Patent Publication Number: US-10309339-B2

Title: Internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to a structure of an internal combustion engine. 
     BACKGROUND ART 
     Internal combustion engines include, for example, an internal combustion engine having a head-block separation structure, as described in PTL 1. The head-block separation structure is a structure in which a cylinder block that forms cylinders and a cylinder head that forms combustion chambers in conjunction with the cylinder block are formed by casting separately and are joined to each other by cylinder head bolts. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 5-187307 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in an internal combustion engine with the head-block separation structure as described in the above-described PTL 1, strength and the like required for the internal combustion engine restrict positions where cylinder head bolts are to be secured to positions where interference with the combustion chambers can be avoided. For this reason, positions where cam journals that support a cam shaft in a rotatable manner are disposed are influenced by positions where the cylinder head bolts are secured, which may cause a problem in that a degree of freedom in designing the cylinder head and cylinder block is reduced. 
     The present invention has been made in view of the problem as described above, and an object of the present invention is to provide an internal combustion engine that is capable of improving a degree of freedom in designing a cylinder head and cylinder block. 
     Solution to Problem 
     In order to achieve the object mentioned above, according to one aspect of the present invention, there is provided an internal combustion engine in which a cylinder block and a cylinder head are formed into one body and an upper surface of the cylinder head is divided, along a direction in which a plurality of cylinders are arranged, into first regions and a second region. Furthermore, the plurality of cylinders are formed in the cylinder block, and the cylinder block and the cylinder head form a plurality of combustion chambers. In addition, at least either an intake-side cam journal or an exhaust-side cam journal included in the cylinder head is disposed in the second region. 
     The first regions are regions that overlap the combustion chambers as viewed from an axial direction of the cylinders. The second region is a region located between two of the first regions adjacent to each other. The intake-side cam journal supports, in a rotatable manner, an intake-side camshaft that displaces intake valves that open and close intake passages. The exhaust-side cam journal supports, in a rotatable manner, an exhaust-side camshaft that displaces exhaust valves that open and close exhaust passages 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrative of a schematic configuration of a vehicle including an internal combustion engine of a first embodiment of the present invention; 
         FIG. 2  is a plan view illustrative of a schematic configuration of the internal combustion engine of the first embodiment of the present invention; 
         FIG. 3  is a cross sectional view taken along the line in  FIG. 2 ; 
         FIG. 4  is a cross sectional view taken along the line IV-IV in  FIG. 2 ; 
         FIG. 5  is a conceptual diagram illustrative of positional relationships among a nozzle fitting hole, an exhaust valve hole, an intake valve hole, and a plug fitting hole that are formed to an identical combustion chamber; 
         FIG. 6  is a conceptual diagram illustrative of a state in which an upper surface of a cylinder head is divided into first regions and second regions; 
         FIG. 7  is a diagram illustrative of a variation of the first embodiment of the present invention; 
         FIG. 8  is a diagram illustrative of another variation of the first embodiment of the present invention; and 
         FIG. 9  is a diagram illustrative of still another variation of the first embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In detailed description below, to provide full understanding of the embodiments of the present invention, specific details are described. However, it is obviously possible to implement one or more embodiments without such specific details. Moreover, to simplify the drawings, known structures and devices are sometimes illustrated schematically. 
     First Embodiment 
     A first embodiment of the present invention will be described below with reference to the drawings. 
     (Schematic Configuration of Vehicle) 
     Using  FIG. 1 , a schematic configuration of a vehicle including an internal combustion engine (engine)  1  of the first embodiment will be described. 
     As illustrated in  FIG. 1 , the internal combustion engine  1  burns, in a combustion chamber (not illustrated), an air-fuel mixture into which air taken in from an intake pipe  2  to which a charger CH is connected and fuel supplied from the inside of a fuel tank  4  are mixed. Energy generated in the combustion of an air-fuel mixture is transmitted to a drive unit  6  including a transmission and the like. Furthermore, gas generated after combustion is exhausted from the combustion chamber to the outside via an exhaust pipe  8 . 
     The charger CH pressurizes or accelerates air taken in from the outside air and supplies it to the intake pipe  2 . 
     The types of the charger CH include an exhaust turbine driven type charger (turbocharger) or a mechanically driven type charger (supercharger). 
     (Configuration of Internal Combustion Engine  1 ) 
     Using  FIGS. 2 to 6 , while referring to  FIG. 1 , a configuration of the internal combustion engine  1  of the first embodiment will be described. 
     As illustrated in  FIGS. 2 to 4 , the internal combustion engine  1  includes a cylinder block  10  and a cylinder head  20 . 
     The cylinder block  10  and the cylinder head  20  are, using a metal material such as an aluminum alloy, formed into one body, for example, by casting. In other words, the internal combustion engine  1  of the first embodiment has a structure in which the cylinder head  20  and the cylinder block  10  are formed into one body by casting (head-block integral structure). 
     Therefore, with regard to the internal combustion engine  1  of the first embodiment, the cylinder block  10  forms the lower portion of the internal combustion engine  1 . In addition, with regard to the internal combustion engine  1  of the first embodiment, the cylinder head  20  forms the upper portion of the internal combustion engine  1 . 
     In the cylinder block  10 , a plurality of cylinders  12  are formed. 
     In the first embodiment, a case where three cylinders  12  are formed in the cylinder block  10  is described. 
     The respective cylinders  12  are arranged with the stroke directions of pistons  14  in the respective cylinders  12  directed in parallel with one another. In  FIGS. 3 and 4 , for purposes of description, the piston  14  is not illustrated in cross section. 
     Each piston  14  moves reciprocally in a cylinder  12  in the axial direction of the cylinder  12  in response to combustion of an air-fuel mixture inside a combustion chamber  22 . 
     Each cylinder  12 , in conjunction with a con rod (not illustrated) and a crankshaft (not illustrated), is formed in such a way that a stroke of a piston  14  is set to be not less than a bore inner diameter of the cylinder  12 . In  FIG. 4 , the stroke of the piston  14  and the bore inner diameter of the cylinder  12  are indicated by a reference symbol “St” and a reference symbol “BID”, respectively. Therefore, each cylinder  12  is formed into such a shape that the conditional expression (1) below holds.
 
 St ≥BID  (1)
 
     In particular, in the first embodiment, each cylinder  12  is formed into such a shape that the conditional expression (2) below holds.
