Patent Publication Number: US-6662786-B2

Title: Vapor separator for outboard motor

Description:
PRIORITY INFORMATION 
     This application is based on and claims priority to Japanese Patent Application No. 2001-030661, filed Feb. 7, 2001, the entire contents of which is hereby expressly incorporated by reference. This application further claims the benefit of U.S. Provisional Application No. 60/322,510, filed Sep. 13, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a vapor separator for an outboard motor and, more particularly, to an improved vapor separator that has a fuel drainage mechanism. 
     2. Description of Related Art 
     Internal combustion engines for outboard motors may employ fuel injection systems to improve emission control and fuel economy. Fuel injection systems typically comprise a vapor separator that removes vapor before supplying the fuel to a fuel injector. The vapor separator can have a drainage mechanism to drain fuel accumulating therein for maintenance of the engine or for storage of the outboard motor. For example, U.S. Pat. Nos. 5,375,578 and No. 5,404,858 disclose such drain mechanisms. 
     Typically, the drain mechanisms include a drain plug threaded into a drain hole located at a lower portion of the vapor separator. In some arrangements, the vapor separator can have a vapor delivery conduit that is directly or indirectly connected to a plenum chamber of an air intake system of the engine to deliver vapor to the intake system for induction. This arrangement is beneficial not only for inhibiting vapor emissions to the atmosphere but also for expediting the drainage of the fuel. 
     Preferably, the vapor delivery conduit is formed as narrow as possible so that the vapor can be gradually delivered to the intake system such that the impact on the air/fuel ratio is minimized. The narrow conduit, however, decreases the rate of drainage. 
     SUMMARY OF THE INVENTION 
     A need therefore exists for an improved vapor separator for an outboard motor that permits a vapor separator to communicate with the atmosphere other than through a vapor delivery conduit. 
     One mechanism could be an atmosphere introduction mechanism that directly introduces atmosphere into the vapor separator. Such a mechanism could be manually operated by a user, operator, mechanic or repairperson. Such an arrangement creates further difficulties. 
     Typically, the air intake system for an outboard motor comprises one or more intake conduits extending generally horizontally along an engine body of the engine. The vapor separator can be disposed between the engine body and the intake conduit(s). In this arrangement, however, access to the operating member is inhibited because the vapor separator that has the operating member is positioned behind the intake conduit(s). 
     Another need therefore exists for an improved vapor separator for an outboard motor that can have an atmosphere introduction mechanism that is easily accessible or operable even if the vapor separator is positioned behind an intake conduit(s). 
     In accordance with one aspect of the present invention, an internal combustion engine for an outboard motor comprises an engine body. A moveable member is moveable relative to the engine body. The engine body and the moveable member together define at least one combustion chamber. An air intake system is arranged to introduce air to the combustion chamber. The intake system includes an intake conduit extending along at least part of the engine body. A fuel delivery system is arranged to deliver fuel to the combustion chamber. The fuel delivery system includes a vapor separator that contains the fuel and removes vapor from the fuel. The vapor separator is disposed between the engine body and the intake conduit. The vapor separator has a drainage mechanism to drain the fuel and an atmosphere introduction mechanism to introduce atmosphere that replaces a volume of the drained fuel. At least the atmosphere introduction mechanism includes an operating member that has an axis along which the operating member moves. The axis does not intersect with the intake conduit. 
     In accordance with another aspect of the present invention, an internal combustion engine for an outboard motor comprises an engine body. A moveable member is moveable relative to the engine body. The engine body and the moveable member together define at least one combustion chamber. A fuel injection system is arranged to spray fuel for combustion in the combustion chamber. The fuel injection system includes a vapor separator that contains the fuel and removes vapor from the fuel. The vapor separator has a drainage mechanism to drain the fuel and an atmosphere introduction mechanism that is manually operable to introduce atmosphere that replaces a volume of the drained fuel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will now be described with reference to the drawings of a preferred embodiment, which embodiment is intended to illustrate and not to limit the present invention. The drawings comprise five figures. 
