Patent Publication Number: US-8113893-B2

Title: Exhaust device for outboard motor multi-cylinder engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust device of a multi-cylinder engine including a catalyst arranged to purify an exhaust gas discharged from an outboard motor multi-cylinder engine. 
     2. Description of Related Art 
     One example of an exhaust device of a multi-cylinder engine is described in Japanese Published Unexamined Patent Application No. 2005-22509. This exhaust device is provided for a 4-cylinder engine for a two-wheeled motor vehicle, and includes an exhaust pipe which collects exhaust passages of cylinders in one passage. A catalyst is provided on the upstream side of the collecting portion in the exhaust pipe. 
     On the other hand, in U.S. Patent Application Publication No. 2008/022669A1, an example of an outboard motor is disclosed. This outboard motor includes an engine which supports a crankshaft in an up-down direction, a casing positioned below the engine, and a cowling surrounding the engine. The engine is a multi-cylinder engine. The outboard motor includes an exhaust pipe which connects the engine and a main exhaust passage inside the casing for preventing exhaust interference among cylinders of the engine. 
     The lower end of the crankshaft is connected to a drive shaft. The drive shaft is housed in the casing and extends in an up-down direction. The lower end of the drive shaft is coupled to a propeller supported rotatably on the lower end of the casing. 
     The main exhaust passage is formed to penetrate the inside of the casing in the up-down direction. An inlet end of the main exhaust passage is connected to exhaust outlets of the cylinder heads via an exhaust pipe. Also, the outlet end of the main exhaust passage opens in water at the shaft center of the propeller. 
     The exhaust pipe has a tubular portion forming exhaust passages of the cylinders. The plurality of exhaust passages are connected commonly to an exhaust chamber and collected together. The exhaust chamber is connected to the main exhaust passage. 
     The tubular portion includes an exhaust manifold portion positioned between two cylinder heads. The tubular portion further includes a horizontal pipe portion extending from the exhaust manifold portion. In this horizontal pipe portion, two catalysts are disposed in series. 
     SUMMARY OF THE INVENTION 
     The inventor 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 device for a multi-cylinder engine including catalysts provided in the exhaust passages communicating with a plurality of cylinders, a self-ignition phenomenon and knocking may easily occur. The self-ignition phenomenon is a phenomenon in which ignition naturally occurs before ignition is performed with an ignition plug in the engine. 
     If the self-ignition phenomenon occurs, the output of the engine lowers. If knocking frequently occurs, a shock wave generated inside the combustion chamber due to knocking breaks a gas film (boundary layer) on the surface inside the cylinder. 
     If this gas film is broken, flames produced by combustion come into direct contact with the metal surfaces (cylinder inner peripheral surface, piston top surface, and cylinder head surface, etc.) inside the cylinder. These metal surfaces are easily melted by heat when they are directly exposed to the flame. If these metal surfaces are melted, this can finally result in breakage of the engine. 
     If the ignition timing of the engine is delayed to prevent the occurrence of knocking, the torque of the engine lowers, and the temperature of the exhaust gas rises, so that the temperature of the catalyst may become excessively high. 
     One problem that was known was the problem of the catalyst being deteriorated by a so-called sintering phenomenon if it is continuously exposed to an excessively high temperature, and the purifying efficiency is deteriorated. This sintering phenomenon is a phenomenon in which the catalyst is held at a high temperature not less than 850° C. for a long period and noble metals in the catalyst thermally adhere to each other and reduce the surface area of the noble metals. 
     The reason for the easy occurrence of the self-ignition phenomenon and knocking is considered to be the configuration of the exhaust passage. In other words, pressures of exhaust gases discharged from the cylinders are transmitted to the exhaust passages of other cylinders via the exhaust hole, and the pressures in the exhaust passages of the respective cylinders become relatively high. The exhaust resistance is made higher by the catalyst, which is also considered as one of the causes of the higher pressures in the exhaust passages. 
     If the pressures inside the exhaust passages are high, in each cylinder, the exhaust gas becomes more difficult to discharge into the exhaust passage in the exhaust stroke, and the amount of exhaust gas remaining in the cylinder increases. That is, inside the cylinder, a large amount of exhaust gas is introduced due to so-called internal EGR (Exhaust Gas Recirculation). Then, while an exhaust gas at a high temperature remains in the cylinder, in an intake stroke, new air is suctioned into the cylinder. This new air (intake air) is mixed with the exhaust gas at a high temperature inside the cylinder, and as a result, the temperature of the intake air is raised by the heat of the exhaust gas. 
     If the temperature of the intake air inside the cylinder becomes excessively high, abnormal combustion such as self-ignition and knocking easily occurs. 
     To reduce the pressure in the exhaust passage, reduction of the exhaust resistance in the exhaust passage was found to be effective. However, in the outboard motor, the space which can be secured for exhaust inside the outboard motor is limited. In this limited space, it is not easy to realize a layout of exhaust passages in which the exhaust resistance can be reduced. 
     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 exhaust device for an outboard motor engine having a plurality of cylinders. The exhaust device includes an exhaust passage having a first end that is connected to the engine and a second end, and an exhaust chamber which is connected to the second end of the exhaust passage and to a main exhaust passage positioned below the engine. The exhaust passage includes a plurality of upstream portions each provided for a respective one of the plurality of cylinders and including inlet ends connected to exhaust gas outlets of the plurality of cylinders whose exhaust valve opening periods are different, a collecting portion arranged to connect outlet ends of the upstream portions to each other, and a plurality of downstream portions which are branched from the collecting portion and connected commonly to the exhaust chamber. A plurality of catalysts is provided and one of the catalysts is provided in each of the plurality of downstream portions. 
     With this configuration, exhaust gases discharged to the upstream portions of the exhaust passage from the respective cylinders of the engine pass through the collecting portion and are distributed to the plurality of downstream portions, and flow into the catalysts. Therefore, as compared to an exhaust device in which the total amount of the exhaust gas discharged from one cylinder flows into one catalyst, the exhaust resistance can be reduced. Exhaust gases do not flow simultaneously into the downstream portions from the plurality of cylinders, so that the influence of the pressures of exhaust gases of other cylinders, that is, exhaust interference, can be minimized. 
