Abstract:
An outboard motor has an internal combustion engine installed in an engine compartment defined by an engine cover. The engine&#39;s intake air inlet is formed by an air intake duct, which communicates with an air intake space outside the engine compartment. The engine cover includes an upper cover with a receiving ring fitted on the air intake duct. The receiving ring and the intake duct form an overlapping part in which the ring and the duct overlap each other in the direction of flow of combustion air. The overlapping part has a sealing member therein which forms a seal between the air intake space and the interior space of the engine compartment. The sealing structure does not require high dimensional precision to form a required sealing property between the engine cover and the air inlet of the engine intake system. The sealing structure minimizes any influence of engine vibrations.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an internal combustion engine installed in an engine compartment covered with an engine cover. The invention relates more specifically to a structure including the engine cover and an air intake part of the engine. The engine is used typically in marine propulsion machines, for example, outboard motors. 
     2. Description of the Related Art 
     A typical outboard motor has an internal combustion engine housed in an engine compartment defined by an engine cover. If an intake air inlet of the intake system of the engine opens into the engine compartment, air to be sucked for combustion into the engine will be heated by the heat generated in the engine and sucked into the engine as heated combustion air. As a consequence, the charging efficiency of the engine will be lowered with resultant reduction in engine output. JP 59-120598 A and JP 05-286490 A disclose a technique for improving the charging efficiency, in which it is attempted to suck air outside the engine compartment into the engine. 
     When the intake air inlet of the intake system of the engine is in communication with the engine compartment, the air pressure within the engine compartment is caused to vary due to intake pulsation of the engine, and the varying air pressure causes the engine cover to vibrate with resultant generation of noises. The vibration can be prevented by providing a sealing member that shuts off communication between the interior of the engine compartment and the intake air inlet of the intake system. In case the sealing member is to be fixedly secured between members which are adjacently disposed in the direction of flow of the combustion air in the intake air inlet, it is required to increase the dimensional accuracy of the adjacent members with respect to the flow direction of the combustion air, for the purpose of providing a reliable sealing property and required sealing forces in the flow direction of the combustion air. Moreover, the sealing property is susceptible to the influence of the vibration since the intake system and the engine cover are caused to vibrate due to the engine operation. 
     The present invention has been made in view of the above circumstances, and it is an object of the invention to provide a sealing structure between the engine cover and the intake air inlet of the intake system, in which a high dimensional accuracy is not required to secure a required sealing property and in which the sealing property is not susceptible to the influence of the vibration. It is a further object of the invention to provide a sealing structure which can be easily assembled between the engine cover and the intake air inlet of the intake system, by utilizing a position determining means on the engine cover made up of divided cover elements. 
     SUMMARY OF THE INVENTION 
     To attain the above objects, the present invention provides an internal combustion engine installed in an engine compartment defined by an engine cover, comprising an engine body having a combustion chamber therein, and an intake system having an intake air passage with an intake air inlet and an intake air outlet to carry combustion gas to the combustion chamber, wherein the intake air inlet is formed by an air intake duct and is in communication with an exterior of the engine compartment; the engine cover is formed with a receiving ring which is fitted on the air intake duct; the receiving ring and the intake duct cooperate to form an overlapping part in which the receiving ring and the intake duct overlap each other in a direction of flow of combustion air; and the overlapping part has a sealing member therein which forms a seal between an exterior of the engine compartment and an interior of the engine compartment. 
     According to the present invention, the sealing member is provided in the overlapping part in which the receiving ring and the intake duct overlap each other with respect to the direction of flow of the combustion air. Therefore, even when vibration occurs and the receiving ring and the intake duct are vibrated in the direction of flow of the combustion air in a manner to induce relative movement between the receiving ring and the intake duct in the direction of flow of the combustion air, a reliable sealing property of the sealing member can be maintained without requiring a high dimensional accuracy because the dimensions of the receiving ring and the intake duct are predetermined to form the overlapping part and because the relative dimensional restriction to the receiving ring and the intake duct prevents relative movement in the overlapping part and the sealing member secured to the overlapping part maintains a sealing condition between the receiving ring and the intake duct. The relative dimensional restriction serves to provide a sealing structure which is not susceptible to the influence of vibrations. Further, since there is maintained a stable sealing property between the engine cover and the intake duct, noises caused by engine cover vibration due to the intake pulsation are prevented from being transmitted to the surrounding since the good sealing property is stably maintained between the engine cover and the intake duct. 
     In a preferred embodiment of the invention, the engine cover includes a first cover secured to the engine body to which the intake system is attached, and a second cover detachably attached to the first cover at a predetermined position in a manner guided by a positioning guide means, the receiving ring is provided on the second cover, and the sealing member is provided on at least one of the intake duct and the receiving ring, arrangement being such that the second cover is guided by the positioning guide means toward the first cover to cause the receiving ring to fit on the intake duct to form the overlapping part when the second cover reaches the predetermined position. 
     According to this feature, the second cover is guided by the positioning guide means toward the first cover when the second cover is attached to the first cover secured to the engine body. When this assembling work is being performed, the receiving ring forming part of the second cover is automatically positioned relative to the intake duct of the intake system, to form the overlapping part. The overlapping part is automatically formed during the course in which the second cover is being guided by the positioning guide means to the predetermined position. During this course, the sealing structure is automatically obtained in which the sealing member forms a seal between the receiving ring and the intake duct. It will be noted that the sealing structure is automatically formed during the assembling work of the first and second covers and the assembling work is easy. 
     The sealing member may include a base part and flexible lips formed on the base part, and the base part may be fitted on the receiving ring and the flexible lips are in sealing contact with the intake duct. 
     In a preferred form, the receiving ring is positioned outside the intake duct, and the air intake duct extends into the exterior of the engine compartment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation of an outboard motor embodying the present invention taken from the right-hand-side of the outboard motor; 
         FIG. 2  is a sectional view of an essential part of the outboard motor shown in  FIG. 1 ; 
         FIG. 3  is a sectional view taken substantially on the line III-III in  FIG. 2 ; 
         FIG. 4  is an enlarged view of a part, including an engine cover locking device, of  FIG. 2 ; 
         FIG. 5  is a sectional view taken substantially on the line V-V in  FIG. 2 ; 
         FIG. 6  is a sectional view taken substantially on the line VI-VI in  FIG. 2 ; 
         FIG. 7  is an enlarged view of a part, including an intake duct of an intake system, of  FIG. 2 ; and 
         FIG. 8  is a sectional view taken on the line VIII-VIII in  FIG. 7 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An outboard motor in a preferred embodiment of the present invention will be described with reference to  FIGS. 1 to 8 . 
