Patent Publication Number: US-6703770-B2

Title: Spark plug attachment structure and spark plug therefor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a spark plug for an internal combustion engine and particularly a spark plug adapted to be fixed in a plug hole of a cylinder head without the necessity of being screwed into the plug hole. The present invention further relates to a structure for attaching such a spark plug to a cylinder head of an internal combustion engine. Hereinafter, the terms “front” and “rear” are used to indicate opposite sides of the spark plug that are nearer to and further from a spark gap thereof, respectively. 
     A spark plug is used in an internal combustion engine for a vehicle so as to ignite an air-fuel mixture in a combustion chamber of the engine. 
     An example of the spark plug is shown in FIG. 8. A spark plug  300  comprises a central electrode  303  extending axially of the spark plug  300 , an insulator  302  surrounding the central electrode  303  and a tubular metallic shell  301  accommodating therein the insulator  302 . The metallic shell  301  includes a front end portion  312  provided with a male thread  312   a  on an outer surface thereof and a protrusion  312   b  on an inner surface thereof, and a hexagon head portion  313 . The insulator  302  has a step  302   a  engaged with the protrusion  312   b  via a packing  351 . The metallic shell  301  thus retains therein the insulator  302  so as to prevent the insulator  302  from slipping out of the metallic shell  301  from the front side, and at the same time, to prevent the passage of air. The spark plug  300  further comprises a ground electrode  304  having one end joined to a front end of the front end portion  312  and the other end opposed to the central electrode  303  with a spark gap g therebetween. Herein, the section at which the ground electrode  304  is joined to the metallic shell  1  is referred to as a joint section  317 . 
     The spark plug  300  is attached to a cylinder head of the engine. More specifically, the spark plug  300  is screwed into a plug hole of the cylinder head by a predetermined torque applied to the hexagon head portion  313  with the use of a wrench (not shown). Then, the spark plug  300  is fixed in the plug hole through engagement of the male thread  312   a  and a female thread of the plug hole. 
     In the above arrangement, when heat is applied to the central electrode  303  and the insulator  302 , the heat is transmitted from the step  302   a  to the protrusion  312   b  via the packing  351 , and then, transmitted from the metallic shell  301  to the cylinder head through the male thread  312   a  and the female thread. 
     However, if the spark plug  300  is screwed into the plug hole in such a way that the spark plug  300  forms an angle with respect to an axis of the plug hole, the male thread  312   a  and the female thread do not engage appropriately and causes so-called “galling”. As a result, the male thread  312   a  and the female thread are damaged. In the worst case, a passage leaking combustion gas from the combustion chamber is formed, and the front end portion  312  of the metallic shell  301  and the central electrode  303  are heated excessively to high temperature. Various problems, such as preignition and electrode fusing, are thus caused. 
     In addition, if the ground electrode  304  is positioned so as to prevent swirl from flowing into the spark gap g in the combustion chamber at compression stroke, the spark plug  300  only ignites a relatively rich mixture. For this reason, the spark plug  300  is generally attached to the cylinder head in consideration of a positional relationship between the joint section  317  and the flow direction of swirl so that the ground electrode  304  is placed in a proper position and orientation. However, with the spark plug  300  screwed into the plug hole, it is difficult to determine the position of the joint section  317  (i.e., to determine the position and orientation of the ground electrode  304 ) in the combustion chamber. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is proposed to fix the spark plug in the plug hole by means of a plug fixing member without providing the spark plug and the plug hole with threads. This makes it possible to attach the spark plug to the cylinder head without any problems resulting from the engagement of the threads. In this case, some clearance needs to be created between the spark plug and the plug hole so that the spark plug is smoothly inserted into and detached from the plug hole. However, there arises a problem that heat cannot be easily transmitted from the spark plug to the cylinder head. More specifically, air remains in the clearance between the spark plug and the plug hole and functions as a thermal barrier. When the spark plug (especially, its central electrode and insulator) is subjected to high temperature owing to, e.g., combustion gas, the air in the clearance prevents the heat from being transmitted to the cylinder head. The spark plug is therefore excessively heated, and faces the problems of preignition, electrode fusing and the like. 
