Patent Publication Number: US-6655334-B2

Title: Ignition apparatus for use in internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ignition apparatus for use in an internal combustion engine, the apparatus including one or more spark plugs attached to corresponding cylinders of the internal combustion engine and being adapted to induce spark discharge at the spark plugs for igniting an air-fuel mixture. 
     2. Description of the Related Art 
     Conventionally, in an internal combustion engine for use as an automobile engine or a like engine, a spark plug used for igniting an air-fuel mixture generally includes a center electrode inserted into a front end portion of an axial hole formed in an insulator, a metallic shell provided outside the insulator, and a ground electrode whose one end is joined to the metallic shell and whose opposite end faces the center electrode across a spark discharge gap. 
     Such spark plugs, together with ignition coils, among other components, constitute an ignition apparatus for use in an internal combustion engine. In the ignition apparatus for use in an internal combustion engine, high voltage for ignition is generated at an ignition coil and applied to a spark plug, whereby spark discharge is induced in a spark discharge gap to thereby ignite an air-fuel mixture within a combustion chamber. 
     A known spark plug is configured such that thread grooves are formed on the outer circumferential surface of the metallic shell for fixedly attaching the spark plug to a cylinder head through screw engagement. 
     In order to meet recent demands for high efficiency (improvement of fuel economy) and clean emission, some internal combustion engines employ a high air-fuel ratio (lean air-fuel ratio) for an air-fuel mixture. Such an internal combustion engine employing a lean air-fuel ratio involves a problem in that ignition of an air-fuel mixture is impaired when the amount of fuel present in the vicinity of the spark discharge gap is small. In order to cope with this problem, the internal combustion engine employing a lean air-fuel ratio utilizes a turbulent flow (swirl flow or tumble flow) of an air-fuel mixture within a combustion chamber for stabilizing ignition of an air-fuel mixture. 
     However, when turbulent flow of an air-fuel mixture is generated, the orientation of the ground electrode of a spark plug within a combustion chamber is known to influence ignition of the air-fuel mixture; i.e., when the ground electrode is oriented in a certain direction within the combustion chamber, ignition may be impaired. In connection with ignition of an air-fuel mixture through contact with spark discharge induced in the spark discharge gap of a spark plug, for example, when the ground electrode is located upstream of the spark discharge gap with respect to a turbulent flow of the air-fuel mixture, the ground electrode serves as a block and hinders contact between the air-fuel mixture and spark discharge. As a result, ignition of the air-fuel mixture may be impaired. 
     In the case of a spark plug which is attached to a cylinder head through screw engagement, the orientation of a ground electrode within a combustion chamber differs among cylinders and is thus uncertain; therefore, difficulty is encountered with such a spark plug in orienting a ground electrode in a specific direction. Also, in the case of a spark plug which is attached to a cylinder head through screw engagement, a slight dimensional error of thread grooves or individual differences cause variation of the orientation of a ground electrode within a combustion chamber at the final stage of attachment. Therefore, such a spark plug encounters difficulty in orienting a ground electrode in a specific direction. 
     Thus, according to a contemplated structure, a spark plug is loosely fitted into a cylinder head without forming thread grooves for fixation on a metallic shell of the spark plug and without forming internal thread grooves in the cylinder head for engaging the spark plug. Employment of a structure in which a spark plug is loosely fitted into a cylinder head for attachment can avoid variations in the orientation of a ground electrode within a combustion chamber among spark plugs and can facilitate orientation of a ground electrode in a specific direction within a combustion chamber. However, in contrast to a conventional structure in which a spark plug is attached to a cylinder head through screw engagement, in the devised structure, gastightness (seal) must be maintained between a spark plug and a cylinder head, and a spark plug must be held fixedly attached to the cylinder head in a stable condition when combustion pressure is applied to the spark plug from a combustion chamber. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an ignition apparatus for use in an internal combustion engine, which facilitates orientation of a ground electrode within a combustion chamber at the final stage of attachment of a spark plug to a cylinder head, exhibits good gastightness between the spark plug and the cylinder head, and is capable of fixedly attaching the spark plug to the cylinder head in a stable condition by a simple structure. 
     In order to achieve the above object, the invention provides an ignition apparatus for use in an internal combustion engine, comprising a spark plug, which comprises a center electrode inserted into a front end portion of an axial hole formed in an insulator, a metallic shell provided outside the insulator, and a ground electrode joined to the metallic shell and forming a spark discharge gap between the ground electrode and the center electrode and which is disposed in a plug disposition hole formed in a cylinder head of the internal combustion engine, the ignition apparatus being adapted to induce spark discharge in the spark discharge gap of the spark plug for igniting an air-fuel mixture. The ignition apparatus is characterized in that the metallic shell of the spark plug comprises a flange portion having a plug seat formed on a front end face thereof and adapted to come in contact with the cylinder head directly or indirectly via another member, and a cylindrical front end portion extending axially frontward from the plug seat of the flange portion and having no thread grooves formed on an outer circumferential surface thereof, and in that a plug hole pipe, which is disposed rearward in relation to the plug seat of the metallic shell and is fixedly attached to the cylinder head, causes the plug seat of the metallic shell to be pressed against the cylinder head, whereby the spark plug is fixedly attached to the cylinder head. 
     A spark plug to be mounted on this ignition apparatus for use in an internal combustion engine has no thread grooves formed on the outer circumferential surface of the front end portion of the metallic shell; i.e., the ignition apparatus does not employ screw engagement for attaching the same to a cylinder head. The front end portion of the metallic shell has no thread grooves formed on the outer circumferential surface thereof, but is formed into a cylindrical shape, whereby the spark plug can be attached to the cylinder head while being loosely fitted into the cylinder head. Thus, when the spark plug is attached to the cylinder head, a position on the metallic shell where the ground electrode is joined can be readily set to a circumferentially specific position about the axis of the plug disposition hole (i.e., the ground electrode can be oriented in a specific direction within a combustion chamber), thereby preventing uncertain orientation of a ground electrode as is the case where a spark plug is screw-engaged with a cylinder head. For example, this spark plug allows orientation of the ground electrode in the following manner: the orientation of the ground electrode within a combustion chamber is set beforehand; and the spark plug is inserted into a cylinder head while the orientation of the ground electrode is maintained, and is then fixed, whereby at the final stage of attachment the ground electrode can be oriented in a specific direction within the combustion chamber. 
     The direction of a turbulent flow of an air-fuel mixture in a combustion chamber depends on the structure of an inlet pipe and that of a combustion chamber and is thus substantially constant with respect to a cylinder head. Therefore, when the spark plug is to be attached to the cylinder head, by setting beforehand the orientation of the ground electrode within the combustion chamber in view of the direction of a turbulent flow of an air-fuel mixture, the ground electrode can be oriented within the combustion chamber in a specific direction in relation to the direction of a turbulent flow of an air-fuel mixture. In contrast to a conventional practice, the plug disposition hole does not require thread grooves, and therefore the diameter (inside diameter) of the plug disposition hole can be reduced, thereby facilitating disposition of large-area inlet and exhaust valves on the cylinder head and facilitating design for such disposition. 
     The ignition apparatus for use in an internal combustion engine of the present invention is characterized in that, when a spark plug including a metallic shell whose front end portion has no thread grooves formed on an outer circumferential surface thereof and is formed into a cylindrical shape as described above is to be fixedly attached to a cylinder head, a plug hole pipe, which is disposed rearward in relation to the plug seat of the metallic shell and fixedly attached to the cylinder head, is used for the fixation, and is further characterized in that the plug hole pipe causes the plug seat to be pressed against the cylinder head. Notably, the term “rearward” used in relation to a spark plug refers to the side toward an axial end of the spark plug opposite the front end of the spark plug. 
     Incidentally, an intake-valve-exhaust-valve drive mechanism including cam shafts and adapted to drive intake and exhaust valves is provided on an upper portion of a cylinder head of an internal combustion engine. In order to smoothly operate the intake-valve-exhaust-valve drive mechanism, lubricating oil is circulated within the upper portion of the cylinder head. Since high voltage for ignition generated by an ignition coil is applied to a spark plug for induction of spark discharge, adhesion of lubricating oil raises a problem in that spark discharge cannot be induced normally, because of electrical leakage or a like cause. In order to cope with this problem, the internal combustion engine employs a plug hole pipe whose inside diameter is determined so as to allow insertion of a spark plug thereinto and which is provided on the upper portion of the cylinder head where the intake-valve-exhaust-valve drive mechanism is disposed, thereby providing a spark plug disposition space and isolating the spark plug from lubricating oil. The plug hole pipe is fixedly attached to the cylinder head such that the axis thereof is aligned with that of the spark plug. 
