Abstract:
An internal combustion engine which includes two parallel crankshafts positioned in crankcases, a cylinder positioned on a side of the crankcases, a piston movably disposed within the cylinder, and connecting rods linked to the piston and extending from respective crankshafts. Positions at which the two connecting rods are linked to the piston and the crankshafts are constantly kept symmetrical with respect to a cylinder axis when the two crankshafts rotate in opposite directions.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an internal combustion engine that is provided with two crankshafts. 
   BACKGROUND OF THE INVENTION 
   An example of an internal combustion engine that is provided with two crankshafts is proposed in JP-A-62-107238, wherein each crankshaft is linked to a piston via connecting rods. This internal combustion engine will be described with reference to  FIGS. 11 and 12  hereof. 
   In an engine  200  as shown in  FIG. 11 , a first crankshaft  201  and a second crankshaft  202  are positioned horizontally and in parallel. The first crankshaft  201  is connected to a first piston  206  by a first connecting rod  203 , and the second crankshaft  202  is connected to a second piston  207  by a second connecting rod  204 . A first cylinder  211  and a second cylinder  212  are positioned so that the cylinder axes are perpendicular to the first crankshaft  201  and the second crankshaft  202 . The first piston  206  and the second piston  207  are movably disposed within the first cylinder  211  and the second cylinder  212 , respectively. 
   As shown in  FIG. 12 , the first crankshaft  201  and the second crankshaft  202  are connected via a synchronizing device  221  for inducing synchronized rotation. The synchronizing device  221  is composed of: a first drive-gear  225  that is attached to the first crankshaft  201 ; a second drive-gear  226  that is attached to the second crankshaft  202 ; and a driven-gear  227  that engages the first drive-gear  225  and the second drive-gear  226 . The driven-gear  227  is attached to an output axle  228 . 
   The first connecting rod  203  and the second connecting rod  204  slant towards the cylinder axes of the first cylinder  211  and the second cylinder  212  when the first crankshaft  201  and the second crankshaft  202  rotate in  FIG. 11 . The first piston  206  and the second piston  207  are pushed against the cylinder walls of the first cylinder  211  and the second cylinder  212  due to thrust-generated on the first piston  206  and the second piston  207 , resulting in increased resistance to sliding, noise generation, and the like. 
   The distance between the first crankshaft  201  and the second crankshaft  202  in  FIGS. 11 and 12  is theoretically reduced when a smaller, more compact engine  200  is designed, but since the first drive-gear  225  and the second drive-gear  226  have prescribed diameters, the distance between the first crankshaft  201  and the second crankshaft  202  only shortens to the combined length of the radius of the first drive-gear  225  and the radius of the second drive-gear  226 , at a minimum. 
   Accordingly, a smaller, more compact internal combustion engine wherein thrust does not act on the pistons has been needed. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention, there is provided an internal combustion engine comprising: two parallel crankshafts positioned in crankcases; a cylinder positioned on a side of the crankcases; a piston movably disposed within the cylinder; and connecting rods linked to the piston and extending from each of the two crankshafts, wherein positions at which the crankshafts and the piston are linked to the two corresponding connecting rods are constantly kept symmetrical with respect to a cylinder axis when the two crankshafts rotate in opposite directions. 
   The slants of the two connecting rods are therefore symmetrical with respect to the cylinder axis in the internal combustion engine of the present invention. Thrust can be prevented from acting on the piston; resistance to sliding of the piston and the cylinder can be reduced; the piston can be lightened without the necessity of a skirt part for maintaining the alignment thereof; and high-speed rotation and fuel-consumption economy can be achieved. Additionally, wear on the piston can be reduced, and the occurrence of piston slap, which is produced when the piston hits the cylinder, can be minimized. 
   According to a second aspect of the present invention, there is provided an internal combustion engine comprising: two parallel crankshafts positioned in crankcases; a pair of cylinders provided to sides of both crankshafts and to a side of the crankcases so as to be aligned with a cylinder axis; pistons movably disposed within each of the cylinders; and connecting rods linked to each of the pistons and extending from the two crankshafts to each of the cylinders in the pair, wherein positions at which the crankshafts and the pistons are linked to the corresponding connecting rods linked to the corresponding pistons are constantly kept symmetrical with respect to a cylinder axis when the two crankshafts rotate in opposite directions, and the pistons in the pair each move horizontally. 
