Patent Publication Number: US-7909022-B2

Title: Fuel supply system for boat and outboard motor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel supply system for a boat and an outboard motor, and specifically relates to a fuel supply system for a boat including a fuel injection device for injecting fuel into an intake passage and an outboard motor. 
     2. Description of the Related Art 
     Conventionally, a fuel supply system for a boat including a fuel injection device for injecting fuel into an intake passage is known (See, for example, JP-A-2001-140720 and JP-A-Hei 9-88623). 
     The fuel supply system for a boat described in JP-A-2001-140720 and JP-A-Hei 9-88623 is a fuel supply system for an outboard motor provided on a boat. In JP-A-2001-140720 and JP-A-Hei 9-88623, fuel is pumped up from a fuel tank mounted on a hull and reserved in a vapor separator tank. The fuel reserved in the vapor separator tank is supplied to a fuel injection device by a fuel supply pump. The fuel supply system for a boat described in JP-A-2001-140720 and JP-A-Hei 9-88623 includes a throttle body including a throttle valve for adjusting a flow rate of air supplied to an engine and an intake passage including a plurality of intake pipes, first ends of which are connected to the throttle body and the second ends of which are respectively connected to a plurality of cylinders. The fuel injection device is disposed in the vicinity of a combustion chamber and configured to inject fuel toward a direction in which the air in the intake passage flows (from upstream to downstream). 
     As in JP-A-2001-140720 and JP-A-Hei 9-88623, in the case where the fuel injection device is disposed in the vicinity of the combustion chamber, it is effective to inject fuel toward a direction of air flow in order to generate a swirl or a tumble in the combustion chamber. 
     However, in the case where the fuel injection device is spaced from the combustion chamber, if fuel is injected toward the direction in which the air in the intake passage flows, the fuel adheres to a wall surface of the intake passage. Accordingly, it is difficult to spread fuel evenly in the intake passage. Thus, there occurs an uneven distribution of air-fuel ratio in the air-fuel mixture, resulting in deterioration in combustion efficiency of the engine. Specifically, when the configuration in which fuel is injected toward the direction in which the air in the intake passage flows is applied to a configuration in which a single fuel injection device is provided to a plurality of intake pipes of an engine having a plurality of cylinders, the air-fuel ratio of the air-fuel mixture suctioned into each intake pipe fluctuates due to uneven distribution of the fuel, thereby fluctuating the air-fuel ratio of the air-fuel mixture supplied to each cylinder. As a result, it becomes difficult to supply an air-fuel mixture with an appropriate air-fuel ratio evenly to each cylinder, resulting in further deterioration in combustion efficiency of the engine. 
     SUMMARY OF THE INVENTION 
     In view of the above, preferred embodiments of the present invention provide a fuel supply system for a boat and an outboard motor that prevents deterioration in combustion efficiency of an engine. 
     A fuel supply system for a boat according to a first preferred embodiment of the present invention includes: an intake passage that is connected to an engine and is arranged to supply air to the engine; and a fuel injection device arranged to inject fuel to the intake passage. The fuel injection device is configured to inject fuel to a direction opposite to an airflow direction in the intake passage. 
     In the fuel supply system for a boat according to the first preferred embodiment, as described above, fuel is injected in a direction (from downstream to upstream) opposite to the airflow direction in the intake passage. Therefore, by colliding with air, the fuel can be atomized and distributed evenly in the air. This minimizes and prevents the occurrence of uneven distribution of the air-fuel ratio in the air-fuel mixture, thereby preventing deterioration in combustion efficiency of the engine. Specifically, in the case where this system is applied to the configuration in which a single fuel injection device is provided to a plurality of intake pipes of an engine having a plurality of cylinders, the single fuel injection device can inject and distribute fuel evenly to the plurality of intake pipes, thereby supplying a air-fuel mixture with the same air-fuel ratio to each of the plurality of cylinders. This prevents deterioration in combustion efficiency of the engine. 
     In the fuel supply system for a boat according to the first preferred embodiment, preferably, the intake passage includes a throttle body including a throttle valve arranged to adjust a flow rate of air supplied to the engine, and the fuel injection device is configured to inject fuel in the vicinity of the throttle body. With this configuration, fuel is injected in the vicinity of the throttle body where air flows fastest in the intake passage. This further facilitates fuel atomization and facilitates even distribution of fuel in the air. 
