Patent Publication Number: US-9840952-B2

Title: Breather chamber of internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2014-198375 filed Sep. 29, 2014 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a breather chamber of an internal combustion engine for suppressing an increase in the size of the internal combustion engine. 
     2. Description of Background Art 
     Some internal combustion engines including a cam chain chamber disposed along a side portion of a cylinder block portion which intersects the direction of a crankshaft and includes a looped cam chain for transmitting the power of the crankshaft to a camshaft provided in a cylinder head having a structure in which a breather chamber is provided outside a rotation locus of the cam chain. See, for example, Japanese Patent Application Publication No. 2008-248806 (FIGS. 1 to 4). 
     However, in the structure disclosed in Japanese Patent Application Publication No. 2008-248806, the breather chamber bulges outside the cam chain. Accordingly, the size of the cam chain chamber increases. Therefore the size of the internal combustion engine may increase. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     An object of an embodiment of the present invention is to provide a breather chamber of an internal combustion engine which includes a looped cam chain for transmitting the power of a crankshaft to a camshaft provided in a cylinder head and includes a cam chain chamber disposed along a side portion of a cylinder block portion intersecting the direction of the crankshaft. Thus, the breather chamber having a large size can be formed while an increase in the size of the internal combustion engine is suppressed. 
     To solve the above-described problem, according to an embodiment of the present invention, a breather chamber of an internal combustion engine includes a looped cam chain for transmitting power of a horizontally disposed crankshaft to a camshaft provided in a cylinder head with a cam chain chamber disposed along a side portion of a cylinder block portion and the side portion intersecting a direction of the crankshaft. The breather chamber is located on a side of a plane formed by a rotation locus of the cam chain in the direction of the crankshaft in the cam chain chamber. 
     According to an embodiment of the present invention, the cam chain chamber is formed by attaching a cam chain chamber cover to the side portion of the cylinder block portion. In addition, a partitioning member for dividing the breather chamber from the cam chain chamber is formed of a flat plate and attached to an inside of the cam chain chamber cover. 
     According to an embodiment of the present invention, the breather chamber has an inlet provided in a lower portion of the cam chain chamber and an outlet provided in an upper portion of the cam chain chamber. 
     According to an embodiment of the present invention, a rib protruding from the cam chain chamber cover into the breather chamber is formed downwardly along an inner surface of the cam chain chamber cover between the inlet and the outlet. 
     According to an embodiment of the present invention, the internal combustion engine is an in-vehicle engine, the crankshaft is directed in a longitudinal direction of a vehicle, the cam chain chamber is disposed on a front surface of the internal combustion engine, and a front portion of the cam chain chamber is divided to form the breather chamber. 
     In the breather chamber of the internal combustion engine, according to an embodiment of the present invention, since a large-area space located on the side of the plane formed by the rotation locus of the cam chain in the direction of the crankshaft is utilized to provide the breather chamber, the breather chamber having a large size can be formed while an increase in the size of the internal combustion engine is suppressed. 
     According to an embodiment of the present invention, with a simple configuration, the breather chamber can be formed between the cam chain chamber cover and the partitioning member, and the cam chain chamber in which oil is scattered can be divided from the breather chamber. Further, since the partitioning member is formed of a flat plate, an increase in the size of the internal combustion engine with respect to the direction of the crankshaft can be suppressed. 
     According to an embodiment of the present invention, a layout which allows oil to be easily discharged from the breather chamber can be obtained using the vertical height of the cam chain chamber. 
     According to an embodiment of the present invention, oil separated from the breather gas to adhere to the rib is caused to flow downwardly, and oil is easily discharged from the breather chamber. 
