Patent Publication Number: US-7584725-B2

Title: Internal combustion engine

Description:
FIELD OF THE INVENTION 
   The present invention relates to an internal combustion engine having a combustion chamber that is formed using a bottomed tubular moveable sleeve and a stationary piston with an internally held valve mechanism. 
   BACKGROUND OF THE INVENTION 
   In GB Patent No. 558115 there is proposed an opposed-piston internal combustion engine in which two pistons are moveably fitted in a cylinder so as to oppose one another, and two crankshafts are connected to the pistons via connecting rods. 
   As shown in  FIG. 12  hereof, the two crankshafts  201 ,  202  are disposed in parallel in a crankcase  200 . The cylinder  203  is provided to the crankcase  200  so as to be disposed between the crankshafts  201 ,  202 , so that the cylinder axis is perpendicular to the crankshafts  201 ,  202 . Pistons  204 ,  206  are moveably inserted into the cylinder  203  from openings on either end thereof. A yoke  207  is integrally formed with an end part of the piston  204 . The yoke  207  is connected to the two crankshafts  201 ,  202  via connecting rods  208 ,  208 . A yoke  211  is integrally formed with the end part of the piston  206 . The yoke  211  is connected to the two crankshafts  201 ,  202  via connecting rods  212 ,  212 . The spaces between the cylinder  203  and the pistons  204 ,  206  are sealed using a plurality of piston rings  214  attached on the pistons  204 ,  206 . A combustion chamber  216  is formed between the two pistons  204 ,  206 . 
   Since the piston rings  214  are attached on the pistons  204 ,  206 , ring flutter occurs at high engine speeds from the piston rings  214  that vibrate within the ring grooves of the reciprocating pistons  204 ,  206 . As a result of the ring flutter, during the power stroke, combustion gas in the combustion chamber  216  passes between the cylinder  203  and the pistons  204 ,  206 ; i.e., between the cylinder  203  and the piston rings  214 . The gas is blown into the crankcase  200 , and the amount of blow-by gas increases. 
   When the pistons  204 ,  206  move in a reciprocating manner, the volume of the crankcase  200  varies, and the pressure inside the crankcase  200  fluctuates. Therefore, when ring flutter occurs, oil mist inside the crankcase  200  passes between the cylinder  203  and the pistons  204 ,  206 , i.e., between the piston rings  214  and the cylinder  203  during the intake stroke, and readily penetrates into the combustion chamber  216 . 
   A demand has accordingly arisen for an internal combustion engine in which it is possible to prevent the incidence of ring flutter, and oil mist penetrating into the combustion chamber. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided an internal combustion engine which comprises: two rotatable crankshafts horizontally disposed in an engine case in vertically spaced relation to each other; two stationary pistons disposed between the two crankshafts and extending perpendicularly to a plane that passes over axial lines of the two crankshafts; moveable sleeves slidably attached to the respective stationary pistons; and combustion chambers surrounded by the stationary pistons and the moveable sleeves, wherein each of the stationary pistons has a piston ring disposed on a exterior surface thereof for sealing between the stationary piston and the respective moveable sleeve, and the two crankshafts and the two moveable sleeves are connected via respective connecting rods. 
   Since piston rings are mounted on the stationary pistons, inertial force does not act on the piston rings when the pistons move back and forth, ring flutter does not occur, and it is possible to prevent an increase in blow-by gas, and oil mist from penetrating to the combustion chamber. 
   In a preferred form, the engine case includes a case cylinder in which the moveable sleeves are moveably fitted, and an upper wall for blocking an end part of the case cylinder. Each of the moveable sleeves desirably includes a seal ring disposed on an outer surface thereof for sealing between the moveable sleeve and the case cylinder. The moveable sleeve, the case cylinder and the upper wall jointly define an intake chamber for admitting a mixed gas containing fuel and air, so that the mixed gas is supplied from the intake chamber to the combustion chamber. 