     In the configuration in which the fuel injection device injects fuel in the vicinity of the throttle body, preferably, the engine includes a plurality of cylinders and the intake passage further includes a plurality of intake pipes, first ends of which are connected to the throttle body and second ends of which are respectively connected the plurality of cylinders. With this configuration, when the engine has a plurality of cylinders, it is possible to introduce a air-fuel mixture into a plurality of intake pipes connected to the respective cylinders under the condition that fuel is injected and evenly distributed in the air in the throttle body. This minimizes and prevents fluctuation in the air-fuel ratio of the air-fuel mixture introduced to each intake pipe between the plurality of intake pipes. Thus, an air-fuel mixture in the same air-fuel ratio can be supplied to each of the cylinders. Therefore, deterioration in combustion efficiency of the engine can be prevented. 
     In this case, only a single fuel injection device is preferably provided and the single fuel injection device is connected to each of the plurality of intake pipes. With this configuration, when the engine has a plurality of cylinders, the single fuel injection device can supply an air-fuel mixture in an even air-fuel ratio to each of the cylinders. That is, there is no need to provide a plurality of fuel injection devices. Accordingly, there is no need to provide a delivery pipe for distributing fuel to the plurality of fuel injection devices when the plurality of fuel injection devices are used, thereby decreasing the number of components and reducing weight of the fuel supply system for a boat. 
     In the configuration in which the fuel injection device injects fuel in the vicinity of the throttle body, the fuel injection device is preferably located in a downstream vicinity in an airflow direction relative to the throttle valve of the throttle body. With this configuration, fuel can be injected into a portion where air flows fastest in the throttle body. This further facilitates fuel atomization and facilitates even distribution of fuel in the air. 
     In the configuration in which the fuel injection device injects fuel in the vicinity of the throttle body, the fuel injection device is preferably configured to inject fuel toward the throttle valve. With this configuration, the fuel that is injected and is not taken into the air does not hit an inner peripheral surface of the throttle body but hits the throttle valve. This prevents the injected fuel from adhering to the inner peripheral surface of the throttle body. 
     In this case, the throttle valve preferably includes a butterfly-type throttle valve. With this configuration, since there is a flow of air between the throttle valve and the inner peripheral surface of the throttle body, even if fuel adheres to the throttle valve, the adherent fuel can be taken into the flow of air when the adherent fuel moves to an end (an end on the side of the inner peripheral surface of the throttle body) of the throttle valve. Thus, differing from the case where fuel is injected toward the inner peripheral surface of the throttle body, a portion of the injected fuel can be prevented from adhering to the inner peripheral surface of the throttle body without being taken into the air. 
     In the configuration in which the fuel injection device injects fuel in the vicinity of the throttle body, preferably, the throttle body includes: a main air passage in which the throttle valve arranged to adjust a flow rate of air supplied to the engine is provided; and a bypass air passage that connects an upstream side and a downstream side of the main air passage relative to the throttle valve. The fuel injection device is disposed such that an injection nozzle of the fuel injection device is positioned in the vicinity of an air exit of the bypass air passage positioned downstream of the throttle valve. With this configuration, fuel can be injected into a portion where air flows relatively fast in the vicinity of the air exit of the bypass air passage. This further facilitates fuel atomization and facilitates even fuel distribution in the air. 
     In the configuration in which the fuel injection device injects fuel in the vicinity of the throttle body, preferably, the fuel supply system further includes: a fuel tank arranged to hold fuel to be supplied to the fuel injection device; and a fuel supply pump arranged to supply the fuel from the fuel tank to the fuel injection device. The fuel tank is disposed adjacent to the throttle body. With this configuration, the temperature of the throttle body is decreased by fuel vaporization when fuel is injected to the throttle body. Therefore, an increase in the temperature in the fuel tank can be minimized and prevented by arranging the fuel tank adjacent to the low-temperature throttle body. This prevents generation of vapor (vaporized fuel) in the fuel tank. 
     In the configuration in which the fuel injection device injects fuel in the vicinity of the throttle body, preferably, the throttle body includes a main air passage in which the throttle valve arranged to adjust a flow rate of air supplied to the engine is provided, and the fuel injection device is configured to inject fuel obliquely relative to the vertical direction on a plane that is perpendicular or substantially perpendicular to a direction in which the main air passage of the throttle body extends. With this configuration, the height of the top of the fuel injection device can be lowered compared with the case where the fuel injection device is configured to vertically inject fuel from just above. This makes it possible to provide a unit including the throttle body and the fuel injection device compact. 
     An outboard motor according to a second preferred embodiment of the present invention includes an engine, an intake passage that is connected to the engine and is arranged to supply air and a fuel injection device arranged to inject fuel to the intake passage, and the fuel injection device is configured to inject fuel to a direction opposite to an airflow direction in the intake passage. 