     FIG. 1 is a side elevational view of an outboard motor configured in accordance with a preferred embodiment of the present invention. An associated watercraft is partially shown in section. 
     FIG. 2 is an enlarged side elevational view of an engine of the outboard motor. A protective cowling is shown in phantom line. 
     FIG. 3 is a top plan view of the engine. An engine cover and a flywheel magneto are shown in phantom line. 
     FIG. 4 is a partial top plan view of a vapor separator showing a positioning of an atmosphere introduction mechanism. A dust cap is shown in phantom line. 
     FIG. 5 is an enlarged view of a manually operated valve of the atmosphere introduction mechanism. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     With reference to FIGS. 1-3, an overall construction of an outboard motor  30  that employs an internal combustion engine  32  will be described. The engine  32  preferably comprises an improved vapor separator  34  that is arranged and configured in accordance with certain features, aspects and advantages of the present invention. 
     In the illustrated arrangement, the outboard motor  30  generally comprises a drive unit  35  and a bracket assembly  36 . The bracket assembly  36  supports the drive unit  35  on a transom  38  of an associated watercraft  40  and places a marine propulsion device in a submerged position with the watercraft  40  resting relative to a surface of a body of water. The bracket assembly  36  preferably comprises a swivel bracket  44 , a clamping bracket  46 , a steering shaft and a pivot pin  50 . 
     The steering shaft typically extends through the swivel bracket  44  and is affixed to the drive unit  35 . The steering shaft is pivotally journaled for steering movement about a generally vertically extending steering axis defined within the swivel bracket  44 . The clamping bracket  46  comprises a pair of bracket arms that preferably are laterally spaced apart from each other and that are attached to the watercraft transom  38 . 
     The pivot pin  50  completes a hinge coupling between the swivel bracket  44  and the clamping bracket  46 . The pivot pin  50  preferably extends through the bracket arms so that the clamping bracket  46  supports the swivel bracket  44  for pivotal movement about a generally horizontally extending tilt axis defined by the pivot pin  50 . The drive unit  35  thus can be tilted or trimmed about the pivot pin  50 . 
     As used in this description, the terms “forward,” “forwardly” and “front” mean at or to the side where the bracket assembly  36  is located, unless indicated otherwise or otherwise readily apparent from the context of use. The terms “rear,” “reverse,” “backwardly” and “rearwardly” mean at or to the opposite side of the front side. 
     A hydraulic tilt and trim adjustment system preferably is provided between the swivel bracket  44  and the clamping bracket  46  for tilt movement (raising or lowering) of the swivel bracket  44  and the drive unit  35  relative to the clamping bracket  46 . Otherwise, the outboard motor  30  can have a manually operated system for tilting the drive unit  35 . 
     The illustrated drive unit  35  comprises a power head  58  and a housing unit  60 , which includes a driveshaft housing  62  and a lower unit  64 . The power head  58  is disposed atop the housing unit  60  and includes the internal combustion engine  32  that is positioned within a protective cowling assembly  66 , which preferably is made of plastic. In most arrangements, the protective cowling assembly  66  defines a generally closed cavity  68  in which the engine  32  is disposed. The engine, thus, is generally protected from environmental elements by the enclosure defined by the cowling assembly  66 . 
     The protective cowling assembly  66  preferably comprises a top cowling member  70  and a bottom cowling member  72 . The top cowling member  70  preferably is detachably affixed to the bottom cowling member  72  by a coupling mechanism to provide access to the engine  32  for maintenance or for other purposes. 
     The top cowling member  70  preferably has a rear intake opening (not shown) defined through an upper rear portion. A rear intake member with one or more air ducts can be unitarily formed with or affixed to the top cowling member  70 . The rear intake member, together with the upper rear portion of the top cowling member  70 , generally defines a rear air intake space. Ambient air is drawn into the closed cavity  68  via the rear intake opening and the air ducts of the rear intake member. Typically, the top cowling member  70  tapers in girth toward its top surface, which is in the general proximity of the air intake opening. The taper helps to reduce the lateral dimension of the outboard motor, which helps to reduce the air drag on the watercraft  40  during movement. 