 
 St &gt;(BID×1.2)  (2)
 
     In other words, in the first embodiment, the stroke St of a piston  14  exceeds 1.2 times the bore inner diameter BID of a cylinder  12 . 
     It is assumed that the shape of the cylinder head  20  is a shape that covers the upper ends of the respective cylinders  12 . The above configuration causes the cylinder head  20 , in conjunction with the cylinder block  10 , to form a plurality of combustion chambers  22 . 
     The plurality of combustion chambers  22  are arranged with the stroke directions of the pistons  14  inside the respective cylinders  12  directed in parallel with one another. 
     In the first embodiment, three cylinders  12  are formed in the cylinder block  10 , as described above. Thus, a case where the cylinder head  20 , in conjunction with the cylinder block  10 , forms three combustion chambers  22  is described. 
     In other words, in the first embodiment, a case where the internal combustion engine  1  is configured as an internal combustion engine with three cylinders arranged in a straight line (straight 3-cylinder engine) is described. 
     The cylinder head  20  includes intake passages  30 , exhaust passages  40 , nozzle fitting holes  24 , and plug fitting holes  26 . 
     In addition to the above, on the cylinder head  20 , an out frame  50 , intake-side cam frames  52 , and exhaust-side cam frames  54  are formed. 
     The intake passages  30  are passages that communicate the intake pipe  2  with the combustion chambers  22 . The intake passages  30  are formed in the internal space of the cylinder head  20 . 
     In the first embodiment, a case where one combustion chamber  22  is communicated with the intake pipe  2  by way of two intake passages  30  is described. Therefore, in the first embodiment, the cylinder head  20  includes six intake passages  30 . 
     Two intake passages  30  that communicate one combustion chamber  22  with the intake pipe  2  are arranged along the direction in which the three cylinders  12  are arranged (in the vertical direction of the plane of illustration of  FIG. 2 ). In addition, two intake passages  30  that communicate one combustion chamber  22  with the intake pipe  2  are formed with the length directions thereof directed in parallel with a radial direction of a cylinder  12  as viewed from the axial direction of the cylinder  12 . 
     One open end of each intake passage  30  opens to the outer surface of the internal combustion engine  1  and communicates with the intake pipe  2 . The other open end of the intake passage  30  opens to a combustion chambers  22  and communicates with the combustion chamber  22 . 
     An intake valve  34  comes into contact with the opening of each intake passage  30  that opens to a combustion chamber  22 . Therefore, the opening of the intake passage  30  that opens to the combustion chamber  22  forms an intake valve hole  32  that is opened and closed by the intake valve  34 . 
     Each intake valve hole  32  opens at a portion of an intake passage  30  that forms an upper surface of a combustion chamber  22 . 
     In the first embodiment, one combustion chamber  22  and the intake pipe  2  are communicated with each other by way of two intake passages  30 . For this reason, in the first embodiment, two intake valve holes  32  are opened at portions of two intake passages  30  that form the upper surface of a combustion chamber  22 . Therefore, in the first embodiment, the cylinder head  20  includes six intake valve holes  32 . 
     In the first embodiment, all the intake valve holes  32  are formed into the same shape. 
     Two intake valve holes  32  that open to one combustion chamber  22  are arranged along the direction in which the three cylinders  12  are arranged. 
     Each intake valve  34  includes an intake valve stem  34   a  and an intake valve head  34   b . In  FIG. 3 , for purposes of description, the intake valve stem  34   a  and the intake valve head  34   b  are not illustrated in cross section. 
     Each intake valve stem  34   a  is formed into a bar shape. One end of the intake valve stem  34   a  is configured to project out of an intake valve guide hole  36 . 
     In addition, the intake valve stem  34   a  is supported to the cylinder head  20  via an intake valve spring  34   c . In  FIG. 3 , for purposes of description, the intake valve spring  34   c  is not illustrated in cross section. 
     Each intake valve spring  34   c  is expandable and contractible in the axial direction of an intake valve stem  34   a  in response to rotation of an intake-side cam shaft  38 , which will be described later. The intake valve spring  34   c  expands due to elastic force to bring an intake valve head  34   b  into contact with an intake valve hole  32  from the side where a combustion chamber  22  is located. 
     Each intake valve guide hole  36  is a through hole that is formed on an upper surface (upper deck)  20   a  of the cylinder head  20 . 
     Each intake valve head  34   b  is formed into a shape (round shape) that enables an intake valve hole  32  to be closed. The intake valve head  34   b  is attached to the other end of an intake valve stem  34   a  and is disposed inside a combustion chamber  22 . 
     The above configuration enables expansion of an intake valve spring  34   c  and contact of an intake valve head  34   b  with an intake valve hole  32  from the side where a combustion chamber  22  is located to cause the intake valve head  34   b  to close an intake passage  30 . 
     The intake-side cam shaft  38  includes an intake-side shaft  38   a  and a plurality of intake-side cams  38   b.    
     The intake-side shaft  38   a  is a cylindrical member. The intake-side shaft  38   a  is, with the axial direction thereof being parallel to the direction in which the three cylinders  12  are arranged, disposed at a position that causes the intake-side shaft  38   a  to overlap all the intake valve holes  32  as viewed in plan. Both ends of the intake-side shaft  38   a  are inserted into through holes (not illustrated) that are formed to the out frame  50 . 
     Each intake-side cam  38   b  is disposed on the outer peripheral surface of the intake-side shaft  38   a . In addition, each intake-side cam  38   b  is disposed at a position where the intake-side cam  38   b  overlaps an intake valve hole  32  as viewed in plan. Furthermore, each intake-side cam  38   b  is formed into an egg shape having a long radius and a short radius as viewed from the axial direction of the intake-side shaft  38   a.    
     In the first embodiment, the cylinder block  10  and the cylinder head  20  form three combustion chambers  22 , and each combustion chamber  22  is communicated with the intake pipe  2  by way of two intake passages  30 . For this reason, in the first embodiment, the intake-side cam shaft  38  includes six intake-side cams  38   b.    
     Pressing one end of each intake valve stem  34   a  by means of a long radius portion of an intake-side cam  38   b  causes the intake valve spring  34   c  to contract. The contraction of the intake valve spring  34   c  causes the intake valve head  34   b  to come off the intake valve hole  32  and to open an intake passage  30 . 
     Consequently, the intake valves  34  are displaced in response to the rotation of the intake-side camshaft  38  to open and close the intake passages  30 . 