     In the outboard motor according to the second preferred embodiment, as described above, fuel is injected in a direction (from downstream to upstream) opposite to the airflow direction in the intake passage. Therefore, by colliding with air, the fuel can be atomized and distributed evenly in the air. This minimizes and prevents the occurrence of uneven distribution of the air-fuel ratio in the air-fuel mixture, thereby preventing deterioration in combustion efficiency of the engine. Specifically, in the case where this system is applied to the configuration in which a single fuel injection device is provided to a plurality of intake pipes of an engine having a plurality of cylinders, the single fuel injection device can inject and distribute fuel evenly to the plurality of intake pipes, thereby supplying a air-fuel mixture with the same air-fuel ratio to each of the plurality of cylinders. This prevents deterioration in combustion efficiency of the engine. 
     Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing a general construction of an outboard motor according to a preferred embodiment of the present invention. 
         FIG. 2  is a perspective view showing an engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 3  is a side view showing the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 4  is a top view showing the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 5  is a front view showing the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 6  is a perspective view showing a throttle body and its vicinity in the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 7  is a top view showing the throttle body and its vicinity in the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 8  is a front view showing the throttle body and its vicinity in the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 9  is a partial sectional view showing the internal structure of a high-pressure fuel pump in the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 10  is a schematic view showing the high-pressure fuel pump in the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 11  is a hydraulic circuit diagram of the high-pressure fuel pump in the engine section of the outboard motor shown in  FIG. 1 . 
         FIG. 12  is a schematic view showing a fuel supply system of the outboard motor of  FIG. 1 . 
         FIG. 13  is a side view of an engine section according to a variation of preferred embodiments of the present invention. 
         FIG. 14  is a plan view of the engine section according to the variation of preferred embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described below with reference to the accompanying drawings. 
       FIG. 1  is a side view showing a configuration of an outboard motor that includes a fuel supply system for a boat according to a preferred embodiment of the present invention.  FIGS. 2 to 12  are illustrations showing the detailed structure of an engine of the outboard motor shown in  FIG. 1 .  FIG. 12  is a schematic diagram showing functions of each component constituting the fuel supply system for a boat. The arrangement of each component (especially the location of a high-pressure fuel pump) in  FIG. 12  is different from that in  FIGS. 2 to 8 . First, referring to  FIGS. 1 to 12 , a structure of an outboard motor  1  provided with a fuel supply system for a boat according to a preferred embodiment of the present invention will be described. 
     As shown in  FIG. 1 , the outboard motor  1  includes an engine section  2 , a drive shaft  3  that is rotated by the driving force of the engine section  2  and extends vertically, a forward/reverse changing mechanism  4  connected to a lower end of the drive shaft  3 , a propeller shaft  5  that is connected to the forward/reverse changing mechanism  4  and extends horizontally, and a propeller  6  attached to a rear end portion of the propeller shaft  5 . The engine section  2  is housed in a cowling  7 . In an upper case  8  and a lower case  9  arranged below the cowling  7 , the drive shaft  3 , the forward/reverse changing mechanism  4 , and the propeller shaft  5  are housed. The outboard motor  1  is mounted to a transom plate  101  provided on a reverse direction (direction of an arrow “A”) side of a hull  100  via a clamp bracket  10 . The clamp bracket  10  supports the outboard motor  1  pivotally around a tilt shaft  10   a  in a vertical direction with respect to the hull  100 . A fuel tank  102  for reserving fuel (gasoline) is provided on the hull  100 . The fuel tank  102  and the engine section  2  of the outboard motor  1  are connected by a fuel pipe (not shown). The engine section  2  of the outboard motor  1  is driven using fuel supplied from the fuel tank  102 . The propeller  6  is driven by the driving force of the engine section  2  and a rotational direction of the propeller  6  is changed by the forward/reverse changing mechanism  4 , thereby propelling the hull  100  in a forward direction (direction of an arrow “B”) or in a reverse direction (direction of the arrow “A”). A vent  7   a  is provided on a reverse direction (direction of the arrow “A”) side portion of the cowling  7 . Air supplied to the engine section  2  is taken in via the vent  7   a  into the engine section  2  in the cowling  7 . 
     As shown in  FIGS. 2 to 5 , the engine section  2  includes an engine  20 , an intake system  30  arranged to supply air to the engine  20 , a fuel system  40  arranged to supply fuel to the engine  20 , and an ECU (Engine Control Unit)  50  (see  FIG. 12 ). The engine  20  is an example of the “engine” according to a preferred embodiment of the present invention. 