     The bottom cowling member  72  preferably has an opening through which an upper portion of an exhaust guide member  80  extends. The exhaust guide member  80  preferably is made of aluminum alloy and is affixed atop the driveshaft housing  62 . The bottom cowling member  72  and the exhaust guide member  80  together generally define a tray. The engine  32  is placed onto this tray and can be affixed to the exhaust guide member  80 . The exhaust guide member  80  also defines an exhaust discharge passage through which burnt charges (e.g., exhaust gases) from the engine  32  pass. 
     The engine  32  in the illustrated embodiment preferably operates on a four-cycle combustion principle. With reference now to FIGS. 2 and 3, the presently preferred engine  32  is a DOHC six cylinder engine and has a cylinder block  84  configured as a V shape. The cylinder block  84  thus defines two cylinder banks which extend side by side with each other. In the illustrated arrangement, each cylinder bank has three cylinder bores such that the cylinder block  84  has six cylinder bores in total. The cylinder bores of each bank extend generally horizontally and are generally vertically spaced from one another. This type of engine, however, merely exemplifies one type of engine. Engines having other numbers of cylinders, having other cylinder arrangements (in-line, opposing, etc.), and operating on other combustion principles (e.g., crankcase compression two-stroke or rotary) also can be used. The illustrated engine  32  generally is symmetrical about a longitudinal center plane  88  (FIG. 3) that extends generally vertically and fore to aft of the outboard motor  30 . 
     As used in this description, the term “horizontally” means that the subject portions, members or components extend generally in parallel to the water surface (i.e., generally normal to the direction of gravity) when the associated watercraft  40  is substantially stationary with respect to the water surface and when the drive unit  35  is not tilted (i.e., is placed in the position shown in FIG.  1 ). The term “vertically” in turn means that portions, members or components extend generally normal to those that extend horizontally. 
     A moveable member, such as a reciprocating piston, moves relative to the cylinder block  84  in a suitable manner. In the illustrated arrangement, one piston (not shown) reciprocates within each cylinder bore. 
     Because the cylinder block  84  is split into the two cylinder banks, each cylinder bank extends outward at an angle to an independent first end in the illustrated arrangement. A pair of cylinder head members  92  are affixed to the respective first ends of the cylinder banks to close those ends of the cylinder bores. The cylinder head members  92 , together with the associated pistons and cylinder bores, preferably define six combustion chambers (not shown). Of course, the number of combustion chambers can vary, as indicated above. Each of the cylinder head members  92  is covered with a cylinder head cover member  94  in the illustrated arrangement. 
     A crankcase member  96  is coupled with the cylinder block  84  and a crankcase cover member  98  is connected to the crankcase member  96 . The crankcase member  96  and the crankcase cover member  98  close the other end of the cylinder bores and, together with the cylinder block  84 , define a crankcase chamber. 
     A crankshaft  100  extends generally vertically through the crankcase chamber and can be journaled for rotation about a rotational axis by several bearing blocks. The rotational axis of the crankshaft  100  preferably is positioned along the longitudinal center plane  88 . Connecting rods couple the crankshaft  100  with the respective pistons in any suitable manner. Thus, the reciprocal movement of the pistons rotates the crankshaft  100 . 
     Preferably, the crankcase cover member  98  is located at the forward-most position of the engine  32 , with the crankcase member  96 , the cylinder block  84 , the cylinder head members  92  and the cylinder head cover members  94  being disposed rearward from the crankcase cover member  98 , one after another. In the illustrated arrangement, the cylinder block  84 , the cylinder head members  92 , the cylinder head cover members  94 , the crankcase member  96  and the crankcase cover member  98  together define an engine body  102 . Preferably, at least these major engine portions  84 ,  92 ,  94 ,  96 ,  98  are made of aluminum alloy. In some arrangements, the cylinder head cover members  94  can be unitarily formed with the respective cylinder head members  92 . Also, the crankcase cover member  98  can be unitarily formed with the crankcase member  96 . 
