Patent Publication Number: US-2023150634-A1

Title: Outboard motor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-185988 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 an outboard motor. 
     2. Description of the Related Art 
     An outboard motor disclosed in Japanese Unexamined Patent Publication No. 2005-16526 includes an engine holder in which an exhaust-gas emission passage is formed, an engine installed in an upper portion of the engine holder, a propeller driven by the engine, and an exhaust manifold. The engine includes a cylinder block in which a plurality of cylinders arranged in series along a vertical direction are formed and a cylinder head connected to the cylinder block from the rear. A plurality of combustion chambers that respectively correspond to the plurality of cylinders and an exhaust port connected to each of the combustion chambers are formed in the cylinder head. The exhaust manifold is located so as to straddle between a side surface of the cylinder head and a side surface of the engine holder. An exhaust passage that connects each of the exhaust ports of the cylinder block and the exhaust-gas emission passage of the engine holder together is formed in the exhaust manifold. The exhaust passage extends from each of the exhaust ports, is gathered in the exhaust manifold, and is connected to the exhaust-gas emission passage. Exhaust gases generated in each of the combustion chambers in accordance with the operation of the engine flow through the corresponding exhaust port, the exhaust passage, and the exhaust-gas emission passage, and then are discharged outwardly from the outboard motor. 
     SUMMARY OF THE INVENTION 
     The inventor of preferred embodiments of the present invention described and claimed in the present application conducted an extensive study and research regarding an outboard motor, 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. 
     In the outboard motor disclosed in Japanese Unexamined Patent Publication No. 2005-16526, a lower end portion of the exhaust passage in the exhaust manifold is bent, and is connected to the exhaust-gas emission passage of the engine holder from the lateral side because of the structure of the exhaust manifold located so as to straddle between the side surface of the cylinder head and the side surface of the engine holder. In this case, exhaust gas that has flowed downwardly through the exhaust passage and reached the lower end portion of the exhaust passage flow into the exhaust-gas emission passage, and thus is required to change its direction so as to be directed toward the lateral side, and therefore there is a possibility that exhaust pressure loss will be increased. An increase in exhaust pressure loss is a factor that degrades engine performance, and therefore it is preferable to reduce or prevent exhaust pressure loss. 
     Preferred embodiments of the present invention provide outboard motors that each reduce or prevent exhaust pressure loss in an engine. 
     In order to overcome the previously unrecognized and unsolved challenges described above, a preferred embodiment of the present invention provides an outboard motor that includes an engine, a relay exhaust pipe, a downstream exhaust pipe connected to the relay exhaust pipe, and a propeller to be driven by the engine. The engine includes a plurality of cylinders arranged in series along a vertical direction and each including a combustion chamber, a plurality of exhaust ports respectively extending from the combustion chambers of the plurality of cylinders in a sideward direction, and an upstream exhaust pipe connected to the plurality of exhaust ports and extending downwardly. The upstream exhaust pipe includes a lower end portion and an upstream portion at a higher location than the lower end portion and extending linearly downwardly. The lower end portion of the upstream exhaust pipe is connected to the relay exhaust pipe from above. The lower end portion of the upstream exhaust pipe is located closer to the combustion chamber than the upstream portion of the upstream exhaust pipe with respect to a left-right direction. 
     With this structural arrangement, in the outboard motor, the propeller is driven when the engine generates a driving force, and therefore the outboard motor generates a thrust. Exhaust gases generated in each of the combustion chambers of the plurality of cylinders arranged in series in the engine flow sideways through a corresponding exhaust port, and then downwardly flow through the upstream exhaust pipe, and then flow through the relay exhaust pipe and through the downstream exhaust pipe in this order, and are discharged outwardly from the outboard motor. In the upstream exhaust pipe, the lower end portion is located closer to the combustion chamber than the upstream portion, and therefore it is possible to locate a connector portion between the lower end portion of the upstream exhaust pipe and the relay exhaust pipe close to the combustion chamber. Thus, it is possible to prevent the connector portion from protruding outwardly from the outboard motor in the left-right direction, thus making it possible to make the outboard motor compact in the left-right direction. 
