Patent Publication Number: US-8118011-B2

Title: Marine vessel propulsion device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a marine vessel propulsion device including a vapor pathway connecting a vapor separator tank to an intake system, and a valve disposed in the vapor pathway. 
     2. Description of Related Art 
     An outboard motor is an example of a marine vessel propulsion device. An outboard motor of one prior art is disclosed in U.S. Patent Application Publication No. US2005/0016504 A1. The outboard motor includes a vapor pathway connecting a vapor separator tank and an intake system, a valve interposed in the vapor pathway, and an engine controller which controls the valve. Fuel is supplied from a fuel tank to the vapor separator tank by a low pressure fuel pump. The fuel in the vapor separator tank is supplied to a fuel injection device (fuel injector) by a high pressure fuel pump. 
     The engine controller closes the valve while the engine is stopped, and controls the valve to gradually open the valve when starting the engine. Therefore, during operation of the engine, the valve is kept open. When the engine operates, the temperature of the fuel in the vapor separator tank is raised by radiation heat from the engine, and generates fuel vapor. The vapor is allowed to escape to the intake system through the vapor pathway, so that the vapor can be burned. When the engine is stopped, the engine controller closes the valve. Accordingly, when the engine is stopped, the vapor can be prevented from being allowed to escape to the outside. 
     SUMMARY OF THE INVENTION 
     The inventors of the present invention described and claimed in the present application conducted an extensive study and research regarding a marine vessel propulsion device, such as the one described above, and in doing so, discovered and first recognized new unique challenges and problems as described in greater detail below. 
     More specifically, as a result of studying marine vessel propulsion devices including the one described above, the inventors of the present invention described and claimed in the present application discovered that, in such a marine vessel propulsion device, immediately after the engine is stopped, the temperature of the engine is high, so that vapor is generated inside the vapor separator tank due to radiation heat from the engine. Therefore, the inner pressure of the vapor separator tank increases. When the engine is started in this state, if the valve is opened too fast, the vapor inside the vapor separator tank is taken all at once into the engine via the vapor pathway and the intake system. As a result, air/fuel mixture to be supplied to the engine becomes fuel-rich, so that the engine may stall. 
     When the valve is opened too fast, the pressure inside the vapor separator tank suddenly lowers, so that the fuel in the vapor separator tank bubbles due to vacuum boiling. In this case, the high pressure fuel pump which transports the fuel to the fuel injection device suctions bubbles of the fuel and does not normally operate. This also causes the engine to stall. 
     On the other hand, when the valve is opened too slowly, the pressure inside the vapor separator tank does not lower, so that it becomes difficult for the low pressure fuel pump to transport new fuel to the vapor separator tank. If this state is continued for an extended period of time, during the operation of the engine, the fuel in the vapor separator tank runs short, and the engine stalls. 
     In order to overcome the previously unrecognized and unsolved problems described above, a preferred embodiment of the present invention provides a marine vessel propulsion device including an engine, a fuel injection device arranged to inject fuel to the engine, an intake system which includes an air passage of air to be supplied to the engine, a vapor separator tank arranged to separate fuel vapor from liquid fuel to be supplied to the engine, a pump unit arranged to transport the fuel from the vapor separator tank to the fuel injection device, a fuel pipe arranged to connect the fuel injection device and the pump section, a vapor pathway arranged to connect the vapor separator tank and the intake system, a valve disposed in the vapor pathway, and an engine control unit arranged to control an opening degree of the valve. The engine control unit maybe arranged to control the opening degree of the valve based on a valve opening speed set based on at least the pressure of fuel inside the fuel pipe when starting the engine. 
     “When starting engine” is a wide-ranging concept including not only the time to start the engine and the time immediately after starting the engine, but also a period until a normal operation state is reached after the start of the engine. 
     By controlling the opening degree of the valve based on the valve opening speed set based on at least the pressure of the fuel inside the fuel pipe, the valve can be opened so as not to lower the pressure of the fuel inside the fuel pipe. Accordingly, the engine can be prevented from stalling due to lowering of the pressure of the fuel inside the fuel pipe. 
     When the pump unit suctions the fuel bubbled by vacuum boiling inside the vapor separator tank (that is, bubble clogging) and does not normally operate, the pressure of the fuel inside the fuel pipe lowers. Therefore, based on the pressure of the fuel inside the fuel pipe, the valve opening speed is set. Accordingly, the inner pressure lowering in the vapor separator tank can be prevented, so that vacuum boiling can be prevented. As a result, the engine can be prevented from stalling due to bubble clogging. 
     If the fuel in the vapor separator tank runs short (hereinafter, referred to as “fuel shortage”), the pressure of the fuel inside the fuel pipe also lowers. Therefore, by setting the opening degree of the valve based on the pressure of the fuel inside the fuel pipe, the engine can be prevented from stalling due to fuel shortage. 
     In a preferred embodiment of the present invention, the valve opening speed is set based on, in addition to the pressure of the fuel inside the fuel pipe, the air/fuel ratio of the air/fuel mixture to be supplied to the engine. With this arrangement, the opening degree of the valve is properly controlled based on the air/fuel ratio in addition to the pressure of the fuel inside the fuel pipe. Accordingly, the air/fuel mixture to be supplied to the engine can be prevented from becoming excessively fuel-rich. Accordingly, the engine can also be prevented from stalling due to an excessively fuel-rich state. 
     In a preferred embodiment of the present invention, the valve opening speed is set based on, in addition to the pressure of the fuel inside the fuel pipe, a remaining amount of the fuel in the vapor separator tank. With this arrangement, the valve opening degree is controlled based on, in addition to the pressure of the fuel inside the fuel pipe, the remaining fuel amount. In the case where the pressure of the fuel inside the fuel pipe is low, when the remaining amount of the fuel in the vapor separator tank is large, it can be determined that lowering of the pressure of the fuel inside the pipe is caused by bubble clogging. In the case where the pressure of the fuel inside the pipe is low, when the remaining amount of the fuel in the vapor separator tank is small, it can be determined that lowering of the pressure of the fuel inside the fuel pipe is caused by fuel shortage. By thus identifying the cause of lowering of the pressure of the fuel inside the fuel pipe, the valve opening speed can be more properly set. Accordingly, the engine can be further prevented from stalling. 