     The engine  32  also comprises an air intake system  106  (see FIG.  3 ). The air intake system  106  draws air from within the cavity  68  and supplies the air to the combustion chambers. The air intake system  106  preferably comprises six intake passages  108  and a pair of plenum chambers  110 . In the illustrated arrangement, each cylinder bank is allotted with three intake passages  108  and one plenum chamber  110 . 
     The most-downstream portions of the intake passages  108  preferably are defined within the cylinder head members  92  as inner intake passages. Thus, these portions can be integrally formed in the cylinder head members  92 . The inner intake passages communicate with the combustion chambers through intake ports, which are formed at inner surfaces of the cylinder head members  92 . Typically, each of the combustion chambers has one or more intake ports. 
     Intake valves can be slideably disposed at each cylinder head member  92  to move between an open position and a closed position. As such, the valves act to open and close the ports to control the flow of air into the combustion chamber. Biasing members, such as springs, are used to urge the intake valves toward the respective closed positions by acting between a mounting boss formed on each cylinder head member  92  and a corresponding retainer that is affixed to each of the valves. When each intake valve is in the open position, the inner intake passage that is associated with the intake port communicates with the associated combustion chamber. 
     Outer portions of the intake passages  108 , which are disposed outside of the cylinder head members  92 , preferably are defined with intake conduits  114 . In the illustrated arrangement, each intake conduit  114  is formed with two pieces. One piece is a throttle body  116  in which a throttle valve assembly  118  (see FIG. 2) is positioned. The throttle valve assemblies  118  are schematically illustrated in FIG.  2 . The throttle bodies  116  are connected to the inner intake passages. 
     Another piece is an intake runner  120  disposed upstream of the throttle body  116 . The respective intake conduits  114  extend forwardly along side surfaces of the engine body  102  on both the port side and the starboard side from the respective cylinder head members  92  toward the front of the crankcase cover member  98 . The intake conduits  114  on the same side preferably extend generally in parallel to each other and, more preferably, are vertically spaced apart from one another to define spaces S 1  therebetween. 
     Each throttle valve assembly  118  preferably includes a throttle valve. Preferably, the throttle valves are butterfly valves that have valve shafts journaled for pivotal movement about a generally vertical axis. In some arrangements, the valve shafts are linked together and are connected to a control linkage. The control linkage would be connected to an operational member, such as a throttle lever, that is provided on the watercraft or otherwise proximate the operator of the watercraft  40 . The operator can control the opening degree of the throttle valves in accordance with operator demand through the control linkage. That is, the throttle valve assemblies  118  can measure or regulate amounts of air that flow through the intake passages  108  to the combustion chambers in response to the operation of the operational member by the operator. Normally, the greater the opening degree, the higher the rate of airflow and the higher the engine speed. 
     The respective plenum chambers  110  preferably are defined with plenum chamber units  124  which are disposed side by side in front of the crankcase cover member  98  and are affixed thereto. Preferably, the plenum chamber units  124  are arranged substantially symmetrically relative to the longitudinal center plane  88 . In the illustrated arrangement, each forward end portion of the intake runners  120  is housed within each plenum chamber unit  124 . 
     As shown in FIG. 2, each plenum chamber unit  124  preferably has two air inlets  126 , which extend generally rearwardly between the respective intake runners  120 . The respective air inlets  126  define inlet openings  128  through which air is drawn into the plenum chambers  110 . In one arrangement, the intake runners  120  and the air inlets  126  can be unitarily formed with the associated plenum chamber unit  124  and those three components  120 ,  124 ,  126  can be made of plastic. The respective plenum chamber units  124  preferably can be connected with each other through one or more connecting pipes  130  (see FIG. 3) to substantially equalize the internal pressures between the chamber units  124 . The plenum chambers  110  coordinate or smooth air delivered to each intake passage  108  and also act as silencers to reduce intake noise. 
