Patent Publication Number: US-8118630-B2

Title: Outboard motor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an outboard motor including a catalyst and an exhaust gas concentration sensor. 
     2. Description of Related Art 
     An outboard motor according to a prior art is described in U.S. Patent Application Publication No. 2008/0022669A1. This outboard motor includes an exhaust device. Also, the exhaust device includes a catalytic converter (hereinafter, referred to as “catalyst”) which purifies exhaust gas and an oxygen sensor which detects a concentration of oxygen in exhaust gas. This exhaust device is arranged to purify exhaust gas of an engine by the catalyst and exhaust the exhaust gas into water from a boss of a propeller. This exhaust device is arranged to control an opening degree of an intake passage and a supply amount of fuel, etc., based on results of detection by the oxygen sensor so as to operate the engine with an air-fuel ratio which brings a high purification effect in the catalyst. 
     SUMMARY OF THE INVENTION 
     The inventor of preferred embodiments of the invention described and claimed in the present application conducted an extensive study and research regarding the design and development of an outboard motor, and in doing so, discovered and first recognized new unique challenges and problems created by the interplay and trade-off relationships of the combination of various problems with outboard motors. In view of the inventor&#39;s discovery of these new unique challenges and problems, the inventor further discovered and developed the preferred embodiments of the present invention, described in greater detail below, to provide unique solutions to previously unrecognized and unsolved problems. 
     More specifically, in an exhaust passage of the exhaust device, water or water vapor may be produced. Water is primarily produced by liquefaction of water vapor inside the exhaust passage. Also, water vapor is produced when water entering the exhaust passage from an outlet of the exhaust passage comes into contact with exhaust gas or a wall of the exhaust passage heated by high-temperature exhaust gas. An amount of produced water vapor increases when a pressure inside the exhaust passage excessively decreases. 
     Excessive decreasing of the pressure inside the exhaust passage is caused, for example, when a throttle valve of the engine is rapidly fully closed from a fully opened state and the engine misfires. When the engine misfires, the pressure of exhaust gas to be exhausted from the engine into the exhaust passage decreases. Also, when the engine misfires during running of a hull including the outboard motor, a suctioning force toward the engine is applied to the exhaust gas inside the exhaust passage, such that the pressure inside the exhaust passage further decreases. 
     Liquefaction of water vapor occurs when the pressure inside the exhaust passage becomes relatively high. For example, when the pressure inside the exhaust passage excessively decreases due to misfiring of the engine, a large amount of water vapor is produced. Immediately after this, when the operation of the engine is restarted and the pressure of the exhaust gas increase, the water vapor is liquefied in the exhaust passage. 
     The catalyst and the oxygen sensor may include ceramics. In this case, a portion of the oxygen sensor to be exposed to exhaust gas is made of ceramics. The resistance of ceramics to water is deteriorated by a high temperature. In detail, when water is attached to ceramics at an excessively high temperature, the ceramics may be damaged. Also, the catalyst and the oxygen sensor may have a high temperature during running of an outboard motor. Therefore, water inside the exhaust passage produced by liquefaction of water vapor may be attached to the catalyst and the oxygen sensor at a high temperature. However, when water is attached to the catalyst and the oxygen sensor at a high temperature, these components may be damaged. 
     Thus, the inventor discovered and carefully studied the many varying problems described above, and recognized certain unique and unsolved interrelationships and trade-offs, and the effects of various unique solutions on such diverse and numerous problems. After diligent research and work on such unique problems and novel potential solutions, the preferred embodiments of the present invention were discovered and developed. 
     A preferred embodiment of the present invention provides an outboard motor including an engine, a first exhaust passage, a partition, a communication portion, an exhaust gas concentration sensor, and a catalyst. The engine is arranged to support a crankshaft extending along an up-down direction. The first exhaust passage is connected to the engine and is arranged to expel exhaust gas of the engine into water. The partition is arranged to partition an inside of the first exhaust passage into an upstream side and a downstream side. The communication portion is arranged to cause the upstream side to communicate with the downstream side of the partition in the first exhaust passage. The exhaust gas concentration sensor is arranged on the upstream side of the partition in the first exhaust passage. The catalyst is arranged on an upstream side of the exhaust gas concentration sensor in the first exhaust passage. The exhaust gas concentration sensor detects a concentration of a component of exhaust gas. The exhaust gas concentration sensor detects a concentration of, for example, oxygen (O 2 ), carbon monoxide (CO), carbon dioxide (CO 2 ), total hydrocarbons (THC), or nitrogen oxide (NO x ), for example. The exhaust gas concentration sensor may be primarily made of ceramics, for example. 
     With this configuration, the inside of the first exhaust passage is partitioned by the partition into an upstream side and a downstream side, such that the partition functions as a dam. Therefore, even when water and water vapor are produced in the first exhaust pipe, the amount of water and water vapor to flow into the upstream side of the partition is small. Therefore, it is difficult for water to attach to the exhaust gas concentration sensor and catalyst provided on the upstream side of the partition in the first exhaust passage. As a result, the exhaust gas concentration sensor and the catalyst are prevented from being damaged by attachment of water. 
     In a preferred embodiment of the present invention, to a portion on the downstream side of the partition in the first exhaust passage, an inlet end of a second exhaust passage is connected. An outlet end of the second exhaust passage may communicate with the atmosphere. In this case, exhaust gas in the first exhaust passage is exhausted into an atmosphere through the second exhaust passage. 
     In a preferred embodiment of the present invention, the first exhaust passage includes an exhaust chamber, the inside of which is partitioned by the partition into an upstream side exhaust gas chamber and a downstream exhaust gas chamber. The exhaust gas concentration sensor may be arranged in an upstream side exhaust gas chamber. Also, an inlet end of the second exhaust passage may be connected to the downstream exhaust gas chamber. Also, the first exhaust passage may include an exhaust pipe arranged on an upstream side of the exhaust chamber. In this case, the catalyst may be arranged in the exhaust pipe. 
     In a preferred embodiment of the present invention, the communication portion includes a communication hole provided in the partition and arranged to communicate between both the upstream side and the downstream side of the partition, and an on-off valve provided in the communication hole. In this case, the outboard motor may include a drive mechanism which is arranged to drive the on-off valve, a pressure sensor which is arranged to detect a pressure inside the first exhaust passage, and a control device which is arranged to control the drive mechanism. Also, a detection value of the pressure sensor may be input into the control device. The control device may control the drive mechanism so as to close the on-off valve when the pressure inside the first exhaust passage is not more than a predetermined value. 
     In a preferred embodiment of the present invention, the first exhaust passage includes a downstream side end portion arranged to allow entrance of water from an outside of the outboard motor during idling of the engine. A portion which is higher than a water surface at a time of the idling and near the water surface at a downstream side end portion of the first exhaust passage and the second exhaust passage may be made to communicate with each other by a communication path. Further, the inlet end of the second exhaust passage may be connected to a highest portion in the first exhaust passage. When the first exhaust passage includes an exhaust chamber, the inside of which is partitioned by a partition into an upstream side exhaust gas chamber and a downstream side exhaust gas chamber, the highest portion in the first exhaust passage may be defined by the exhaust chamber. 
     In a preferred embodiment of the present invention, the second exhaust passage includes a sound absorbing chamber which is arranged at a downstream side end portion of the second exhaust passage and is removable from the outboard motor. An inside of the sound absorbing chamber may communicate with the first exhaust passage via the communication path. 
     In a preferred embodiment of the present invention, the second exhaust passage includes a sound absorbing chamber arranged at the downstream side end portion of the second exhaust passage. The sound absorbing chamber may include a plurality of expansion chambers partitioned by partitioning members, and expansion chamber communication holes which are provided in the partition members and communicate the plurality of expansion chambers with each other. Also, the inside of the sound absorbing chamber may communicate with the first exhaust passage via the communication path. Opening areas of the expansion chamber communication holes may be smaller than a smallest passage cross-section area of an upstream side of the sound absorbing chamber. 
     Also, in a preferred embodiment of the present invention, the inlet end of the second exhaust passage is connected to a highest portion in the first exhaust passage. 
