Patent Publication Number: US-2023151786-A1

Title: Vessel propulsion apparatus, vessel, and vessel engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-185986 filed on Nov. 15, 2021. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vessel propulsion apparatus, a vessel, and a vessel engine. 
     2. Description of the Related Art 
     US 2002/0072281A1 discloses an outboard motor that is an example of a vessel propulsion apparatus. An engine for use in the outboard motor includes a cylinder block provided with a plurality of cylinders, a piston disposed in each of the cylinders, and a cylinder head connected to the cylinder block. Combustion concave portions, which form a combustion chamber together with corresponding cylinders and pistons, are provided one by one at a portion, which faces each of the cylinders, of the cylinder head. A pair of intake valve openings and an exhaust valve opening are formed in each of the combustion concave portions. A pair of intake ports that extend from an intake manifold are connected to the pair of intake valve openings, respectively. An exhaust port is connected to the exhaust valve opening. With respect to intake/exhaust in the each of the cylinders, air that has flowed into the intake manifold divides and flows into the pair of intake ports, and then flows from the pair of intake valve openings into the combustion chamber, and then is combusted together with fuel in the combustion chamber, and, as a result, turns into an exhaust gas, and then flows through the exhaust valve opening and the exhaust port, and then is discharged outwardly from the outboard motor. 
     SUMMARY OF THE INVENTION 
     In a vessel propulsion apparatus, such as the outboard motor disclosed by US 2002/0072281A1, it is necessary to consider a so-called ω swirl caused by air that has flowed from the pair of intake valve openings into the combustion chamber in the engine. The ω swirl is a swirl making an ω-shaped trace seen from a moving direction of the piston in such a manner that air that has flowed into the combustion chamber from each of the pair of intake valve openings flows toward the exhaust valve opening so that a distance between the flows of air becomes longer immediately after entering into the combustion chamber and then decreases while the flows of air are approaching each other. If it is possible to prevent the occurrence of the co swirl, it is possible to improve engine performance by increasing a combustion speed. 
     The inventor of preferred embodiments of the present invention described and claimed in the present application conducted an extensive study and research regarding a vessel propulsion apparatus, a vessel, and a vessel engine, such as the one described above, and in doing so, discovered and first recognized new unique challenges and previously unrecognized possibilities for improvements as described in greater detail below. 
     Preferred embodiments of the present invention provide vessel propulsion apparatuses each able to prevent the occurrence of an ω swirl in a combustion chamber of an engine, vessels including the vessel propulsion apparatuses, and vessel engines included in the vessel propulsion apparatuses. 
     In order to overcome the previously unrecognized and unsolved challenges described above, a preferred embodiment of the present invention provides a vessel propulsion apparatus including an engine and a propulsion unit to be driven by the engine. The engine includes a cylinder head including a combustion chamber and a pair of intake openings to communicate with the combustion chamber and a pair of intake ports respectively connected to the pair of intake openings. The pair of intake ports include a pair of downstream portions respectively connected to the pair of intake openings. A distance between the pair of downstream portions decreases toward the pair of intake openings. 
     With this structural arrangement, in the engine of the vessel propulsion apparatus, air divides and flows into the pair of intake ports, flows into the combustion chamber from the pair of intake openings, and is combusted in the combustion chamber together with fuel. Thus, the engine generates a driving force, the propulsion unit is driven in the vessel propulsion apparatus, and therefore the vessel propulsion apparatus generates a thrust. The distance between the downstream portions connected to the intake openings of the pair of intake ports decreases toward the intake openings, and therefore air immediately after having flowed into the combustion chamber from the pair of intake openings is able to flow so that the distance between each other does not increase. This makes it possible to prevent the occurrence of an ω swirl in the combustion chamber of the engine. 
     In a preferred embodiment of the present invention, the vessel propulsion apparatus further includes a pressure charger and an intercooler to cool air compressed by the pressure charger. Air cooled by the intercooler flows into the pair of intake ports. The pair of intake ports include a pair of independent intake ports that individually extend from the intercooler to the pair of intake openings. 
     With this structural arrangement, as each of the pair of intake ports is an independent intake port, it is possible to easily arrange the distance between the downstream portions connected to the intake openings so as to decrease in proportion as the downstream portions approach the intake openings. 
     A preferred embodiment of the present invention provides a vessel propulsion apparatus including an engine and a propulsion unit to be driven by the engine. The engine includes a cylinder head including a combustion chamber and a pair of intake openings to communicate with the combustion chamber and a pair of intake ports respectively connected to the pair of intake openings. The cylinder head includes a pair of inner surfaces that respectively extend from peripheral edges of the pair of intake openings and that face each other in an arrangement direction in which the pair of intake openings are arranged. The combustion chamber is defined between the pair of inner surfaces. The pair of inner surfaces include a pair of upstream portions respectively connected to the peripheral edges of the pair of intake openings. A distance between the pair of upstream portions decreases in a direction moving away from the peripheral edges of the pair of intake openings. 