     It is therefore an object of the present invention to provide a spark plug adapted to be easily fixed in a plug hole of a cylinder head without the necessity of being screwed into the plug hole, and at the same time, having a good heat resistance so as to transmit heat rapidly to the cylinder head. 
     It is also an object of the present invention to provide a structure for attaching a spark plug to a cylinder head. 
     According to one aspect of the present invention, there is provided a spark plug attachment structure, comprising: a spark plug comprising a central electrode extending axially of the spark plug, an insulator surrounding the central electrode, and a metallic shell retaining therein the insulator and provided with a plug bearing surface and a front end portion on a front side of the plug bearing surface, the front end portion having a smooth cylindrical outer surface; a cylinder head defining therein a plug hole provided with a shoulder and a front end section having a smooth cylindrical inner surface, the spark plug having a clearance fit with the plug hole, with the plug bearing surface supported on the shoulder and the front end portion accommodated in the front end section, so as to provide a clearance between the cylindrical outer surface of the front end portion of the metallic shell and the cylindrical inner surface of the front end section of the plug hole; and a sealant provided in the clearance. 
     According to another aspect of the present invention, there is provided a spark plug for attachment to a cylinder head, the cylinder head defining therein a plug hole provided with a shoulder and a front end section having a smooth cylindrical inner surface, the spark plug comprising: a central electrode extending axially of the spark plug; an insulator surrounding the central electrode; and a metallic shell retaining therein the insulator and provided with a plug bearing surface and a front end portion on a front side of the plug bearing surface, the front end portion having a smooth cylindrical outer surface, the spark plug being adapted to have a clearance fit with the plug hole, with the plug bearing surface supported on the shoulder and the front end portion accommodated in the front end section, so as to provide a clearance between the cylindrical outer surface of the front end portion of the metallic shell and the cylindrical inner surface of the front end section of the plug hole to be provided with a sealant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a half sectional view of a spark plug according to a first embodiment of the invention. 
     FIG. 2 is a sectional view of the spark plug according to the first embodiment of the invention. 
     FIG. 3A is a partial plan view of a cylinder head used in to the first embodiment of the invention. 
     FIG. 3B is a partial sectional view of the cylinder head used in the first embodiment of the invention. 
     FIG. 4 shows a structure for fixing the spark plug in a plug hole of the cylinder head according to the first embodiment of the invention. 
     FIG. 5 shows a spark plug fitted in a plug hole of a cylinder head according to a second embodiment of the invention. 
     FIG. 6 shows a test system for preignition. 
     FIG. 7 is a graph showing an improvement in heat resistance of the spark plug with respect to the clearance between the spark plug and the plug hole. 
     FIG. 8 is a partial sectional view of a conventional spark plug. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be more clearly understood from the following description. 
     Firstly, a spark plug  100 , a cylinder head SH 1  and an attachment structure thereof according to a first embodiment of the invention will be described with reference to FIGS. 1 to  4 . 
     As shown in FIGS. 1 and 2, the spark plug  100  comprises a metallic shell  1 , an insulator  2  retained in the metallic shell  1 , a central electrode  3  and a terminal electrode  5  both extending in the insulator  2  axially of the spark plug  100 , and a single ground electrode  4 . 