     Therefore, as compared to an exhaust device configured such that exhaust gases of all cylinders pass through one catalyst, the exhaust resistance is greatly reduced and minimized, and in addition, exhaust interference does not occur. Accordingly, the pressure in the exhaust passage is effectively lowered, so that the amount of exhaust gas remaining inside the cylinders due to the internal EGR is greatly reduced and minimized. 
     As a result, the temperature of intake air suctioned into the cylinders in the intake stroke becomes relatively low, so that an occurrence of abnormal combustion such as the above-described self-ignition and knocking is reliably prevented. 
     In addition, the structure of the exhaust passage having the upstream portions for the cylinders, a collecting portion which collects the upstream portions, and a plurality of downstream portions branched from the collecting portion, can be provided without requiring a large space. Therefore, the exhaust passage which can sufficiently lower the exhaust resistance can be provided in a limited space inside the outboard motor. 
     In a preferred embodiment of the present invention, the engine preferably is arranged to support a crankshaft extending along an up-down direction of the outboard motor, exhaust gas outlets of the cylinders are located on the side portion of the engine, and the exhaust passage is arranged to extend from the exhaust gas outlets to a vicinity of the crankshaft in a plan view. 
     The exhaust chamber may be disposed on the opposite side of the exhaust gas outlets with respect to the engine. The exhaust passage may extend from the exhaust gas outlets and bypass the engine along the crank case in a plan view, and is connected to the exhaust chamber. 
     In a preferred embodiment of the present invention, an air introducing passage is connected to the collecting portion of the exhaust passage. Into this air introducing passage, air which has not passed through the insides of the cylinders of the engine is introduced. Such air may be referred to as “secondary air” in this specification. 
     In a preferred embodiment of the present invention, nozzles are disposed on the upstream sides of the catalysts in the downstream portions of the exhaust passage. Each of the nozzles may have a narrowing portion at which the cross-section area of the exhaust path is gradually reduced toward the downstream side thereof, and an expanding portion at which the cross-section area of the exhaust path gradually increases toward the downstream side between the narrowing portion and the catalyst. The nozzle may be a supersonic nozzle. In the supersonic nozzle, when a ratio of the pressure of the upstream of the narrowing portion and the pressure of the downstream of the expanding portion becomes less than a critical pressure ratio, the exhaust gas flow rate in the throat portion reaches the sonic speed, and in the expanding portion, the exhaust gas is accelerated to a supersonic speed. 
     An outboard motor according to another preferred embodiment of the present invention includes an exhaust passage having a first end that is connected to the engine and a second end, and an exhaust chamber which is connected to the second end of the exhaust passage and to a main exhaust passage positioned below the engine. The exhaust passage includes a plurality of upstream portions each provided for a respective one of the plurality of cylinders and including inlet ends connected to exhaust gas outlets of the plurality of cylinders whose exhaust valve opening periods are different, a collecting portion arranged to connect outlet ends of the upstream portions to each other, and a plurality of downstream portions which are branched from the collecting portion and connected commonly to the exhaust chamber. A plurality of catalysts is provided and one of the catalysts is provided in each of the plurality of downstream portions. 
     Other elements, features, arrangements, 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 sectional view showing a configuration of an exhaust device for an outboard motor multi-cylinder engine of a first preferred embodiment of the present invention. 
         FIG. 2  is a side view of an outboard motor equipped with an exhaust device for an outboard motor multi-cylinder engine of a second preferred embodiment of the present invention. 
         FIG. 3  is an enlarged side view of an engine portion of the outboard motor. 
         FIG. 4  is an enlarged plan view of the engine portion of the outboard motor. 
         FIG. 5  is a sectional view of an intake surge tank portion. 
         FIG. 6  is a side view of an intake duct. 
         FIG. 7  is a sectional view for describing a configuration of an exhaust system. 
         FIG. 8  is a sectional view of an exhaust pipe, along VIII-VIII of  FIG. 3 . 
         FIG. 9  is a sectional view of an exhaust chamber. 
         FIG. 10  is a view showing a configuration of a supersonic nozzle. 
         FIG. 11  is a graph showing the relationship between the pressure ratio of the upstream and the downstream of the supersonic nozzle and the Mach number. 
         FIGS. 12A ,  FIG. 12B , and  FIG. 12C  are sectional views for describing advancing states of a shock wave and an exhaust gas, wherein  FIG. 12A  shows an initial state of an exhaust stroke,  FIG. 12B  shows a state in which the shock wave propagates into a branched passage, and  FIG. 12C  shows a state in which the shock wave reflected by the branched passage and the exhaust gas collides with each other. 
         FIG. 13  is a graph showing changes in speed of the exhaust gas inside the supersonic nozzle. 
         FIG. 14  is a graph showing changes in temperature and pressure of the exhaust gas before the throat. 
         FIG. 15  is a graph showing changes in temperature and pressure of the exhaust gas after the throat. 
         FIG. 16  is a graph showing changes in temperature and pressure of the exhaust gas at the rear end of the supersonic nozzle. 
         FIG. 17  is a side view of an outboard motor of a third preferred embodiment of the present invention, showing a configuration of an exhaust pipe to which a secondary air introducing pipe is connected. In  FIG. 17 , the exhaust pipe and the secondary air introducing pipe are illustrated as partially broken. 
         FIG. 18  is a plan view of the outboard motor of the third preferred embodiment, showing a configuration of an exhaust pipe to which a secondary air introducing pipe is connected. In  FIG. 18 , the secondary air introducing pipe, the reed valves, and a portion of the communicating pipe are illustrated as partially broken. 