     With this structural arrangement, in the engine of the vessel propulsion apparatus, air divides and flows into the pair of intake ports, flows into the combustion chamber from the pair of intake openings, and is combusted in the combustion chamber together with fuel. Thus, the engine generates a driving force, the propulsion unit is driven in the vessel propulsion apparatus, and therefore the vessel propulsion apparatus generates a thrust. In the pair of inner surfaces that respectively extend from the peripheral edges of the pair of intake openings in the cylinder head and that face each other across the combustion chamber in an arrangement direction of the pair of intake openings, the distance between the pair of upstream portions connected to the peripheral edges of the intake openings decreases in the direction moving away from the peripheral edge of the intake openings. Therefore, air immediately after having flowed into the combustion chamber from the pair of intake openings is able to flow so that the distance between the air flows does not increase. This makes it possible to prevent the occurrence of an ω swirl in the combustion chamber of the engine. 
     In a preferred embodiment of the present invention, the cylinder head includes a pair of exhaust openings to communicate with the combustion chamber and arranged side by side along the arrangement direction. The engine includes a pair of exhaust ports respectively connected to the pair of exhaust openings. A first inner surface of the pair of inner surfaces connects a peripheral edge of a first intake opening of the pair of intake openings and a peripheral edge of a first exhaust opening of the pair of exhaust openings. A second inner surface of the pair of inner surfaces connects a peripheral edge of a second intake opening of the pair of intake openings and a peripheral edge of a second exhaust opening of the pair of exhaust openings. The first inner surface and the second inner surface are curved toward each other. 
     With this structural arrangement, the distance between the upstream portions in the pair of inner surfaces of the cylinder head decreases in the direction moving away from the peripheral edges of the intake openings. 
     In a preferred embodiment of the present invention, the engine includes a crankshaft extending along a vertical direction. The vessel propulsion apparatus is an outboard motor that includes a drive shaft, a propeller shaft, a propeller, and a transmission. The drive shaft is joined to the crankshaft, and extends along the vertical direction. The propeller shaft extends along a horizontal direction. The propeller defines the propulsion unit, and is joined to the propeller shaft. The transmission transmits rotation of the drive shaft to the propeller shaft. 
     With this structural arrangement, in the outboard motor, the rotation of the crankshaft of the engine is transmitted to the propeller shaft through the drive shaft and the transmission, and, as a result, the propeller shaft rotates together with the propeller, and therefore the propeller generates a thrust. In the combustion chamber of the engine of the outboard motor, it is possible to prevent the occurrence of an ω swirl as described above. 
     A preferred embodiment of the present invention provides a vessel including a hull and the vessel propulsion apparatus mounted in the hull to provide a thrust to the hull. 
     With this structural arrangement, in the combustion chamber of the engine of the vessel propulsion apparatus included in the vessel, it is possible to prevent the occurrence of an ω swirl as described above. 
     A preferred embodiment of the present invention provides a vessel engine including a cylinder head including a combustion chamber, a pair of intake openings to communicate with the combustion chamber, and a pair of intake ports respectively connected to the pair of intake openings. The pair of intake ports include a pair of downstream portions respectively connected to the pair of intake openings. A distance between the pair of downstream portions decreases toward the pair of intake openings. 
     With this structural arrangement, in the combustion chamber of the vessel engine, it is possible to prevent the occurrence of an ω swirl as described above. 
     A preferred embodiment of the present invention provides a vessel engine including a cylinder head including a combustion chamber, a pair of intake openings to communicate with the combustion chamber, and a pair of intake ports respectively connected to the pair of intake openings. The cylinder head includes a pair of inner surfaces that respectively extend from peripheral edges of the pair of intake openings and that face each other in an arrangement direction in which the pair of intake openings are arranged. The combustion chamber is defined between the pair of inner surfaces. The pair of inner surfaces include a pair of upstream portions respectively connected to the peripheral edges of the pair of intake openings. A distance between the pair of upstream portions decreases in a direction moving away from the peripheral edges of the pair of intake openings. 
     With this structural arrangement, in the combustion chamber of the vessel engine, it is possible to prevent the occurrence of an ω swirl as described above. 
     The above and 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 schematic plan view of a vessel according to a preferred embodiment of the present invention. 
         FIG.  2    is a schematic side view of an outboard motor included in the vessel. 
         FIG.  3    is a schematic view shown to describe an air intake/exhaust system of the outboard motor. 
         FIG.  4    is a perspective view of an air intake structure in the air intake/exhaust system. 
         FIG.  5    is a rear view of a main portion in the air intake structure. 
         FIG.  6    is a front view of the main portion in the air intake structure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG.  1    is a schematic plan view of a vessel  1  according to a preferred embodiment of the present invention. The vessel  1  includes a hull  2 , a vessel operation device  3 , and an outboard motor  4 , and the hull  2  is equipped with both the vessel operation device  3  and the outboard motor  4 . An example of the vessel operation device  3  includes a steering wheel  6  and a throttle lever  7  both of which are provided at an operational platform  5  around a vessel operation seat of the hull  2 , and a communication bus  9  by which an ECU (electronic control unit)  8  built into the outboard motor  4 , a steering wheel  6 , and the throttle lever  7  are connected together. A vessel operator turns the steering wheel  6  in a left-right direction to steer. The vessel operator turns the throttle lever  7  in a front-rear direction to adjust the output of the outboard motor  4 . A joystick  10  that is operated by the vessel operator to steer and adjust the output of the outboard motor  4  may be provided at the operational platform  5 . 