     In a preferred embodiment of the present invention, the engine control unit has a storage section which stores a preset valve opening speed. When starting the engine, the engine control unit controls the opening degree of the valve based on the set value stored in the storage section. For example, a set value of the valve opening speed which at least does not cause lowering of the pressure of the fuel inside the pipe is determined in advance through an experiment and stored in the storage section. Accordingly, when starting the engine, the opening degree of the valve is controlled based on the set value, so that without providing sensors, the opening degree of the valve can be easily controlled so as not to cause the engine to stall. 
     In this case, preferably, when starting the engine, the engine control unit may open the valve at a fixed speed based on the valve opening speed (set value) stored in the storage section. With this arrangement, the opening degree of the valve can be more easily controlled. 
     Another preferred embodiment of the present invention provides a marine vessel propulsion device that includes an engine, a fuel injection device arranged to inject fuel to the engine, an intake system including an air passage of air to be supplied to the engine, a vapor separator tank arranged to separate fuel vapor to be supplied to the engine from liquid fuel, a pump unit arranged to transport fuel from the vapor separator tank to the fuel injection device, a fuel pipe arranged to connect the fuel injection device and the pump unit, a fuel pressure sensor arranged to detect the pressure of fuel inside the fuel pipe, a vapor pathway arranged to connect the vapor separator tank and the intake system, a valve disposed in the vapor pathway, and an engine control unit arranged to control the opening degree of the valve. In this case, when starting the engine, the engine control unit may preferably control the opening degree of the valve based on at least a detected value of the fuel pressure sensor. With this arrangement, the opening degree of the valve can be controlled in real time so as not to cause lowering of the pressure of fuel inside the fuel pipe. 
     For example, an engine control unit may be arranged to correct a prescribed value of the valve opening degree to be applied during normal operation based on at least a detected value of the fuel pressure sensor. 
     A marine vessel propulsion device of a preferred embodiment further includes, in addition to the fuel pressure sensor, an air/fuel ratio sensor arranged to detect an air/fuel ratio of an air/fuel mixture to be supplied to the engine. In this case, when starting the engine, preferably, the engine control unit may control the opening degree of the valve based on, in addition to the detected value of the fuel pressure sensor, a detected value of the air/fuel ratio sensor. With this arrangement, the opening degree of the valve can be controlled in real time so as not to cause the air/fuel mixture to become excessively fuel-rich due to escape of vapor in the vapor separator tank to the air intake system. 
     A marine vessel propulsion device according to a preferred embodiment of the present invention further includes, in addition to the fuel pressure sensor, a remaining amount sensor arranged to detect a remaining amount of fuel in the vapor separator tank. In this case, when starting the engine, preferably, the engine control unit may control the opening degree of the valve also based on a detected value of the remaining amount sensor in addition to the detected value of the fuel pressure sensor. With this arrangement, the valve opening degree is controlled based on the fuel remaining amount in addition to the pressure of the fuel inside the fuel pipe. In the case where the pressure of fuel inside the fuel pipe is low, when the remaining amount of fuel in the vapor separator tank is large, it can be determined that lowering of the pressure of fuel inside the fuel pipe is caused by bubble clogging. In the case where the pressure of fuel inside the pipe is low, when the remaining amount of the fuel in the vapor separator tank is small, it can be determined that lowering of the pressure of the fuel inside the fuel pipe is caused by fuel shortage. By thus identifying the cause of lowering of the pressure of the fuel inside the fuel pipe, the valve opening degree can be more properly set. Accordingly, the engine can be more effectively prevented from stalling. 
     A marine vessel propulsion device according to a preferred embodiment of the present invention further includes, in addition to the fuel pressure sensor, a fuel temperature sensor arranged to detect the temperature of the fuel in the vapor separator tank. When starting the engine, preferably, the engine control unit may set the opening degree of the valve to a prescribed value in the case where the temperature of the fuel detected by the fuel temperature sensor is lower than a predetermined temperature, and in the case where the temperature of the fuel detected by the fuel temperature sensor is not lower than the predetermined temperature, the engine control unit may control the opening degree of the valve based on at least the detected value of the fuel pressure sensor. When the temperature of the fuel in the vapor separator tank is high, a lot of vapor is generated. Therefore, when the temperature of the fuel in the vapor separator tank is high, by controlling the opening degree of the valve according to the pressure of the fuel inside the fuel pipe, engine stall caused by the vapor can be prevented. 
     In the arrangement including the fuel pressure sensor, preferably, when starting the engine, the engine control unit may acquire a detected value of the fuel pressure sensor every predetermined time period, and control the opening degree of the valve based on at least the detected value of the fuel pressure sensor. With this arrangement, the opening degree of the valve can be set in real time suitably for the state of the marine vessel propulsion device. 
     In this case, preferably, when starting the engine, the engine control unit may change the opening speed of the valve by increasing, reducing, or keeping the opening degree of the valve every predetermined time period. With this arrangement, the opening speed of the valve can be properly controlled in real time so as not to cause the engine to stall. 
     Other elements, features, steps, characteristics, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing an entire arrangement of an outboard motor according to a first preferred embodiment of the present invention. 
         FIG. 2  is a perspective view showing an engine section of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 3  is a plan view showing the engine section of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 4  is a side view showing the engine section of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 5  is a system view of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 6  is a block diagram showing an electric arrangement of major portions of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 7  is a map used for controlling the valve opening degree during normal operation of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 8  is a graph for describing the valve opening degree and a state of the outboard motor when starting the engine of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 9  is a plan view showing an outboard motor to be used in an experiment for determining a set value of a valve opening speed of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 10  is a system view showing an outboard motor to be used in an experiment for determining a set value of the valve opening speed of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 11  is a flowchart for describing steps of determining a set value of the valve opening speed of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 12  is a graph showing a state of the outboard motor when the opening speed of the valve is too fast in the steps of determining the set value of the valve opening speed of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 13  is a graph showing a state of the outboard motor when the opening speed of the valve is too slow in the steps of determining the set value of the valve opening speed of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 14  is a graph showing a state of the outboard motor when the opening speed of the valve is proper in the steps of determining the set value of the valve opening speed of the outboard motor of the first preferred embodiment of the present invention. 
