Patent Publication Number: US-6705269-B2

Title: Four-cycle engine

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2000-349951 filed in Japan on Nov. 16, 2000, and Patent Application No. 2001-333344 filed in Japan on Oct. 30, 2001, the entirety of each of which is herein incorporated by reference. This nonprovisional application further claims priority under 35 U.S.C. § 119(e) on U.S. Provisional Application 60/248,549, filed on Nov. 16, 2000, the entirety of which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a four-cycle engine in which a pair of intake valve ports is openable/closable by respective intake valves, and a pair of exhaust valve ports is openable/closable by respective exhaust valves. The pair of intake valve ports and the pair of exhaust valve ports are provided in a cylinder head of the engine such that they are positioned on both sides of a first virtual plane which contains the axial line of a cylinder bore and passes through an approximately central portion of a combustion chamber. Furthermore, a pair of ignition plugs are mounted in the cylinder head. 
     2. Description of Background Art 
     A four-cycle engine of the above type has been known, for example, from Japanese Utility Model Publication No. Hei 3-6826. 
     In the above-described four-cycle engine, a pair of ignition plugs is disposed in an outer peripheral portion of a combustion chamber. Alternatively, one of a pair of ignition plugs is disposed at the center of a combustion chamber and the other is disposed in an outer peripheral portion of the combustion chamber. Accordingly, in the case where both of the ignition plugs are disposed in the outer peripheral portion of the combustion chamber, a flame propagation distance in the combustion chamber due to accidental firing of either of the ignition plugs increases. Therefore, a flame propagation condition is degraded. In the case where one of the ignition plugs is disposed at the center of the combustion chamber, the flame propagation distance in the combustion chamber due to accidental firing of the ignition plug also increases. Therefore, the flame propagation condition is also degraded. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a four-cycle engine capable of minimizing the degradation of the flame propagation condition, even if accidental firing of either of a pair of ignition plugs occurs. 
     To achieve the above object, according to a first aspect of the present invention, a four-cycle engine includes a pair of intake valve ports openable/closable by respective intake valves and a pair of exhaust valve ports openable/closable by respective exhaust valves. The pair of intake valves and the pair of exhaust valves are provided in a cylinder head such that they are positioned on both sides of a first virtual plane which contains the axial line of a cylinder bore and passes through an approximately central portion of a combustion chamber. Furthermore, a pair of ignition plugs are mounted in the cylinder head. The pair of ignition plugs are disposed substantially symmetrically with respect to a second virtual plane which passes through the center of the combustion chamber and is perpendicular to the first virtual plane. The pair of ignition plugs is mounted in the cylinder head such that they have axial lines extending substantially parallel to the first virtual plane. The pair of ignition plugs is tilted with a distance therebetween becoming smaller toward the combustion chamber. Furthermore, end portions of the ignition plugs, which project into the combustion chamber, are disposed in a region surrounded by the intake valve ports and the exhaust valve ports. 
     With this configuration, the ends of the pair of ignition plugs which project into the combustion chamber, are disposed in proximity to an approximately central portion in the combustion chamber. Accordingly, it is possible to ideally propagate flame in the combustion chamber. Furthermore, even if accidental firing of either of the ignition plugs occurs, the other ignition plug is located at the approximately central portion of the combustion chamber. Accordingly, it is possible to minimize the deterioration of a flame propagation condition. Both of the ignition plugs are disposed, as seen from the direction perpendicular to the first virtual plane, in an approximately V-shape opened in the direction opposite to the combustion chamber. Accordingly, both of the ignition plugs can be easily mounted in the cylinder head with the ends of the ignition plugs, which project into the combustion chamber, disposed in proximity to an approximately central portion of the combustion chamber. Both of the ignition plugs are collectively disposed in the vicinity of the central portion of the combustion chamber. Accordingly, it is possible to enhance the degree of freedom of the shape of a water jacket on the cylinder head side and the degree of freedom of disposition of fastening bolts for fastening the cylinder head to the cylinder block. Accordingly, the sealing performance between the cylinder head and the cylinder block as well as the cooling performance can be improved. 
     According to a second aspect of the present invention, in addition to the configuration of the first aspect of the present invention, a pair of rocker shafts has axial lines extending in parallel to the first virtual plane. The pair of rocker shafts is mounted to the cylinder head such that it is positioned on both sides of the pair of ignition plugs. An intake side rocker arm corresponding to both of the intake valves is rockably supported by the rocker shaft. An exhaust side rocker arm corresponding to both of the exhaust valves is rockably supported by the rocker shaft. With this configuration, it is possible to set the width of the cylinder head at a relatively small value in the direction along the second virtual plane. Accordingly, the engine can be made more compact. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a vertical sectional rear view of an engine; 
     FIG. 2 is an enlarged sectional view along line  2 — 2  of FIG. 1; 
     FIG. 3 is an enlarged sectional view along line  3 — 3  of FIG. 2; 
     FIG. 4 is an enlarged sectional view along line  4 — 4  of FIG. 2; 
     FIG. 5 is a sectional view along line  5 — 5  of FIG. 1 showing a cylinder head; 
     FIG. 6 is a sectional view along line  6 — 6  of FIG. 5 showing the cylinder head in a state in which a head cover is removed; 
     FIG. 7 is a view along an arrow  7  of FIG. 6; 
     FIG. 8 is a sectional view along line  8 — 8  of FIG. 7; 
     FIG. 9 is a sectional view along line  9 — 9  of FIG. 1; 
     FIG. 10 is an exploded perspective view of a structure for connecting a pull rod to a cam follower; 
     FIG. 11 is a side elevational view showing an engine installed in an airplane; 
     FIG. 12 is a sectional view along line  12 — 12  of FIG. 11; 
     FIG. 13 is an enlarged sectional view along line  13 — 13  of FIG. 11; and 
     FIG. 14 is an exploded perspective view, corresponding to FIG. 10, showing another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIGS. 1 to  10  show one embodiment in which the present invention is applied to a four-cycle horizontally opposed type two-cylinder engine, wherein FIG. 1 is a vertical sectional rear view of the engine; FIG. 2 is an enlarged sectional view along line  2 — 2  of FIG. 1; FIG. 3 is an enlarged sectional view along line  3 — 3  of FIG. 2; FIG. 4 is an enlarged sectional view along line  4 — 4  of FIG. 2; FIG. 5 is a sectional view along line  5 — 5  of FIG. 1 showing a cylinder head; FIG. 6 is a sectional view along line  6 — 6  of FIG. 5 showing the cylinder head in a state in which a head cover is removed; FIG. 7 is a view along an arrow  7  of FIG. 6; FIG. 8 is a sectional view along line  8 — 8  of FIG. 7; FIG. 9 is a sectional view along line  9 — 9  of FIG. 1; FIG. 10 is an exploded perspective view of a structure for connecting a pull rod to a cam follower; FIG. 11 is a side elevational view showing an engine installed in an airplane; FIG. 12 is a sectional view along line  12 — 12  of FIG. 11; and FIG. 13 is an enlarged sectional view along line  13 — 13  of FIG.  11 . 
