Patent Publication Number: US-8118631-B2

Title: Outboard motor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an outboard motor to be mounted in a boat. 
     2. Description of the Related Art 
     In general, an outboard motor mounted in a boat has an upper casing and a lower casing, and an engine is disposed in the upper casing. An exhaust passage connected to a plurality of cylinders in the engine is disposed to extend from the inside of the upper casing to a bottom portion of the lower casing. The exhaust passage is provided with a catalyst that purifies exhaust gas. 
     In such a construction, exhaust gas flowing out from each cylinder to the exhaust passage is purified in the catalyst, and then discharged into water from the bottom portion of the lower casing. 
     A lower end portion of the exhaust passage is immersed in water. Therefore, water in the lower end portion of the exhaust passage may flow backward to an engine side as a result of negative pressure or the like that is generated in the engine. Especially, a four-stroke engine is largely affected by exhaust pulsation, so water is sucked into an engine side by strong force in the exhaust passage. 
     In order to prevent deterioration of the catalyst, water flowing backward in the exhaust passage must be prevented from adhering to the catalyst. To prevent water adhesion to the catalyst, an outboard motor in which a catalyst is disposed in an upper casing has been developed (for example, refer to JP-A-2000-356123). 
     However, even if a catalyst is simply disposed in the upper casing as indicated in JP-A-2000-356123, water adhesion to the catalyst cannot be sufficiently prevented when water flows backward in the exhaust passage. In addition, with the construction of the outboard motor described in JP-A-2000-356123, the catalyst must be disposed at an even higher position to securely prevent water adhesion to the catalyst. Accordingly, the size of the outboard motor is disadvantageously increased. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems described above, preferred embodiments of the present invention provide an outboard motor that can sufficiently prevent water adhesion to a catalyst while avoiding any increase in size. 
     An outboard motor according to a preferred embodiment of the present invention preferably includes a cowling, an engine body having a plurality of cylinders disposed to be lined vertically in the cowling, a discharge section that is disposed below the cowling and discharges burned gas generated in the plurality of cylinders, a discharge passage that guides burned gas from the plurality cylinders to the discharge section, and a catalyst that purifies the burned gas in the exhaust passage. The exhaust passage includes a plurality of first passages connected to the plurality of cylinders, and disposed to be joined at a flow-joining portion located below the topmost cylinder, a second passage connected to the flow-joining portion and extending above the flow-joining portion, and a third passage that passes above the topmost cylinder from an upper end of the second passage and is connected to the discharge section. The catalyst is disposed in the second passage. 
     In this outboard motor, the engine body is disposed in the cowling. The discharge section, which discharges burned gas in a plurality of cylinders of the engine body to the outside, is disposed in a lower portion of the cowling. Burned gas discharged from a plurality of cylinders of the engine body is guided to the discharge section through the exhaust passage. 
     The exhaust passage includes the first passage, the second passage, and the third passage. Burned gas discharged from each cylinder is guided to the discharge section while sequentially passing through the first to third passages. The catalyst, which purifies burned gas, is disposed in the second passage. 
     In this outboard motor, the third passage is arranged to pass above the topmost cylinder. That is, a portion of the third passage is disposed at a sufficiently high position in the cowling. 
     In this case, when water intrudes from the discharge section into the third passage, the water can be prevented from passing the third passage and intruding into the second passage. Accordingly, water adhesion to the catalyst can be prevented. As a result, lowering of catalyst purification performance can be prevented. 
     The second passage is disposed to be connected to a joining portion of the first passage and to extend higher than the flow-joining portion. The flow-joining portion is located lower than the topmost cylinder. In this case, the catalyst can be disposed lower than the topmost cylinder by disposing the catalyst in the second passage. Accordingly, the catalyst can be disposed in the outboard motor while a vertical height of the outboard motor is prevented from being increased. 
     As a result, water adhesion to the catalyst can be sufficiently prevented while the outboard motor avoids any increase in size. 
     The second passage preferably has a first straight passage disposed to extend vertically on the side of a plurality of cylinders. The catalyst may be disposed in the first straight passage. 
     In this case, an increase in the width of the outboard motor can be prevented by disposing the catalyst in the first straight passage. 
     The third passage may preferably have a second straight passage disposed on the opposite side of the plurality of cylinders from the first straight passage, and a connection passage that is disposed above the topmost cylinder and that connects the second passage and the second straight passage. 
     In this case, the second straight passage is disposed on the opposite side of the plurality of cylinders from the first straight passage. Thus, the plurality of cylinders can be disposed in the center or approximate center of the cowling. Accordingly, stability of the outboard motor can be improved. 
     The connection passage is disposed above the topmost cylinder. Thus, water can be securely prevented from flowing into the second passage from the second straight passage. Accordingly, water adhesion to the catalyst can be securely prevented. 
     A plurality of cylinders and the second straight passage preferably may be provided in a common cylinder block. 
     In this case, the exhaust passage can be integral with the cylinder block. Thus, structure around the engine body can be simplified. 
     The outboard motor may further include a first oxygen sensor disposed at an upstream side of the catalyst in the exhaust passage. 
     In this case, water adhesion to the first oxygen sensor can be securely prevented. Accordingly, the first oxygen sensor can be improved in reliability. As a result, an air-fuel ratio of burned gas can be detected in high precision based on a detected value of the first oxygen sensor. 
     The outboard motor may further include a second oxygen sensor disposed at a downstream side of the catalyst in the second passage. 
     In this case, the second passage is disposed upstream of the third passage. Thus, water adhesion to the second oxygen sensor can be securely prevented. Accordingly, the second oxygen sensor can be improved in reliability. As a result, an air-fuel ratio of burned gas, which has been purified through the catalyst, can be detected with high precision based on a detected value of the second oxygen sensor. Accordingly, a purification rate of burned gas by the catalyst can be detected in high precision. 
     The outboard motor preferably may further include a moisture capture member disposed in the third passage above the topmost cylinder. 
     In this case, when droplets are created by the water intruded from the discharge section into the discharge passage, the droplets can be captured by the moisture capture member. Accordingly, droplet adhesion to the catalyst can be prevented. 
     In the exhaust passage, when a sensor such as an oxygen sensor is disposed at an upstream side of the moisture capture member, droplet adhesion to the sensor can be prevented. Accordingly, the sensor can be sufficiently improved in reliability. 
     The outboard motor may preferably further include a first temperature sensor disposed in a downstream side of the catalyst in the second passage. 
     In this case, the second passage is disposed upstream of the third passage. Thus, water adhesion to the first temperature sensor can be securely prevented. Accordingly, the first temperature sensor can be improved in reliability. As a result, a purification state of burned gas in the catalyst can be detected with high precision based on a detected value of the first temperature sensor. Accordingly, a determination can be easily made whether the catalyst is functioning normally or not. 
     The outboard motor preferably may further include a second temperature sensor disposed in the third passage further below than the topmost cylinder. 
     In this case, water intrusion into the exhaust passage can be detected by the second temperature sensor. 
     The outboard motor may further include a controller arranged to perform water intrusion suppression control to suppress water intrusion from the discharge section into the discharge passage based on a detected value of the second temperature sensor. 
     In this case, water intrusion suppression control can be performed quickly based on the detected value of the second temperature sensor. Accordingly, water is securely prevented from flowing backward in the exhaust passage. 
     The outboard motor preferably may further include a cooling water passage disposed to cover the exhaust passage, and an air vent disposed generally at the highest portion of the cooling water passage. 
     In this case, the exhaust passage can be cooled by cooling water in the cooling water passage. Thus, temperature increases in the catalyst can be prevented. Accordingly, temperature increases in the cowling and components of the engine body can be prevented. 