     According to an embodiment of the present invention, even in the case where the internal combustion engine with the crankshaft directed in the longitudinal direction of a vehicle is mounted on a vehicle having space limitations with respect to the longitudinal direction thereof, partitioning a front-side space of the cam chain chamber with respect to the direction of the crankshaft suppresses an increase in the size of the internal combustion engine with respect to the longitudinal direction of the vehicle while achieving a large volume of the breather chamber, and facilitates the installation of the internal combustion engine. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a right side view of a principal part of a motorcycle including a power unit in which a breather chamber of an internal combustion engine according to one embodiment of the present invention is provided; 
         FIG. 2  is a front view of the power unit as seen from arrows II-II of  FIG. 1 ; 
         FIG. 3  is a vertical cross-sectional view of a principal part of a front portion of the power unit taken in the direction of a crankshaft as seen from arrows III-III of  FIGS. 2, 4, and 5 ; 
         FIG. 4  is a front view of a portion around an opening of the cam chain chamber with a cam chain chamber cover of  FIG. 2  removed, a partitioning member being shown at a predetermined position, as seen from arrows IV-IV of  FIG. 3 ; and 
         FIG. 5  is a front view of a portion around the opening of the cam chain chamber as seen from arrows V-V of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A breather chamber of an internal combustion engine according to one embodiment of the present invention will be described with reference to  FIGS. 1 to 5 . 
     In the appended claims and this specification, directions such as front, rear, left, right, upward, and downward directions are based on the orientation of a vehicle including a power unit in which the breather chamber the internal combustion engine of the present embodiment is provided. In the present embodiment, a vehicle is a saddle-type vehicle such as a motorcycle. 
     In the drawings, arrows FR, LH, RH, and UP indicate front, left, right, and upward directions, respectively. 
       FIG. 1  is a right side view of a principal part of a motorcycle  1  including a power unit  3  in which a breather chamber  7  of an internal combustion engine  4  according to one embodiment of the present invention is provided. In the motorcycle  1  of  FIG. 1 , a body cover  10  is indicated by a two-dot chain line in a simplified manner, part of which is omitted, and only a principal part is shown with an intake system, an exhaust system, a fuel system, and the like being omitted. 
     A body frame  2  of the motorcycle  1  includes a head pipe  20  by which a front fork  12  pivotally supporting a front wheel  11  is movably supported so that steering can be performed, a main frame  21  extending from the head pipe  20  and downward sloping toward the back, seat rails  22  extending from upper portions of rear ends of the main frame  21  and upward sloping toward the back, and a back stay  23  connecting lower portions of rear ends of the main frame  21  and rear-side portions of the seat rails  22 . 
     A steering handle  13  is connected to an upper portion of the front fork  12 . Moreover, a front end portion of a swing arm  14  is movably supported by rear end portions of the main frame  21  to be vertically swingable with a rear drive wheel  15  being pivotally supported by a rear end portion of the swing arm  14 . 
     Further, an unillustrated rear shock absorber is provided between the upper portions of the rear ends of the main frame  21  and the swing arm  14 , and a riding seat  16  is attached to upper portions of the seat rails  22 . 
     A power unit  3  for driving the rear wheel  15  is disposed in a space below the main frame  21 . The power unit  3  is supported by the main frame  21  with a plurality of hanger members  17  interposed therebetween. An output shaft  32  of the power unit  3  is connected to the rear wheel  15  through a drive shaft  33  installed along the swing arm  14  for transmitting rotational power to the rear wheel  15 . 
       FIG. 2  is a front view of the power unit  3  as seen from arrows II-II of  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , the power unit  3  includes the water-cooled, horizontally-opposed, six-cylinder, four-stroke internal combustion engine  4 , and a transmission  5  which changes the speed of the rotational power of the internal combustion engine  4  and which is provided with a reverse transmission system for reversing the direction of rotation. 
     A shell of the internal combustion engine  4  includes a crankcase  42 , which includes a left crankcase  42 L disposed on a left side as seen in the front direction of travel of the motorcycle  1  and a right crankcase  42 R disposed on a right side as seen in the front direction of travel thereof; left and right cylinder heads  43 L and  43 R respectively connected to outer ends of left and right crankcases  42 L and  42 R and a rear cover  44  connected to the left and right crankcases  42 L and  42 R (see  FIG. 1 ). 
     The rear cover  44  is connected to the left and right crankcases  42 L and  42 R to close rear portions of the left and right crankcases  42 L and  42 R disposed along the direction of travel of the motorcycle  1 . 
     It should be noted that left and right cylinder head covers  45 L and  45 R are respectively fastened to outer ends of the left and right cylinder heads  43 L and  43 R to cover left and right valve trains provided in the left and right cylinder heads  43 L and  43 R and driven sprockets  62 L and  62 R of camshafts  61 L and  61 R thereof. 