     Referring to  FIGS. 1 and 2 , an outboard motor S, namely, a marine propulsion apparatus as a machine to which the present invention is applied, includes a propulsion unit, namely, a power unit, and a mounting device  23  for holding the propulsion unit on a hull  24 . The propulsion unit includes an internal combustion engine E, a propeller unit driven by the internal combustion engine E to generate thrust, an alternator G, cases  10 ,  11  and  12 , and an engine cover C. 
     Referring also to  FIG. 3 , the internal combustion engine E is a vertical, water-cooled multicylinder 4-stroke internal combustion engine provided with a vertical crankshaft  7  disposed with its center axis Le set in a vertical position. In this embodiment, the internal combustion engine E is a V-6 internal combustion engine. The internal combustion engine E has an engine body Ea including a cylinder block  1  provided with two banks  1   b  and  1   c  set at an angle to form a V, a crankcase  2  joined to the front end of the cylinder block  1 , a cylinder head  3  joined to the respective rear ends of the banks  1   b  and  1   c  of the cylinder block  1 , and a head cover  4  joined to the rear end of the cylinder head  3 . The crankshaft  7  is supported for rotation on the cylinder block  1  and is connected to pistons  5  by connecting rods  6 . 
     In the description, an expression: “as seen in plan view” signifies viewing in a vertical direction. In a state shown in  FIG. 1 , the center axis Le of the crankshaft  7  is vertical, horizontal directions perpendicular to the vertical direction include forward and rearward longitudinal directions and rightward and leftward lateral directions. The vertical directions, the longitudinal directions and the lateral directions coincide with those with respect to the hull  24 , respectively. The longitudinal directions and the lateral directions perpendicular to the longitudinal directions are first and second horizontal directions, respectively. 
     The engine body Ea is joined to the upper end of the mount case  10 . An oil pan  8  and the extension case  11  surrounding the oil pan  8  are joined to the lower end of the mount case  10 . A gear case  12  is joined to the lower end of the extension case  11 . A lower end part of the internal combustion engine E, the mount case  10  and an upper part of the extension case  11  are covered with a lower cover  13 , namely, a first cover, connected to the extension case  11 . An upper cover  14 , namely, a second cover, covering most part of an upper portion of the internal combustion engine E, is connected to the upper end of the lower cover  13 . The lower cover  13  and the upper cover  14  forms the split engine cover C defining an engine compartment  15  encasing the internal combustion engine E. Installed in addition to the internal combustion engine E in the engine compartment  15  are a ventilation system  70  for supplying ventilation air into the engine compartment  15 , and the alternator G. 
     Referring to  FIG. 2 , the lower cover  13  is fixedly held on the engine body Ea by the extension case  11  and the mount case  10 . The upper cover  14  is detachably attached to the lower case  13  and held in place by plural locking devices  16  serving also as positioning devices. In this embodiment the number of the locking devices  16  is four. The four locking devices  16  are arranged at intervals on the joint of the lower cover  13  and the upper cover  14 . As shown in  FIG. 4 , each of the locking devices  16  includes a first locking member  16   a  projecting from the inside surface of an upper end part of the lower cover  13  and provided with a guide hole  16   b , and a second locking member  16   c  projecting from the inside surface of a lower end part of the upper cover  14  so as to be inserted into the guide hole  16   b . The second locking member  16   c  has a fixed part  16   d  having a support part  16   d   1  and fastened to the upper cover  14  with screws  16   k , a cylindrical moving part  16   e  axially slidably put on the support part  16   d   1  of the fixed part  16   d , a bolt  16   f  extending through the support part  16   d   1  and the moving part  16   e , a nut  16   g  screwed on the bolt  16   f , and a spring  16   h  extending between the fixed part  16   d  and the moving part  16   e  to push the moving part  16   e  away from the fixed part  16   d.    
     The second locking members  16   c  attached to the upper cover  14  are inserted in the guide holes  16   b  to join the upper cover  14  to the lower cover  13 . Then, the moving parts  16   e  is guided by and moved in the guide holes  16   b  to position the upper cover  13  in place on the lower cover  13  and to join the upper cover  14  to the lower cover  13 . A gap between the lower cover  13  and the upper cover  14  is sealed by a sealing member  17 . The vertical size of the gap can be adjusted by adjusting the respective positions of the support parts  16   d   1  relative to the corresponding moving parts  16   e  by turning the nuts  16   g.    
     Referring to  FIGS. 1 and 2 , a flywheel  18  is mounted on the lower end part of the crankshaft  7 , namely, the output shaft of the internal combustion engine E, and a drive shaft  19  is coupled with the lower end part of the crankshaft  7 . The drive shaft  19  driven for rotation by the internal combustion engine E extends downward through the mount case  10  and the extension case  11  into the gear case  12 . The drive shaft  19  is interlocked with a propeller shaft  21  by a reversing mechanism  20  held in the gear case  12 . The power of the internal combustion engine E is transmitted by the crankshaft  7 , the drive shaft  19 , the reversing mechanism  20  and the propeller shaft  21  to a propeller  22  mounted on the propeller shaft  21  to rotate the propeller  22 . The drive shaft  19 , the reversing mechanism  20 , the propeller shaft  21  and the propeller  22  constitute the propulsion unit. 
     The mounting device  23  includes a swivel case  23   d  mounted so as to be turnable on a swivel shaft  23   c  fixedly held by mount rubbers  23   a  and  23   b  on the mount case  10  and the extension case  11 , a tilt shaft  23   e  supporting the swivel case  23   d  so as to be tiltable, and a bracket  23   f  holding the tilt shaft  23   e  and fixed to the stem of the hull  24 . The propulsion unit of the outboard motor S is held by the mounting device  23  on the hull  24 . The propulsion unit can be turned on the tilt shaft  23   e  in a vertical plane and is turnable on the swivel shaft  23   d  in a horizontal plane. 