         FIG. 19A  is a cross-sectional view of the reed valves, and  FIG. 19B  is a longitudinal sectional view of the same. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
     Hereinafter, an exhaust device for an outboard motor multi-cylinder engine of a first preferred embodiment of the present invention will be described in detail with reference to  FIG. 1 . 
     The exhaust device  1  shown in  FIG. 1  is attached to the cylinder head  3  of a 4-cycle multi-cylinder engine  2 , and connected to a first cylinder  4  and a second cylinder  5  of the engine  1 . The multi-cylinder engine  2  preferably is an outboard motor engine, for example. 
     The first cylinder  4  and the second cylinder  5  are cylinders whose exhaust valves  6  open in different time periods (without overlap). For example, it is assumed that the engine  2  is a 4-cylinder engine and the cylinder # 1 , the cylinder # 3 , the cylinder # 4 , and the cylinder # 2  are ignited in this order, for example. In this case, the pair of the cylinder # 1  and the cylinder # 4  corresponds to the pair of two cylinders  4  and  5 . Other pair of the cylinder # 2  and the cylinder # 3  corresponds to the pair of the two cylinders  4  and  5 . In other words, in the 4-cylinder engine, two pairs of cylinders can correspond to the two cylinders  4  and  5 . 
     In the cylinder head  3 , one exhaust port  7  is provided per one cylinder, and exhaust valves  6  for opening and closing the exhaust ports  7  are provided. The downstream end of the exhaust ports  7  open on the outer side surface of the cylinder head  3 . The openings define exhaust gas outlets  8 . 
     The exhaust device  1  of this preferred embodiment includes an exhaust pipe  12  that defines an exhaust passage  11  connected to the two cylinders  4  and  5 , and an exhaust chamber  13  connected to the downstream side end of the exhaust pipe  12 . 
     The exhaust pipe  12  includes (two) upstream pipes  14  and  14  for the respective cylinders each connected to the exhaust gas outlets  8  and  8  of the two cylinders  4  and  5 , a collecting pipe  15  which connects the downstream ends of the upstream pipes  14  to each other, and two downstream pipes  16  and  16  branched from the collecting pipe  15 . 
     The space formed inside the exhaust pipe  12  defines the exhaust passage  11 . The spaces inside the upstream pipes  14  define upstream portions  17  of the exhaust passage  11 . The space inside the collecting pipe  15  defines a collecting portion  18  of the exhaust passage  11 . The spaces inside the downstream pipes  16  define downstream portions  19  of the exhaust passage  11 . 
     At the inlet ends of the upstream pipes  14 , an attaching flange  14   a  is provided. The upstream pipes  14  are attached to the cylinder head  3  via the flange  14   a . The flange  14   a  is attached to the cylinder head  3  preferably via attaching bolts  20 , for example. Two upstream pipes  14  and  14  preferably have lengths equal to each other. 
     The upstream side end of the collecting pipe  15  preferably has a substantially U-sectional shape opening to the upstream side, and is connected to two upstream pipes  14 . The downstream side end of the collecting pipe  15  preferably has a substantially U-sectional shape opening to the downstream side, and is connected to two downstream pipes  16 . The shapes of the connecting portions between the collecting pipe  15  and the upstream pipes  14  and the downstream pipes  16  are not limited to the U sectional shapes show in  FIG. 1 , and can be modified as appropriate. 
     The inner diameter of the collecting pipe  15  shown in  FIG. 1  preferably is substantially equal to the inner diameter of the upstream pipes  14  and the inner diameter of the downstream pipes  16 . However, the inner diameter of the collecting pipe  15  can be larger than the inner diameter of the upstream pipes  14  and the inner diameter of the downstream pipes  16 , for example. 
     The downstream pipes  16  have an attaching flange  16   a  on the downstream side ends. The downstream pipes  16  are attached commonly to one exhaust chamber  13  by the flange  16   a . Two downstream pipes  16  and  16  preferably have lengths equal to each other. In the two exhaust pipes  16  and  16 , catalysts  21  are provided, respectively. In this preferred embodiment, an example including two downstream pipes  16  is shown, however, three or more downstream pipes  16  can be provided, for example. 
     The number of the exhaust pipes  12  is increased at a rate of one per two cylinders when the number of cylinders is larger than two as in the case of the engine  2 , which in the present preferred embodiment preferably is a 4-cylinder engine, for example. In each case, the exhaust pipe  12  is connected to two cylinders whose exhaust valves  5  open in different time periods. 
     The exhaust chamber  13  has an expansion chamber  22  to which exhaust gases are discharged from the plurality of downstream pipes  16 , and discharges the exhaust gases to the main exhaust passage  23  on the downstream side via the expansion chamber  22 . The main exhaust passage  23  is provided with a muffler although this is not shown. 
     As in the case where the number of cylinders of the engine  2  is not less than three, when a plurality of the exhaust pipes  12  are provided, downstream pipes  16  of all of these exhaust pipes  12  can be connected to one exhaust chamber  13 . 
     In the exhaust device  1  of the multi-cylinder engine  2  thus configured, the exhaust gas of the engine  2  is led to the catalysts  21  by the exhaust pipe  12  from the insides of the cylinders (the first and second cylinders  4  and  5 ). The exhaust gas is purified by the catalysts  21 , and then discharged through the exhaust chamber  13  and the muffler (not shown). The exhaust gases discharged to the upstream portions  17  of the exhaust passage  11  from the cylinders  4  and  5  of the engine  2  pass through the collecting portion  18  and are then distributed to the two downstream portions  19 , and flow into the catalysts  21 , respectively. 
     Therefore, according to this preferred embodiment, as compared to an exhaust device configured such that the total amount of the exhaust gas discharged from one cylinder flows into one catalyst, the exhaust resistance can be reduced. In addition, exhaust gases do not flow simultaneously into the downstream portions  19  from the plurality of cylinders, so that the influence of the pressures of the exhaust gases of other cylinders, that is, exhaust interference, does not occur. 