     Also, in a preferred embodiment of the present invention, the inlet end of the second exhaust passage is connected to the highest portion in the first exhaust passage, and the highest portion in the first exhaust passage is defined by the exhaust chamber. 
     Other elements, features, steps, characteristics, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an outboard motor of a preferred embodiment of the present invention, drawn in a state in which a portion of an engine cover is omitted. 
         FIG. 2  is an enlarged side view of an engine section of the outboard motor. 
         FIG. 3  is an enlarged plan view of the engine section of the outboard motor. 
         FIG. 4  is a sectional view of an intake surge tank section. 
         FIG. 5  is a side view of an intake duct. 
         FIG. 6  is a sectional view for describing a configuration of an exhaust system. 
         FIG. 7  is a sectional view of an exhaust pipe, along VII-VII line of  FIG. 2 . 
         FIG. 8  is a sectional view of an exhaust chamber. 
         FIG. 9  is an enlarged longitudinal sectional view of a portion of a second exhaust passage, along the IX-IX line of  FIG. 10 . 
         FIG. 10  is an enlarged transverse sectional view of a portion of the second exhaust passage. 
         FIG. 11  is an enlarged longitudinal sectional view of a downstream side end portion of the first exhaust passage and a communication path, along the XI-XI line of  FIG. 10 . 
         FIG. 12  is an enlarged sectional view of a rear end portion of a sound absorbing chamber and a coolant chamber. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an outboard motor  1  of a preferred embodiment of the present invention will be described in detail with reference to  FIG. 1  to  FIG. 12 . 
     The outboard motor  1  of this preferred embodiment is to be attached to a transom board of a hull not shown so as to be steered and tilted via a bracket  2 . Therefore, the outboard motor  1  can be in various postures with respect to the hull in an actual use state; however, in this specification, for the sake of convenience, based on a predetermined reference posture of the outboard motor  1 , up-down, left-right, and front-rear directions are defined. The reference posture is a posture of the outboard motor  1  at a steering angle of zero and a tilt angle of zero with respect to the hull in the horizontal posture. In this condition, when a propulsive force in the forward drive direction is generated from the outboard motor  1 , the hull moves straight ahead. In other words, in this specification, as expressions of directions of the outboard motor  1  and the respective members, the heading direction of a hull when it moves ahead, that is, when it moves straight ahead is simply referred to as the front of the outboard motor  1 , and the side 180 degrees opposite to the front is referred to as the rear side. In addition, the left side of the hull with respect to the heading direction of the hull when the hull moves ahead is referred to as the outboard motor left side or the left side simply, the right side of the hull with respect to the heading direction when the hull moves ahead is referred to as the outboard motor right side or the right side simply. Further, the left-right direction of the outboard motor  1  when the hull moves ahead is referred to as the left-right direction of the outboard motor  1 . 
     Also, in the drawings, an arrow F indicating the forward side of the outboard motor  1  is shown as is appropriate. 
       FIG. 1  is a side view of the outboard motor  1  of a preferred embodiment of the present invention. The outboard motor  1  includes an engine support member  3 , an engine  4 , an upper casing  5 , a lower casing  6 , a propeller  7 , and an engine cover  8 . The engine support member  3  is a plate-shaped member joined to the upper end of a bracket  2 . On the engine support member  3 , the engine  4  is mounted. In addition, to the lower portion of the engine support member  3 , the upper casing  5  is attached. To the lower end of the upper casing  5 , the lower casing  6  is attached. Onto the lower casing  6 , the propeller  7  is supported rotatably. The engine cover  8  covers the engine  4 . In  FIG. 1 , etc., the external shape of the engine cover  8  is indicated by a phantom line, and the internal structure is shown by a see-through engine cover  8 . 
     The engine  4  preferably is a four-cycle four-cylinder engine in this preferred embodiment. The engine  4  is mounted on the engine support member  3  in a posture in which the axis line of the crankshaft  11  extends along the up-down direction. Four cylinders of the engine  4  are positioned behind the crankshaft  11  (opposite side of the hull with respect to the crankshaft  11 ), and are aligned in series along the up-down direction. In the present preferred embodiment, among the four cylinders of the engine  4 , the highest cylinder is referred to as a first cylinder # 1 , and cylinders below the first cylinder # 1  are referred to as, in order from the top, a second cylinder # 2 , a third cylinder # 3 , and a fourth cylinder # 4 . In the engine  4 , the first cylinder # 1 , the third cylinder # 3 , the fourth cylinder # 4 , and the second cylinder # 2  are ignited in this order, for example. 
     The crankshaft  11  is arranged so as to penetrate through the engine  4  in the up-down direction. At an upper end portion of the crankshaft  11 , a flywheel magneto  16  is provided. To the lower end of the crankshaft  11 , a drive shaft  17  is coupled. The drive shaft  17  extends along the up-down direction from the lower end of the engine  4  to the inside of the lower casing  6 . The drive shaft  17  is supported rotatably onto the engine support member  3 , the upper casing  5  and the lower casing  6  via bearings (not shown). The lower end of the drive shaft  17  is coupled to a propeller shaft  19  via a forward-reverse switching mechanism  18 . The propeller  7  rotates integrally with the propeller shaft  19 . 
       FIG. 2  is an enlarged side view of the engine portion, and  FIG. 3  is an enlarged plan view of the engine portion. The engine  4  includes a crank case  12 , a cylinder body  13 , a cylinder head  14 , and a head cover  15 . The crank case  12  and the cylinder body  13  rotatably support the crankshaft  11 . The cylinder head  14  is attached to the cylinder body  13 . The head cover  15  is attached to the cylinder head  14 . The crank case  12 , the cylinder body  13 , the cylinder head  14 , and the head cover  15  are arranged in the front-rear direction of the outboard motor  1  in this order from the forward side of the outboard motor  1 . In addition, the crank case  12 , the cylinder body  13 , the cylinder head  14 , and the head cover  15  are mounted on the engine support member  3 , respectively. 
     In the cylinder body  13 , cylinders  21  (see  FIG. 3 ) constituting first cylinder # 1  to fourth cylinder # 4  are provided and lined up in the up-down direction. As shown in  FIG. 3 , in the cylinder head  14 , an intake port  22  and an exhaust port  23  are preferably provided for each of the cylinders. Further, the cylinder head  14  is provided with intake valves  24  and exhaust valves  25  arranged to open and close these ports  22  and  23 , respectively. The cylinder head  14  is further provided with a valve operating device  26  arranged to drive the intake valve  24  and exhaust valve  25  and an injector  27  for each cylinder arranged to inject fuel into the intake port  22 . 
     The intake ports  22  are provided at the side portion on the outboard motor right side of the cylinder head  14 , that is, at the side portion on the opposite side of the exhaust ports  23  with respect to the left-right direction of the outboard motor  1  as shown in  FIG. 3 . The intake ports  22  extend toward the outboard motor rear side, that is, toward the head cover  15  side, so as to separate from the crank case  12 . The respective inlet end of the intake ports  22  is connected to corresponding intake pipes  32  inside an intake surge tank  31  arranged behind the head cover  15 . The intake surge tank  31  is arranged at the rear end of the engine  4 . The rear end of the engine  4  is an end on the opposite side of the crank case  12  in a plan view. 
     The exhaust ports  23  open on the outer portion (side portion on the outboard motor left side) in the left-right direction of the outboard motor  1  of the cylinder head  14 , and are connected to an exhaust device  51  as shown in  FIG. 3 . The openings of the exhaust ports  23  define exhaust gas outlets  29 . 
     As shown in  FIG. 3 , the exhaust device  51  includes a first exhaust pipe  52 , a second exhaust pipe  53 , a third exhaust pipe  54 , an exhaust chamber  55 , and a main exhaust passage  56 . An upstream end of the first exhaust pipe  52  is connected to the exhaust gas outlets  29 . The second exhaust pipe  53  is connected to a downstream end of the first exhaust pipe  52 . The third exhaust pipe  54  is connected to a downstream end of the second exhaust pipe  53 . The exhaust chamber  55  is connected to a downstream end of the third exhaust pipe  54 . The main exhaust passage  56  is arranged so as to extend downward from a downstream end portion of the exhaust chamber  55 . 