   The slants of the connecting-rod pairs of each piston are therefore symmetrical with respect to the cylinder axis in the horizontally-opposed internal combustion engine of the internal combustion engine of the present invention. Thrust can be prevented from acting on the pistons; resistance to sliding of the pistons and the cylinders can be reduced; the pistons can be lightened without the necessity of a skirt part for maintaining the alignment thereof; and high-speed rotation and fuel-consumption economy can be achieved. Additionally, wear on the sides of the pistons can be reduced, and the occurrence of piston slap, which is produced when the pistons hit the cylinders, can be minimized. Since the opposing cylinders are positioned on the same cylinder axis, there is no cylinder offset, a couple is not generated on the crankshafts during the respective strokes of the opposing pistons, and mechanical vibration can be completely cancelled out. 
   It is preferable for the piston and one of the two crankshafts to be linked with one connecting rod, for the piston and the other of the two crankshafts to be linked with two connecting rods and for one connecting rod to be positioned so as to be sandwiched by two connecting rods. 
   Since the pistons are supported by three connecting rods, the pistons can be stably supported and the generation of a couple in the pistons can be prevented. 
   According to a third aspect of the present invention, there is provided an internal combustion engine comprising: first and second cylinders provided to the side of crankcases; first and second crankshafts positioned in parallel in the crankcases; a first connecting rod linked at one end to the first crankshaft via a first crankpin; a first piston movably disposed within the first cylinder and linked to another end of the first connecting rod; a second connecting rod linked at one end to the second crankshaft via a second crankpin; and a second piston movably disposed within the second cylinder and linked to another end of the second connecting rod, a distance from an axis of the first crankshaft to the first crankpin and a distance from an axis of the second crankshaft to the second crankpin being equivalent, wherein the internal combustion engine further comprises pairs of first crank arms provided to the first crankshaft, formed in a tapered shape and supported on a tip by the first crankpin, and pairs of second crank arms provided to the second crankshaft, formed in a tapered shape and supported on a tip by the second crankpin, and wherein the first crank arms and the second crank arms are positioned in parallel, a length of the second crankpin is equivalent to a length of the first crankpin or is longer than the length of the first crankpin, or, alternatively, the length of the first crankpin is equivalent to the length of the second crankpin or is longer than the length of the second crankpin, and the first crank arms are capable of passing through an inner side of the second crank arms and the second crank arms are capable of passing through an inner side of the first crank arms when the first crankshaft and the second crankshaft are rotating. 
   The distance between the first crankshaft and the second crankshaft can thus be shortened in the internal combustion engine of the present invention, and the internal combustion engine can be made smaller and more compact. 
   An axis of the first crankpin is preferably able to be positioned further toward an axis of the second crankshaft and away from a straight line that passes through a center between the axis of the first crankshaft and the axis of the second crankshaft; and an axis of the second crankpin is preferably able to be positioned further toward the axis of the first crankshaft and away from the straight line. 
   The distance between the first crankshaft and the second crankshaft can thus be reduced further than in the conventional art, and the internal combustion engine can be made smaller and more compact. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a sectional view of an internal combustion engine according to the present invention; 
       FIG. 2  is an oblique view showing an assembly of crankshafts, connecting rods, and pistons according to the present invention; 
       FIG. 3  is a side elevational view showing an assembly of crankshafts, connecting rods, and pistons according to the present invention; 
       FIG. 4  is a sectional view of the first crankshaft and the second crankshaft according to the present invention; 
       FIG. 5  is a schematic view of the first crankshaft and the second crankshaft according to the present invention; 
       FIGS. 6A through 6C  illustrates a piston of the internal combustion engine according to the present invention, wherein  FIG. 6A  is a sectional view of the piston,  FIG. 6B  is a sectional view taken along line b-b of  FIG. 6A , and  FIG. 6C  is a view from the bottom of  FIG. 6A ; 
       FIGS. 7A and 7B  are views showing a first stage operation of the crankshaft, connecting rod, and piston according to the present invention, wherein  FIG. 7A  is a diagram viewed from the direction extending out from the crankshafts, and  FIG. 7B  is a diagram viewed from arrow b of  FIG. 7A ; 
       FIGS. 8A and 8B  are views showing a second stage operation of the crankshaft, connecting rod, and piston according to the present invention, wherein  FIG. 8A  is a diagram viewed from the direction extending out from the crankshafts, and  FIG. 8B  is a diagram viewed from arrow b of  FIG. 8A ; 
       FIGS. 9A and 9B  are views showing a third stage operation of the crankshaft, connecting rod, and piston according to the present invention, wherein  FIG. 9A  is a diagram viewed from the direction extending out from the crank axis, and  FIG. 9B  is a diagram viewed from arrow b of  FIG. 9A ; 
       FIGS. 10A and 10B  are views showing a fourth stage operation of the crankshaft, connecting rod, and piston according to the present invention, wherein  FIG. 10A  is a diagram viewed from the direction extending out from the crankshafts, and  FIG. 10B  is a diagram viewed from arrow b of  FIG. 10A ; 
       FIG. 11  is a sectional view of a conventional internal combustion engine; and 
       FIG. 12  is a schematic view showing a synchronizing device of a conventional internal combustion engine. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An internal combustion engine  10  is horizontally-opposed with two cylinders and is composed of a left cylinder  12  as a first cylinder and a right cylinder  14  as a second cylinder, as shown in  FIG. 1 . Two first and second crankshafts  16 ,  17  are rotatably attached between the left cylinder  12  and the right cylinder  14 . 