     Therefore, with the exhaust device  1  of this preferred embodiment, the exhaust resistance is greatly reduced and minimized as compared to that in the exhaust device configured such that exhaust gases of all cylinders are made to pass through one catalyst. In addition, exhaust interference does not occur. Therefore, the pressure inside the exhaust passage  11  can be relatively lowered. 
     By thus relatively lowering the pressure inside the exhaust passage  11 , the exhaust gas is efficiently discharged into the exhaust pipe  12  from the combustion chamber (not shown) in the exhaust stroke of the engine  2 . As a result, the amount of exhaust gas remaining in the cylinders can be reduced. 
     Thus, the temperature of intake air suctioned into the cylinders in the intake stroke becomes relatively low, so that an occurrence of abnormal combustion such as the above-described self-ignition and knocking can be reliably prevented. 
     In addition, the exhaust pipe  12  including the upstream pipes  14  for the respective cylinders, the collecting pipe  15  which collects the upstream pipes, and the downstream pipes  16  branched from the collecting portion  15 , can be provided without requiring a large space. Therefore, the exhaust passage which can sufficiently reduce the exhaust resistance can be provided in a limited space inside the outboard motor. 
     Second Preferred Embodiment 
     An exhaust device of a multi-cylinder engine according to a second preferred embodiment of the present invention can be configured as shown in  FIG. 2  to  FIG. 16 . Here, an example in which a preferred embodiment of the present invention is provided in an outboard motor 4-cylinder engine is described. In  FIG. 2  to  FIG. 16 , members identical or equivalent to those described in  FIG. 1  will be assigned with the same reference numerals, and detailed description thereof will be omitted as appropriate. 
     The outboard motor  31  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  32 . Therefore, the outboard motor  31  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  31 , up-down, left-right, and front-rear directions are defined. The reference posture is a posture of the outboard motor  31  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  31 , the hull moves straight ahead. In other words, in this specification, as expressions of directions of the outboard motor  31  and the respective members, the heading direction of a hull with the outboard motor  31  when it moves ahead, that is, when it moves straight ahead is simply referred to as the front of the outboard motor  31 , and the side 180-degree opposite to the front is referred to as the rear side. 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  31  when the hull moves ahead is referred to as the width direction of the outboard motor  31 .  FIG. 2  shows the outboard motor  31  viewed from the left side, and  FIG. 5  and  FIG. 6  show the outboard motor  31  viewed from the outboard motor right side. In these figures, the front of the outboard motor  31  is indicated by an arrow F. 
       FIG. 2  is a side view of the outboard motor  31  of the first preferred embodiment of the present invention. The outboard motor  31  according to the present preferred embodiment includes an engine support member  33 , an engine  2 , an upper casing  34 , a lower casing  35 , a propeller  36 , and a engine cover  37 . The engine support member  33  is a plate-shaped member joined to the upper end of the bracket  32 . On the engine support member  33 , the engine  2  is mounted. To the lower portion of the engine support member  33 , the upper casing  34  is attached. To the lower end of the upper casing  34 , the lower casing  35  is attached. Onto this lower casing  35 , the propeller  36  is supported rotatably. The engine cover  37  covers the engine  2 . In  FIG. 2 , etc., the external shape of the engine cover  37  is indicated by a phantom line, and the internal structure is shown. 
     The engine  2  of this preferred embodiment preferably is a multi-cylinder engine, specifically, a 4-cycle 4-cylinder engine, for example. The engine  2  is mounted on an engine support member  33  while taking a posture in which the axis line of the crankshaft  41  is along the up-down direction. The first cylinder # 1  to the fourth cylinder # 4  of the engine  2  are positioned behind the crankshaft  41  (on the opposite side of the hull with respect to the crankshaft  41 ), and are aligned in series along the up-down direction. In this preferred embodiment, among the four cylinders of the engine  2 , the cylinder positioned highest is the first cylinder # 1 , and cylinders below the first cylinder # 1  are the second cylinder # 2 , the third cylinder # 3 , and the fourth cylinder # 4  in order downward. As to the order of ignition of the engine  2 , 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  41  is arranged so as to penetrate through the engine  2  in the up-down direction. To the lower end of the crankshaft  41 , a drive shaft  45  is coupled. The drive shaft  45  extends along the up-down direction from the lower end of the engine  2  to the inside of the lower casing  35 . The drive shaft  45  is supported rotatably onto the engine support member  33 , the upper casing  34  and the lower casing  35  by bearings (not shown). The lower end of the drive shaft  45  is coupled to a propeller shaft  47  via a forward-reverse switching mechanism  46 . The propeller  36  is arranged so as to rotate integrally with the propeller shaft  47 . 
       FIG. 3  is an enlarged side view of the engine portion, and  FIG. 4  is an enlarged plan view of the engine portion. The engine  2  includes a crank case  42  that rotatably supports the crankshaft  41 , a cylinder body  43 , a cylinder head  3 , and a head cover  44 . The crank case  42  and the cylinder body  43  rotatably support the crankshaft  41 . The cylinder head  3  is attached to the cylinder body  43 . The head cover  44  is attached to the cylinder body  43 . The crank case  42 , the cylinder body  43 , the cylinder case  3 , and the head cover  44  are mounted on the engine support member  33  in a state in which these elements are lined up in this order in the front-rear direction of the outboard motor  31 . The crank case  42  is positioned on the forefront side of the outboard motor  1 . 
     In the cylinder body  43 , four cylinders  48  (see  FIG. 4 ) constituting first cylinder # 1  to fourth cylinder # 4  are provided and lined up in the up-down direction. 
     In the cylinder head  3 , as shown in  FIG. 4 , an intake port  51  and an exhaust port  7  are provided for each of the cylinders. Further, the cylinder head  3  is provided with intake valves  52  and exhaust valves  6  arranged to open and close the ports  51  and  7 . The cylinder head  3  is further provided with a valve operating device  53  arranged to drive the intake and exhaust valves  52  and  6  and an injector  54  for each cylinder arranged to inject fuel into the corresponding intake port  51 . 