     In the present invention, the plug hole pipe is used not only for isolating a spark plug from lubricating oil by determining an inside diameter thereof such that the same can enclose the spark plug, but also as a fixation member for fixedly attaching the spark plug to a cylinder head. 
     By using a plug hole pipe to be attached to a cylinder head as a fixation member for fixedly attaching, to the cylinder head, a spark plug whose metallic shell has no thread grooves formed thereon, the number of components can be reduced as compared with the case where a fixation member for fixing a spark plug is newly added. Since a special fixation member is not employed, manufacturing cost can be reduced, and a spark plug whose metallic shell has no thread grooves formed thereon can be fixedly attached to a cylinder head by use of a simple structure. 
     In contrast to a conventional structure in which a spark plug is attached to a cylinder head through screw engagement, when a spark plug whose metallic shell has no thread grooves formed thereon is fixedly attached to a cylinder head, gastightness (seal) must be maintained between the spark plug and the cylinder head, and the spark plug must be held fixedly attached to the cylinder head in a stable condition even when combustion pressure is applied to the spark plug from a combustion chamber, or the internal combustion engine itself undergoes strong vibration. 
     According to the present invention, in order to maintain gastightness between a spark plug and a cylinder head, a plug hole pipe is fixedly attached to the cylinder head such that the plug seat of a metallic shell is pressed against the cylinder head, and the plug hole pipe causes the spark plug to be fixedly attached to the cylinder head. As a result, the plug seat of the flange portion of the metallic shell is uniformly pressed against the cylinder head along the circumferential direction, whereby the plug seat and the cylinder head can maintain stable contact with each other. Further, since the plug hole is fixedly attached to the cylinder head such that the plug seat of the metallic shell is pressed against the cylinder head, even when combustion pressure is applied to the spark plug from a combustion chamber, the spark plug can be held fixedly attached to the cylinder head in a stable condition. 
     Preferably, in order to fixedly attach the spark plug to the cylinder head in a stable condition by use of the plug hole pipe, the plug hole pipe is attached to the cylinder head such that the plug seat of the metallic shell is pressed against the cylinder head by a pressing force greater than combustion pressure generated by combustion of an air-fuel mixture; specifically, by a pressing force of not less than 10 MPa. 
     Thus, according to the ignition apparatus for use in an internal combustion engine of the present invention, at the final stage of attachment of the spark plug, the ground electrode can be oriented within the combustion chamber in a specific direction in relation to the direction of a turbulent flow of an air-fuel mixture. Also, the plug hole pipe is used not only for isolating the spark plug from lubricating oil, but also as a fixation member for fixedly attaching the spark plug to the cylinder head, whereby the spark plug, whose metallic shell has no thread grooves formed thereon, can be fixedly attached to the cylinder head in a stable condition and in a manner such that contact between the spark plug and the cylinder head is favorably maintained even when vibration of the internal combustion engine is propagated to the spark plug. 
     Preferably, when the plug hole pipe is to be fixedly attached to the cylinder head such that the plug seat of the metallic shell is pressed against the cylinder head, the plug hole pipe to be disposed rearward in relation to the plug seat of the metallic shell abuts and presses a portion of the metallic shell to thereby fixedly attach the spark plug to the cylinder head. 
     By fixedly attaching the plug hole pipe to the cylinder head such that the plug hole pipe abuts and presses a portion of the metallic shell as described above, the plug seat of the metallic shell can be reliably pressed against the cylinder head. Thus, the spark plug, whose metallic shell has no thread grooves formed thereon, can be fixedly attached to the cylinder head in a stable condition and in a manner such that contact between the spark plug and the cylinder head is favorably maintained even when vibration of the internal combustion engine is propagated to the spark plug. 
     Preferably, when the plug hole pipe is caused to abut a portion of the metallic shell, the plug hole pipe accommodates a portion of the spark plug which extends rearward from the rear end face of the flange portion of the metallic shell, the rear end face being an end face located opposite the plug seat, and abuts and presses the rear end face of the flange portion of the metallic shell to thereby fixedly attach the spark plug to the cylinder head. Thus, the plug seat of the metallic shell can be reliably pressed against the cylinder head by use of the plug hole pipe. 
     Preferably, in the case of a spark plug whose metallic shell comprises a crimped portion, which is formed by crimping a circumferential edge of a rear end portion thereof toward an outer circumferential portion of an insulator, the plug hole pipe accommodates a portion of the spark plug which extends rearward from the crimped portion of the metallic shell, and abuts and presses the crimped portion to thereby fixedly attach the spark plug to the cylinder head. Thus, the plug seat of the metallic shell can be reliably pressed against the cylinder head by use of the plug hole pipe. Additionally, as compared with the above-described structure in which the plug hole pipe is caused to abut the end face of the flange portion, the outside diameter of the plug hole pipe can be reduced, thereby reducing a space which the plug hole pipe occupies within the cylinder head, and thus increasing a space for disposing other devices, such as an intake-valve-exhaust-valve mechanism. 
     Preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, the plug hole pipe is fixedly attached to the cylinder head through press fitting. 
     In conventional internal combustion engines, press fitting is the most widely used method for fixedly attaching a plug hole pipe to a cylinder head, and the plug hole pipe can be firmly attached to the cylinder head through press fitting. Thus, the spark plug can be pressed against the cylinder head by a pressing force greater than combustion pressure, and the spark plug, whose metallic shell has no thread grooves formed thereon, can be fixedly attached to the cylinder head in a reliable condition. 
     Once a plug hole pipe is fixedly attached to a cylinder head through press fitting, removal of the attached plug hole pipe is difficult. Therefore, this press-fitting method is not suited for an internal combustion engine which requires periodic replacement of spark plugs. 
     Thus, preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, the plug hole pipe comprises a flange portion projecting outward from an outer circumferential surface thereof, and the flange portion is fixedly attached the cylinder head, whereby the plug hole pipe is fixedly attached to the cylinder head. 
     Utilizing the flange portion for fixedly attaching the plug hole pipe to the cylinder head eliminates the need to press-fit the plug hole pipe into the cylinder head. By fixedly attaching the flange portion to the cylinder by a method that allows removal of the attached flange portion, the fixedly attached plug hole pipe can be removed from the cylinder head. 
     The flange portion may be fixedly attached to the cylinder head, for example, by use of bolts or the like or by a method in which an engagement portion is provided on the cylinder head and used such that, after a spark plug is held between the plug hole pipe and the cylinder head, the plug hole pipe is turned about the center axis thereof to thereby engage the flange portion with the engagement portion. 
     Preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, the plug hole pipe has thread grooves formed on an outer circumferential surface thereof, and the thread grooves are engaged with thread grooves formed on the cylinder head, whereby the plug hole pipe is fixedly attached to the cylinder head. 
     This fixation method allows removal of the fixedly attached plug hole pipe from the cylinder head and enables fixation of the plug hole pipe onto the cylinder head without use of additional members such as bolts. Also, through control of torque applied for tightening the plug hole pipe into the cylinder head, a force (load) to be applied to the spark plug by the plug hole pipe can be rendered substantially constant, whereby the plug hole pipe can cause the plug seat of the metallic shell to be pressed against the cylinder head stably and without variations among spark plugs. 
     Thus, according to the ignition apparatus for use in an internal combustion engine of the present preferred aspect of the invention, the spark plug can be replaced and can be fixedly attached to the cylinder head by use of a small number of components, thereby reducing the cost of manufacturing the internal combustion engine. 
     Preferably, in the above-described ignition apparatus for use in an internal combustion engine, in which the spark plug is fixedly attached to the cylinder head by use of the plug hole pipe, a cylinder head cover is provided on the cylinder head, and the plug hole pipe is held between the cylinder head cover and the cylinder head to thereby be fixedly attached to the cylinder head. 