     The outboard motor  4  is an example of a vessel propulsion apparatus that provides a thrust to the hull  2 , and is provided as a single outboard motor or as a plurality of outboard motors. The single outboard motor  4  is attached to a transom stern  2 A on a virtual center line C along the front-rear direction through the transom stern  2 A and a bow  2 B of the hull  2 . The plurality of outboard motors  4  are attached to the transom stern  2 A at bilaterally symmetrical positions with respect to the center line C. 
       FIG.  2    is a schematic right side view of the outboard motor  4 . The left side in  FIG.  2    is the front side of the outboard motor  4 , and the right side in  FIG.  2    is the rear side of the outboard motor  4 . The upper side in  FIG.  2    is the upper side of the outboard motor  4 , and the lower side in  FIG.  2    is the lower side of the outboard motor  4 . An up-down direction is also a vertical direction. A direction perpendicular to the plane of paper of  FIG.  2    is the left-right direction of the outboard motor  4 . In the following description, a leftward or rightward direction of the outboard motor  4  is determined based on a direction given when the outboard motor  4  is seen from the front side. Therefore, the near side in the direction perpendicular to the plane of paper of  FIG.  2    is the right side of the outboard motor  4 , and the far side in the direction perpendicular to the plane of paper of  FIG.  2    is the left side of the outboard motor  4 . 
     The outboard motor  4  includes a mount  11  to attach the outboard motor  4  to the transom stern  2 A and an outboard motor main body  12 . The mount  11  includes a clamp bracket  13  fixed to the transom stern  2 A and a swivel bracket  15  coupled to the clamp bracket  13  through a tilt shaft  14  horizontally extending in the left-right direction. The swivel bracket  15  is coupled to the outboard motor main body  12  through a steering shaft  16  extending in the up-down direction. Thus, the outboard motor main body  12  is attached to the transom stern  2 A by the mount  11  in a vertical or substantial vertical attitude. 
     The outboard motor main body  12  and the swivel bracket  15  are turnable in the up-down direction around the tilt shaft  14  with respect to the clamp bracket  13 . The outboard motor main body  12  is turned around the tilt shaft  14 , and, as a result, the outboard motor main body  12  is tilted with respect to the hull  2  and the clamp bracket  13 . The outboard motor main body  12  is turnable in the left-right direction together with the steering shaft  16  with respect to the clamp bracket  13  and the swivel bracket  15 . When the outboard motor main body  12  turns in the left-right direction, the vessel  1  is steered. 
     The outboard motor main body  12  includes a box-shaped engine cover  17 , a hollow casing  18  extending downwardly from the engine cover  17 , and a plate-shaped exhaust guide  19  attached to a lower end portion of the engine cover  17  so as to close an internal space of the casing  18  from above. A lower end portion of the casing  18  is a lower case  18 A. The outboard motor main body  12  includes an engine  20  mounted on an upper surface of the exhaust guide  19  in the engine cover  17 , a drive shaft  21  extending along the up-down direction in the casing  18 , and a propeller shaft  22  and a transmission  23  both of which are located in the lower case  18 A. 
     The engine  20  is a vessel engine, and includes an internal combustion engine that burns fuel, such as gasoline, and generates power. The engine  20  includes a cylinder block  25  including a single or a plurality of cylinders  24 , a piston  26  located in the cylinder  24  one by one, and a crankshaft  27  extending along the up-down direction in the cylinder block  25  and that is coupled to the piston(s)  26 . The engine  20  in the present preferred embodiment is a straight-type four-cylinder engine in which four cylinders  24  are located in series along the up-down direction. 
     An internal space of each of the cylinders  24  includes a circular cylindrical shape extending along the front-rear direction. A combustion chamber  28  is defined in a region behind the piston  26  in the internal space of each of the cylinders  24 . A front portion that houses the crankshaft  27  in the cylinder block  25  is a crank case  25 A. 
     The engine  20  includes a cylinder head  29  attached to the cylinder block  25  from behind and a head cover  30  attached to the cylinder head  29  from behind. The cylinder head  29  and the head cover  30  may be regarded as elements of the cylinder block  25 . Concave portions  29 A each of which is rearwardly hollowed as a portion of the combustion chamber  28  are provided one by one at a portion, which faces the combustion chamber  28  of each of the cylinders  24 , of a front surface of the cylinder head  29 . The engine  20  includes an intake valve  31  and an exhaust valve  32  that are exposed to each of the concave portions  29 A and a camshaft  33  that extends along the up-down direction and that is rotatably supported by the head cover  30 . The camshaft  33  may be provided as a pair of camshafts in accordance with each of the intake valve  31  and the exhaust valve  32 . 
     The crankshaft  27  has a crankshaft axis  27 A extending in the up-down direction. An upper end portion of the crankshaft  27  protrudes upwardly from the crank case  25 A. A lower end portion of the crankshaft  27  is joined to the upper end portion of the drive shaft  21 . The engine  20  includes a flywheel magneto  34  fixed to the upper end portion of the crankshaft  27  and a cam chain  35  connecting the lower end portion of the crankshaft  27  and a lower end portion of the camshaft  33 . The flywheel magneto  34  is located at a higher position than the crank case  25 A. The cam chain  35  is located below the four cylinders  24  in the cylinder block  25 . 