     The intake ports  51  are provided at the side portion on the outboard motor right side of the cylinder head  3 , that is, at the side portion on the opposite side of the exhaust ports  7  in the width direction of the outboard motor  31  as shown in  FIG. 4 . The intake ports  51  extend toward the outboard motor rear side, that is, toward the head cover  44  side so as to separate from the crank case  42 . The respective inlet ends of the intake ports  51  are connected to corresponding intake pipes  57  inside a surge tank  56  arranged behind the head cover  44 . The intake surge tank  56  is arranged at the rear end of the engine  2 . The rear end of the engine  2  is an end on the opposite side of the crank case  42  in a plan view. 
     The exhaust ports  7  open on the outer portion (side portion on the outboard motor left side) in the width direction of the outboard motor  31  of the cylinder head  3 , and are connected to an exhaust device  72  as shown in  FIG. 4 . The openings of the exhaust ports  7  define exhaust gas outlets  8 . The exhaust gas outlets  8  open toward the left side of the outboard motor  31  on the left side surface of the cylinder head  3 . In other words, the exhaust gas outlets  8  are arranged so as to open in the opposite direction of the intake ports  51  in the width direction of the outboard motor  31 . 
     The exhaust device  72  includes a first exhaust pipe  73  whose upstream end is connected to the exhaust gas outlet  8 , a second exhaust pipe  74  connected to the downstream end of the first exhaust pipe  73 , and a third exhaust pipe  75  connected to the downstream end of the second exhaust pipe  74 . The exhaust device  72  further includes an exhaust chamber  76  connected to the downstream end of the third exhaust pipe  75 , and a main exhaust passage  77  formed so as to extend downward from the lower end of the exhaust chamber  76 . The exhaust device  72  also includes a first catalyst  78  provided in the connecting portion between the first exhaust pipe  73  and the second exhaust pipe  74 , and a second catalyst  79  provided in the connecting portion between the second exhaust pipe  74  and the third exhaust pipe  75 . In this preferred embodiment, the space formed inside the first to third exhaust pipes to  75  defines the exhaust passage  11 . 
       FIG. 5  is a sectional view for describing a configuration relating to the intake surge tank  31 . The inlet ends of the intake ports  51  open on the end on the outboard motor right side of the rear surface  3   a  of the cylinder head  3  (rear surface to which the head cover is connected). The openings of the inlet ends of the intake ports define intake inlets  55  of the engine  2 . The intake inlets  55  are arranged on the opposite side of the exhaust gas outlets  8  of the cylinder head  3  in the width direction of the outboard motor  31 . The intake inlets  55  are connected to respective intake holes  56   a  of the intake surge tank  56  attached to the rear surface  3   a  of the cylinder head  3 . The intake holes  56   a  are connected to the respective intake pipes inside the intake surge tank  56 . 
     The intake surge tank  56  has a box-shaped intake surge tank main body  56   b  opening toward the front of the outboard motor  56  (head cover  44  side), and an attaching member  56   c  which closes the opening portion of the intake surge tank main body  56   b . The intake surge tank  56  is attached to the head cover  44  preferably via attaching bolts  56   d,  for example. 
     The intake pipes  57  are arranged so as to extend while curving in an arc shape in a plan view. In detail, the intake pipes  57  curve so as to project to the rear side (upper side in  FIG. 5 ) of the outboard motor  56 , that is, in the opposite direction of the crank case  42  with respect to the cylinder head  3  from the intake inlets  55 . The intake pipes  57  curve so as to project to the left side (right side in  FIG. 5 ) of the outboard motor  31 , that is, come closer to the exhaust ports  7  in the width direction of the outboard motor  31 . The intake pipes  57  are arranged so as to extend across the region from the side wall  56   e  on the outboard motor right side to the rear wall  56   f  of the suction surge tank main body  56   b . The intake pipes  57  open at positions on the outboard motor rear side inside the intake surge tank  56 . 
     The intake hole  56   a  and the intake pipe  57  are provided for each cylinder, and define an intake passage for each cylinder in cooperation with the intake port  51  of each cylinder. The inlet ends of the intake pipes  57  define an intake ports for intake to the engine  2 . Intake passages are arranged so as to extend to the head cover  44  side, so that the length of the intake passages can be secured while the exhaust passage  11  is formed to be long. 
     At the inlet ends of the intake pipes  57 , a variable intake pipe mechanism  58  is provided. The variable intake pipe mechanism  58  includes auxiliary intake pipes  59  removably connected to the intake pipes  57 , and servo motors  61  which drives the auxiliary intake pipes  59 . The auxiliary intake pipe  59  is provided for each intake pipe  57  of each cylinder. These auxiliary intake pipes  59  are pivotally supported on a support bracket  62  of the head cover  44  such that they move between the connecting position shown by the solid line in  FIG. 5  and the separated position shown by the alternate long and two short dashed line in  FIG. 5 . 
     These auxiliary intake pipes  59  are joined to the servo motors  61  via links  63 , and are driven to turn by the servo motors  61  to be arranged to the connecting position or the separated position. By disposing the auxiliary intake pipes  59  at the connecting position, the substantial intake pipe length becomes relatively long. By moving the auxiliary intake pipes  59  to the separated position, the substantial intake pipe length becomes relatively short. The servo motors  61  are provided at the upper portion and the lower portion of the head cover  44 , respectively, as shown in  FIG. 7 . The servo motor  61  positioned on the upper side drives the first cylinder auxiliary intake pipe  59  and the second cylinder auxiliary intake pipe  59 , and the servo motor  61  positioned on the lower side drives the third cylinder auxiliary intake pipe  59  and the fourth cylinder auxiliary intake pipe  59 . 
     To the upper end of the intake surge tank  56 , as shown in  FIG. 4 , an intake duct  64  arranged to lead air of the inside the engine cover  37  is connected. The intake duct  64  leads the air inside the engine cover  37  to the intake port of the engine  2  (the inlet end of the intake pipes  57  opening inside the intake surge tank  56 ). 