     In the first embodiment, one combustion chamber  22  is communicated with the intake pipe  2  by way of two intake passages  30 . For this reason, with respect to one combustion chamber  22 , two intake valve holes  32  are formed. Therefore, in the first embodiment, with respect to one combustion chamber  22 , two intake valve guide holes  36  are formed. The two intake valve guide holes  36  are arranged along the direction in which the three cylinders  12  are arranged. 
     The exhaust passages  40  are passages that communicate the exhaust pipe  8  with the combustion chambers  22 . Each exhaust passage  40  is formed in a different space from the intake passages  30  in the internal space of the cylinder head  20 . 
     In the first embodiment, a case where one combustion chamber  22  is communicated with the exhaust pipe  8  by way of two exhaust passages  40  is described. Therefore, in the first embodiment, the cylinder head  20  includes six exhaust passages  40 . 
     Two exhaust passages  40  communicating one combustion chamber  22  with the exhaust pipe  8  are arranged along the direction in which the three cylinders  12  are arranged. In addition, two exhaust passages  40  that communicate one combustion chamber  22  with the exhaust pipe  8  are formed with the length directions thereof directed in parallel with a radial direction of a cylinder  12  as viewed from the axial direction of the cylinder  12 . 
     One open end of each exhaust passage  40  opens to the outer surface of the internal combustion engine  1  and communicates with the exhaust pipe  8 . The other open end of the exhaust passage  40  opens to a combustion chamber  22  and communicates with the combustion chamber  22 . 
     An exhaust valve  44  comes into contact with the opening of each exhaust passage  40  that opens to a combustion chamber  22 . Therefore, the opening of the exhaust passage  40  that opens to the combustion chamber  22  forms an exhaust valve hole  42  that is opened and closed by the exhaust valve  44 . 
     Each exhaust valve hole  42  opens at a portion of an exhaust passage  40  that forms an upper surface of a combustion chamber  22  and is different from the respective intake valve holes  32 . 
     In the first embodiment, one combustion chamber  22  is communicated with the exhaust pipe  8  by way of two exhaust passages  40 . For this reason, two exhaust valve holes  42  are opened at portions of two exhaust passages  40  that form the upper surface of a combustion chamber  22 . Therefore, in the first embodiment, the cylinder head  20  includes six exhaust valve holes  42 . 
     In the first embodiment, all the exhaust valve holes  42  are formed into the same shape. 
     In addition, in the first embodiment, the exhaust valve holes  42  and the intake valve holes  32  are formed into such shapes that the conditional expression (3) below holds.
 
EXHvdi&gt;INTvdi  (3)
 
     In the conditional expression (3), “EXHvdi” and “INTvdi” indicate an inner diameter of an exhaust valve hole  42  and an inner diameter of an intake valve hole  32 , respectively. Therefore, in the first embodiment, the opening area of an exhaust valve holes  42  is set to be larger than the opening area of an intake valve holes  32 . 
     In  FIG. 5 , for purposes of description, only four holes (an exhaust valve hole  42 , an intake valve hole  32 , a nozzle fitting hole  24 , and a plug fitting hole  26 ) that are formed to one combustion chamber  22  are illustrated. 
     As described above, in the first embodiment, the cylinder head  20  includes six intake valve holes  32  and six exhaust valve holes  42 . Furthermore, in the first embodiment, all the intake valve holes  32  are formed into the same shape. In addition to the above, in the first embodiment, all the exhaust valve holes  42  are formed into the same shape. 
     Therefore, in the first embodiment, the total value of opening areas of two exhaust valve holes  42  opening to one combustion chamber  22  is set to be larger than the total value of opening areas of two intake valve holes  32  opening to the one combustion chamber  22 . 
     In addition, in the first embodiment, since the total value of opening areas of all the exhaust valve holes  42  is set to be larger than the total value of opening areas of all the intake valve holes  32 , the conditional expression (4) below holds.
 
(EXHvdix6)&gt;(INTvdix6)  (4)
 
     Two exhaust valve holes  42  opening at a portion of an exhaust passage  40  that forms a roof of a combustion chamber  22  are arranged along the direction in which the three cylinders  12  are arranged. 
     Each exhaust valve  44  includes an exhaust valve stem  44   a  and an exhaust valve head  44   b . In  FIG. 3 , for purposes of description, the exhaust valve stem  44   a  and the exhaust valve head  44   b  are not illustrated in cross section. 
     Each exhaust valve stem  44   a  is formed into a bar shape. One end of the exhaust valve stem  44   a  is configured to project out of an exhaust valve guide hole  46 . 
     In addition, the exhaust valve stem  44   a  is supported to the cylinder head  20  via an exhaust valve spring  44   c . In  FIG. 3 , for purposes of description, the exhaust valve spring  44   c  is not illustrated in cross section. 
     Each exhaust valve spring  44   c  is expandable and contractible in the axial direction of an exhaust valve stem  44   a  in response to rotation of an exhaust-side camshaft  48 , which will be described later. The exhaust valve spring  44   c  expands due to elastic force to bring an exhaust valve head  44   b  into contact with an exhaust valve hole  42  from the side where a combustion chamber  22  is located. 
     Each exhaust valve guide hole  46  is a through hole that is formed on the upper surface  20   a  of the cylinder head  20 . 
     Each exhaust valve head  44   b  is formed into a shape (round shape) that enables an exhaust valve hole  42  to be closed. The exhaust valve head  44   b  is attached to the other end of an exhaust valve stem  44   a  and is disposed inside a combustion chambers  22 . The above configuration enables expansion of an exhaust valve spring  44   c  and contact of an exhaust valve head  44   b  with an exhaust valve hole  42  from the side where a combustion chamber  22  is located to cause the exhaust valve head  44   b  to close an exhaust passage  40 . 
     As described above, in the first embodiment, the inner diameter EXHvdi of an exhaust valve hole  42  is set to be larger than the inner diameter INTvdi of an intake valve hole  32 . Therefore, in the first embodiment, the outer diameter of an exhaust valve head  44   b  (the outer diameter of a portion coming into contact with an exhaust valve hole  42 ) is set to be larger than the outer diameter of an intake valve head  34   b  (the outer diameter of a portion coming into contact with an intake valve hole  32 ). In other words, the mass of an exhaust valve head  44   b  is set to be larger than the mass of an intake valve head  34   b.    
     The exhaust-side cam shaft  48  includes an exhaust-side shaft  48   a  and a plurality of exhaust-side cams  48   b.    