     This fixation method utilizes the cylinder head cover to be provided on the cylinder head, for fixing the plug hole pipe within the cylinder head. Specifically, a force associated with fixation of the cylinder head cover onto the cylinder head with the plug hole pipe being held therebetween is used for fixedly attaching the plug hole pipe to the cylinder head and transmitted to the plug hole pipe, whereby the plug seat of the metallic shell can be reliably pressed against the cylinder head by use of the plug hole pipe. 
     As discussed above, in the case of the spark plug whose metallic shell has no thread grooves formed thereon, when the spark plug is attached to the cylinder head, a position on the metallic shell where the ground electrode is joined can be readily set to a circumferentially specific position about the axis of the plug disposition hole (i.e., the ground electrode can be oriented in a specific direction within a combustion chamber). However, after the spark plug is inserted into the plug disposition hole, the spark plug may turn with a resultant change in the orientation of the ground electrode within the combustion chamber. 
     Thus, preferably, in the ignition apparatus for use in an internal combustion engine described above, the spark plug is configured such that the metallic shell comprises an anti-rotation portion which is engaged with the cylinder head at one or more positions when the spark plug is turned about the axis of the plug disposition hole in the course of attachment to the cylinder head; the cylinder head comprises an anti-rotation reception portion corresponding to the anti-rotation portion; and when the spark plug is fixedly attached to the cylinder head, the position of the joint between the metallic shell and the ground electrode is aligned with a predetermined circumferential position about the axis of the plug disposition hole. 
     The anti-rotation portion formed on the metallic shell and the anti-rotation reception portion formed on the cylinder head are fitted to or engaged with each other to thereby disable the spark plug turning about its axis and to thereby specify the orientation of the ground electrode within the combustion chamber when the spark plug is attached to the cylinder head. The anti-rotation portion may assume the form of, for example, a protrusion extending axially on the outer surface of a flange portion or front end portion of the metallic shell or a groove formed axially on the outer surface of the flange portion or front end portion of the metallic shell. Alternatively, the flange portion or front end portion of the metallic shell may include a special-section portion whose section taken along a plane perpendicular to the axial direction of the spark plug assumes a shape other than circle (polygon (such as hexagon), oval, etc.) and which serves as an anti-rotation portion. 
     In this case, the anti-rotation reception portion capable of being fitted to or engaged with the above-described anti-rotation portion is provided on the cylinder head such that the ground electrode is oriented in a direction of good ignition within the combustion chamber. Thus, merely by fitting (engaging) the anti-rotation portion of the metallic shell to (with) the anti-rotation reception portion formed on the cylinder head, the ground electrode can be reliably and readily oriented in a predetermined direction of good ignition within the combustion chamber. 
     Thus, according to the ignition apparatus for use in an internal combustion engine of the present preferred aspect of the invention, after the ground electrode is oriented in a certain direction within the combustion chamber in the course of attachment of the spark plug to the cylinder head, a change in the orientation of the ground electrode can be prevented, and the ground electrode can be readily and reliably oriented in a specific direction within the combustion chamber, whereby good ignition can be attained. 
     Incidentally, a plug hole pipe is originally intended to isolate a spark plug from lubricating oil. As a result of adding to the plug hole pipe a function for fixedly attaching a spark plug to a cylinder head, the structure of the plug hole pipe becomes complicated to a certain degree as compared with a conventional plug hole pipe. Therefore, a dimensional error or a like factor may cause formation of clearance between the plug hole pipe and the cylinder head, potentially resulting in penetration of lubricating oil. 
     Thus, preferably an oil seal member is provided between the plug hole pipe and the cylinder head. The oil seal member prevents adhesion of lubricating oil to the spark plug, thereby preventing occurrence of an anomaly, such as the spark plug&#39;s failing to induce normal spark discharge, which would otherwise result from adhering lubricating oil. The oil seal member is formed of, for example, an elastically deformable material (rubber, etc.). The sectional shape of the oil seal member is deformed according to clearance formed between the plug hole pipe and the cylinder head to thereby fill the clearance, thereby favorably preventing penetration of lubricating oil. 
     Preferably, in the ignition apparatus for use in an internal combustion engine described above, the plug seat of the metallic shell of the spark plug is in contact with the cylinder head via a gasket. 
     By employing a gasket, when the spark plug is fixedly attached to the cylinder head such that the plug hole causes the plug seat of the metallic shell to be pressed against the cylinder head, gastightness between the spark plug and the cylinder head can be maintained more reliably. Also, since a heat release path via the gasket is established, the spark plug&#39;s performance (particularly heat resistance) can be retained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory view showing the state of attachment, to a cylinder head, of a spark plug and a plug hole pipe included in an ignition apparatus for use in an internal combustion engine of a first embodiment; 
     FIG. 2 is a sectional view of a cylinder head in which the spark plug and the plug hole pipe are disposed; 
     FIG. 3 is a sectional view of a cylinder head in which a second plug hole pipe having a flange portion and a spark plug are disposed; 
     FIG. 4 is a sectional view of a cylinder head in which a third plug hole pipe having thread grooves formed on an outer circumferential surface thereof and a spark plug are disposed; 
     FIG.  5 ( a ) is an exterior view of a second spark plug as viewed laterally; 
     FIG.  5 ( b ) is an exterior view of the second spark plug as viewed from the front end side; 
     FIG.  6 ( a ) is an exterior view of a third spark plug as viewed laterally; 
     FIG.  6 ( b ) is an exterior view of the third spark plug as viewed from the front end side; 
     FIG.  7 ( a ) is an exterior view of a fourth spark plug as viewed laterally; 
     FIG.  7 ( b ) is an exterior view of the fourth spark plug as viewed from the front end side; 
     FIG. 8 is a sectional view of a cylinder head in which a fourth plug hole pipe and a spark plug are disposed, showing a structure in that a front end portion of the fourth plug hole pipe abuts a crimped portion of the spark plug; 
     FIG. 9 is an explanatory view showing the state of attachment, to a cylinder head, of a spark plug and a plug hole pipe included in an ignition apparatus for use in an internal combustion engine of a fifth embodiment. 
    
    
     Reference numerals are used to identify items shown in the drawings as follows: 
       1 : ignition apparatus for use in internal combustion engine 
       11 : spark plug 
       11   a : second spark plug 
       11   b:  third spark plug 
       11   c:  fourth spark plug 
       13 : terminal electrode 
       15 : insulator 
       17 : center electrode 
       19 : metallic shell 
       21 : ground electrode 
       23 : flange portion 
       23   a:  plug seat 
       23   b:  rear end face 
       25 : front end portion 
       27 : crimped portion 
       31 : plug hole pipe 
       51 : cylinder head 
       51   a:  plug disposition hole 
       53 : cylinder head cover 
       63 : oil seal 
       65 : protrusion 
       67 : groove 
       69 : flat portion 
       71 : gasket 
       73 : second plug hole pipe 
       75 : flange portion 
       79 : third plug hole pipe 
       81 : thread grooves 
       91 : fourth plug hole pipe 
       93 : fifth plug hole pipe 
     g: spark discharge gap 
     Detailed Description of the Preferred Embodiments 
     FIG. 1 is an explanatory view showing the state of attachment, to a cylinder head  51 , of a spark plug  11  and a plug hole pipe  31  included in an ignition apparatus for use in an internal combustion engine  1  of a first embodiment of the present invention. 
     As shown in FIG. 1, the spark plug  11  is disposed in a plug disposition hole  51   a  formed in the cylinder head  51  of an internal combustion engine, and the plug hole pipe  31  is disposed while extending between the cylinder head  51  and a cylinder head cover  53  of the internal combustion engine. 
     Notably, in a space defined by the cylinder head  51  and the cylinder head cover  53 , an intake-valve-exhaust-valve drive mechanism (not shown in FIG. 1) including cam shafts and adapted to drive intake and exhaust valves is provided, and lubricating oil for smoothly operating the intake-valve-exhaust-valve drive mechanism is circulated. 
     The ignition apparatus for use in an internal combustion engine  1  includes an ignition coil having a primary winding and a secondary winding and an igniter having a power switching element, which are not shown in FIG.  1 . The igniter controls supply and cutoff of power to the primary winding to thereby generate high voltage for ignition at the secondary winding of the ignition coil. When the high voltage for ignition is applied to the spark plug  11 , spark discharge is induced in a spark discharge gap g (see FIG. 2) formed between a center electrode  17  and a ground electrode  21  of the spark plug  11 . That is, the ignition apparatus for use in an internal combustion engine  1  is adapted to induce spark discharge in the spark discharge gap g of the spark plug  11  to thereby ignite an air-fuel mixture by the spark discharge. 