     In this outboard motor, the air vent preferably is disposed generally at the highest portion of the cooling water passage. In this case, air collected in an upper portion of the cooling water passage can be efficiently discharged. Thus, cooling water can be efficiently supplied to the entire cooling water passage. As a result, the exhaust passage can be efficiently cooled. 
     The outboard motor may further include an intake passage that guides air to a plurality of cylinders, and the intake passage may be disposed to pass between the third passage and the cowling. In this case, the intake passage can be provided without any increase in size of the cowling. 
     The outboard motor preferably may further include a timing belt disposed above the engine body, and a belt tensioner that is disposed above the engine body and applies tension to the timing belt. The third passage may be disposed to pass above the belt tensioner. 
     In this case, expansion of the timing belt in the width direction can be sufficiently limited by the belt tensioner. 
     The third passage is preferably arranged to pass above the belt tensioner. Thus, the third passage can be arranged to pass above a position where the expansion of the timing belt in the width direction is sufficiently squeezed. In this case, when a portion of the third passage is disposed on the opposite side of a plurality of cylinders from the second passage, a portion of the third passage and the second passage can be prevented from being widely separated. Accordingly, an increase in the width of the outboard motor can be prevented. 
     The outboard motor preferably may further include a flywheel magneto cover disposed above the engine body and the timing belt, and the third passage may be disposed to pass between the timing belt and the flywheel magneto cover. 
     In this case, the third passage is cooled by an air current generated in the flywheel magneto cover. Accordingly, a temperature increase in the exhaust passage can be prevented. Thus, a temperature increase of the catalyst can be prevented. 
     The outboard motor preferably may further include the cooling water passage disposed to cover the exhaust passage, and a cooling water supply portion disposed in an area that covers a lower end portion of the first passage of the cooling water passage or a lower end portion of the second passage of the cooling water passage. 
     In this case, the exhaust passage can be cooled by the cooling water passage. The cooling water supply portion is disposed in an area of the cooling water passage that covers the lower end portion of the first passage or the lower end portion of the second passage. That is, the cooling water supply portion is disposed in a lower end portion of the cooling water passage. In this case, the cooling water supply portion is utilized as a discharge section of cooling water, so that cooling water in the cooling water passage can be efficiently discharged from the cooling water supply portion. 
     The lower end portion of the first or second passage preferably may be located below the bottommost cylinder. 
     In this case, when water is collected in the lower end portion of the first or second passage due to condensation or the like, the water flow to the downstream side by burned gas discharged from each cylinder can be prevented. Accordingly, water adhesion to the catalyst can be securely prevented. 
     According to various preferred embodiments of the present invention, when water intrudes from the discharge section into the third passage, the water can be prevented from passing the third passage and intruding into the second passage. Accordingly, water adhesion to the catalyst can be prevented. As a result, a decrease in the catalyst purification performance can be prevented. 
     The catalyst can be disposed lower than the topmost cylinder. Accordingly, the catalyst can be disposed in the outboard motor while an increase in a vertical height of the outboard motor is prevented. 
     As a result, water adhesion to the catalyst can be sufficiently prevented while preventing any increase in size of the outboard motor. 
     Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing an outboard motor according to a first preferred embodiment of the present invention. 
         FIG. 2  is a schematic perspective view of an engine. 
         FIG. 3  is a schematic perspective view of the engine. 
         FIG. 4  is a top view showing a construction of the engine. 
         FIG. 5  is a partial cross-section of the inside of the upper casing as seen from a −Y side. 
         FIG. 6  is a partial cross-section of the upper casing as seen from a +Y side. 
         FIG. 7  is a front view of an engine. 
         FIG. 8  is a rear view of a cylinder block. 
         FIG. 9  is a sectional view taken along the line A-A of FIG.  4 . 
         FIG. 10  is a sectional view taken along the line C-C of  FIG. 9 . 
         FIG. 11  is a sectional view taken along the line B-B of  FIG. 4 . 
         FIG. 12  is a top view of a flywheel magneto cover. 
         FIG. 13  is a partial cross-section showing an inner structure of a flywheel magneto cover. 
         FIG. 14  shows the construction in an upper casing of an outboard motor according to the second preferred embodiment of the present invention. 
         FIG. 15  is a schematic top view of an outboard motor according to the third preferred embodiment of the present invention. 
         FIG. 16  is a schematic top view of an outboard motor according to the fourth preferred embodiment of the present invention. 
         FIG. 17  is a block diagram showing an example of a control system of an outboard motor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an outboard motor according to preferred embodiments of the present invention is described with reference to drawings. 
     In a preferred embodiment described below, a downstream end opening  7   a  is an example of discharge section; a flow-joining pipe  134 , a first exhaust pipe  71 , a second exhaust pipe  72 , third exhaust pipe  73 , an exhaust passage  70 , and an exhaust passage  7  are examples of an exhaust passage; a flow-joining pipe  134  is an example of a first passage; a first exhaust pipe  71  and a second exhaust passage  72  are examples of second passage; a third exhaust pipe  73 , an exhaust passage  70 , and an exhaust passage  7  are examples of a third passage; a second exhaust pipe  72  is an example of a first straight passage; an exhaust passage  70  is an example of a second straight passage; a third exhaust pipe  73  is an example of a connection passage; a flow path  700  is an example of a cooling water passage; an extension pipe  731  is an example of an air vent; an intake pipe  56  is an example of an intake passage; and an ECU  103  is an example of a controller. 
     The above description merely provides non-limiting elements of preferred embodiments of the present invention. Other various elements that have the same or similar constitution or function as described herein may be used. 
     First Preferred Embodiment 
     (1) General Construction of Outboard Motor 
       FIG. 1  is a side view showing an outboard motor according to a first preferred embodiment of the present invention. 
     As shown in  FIG. 1 , an outboard motor  100  according to a preferred embodiment of the present invention preferably includes an upper casing  1 , a lower casing  2 , a clamp bracket  3 , and an exhaust guide  4 . The upper casing  1 , the lower casing  2 , and the clamp bracket  3  are fixed to the exhaust guide  4 . 
     The outboard motor  100  is mounted to a hull  901  of a boat  900  through the clamp bracket  3 . In  FIG. 1  and  FIGS. 2 to 16  described below, as indicated by arrows X, Y, and Z, three directions that are perpendicular to one another are defined as X direction, Y direction, and Z direction. The direction that the X direction arrow points is the front, and its opposite is the rear. The direction that the Z direction arrow points is the top, and its opposite is the bottom. The direction that the respective arrows of X direction, Y direction, and Z direction point is a + side, and its opposite is a − side. 
     An engine  5  is disposed in the upper casing  1 . The engine  5  is fixed to the exhaust guide  4 . A propeller  6  is disposed in a lower portion of the lower casing  2 . An exhaust passage  7  is disposed in the lower casing  2 . The exhaust passage  7  is arranged to extend from the engine  5  through the exhaust guide  4  and the lower casing  2  to a rear end of the propeller  6 . An upper end of the exhaust passage  7  is connected to the after-mentioned exhaust passage  70  (refer to  FIG. 2  and  FIG. 3 ) of the engine  5 . 
     A drive shaft  8  is disposed in the lower casing  2  along a vertical direction. The drive shaft  8  is fixed to a crankshaft  142  (refer to  FIG. 11 ) of the engine  5 . A propeller shaft  9  is fixed to the inside of the propeller  6 . The propeller shaft  9  is connected to a lower portion of the drive shaft  8  through a bevel gear  10 . 
     According to the structure described above, the driving force generated by the engine  5  is transmitted through the drive shaft  8  and the propeller shaft  9  to the propeller  6 . Thus, the propeller rotates in a normal direction or a reverse direction. As a result, a propulsive force to propel the boat  900  forward or backward is generated. Exhaust gas (burned gas) discharged from the engine  5  is discharged into the water from a downstream end opening  7   a  of the exhaust passage  7 . 