     The exhaust-side shaft  48   a  is a cylindrical member. The exhaust-side shaft  48   a  is, with the axial direction thereof being parallel to the direction in which the three cylinders  12  are arranged, disposed at a position that causes the exhaust-side shaft  48   a  to overlap all the exhaust valve holes  42  as viewed in plan. Both ends of the exhaust-side shaft  48   a  are inserted into through holes (not illustrated) that are formed to the out frame  50 . 
     Each exhaust-side cam  48   b  is disposed on the outer peripheral surface of the exhaust-side shaft  48   a . In addition, each exhaust-side cam  48   b  is disposed at a position where the exhaust-side cam  48   b  overlaps an exhaust valve hole  42  as viewed in plan. Furthermore, each exhaust-side cam  48   b  is formed into an egg shape having a long radius and a short radius as viewed from the axial direction of the exhaust-side shaft  48   a.    
     In the first embodiment, the cylinder block  10  and the cylinder head  20  form three combustion chambers  22 , and each combustion chamber  22  is communicated with the exhaust pipe  8  by way of two exhaust passages  40 . For this reason, in the first embodiment, the exhaust-side cam shaft  48  includes six exhaust-side cams  48   b.    
     Pressing one end of each exhaust valve stem  44   a  by means of a long radius portion of an exhaust-side cam  48   b  causes the exhaust valve spring  44   c  to contract. The contraction of the exhaust valve spring  44   c  causes the exhaust valve head  44   b  to come off the exhaust valve hole  42  and to open an exhaust passage  40 . 
     Consequently, the exhaust valves  44  are displaced in response to the rotation of the exhaust-side cam shaft  48  to open and close the exhaust passages  40 . 
     In the first embodiment, since one combustion chamber  22  is communicated with the exhaust pipe  8  by way of two exhaust passages  40 , two exhaust valve holes  42  are formed with respect to one combustion chamber  22 . Therefore, in the first embodiment, with respect to one combustion chamber  22 , two exhaust valve guide holes  46  are formed. The two exhaust valve guide holes  46  are arranged along the direction in which the three cylinders  12  are arranged. 
     Each nozzle fitting hole  24  is a hole through which a fuel injection nozzle  16  is inserted into a combustion chambers  22 . The nozzle fitting hole  24  is formed by a through hole that penetrates the upper surface  20   a  of the cylinder head  20 . In  FIG. 4 , for purposes of description, the fuel injection nozzle  16  is not illustrated in cross section. 
     In the first embodiment, the cylinder head  20 , in conjunction with the cylinder block  10 , forms three combustion chambers  22 . For this reason, the cylinder head  20  includes three nozzle fitting holes  24 . 
     In addition, each nozzle fitting hole  24  is formed at such a position that the conditional expression (5) below holds.
 
INJ-EXTr&gt;INJ-INTr  (5)
 
     In the conditional expression (5), “INJ-EXTr” indicates a distance between the centers of a nozzle fitting hole  24  and an exhaust valve hole  42  that are formed to an identical combustion chamber  22 . In the conditional expression (5), “INJ-INTr” indicates a distance between the centers of the nozzle fitting hole  24  and an intake valve hole  32  that are formed to the identical combustion chamber  22 . 
     Therefore, in the first embodiment, the distance between a nozzle fitting hole  24  and an exhaust valve hole  42  is set to be longer than the distance between the nozzle fitting hole  24  and an intake valve hole  32 . 
     Each fuel injection nozzle  16  is coupled to the fuel tank  4 . 
     In addition, each fuel injection nozzle  16  is controlled by an ECU (Engine Control Unit) and the like to inject fuel (gasoline and the like) in the fuel tank  4  into a combustion chambers  22 . 
     Each plug fitting hole  26  is a hole through which a spark plug  18  is inserted into a combustion chamber  22 . The plug fitting hole  26  is formed penetrating the upper surface  20   a  of the cylinder head  20 . In  FIG. 4 , for purposes of description, the spark plug  18  is not illustrated in cross section. 
     In the first embodiment, the cylinder head  20 , in conjunction with the cylinder block  10 , forms three combustion chambers  22 . For this reason, the cylinder head  20  includes three plug fitting holes  26 . 
     Each plug fitting hole  26  is formed at such a position that the conditional expression (6) below holds.
 
SP-EXTr≥SP-INTr  (6)
 
     In the conditional expression (6), “SP-EXTr” indicates a distance between the centers of a plug fitting hole  26  and an exhaust valve hole  42  that are formed to an identical combustion chamber  22 . In the conditional expression (6), “SP-INTr” indicates a distance between the centers of the plug fitting hole  26  and an intake valve hole  32  that are formed to the identical combustion chamber  22 . 
     Therefore, in the first embodiment, the distance between a plug fitting hole  26  and an exhaust valve hole  42  is set to be longer than the distance between the plug fitting hole  26  and an intake valve hole  32 . 
     Each plug fitting hole  26  is disposed, as viewed from the axial direction of a cylinder  12 , at the center of a combustion chamber  22  into which a spark plug  18  is inserted therethrough. 
     Each spark plug  18  is controlled by the ECU and the like to generate a spark inside a combustion chamber  22 . 
     The out frame  50  is formed by combining four plate-shaped members into a frame shape and is disposed on the upper surface  20   a  of the cylinder head  20 . The out frame  50  is formed into a shape enclosing the circumference of the cylinder head  20  as viewed in plan and forms an outer frame of the cylinder head  20 . 
     The upper surface  20   a  of the cylinder head  20  is now divided into first regions E 1  and second regions E 2 , as illustrated in  FIG. 6 . 
     The first regions E 1  are regions that are arranged along the direction in which the plurality of cylinders  12  are arranged and overlap the combustion chambers  22  as viewed form the axial direction of a cylinder  12 . 
     The second regions E 2  are regions each of which is arranged between two first regions E 1  that are adjacent to each other. 
     In the first embodiment, the cylinder head  20 , in conjunction with the cylinder block  10 , forms three combustion chambers  22 . For this reason, the upper surface  20   a  of the cylinder head  20  is divided into three first regions E 1  and two second regions E 2 . 
     Each intake-side cam frame  52  is formed by a plate-shaped member and has side surfaces opposed to the upper surface  20   a  of the cylinder head  20  and the inner side surfaces of the out frame  50 , respectively. 
     In the first embodiment, a case where two intake-side cam frames  52  are formed on the upper surface  20   a  of the cylinder head  20  is described. 