     Referring first to FIG. 1, a four-cycle horizontally opposed type engine E is shown. The engine is mountable on an automobile, a motorcycle, aircraft, and the like. A main body  11  of the engine E includes a left engine block  12 L disposed on the left side as seen from the rear side of the engine E and a right engine block  12 R disposed on the right side as seen from the rear side of the engine E. 
     The left engine block  12 L includes a left cylinder block  13 L, a left crankcase  14 L formed integrally with the left cylinder block  13 L, and a left cylinder head  15 L connected to the side, opposite to the left crankcase  14 L, of the left cylinder block  13 L. Similarly, the right engine block  12 R includes a right cylinder block  13 R, a right crankcase  14 R formed integrally with the right cylinder block  13 R, and a right cylinder head  15 R connected to the side, opposite to the right crankcase  14 R, of the right cylinder block  13 R. 
     The cylinder block  13 L (or  13 R) has a cylinder bore  16 L (or  16 R). A piston  18 L (or  18 R) is slidably fitted in the cylinder bore  16 L (or  16 R) in such a manner as to form a combustion chamber  17 L (or  17 R) between the cylinder bore  16 L (or  16 R) and the cylinder head  15 L (or  15 R). 
     Both of the engine blocks  12 L and  12 R are oppositely disposed with axial lines of the cylinder bores  16 L and  16 R arranged substantially in the horizontal direction. The left and right crankcases  14 L and  14 R are fastened to each other to form a crankcase  19  in cooperation with each other. A crankshaft  21  connected to the pistons  18 L and  18 R via connecting rods  20 L and  20 R is rotatably supported between the left and right crankcases  14 L and  14 R. 
     Referring to FIG. 2, the crankcase  19  is provided with a front journal wall  22 F, an intermediate journal wall  22 M, and a rear journal wall  22 R, which are spaced from each other in the longitudinal direction. The journal walls  22 F,  22 M and  22 R rotatably support three portions spaced from each other in the axial direction of the crankshaft  21 . The crankshaft  21  is housed in a crank chamber  24  formed in the crankcase  19 . A partition wall  25  defining the bottom of the crank chamber  24  is provided on the inner wall of the crankcase  19 . 
     A rear end portion (left end portion in FIG. 2) of the crankshaft  21  projects rearwardly from the rear journal wall  22 R. A rotor  27  of a generator  26  is coaxially connected to the rear end portion of the crankshaft  21 . A stator  28  of the generator  26  is disposed behind the rear journal wall  22 R and is fixedly supported by a supporting plate  29  fixed to the crankcase  19 . A cover  30  for covering the generator  26  is fastened to a rear portion of the crankcase  19 . 
     A drive gear  31  is fixed to the crankshaft  21  at a position between the rear journal wall  22 R and the supporting plate  29 . A rotating shaft  33 , to which a first intermediate gear  32  meshing with the drive gear  31 , is rotatably supported by the rear journal wall  22 R and the supporting plate  29 . A second intermediate gear  34 , which is integrally provided on the rotating shaft  33 , meshes with a gear  35  provided on a camshaft  36 . The camshaft  36  having an axial line parallel to the crankshaft  21  is rotatably supported by the crankcase  19  at a position under the partition wall  25 . 
     In this way, power is transmitted from the crankshaft  21  to the camshaft  36 , at a reduction ratio of 1/2, via the drive gear  31 , first intermediate gear  32 , second intermediate gear  34 , and gear  35 . 
     A water pump  37  is mounted to the cover  30 . A pump shaft  38  of the water pump  37  is coaxially connected to the rotating shaft  33  such that it does not rotate relative to the rotating shaft  33 , whereby rotational power is transmitted from the crankshaft  21  to the water pump  37 . 
     Referring to FIGS. 3 and 4, an oil pan  42  is connected to a lower portion of the crankcase  19  such that an oil reservoir chamber  43  is formed under the camshaft  36 . An oil pump  44  is configured as a trochoide pump and is housed in the oil pan  42 . 
     A pump housing  45  of the oil pump  44  is formed by connecting a pair of housing halves  46  and  47  to each other. A drive shaft  48  having an axial line parallel to the crankshaft  21  and the camshaft  36  is rotatably supported by the housing half  46 . The drive shaft  48  is connected to a rotor  49  disposed between the housing halves  46  and  47 . 
     A partition wall  46   a  is integrally provided on the housing half  46 , whereby a power transmission chamber  50  partitioned from the oil reservoir chamber  43  formed in the oil pan  42  is formed between the partition wall  46   a  and a side wall of the oil pan  42 . A gear  51  meshing with the gear  35  of the camshaft  36 , which is rotated by power transmitted from the crankshaft  21 , is fixed to an end portion, on the power transmission chamber  50  side, of the drive shaft  48 . In this way, rotational power is transmitted from the crankshaft  21  to the oil pump  44 . 