     As shown in  FIG. 3 , the engine  20  preferably includes two cylinders  21  disposed parallel or substantially parallel in a vertical direction (“Z” direction) and two pistons  22  respectively reciprocating horizontally in each of the cylinders  21 . Each of the pistons  22  is connected to a crankshaft  24  extending in a vertical direction (“Z” direction) via a connecting rod  23 . A horizontal reciprocating motion of the piston  22  is converted to a rotational motion by the connecting rod  23  and the crankshaft  24 . A lower end  24   a  of the crankshaft  24  is connected to the drive shaft  3  (see  FIG. 1 ). As shown in  FIGS. 2 to 5 , rotation of the crankshaft  24  is transmitted to a camshaft  27  by a pulley  25  fixed at the top of the crankshaft  24 , a belt  26 , and a pulley  28  fixed to the camshaft  27 . An intake valve (not shown) and an exhaust valve (not shown) of each cylinder  21  are driven at predetermined timings by the rotation of the camshaft  27 . 
     As shown in  FIGS. 2 and 5 , the intake system  30  is disposed along a right side of the engine  20  when seen in a forward direction (direction of the arrow “B”) of the engine  20 . The intake system  30  includes a silencer case  31  that is disposed in a forward direction (direction of the arrow “B”) side and has an inlet  31   a  (see  FIG. 3 ), a throttle body  32  connected to the silencer case  31 , and two intake pipes  33  respectively connected to an intake port (not shown) of each of the two cylinders  21  of the engine  20 . Note that the silencer case  31 , the throttle body  32 , and the intake pipes  33  constitutes an example of the “intake passage” according to a preferred embodiment of the present invention. 
     As shown in  FIGS. 6 to 8  and  FIG. 12 , the throttle body  32  is preferably formed of resin or metal and has a cylindrical air passage  32   a . Note that the air passage  32   a  is an example of the “main air passage” according to a preferred embodiment of the present invention. A butterfly-type throttle valve  32   b  (see  FIGS. 8 and 12 ) is provided in the air passage  32   a . The butterfly-type throttle valve  32   b  includes a rotational shaft  321  extending perpendicular or substantially perpendicular to the air passage  32   a  and extending horizontally and a discoid valve plate  322  attached to the rotational shaft  321 . When the valve plate  322  is generally vertical, the air passage  32   a  is fully closed by the valve plate  322 . When the valve plate  322  is generally horizontal, the air passage  32   a  is fully opened. As shown in  FIG. 12 , a bypass air passage  32   c  that connects an upstream side and a downstream side of the air passage  32   a  relative to the throttle valve  32   b  is provided in the throttle body  32 . The bypass air passage  32   c  provides an air flow rate at idling when the throttle valve  32   b  is completely closed. Also, in the bypass air passage  32   c , an ISC (Idle Speed Control) unit  34  having a valve arranged to control the air flow rate in the bypass air passage  32   c  is provided. Engine speed at idling can be controlled by adjusting the opening degree of the valve of the ISC unit  34 . The throttle body  32  also has a throttle opening sensor  35  arranged to detect the opening degree of the throttle valve  32   b , an intake air pressure sensor  36  arranged to detect air pressure in the air passage  32   a , and an intake air temperature sensor  37  arranged to detect air temperature in the air passage  32   a . The ISC unit  34  and a sensor section  38  including the throttle opening sensor  35 , the intake air pressure sensor  36 , and the intake air temperature sensor  37  are attached to an upper portion of the throttle body  32 . 
     As shown in  FIGS. 2 to 6  and  FIG. 12 , the fuel system  40  includes a filter  41  connected to the fuel tank  102  disposed on the hull  100 , a low-pressure fuel pump  43  connected to the filter  41  via a fuel pipe  42  preferably made of rubber or resin, a vapor separator tank  45  connected to the low-pressure fuel pump  43  via a fuel pipe  44  preferably made of rubber or resin, a high-pressure fuel pump  46  (see  FIG. 6 ) arranged to transport fuel reserved in the vapor separator tank  45 , and an injector  47  arranged to inject the fuel transported by the high-pressure fuel pump  46 . Note that the vapor separator tank  45 , the high-pressure fuel pump  46 , and the injector  47  respectively are examples of the “fuel tank,” the “fuel supply pump,” and the “fuel injection device” according to a preferred embodiment of the present invention. 