     To each intake-side cam frame  52 , an intake-side frame through hole  52   a  is formed. 
     Each intake-side frame through hole  52   a  is a through hole that passes through an intake-side cam frame  52  in the thickness direction. 
     In addition, each intake-side frame through hole  52   a  is formed into a shape through which a portion of the intake-side shaft  38   a  at which no intake-side cam  38   b  is disposed can be inserted in a freely rotatable manner. The above configuration causes the inner wall surface of each intake-side frame through hole  52   a  to form an intake-side cam journal  56  that supports the intake-side cam shaft  38  in a rotatable manner. 
     In the first embodiment, a case where two intake-side cam frames  52  are formed on the upper surface  20   a  of the cylinder head  20  is described. Therefore, in the first embodiment, the cylinder head  20  includes two intake-side cam journals  56 . 
     In the first embodiment, each of the two intake-side cam frames  52  is disposed in one of the second regions E 2  of the upper surface  20   a  of the cylinder head  20 . 
     Therefore, in the first embodiment, each of the two intake-side cam journals  56  is disposed in one of the second regions E 2  of the upper surface  20   a  of the cylinder head  20 . 
     Each exhaust-side cam frame  54  is formed by a plate-shaped member and has side surfaces opposed to the upper surface  20   a  of the cylinder head  20  and the inner side surfaces of the out frame  50 , respectively. 
     The exhaust-side cam frames  54  are formed into the same shape as that of the intake-side cam frames  52 . 
     In the first embodiment, a case where three exhaust-side cam frames  54  are formed on the upper surface  20   a  of the cylinder head  20  is described. 
     To each exhaust-side cam frame  54 , an exhaust-side frame through hole  54   a  is formed. 
     Each exhaust-side frame through hole  54   a  is a through hole that passes through an exhaust-side cam frame  54  in the thickness direction. 
     In addition, each exhaust-side frame through hole  54   a  is formed into a shape through which a portion of the exhaust-side shaft  48   a  at which no exhaust-side cam  48   b  is disposed can be inserted in a freely rotatable manner. The above configuration causes the inner wall surface of each exhaust-side frame through hole  54   a  to form an exhaust-side cam journal  58  that supports the exhaust-side cam shaft  48  in a rotatable manner. 
     In the first embodiment, a case where three exhaust-side cam frames  54  are formed on the upper surface  20   a  of the cylinder head  20  is described. In other words, in the first embodiment, the cylinder head  20  includes three exhaust-side cam journals  58 . 
     Therefore, in the first embodiment, the intake-side cam frames  52  and the exhaust-side cam frames  54  are formed into the same shape, and, furthermore, one more exhaust-side cam frame  54  than the number of intake-side cam frames  52  is formed on the upper surface  20   a  of the cylinder head  20 . 
     In the first embodiment, each of the three exhaust-side cam frames  54  is disposed in one of the first regions E 1  of the upper surface  20   a  of the cylinder head  20 . 
     Therefore, in the first embodiment, each of the three exhaust-side cam journals  58  is disposed in one of the first regions E 1  of the upper surface  20   a  of the cylinder head  20 . 
     (Regarding Position of Intake-Side Cam Frame  52 ) 
     With reference to  FIGS. 1 to 6 , the reason for disposing the intake-side cam frames  52  in the second regions E 2  of the upper surface  20   a  of the cylinder head  20  will be described. 
     On an internal combustion engine with a head-block separation structure, each intake-side cam frame  52  is disposed, as viewed from the axial direction of a cylinder  12 , between two intake valve holes  32  that are formed for one combustion chamber  22  in the upper surface  20   a  of the cylinder head  20 . In other words, on an internal combustion engine with the head-block separation structure, the intake-side cam frames  52  are disposed in the first regions E 1  of the upper surface  20   a  of the cylinder head  20 . 
     The head-block separation structure is a structure in which the cylinder head  20  and the cylinder block  10  are formed by casting separately. The cylinder head  20  and the cylinder block  10  are subsequently joined to each other using cylinder head bolts. In  FIG. 2 , for purposes of description, a virtual securing position of a cylinder head bolt on an internal combustion engine with the head-block separation structure is indicated by assigning a reference symbol “VSP”. 
     The reason for disposing the intake-side cam frames  52  in the first regions E 1  of the upper surface  20   a  of the cylinder head  20  on the internal combustion engine with the head-block separation structure is as follows. 
     On the internal combustion engine with the head-block separation structure, a position where a cylinder head bolt is secured is, restricted by strength and the like that an internal combustion engine is required to have, located between intake valve holes  32  formed separately for combustion chambers  22  adjacent to each other in the upper surface  20   a  of the cylinder head  20 . 
     The internal combustion engine  1  of the first embodiment has a head-block integral structure and does not require a cylinder head bolt. Therefore, in the first embodiment, to the cylinder head  20  and the cylinder block  10 , neither opening nor space for insertion of a cylinder head bolt is formed. 
     For this reason, in the first embodiment, an intake-side cam frame  52  can be disposed at a position where a cylinder head bolt would be disposed if the internal combustion engine  1  had the head-block separation structure. 
     (Regarding Position of Nozzle Fitting Hole  24 ) 
     With reference to  FIGS. 1 to 5 , the reason for forming each nozzle fitting hole  24  at such a position that the conditional expression (5) holds will be described. 
     As described above, on an internal combustion engine with the head-block separation structure, each intake-side cam frame  52  is disposed, as viewed from the axial direction of a cylinder  12 , between two intake valve holes  32  that are formed for one combustion chamber  22  in the upper surface  20   a  of the cylinder head  20 . For this reason, on the internal combustion engine with the head-block separation structure, each nozzle fitting hole  24  is required to be formed on the top of a combustion chamber  22  (top injection structure). 
     This is because the intake-side cam frames  52  are disposed on the side of the combustion chambers  22  where the intake pipe  2  is located, which makes it difficult to secure spaces for disposing the fuel injection nozzles  16 . Similarly, this is because, on the side of the combustion chambers  22  where the exhaust pipe  8  is located, the exhaust-side cam frames  54  are disposed, which makes it difficult to secure spaces for disposing the fuel injection nozzles  16 . 
     On the internal combustion engine  1  of the first embodiment, as described above, the intake-side cam frames  52  can be disposed at positions where cylinder head bolts would be disposed if the internal combustion engine  1  had the head-block separation structure. 