     The metallic shell  1  is made of, e.g., carbon steel, and includes a plug bearing portion  11  of a maximum outer diameter, a front end portion  12  on the front side of the plug bearing portion  11 , and a tool engaging portion  15  on the rear side of the plug bearing portion  11 . The plug bearing portion  11  has a plug bearing surface  13  at a front end face thereof, which is generally made flat. A ring-shaped gasket G is mounted on the plug bearing surface  13 . The front end portion  12  is generally formed into a cylindrical shape so as to have a smooth cylindrical outer surface extending from the plug bearing surface  13  to a front end of the metallic shell  1  without any thread thereon. Further, a protrusion  18  is formed on an inner surface of the front end portion  12  of the metallic shell  1  so as to protrude inwardly, and has a retaining surface  18   a . A pair of ring-shaped packings  22  is provided, with talc  23  being disposed between the metallic shell  1  and the insulator  2 . A rear edge of the metallic shell  1  is caulked to the insulator  2  via the packing  22  to form a caulked portion  16 , while the metallic shell  1  is caulked between the plug bearing portion  11  and the tool engaging portion  15  to the insulator  2  to form a caulked portion  14 . 
     The insulator  2  includes a protruding portion  2   e  formed in the middle of the insulator  2  so as to protrude outwardly toward the metallic shell  1 , a shell portion  2   b  on the rear side of the protruding portion  2   e  to be smaller in diameter than the protruding portion  2   e  and formed with corrugation  2   c , and on the front side of the protruding portion  2   e  a first stem portion  2   g  smaller in diameter than the protruding portion  2   e  and a second stem portion  2   i  smaller in diameter than the first stem portion  2   g . The shell portion  2   b  is covered with a glazing layer  2   d . The second stem portion  2   i  is generally conical in shape and has an outer diameter gradually decreasing toward the front end. A step  2   h  is formed between the first stem portion  2   g  and the second stem portion  2   i , and engaged with the retaining surface  18   a  of the protrusion  18  via a plate-shaped packing  51  so as to prevent the insulator  2  from slipping out of the metallic shell  1  from the front side. Further, the insulator  2  is formed with a through hole  6  throughout its length. The through hole  6  includes a first hole section  6   a  on the front side and a second hole section  6   b  having a diameter larger than that of the first hole section  6   a  on the rear side. An abutment surface  6   c  is formed between the first hole section  6   a  and the second hole section  6   b . The abutment surface  6   c  may be tapered or curved. 
     The central electrode  3  has a projecting portion  3   a  at a rear end thereof, and is fitted in the first hole section  6   a  with the projecting portion  3   a  held by the abutment surface  6   c  and a front end thereof protruding from the insulator  2 . The terminal electrode  5  is fitted in the second hole section  6   b  and electrically connected to the central electrode  3  via a ceramic resistor  7  and conductive glass seal layers  8  and  9  within the second hole section  6   b . Alternately, the central electrode  3  and the terminal electrode  5  may be connected to each other via a single conductive glass seal layer without the ceramic resistor  7 . 
     One end of the ground electrode  4  is joined to the front end of the metallic shell  1  at a joint section  17 , and the other end of the ground electrode  4  faces the front end of the central electrode  3  so as to form a spark gap g therebetween. 
     As shown in FIGS. 3A,  3 B and  4 , the cylinder head SH 1  has a head body  31  provided with a plug hole P 1  extending from a top surface  32  to a chamber-side surface  33  thereof. Threaded holes  32   a  to  32   d  are formed in the top surface  32  around the plug hole P 1 , and used for securing a plug fixing member  41  (described later) by bolts (only two bolts  49   a  and  49   c  are shown in FIG.  4 ). The plug hole P 1  has such a depth that is large enough to accommodate therein the whole of the spark plug  100 , and includes three sections: an insert section  34  near to the top surface  32 , a receiving section  35  smaller in diameter than the insert section  34 , and a front end section  36  smaller in diameter than the receiving section  35  and near to the chamber-side surface  33 . The insert section  34  is generally circular and extends from the top surface  32  to a point near to the chamber-side surface  33 . The diameter of the insertion section  34  is made larger than the outer diameter of the plug bearing portion  11  of the metallic shell  1 . The receiving section  35  and the front end section  36  are shaped corresponding to the plug bearing portion  11  and the front end portion  12  of the metallic shell  1 , respectively. The front end section  36  has a smooth cylindrical inner surface without any thread formed thereon. A shoulder  37  is formed between the receiving section  35  and the front end section  36  so as to receive the plug bearing surface  13  via the gasket G for sealing the spark plug  100  in the plug hole P 1 . Further, a cooling channel  38  is formed in the cylinder head SH 1  for circulation of coolant therethrough. 