     Referring to  FIG. 2 , the cylinder head  3  is provided with combustion chambers  30  ( FIG. 5 ) respectively axially corresponding to the pistons  5  fitted in cylinders  1   a , intake ports  31  ( FIG. 3 ) opening respectively into the combustion chambers  30 , exhaust ports opening respectively into the combustion chambers, and spark plugs exposed respectively to the combustion chambers  30 . Intake valves and exhaust valves incorporated into the cylinder head  3  to open and close the intake ports and the exhaust ports, respectively, are driven for opening and closing operations in synchronism with the rotation of the crankshaft  7  by an overhead-camshaft valve moving mechanism  32  installed in a valve chamber defined by the cylinder head  3  and the head cover  4 . 
     The valve moving mechanism  32  includes camshafts  32   a  driven for rotation by the power of the crankshaft  7  transmitted thereto by a transmission mechanism  33 , intake cams  32   b  and exhaust cams  32   c  formed on the camshafts  32   a , a pair of rocker arm shafts  32   d , intake rocker arms and exhaust rocker arms supported for turning on the rocker arm shafts  32   d . The intake cams  32   b  and the exhaust cams  32   c  drive the intake valves and the exhaust valves for opening and closing operation through the intake rocker arms and the exhaust rocker arms, respectively. 
     Referring to  FIG. 3 , a drive pulley  33   a  and a drive pulley  34   a  are mounted in that order in an upward arrangement on the upper end part of the crankshaft  7 . The transmission mechanism  33  including the drive pulley  33   a , a cam pulley  33   b  mounted on the camshaft  32   a  and a belt  33   c  extending between the drive pulley  33   a  and the cam pulley  33   b , and a transmission mechanism  34  including the drive pulley  34   a , a driven pulley  34   b  mounted on the shaft  81  of the alternator G and a belt  34   c  extending between the drive pulley  34   c  and the driven pulley  34   b , are disposed in a transmission chamber covered with a belt cover, namely, a transmission cover, attached to the upper end of the engine body Ea. The belt cover includes first belt covers  35  disposed above the upper ends of the cylinder heads  3  mainly for covering the cam pulleys  33   b , and a lower case  50   a  serving also as a second belt cover disposed above the upper end parts of the cylinder blocks  1  to cover the drive pulleys  33   a  and  34   a  and driven pulley  34   b.    
     The shaft  81  driven for rotation through the transmission mechanism  34  by the crankshaft  7  is disposed with the center axis Lg of the shaft  81  spaced a predetermined center distance d apart from the center axis Le of the crankshaft  7 . 
     Fuel sprayed out by a fuel injection valve, namely, an air-fuel mixture producing means, attached to the cylinder head  3  is mixed with the combustion air flowing through an intake air passage P ( FIGS. 2 and 3 ) formed in an intake system N installed in the engine compartment  15  to produce an air-fuel mixture. The air-fuel mixture burns in the combustion chamber  30  when the same is ignited by the spark plug attached to the cylinder head  3 . The piston  5  is driven for reciprocation by the pressure of a combustion gas produced in the combustion chamber  30  to drive the crankshaft  7  for rotation through the connecting rod  6 . The combustion gas discharged as an exhaust gas through the exhaust port from the combustion chamber  30  flows through an exhaust manifold  25  ( FIG. 3 ) into an exhaust pipe  26  ( FIG. 1 ). Then the exhaust gas flows from the exhaust pipe  26  through an exhaust passage formed in the extension case  11 , the gear case  12  and the boss of the propeller  22  and is discharged to the outside of the outboard motor S. 
     Referring to  FIGS. 2 ,  5  and  6 , particularly to  FIG. 6 , an air supply and exhaust system includes an outside-air intake structure Ai for taking outside air surrounding the outboard motor S into the outboard motor S and an air exhaust structure Ae for discharging air from the outboard motor S to the outside. The air supply and exhaust system is disposed outside the upper cover  14  (or the engine compartment  15 ). The air supply and exhaust system includes an exterior cover  40  extended over and detachably attached to the top wall  14   a  ( FIG. 2 ) of the upper cover  14 . The exterior cover  40  may also be referred to as an exterior cap member  40 , due to its placement as a cap over the top of the upper cover  14 . The air supply and exhaust system further includes a wall member  41  dividing a space defined by the exterior cap member  40  and the top wall  14   a  into an air intake space  42  ( FIG. 2 ) and an air exhaust space  43 . The wall member  41  is connected to the exterior cover  40  and the top wall  14   a  by a fitting structure. The wall member  41  is fixedly joined to the top wall  14   a  when the exterior cap member  40  is detachably fastened to the top wall  14  with screws. The wall member  41  has a front lateral wall  41   a  extending in a front zone of the space, and a longitudinal partition wall  41   b  laterally separating the air intake space  42  ( FIG. 2 ) and the air exhaust space  43  from each other. The wall member  41  may be formed integrally with the exterior cap member  40  or the top wall  14   a.    
     The outside-air intake structure Ai includes the exterior cover  40 , the front wall  41   a , the partition wall  41   b , an entrance louver  45 , namely, an air current straightening member, disposed at an air inlet  44  ( FIG. 2 ), and a deflector  46  dividing the air intake space  42  into a first space  42   a  through which the combustion air is supplied to the internal combustion engine E and a second space  42   b  ( FIG. 2 ) through which ventilation air flows into the engine compartment  15 . The entrance louver  45  is joined to the partition wall  41   b  and the top wall  14   a . The deflector  46  is formed integrally with the partition wall  41   b.    
     The air intake space  42  is defined by the exterior cover  40 , the top wall  14   a , the front wall  41   a  and the partition wall  41   b . The air inlet  44  ( FIG. 2 ) of the air intake space  42  opens rearward. The entrance louver  45  has a wall  45   a  ( FIG. 2 ) which determines the vertical size of the air inlet  44  such that the passage area of the air inlet  44  is set to be smaller than the passage area of the first space  42   a . Thus the air intake space  42  constitutes an intake silencing chamber  40   r  having the air inlet  44  as air introducing means and an air expansion chamber connected to the air inlet  44 . The exterior cover  40 , the upper cover  14  having the top wall  14   a , the entrance louver  45  and the wall member  41  including the front wall  41   a  and the partition wall  41   b  constitute an exterior intake silencer  40   s  located outside the engine compartment  15  and including the intake silencing chamber  40   r  communicating with an intake air inlet Pi to be described later to conduct combustion air to the air intake passage P. The exterior intake silencer  40   s  can be detached together with the upper cover  14  from the intake system N. The exterior intake silencer  40   s  overlaps a major portion of an intake silencer  50  to be described later when seen in plan view. 