     The cylinder head  51  includes the plug disposition hole  51   a  in which the spark plug  11  is disposed. The plug disposition hole  51   a  includes a step face  51   b,  which comes in contact with a plug seat  23   a  (see FIG. 2) of a metallic shell  19  of the spark plug  11  directly or indirectly via another member; a first wall surface  51   c,  which is located on the side toward a combustion chamber  55  with respect to the step face  51   b ; and a second wall surface  51   d  and a third wall surface  51   e,  which are located on the side toward the cylinder head cover  53  with respect to the step face  51   b.    
     As shown in FIG. 1, the plug hole pipe  31  is a cylindrical member formed of a metallic material, such as stainless steel or aluminum, opens at a rear end  31   a  and at a front end  31   b,  and can accommodate a portion of the spark plug  11  which extends rearward (upward in FIG. 1) from the plug seat  23   a  of a flange portion  23  of the metallic shell  19 . After the spark plug  11  is disposed in the plug disposition hole  51   a  formed in the cylinder head  51 , the plug hole pipe  31  is press-fitted into the plug disposition hole  51   a  while a rear end portion of the spark plug  11  is inserted therein through the front end  31   b , to thereby be fixedly attached to the cylinder head  51 . 
     FIG. 2 is a sectional view of the cylinder head  51  in which the spark plug  11  and the plug hole pipe  31  are disposed. As shown in FIG. 2, an intake valve  57  and an exhaust valve  59  are provided in the cylinder head  51 ; and in an upper portion of the cylinder head  51 , an intake-valve-exhaust-valve drive mechanism including cam shafts  61  and adapted to drive the intake valve  57  and the exhaust valve  59  is provided, and lubricating oil for smoothly operating the intake-valve-exhaust-valve drive mechanism is present. 
     As shown in FIG. 2, the spark plug  11  includes the center electrode  17  inserted into a front end portion (a lower end portion in FIG. 2) of an axial hole formed in an insulator  15 ; the metallic shell  19  provided outside the insulator  15 ; the ground electrode  21 , whose one end is joined to the metallic shell  19  and whose opposite end forms the spark discharge gap g between the same and the center electrode  17 ; and a terminal electrode  13  inserted into a rear end portion (an upper end portion in FIG. 2) of the axial hole formed in the insulator  15  and electrically connected to the center electrode  17 . The metallic shell  19  includes a crimped portion  27 , which is formed by crimping the circumferential edge of a rear end portion thereof toward an outer circumferential portion of the insulator  15 . The crimped portion  27  prevents the insulator  15  from coming-off, which is engaged with an engagement portion (not shown) formed on the inside of the metallic shell  19 , whereby the insulator  15  is held by the metallic shell  19 . 
     The metallic shell  19  includes the flange portion  23  having the plug seat  23   a  formed on the front end face thereof and adapted to come in contact with the step face  51   b  of the plug disposition hole  51   a  formed in the cylinder head  51  directly or indirectly via another member (via a gasket  71  in the present embodiment), and a cylindrical front end portion  25  extending axially frontward from the plug seat  23   a  of the flange portion  23  and having no thread grooves formed on the outer circumferential surface thereof. A rear end face  23   b  is formed on an end face of the flange portion  23  located opposite the plug seat  23   a.    
     The front end portion  25  of the metallic shell  19  has an axial length equal to the length as measured between the step face  51   b  of the plug disposition hole  51   a  and the combustion chamber  55 . Thus, when the spark plug  11  is disposed in the plug disposition hole  51   a , the spark discharge gap g is located in the combustion chamber  55 . 
     As shown in FIG. 2, the spark plug  11  fixedly rests in the plug disposition hole  51   a  such that the plug seat  23   a  of the flange portion  23  is in contact with the step face  51   b  of the plug disposition hole  51   a  via the gasket  71 , and the rear end face  23   b  of the flange portion  23  abuts the front end  31   b  of the plug hole pipe  31 . Further, the plug hole pipe  31  is attached to the cylinder head  51  such that the front end  31   b  of the plug hole pipe  31  presses the rear end face  23   b  of the flange portion  23  of the metallic shell  19 , whereby the plug seat  23   a  of the metallic shell  19  of the spark plug  11  is pressed against the cylinder head (specifically, the step face  51   b  of the plug disposition hole  51   a ), and in this state the spark plug  11  is fixedly attached to the cylinder head  51 . 
     The plug hole pipe  31  is formed such that the inside diameter thereof is smaller than the outside diameter of the flange portion  23  of the spark plug  11 , and thus the front end  31   b  abuts the rear end face  23   b  of the flange portion  23  of the spark plug  11 . Thus, the plug hole pipe  31  is smaller in outside diameter than a conventional plug hole pipe. 
     In the plug disposition hole  51   a , the first wall surface  51   c  faces the front end portion  25  of the metallic shell  19 , and the second wall surface  51   d  faces the outer circumferential surface of the flange portion  23  of the spark plug  11  and the outer circumferential surface of the plug hole pipe  31 . Further, the plug disposition hole  51   a  includes the third wall surface  51   e , which faces the outer circumferential surface of the plug hole pipe  31  via an oil seal  63 . Notably, clearance is provided between the first wall surface  51   c  of the plug disposition hole  51   a  and the front end portion  25  of the metallic shell  19  so that the spark plug  11  can be inserted into the cylinder head  51  in a loose fit condition. Preferably, the clearance satisfies the relation as represented by 0 mm&lt;D−d≦1.0 mm, where D is the inside diameter of a portion of the plug disposition hole  51   a  corresponding to the first surface  51   c , and d is the outside diameter of the front end portion  25  of the metallic shell  19 . 
     When the plug hole pipe  31  is press-fitted into the plug disposition hole  51   a , the plug hole pipe  31  is tightly held by the second wall surface  51   d  to thereby be fixedly attached to the cylinder head  51 . The plug hole pipe  31  tightly held by the second wall surface  51   d  presses the plug seat  23   a  of the spark plug  11  against the cylinder head  51  by a force (not less than 10 MPa) greater than the combustion pressure generated by combustion of an air-fuel mixture, thereby fixedly attaching the spark plug  11  to the cylinder head  51 . Thus, even when combustion pressure is generated by combustion of an air-fuel mixture, the spark plug  11 , which includes the metallic shell  19  having no thread grooves formed thereon, is held fixedly attached to the cylinder head  51  in a stable condition, and good gastightness is maintained between the spark plug  11  and the cylinder head  51 . 
     In the course of press-fitting the plug hole pipe  31  into the plug disposition hole  51   a , the oil seal  63  is disposed between the plug hole pipe  31  and the third wall surface  51   e  of the plug disposition hole  51   a . The oil seal  63  is an annular member formed of a heat resistant, elastically deformable material. While the oil seal  63  is held between the plug hole pipe  31  and the third wall surface  51   e,  the sectional shape of the same is deformed to thereby fill clearance formed between the plug hole pipe  31  and the cylinder head  51  (specifically, the third wall surface  51   e ). Thus, penetration of lubricating oil through the clearance between the plug hole pipe  31  and the cylinder head  51  can be prevented. In FIG. 1, the oil seal  63  is not illustrated. The oil seal  63  is disposed while being fitted into a groove  31   c  formed circumferentially on the outer circumferential surface of the plug hole pipe  31 . In the course of press-fitting the plug hole pipe  31  into the plug disposition hole  51   a , the oil seal  63  is elastically deformed while being held between the plug hole pipe  31  and the third wall surface  51   e  of the plug disposition hole  51   a.    
     After the plug hole pipe  31  is fixedly attached to the cylinder head  51 , the cylinder head cover  53  is fixedly attached to an upper portion of the cylinder head  51 , whereby the rear end  31   a  of the plug hole pipe  31  abuts an opening portion  53   a  of the cylinder head cover  53  (see FIG.  1 ). At this time, an oil seal (not shown) for preventing penetration of lubricating oil is disposed between the rear end  31   a  of the plug hole pipe  31  and the opening portion  53   a  of the cylinder head cover  53 . Thus, the plug hole pipe  31  can isolate the spark plug  11  from lubricating oil, thereby preventing adhesion of lubricating oil to the spark plug  11 . 