   Since the moveable sleeves merely slide and move along stationary pistons and case cylinders, the volume inside the crankcase does not vary. Therefore, the pressure within the crankcase does not vary. It is therefore possible to prevent oil mist from penetrating through to the combustion chamber from the crankcase. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view showing an internal combustion engine according to the present invention; 
       FIG. 2  is a perspective view showing a state in which an engine case of the internal combustion engine has been removed; 
       FIG. 3  is a rear view of the internal combustion engine; 
       FIG. 4  is a cross-sectional view as seen from the rear of the internal combustion engine; 
       FIG. 5  is a cross-sectional view as seen from the top of the internal combustion engine; 
       FIG. 6  is a partial cross-sectional view showing a seal structure of the moving parts of the internal combustion engine; 
       FIG. 7  is a cross-sectional view of the internal combustion engine as seen from the side; 
       FIG. 8  is a cross-sectional view taken along line  8 - 8  of  FIG. 1 ; 
       FIG. 9  is a cross-sectional view showing a coolant channel of the internal combustion engine; 
       FIG. 10A  is an operation diagram showing an exhaust stroke in the left cylinder and a compression stroke in the right cylinder in the internal combustion engine; 
       FIG. 10B  is an operation diagram showing an intake stroke in the left cylinder and a combustion stroke in the right cylinder in the combustion engine; 
       FIG. 11A  is an operation diagram showing a compression stroke in the left cylinder and an exhaust stroke in the right cylinder in the internal combustion engine; 
       FIG. 11B  operation diagram showing a combustion stroke in the left cylinder and an intake stroke in the right cylinder in the internal combustion engine; and 
       FIG. 12  is a cross-sectional view of a conventional internal combustion engine as seen from the front. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As shown in  FIG. 1 , an internal combustion engine  10  is a drive source having the following configuration. A vertically disposed upper crankshaft  14  and lower crankshaft  16  are rotatably attached via a bearing so as to be held at a surface  10 A in which two horizontally separated left and right fixed blocks  36 ,  37  are joined. Engine cases  41  are attached to side surfaces of the left fixed block  36  and the right fixed block  37 . The left and right engine cases  41  constitute a left cylinder  12  and a right cylinder  13  that extend horizontally leftward and rightward. An upper crank output gear  103  is attached on a distal end of the upper crankshaft  14 . An idler gear  112  that is rotatably supported on the lower crankshaft  16  meshes with the upper crank output gear  103 . A lower crank output gear  107  is attached on a distal end of the lower crankshaft  16 . 
   The reference numbers  36   a ,  37   a  and  36   b ,  37   b  in  FIG. 1  designate coolant inlets and coolant outlets provided to upper surfaces of the left fixed block  36  and the right fixed block  37 . The coolant inlets  36   a ,  37   a  and the coolant outlets  36   b ,  37   b  are connected to a water pump and a radiator (not shown), and coolant is circulated through these inlets and outlets within the internal combustion engine  10  (i.e., inside the left fixed block  36  and the right fixed block  37 ) via the water pump and the radiator. 
   Moveable sleeves that are connected to connecting rods are disposed on the upper crankshaft  14  and the lower crankshaft  16  in the left and right engine cases  41 ,  41  so as to be able to move in the horizontal direction. A throttle body, an air cleaner, and other intake devices are connected to rear portions of end parts of the left and right engine cases  41 ,  41  via an intake manifold. Inner teeth of a ring gear mesh with the idler gear  112  and the lower crank output gear  107 . An output shaft via which output is transferred to the exterior is attached on the ring gear. 
     FIG. 2  shows a state in which the left and right engine cases  41 ,  41  shown in  FIG. 1  have been removed. 