     The deflector  46  disposed in the air intake space  42  is a box-shaped member having an inclined deflecting wall  46   a  having a flat surface inclined so as to deflect the flow of outside air that has passed through the entrance louver  45 , namely, the combustion air, obliquely upward. A ventilation duct  71  is disposed in the second space  42   b  demarcated by the deflector  46 . The ventilation duct  71  has an inclined deflecting wall  71   a  having a flat surface inclined so as to deflect obliquely upward the flow of the outside air that has flowed through the lowermost part of the air inlet  44 , which is vertically divided into parts by the entrance louver  45 . Water contained in the outside air impinges on the deflecting walls  46   a  and  71   a  and is separated from the outside air. Consequently, the amount of water contained in the combustion air flowing downstream from the deflector  46  is reduced, the flow of water into the intake air passage P is suppressed, the amount of water contained in the ventilation air that flows into the ventilation duct  71  is reduced, and the flow of water into the engine compartment  15  is suppressed. 
     Thus the ventilation air flows into the engine compartment  15  separately from the combustion air that is supplied to the internal combustion engine E. 
     The air exhaust structure Ae includes the exterior cover  40 , the front wall  41   a , the partition wall  41   b , and an exit louver  48 . The exit louver  48  serves as an air guide member disposed at an air exit  47  through which air in the air exhaust space  43  is discharged. The air exhaust space  43  is defined by the exterior cover  40 , the top wall  14   a , the front wall  41   a  and the partition wall  41   b . The air exit  47  of the air exhaust space  43  is formed in the left side wall  40   a  so as to open leftward. The exit louver  48  is formed integrally with the left side wall  40   a . A rear part of the air exhaust space  43  excluding a drain passage  49   c  ( FIG. 5 ) formed in the lowermost part of the entrance louver  45  is closed by a rear part  41   b   1  of the partition wall  41   b.    
     The air intake structure Ai and the air exhaust structure Ae are provided with drain passages for draining water collected in the air intake space  42  and the air exhaust space  43  to the outside of the outboard motor S. The drain passage formed in the air intake structure Ai has a rear drain passage  49   a  ( FIG. 5 ) formed in the lowermost part of the air inlet  44 , and a front drain passage  49   b  formed in the front wall  41   a  and the top wall  14   a . When the outboard motor S is tilted up, water is drained through the front drain passage  49   b . Referring also to  FIG. 7 , the drain passage formed in the air exhaust structure Ae includes a rear drain passage  49   c  ( FIG. 5 ), and a front drain passage  49   d  ( FIGS. 2 and 6 ) formed in the front wall  41   a  and the top wall  14   a . When the outboard motor S is tilted up, water is drained through the front drain passage  49   d . The front drain passages  49   b  and  49   d  have openings  49   b   1  and  49   d   1 , respectively. The openings  49   b   1  and  49   d   1  open into the atmosphere. The front drain passages  49   b  and  49   d  are provided with one-way valves  49   e , respectively. One-way valves  49   e  allow water to flow out only from the air intake space  42  and the air exhaust space  43 . Each of the one-way valves  49   e  is, for example, a reed valve provided with a flexible valve element formed by processing a thin sheet. 
     Referring to  FIG. 2 , the intake system N forming the intake air passage P for carrying the combustion air from the air intake space  42  into the combustion chambers  30  is joined to the upper end of the cylinder block  1 . As shown in  FIGS. 2 and 3 , the intake system N includes an intake silencer  50  disposed above the engine body Ea, a reversing pipe  51  for reversing the flowing direction of the combustion air, a throttle device  52  provided with a throttle valve  52   a  for controlling the flow of the combustion air that has flowed through the reversing pipe  51 , and an intake manifold  53 . The reversing pipe  51  is connected to the intake silencer  50 , disposed behind the engine body Ea and bent in a U-shape in a vertical plane ( FIG. 5 ). The throttle device  52  is disposed above the engine body Ea. The intake manifold  53  is disposed between the reversing pipe  51  and the engine body Ea with respect to the longitudinal direction. The intake silencer  50  includes the lower case  50   a  ( FIG. 2 ) covering the transmission mechanism  34 , and an upper case  50   b  ( FIG. 7 ) hermetically fastened to the lower case  50   a  with screws. The intake manifold  53  is disposed over and attached to both the right and left cylinder heads  3 . 
     Referring to  FIG. 7 , the intake silencer  50  defines an intake silencing chamber including an upstream first silencing chamber  61  into which the intake air inlet Pi of the intake air passage P opens, and a second silencing chamber  65  on the downstream side of the first silencing chamber  61 . The intake silencer  50  is an interior intake silencer located within the engine compartment  15 , and the first and second silencing chambers  61  and  65  are interior silencing chambers provided within the engine compartment  15  to form a part of the intake air passage P. 
     Referring to  FIG. 5 , the reversing pipe  51  is a one-piece member and forms a first down passage  62  in which the combustion air coming from the first silencing chamber  61  flows down, a first reversing passage  63  in which the flowing direction of the combustion air that has flowed down through the first down passage  62  is reversed in a vertical plane such that the combustion air flows upward, and an up passage  64  in which the combustion air coming from the first reversing passage  63  flows upward. As shown in  FIG. 2 , the throttle device  52  forms a throttle passage  66  in which the throttle valve  52   a  is disposed. The combustion air that has flowed through the up passage  64  and the second silencing chamber  65  ( FIG. 7 ) flows into the throttle passage  66 . The intake manifold  53  forms a manifold passage  67  ( FIG. 2 ) having a pair of distribution chambers, namely, a second down passage through which the combustion air that has been metered by the throttle valve  52   a  and has flowed through the throttle passage  66  flows down. The opening of the throttle valve  52   a  is controlled by a throttle operating mechanism. The combustion air that has flowed through the manifold passage  67  flows through the intake air outlets Pe ( FIG. 5 ) of the intake air passage P, and the intake ports  31  of the engine body Ea into the combustion chambers  30 . 