     As shown in  FIG. 5 , the low-pressure fuel pump  43  preferably is a so-called diaphragm type fuel pump including a piston (not shown) and a diaphragm (not shown). The piston of the low-pressure fuel pump  43  is arranged to be reciprocated in conjunction with rotation of a cam (not shown) attached to the camshaft  27  of the engine  20  (see  FIG. 2 ). The diaphragm is arranged to be reciprocated corresponding to the reciprocation of the piston, thereby transporting fuel. A water-cooling section  43   a  is provided on a side portion of the low-pressure fuel pump  43 . The water-cooling section  43   a  has a pipe  43   b  extending along the side portion of the low-pressure fuel pump  43  and allows the pipe  43   b  to flow sea water, thereby cooling the low-pressure fuel pump  43 . Since the fuel pumped up from the fuel tank  102  on the hull  100  by the low-pressure fuel pump  43  passes through the filter  41 , foreign matters and the like contained in the fuel are eliminated. 
     The fuel sent out by the low-pressure fuel pump  43  via the fuel pipe  44  is discharged from an outlet  45   a  (see  FIG. 12 ) into the vapor separator tank  45  to be reserved therein. The vapor separator tank  45  is preferably formed of resin and disposed adjacent to and below the throttle body  32  to contact with the throttle body  32 . In this embodiment, as shown in  FIGS. 6 to 8 , the throttle body  32  and the vapor separator tank  45  are fixedly joined preferably by four screws  200 , for example. 
     The vapor separator tank  45  reserves the fuel pumped up from the fuel tank  102  and separates the vaporized fuel (vapor) or air from the liquid fuel. As shown in  FIG. 12 , the vapor separator tank  45  is configured such that the reserved quantity of fuel in the tank is kept constant and the fuel in the tank is kept at a predetermined level “P.” Specifically, a float  45   c  pivotable about a pivot shaft  45   b  in a vertical direction (“Z” direction) is provided in the vapor separator tank  45 . A needle valve  45   d  is provided in the float  45   c  at a position corresponding to the outlet  45   a . Since the float  45   c  moves in a vertical direction as the fuel level in the vapor separator tank  45  moves, the needle valve  45   d  moves in a vertical direction corresponding to the movement of the float  45   c . If the fuel level in the vapor separator tank  45  becomes higher than the predetermined level “P,” the float  45   c  ascends to insert the needle valve  45   d  into the outlet  45   a , thereby automatically stopping inflow of fuel into the vapor separator tank  45 . If the fuel level in the vapor separator tank  45  is lower than the predetermined level “P,” the float  45   c  descends to separate the needle valve  45   d  from the outlet  45   a , thereby automatically allowing inflow of fuel into the vapor separator tank  45 . With the above described mechanism, the reserved quantity of fuel in the vapor separator tank  45  is kept constant and the fuel in the tank is kept at the predetermined level “P.” 
     At the bottom of the vapor separator tank  45 , there is provided a water sensor  45   e  arranged to detect water collected at the bottom of the vapor separator tank  45 . Specifically, a central portion  45   f  of the bottom of the vapor separator tank  45  is protruded upward. The protruded portion defines a recess as seen from the outside below the vapor separator tank  45 . Two leads  451 ,  452  are disposed in the recess and tips of the leads  451 ,  452  are connected. Also, a pair of floats  45   g  that are floatable in water are provided at the bottom of the vapor separator tank  45 . Each of the pair of floats  45   g  has a built-in magnet (not shown). When water is collected in the bottom of the vapor separator tank  45 , the float  45   g  having a magnet ascends as a water level “Q” ascends. When the floats  45   g  ascend up to a predetermined position, the tip of the lead  451  and the tip of the lead  452  are separated from each other by magnetic forces from the magnets. Accordingly, connection between the leads  451 ,  452  is interrupted. With the above configured water sensor  45   e , it is possible to detect whether or not water is collected equal to or more than a predetermined quantity in the bottom of the vapor separator tank  45 . 
     A leading end  46   h  of a pipe  46   f  is inserted into an upper portion of the vapor separator tank  45 . The pipe  46   f  is connected to the high-pressure fuel pump  46 , which will be described later. The fuel returned from the high-pressure fuel pump  46  is discharged from the leading end  46   h  of the pipe  46   f  into the vapor separator tank  45 . A buffer plate  45   h  is disposed below the leading end  46   h  of the pipe  46   f  and above the float  45   c  in the vapor separator tank  45 . A plurality of small holes are provided in the buffer plate  45   h . Fuel is discharged from the leading end  46   h  of the pipe  46   f  via the holes of the buffer plate  45   h  into the vapor separator tank  45  to be reserved therein again. When the fuel discharged from the leading end  46   h  of the pipe  46   f  bubbles, the buffer plate  45   h  can drip the liquid fuel into the vapor separator tank  45  without dropping vapor. 
