Patent Publication Number: US-11041417-B2

Title: Internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase Application of PCT/JP2017/044430, filed Dec. 11, 2017, the contents of which are hereby expressly incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to an internal combustion engine provided with a breather chamber. 
     BACKGROUND ART 
     An internal combustion engine provided with a breather chamber for separating oil from blow-by gas in a cylinder block is known (for example, Patent Document 1). In the internal combustion engine disclosed in Patent Document 1, a first recess is formed on one side of a cylinder block, and a first breather chamber is formed by closing the first recess with a first cover. Further, a second recess is formed on another side of the cylinder block, and a second breather chamber is formed by closing the second recess with a second cover. The first breather chamber is communicated with the second breather chamber via a communicating passage extending laterally along a side of the cylinder. The breather gas flows through the first breather chamber, the communication passage and the second breather chamber, in that order, before flowing into an intake device. 
     PRIOR ART DOCUMENT(S) 
     Patent Document(s) 
     
         
         Patent Document 1: JP3819685B2 
       
    
     SUMMARY OF THE INVENTION 
     Task to be Accomplished by the Invention 
     The internal combustion engine mentioned above has a problem that the number of component parts increases because additional covers are required to form the breather chambers. 
     In view of such a back ground, an object of the present invention is to form a breather chamber without increasing the number of component parts. 
     Means to Accomplish the Task 
     To achieve such an object, the present invention provides an internal combustion engine ( 1 ), comprising: an engine block ( 30 ) defining a cylinder ( 2 ); a case member ( 19 ) fastened to a lower part of the engine block to define a crank chamber ( 4 ) jointly with the engine block; and a bearing member ( 50 ) fastened to the engine block in the crank chamber to rotatably support a crankshaft; wherein the engine block is provided with a central recess ( 134 ) formed in a part thereof to which the bearing member is fastened, a blow-by gas inlet passage ( 112 ,  116 ,  121 ) communicating the central recess with the crank chamber, and a blow-by gas return passage ( 122 ,  118 ) forming a part of a passage communicating the central recess with an intake device, the central recess and the bearing member jointly defining a central breather chamber. 
     According to this arrangement, the central breather chamber can be formed above the bearing member in the cylinder block. The central breather chamber can be formed by the recess formed in the cylinder block and the bearing member fastened to the cylinder block. Since the recess is exposed to the outside before being closed by the bearing member, the necessary processing such as deburring can be performed during the process of forming the engine block. 
     In this arrangement, preferably, the bearing member extends orthogonally to an axis of the crankshaft as viewed in a direction extending along an axis of the cylinder, and is provided with a pair of bolt holes ( 59 ) through which bolts ( 62 ) for fastening the bearing member to the engine block pass in either end part thereof, and 
     the engine block is provided with a pair of female screw holes ( 61 ) into which the respective bolts are threaded in parts thereof opposing the respective bolt holes, 
     the central recess being positioned between the female screw holes. 
     Thus, the central breather chamber can be formed by making use of the part of the cylinder block located between the female screw holes. 
     In this arrangement, preferably, the central recess extends along the bearing member. 
     Thereby, the central breather chamber can be conveniently positioned between the female screw holes. 
     In this arrangement, preferably, the central recess is formed deeper than the female screw holes, and at least one of the blow-by gas inlet passage and the blow-by gas return passage extends above one of the female screw holes. 
     Thereby, the blow-by gas inlet passage and the blow-by gas return passage can be arranged around the cylinder in a compact fashion while avoiding the female screw holes. 
     In this arrangement, preferably, at least one baffle wall ( 135 ) projects into the central recess. 
     Thereby, oil can be separated from the blow-by gas in the first breather chamber. 
     In this arrangement, preferably, at least one of the engine block and the bearing member is provided with an oil return passage communicating the central breather chamber with the crank chamber. 
     Thereby, the oil collected in the central breather chamber can be returned to the crank chamber. 
     In this arrangement, preferably, as viewed in a direction extending along an axis of the crankshaft, the blow-by gas inlet passage is located on one side of the cylinder, and the blow-by gas return passage is located on another side of the cylinder, 