     The first silencing chamber  61  is defined by the first intake silencer formed by only the upper case  50   b  right above the engine body Ea and the transmission mechanism  33  and forms an upstream part of the intake air passage P. As shown in  FIG. 3 , the first silencing chamber  61  has an inlet part  61   a  defined by a cylindrical intake duct  54 , an outlet part  61   b  connecting with an inlet part  62   a  of the first down passage  62 , and an expansion chamber  61   c  of a passage area greater than those of the inlet part  61   a  and the outlet part  61   b . As shown in  FIG. 2 , the cylindrical intake duct  54  extends upward through the top wall  14   a  of the upper cover  14  into the first space  42   a . Thus the intake duct  54  or the inlet part  61   a  extends between the exterior of the engine compartment  15  and the interior of the same. 
     The inlet part  61   a  has the intake air inlet Pi. The intake air inlet Pi does not open into the engine compartment  15  and opens into the first space  42   a  which is outside the engine compartment  15 . Referring  FIGS. 6 and 7 , the intake duct  54  through which the combustion air from the first space  42   a  flows down, and a receiving ring  14   b  formed integrally with the upper cover  14  and receiving an end part of the intake duct  54  overlap each other with respect to a flowing direction F in which the combustion air flows to form an overlapping part W. The overlapping part W is provided with an annular sealing member  55  to seal the gap between the engine compartment  15  and the first space  42   a . The intake duct  54  extends upward through the receiving ring  14   b  into the first space  42   a.    
     The sealing member  55  has a base  55   a  hermetically engaged with the receiving ring  14   b  defining a circular opening for receiving the intake duct  54 , and annular, flexible lips  55   b  extending from the base  55   a  toward the intake duct  54  and in close contact with the outside surface of the intake duct  54 . In this embodiment the number of the flexible lips  55   b  is three. The flexible lips  55   b  are arranged in the flowing direction F. 
     The sealing function of the sealing member  55  becomes effective when the upper cover  14  is put from above on the intake system N attached to the engine body Ea fixed to the mount case  10 , and the end part of the intake duct  54  is received in the receiving ring  14   b  to form the overlapping part W. 
     Referring also to  FIG. 2 , the upper cover  14  is guided by the locking devices  16  and moves to its working position where the upper cover  14  is joined to the lower cover  13  before the overlapping part W is formed, and the receiving ring  14   b  receives the end part of the intake duct  54  to form the overlapping part W. More concretely, the locking devices  16  guide the upper cover  14  toward the lower cover  13  when the upper cover  14  is moved to join the same to the lower cover  13  fixed to the engine body Ea such that the axis of the receiving ring  14   b  of the upper cover  14  is aligned with the vertical axis of the intake duct  54  of the intake system N attached to the cylinder block  1 , and the receiving ring  14   b  moves vertically toward the intake duct  54  along the vertical axis of the intake duct  54 . Thus the overlapping part W is formed and the sealing member  55  is closely engaged with the intake duct  54  and the receiving ring  14   b  when the upper cover  14  is joined to the lower cover  13 . 
     Thus the intake duct  54  cooperates with the receiving ring  14   b  of the upper cover  14  of the intake silencer  40   s  to form a separable connecting structure so that the intake silencer  40   s  can be detachably connected to the intake system N. The detachable connecting structure includes the overlapping part W and the sealing member  55 . 
     Referring to  FIGS. 3 and 5 , the first down passage  62  formed at the rear of the engine body Ea has an inlet part  62   a  connected to the outlet part  61   b  at a position above the engine body Ea, and a vertical down part  62   c  of a cross-sectional area greater than that of the inlet part  62   a . The combustion air flowing substantially horizontally rearward through the outlet part  61   b  and the inlet part  62   a  flows downward through the down part  62   c    
     The up passage  64  formed at the rear of the engine body Ea has an outlet part  64   b  at substantially the same position as the inlet part  62   a  with respect to the vertical direction, and a vertical up part  64   c  of a cross-sectional area greater than that of the outlet part  64 . 
     The up passage  64  and the first down passage  62  are substantially symmetrical with respect to a vertical plane containing the center axis Le of the crankshaft  7  and perpendicular to the lateral direction on the outboard motor S. 
     The reversing passage  63  formed at the rear of the engine body Ea reverses the flowing direction of the combustion air flowing downward at a position overlapping the engine body Ea with respect to the vertical direction to make the combustion air flow upward. A drain passage  68  is connected to a bottom part of the reversing pipe  51  so as to communicate with a bottom part  63   d  of the reversing passage  63 . The drain passage  68  opens into the engine compartment  15  in the flowing direction of the combustion air in the bottom part  63   d . The drain passage  68  is provided with a one-way valve  68   e  ( FIG. 5 ) that is opened by the weight of water collected in the bottom part  63   d  to permit only discharging the water into the engine compartment  15 . The one-way valve  68   e , similarly to the one-way valve  49   c , is a reed valve. 
     The first down passage  62 , the reversing passage  63  and the up passage  64  form a U-shaped passage as viewed in a longitudinal direction. The U-shaped passage extending down from the inlet part  62   a  above the upper end of the engine body Ea to the lower end of the engine body Ea, curves in an upwardly concave U-shape and extends upward to the outlet part  64   b  above the upper end of the engine body Ea. The combustion air flowing through the intake air passage P flows downward first, and then flows upward between the first silencing chamber  61  and the second silencing chamber  65 . The first down passage  62 , the reversing passage  63  and the up passage  64  form a water separating unit. Water contained in combustion chamber is separated from the combustion air by centrifugal force while the combustion air is flowing through the reversing passage  63 . Therefore, the first silencing chamber  61  and the second silencing chamber  65  are disposed on the upstream side and the downstream side, respectively, of the water separating unit. 
     Referring to  FIG. 3 , the second silencing chamber  65  of the second intake silencer is made up of the lower case  50   a  and the upper case  50  and is disposed right above the engine body Ea and the transmission mechanisms  33  and  34 . The second silencing chamber  65  has an inlet part  65   a  connected to the outlet part  64   b , an outlet part  65   b  connected to the throttle passage  66 , and an expansion part  65   c  of a cross-sectional area greater than those of the inlet part  65   a  and the outlet part  65   b.    