         FIG. 15  is a flowchart for describing control of the valve opening degree of an outboard motor of a second preferred embodiment of the present invention. 
         FIG. 16  is a graph for describing changes in valve opening degree when starting the engine of the outboard motor of the second preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
       FIG. 1  is a side view showing an entire arrangement of an outboard motor of a first preferred embodiment of the present invention. The outboard motor  1  of the present preferred embodiment is mounted on a transom  101  of a hull  100  via a clamp bracket  10  so as to be steered and tilted. Therefore, the outboard motor  1  assumes various postures with respect to the hull  100  in actual use; however, in the present specification, for the sake of convenience, up-down, left-right, and front-rear directions are determined based on a predetermined reference posture of the outboard motor  1 . The reference posture is a posture of the outboard motor  1  at a steering angle of zero and a tilt angle of zero with respect to the hull  100  in a horizontal posture. In this state, when a forward propulsive force is generated from the outboard motor  1 , the hull  100  moves straight ahead. In other words, in the present specification, as expressions showing the directions of the outboard motor  1  and the members, the heading direction of the hull  100  when it moves ahead, in other words, when it moves straight ahead will be referred to as “front,” and the direction  180  degrees opposite to the front will be referred to as “rear.” The left side of the heading direction when the hull  100  moves ahead will be referred to as “left,” and the right side of the heading direction when the hull  100  moves ahead will be referred to as “right.” 
     The outboard motor  1  includes an engine section  2 , a drive shaft  3 , a forward-reverse switching mechanism  4 , a propeller shaft  5 , and a propeller  6 . The drive shaft  3  is arranged to extend in the vertical direction (Z direction), and is rotated by a driving force of the engine section  2 . The forward-reverse switching mechanism  4  is coupled to the lower end of the drive shaft  3 . The propeller shaft  5  extends in the horizontal direction, and is coupled to the forward-reverse switching mechanism  4 . The propeller  6  is attached to the rear end of the propeller shaft  5 . 
     The engine section  2  is housed inside an engine cover  7 . Inside an upper case  8  and a lower case  9  which are arranged below the engine cover  7 , the drive shaft  3 , the forward-reverse switching mechanism  4 , and the propeller shaft  5  are housed. 
     The outboard motor  1  is attached to the transom  101  provided on the reverse (arrow A direction) side of the hull  100  via a clamp bracket  10 . The clamp bracket  10  supports the outboard motor  1  such that the outboard motor  1  can swing up and down around the tilt shaft  10   a  with respect to the hull  100 . In the hull  100 , a fuel tank  102  for storing fuel (gasoline) is provided. 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 by using fuel supplied from the fuel tank  102 . 
     The drive shaft  3  is rotated by a driving force of the engine section  2 . The rotation of the drive shaft  3  is transmitted to the propeller shaft  5  via the forward-reverse switching mechanism  4 . Accordingly, the propeller  6  is rotated. The forward-reverse switching mechanism  4  can switch the rotation direction of the propeller shaft  5 . Accordingly, the rotation direction of the propeller  6  is switched. As a result, the hull  100  is propelled in the forward drive direction (arrow B direction) or in the reverse drive direction (arrow A direction) On the side portion on the reverse drive direction side (arrow A direction) of the engine cover  7 , a vent hole  7   a  is provided. Air taken into the inside of the engine cover  7  via the vent hole  7   a  is supplied to the engine section  2 . 
       FIG. 2  is a perspective view of the engine section  2 ,  FIG. 3  is a plan view of the same, and  FIG. 4  is a left side view of the same. Further,  FIG. 5  shows a system arrangement of the outboard motor  1 , and  FIG. 6  shows an electric arrangement of major portions. 
     The engine section  2  includes an engine main body  20  (internal combustion engine), an intake system  30 , a fuel system  40 , and an ECU (Engine Control Unit)  50  (see  FIG. 5  and  FIG. 6 ). The intake system  30  supplies air to the engine main body  20 . The fuel system  40  supplies fuel to the engine main body  20 . For example, the engine main body  20  may be a four-stroke cycle engine with gasoline used as fuel. The engine main body  20  and the ECU  50  are examples of “engine” and “engine control unit” according to a preferred embodiment of the present invention, respectively. 
     As shown in  FIG. 3 , the engine main body  20  preferably includes three cylinders  21 , for example, arranged in the up-down direction (Z direction of  FIG. 2 ), and pistons  22  which move to reciprocate horizontally inside the cylinders  21 . The pistons  22  are coupled to the crankshaft  24  via connecting rods  23 . The crankshaft  24  extends in the up-down direction (Z direction). The horizontal reciprocation of the piston  22  is converted into rotating motion by the connecting rod  23  and the crankshaft  24 . The lower end of the crankshaft  24  is connected to the drive shaft  3  (see  FIG. 1 ). 
     The rotation of the crankshaft  24  is transmitted to a cam shaft  26 . In detail, around a pulley (not shown) fixed to the upper portion of the crankshaft  24  and a pulley  27  (see  FIG. 2 ) fixed to the cam shaft  26  (see  FIG. 3 ), a belt  25  (see  FIG. 2 ) is wound. Accordingly, the rotation of the crankshaft  24  is transmitted to the cam shaft  26 . By the rotation of the cam shaft  26 , intake valves  28   a  and exhaust valves  28   b  (see  FIG. 3 ) of the respective cylinders  21  are driven at predetermined timings. Exhaust gas exhausted from the exhaust valve  28   b  is released to the outside via an exhaust passage  29 . 