     The air within the closed cavity  68  is drawn into the plenum chambers  110  through the inlet openings  128  of the air inlets  126 . The air expands within the plenum chambers  110  to reduce pulsations and then enters the outer intake passages  108 . The air passes through the outer intake passages  108  and flows into the inner intake passages. The level of airflow is measured by the throttle valve assemblies  118  before the air enters the inner intake passages. 
     The engine  32  further comprises an exhaust system that routes burnt charges, i.e., exhaust gases, to a location outside of the outboard motor  30 . In one preferred arrangement, each cylinder head member  92  defines a set of inner exhaust passages that communicate with the combustion chambers through one or more exhaust ports, which may be defined at the inner surfaces of the respective cylinder head members  92 . The exhaust ports can be selectively opened and closed by exhaust valves. The construction of each exhaust valve and the arrangement of the exhaust valves are substantially the same as the intake valve and the arrangement thereof, respectively. Thus, further description of these components is deemed unnecessary. 
     Exhaust manifolds preferably are defined generally vertically within the cylinder block  84  between the cylinder bores of both the cylinder banks (i.e. in the valley of the v-shape). The exhaust manifolds communicate with the combustion chambers through the inner exhaust passages and the exhaust ports to collect exhaust gases therefrom. The exhaust manifolds are coupled with the exhaust discharge passage of the exhaust guide member  80 . When the exhaust ports are opened, the combustion chambers communicate with the exhaust discharge passage through the exhaust manifolds. 
     A valve cam mechanism preferably is provided for actuating the intake and exhaust valves in each cylinder bank. Preferably, the valve cam mechanism includes a pair of camshafts  132  per cylinder bank, although one of them is not shown in the figures. The camshafts  132  are intake and exhaust camshafts. The illustrated camshafts  132  extend generally vertically and are joumaled for rotation between the cylinder head members  92  and the cylinder head cover members  94 . The camshafts  132  have cam lobes to push valve lifters that are affixed to the respective ends of the intake and exhaust valves in any suitable manner. The cam lobes repeatedly push the valve lifters in a timed manner, which is in proportion to the engine speed. The movement of the lifters generally is timed by rotation of the camshafts  132  to appropriately actuate the intake and exhaust valves. 
     A camshaft drive mechanism preferably is provided for driving the valve cam mechanism. As illustrated in FIG. 3, the camshaft drive mechanism preferably comprises driven sprockets  136  positioned atop the camshafts  132 , a drive sprocket  138  positioned atop the crankshaft  100  and a timing belt or chain  140  wound around the driven sprockets  136  and the drive sprocket  138 . The crankshaft  100  thus drives the respective camshafts  132  through the timing belt  140  in the timed relationship. A belt tensioner  142  keeps the timing belt  140  tight on the sprockets  136 ,  138 . The other camshaft on each bank is driven by the camshaft driven by the crankshaft  100  or the first camshaft via another belt or chain. Because the camshafts  132  must rotate at half of the rotational speed of the crankshaft  100  in a four-cycle engine, a diameter of the driven sprockets  136  is twice as large as a diameter of the drive sprocket  138 . 
     The engine  32  further comprises indirect, port or intake passage fuel injection as a fuel delivery system. The fuel injection system preferably comprises six fuel injectors  144  with one fuel injector allotted for each one of the respective combustion chambers. The fuel injectors  144  preferably are mounted on the throttle bodies  116  of the respective banks with a pair of fuel rails  146 . The fuel rails  146  connect the fuel injectors  144  on the same bank with each other and also define portions of fuel conduits to deliver fuel to the injectors  144 . 
     Each fuel injector  144  preferably has an injection nozzle directed downstream within the associated intake passage  108 , which is downstream of the throttle valve assembly  118 . The fuel injectors  144  spray fuel into the intake passages  108  under control of an electronic control unit (ECU) (not shown). The ECU controls both the initiation timing and the duration of the fuel injection cycle of the fuel injectors  144  so that the nozzles spray a proper amount of fuel each combustion cycle. 