     The left and right crankcases  42 L and  42 R are fastened to each other to form the crankcase  42  wherein a crankshaft  31  horizontally disposed with an axis X thereof directed in the longitudinal direction of the motorcycle  1  is rotatably supported at mating surfaces of the left and right crankcases  42 L and  42 R to demarcate a crank chamber  30  around the crankshaft  31 . 
     Further, the left and right crankcases  42 L and  42 R have left and right cylinder block portions  46 L and  46 R incorporated therein on left and right sides of the crank chamber  30 , respectively. Each of the left and right cylinder block portions  46 L and  46 R has three parallel cylinder bores  46   a  (see  FIG. 3 ) formed therein through which unillustrated pistons connected to the crankshaft  31  through unillustrated connecting rods in common are inserted. 
     In the left and right crankcases  42 L and  42 R, below the crank chamber  30 , a main shaft  51  and a counter shaft  52  of the transmission  5  which are disposed parallel to the crankshaft  31  and directed in the longitudinal direction of the vehicle are supported, and a transmission chamber  50  is demarcated. In the left and right crankcases  42 L and  42 R, further below the transmission chamber  50 , an oil pan portion  47  is formed. 
     A clutch cover  53  disposed concentrically with the main shaft  51  of the transmission  5  to cover an unillustrated clutch mechanism is attached to a rear surface of the rear cover  44 , and the output shaft  32  of the power unit  3  protrudes from the rear cover  44  toward the rear (see  FIG. 1 ). 
     The output shaft  32  is connected to the drive shaft  33  (see  FIG. 1 ), which extends along the swing arm  14  and is connected to the rear wheel  15 , and transmits the rotational power of the internal combustion engine  4  to the rear wheel  15 . 
     As shown in  FIG. 2 , a cam chain chamber opening  48  is provided to straddle left and right crankcase front walls  42 La and  42 Ra, which are front portions of the left and right crankcases  42 L and  42 R fastened to each other, and to be located from a region around the crankshaft  31  to regions near the left and right cylinder heads  43 L and  43 R. 
     An opening circumferential wall  48   a  protruding toward the front is formed on a circumferential edge of the cam chain chamber opening  48 . A cam chain chamber cover  49  is fastened to the opening circumferential wall  48   a  with fastening bolts  49   b . Thus, the cam chain chamber cover  49  closes the cam chain chamber opening  48  to close a front portion of the crank chamber  30 . 
     Moreover, a transmission holder  55  is provided around the main shaft  51  and the counter shaft  52  of the transmission  5 , which is disposed below the crankshaft  31 , with a shift drum  54 , and the like (the positions of the central axes thereof are shown in  FIG. 2 ), to be connected to the left and right crankcases  42 L and  42 R and to close a front portion of the transmission chamber  50 . 
     The transmission chamber  50  is formed from the transmission holder  55  to the insides of the left and right crankcases  42 L and  42 R, and houses the transmission  5 . 
     The left and right crankcases  42 L and  42 R have left and right cam chain chambers  63 L and  63 R demarcated therein along front-side (on a front side in  FIG. 2 ) side portions of the respective cylinder block portions  46 L and  46 R with respect to the direction of the crankshaft  31  to communicate with insides of the left and right cylinder heads  43 L and  43 R. The cam chain chamber cover  49 , together with the left and right crankcase front walls  42 La and  42 Ra, constitutes part of a front wall which covers the left and right cam chain chambers  63 L and  63 R. 
     It should be noted that in  FIG. 2 , the cam chain chamber cover  49  is shown with part of a left-side portion (right-side portion in the drawing) being cut away. A front side portion (“side portion” in the present invention)  46 La of the left cylinder block portion  46 L and the left cam chain chamber  63 L located ahead of the front side portion  46 La in the drawing are shown behind the cut-away portion in the drawing. Part of a left cam chain  65 L extending in the cam chain chamber  63 L is shown in  FIG. 2 . 
     Similarly, a front side portion (“side portion” in the present invention)  46 Ra of the right cylinder block portion  46 R, a right cam chain chamber  63 R, and a right cam chain  65 R are disposed behind a right-side portion (left-side portion in the drawing) of the cam chain chamber cover  49 . 