     As shown in  FIG. 5 , a crank angle sensor  24   a  is attached to the vicinity of the crankshaft  24 . The ECU  50  can calculate the engine speed based on an output value of the crank angle sensor  24   a.    
     As shown in  FIG. 2  to  FIG. 4 , the intake system  30  is arranged lateral to the engine main body  20 . In the present preferred embodiment, the intake system  30  is arranged along a side portion on the right side of the engine main body  20 . The intake system  30  includes a silencer case  31 , a throttle body  32 , a surge tank  33 , and preferably three intake pipes  34 , for example. The silencer case  31  has an intake port  31   a  arranged on the forward drive direction (arrow B direction) side (see  FIG. 3 ). To the silencer case  31 , the throttle body  32  is connected. Further, to the throttle body  32 , the surge tank  33  is connected. Three intake pipes  34  extend from the surge tank  33 , and are respectively connected to intake ports of the three cylinders  21  of the engine main body  20 . The throttle body  32  is coupled to the surge tank  33  with screws  150 , and further coupled to the silencer case  31  with other screws  150 . 
     The throttle body  32  may be made of resin or metal, and has an air passage  32   a  having an inner surface formed into a cylindrical shape as shown in  FIG. 3  to  FIG. 5 . In the air passage  32   a , a butterfly throttle valve  32   b  is provided. As shown in  FIG. 5 , a bypass air passage  32   c  is preferably integrally provided with the throttle body  32 . The bypass passage  32   c  connects the upstream side and downstream side with respect to the throttle valve  32   b  of the air passage  32   a . The bypass air passage  32   a  secures an air flow rate for maintaining an idling state when the throttle valve  32   b  is fully closed. 
     As shown in  FIG. 5 , to the throttle body  32 , three sensors (a throttle opening degree sensor  35 , an intake air pressure sensor  36 , and an intake air temperature sensor  37 ) arranged to control the fuel injection amount of the injector  45  are attached. Further, to the throttle body  32 , an idle speed control unit  38  (hereinafter, referred to as “ISC unit  38 ”) arranged to adjust the air flow rate while idling is attached. The ISC unit  38  is arranged in the middle of the bypass air passage  32   c . The ISC unit  38  arranged to control the flow rate of air passing through the bypass air passage  32   c  to control the engine speed while idling. 
     As shown in  FIG. 2  to  FIG. 5 , the fuel system  40  includes a filter  41  connected to a fuel tank  102  (see  FIG. 1  and  FIG. 5 ) arranged in the hull  100 , a low pressure fuel pump  42  connected to the filter  41 , and a vapor separator tank  43  connected to the low pressure fuel pump  42 . The fuel system  40  further includes a high pressure fuel pump  44  (see  FIG. 5 ) which transports fuel in the vapor separator tank  43 , and an injector  45  which injects the fuel transported by the high pressure fuel pump  44 . The high pressure fuel pump  44  and the injector  45  are connected via a pipe  44   a  and a delivery pipe  44   b  (see  FIG. 4 ). 
     The pipe  44   a  and the delivery pipe  44   b  are an example of “fuel pipe” according to a preferred embodiment of the present invention. Also the high pressure fuel pump  44  and the injector  45  are examples of “pump unit” and “fuel injection device” according to a preferred embodiment of the present invention, respectively. 
     The low pressure fuel pump  42  has a function to transport fuel into the vapor separator tank  43  from the fuel tank  102 . The low pressure fuel pump  42  preferably is a mechanical driving pump which is driven in conjunction with the rotation of the crankshaft  24 . When the fuel suctioned from the fuel tank  102  of the hull  100  by the low pressure fuel pump  42  passes through the filter  41 , foreign matter contained in the fuel is removed. 
     The fuel fed by the low pressure fuel pump  42  is stored in the vapor separator tank  43 . As shown in  FIG. 3  and  FIG. 4 , the vapor separator tank  43  is arranged between the engine main body  20 , surge tank  33 , and the air intake pipe  34  as viewed in plan. 
     The vapor separator tank  43  stores the fuel suctioned up from the fuel tank  102 , and separates fuel vapor or air and liquid fuel from each other. As shown in  FIG. 5 , the vapor separator tank  43  is configured such that the amount of the fuel stored in the vapor separator tank  43  is kept constant and the liquid level position of the fuel inside the vapor separator tank  43  is kept at a predetermined height position. In detail, a float  43   a  having a needle valve  43   b  is provided inside the vapor separator tank  43 . When the liquid level position of the fuel in the vapor separator tank  43  becomes not lower than the predetermined height, the flow of the fuel into the vapor separator tank  43  is automatically stopped by the needle valve  43   b  of the float  43   a . In addition, when the liquid level position of the fuel in the vapor separator tank  43  becomes lower than the predetermined height, the flow of the fuel into the vapor separator tank  43  is automatically started. With this arrangement, the amount of fuel stored in the vapor separator tank  43  is kept constant, and the liquid level position of the fuel in the vapor separator tank  43  is kept at the predetermined height. 
     The high pressure fuel pump  44  is arranged inside the vapor separator tank  43 , and has a function to transport fuel with a predetermined pressure to the injector  45 . The injector  45  has a function to inject the fuel fed at a predetermined pressure by the high pressure fuel pump  44  to the vicinity of the intake port of the cylinder  21  (see  FIG. 3 ) at a predetermined timing. Also, a portion of fuel transported to the injector  45  from the high pressure fuel pump  44  is returned into the vapor separator tank  43  via a cooling device (not shown) which cools the fuel. 
     In addition, as shown in  FIG. 2 ,  FIG. 4 , and  FIG. 5 , the upper portion of the vapor separator tank  43  is connected to the throttle body  32  via a pipe  46   a  and a pipe  46   b . Accordingly, vapor in the vapor separator tank  43  is allowed to escape to the air passage  32   a  of the throttle body  32 . Between the pipe  46   a  and the pipe  46   b , a vapor shut valve  47  (hereinafter, referred to as “VSV  47 ”) is provided. By controlling the VSV  47 , the timing of allowing vapor to escape can be controlled. 