     In the present preferred embodiment, the first exhaust passage  59  is defined by spaces inside the first to third exhaust pipes  52  to  54 , a space inside the exhaust chamber  55 , and the main exhaust passage  56 . Also, in the present preferred embodiment, an exhaust pipe is defined by the first to third exhaust pipes  52  to  54 .  FIG. 2  and  FIG. 3  are drawn such that only an external form of the insides of the first to third exhaust pipes  52  to  54  (corresponding to a portion of the first exhaust passage  59 ) is shown. 
       FIG. 4  is a sectional view for describing a configuration relating to the intake surge tank  31 . The inlet ends of the intake ports  22  open on the end on the outboard motor right side of the rear surface  14   a  of the cylinder head  14  (a surface to which the head cover  15  is connected). The openings of the inlet ends of the intake ports  22  define intake inlets  28  of the engine  4 . The intake inlets  28  are provided on the opposite sides of the exhaust gas outlets  29  in the left-right direction of the outboard motor  1 . The intake inlets  28  are connected to intake holes  31   a  of the intake surge tank  31  attached to the rear surface  14   a  of the cylinder head  14 . The intake holes  31   a  are connected to the respective intake pipes  32  inside the intake surge tank  31 . 
     The intake surge tank  31  has a box-shaped intake surge tank main body  31   b  opening toward the front of the outboard motor  1  (head cover  15  side), and an attaching member  31   c  which closes the opening portion of the intake surge tank main body  31   b . The intake surge tank  31  is attached to the head cover  15  with attaching bolts  31   d.    
     The intake pipes  32  are arranged so as to extend while curving in an arc shape in a plan view. In detail, the intake pipes  32  curve so as to project to the rear side (upper side in  FIG. 4 ) of the outboard motor  1 , that is, in the opposite direction of the crank case  12  with respect to the cylinder head  14  from the intake inlets  28 . Also, the intake pipes  32  curve so as to project to the left side (right side in  FIG. 4 ) of the outboard motor  1 , that is, come closer to the exhaust ports  23  (see  FIG. 3 ) in the left-right direction of the outboard motor  1 . The intake pipes  32  are extended along the side wall  31   e  on the outboard motor right side and the rear wall  31   f  of the suction surge tank main body  31   b  inside the intake surge tank  31 . The intake pipes  32  open within an end portion on the outboard motor rear side inside the intake surge tank  31 . 
     The intake hole  31   a  and the intake pipe  32  are provided for each cylinder, and define an intake passage for each cylinder in cooperation with the intake port  22  of each cylinder. The inlet ends of the intake pipes  32  define intake ports of the engine  4 . As described below, intake passages extend to the head cover  15  side, such that the length of the intake passages can be secured while the first exhaust passage  59  is formed to be long. 
     At the inlet ends of the intake pipes  32 , a variable intake pipe mechanism  33  is provided. The variable intake pipe mechanism  33  includes an auxiliary intake pipe  34  removably connected to the intake pipes  32 , and a pair of servo motors  35  which drives the auxiliary intake pipe  34 . The auxiliary intake pipe  34  is provided for each intake pipe  32  of each cylinder. These auxiliary intake pipes  34  are pivotally supported on a support bracket  36  such that they can move between the connecting position shown by the solid line in  FIG. 4  and the separated position shown by a phantom line in  FIG. 4 . 
     These auxiliary intake pipes  34  are joined to the servo motors  35  via links  37 . These auxiliary intake pipes  34  are driven to turn by the servo motors  35  to be in the connecting position or the separated position. By disposing the auxiliary intake pipes  34  at the connecting position, the intake pipe length becomes relatively long. Also, by moving the auxiliary intake pipes  34  to the separated position, the intake pipe length becomes relatively short. The servo motors  35  are provided at the upper portion and the lower portion of the head cover  15 , respectively, as shown in  FIG. 6 . The servo motor  35  positioned on the upper side drives the first cylinder auxiliary intake pipe  34  and the second cylinder auxiliary intake pipe  34 , and the servo motor  35  positioned on the lower side drives the third cylinder auxiliary intake pipe  34  and the fourth cylinder auxiliary intake pipe  34 . 
     As shown in  FIG. 3 , an intake duct  41  is connected to the upper end of the intake surge tank  31 . The intake duct  41  is arranged to lead the air inside the engine cover  8  to the intake port of the engine  4  (the inlet end of the intake pipes  32  opening inside the intake surge tank  31 ). The intake duct  41  preferably has a U shape as viewed from the outboard motor right side as shown in the side view of  FIG. 5 . That is, the intake duct  41  includes a downstream side horizontal portion  42 , a downstream side vertical portion  43 , an upstream side horizontal portion  44 , and an upstream side vertical portion  45 . 
     As shown in  FIG. 3 , the downstream side horizontal portion  42  extends in the front-rear direction on the upper rear right side of the engine  4 , and the downstream side end portion of the downstream side horizontal portion  42  is connected to an upper end portion of the intake surge tank  31 . In the downstream side horizontal portion  42 , a throttle valve  46  is provided. Also, as shown in  FIG. 3 , the downstream side vertical portion  43  is positioned on the lateral right side of the engine  4 . The downstream side vertical portion  43  extends downward to the vicinity of the lower end portion of the engine  4  from the front end portion of the downstream side horizontal portion  42 . As shown in  FIG. 5 , the upstream side horizontal portion  44  extends forward from the lower end portion of the downstream side vertical portion  43 . In addition, the upstream side vertical portion  45  extends upward from the front end portion of the upstream side horizontal portion  44  to the height of the vicinity of the upper end portion of the engine  4 . 
     As shown in  FIG. 5 , at the upper end portion of the upstream side vertical portion  45 , an air suction port  47  is provided. The air suction port  47  has a tubular shape extending in the up-down direction. An opening shape of the air suction port  47  is has an elongated oval shape in the left-right direction of the outboard motor  1  as shown in  FIG. 3 . 
     In addition, the air suction port  47  is arranged at an upper end portion on the outboard motor front side inside the engine cover  8  surrounding the engine  4 . As shown in  FIG. 1 , the space inside the engine cover  8  communicates with the atmosphere via an air inlet  48  provided on the rear portion on the outboard motor left side of the engine cover  8 . 
       FIG. 6  is a sectional view for describing a configuration of an exhaust system. The main exhaust passage  56  opens in water at the shaft center of the propeller  7 . The main exhaust passage  56  preferably includes a plurality of members. Specifically, the plurality of members of the main exhaust passage  60  include a cylinder body  13  of the engine  2 , an engine support member  3 , an oil pan  65  attached to the lower end of the engine support member  3 , and a pipe  66  attached to the oil pan  65 . Further, the plurality of members of the main exhaust passage  60  include a muffler  67  which is attached to the lower end of the oil pan  65  and extends downward, the upper casing  5  which houses the muffler  67 , and the lower casing  6 . 
       FIG. 7  is a sectional view of an exhaust pipe along VII-VII of  FIG. 2 . The first to third exhaust pipes  52  to  54  are molded preferably by casting into pipe shapes, respectively, for example. The first exhaust pipe  52  has a double pipe structure such that the first exhaust passage  59  is covered by a coolant passage  61 . Also, the second and third exhaust pipes  53  and  54  also have the same double pipe structure as the first exhaust pipe  52 , formed in to a pipe shape preferably by casting, for example. The coolant passage  61  formed inside the first exhaust pipe  52  communicates with a coolant passage (not shown) of the cylinder head  3 . Also, the coolant passage  61  is connected to the coolant passage  64  (see  FIG. 8 ) inside the exhaust chamber  55  via the coolant passages  62  and  63  inside the second exhaust pipe  53  and the third exhaust pipe  54 . 
     Inside the connecting portion between the first exhaust pipe  52  and the second exhaust pipe  53 , a first catalyst  57  is provided. In addition, inside the connecting portion between the second exhaust pipe  53  and the third exhaust pipe  54 , a second catalyst  58  is provided. The first and second catalysts  57  and  58  preferably are made of a so-called ternary catalyst. The ternary catalyst can detoxify hydrocarbon, nitrogen oxide, and carbon monoxide at the time of combustion near a theoretical air-fuel ratio at the same time. The first catalyst  57  is arranged on the opposite side of the crank case  12  across the air suction port  47  as shown in  FIG. 3 . In other words, the first catalyst  57  is arranged on the further front of the outboard motor  1  than the air suction port  47  in a plan view. 