   The moveable sleeves  43  are moveably fitted on stationary pistons  61 A (only the stationary piston  61 A in the left fixed block  36  is shown) that horizontally protrude from either side of the left fixed block  36  and the right fixed block  37 . The left moveable sleeve  43  is connected via connecting pins  18 ,  19  to small end parts  26   a ,  27   a ,  28   a  of connecting rods  26 ,  27 ,  28 . The connecting rods extend leftward from within the left fixed block  36  and the right fixed block  37 . Similarly, the right moveable sleeve  43  is connected via connecting pins  18 ,  19  to small end parts  26   a ,  27   a ,  28   a  of the connecting rods  26 ,  27 ,  28  (these connecting rods  26 ,  27 ,  28  are not shown). The connecting rods extend rightward from within the left fixed block  36  and the right fixed block  37 . 
   Three intake valves  82  (only one intake valve  82  is shown in the right fixed block  37 ) are provided to a top part  43   b  of the moveable sleeve  43 . One end of each of rocker arms  86  (only two rocker arms  86  are shown in the right fixed block  37 ), which have a middle part rotatably attached to the top part  43   b , are connected to each end part of the intake valves  82 . Weights  87  (only two weights  87  are shown in the right fixed block  37 ) for adjusting balance are attached on each of the other ends of the rocker arms  86 . 
   As shown in  FIG. 3 , ball-bearings  106 ,  116  for rotatably supporting the upper crankshaft  14  and the lower crankshaft  16  are attached so as to be held at the joining surface  10 A of the left and right fixed blocks  36 ,  37 . A rectangular plug cord insertion opening  64  is formed along each of the left fixed block  36  and the right fixed block  37 . A plug cord (not shown) connected to a spark plug (not shown) disposed inside each of the left and right fixed blocks is inserted via the plug cord insertion openings. Elliptical exhaust outlets  36   c ,  37   c  for discharging exhaust gas are formed in the left fixed block  36  and the right fixed block  37  below the plug cord insertion holes  64 . 
   Exhaust pipes are connected to the exhaust outlets  36   c ,  37   c , and a muffler is connected to the exhaust pipes. 
   As shown in  FIG. 4 , the internal combustion engine  10  has the left cylinder  12 , which is disposed to the left of a vertically extending center line  11  (the center line  11  passes through the matched surface  10 A); the right cylinder  13 , which is disposed to the right of the center line  11 ; the upper crankshaft  14  and the lower crankshaft  16 , which are disposed in parallel to one another so as to be along and perpendicular to the center line  11 ; a first connecting rod  26 , a second connecting rod  27 , and a third connecting rod  28  (not shown; see  FIG. 7 ), in which large end parts  26   b ,  27   b    28   b  (the large end part  28   b  is not shown; see  FIG. 7 ) thereof are rotatably connected to crank pins  20 ,  21 ,  22  (crank pin  22  is not shown; see  FIG. 7 ) of the upper crankshaft  14  via bearings  24  (the bearing  24  of the large end part  28   b  is not shown); a first connecting rod  26 , a second connecting rod  27 , and a third connecting rod  28  (not shown; see  FIG. 7 ), in which large end parts  26   b ,  27   b    28   b  (the large end part  28   b  is not shown; see  FIG. 7 ) thereof are rotatably connected to crank pins  30 ,  31 ,  32  (crank pin  30  is not shown; see  FIG. 7 ) of the lower crankshaft  16  via bearings  24  (the bearing  24  of the large end part  28   b  is not shown); the left fixed block  36  and the right fixed block  37 , which are divided in two along the center line  11  in order to rotatably support the upper crankshaft  14  and the lower crankshaft  16  via the bearings (not shown); and a cam drive mechanism  39  for driving a camshaft  38  disposed between the upper crankshaft  14  and the lower crankshaft  16 . Reference number  14 A denotes an axial line that extends in the axial direction through the center of the upper crankshaft  14 , and reference symbol  16 A denotes an axial line that extends in the axial direction and passes through the center of the lower crankshaft  16 . 
   The left cylinder  12  and the right cylinder  13  have the same basic structure, and only the left cylinder  12  is described below. 