     The piston  26  is rectilinearly reciprocated in the front-rear direction perpendicular to the crankshaft axis  27 A by combustion of an air-fuel mixture in each of the combustion chambers  28 . When the piston  26  is rectilinearly reciprocated, the crankshaft  27  is driven and rotated around the crankshaft axis  27 A along with the drive shaft  21 . In accordance with the rotation of the crankshaft  27 , the flywheel magneto  34  rotates and generates electricity, and the cam chain  35  moves in a circular motion. The camshaft  33  is rotated in accordance with the circular movement of the cam chain  35 . The intake valve  31  and the exhaust valve  32  are actuated interlockingly with the rotation of the camshaft  33 . Thus, intake/exhaust is performed in each of the combustion chambers  28 . 
     The propeller shaft  22  horizontally extends along the front-rear direction in the lower case  18 A. A lower end portion of the drive shaft  21  is coupled to a front end portion of the propeller shaft  22  by the transmission  23 . A rear end portion of the propeller shaft  22  protrudes rearwardly from the lower case  18 A. A propeller  36  as an example of a propulsion unit that is an element of the outboard motor  4  is joined to the rear end portion of the propeller shaft  22 . The propeller shaft  22  rotates together with the propeller  36  around a rotational axis  22 A that extends in the front-rear direction. 
     The transmission  23  is used to transmit the rotation of the drive shaft  21  to the propeller shaft  22 . The transmission  23  includes a driving gear  38  fixed to the lower end portion of the drive shaft  21  and a rotary body  39  and a dog clutch  40  both of which are attached to the front end portion of the propeller shaft  22 . The driving gear  38  is a bevel gear. The propeller shaft  22  is located below the driving gear  38 . The rotary body  39  includes a first rotary body  41  and a second rotary body  42  that are located side by side in the front-rear direction along the propeller shaft  22 . The first rotary body  41  and the second rotary body  42  are, for example, cylindrical bevel gears, respectively. 
     In the present preferred embodiment, the first rotary body  41  is located at a more forward position than the driving gear  38 , and the second rotary body  42  is located at a more rearward position than the driving gear  38 , and yet the front-rear positional relationship between the first rotary body  41  and the second rotary body  42  may be opposite to that of the present preferred embodiment. In a rear surface of the first rotary body  41 , a tooth portion  41 A is provided at a tapered outer peripheral portion, and a claw portion  41 B is provided at an inner peripheral portion. In a front surface of the second rotary body  42 , a tooth portion  42 A is provided at a tapered outer peripheral portion, and a claw portion  42 B is provided at an inner peripheral portion. 
     The first rotary body  41  surrounds a portion, which is at a more forward position than the driving gear  38 , of the front end portion of the propeller shaft  22 , and the second rotary body  42  surrounds a portion, which is at a more rearward position than the driving gear  38 , of the front end portion of the propeller shaft  22 . The first rotary body  41  and the second rotary body  42  are located so that their tooth portions  41 A and  42 A face each other at a distance from each other in the front-rear direction, and engage with the driving gear  38 . When the driving gear  38  rotates together with the drive shaft  21  in response to the driving of the engine  20 , the rotation of the driving gear  38  is transmitted to the first rotary body  41  and to the second rotary body  42 . Thus, the first rotary body  41  and the second rotary body  42  rotate around the rotational axis  22 A of the propeller shaft  22  in mutually opposite directions. 
     The dog clutch  40  is located between the first rotary body  41  and the second rotary body  42 . The dog clutch  40  is, for example, cylindrical, and surrounds the front end portion of the propeller shaft  22 . A first claw portion  40 A is provided at a front end surface of the dog clutch  40 , and a second claw portion  40 B is provided at a rear end surface of the dog clutch  40 . The dog clutch  40  is coupled to the front end portion of the propeller shaft  22  by, for example, a spline. Therefore, the dog clutch  40  rotates together with the front end portion of the propeller shaft  22 . Additionally, the dog clutch  40  is movable in the front-rear direction with respect to the front end portion of the propeller shaft  22 . In other words, the dog clutch  40  is rotatable together with the propeller shaft  22 , and is movable along the front-rear direction relatively with the propeller shaft  22 . 
     The transmission  23  also includes a shifter  43  located at a more forward position than the propeller shaft  22  in the lower case  18 A. The shifter  43  includes, for example, a shift rod  44  extending in the up-down direction and an electric shift actuator  45  joined to the shift rod  44 . A lower end portion of the shift rod  44  is coupled to the dog clutch  40 . When the shift actuator  45  is operated by the control of the ECU  8  (see  FIG.  1   ), the shift rod  44  turns around an axis of the shift rod  44 . The shift rod  44  turns, and, as a result, the dog clutch  40  is moved along the front-rear direction between a disconnection position and a connection position. 
     The disconnection position is a position in which the dog clutch  40  is spaced apart from the first rotary body  41  and the second rotary body  42 , and does not engage with either of these rotary bodies of the rotary body  39  as shown in  FIG.  2   . In a state in which the dog clutch  40  is located in the disconnection position, each of the rotary body  39  to which the rotation of the drive shaft  21  is transmitted runs idle, and therefore the rotation of the drive shaft  21  is not transmitted to the propeller shaft  22 . In the following description, the shift position of the outboard motor  4  at this time is referred to as “neutral.” 