     The above feature enables the internal combustion engine  1  of the first embodiment to secure spaces for disposing the fuel injection nozzles  16  on the side of the combustion chambers  22  where the intake pipe  2  is located. Therefore, in the first embodiment, it becomes possible to form each nozzle fitting hole  24  at such a position that the conditional expression (5) holds. 
     (Regarding Position of Plug Fitting Hole  26 ) 
     With reference to  FIGS. 1 to 6 , the reason for forming each plug fitting hole  26  at such a position that the conditional expression (6) holds will be described. 
     As described above, on an internal combustion engine with the head-block separation structure, each nozzle fitting hole  24  is formed on the top of a combustion chamber  22 . For this reason, on the internal combustion engine with the head-block separation structure, each plug fitting hole  26  is formed on the side of a combustion chamber  22  where the exhaust pipe  8  is located. This is because interference between a spark plug  18  and a fuel injection nozzle  16  is to be avoided. 
     On the internal combustion engine  1  of the first embodiment, as described above, spaces for disposing the fuel injection nozzles  16  can be secured on the side of the combustion chambers  22  where the intake pipe  2  is located. Therefore, in the first embodiment, it becomes possible to form each plug fitting hole  26  at such a position that the conditional expression (6) holds. 
     (Regarding Opening Area of Exhaust Valve Hole  42  and Opening Area of Intake Valve Hole  32 ) 
     With reference to  FIGS. 1 to 6 , the reason for setting the opening area of an exhaust valve holes  42  to be larger than the opening area of an intake valve holes  32  will be described. 
     As described above, on an internal combustion engine with the head-block separation structure, each intake-side cam frame  52  is disposed, as viewed from the axial direction of a cylinder  12 , between two intake valve holes  32  that are formed for one combustion chamber  22  in the upper surface  20   a  of the cylinder head  20 . In addition to the above, on the internal combustion engine with the head-block separation structure, each exhaust-side cam frame  54  is disposed, as viewed from the axial direction of a cylinder  12 , between two exhaust valve holes  42  that are formed for one combustion chamber  22  in the upper surface  20   a  of the cylinder head  20 . 
     This is because a position where a cylinder head bolt is secured is restricted to, in the upper surface  20   a  of the cylinder head  20 , a position between pairs of two exhaust valve holes  42  formed for one combustion chamber  22  because of required strength and the like. 
     On the internal combustion engine  1  of the first embodiment, as described above, spaces for disposing the fuel injection nozzles  16  can be secured on the side of the combustion chambers  22  where the intake pipe  2  is located. In addition to the above, on the internal combustion engine  1  of the first embodiment, each plug fitting holes  26  can be formed at such a position that the conditional expression (5) holds. In the first embodiment, the above feature enables a space margin to be secured on the side of the combustion chambers  22  where the exhaust pipe  8  is located more easily than on the side of the combustion chambers  22  where the intake pipe  2  is located. 
     Therefore, in the first embodiment, it becomes possible to set the opening area of an exhaust valve holes  42  to be larger than the opening area of an intake valve holes  32 . 
     (Operation) 
     With reference to  FIGS. 1 to 6 , an example of an operation performed using the internal combustion engine  1  of the first embodiment will be described. 
     When the internal combustion engine  1  is operating, such as while a vehicle is in use, air taken in from the intake pipe  2  and fuel injected through the nozzle fitting holes  24  into the combustion chambers  22  are mixed in the combustion chambers  22 . Air-fuel mixtures mixed in the combustion chambers  22  are ignited by sparks generated by the spark plugs  18  and are burned in the combustion chambers  22 . The above operation causes energy generated by combustion of the air-fuel mixtures to be transmitted to the drive unit  6  and gas after combustion to be exhausted to the outside via the exhaust pipe  8 . 
     In the first embodiment, the charger CH is connected to the intake pipe  2 . Thus, when an amount of air taken in from the intake pipe  2  into the combustion chambers  22  (intake amount) is to be increased in acceleration of the vehicle and the like, the intake amount is forcibly increased by the charger CH. The above operation causes filling efficiency of air supplied into the combustion chambers  22  to be increased. 
     Regarding the internal combustion engine  1  of the first embodiment, the opening area of an exhaust valve holes  42  is larger than the opening area of an intake valve holes  32 . 
     For this reason, it becomes possible to set an amount of air (exhaust) that is able to pass the exhaust valve holes  42  per unit time to be larger than an amount of air (intake) that is able to pass the intake valve holes  32  per unit time. 
     Even when the intake amount is increased by the charger CH, the above configuration enables a reduction in a ratio of the exhaust amount to the intake amount to be suppressed and an increase in the intake amount by the charger CH to be offset. 
     Therefore, in the first embodiment, it becomes possible to, with respect to the internal combustion engine  1 , suppress a reduction in exhaust efficiency to suppress a reduction in combustion efficiency. 
     It should be noted that the first embodiment mentioned above is one example of the present invention, the present invention is not limited to the first embodiment mentioned above, and, even when the present invention may be carried out in modes other than the embodiment, depending on designs, various changes may be made to the present invention within a scope not departing from the technical idea of the present invention. 
     (Advantageous Effects of First Embodiment) 
     The internal combustion engine  1  according to the first embodiment enables advantageous effects described below to be attained. 
     (1) The opening area of an exhaust valve holes  42  is set to be larger than the opening area of an intake valve holes  32 . 
     This feature enables an exhaust amount per unit time to be set to be greater than an intake amount per unit time. 
     As a consequence, even when the intake amount is increased by the charger CH, it becomes possible to suppress a reduction in a ratio of the exhaust amount to the intake amount to offset an increase in the intake amount by the charger CH. 
     The above configuration enables the internal combustion engine  1  to suppress a reduction in exhaust efficiency to suppress a reduction in combustion efficiency. For this reason, it becomes possible to improve torque and output power that the internal combustion engine  1  generates. 
     (2) The stroke St of each piston  14  is set to be not less than the bore inner diameter BID of each cylinder  12 . 
     As a consequence, compared with an internal combustion engine  1  having the same exhaust amount and including cylinders  12  each of which has a stroke St less than a bore inner diameter BID, it becomes possible to maintain speed-up of the pistons  14  and, in conjunction therewith, to improve exhaust efficiency. 
     (3) The distance INJ-EXTr between a nozzle fitting hole  24  and an exhaust valve hole  42  is set to be longer than the distance INJ-INTr between the nozzle fitting hole  24  and an intake valve hole  32 . 