     Typically, a fuel supply tank disposed on a hull of the associated watercraft  40  contains fuel for the outboard motor  30 . The fuel is delivered to the fuel rails  146  through the fuel conduits. The vapor separator  34  preferably is disposed along the conduits to separate vapor from the fuel and can be mounted on the engine body  102  along the port side surface. In the illustrated embodiment, the fuel injection system employs at least two fuel pumps to deliver the fuel to and from the vapor separator  34 . More specifically, a lower pressure pump  150 , which is affixed to the vapor separator  34 , pressurizes the fuel for delivery toward the vapor separator  34 , while a high pressure pump (not shown) which is disposed within the vapor separator  34  is applied to pressurize the fuel from the vapor separator  34 . FIGS. 2 and 3 show a fuel delivery conduit  151  through which the fuel is pumped out to the fuel rails  146  from the vapor separator  34 . 
     A vapor delivery conduit  152  couples the vapor separator  34  with at least one of the plenum chambers  110 . The vapor thus can be delivered to the plenum chamber  110  for delivery to the combustion chambers together with the air for combustion. In other applications, the engine  32  can be provided with a ventilation system arranged to send lubricant vapor to the plenum chambers. In such applications, the fuel vapor also can be sent to the plenum chambers via the ventilation system. 
     The fuel injection system, particularly, the illustrated vapor separator  34  and also the vapor delivery conduit  152  will be described in greater detail below. However, similar fuel injection systems are disclosed, for example, in U.S. Pat. Nos. 5,375,578, 5,404,858, 5,797,378, 5,865,160, 5,873,347, 5,915,363 and 5,924,409, and the disclosures of which are hereby expressly incorporated by reference. It should be noted that a direct fuel injection system that sprays fuel directly into the combustion chambers can replace the indirect fuel injection system described above. Moreover, other charge forming devices, such as carburetors, can be used instead of the fuel injection systems. 
     The engine  32  further comprises an ignition system. Each combustion chamber is provided with a spark plug, which preferably is disposed between the intake and exhaust valves. Each spark plug has electrodes that are positioned in the associated combustion chamber and that are spaced apart from each other by a small gap. The spark plugs are connected to the ECU through ignition coils. The spark plugs generate a spark between the electrodes to ignite an air/fuel charge in the combustion chamber at a selected ignition timing under the control of the ECU. 
     Generally, during an intake stroke of the engine  32 , air is drawn into the combustion chambers through the air intake passages  108  and fuel is injected into the intake passages  108  by the fuel injectors  144 . The air and the fuel thus are mixed to form the air/fuel charge in the combustion chambers. At a beginning of a power stroke, the respective spark plugs ignite the compressed air/fuel charge in the respective combustion chambers. The air/fuel charge thus rapidly burns during the power stroke to move the pistons. The burnt charge, i.e., exhaust gases, then are discharged from the combustion chambers during an exhaust stroke following the power stroke. 
     The engine  32  may comprise a cooling system, a lubrication system and other systems, mechanisms or devices other than the systems described above. Such systems can be arranged in any suitable manner. 
     A flywheel assembly  156 , which is schematically illustrated with phantom line in FIG. 3, preferably is positioned atop the crankshaft  100  and is mounted for rotation with the crankshaft  100 . The flywheel assembly  156  comprises a flywheel magneto or AC generator that supplies electric power directly or indirectly (e.g. via a battery) to various electrical components, such as the fuel injection system, the ignition system and the ECU, for instance. An engine cover  158  extends over almost all of the engine  32 , including the flywheel assembly  156 . 
     With reference again to FIG. 1, the driveshaft housing  62  depends from the power head  58  and supports a driveshaft, which is coupled with the crankshaft  100  and which extends generally vertically through the driveshaft housing  62 . The driveshaft is journaled for rotation and is driven by the crankshaft  100 . The driveshaft housing  62  preferably defines an internal section of the exhaust system that leads the majority of exhaust gases to the lower unit  64 . The internal section includes an idle discharge portion that branches off of a main portion of the internal section such that idle exhaust gases can be discharged directly out to the atmosphere through a discharge port that is formed on a rear surface of the driveshaft housing  62 . 