     Hereinafter, the engine  5  and its surrounding structure are described in detail with reference to the drawings. 
     (2) Arrangement of Peripheral Devices of Engine 
     Hereinafter, an arrangement of peripheral devices of the engine  5  is described with reference to drawings. 
       FIG. 2  and  FIG. 3  are schematic perspective views showing the engine  5 . 
     As shown in  FIG. 2  and  FIG. 3 , the engine  5  has an engine body  51 . In  FIG. 2 , the engine body  51  is shown in a simplified manner for easier description. 
     A drive pulley  52  is disposed above a front portion of the engine body  51 . The drive pulley  52  is fixed to the crankshaft  142  (refer to  FIG. 11 ). Driven pulleys  53 ,  54  are disposed above a rear portion of the engine body  51 . The driven pulleys  53 ,  54  are fixed to a camshaft (not shown) of the engine  5 . A timing belt  55  is wound around the drive pulley  52  and the driven pulleys  53 ,  54 . In the present preferred embodiment, a belt tensioner  551  is disposed above a center portion of the engine body  51 . The belt tensioner  551  maintains the tension of the timing belt  55 . 
     The exhaust passage  70  is located on a +Y side of the engine body  51 . One end of the first exhaust pipe  71  ( FIG. 2 ) generally in the shape of L is connected to a side surface of the engine body  51  on a −Y side. One end of the second exhaust pipe  72  in the shape of a cylinder is connected to the other end of the first exhaust pipe  71 . A catalyst  11 , described in more detail below, is housed in the first exhaust pipe  71  and the second exhaust pipe  72  (refer to  FIG. 5  and  FIG. 9 ). 
     One end of the third exhaust pipe  73  in the shape of inverted U is connected to the other end of the second exhaust pipe  72 . The other end of the third exhaust pipe  73  is connected to one end of the exhaust passage  70 . The third exhaust pipe  73  is disposed to pass above the timing belt  55 . The extension pipe  731  is disposed in the third exhaust pipe  73 . The extension pipe  731  is described later. 
     In this way, the first and second exhaust pipes  71 ,  72  are disposed on one side of the engine body  51 , and the exhaust passage  70  is disposed on the other side thereof. The third exhaust pipe  73  is arranged to pass above the engine body  51  to connect the second exhaust pipe  72  and the exhaust passage  70 . Accordingly, when water flows backward in the exhaust passage  7  in  FIG. 1 , the water can be prevented from passing in the third exhaust pipe  73  toward an upstream side. 
     As described above, the catalyst  11  (refer to  FIG. 5 ) is housed in the first exhaust pipe  71  and the second exhaust pipe  72 . In other words, in the present preferred embodiment, the catalyst  11  is disposed upstream of the third exhaust pipe  73 . Accordingly, when water flows backward in the exhaust passage  7  in  FIG. 1 , the catalyst  11  can be sufficiently protected against water adhesion. 
     First ends of a plurality of intake pipes  56  (for example, four pipes in the present preferred embodiment) are connected to a side surface of the engine body  51  on the +Y side. Second ends of the plurality of intake pipes  56  are connected to a surge tank  57  disposed on the +Y side of the engine body  51 . A throttle body  58  and a throttle drive motor  59  are disposed on a lower portion of the surge tank  57 . 
       FIG. 4  is a top view showing the construction of the engine  5 .  FIG. 5  is a partial cross-section of the upper casing  1  as seen from the −Y side.  FIG. 6  is a partial cross-section of the upper casing  1  as seen from the +Y side.  FIG. 7  is a front view of the engine  5 . 
     As shown in  FIG. 4  to  FIG. 6 , the engine body  51  includes a cylinder block  101  and a cylinder head  102 . As shown in  FIG. 5  to  FIG. 7 , the ECU (Engine Control Unit)  103  is disposed in front of the cylinder block  101 . 
     As shown in  FIG. 6  and  FIG. 7 , a first end of a communication pipe  104  generally in the shape of L is connected to the throttle body  58  in front of the cylinder block  101 . A second end of the communication pipe  104  is connected to an intake duct  105  of a flywheel magneto cover  200 . The flywheel magneto cover  200  and the intake duct  105  are described later in detail. In  FIG. 7 , a cross-section of the communication pipe  104  is shown. 
     As shown in  FIG. 5 , a starter motor  106  and a starter relay  107  are disposed in an upper portion of aside surface of the cylinder block  101  on the −Y side. An accelerator operation amount sensor  108  and a shift slider  109  are disposed below the starter relay  107 . The shift slider  109  is connected to a shift lever (not shown) through a connection mechanism  110  formed with a shift rod and the like. A rectifier regulator unit  121  is disposed in a side surface of the cylinder head  102  on the −Y side. 
     As shown in  FIG. 5  and  FIG. 6 , a fuel filter  122  ( FIG. 5 ), a high-pressure filter  123  ( FIG. 5 ), a vapor separator tank  124  ( FIG. 6 ), and a canister  125  ( FIG. 6 ) are disposed behind the cylinder head  102 . 
     As shown in  FIG. 4 , a valve timing mechanism (not shown) and an oil control valve  126  to adjust an amount of oil supplied to the valve timing mechanism are disposed in the cylinder head  102 . A thermostat  127 , which controls the temperature of cooling water in the engine  5 , is disposed in an upper surface of the cylinder head  102  on the −X side. An electrical component box  128 , in which various electrical devices are housed, is disposed above the throttle drive motor  59 . 
     (3) Construction of Engine 
     Now, a construction of the engine  5  is described in detail with reference to the drawings. 
       FIG. 8  is a rear view of the cylinder block  101 .  FIG. 9  is a cross-sectional view taken along the line A-A in  FIG. 4 .  FIG. 10  is a cross-sectional view taken along the line C-C in  FIG. 9 .  FIG. 11  is a cross-sectional view taken along the line B-B in  FIG. 4 . 
     As shown in  FIG. 8  and  FIG. 9 , four cylinders  131  are disposed to be lined vertically in a rear portion of the cylinder block  101 . As shown in  FIG. 8 , the intake port  132  and the exhaust port  133  are disposed in each cylinder  131 . The intake port  132  and the exhaust port  133  are provided in the cylinder head  102  (refer to  FIG. 4  to  FIG. 6 ). 
     The intake pipe  56  is connected to each intake port  132 . The flow-joining pipe  134  is connected to the four exhaust ports  133 . As shown in  FIG. 5  and  FIG. 8 , the flow-joining pipe  134  preferably has four branch portions  91  to  94  disposed to extend in the +Y direction and a flow-joining portion  95  disposed to extend in the +X direction. 
     The branch portions  91  to  94  are disposed to be lined in the vertical direction. The flow-joining portion  95  is disposed generally at the same height as the branch portion  94 , which is the bottommost of the branch portions  91  to  94 . The branch portions  91  to  94  are connected to the exhaust port  133 , and the flow-joining portion  95  is connected to the first exhaust pipe  71 . 
     As shown in  FIG. 5  and  FIG. 9 , the catalyst  11  is disposed in a connection portion of the first exhaust pipe  71  and the second exhaust pipe  72 . The catalyst  11  is fixed in the first and second exhaust pipes  71 ,  72 . As the catalyst  11 , a three-way catalyst is preferably used, for example. 
     As shown in  FIG. 5 , in the present preferred embodiment, the first exhaust pipe  71  is attached to the cylinder block  101  through an elastic member  135 . Similarly, the second exhaust pipe  72  is attached to the cylinder block  101  through an elastic member  136 . Accordingly, vibration transmitted from the cylinder block  101  to the catalyst  11  can be dampened. As a result, the reliability of the catalyst  11  can be improved. As the elastic members  135 ,  136 , elastic rubber can be used, for example. 