     The cam chain  65 L for transmitting the power of the crankshaft  31  to the left camshaft  61 L is passed through the left cam chain chamber  63 L and looped around a drive sprocket  64 L for the left camshaft  61 L fitted to a front-end side of the crankshaft  31  and the driven sprocket  62 L of the camshaft  61 L of the left valve train provided in the left cylinder head  43 L. 
     Moreover, the cam chain  65 R for transmitting the power of the crankshaft  31  to the right camshaft  61 R is passed through the right cam chain chamber  63 R and looped around a drive sprocket  64 R for the right camshaft  61 R fitted to the front-end side of the crankshaft  31  and the driven sprocket  62 R of the camshaft  61 R of the right valve train provided in the right cylinder head  43 R. 
     These components drive the left and right valve trains. An inlet valve and an exhaust valve, both unillustrated, corresponding to each of the cylinder bores  46   a  (see  FIG. 3 ) are opened and closed with a predetermined timing in synchronization with the rotation of the crankshaft  31 . 
     It should be noted that a water pump drive gear  31   a  is also fitted to the front-end side of the crankshaft  31  to mesh with a driven gear  56   a  of a water pump  56 . 
     More specifically, as shown in  FIG. 2 , in the left and right crankcases  42 L and  42 R, the left and right cam chain chambers  63 L and  63 R are demarcated which are covered by the left and right crankcase front walls  42 La and  42 Ra along the front side portions  46 La and  46 Ra of the respective cylinder block portions  46 L and  46 R intersecting the direction of the crankshaft  31 . The left and right crankcase front walls  42 La and  42 Ra have the cam chain chamber opening  48  which straddles both the left and right crankcase front walls  42 La and  42 Ra to include a region around the crankshaft  31 . The cam chain chamber cover  49  is fastened to the opening circumferential wall  48   a , which protrudes from the circumferential edge of the cam chain chamber opening  48  to the front, to cover the left and right cam chain chambers  63 L and  63 R. 
     As shown in  FIG. 3  showing a vertical cross section of a principal part of a front portion of the power unit  3  taken along the axis X of the crankshaft  31  as seen from arrows of  FIG. 2 , an outer circumferential edge of the cam chain chamber cover  49  also has a cover circumferential wall  49   a  protruding to the rear, and the cover circumferential wall  49   a  is fastened to the opening circumferential wall  48   a  with cover fastening bolts  49   b  (also see  FIG. 4 ). 
     Accordingly, portions of the left and right cam chain chambers  63 L and  63 R which are covered with the cam chain chamber cover  49  have spaces larger than those of portions thereof covered with the left and right crankcase front walls  42 La and  42 Ra by an amount equal to an inside height of the cam chain chamber cover  49 , i.e., an amount approximately equal to the height of the cover circumferential wall  49   a  in the direction of the axis X of the crankshaft. 
     Accordingly, as shown in  FIG. 3 , in the present embodiment, in the cam chain chamber  63 R on the right crankcase  42 R side, a partitioning member  71  formed of a flat plate is disposed on the front side of a plane P formed by a rotation locus L (see  FIG. 4 ) of the cam chain  65 R in the direction of the crankshaft  31 . Thus, the breather chamber  7  is demarcated between the partitioning member  71  and an inner surface (rear surface)  49   g  of the cam chain chamber cover  49  to be divided from the cam chain chamber  63 R. 
     More specifically, a portion of the cam chain chamber  63 R which is covered with the cam chain chamber cover  49  is formed by fastening the cam chain chamber cover  49  to the opening circumferential wall  48   a  of the cam chain chamber opening  48 . Since the cam chain chamber cover  49  having the partitioning member  71  attached to the inside (rear side) thereof is fastened, the breather chamber  7  is formed on the front side of the plane P formed by the rotation locus L of the right cam chain  65 R in the direction of the crankshaft  31  to be divided from the right cam chain chamber  63 R. 
     Accordingly, since a large-area space in the cam chain chamber opening  48  which is located on the front side of the plane P formed by the rotation locus L of the right cam chain  65 P in the direction of the crankshaft  31  is utilized to provide the breather chamber  7 , a breather chamber  7  having a large size is formed while an increase in the size of the internal combustion engine  4  is suppressed. 