     The pipe  46   a  and the pipe  46   b  are an example of “vapor pathway” according to a preferred embodiment of the present invention. In the first preferred embodiment, the opening degree of the VSV  47  preferably is finely controllable by a stepping motor  47 M. 
     As shown in  FIG. 5  and  FIG. 6 , the ECU  50  electrically controls the high pressure fuel pump  44 , the injector  45 , the VSV  47  (stepping motor  47 M) and the ISC unit  38 . The fuel injection amount of the injector  45  is controlled based on the results of detections by the throttle opening degree sensor  35 , the intake air temperature sensor  37 , and the intake air pressure sensor  36  attached to the throttle body  32 . 
     The ECU  50  closes the VSV  47  when stopping the engine section  2 . In the first preferred embodiment, when starting the engine section  2 , the ECU  50  controls the VSV  47  (stepping motor  47 M) so as to gradually open the VSV  47  at a fixed speed. 
     In detail, the ECU  50  has a storage section  51  (see FIG.  6 ). In the storage section  51 , a set value of the speed of opening the VSV  47  (valve opening speed) when starting the engine section  2  is stored. The valve opening speed is set through an experiment in advance such that the engine speed does not lower when opening the VSV  47  with the valve opening speed. 
     In the storage section  51 , a map for controlling the opening degree of the VSV  47  during normal operation (except for the time required to start the engine section  2 ) is also stored. An example of the map is shown in  FIG. 7 . In the normal operation, the ECU  50  reads out, from the map of  FIG. 7 , an opening degree set value of the VSV  47  based on a detected value detected by the intake air pressure sensor  36 , and an engine speed. 
     “When starting the engine section  2 ” is a wide-ranging concept including not only the time to start the engine section  2  and the time immediately after starting the engine section  2 , but also a period until reaching a normal operation state after the start of the engine section  2 . In detail, “when starting the engine section  2 ” means a period until the fuel at a high temperature in the vapor separator tank  43  is replaced by fuel at a low temperature supplied from the fuel tank  102 . Therefore, “normal operation state” means a state that the fuel at a high temperature in the vapor separator tank  43  has been discharged and replaced by the fuel at a low temperature supplied from the fuel tank  102 . 
     The fuel at a high temperature is, for example, fuel at about 60° C. to about 65° C. The fuel at a low temperature is, for example, fuel at approximately 35° C. However, the fuel at a low temperature means fuel in the fuel tank  102 , and the temperature thereof changes depending on the temperature inside the fuel tank  102 . 
     The temperature of fuel in the vapor separator tank  43  can be detected by the fuel temperature sensor. Therefore, by providing a fuel temperature sensor, the period in which “control when starting the engine section  2 ” is performed can be determined based on an output of the fuel temperature sensor. Alternatively, a time necessary for replacing the fuel in the vapor separator tank  43  may be determined in advance. In the period until a predetermined time longer than such necessary time elapses immediately after the engine starts, the “control when starting the engine section  2 ” may be performed. 
     Next, referring to  FIG. 8 , control of the VSV  47  when starting the engine of the outboard motor  1  of the first preferred embodiment will be described.  FIG. 8  shows a situation when restarting the engine section  2 . “When restarting” means a time at which the engine section  2  after being driven is stopped and then restarted while the temperature of the engine section  2  is high.  FIG. 8  shows time changes of the opening degree of the VSV  47 , the engine speed, the discharge pressure of the low pressure fuel pump  42 , the temperature inside the vapor separator tank  43 , and the inner pressure of the vapor separator tank  43  when throttle is fully open. The reason for full throttle is that a case is assumed such that the marine vessel is driven immediately after the engine section  2  is started. 
     As shown in  FIG. 8 , when the engine section  2  stops, a fuel cooling device (not shown) does not operate, and the vapor separator tank  43  is subjected to radiation heat from the engine main body  20  with high temperature. Therefore, the temperature inside the vapor separator tank  43  is high. Along with this, the fuel in the vapor separator tank  43  vaporizes, and vapor (fuel vapor) is generated in the vapor separator tank  43 . While the engine section  2  is stopped, the VSV  47  is closed, so that the pressure inside the vapor separator tank  43  is increased by the generated vapor. 
     When the engine section  2  is started and the throttle is fully open, the engine speed rises to a predetermined speed, and is then kept at the speed. The discharge pressure of the low pressure fuel pump  42  which is driven in conjunction with the engine section  2  increases with an increase in engine speed, and is kept at a fixed discharge pressure thereafter. Immediately after starting, the inner pressure of the vapor separator tank  43  is higher than the discharge pressure of the low pressure fuel pump  42 , so that new fuel is not supplied to the vapor separator tank  43 . 
     In the first preferred embodiment, after the engine section  2  is started, the VSV  47  is opened at a fixed speed based on the valve opening speed (set value) stored in the storage section  51 . As the VSV  47  opens, the vapor inside the vapor separator tank  43  is allowed to escape to the intake system  30  via the pipes  46   a  and  46   b . Accordingly, the inner pressure of the vapor separator tank  43  gradually lowers. After time T 1 , the discharge pressure of the low pressure fuel pump  42  becomes higher than the inner pressure of the vapor separator tank  43 , so that new fuel is supplied into the vapor separator tank  43 . After starting the engine section  2 , a fuel cooling device is driven, so that the temperature of the fuel in the vapor separator tank  43  lowers, and generation of vapor inside the vapor separator tank  43  is reduced. Thereafter, the operation shifts to normal operation. 
     In the first preferred embodiment, the speed of opening the VSV  47  is set to a suitable value, so that when starting the engine section  2 , lowering of the engine speed and an occurrence of stall of the engine section  2  are prevented. 
     Next, with reference to  FIG. 9  to  FIG. 14 , a method for setting the speed of opening the VSV  47  (valve opening speed) of the outboard motor  1  will be described. 