     The connection position is a position in which the dog clutch  40  engages with either one of the first rotary body  41  or the second rotary body  42 . The connection position includes a first connection position in which the first claw portion  40 A of the dog clutch  40  engages with only the claw portion  41 B of the first rotary body  41  and a second connection position in which the second claw portion  40 B of the dog clutch  40  engages with only the claw portion  42 B of the second rotary body  42 . The disconnection position is a position between the first connection position and the second connection position. The first connection position is more forward than the disconnection position, and the second connection position is more rearward than the disconnection position. 
     In a state in which the dog clutch  40  is located in the first connection position and is coupled to only the first rotary body  41 , the rotation of the first rotary body  41  is transmitted to the propeller shaft  22 , and therefore the shift position of the outboard motor  4  is shifted into “forward.” Thereupon, the rotation of the drive shaft  21  is transmitted to the propeller shaft  22  through the first rotary body  41  and the dog clutch  40 , and, as a result, the propeller  36  rotates in a forward rotational direction (for example, a clockwise direction when seen from the rear side). Thus, the propeller  36  is driven by the engine  20 , and a forward thrust is generated. 
     In a state in which the dog clutch  40  is located in the second connection position and is coupled to only the second rotary body  42 , the rotation of the second rotary body  42  is transmitted to the propeller shaft  22 , and therefore the shift position of the outboard motor  4  is shifted into “reverse.” Thereupon, the rotation of the drive shaft  21  is transmitted to the propeller shaft  22  through the second rotary body  42  and the dog clutch  40 , and, as a result, the propeller  36  rotates in a reverse rotational direction opposite to the forward rotational direction. Thus, the propeller  36  is driven by the engine  20 , and a reverse thrust is generated. As thus described, in the present preferred embodiment, the first rotary body  41  is a gear for forward movement, and the second rotary body  42  is a gear for reverse movement. Of course, the first rotary body  41  may be a gear for reverse movement, and the second rotary body  42  may be a gear for forward movement. 
     The outboard motor main body  12  includes an exhaust passage  46  provided inside the outboard motor main body  12  and connected to the engine  20 . The exhaust passage  46  passes through the exhaust guide  19  in the up-down direction, and extends downwardly in the casing  18  and rearwardly in the propeller  36 . The exhaust passage  46  includes an outlet  46 A provided at a rear end surface of the propeller  36 . In a state in which the vessel  1  is floating on water and in which the propeller  36  is located below a water surface, the outlet  46 A is located in the water, and therefore water that has passed through the outlet  46 A enters a downstream portion of the exhaust passage  46 . On the other hand, when the engine  20  rotates at a high speed, water in the exhaust passage  46  is pushed by the pressure of an exhaust gas emitted from the engine  20 , and is discharged from the outlet  46 A together with the exhaust gas. Thus, the exhaust gas generated by the engine  20  is discharged into the water. 
     A steering rod  47  that forwardly extends is fixed to the outboard motor main body  12 . An electric steering actuator  48  that is controlled by the ECU  8  is joined to the steering rod  47 . The outboard motor main body  12  is able to turn around the steering shaft  16  by allowing the steering actuator  48  to operate, thus making it possible to perform steering. 
       FIG.  3    is a schematic view shown to describe an air intake/exhaust system  49  of the outboard motor  4 . The air intake/exhaust system  49  includes the engine  20 , a pressure charger  50  that compresses air and supplies the air to the engine  20 , and an intercooler  51  that cools air compressed by the pressure charger  50 . 
     With respect to the air intake/exhaust system  49 , the engine  20  includes the exhaust passage  46 , an air intake passage  52 , and an electric throttle valve  53  located in the air intake passage  52 . The exhaust passage  46  is connected to each of the combustion chambers  28  through a plurality of exhaust ports  54  provided in the cylinder head  29  of the engine  20 . The air intake passage  52  is connected to each of the combustion chambers  28  through a plurality of intake ports  55  provided in the cylinder head  29 . An inlet  52 A is provided at an end portion, which is opposite to the intake port  55 , of the air intake passage  52 . The ECU  8  controls the throttle valve  53 , and, as a result, the opening degree of the throttle valve  53  is adjusted. 
     The pressure charger  50  is interposed between the ends of the air intake passage  52 . The pressure charger  50  is a supercharger driven by the rotation of the crankshaft  27  of the engine  20 . The pressure charger  50  includes a housing  50 A including an internal space defining a portion of the air intake passage  52 , a compressor wheel  50 B located in the housing  50 A, and a rotational shaft  50 C coaxially fixed to the compressor wheel  50 B. An end portion, which is spaced apart from the compressor wheel  50 B, of the rotational shaft  50 C is located outside the housing  50 A, and a rotor  56  is coaxially fixed to this end portion. 
     The air intake/exhaust system  49  includes a power transmission by which the crankshaft  27  and the pressure charger  50  are joined together. An example of the power transmission includes the rotor  56 , another rotor  57  attached to the crankshaft  27 , and a belt  58  by which the rotor  56  and the rotor  57  are connected together. An example of each of the rotors  56  and  57  is a pulley. The rotor  57  is attached to a portion, which is located at a higher position than the flywheel magneto  34 , of the upper end portion of the crankshaft  27  (see  FIG.  2   ). 
     When the crankshaft  27  rotates, the rotor  57  rotates together with the crankshaft  27 . The rotation of the rotor  57  is transmitted to the rotor  56  through the belt  58 . Thereupon, the rotational shaft  50 C rotates together with the compressor wheel  50 B, and, as a result, the pressure charger  50  is driven. A sprocket may be used as each of the rotors  56  and  57  instead of the pulley, and a chain may be used instead of the belt  58 . 