     More particularly, even in the case where the internal combustion engine  4  of the power unit  3  is an in-vehicle engine which has the crankshaft  31  directed in the longitudinal direction of the vehicle and which is mounted on a vehicle having space limitations with respect to the longitudinal direction, disposing the cam chain chambers  63 L and  63 R on front surfaces of the crankcases  42 L and  42 R of the internal combustion engine  4  and forming the breather chamber  7  in the front portion of the cam chain chamber  63 R by dividing a front-side space of the right cam chain chamber  63 R suppresses an increase in the size of the internal combustion engine  4  with respect to the longitudinal direction of the vehicle while achieving a large volume of the breather chamber  7 , and facilitates the installation of the internal combustion engine  4  of the power unit  3 . 
     Moreover, with a simple configuration in which the partitioning member  71  formed of a flat plate is disposed, the breather chamber  7  can be formed between the cam chain chamber cover  49  and the partitioning member  71 . Thus, the cam chain chamber  63 R in which oil is scattered can be divided from the breather chamber  7 . Further, since the partitioning member  71  is formed of a flat plate, an increase in the size of the internal combustion engine  4  with respect to the direction of the crankshaft  31  can be suppressed. 
     It should be noted that in the present embodiment, the partitioning member  71  is attached to the cam chain chamber cover  49  by screwing, from the rear, partitioning member fastening bolts  71   a  into partitioning member fastening bosses  49   c  (see  FIG. 5 ) provided upright on the inner surface  49   g  of the cam chain chamber cover  49  toward the rear. However, the partitioning member  71  may be attached to the opening circumferential wall  48   a  such that when the cam chain chamber cover  49  is fastened to the opening circumferential wall  48   a , the partitioning member  71  and the cam chain chamber cover  49  are fastened together. 
     In  FIG. 3 , a breather outlet flow path  72  is provided which also serves as a downstream-side portion of the breather chamber  7 . An upstream side of the breather outlet flow path  72  is opened at a front end of the opening circumferential wall  48   a  to communicate with a breather chamber outlet  7   b  provided in an upper portion of the cam chain chamber  63 R with a downstream side thereof extending in the cylinder block portion  46 R to the rear to communicate with an outlet nozzle  73  directed outward. The outlet nozzle  73  communicates with an unillustrated air cleaner through an unillustrated breather return pipe. 
       FIG. 4  shows a front surface around the cam chain chamber opening  48  with the cam chain chamber cover  49  in  FIG. 2  removed as seen from arrows IV-IV of  FIG. 3 . It should be noted that the partitioning member  71  is shown at a predetermined position. 
     The partitioning member  71  is located ahead of the right cam chain  65 R (on a front side in the drawing) in the right cam chain chamber  63 R to be in contact with the inner circumference of the opening circumferential wall  48   a  of the cam chain chamber opening  48  on the right crankcase front wall  42 Ra side and with the circumference of the crankshaft  31 . In other words, the partitioning member  71  is disposed on the front side of the plane P formed by the rotation locus L of the right cam chain  65 R in the direction of the crankshaft  31 , and divides the breather chamber  7  from the right cam chain chamber  63 R. 
     As shown in  FIG. 4 , the breather chamber outlet  7   b , communicating with the breather outlet flow path  72  located in an upper portion of the right cam chain chamber  63 R, is provided in the opening circumferential wall  48   a  and is formed in a right upper portion (left upper portion in the drawing) of the breather chamber  7 . 
     In  FIG. 4 , partitioning member fastening holes  71   b  allow the partitioning member fastening bolts  71   a  for fastening the partitioning member  71  to be inserted through the partitioning member fastening bosses  49   c  provided upright on the inner surface  49   g  of the aforementioned cam chain chamber cover  49  in  FIG. 3  toward the rear. 
     Moreover, a top surface  48   aa , straddling the left and right crankcase front walls  42 La and  42 Ra, of the opening circumferential wall  48   a  of the cam chain chamber opening  48  serves as a mating surface to which the cam chain chamber cover  49  is fastened. The cover fastening bolts  49   b  (see  FIG. 3 ) screwed into the cam chain chamber cover  49  from the front portion side (on a front side in the drawing) are indicated by two-dot chain lines in  FIG. 4 . 