     As shown in  FIG. 2 , the first exhaust pipe  52  collects exhaust gases exhausted from the four exhaust gas outlets  29  of the cylinder head  14  at two points, and further distributes the exhaust gases to four points (four second exhaust pipes  53 ). In detail, the first exhaust pipe  52  includes four upstream portions  52   a  to  52   d , two collecting portions (first and second collecting portions  52   e  and  52   f ), and four downstream portion (first to fourth downstream portions  52   g  to  52   j ). 
     Inlet ends of the four upstream portions  52   a  to  52   d  are respectively connected to the exhaust gas outlets  29  of the four cylinders. An outlet end of the first cylinder upstream portion  52   a  and an outlet end of the fourth cylinder upstream portion  52   d  are connected to the first collecting portion  52   e . Also, an outlet end of the second cylinder upstream portion  52   b  and an outlet end of the third cylinder upstream portion  52   c  are connected to the second collecting portion  52   f . In other words, to the first collecting portion  52   e , the first and fourth cylinder upstream portions  52   a  and  52   d  which are respectively connected to the first cylinder # 1  and the fourth cylinder # 4  to be ignited in ignition periods 360 degrees different from each other are connected. In addition, to the second collecting portion  52   f , the second and third cylinder upstream portions  52   b  and  52   c  respectively connected to the second cylinder # 2  and the third cylinder # 3  to be ignited in ignition periods 360 degrees different from each other are connected. The first and second downstream portions  52   g  and  52   h  are connected to the first collecting portion  52   e  so as to branch from the first collecting portion  52   e . Also, the third and fourth downstream portions  52   i  and  52   j  are connected to the second collecting portion  52   f  so as to branch from the second collecting portion  52   f.    
     As shown in  FIG. 3 , the first and fourth cylinder upstream portions  52   a  and  52   d  are arranged closer to the engine  4  in the left-right direction of the outboard motor  1  than the second and third cylinder upstream portions  52   b  and  52   c . Therefore, the first collecting portion  52   e  is provided at a position closer to the engine  4  than the second collecting portion  52   f . As shown in  FIG. 2 , the first collecting portion  52   e  and the second collecting portion  52   f  are arranged at substantially the same height as that of the central portion in the up-down direction of the cylinder body  13 . Accordingly, the pipe length of the first cylinder upstream portion  52   a  and the pipe length of the fourth cylinder upstream portion  52   d  can be made equal to each other. Also, the pipe length of the second cylinder upstream portion  52   b  and the pipe length of the third cylinder upstream portion  52   c  can be made equal to each other. 
     The first and fourth cylinder upstream portions  52   a  and  52   d  are preferably longer than the second and third cylinder upstream portions  52   b  and  52   c  in a side view shown in  FIG. 2 . On the other hand, the second and third cylinder upstream portions  52   b  and  52   c  are preferably constructed such that the radius of curvature of the bent portions for connection to the cylinder head  14  become higher than the radius of curvature of the first and fourth cylinder upstream portions  52   a  and  56   d  as shown in  FIG. 3 . With this configuration, pipe lengths of the four upstream portions  52   a  to  52   d  are matched with each other. 
     At the inlet ends of the first to fourth cylinder upstream portions  52   a  to  52   d , as shown in  FIG. 7 , an upstream side attaching flange  52   k  arranged to attach the first exhaust pipe  52  to the cylinder head  14  is formed integrally. The inlet ends of the first to fourth cylinder upstream portions  52   a  to  52   d  are connected to each other by the upstream side attaching flange  52   k.    
     On the other hand, the first and second downstream portions  52   g  and  52   h  extend upward and downward as they go to the downstream side (forward of the outboard motor  1 , and toward the crank case  12  side in a side view shown in  FIG. 2 ) from the first collecting portion  52   e  as shown in  FIG. 2 . These first and second downstream portions  52   g  and  52   h  are bent forward of the outboard motor  1  such that their inclination angles with respect to the horizontal become smaller at positions corresponding to a connection portion between the crank case  12  and the cylinder body  13  as viewed from the lateral. A tip portion from the bent portion of the first downstream portion  52   g  which is the upper one of the first and second downstream portions  52   g  and  52   h  inclines forward and downward, and extends straight in a side view. A tip portion from the bent portion of the second downstream portion  52   h  positioned on the lower side inclines forward and upward, and extends straight in a side view. 
     The third and fourth downstream portions  52   i  and  52   j  connected to the second collecting portion  52   f  extend upward and downward, respectively, as they go to the downstream side (forward) from the second collecting portion  52   f  as shown in  FIG. 2 . These third and fourth downstream portions  52   i  and  52   j  are bent such that their inclination angles with respect to the horizontal become smaller than those of the upstream sides at positions corresponding to the connection portion between the crank case  12  and the cylinder body  13  as viewed from the lateral. 
     The inclination angles with respect to the horizontal of tip portions from the bent portions of the third and fourth downstream portions  52   i  and  52   j  are larger than the inclination angles with respect to the horizontal of the first and second downstream portions  52   g  and  52   h . A tip portion from the bent portion of the third downstream portion  52   i  which is the upper one of the third and fourth downstream portions  52   i  and  52   j  inclines forward and upward, and extends straight in a side view. A tip portion from the bent portion of the fourth downstream portion  52   j  positioned on the lower side is inclined forward and downward, and extends straight in a side view. 
     As shown in  FIG. 2 , an outlet end portion of the third downstream portion  52   i  is positioned above an outlet end portion of the first downstream portion  52   g . Also, an outlet end portion of the fourth downstream portion  52   j  is positioned below an outlet end portion of the second downstream portion  52   h . As shown in  FIG. 3 , the outlet end portions of the first to fourth downstream portions  52   g  to  52   j  are bent toward the center in the left-right direction of the outboard motor  1 . 
     The second exhaust pipe  53  is connected to the first exhaust pipe  52  ahead of the crank case  12 , that is, on the opposite side of the cylinder head  3  with respect to the crank case  12  as shown in  FIG. 3 . The second exhaust pipe  53  is configured to extend to the diagonally right front of the engine  2 . The second exhaust pipe  53  is formed preferably by integrally molding by casting the four tubular portions  53   a  and two flanges  53   b  and  53   c  respectively positioned on the upstream side ends and the downstream side ends of these tubular portions  53   a  as shown in  FIG. 6  and  FIG. 7 , for example. 
     The third exhaust pipe  54  is arranged on the lateral right side of the engine  2 , that is, at a position adjacent aside the crank case  12  as shown in  FIG. 3 . The third exhaust pipe  54  extends in the front-rear direction of the outboard motor  1 , that is, a direction in which the crank case  12  and the cylinder body  13  are lined up. Then, the third exhaust pipe  54  connects the second exhaust pipe  53  to the exhaust chamber  55 . The exhaust chamber  55  is positioned on the lateral right side of the cylinder body  13 , that is, on the opposite side of the first exhaust pipe  52  in the left-right direction of the outboard motor  1 . The third exhaust pipe  54  is formed preferably by integrally molding by casting the four tubular portions  54   a  and two flanges  54   b  and  54   c  respectively positioned on the upstream side ends and the downstream side ends of these tubular portions  54   a  as shown in  FIG. 6  and  FIG. 7 , for example. 
     As shown in  FIG. 3 , these first to third exhaust pipes  52  to  54  extend from the exhaust gas outlets  29  in a plan view. Further, the first to third exhaust pipes  52  to  54  define a bypass exhaust pipe which extends along the crank case  12  in the vicinity of the outside (vicinity of the front) of the crank case  12 , and bypasses the engine  4  and extends to the opposite side in the left-right direction of the outboard motor  1  (right side of the outboard motor  1 ). Preferably, the length of the first to third exhaust pipes  52  to  54  (the bypass exhaust pipe) is designed so as to surround the crankshaft  11  at angles not less than 90 degrees in the rotation direction of the crankshaft  11 . 