     the blow-by gas inlet passage includes an upstream breather chamber ( 116 ), and a first connection passage ( 121 ) extending linearly from the upstream breather chamber to the central breather chamber, and 
     the blow-by gas return passage includes a downstream breather chamber ( 118 ), and a second connection passage ( 122 ) extending linearly from the downstream breather chamber to the central breather chamber. 
     Since the upstream breather chamber and the downstream breather chamber are provided in addition to the central breather chamber, the oil can be separated from the blow-by gas in a more reliable manner. 
     In this arrangement, preferably, the upstream breather chamber is formed by a first recess ( 131 ) formed on one side surface of the engine block and a first cover ( 132 ) closing the first recess, and the upstream breather chamber is formed by a second recess ( 132 ) formed on another side surface of the engine block and a second cover ( 138 ) closing the second recess, and 
     wherein an end of the first connection passage opens at a bottom portion of the first recess, and an end of the second connection passage opens at a bottom portion of the second recess. 
     Thereby, when forming the cylinder block, the first connection passage and the second connection passage can be formed by a mold. Since the first recess is formed between the first connection passage and the second connection passage, the stroke or the travel of the mold that is required to form the first connection passage and the second connection passage can be minimized so that the size of the molding device or the casting device can be reduced. 
     In this arrangement, preferably, a transmission mechanism case accommodating a transmission mechanism therein is provided on a side part of the cylinder block, and the central recess is formed in a part forming both a part of both the cylinder block and a part of the transmission mechanism case. 
     Thereby, the central breather chamber can be formed in a boundary part between the cylinder block and the transmission mechanism case. 
     Effect of the Invention 
     According to the above configuration, it is possible to form a breather chamber without increasing the number of component parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view of an internal combustion according to the present embodiment as viewed from behind; 
         FIG. 2  is a section view of an internal combustion according to the present embodiment as viewed from right; 
         FIG. 3  is an enlarged sectional view of an engine block as viewed from right showing a blow-by gas passage; 
         FIG. 4  is a perspective of the internal combustion engine of the present embodiment; 
         FIG. 5  is an exploded perspective of the internal combustion engine of the present embodiment; 
         FIG. 6  is a sectional view taken along line VI-VI of  FIG. 3 ; 
         FIG. 7  is a bottom view of the engine block; and 
         FIG. 8  is a side view of a front recessed part of the engine block as viewed from front. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     An internal combustion engine according to a preferred embodiment of the present invention is described in the following with reference to the appended drawings. 
     (Overall Structure of Internal Combustion Engine) 
     As shown in  FIGS. 1 to 4 , the internal combustion engine  1  consists of a single cylinder, overhead camshaft (OHC), four-stroke reciprocating engine. The internal combustion engine  1  is provided with a cylinder block  3  defining a cylinder  2 , a crankcase  5  provided under the cylinder block  3  to define a crank chamber  4 , a cylinder head  7  provided above the cylinder block  3  to close an upper end of the cylinder  2 , a valve actuating chamber case  9  provided above the cylinder head  7  to define a valve actuating chamber, and a belt case  11  (a transmission mechanism case) connecting the crankcase  5  and the valve actuating chamber case  9  with each other. The crank chamber  4  receives a crankshaft  12  therein, the valve actuating chamber receives a valve actuating mechanism  14  including a camshaft  13  therein, and the belt case  11  receives a power transmission mechanism  15  that transmits the rotational force of the crankshaft  12  to the camshaft  13  therein. 
     The crankcase  5  consists of a first case half  18  and a second case half  19  (case member) which are divided from each other by a parting surface  17  that is inclined with respect to the axis of the crankshaft  12  that extends in the lateral direction. The parting surface  17  is inclined upward (to the right) toward the belt case  11 . The first case half  18  forms a left end wall  21  of the crankcase  5  and upper portions of a front side wall  22  and a rear side wall  23 . The second case half  19  forms a right end wall  24  of the crankcase  5 , lower portions of the front side wall  22  and the rear side wall  23 , and a bottom wall  25 . The first case half  18 , the cylinder block  3 , the cylinder head  7 , the belt case  11 , and the valve actuating chamber case lower part  27  forming a lower part of the valve actuating chamber case  9  are formed as an engine block  30  formed integrally by metal casting. The second case half  19  may be, for example, a stamp formed product of a metal plate or a molded plastic product. The second case half  19  is fastened to the first case half  18  by bolts. A valve actuating chamber case upper part  31 , which is an upper part of the valve actuating chamber case  9 , is, for example, a stamp formed product or a molded plastic product, and is fastened to a valve actuating chamber case lower part  27 , which is a lower part of the valve actuating chamber case  9 , by bolts or the like. 