     Referring to  FIG. 8 , the expansion chamber  65   c  is divided by a partition wall  56  extending downward and forward from the upper case  50   b  into a front passage  65   c   1  through which the combustion air from the inlet part  65   a  flows forward, a reversing part  65   c   2  ( FIG. 3 ) in which the flowing direction of the combustion air is reversed, and a rear passage  65   c   3  through which the combustion air flows rearward to the outlet part  65   b . Thus the second silencing chamber  65  serves as a second reversing passage for reversing the flowing direction of the combustion gas flowing in the forward direction in a horizontal plane. The partition wall  56  is formed integrally with a separator wall  92  and is attached to the intake silencer  50 . 
     A flame arrester  57  is disposed on the upstream side of the outlet part  65   b . The flame arrestor  57  is provided with a wire net that plays a quenching function when back fire occurs. 
     The throttle device  52  has a throttle body  52   b  defining the throttle passage  66  and connected by a flexible conduit  58  to the outlet part  65   b . The throttle valve  52   a  is disposed in the intake air passage P on the downstream side of the up passage  64  and on the upstream side of the second down passage  67 . Thus the throttle valve  52   a  is on the downstream side of the water separating unit. As shown in  FIGS. 3 and 5 , in the intake air passage P, the outlet part  61   b , namely, an inlet passage having an upstream end connecting with the inlet part  62   a  of the first down passage  62 , and the inlet part  65   a , namely, an outlet passage having a downstream end connecting with the outlet part  64   b  of the up passage  64  are on the opposite sides, respectively, of the throttle device  52  as seen in plan view. The inlet parts  62   a  and  65   a , and the outlet parts  61   b  and  64   b  are substantially horizontal passages. 
     Referring to  FIGS. 2 and 5 , the manifold passage  67 , namely, an outlet part of the intake air passage P, has an inlet part  67   a  into which the combustion air from the throttle passage  66  flows, a pair of distribution chambers  67   c  separated by a partition wall  53   a , branching off from the inlet part  67   a  and respectively corresponding to the banks  1   b  and  1   c  ( FIG. 3 ), and three runner passages  67   b  branching off from each of the distribution chambers  67   c . The partition wall  53   a  is provided with shutoff valves  53   b  that opens or closes depending on engine speed. The shutoff valves  53   b  close to disconnect the distribution chambers  67   c  while engine speed is in a low speed range to improve volumetric efficiency by resonance supercharge. The shutoff valves  53   b  open to connect the distribution chambers  67   c  while engine speed is in a high speed range to improve volumetric efficiency by inertia supercharge. 
     Each of the runner passages  67   b  has an intake air outlet Pe at its downstream end. In the manifold passage  67 , the combustion air flows from the distribution chambers  67   c  through the runner passages  67   b  and the intake ports  31  into the combustion chambers  30 . In  FIG. 5 , the manifold passage  67  is indicated by broken lines, and the intake ports  31  and the combustion chambers  30  are indicated by chain lines for convenience. The upper end of the up passage  64  is at a level higher than that of the uppermost intake air outlet Pe 1  at the highest position among the intake air outlets Pe. 
     Referring to  FIGS. 2 ,  3  and  5 , the intake air passage P extends continuously from the intake air inlet Pi to the intake air outlets Pe in the engine compartment  15 . The intake air passage P has the first silencing chamber  61 , the first down passage  62 , the reversing passage  63 , the up passage  64 , the second silencing chamber  65 , the throttle passage  66  and the distribution chambers  67   c , namely, down passages, arranged in that order from the upstream end to the downstream end. The combustion air taken in through the air inlet  44 , the first space  42   a  and the intake air inlet Pi flows down through the duct  54 , flows rearward in a horizontal plane through the expansion part  61   c , flows rearward through the outlet part  61   b  and the inlet part  62   a  in a horizontal plane, flows down through the down part  62   c , the flowing direction of the combustion air is reversed by the reversing passage  63  so that the combustion air flows upward through the up part  64   c  to a position at a level higher than that of the uppermost intake air outlet Pe 1 , flows forward in a horizontal plane through the outlet part  61   b  and the inlet part  65   a , flows rearward through the second silencing chamber  65 , flows rearward in a horizontal plane through the outlet part  65   b  and the throttle passage  66 , and flows down through the distribution chambers  67   c . Then the combustion air flows through the intake air outlets Pe of the runner passages  67   b  and the intake ports  31  into the combustion chambers  30 . 
     The ventilation system  70  for carrying air in the second space  42   b  as ventilating air into the engine compartment  15  is disposed behind the engine body Ea and near the cylinder head  3 . The ventilation system  70  includes the ventilation duct  71  defining an inlet passage  76  ( FIG. 5 ) having an air inlet  75  ( FIG. 6 ), and guide ducts  72  ( FIGS. 3 and 5 ) defining right and left guide passages  77  on the laterally opposite sides, respectively, of the first down passage  62  and the up passage  64 . Each of the guide passages  77  has an air outlet  78  opening downward in the engine compartment  15  at a position corresponding to the engine body Ea and the reversing passage  63  with respect to the vertical direction. The guide ducts  72  is attached to brackets  73  ( FIG. 3 ) fastened to the head cover  4 . 
     The ventilation air that has flowed down through the guide passages  77  into the engine compartment  15  cools the engine body Ea, the intake system N and the exhaust manifold  25  installed in the engine compartment  15 . Then, most part of the ventilation air is sucked as cooling air into the alternator G attached to a brackets  2   a  ( FIG. 1 ) fastened to the crankcase  2  on the front end of the engine body Ea. The ventilation system N and the alternator G are disposed at the rear and the front end, respectively, of the engine body Ea. The engine body Ea is cooled substantially entirely by the ventilation air that flows forward from behind the engine body Ea. Thus the ventilation air used efficiently as the cooling air flows into the alternator G. 
     Referring to  FIGS. 1 to 3 , the alternator G has the shaft  81  ( FIG. 3 ) driven for rotation by the crankshaft  7 , and a housing  82  housing a rotor fixedly mounted on the shaft  81  and a stator. The rotor is provided with cooling air blades (fan) for taking air into the housing  82 . The housing  82  is provided with air inlets  83  through which cooling air taken by the fan flows into the housing  82 , and air outlets  84  through which the cooling air used for cooling the alternator G is discharged from the housing  82 . A louver  85  placed on the lower case  50   a  straightens the flow of the ventilation air. The straightened ventilation air flows through the air inlets  83  into the housing  82 . 
     Exhaust air discharged through the air exit  47  flows scarcely into the engine compartment  15 , is guided by an exhaust air guide structure  90  ( FIG. 2 ) to the exhaust structure Ae, and then is discharged to the outside of the outboard motor S. 