     As shown in  FIG. 3 , the upstream portion of the first exhaust passage  59  defined inside the first to third exhaust pipes  52  to  54  and the intake passage on the downstream side of the intake surge tank  31  (intake passage defined inside the intake pipe  32 , inside the intake hole  31   a , and inside the intake port  22 ) have a substantially S shape in a plan view. The intake passage on the downstream side of the intake surge tank  31  preferably is an intake passage formed inside the intake pipe  32 , the intake hole  31   a , and the intake port  22 . The first to third exhaust pipes  52  to  54  and the intake passage may have a mirror-reversed S shape in a plan view (that is, an S shape in a bottom view). This mirror-reversed S shape is also included in one mode of “S shape.” In other words, the first to third exhaust pipes  52  to  54  and the intake passage extend opposite to each other in the left-right direction of the outboard motor from the cylinder head  14 . Then, the intake passage curves so as to bypass the cylinder head  14  at the rear portion of the outboard motor. On the other hand, the bypass exhaust pipe defined by the first to third exhaust pipes  52  to  54  curves so as to bypass the engine  4  to the front of the crank case  12  at the front portion of the outboard motor. 
       FIG. 8  is a sectional view of the exhaust chamber  55 . The exhaust chamber  55  preferably has a box shape which opens to the cylinder body  13 . The exhaust chamber  59  is attached to the side portion on the outboard motor right side of the cylinder body  13  such that the opening portion of the exhaust chamber is closed by the cylinder body  13 . On the side portion of the cylinder body  13 , a recess portion  72  which opens to the exhaust chamber  55  (to the right side of the outboard motor  1 ) is provided. The recess portion  72  defines an expansion chamber  71  in conjunction with the exhaust chamber  55 . Accordingly, the expansion chamber  71  has a capacity larger than the inner space of the exhaust chamber  55 . On the lower wall  13   a  of the cylinder body  13  which defines the side wall on the lower side of the recess portion  72 , as shown in  FIG. 6  and  FIG. 8 , the main exhaust passage  56  opens. 
     As shown in  FIG. 5 , near the lower side of the exhaust chamber  55 , the upstream side horizontal portion  44  of the intake duct  41  is positioned. Also, as shown in  FIG. 8 , on the opposite side (near the rear side) of the third exhaust pipe  54  of the exhaust chamber  55 , the downstream side vertical portion  43  of the intake duct  41  is positioned. As shown in  FIG. 6 , the exhaust chamber  55  preferably has a height in the up-down direction that is longer than the width in the front-rear direction to allow the four third exhaust pipes  54  to be connected thereto. 
     As shown in  FIG. 8 , inside the outer wall of the exhaust chamber  55 , a coolant passage  64  is provided. The coolant passage  64  is arranged such that a coolant is supplied from the coolant passage  63  of the third exhaust pipe  54 . Also, the coolant passage  64  is configured to discharge a coolant supplied from the coolant passage  63  of the third exhaust pipe  54  to a coolant discharge passage (not shown) of the cylinder body  13 . 
     As shown in  FIG. 8 , inside the exhaust chamber  55 , a partition  75  is arranged to partition the expansion chamber  71  into an upstream exhaust gas chamber  73  and a downstream exhaust gas chamber  74 . The partition  75  partitions the expansion chamber  71  into the two chambers  73  and  74  in cooperation with a longitudinal wall  76  stood on the cylinder body  13 . In the present preferred embodiment, a partition is preferably defined by a division wall  75  and a longitudinal wall  76 , for example. 
     In the partition  75 , a communicating hole  77  which makes communication between both the gas chambers  73  and  74  is provided. Further, the partition  75  is provided with an on-off valve  78  which opens and closes the communicating hole  77 . The communication hole  77  is positioned at the central portion in the up-down direction of the division wall  75 , that is, at a position spaced downward from the upper wall  79  (see  FIG. 6  and  FIG. 9 ) inside the exhaust chamber  55  in the first exhaust passage  59 . Further, the communication hole  77  is positioned at a central portion of the division wall  75  in the left-right direction of the outboard motor  1 . The opening shape of the communicating hole  77  preferably is an ellipse shape that allows the valve body  80  of the on-off valve  78  to be inserted therein. 
     As shown in  FIG. 6 , the exhaust chamber  55  is arranged at substantially the same height as the engine  4 . The exhaust chamber  55  is provided at the highest position in the first exhaust passage  59 . In other words, in the present preferred embodiment, the highest portion of the first exhaust passage  59  is inside the exhaust chamber  55 . The highest portion of the first exhaust passage  59  has an upstream side exhaust gas chamber  73  and a downstream side exhaust gas chamber  74  partitioned by the division wall  75  and the longitudinal wall  76 , and a communication hole  77  which causes the upstream side exhaust gas chamber  73  to communicate with the downstream side exhaust gas chamber  74 . 
     On the other hand, as shown in  FIG. 8 , the on-off valve  78  preferably is a butterfly valve having a disk-shaped valve body  80  inserted inside the communicating hole  77 . The valve body  80  preferably includes an elongated oval plate in the left-right direction of the partition  75 . The valve body  89  is attached to a valve shaft  81  extending along the partition  75 . The valve shaft  81  is pivotally supported by a bearing  82  and a cover  83  fixed to the partition  75 . In addition, the valve shaft  81  is connected to a drive device  81   a  via a wire. The valve body  89  and the valve shaft  81  rotate according to driving of the drive device  81   a . The drive device  81   a  is controlled by an ECU (Electronic Control Unit)  81   b . For example, the ECU  81   b  controls the drive device  81   a  based on a detection value of the pressure sensor  81   c . In the present preferred embodiment, the ECU  81   b  functions as a control device. 
     The on-off valve  78  is driven by the drive device  81   a  so as to close when the crankshaft  11  rotates in reverse or the pressure inside the first exhaust passage  59  excessively decreases and a high negative pressure is generated inside the exhaust chamber  55 , and to open at other times. A sensor (not shown) for detecting the rotating speed of the crankshaft  11  detects whether the crankshaft  11  has rotated in reverse. Also, the pressure inside the exhaust chamber  55  is detected by a pressure sensor  81   c . For example, when a high negative pressure is generated inside the exhaust chamber  55 , the ECU  81   b  controls the drive device  81   a  to close the on-off valve  78 . Accordingly, the communication hole  77  is closed and a fluid is prevented from flowing between the upstream side exhaust gas chamber  73  and the downstream side exhaust gas chamber  74 . 
     As shown in  FIG. 6 , at the upper end of the exhaust chamber  55 , an oxygen sensor  84  is provided to detect the amount of oxygen in the exhaust gas. In the present preferred embodiment, the oxygen sensor  84  functions as an exhaust gas concentration sensor. The oxygen sensor  84  detects, for example, an amount of oxygen in the exhaust gas from a voltage generated by a difference between an oxygen partial pressure of exhaust gas and an oxygen partial pressure of the atmosphere. The oxygen sensor  84  preferably is a sensor primarily made of ceramic (for example, zirconia). The oxygen sensor  84  may detect an oxygen concentration in exhaust gas as an electric resistance. In this sensor, for example, cerium oxide (CeO 2 ) is preferably used as ceramic. The oxygen sensor  84  is positioned at an upper end portion of the upstream exhaust gas chamber  73 . The oxygen sensor  84  detects an amount of oxygen in the exhaust gas flowing inside the upstream exhaust gas chamber  73 . The oxygen sensor  84  sends the detected amount of oxygen as detection data to an ECU  81   b  (see  FIG. 8 ) of the engine  2 . The ECU  81   b  controls the fuel injection amount of the injector  27  and the ignition timing of the ignition plug (not shown), etc., based on the rotation speed of the engine  2 , the opening degree of the throttle valve  46 , and the amount of oxygen in the exhaust gas detected by the oxygen sensor  84 , etc. 