     The partition wall  46   a  has an approximately U-shaped transverse cross-section, which is opened upwardly. The upper end of the partition wall  46   a  is located at a position higher than the oil level L of oil in the oil reservoir chamber  43 , so that oil does not flow from the oil reservoir chamber  43  side to the power transmission chamber  50  side. On the other hand, oil flows from the crank chamber  24  side into the power transmission chamber  50  via a gear train disposed in the power transmission route from the crankshaft  21  to the gear  51 . The oil in the power transmission chamber  50  splashes to the oil reservoir chamber  43  side across the upper end of the partition wall  46   a  by rotation of the gear  51 . 
     A pair of boss-like mounting portions  52  are integrally provided on a portion, corresponding to the housing half  46 , of the bottom of the oil pan  42  in such a manner as to project therefrom. The housing half  46  is removably mounted on the mounting portions  52  with bolts  53 . Similarly, a pair of boss-like mounting portions  52  are integrally provided on a portion, corresponding to the housing half  47 , of the bottom of the oil pan  42  in such a manner as to project therefrom. The housing half  47  is removably mounted on the mounting portions  52  with bolts  53 . Specifically, the pump housing  45  is removably mounted on the mounting portions  52  provided on the bottom of the oil pan  42 . 
     An inlet  54  is provided in the housing half  46  of the pump housing  45 . An oil strainer  55  connected to the inlet  54  is fixedly held between the housing half  46  and the oil pan  42 . To be more specific, an upper portion of the oil strainer  55  is inserted from below in a lower portion of the housing half  46  such that it is continuous with the inlet  54 . A lower peripheral edge of the oil strainer  55  is received on a receiving portion  56  provided on the bottom of the oil pan  42 . 
     An outlet  57  is provided in the housing half  47  of the pump housing  45 . A relief valve  58  connected to the outlet  57  is fixedly held between the housing half  47  and the oil pan  42 , while being kept in a posture parallel to that of the oil strainer  55 . To be more specific, an upper portion of the relief valve  58  is inserted from below in a lower portion of the housing half  47  such that it is continuous with the outlet  57 . A lower end of the relief valve  58  is received by a raised portion  59  provided on the bottom of the oil pan  42 . 
     An oil passage  61  in communication with the outlet  57  is provided in the housing half  47 . An oil passage  62  in communication with the oil passage  61  is provided in the lower portion of the oil pan  42  when the pump housing  45  is mounted to the oil pan  42 . An oil filter  63  connected to the oil passage  62  is removably mounted to an outer surface of a side wall of the oil pan  42 . An oil passage  64  for oil cleaned by passing through the oil filter  63  is provided in the oil pan  42  and the crankcase  19 . The oil passage  64  is in communication with a main gallery  65  provided in the crankcase  19 . 
     A front portion of the crankshaft  21  is formed into a cylindrical hollow shape for reducing the weight of the crankshaft  21 . A cylindrical spacer  66 , having an annular chamber  67  formed between the inner surface of the crankshaft  21  and the outer surface of the spacer  66 , is fitted in the cylindrical hollow portion of the crankshaft  21 . The annular chamber  67  extends at least between portions corresponding to the front and intermediate journal walls  22 F and  22 M of the crankcase  19 . Both axial ends of the annular chamber  67  are sealed in a fluid tight manner by mounting seal members to both ends of the spacer  66  or press-fitting both the ends of the spacer  66  in the crankshaft  21 . 
     An oil passage  68  for supplying oil to a portion to be lubricated between the intermediate journal wall  22 M and the crankshaft  21  is provided in the crankcase  19  in communication with the main gallery  65 . The crankshaft  21  has a passage hole  69  for supplying oil from the portion to be lubricated between the intermediate journal wall  22 M and the crankshaft  21  to the annular chamber  67 . The crankshaft  21  also has a passage hole  70  for supplying oil from the annular chamber  67  to a portion to be lubricated between the front journal wall  22 F and the crankshaft  21 . 
     The crankshaft  21  integrally includes a crank pin  21 L connected to the connecting rod  20 L on the left engine block  12 L side and a crank pin  21 R connected to the connecting rod  20 R on the right engine block  12 R side. An oil passage  71  for supplying oil from the annular chamber  67  to a portion to be lubricated between the connecting rod  20 L and the crank pin  21 L is provided in the crankshaft  21 . Oil is supplied from the main gallery  65  to a portion to be lubricated between the rear journal wall  22 R and the crankshaft  21 . An oil passage  72  for supplying oil from the portion to be lubricated between the rear journal wall  22 R and the crankshaft  21  to a portion to be lubricated between the connecting rod  20 R and the crank pin  21 R is provided in the crankshaft  21 . 
     In addition, in order to supply oil to the portion to be lubricated between the connecting rod  20 L and the crank pin  21 L, the entire cylindrical hollow portion of the crankshaft  21  can be used as an oil passage. However, in this case, since the volume of the oil passage may become excessively large, the hydraulic pressure rising time upon start-up of the engine E may be retarded and a residual amount of oil upon oil exchange may be increased. According to this embodiment, the annular chamber  67  formed between the cylindrical hollow portion of the crankshaft  21  and the spacer  66  is, as described above, used as the oil passage. Accordingly, the passage volume can be set to a suitable value, to prevent the retardation of the hydraulic pressure rising time and to prevent the increase in residual amount of oil upon oil exchange. Also, since the inside diameter of the cylindrical hollow portion of the crankshaft  21  can be set to a relatively large value without increasing the passage volume, it is not required to increase the accuracy of the penetrating depths of the passage holes  69  and  70 . Furthermore, by making the spacer  66  from a material lighter in weight than that of the crankshaft  21 , the entire crankshaft  21  can be made lighter in weight. 