   The left cylinder  12  is provided with: a left cylinder block  21 ; a first piston  23  that is movably disposed within a left cylinder bore  22  provided to the left cylinder block  21 ; a first connecting rod  26  and a third connecting rod  28  (not shown) that are linked to both the first piston  23  and the second crankshaft  17 ; and a second connecting rod  27  that is linked to both the first piston  23  and the first crankshaft  16 . 
   The left cylinder block  21  is composed of: a main body  31  of the left cylinder; a cylindrical left sleeve  32  that forms the left cylinder bore  22  and is fit within the main body  31  of the left cylinder; and a left crankcase  33  that is attached to the main body  31  of the left cylinder. 
   Reference number  41  designates a left cylinder head that is attached to the side of the left cylinder block  21  via a head gasket (not shown). Reference number  42  designates a combustion chamber; reference number  43 , an intake port; reference number  44 , an intake valve; reference number  46 , an exhaust port; and reference number  47 , an exhaust valve. 
   The right cylinder  14  has the same basic structure as the left cylinder  12 , and is provided with: a right cylinder block  51 ; a right cylinder bore  52 ; a second piston  53 ; a fourth connecting rod  54  and a sixth connecting rod  56  (not shown) that are linked to both the second piston  53  and the first crankshaft  16 ; and a fifth connecting rod  55  that is linked to both the second piston  53  and the second crankshaft  17 . 
   The right cylinder block  51  is composed of a main body  61  of the right cylinder, a right sleeve  63 , and a right crankcase  64 . Reference number  71  designates a right cylinder head; reference number  72 , a combustion chamber; reference number  73 , an intake port; reference number  74 , an intake valve; reference number  76 , an exhaust port; and reference number  77 , an exhaust valve. Reference number  78  designates a bolt that fastens together the left crankcase  33  and the right crankcase  64 . Reference number  80  designates a cylinder axis that passes through the centers of the left cylinder bore  22  and the right cylinder bore  52 . 
   As shown in  FIG. 2 , a large end  54   a  of the fourth connecting rod  54  (designated “# 4 ”) is linked to a first crankpin  81  of the first crankshaft  16 , a large end  27   a  of the second connecting rod  27  (designated “# 2 ”) is linked to a second crankpin  82  of the first crankshaft  16 , and a large end  56   a  of the sixth connecting rod  56  (designated “# 6 ”) is linked to a third crankpin  83  of the first crankshaft  16 . A large end  26   a  of the first connecting rod  26  (designated “# 1 ”) is linked to a first crankpin  86  of the second crankshaft  17 , a large end  55   a  of the fifth connecting rod  55  (designated “# 5 ”) is linked to a second crankpin  87  of the second crankshaft  17 , and a large end  28   a  of the third connecting rod  28  (designated “# 3 ”) is connected to a third crankpin  88  of the second crankshaft  17 . Each of the small ends of the first connecting rod  26 , the second connecting rod  27 , and the third connecting rod  28  are linked to the first piston  23  via piston pins (described in detail hereinafter), and each of the small ends of the fourth connecting rod  54 , the fifth connecting rod  55 , and the sixth connecting rod  56  are connected to the second piston  53  via pistons pins (described in detail hereinafter). 
   The first piston  23  is thus supported by three connecting rods, the first connecting rod  26 , the second connecting rod  27 , and the third connecting rod  28 ; and the second piston  53  is supported by three connecting rods, the fourth connecting rod  54 , the fifth connecting rod  55 , and the sixth connecting rod  56 . The first piston  23  and the second piston  53  are stably supported as a result. 
   The first piston  23  is a component that is linked to the first connecting rod  26  and the third connecting rod  28  (in the diagram, the third connecting rod  28  is positioned in front of the first connecting rod  26 ) by a first piston pin  66 , and is linked to the second connecting rod  27  by a second piston pin  67 , as shown in  FIG. 3 . 
   The second piston  53  is linked to the fourth connecting rod  54  and the sixth connecting rod  56  (the sixth connecting rod  56  is positioned in front of the fourth connecting rod  54 ) by the first piston pin  66 , and is linked to the fifth connecting rod  55  by the second piston pin  67 . 