     As shown in  FIG. 9  and  FIG. 10 , a catalyst cover  137  is attached to cover a side surface of the first exhaust pipe  71  and that of the second exhaust pipe  72  on the −Y side. As shown in  FIG. 10 , the catalyst cover  137  is fixed to the second exhaust pipe  72  (and the third exhaust pipe  73 ) preferably by bolts  750 , for example. The catalyst cover  137  is disposed to cover at least the −Y side of the catalyst from its center. Accordingly, when the engine  5  is under maintenance or the like, a user can be prevented from touching the first and second exhaust pipes  71 ,  72  that are heated by radiant heat of the catalyst  11 . Other effects of the catalyst cover  137  are described later. 
     As shown in  FIG. 5 ,  FIG. 9  and  FIG. 10 , the flow-joining pipe  134 , the first exhaust pipe  71 , the second exhaust pipe  72 , and the third exhaust pipe  73  has the flow path  700 . The flow paths  700  of the flow-joining pipe  134 , the first exhaust pipe  71 , the second exhaust pipe  72 , and the third exhaust pipe  73  are communicated with one another. When the engine  5  is operated, cooling water is supplied in the flow path  700 . Accordingly, a temperature increase of the flow-joining pipe  134 , the first exhaust pipe  71 , the second exhaust pipe  72 , and the third exhaust pipe  73  is prevented. 
     As shown in  FIG. 5 , a cooling water supply portion  711  is located in the lower end portion of the first exhaust pipe  71 . An extension pipe  712  is disposed in the cooling water supply portion  711 . In the present preferred embodiment, cooling water is supplied from a cooling water supply source (not shown) through the extension pipe  712  and the cooling water supply portion  711  to the flow path  700  of the first exhaust pipe  71 . 
     When the engine  5  is not operated, cooling water in the flow path  700  is discharged through the cooling water supply portion  711  and the extension pipe  712 . In the present preferred embodiment, the cooling water supply portion  711  is disposed in the lower end portion of the first exhaust pipe  71 . Accordingly, cooling water in the flow path  700  can be discharged efficiently and securely. As a result, cooling water is sufficiently prevented from remaining in the flow path  700 . 
     As shown in  FIG. 9 , the extension pipe  731  is disposed in the upper surface of the third exhaust pipe  73  so as to communicate between the flow path  700  and the outside of the third exhaust pipe  73 . The extension pipe  731  is communicated to the outside of the upper casing  1  by a hose (not shown). Accordingly, air in the flow path  700  is discharged to the outside of the upper casing  1 . As a result, cooling water can be efficiently circulated in the flow path  700 . 
     As shown in  FIG. 5  and  FIG. 9 , a first oxygen sensor OS 1  is disposed in the first exhaust pipe  71 . The first oxygen sensor OS 1  is disposed on the upstream side of the catalyst  11 . A second oxygen sensor OS 2  and a first temperature sensor TS 1  ( FIG. 5 ) are disposed in the second exhaust pipe  72 . The second oxygen sensor OS 2  and the first temperature sensor TS 1  are disposed on the downstream side of the catalyst  11 . 
     As the first and second oxygen sensors OS 1 , OS 2 , a sensor using a ceramic component can be preferably used, for example. An oxygen sensor including zirconia ceramics can be preferably used, for example. 
     The first oxygen sensor OS 1  detects an oxygen concentration in the first exhaust pipe  71 . The second oxygen sensor OS 2  detects an oxygen concentration in the second exhaust pipe  72 . The first temperature sensor TS 1  detects temperature in the second exhaust pipe  72 . Detected values of the first oxygen sensor OS 1 , the second oxygen sensor OS 2 , and the first temperature sensor TS 1  are supplied to the ECU  103  in  FIG. 7 . 
     The ECU  103  adjusts an air-fuel ratio of mixture in the cylinder  131  ( FIG. 9 ) by controlling a fuel injection device (not shown) or a valve timing mechanism (not shown) based on a detected value of the first oxygen sensor OS 1 . 
     The ECU  103  determines whether or not exhaust gas is properly purified in the catalyst  11  based on a detected value of the second oxygen sensor OS 2 . 
     The ECU  103  drives a fan  226  ( FIG. 9 ) based on a detected value of the first temperature sensor TS 1 . 
     The first oxygen sensor OS 1  is preferably disposed above a bottom cowling  303  ( FIG. 5 ). Accordingly, when water flows in the bottom cowling  303 , water adhesion to the first oxygen sensor OS 1  can be securely prevented. As a result, the reliability of the first oxygen sensor OS 1  can surely be improved. 
     As shown in  FIG. 9 , the exhaust passage  70  is provided in a side portion of the cylinder block  101  on the +Y side. The exhaust passage  70  is arranged to extend vertically on the side of the cylinder  131 . An upper end of the exhaust passage  70  is connected to the third exhaust pipe  73 . A lower end of the exhaust passage  70  is connected to the exhaust passage  7  provided in the exhaust guide  4 . 
     The second temperature sensor TS 2  is disposed in a lower end portion of the exhaust passage  70 . The second temperature sensor TS 2  detects temperature in the exhaust passage  70 . A detected value of the second temperature sensor TS 2  is supplied to the ECU  103 . The ECU  103  determines whether or not water is intruded into the exhaust passage  70  based on a detected value of the second temperature sensor TS 2 . 
     As shown in  FIG. 5  and  FIG. 9 , a communication passage  713 , which communicates between the first exhaust pipe  71  and spaces  401 ,  402  in the exhaust guide  4 , is disposed in the lower end portion of the first exhaust pipe  71 . In this case, when condensation occurs in the first exhaust pipe  71  when the engine  5  is not operating, water can be discharged from the communication passage  713  to the outside of the first exhaust pipe  71 . Accordingly, water adhesion on the first oxygen sensor OS 1  can be prevented. As a result, the reliability of the first oxygen sensor OS 1  can be improved. The space  402  is used for an exhaust passage when the engine  5  idles. 
     As shown in  FIG. 11 , a crankcase  141  is disposed in front of the cylinder block  101 . The crankshaft  142  is arranged to extend vertically in the crankcase  141 . One end of a connecting rod  143 , which is disposed in each cylinder  131  ( FIG. 9 ), is connected to the crankshaft  142 . The other end of connecting rod  143  is connected to a piston (not shown) disposed in each cylinder  131 . 
     An upper end portion of the drive shaft  8  is connected to a lower end portion of the crankshaft  142 . Accordingly, the torque of the crankshaft  142  is transmitted to the drive shaft  8 . 
     As shown in  FIG. 5  and  FIG. 11 , the flywheel magneto  144  is disposed above the crankcase  141 . A rotor (flywheel)  145  of the flywheel magneto  144  is fixed to the crankshaft  142 . The rotor  145  is rotated with the rotation of the crankshaft  142 . Accordingly, electric power is generated in the flywheel magneto  144 . 
     A fin  146  is attached to an upper end portion of the crankshaft  142 . The fin  146  is rotated with the rotation of the crankshaft  142 . Accordingly, heat in the upper casing  1  is discharged to the outside. A heat discharge pathway in the upper casing  1  is described later. 
     The flywheel magneto cover  200  is disposed above the crankcase  141  so as to cover the flywheel magneto  144  and the fin  146 . The flywheel magneto cover  200  is described in detail in later paragraph. 
     As shown in  FIG. 11 , the cylinder block  101  is fixed on the exhaust guide  4 . An upper mount  147  is disposed between the cylinder block  101  and the exhaust guide  4 . Accordingly, the cylinder block  101  can be stabilized on the exhaust guide  4 . An oil pump  148 , which supplies oil to the engine  5 , is disposed between the cylinder block  101  and the exhaust guide  4 . 