     The cylinder  2  extends vertically inside the cylinder block  3 . The cylinder head  7  is formed integrally with the cylinder block  3  and closes the upper end of the cylinder  2 . The cylinder head  7  has an intake port  33  and an exhaust port  34  opening at the upper end of the cylinder  2 . The intake port  33  is provided with an intake valve, and the exhaust port  34  is provided with an exhaust valve. The cylinder head  7  is provided with a spark plug. 
     The left end wall  21  of the crankcase  5  is provided with a first bearing  38  that rotatably supports a left journal of the crankshaft  12 . The first bearing  38  may be a ball bearing or a plain bearing. The left end of the crankshaft  12  projects leftward from the left end wall  21  of the crankcase  5 , and is connected to a generator  41 . The generator  41  has a stator including a coil mounted on the outer surface of the left end wall  21 , and a rotor including a permanent magnet coupled to the left end of the crankshaft  12 . 
     A support wall  48  protruding downward is provided on a part of the lower surface of the cylinder block  3  located to the right of the cylinder  2 . The support wall  48  is arranged on the left side of the lower end of the belt case  11 . The support wall  48  forms a part of the wall of the cylinder block  3  and also forms a part of the wall of the belt case  11 . Further, the support wall  48  is disposed at a boundary between the cylinder block  3  and the belt case  11 . 
     A bearing member  50  that rotatably supports the right end of the crankshaft  12  is attached to the lower end of the support wall  48 . The bearing member  50  has a support body  51  fastened to the support wall  48 . The lower end of the support wall  48  is formed with a flat block-side fastening surface  52 , and the upper end of the support body  51  is formed with a bearing-side fastening surface  53  which is in contact with the block-side fastening surface  52 . The support wall  48  and the support body  51  extend in a direction (fore and aft direction) perpendicular to the crankshaft  12  when viewed from a direction extending along the axis of the cylinder  2  (vertical direction), and are each formed in a plate shape having a major plane facing in the lateral direction. 
     As shown in  FIG. 5 , a flange  54  protrudes laterally from the right edge of the upper end of the support body  51 . A pair of first pin holes  56  are passed vertically through the flange  54 . As shown in  FIGS. 6 and 7 , a pair of second pin holes  57  are passed into a part of the block-side fastening surface  52  facing the two first pin holes  56 . The position of the support body  51  with respect to the cylinder block  3  is determined by inserting positioning pins  58  into the first pin holes  56  and the second pin holes  57  in a corresponding manner. As shown in  FIG. 3 , when the positioning pins  58  are inserted into the first pin hole  56  and the second pin hole  57  in a corresponding manner, the lower ends of the positioning pins  58  protrude downward from the corresponding first pin holes  56 . 
     As shown in  FIGS. 3 and 5 , a bolt hole  59  is passed vertically through each of a front end part and a rear end part of the support body  51 . A pair of female screw holes  61  are formed in the block-side fastening surface  52  at parts thereof facing the respective bolt holes  59 . The support body  51  of the bearing member  50  is fastened to the cylinder block  3  by a pair of bolts  62  that are inserted through the respective bolt holes  59  and threaded into the respective female screw holes  61 . 
     A bearing hole  63  extends laterally through the support body  51 . A second bearing  65  is fitted in the bearing hole  63 . The second bearing  65  consists of a ball bearing in the present embodiment, but may be either a ball bearing or a plain bearing. The right end edge part of the bearing hole  63  is formed with a plurality of engagement pieces  67  for engaging a retaining ring  66 . The engagement pieces  67  engage the outer peripheral portion of the retaining ring  66  so as to keep the inner peripheral portion of the retaining ring  66  protruding into the bearing hole  63 . As shown in  FIG. 1 , the second bearing  65  is held in the bearing hole  63  by abutting against a shoulder surface formed on the crankshaft  12  and the retaining ring  66 . As shown in  FIG. 3 , the upper peripheral parts of the retaining ring  66  are in contact with the lower ends of the positioning pins  58  so as to prevent the positioning pins  58  from coming out of the first pin holes  56  and the second pin holes  57 . 