   The left cylinder  12  has the engine case  41 , which is attached on the left fixed block  36 ; a center head  42 , which protrudes from a side surface of the left fixed block  36  so as to be perpendicular to the center line  11 ; the moveable sleeve  43 , which is configured as a bottomed tube, and is moveably fitted on the center head  42 ; a connecting rod connecting member  44 , which is provided in order to connect the second connecting rod  27  to an outer surface  43   a  of the moveable sleeve  43 ; a connecting rod connecting member  46 , which is provided in order to connect the first connecting rod  26  and the third connecting rod  28  (not shown; see  FIG. 7 ) to the outer surface  43   a  of the moveable sleeve  43 ; and an intake valve mechanism  47 , which is provided to a top part  43   b  of the moveable sleeve  43 . Symbol  12   a  denotes a left cylinder axis, and symbol  13   a  denotes a right cylinder axis. These axes are perpendicular to a plane that passes through the axial line  14 A of the upper crankshaft  14  and the axial line  16 A of the lower crankshaft  16 , and are provided so as to extend toward either side of the upper crankshaft  14  and the lower crankshaft  16 . The left cylinder axis  12   a  is aligned with the center axes of the center head  42  and the moveable sleeve  43 . Symbol  48  denotes a combustion chamber formed by the center head  42  and the moveable sleeve  43 . 
   The engine case  41  has a case main body  51  and a liner cap that is fitted into a tubular part  51   a . The tubular part  51   a  is formed in an end part of the case main body  51 . The liner cap  52  has a liner part  52   a  that slides along the outer surface  43   a  of the moveable sleeve  43 , and an upper wall  52   b  that is integrally formed with the liner part  52   a  in order to block an end part of the liner part  52   a.    
   The center head  42  has a head main body  61 , which is integrally formed with the left fixed block  36 ; and a valve mechanism (not shown) described below and a spark plug (not shown), which are provided to the head main body  61 . 
   The head main body  61  has a stationary piston  61 A formed on an outer peripheral part, and a coolant channel  61   b  through which coolant flows. 
   The stationary piston  61 A is a bottomed tubular portion configured from the outer peripheral part and end part of the head main body  61 . A concave crown surface  61   d  is formed in the bottom of the stationary piston. A plurality of piston ring grooves is formed in an end part of an outer peripheral surface  61   c  near the crown surface  61   d , and piston rings are installed in the piston ring grooves. 
   The camshaft  38  is securely held between the left fixed block  36  and the right fixed block  37 , and is rotatably supported by bearings  63 . 
   The cam drive mechanism  39  has a camshaft drive gear  65  attached on the upper crankshaft  14 , and a camshaft driven gear  66  attached on the camshaft  38  so as to mesh with the camshaft drive gear  65 . 
   The camshaft driven gear  66  has twice as many teeth as the camshaft drive gear  65 , and rotates at ½ the rate at which the camshaft drive gear  65  rotates. 
   Since the internal combustion engine  10  is a four-cycle engine, the camshaft  38  thus rotates once for every two rotations of the upper crankshaft  14 . 
   For example, if the internal combustion engine  10  is a two-cycle engine, the camshaft  38  will rotate once for every rotation of the upper crankshaft  14 . 
   As shown in  FIG. 5 , the center head  42  has a valve mechanism  71  and a spark plug  72 . The head main body  61  of the center head  42  has an exhaust port  61   e  that is formed in the crown surface  61   d , and a thread  61   f  and a plug insertion hole  61   g  into which the spark plug  72  is inserted. 
   The valve mechanism  71  has an exhaust valve  74  for opening and closing an inlet of the exhaust port  61   e ; a valve guide  75  attached on the head main body  61  in order to moveably support the exhaust valve  74 ; a valve spring  77  interposed between a bottom of an empty space  61   h  formed in the head main body  61 , and a spring hanger member  76  formed on the end of the shaft of the exhaust valve  74 , in order to urge the exhaust valve  74  to the closing side; and a hollow camshaft  38  for directly driving the exhaust valve  74  via a cam  38   b . Reference number  78  denotes an annular valve seat on which the exhaust valve  74  rests, and the opening of the exhaust port  61   e  is formed in the valve seat. 