   The first and third connecting rods  26 ,  28  and the fourth and sixth connecting rods  54 ,  56  have identical shapes. The second and fifth connecting rods  27 ,  55  also have identical shapes. Since the first and third connecting rods  26 ,  28  and the fourth and sixth connecting rods  54 ,  56  support larger loads, the cross-sectional area thereof is increased. 
   A point  91  is the center of the linkage between the large ends  26   a ,  28   a  of the first and third connecting rods  26 ,  28  and the first and third crankpins  86 ,  88  of the second crankshaft  17 . A point  92  is the center of the linkage between small ends  26   b ,  28   b  of the first and third connecting rods  26 ,  28  and a first pin boss  23   a  of the first piston  23 . 
   A point  93  is the center of the linkage between the large end  27   a  of the second connecting rod  27  and the second crankpin  82  of the first crankshaft  16 . A point  94  is the center of the linkage between a small end  27   b  of the second connecting rod  27  and a second pin boss  23   b  of the first piston  23 . 
   A point  95  is the center of the linkage between the large ends  54   a ,  56   a  of the fourth and sixth connecting rods  54 ,  56  and the first and third crankpins  81 ,  83  of the first crankshaft  16 . A point  96  is the center of the linkage between small ends  54   b ,  56   b  of the fourth and sixth connecting rods  54 ,  56  and a first pin boss  53   a  of the second piston  53 . 
   A point  97  is the center of the linkage between the large end  55   a  of the fifth connecting rod  55  and the second crankpin  87  of the second crankshaft  17 . A point  98  is the center of the linkage between a small end  55   b  of the fifth connecting rod  55  and a second pin boss  53   b  of the second piston  53 . 
   A distance L 1  between the point  91  and the point  92  as indicated above, a distance L 2  between the point  93  and the point  94 , a distance L 3  between the point  95  and the point  96 , and a distance L 4  between the point  97  and the point  98  are equivalent (L 1 =L 2 =L 3 =L 4 ). 
   In other words, the mounting lengths of the connecting rods  26  through  28 ,  54  through  56  are equal to each other. 
   Furthermore, the point  91  and the point  93 , the point  92  and the point  94 , the point  95  and the point  97 , and the point  96  and the point  98  described above are positioned symmetrically in relation to the cylinder axis  80 . 
   Therefore, a distance L 5  between the cylinder axis  80  and the point  92 , a distance L 6  between the cylinder axis  80  and the point  94 , a distance L 7  between the cylinder axis  80  and the point  96 , and a distance L 8  between the cylinder axis  80  and the point  98  are all equivalent (L 5 =L 6 =L 7 =L 8 ). A distance L 11 , from crank axes  101 ,  102  of the first crankshaft  16  and second crankshaft  17 , respectively, to the points  92 ,  94 , and a distance L 12 , from the crank axes  101 ,  102  to the points  96 ,  98 , are also equivalent (L 11 =L 12 ). 
   A distance L 13 , from the cylinder axis  80  to the crank axis  101 , and a distance L 14 , from the cylinder axis  80  to the crank axis  102 , are also equivalent (L 13 =L 14 ). The distances L 13 , L 14  are equivalent to the aforementioned distances L 5 , L 6 , L 7 , L 8 . 
   The cylinder axis of the first piston  23  and the cylinder axis of the second piston  53  are aligned along an arrow Z in  FIG. 3  (in other words, the first piston  23  and the second piston  53  have a common cylinder axis  80 ). The left cylinder bore  22  (see  FIG. 1 ) and the right cylinder bore  52  (see  FIG. 1 ) have no cylinder offset along the direction extending along the crank axes  101 ,  102  (the direction perpendicular to the page in  FIG. 3 ). 
   The first crankshaft  16  and the second crankshaft  17  have identical shapes, as shown in  FIG. 4 . The crank axis  101  and the crank axis  102  are positioned in parallel and in closer proximity than in the conventional art. The first crankshaft  16  will be described in detail below, but only the essentials of the second crankshaft  17  will be described. 
   The first crankshaft  16  is composed of: a first member  112  that has a journal  111  formed on an end thereof; a second member  116  that is attached to the first member  112  by a hollow bolt  113  and a nut  114 ; a third member  121  that is attached to the second member  116  by a hollow bolt  117  and nuts  118 ,  119 ; and a fourth member  126  that has a journal  122  formed on an end thereof and is attached to the third member  121  by a hollow bolt  123  and a nut  124 . Reference number  128  designates a bearing that is provided between the left crankcase  33  (see  FIG. 1 ) and the right crankcase  64  (see  FIG. 1 ) on one side and the journals  111 ,  122  on the other side. 
   The first member  112  and the second member  116  are members that form a first arm  131 , the first crankpin  81 , a second arm  132 , and a third arm  133 . The large end  54   a  of the fourth connecting rod  54  is swingably linked to the first crankpin  81 . 