     This feature enables the positions of the nozzle fitting holes  24  to be located on the intake side of the internal combustion engine  1  rather than the exhaust side. The above configuration enables the fuel injection nozzles  16  to be disposed on the intake side where the temperature is lower than the exhaust side. 
     As a consequence, it becomes possible to reduce a deposit (carbon deposit) produced on the fuel injection nozzles  16 . 
     (4) The distance SP-EXTr between a plug fitting hole  26  and an exhaust valve hole  42  is set to be not shorter than the distance SP-INTr between the plug fitting hole  26  and an intake valve hole  32 . 
     As a consequence, it becomes possible to locate the positions of the plug fitting holes  26  at positions located on the intake side between the exhaust side and the intake side of the internal combustion engine  1 . In other words, the degree of freedom in designing positions where the spark plugs  18  are to be disposed has been improved. 
     (5) Each plug fitting hole  26  is disposed at the center of a combustion chamber  22 . 
     This feature enables sparks that the spark plugs  18  generate to be generated at the centers of the combustion chambers  22 . The above configuration enables combustion performance of air-fuel mixtures in the combustion chambers  22  to be improved. 
     As a consequence, it becomes possible to improve torque and output power that the internal combustion engine  1  generates. 
     (6) The total value of the opening areas of a plurality of exhaust valve holes  42  opening to one combustion chamber  22  is set to be larger than the total value of the opening areas of a plurality of intake valve holes  32  opening to the one combustion chamber  22 . 
     This feature enables, even when the intake amount is increased by the charger CH, a reduction in a ratio of the exhaust amount to the intake amount to be suppressed and an increase in the intake amount by the charger CH to be offset. 
     As a consequence, with respect to the internal combustion engine  1 , it becomes possible to suppress a reduction in exhaust efficiency to suppress a reduction in combustion efficiency. For this reason, it becomes possible to improve torque and output power that the internal combustion engine  1  generates. 
     (7) To the cylinder block  10 , a plurality of cylinders  12  that are arranged with the stroke directions of the pistons  14  directed in parallel with one another are formed. In addition, the cylinder head  20  and the cylinder block  10  that are formed into one body by casting form a plurality of combustion chambers  22  that are arranged with the stroke directions of the pistons  14  directed in parallel with one another. 
     Furthermore, the upper surface  20   a  of the cylinder head  20  is divided, along the direction in which the plurality of cylinders  12  are arrange, into the first regions E 1  that overlap the combustion chambers  22  as viewed from the axial direction of a cylinder  12  and the second regions E 2  each of which is arranged between two first regions E 1  adjacent to each other. In addition to the above, the intake-side cam journals  56  are disposed in the second regions E 2  of the upper surface  20   a  of the cylinder head  20 . 
     The above configuration enables, without increasing the distance between the intake-side cam frames  52 , the positions of the intake-side cam journals  56  to be shifted from, as viewed from the axial direction of a cylinder  12 , positions each between two intake valve holes  32  formed for one combustion chamber  22 . 
     As a consequence, it becomes possible to improve a degree of freedom in designing the cylinder head  20 , such as determining layouts of the nozzle fitting holes  24  and the plug fitting holes  26  and shapes, dimensions, and the like of the exhaust valve holes  42  and the intake valve holes  32 . 
     In addition, positions where the intake-side cam journals  56  are disposed are not influenced by positions where cylinder head bolts would be secured if the internal combustion engine  1  had the head-block separation structure. 
     Since the above configuration enables the degree of freedom in designing the cylinder head  20  and the cylinder block  10  to be improved, it becomes possible to improve the degree of freedom in designing the internal combustion engine  1 . 
     (8) The intake-side cam journals  56  are disposed in the second regions E 2  of the upper surface  20   a  of the cylinder head  20 . 
     This feature enables, without increasing the distance between the intake-side cam frames  52 , the positions of the intake-side cam journals  56  to be shifted from, as viewed from the axial direction of a cylinder  12 , positions each between two intake valve holes  32  formed for one combustion chamber  22 . 
     As a consequence, compared with an internal combustion engine  1  with a configuration in which the positions of the intake-side cam journals  56  are shifted by increasing the distance between the intake-side cam frames  52 , it becomes possible to suppress an increase in the size and weight of the internal combustion engine  1 . 
     (9) The intake-side cam journals  56  are disposed in the second regions E 2  of the upper surface  20   a  of the cylinder head  20 . 
     This feature enables distances between the intake-side cam frames  52  and the plug fitting holes  26  to be increased compared with a case in which each intake-side cam journal  56  is disposed between two intake valve holes  32  formed for one combustion chamber  22 . 
     As a consequence, compared with a case in which each intake-side cam journal  56  is disposed between two intake valve holes  32  formed for one combustion chamber  22 , it becomes possible to suppress deformations of the intake-side cam journals  56  due to the influence from heat generated by the spark plugs  18 . 
     (10) The masses of the exhaust valve heads  44   b  are set to be larger than the masses of the intake valve heads  34   b.    
     The intake-side cam frames  52  and the exhaust-side cam frames  54  are formed into the same shape. In addition to the above, the exhaust-side cam shaft  48  is supported in a rotatable manner by more exhaust-side cam journals  58  than intake-side cam journals  56 . 
     These features enable the exhaust-side cam shaft  48  that, in response to rotation thereof, displaces the exhaust valves  44  with larger masses than the intake valves  34  to be supported in a rotatable manner by more exhaust-side cam journals  58  than intake-side cam journals  56 . 
     As a consequence, the exhaust-side cam shaft  48  that is required to have more strength than the intake-side cam shaft  38  is supported by more exhaust-side cam journals  58  than intake-side cam journals  56 , and which enables a load imposed on the exhaust-side cam journals  58  to be distributed. The above configuration enables durability of the exhaust-side cam frames  54  to be increased. In addition, it becomes possible to improve stability in supporting the exhaust-side cam shaft  48 . 
     (Variations) 
     (1) Although, in the first embodiment, the intake-side cam journals  56  were disposed in the second regions E 2  of the upper surface  20   a  of the cylinder head  20 , the present invention is not limited to the configuration. 
     In other words, as illustrated in  FIG. 7 , the exhaust-side cam journals  58  may be disposed in the second regions E 2  of the upper surface  20   a  of the cylinder head  20 . 