     The oil, which has lubricated the portion to be lubricated between the connecting rod  20 L and the crank pin  21 L and the portion to be lubricated between the connecting rod  20 R and the crank pin  21 R, is released into the crank chamber  24  and is accumulated on the partition wall  25 . Oil through-holes  73  for directing the oil accumulated on the partition wall  25  to portions of the crankcase  19  for supporting both ends of the camshaft  36  are provided in the partition wall  25 . As a result, the oil is supplied to portions to be lubricated between the camshaft  36  and the crankcase  19 . 
     Referring to FIGS. 5 and 6, a pair of intake valve ports  76  and a pair of exhaust valve ports  77  are provided in the cylinder head  15 R of the right engine block  12 R in such a manner as to be positioned on both sides of a first virtual plane  78  containing the axial line of the cylinder bore  16 R and passing through the center of the combustion chamber  17 R. The pair of intake valve ports  76  and the pair of exhaust valve ports  77  face toward the combustion chamber  17 R. The first virtual plane  78  crosses an axial line C of the crankshaft  21  at an angle on the projection plane perpendicular to the axial line of the cylinder bore  16 R (parallel to the paper in FIG.  5 ). 
     Referring to FIGS. 7 and 8, a pair of ignition plugs  80  is mounted in the cylinder head  15 R in such a manner that the end portions thereof project into the combustion chamber  17 R. The axial lines of the ignition plugs  80  pass through the center of the combustion chamber  17 R and are disposed on the first virtual plane  78 . 
     The ignition plugs  80  are disposed symmetrically with respect to the second virtual plane  79  perpendicular to the first virtual plane  78 , and are mounted in the cylinder head  15 R such that they are tilted with a distance therebetween becoming smaller towards the combustion chamber  17 R. The end portions, projecting in the combustion chamber  17 R, of both the ignition plugs  80  are disposed in a region surrounded by both of the intake valve ports  76  and both of the exhaust valve ports  77 . 
     Both of the ignition plugs  80  are connected to an ignition circuit (not shown) and are usually operated in synchronization with each other by the ignition circuit. 
     Inner ends of plug insertion cylinders  81 , in which the ignition plugs  80  are to be inserted, are fixedly fitted in the cylinder head  15 R. Outer ends of the plug insertion cylinders  81  are located in opening portions  83  formed in the head cover  82 R fastened to the cylinder head  15 R. Spaces between the outer ends of the plug insertion cylinders  81  and the head cover  82 R are sealed. 
     A single intake port  84 , in communication with both of the intake valve ports  76  and having its axial line disposed on the second virtual plane  79 , is provided in the cylinder head  15 R such that it is opened in an upper side surface of the cylinder head  15 R. A single exhaust port  85 , in communication with both of the exhaust valve ports  77  and having its axial line disposed on the second virtual plane  79 , is provided in the cylinder head  15 R such that it is opened in a lower side surface of the cylinder head  15 R. 
     An intake pipe  74 R is connected to the upper side surface of the cylinder head  15 R such that it is in communication with the intake port  84 . A fuel injection valve  75 R is provided in the intake pipe  74 R. 
     Each of the intake valve ports  76  is openable/closable by an intake valve VI as an engine valve. A valve stem  86  of the intake valve VI is slidably fitted in a guide cylinder  87  provided in the cylinder head  15 R. The intake valve VI is elastically biased in the valve closing direction by a valve spring  89  provided between the cylinder head  15 R and a retainer  88  fixed to an end, projecting from the guide cylinder  87 , of the valve stem  86 . 
     Each of the exhaust valve ports  77  is openable/closable by an exhaust valve VE as an engine valve. A valve stem  90  of the exhaust valve VE is slidably fitted in a guide cylinder  91  provided in the cylinder head  15 R. The exhaust valve VE is elastically biased in the valve closing direction by a valve spring  93  provided between the cylinder head  15 R and a retainer  92  fixed to an end, projecting from the guide cylinder  91 , of the valve stem  90 . 
     Like the right cylinder head  15 R, the left cylinder head  15 L on the left engine block  12 L side is provided with a pair of intake valves VI and a pair of exhaust valves VE, and is also provided with a pair of ignition plugs  80 . A head cover  82 L is fastened to the cylinder head  15 L. An intake pipe  74 L provided with a fuel injection valve  75 L is connected to an upper side surface of the cylinder head  15 L. 
     The pair of the intake valves VI and the pair of the exhaust valves VE disposed in the right cylinder head  15 R are opened/closed by a valve system  94 R. The pair of the intake valves VI and the pair of the exhaust valves VE disposed in the left cylinder head  15 L are opened/closed by a valve system  94 L. The configuration of the valve system  94 R is the same as that of the valve system  94 L. Accordingly, only the configuration of the valve system  94 R on the right cylinder head  15 R side will be hereinafter described. 
     The valve system  94 R includes a holder  97  which integrally includes cylindrical lifter housings  95  coaxial with valve stems  86  of both of the intake valves VI and cylindrical lifter housings  96  coaxial with valve stems  90  of both of the exhaust valves VE. The holder  97  is fastened to the cylinder head  15 R. An intake side rocker shaft  98  and an exhaust side rocker shaft  99  have axial lines parallel to each other and are fixedly supported by the holder  97 . An intake side rocker arm  100  is rockably supported by the intake side rocker shaft  98  and an exhaust side rocker arm  101  is rockably supported by the exhaust side rocker shaft  99 . Lifters  102  are slidably fitted in the lifter housings  96  such that they are interposed between the intake rocker arm  100  and both of the intake valves VI. Lifters  103  are slidably fitted in the lifter housings  97  such that they are interposed between the exhaust side rocker arm  101  and both of the exhaust valves VE. The camshaft  36  is interlocked with the crankshaft  21  at a reduction ratio of 1/2. A push rod  104  imparts a valve opening force to the intake side rocker arm  100  according to the rotation of the camshaft  36 . A pull rod  105  imparts a valve opening force to the exhaust side rocker arm  101  according to the rotation of the camshaft  36 . 