     As shown in  FIG. 5 ,  FIG. 6 ,  FIG. 9 , and  FIG. 11 , the upper casing  1  has a top cover  301 , a top cowling  302 , and a bottom cowling  303 . The bottom cowling  303  is fixed to the exhaust guide  4  so as to cover an outer periphery of a lower portion of the engine  5 . The top cowling  302  is fixed to the bottom cowling  303  so as to cover the side and top of the engine  5 . The top cover  301  is attached to an upper surface of the top cowling  302 . 
     As shown in  FIG. 9 , a partition wall  311  is disposed in a center portion of the top cover  301  in the Y direction. The partition wall  311  defines a space  312  and a space  313  between the top cover  301  and the top cowling  302 . 
     In the space  312 , an inlet opening  314  is disposed in an upper surface of the top cowling  302 . In the space  313 , a ventilation opening  315  is disposed in an upper surface of the top cowling  302 . 
     In the present preferred embodiment, air in the outside of the upper casing  1  is supplied through the space  312 , the inlet opening  314 , and the flywheel magneto cover  200  to the communication pipe  104  ( FIG. 7 ). Air in the top cowling  302  is discharged through the flywheel magneto cover  200 , ventilation opening  315 , and the space  313  to the outside of the upper casing  1 . 
     (4) Flywheel Magneto Cover 
     (4-1) Construction of Flywheel Magneto Cover 
     Construction of the flywheel magneto cover  200  is described in detail with reference to the drawings. 
       FIG. 12  is a top view of the flywheel magneto cover  200 .  FIG. 13  is a partial cross-sectional view showing an inner structure of the flywheel magneto cover  200 . 
     As shown in  FIG. 5  to  FIG. 7 ,  FIG. 9 , and  FIG. 11  to  FIG. 13 , the flywheel magneto cover  200  has an upper cover  201  and a lower cover  202 . In  FIG. 13 , a cross-section of the upper cover  201  is shown by hatch pattern. 
     As shown in  FIG. 12 , convex portions  211 ,  212 , which are generally in the shape of U in an XY plane, are disposed on the −X side of the upper cover  201 . The convex portions  211 ,  212  are arranged in a way that the both ends face the +Y side. An elastic member  213  is fitted between the convex portion  211  and the convex portion  212 . 
     As shown in  FIG. 9 , the elastic member  213  is in tight contact with a ceiling surface of top cowling  302 . In the present preferred embodiment, positions of the inlet opening  314  and the convex parts  211 ,  212  are set such that the inlet opening  314  is located inside the elastic member  213  in a XZ plane. 
     As shown in  FIG. 7 ,  FIG. 9 , and  FIG. 11  to  FIG. 13 , an outer wall  203  is arranged to extend in the −Z direction on a lower surface side of the lower cover  202 . As shown in  FIG. 9 , a lower end portion of the outer wall  203  is fixed to the third exhaust pipe  73 . Accordingly, the flywheel magneto cover  200  is fixed to the engine  5 . The driven pulleys  53 ,  54  ( FIG. 2  to  FIG. 4 ), the third exhaust pipe  73 , and the top and side of the flywheel magneto  144  ( FIG. 11 ) are covered by the lower cover  202  and the outer wall  203 . 
     As shown in  FIG. 11  and  FIG. 13 , in the lower cover  202 , an opening  221  is formed on an axial extension of the crankshaft  142 . A space  222  generally in the shape of a cylinder is formed on the opening  221  of the flywheel magneto cover  200 . The crankshaft  142  is inserted in the opening  221 . In the space  222 , the fin  146  is attached to the crankshaft  142 . As shown in  FIG. 11  and  FIG. 12 , on top of the fin  146  in the upper cover  201 , a fin cover  210  in the shape of net having a plurality of openings is disposed. 
     As shown in  FIG. 9  and  FIG. 13 , in the lower cover  202 , an opening  223  is formed in an upper portion of the second exhaust pipe  72 . In the upper cover  201 , an opening  224  ( FIG. 12 ), which has a larger area than the opening  223 , is formed in an upper portion of the opening  223 . As shown in  FIG. 12  and  FIG. 13 , a space  2234  is formed between the opening  223  ( FIG. 13 ) and the opening  224  ( FIG. 12 ). 
     As shown in  FIG. 9  and  FIG. 13 , a first ventilation duct  225  is formed to extend from the space  222  ( FIG. 13 ) to the space  2234  ( FIG. 13 ). As shown in  FIG. 9 , the electric fan  226  is disposed above the opening  223 . 
     As shown in  FIG. 9 , a divider  227  is disposed between the top cowling  302  and the flywheel magneto cover  200  so as to form a space that connects the opening  224  and the ventilation opening  315  (this space is referred to as a second ventilation duct  228 ). 
     As shown in  FIG. 9  and  FIG. 12 , dimensions of the opening  224  and the fan  226  are set so as to form a gap  229  between an inner peripheral surface of the opening  224  and an outer peripheral surface of the fan  226 . The first ventilation duct  225  and the second ventilation duct  228  are communicated by the gap  229 . 
     As shown in  FIG. 9  and  FIG. 13 , the intake duct  105  is arranged to cover a portion of an outer periphery of the first ventilation duct  225 . As shown in  FIG. 7 , an end portion of the intake duct  105  on the +X side is connected to the communication pipe  104 . 
     As shown in  FIG. 5  and  FIG. 13 , an inflow opening  231  is formed between an end portion of the upper cover  201  on the −X side and an end portion of the lower cover  202  on the −X side. The inflow opening  231  communicates the intake duct  105  with the outside of the flywheel magneto cover  200 . 
     (4-2) Intake Passage 
     Hereinafter, an intake passage from the inlet opening  314  to the engine  5  is described. 
     As described above, in the present preferred embodiment, the elastic member  213  ( FIG. 9 ) and the ceiling surface of the top cowling  302  ( FIG. 9 ) are in tight contact. In this case, airflow from the inlet opening  314  ( FIG. 9 ) to the ±X side and the −Y side is prevented by the elastic member  213 . Thus, as indicated by the arrows in  FIG. 9  and  FIG. 12 , air introduced into the intake opening  314  flows to a +Y side of the flywheel magneto cover  200 . 
     As indicated by the arrows in  FIG. 12 , the air, which has flown to the +Y side of the flywheel magneto cover  200 , flows to the −X side of the flywheel magneto cover  200 . As indicated by the arrows in  FIG. 5  and  FIG. 13 , the air flows from the inflow opening  231  into the intake duct  105 . Thereafter, as indicated by the arrows in  FIG. 7 , the air is supplied from the intake duct  105  through the communication pipe  104  and the surge tank  57  to the intake pipe  56 . 
     (4-3) Ventilation Passage 
     A ventilation passage in the top cowling  302  ( FIG. 11 ) is described. 
     The fin  146  ( FIG. 11 ) rotates when the engine  5  is operated. In this case, as indicated by the arrows in  FIG. 11 , air in the top cowling  302  is introduced from the fin cover  210  into the space  222  by the rotation of the fin  146 . 
     As indicated by the arrows in  FIG. 9  and  FIG. 13 , the air in the space  222  ( FIG. 13 ) is discharged into the space  313  ( FIG. 9 ) by the fin  146  through the first ventilation duct  225 , the gap  229  ( FIG. 9 ), the second ventilation duct  228  ( FIG. 9 ), and the ventilation opening  315  ( FIG. 9 ). 
     On the other hand, when the engine  5  stops, the fan  226  ( FIG. 9 ) is driven by the control of the ECU  103  ( FIG. 7 ) if the temperature in the second exhaust pipe  72  detected by the first temperature sensor TS 1  ( FIG. 5 ) increases to be a certain value or more. In this case, as indicated by the arrows in  FIG. 9 , air around the first exhaust pipe  71  and the second exhaust pipe  72  is discharged into the space  313  through the fan  226 , the second ventilation duct  228 , and the ventilation opening  315 . 