     Next, a second exhaust passage  91  will be described. As shown in  FIG. 6 , the exhaust device  51  further includes a second exhaust passage  91  which exhausts exhaust gas inside the first exhaust passage  59  into the atmosphere. The second exhaust passage  91  is a passage arranged to expel exhaust gas to the outside of the outboard motor  1  when the speed of the engine  4  is low as in the case during idling. In other words, during idling of the engine  4 , the pressure of exhaust gas to be exhausted from the engine  4  is relatively low. Therefore, when idling the engine  4 , water (indicated by a reference symbol W in  FIG. 6  and  FIG. 11 ) which has entered the inside of the first exhaust passage  59  from an outlet of the first exhaust passage  59  (outlet of the main exhaust passage  56 ) cannot be discharged by the pressure of the exhaust gas. Therefore, in this case, the exhaust gas is exhausted exclusively through the second exhaust passage  91 . 
     As shown in  FIG. 6 , an inlet end portion of the second exhaust passage  91  is connected to the highest portion of the exhaust chamber  55 . The second exhaust passage  91  includes a passage inlet  92 , a first vertical portion  93 , and a sound absorbing chamber  95 . The first vertical portion  93  extends from the passage inlet  92  to a lower end portion of the engine  4  along the exhaust chamber  55 . In addition, the sound absorbing chamber  95  communicates with a lower end of the first vertical portion  93  via a first connecting passage  94  formed in the engine support member  3 . The sound absorbing chamber  95  is arranged at the downstream side end portion of the second exhaust passage  91 . The sound absorbing chamber  95  is connected to a muffler  97  via a communication path  96 . The communication path  96  is a path which causes the inside of the sound absorbing chamber  95  and the inside of the muffler  67  to communicate. 
       FIG. 9  is an enlarged longitudinal sectional view of a portion of the second exhaust passage  91 , along the IX-IX line of  FIG. 10 . The passage inlet  92  is defined by a through hole penetrating through a wall  72   a  on the outboard motor rear side of a recessed portion  72  provided on the cylinder body  13 . The passage inlet  92  is formed at the highest position of the wall  72   a . In other words, the inlet end portion of the second exhaust passage  91  is connected to the highest portion which is on the downstream side of the first and second catalysts  57  and  58  and the oxygen sensor  84  in the first exhaust passage  59 . 
     The first vertical portion  93  includes a first recessed groove  93   a , a second recessed groove  93   b , and a passage hole  93   c . The first recessed groove  93   a  is arranged on the cylinder body  13  so as to open toward the cylinder head  14 . Also, the second recessed groove  93   b  is arranged on the cylinder head  14  so as to be opposed to the first recessed groove  93   a . The passage hole  93   c  is formed at a lower end portion of the cylinder body  13  so as to be connected to lower end portions of the recessed grooves  93   a  and  93   b.    
       FIG. 10  is an enlarged transverse sectional view of a portion of the second exhaust passage  91 . The sound absorbing chamber  95  includes a plurality of partitioning plates  111  to  115 , a plurality of expansion chambers (first to third expansion chambers  116  to  118 ), and a plurality of communication holes (first to third communication holes  119  to  121 ). The inside of the sound absorbing chamber  95  is partitioned by the plurality of partitioning plates  111  to  115  into the first to third expansion chambers  116  to  118 . In addition, the first to third expansion chambers  116  to  118  are made to communicate with each other by the first to third communication holes  119  to  121 . In the present preferred embodiment, the first to third communication holes  119  to  121  define expansion chamber communication holes. 
     The sound absorbing chamber  95  is formed preferably by casting so as to have a hollow box shape as shown in  FIG. 9  and  FIG. 10 . The sound absorbing chamber  95  is placed on a rear end portion of the engine support member  3 . As shown in  FIG. 9 , the sound absorbing chamber  95  is fixed to the engine support member  3  by an attaching bolt  97 . The attaching bolt  97  is threaded to the engine support member  3  while penetrating through an attaching bracket  95   a  of the sound absorbing chamber  95  from the rearward of the sound absorbing chamber  95 . 
     As shown in  FIG. 10 , to both sides in the left-right direction of the outboard motor  1  at a front end portion of the sound absorbing chamber  95 , first and second connecting pipes  101  and  102  arranged to connect the front end portion of the sound absorbing chamber  95  to the engine support member  3  are attached. These connecting pipes  101  and  102  are attached to the sound absorbing chamber  95  such that their center lines extend along the front-rear direction of the outboard motor  1 . The first connecting pipe  101  is arranged on the outboard motor right side, and the second connecting pipe  102  is arranged on the outboard motor left side. The first and second connecting pipes  101  and  102  project forward from the front end surface of the sound absorbing chamber  95 . 
     The projecting portions of the connecting pipes  101  and  102  are removably fitted into two circular holes  103  formed in the engine support member  3 . The sound absorbing chamber  95  is removed from the engine support member  3  by being pulled rearward in a state in which the attaching bolt  97  is removed from the attaching bracket  95   a . By fitting the projecting portions of the two connecting pipes  101  and  102  into the two circular holes  103  from the rearward, the sound absorbing chamber  95  is attached to the engine support member  3  from the rearward. 
     As shown in  FIG. 9 , the first connecting pipe  101  causes the inside of the sound absorbing chamber  95  to communicate with a first connecting passage  94  formed in the engine support member  3 . The first connecting passage  94  includes a circular hole  103  into which the first connecting pipe  101  is fitted and a passage hole  104  extending upward to the passage hole  93   c  of the cylinder body  13  from the front end of the circular hole  103 . 
     As shown in  FIG. 10 , the second connecting pipe  102  causes the inside of the sound absorbing chamber  95  to communicate with a second connecting passage  105  formed in the engine support member  3 . The second connecting passage  105  constitutes a portion of the communication path  96 . The second connecting passage  105  includes a circular hole  103  into which the second connecting pipe  102  is fitted and a passage hole  106  extending downward from the front end of the circular hole  103 . 
     Inside the sound absorbing chamber  95 , as shown in  FIG. 9  and  FIG. 10 , first to third expansion chambers  116  to  118  partitioned by the plurality of partitioning plates  111  to  115  are provided. The first to third expansion chambers  116  to  118  are arranged in the front-rear direction of the outboard motor  1  in this order from the outboard motor forward side. 
     Between the first expansion chamber  116  and the second expansion chamber  117 , a partition plate  111  and a partition plate  112  are arranged. Between the partition plate  111  and the partition plate  112 , a first communication hole  119  arranged to lead exhaust gas from the first expansion chamber  116  to the second expansion chamber  117  is provided. Also, between the second expansion chamber  117  and the third expansion chamber  118 , the partition plate  113 , the partition plate  114 , and the partition plate  115  are arranged. Between the partition plate  113  and the partition plate  114 , a second communication hole  120  arranged to lead exhaust gas from the second expansion chamber  117  to the third expansion chamber  118  is provided. Also, between the partition plate  114  and the partition plate  115 , a third communication hole  121  arranged to lead exhaust gas from the second expansion chamber  117  to the third expansion chamber  118  is provided. 
     The first to third communication holes  119  to  121  preferably are formed like slits, respectively, for example. The opening areas of the first to third communication holes  119  to  121  are smaller than a passage cross-section area of a narrowest portion of the passage on the upstream side of the sound absorbing chamber  95  (the smallest passage cross-section area on the upstream side of the sound absorbing chamber  95 ). In other words, a resistance when exhaust gas passes through the sound absorbing chamber  95  is higher than a resistance when exhaust gas flows in the two passages on the upstream side of the sound absorbing chamber  95 . Accordingly, a concentrated flow of exhaust gas into one passage with a smaller resistance when exhaust gas flows of the two passages can be prevented. In other words, a concentrated flow of exhaust gas into one passage with a smaller resistance when exhaust gas flows of a passage from the exhaust chamber  55  to the sound absorbing chamber  95  and a passage from the muffler  67  to the sound absorbing chamber  95  (corresponding to the communication passage  96 ) can be prevented. 