   The second member  116  and the third member  121  are members that form the second crankpin  82 , a fourth arm  134 , and a fifth arm  135 . The large end  27   a  of the second connecting rod  27  is swingably linked to the second crankpin  82 . 
   The third member  121  and the fourth member  126  are members that form the third crankpin  83  and a sixth arm  136 . The large end  56   a  of the sixth connecting rod  56  is swingably linked to the third crankpin  83 . 
   The first member  112  and the second member  116  of the second crankshaft  17  are members that form the first crankpin  86 . The large end  26   a  of the first connecting rod  26  is swingably linked to the first crankpin  86 . 
   The second member  116  and the third member  121  of the second crankshaft  17  are members that form the second crankpin  87 . The large end  55   a  of the fifth connecting rod  55  is swingably linked to the second crankpin  87 . 
   The third member  121  and the fourth member  126  of the second crankshaft  17  are members that form the third crankpin  88 . The large end  28   a  of the third connecting rod  28  is swingably linked to the third crankpin  88 . 
   A distance L 16  between the first crankshaft  16  and the second crankshaft  17  is the distance between the crank axes  101 ,  102  in the drawings and is a combination of the distance L 13  and the distance L 14  shown in  FIG. 3 . In other words, L 16 =L 13 +L 14 . 
   L 17  is identical both to the distance from the crank axis  101  to an axis  137   a  of the first crankpin  81  and the third crankpin  83  and to the distance from the crank axis  101  to an axis  137   b  of the second crankpin  82 . The distance L 17  is half the length of the stroke of the first and second pistons  23 ,  53  (see  FIG. 3 ). 
   The distance from the crank axis  102  to an axis  138   a  of the first crankpin  86  and the third crankpin  88 , as well as the distance from the crank axis  102  to an axis  138   b  of the second crankpin  87  are also identical to L 17 . 
   The axis  138   b  of the second crankshaft  17  is positioned further to the side of the first crank axis  101  than the axis  137   a  of the first crankshaft  16  and is also positioned further to the side of the first crank axis  101  than a straight line  139  that passes through the center of the first crank axis  101  and the second crank axis  102 . 
   When the first crankshaft  16  and the second crankshaft  17  rotate 180 20  around the first crank axis  101  and the second crank axis  102 , respectively, the axis  137   b  of the first crankshaft  16  is positioned further to the side of the second crank axis  102  than the axis  138   a  of the second crankshaft  17  and is also positioned further to the side of the second crank axis  102  than the straight line  139 . 
   The distance L 16  between the first crankshaft  16  and the second crankshaft  17  can thus be made shorter than in the conventional art, and the internal combustion engine can be made smaller and more compact. 
   The smallest distance between the first crankshaft  16  and the second crankshaft  17  is determined by the outside diameters of the large ends  26   a ,  28   a ,  54   a ,  56   a , error in component manufacture, and the clearance between the adjoining large ends (in the drawings, the clearance between the large ends  26   a ,  54   a  and the clearance between the large ends  28   a ,  56   a ) that are subjected to the effects of heat expansion. 
   The shapes of the first crankshaft  16  and the second crankshaft  17  are schematically shown by the thick lines in  FIG. 5 . The description below will focus on the parts designated by thick lines. When it proves necessary in order to distinguish the parts of the first crankshaft  16  and the parts of the second crankshaft  17 , “A” will be appended to the end of notation referring to components of the first crankshaft  16 , and “B” will be appended to the end of notation referring to components the second crankshaft  17 . 
   The first crankshaft  16  is composed of: the journal  111 A; the first arm  131 A that extends diagonally from the journal  111 A; the first crankpin  81  that is provided to the end of the first arm  131 A; the second arm  132 A and the third arm  133 A that extend diagonally from the first crankpin  81  in a substantially straight line; the second crankpin  82  that is provided to the end of the third arm  133 A; the fourth arm  134 A and the fifth arm  135 A that extend diagonally from the second crankpin  82  in a substantially straight line; the third crankpin  83  that is provided to the end of the fifth arm  135 A; the sixth arm  136 A that extends diagonally from the third crankpin  83 ; and the journal  122 A that is provided to the end of the sixth arm  136 A. 
   The crank axis  101  serves as the boundary of the aforementioned second arm  132 A and third arm  133 A, as well as of the fourth arm  134 A and the fifth arm  135 A. 
   The journal  111 A, the first crankpin  81 , the second crankpin  82 , the third crankpin  83 , and the journal  122 A are designated by a thick line that is drawn to pass through the centers of the outside diameters thereof. 