     The spark plug  11  of the present embodiment is configured such that thread grooves are not formed on the outer circumferential surface of the front end portion  25  of the metallic shell  19 , and thus does not employ screw engagement as a method for fixedly attaching the same to the cylinder head  51 . Therefore, a position on the metallic shell  19  where the ground electrode  21  is joined can be readily set to a circumferentially specific position about the axis of the plug disposition hole  51   a  (i.e., the ground electrode  21  can be oriented in a specific direction within the combustion chamber  55 ). Since the front end portion  25  of the metallic shell  19  has no thread grooves formed on the outer circumferential surface thereof and is formed into a cylindrical shape, the spark plug  11  can be inserted, in a loose fit condition, into the plug disposition hole  51   a  formed in the cylinder head  51 . Therefore, as compared with a case where a spark plug is attached to a cylinder head through screw engagement, the spark plug  11  can be easily attached. 
     Thus, the spark plug  11  allows easy orientation of the ground electrode  21  in the following manner: the orientation of the ground electrode  21  within the combustion chamber  55  is set beforehand; and the spark plug  11  is inserted into the plug disposition hole  51   a  while the orientation of the ground electrode  21  is maintained, whereby at the final stage of attachment the ground electrode  21  can be oriented in a specific direction within the combustion chamber  55 . Subsequently, the plug hole pipe  31  is press-fitted into the plug disposition hole  51   a  and thereby causes the plug seat  23   a  of the metallic shell  19  to be pressed against the cylinder head  51 , whereby the spark plug  11  can be fixedly attached to the cylinder head  51 . 
     In the ignition apparatus for use in an internal combustion engine  1  of the present embodiment, the spark plug  11  is fixedly attached to the cylinder head  51  such that the ground electrode  21  is oriented, in relation to the cylinder head  51 , within the combustion chamber  55  in view of the direction of a turbulent flow of an air-fuel mixture within the combustion chamber so as to attain good ignition of an air-fuel mixture. 
     As described above, in the ignition apparatus for use in an internal combustion engine  1  of the first embodiment, the plug hole pipe  31  causes the plug seat  23   a  of the metallic shell  19  to be pressed against the cylinder head  51 , whereby the spark plug  11  is fixedly attached to the cylinder head  51 . Since screw engagement is not employed as a method for attachment of the spark plug  11 , in the course of fixedly attaching the spark plug  11  to the cylinder  51 , a position on the metallic shell  19  where the ground electrode  21  is joined can be readily set to a circumferentially specific position about the axis of the plug disposition hole  51   a  (i.e., the ground electrode  21  can be oriented in a specific direction within the combustion chamber  55 ). 
     In the ignition apparatus for use in an internal combustion engine  1  of the first embodiment, the plug hole pipe  31  is used not only for isolating the spark plug  11  from lubricating oil, but also as a fixation member for fixedly attaching the spark plug  11  to the cylinder head  51 . Therefore, when the spark plug  11 —whose metallic shell  19  has no thread grooves formed thereon in order to allow orientation of the ground electrode  21  in a specific direction within the combustion chamber  55 —is to be fixedly attached to the cylinder head  51 , there is no need to newly add a fixation member, thereby preventing increase in the number of components. 
     According to the ignition apparatus for use in an internal combustion engine  1  of the first embodiment, the oil seal  63  seals clearance between the plug hole pipe  31  and the plug disposition hole  51   a  formed in the cylinder head  51 , thereby preventing penetration of lubricating oil. Thus, adhesion of lubricating oil to the spark plug  11  can be prevented, thereby preventing the spark plug&#39;s failing to induce normal spark discharge, which would otherwise result from adhering lubricating oil. 
     The spark plug  11  may assume the form of a second spark plug  11   a , a third spark plug  11   b,  or a fourth spark plug  11   c  as shown in FIGS. 5,  6  and  7  in order that a position on the metallic shell  19  where the ground electrode  21  is joined is reliably set to a circumferentially specific position about the axis of the plug disposition hole  51   a  (i.e., the ground electrode  21  is reliably oriented in a specific direction within the combustion chamber  55 ). 
     First, in the second spark plug  11   a , an axially extending protrusion  65  is formed on the outer circumferential side surface of the flange portion  23  of the metallic shell  19  and serves as an anti-rotation portion. FIG.  5 ( a ) is an exterior view of the second spark plug  11   a  as viewed laterally, and FIG.  5 ( b ) is an exterior view of the second spark plug  11   a  as viewed from the front end side (as viewed from below in FIG.  5 ( a )). Since the protrusion  65  is formed such that its position in relation to the ground electrode  21  is constant as shown in FIG.  5 ( b ), by providing a mating groove, which is in an engaging relation with the protrusion  65  and serves as an anti-rotation reception portion, on the second wall surface  51   d  (see FIG. 1) of the plug disposition hole  51   a  of the cylinder head  51 , the ground electrode  21  can be oriented in a constant direction within the combustion chamber  55  as follows: the second spark plug  11   a  is inserted into the plug disposition hole  51   a  while the protrusion  65  formed on the metallic shell  19  is engaged with the above-mentioned mating groove formed on the cylinder head  51 . The position of the mating groove to be formed on the cylinder head  51  is determined beforehand such that a position on the metallic shell  19  where the ground electrode  21  is joined is set to a circumferentially specific position about the axis of the plug disposition hole  51   a , and in view of the relative position between the ground electrode  21  and the protrusion  65 . 
     As is apparent from FIG.  5 ( b ), in the second spark plug  11   a,  the position of the protrusion  65  formed on the outer circumferential surface of the flange portion  23  of the metallic shell  19  is circumferentially aligned with the position of the joint between the ground electrode  21  and the metallic shell  19 . However, the position of the protrusion  65  formed on the flange portion  23  is not necessarily circumferentially aligned with the position of the joint between the ground electrode  21  and the metallic shell  19 . In this case, as mentioned above, the position of the mating groove to be formed on the cylinder head  51  must be determined as appropriate in view of the relative position between the ground electrode  21  and the protrusion  65  and specific orientation of the ground electrode  21  within the combustion chamber  55 . In the second spark plug  11   a , the protrusion  65  is formed on the outer circumferential side surface of the flange portion  23  of the metallic shell  19 . However, the protrusion  65  may be formed on the outer circumferential surface of the front end portion  25  of the metallic shell  19 . In this case, in order to correspond to a protrusion to be formed on the front end portion  25 , a mating groove may be formed as appropriate on the first wall surface  51   c  of the plug disposition hole  51   a  formed in the cylinder head  51 . 
     In the third spark plug  11   b,  a groove  67  is formed on the side surface of the flange portion  23  of the metallic shell  19  and serves as an anti-rotation portion. FIG.  6 ( a ) is an exterior view of the third spark plug  11   b  as viewed laterally, and FIG.  6 ( b ) is an exterior view of the third spark plug  11   b  as viewed from the front end side (as viewed from below in FIG.  6 ( a )). Since the groove  67  is formed such that its position in relation to the ground electrode  21  is constant as shown in FIG.  6 ( b ), by providing a mating protrusion, which is in an engaging relation with the groove  67  and serves as an anti-rotation reception portion, on the second wall surface  51   d  (see FIG. 1) of the plug disposition hole  51   a  formed in the cylinder head  51 , the ground electrode  21  can be oriented in a constant direction within the combustion chamber  55  as follows: the third spark plug  11   b  is inserted into the plug disposition hole  51   a  while the groove  67  formed on the metallic shell  19  is engaged with the above-mentioned mating protrusion formed on the cylinder head  51 . The position of the mating protrusion to be formed on the cylinder head  51  is determined beforehand such that a position on the metallic shell  19  where the ground electrode  21  is joined is set to a circumferentially specific position about the axis of the plug disposition hole  51   a , and in view of the relative position between the ground electrode  21  and the groove  67 . 