     For determining the opening speed of the VSV  47  (valve opening speed) which should be stored in the storage section  51  (see  FIG. 6 ), an experiment is performed by using an experimental outboard motor  1   a  having the same arrangement as that of the outboard motor  1 . As shown in  FIG. 9  and  FIG. 10 , to the experimental outboard motor  1   a , a remaining amount sensor  61 , a fuel temperature sensor  62 , a fuel pressure sensor  63 , an A/F sensor  64  (see  FIG. 9 ), and a VST inner pressure sensor  65  are attached. The remaining amount sensor  61  detects the remaining amount of fuel in the vapor separator tank  43 . The fuel temperature sensor  62  detects the temperature of the fuel in the vapor separator tank  43 . The fuel pressure sensor  63  detects the pressure of the fuel (fuel pressure) to be supplied to the injector  45  from the high pressure fuel pump  44 . The A/F sensor  64  detects the air/fuel ratio of an air/fuel mixture supplied to the cylinder  21  of the engine main body  20 . The VST inner pressure sensor  65  detects the inner pressure of the vapor separator tank  43 . All output signals of these sensors are input into the ECU  50 , for example. 
     The A/F sensor  64  is an example of the “air/fuel ratio sensor” according to a preferred embodiment of the present invention. As the remaining amount sensor  61 , a sensor which detects motion of the float  43  by using electric resistance changes, a sensor which detects the position of the float  43   a  by using magnetism, or a sensor which detects the liquid level position by using ultrasonic waves can be used by way of example. 
       FIG. 11  is a flowchart showing detailed steps of determining the valve opening degree. An experiment is performed by using the experimental outboard motor la as follows. That is, the outboard motor la is operated, and the engine section  2  is stopped in a state that the temperature of the engine section  2  is high. Then, the engine section  2  is restarted, and the ECU  50  performs control to open the VSV  47  at a temporarily set value of opening speed (Step S 1 ). Then, changes of output values of the sensors (the remaining amount sensor  61 , the fuel temperature sensor  62 , the fuel pressure sensor  63 , the A/F sensor  64 , and the VST inner pressure sensor  65 ) with the lapse of time are recorded. 
     In detail, a computer with a predetermined tool program installed is connected to the ECU  50 . The computer can acquire and record the output signals of the sensors via the ECU  50 . An operator of the experiment can set the opening speed of the VSV  47  (specifically, the pulse interval to be applied to the stepping motor  47 M) to the ECU  50  by operating the computer. 
     The operator determines whether the fuel pressure or A/F is abnormal by referring to output values of the sensors recorded as described above. In detail, first, based on the output of the fuel temperature sensor  62 , it is determined whether the temperature of fuel in the vapor separator tank  43  when starting the engine section  2  is equal to or higher than a predetermined temperature (Step S 2 ). Here, the predetermined temperature is, for example, approximately 45° C. which causes the fuel to boil and vaporize. When the temperature of the fuel in the vapor separator tank  43  is not lower than the predetermined temperature, vapor is generated in the vapor separator tank  43  and may cause the engine section  2  to stall. On the other hand, when the temperature of the fuel in the vapor separator tank  43  is lower than the predetermined temperature (when the engine section  2  is cooled), very little vapor is generated in the vapor separator tank  43 , and the influence of the opening speed of the VSV  47  on the engine section  2  is small. Therefore, data when the engine section  2  is cooled is not used for setting the opening speed of the VSV  47  in the case of the engine section  2  at a high temperature, and the experiment is made again (Step S 2 : NO). 
     When the temperature of the fuel in the vapor separator tank  43  is equal to or higher than the predetermined temperature (Step S 2 : YES), at Step S 3 , the operator determines whether the fuel pressure has decreased on the data acquired by the fuel pressure sensor  63 . When the fuel pressure lowers, it becomes difficult for the injector  45  to inject a proper amount of fuel, and causes the engine section  2  to stall. Therefore, the process advances to Step S 4 , and the operator adjusts the opening speed of the VSV  47  (Steps S 5  and S 6 ). The fuel pressure lowering is judged based on not only the output of the fuel pressure sensor  63  but also the output of the VST inner pressure sensor  65  and the output of the fuel temperature sensor  62 . 
     At Step S 4 , the operator determines whether fuel remains in the vapor separator tank  43  based on an output value of the remaining amount sensor  61 . When fuel remains (Step S 4 : YES), it is determined that the high pressure fuel pump  44  has been clogged with bubbles. In other words, when the opening speed of the VSV  47  is too fast, vapor in the vapor separator tank  43  is rapidly allowed to escape to the intake system. Therefore, the pressure inside the vapor separator tank  43  suddenly lowers, so that the fuel bubbles due to vacuum boiling. If the high pressure fuel pump  44  suctions bubbles, it becomes difficult for the high pressure fuel pump  44  to transport the fuel normally, so that the fuel pressure lowers (Step S 3 : YES). 
       FIG. 12  is a graph of data actually acquired when the opening speed of the VSV  47  is too fast. In  FIG. 12 , the interval t 1  of pulses to be input into the stepping motor  47 M of the VSV  47  is preferably set to about 3 seconds, for example. In the graph of  FIG. 12 , lowering of the fuel pressure is observed at the portion indicated by the arrow P. In  FIG. 12 , lowering of the engine speed is not observed; however, when the fuel pressure greatly lowers, it can result in lowering of the engine speed and an occurrence of stall of the engine section  2 . 
     Therefore, when fuel remains at Step S 4  of  FIG. 11 , it is determined that the opening speed of the VSV  47  is too fast. Then, at Step S 5 , the operator sets the opening speed of the VSV  47  to be slower by increasing the interval of pulses to be input into the stepping motor  47 M. Thereafter, the process returns to Step S 1 , and the same experiment as described above is performed at the opening speed of the VSV  47  that has been made slower. 