     In the present invention, the spark plug  100  has a clearance fit with the plug hole P 1  so as to provide a clearance between the cylindrical outer surface of the front end portion  12  and the cylindrical inner surface of the front end section  36 . A sealant  25  is provided in the clearance around the tip end portion  12  and so as to transmit the heat from the spark plug  100  to the cylinder head SH 1 . To be provided with the sealant  25 , the clearance is preferably given by the following expression: 
     
       
         0.005 ≦D−d   1 ≦0.50 
       
     
     wherein D (mm) is an inner diameter of the front end section  36  and d 1  (mm) is an outer diameter of the front end portion  12 . 
     By such a clearance provided with the sealant  25 , the spark plug  100  can transmit heat efficiently to the cylinder head SH 1 , and can have improved heat resistance and is therefore useful in various internal combustion engines including a high-powered engine. More preferably, for attaining further improved heat resistance, the clearance is given by the following expression: 
     
       
         0.075 ≦D−d   1 ≦0.40, 
       
     
     and most preferably 
     
       
         0.075 ≦D−d   1 ≦0.15. 
       
     
     More specifically, the heat is diffused from the central electrode  3  and the insulator  2  to the metallic shell  1  via the protrusion  18  and then transmitted to the cylinder head SH 1  via the sealant  25  and the contact between the plug bearing surface  13  of the metallic shell  1  and the shoulder  37  of the plug hole P 1 . In order to transmit the heat more efficiently, it is preferable that the plug bearing surface  13  and the protrusion  18  have a positional relationship given by the following expression: 
     
       
           L≧ 0, 
       
     
     and more perferably 
     
       
         0 ≦L≦ 6, 
       
     
     wherein L is an axial distance (mm) from the plug bearing surface  13  to a rear edge of the protrusion  18 , and at the same time, the sealant  25  is placed on at least a part  12   a  of the outer surface of the front end portion  12  that axially corresponds in position to the protrusion  18 . 
     The sealant  25  is required to have thermal properties of withstanding the service conditions (especially, high temperature conditions) of internal combustion engine. Preferably, the sealant  25  has heat resistance of 400° C. or higher. Also, the sealant  25  preferably contains at least one of metal powder, alloy powder and ceramic powder each having a thermal conductivity of 20 W/m·K or higher in order to rapidly transmit heat. The metal, alloy and/or ceramic powders may be mixed with viscous and/or elastic material. Specific examples of the metal powder include copper powder, aluminum powder, molybdenum powder or the like. Specific examples of the alloy powder include molybdenum disulfide and the like. Specific examples of the ceramic powder include aluminum nitride, boron nitride and the like. The metal, alloy and ceramic powders are not limited to the above, and may be any suitable powder having a thermal conductivity of 20 W/m·K or higher. Specific examples of the viscous material include lubricant such as grease, silicone oil, machine oil, mineral oil, dibasic acid ester and the like. Specific examples of the elastic material include heat-resistance gum material and the like. The total amount of the metal, alloy and ceramic powders contained in the sealant  25  is preferably within a range from 20 to 80% by weight based on the weight of the sealant  25 . When the total amount of the metal, alloy and ceramic powders is less than 20% by weight, the sealant  25  may not exhibit sufficient ability to transmit heat from the metallic shell  1  to the cylinder head SH 1 . On the other hand, when the total amount of the metal, alloy and ceramic powders exceeds 80% by weight, the sealant  25  can transfer the heat rapidly to the cylinder head SH 1 , but not seal the clearance between the spark plug  100  and the plug hole P 1  owing to deteriorated viscosity or elasticity thereof for lack of the viscous or elastic materials. Furthermore, each of the metal, alloy and ceramic powders preferably has an average particle size smaller than or equal to half of the clearance so that the sealant  25  maintains good viscosity or elasticity. For example, the sealant  25  has heat resistant of about 400° C., and comprises copper powder contained in mineral oil in an amount of 40% by weight based on the total weight of the sealant  25 . 