     The vapor separator tank  45  and the throttle body  32  are connected via a check valve  45   i . The check valve  45   i  is configured to pass vapor (vaporized fuel) or air only in one direction from the vapor separator tank  45  to the throttle body  32 . When vapor occurs to increase an internal pressure of the vapor separator tank  45 , the check valve  45   i  opens to discharge the vapor from the vapor separator tank  45  to the throttle body  32 . Also, when the engine (engine section  2 ) is operated, the negative pressure in the throttle body  32  opens the check valve  45   i  to discharge the vapor from the vapor separator tank  45  to the throttle body  32 . 
     As shown in  FIGS. 6 to 8 , the high-pressure fuel pump  46  is a so-called in-line type fuel pump that is disposed outside the vapor separator tank  45  and interposed between fuel pipes. The high-pressure fuel pump  46  is fixed to a side of the vapor separator tank  45  at two locations preferably by screws  201 , for example. The high-pressure fuel pump  46  is preferably formed of resin as a base material. More specifically, as shown in  FIG. 9 , the high-pressure fuel pump  46  is configured such that a pump main portion  46   a  through which fuel passes is retained by an outer frame  46   b  preferably made of resin. The outer frame  46   b  is fixed to the vapor separator tank  45  preferably by screws  201  (see  FIGS. 7 and 8 ), for example. The pump main portion  46   a  is configured to transport fuel by rotating a rotary shaft  46   c . In this preferred embodiment, as shown in  FIGS. 2 to 5 , a pulley  46   d  is fixed at an upper end of the rotary shaft  46   c . The pulley  46   d  is meshed with the belt  26  together with the pulley  25  of the crankshaft  24  and the pulley  28  of the camshaft  27 . Thus, as the crankshaft  24  is rotated by the driving force of the engine  20 , the pulley  46   d  and the rotary shaft  46   c  are rotated to drive the pump main portion  46   a.    
     As shown in  FIGS. 9 to 11 , the pump main portion  46   a  preferably includes: an inlet  461  connected to the vapor separator tank  45  via a resinous pipe  46   e ; a swash plate  462  fixed aslant to the rotary shaft  46   c ; a plunger  463 ; a filter  464 ; a reserve chamber  465  arranged to temporarily hold reserve fuel; a reserve chamber  467  containing a fuel pressure retaining valve  466 ; a relief valve  468  connected to the vapor separator tank  45  via a resinous pipe  46   f ; and an outlet  469  connected to the injector  47  (see  FIG. 12 ) via a resinous pipe  46   g . An upper end of the plunger  463  abuts on a lower surface of the swash plate  462 . As the swash plate  462  together with the rotary shaft  46   c  rotates, the plunger  463  moves in a vertical direction. When the plunger  463  moves upward, fuel is drawn from the vapor separator tank  45  into the reserve chamber  465  via the inlet  461  and the filter  464 . When the plunger  463  moves downward, fuel is pushed out from the reserve chamber  465  to the reserve chamber  467 . There are provided a lead valve  465   a  and a lead valve  465   b  respectively between the filter  464  and the reserve chamber  465  and between the reserve chamber  465  and the reserve chamber  467 . These valves open when fuel flows in a transport direction (direction from the inlet  461  to the outlet  469 ) and close when fuel attempts to flow in the opposite direction. When fuel is drawn from the filter  464  into the reserving chamber  465 , the lead valve  465   a  opens and the lead valve  465   b  closes at the same time as the plunger  463  moves upward. When fuel is pushed out from the reserving chamber  465  to the reserving chamber  467 , the lead valve  465   a  closes and the lead valve  465   b  opens at the same time as the plunger  463  moves downward. When the pressure of the fuel reserved in the reserving chamber  467  becomes equal to or larger than a predetermined value, the fuel is discharged from the outlet  469  via the fuel pressure retaining valve  466 . The outlet  469  is connected to the relief valve  468 . When pressure at the outlet  469  excessively increases in such a case that the injector  47  (see  FIG. 12 ) is plugged with fuel, fuel is discharged into the vapor separator tank  45  (see  FIG. 12 ) via the relief valve  468  and the pipe  46   f.    