     As shown in  FIG. 1 , the crankshaft  12  is provided with a pair of crank webs located between the first bearing  38  and the second bearing  65  and a crankpin extending between the crank webs. The crankpin is connected to a piston  74  slidably received in the cylinder  2  via a connecting rod  73 . The right end of the crankshaft  12  protrudes to the right of the second bearing  65  and is spaced from the inner surface of the right end wall  24  of the crankcase  5  by a certain distance. 
     A support shaft  81  extends in parallel with the crankshaft  12  in a lower part of the valve actuating chamber case  9 . A cylindrical camshaft  13  is rotatably supported on the support shaft  81 , and a cam pulley  82  is coaxially attached to an end of the camshaft  13 . The cam pulley  82  rotates integrally with the camshaft  13  relative to the support shaft  81 . A cam  83  protrudes from the outer peripheral surface of the camshaft  13  in a radial direction. The camshaft  13  and the cam pulley  82  are arranged in the valve actuating chamber. 
     The crank pulley  78  and the cam pulley  82  are each provided with teeth on the outer periphery thereof. A toothed belt  85  formed as an endless transmission belt is passed around the crank pulley  78  and the cam pulley  82 . The toothed belt  85  extends from the crank chamber  4  in which the crank pulley  78  is disposed, passes through a belt chamber  86  (transmission mechanism chamber) defined in the belt case  11 , and extends into a valve actuating chamber in which the cam pulley  82  is disposed. The belt case  11  extends vertically on one side of the cylinder block  3 , and the belt chamber  86  communicates with an upper part of the crank chamber  4  at a lower end thereof and with a lower part of the valve actuating chamber at an upper end thereof. The crank pulley  78 , the cam pulley  82 , and the toothed belt  85  constitute the power transmission mechanism  15  that transmits the rotation of the crankshaft  12  to the camshaft  13 . 
     The valve actuating mechanism  14  opens and closes the intake valve and the exhaust valve at a predetermined timing according to the rotation of the crankshaft  12 . The valve actuating mechanism  14  includes, in addition to the camshaft  13 , an intake rocker arm and an exhaust rocker arm rotatably supported by a rocker shaft provided in the valve actuating chamber case lower part  27 , and actuated by the cam  83  so as to selectively push the intake valve and the exhaust valve in the opening direction, and a pair of valve springs urging the intake valve and the exhaust valve, respectively, in the closing direction. 
     As shown in  FIG. 2 , an intake device  90  is connected to the intake port  33 . The intake device  90  is arranged on the front side of the cylinder block  3 , and is provided with an intake inlet (not shown the drawings), an air cleaner  91 , and a carburetor  92 , in that order from the upstream end. The carburetor  92  is supplied with fuel from a fuel tank  93  provided above the valve chamber  9 . An exhaust muffler  95  is connected to the exhaust port  34 . The exhaust muffler  95  is covered by a muffler cover supported by the cylinder block  3 . 
     As shown in  FIG. 1 , oil (lubricating oil) is stored in a lower part of the crank chamber  4 . The big end of the connecting rod  73  is provided with an oil dipper  98  (oil scraper) that projects in the radial direction of the crankshaft  12 . A stirring wheel  99  is rotatably supported in a lower part of the right end wall  24  formed by the second case half  19 . The stirring wheel  99  has a disk portion, a plurality of blades protruding from the outer peripheral surface of the disk portion, and a gear formed on the outer peripheral surface of the disk portion. The gear of the stirring wheel  99  meshes with a gear formed on the outer peripheral surface of the crank pulley  78  so that the stirring wheel  99  rotates jointly with the crank pulley  78 . When the internal combustion engine  1  is operated, the oil surface  100  of the oil in the crank chamber  4  is arranged so as to be above the lower end of the stirring wheel  99  so that the oil is splashed by the stirring wheel  99 . An oil level sensor  102  is provided on the bottom wall  25  of the crankcase  5 . 
     (Blow-by Gas Passage) 
     As shown in  FIG. 3 , the internal combustion engine  1  is provided with a blow-by gas passage  110  for returning the blow-by gas in the crank chamber  4  to the intake device  90 . The blow-by gas passage  110  includes a gas inlet  111 , an inlet passage  112 , a breather chamber  113 , and a connection pipe  114 , in series in that order from the upstream end. The blow-by gas passage  110  opens to the crank chamber  4  at the gas inlet  111  and is communicated with the air cleaner  91  via the connection pipe  114 . The breather chamber  113  is partitioned into an upstream breather chamber  116 , a central breather chamber  117 , and a downstream breather chamber  118 , in that order from the upstream end. The upstream breather chamber  116  and the central breather chamber  117  are communicated with each other via a first connection passage  121 , and the central breather chamber  117  is communicated with the downstream breather chamber  118  via a second connection passage  122 . 