     In addition, the metallic shell  1  is adapted to be engaged in the plug hole P 1  at one or more places for determining a rotational position of the spark plug  100  relative to the plug hole P 1 . More specifically, the front end portion  12  and the front end section  36  have a key projection  19  (or ridge) and a key depression  39 , respectively, formed so as to allow the joint section  17  to be disposed at a predetermined position relative to the flow direction of swirl in the combustion chamber upon engagement therebetween. Alternatively, the metallic shell  1  and the plug hole P 1  may be provided with a plurality of projections  19  and a plurality of depressions  39  engageable with the projections  19 , respectively. The metallic shell  1  and the plug hole P 1  may be provided with at least one depression and at least one projection, respectively. The front end portion  12  of the metallic shell  1  may be chamfered, and the front end section  36  of the plug hole P 1  may be shaped to fit with the chamfered front end portion  12 . The metallic shell  1  and the plug hole P 1  may be egg-shaped, oval-shaped or polygonal-shaped. The joint section  17  is formed in a particular position relative to the rotational position of the spark plug  100  so that the ground electrode  4  does not prevent the swirl from flowing into the spark gap g. This makes it possible to determine the position and orientation of the ground electrode  4  in the combustion chamber easily and properly. 
     Hereinafter, how to attach the spark plug  100  to the cylinder head SH 1  will be explained below. 
     Before fitting the spark plug  100  in the plug hole P 1 , the sealant  25  is applied to the outer surfaces of the front end portion  12  and the key projection  19 . It is not necessary to apply the sealant  25  to the whole of the outer surface of the front end portion  12 , and the sealant  25  may be applied to a part of the outer surface of the front end portion  12  as described above. In this case, it is preferable to apply the sealant  25  to a front part of the outer surface of the front end portion  12  so that the sealant  25  spreads through the clearance when the spark plug  100  is being inserted in the plug hole P 1 . 
     Then, the spark plug  100  is inserted into the plug hole P 1  so that the key projection  19  is engaged in the key depression  39  and the plug bearing portion  11  and the front end portion  12  of the metal shell  1  are accommodated in the receiving section  35  and the front end section  36  of the plug hole P 1 , respectively. As described above, the ground electrode  304  is positioned so as not to interfere with the flow of swirl by the engagement between the key projection  19  and the key depression  39 . Further, the plug bearing surface  13  is seated on the shoulder  37  via the gasket G. 
     While the spark plug  100  is inserted into the plug hole P 1 , the sealant  25  applied to the outer surface of the front end portion  12  spreads because of viscosity thereof so as to fill in the clearance between the outer surface of the front end portion  12  and the inner surface of the front end section  36  in this embodiment. 
     Then, the spark plug  100  is secured to the cylinder head SH 1  by means of the plug fixing member  41 . 
     The plug fixing member  41  is generally made of metal, and includes a flange portion  42  used for securing the plug fixing member  41  to the cylinder head SH 1  and a tube portion  43  generally formed into a cylindrical shape as shown in FIG.  4 . Holes (not shown) are formed in the flange portion  42  so as to correspond to the threaded holes  32   a  to  32   d . The tube portion  43  has an outer diameter slightly smaller than the diameter of the insert section  34  of the plug hole P 1  as well as an inner diameter larger than the outer diameter of the tool engaging portion  15 , and accommodates therein an ignition coil unit  50 . The ignition coil unit  50  has a core coil  44 , a primary winding L 1 , a secondary winding L 2 , a connection terminal  45  in the form of a coil spring, and an insulating element  46  generally made of heat-resistant rubber. The primary and secondary windings L 1  and L 2  are wound around the core coil  44 , while one end of the secondary winding L 2  is connected to the connection terminal though so not shown. Further, the plug fixing member  41  is provided with a connector  47  connected to external devices (such as a power source and a ECU, not shown) and an ignition switch  48  accommodating therein a switching element (not shown) for controlling current supply from the power source. 