     As is apparent from FIG.  6 ( b ), in the third spark plug  11   b,  the position of the groove  67  formed on the outer circumferential surface of the flange portion  23  of the metallic shell  19  is circumferentially aligned with the position of the joint between the ground electrode  21  and the metallic shell  19 . However, the position of the groove  67  formed on the flange portion  23  is not necessarily circumferentially aligned with the position of the joint between the ground electrode  21  and the metallic shell  19 . In this case, as mentioned above, the position of the mating protrusion to be formed on the cylinder head  51  must be determined as appropriate in view of the relative position between the ground electrode  21  and the groove  67  and specific orientation of the ground electrode  21  within the combustion chamber  55 . In the third spark plug  11   b,  the groove  67  is formed on the outer circumferential side surface of the flange portion  23  of the metallic shell  19 . However, the groove  67  may be formed on the outer circumferential surface of the front end portion  25  of the metallic shell  19 . In this case, in order to correspond to a groove to be formed on the front end portion  25 , a mating protrusion may be formed as appropriate on the first wall surface  51   c  of the plug disposition hole  51   a  formed in the cylinder head  51 . 
     In the fourth spark plug  11   c,  a flat portion  69  is formed on the side surface of the flange portion  23  of the metallic shell  19  and serves as an anti-rotation portion. FIG.  7 ( a ) is an exterior view of the fourth spark plug  11   c  as viewed laterally, and FIG.  7 ( b ) is an exterior view of the fourth spark plug  11   c  as viewed from the front end side. Since the flat portion  69  is formed such that its position in relation to the ground electrode  21  is constant as shown in FIG.  7 ( b ), by providing a mating flat portion, which faces the flat portion  69  in an engaging relation with the flat portion  69  and serves as an anti-rotation reception portion, on the second wall surface  51   d  (see FIG. 1) of the plug disposition hole  51   a  formed in the cylinder head  51 , the ground electrode  21  can be oriented in a constant direction within the combustion chamber  55  as follows: the fourth spark plug  11   c  is inserted into the plug disposition hole  51   a  while the flat portion  69  formed on the metallic shell  19  is engaged with the above-mentioned mating flat portion formed on the cylinder head  51 . The position of the mating flat portion to be formed on the cylinder head  51  is determined beforehand such that a position on the metallic shell  19  where the ground electrode  21  is joined is set to a circumferentially specific position about the axis of the plug disposition hole  51   a , and in view of the relative position between the ground electrode  21  and the flat portion  69 . 
     As is apparent from FIG.  7 ( b ), in the fourth spark plug  11   c,  the position of the flat portion  69  formed on the outer circumferential surface of the flange portion  23  of the metallic shell  19  is circumferentially aligned with the position of the joint between the ground electrode  21  and the metallic shell  19 . However, the position of the flat portion  69  formed on the flange portion  23  is not necessarily circumferentially aligned with the position of the joint between the ground electrode  21  and the metallic shell  19 . In this case, as mentioned above, the position of the mating flat portion to be formed on the cylinder head  51  must be determined as appropriate in view of the relative position between the ground electrode  21  and the flat portion  69  and specific orientation of the ground electrode  21  within the combustion chamber  55 . 
     In the fourth spark plug  11   c , the flange portion  23  of the metallic shell  23  assumes a section (a section taken along a plane perpendicular to the axial direction of the fourth spark plug  11   c ) whose shape is a circle which is partially cut off. However, the section of the flange portion  23  may assume a shape other than circle, such as polygon (hexagon, etc.) or oval. A corresponding portion of the plug disposition hole  51   a  (see FIG. 1) formed in the cylinder head  51  may assume a sectional shape such that the flange portion  23  of a spark plug can be disposed therein. 
     Next will be described a second embodiment; i.e., an ignition apparatus for use in an internal combustion engine which includes a second plug hole pipe  73  having a flange portion  75 . The spark plug  11  included in the ignition apparatus for use in an internal combustion engine of the second embodiment is configured in a manner similar to that of the spark plug of the first embodiment. An intake-valve-exhaust-valve drive mechanism to be provided on the cylinder head  51  is configured in a manner similar to that of the first embodiment, and therefore redundant description thereof is omitted. 
     FIG. 3 is a sectional view of the cylinder head  51  in which the second plug hole pipe  73  having the flange portion  75  and the spark plug  11  are disposed. 
     As shown in FIG. 3, the second plug hole pipe  73  includes the flange portion  75 , which projects radially outward from the outer circumferential surface of the same. Through-holes  75   a  for allowing insertion of corresponding bolts  77  are formed in the flange portion  75 . The flange portion  75  may be formed along the entire circumference of the second plug hole pipe  73  in such a manner as to project radially outward as in the case of the present embodiment, or one or more flange portions  75  may be formed intermittently along the entire circumference in such a manner as to project outward. 
     The length of a portion of the second plug hole pipe  73  extending between a front end  73   b  and the front end face of the flange portion  75  is set longer than a length obtained by subtracting the axial length of the flange portion  23  of the spark plug  11  and the axial length (thickness) of the gasket  71  as measured before elastic deformation thereof from the axial length of the second wall surface  51   d  of the plug disposition hole  51   a . By so doing, when the flange portion  75  is fixedly attached to the cylinder head  51  by use of the bolts  77 , at least the front end  73   b  of the second plug hole pipe  73  can abut the rear end face  23   b  of the flange portion  23  and thus can cause the plug seat  23   a  of the spark plug  11  to be pressed against the cylinder head  51  (specifically, the step face  51   b  of the plug disposition hole  51   a ). By fixedly attaching the second plug hole pipe  73  to the cylinder head  51 , the spark plug  11 , whose metallic shell  19  has no thread grooves formed thereon, is fixedly attached to the cylinder head  51  while the plug seat  23   a  of the spark plug  11  is pressed against the cylinder head  51  as described above. 
     The flange portion  75  of the second plug hole pipe  73  is fixedly attached to the cylinder head  51  by use of the bolts  77 , which are screw-engaged with the cylinder head  51  through the corresponding through-holes  75   a.  Female-threaded portions  51   g,  which can be screw-engaged with the corresponding bolts  77 , are formed on the cylinder head  51 . 
     The oil seal  63  is disposed between an outer circumferential surface of the second plug hole pipe  73  extending between the flange portion  75  and the front end  73   b  and the second wall surface  51   d  of the plug disposition hole  51   a  formed in the cylinder head  51 , to thereby prevent penetration of lubricating oil. Notably, the plug disposition hole  51   a  formed in the cylinder head  51  of the second embodiment includes the first wall surface  51   c  and the second wall surface  51   d,  but does not include a third wall surface in contrast to the plug disposition hole  51   a  of the first embodiment. The oil seal  63  is disposed while being fitted into a groove  73   c  formed circumferentially on the outer circumferential surface of the second plug hole pipe  73 . In the course of fixedly attaching the second hole pipe  73  to the cylinder head  51  by use of the flange portion  75 , the oil seal  63  is elastically deformed while being held between the second plug hole pipe  73  and the second wall surface  51   d  of the plug disposition hole  51   a.    
     As described above, the ignition apparatus for use in an internal combustion engine of the second embodiment is configured such that, by fixedly attaching the flange portion  75  to the cylinder head  51  by use of the bolts  77 , the second plug hole pipe  73  is fixedly attached to the cylinder head  51 . Removal of the bolts  77  allows the fixedly attached second plug hole pipe  73  to be removed from the cylinder head  51 . 
     Thus, as in the case of the ignition apparatus for use in an internal combustion engine of the first embodiment, according to the ignition apparatus for use in an internal combustion engine of the second embodiment, in the course of attachment of the spark plug  11  to the cylinder head  51  the ground electrode  21  can be oriented in a direction of optimal ignition within the combustion chamber  55 , and the spark plug  11  can be replaced. Therefore, the ignition apparatus of the second embodiment is applicable to various kinds of internal combustion engines. 
     Next will be described a third embodiment; i.e., an ignition apparatus for use in an internal combustion engine which includes a third plug hole pipe  79  having thread grooves  81  formed on the outer circumferential surface thereof. The spark plug  11  included in the ignition apparatus for use in an internal combustion engine of the third embodiment is configured in a manner similar to that of the spark plug of the first embodiment. An intake-valve-exhaust-valve drive mechanism to be provided on the cylinder head  51  is configured in a manner similar to that of the first embodiment, and therefore redundant description thereof is omitted. 
     FIG. 4 is a sectional view of the cylinder head  51  in which the third plug hole pipe  79  and the spark plug  11  are disposed. 