     As shown in  FIG. 9  and  FIG. 10 , on a rear end portion of the sound absorbing chamber  95 , at the central portion in the width direction of the outboard motor  1 , an exhaust pipe  122  is provided. A projecting side end portion of the exhaust pipe  122  is inserted into an idling exhaust port  8   a  located at the rear end portion of the engine cover  8  as shown in  FIG. 9 . The third expansion chamber  118  of the sound absorbing chamber  95  communicates with the outside of the sound absorbing chamber  95  via the exhaust pipe  122 . Therefore, exhaust gas which entered into the sound absorbing chamber  95  through the first and second connecting pipes  101  and  102  passes through the first to third expansion chambers  116  to  118  and is exhausted to the rearward of the engine cover  8  from the exhaust pipe  122 . Also, when exhaust gas which entered into the sound absorbing chamber  95  passes through the first to third expansion chambers  116  to  118 , noise of the exhaust gas is absorbed. 
     As shown in  FIG. 9  and  FIG. 10 , inside the outer wall (an upper wall  95   b , a lower wall  95   c , a left side wall  95   d , a right side wall  95   e , a front wall  95   f , and a rear wall  95   g ) of the sound absorbing chamber  95 , a coolant passage  123  is defined. Into the coolant passage  123 , a portion of coolant which cooled the engine  1  is supplied by a coolant hose  124  (see  FIG. 12 ). The coolant hose  124  is connected to an end portion on the outboard motor rear side at an upper end portion of the sound absorbing chamber  95 . 
       FIG. 11  is an enlarged longitudinal sectional view of the downstream side end portion of the first exhaust passage  59  and the communication path  96 , along the XI-XI line of  FIG. 10 . The communication path  96  includes the second connecting pipe  102  (see  FIG. 10 ), a second connecting passage  105 , and a communicating pipe  107 . The second connecting passage  105  includes a passage hole  106  extending along the up-down direction. The passage hole  106  opens in the lower surface of the engine support member  3 . A lower end of the passage hole  106  is connected to the muffler  67  positioned below the passage hole  106  by a communicating pipe  107 . Accordingly, the inside of the muffler  67  is made to communicate with the inside of the sound absorbing chamber  95  via the communication path  96 . The communicating pipe  107  is positioned between a side wall  65   a  of an oil pan  65  and the upper casing  5 , and extends along the up-down direction. The communicating pipe  107  is sandwiched by the lower end portion of the engine support member  3  and an upper end portion of the muffler  67 . 
     Water enters a portion lower than the muffler  67  in the first exhaust passage  59  from the propeller  7  side in an operation state in which the pressure of the exhaust gas decreases as in the case during idling. The water surface W 1  of water W which entered the inside of the first exhaust passage  59  reaches a portion near the lower side of the muffler  67 . Therefore, the communication path  96  communicates a portion which is higher than the water surface W 1  in the first exhaust passage  59  and near the water surface W 1  and the second exhaust passage  91  with each other. 
       FIG. 12  is an enlarged sectional view of the rear end portion of the sound absorbing chamber  95  and a coolant chamber  125 . The coolant passage  123  formed in the sound absorbing chamber  95  is connected to the coolant chamber  125  at the lower end portion of the sound absorbing chamber  95 . The coolant chamber  125  is defined by two recesses respectively formed on the rear end portion of the engine support member  3  and the rear end portion of the upper casing  5 . In other words, the rear end portion of the engine support member  3  and the rear end portion of the upper casing  5  are overlapped with each other in the up-down direction. In addition, a recess recessed upward is formed on the portion overlapped with the upper casing  5  at the rear end portion of the engine support member  3 . A recess recessed downward is formed at the portion overlapped with the engine support member  3  at the rear end portion of the upper casing  5 . The spaces inside these two recesses communicate with each other. Accordingly, the coolant chamber  125  is formed. At the lower end portion of the coolant chamber  125 , a pipe  126  arranged to make a coolant as pilot water flow out rearward of the outboard motor  1  is provided. In other words, the coolant passage  123  provided in the sound absorbing chamber  95  is provided at an intermediate portion of the passage which leads pilot water from the engine  1  to the pipe  126 . 
     The pipe  126  is inserted in a through hole  8   b  opened in the rear surface of the engine cover  8 . The pipe  126  projects to the outside of the engine cover  8 . As shown in  FIG. 9 , the pipe  126  is arranged near the lower side of the exhaust pipe  122 . 
     Portions in which the exhaust pipe  122  and the pilot water pipe  126  are exposed of the rear surface of the engine cover  8  are exposed to exhaust gas exhausted from the exhaust pipe  122  and stained with, for example, carbon. In addition, the exposed portions are stained with seawater flowing out from the pilot water pipe  126  and whitened by salt. In the present preferred embodiment, the exhaust pipe  122  and the pilot water pipe  126  are provided close to each other, such that the area to be stained on the engine cover  8  is small. Therefore, it becomes difficult for the external appearance of the outboard motor  1  to be deteriorated, and it is easily cleaned. 
     Technical effects and advantages of the outboard motor  1  of the present preferred embodiment will be illustrated hereinafter. 
     When a hull including the outboard motor  1  runs, most of the exhaust gas of the engine  4  is exhausted into water from an axis portion of the propeller  7  through the first to third exhaust pipes  52  to  54 , the exhaust chamber  55 , and the main exhaust passage  56 . In other words, most of exhaust gas of the engine  4  is exhausted into water from the axis portion of the propeller  7  through the first exhaust passage  59 . In addition, a portion of the exhaust gas which entered the first exhaust passage  59  is exhausted rearward of the engine cover  8  through the second exhaust passage  91 . Also, in an operation state in which the speed of the engine  4  is relatively low as in the case during idling, the pressure of the exhaust gas to be exhausted from the engine  4  is relatively low. Therefore, in this operation state, the outlet of the first exhaust passage  59  is closed by water, and the exhaust gas which entered the first exhaust passage  59  is exhausted to the outside of the outboard motor  1  exclusively through the second exhaust passage  91 . 
     At the lower end portion of the first exhaust passage  59 , water comes into contact with the exhaust gas and the wall (the upper casing  5  and the muffler  6 , etc.,) of the first exhaust passage  59  heated by the exhaust gas at a high temperature, such that water vapor is produced. Water vapor produced inside the first exhaust passage  59  ascends toward the highest portion of the first exhaust passage  59  when the amount of exhaust gas to be exhausted into the first exhaust passage  59  from the engine  4  is relatively small as in the case during idling. Therefore, water vapor at the lower end portion of the first exhaust passage  59  ascends toward the downstream side exhaust gas chamber  74  of the exhaust chamber  55 . In other words, when the amount of exhaust gas to be exhausted into the first exhaust passage  59  from the engine  4  is relatively small, water vapor stagnates in a range from the lower end portion to the upper end portion of the first exhaust passage  59 . 
     In the present preferred embodiment, when the engine  4  is operated at a low engine speed as in the case during idling, water vapor at the lower end portion of the first exhaust passage  59  is pushed out into the communication path  96  by the exhaust gas. Therefore, water vapor at the lower end portion of the first exhaust passage  59  is discharged to the outside of the outboard motor  1  through the communication path  96  and the second exhaust passage  91 . Also, water vapor at the upper end portion of the first exhaust passage  59  is pushed out into the passage inlet  92  of the second exhaust passage  91  from the inside of the exhaust chamber  55  by the exhaust gas. Therefore, water vapor at the upper end portion of the first exhaust passage  59  is discharged to the outside of the outboard motor  1  through the second exhaust passage  91 . Accordingly, the amount of water vapor to flow into the first and second catalysts  57  and  58  and the vicinity of the oxygen sensor  84  is reduced. 
     Also, during operation of the engine at a high engine speed, when the throttle valve  46  is rapidly returned to a fully closed state from a fully opened state, the engine  4  misfires, and the pressure inside the first exhaust passage  59  may decrease excessively. In this case, the atmosphere is suctioned into the first exhaust passage  59  through the second exhaust passage  91  from the outside of the outboard motor  1 . Therefore, the pressure inside the first exhaust passage  59  can be prevented from becoming excessively negative. Therefore, even if water vapor stagnates inside the first exhaust passage  59 , the water vapor can be prevented from being liquefied by decreasing of the pressure. 