   Annular coolant channels  61   b  are formed around each of the exhaust port  61   e , the exhaust valve  74 , and the spark plug  72 ; and portions that reach high temperatures are better able to be cooled. 
   The intake valve mechanism  47  has a valve supporting part  43   d  integrally formed in the top part  43   b  of the moveable sleeve  43 ; three valve guides  81  (two valve guides  81  are shown in the present embodiment) attached on the valve support part  43   d ; intake valves  82  (two intake valves  82  are shown in the present embodiment) moveably inserted in the valve guides  81  in order to open and close three intake holes  43   e  (one intake hole  43   e  is shown here) formed in the top part  43   b  of the moveable sleeve  43 ; a single rocker shaft  83  attached on the valve support part  43   d ; three rocker arms  86  (one rocker arm  86  is shown here) that are pivotably attached on the rocker shaft  83 , and that have one end connected to the intake valves  82  via connecting pins  84 ; weights  87  attached on the other ends of the rocker arms  86 ; and three torsion coil springs  88  (one torsion coil spring  88  is shown here) provided between the valve support part  43   d  and the rocker arms  86  in order to close the intake valves  82  using a small amount of urging force. 
   The weight  87  is used in order to balance the intake valves  74  so that they do not move as a result of inertia when the moveable sleeve  43  is caused to move back and forth. 
   The liner cap  52  and the top part  43   b  of the moveable sleeve  43  are components that form an intake chamber  90  into which a mixed gas containing fuel and air is drawn. An intake manifold  91  is connected to the intake chamber  90  of the left cylinder  12  and the intake chamber  90  of the right cylinder  13 . A pair of leaf valves  92 ,  92  is provided to the inlet  91   a  of the intake manifold  91  as a one-way valve for only allowing the mixed gas to flow from a throttle body (not shown) connected to the intake manifold  91  to the intake chambers  90 ,  90 . 
   As shown in  FIG. 6 , an annular top ring groove  61   j , an annular secondary ring groove  61   k , and an annular oil ring groove  61   m  are formed in the stated order from the crown surface  61   d  on the outer surface  61   c  of the stationary piston  61 A. An annular top ring  95  is fitted in the top ring groove  61   j . An annular secondary ring  96  is fitted in the secondary ring groove  61   k . An annular oil ring  97  is fitted in the oil ring groove  61   m . The space between the stationary piston  61 A and the moveable sleeve  43  is sealed and lubricating oil is scraped off by the top ring  95 , the secondary ring  96 , and the oil ring  97 . 
   A tubular land part  43   f  is integrally formed with the moveable sleeve  43  closer to the upper wall  52   b  of the liner cap  52  than the top part  43   b . Annular seal ring grooves  43   h ,  43   j  are formed in an outer peripheral surface  43   g  of the land part  43   f . An annular seal ring  101  is fitted in the seal ring groove  43   h , and a seal ring  102  is fitted in the seal ring groove  43   j . The space between the moveable sleeve  43  and the liner part  52   a  of the liner cap  52  is sealed and lubricating oil is scraped off by the seal rings  101 ,  102 . 
   As shown in  FIG. 7 , the upper crankshaft  14  has a tapered shaft  14   a , a front journal shaft  14   b , a crank part  14   c , and a rear journal shaft  14   d . An upper crank output gear  103  is attached to the taper shaft  14   a  by a nut  104 . The front journal shaft  14   b  is rotatably attached to the left fixed block  36  (not shown) and the right fixed block  37  via a roller bearing  105 . A first connecting rod  26 , a second connecting rod  27 , and a third connecting rod  28  are connected to crank pins  20 ,  21 ,  22  provided to the crank part  14   c . The rear journal shaft  14   d  is rotatably attached to the left fixed block  36  and the right fixed block  37  via a ball bearing  106 . 