     The air discharged into the space  313  is discharged to the outside of the top cover  301  from a discharge section provided in the space  313  or from a gap between the top cover  301  and the top cowling  302 . 
     As described above, ventilation is performed in the top cowling  302 . The ECU  103  stops the drive of the fan  226 , if the temperature in the second exhaust pipe  72  ( FIG. 5 ) detected by the first temperature sensor TS 1  ( FIG. 5 ) falls to be a certain value or less, or if the operation period of the fan  226  reaches a certain duration or more. 
     As described above, in the present preferred embodiment, the catalyst cover  137  is disposed to cover the first and second exhaust pipes  71 ,  72  ( FIG. 9 ) on the −Y side. The catalyst cover  137  is disposed to extend to a lower end portion of the outer wall  203  of the flywheel magneto cover  200 . In this case, the catalyst cover  137  is used as a guide wall to efficiently flow the air around the first and second exhaust pipes  71 ,  72  to the fan  226 , when the top cowling  302  is ventilated. Accordingly, the air heated by the radiant heat of the catalyst  11  can be efficiently discharged to the outside of the top cowling  302 . 
     (5) Effects of the Present Preferred Embodiment 
     (5-1) Effects of the Engine  5   
     (a) Effects Caused By Positional Arrangement of the Catalyst  11  and the First and Second Oxygen Sensors OS 1 , OS 2   
     As shown in  FIG. 9 , the first and second exhaust pipes  71 ,  72  are disposed on one side of the cylinder block  101 , and the exhaust passage  70  is disposed on the other side of the cylinder block  101 . The third exhaust pipe  73  is disposed to connect the second exhaust pipe  72  and the exhaust passage  70 . In the construction described above, the catalyst  11  is disposed to be housed in the first exhaust pipe  71  and the second exhaust pipe  72 . The first oxygen sensor OS 1  is disposed in the first exhaust pipe  71 , and the second oxygen sensor OS 2  is disposed in the second exhaust pipe  72 . 
     In the present preferred embodiment, the third exhaust pipe  73  is arranged to pass above the cylinder block  101 . That is, the third exhaust pipe  73  is disposed sufficiently high in the upper casing  1 . 
     In this case, in a case where water flows in reverse in the exhaust passage  7  ( FIG. 1 ), the water can be securely prevented from passing through the third exhaust pipe  73  toward the upstream side. Accordingly, water adhesion to the catalyst  11 , the first oxygen sensor OS 1 , and the second oxygen sensor OS 2  can be sufficiently prevented. As a result, the catalyst  11 , the first oxygen sensor OS 1 , and the second oxygen sensor OS 2  can be improved in reliability. 
     (b) Effects of Flow-Joining Pipe  134   
     As shown in  FIG. 8 , exhaust gas discharged from each cylinder  131  is collected in the lower portion of the upper casing  1  by the flow-joining pipe  134 . Accordingly, the first and second exhaust pipes  71 ,  72  can be disposed on the side of the cylinder block  101 . As a result, the catalyst  11  can be disposed on the side of the cylinder block  101 , thus upsizing of the outboard motor  100  can be prevented. 
     (c) Effects of Shapes of First Exhaust Pipe  71  and Second Exhaust Pipe  72   
     As shown in  FIG. 9 , a portion of the first exhaust pipe  71  and the second exhaust pipe  72  are disposed to extend vertically on the side of the cylinder  131 . Accordingly, an increase in the width of the engine  5  can be prevented. 
     The first and second exhaust pipes  71 ,  72  and the exhaust passage  70  face each other while interposing the plurality of cylinders  131 . In this case, the plurality of cylinders  131  can be disposed in the center of the upper casing  1 . Accordingly, stability of the outboard motor  100  can be improved. 
     (d) Effect of Shape of Exhaust Passage  70   
     As shown in  FIG. 9 , the exhaust passage  70  is arranged to extend vertically on the side of the cylinder  131  in the cylinder block  101 . Accordingly, an increase in the width of the cylinder block  101  can be prevented. 
     (e) Effects of Positional Arrangement of Third Exhaust Pipe  73   
     As shown in  FIG. 9 , the third exhaust pipe  73  is disposed to pass above the timing belt  55  and below the flywheel magneto cover  200 . In this case, the drive pulley  52 , the driven pulleys  53 ,  54 , the timing belt  55 , and the third exhaust pipe  73  shown in  FIG. 2  and  FIG. 3  do not have to be spaced out. Thus, upsizing of the engine  5  can be prevented. 
     As shown in  FIG. 13 , the third exhaust pipe  73  is disposed to be covered by the flywheel magneto cover  200 . In this case, the third exhaust pipe  73  can be cooled by the air current generated by the fin  146  ( FIG. 11 ) and the fan  226  ( FIG. 9 ) of the flywheel magneto cover  200 . Accordingly, excessive temperature increases in the catalyst  11  can be prevented. 
     (f) Effects of Positional Arrangement of Belt Tensioner  551   
     As shown in  FIG. 9 , the third exhaust pipe  73  is disposed to pass above the belt tensioner  551 . In this case, the third exhaust pipe  73  and the exhaust passage  70  can be connected at the position where the expansion of the timing belt  55  in the width direction is sufficiently limited. Accordingly, the exhaust passage  70  can be formed in the proximity of the cylinder  131 . As a result, downsizing of the cylinder block  101  in the width direction becomes possible. 
     (g) Effects of Positional Arrangement of First Exhaust Pipe  71   
     As shown in  FIG. 5  and  FIG. 8 , the first exhaust pipe  71  is disposed in the way that a bottommost portion of an inner peripheral surface of the first exhaust pipe  71  is positioned lower than the bottommost cylinder  131 . In this case, when water is produced in the first exhaust pipe  71  due to condensation or the like, downstream water flow caused by exhaust gas discharged from each cylinder  131  can be prevented. Accordingly, water adhesion to the catalyst  11  and the oxygen sensor OS 1  can be securely prevented. 
     (5-2) Effects of Flywheel Magneto Cover  200   
     (a) Effects of Fan  226   
     As shown in  FIG. 9 , the electric fan  226  is disposed above the catalyst  11 . In this case, heat generated in the catalyst  11  can be efficiently discharged to the outside of the upper casing  1 . For example, even if ventilation is not performed by the fin  146  when the engine  5  stops operation, heat generated in the catalyst  11  can be efficiently discharged to the outside of the upper casing  1 . Accordingly, a temperature increase in the top cowling  302  can be prevented. As a result, electric components (rectifier regulator unit  121 , etc.) and fuel system components (vapor separator tank  124  etc.) can be prevented from causing defects by heat. 
     (b) Effects of Ventilation Passage 
     In the present preferred embodiment, when the engine  5  is operates, ventilation in the top cowling  302  is performed by the fin  146 . When the engine  5  stops operation, ventilation in the top cowling  302  is performed by the fan  226 . 
     As shown in  FIG. 9 , the second ventilation duct  228  and the ventilation opening  315  are used as a common ventilation passage regardless of ventilation performed by the fin  146  ( FIG. 12 ) or the fan  226  ( FIG. 13 ). 
     In this case, the number of passages used for ventilation can be reduced. Thus, the flywheel magneto cover  200  can be downsized. 
     (c) Effects of Elastic Member  213   
     As shown in  FIG. 9 , the elastic member  213  can prevent air introduced into the intake opening  314  from flowing in the ±X direction. Accordingly, air introduced into the intake opening  314  can be prevented from immediately flowing from the inflow opening  231  ( FIG. 13 ) into the intake duct  105 . 
     In this case, when water flows into the ventilation opening  315  together with air, the water can be prevented from flowing into the intake duct  105 . Accordingly, reliability of the engine  5  can be improved. 