     The upstream portion of the upstream exhaust pipe extends linearly downwardly, and therefore air that downwardly flows in the upstream portion is able to reach the lower end portion of the upstream exhaust pipe substantially without changing its direction. Additionally, the lower end portion of the upstream exhaust pipe is connected to the relay exhaust pipe from above, and therefore exhaust gas that has flowed downwardly and reached the lower end portion of the exhaust passage is able to flow into the relay exhaust pipe substantially without changing its direction. This enables exhaust gases from the upstream exhaust pipe to flow smoothly toward the relay exhaust pipe, thus making it possible to reduce or prevent exhaust pressure loss in the engine. 
     In a preferred embodiment of the present invention, all of the plurality of exhaust ports are connected to the upstream portion of the upstream exhaust pipe. 
     With this structural arrangement, it is possible to enable exhaust gases generated in all of the combustion chambers to smoothly flow from the upstream exhaust pipe toward the relay exhaust pipe, and therefore it is possible to reduce or prevent exhaust pressure loss in the engine more advantageously. 
     In a preferred embodiment of the present invention, the engine includes a camshaft operable to intake/exhaust air in the combustion chamber and a cam chain located below the plurality of cylinders to rotate the camshaft. The upstream portion of the upstream exhaust pipe is located at a more sideward location than the cam chain. At least a portion of the lower end portion of the upstream exhaust pipe is located directly under the cam chain. 
     With this structural arrangement, in the upstream exhaust pipe, it is possible to locate the lower end portion closer to the combustion chamber than the upstream portion. 
     In a preferred embodiment of the present invention, an inner surface of the upstream portion of the upstream exhaust pipe includes a first region that is farther away from the combustion chamber than a central axis of the upstream exhaust pipe. An inner surface of the lower end portion of the upstream exhaust pipe includes a second region that is farther away from the combustion chamber than the central axis. The first region and the second region are flush or substantially flush with each other. 
     With this structural arrangement, exhaust gas of the upstream exhaust pipe is able to downwardly flow along the first region of the upstream portion and along the second region of the lower end portion, and thus is able to reach the lower end portion substantially without changing its direction, thus making it possible to reduce or prevent exhaust pressure loss in the engine more advantageously. 
     In a preferred embodiment of the present invention, the engine includes a cylinder head that includes the plurality of exhaust ports and is integral with the upstream exhaust pipe. 
     With this structural arrangement, in the cylinder head of the engine, the plurality of exhaust ports are integral with the upstream exhaust pipe, thus making it possible to make the engine compact. 
     In a preferred embodiment of the present invention, the relay exhaust pipe includes an inner pipe into which the lower end portion of the upstream exhaust pipe is inserted and an outer pipe surrounding the inner pipe. A flow passage in which cooling water flows is defined between the inner pipe and the outer pipe. 
     With this structural arrangement, a portion, to which the lower end portion of the upstream exhaust pipe is connected, of the relay exhaust pipe includes a dual structure provided by the inner pipe and the outer pipe, and therefore the connector portion between the lower end portion of the upstream exhaust pipe and the relay exhaust pipe are thickened. However, it is possible to prevent the connector portion from protruding outwardly from the outboard motor in the left-right direction by disposing the connector portion close to the combustion chamber as described above, and therefore the outboard motor is able to be compact in the left-right direction even if the relay exhaust pipe having a dual structure is used. 
     In a preferred embodiment of the present invention, the engine includes an air intake passage connected to the combustion chamber, a crankshaft extending along a vertical direction, and a rotor attached to an upper end portion of the crankshaft so as to rotate together with the crankshaft. The outboard motor further includes a pressure charger to be driven by transmission of rotation of the rotor so as to compress air flowing through the air intake passage. 
     With this structural arrangement, the rotor is located above the engine, and not below the engine, thus making it possible to secure a space, which is used to locate the lower end portion of the upstream exhaust pipe closer to the combustion chamber than the upstream portion, below the engine. 
     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 rear view of a main portion of an engine in the outboard motor. 
         FIG.  5    is a longitudinal sectional rear view of the outboard motor. 
         FIG.  6    is an enlarged view in which the main portion has been extracted from  FIG.  5   . 