     The above-structured plug fixing member  41  is set by inserting the tube portion  43  into the plug hole P 1  until the front end face  43   a  contacts with the rear end face of the plug bearing portion  11  of the metallic shell  1 . In this state, the connection terminal  45  electrically connects the secondary winding L 2  to the terminal electrode  5  so as to supply high discharge voltage while being held against the spring tension. Further, the insulating element  46  is brought into contact with the shell portion  2   b  of the insulator  2 , thereby providing electrical insulation between the spark plug  100  and the ignition coil unit  50 . Then, the plug fixing member  41  is disposed so that the holes of the flange portion  42  are positioned correspondingly to the threaded holes  32   a  to  32   d , and is then secured by screwing the bolts (including the bolts  49   a  and  49   c ) into the threaded holes  32   a  to  32   d  through the holes. The plug bearing portion  11  of the metallic shell  1  is pushed down with the front end face  43   a  of the tube portion  43 , thereby sealing a gap between the plug bearing surface  13  and the shoulder  37  with the gasket G. Herein, the length of the tube portion  43  has been previously adjusted according to the dimensions of the spark plug  100  and the plug hole P 1 , so that the front end face  43  pushes the rear end face of the plug bearing portion  11  with a predetermined pressure. Simultaneously with securing the plug fixing member  41 , the ignition coil unit  50  can be arranged in the plug hole P 1 . Since a high-tension cable is not needed for electrically connecting the spark plug  100  with the ignition coil unit  50 , noise reduction will result. 
     As described above, the spark plug  100  is clearance fitted in the plug hole P 1  with the clearance between the outer surface of the front end portion  12  and the inner surface of the front end section  36  of the plug hole P 1  being given by the following expression: 0.005≦D−d 1  ≦0.50 and filled with the sealant  25 . For example, the clearance (D−d 1 ) is 0.15 mm, when D and d 1  are 13.70 mm and 13.55 mm, respectively. Herein, the diameters D and d 1  are determined without including the depression  39  and the projection  19 , respectively. The protrusion  18  is disposed at a position given by the following expression: L≧0. For example, the distance L is set to 0.5 mm. At the same time, the sealant  25  is placed on at least the part  12   a  of the outer surface of the front end portion  12  that axially corresponds in position to the protrusion  18 . 
     With the above-described arrangement, heat applied to the central electrode  3  and the insulator  2  (especially, the second stem portion  2   i ) is transferred to the metallic shell  1  via the step  2   h , the protrusion  18  and the packing  51 . Heat is also applied to the ground electrode  4  and transferred to the metallic shell  1 . Then, the heat is diffused through the metallic shell  1  and transmitted from the front end portion  12  and the plug bearing surface  13  to the cylinder head SH 1  via the sealant  25 . This makes it possible to keep the central electrode  3 , the insulator  2  and the ground electrode  4  at low temperature. The spark plug  100  can therefore attain heat resistance equal to or higher than conventional with great ability to transmit the heat to the cylinder head SH 1 . Upon the engagement between the key projection  19  and the key depression  39 , the ground electrode  4  is positioned so as not to prevent swirl from flowing into the spark gap g. Thus, the spark plug  100  can attain high and stable ingitionability, thus being capable of igniting even a lean mixture assuredly. 
     Next, a spark plug  200  and a cylinder head SH 2  according to a second embodiment of the invention will be described with reference to FIG.  5 . Herein, like parts and portions are designated by like reference numerals in the drawings and repeated descriptions thereof are omitted. 
     The metallic shell  1  of the spark plug  200  has a front end portion  212  shaped differently from the front end portion  12  of the first embodiment. More specifically, the front end portion  212  includes a first cylindrical part  212   a  extending from the plug bearing surface  13 , a tapered part  212   b  on the front side of the first end part  212   a , and a second cylindrical part  212   c  on the front side of the tapered part  212   b . The tapered part  212   b  is generally conical with an outer diameter gradually decreasing toward the front end. Further, the outer diameter of the plug bearing portion  11  is smaller than that of the tool engaging portion  15 . 