       FIG. 5  is a front view around the cam chain chamber opening  48   a  in  FIG. 2  as seen from arrows V-V of  FIG. 3 , with the cam chain chamber cover  49  being cut. The cover circumferential wall  49   a  provided on the outer circumference of the cam chain chamber cover  49 , the partitioning member fastening bosses  49   c  provided to protrude from the inner surface  49   g , ribs  49   d , and a crankshaft circumferential wall  49   e  surrounding the crankshaft  31  are shown as a cross section perpendicular to the axis X of the crankshaft  31 . 
     The breather chamber  7  is surrounded by the cover circumferential wall  49   a  and the crankshaft circumferential wall  49   e  to be demarcated between the partitioning member  71  and the inner surface  49   g  of the cam chain chamber cover  49 . A gap  49   f  is provided between the cover circumferential wall  49   a  and the crankshaft circumferential wall  49   e  so that the breather chamber  7  may communicate with the cam chain chamber opening  48  in a lower portion of the cam chain chamber  63 R, and constitutes a breather chamber inlet (“inlet” in the present invention)  7   a.    
     Moreover, in a right upper portion (left upper portion in the drawing) of the cover circumferential wall  49   a , the breather chamber outlet (“outlet” in the present invention)  7   b  overlapping an upstream end of the breather outlet flow path  72  is provided in the opening circumferential wall  48   a  and a passage  49   h  (see  FIG. 3 ) for allowing the inside of the breather chamber  7  to communicate with the breather chamber outlet  7   b  are formed. 
     Accordingly, breather gas flows into the breather chamber inlet  7   a  from the crank chamber  30  side through the left and right cam chain chambers  63 L and  63 R and the cam chain chamber opening  48   a . Breather gas from the breather chamber  7  flows out from the breather chamber outlet  7   b  into the breather outlet flow path  72 , and is further sent from the outlet nozzle  73  through the unillustrated breather return pipe to the unillustrated air cleaner. 
     It should be noted that the breather chamber inlet  7   a  is located in lower portions of the left and right cam chain chambers  63 L and  63 R and that the breather chamber outlet  7   b  is located in an upper portion of the right cam chain chamber  63 R. Accordingly, breather gas flowing in through the breather chamber inlet  7   a  flows toward the breather chamber outlet  7   b  in the breather chamber  7  as a rising stream. Accordingly, oil in liquid phase mixed in the breather gas is easily separated downwardly from the gas by the difference in weight between the oil and the gas. Further, oil flowing downwardly from the breather chamber inlet  7   a  located at a lower position flows through the cam chain chambers  63 L and  63 R into the oil pan portion  47  (see  FIG. 2 ) demarcated below the crank chamber  30 . 
     Thus, a layout which allows oil in the breather gas to be easily discharged from the breather chamber  7  is obtained by providing the breather chamber  7  over the entire height of the cam chain chamber opening  48  using the vertical heights of the cam chain chambers  63 L and  63 R. 
     Moreover, as shown in  FIG. 5 , each of the ribs  49   d  (see  FIG. 3 ) protruding from the inner surface  49   g  of the cam chain chamber cover  49  into the breather chamber  7  is formed to be inclined downwardly from an end connected to the opening circumferential wall  48   a  or the crankshaft circumferential wall  49   e  to an open end in the breather chamber  7  along the inner surface  49   g  of the cam chain chamber cover  49 , as seen from the front surface. 
     Accordingly, while flowing toward the breather chamber outlet  7   b  in the breather chamber  7  as a rising stream, breather gas flowing in through the breather chamber inlet  7   a  passes through a labyrinthine flow path formed by the ribs  49   d  extending from the opening circumferential wall  48   a  and the crankshaft circumferential wall  49   e . This facilitates the separation of oil from breather gas. Oil separated from breather gas to adhere to the ribs  49   d  flows downwardly toward the open ends of the ribs  49   d . Thus, oil is easily discharged from the breather chamber  7 . 
     Hereinafter, characteristic configurations and advantageous effects of the breather chamber  7  of the internal combustion engine  4  of the present embodiment will be described together. 
