectangle shown in  FIG. 17 , for example. In addition, the sectional shape of the ground electrode  30  is not limited to the examples shown in  FIGS. 18 and 19 , and hence, in addition to those described heretofore, for example, an oval shape, a trapezoidal shape and other polygonal shapes can be adopted as the cross-sectional shape of the ground electrode  30 . 
     D-3 Modified Example 3 
     In the embodiment, as is shown in  FIG. 2 , the distal end portion of the ground electrode  30  is described as facing the front end portion of the center electrode  20  on the axis O. However, the positional relationship between the distal end portion of the ground electrode  30  and the front end portion of the center electrode  20  is not limited thereto. 
       FIG. 20  is an explanatory view showing another positional relationship between a distal end portion of a ground electrode  30  and a front end portion of a center electrode  20 . As is shown in the figure, in this modified example, the distal end portion of the ground electrode  30  is made to face the front end portion of the center electrode  20  at the front end portion of the center electrode  20  on an axis Q which intersects an axis O. In this mode, a spark discharge is generated not on the axis O but on the axis Q. In addition to this positional relationship, a distal end portion of a ground electrode  30  may be made to face a front end portion of a center electrode  20  at a predetermined angle formed relative to an axis O. In either of the cases, a front end portion of an insulator  10  is made not to exist on the axis on which the front end portion of the center electrode  20  and the distal end portion of the ground electrode  30  face each other. The positional relationship between the distal end portion of the ground electrode  30  and the front end portion of the center electrode  20  can be set as required in accordance with applications of the spark plug or performances required.