     In the cylinder head SH 2 , a plug hole P 2  is formed so as to correspond to the front end portion  212  of the metallic shell  1 . The plug hole P 2  includes only two section: a receiving section  35  and a front end section  236 . The front end section  236  has a first cylindrical region  236   a , a tapered region  236   b  and a second cylindrical region  236   c  that correspond to the first cylindrical part  212   a , the tapered part  212   b  and the second cylindrical part  212   c , respectively. A clearance is created between the outer surface of the first cylindrical part  212   a  and the inner surface of the first cylindrical region  236   b . The clearance (D−d 1 ) is controlled to from 0.005 to 0.50 mm. On the other hand, the tapered part  212   b  and the tapered region  236  are formed so as to come in contact with each other. Likewise, the second cylindrical part  212   c  and the second cylindrical region  236   c  are formed so as to come in contact with each other. 
     The spark plug  200  is secured to the cylinder head SH 2  by a plug fixing member (not shown) in such a manner that the sealant  25  is interposed between the outer surface of the first cylindrical part  212   a  and the inner surface of the first cylindrical region  236   b.    
     In the above arrangement, when heat is applied to the central electrode  3 , the insulator  2  and the ground electrode  4  and flows into the metallic shell  1 , the sealant  25  rapidly transmits the heat to the cylinder head SH 2 . It is thus possible to attain improved heat resistance of the spark plug  200 . Further, the sealant  25  is prevented from falling off to enter the combustion chamber during the use of the spark plug  200  by the contacts between the tapered part  212   b  and the tapered region  236   b  and between the second cylindrical part  212   c  and the second cylindrical region  236   c . The spark plug  200  is therefore more reliable. 
     The present invention will be more specifically illustrated by way of the following example. 
     EXAMPLE 
     Herein, how the heat resistance (i.e., the resistance to preignition) of the spark plug  100  varies with changes in the clearance (D−d 1 ) was evaluated using a preignition tester (not shown). 
     The following are the dimensions of the plug hole P 1  and samples of the spark plug  100  used in the evaluation (refer to FIG.  1 ). 
     Plug hole P 1   
     Inner diameter D of the front end section  36 : 13.7 mm (constant) 
     Spark plug  100   
     Outer diameter d 1  of the front end portion  12 : variable 
     Length T 1  of the front end portion  12 : 15.0 mm 
     Inner diameter d 2  of the front end portion  12 : 8.4 mm 
     Minimum inner diameter d 3  of the protrusion  18 : 7.5 mm 
     Length L between the plug bearing surface  13  and the rear end of the protrusion  18 : 0.5 mm 
     Length T 2  of the second stem portion  2   i:  17 mm 
     Outer diameter d 4  of the front end of the second stem portion  2   i:  5.1 mm 
     As shown in FIG. 6, a test system was prepared. More specifically, the spark plug  100  was secured to the cylinder head SH 1  by means of a plug fixing member  141 , while the sealant  25  is provided in the clearance between the outer surface of the front end portion  12  and the inner surface of the front end section  36 . The plug fixing member  141  is the same in structure as the above-mentioned plug fixing member  41 , except that part of ignition coil unit  50  was removed and a hole  43   b  is formed with an opening in the flange portion  42  so as to continue to the inner surface of the tube portion  43 . Two ends of the connection terminal  45  were electrically connected to the terminal electrode  5  and one end of a cable  51 , respectively, under spring tension thereof. The other end of the cable  51  was electrically connected to an ignition circuit (not shown) and the preignition tester via diodes  52  and  53 , respectively. 
     The preignition tester is for measuring so-called ion current. When preignition occurs thereby generating a flame at the spark gap g, the spark gap g is brought into conduction by means of ions in the flame. Thus, the ion current is fed upon applying voltage of several hundreds volts. The preignition tester determines that preignition occurs by detecting the ion current before the ignition occurred through the ignition circuit. 