     The intake side and exhaust side rocker shafts  98  and  99  are mounted to the cylinder head  15  such that they are disposed on both sides of the pair of ignition plugs  80 . To be more specific, the intake side rocker shaft  98  is disposed between the pair of the intake valves VI, i.e., the lifter housings  95  and both of the ignition plugs  80 . The exhaust side rocker shaft  99  is disposed between the pair of exhaust valves VE, i.e., the lifter housings  96  and both of the ignition plugs  80 . On the projection plane perpendicular to the axial line of the cylinder bore  16 R (parallel to the paper in FIG.  7 ), the postures of both of the rocker shafts  98  and  99  are set such that axial lines thereof extend in parallel to the first virtual planes  78  on both sides of the first virtual plane  78  while crossing the axial line C of the crankshaft  21 . 
     The lifter  102  (or  103 ) is formed into a cylindrical shape with its bottom closed. The lifter  102  (or  103 ) has a diameter larger than an outside diameter of the valve stem  86  of the intake valve VI (or the valve stem  90  of the exhaust valve VE). The lifter  102  (or  103 ) is slidably fitted in the lifter housing  95  (or  96 ) with the closed end thereof directed toward the rocker arm  100  (or  101 ) side. The closed end of the lifter  102  (or  103 ) has a plurality of through-holes  106  (or  107 ) arranged along a circular line for reducing the weight of the lifter  102  (or  103 ). 
     A pair of drive arms  100   a  and  100   b  extending to the lifters  102  is integrally provided on the intake side rocker arm  100 . The leading ends of the drive arms  100   a  and  100   b  are in contact with the outer surfaces of the closed ends of the lifters  102  in order to impart driving forces for pressing the intake valves VI in the valve opening direction to the valve stems  86  of the intake valves VI via the lifters  102 . 
     A pair of drive arms  101   a  and  101   b  extending to the lifters  103  are integrally provided on the exhaust side rocker arm  101 . The leading ends of the drive arms  101   a  and  100   b  are in contact with the outer surfaces of the closed ends of the lifters  103  in order to impart driving forces for pressing the exhaust valves VE in the valve opening direction to the valve stems  90  of the exhaust valves VE via the lifters  103 . 
     It should be noted that according to this embodiment, to adjust a tappet clearance, as shown in FIG. 6, a shim  121  is held between the valve stem  86  and the lifter  102  and a shim  122  is held between the valve stem  90  and the lifter  103 . In place of the shim  121  (or  122 ), a tappet screw screwed in the leading end of the drive arm  100   a  (or  100   b ,  101   a , or  101   b ) such that it is adjustable in the forward or backward direction may be brought into contact with the lifter  102  (or  103 ). 
     An oil passage  108 , to which oil is supplied from the oil pump  44 , is provided in both the cylinder head  15 R and the holder  97  connected to the cylinder head  15 R. An oil hole  109 , which is in communication with the oil passage  108  and annular recesses  110  and  111  provided in inner surfaces of the lifter housings  95  and  96 , is provided in the holder  97  and in the lifter housings  95  and  96 . 
     Referring to FIG. 9, the camshaft  36  disposed under the crankshaft  21  is provided with an intake side cam  112 R corresponding to the intake valves VI on the right engine block  12 R side, an intake side cam  112 L corresponding to the intake valves VI on the left engine block  12 L side, an exhaust side cam  113 R corresponding to the exhaust valves VE on the right engine block  12 R side, and an exhaust side cam  113 L corresponding to the exhaust valves VE on the left engine block  12 L side. 
     Cam followers  114 R and  114 L following the intake side cams  112 R and  112 L and cam followers  115 R and  115 L following the exhaust side cams  113 R and  113 L are rockably supported by the crankcase  19 . The cam followers  114 R and  115 L are disposed on the right engine block  12 R side with respect to the camshaft  36 , and are rockably supported by a common supporting shaft  118  mounted to the crankcase  19 . The cam followers  114 L and  115 R are disposed on the left engine block  12 L side with respect to the camshaft  36 , and are rockably supported by a common supporting shaft  119  mounted to the crankcase  19 . 
     Referring to FIG. 7, input arms  100   c  and  101   c  extending, on the projection plane perpendicular to the axial line of the cylinder bore  16 R, from the intake side rocker arm  100  and the exhaust side rocker arm  101  to the camshaft  36  side (lower side of FIG.  7 ), are provided on the rocker arms  100  and  101 , respectively. The input arm  100   c  of the intake side rocker arm  100  is connected to the cam follower  114 R by means of a push rod  104 . The input arm  101   c  of the exhaust side rocker arm  101  is connected to the cam follower  115 R by means of the pull rod  105 . The push rod  104  acts, upon movement thereof in the direction opposite to the camshaft  36 , to push up the input arm  100   c  for rocking the intake side rocker arm  100  in the valve opening direction. The pull rod  105  acts, upon movement thereof on the camshaft  36  side, to pull the input arm  101   c  for rocking the exhaust side rocker arm  101  in the valve opening direction. 
     A rod chamber  120  extending from the crankcase  19  to both of the cylinder heads  15 R and  15 L is formed under the engine main body  11 . The push rod  104  and the pull rod  105  are contained and disposed in the rod chamber  120 . In addition, since the tensile strength of a material for forming both of the rods  104  and  105  is higher than the compression strength thereof, the diameter of the pull rod  105  is set to be smaller than that of the push rod  104 . 
     Spherical portions  104   a  and  104   b  are provided on both ends of the push rod  104 . The spherical portion  104   a  at one end of the push rod  104  is swingably received on the cam follower  114 R. The spherical portion  104   b  at the other end of the push rod  104  is swingably received on the leading end of the input arm  100   c  provided on the intake side rocker arm  100 . 