     The intake duct  64  preferably has a U shape as viewed from the outboard motor right side as shown in the side view of  FIG. 6 . In other words, the intake duct  64  has a downstream side horizontal portion  65  which has a downstream side end connected to the upper end of the intake surge tank  56  and extends in the front-rear direction at the upper right rear of the engine  2 . Further, the intake duct  64  has a downstream side vertical portion  66  which extends downward to the vicinity of the lower end of the engine  2  on the lateral right side of the engine  2  from the front end of the downstream side horizontal portion  65 . Further, the intake duct  64  has an upstream side horizontal portion  67  which extends forward from the lower end of the downstream side vertical portion  66 . Further, the intake duct  64  has an upstream side vertical portion  68  extending upward to the height of the vicinity of the upper end of the engine  2  from the front end of the upstream side horizontal portion  67 . The downstream side horizontal portion  65  is provided with a throttle valve  69  (also see  FIG. 4 ). 
     At the upper end of the upstream side vertical portion  68 , an air suction port  70  opening inside the engine cover  37  is provided. The space inside the engine cover  37  communicates with the atmosphere via the air inlet  71  provided at the rear portion of the outboard motor left side of the engine cover  37  as shown in  FIG. 2 . The air introduced into the engine cover  37  from the air inlet  71  is suctioned into intake passages of the respective cylinders through the intake duct  64  and the intake surge tank  56 . 
       FIG. 7  is a sectional view for describing a configuration of an exhaust system. The main exhaust passage  77  opens inside water at the shaft center of the propeller  36 . The main exhaust passage  77  preferably includes a plurality of members. Specifically, the plurality of members of the main exhaust passage  77  include a cylinder body  43 , an engine support member  33 , an oil pan  84  attached to the lower end of the engine support member  33 , and a pipe  85  attached to the oil pan  84 . Further, the plurality of members of the main exhaust passage  77  include a muffler  86  which is attached to the lower end of the oil pan  84  and extends downward, the upper casing  34  which houses the muffler  86 , and the lower casing  35 . 
     The first to third exhaust pipes  73  to  75  shown in  FIG. 2  to  FIG. 4  are drawn such that only the external form or contour of the exhaust passage  11  formed inside the first to third exhaust pipes is shown. These first to third exhaust pipes  73  to  75  are preferably molded by casting into a pipe shape in actuality. 
     As shown in the sectional view of  FIG. 8 , the first exhaust pipe  73  preferably has a double pipe structure in which the exhaust passage  11  is covered by a coolant passage  80 . The second and third exhaust pipes  74  and  75  are also preferably molded by casting into a pipe shape, and have the same double pipe structure as that of the first exhaust pipe  73 . The coolant passage  80  formed inside the first exhaust pipe  73  communicates with a coolant passage (not shown) of the cylinder head  3 . The coolant passage  80  is connected to the coolant passage  83  (see  FIG. 9 ) inside the exhaust chamber  76  via the coolant passages  81  and  82  inside the second exhaust pipe  74  and the third exhaust pipe  75 . 
     The first and second catalysts  78  and  79  preferably are made of a so-called ternary catalyst. The ternary catalyst can simultaneously detoxify hydrocarbon, nitrogen oxide, and carbon monoxide at the time of combustion near a theoretical air-fuel ratio. The first catalyst  78  is arranged on the opposite side of the crank case  42  across the air suction port  70  of the intake duct  64  as shown in  FIG. 4 . In other words, the first catalyst  78  is arranged on the further front of the outboard motor  31  than the air suction port  70  in a plan view. 
     The first exhaust pipe  73  preferably has a structure obtained by substantially combining two exhaust pipes  12  shown in  FIG. 1 , that is, exhaust pipes  12  connected to two cylinders whose exhaust valves  6  open in different time periods. One of the two exhaust pipes of the first exhaust pipe  73  corresponds to the first cylinder # 1  and the fourth cylinder # 2  whose ignition timings are  360  degrees different from each other. In detail, this exhaust pipe has, as shown in  FIG. 3 ,  FIG. 4 , and  FIG. 8 , a first cylinder upstream portion  73   a  and a fourth cylinder upstream portion  73   d,  and a first collecting portion  73   e  (see  FIG. 3  and  FIG. 8 ) which connects the downstream ends of these upstream portions  73   a  and  73   d  to each other. This exhaust pipe further has first and second downstream portions  73   g  and  73   h  branched from the first collecting portion  73   e.    
     The other of the two exhaust pipes of the first exhaust pipe  73  corresponds to the second cylinder # 2  and the third cylinder # 3  whose ignition timings are  360  degrees different from each other. In detail, this exhaust pipe includes, as shown in  FIG. 3 ,  FIG. 4 , and  FIG. 8 , a second cylinder upstream portion  73   b  and a third cylinder upstream portion  73   c,  and a second collecting portion  73   f  which connect the downstream ends of these upstream portions  73   b  and  73   c . This exhaust pipe further has third and fourth downstream portions  73   i ; and  73   j  branched from the second collecting portion  73   f.    
     At the inlet ends of the first to fourth upstream portions  73   a  to  73   d,  as shown in  FIG. 8 , an upstream side attaching flange  73   k  arranged to attach the first exhaust pipe  73  to the cylinder head  3  is integrally provided. The inlet ends of the first to fourth upstream portions  73   a  to  73   d  are connected to each other by the upstream side attaching flange  73   k.    
     At the outlet ends of the first to fourth downstream portions  73   g  to  73   i,  a downstream side attaching flange  731  arranged to attach the second exhaust pipe  74  is integrally provided. The outlet ends of the first to fourth downstream portions  73   g  to  73   i  are connected to each other by the downstream side attaching flange  731 . 
     The spaces formed inside the first cylinder upstream portion  73   a  to the fourth cylinder upstream portion  73   d  define the upstream portions  17  of the exhaust passages  11 . The spaces formed inside the first collecting portion  73   e  and the second collecting portion  73   f  define the collecting portions  18  of the exhaust passages  11 . Further, the spaces formed inside the first downstream portion  73   g  to the fourth downstream portion  73   i  and the space formed inside the second exhaust pipe  74  and the third exhaust pipe  75  define the downstream portions  19  of the exhaust passages  11 . 