     As shown in FIG. 4, the third plug hole pipe  79  has the thread grooves  81  formed on the outer circumferential surface of a portion thereof located in the vicinity of a front end  79   b  thereof. Thread grooves  51   f,  which can be engaged with the thread grooves  81  formed on the third plug hole pipe  79 , are formed on the second wall surface  51   d  of the plug disposition hole  51   a  formed in the cylinder head  51 . 
     The axial length of the threaded grooves  81  formed on the outer circumferential surface of the third plug hole pipe  79  and extending axially rearward (upward in FIG. 4) from the front end  79   b  is set longer than the axial length of the threaded grooves  51   f  of the plug disposition hole  51   a . By so doing, when the third plug hole pipe  79  is screw-engaged with the cylinder head  51 , at least the front end  79   b  of the third plug hole pipe  79  can abut the rear end face  23   b  of the flange portion  23  and thus can cause the plug seat  23   a  of the spark plug  11  to be pressed against the cylinder head  51  (specifically, the step face  51   b  of the plug disposition hole  51   a ). By fixedly attaching the third plug hole pipe  79  to the cylinder head  51  through screw engagement, the spark plug  11 , whose metallic shell  19  has no thread grooves formed thereon, is fixedly attached to the cylinder head  51  while the plug seat  23   a  of the spark plug  11  is pressed against the cylinder head  51  as described above. 
     The oil seal  63  is disposed between an outer circumferential surface of the third plug hole pipe  79  located rearward in relation to the threaded grooves  81  and the third wall surface  51   e  of the plug disposition hole  51   a  formed in the cylinder head  51 , to thereby prevent penetration of lubricating oil. The oil seal  63  is disposed while being fitted into a groove  79   c  formed circumferentially on the outer circumferential surface of the third plug hole pipe  79 . In the course of fixedly attaching the third plug hole pipe  79  to the cylinder head  51  through screw engagement, the oil seal  63  is elastically deformed while being held between the third plug hole pipe  79  and the third wall surface  51   e  of the plug disposition hole  51   a.    
     As described above, the ignition apparatus for use in an internal combustion engine of the third embodiment employs screw engagement which is effected by use of the threaded grooves  81 , as a method for fixedly attaching the third plug hole pipe  79  to the cylinder head  51 . This fixation method allows removal of the fixedly attached third plug hole pipe  79  from the cylinder head  51  and enables fixation of a plug hole pipe onto a cylinder head without use of additional members such as bolts. 
     The third plug hole pipe  79  is fixedly attached to the cylinder head  51  through screw engagement of the threaded grooves  81  with the cylinder head  51 ; i.e., the plug hole pipe itself has a portion in direct contact with the cylinder head. Therefore, the oil seal  63  can be omitted. 
     According to the employed method, a plug hole pipe is fixedly attached to a cylinder head through screw engagement. Therefore, through control of torque applied for tightening the third plug hole pipe  79  into the cylinder head  51 , a force (load) to be applied to the spark plug  11  by the third plug hole pipe  79  can be rendered substantially constant, whereby the third plug hole pipe  79  can cause the plug seat  23   a  of the metallic shell  19  to be pressed against the cylinder head  51  stably and without variations among spark plugs. 
     Notably, when the third plug hole pipe  79  is fixedly attached to the cylinder head  51 , the third plug hole pipe  79  rotates. Accordingly, if a spark plug is of a type that is rotatable in the plug disposition hole  51   a , the spark plug will rotate with the third plug hole pipe  79 , potentially resulting in change in the orientation of the ground electrode  21  in relation to the cylinder head  51 . Therefore, preferably, when the third plug hole pipe  79  is to be used, a spark plug is of a type that does not rotate in the plug disposition hole  51   a . Specifically, by use of a spark plug of a type that is shown in FIG. 5, a change in the orientation of a ground electrode can be prevented. 
     Next will be described a fourth embodiment; i.e., an ignition apparatus for use in an internal combustion engine which includes a fourth plug hole pipe  91  having thread grooves formed on the outer circumferential surface thereof as in the case of the third embodiment and including a front end portion  91   c,  which abuts the crimped portion  27  of the metallic shell  19  of the spark plug  11 . The spark plug  11  included in the ignition apparatus for use in an internal combustion engine of the fourth embodiment is configured in a manner similar to that of the spark plug of the first embodiment except that the flange portion  23  is smaller in size. An intake-valve-exhaust-valve drive mechanism to be provided on the cylinder head  51  is configured in a manner similar to that of the first embodiment, and therefore redundant description thereof is omitted. 
     FIG. 8 is a sectional view of the cylinder head  51  in which the fourth plug hole pipe  91  and the spark plug  11  are disposed. 
     As shown in FIG. 8, the fourth plug hole pipe  91  has the thread grooves  81  formed on the outer circumferential surface of a portion thereof located in the vicinity of the front end portion  91   c  thereof. The thread grooves  51   f , which can be engaged with the thread grooves  81  formed on the fourth plug hole pipe  91 , are formed on the third wall surface  51   e  of the plug disposition hole  51   a  formed in the cylinder head  51 . The front end portion  91   c  of the fourth plug hole pipe  91  is formed in such a manner as to project inward from the inner wall of the fourth plug hole pipe  91  and adjusted in shape such that the same can abut the crimped portion  27  of the metallic shell  19  as will be described later in this specification. 
     The axial length of a portion of the outer circumferential surface of the fourth plug hole pipe  91  on which the threaded grooves  81  are formed is set longer than the axial length of the threaded grooves  51   f  of the plug disposition hole  51   a . By so doing, when the fourth plug hole pipe  91  is screw-engaged with the cylinder head  51 , at least the front end portion  91   c  of the fourth plug hole pipe  91  can abut the crimped portion  27  of the flange portion  23  and thus can cause the plug seat  23   a  of the spark plug  11  to be pressed against the cylinder head  51  (specifically, the step face  51   b  of the plug disposition hole  51   a ). By fixedly attaching the fourth plug hole pipe  91  to the cylinder head  51  through screw engagement, the spark plug  11 , whose metallic shell  19  has no thread grooves formed thereon, is fixedly attached to the cylinder head  51  while the plug seat  23   a  of the spark plug  11  is pressed against the cylinder head  51  as described above. 
     The oil seal  63  is disposed between an outer circumferential surface of the fourth plug hole pipe  91  located frontward in relation to the threaded grooves  81  and the second wall surface  51   d  of the plug disposition hole  51   a  formed in the cylinder head  51 , to thereby prevent penetration of lubricating oil. The oil seal  63  is disposed while being fitted into a groove  91   d  formed circumferentially on the outer circumferential surface of the fourth plug hole pipe  91 . In the course of fixedly attaching the fourth plug hole pipe  91  to the cylinder head  51  through screw engagement, the oil seal  63  is elastically deformed while being held between the fourth plug hole pipe  91  and the second wall surface  51   d  of the plug disposition hole  51   a.    
     The fourth plug hole pipe  91  is fixedly attached to the cylinder head  51  through screw engagement of the threaded grooves  81  with the cylinder head  51 ; i.e., the plug hole pipe itself has a portion in direct contact with the cylinder head. Therefore, the oil seal  63  can be omitted. 
     As described above, in addition to the above-described effect yielded by the ignition apparatus for use in an internal combustion engine of the third embodiment, the ignition apparatus for use in an internal combustion engine of the fourth embodiment allows reduction in the outside diameter of a plug hole pipe (the fourth plug hole pipe  91 ) as compared with a structure in that a plug hole pipe is caused to abut the rear end face  23   b  of the flange portion  23  of the metallic shell  19  as in the case of the internal combustion engine of the third embodiment. Thus, a space which a plug hole pipe itself occupies within the cylinder head  51  can be reduced, and a space for disposing other devices, such as an intake-valve-exhaust-valve mechanism, can be increased accordingly. Therefore, the ignition apparatus for use in an internal combustion engine becomes an optimum ignition apparatus for use in an internal combustion engine of high output. 
     Next will be described a fifth embodiment; i.e., an ignition apparatus for use in an internal combustion engine which includes a fifth plug hole pipe  93  formed into a cylindrical shape as shown in FIG.  9  and in which a force associated with fixation of the cylinder head cover  53  onto the cylinder head  51  with the fifth plug hole pipe  93  being held therebetween is used for fixedly attaching the fifth plug hole pipe  93  to (for retaining the same in) the cylinder head  51 . The spark plug  11  included in the ignition apparatus for use in an internal combustion engine of the fifth embodiment is configured in a manner similar to that of the spark plug of the first embodiment. An intake-valve-exhaust-valve drive mechanism to be provided on the cylinder head  51  is configured in a manner similar to that of the first embodiment, and therefore redundant description thereof is omitted. 