     (d) Effect of Shape of Inflow Opening  231   
     As shown in  FIG. 5 , the inflow opening  231  opens downward. Accordingly, water is securely prevented from flowing into the intake duct  105 . 
     (6) Other Examples 
     In the above preferred embodiment, as shown in  FIG. 9 , the first oxygen sensor OS 1  is preferably disposed in the first exhaust pipe  71 . However, a positional arrangement of the first oxygen sensor OS 1  is not limited to the above example. For example, the first oxygen sensor OS 1  can be disposed in the flow-joining portion  95  ( FIG. 8 ) of the flow-joining pipe  134 . 
     The first oxygen sensor OS 1  is preferably disposed upstream of the catalyst  11  and downstream of the branch portion  94  of the flow-joining pipe  134 . In this case, an average value of air-fuel ratio of exhaust gas discharged from each cylinder  131  can be detected with high precision. 
     In the above preferred embodiment, the second oxygen sensor OS 2  is preferably disposed in the second exhaust pipe  72 . However, the second oxygen sensor OS 2  may not be disposed necessarily. In this case, the ECU  103  may determine whether or not exhaust gas is properly purified in the catalyst  11  based on a detected value of the first temperature sensor TS 1 . 
     In the above-described preferred embodiment, the cooling water supply portion  711  and the extension pipe  712  are disposed in the lower end portion of the first exhaust pipe  71 . However, the cooling water supply portion  711  and the extension pipe  712  may be disposed in the lower end portion of the flow-joining pipe  134 . 
     In the above-described preferred embodiment, the communication passage  713  is disposed in the lower end portion of the first exhaust pipe  71 . However, the communication passage  713  may be disposed in the lower end portion of the flow-joining portion  95 . 
     The third exhaust pipe  73  does not have to pass above the topmost cylinder  131 . It is acceptable as long as a portion of the third exhaust pipe  73  is located above the cylinder  131 . 
     The number of the cylinders  131  does not have to be four, but may be less than or more than four, for example. 
     Two or more of the flow-joining pipe  134 , the first exhaust pipe  71 , the second exhaust pipe  72 , the third exhaust pipe  73 , and the exhaust passage  70  may be integrally formed. 
     In the above-described preferred embodiment, when the temperature in the second exhaust pipe  72  reaches a certain degree or more, the fan  226  is driven by the ECU  103 . However, the condition for driving the fan  226  is not limited to the above example. For example, a temperature sensor may be disposed in the engine body  51 , and the fan  226  may be driven by the ECU  103  when the temperature detected by the temperature sensor reaches a certain degree or more. 
     Second Preferred Embodiment 
       FIG. 14  shows a construction of the upper casing  1  of the outboard motor according to the second preferred embodiment. 
     The outboard motor according to the present preferred embodiment differs from the outboard motor  100  according to the first preferred embodiment in the following points. 
     As shown in  FIG. 14 , in the present preferred embodiment, a moisture capture member  400  is preferably disposed in the third exhaust pipe  73 . The moisture capture member  400  is preferably in the shape of a honeycomb, for example. The moisture capture member  400  is preferably made of metal or ceramic, for example. 
     In the present preferred embodiment, since the moisture capture member  400  is disposed in the third exhaust pipe  73 , moisture in the third exhaust pipe  73  can be surely removed in the moisture capture member  400 . Accordingly, droplets, which are created by water flown into the exhaust passage  70 , can be securely prevented from flowing into the second exhaust pipe  72  and the first exhaust pipe  71  through the third exhaust pipe  73 . As a result, the catalyst  11 , the first oxygen sensor OS 1 , and the second oxygen sensor OS 2  can be sufficiently improved in reliability. 
     Third Preferred Embodiment 
       FIG. 15  is a schematic top view of an outboard motor according to the third preferred embodiment. 
     The outboard motor according to the present preferred embodiment differs from the outboard motor  100  according to the first preferred embodiment in the following points. 
     As shown in  FIG. 15 , in the present preferred embodiment, a first branch portion  1011  and a second branch portion  1012  are preferably formed in the shape of V on the −X side of the cylinder block  101 . In the first branch portion  1011 , a plurality of cylinders (not shown) are disposed to be lined vertically. Similarly, in the second branch portion  1012 , a plurality of cylinders (not shown) are disposed to be lined vertically. 
     A first cylinder head  1021  and a second cylinder head  1022  are disposed on the −X side of the first branch portion  1011  and that of the second branch portion  1012 , respectively. In the same way as in  FIG. 2 , the driven pulleys  53 ,  54  are disposed in the first cylinder head  1021  and the second cylinder head  1022 . In the same way as in  FIG. 2 , the driven pulley  52  is disposed on the +X side of the cylinder block  101 . The timing belt  55  is wound around the drive pulley  52  and the driven pulleys  53 ,  54 . 
     Idler pulleys  561 ,  562  and the belt tensioner  563  are disposed in a center portion of a top surface of the cylinder block  101 . The outer peripheral surface of the timing belt  55  is abutted on the idler pulley  561  in a position between the drive pulley  52  and the driven pulley  54  on the first cylinder head  1021 . The outer peripheral surface of the timing belt  55  is abutted on the idler pulley  562  in a position between the driven pulley  53  on the first cylinder head  1021  and the driven pulley  53  on the second cylinder head  1022 . The outer peripheral surface of the timing belt  55  is abutted on the belt tensioner  563  in a position between the driven pulley  54  on the second cylinder head  1022  and the driven pulley  52 . 
     The surge tank  57  is disposed on the −X side of the first and second cylinder heads  1021 ,  1022 . The surge tank  57  is provided with the throttle body  58  and the plurality of intake pipes  56 . 
     In the same way as in  FIG. 8 , the plurality of intake ports  132  are disposed on the +Y side of the first cylinder head  1021 . In the same way as in  FIG. 8 , the plurality of intake ports  132  are disposed on the −Y side of the second cylinder head  1022 . The intake pipes  56  are connected to the intake ports  132  respectively between the first cylinder head  1021  and the second cylinder head  1022 . 
     The flow-joining pipe  134  similar to that of  FIG. 8  is disposed on a side surface of the first cylinder head  1021  on the −Y side and on a side surface of the second cylinder head  1022  on the +Y side. 
     The flow-joining pipes  134  are connected with the first and second exhaust pipes  71 ,  72  respectively in the same way as in  FIG. 9 . In the same way as in  FIG. 9 , the catalyst  11  (not shown) is disposed in the first and second exhaust pipes  71 ,  72 . 
     Two exhaust passages  70  are provided in the cylinder block  101  between the first cylinder head  1021  and the second cylinder head  1022  in the same way as in  FIG. 9 . 
     In the same way as in  FIG. 9 , the third exhaust pipe  73  is disposed to communicate each of the exhaust passages  70  with each of the second exhaust pipes  72 . In the present preferred embodiment, the third exhaust pipe  73  on the first cylinder head  1021  side is provided to pass above the first branch portion  1011  and the timing belt  55 , and the third exhaust pipe  73  on the second cylinder head  1022  side is disposed to pass above the second branch portion  1012  and the timing belt  55 . 
     Fourth Preferred Embodiment 
       FIG. 16  is a schematic top view of an outboard motor according to the fourth preferred embodiment. 
     The outboard motor according to the present preferred embodiment differs from the outboard motor according to the third preferred embodiment in the following points. 
     As shown in  FIG. 16 , in the present preferred embodiment, the surge tank  57  is disposed on the +X side of the cylinder block  101 . The plurality of intake pipes  56  are disposed on the −Y side of the cylinder block  101  to connect the surge tank  57  and a side surface of the first cylinder head  1021  on the −Y side. The plurality of intake pipes  56  are disposed on the +Y side of the cylinder block  101  to connect the surge tank  57  and a side surface of the second cylinder head  1022  on the +Y side. 