     When the pressure charger  50  operates in a state in which the throttle valve  53  has been opened, air that has been taken from the inlet  52 A and that flows through the air intake passage  52  is compressed by the compressor wheel  50 B rotating in the housing  50 A. Another arrangement, such as a Lysholm-type device, may be used as the pressure charger  50  without being limited to the centrifugal-type device shown in  FIG.  3   . 
     The intercooler  51  is interposed between each of the intake ports  55  of the engine  20  and the pressure charger  50  in the air intake passage  52 . The intercooler  51  includes a housing  51 A including an internal space defining a portion of the air intake passage  52  and a cooling fin (not shown). Either of an air-cooled intercooler or a water-cooled intercooler may be used as the intercooler  51 . The intercooler  51  includes an intake manifold  51 B that extends from the housing  51 A and is connected to the intake port  55 . The intake manifold  51 B is integral with the housing  51 A. 
     Air compressed by the compressor wheel  50 B in the housing  50 A of the pressure charger  50  continuously flows through the air intake passage  52 , and thus is guided to the intercooler  51 , and is cooled by heat exchange with the cooling fin in the housing  51 A of the intercooler  51 . The air cooled by the intercooler  51  flows through the intake manifold  51 B, and then is turned into an air-fuel mixture, supplied from the intake port  55  to the combustion chamber  28  in the cylinder  24 , and combusted. Exhaust gas generated by the combustion flows from the exhaust port  54  through the exhaust passage  46 , and then is discharged from the outlet  46 A into the water as described above. 
       FIG.  4    is a perspective view of an air intake structure  60 , seen from the right front side, which excludes the exhaust passage  46  in the air intake/exhaust system  49 . The intercooler  51  is located lateral to (more specifically, on the left side of) the cylinder block  25  of the engine  20 . The housing  51 A of the intercooler  51  is made of metal, such as aluminum, and is a hollow body elongated in the up-down direction. The intake manifold  51 B integral with the housing  51 A extends rearwardly from the housing  51 A, and is bent rightwardly. 
     The pressure charger  50  is located in front of the cylinder block  25 . An upstream region  52 B, that is closer to the inlet  52 A than the pressure charger  50 , of the air intake passage  52  extends downwardly from the housing  50 A of the pressure charger  50 , and then extends right-rearwardly, and is bent on the right side of the cylinder block  25 , and extends upwardly. The inlet  52 A is provided at an upper end portion of the upstream region  52 B and faces forwardly. A filter  61  made of mesh, etc., is provided at the inlet  52 A. A portion, which is located on the right side of the cylinder block  25 , of the upstream region  52 B may be referred to as an air intake duct. The rotor  56  is located at an upper surface of the housing  50 A of the pressure charger  50 . 
       FIG.  5    is a rear view of a main portion of the air intake structure  60 . A portion of the cylinder head  29  and a pair of the intake ports  55  are shown in  FIG.  5   . The concave portion  29 A, i.e., a portion of the combustion chamber  28  is provided at the cylinder head  29  as a single or a plurality of concave portions whose number is the same as the number of cylinders  24 , and, in the present preferred embodiment, four concave portions  29 A are arranged side by side in the up-down direction (see  FIG.  2   ). The intake port  55  is a circular tubular pipe passage provided as a pair for each combustion chamber  28 , and extends rightwardly from the intake manifold  51 B (in a rear view, leftwardly), and is connected to a corresponding combustion chamber  28 . A valve guide  62  that supports the intake valve  31  (see  FIG.  2   ) is provided at each of the intake ports  55 . The intake valve  31  and the exhaust valve  32  are omitted and are not shown in  FIG.  5    and in the drawings subsequent to  FIG.  5   . The exhaust port  54  is a circular tubular pipe passage provided as a pair for each combustion chamber  28  (see  FIG.  6   ). 
     A pair of air intake openings  63  and a pair of exhaust openings  64  are provided in a region, which coincides with each of the combustion chambers  28  in a rear view, of a rear surface of the cylinder head  29 . Each of the air intake opening  63  and the exhaust opening  64  is a round hole that passes through the cylinder head  29  in the front-rear direction or substantially in the front-rear direction (see also  FIG.  6   ). A single intake valve  31  is located in a single air intake opening  63 , and a single exhaust valve  32  is located in a single exhaust opening  64  (not shown). A through-hole  29 B that exposes an ignition plug (not shown) to each of the combustion chambers  28  is also provided in the cylinder head  29 . 
     An arrangement relative to each of the combustion chambers  28  will be hereinafter described with reference to a single combustion chamber  28 . For the air intake opening  63  and the exhaust opening  64  corresponding to the single combustion chamber  28 , the pair of air intake openings  63  arranged side by side along the up-down direction are located on the left side (in a rear view, on the right side) closer to the intake manifold  51 B than the pair of exhaust openings  64  arranged side by side along the up-down direction. In other words, in the present preferred embodiment, an arrangement direction in which the pair of air intake openings  63  are arranged is the up-down direction (the arrangement direction), and an arrangement direction in which the pair of exhaust openings  64  are arranged is likewise the up-down direction (the arrangement direction). 