     As shown in  FIGS. 6 to 8  and  12 , the injector  47  has a function to inject the fuel, which is sent out at a predetermined pressure by the high-pressure fuel pump  46 , at the predetermined timing. In this preferred embodiment, the injector  47  is inserted into amounting hole  32   d  of the throttle body  32  for mounting. The injector  47  is configured to inject fuel in a direction (from downstream to upstream) opposite to an airflow direction in the air passage  32   a  of the throttle body  32 . On a plane defined by the direction of fuel injection and the air flow direction, the direction of fuel injection is tilted at an angle of α (about 20 degrees to about 60 degrees, for example) relative to the airflow direction. Also, as shown in  FIG. 8 , on a plane perpendicular or substantially perpendicular to a direction in which the air passage  32   a  extends, the injector  47  is tilted upward at an angle of β relative to the horizontal direction (direction in which the rotational shaft  321  of the throttle valve  32   b  extends) so as to inject fuel obliquely downward into the air passage  32   a  from above. Further, as shown in  FIG. 7 , on a horizontal plane, the injector  47  is tilted by an angle of γ relative to the direction in which the air passage  32   a  extends. Thus, the injector  47  is disposed at angles (angle of α, β, and γ) relative to the air passage  32   a . Accordingly, the height of projection of the injector  47  from the throttle body  32  can be minimized, which makes it possible to provide a unit including the throttle body  32  and the injector  47  that is very compact. As shown in  FIG. 12 , an injection nozzle  47   a  of the injector  47  is disposed in a downstream vicinity of the throttle valve  32   b . Fuel is injected from the injection nozzle  47   a  of the injector  47  toward the throttle valve  32   b . Also, the injection nozzle  47   a  of the injector  47  is positioned at an exit of the bypass air passage  32   c.    
     In this preferred embodiment, as described above, fuel is injected in a direction (from downstream to upstream) opposite to an airflow direction in the throttle body  32 . Therefore, by colliding with air, the fuel can be atomized and distributed evenly in the air. This minimizes and prevents the occurrence of uneven distribution of the air-fuel ratio in the air-fuel mixture, thereby preventing deterioration in combustion efficiency of the engine  20 . 
     In this preferred embodiment, as described above, the injector  47  is configured to inject fuel in the throttle body  32 . Therefore, fuel is injected in the throttle body  32  where air flows fastest, which further facilitates fuel atomization and facilitates even fuel distribution in the air. 
     In this preferred embodiment, as described above, fuel is injected into the throttle body  32  under the condition that one end of each of the two intake pipes  33  is connected to the throttle body  32  and the other end of each of the two intake pipes  33  is connected to each of the two cylinders  21 . Thus, the two intake pipes  33  respectively connected to the cylinders  21  can introduce an air-fuel mixture under the condition that fuel is injected and evenly distributed in the air in the throttle body  32 . This prevents fluctuation in the air-fuel ratio of the air-fuel mixture introduced to each intake pipe  33  between the two intake pipes  33 . Thus, an air-fuel mixture in the same air-fuel ratio can be supplied to each of the cylinders  21 . Therefore, deterioration in combustion efficiency of the engine  20  can be prevented. 
     In this preferred embodiment, as described above, a single injector  47  is provided for and connected to the two intake pipes  33 . Thus, the single injector  47  can supply an air-fuel mixture in an even air-fuel ratio to each of the cylinders  21  in the two-cylinder engine  20 . That is, there is no need to provide a plurality of injectors. Accordingly, there is no need to provide a delivery pipe for distributing fuel to the plurality of injectors, thereby decreasing the number of components and reducing weight of the outboard motor  1 . 
     In this preferred embodiment, as described above, the injector  47  is preferably disposed in a downstream vicinity in the airflow direction relative to the throttle valve  32   b  of the throttle body  32 . Therefore, fuel can be injected into a portion where air flows fastest in the throttle body  32 . This further facilitates fuel atomization and facilitates even distribution of fuel in the air. 
     In this preferred embodiment, as described above, the injector  47  is configured to inject fuel toward the throttle valve  32   b . Therefore, the fuel that is injected and is not taken into the air does not hit an inner peripheral surface of the throttle body  32  but hits the throttle valve  32   b . This prevents the injected fuel from adhering to the inner peripheral surface of the throttle body  32 . 
     In this preferred embodiment, as described above, fuel is injected toward the butterfly-type throttle valve  32   b . Accordingly, since there is a flow of air between the throttle valve  32   b  and the inner peripheral surface of the throttle body  32 , even if fuel adheres on the throttle valve  32   b , the adherent fuel can be taken into the flow of air when the adherent fuel moves to an end (an end on the side of the inner peripheral surface of the throttle body  32 ) of the throttle valve  32   b . Thus, differing from the case where fuel is injected toward the inner peripheral surface of the throttle body  32 , a portion of the injected fuel can be prevented from adhering to the inner peripheral surface of the throttle body  32  without being taken into the air. 