     The first and fourth cylinder upstream portions  73   a  and  73   d  are arranged closer to the engine  2  in the width direction of the outboard motor  31  than the second and third cylinder upstream portions  73   b  and  73   c  as shown in  FIG. 4  and  FIG. 8 . Therefore, the first collecting portion  73   e  is provided at a position closer to the engine  2  than the second collecting portion  73   f.    
     These first collecting portion  73   e  and second collecting portion  73   f  are arranged at substantially the same height as that of the central portion in the up-down direction of the cylinder body  43  as shown in  FIG. 3 . Accordingly, the pipe length of the first cylinder upstream portion  73   a  and the pipe length of the fourth cylinder upstream portion  73   d  can be made equal to each other. The pipe length of the second cylinder upstream portion  73   b  and the pipe length of the third cylinder upstream portion  73   c  can be made equal to each other. 
     Further, the first cylinder upstream portion  73   a  and the fourth cylinder upstream portion  73   d  are preferably longer than the second cylinder upstream portion  73   b  and the third cylinder upstream portion  73   c  in the side view of  FIG. 3 . On the other hand, as shown in  FIG. 4 , the second cylinder upstream portion  73   b  and the third cylinder upstream portion  73   c  are preferably constructed such that the radius of curvature of the bent portion for connection to the cylinder head  3  is larger than the radius of curvature of the first and fourth cylinder upstream portions  73   a  and  73   d . With this configuration, the first cylinder and fourth cylinder upstream portions  73   a  and  73   d  and the second cylinder and third cylinder upstream portions  73   b  and  73   c  are preferably constructed such that their pipe lengths match each other. 
     The first and second downstream portions  73   g  and  73   h  connected to the first collecting portion  73   e  extend upward and downward, respectively, as they go to the downstream side (the front of the outboard motor  31 , the crank case  42  side in the side view of  FIG. 3 ) from the first collecting portion  73   e  as shown in  FIG. 3 . These first and second downstream portions  73   g  and  73   h  bend toward the front of the outboard motor  31  at positions corresponding to the connecting portion between the crank case  42  and the cylinder body  43  as viewed laterally. The inclination angles of the parts  73   g  and  73   h  with respect to the horizontal become smaller at those positions. A portion on the tip side of the bent portion of the first downstream portion  73   g,  which is positioned higher than the second downstream portion  73   h,  inclines forward and downward, and extends straight in a side view. A portion on the tip side of the bent portion of second downstream portion  73   h  positioned lower inclines forward and upward, and extends straight in a side view. 
     The third and fourth downstream portions  73   i  and  73   j  connected to the second collecting portion  73   f  extend upward and downward, respectively, as they go to toward the downstream side (forward) from the second collecting portion  73   f  as shown in  FIG. 3 . These third and fourth downstream portions  73   i  and  73   j  bend such that their inclination angles with respect to the horizontal become smaller than those of the upstream sides at a position corresponding to the connecting portion between the crank case  42  and the cylinder body  43  as viewed laterally. The inclination angles with respect to the horizontal of the portions on the tip sides of the bent portions of these downstream portions are larger than the inclination angles of the first and second downstream portions  73   g  and  73   h  with respect to the horizontal. A portion on the tip side of the bent portion of the third downstream portion  73   i,  which is positioned higher than the fourth downstream portion  73   j,  inclines forward and upward, and extends straight in the side view. The portion on the tip side of the bent portion of the fourth downstream portion  73   j  positioned lower inclines forward and downward, and extends straight in the side view. 
     The outlet end of the third downstream portion  73   i  is positioned above the outlet end of the first downstream portion  73   g.  The outlet end of the fourth downstream portion  73   j  is positioned below the outlet end of the second downstream portion  73   h.    
     The outlet ends of the first to fourth downstream portions  73   g  to  73   k  bend toward the center in the width direction of the outboard motor  31  as shown in  FIG. 4 . In the portions assuming straight shapes in the side view of these first to fourth downstream portions  73   g  to  73   j,  supersonic nozzles  87  described later are preferably provided, respectively. 
     The second exhaust pipe  74  is connected to the first exhaust pipe  73  ahead of the crank case  42 , that is, on the opposite side of the cylinder head  3  with respect to the crank case  42  as shown in  FIG. 4 . The second exhaust pipe  74  is arranged so as to extend to the diagonally right front of the engine  2 . This second exhaust pipe  74  is preferably formed by integrally molding by casting the four tubular portions  74   a  and flanges  74   b  and  74   c  positioned on the upstream side ends and the downstream side ends of these tubular portions  74   a  as shown in  FIG. 7  and  FIG. 8 . 
     The third exhaust pipe  75  is arranged on the lateral right side of the engine  2 , that is, at a position adjacent aside the crank case  42  as shown in  FIG. 4 . The third exhaust pipe  75  extends in the front-rear direction of the outboard motor  31 , that is, a direction in which the crank case  42  and the cylinder body  43  are lined up. The third exhaust pipe  75  connects the second exhaust pipe  74  to the exhaust chamber  76 . The exhaust chamber  76  is positioned on the lateral right side of the cylinder body  43 , that is, on the opposite side of the first exhaust pipe  73  in the width direction of the outboard motor  31 . This third exhaust pipe  75  is preferably formed by integrally molding by casting the four tubular portions  75   a  and flanges  75   b  and  75   c  positioned on the inlet ends and the outlet ends of these tubular portions  75   a  as shown in  FIG. 7  and  FIG. 8 . 