         FIG.  7    is a rear view of the main portion of the outboard motor. 
     
    
    
     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 arranged 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 rear view of a main portion of the engine  20  in a state in which the head cover  30  has been detached.  FIG.  4    shows an exhaust structure  80  included in the air intake/exhaust system  49  of the engine  20 . The exhaust structure  80  includes the exhaust ports  54  and an upstream exhaust pipe  81  located on the left side of these exhaust ports  54 . The cylinder head  29  is elongated in the up-down direction so as to straddle the plurality of (in the present preferred embodiment, four) cylinders  24 . The cylinder head  29  is provided with the concave portions  29 A (a portion of the combustion chamber  28 ) whose number is equal to that of the cylinders  24 . 
     In the following description, the uppermost cylinder  24  among the four cylinders  24  in the up-down direction will be referred to as a first cylinder  24 A if necessary, and the cylinder  24  downward of and next to the first cylinder  24 A will be referred to as a second cylinder  24 B if necessary. The cylinder  24  downward of and next to the second cylinder  24 B will be referred to as a third cylinder  24 C if necessary, and the cylinder  24  downward of and next to the third cylinder  24 C, i.e., the lowermost cylinder  24  will be referred to as a fourth cylinder  24 D if necessary. 
     Referring to the first cylinder  24 A, a pair of intake openings  82  and a pair of exhaust openings  83  are provided in a region, which coincides with the combustion chamber  28  in a rear view, of a rear surface of the cylinder head  29 . The intake opening  82  and the exhaust opening  83  are each a round hole that passes through the cylinder head  29  in the front-rear direction or substantially in the front-rear direction. The single intake valve  31  is located at the single intake opening  82 , and the single exhaust valve  32  is located at the single exhaust opening  83  (not shown). 
     For the intake opening  82  and the exhaust opening  83  that correspond to the single combustion chamber  28 , the pair of exhaust openings  83  arranged side by side along the up-down direction are located at a more leftward location than the pair of intake openings  82  arranged side by side along the up-down direction. The pair of intake openings  82  and the pair of exhaust openings  83  communicate with the combustion chamber  28  from the rear. The intake port  55  is provided as a plurality of (in the present preferred embodiment, eight) intake ports whose number is equal to that of the intake openings  82 , and these intake ports  55  are each, for example, a circular tubular pipe passage, and the single intake port  55  is connected to the single intake opening  82  (not shown). 
     The exhaust port is, for example, a circular tubular pipe passage, and is provided as a pair of exhaust ports for each cylinder  24 , i.e., for each combustion chamber  28  in the cylinder head  29 , and eight exhaust ports  54  in total are provided in the present preferred embodiment. Each of these exhaust ports  54  is connected to the single exhaust opening  83 . Thus, the plurality of exhaust ports are respectively connected to the plurality of combustion chambers  28 , and extend from these combustion chambers  28  in the lateral direction, i.e., more specifically, in the leftward direction. The pair of exhaust ports  54  connected to each of the combustion chambers  28  are arranged side by side in the up-down direction. 
     In the following description, the pair of exhaust ports  54  connected to the combustion chamber  28  of the first cylinder  24 A will be referred to as a pair of first exhaust ports  54 A if necessary, and the pair of exhaust ports  54  connected to the combustion chamber  28  of the second cylinder  24 B will be referred to as a pair of second exhaust ports  54 B if necessary. The pair of exhaust ports  54  connected to the combustion chamber  28  of the third cylinder  24 C will be referred to as a pair of third exhaust ports  54 C if necessary, and the pair of exhaust ports  54  connected to the combustion chamber  28  of the fourth cylinder  24 D will be referred to as a pair of fourth exhaust ports  54 D if necessary. The pair of second exhaust ports  54 B are located downward of and next to the pair of first exhaust ports  54 A, and the pair of third exhaust ports  54 C are located downward of and next to the pair of second exhaust ports  54 B, and the pair of fourth exhaust ports  54 D are located downward of and next to the pair of third exhaust ports  54 C. 