     The pair of air intake openings  63  communicate with a first region  28 A, which occupies substantially a left half of the combustion chamber  28 , from behind, and the pair of exhaust openings  64  communicate with a second region  28 B, which occupies substantially a right half of the combustion chamber  28 , from behind. A region surrounded by the pair of air intake openings  63  and by the pair of exhaust openings  64  in a rear view is a central region  28 C of the combustion chamber  28 . The central region  28 C is a boundary region between the first region  28 A and the second region  28 B. The through-hole  29 B is located at a position that coincides with the central region  28 C in a rear view. 
     In the following description, an upper one of the pair of air intake openings  63  will be referred to as a first air intake opening  63 A if necessary, and a lower one of the pair of air intake openings  63  will be referred to as a second air intake opening  63 B if necessary. Additionally, an upper one of the pair of exhaust openings  64 , i.e., the exhaust opening  64  located on the first air intake opening  63 A side in the up-down direction will be referred to as a first exhaust opening  64 A if necessary, and a lower one of the pair of exhaust openings  64 , i.e., the exhaust opening  64  located on the second air intake opening  63 B side in the up-down direction will be referred to as a second exhaust opening  64 B if necessary. 
     A pair of intake ports  55  are respectively connected to the pair of air intake openings  63  communicating with the single combustion chamber  28 . A pair of exhaust ports  54  are respectively connected to the pair of exhaust openings  64  communicating with this combustion chamber  28  (see  FIG.  6   ). 
     Each of the pair of intake ports  55  is an independent intake port that individually extends from the intake manifold  51 B integral with the intercooler  51  to the air intake opening  63 . A portion, which is connected to the air intake opening  63 , i.e., a portion closer to the air intake opening  63 , of each of the pair of intake ports  55 , is referred to as a downstream portion  55 A. A distance D between the downstream portions  55 A of the pair of intake ports  55  decreases toward the air intake opening  63 . 
     A virtual line passing through the center of the intake port  55  (more specifically, the center in a flow-passage cross section of the intake port  55 ) is referred to as a tube axis  55 B. A distance D′ between the tube axes  55 B in the downstream portions  55 A of the pair of intake ports  55  likewise decreases toward the air intake opening  63  in the same way as the distance D between the downstream portions  55 A. A distance between the entire areas of the pair of intake ports  55  may also become shorter toward the air intake opening  63  without being limited to the distance D between the downstream portions  55 A. 
     Air cooled by the intercooler  51  divides and flows into each of the pair of intake ports  55  in the engine  20  of the outboard motor  4 . The air that has been divided and flowed into each of the pair of intake ports  55  flows into a corresponding one of the combustion chambers  28  from each of the pair of air intake openings  63 , and is combusted in the combustion chamber  28  together with fuel. Thus, the engine  20  generates a driving force, and the propeller  36  is driven in the outboard motor  4 , and therefore the outboard motor  4  generates a thrust. 
     In the pair of intake ports  55 , the distance D between the downstream portions  55 A connected to the air intake openings  63  decreases toward the air intake openings  63  as described above. This arrangement enables air immediately after having flowed into the combustion chamber  28  from each of the pair of air intake openings  63  to flow toward the central region  28 C side of the combustion chamber  28  (i.e., toward the ignition plug exposed through the through-hole  29 B) so that the distance between the air flows does not increase (see the thick solid arrow in  FIG.  5   ). This makes it possible to prevent the occurrence of an ω swirl (see the thick alternate long and two short dashed line arrow in  FIG.  5   ) in the combustion chamber  28  of the engine  20 . If the occurrence of an ω swirl is prevented, it is possible to prevent the occurrence of knocking, and it is possible to raise a combustion speed particularly during ignition advance, thus making it possible to improve the performance of the engine  20 . Additionally, if each of the pair of intake ports  55  is an independent intake port, it is possible to easily arrange the distance D between the downstream portions  55 A connected to the air intake openings  63  so as to become shorter toward the air intake openings  63 . 
       FIG.  6    is a front view of a main portion of the air intake structure  60 . A portion of a front surface  29 C, at which the combustion chamber  28  (concave portion  29 A) is provided, of the cylinder head  29  and the pair of exhaust ports  54  are shown in  FIG.  6   . The pair of exhaust ports  54  extend rightwardly. 
     A pair of inner surfaces  65  facing each other in an arrangement direction in which the pair of air intake openings  63  are arranged (in the present preferred embodiment, in the up-down direction) and another pair of inner surfaces  66  facing each other in the left-right direction perpendicular to the arrangement direction are provided for each of the combustion chambers  28  in the cylinder head  29 . 
     In the following description, an upper inner surface  65 , which is located on the first air intake opening  63 A side in the up-down direction, of the pair of inner surfaces  65  will be referred to as a first inner surface  65 A if necessary, and a lower inner surface  65 , which is located on the second air intake opening  63 B side in the up-down direction, of the pair of inner surfaces  65  will be referred to as a second inner surface  65 B if necessary. Additionally, a left inner surface  66 , which is located on the pair of air intake ports  63  side in the left-right direction, of the pair of inner surfaces  66  will be referred to as a first inner surface  66 A if necessary, and a right inner surface  66 , which is located on the pair of exhaust openings  64  side in the left-right direction, of the pair of inner surfaces  66  will be referred to as a second inner surface  66 B if necessary. 