   The upper crankshaft  16  has a tapered shaft  16   a , a front journal shaft  16   b , a crank part  16   c , and a rear journal shaft  16   d . A lower crank output gear  107  is attached to the taper shaft  16   a  by a nut  108 . An idler gear  112  is rotatably attached to a front part of the front journal shaft  16   b  via ball bearings  111 ,  111 . A rear part of the front journal shaft  16   b  is rotatably attached to the left fixed block  36  and the right fixed block. A first connecting rod  26 , a second connecting rod  27 , and a third connecting rod  28  are connected to crank pins  30 ,  31 ,  32  provided to the crank part  16   c . The rear journal shaft  16   d  is rotatably attached to the left fixed block  36  and the right fixed block  37  via a ball bearing  116 . 
   The upper crank output gear  103  meshes with the idler gear  112 . The lower crank output gear  107  and the idler gear  112  mesh with inner teeth of a ring gear  118  disposed in front of the upper crank output gear  103  and the lower crank output gear  107 . 
   The output of the upper crankshaft  14  is outputted via the upper crank output gear  103 , the idler gear  112  and the ring gear  118  to an output shaft  120 , which is attached on the ring gear  118 . The output of the lower crankshaft  16  is output to an output shaft  120  via the lower crank output gear  107  and the ring gear  118 . The output shaft  120  is rotatably supported by the left fixed block  36  and the right fixed block  37  via a bearing (not shown). 
   The camshaft  38  is rotatably supported by the left fixed block  36  and the right fixed block  37  via the bearings  63 ,  122 . Reference number  123  denotes a nut for attaching the camshaft driven gear  66  to a tapered part  38   c  of the camshaft  38 . 
   The following is a summary of the operation of the internal combustion engine  10  described above. 
   In  FIG. 4 ,  FIG. 5 , and  FIG. 7 , when, for example, a mixed gas comprising fuel and air is supplied to the combustion chamber  48  via the intake manifold  91  and the intake chamber  90  in the left cylinder  12 , and the mixed gas is ignited in the combustion chamber. The pressure inside the combustion chamber  48  increases, and the moveable sleeve  43  moves toward the bottom dead center position; i.e., toward the upper wall  52   b  of the liner cap  52 , with respect to the center head  42 . 
   At this time, the upper crankshaft  14  and the lower crankshaft  16  are made to rotate by the first through third connecting rods  26 ,  27 ,  28 , which are attached to the moveable sleeve  43  via the connecting rod connecting members  44 ,  46 . The upper crankshaft  14  rotates in the opposite direction of the lower crankshaft  16 . 
   The rotation of the upper crankshaft  14  and the lower crankshaft  16  is transferred to the exterior of the internal combustion engine  10  from the output shaft  120  via the upper crank output gear  103 , the idler gear  112 , the lower crank output gear  107 , and the ring gear  118 . The rotation is maintained by the moment of inertia of the upper crankshaft  14  and lower crankshaft  16 , the upper crank output gear  103 , the idler gear  112 , the lower crank output gear  107 , and the ring gear  118 . The camshaft driven gear  66  is made to rotate by the rotation of the camshaft drive gear  65 . The cam  38   b  of the camshaft  38  drives the exhaust valves  74 ,  74 , and combustion gas is discharged at a prescribed timing. When the moveable sleeve  43  described above moves toward bottom dead center, the mixed gas in the intake chamber  90  is compressed by the moveable sleeve  43 , and passes through the intake manifold  91  to the intake chamber  90  of the right cylinder  13 . The intake valves  74  are opened, and the mixed gas is charged into the combustion chamber  48 . The moveable sleeve  43  is thereby caused to continuously move back and forth. 