     A space  84  that is elongated in the left-right direction is provided at a lower end portion, which is located at a more downward location than the combustion chamber  28  of the fourth cylinder  24 D, of the cylinder head  29 . A portion of the cam chain  35  and lubricating oil for the cam chain  35  are located in the space  84  as shown in  FIG.  5    that is a longitudinal sectional rear view of the outboard motor  4 . Structures other than the exhaust structure  80  are omitted, and are not shown in  FIG.  5   . The flywheel magneto  34  is attached to the upper end portion of the crankshaft  27  as described above (see  FIG.  2   ). The flywheel magneto  34  is a heavy device, and thus is required to provide the flywheel magneto  34  at a location as low as possible so as not to generate torsional resonance in the crankshaft  27 , and, in accordance with this, the cam chain  35  is located in the space  84  at the lower end portion of the cylinder head  29  in the present preferred embodiment. 
       FIG.  6    is an enlarged view in which the main portion has been extracted from  FIG.  5   . The upstream exhaust pipe  81  extends downwardly along the up-down direction, and its inner surface has a cylindrical shape (for example, a circular cylindrical shape or a substantially circular cylindrical shape) extending in the up-down direction. The upstream exhaust pipe  81  is integral with each of the exhaust ports  54  of the cylinder head  29 . This makes it possible to make the engine  20  compact (particularly in the left-right direction in the present preferred embodiment). In relation to the upstream exhaust pipe  81 , the exhaust structure  80  includes a relay exhaust pipe  85  extending in the up-down direction through the exhaust guide  19  supporting the engine  20  from below and a downstream exhaust pipe  86  that is connected to a lower end of the relay exhaust pipe  85  and that extends downwardly. The upstream exhaust pipe  81 , the relay exhaust pipe  85 , and the downstream exhaust pipe  86  are elements of the exhaust passage  46  discussed above. 
     The upstream exhaust pipe  81  includes a lower end portion  81 A and an upstream portion  81 B located at a higher location than the lower end portion  81 A. The lower end portion  81 A may be an angular cylindrical portion, although the lower end portion  81 A is described definitively as a circular cylindrical portion in the following description. The same applies to the upstream portion  81 B. The lower end portion  81 A is connected to the relay exhaust pipe  85  from above. The relay exhaust pipe  85  includes an inner pipe  85 A into which the lower end portion  81 A is inserted and an outer pipe  85 B surrounding the inner pipe  85 A, and is integral with a left end portion of the exhaust guide  19 . A flow passage  85 C in which cooling water flows is defined between the inner pipe  85 A and the outer pipe  85 B. 
     The flow passage  85 C includes an inlet (not shown) and an outlet (not shown) that are provided in an outer surface of the casing  18 , and extends along the engine  20  as a cooling jacket. A pump (not shown) that operates interlockingly with the rotation of the drive shaft  21  takes in external water from the inlet, and allows the water to flow to the flow passage  85 C, and discharges it from the outlet. Water flowing through the flow passage  85 C cools the engine  20  as cooling water. 
     The lower end portion  81 A of the upstream exhaust pipe  81  includes a circular cylindrical small diameter portion  81 AA defining a lower end of the upstream exhaust pipe  81  and a large diameter portion  81 AB larger in diameter than the small diameter portion  81 AA. A circular annular first stepped portion  85 D is provided at a location slightly lower than an upper end of the inner pipe  85 A in an inner peripheral surface of the inner pipe  85 A of the relay exhaust pipe  85 . An upper end portion of the outer pipe  85 B protrudes more upwardly than an upper end portion of the inner pipe  85 A. The relay exhaust pipe  85  is provided with a circular annular second stepped portion  85 E that extends horizontally from the upper end of the inner pipe  85 A and that is connected to the outer pipe  85 B. 