     As shown in  FIGS. 3 and 7 , an upper part of the rear side wall  23  of the crankcase  5  formed by the first case half  18  has a bulging portion  124  protruding forward. The gas inlet  111  is formed on the lower surface of the bulging portion  124  and opens downward. The inlet passage  112  extends upward from the gas inlet  111  inside the bulging portion  124 . In the present embodiment, the gas inlet  111  and the inlet passage  112  are each partitioned into a pair of parallel passage parts by a partition wall  125 . 
     As shown in  FIGS. 3, 5, and 6 , a rear recess  131  which is recessed toward the cylinder  2  is formed on the rear side surface of the cylinder block  3 . The upper end of the inlet passage  112  opens at a lower part of the rear recess  131 . The rear recess  131  is closed by a rear cover  132  fastened to the cylinder block  3  so as to define the upstream breather chamber  116  jointly with the rear cover  132 . One end of the first connection passage  121  opens at an upper part of the bottom surface of the rear recess  131 . A plurality of baffle walls  133  project from the bottom surface of the rear recess  131  toward the rear cover  132 . The baffle walls  133  meander the flow path extending in the upstream breather chamber  116  from the inlet passage  112  provided in the lower part thereof to the first connection passage  121  provided in the upper part thereof. 
     As shown in  FIGS. 3, 6, and 7 , the block-side fastening surface  52  of the support wall  48  on the lower surface of the cylinder block  3  is formed with a central recess  134  which is recessed upward. The central recess  134  is formed in a part forming a part of the cylinder block  3  which also forms a part of the belt case  11 . In other words, the central recess  134  is formed in a wall part that separates the cylinder block  3  and the belt case  11  from each other. The central recess  134  extends between the female screw holes  61  in a direction orthogonal to the axis of the crankshaft  12  in a plan view. The central recess  134  is disposed on the left side of the second pin holes  57 . The central recess  134  is closed by the support body  51  fastened to the block-side fastening surface  52 , and forms the central breather chamber  117  jointly with the support body  51 . The central breather chamber  117  is arranged on one side of the cylinder  2  and above the bearing member  50 . The central breather chamber  117  extends along the support body  51  of the bearing member  50 . The central recess  134  is formed deeper than the female screw holes  61 . In other words, the upper end of the central recess  134  is located above the upper ends of the female screw holes  61 . 
     The first connection passage  121  extends in the fore and aft direction above the rear second pin hole  57 , and communicates an upper part of the upstream breather chamber  116  with an upper part of the front end of the central breather chamber  117 . The first connection passage  121  extends linearly, and has an increased width in a part thereof extending from the central breather chamber  117  to the upstream breather chamber  116 . One end of the second connection passage  122  opens at an upper end part of the rear end of the central breather chamber  117 . At least one baffle wall  135  protrudes downward from the bottom surface of the central recess  134 . The baffle wall  135  has a major plane facing in the fore and aft direction, and the left and right ends of the baffle wall  135  are connected to the left and right wall surfaces of the central recess  134 , respectively. The baffle wall  135  causes the flow path extending from the first connection passage  121  provided in an upper rear end part of the central breather chamber  117  to the second connection passage  122  provided in an upper front end part of the central breather chamber  117  to meander as the flow path extends downward. 
     With reference to the central breather chamber  117 , the inlet passage  112 , the upstream breather chamber  116 , and the first connection passage  121  function as a blow-by gas inlet passage communicating the crank chamber  4  with the central breather chamber  117 . In addition, the second connection passage  122  and the downstream breather chamber  118  function as a blow-by gas return passage which forms a part of a passage communicating the central breather chamber  117  with the intake device  90 . 
     As shown in  FIGS. 3, 6 and 8 , the front side surface of the cylinder block  3  is formed with a front recess  137  which is recessed toward the cylinder  2 . The front recess  137  is closed by a front cover  138  fastened to the cylinder block  3  to define a downstream breather chamber  118  jointly with the front cover  138 . The downstream end of the second connection passage  122  opens at an upper right part of the bottom surface of the front recess  137 . The second connection passage  122  extends in the fore and aft direction above the front female screw hole  61 , and communicates an upper part of the front end of the central breather chamber  117  with an upper part of the downstream breather chamber  118 . The second connection passage  122  extends linearly, and has an increased width in a part thereof extending from the central breather chamber  117  to the downstream breather chamber  118 . A one-way valve  141  is provided at the open end of the second connection passage  122  in the front recess  137 . The one-way valve  141  consists of a reed valve, and allows the flow from the second connection passage  122  to the downstream breather chamber  118 , while blocking the flow from the downstream breather chamber  118  to the second connection passage  122 . 