   When the moveable sleeve  43  moves back and forth, the center head  42  in particular reaches high temperatures due to the combustion heat generated in the combustion chamber  48  and heat generated by the sliding of the components. Coolant is accordingly made to circulate through the coolant channel  61   b , whereby cooling is performed. 
   As shown in  FIG. 8 , the left fixed block  36  has a coolant channel  36   d , which extends downward from the coolant inlet  36   a ; a coolant channel  36   f , which is connected to the coolant channel  36   d  so as to be perpendicular thereto, and which is connected to the coolant channel  61   b , which annularly extends around the spark plug  72  (see  FIG. 5 ), and the intake valves  74  (see  FIG. 5 ); a horizontally extending coolant channel  36 , which is connected to the coolant channel  61   b ; and a coolant channel  36   h , which extends upward to the coolant inlet  36   b  from a coolant channel  36   g  so as to be perpendicular thereto. The right fixed block  37  is provided with similar coolant channels. 
   As shown in  FIG. 9A , the coolant channel  61   b  has a first channel  61   p , which surrounds the plug insertion hole  61   g  through which the spark plug  72  is inserted (see  FIG. 5 ); a second channel  61   q , which surrounds the exhaust valve  74 ; and a third channel  61   r  annularly inside the stationary piston  61 A. 
   An exhaust channel  79  is formed inside an inner wall  61   u  positioned inside the first channel  61   p , the second channel  61   q , and the third channel  61   r . An exhaust port  61   e , which extends from the exhaust valve  74 , is connected to the exhaust channel  79  via two exhaust port through-holes  61   v  that pass through the exhaust port  61   e.    
   The exhaust channel  79  is connected to the exhaust outlets  36   c ,  37   c  (both shown in  FIG. 3 ) described above. 
   As shown in  FIG. 9B , the first channel  61   p  is formed around the plug insertion hole  61   g . The second channel  61   q  is formed around the exhaust port  61   e , a valve guide insertion hole  61   s , and the empty space  61   h . The third channel  61   r  is formed inside the side wall  61   t  and the crown surface  61   d  of the stationary piston  61 A. 
   As shown in  FIG. 9C , the third channel  61   r  is a portion formed along the crown surface  61   d  in the vicinity thereof. The crown surface  61   d , which reaches high temperatures as a result of being subjected to combustion heat from the combustion chamber  48  (see  FIG. 5 ), can be effectively cooled by coolant that flows through the third channel  61   r.    
   The action of each stroke of the internal combustion engine  10  described above will be described below. The letter “L” has been added at the end of the symbols of the components in the left cylinder  12 , and the letter “R” has been added at the end of the symbols of the components in the right cylinder  13 . 
     FIG. 10A  shows a state in which the moveable sleeves  43 L,  43 R of the left cylinder  12  and right cylinder  13  are moved toward the center of the internal combustion engine  10 , and the moveable sleeves  43 L,  43 R reach top dead center. 
   The exhaust valve  74 L is open and the intake valve  82 L is closed in the left cylinder  12  until top dead center is reached, and combustion gas that has exploded within the combustion chamber  48 L is discharged. The exhaust valve  74 L is and the intake valve  82 L are closed in the right cylinder  13 , the spark plug  72 R is ignited before the engine reaches top dead center, and the mixed gas is caused to explode. The moveable sleeve  43 R moves from top dead center to bottom dead center as result of the increase of pressure within the combustion chamber  48 R. 
   As a consequence of the moveable sleeves  43 L,  43 R moving toward top dead center, pressure decreases within the intake chambers  90 L,  90 R of the left cylinder  12  and the right cylinder  13 . Therefore, the leaf valves  92 ,  92  in the intake manifold  91  open, and the mixed gas flows into the intake chambers  90 L,  90 R as shown by the arrow. 