     The small diameter portion  81 AA faces the first stepped portion  85 D from above in a state of being surrounded by the upper end portion of the inner pipe  85 A. The large diameter portion  81 AB faces the second stepped portion  85 E from above in a state of being surrounded by the upper end portion of the outer pipe  85 B. A gap between the small diameter portion  81 AA and the upper end portion of the inner pipe  85 A is sealed with a sealing member  87 , such as an O ring, and a gap between the large diameter portion  81 AB and the upper end portion of the outer pipe  85 B is sealed with a sealing member  88 , such as an O ring. This prevents water flowing through the flow passage  85 C from leaking from the gap between the lower end portion  81 A and the relay exhaust pipe  85 . Note that the inner diameter of a portion, which is at a more downward location than the first stepped portion  85 D, of the inner pipe  85 A is equal to the inner diameter of the small diameter portion  81 AA, i.e., is equal to the inner diameter of the lower end portion  81 A of the upstream exhaust pipe  81 , or is slightly larger than the inner diameter of the small diameter portion  81 AA. 
     The upstream portion  81 B of the upstream exhaust pipe  81  extends linearly downwardly. The pair of first exhaust ports  54 A are downwardly curved and are connected to the upper end of the upstream portion  81 B from above in a state of having merged together. The pair of second exhaust ports  54 B are downwardly curved and are connected to the upstream portion  81 B from the right side in a state of having merged together. A downstream portion  54 H of the pair of third exhaust ports  54 C is downwardly curved and is connected to the upstream portion  81 B from the right side in a state of having merged together. The downstream portion  54 H of the pair of fourth exhaust ports  54 D is downwardly curved and is connected to the upstream portion  81 B from the right side in a state of having merged together. In other words, all of the plurality of exhaust ports are connected to the upstream portion  81 B. The inner surface of the upstream portion  81 B includes an adjacent region  81 K which extends in the up-down direction and is located near or adjacent to combustion chamber  28  in the left-right direction. 
     In the upstream exhaust pipe  81 , the upstream portion  81 B is at a more leftward location than the cam chain  35  in the space  84  at the lower end portion of the cylinder head  29 , and at least a portion of the lower end portion  81 A is located directly under the cam chain  35 . Therefore, the lower end portion  81 A is located about 5 mm to about 10 mm, for example, closer to the combustion chamber  28  than the upstream portion  81 B in the left-right direction, i.e., the lower end portion  81 A is located at a more rightward location than the upstream portion  81 B. 
     As described above, according to the present preferred embodiment, exhaust gases generated in each of the combustion chambers  28  of the plurality of cylinders  24  arranged in series in the engine  20  leftwardly flow through a corresponding exhaust port  54 , and then downwardly flow through the upstream exhaust pipe  81 , and then flow through the relay exhaust pipe  85  and through the downstream exhaust pipe  86  in this order, and are discharged from the outlet  46 A (see  FIG.  2   ) of the exhaust passage  46 , and are discharged outwardly from the outboard motor  4 . 
     In the upstream exhaust pipe  81 , the lower end portion  81 A is located closer to the combustion chamber  28  than the upstream portion  81 B, and therefore it is possible to locate a connector portion  89  between the lower end portion  81 A of the upstream exhaust pipe  81  and the relay exhaust pipe  85  close to the combustion chamber  28 . This makes it possible to prevent the connector portion  89  from protruding outwardly from the outboard motor  4  in the left-right direction. In this case, it is possible to prevent a portion  90  (see  FIG.  7   ), which is near or adjacent to the connector portion  89 , of the engine cover  17  from protruding outwardly from the outboard motor  4  in the left-right direction, thus making it possible to make the outboard motor  4  compact in the left-right direction. Therefore, in a case in which a plurality of outboard motors  4  are arranged side by side in the left-right direction and are mounted on the vessel  1 , it is possible to prevent interference between the adjacent outboard motors  4 . 
     The rotor  57  is located above the engine  20  as described above, and not below the engine  20 . This makes it possible to secure the space  91 , which is used to locate the lower end portion  81 A of the upstream exhaust pipe  81  closer to the combustion chamber  28  than the upstream portion  81 B, below the engine  20 . 