     Left ends of the pair of inner surfaces  65  are provided between the first inner surfaces  66 A, and right ends of the pair of inner surfaces  65  are provided between the second inner surfaces  66 B. The single combustion chamber  28  is defined between the pair of inner surfaces  65  and between the pair of inner surfaces  66 . The combustion chamber  28  includes a rectangular or substantially rectangular shape whose four corners are rounded in a front view. Either the pair of air intake openings  63  or the pair of exhaust openings  64  are located one by one at the four corners of the combustion chamber  28  in a front view. 
     A circular arc, by which a left end portion of the first inner surface  65 A and an upper end portion of the first inner surface  66 A are connected together, of a circular peripheral edge  63 AA fringing the first air intake opening  63 A is referred to as a peripheral edge  63 AB. A circular arc, by which a left end portion of the second inner surface  65 B and a lower end portion of the first inner surface  66 A are connected together, of a circular peripheral edge  63 BA fringing the second air intake opening  63 B is referred to as a peripheral edge  63 BB. A circular arc, by which a right end portion of the first inner surface  65 A and an upper end portion of the second inner surface  66 B are connected together, of a circular peripheral edge  64 AA fringing the first exhaust opening  64 A is referred to as a peripheral edge  64 AB. A circular arc, by which a right end portion of the second inner surface  65 B and a lower end portion of the second inner surface  66 B are connected together, of a circular peripheral edge  64 BA fringing the second exhaust opening  64 B is referred to as a peripheral edge  64 BB. 
     The first inner surface  65 A connects the peripheral edge  63 AB of the first air intake opening  63 A and the peripheral edge  64 AB of the first exhaust opening  64 A together. The second inner surface  65 B connects the peripheral edge  63 BB of the second air intake opening  63 B and the peripheral edge  64 BB of the second exhaust opening  64 B together. In other words, the pair of inner surfaces  65  extend from the peripheral edge  63 AB and the peripheral edge  63 BB of the pair of air intake openings  63  to the peripheral edge  64 AB and the peripheral edge  64 BB of the pair of exhaust openings  64 , respectively. 
     A portion, which is connected to the peripheral edge  63 AB or the peripheral edge  63 BB of a corresponding air intake opening  63 , of each of the pair of inner surfaces  65 , i.e., a portion, which is substantially a left half closer to the air intake opening  63 , of each of the pair of inner surfaces  65  is referred to an upstream portion  65 C. The first inner surface  65 A and the second inner surface  65 B of the pair of inner surfaces  65  are curved so as to approach each other. Therefore, a distance E between the upstream portions  65 C of the pair of inner surfaces  65  decreases in a direction moving rightwardly away from the peripheral edge  63 AB and from the peripheral edge  63 BB. Likewise, this arrangement enables air immediately after having flowed into the combustion chamber  28  from each of the pair of air intake openings  63  to flow so that the distance between each other does not increase (see the thick solid arrow in  FIG.  6   ). This makes it possible to prevent the occurrence of an ω swirl (see the thick alternate long and two short dashed line arrow in  FIG.  6   ) in the combustion chamber  28  of the engine  20 . The pair of inner surfaces  66  may be curved so as to approach each other in the same way as the pair of inner surfaces  65 , or may extend linearly in a parallel or substantially parallel manner, or one of the pair of inner surfaces  66  may be curved while the other inner surface  66  may extend linearly. 
     Preferred embodiments of the present invention have been described above, and yet the present invention is not limited to the contents of the above-described preferred embodiments, and various modifications can be made within the scope of the present invention. 
     For example, an arrangement in which the distance D decreases toward the air intake opening  63  (see  FIG.  5   ) and an arrangement in which the distance E decreases in a direction moving away from the peripheral edge  63 AB and the peripheral edge  63 BB of the pair of air intake openings  63  (see  FIG.  6   ) may be used in combination with each other, or only one of the arrangements may be used. 
     For example, the pressure charger  50  includes only the supercharger in the above-described preferred embodiments. The outboard motor  4  may be provided with a turbocharger (not shown) driven by exhaust gas passing through the exhaust passage  46 . The pressure charger  50  may include only a turbocharger, or may include both a supercharger and a turbocharger. 
     Additionally, an inboard/outboard motor, or an inboard motor, or a waterjet drive may be used as an example of a vessel propulsion apparatus other than the outboard motor  4 . In the inboard/outboard motor, a vessel engine arranged in the same way as the engine  20  is located inside the vessel, and a drive unit including a propulsion unit (propeller  36 , etc.) and a steering assembly is located outside the vessel. The inboard motor is a vessel engine and a drive unit that are built into the hull  2  and in which the propeller  36  is attached to a propeller shaft extending from the drive unit and outwardly from the vessel. In this case, the steering assembly is separately provided. The waterjet drive is arranged to accelerate water taken in from a vessel bottom by a pump, and jet the water from a jet nozzle of a transom stern, and thus obtain a thrust. In this case, the pump is driven by the vessel engine, and the steering assembly includes the jet nozzle and a mechanism that turns the jet nozzle along a horizontal plane. In the vessel engine, a plurality of cylinders  24  may be located in series along the horizontal direction, etc., so that the crankshaft  27  extends in the front-rear direction. 
     Various features described above may be appropriately combined together. 
     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 from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.