     A through hole  142  is formed on an upper left side part of the front cover  138 , and one end of the connection pipe  114  is inserted therein. The connection pipe  114  may be, for example, a flexible hose. The end of the connection pipe  114  is in contact with an abutment wall  143  protruding from the bottom surface of the front recess  137  so that the insertion depth is fixed. Further, the outer periphery of the connection pipe  114  and the edge of the through hole  142  are in closed contact with each other in an airtight manner. The thickness of the abutment wall  143  is selected to be smaller than the inner diameter of the connection pipe  114  so the interior of the connection pipe  114  and the downstream breather chamber  118  communicate with each other. The other end of the connection pipe  114  communicates with the inside of the air cleaner  91 . 
     As shown in  FIG. 8 , a plurality of baffle walls  144  protrude from the bottom surface of the front recess  137  toward the front cover  138 . At least one of the baffle walls  144  is disposed between the downstream end of the second connection passage  122  and the upstream end of the connection pipe  114 , and extends vertically. The baffle walls  144  cause the flow path from the second connection passage  122  provided in the upper right part of the downstream breather chamber  118  to the connection pipe  114  provided in the upper left part of the downstream breather chamber  118  to meander as the flow path extends downward. 
     As shown in  FIG. 3 , the lower end of the front recess  137  communicates with the upper end of the front bolt hole  59  of the support body  51  via a connection passage  146 . The connection passage  146  is formed at least in the cylinder block  3 , opens at the block-side fastening surface  52 , and communicates with the bolt hole  59 . Further, the connection passage  146  may be formed in the support body  51  in addition to the cylinder block  3 . Further, the connection passage  146  may communicate with the front female screw hole  61 . The support body  51  is formed with an oil return port  147  that opens from a lower part of the front bolt hole  59  to the side surface (left side surface) of the support body  51  on the side of the cylinder  2 . The oil return port  147  is provided in a lower part of the support body  51 , and in particular in a lower end part of the support body  51  or an adjacent part thereto. The lower part of the support body  51  refers to a part located below the center of the bearing hole, and the lower end part of the support body  51  refers to a part located below the lower end of the bearing hole  63 . The outer diameter of the bolt  62  is smaller than the inner diameter of the bolt hole  59 . so that a space through which oil can flow is defined between the inner circumferential surface of the bolt hole  59  and the outer circumferential surface of the bolt  62 . Further, in order to enlarge the flow path, a groove extending in the longitudinal direction (vertical direction) may be formed on one of the inner circumferential surface of the bolt hole  59  and the outer circumferential surface of the bolt  62 . The connection passage  146 , the space between the inner peripheral surface of the bolt hole  59  and the outer peripheral surface of the bolt  62 , and the oil return port  147  communicate with one another so as to define a first oil return passage  150  that returns the oil collected at the bottom of the downstream breather chamber  118  to the crank chamber  4 . 
     The cylinder block  3  is formed with a second oil return passage  151  which communicates the lower end of the central breather chamber  117  with the crank chamber  4 . The second oil return passage  151  may be formed as a small through hole. In another embodiment, the second oil return passage  151  communicates the lower end of the central breather chamber  117  with the front bolt hole  59 . In this case, the space between the inner circumferential surface of the bolt hole  59  and the outer circumferential surface of the bolt  62  serves both as the first oil return passage  150  and the second oil return passage  151 . 