     FIG. 9 is a sectional view of the cylinder head  51  in which the fifth plug hole pipe  93  and the spark plug  11  are disposed and on which the cylinder head cover  53  is provided. 
     As shown in FIG. 9, the fifth plug hole pipe  93  is formed into a cylindrical shape in such a manner as to open at a front end  93   b  and a rear end  93   a.  The rear end  93   a  is formed such that the outside diameter thereof is rendered greater than that of an axially intermediate, cylindrical portion of the fifth plug hole pipe  93 , thereby being able to abut the circumferential inner wall of the opening portion  53   a  of the cylinder head cover  53 . A front end portion of the fifth plug hole pipe is formed such that the diameter thereof is rendered smaller than that of the cylindrical portion, and the front end  93   b  can abut the rear end face  23   b , located opposite the plug seat  23   a,  of the flange portion  23  of the metallic shell  19  of the spark plug  11 . The diameter-reduced front end portion of the fifth plug hole pipe  93  assumes an outside diameter smaller than the inside diameter of the second wall surface  51   d  of the plug disposition hole  51   a  formed in the cylinder head  51 . After the spark plug  11  is disposed in the cylinder head  51 , the fifth plug hole pipe  93  is loosely fitted into the plug disposition hole  51   a  such that the front end  93   b  abuts the rear end face  23   b  of the metallic shell  19  of the spark plug  11 . 
     The ignition apparatus for use in an internal combustion engine of the fifth embodiment is assembled in the following manner. The spark plug  11 , whose metallic shell  19  has no thread grooves formed thereon, is disposed in the plug disposition hole  51   a  formed in the cylinder head  51 . Next, the above-described fifth plug hole pipe  93  is inserted into the plug disposition hole  51   a . Subsequently, the cylinder head cover  53  is fixedly attached to an upper portion (located at the upside of FIG. 9) of the cylinder head  51 . In this case, the axial length of the fifth plug hole pipe  93  must be adjusted beforehand such that, when the cylinder head cover  53  is fixedly attached onto the cylinder head  51 , the fifth plug hole pipe  93  and the spark plug  11  (including the gasket  71  if provided) can be held together between the cylinder head cover  53  and the cylinder head  51 . 
     By determining beforehand the axial length of the fifth plug hole pipe  93  in view of the axial length of the flange portion  23  of the spark plug  11  (including the thickness of the gasket  71  when the gasket  71  is provided as in the case of the present embodiment), the dimension of the cylinder head  51 , and the dimension of the cylinder head cover  53 , when the cylinder head cover  53  is fixedly attached onto the cylinder head  51 , the fifth plug hole pipe  93 , together with the spark plug  11 , is held between the cylinder head cover  53  and the cylinder head  51 . As a result, the fifth plug hole pipe  93  causes the plug seat  23   a  of the spark plug  11  to be pressed against the cylinder head  51  (specifically, the step face  51   b  of the plug disposition hole  51   a ) while the front end  93   b  of the fifth plug hole pipe  93  abuts the rear end face  23   b  of the flange portion  23  of the metallic shell  19  of the spark plug  11 , whereby the spark plug  11 , whose metallic shell  19  has no thread grooves formed thereon, is fixedly attached to the cylinder head  51  by use of the fifth plug hole pipe  93 . 
     The oil seal  63  is disposed between the outer circumferential surface of the diameter-reduced front end portion of the fifth plug hole pipe  93  and the third wall surface  51   e  of the plug disposition hole  51   a  formed in the cylinder head  51 , thereby preventing penetration of lubricating oil. The oil seal  63  is disposed while being fitted into a groove (not shown) formed circumferentially on the outer circumferential surface of the fifth plug hole pipe  93 . In the course of insertion of the fifth plug hole pipe  93  into the plug disposition hole  51   a  formed in the cylinder head  51 , the oil seal  63  is elastically deformed while being held between the fifth plug hole pipe  93  and the third wall surface  51   e  of the plug disposition hole  51   a.    
     A second oil seal  64  for preventing penetration of lubricating oil is also provided between the rear end  93   a  of the fifth plug hole pipe  93  and the circumferential inner wall of the opening portion  53   a  of the cylinder head cover  53 . The second oil seal  64  prevents leakage of lubricating oil along the interface between the rear end  93   a  of the fifth plug hole pipe  93  and the cylinder head cover  53 , thereby preventing lubricating oil from entering from the rear end  93   a  and reaching the spark plug  11 . Although, in this embodiment, the plug hole pipe  93  and the cylinder head cover are separate parts, but they can be formed of a single part, or can be jointed together by welding or brazing. 
     As described above, the ignition apparatus for use in an internal combustion engine of the fifth embodiment employs the following method for fixedly attaching the fifth plug hole pipe  93  to the cylinder head  51 : the fifth plug hole pipe  93  is held between the cylinder head  51  and the cylinder head cover  53 , which is fixedly attached onto the cylinder head  51 . This fixation method allows removal of the fixedly attached fifth plug hole pipe  93  from the cylinder head  51  and enables fixation of a plug hole pipe onto a cylinder head without use of additional members such as bolts. Since the cylinder head cover  53  is fixedly attached onto the cylinder head  51  to thereby hold the fifth plug hole pipe  93  and the spark plug  11  together therebetween, the fifth plug hole pipe  93  can cause the plug seat  23   a  of the metallic shell  19  of the spark plug  11  to be stably pressed against the cylinder head  51 , thereby maintaining good seal between the spark plug  11  and the cylinder head  51  and enabling stable contact between the plug seat  23   a  (specifically, the plug seat  23   a  via the gasket  71 ) of the spark plug  11  and the cylinder head  51  even when vibration of an internal combustion engine is propagated to the spark plug  11 . As a modified embodiment, a part of the cylinder head cover may be held between a cylinder head and a rear end of a plug hole pipe. By such a structure, the cylinder head cover can be pressed onto the cylinder head by the rear end of the plug hole pipe. 
     While the present invention has been described with reference to the first through fifth embodiments, the present invention is not limited thereto, but may be embodied in various forms. 
     For example, the second plug hole pipe  73  of the second embodiment uses bolts for fixedly attaching the flange portion  75  to the cylinder head  51 . However, the fixation method is not limited thereto. Any method that allows removal after fixation may be employed. Specifically, an engagement portion may be formed on the cylinder head  51  such that, after the spark plug  11  is held between the second plug hole pipe  73  and the cylinder head  51 , turning of the second plug hole pipe  73  about the center axis causes the flange portion  75  to be engaged with the same. 
     In the ignition apparatus for use in an internal combustion engine of the first through fourth embodiments, the gasket  71  is held between the plug seat  23   a  of the spark plug  11  and the cylinder head  51 . However, the spark plug  11  may be of a type that a plug seat tapers frontward between the flange portion  23  and the front end portion  25  of the metallic shell  19 , whereby the plug seat is brought in direct contact with the cylinder head  51  without use of the gasket  71  (so-called conical seat type). Furthermore, the flange portion  23  or crimped portion  27  of the metallic shell  19  and the plug hole pipe  93  may be jointed together by welding or brazing. 
     The ignition apparatus for use in an internal combustion engine of the fourth embodiment is described while describing a fixation method for a spark plug in which a plug hole pipe is caused to abut the crimped portion  27  of the metallic shell  19 . Application of this fixation method for a spark plug is not limited to the ignition apparatus of the fourth embodiment, in which the plug hole pipe is fixedly attached to the cylinder head  51  through screw engagement. Needless to say, this fixation method may be applicable to an ignition apparatus in which a plug hole pipe is press-fitted into the cylinder head  51  as in the case of the first embodiment, an ignition apparatus in which a plug hole pipe is fixedly attached to the cylinder head  51  by use of the flange portion  75  as in the case of the second embodiment, or the ignition apparatus of the fifth embodiment. 
     This application is based on Japanese Patent Application No. 2001-131704 filed Apr. 27, 2001, the disclosure of which is incorporated herein by reference in its entirety.