     As shown in FIG. 10, an approximately U-shaped fork  116  opened toward the side opposite to the camshaft  36  is integrally provided on the cam follower  115 R. A pin  123  fixed in one end of the pull rod  105  by press-fitting or the like is engaged with the fork  116 . Furthermore, an approximately U-shaped fork  117  opened on the side opposite to the camshaft  36  is integrally provided on the leading end of the input arm  101   c  provided on the exhaust side rocker arm  101 . A pin  124  fixed in the other end of the pull rod  105  is engaged with the fork  117 . With this configuration, both of the ends of the pull rod  105  can be connected to the input arm  101   c  provided on the exhaust side rocker arm  101  and the cam follower  115 R only by engaging both of the ends of the pull rod  105  with the forks  116  and  117 . Accordingly, one end of the pull rod  105  can be connected to the cam follower  115 R from the cylinder head  15 R side without disassembly of the oil pan  42 . This results in the maintenance being improved. 
     It should be noted that when an engine E as described above is installed in an air plane  150  as shown in FIG. 11, the engine E is accommodated in a cowl  152  attached to a front portion of a body  151  such that an axial line of the crankshaft  21  extends in the forward and backward direction. Furthermore, the engine E is resiliently supported on a support frame  153  disposed in the cowl  152 . 
     A spinner  155  having a plurality of propellers  154  is disposed forwardly of the cowl  152 , and the crankshaft  21  of the engine E is coupled coaxially to the spinner  155 . 
     Referring also to FIG. 12, an intake manifold  156  is disposed above the engine E and extends in the forward and backward direction. A pair of intake pipes  74 L and  74 R are connected to the opposite sides of a front portion of the intake manifold  156  such that they communicate with the intake ports  84  of the cylinder heads  15 L and  15 R of the cylinder blocks  12 L and  12 R of the engine E. 
     An air cleaner  157  is disposed below a rear portion of the intake manifold  156  on the rear side of the engine E and is connected to a rear portion of the intake manifold  156 . In addition, a suction pipe  158  is connected to a lower portion of the air cleaner  157  and extends forwardly below the engine E. The forward end of the suction pipe  158  is open to a screen  159  provided at a lower portion of the front end of the cowl  152 . 
     A pair of radiators  160 ,  160  is disposed on the opposite left and right sides of a lower portion of the engine E. The radiators  160 ,  160  are accommodated in a pair of first air ducts  161 ,  161 , which extends forwardly upwards. The lower ends of the first air ducts  161 ,  161  are open obliquely rearwards in the cowl  152 . A second air duct  162  is connected in common to the upper ends of the two first air ducts  161 ,  161 . The second air duct  162  includes a common duct portion  162   a  extending leftwardly and rightwardly below a front portion of the engine E and having, at a front and central portion thereof, and air intake opening  163  opposed to the screen  159 . A pair of branch duct portions  162   b ,  162   b  extend rearwardly upwards from the opposite left and right end portions of the common duct portion  162   a  and connect to the upper ends of the first air ducts  161 ,  161 . 
     In particular, the radiators  160 ,  160  disposed on the opposite left and right sides of a lower portion of the engine E are cooled by air fed from the screen  159  at the front end of the cowl  152  to the air intake opening  163  by the propellers  154  and flowing through the left and right first air ducts  161 ,  161  separately from the second air duct  162 . 
     The support frame  153  is formed from; for example, a plurality of pipe members combined in such a manner as to embrace the engine E from the rear. In addition; for example, mounting arms  164 ,  164  are inclined such that the distances between them increase rearwardly at four locations of a rear portion of the crankcase  19  of the engine E. The mounting arms  164 ,  164  are provided such that they may be positioned at the corners of an imaginary rectangular parallelepiped centered at the axial line of the crankshaft  21  in a plane perpendicular to the axial line. The mounting arms  164 ,  164 , are mounted on the support frame  153  through resilient mounts  165 ,  165 . 
     Referring to FIG. 13, each resilient mount  165  includes a cylindrical collar  166 , a cylindrical support tube  167  fixed to the support frame  153  and coaxially surrounding the collar  166 , and a mount rubber member  168  interposed between the collar  166  and the support tube  167  with inner and outer peripheries thereof baked to an outer periphery of the collar  166  and an inner periphery of the support tube  167 . Opposite ends of the collar  166  project from the opposite ends of the support tube  167   
     The collar  166  has one end contacting with a mounting arm  164 . The collar  166  contacts, at the other end thereof, with a holding down plate  169 . A bolt  170  has an increased diameter head portion  170   a  for engaging with an outer face of the holding down plate  169  and extending through the holding down plate  169  and the collar  166 . The bolt  170  is screwed in the mounting arm  164  such that the mounting arm  164 , i.e., the engine E, is resiliently mounted on the support frame  153  by tightening the bolt  170 . 
     The function of this embodiment will be described below. Since the pump housing  45  of the oil pump  44  for supplying lubricating oil to various portions of the engine E is removably mounted on the mounting portions  52  provided on the bottom of the oil pan  42  connected to the lower portion of the crankcase  19 , it is possible to set the oil pump  44  at a relatively low position in the engine E. Accordingly, the center of gravity of the engine E can be lowered and the suction efficiency and maintenance performance of the oil pump  44  can be improved. 
     Since the oil strainer  55  connected to the inlet  54  of the oil pump  44  is fixedly held between the oil pan  42  and the pump housing  45 , it is possible to fix the oil strainer  55  between the oil pan  42  and the pump housing  45  without use of parts specialized for fixture thereof such as bolts. Accordingly, the number of parts and the number of assembling steps can be reduced. Furthermore, since an oil suction passage between the inlet  54  of the oil pump  44  and the oil strainer  55  can be shortened, the pumping loss of the oil pump  44  can be reduced. 