   The second crankshaft  17  is similarly composed of the journal  111 B, the first arm  131 B, the first crankpin  86 , the second arm  132 B and the third arm  133 B, the second crankpin  87 , the fourth arm  134 B and the fifth arm  135 B, the third crankpin  88 , the sixth arm  136 B, and the journal  122 B. 
   The crank axis  102  serves as the boundary of the aforementioned second arm  132 B and third arm  133 B, as well as of the fourth arm  134 B and the fifth arm  135 B. 
   The journal  111 B, the first crankpin  86 , the second crankpin  87 , the third crankpin  88 , and the journal  122 B are designated by a thick line that is drawn to pass through the centers of the outside diameters thereof. 
   In the aforementioned first crankshaft  16 , the first arm  131 A, the second arm  132 A, the third arm  133 A, the fourth arm  134 A, the fifth arm  135 A, and the sixth arm  136 A all slant from a direction perpendicular to the crank axes  101 ,  102 . In the second crankshaft  17 , the first arm  131 B, the second arm  132 B, the third arm  133 B, the fourth arm  134 B, the fifth arm  135 B, and the sixth arm  136 B all extend diagonally from a direction perpendicular to the crank axes  101 ,  102 . 
   The first arm  131 A and the second arm  132 A, the third arm  133 A and the fourth arm  134 A, the fifth arm  135 A and the sixth arm  136 A, and the second arm  132 A and the fifth arm  135 A all form tapered shapes. The first arm  131 B and the second arm  132 B, the third arm  133 B and the fourth arm  134 B, the fifth arm  135 B and the sixth arm  136 B, and the second arm  132 B and the fifth arm  135 B all form tapered shapes. 
   The first arm  131 A and the first arm  131 B, the second arm  132 A and the second arm  132 B, the third arm  133 A and the third arm  133 B, the fourth arm  134 A and the fourth arm  134 B, the fifth arm  135 A and the fifth arm  135 B, and the sixth arm  136 A and the sixth arm  136 B are parallel. 
   A length A 1  of the first crankpin  81  is equivalent to a length B 1  of the first crankpin  86 ; a length A 2  of the second crankpin  82  is equivalent to a length B 2  of the second crankpin  87 ; and a length A 3  of the third crankpin  83  is equivalent to a length B 3  of the third crankpin  88 . 
   In  FIG. 5 , the first crankpin  81  is capable of passing through the inner side of the first crankpin  86 , the second crankpin  87  is capable of passing through the inner side of the second crankpin  82 , and the third crankpin  83  is capable of passing through the inner side of the third crankpin  88 , but A 1 &gt;B 1 , B 2 &gt;A 2 , A 3 &gt;B 3  is acceptable. 
   With such an arrangement, the first crankpin  86  is capable of passing through the inner side of the first crankpin  81 , the second crankpin  82  is capable of passing through the inner side of the second crankpin  87 , and the third crankpin  88  is capable of passing through the inner side of the third crankpin  83  even when the first crankshaft  16  and the second crankshaft  17  rotate 180° round the crank axes  101 ,  102 , respectively, from the state shown in  FIG. 5 . A 1 &lt;B 1 , B 2 &lt;A 2 , A 3 &lt;B 3  is also acceptable. 
   The tapered-form third arm  133 B and fourth arm  134 B, as well as the tapered-form second arm  132 B and fifth arm  135 B are inserted more easily in the above manner between the tapered-form third arm  133 A and fourth arm  134 A and between the tapered-form second arm  132 A and fifth arm  135 A, for example. In other words, these components are more easily positioned in close proximity. The distance between the first crankshaft  16  and the second crankshaft  17  can therefore be reduced, and the internal combustion engine can be made smaller. 
   In  FIG. 6A , the first piston  23  is in a raised state, the first piston pin  66  is inserted in the first pin boss  23   a , and the second piston pin  67  is inserted in the second pin boss  23   b  (not shown). The second piston  53  has a structure that is identical to that of the first piston  23 . 
   The first piston  23  is not subject to thrust, and therefore a skirt for controlling piston alignment within the cylinder is not used. The first pin boss  23   a  is composed of a pair of pin-boss halves  23 c,  23 d, which protrude downward. 
   As shown in  FIG. 6B , the first piston  23  is composed of: a head  23   e ; a land  23   f  that is integrally formed with the outer periphery of the head  23   e ; the pin-boss halves  23   c ,  23   d  (only  23   d  is shown in  FIG. 6B ) that protrude downward from the head  23   e  and the land  23   f ; and a longitudinal rib  23   g  that links and reinforces the pin-boss halves  23   c ,  23   d  and the head  23   e.    
   The head  23   e  has a head surface  23   h  that is curved in a concave shape in order to form the combustion chamber; and a tapered part  23   j  that is formed on the outer edge of the head surface  23   h  in order to generate squish. 