     In this case, it becomes possible to, without increasing the distances between the exhaust-side cam frames  54 , shift the positions of the exhaust-side cam journals  58  from, as viewed from the axial direction of a cylinder  12 , positions each between two exhaust valve holes  42  formed for one combustion chamber  22 . 
     The above configuration enables the degree of freedom in designing the cylinder head  20 , such as determining layouts of the nozzle fitting holes  24  and the plug fitting holes  26  and shapes, dimensions, and the like of the exhaust valve holes  42  and the intake valve holes  32 , to be improved. 
     Therefore, in the present invention, positions where the exhaust-side cam journals  58  are disposed are not influenced by positions where cylinder head bolts would be secured if the internal combustion engine  1  had the head-block separation structure. 
     Since the above configuration enables the degree of freedom in designing the cylinder head  20  and the cylinder block  10  to be improved, it becomes possible to improve the degree of freedom in designing the internal combustion engine  1 . 
     When the configuration of the internal combustion engine  1  is the configuration illustrated in  FIG. 7 , the inner diameter EXHvdi of the exhaust valve holes  42  may be set to be less than the inner diameter INTvdi of the intake valve holes  32 , differing from the first embodiment. 
     (2) Although, in the first embodiment, the intake-side cam journals  56  were disposed in the second regions E 2  of the upper surface  20   a  of the cylinder head  20 , the present invention is not limited to the configuration. 
     In other words, as illustrated in  FIG. 8 , the intake-side cam journals  56  and the exhaust-side cam journals  58  may be disposed in the second regions E 2  of the upper surface  20   a  of the cylinder head  20 . 
     In this case, it becomes possible to, without increasing the distance between the intake-side cam frames  52 , shift the positions of the intake-side cam journals  56  from, as viewed from the axial direction of a cylinder  12 , positions each between two intake valve holes  32  formed for one combustion chamber  22 . In addition to the above, it becomes possible to, without increasing the distances between the exhaust-side cam frames  54 , shift the positions of the exhaust-side cam journals  58  from, as viewed from the axial direction of a cylinder  12 , positions each between two exhaust valve holes  42  formed for one combustion chamber  22 . 
     The above configuration enables the degree of freedom in designing the cylinder head  20 , such as determining layouts of the nozzle fitting holes  24  and the plug fitting holes  26  and shapes, dimensions, and the like of the exhaust valve holes  42  and the intake valve holes  32 , to be improved. 
     Therefore, in the present invention, positions where the intake-side cam journals  56  and the exhaust-side cam journals  58  are disposed are not influenced by positions where cylinder head bolts would be secured if the internal combustion engine  1  had the head-block separation structure. 
     Since the above configuration enables the degree of freedom in designing the cylinder head  20  and the cylinder block  10  to be improved, it becomes possible to improve the degree of freedom in designing the internal combustion engine  1 . 
     When the configuration of the internal combustion engine  1  is the configuration illustrated in  FIG. 8 , the inner diameter EXHvdi of the exhaust valve holes  42  and the inner diameter INTvdi of the intake valve holes  32  may be set at the same value, differing from the first embodiment. 
     (3) Although, in the first embodiment, the configuration of the internal combustion engine  1  was a configuration in which air-fuel mixtures in the combustion chambers  22  are ignited by sparks generated by the spark plugs  18  (gasoline engine), the present invention is not limited to the configuration. 
     In other words, the configuration of the internal combustion engine  1  may be a configuration in which air-fuel mixtures in the combustion chambers  22  are ignited without using a spark plug  18  (diesel engine). In this case, the configuration of the internal combustion engine  1  becomes, for example, a configuration in which the cylinder head  20  does not include any plug fitting hole, as illustrated in  FIG. 9 . 
     (4) Although, in the first embodiment, the configuration of the internal combustion engine  1  was an internal combustion engine with three cylinders arranged in a straight line (straight 3-cylinder engine), the present invention is not limited to the configuration. 
     In other words, the internal combustion engine  1  may be configured as an internal combustion engine of V-type (V-type engine) or an internal combustion engine of horizontally opposed type (horizontally opposed engine). 
     (5) Although, in the first embodiment, the configuration of the intake pipe  2  was a configuration in which the charger CH is connected thereto, the present invention is not limited to the configuration. 
     In other words, the configuration of the intake pipe  2  may be a configuration in which no charger is connected (natural intake: Natural Aspiration or Normal Aspiration). 
     REFERENCE SIGNS LIST 
     
         
           1  Internal combustion engine 
           2  Intake pipe 
           4  Fuel tank 
           6  Drive unit 
           8  Exhaust pipe 
           10  Cylinder block 
           12  Cylinder 
           14  Piston 
           16  Fuel injection nozzle 
           18  Spark plug 
           20  Cylinder head 
           20   a  Upper surface of the cylinder head 
           22  Combustion chamber 
           24  Nozzle fitting hole 
           26  Plug fitting hole 
           30  Intake passage 
           32  Intake valve hole 
           34  Intake valve 
           34   a  Intake valve stem 
           34   b  Intake valve head 
           34   c  Intake valve spring 
           36  Intake valve guide hole 
           38  Intake-side cam shaft 
           38   a  Intake-side shaft 
           38   b  Intake-side cam 
           40  Exhaust passage 
           42  Exhaust valve hole 
           44  Exhaust valve 
           44   a  Exhaust valve stem 
           44   b  Exhaust valve head 
           44   c  Exhaust valve spring 
           46  Exhaust valve guide hole 
           48  Exhaust-side cam shaft 
           48   a  Exhaust-side shaft 
           48   b  Exhaust-side cam 
           50  Out frame 
           52  Intake-side cam frame 
           52   a  Intake-side frame through hole 
           54  Exhaust-side cam frame 
           54   a  Exhaust-side frame through hole 
           56  Intake-side cam journal 
           58  Exhaust-side cam journal 
         CH Charger 
         St Stroke of piston 
         BID Bore inner diameter of a cylinder 
         EXHvdi Inside diameter of an exhaust valve hole 
         INTvdi Inside diameter of an intake valve hole 
         INJ-EXTr Distance between the center of a nozzle fitting hole and the center of an exhaust valve hole 
         INJ-INTr Distance between the center of a nozzle fitting hole and the center of an intake valve hole 
         SP-EXTr Distance between the center of a plug fitting hole and the center of an exhaust valve hole 
         SP-INTr Distance between the center of a plug fitting hole and the center of an intake valve hole 
         E 1  First region 
         E 2  Second region 
         VSP Virtual securing position of a cylinder head bolt