     Since the relief valve  58  connected to the outlet  57  of the oil pump  44  is fixedly held between the oil pan  42  and the pump housing  45  while being kept in a posture parallel to that of the oil strainer  55 , the relief valve  58  can be disposed by making effective use of a space which is formed on a side of the oil strainer  55  by holding the oil strainer  55  between the pump housing  45  and the oil pan  42 . In addition, the relief valve is directly connected to the pump housing  45  of the oil pump  44 . Accordingly, it is possible to shorten and simplify the oil discharge passage, composed of the oil passages  61  and  62  extending from the oil filter  63  mounted on the outer surface of the side wall of the oil pan  42 , to the oil pump  44 . Furthermore, since a relief port of the relief valve  58  can be easily set in the oil in the oil pan  42 , it is possible to prevent the oil from bubbling. 
     In addition, the partition wall  46   a  forming the power transmission chamber  50 , which is partitioned from the oil reservoir chamber  43  formed in the oil pan  42 , between the side wall of the oil pan  42  and the partition wall  46   a , is formed on the housing half  46  constituting part of the pump housing  45 . The gear  51  rotated by power transmitted from the crankshaft  21  is fixed to the end portion, on the power transmission chamber  50  side, of the drive shaft  48  rotatably supported by the pump housing  45 . Accordingly, since the gear  51  rotated for transmitting a power from the crankshaft  21  to the drive shaft  48  does not agitate the oil reserved in the oil reservoir chamber  43  in the oil pan  42 , it is possible to prevent the occurrence of friction loss and oil mist due to agitation of the oil. 
     The lifter  102  (or  103 ) is formed into a cylindrical shape with its bottom closed. The lifter  102  (or  103 ) has a diameter larger than that of the valve stem  86  (or  90 ) and is interposed between the valve stem  86  of the intake valve VI (or the valve stem  90  of the exhaust valve VE) and the intake side rocker arm  100  (or exhaust side rocker arm  101 ) which is interlocked to rock with the rotation of the camshaft  36 . The lifter  102  (or  103 ) is slidably fitted in the cylindrical lifter housing  95  (or  96 ), which is integrally provided on the holder  97  fixed to the cylinder heads  15 R and  15 L, and which is coaxial with the valve stem  86  (or  90 ). 
     With this configuration, a drive force from the intake side rocker arm  100  (or the exhaust side rocker arm  101 ) is applied to the valve stem  86  of the intake valve VI (or the valve stem  90  of the exhaust valve VE) via the lifter  102  (or  103 ), so that a bending load is not applied to the valve stem  86  (or  90 ), which has a relatively small diameter. As a result, it is possible to prevent the occurrence of partial wear, galling, and the like in the guide cylinder  87  (or  91 ). Furthermore, the lifter  102  (or  103 ) has a relatively large diameter. Accordingly, even if a bending load is applied from the intake side rocker arm  100  (or exhaust side rocker arm  101 ), it is possible to minimize the occurrence of partial wear, galling, and the like between the lifter housing  95  (or  96 ) and the lifter  102  (or  103 ). Therefore, the reliability of the valve systems  94 R and  94 L can be improved. 
     The oil hole  109  is opened in the inner surfaces of the lifter housings  95  and  96  and is provided in the holder  97  and in the lifter housings  95  and  96 . Accordingly, it is possible to make the sliding motion of the lifter  102  (or  103 ) in the lifter housing  95  (or  96 ) smoother. Therefore, the occurrence of partial wear, galling, and the like between the lifter housing  95  (or  96 ) and the lifter  102  (or  103 ) can be more surely prevented. 
     In this case, if a point of the lifter  102  (or  103 ), to which a drive force is applied from the intake side rocker arm  100  (or  101 ), is offset from the center of the lifter  102  (or  103 ), the lifter  102  (or  103 ) can be rotated around its axial line. Correspondingly, the intake valve VI (or exhaust valve VE) can be rotated, to thereby prevent seizing on one side of the intake valve VI (or exhaust valve VE). From this viewpoint, according to this embodiment, the intake valve VI (or exhaust valve VE) can be easily rotated by smoothly sliding the lifter  102  (or  103 ) in the lifter housing  95  (or  96 ). 
     The pair of intake valve ports  76  and the pair of exhaust valve ports  77  are provided in the cylinder head  15 R (or  15 L) such that they are located on both sides of the first virtual plane  78  containing the axial line of the cylinder bore  16 R (or  16 L) and passing through an approximately center of the combustion chamber  17 R ( 17 L). The pair of intake valve ports  76  and the pair of exhaust valve ports  77  face toward the combustion chamber  17 R ( 17 L). The pair of ignition plugs  80  is mounted in the cylinder head  15 R (or  15 L). Both of the ignition plugs  80  are approximately symmetrical with respect to the second virtual plane  79  passing through the center of the combustion chamber  17 R (or  17 L) and are perpendicular to the first virtual plane  78 . Furthermore, the ignition plugs  80  are disposed in the cylinder head  15 R (or  15 L) in such a manner that the axial lines thereof extend substantially along the first virtual plane  78  and are tilted with a distance therebetween becoming smaller toward the combustion chamber  17 R (or  17 L). The ends of the ignition plugs  80 , projecting in the combustion chamber  17 R (or  17 L), are disposed in the region surrounded by both of the intake valve ports  76  and both of the exhaust valve ports  77 . 
     With this configuration, the ends of the pair of ignition plugs  80 , projecting in the combustion chamber  17 R (or  17 L), are disposed in proximity to an approximately central portion in the combustion chamber  17 R (or  17 L). Accordingly, it is possible to ideally propagate flame in the combustion chamber  17 R (or  17 L). Furthermore, even if an accidental firing of either of the ignition plugs  80  occurs, since the other ignition plug  80  is located at the approximately central portion of the combustion chamber  17 R (or  17 L), it is possible to minimize the deterioration of the flame propagation condition. 