   In order from the side of the tapered part  23   j , the land  23   g  forms a top land  23   m , a top ring groove  23   n , a second land  23   p , a second ring groove  23   q , a third land  23   r , an oil ring groove  23   s , and a bottom land  23   t.    
   As shown in  FIG. 6C , the pin-boss half  23   c  is composed of a first protrusion  23   v  and a second protrusion  23   w  that are bounded by the longitudinal rib  23   g , and the pin-boss half  23   d is composed of a first protrusion  23   x  and a second protrusion  23   y  that are bounded by the longitudinal rib  23   g.    
   The first protrusion  23   v  and the first protrusion  23   x  constitute the first pin boss  23   a , and the second protrusion part  23   w  and the second protrusion part  23   y  constitute the second pin boss  23   b . Reference number  23 A designates a first pin hole through which passes the first piston pin  66 , and reference number  23 B designates a second pin hole through which passes the second piston pin  67 . 
   The first piston pin  66  is fit in the first pin holes  23 A,  23 A via bearings  141 ,  141 . Reference number  142  designates a collar into which the first piston pin  66  is inserted and which is positioned between the bearings  141 ,  141 . 
   The small end  26   b  of the first connecting rod  26  and the small end  28   b  of the third connecting rod  28  are fit (press fit) in the sections of the first piston pin  66  that protrude out farther than the first protrusion part  23   v  and the first protrusion part  23   x.    
   The second piston pin  67  is fit (press fit) in the second pin holes  23 B,  23 B. The small end  27   b  of the second connecting rod  27  is fit between the second protrusion  23   w  and the second protrusion  23   y  of the second piston pin  67  via a bearing  144 . 
   Two connecting rods; i.e., the first connecting rod  26  and the third connecting rod  28 , are linked to the first piston  23  by the first piston pin  66 . One connecting rod; i.e., the second connecting rod  27 , is linked to the first piston  23  by the second piston pin  67 . Thrust generated due to the slant of the second connecting rod  27  (acting in the vertical direction in  FIG. 6C ) and thrust generated due to the slant of the first connecting rod  26  and the third connecting rod  28  (acting in the vertical direction in  FIG. 6C ) pass through the center of the first piston  23 , for example, and can be positioned on the same plane, which extends in the vertical direction in  FIG. 6C , allowing the generation of a couple on the first piston  23  (acting in the direction that causes the first piston  23  to rotate on the page) to be prevented. 
   Of the two crankshafts, the first crankshaft  16  and the second crankshaft  17  described above using  FIGS. 2 and 6 , the first crankshaft  16  is linked to the first piston  23  by one connecting rod; i.e., the second connecting rod  27 . The other of the two crankshafts, the second crankshaft  17 , is linked to the first piston  23  by two connecting rods; i.e., the first and third connecting rods  26 ,  28 . One connecting rod, i.e., the second connecting rod  27 , is sandwiched in position by two connecting rods; i.e., the first and third connecting rods  26 ,  28 . 
   Of the two crankshafts, i.e., the first crankshaft  16  and the second crankshaft  17 , the second crankshaft  17  is linked to the second piston  53  by one connecting rod; i.e., the fifth connecting rod  55 . The other of the two crankshafts; i.e., the first crankshaft  16 , is linked to the second piston  53  by two connecting rods; i.e., the fourth and sixth connecting rods  54 ,  56 . One connecting rod, i.e., the fifth connecting rod  55 , is sandwiched in position by the two connecting rods; i.e., the fourth and sixth connecting rods  54 ,  56 . 
   Since the first piston  23  is accordingly supported by each of the three connecting rods  26  through  28 , and the second piston  53  is supported by each of the three connecting rods  54  through  56 , the first piston  23  and the second piston  53  can be stably supported, and the generation of a couple in the first piston  23  and the second piston  53  can also be prevented. 
   The operation of each of the aforedescribed crankshafts, connecting rods, and pistons will be described next. 
     FIGS. 7A and 7B  show a crank angle of 0°; i.e., a state wherein the first piston  23  is positioned at the top dead center of the compression stroke and the second piston  53  is positioned at the top dead center of the exhaust stroke. 
   When viewed from the direction extending from the crank axes  101 ,  102 , as in  FIG. 7A , the second connecting rod  27 , the first crankshaft  16 , and the fourth and sixth connecting rods  54 ,  56  form a straight line; the first and third connecting rods  26 ,  28 , the second crankshaft  17 , and the fifth connecting rod  55  form a straight line; and the first crankshaft  16  and the second crankshaft  17  do not overlap. The unlabelled arrows in  FIG. 7A  designate the directions of rotation of the first crankshaft  16  and the second crankshaft  17  (the same applies through  FIG. 10  below). In other words, the first crankshaft  16  and the second crankshaft  17  rotate in opposite directions. 