     The lower unit  64  depends from the driveshaft housing  62  and supports a propulsion shaft that is driven by the driveshaft. The propulsion shaft extends generally horizontally through the lower unit  64  and is journaled for rotation. A propulsion device is attached to the propulsion shaft. In the illustrated arrangement, the propulsion device is a propeller  160  that is affixed to an outer end of the propulsion shaft. The propulsion device, however, can take the form of a dual counter-rotating system, a hydrodynamic jet, or any of a number of other suitable propulsion devices. 
     A transmission preferably is provided between the driveshaft and the propulsion shaft, which lie generally normal to each other (i.e., at a 90° shaft angle) to couple together the two shafts by bevel gears. The outboard motor  30  has a clutch mechanism that allows the transmission to change the rotational direction of the propeller  160  among forward, neutral and reverse. 
     The lower unit  64  also defines an internal section of the exhaust system that is connected with the internal exhaust section of the driveshaft housing  62 . At engine speeds above idle, the exhaust gases generally are discharged to the body of water surrounding the outboard motor  30  through a discharge section defined within the hub of the propeller  160 . 
     With reference still to FIGS. 2 and 3 and additional reference to FIGS. 4 and 5, the vapor separator  34  and the vapor delivery conduit  152  will now be described in greater detail. 
     The illustrated vapor delivery conduit  152  connects an upper region inside of the vapor separator  34  with the plenum chamber  110  located on the starboard side. In some arrangements, the connection could be with the port plenum chamber  110 . The illustrated delivery conduit  152  has a solid portion  170  (FIG. 3) that is branched off therefrom. This solid portion  170  is coupled to the plenum chamber unit  124  on the port side to better secured the vapor delivery conduit  152 . 
     The illustrated delivery conduit  152  also has a check valve  172  that can be in an open position when a pressure in the vapor separator  34  is greater than a preset pressure. The check valve  172  regulates the flow of vapor into the plenum chamber  110 . 
     The illustrated vapor delivery conduit  152  also has a filter  174  to remove foreign substances from the vapor. The filter  174  also is useful to trap the vapor and then to gradually release it. This is advantageous because the delivery of the vapor is less likely to affect the air/fuel ratio. 
     The vapor separator  34  preferably comprises a drainage mechanism  178  to drain the fuel in the vapor separator  34  for maintenance or for storage of the outboard motor  30 . The drainage mechanism  178  can comprise, for example, a drain hole and a drain plug  180  threaded into the drain hole. The drain hole preferably is formed in a lowermost portion of the vapor separator  34  when the outboard motor is in the operating position. The illustrated drain hole opens in the space S 1  defined between the two lowermost intake conduits  114 . The drain hole preferably has an axis that extends generally normal to the center plane  88 . The drain plug  180  acts as an operating member of the drainage mechanism  178  and preferably is moveable along the axis. In the illustrated arrangement, the axis does not intersect with either the lowermost positioned intake conduit  114  or the intake conduit  114  positioned next to the lowermost positioned conduit  114 . The illustrated drain plug  180 , thus, is easily operable at a location positioned between those conduits  114 . 
     The illustrator vapor separator  34  further comprises an atmosphere introduction mechanism  184  to introduce air that replaces a displaced volume of the fuel in the vapor separator  34  such as during draining. FIG. 4 illustrates an exemplary construction of the atmosphere introduction mechanism  184 . The introduction mechanism  184  preferably comprises a valve  186  and an opening  188  in which the valve  186  is fitted. 
     In one arrangement, the opening  188  is formed atop of the vapor separator  34  and communicates with a space S 2  defined among the uppermost intake conduit  114 , the engine cover  158  and the inner upper space of the vapor separator  34 . The opening  188  has an axis  190  as shown in FIGS. 2-4. In the illustrated embodiment, the opening  188  has two different diameters such that a first portion which communicates with the inner upper space of the vapor separator  34  has a smaller diameter than a second portion which communicates with the space S 2 . The first portion substantially supports the valve  186 . The second portion defines a recess  192  into which a dust-proof cap  194  can be fitted. The illustrated cap  194  can be removably placed in the recess  192 . The valve  186  is manually operable with a tool by detaching the cap  194 . 