     More specifically, in the breather chamber  7  of the internal combustion engine  4  including the left and right looped cam chains  65 L and  65 R for transmitting the power of the horizontally disposed crankshaft  31  to the camshafts  61 L and  61 R provided in the left and right cylinder heads  43 L and  43 R and including the left and right cam chain chambers  63 L and  63 R disposed along the front side portions  46 La and  46 Ra of the left and right cylinder block portions  46 L and  46 R which intersect the direction of the crankshaft  31 , the breather chamber  7  is disposed on the front side of the plane P formed by the rotation locus L of the right cam chain  65 R in the direction of the crankshaft  31  in the right cam chain chamber  63 R. 
     Accordingly, since a large-area space located on the front side of the plane P formed by the rotation locus L of the cam chain  65 R in the direction of the crankshaft  31  is utilized to provide the breather chamber  7 , the breather chamber  7  having a large size can be formed while an increase in the size of the internal combustion engine  4  is suppressed. 
     Moreover, the left and right cam chain chambers  63 L and  63 R are formed by attaching the cam chain chamber cover  49  to the front side portions  46 La and  46 Ra of the left and right cylinder block portions  46 L and  46 R. The partitioning member  71  for dividing the breather chamber  7  from the right cam chain chamber  63 R is formed of a flat plate and attached to the inside of the cam chain chamber cover  49 . 
     Accordingly, with a simple configuration, the breather chamber  7  can be formed between the cam chain chamber cover  49  and the partitioning member  71 . Thus, the cam chain chambers  63 L and  63 R in which oil is scattered can be divided from the breather chamber  7 . Further, since the partitioning member  71  is formed of a flat plate, an increase in the size of the internal combustion engine  4  with respect to the direction of the crankshaft  31  can be suppressed. 
     Moreover, the breather chamber  7  has the breather chamber inlet  7   a  provided in a lower portion of the right cam chain chamber  63 R and the breather chamber outlet  7   b  provided in an upper portion of the right cam chain chamber  63 R. Thus, a layout which allows oil to be easily discharged from the breather chamber  7  is obtained using the vertical height of the cam chain chamber  63 R. 
     Moreover, the ribs  49   d  protruding from the cam chain chamber cover  49  into the breather chamber  7  are formed downwardly along the inner surface of the cam chain chamber cover  49  between the breather chamber inlet  7   a  and the breather chamber outlet  7   b . Accordingly, oil separated from breather gas to adhere to the ribs  49   d  is caused to flow downwardly. Thus, oil is easily discharged from the breather chamber  7 . 
     Moreover, the internal combustion engine  4  is an in-vehicle engine, the crankshaft  31  is directed in the longitudinal direction of the vehicle, the left and right cam chain chambers  63 L and  63 R are disposed on the front surface of the internal combustion engine  4 , and the breather chamber  7  is formed by partitioning the front portion of the right cam chain chamber  63 R. Accordingly, even in the case where the internal combustion engine  4  with the crankshaft  31  directed in the longitudinal direction of the vehicle is mounted on a vehicle having space limitations with respect to the longitudinal direction thereof, partitioning a front-side space of the right cam chain chamber  63 R with respect to the direction of the crankshaft  31  suppresses an increase in the size of the internal combustion engine  4  with respect to the longitudinal direction of the vehicle while achieving a large volume of the breather chamber  7 , and facilitates the installation of the power unit  3  including the internal combustion engine  4 . 
     While one embodiment of the present invention has been described above, it is a matter of course that aspects of the present invention are not limited to the above-described embodiment, and include various aspects for carrying out the invention within the scope of the spirit of the present invention. 
     For example, the internal combustion engine of the power unit is not limited to the horizontally-opposed, six-cylinder internal combustion engine of the embodiment. Moreover, the internal combustion engine is not limited to an in-vehicle engine, and, if the internal combustion engine is an in-vehicle engine, the crankshaft is not limited to the crankshaft directed in the longitudinal direction of the vehicle. 
     It should be noted that in the embodiment, left and right in the above description of the configurations and arrangements of components of the power unit, the internal combustion engine, and the breather chamber are specified to be left and right in the drawing for convenience of explanation. However, in the present invention, left and right may be reversed. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.