     Using the above-described test system, internal combustion engine (2000 cc displacement, in-line four-cylinder engine) was driven at full throttle. Ignition advance was gradually changed by one degree. While driving the engine for two minutes at each advanced ignition timing, it was monitored whether preignition occurred. The advance by which preignition occurred was determined to be AD 1 . 
     Next, the same procedure was repeated, except that the sealant  25  was not interposed between the outer surface of the front end portion  12  and the inner surface of the front end section  36  of the plug hole P 1 . The advance by which preignition occurred was determined to be AD 2 . 
     Differences between AD 1  and AD 2  were calculated as ΔAD, and are shown in FIG.  7 . The larger value of ΔAD means that the heat resistance of the spark plug  100  was highly improved. 
     It is clear from FIG. 7 that when the clearance (D−d 1 ) was in a range from 0.005 to 0.50 mm, the heat resistance (i.e., resistance to preignition) of the spark plug  100  was improved. When the clearance (D−d 1 ) is in a range from 0.075 to 0.40 mm, the heat resistance of the spark plug  100  was further improved. 
     As is apparent from the foregoing, the spark plug of the present invention is easily and reliably fit in the plug hole without the necessity of being screwed in the plug hole, and is therefore free from problems associated with the engagement of the male and female threads. Particularly, DOHC engine currently in the mainstream is often equipped with intake and exhaust valves that are large in area, and the plug hole is designed to be situated in the recesses of the cylinder head. The spark plug of the present invention is easily and reliably fit even in such a recessed plug hole. If the spark plug needs replacing, the used spark plug is easily removed from the plug hole, and then, a new one is easily fit in the plug hole. In addition, the inner diameter of the plug hole can be made smaller than conventional because of forming the plug hole with no female thread. Thus, the intake and exhaust valves are allowed to stand nearer to the axis of a cylinder, thereby providing a wider range of engine design. Further, the sealant links the front end portion of the metallic shell and the front end section of the plug hole while avoiding the existence of air. That is, the heat applied to the central electrode and the insulator and diffused to the metallic shell therefrom is rapidly transmitted from the front end portion of the metallic shell to the cylinder head via the sealant. The insulator and the central electrode are thus kept at low temperature, whereby the spark plug can attain heat resistance equal to or higher than conventional without the possibility of preignition, electrode fusing and the like. Also, it is possible to minimize variations in the heat resistance of the spark plugs, which results from difference in clearances, and at the same time, to prevent the passage of combustion gas from a combustion chamber unfailingly so as to improve gastightness. Furthermore, the spark plug and the plug hole are shaped so as to position the ground electrode so as not to prevent swirl from flowing into the spark gap g, whereby the swirl can be certainly brought in contact with a flame kernel of spark discharge at the spark gap g. Therefore, the spark plug can ignite not only a rich mixture but also a lean mixture reliably. It is also possible minimize variations in ingitionability from cylinder to cylinder and from engine to engine so that ignition occurs at a constant air-fuel ratio and possible to improve fuel efficiency. 
     Although the invention has been described with reference to the specific embodiments thereof, the invention is not limited to the above-described embodiments. Various modification and variation of the embodiments described above will occur to those skilled in the art in light of the above teaching. For example, the spark plug may comprise a plurality of ground electrodes. Further, the plug bearing surface of the metallic shell may be tapered, and may seated on the shoulder of the plug hole with no gasket therebetween. The front end portion of the metallic shell can be formed into any tubular shape without being limited to a cylindrical shape. It is noted that if the front end portion of the metallic shell is formed into a shape other than a cylindrical shape, “the outer diameter d 1  of the front end portion” and “the inner diameter D of the front end section” mean a maximum outer diameter of the front end portion and an inner diameter of the front end section corresponding to the maximum outer diameter of the front end portion, respectively. The scope of the invention is defined with reference to the following claims.