     Both of the ignition plugs  80  are disposed in an approximately V-shape opened in the direction opposite to the combustion chamber  17 R (or  17 L), as seen from the direction perpendicular to the first virtual plane  78 . Furthermore, both of the ignition plugs  80  can be easily mounted in the cylinder head  15 R (or  15 L) with the ends of the ignition plugs  80 , projecting in the combustion chamber  17 R (or  17 L). Accordingly, the ignition plugs  80  are allowed to be disposed in proximity to an approximately central portion of the combustion chamber  17 R (or  17 L). 
     Both of the ignition plugs  80  are collectively disposed in the vicinity of the central portion of the combustion chamber  17 R (or  17 L). Accordingly, it is possible to enhance the degree of freedom of the shape of a water jacket on the cylinder head  15 R (or  15 L) side and the degree of freedom of disposition of fastening bolts for fastening the cylinder head  15 R (or  15 L) to the cylinder block  13 R (or  13 L). Accordingly, the sealing performance between the cylinder head  15 R (or  15 L) and the cylinder block  13 R (or  13 L) as well as the cooling performance can be improved. 
     The intake side and exhaust side rocker arms  100  and  101  are rockably supported by the intake side and exhaust side rocker shafts  98  and  99  having the axial lines extending along the first virtual plane  78 . The virtual plane  78  contains the axial line of the cylinder bore  16 R and passes through the center of the combustion chamber  17 R and crosses the axial line of the crankshaft  21  at the angle on the projection plane perpendicular to the axial line of the cylinder bore  16 R. The intake side and exhaust side rocker shafts  98  and  99  are mounted to the cylinder head  15 R (or  15 L) such that they are disposed on both of the sides of the ignition plugs  80 . With this configuration, it is possible to set the width of the cylinder head  15 R (or  15 L) at a relatively small value in the direction along the second virtual plane  79 . Accordingly, the engine E can be made more compact. 
     The input arm  101   c  extends on the projection plane perpendicular to the axial line of the cylinder bore  16 R (or  16 L) from the rocker arm  101  to the camshaft  36  side. The input arm  101   c  is provided on the exhaust side rocker arm  101 . The pull rod  105  reciprocates in the axial direction according to the rotation of the camshaft  36 . The pull rod  105  is connected to the input arm  101   c  in order to rock the exhaust side rocker arm  101  in the valve opening direction when the pull rod  105  is moved to the camshaft  36  side. With this configuration, it is not required to enlarge the width of the cylinder head  15 R ( 15 L) in the direction along the axial line of the crankshaft  21  for disposing the pull rod  105 . Accordingly, the size and weight of the engine E in the direction along the axial line of the crankshaft  21  can be reduced. 
     The input arm  100   c  extends on the projection plane perpendicular to the axial line of the cylinder bore  16 R (or  16 L), from the rocker arm  101  to the camshaft  36  side. The input arm  100   c  is provided on the intake side rocker arm  100 . The push rod  104  reciprocates in the axial direction according to the rotation of the camshaft  36 . The push rod  104  is connected to the input arm  100   c  in order to rock the intake side rocker arm  100  in the valve opening direction when the push rod  104  is moved to the camshaft  36  side. 
     The intake and exhaust side rocker shafts  98  and  99  and the intake side and exhaust side rocker arms  100  and  101  are disposed as described above. Furthermore, an opening/closing force is imparted to the intake side rocker arm  100  by the pull rod  105  and to the exhaust side rocker arm  101  by the push rod  104 . Accordingly, the space in the cylinder head  15 R (or  15 L) necessary for disposing the rocker shafts  98  and  99  and the rocker arms  100  and  101  constituting parts of the valve system  94 R (or  94 L) can be can be decreased in size in the direction along the axial line of the crankshaft  21 . 
     Furthermore, it is not required to enlarge the width of the cylinder head  15 R (or  15 L) in the direction along the axial line of the crankshaft  21  for disposing the pull rod  105  and the push rod  104 . Accordingly, a drive system between the camshaft  36  and both the rocker arms  100  and  101  can be disposed in good balance. This makes it possible to reduce the size and weight of the engine E in the direction along the axial line of the crankshaft  21 . 
     As mentioned above, the pair of the intake valves VI and the pair of the exhaust valves VE are disposed in the cylinder head  15 R (or  15 L) in such a manner as to face the combustion chamber  17 R (or  17 L). Accordingly, it is possible to improve the suction efficiency and thereby increase the output torque in a low speed rotational range of the engine E. 
     FIG. 14 shows another embodiment of the present invention. A spherical portion  105   a  is provided at one end of a pull rod  105 . An engagement portion  126  is formed into a bowl shape and has a slit  127  allowing the insertion of the pull rod  105 . The engagement portion  126  is provided on a cam follower  115 R for being connected to the one end of the pull rod  105 . The one end of the pull rod  105  is connected to the cam follower  115 R by engaging the spherical portion  105   a  with the engagement portion  126 . 
     In this embodiment, since the one end of the pull rod  105  can be connected to the cam follower  115 R from the cylinder head  15 R side without disassembly of the oil pan  42 , it is possible to improve the maintenance of the device. 
     While the preferred embodiments have been described above, the present invention is not limited thereto, and it is to be understood that various changes in design may be made without departing from the scope of the claims. 
     For example, the present invention can be widely applied to engines other than the horizontally opposed type two-cylinder engine described above. 
     According to the first aspect of the present invention, it is possible to ideally propagate flame in the combustion chamber. Furthermore, even if accidental firing of either of the ignition plugs occurs, since the other ignition plug is located at the approximately central portion of the combustion chamber, it is possible to minimize the deterioration of a flame propagation condition. Also, the ignition plugs can be easily mounted in the cylinder head with the ends of the ignition plugs projecting in the combustion chamber. Accordingly, the ignition plugs can be disposed in proximity to an approximately central portion of the combustion chamber. Furthermore, it is possible to improve the sealing performance between the cylinder head and the cylinder block as well as the cooling performance. 
     According to the second aspect of the present invention, it is possible to set the width of the cylinder head at a relatively small value in the direction along the second virtual plane. Accordingly, the engine can be made more. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.