   As shown in  FIG. 10B , the mixed gas in the intake chamber  90 R is compressed when the moveable sleeve  43 R of the right cylinder  13  moves to bottom dead center as a result of the high pressure generated by the combustion of the mixed gas in the combustion chamber  48 R. As a result, the mixed gas moves from the intake chamber  90 R, through the channels in the exhaust manifold  91 , and into the intake chamber  90 L in the left cylinder  12 . While the moveable sleeve  43  of the left cylinder  12  moves from top dead center to bottom dead center, the intake valve  82 L opens due to the pressure within the intake chamber  90 L, and the intake gas flows into the combustion chamber  48 L. In other words, the mixed gas continuously flows into the combustion chamber  48 L of the left cylinder  12 . 
     FIG. 11A  shows a state in which the moveable sleeves  43 L,  43 R of the left cylinder  12  and the right cylinder  13  have once again reached top dead center. The exhaust valve  74 L and the intake valve  82 L in the left cylinder  12  are closed until top dead center is reached. The spark plug  72 L is ignited and the mixed gas explodes before top dead center is reached. The moveable sleeve  43 L moves from top dead center to bottom dead center in concert with the increase in pressure in the combustion chamber  48 L. In the right cylinder  13 , the exhaust valve  74 R opens and the intake valve  82 R closes, and combustion gas that has exploded within the combustion chamber  48 R is discharged. 
   As a consequence of the moveable sleeves  43 L,  43 R moving toward top dead center, in pressure decreases within the intake chambers  90 L,  90 R of the left cylinder  12  and the right cylinder  13 . Therefore, the leaf valves  92 ,  92  in the intake manifold  91  open, and the mixed gas flows into the intake chambers  90 L,  90 R as shown by the arrow. 
   As shown in  FIG. 11B , the mixed gas in the intake chamber  90 L is compressed when the moveable sleeve  43 L of the left cylinder  12  moves to bottom dead center as a result of the high pressure generated by the combustion of the mixed gas in the combustion chamber  48 L. As a result, the mixed gas moves from the intake chamber  90 L, through the channels in the exhaust manifold  91 , and into the intake chamber  90 R in the right cylinder  13 . While the moveable sleeve  43  of the right cylinder  13  moves from top dead center to bottom dead center, the intake valve  82 R opens due to the pressure within the intake chamber  90 R, and the intake gas flows into the combustion chamber  48 R. In other words, the mixed gas continuously flows into the combustion chamber  48 R of the right cylinder  13 . 
   The flow of coolant through the coolant channels in the center head  42  described above is illustrated in  FIG. 8  and  FIG. 9 . A description will be provided hereunder of the left fixed block  36  and the left cylinder  12 . The right fixed block  37  and the right cylinder  13  are identical to the left fixed block  36  and the left cylinder  12 , and descriptions thereof have been omitted. 
   In  FIG. 8 , coolant flows from the coolant inlet  36   a  to the coolant channel  61   b  through the coolant channel  36   d  and the coolant channel  36   f , as shown by the arrow. 
   Coolant in the coolant channel  61   b  flows toward the crown surface  61   d  through the first channel  61   p  around the plug insertion hole  61   g , and cools both the spark plug  72  and the area surrounding same, as shown in  FIG. 9B . As shown in  FIG. 9B  and  FIG. 9C , coolant flows through the third channel  61   r , which extends from the first channel  61   p  along the crown surface  61   d , and cools the crown surface  61   d  and the side wall  61   t , and particularly the top ring groove  61   j , the secondary ring groove  61   k , the oil ring groove  61   m , the top ring  95  (see  FIG. 6 ), the secondary ring  96  (see  FIG. 6 ), and the oil ring  97  (see  FIG. 6 ). Coolant also flows through the second channel  61   q  around the exhaust valve  74  and cools the exhaust valve  74  and the area surrounding same. 
   The coolant then flows from the coolant channel  61   b  to the coolant outlet  36   c  through the coolant channel  36   g  and the coolant channel  36   h.    
   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.