     Referring to  FIGS. 2 ,  3  and  6  to  8 , the exhaust air guide structure  90  includes an exhaust air duct  91  ( FIG. 2 ) defining an exhaust air passage  95  ( FIG. 3 ) surrounding the air exit  47  to guide exhaust air to a predetermined position from which the exhaust air is hardly able to flow again through the air inlets  83  into the housing  82  of the alternator G. The exhaust air guide structure  90  also includes a separator wall  92  for separating the exhaust air duct  91  extending down from the upper case  50   b  through the intake silencer  50 , from the second silencing chamber  65 . A condition where the exhaust air is carried to the predetermined position can more effectively suppress or prevent the flow of the exhaust air again through the air inlets  83  into the housing  82  than a condition without the exhaust air duct  91 . In this embodiment, the predetermined position is in the air exhaust space  43  ( FIG. 6 ) outside the engine compartment  15 , and the exhaust air passage has an outlet  95   b  opening into the air exhaust space  43 . A heat insulating space  96  ( FIG. 3 ) defined by the separator wall  92  and the upper case  50   b  is formed between the exhaust air passage  95  and the second silencing chamber  65 , and the exhaust air duct  91  is made to extend in the heat insulating space  96 . Since the heat insulating space  96  is formed between the exhaust air passage  95  and the second silencing chamber  65 , the combustion air flowing through the second silencing chamber  65  is prevented or suppressed from being heated by the heat of exhaust air from the alternator G. 
     The alternator G serves also as an exhaust fan that discharges the ventilation air passing through the engine compartment  15  to the outside of the engine compartment  15  in a manner separated from the combustion air. 
     The operation and effect of the foregoing embodiment will be explained. 
     The intake air passage P of the internal combustion engine E incorporated into the outboard motor S extends continuously from the intake air inlet Pi to the intake air outlets Pe in the engine compartment  15 . The intake air passage P has the first down passage  62 , the reversing passage  63 , the up passage  64  and the distribution chambers  67   c  arranged in that order in the flowing direction of the combustion air. The combustion air taken through the intake air inlet Pi into the intake air passage P flows down through the first down passage  62 , the flowing direction of the combustion air is reversed by the reversing passage  63  so that the combustion air flows upward, and then the combustion air flows up through the up passage  64  to a position at a level higher than that of the intake air outlet Pe 1  at the highest position among the intake air outlets Pe, flows down through the distribution chambers  67   c , and then flows through the intake air outlets Pe into the combustion chambers  30 . Therefore, water contained in the combustion air that has flowed through the intake air inlet Pi into the intake air passage P is separated from the combustion air by centrifugal force as the combustion air flows through the curved reversing passage  63 . The combustion air that has passed through the reversing passage  63  flows to the position at the level higher than that of the intake air outlet Pe 1  at the highest position among the intake air outlets P 3 . The combustion air flows down through the distribution chambers  67   c  and flows through the intake air outlets Pe into the combustion chambers  30 . Thus water can be surely separated from the combustion air while the combustion air is flowing up through the up passage  64  after the flowing direction of the combustion air has been reversed, as compared with a state where the combustion air flows out through intake air outlets formed in intermediate parts of the up passage below the upper end of the up passage. Consequently, the water trapping effect is improved. When the intake air passage P is provided with the plural intake air outlets Pe, the water trapping effect of the air intake air passage P is satisfactory with all the combustion chambers  30  regardless of the positions of the intake air outlets Pe. 
     The intake air inlet Pi does not open into the engine compartment  15  and opens directly into the air intake space  42  outside the engine compartment  15 . Therefore, hot air heated in the engine compartment  15  does not flow through the intake air inlet Pi into the intake air passage P. Thus the rise of the temperature of the combustion air can be suppressed, the charging efficiency is improved, and the generation of noise by the engine cover C due to intake pulsation can be prevented because the pressure of air in the engine compartment is not caused to vary by the intake pulsation. 
     The throttle valve  52   a  of the intake system N is disposed in the intake air passage P on the downstream side of the up passage  64  or the water separating unit and on the upstream side of the distribution chambers  67   c . Since the throttle valve  52   a  controls the flow of the combustion air from which water has been separated in the reversing passage  63  and the up passage  64 , the throttle valve  52   a  is prevented from being wetted with water. When the combustion air contains salt water, adhesion of salt to the throttle valve  52   a  can be prevented. 
     In the intake air passage P, the inlet part  62   a  of the first down passage  62  or the outlet part  61   b , and the outlet part  61   b  of the up passage  64  or the inlet part  65   a  are on the opposite sides, respectively, of the throttle valve  52   a  or the throttle device  52  as seen in plan view. Thus the throttle valve  52   a  or the throttle device  52  is disposed in the space between the inlet part  62   a  or the outlet part  61   b , and the outlet part  64   b  or the inlet part  65   a . Therefore, the throttle valve  52   a  or the throttle device  52 , and the intake air passage P can be formed in a compact arrangement. The down part  62   c  of the first down passage  62  and the up part  64   c  of the up passage  64  can be formed in increased widths and large cross-sectional areas, respectively, by using the space, whereby the water separating effect is enhanced by reducing the flowing speed of the combustion air in the down part  62   c  of the flow passage  62 . An expansion silencing function can be imparted to the first down passage  62 , the reversing passage  63  and the up passage  64 , which contributes to reducing intake noise. 
     The intake silencer of the outboard motor S including the first silencing chamber  61  and the second silencing chamber  65  disposed respectively on the upstream and the downstream side of the water separating unit has an excellent intake noise reducing effect. 
     The intake air passage P is a passage within the engine compartment  15 , extending continuously from the intake air inlet Pi to the intake air outlets Pe, and the intake silencing chamber  40   r  communicating with the intake air inlet Pi is disposed outside the engine compartment  15 , while the intake silencing chamber  61  constituting part of the intake air passage P is disposed in the engine compartment  15 . Thus the plural intake silencing chambers including the intake silencing chamber  40   r  and the intake silencing chamber  61  are arranged in such a disposition allotted in both the inside and outside of the engine compartment  15 . This arrangement enables increasing the total number of the intake silencing chambers to be provided on the engine E without increasing the number of the intake silencing chambers in the engine compartment  15 , thereby preventing the engine cover C from becoming enlarged in size and further reducing the intake noises due to the provision of the plural intake silencing chambers. Thus a small-sized outboard motor having a low intake noise level can be obtained. 