     At Step S 4 , when it is determined that fuel does not remain in the vapor separator tank  43 , it is determined that a fuel shortage has occurred. In other words, when the opening speed of the VSV  47  is too slow, it takes time for the vapor to escape from the vapor separator tank  43 . Therefore, the time (T 1  of  FIG. 8 ) until the discharge pressure of the low pressure fuel pump  42  becomes higher than the inner pressure of the vapor separator tank  43  after the start of the engine section  2  becomes longer. Until the time T 1  elapses after the start of the engine section  2 , new fuel is not supplied into the vapor separator tank  43 . Therefore, when the fuel in the vapor separator tank  43  is consumed before the time T 1  elapses, a fuel shortage occurs. In the case of fuel shortage, it also becomes difficult for the high pressure fuel pump  44  to normally transport fuel, so that the fuel pressure lowers (Step S 3 : YES). 
       FIG. 13  is a graph of data actually acquired when the opening speed of the VSV  47  is too slow. In  FIG. 13 , the interval t 2  of pulses to be input into the stepping motor  47 M of the VSV  47  is set to about 7.5 seconds, for example. In the graph of  FIG. 13 , lowering of the fuel pressure and lowering of the engine speed are observed at the portion indicated by the arrow Q. 
     Therefore, when fuel does not remain at Step S 4 , it is determined that the opening speed of the VSV  47  is too slow. Then, at Step S 6 , the operator increases the opening speed of the VSV  47  by reducing the interval of pulses to be input into the stepping motor  47 M. Thereafter, the process returns to Step S 1 , and the same experiment as described above is made at the increased opening speed of the VSV  47 . 
     In addition, when lowering of the fuel pressure is not found at Step S 3 , the operator determines whether the air/fuel mixture has become excessively fuel-rich (over-rich) based on an output value of the A/F sensor  64  at Step S 7 . That is, when the opening speed of the VSV  47  is too fast, vapor inside the vapor separator tank  43  is taken all at once into the intake system  30 . As a result, the air/fuel mixture may become excessively fuel-rich. In this case, the combustion of the air/fuel mixture becomes abnormal, and this may cause the engine section  2  to stall. When the air/fuel mixture is not excessively fuel-rich at Step S 7 , the operator determines the opening speed of the VSV  47  of Step S 1  as a set value (valve opening speed) and ends the opening speed setting of the VSV  47 . 
     When the air/fuel mixture is excessively fuel-rich at Step S 7 , the opening speed of the VSV  47  is too fast, so that the operator lowers the opening speed of the VSV  47  at Step S 8 . In other words, the operator makes longer the interval of pulses to be applied to the stepping motor  47 M. Thereafter, the process returns to Step S 1 , and the same experiment as described above is made at the decreased opening speed of the VSV  47 . 
     Thus, by repeating Step S 1  to Step S 8  described above, the opening speed of the VSV  47  which does not cause the engine section  2  to stall when starting the engine section  2  can be determined. 
       FIG. 14  is a graph of data actually acquired when the opening speed of the VSV  47  was adjusted to a proper value. In  FIG. 14 , the interval t 3  of pulses to be input into the stepping motor  47 M of the VSV  47  is set to about 5 seconds, for example. In the graph of  FIG. 14 , lowering of the fuel pressure and the lowering of the engine speed are not observed. Therefore, different from the cases of  FIG. 12  (high opening speed) and  FIG. 13  (low opening speed), an occurrence of stall of the engine section  2  is prevented when starting the engine. 
     In this description, the graphs of data acquired when the pulse intervals t 1 , t 2 , and t 3  preferably are respectively set to about 3 seconds, about 7.5 seconds, and about 5 seconds are shown. However, these values are merely examples, and optimum values differ depending on the measurement environment and the used device. 
     As described above, in the present preferred embodiment, the opening speed of the VSV  47  is determined based on data acquired through an experiment made with full throttle after the engine section  2  is started. The reason for this is as follows. That is, when the throttle is fully open, the load on the engine section  2  becomes the highest, and lowering of the fuel pressure and engine stall easily occur. Therefore, by determining the opening speed of the VSV  47  based on results of the experiment made with full throttle, engine stall may not occur even when the opening of the throttle is not full. 
     Examples of technical advantages of the first preferred embodiment are as follows. 
     In the first preferred embodiment, by controlling the opening degree of the VSV  47  such that the opening speed set base on the pressure of the fuel (fuel pressure) inside the pipe  44   a  is attained, the VSV  47  can be opened without lowering of the pressure of the fuel inside the pipe  44   a . Accordingly, the engine section  2  can be prevented from stalling due to lowering of the pressure of the fuel inside the pipe  44   a.    
     In the first preferred embodiment, when starting the engine section  2 , the opening degree of the VSV  47  is controlled such that the opening speed set also based on the air/fuel ratio of the air/fuel mixture to be supplied to the engine section  2 , in addition to the fuel pressure, is attained. Accordingly, the air/fuel mixture to be supplied to the engine section  2  can be prevented from becoming excessively fuel-rich. Accordingly, the engine section  2  can be prevented from stalling due to an excessively fuel-rich state of the air/fuel mixture. 
     In the first preferred embodiment of the present invention, when starting the engine section  2 , the opening degree of the VSV  47  is controlled such that the opening speed set also based on the remaining amount of the fuel in the vapor separator tank  43 , in addition to the fuel pressure, is attained. In the case where the pressure of the fuel inside the pipe  44   a  is low, when the remaining amount of the fuel in the vapor separator tank  43  is large, it can be determined that the lowering of the pressure of the fuel inside the pipe  44   a  is caused by bubble clogging. On the other hand, in the case where the pressure of the fuel inside the pipe  44   a  is low, when the remaining amount of the fuel in the vapor separator tank  43  is small, it can be determined that the lowering of the pressure of the fuel inside the pipe  44   a  is caused by fuel shortage. By thus identifying the cause of the lowering of the pressure of the fuel inside the pipe  44   a , the opening speed of the VSV  47  can be more properly set. Accordingly, the engine section  2  can be further prevented from stalling. In addition, when setting the valve opening speed, a proper valve opening speed can be determined quickly. 