     In this preferred embodiment, as described above, the injection nozzle  47   a  of the injector  47  is preferably disposed in the vicinity of an air exit of the bypass air passage  32   c  positioned downstream of the throttle valve  32   b . Therefore, fuel can be injected into a portion where air flows relatively fast in the vicinity of an air exit of the bypass air passage  32   c . This further facilitates fuel atomization and facilitates even fuel distribution in the air. 
     In this preferred embodiment, as described above, the vapor separator tank  45  is disposed adjacent to the throttle body  32  whose temperature is decreased by fuel vaporization when fuel is injected thereto. Therefore, an increase in the temperature in the vapor separator tank  45  can be prevented by arranging the vapor separator tank  45  adjacent to the low-temperature throttle body  32 . This easily minimizes and prevents generation of vapor (vaporized fuel) in the vapor separator tank  45 . 
     In this preferred embodiment, as described above, the injector  47  is configured to obliquely inject fuel relative to the vertical direction on a plane that is perpendicular or substantially perpendicular to a direction in which the air passage  32   a  of the throttle body  32  extends. Therefore, the height of the top of the injector  47  can be lowered compared with the case where the injector  47  is configured to vertically inject fuel from just above the throttle body  32 . This makes it possible to downsize a unit made up of the throttle body  32  and the injector  47 . 
     It should be understood that the preferred embodiment described above is illustrative in all respects and not restrictive. The scope of the present invention is intended to be defined not by the above description of the above preferred embodiment but by the claims, and to include all equivalents and modifications of the claims. 
     For example, in the above preferred embodiment, the injector  47  is preferably disposed in the throttle body  32 . However, the present invention is not limited thereto. The injector  47  may be disposed in the intake pipe  33 . In this case, the number of the injectors is required to be the same as the number of the cylinder in the case of multi-cylinder engines. 
     In the above preferred embodiment, fuel is preferably injected toward upstream in a downstream side of the throttle valve  32   b . However, the present invention is not limited thereto. Fuel may be injected to a direction opposite to an airflow direction in an upstream side of the throttle valve  32   b.    
     In the above preferred embodiment, as shown in  FIG. 8 , the injector  47  is configured to inject fuel obliquely downward into the air passage  32   a  from above on a plane that is perpendicular or substantially perpendicular to a direction in which the air passage  32   a  extends. However, the present invention is not limited thereto. The injector  47  may be configured to inject fuel upward, laterally (horizontally), or downward into the air passage  32   a  on the plane orthogonal to a direction in which the air passage  32   a  extends. 
     In the above preferred embodiment, the butterfly-type throttle valve  32   b  is preferably used. However, the present invention is not limited thereto. A slide-type throttle valve may be used, for example. 
     In the above preferred embodiment, the direction of fuel injection is tilted at an angle of α (about 20 degrees to about 60 degrees, for example) relative to the airflow direction. However, the present invention is not limited thereto. The tilted angle α shown in  FIG. 12  may be within the range between 0 to 90 degrees, for example. 
     In the above preferred embodiment, the high-pressure fuel pump  46  transports fuel preferably by driving the plunger  463  with the swash plate  462 . However, the present invention is not limited thereto. Other types of high-pressure fuel pump such as a vane-type pump, a screw-type pump or a trochoid-type pump may be used. 
     In the above preferred embodiment, fuel is preferably gasoline. However, the present invention is not limited thereto. Fuel may be alcohol. 
     In the above preferred embodiment, the fuel supply system for a boat of the present invention is preferably applied to the outboard motor  1 . However, the present invention is not limited thereto. The fuel supply system for a boat of the present invention may be applied to an inboard motor in which an engine section is mounted on a hull or to an inboard/outboard motor. 
     In the above preferred embodiment, the present invention is preferably applied to the outboard motor  1  utilizing the two-cylinder engine section  2  having the two cylinders  21 . However, the present invention is not limited thereto. The present invention may be applied to an outboard motor utilizing an engine section having one cylinder or three or more cylinders. For example, a three-cylinder engine section  2   a  according to a variation shown in  FIGS. 13 and 14  includes three cylinders  21   a  each having a piston  22   a  and a connecting rod  23   a . The engine section  2   a  is connected to the throttle body  32  and includes three intake pipes  33   a  respectively connected to an intake port (not shown) of each of the three cylinders  21   a . The construction other than the above mentioned is preferably similar to that of the engine section  2  in the outboard motor  1 . 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.