     The upstream portion  81 B of the upstream exhaust pipe  81  extends linearly downwardly, and therefore air that downwardly flows in the upstream portion  81 B is able to reach the lower end portion  81 A of the upstream exhaust pipe  81  substantially without changing its direction (see the thick solid arrow X 1  in  FIG.  6   ). Additionally, the lower end portion  81 A of the upstream exhaust pipe  81  is connected to the relay exhaust pipe  85  from above, and therefore exhaust gases that have flowed downwardly and have reached the lower end portion  81 A of the exhaust passage are able to flow into the relay exhaust pipe  85  along the shortest path substantially without changing their direction (see the thick solid arrow X 2  in  FIG.  6   ). This enables exhaust gases to flow smoothly toward the relay exhaust pipe  85  from the upstream exhaust pipe  81 , thus making it possible to reduce or prevent exhaust pressure loss in the engine  20 . In an arrangement in which an amount of exhaust gases becomes comparatively large by providing the pressure charger  50  as in the present preferred embodiment, the benefit obtained by reducing or preventing exhaust pressure loss is particularly great. 
     In the present preferred embodiment, all of the plurality of exhaust ports  54  are connected to the upstream portion  81 B. With this arrangement, it is possible to enable exhaust gases generated in all of the combustion chambers  28  to smoothly flow from the upstream exhaust pipe  81  toward the relay exhaust pipe  85 , and therefore it is possible to reduce or prevent exhaust pressure loss in the engine  20  more advantageously. Additionally, it is possible to secure the space  84  used to locate the cam chain  35 . 
     In the present preferred embodiment, a first region  81 C, which is farther away from the combustion chamber  28  than a central axis V of the upstream exhaust pipe  81 , of the inner surface of the upstream portion  81 B and a second region  81 D, which is farther away from the combustion chamber  28  than the central axis V, of the inner surface of the lower end portion  81 A are flush or substantially flush with each other. The central axis V is a virtual line passing through the center of an inner surface  541  of the upstream exhaust pipe  81  (center of a flow-passage cross section of the upstream exhaust pipe  81 ). The adjacent region  81 K is located opposite to the first region  81 C with the central axis V interposed therebetween. 
     With this arrangement, exhaust gas in the upstream exhaust pipe  81  is able to downwardly flow along the first region  81 C of the upstream portion  81 B and along the second region  81 D of the lower end portion  81 A, and thus able to reach the lower end portion  81 A substantially without changing its direction (see the thick solid arrows X 1  and X 2  in  FIG.  6   ). Therefore, it is possible to reduce or prevent exhaust pressure loss in the engine  20  more advantageously. 
     The flow rate becomes larger in proportion to a downward flow in the upstream exhaust pipe  81  because of the fact that exhaust gases emitted from each of the exhaust ports  54  merge together, and therefore an upper portion  81 CA, which has a comparatively small flow rate, of the first region  81 C may be more inclined toward the combustion chamber  28  in proportion to an upward flow. On the other hand, a third region  81 E, which is closer to the combustion chamber  28 , of the inner surface of the upstream portion  81 B curves rightwardly and then extends linearly downwardly. In accordance with this, the central axis V extends along the up-down direction in the upstream portion  81 B, and yet the central axis V downwardly extends in a state of being slightly deviated rightwardly in the lower end portion  81 A. 
     Additionally, the cross-sectional area of the flow passage of the upstream exhaust pipe  81  is invariable or substantially invariable with respect to the upstream portion  81 B and the lower end portion  81 A, and therefore it is possible to reduce or prevent exhaust pressure loss in the engine  20  even more advantageously. The cross section of the flow passage of the lower end portion  81 A may be elongated in, for example, the left-right direction in order to equalize the cross-sectional area of the flow passage of the upstream portion  81 B and the cross-sectional area of the flow passage of the lower end portion  81 A with each other. 
     Additionally, a portion, to which the lower end portion  81 A of the upstream exhaust pipe  81  is connected, of the relay exhaust pipe  85  includes a dual structure provided by the inner pipe  85 A and the outer pipe  85 B, and therefore the connector portion  89  between the lower end portion  81 A of the upstream exhaust pipe  81  and the relay exhaust pipe  85  is thickened. However, it is possible to prevent the connector portion  89  from protruding outwardly from the outboard motor  4  in the left-right direction by locating the connector portion  89  close to the combustion chamber  28  as described above, and therefore the outboard motor  4  is able to be compact in the left-right direction even if the relay exhaust pipe  85  having a dual structure is used. 
     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.