     The intake duct  54  extends through the top wall  14   a  of the upper cover  14  into the first space  42   a . The extension of the intake duct  54  into the first space  42   a  enables arrangement of the intake silencing chambers  40   r  and  61  in mutually adjacent disposition in vertical direction with the top wall  14   a  of the upper cover  14  disposed between the two silencing chambers, so that the intake silencing chambers  40   r  and  61  can be arranged in vertically compact disposition. Thus the intake silencing chambers  40   r  and the engine E can also be arranged in compact disposition, serving to reduce the size of the outboard motor S. 
     The intake silencing chambers  40   r  is formed by the intake silencing chambers  40   s , the inlet part  61   a  of the first silencing chamber  61  is formed by the intake duct  54 , and the intake duct  54  cooperates with the intake silencer  40   s  to form the separable connecting structure so that the intake silencer  40   s  can be separably connected with the intake system N or the intake silencer  50 . Thus the intake silencer  40   s  is separable from the intake silencing chambers  40   r  in the intake duct  54 , whereby it is easy for the intake silencing chambers  40   r  and  61  to be separated with resultant improvement in maintenance work. 
     The separable connecting structure includes the sealing member  55  that provides a hermetical seal between the exterior and interior of the engine compartment  15 , so that intake pulsation within the intake air passage P is prevented from being transmitted to the air in the engine compartment  15 . Thus vibrations of the engine cover C due to air pressure variations in the engine compartment  15  that is caused by the intake pulsation are prevented with resultant reduction in the level of noises of the engine cover C that are produced by the intake pulsation. 
     The intake air inlet Pi of the intake duct  54  of the intake system N is connected to the first space  42   a  of the air intake space  42 , and the sealing member  55  placed in the overlapping part W where the receiving ring  14   b  of the upper cover  14  and the end part of the intake duct  54  overlap each other with respect to the flowing direction F in which the combustion air flows to seal the gap between the engine compartment  15  and the external space. Therefore, even if the intake duct  54  and the receiving ring  14   b  vibrate and move relative to each other in directions parallel to the flowing direction F, the gap between the intake duct  54  and the engine cover C can be sealed by the sealing member  55  by forming the intake duct  54  and the receiving ring  14   b  in sizes such that the overlapping part W can be formed. Thus the components of the sealing structure do not need to be formed in high dimensional accuracy and the sealing performance of the sealing structure is scarcely subject to vibrations. Since the gap between the intake duct  54  and the engine cover C can be stably sealed, noise generation by the engine cover C due to intake pulsation can be surely prevented. 
     The engine cover C includes the lower cover  13  fixed to the engine body Ea holding the intake system N, and the upper cover  14  which is guided by the locking device  16  serving as positioning devices to the joining position and detachably joined to the lower cover  13 . The sealing member  55  is put on the receiving ring  14   b . The upper cover  14  provided with the receiving ring  14   b  is guided toward the lower cover  13  by the locking devices  16 , and the intake duct  54  is received in the receiving ring  14   b  to form the overlapping part W upon the arrival of the upper cover  14  at the joining position. Thus the locking devices  16  guide the upper cover  14  toward the lower cover  13  to join the upper cover  14  to the lower cover  13  to position the receiving ring  14   b  of the upper cover  14  at the position for forming the overlapping part W, the overlapping part W is formed by guiding the upper cover  14  by the locking device  16  to the joining position. When the overlapping part W is thus formed, the sealing member  55  comes into close contact with the intake duct  54  and the receiving ring  14   b  to complete a sealing structure. Thus the sealing structure can be easily formed. 
     In the engine compartment  15  of the outboard motor S, the shaft  81  of the alternator G is disposed with its center axis Lg spaced the predetermined center distance d apart from the center axis Le of the crankshaft  7 . The exhaust air duct  91  surrounds the outlets  84  of the housing  82  of the alternator G and carries the exhaust air to the predetermined air exhaust space  43  from which the exhaust air is hardly able to flow again through the air inlets  83  into the housing  82 . Therefore, it is prevented for the exhaust air, which is discharged from the alternator G and has scarcely undergone temperature drop, to flow again into the alternator G. For this reason, the alternator G disposed in the engine compartment  15  and having the shaft  81  at the center distance d from the output shaft of the internal combustion engine E can be efficiently cooled. 
     The exhaust air duct  91  carries the exhaust air to the air exhaust space  43  outside the engine compartment  15 . Therefore, heating the combustion air by the exhaust air can be suppressed to suppress the reduction of the charging efficiency. 
     The ventilation air and the combustion air flow separately into the engine compartment  15 , and the alternator G serves as an exhaust fan for discharging the ventilation air to the outside of the engine compartment  15 . Since the alternator G serves also as the exhaust fan, an exhaust fan especially for ventilation is unnecessary. Thus the engine compartment  15  can be efficiently ventilated without requiring additional parts, and the internal combustion engine E, devices and the members installed in the engine compartment  15  can be efficiently cooled. Since the ventilation air and the combustion air flow separately into the engine compartment  15 , the flow of the combustion air taken in by the intake system N will not be affected by the ventilation air even if ventilation is promoted. 
     Modifications of the foregoing embodiment will be described. 
     The above described embodiment is provided with one intake silencer outside the engine compartment. However, more than two intake silencers could be provided outside the engine compartment. Further, the intake silencer having the intake silencing chambers could be made detachable from the intake system or the engine together with the engine cover. 
     The intake duct does not extend through the receiving ring. When the receiving ring is cylindrical, the intake duct may be fitted on the receiving ring. When the intake duct is fitted on the receiving ring, the sealing member may be held between the inside surface of the intake duct and the outside surface of the receiving ring. 
     The sealing member  55  may be combined with at least either of the intake duct  54  and the receiving ring  14   b.    
     The internal combustion engine E may be an in-line multicylinder internal combustion engine or a single-cylinder internal combustion engine. When a single-cylinder internal combustion engine has a single intake air outlet, the single intake air outlet corresponds to the uppermost intake air outlet. 
     The internal combustion engine may be applied to marine propulsion machines (for example, inboard or outboard) or machines other than the marine propulsion machines, such as vehicles and working machines.