     The mode of operation of the internal combustion engine  1  configured as described above is discussed in the following. As shown by the white arrows in  FIG. 3 , the blow-by gas flows from the gas inlet  111  to the inlet passage  112 , the upstream breather chamber  116 , the first connection passage  121 , the central breather chamber  117 , the second connection passage  122 , the downstream breather chamber  118 , and the connection pipe  114 , in that order, before flowing into the air cleaner  91  of the intake device  90 . The blow-by gas is mixed with fresh air in the air cleaner  91 , and is supplied to the cylinder  2  via the carburetor  92  and the intake port  33 . The blow-by gas collides with the baffle walls  133 ,  135 ,  144  in the upstream breather chamber  116 , the central breather chamber  117 , and the downstream breather chamber  118  so that oil is separated from the blow-by gas. The oil collected in the upstream breather chamber  116  flows downward by gravity, passes through the inlet passage  112  and the gas inlet  111 , and returns to the crank chamber  4 . As shown by the black arrows in  FIG. 3 , the oil collected in the central breather chamber  117  flows downward by gravity, passes through the second oil return passage  151 , and returns to the crank chamber  4 . The oil collected in the downstream breather chamber  118  flows downward by gravity, passes through the first oil return passage  150 , and returns to the crank chamber  4 . 
     The effect of the internal combustion engine  1  configured as described above is discussed in the following. Since the bearing member  50  that supports the crankshaft  12  is disposed in a region surrounding the lower end of the cylinder  2 , the bearing member  50  is positioned substantially at the center of the crank chamber  4 . Therefore, by forming the oil return port  147  in the support body  51  of the bearing member  50 , the oil return port  147  can be disposed substantially at the center of the crank chamber  4 , and is therefore located above the surface  100  of the oil stored in the lower part of the crank chamber  4 . The bearing member  50  and the oil return port  147  are positioned above the oil surface  100  without regard to the orientation of the internal combustion engine  1 , or even when any one of the front side, the rear side, the left end face, and the right end face of the internal combustion engine  1  is facing down. In particular, since the oil return port  147  is formed on the left side surface (the side surface on the side of the cylinder  2 ) of the support body  51 , even when the right end surface of the internal combustion engine  1  is facing down, an adequate distance can be secured between the oil surface  100  and the oil return port  147 . Thereby, the oil is prevented from flowing backward from the oil return port  147 . 
     The bearing member  50  that supports the crankshaft  12  is an essential component of the internal combustion engine  1 . Therefore, by forming the first oil return passage  150  in the bearing member  50 , an increase in the number of component parts and an increase in the size of the internal combustion engine  1  can be avoided. 
     Since the bolt hole  59  also serves as a part of the first oil return passage  150 , an additional passage is not required to be formed in the support body  51 . Therefore, the support body  51  can be made compact. Further, by using the female screw hole  61  as a part of the first oil return passage  150 , the length of the connection passage  146  can be minimized. 
     In addition, by using the bearing member  50 , the central breather chamber  117  can be formed without increasing the number of component parts. Since the central recess  134  has an open end facing downward when not closed by the bearing member  50 , necessary processing such as deburring can be performed during the process of forming the engine block  30 . 
     Since a part of the cylinder block  3  located between the female screw holes  61  is effectively utilized as a part of the central breather chamber  117 , an increase in the size of the cylinder block  3  can be avoided. Further, by extending the central breather chamber  117  along the bearing member  50 , the central breather chamber  117  can be expanded. By arranging the second connection passage  122  above the female screw holes  61 , the thickness of the wall of the cylinder block  3  can be minimized. 
     By providing the central breather chamber  117  centrally of the cylinder block  3  with respect the fore and aft direction, the lengths of the first connection passage  121  and the second connection passage  122  communicating the upstream breather chamber  116  and the downstream breather chamber  118  with each other in the fore and aft direction can be minimized. Thereby, when forming the engine block  30 , the necessary stroke or travel of the mold in forming the first connection passage  121  and the second connection passage  122  can be minimized. Thereby, the molding device or the casting device of the engine block  30  can be downsized. 
     The present invention has been described in terms of a specific embodiment, but is not limited by such an embodiment, and can be modified in various ways without departing from the scope of the present invention. The toothed belt  85  and the pulleys  76  and  82  of the power transmission mechanism  15  may be replaced by a chain and sprockets. In this case, the transmission mechanism will be a chain case, instead of a belt case. 
     Glossary of Terms 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                  1: internal combustion engine 
                  2: cylinder 
               