   As shown in  FIG. 7B , the large end  27   a  of the second connecting rod is positioned further to the front than the large end  54   a  of the fourth connecting rod and the large end  56   a  of the sixth connecting rod. The large end  55   a  of the fifth connecting rod is positioned further in the interior than the large end  26   a  of the first connecting rod and the large end  28   a  of the third connecting rod. 
     FIGS. 8A and 8B  show a crank angle of 90°; i.e., a state wherein the first piston  23  is positioned in the expansion stroke and the second piston  53  is positioned in the intake stroke. When viewed from the direction extending from the crank axes  101 ,  102 , as in  FIG. 8A , the first and third connecting rods  26 ,  28  and the second connecting rod  27  form an X-shape; the fourth and sixth connecting rods  54 ,  56  and the fifth connecting rod  55  form a V-shape; and the first crankshaft  16  and the second crankshaft  17  overlap. 
   Since the resultant force of the explosive force on the head  23   e  and the inertia force acts in the direction of the cylinder axis  80  on the side of the first piston  23  in the expansion stroke, a thrust is generated in a direction facing further to the bottom of  FIG. 8A  than the slant of the first and third connecting rods  26 ,  28  relative to the cylinder axis  80 , and a thrust is generated in a direction facing further to the top of  FIG. 8A  than the slant of the second connecting rod  27  relative to the cylinder axis  80 . However, these thrusts have the same magnitude, and therefore cancel each other out. As a result, thrust does not act on the first piston  23 . The same applies for the second piston  53 . 
   As shown in  FIG. 8B , the large end  27   a  on the side of the first crankshaft  16  penetrates to the inner side of the second arm  132 , the third arm  133 , the fourth arm  134 , and the fifth arm  135  of the second crankshaft  17 . The large end  26   a  on the side of the second crankshaft  17  penetrates to the inner side of the first arm  131 , the second arm  132 , and the third arm  133  of the first crankshaft  16 . The large end  28   a  on the side of the second crankshaft  17  penetrates to inner side of the fourth arm  134 , the fifth arm  135 , and the sixth arm  136  of the first crankshaft  16 . The space between the first crankshaft  16  and the second crankshaft  17  can therefore be made extremely small. 
     FIGS. 9A and 9B  show a crank angle of 180°; i.e., a state wherein the first piston  23  is positioned at the bottom dead center of the expansion stroke and the second piston  53  is positioned in the bottom dead center of the intake stroke. When viewed from the direction extending from the crank axis  101 ,  102 , as in  FIG. 9A , the second connecting rod  27 , the first crankshaft  16 , and the fourth and sixth connecting rods  54 ,  56  overlap; the first and third connecting rods  26 ,  28 , the second crankshaft  17 , and the fifth connecting rod  55  overlap; but the first crankshaft  16  and the second crankshaft  17  do not overlap. 
   As shown in  FIG. 9B , the large end  27   a  of the second connecting rod is positioned further in the interior than the large end  54   a  of the fourth connecting rod and the large end  56   a  of the sixth connecting rod. The large end  55   a  of the fifth connecting rod is positioned further to the front than the large end  26   a  of the first connecting rod and the large end  28   a  of the third connecting rod. 
     FIGS. 10A and 10B  show a crank angle of 270°; i.e., a state wherein the first piston  23  is positioned in the exhaust stroke and the second piston  53  is positioned in the compression stroke. When viewed from the direction extending from the crank axes  101 ,  102 , as in  FIG. 10A , the first and third connecting rods  26 ,  28  and the second connecting rod  27  form a V-shape; the fourth and sixth connecting rods  54 ,  56  and the fifth connecting rod  55  form a X-shape; and the first crankshaft  16  and the second crankshaft  17  overlap. 
   As shown in  FIG. 10B , the large end  54   a  on the side of the first crankshaft  16  penetrates to the inner side of the first arm  131 , the second arm  132 , and the third arm  133  of the second crankshaft  17 . The large end  56   a  on the side of the first crankshaft  16  penetrates to the inner side of fourth arm  134 , the fifth arm  135 , and the sixth arm  136  of the second crankshaft  17 . The large end  55   a  on the side of the second crankshaft  17  penetrates to inner side of the second arm  132 , the third arm  133 , the fourth arm  134 , and the fifth arm  135  of the first crankshaft  16 . The space between the first crankshaft  16  and the second crankshaft  17  can therefore be made extremely small. 
   The internal combustion engine of the present embodiment is shown in  FIG. 1  as horizontally-opposed with two cylinders however, the present invention is not limited thereto, and may be horizontally-opposed with four cylinders, six cylinders, or another even number of cylinders. 
   Obviously, various minor changes and modifications of the present invention are possible in light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.