     The flow-joining pipes  134  similar to the one in  FIG. 8  are disposed on a side surface of the first cylinder head  1021  on the +Y side and on a side surface of the second cylinder head  1022  on the −Y side. On the −X side of the cylinder block  101 , the first and second exhaust pipes  71 ,  72  are connected to each of the flow-joining pipes  134 . The catalyst  11  (not shown) is disposed in the first and second exhaust pipes  71 ,  72 . 
     The exhaust passages  70  similar to the one in  FIG. 9  are provided in the first branch portion  1011  on the −Y side and in the second branch portion  1012  on the +Y side. The third exhaust pipe  73  is arranged to communicate each of the exhaust passages  70  with each of the second exhaust pipes  72 . In the present preferred embodiment, the third exhaust pipe  73  on the first branch portion  1011  side is arranged to pass above the first branch portion  1011  and the timing belt  55 , and the third exhaust pipe  73  on the second branch portion  1012  side is arranged to pass above the second branch portion  1012  and the timing belt  55 . 
     Control System 
     According to the control system described below, problems pertaining to general outboard motors can be solved. First, problems pertaining to general outboard motors are described. 
     (1) Problems 
     In a case where opening of a throttle valve of an outboard motor engine is reduced quickly when a boat is traveling at high speed, a hull has a large braking force applied thereto and the boat speed is reduced suddenly. This causes, water in the vicinity of a rear portion of the hull to pass the hull (hereinafter, referred to as the following wave effect). 
     If a position of a gear (hereinafter, referred to as a shift gear), which switches between forward travel and backward travel, is switched from a forward traveling position to a backward traveling position in a state where the hull speed is reduced due to the above braking force, a propeller of the outboard motor rotates so as to push water from the rear to the front. 
     Under such a state, water, which is pushed to the front by the following wave effect and the propeller, may intrude into an exhaust passage from an outlet of exhaust gas. However, in a state where the engine is operated, due to exhaust pressure from the engine, water intruded from the outlet is prevented from reaching a top portion of the outboard motor. 
     On the other hand, when the hull is suddenly reduced in speed, water flows from the front to the rear with respect to the propeller since the hull travels forward through inertia. This water flow applies torque to the propeller. If the shift gear is set in a forward traveling position in such a state, engine speed is determined by the torque applied from the engine to the crankshaft and by the torque applied from water flow to the propeller. 
     In a case where the throttle valve is fully closed when the hull is traveling through inertia, the torque applied from water flow to the propeller becomes larger than the torque applied from the engine to the crankshaft. When the shift gear is changed to a backward position in such a state, the propeller is applied with the torque what is in an opposite direction of the torque applied from the engine to the crankshaft and that is larger than the torque applied from the engine to the crankshaft. Accordingly, the engine is caused to miss and stop. 
     In this case, the crankshaft rotates in reverse by the torque provided from the propeller, and exhaust gas in the exhaust passage flows backward. Accordingly, water intruded from the outlet into the exhaust passage may be sucked further. 
     (2) Control System 
       FIG. 17  is a block diagram showing an example of a control system of the outboard motor  100 . 
     As shown in  FIG. 17 , a control system  1000  preferably includes the ECU  103 , a throttle sensor  601 , a hull speed sensor  602 , an engine speed sensor  603 , an intake pressure sensor  604 , a shift position sensor  605 , the first oxygen sensor OS 1 , the second oxygen sensor OS 2 , the first temperature sensor TS 1 , the second temperature sensor TS 2 , the oil control valve (OCV)  126 , the fan  226 , a fuel injection device  501 , an informing lamp  502 , an ignition device  503 , and an electronic throttle  504 . 
     The throttle sensor  601  is disposed in the throttle drive motor  59  ( FIG. 4 ) and detects a throttle opening of the electronic throttle  504 . The hull speed sensor  602  has a GPS function, for example, and detects the speed of the hull  901  ( FIG. 1 ). The engine speed sensor  603  detects the rotational speed of the engine  5  ( FIG. 1 ) by detecting a rotational angle of the crankshaft  142  ( FIG. 11 ), for example. The intake pressure sensor  604  is disposed in the intake pipe  56  ( FIG. 8 ) or the intake port  132  ( FIG. 8 ), for example, and detects the pressure in the intake pipe  56  or the intake port  132 . The shift position sensor  605  is disposed in a shift slider  109 , for example, and detects a shift position (forward, neutral, or backward) of the shift gear. 
     The fuel injection device  501  is disposed in the intake port  132 , for example, and injects fuel into the intake port  132 . The informing lamp  502  is disposed in a position where it can be visually recognized by an operator of the hull  901  ( FIG. 1 ), and is lit under a certain condition as described later. The ignition device  503  is disposed in the cylinder head  102  ( FIG. 4 ) and performs spark-ignition of fuel-air mixture in the engine  5  ( FIG. 1 ). The electronic throttle  504  is disposed in the intake port  132  ( FIG. 8 ) and adjusts an amount of air introduced to the engine  5  by control of the ECU  103 . 
     In the construction described above, if a change amount of a detected value of the second temperature sensor TS 2  exceeds a certain threshold values per unit time (if temperature is decreased suddenly), the ECU  103  executes a water intrusion suppression control described below. 
     In the water intrusion suppression control, when the throttle opening is a certain threshold value or lower, when the speed of the hull  901  is a certain threshold value or higher, and when a shift position is in a forward position, the ECU  103  sets the shortest overlap period of an intake valve (not shown) and an exhaust valve (not shown) by increasing the throttle opening of the electronic throttle  504  to a certain target value and by adjusting an oil amount of the OCV  126 . 
     Accordingly, torque generated in the engine  5  can be increased. At the same time, an amount of burned gas (EGR gas) that flows backward into the engine  5  can be reduced by shortening the overlap period. As a result, when any of the problems as described above occurs to the outboard motor  100 , engine misfire can be prevented. Accordingly, backflow of water to an upper portion of the outboard motor  100  can be prevented. 
     The certain target value of throttle opening described above is set larger than the certain threshold value of throttle opening described above. The certain target value of throttle opening is a variable set in accordance with a load of the engine  5  that is calculated based on the hull speed and a detected value of the intake pressure sensor  604 . 
     In addition to the control described above, the ECU  103  may control the ignition device  503  to advance an ignition timing of fuel-air mixture in the engine  5  to the proximity of engine knock. 
     The certain target value of throttle opening may be calculated by the ECU  103  in accordance with hull speed, so that the engine speed can be reduced as much as possible while misfire is avoided. 
     In the present preferred embodiment, the ECU  103  preferably sets the appropriate target value of throttle opening in accordance with the hull speed, and adjusts an injection amount of fuel injected by the fuel injection device  501  to adjust an air-fuel ratio to an appropriate value. 
     The ECU  103  determines whether or not exhaust gas is properly purified in the catalyst  11  ( FIG. 9 ), based on a detected value of the second oxygen sensor OS 2  and a detected value of the first temperature sensor TS 1 . When the exhaust gas is determined not to be purified properly in the catalyst  11 , the ECU  103  lights the informing lamp  502 . Accordingly, the operator can recognize a state of the catalyst  11 . 
     The ECU  103  controls the fan  226  based on a detected value of the engine speed sensor  603 . In detail, the ECU  103  actuates the fan  226  when the engine  5  stops. Accordingly, a temperature increase in the top cowling  302  ( FIG. 9 ) can be prevented even when the engine  5  is not driven. 
     The ECU  103  may control the fan  226  based on a detected value of the first temperature sensor TS 1 . Accordingly, a temperature increase in the top cowling  302  ( FIG. 9 ) can be securely prevented. 
     The present invention can be effectively utilized in an outboard motor mounted in a boat. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.