     FIG. 5 illustrates an exemplary construction of the valve  186 . The valve  186  generally comprises a valve body  198 , a shaft  200 , a head member  202 , a valve seat  204  and a coil spring  206 . The head member  202  is connected to one end of the body  198 . The valve body  198 , together with the head member  202 , supports the shaft  200 . Both the valve body  198  and the head member  202  comprise a lumen through which the shaft  200  extends. The lumen narrows in the head member  202  but the lumen still defines a small gap between the shaft  200  and the head member  202 . 
     The shaft  200  defines an operating member of the atmosphere introduction mechanism  184  and the shaft  200  is axially moveable within the lumen. Preferably, the valve seat  204  is rigidly mounted to the shaft  200 . The valve seat  204  is made of, for example, a rubber material and can be connected to the shaft  200  by a support member  208 . 
     The spring  206  is confined within the hollow of the valve body  198  and extends around the shaft  200 . An inner shoulder portion is formed in the valve body  198 . The shoulder portion supports one end of the spring  206  while spring retainer  210 , also formed on the shaft  200 , supports the other end of the spring  206 . Thus, the spring  206  urges the shaft  200  in a direction away from the valve seat  204  such that the valve seat  204  abuts on a portion of the valve body  198 . This is a closed position. When the shaft  200  moves within the valve body  198  in a direction toward the valve seat  204 , the valve seat  204  separates from the end of the valve body  198  and a gap is formed therebetween. This is an open position. 
     In the illustrated arrangement, the valve  186  is positioned in the opening  188  such that an axis of the shaft  200  is consistent with the axis  190 . At least the valve seat  204  and the end of the valve body  198  preferably are placed within the inner upper space of the vapor separator  34 . As such, a tip portion of the shaft  200  is disposed within the recess  192 . A seal  212  surrounds the valve body  198  to sealingly secure the valve  186  in the opening  188 . The axis  190  along which the shaft  200  moves is arranged not to intersect with the uppermost intake conduit  114 . Thus, manipulation of the shaft  200  is not prevented by its placement. In other words, the shaft  200  is operable at a location positioned generally above the uppermost intake conduit  114 . 
     For maintenance of the engine  32  or for storage of the outboard motor  30 , the dust-proof cap  194  first is detached from the recess  192  to expose the tip portion of the shaft  200 . Also, the drain plug  180  is removed from the drain hole. The valve  186  is held in the closed position by the bias force of the spring  206  and the air is not introduced into the vapor separator  34 . When the tip portion of the shaft  200  is pushed, such as, with a tool, for instance, the valve  186  is opened against the bias force of the spring  206 . The ambient air is introduced into the vapor separator  34  to replace the volume of the fuel which is being drained. Thus, fuel can be drained quickly and efficiently. In addition, the drain plug  180  and the shaft  200  of the valve  186  have axes that do not intersect with any one of the intake conduits  114  in the illustrated arrangement. Those members  180 ,  200 , thus, are easily accessible or operable even if the vapor separator  34  is positioned behind the intake conduits  114 . Furthermore, because the shaft  200  of the valve  186  is always urged toward the closed position, only one way actuation of the shaft  200  toward the open position is necessary. This can be done one-handed. 
     Of course, the foregoing description is that of a preferred construction having certain features, aspects and advantages in accordance with the present invention. Various changes and modifications may be made to the above-described arrangements without departing from the spirit and scope of the invention, as defined by the appended claims. For instance, the recess and the dust-proof cap are not necessarily provided. In one alternative, the tip portion of the shaft of the valve can expose at a surface of the vapor separator and can be operable without any tool. Accordingly, the scope of the present invention should not be limited to the illustrated configurations, but should only be limited to a fair construction of the claims that follow and any equivalents of the claims.