     Also, in the first preferred embodiment, when starting the engine section  2 , the opening degree of the VSV  47  is controlled based on the set value (valve opening speed) stored in the storage section  51 . In other words, a set value of the opening speed of the VSV  47  which at least does not cause lowering of the pressure of the fuel inside the pipe  44   a  is obtained through an experiment, and based on the set value, the opening degree of the VSV  47  is controlled. Accordingly, without providing a sensor, etc., the opening degree of the VSV  47  can be easily controlled so as not to cause the engine section  2  to stall. 
     In the first preferred embodiment, by controlling the opening degree of the VSV  47  such that the VSV  47  is opened at a fixed speed, the opening degree of the VSV  47  can be easily controlled. 
     Second Preferred Embodiment 
       FIG. 15  is a flowchart for describing control of the VSV when starting an engine of an outboard motor of a second preferred embodiment of the present invention, showing processing to be repeated each predetermined time (control cycle) by the ECU  50  when starting the engine.  FIG. 16  is a graph for describing changes in opening degree of the VSV when starting the engine of the outboard motor of the second preferred embodiment of the present invention. In the first preferred embodiment described above, the opening degree of the VSV  47  preferably is controlled based on the valve opening speed (set value) stored in advance. On the other hand, in the second preferred embodiment, by correcting the opening speed of the VSV  47  in real time based on outputs of sensors, the opening degree of the VSV  47  is preferably controlled. The mechanical structure of the outboard motor of the second preferred embodiment is the same as that of the outboard motor  1   a  shown in  FIG. 9  and  FIG. 10 , so that description thereof will be omitted. 
     In the outboard motor of the second preferred embodiment, when starting the engine, the ECU  50  determines whether the temperature of the fuel in the vapor separator tank  43  is high (not lower than about 45° C., for example) based on an output value of the fuel temperature sensor  62  (Step S 11 ). When the temperature of the fuel is not high, the ECU  50  refers to a VSV opening degree map for normal operation (see  FIG. 7 ) based on an engine speed obtained from an output of the crank angle sensor  24   a  and an output value of the intake air pressure sensor  36 . The ECU  50  reads out a corresponding valve opening degree and applies it. In other words, the ECU  50  controls the opening degree of the VSV  47  to the value of normal operation (VSV opening degree prescribed value). 
     When the temperature of the fuel is high (Step S 11 : YES), at Step S 13 , the ECU  50  determines whether the fuel pressure has decreased based on output values of the VST inner pressure sensor  65 , the fuel temperature sensor  62 , and the fuel pressure sensor  63 . The VST inner pressure sensor  65  is an example of “vapor separator tank inner pressure sensor” according to a preferred embodiment of the present invention. 
     When it is determined at Step S 13  that the fuel pressure has decreased, at Step S 14 , the ECU  50  determines whether the fuel remains in the vapor separator tank  43  based on an output value of the remaining amount sensor  61 . When fuel does not remain, it is determined that fuel shortage occurs, so that vapor inside the vapor separator tank  43  must be allowed to escape quickly. Therefore, at Step S 15 , the ECU  50  opens the VSV  47  by one step as shown by the arrows R 1  in  FIG. 16 . Thereafter, the process of the ECU  50  returns to Step S 11 . 
     When it is determined at Step S 14  that fuel remains in the vapor separator tank  43 , it is determined that bubble clogging occurs, so that an occurrence of foaming due to vacuum boiling must be prevented. Therefore, at Step S 16 , the ECU  50  closes the VSV  47  by one step as shown by the arrows R 2  in  FIG. 16 . Thereafter, the process of the ECU  50  returns to Step S 11 . 
     When it is determined at Step S 13  that the fuel pressure has not decreased, at Step S 17 , the ECU  50  determines whether the air/fuel ratio is excessively high based on an output value of the A/F sensor  64 . When the air/fuel ratio is excessively high, it is determined that the VSV  47  is open excessively, so that at Step S 18 , the ECU  50  closes the VSV  47  by one step. Also, when the air/fuel ratio is not excessively high, the opening degree of the VSV  47  is proper, so that at Step S 19 , the ECU  50  keeps the opening degree of the VSV  47  without change as shown by the arrows R 3  in  FIG. 16 . 
     Thus, in the second preferred embodiment, the ECU  50  repeats Step S 11  to Step S 19  described above each predetermined time period. Accordingly, the ECU  50  increases, reduces, or keeps the opening degree of the VSV  47  to correct the opening speed of the VSV  47  each predetermined time. Accordingly, the opening degree of the VSV  47  is controlled in real time. 
     In the second preferred embodiment, when starting the engine section  2 , the ECU  50  acquires the pressure of the fuel inside the pipe  44   a  detected by the fuel pressure sensor  63  each predetermined time. Then, the ECU  50  controls the opening degree of the VSV  47  based on the detected value detected by the fuel pressure sensor  63 . Accordingly, the opening degree of the VSV  47  can be set in real time to an opening degree suitable for the state of the outboard motor. 
     Other advantages of the second preferred embodiment are the same as those of the first preferred embodiment described above. 
     A detailed description has been provided of the preferred embodiments of the present invention. However, the preferred embodiments are only specific examples to describe the technical content of the present invention, and the present invention is not to be construed as limited to these specific examples. The spirit and scope of the present invention are restricted only by the appended claims. 
     For example, the first preferred embodiment described above shows an example in which the VSV  47  is preferably controlled to open at a fixed opening speed when starting the engine. However, the present invention is not limited to this, and the speed of opening the VSV  47  may not be fixed. 
     In the first and second preferred embodiments described above, an example in which the present invention is applied to the outboard motor  1  is shown. However, the present invention is not limited to this, and the present invention is also applicable to an inboard motor or an inboard/outboard motor. 
     In the first preferred embodiment described above, an example in which an experiment for determining the set value of the opening speed of the VSV  47  is preferably made by a person. However, the present invention is not limited to this, and the set value may be automatically determined by a computer installed with a program for performing the experiment for determining the set value of the opening speed of the VSV  47 . 
     The present application corresponds to Japanese Patent Application No. 2008-205027 filed in the Japan Patent Office on Aug. 8, 2008, and the entire disclosure of the application is incorporated herein by reference. 
     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.