               
                  4: crank chamber 
                  12: crankshaft 
               
               
                  19: second case half (case member) 
                  30: engine block 
               
               
                  50: bearing member 
                  51: support body 
               
               
                  52: block-side fastening surface 
                  53: bearing-side fastening surface 
               
               
                  56: first pin hole 
                  57: second pin hole 
               
               
                  58: positioning pin 
                  59: bolt hole 
               
               
                  61: female screw hole 
                  62: bolt 
               
               
                  63: bearing hole 
                  65: second bearing 
               
               
                  66: retaining ring 
                  67: locking piece 
               
               
                  90: intake device 
                 100: oil surface 
               
               
                 110: blow-by gas passage 
                 111: gas inlet 
               
               
                 112: inlet passage 
                 113: breather chamber 
               
               
                 114: connection pipe 
                 116: upstream breather chamber 
               
               
                 117: central breather chamber 
                 118: downstream breather chamber 
               
               
                 121: first connection passage 
                 122: second connection passage 
               
               
                 131: rear recess 
                 132: rear cover 
               
               
                 134: central recess 
                 135: baffle wall 
               
               
                 137: front recess 
                 138: front cover 
               
               
                 146: connection passage 
                 147: oil return port 
               
               
                 150: first oil return passage 
                 151: second oil return passage