     Thus, according to the present preferred embodiment, water vapor produced inside the first exhaust passage  59  is discharged to the outside of the outboard motor  1  through the second exhaust passage  91 . In addition, the pressure inside the first exhaust passage  59  can be prevented from excessively decreasing by suctioning the outside air from the second exhaust passage  91 . As a result, production of water inside the first exhaust passage  59  due to liquefaction of water vapor inside the first exhaust passage  59  can be prevented. Therefore, it is difficult for water to attach to the first and second catalysts  57  and  58  and the oxygen sensor  84 . Therefore, these members can be prevented from being damaged by attachment of water. Therefore, an exhaust device  51  in which it is difficult for water to attach to the first and second catalysts  57  and  58  and the oxygen sensor  84  and these members can be prevented from being damaged by attachment of water, can be provided. 
     In addition, the highest portion of the first exhaust passage  59  is formed by the upstream side exhaust gas chamber  73  and the downstream side exhaust gas chamber  74  partitioned by the division wall  75  and the longitudinal wall  76  and the communication hole  77  which causes the upstream side exhaust gas chamber  73  to communicate with the downstream side exhaust gas chamber  74 . Also, the oxygen sensor  84  is provided in the upstream side exhaust gas chamber  73 , and the inlet end portion of the second exhaust passage  91  is connected to the downstream side exhaust gas chamber  74 . Therefore, even in a state in which the on-off valve  78  in the communication hole  77  is open, the division wall  75  and the longitudinal wall  76  substantially function as a dam, and the amount of water vapor to flow to the oxygen sensor  84  side of the division wall  75  and the longitudinal wall  76  is reduced. As a result, the catalysts  57  and  58  and the oxygen sensor  84  are more reliably prevented from being damaged. 
     By closing the on-off valve  78 , the flow of water from the upstream side exhaust gas chamber  73  to the downstream side exhaust gas chamber  74  can be blocked. Therefore, even if water stagnating at the lower end portion of the first exhaust passage  59  is suctioned by the negative pressure and ascends, this water can be prevented from flowing backward to the upstream side of the on-off valve  78 . 
     The phenomenon in which water ascends inside the first exhaust passage  59  occurs infrequently when the shift position is switched to “reverse” by the forward-reverse switching mechanism  18  during advancing of the hull in order to brake the hull, for example. In other words, when the forward-reverse switching mechanism  18  is switched to the reverse side during advancing of the hull, in a case in which the speed of the hull is high, the propeller  7  is pushed by a strong force of water and cannot rotate in reverse. Instead the drive shaft  17  (engine  2 ) may be rotated in reverse. 
     When the engine  4  is thus rotated in reverse, the piston moves down while the exhaust valve  25  opens, and exhaust gas in the first exhaust passage  59  is suctioned into the cylinders  21 . In addition, the longer the time during which the engine  2  rotates in reverse, the larger the amount of exhaust gas to be suctioned into the engine  4 . However, if the amount of exhaust gas to be suctioned into the engine  4  increases, the negative pressure inside the first exhaust passage  59  becomes higher, and water ascends inside the first exhaust passage  59 . 
     When the hull including the outboard motor  1  is used at sea, seawater enters the inside of the first exhaust passage  59 . When the seawater comes into contact with the catalysts  57  and  58 , the catalysts  57  and  58  are poisoned and deteriorated by Na, Mg, and Cl, etc., of seawater components. When the catalysts  57  and  58  at a high temperature are splashed with water, sudden shrinkage may cause the catalysts  57  and  58  to crack. Further, when water goes upstream in the first exhaust passage  59  and is suctioned into the engine  4 , a so-called water hammer phenomenon occurs and may damage the engine  4 . 
     In the present preferred embodiment, when water ascends inside the first exhaust passage (when the engine  4  rotates in reverse or the pressure inside the exhaust chamber  55  becomes excessively low), the on-off valve  78  in the exhaust chamber  55  is closed. Accordingly, water going upstream can be stopped by the exhaust chamber  55 . Therefore, suctioning of water into the engine  4  through the first to third exhaust pipes  52  to  54  can be reliably prevented. Therefore, the catalysts  57  and  58  can be reliably prevented from being deteriorated by contact with seawater. Further, the catalysts  57  and  58  at a high temperature can be reliably prevented from being suddenly cooled by water and damaged. In addition, an occurrence of a water hammer phenomenon can also be prevented. 
     Water vapor inside the first exhaust passage  59  is produced most near the water surface W 1 . The second exhaust passage  91  communicates with the portion near the upper side of the water surface W 1  inside the first exhaust passage  59  via the communication path  96 . Therefore, water vapor inside the first exhaust passage  59  is exhausted to the outside of the outboard motor  1  from the lower portion of the first exhaust passage  59  near the source of water vapor in addition to the highest portion of the first exhaust passage  59 . Accordingly, the amount of water vapor inside the first exhaust passage  59  can be reliably reduced. 
     It is known that carbon adheres to the wall of the first exhaust passage  59 , and when water vapor comes into contact with sulfur contained in the carbon, sulfuric acid is produced. When sulfuric acid is produced on the wall surface of the first exhaust passage  59 , a member forming the wall surface of the first exhaust passage  59  corrodes. According to the present preferred embodiment, the amount of water vapor inside the first exhaust passage  59  is greatly reduced, such that the corrosion of the wall surface of the first exhaust passage  59  can be minimized as much as possible. 
     Exhaust gas during idling passes through the second exhaust passage  91 , and is exhausted to the outside of the outboard motor  1  from the sound absorbing chamber  95 . Into the sound absorbing chamber  95 , exhaust gas is introduced from both the highest portion and the lower portion of the first exhaust passage  59 , such that the amount of exhaust gas to flow into the sound absorbing chamber  95  is larger than that into other portions. Therefore, the sound absorbing chamber  95  more easily corrodes than other portions. 
     In the present preferred embodiment, the sound absorbing chamber  95  is preferably separate from other members forming the first exhaust passage  59 . Therefore, the sound absorbing chamber  95  can be made of an exclusive material. Further, exclusive surface treatment can be applied to the sound absorbing chamber  95 . Specifically, the sound absorbing chamber  95  can be made of a material with high corrosion resistance. Further, surface treatment for improving corrosion resistance can be applied to the wall surfaces of the first to third expansion chambers  116  to  118 . Accordingly, the corrosion resistance of the sound absorbing chamber  95  can be improved. Also, the sound absorbing chamber  95  is removably attached to the engine support member  3 , such that when the sound absorbing chamber  95  greatly corrodes, the sound absorbing chamber  95  can be replaced with a new one. Therefore, the life of the entirety of the outboard motor  1  can be improved. 
     In addition, the opening areas of the first to third communication holes  119  to  120  provided in the sound absorbing chamber  95  are smaller than the passage cross-section area of the narrowest portion of the passage on the upstream side of the sound absorbing chamber  95 . Therefore, a concentrated flow of the exhaust gas into one of the two passages connected to the sound absorbing chamber  95  (a passage from the exhaust chamber  55  to the sound absorbing chamber  95  and a communication passage  96  from the muffler  67  to the sound absorbing chamber  95 ) can be prevented. Therefore, an amount of sulfuric acid produced in the one passage can be prevented from being increased by the concentrated flow of the exhaust gas. Thus, the advance of corrosion in the one passage can be prevented by the concentrated flow of the exhaust gas. Accordingly, the advance of corrosion can be made slower than in the configuration in which exhaust gas flows into one of the two passages in a concentrated manner. 
     As described above, the exhaust device  51  can prevent the first and second catalysts  57  and  58  and the oxygen sensor  84  from being damaged by attachment of water. Therefore, the outboard motor  1  including the exhaust device  51  can sufficiently purify exhaust gas by the first and second catalysts  57  and  58 , and can exhaust clean exhaust gas for a long period of time. 
     A detailed description was provided of the preferred embodiments of the present invention. However, the preferred embodiments are only specific examples to describe the technical content of the present invention, and the present invention is not to be construed as limited to these specific examples. The spirit and scope of the present invention is restricted only by the appended claims. 
     The present application corresponds to Japanese Patent Application No. 2008-221506 filed in the Japan Patent Office on Aug. 29, 2008, and the entire disclosure of the application is incorporated in its entirety herein by reference. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.