     These first to third exhaust pipes  73  to  75  extend from the exhaust gas outlets  8  in a plan view as shown in  FIG. 4 . Further, the first to third exhaust pipes  73  to  75  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  42 , and bypasses the engine  4  and extends to the opposite side in the width direction of the outboard motor  31  (right side of the outboard motor  31 ). Preferably, the length of the first to third exhaust pipes  73  to  75  (the bypass exhaust pipe) is designed so as to surround the crankshaft  41  at angles not less than 90 degrees in the rotation direction of the crankshaft  41 , for example. 
     The exhaust passage  11  inside the first to third exhaust pipes  73  to  75  and the intake passage on the downstream side of the intake surge tank  56  are preferably formed into a substantially S shape in a plan view as shown in  FIG. 4 . The intake passage on the downstream side of the intake surge tank  56  means an intake passage formed inside the intake pipe  57 , the intake hole  56   a,  and the intake port  51 . Of course, the first to third exhaust pipes  73  to  75  and the intake passage may be formed into 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  73  to  75  and the intake passage extend opposite to each other in the width direction of the outboard motor from the cylinder head  14 . The intake passage curves so as to bypass the cylinder head at the rear portion of the outboard motor. On the other hand, the bypass exhaust pipe formed of the first to third exhaust pipes  73  to  75  curves so as to bypass the engine  4  to the front of the crank case  12  at the front portion of the outboard motor. 
     The supersonic nozzle  87  provided on the first exhaust pipe  73  is arranged to accelerate the flow rate of the exhaust gas from a speed not more than the sonic speed to a supersonic speed. This supersonic nozzle  87  may be a De Laval nozzle invented by De Laval. A De Laval nozzle has a flow passage structure in which a sectional area of a flow path is first reduced and then increased. 
     The supersonic nozzle  87  has, as shown in  FIG. 8 , a narrowing portion  111 , an expanding portion  112 , and a throat portion  113 . The narrowing portion  111  is formed such that the passage cross-section area is gradually reduced toward the downstream side of the flow direction of the exhaust gas. The expanding portion  112  is formed such that the passage cross-section area gradually increases toward the downstream side. The throat portion  113  is positioned between the narrowing portion  111  and the expanding portion  112 , and has the smallest passage cross-section area among these elements. 
     The downstream side end of the first exhaust pipe  73  bends toward the center in the width direction of the outboard motor  1 , that is, toward the first catalyst  78 . The inner diameter at the downstream side end of the first exhaust pipe  73 , that is, the inner diameter of the portion on the downstream side of the expanding portion  112  gradually increases toward the downstream side. Accordingly, the exhaust pipe inner surface can be connected to the first catalyst  78  with a relatively large outer diameter without steps. 
     The inner diameter at the upstream end of the narrowing portion  111  matches the inner diameter of the first and second collecting portions  73   e  and  73   f  of the first exhaust pipe  73 . The inner diameter at the downstream end of the expanding portion  112  matches the inner diameter of the downstream portion  73   g  to  73   j  of the first exhaust pipe  73 . 
       FIG. 9  is a sectional view of the exhaust chamber  76 . The exhaust chamber  76  preferably has a box shape which opens to the cylinder body  43 . The exhaust chamber  76  is attached to the side portion on the outboard motor right side of the cylinder body  43  such that the opening portion of the exhaust chamber is closed by the cylinder body  43 . On the side portion of the cylinder body  43 , a recess portion  92  which opens to the exhaust chamber  76  (to the right side of the outboard motor  31 ) is formed. The recess portion  92  defines an expansion chamber  91  in conjunction with the exhaust chamber  76 . Accordingly, the expansion chamber  91  has a capacity larger than the inner space of the exhaust chamber  76 . On the lower wall  43   a  of the cylinder body  43  which defines the side wall on the lower side of the recess portion  92 , as shown in  FIG. 7  and  FIG. 9 , the main exhaust passage  77  opens. 
     Near the lower side of the exhaust chamber  76 , as shown in  FIG. 6 , the upstream side horizontal portion  67  of the intake duct  64  is positioned. On the opposite side (near the rear side) of the third exhaust pipe  75  of the exhaust chamber  76 , as shown in  FIG. 9 , the downstream side vertical portion  66  of the intake duct  64  is positioned. 
     The exhaust chamber  76  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  75  to be connected thereto (see  FIG. 7 ). 
     Inside the outer wall of the exhaust chamber  76 , as shown in  FIG. 9 , a coolant passage  83  is formed. The coolant passage  83  is arranged such that a coolant is supplied from the coolant passage  82  of the third exhaust pipe  75 , and this coolant is discharged to a coolant discharge passage (not shown) of the cylinder body  43 . 
     Inside the exhaust chamber  76 , a partition  95  for partitioning the expansion chamber  91  into an upstream exhaust gas chamber  93  and a downstream exhaust gas chamber  94  is provided. This partition  95  partitions the expansion chamber  91  into the above-described two chambers  93  and  94  in cooperation with a longitudinal wall  96  extending from the cylinder body  43 . In the partition  95 , a communicating hole  97  is formed so as to make communication between both the gas chambers  93  and  94 . Further, the partition  95  is provided with an on-off valve  98  which opens and closes the communicating hole  97 . The communicating hole  97  is positioned at the central portion in the up-down direction of the partition  95  and also at the central portion of the partition  95  in the width direction of the outboard motor  31 . The opening shape of the communicating hole  97  preferably is ellipse shape that allows the valve body  99  of the on-off valve  98  to be inserted therein. 
     The on-off valve  98  preferably is a butterfly valve having a disk-shaped valve body  99  inserted inside the communicating hole  97 . The valve body  99  preferably includes an oval plate long in the width direction of the partition  95 . This valve body  99  is attached to a valve shaft  100  extending along the partition  95 . The valve shaft  100  is pivotally supported by a bearing  101  and a cover  102  fixed to the partition  95 . The valve shaft  100  is connected to a drive device not shown via a wire, and rotates according to driving of the drive device. 
     The on-off valve  98  is driven by the drive device so as to close when the crankshaft  41  rotates in reverse or a high negative pressure is generated in the exhaust chamber  76 , and opens in other cases. A sensor (not shown) for detecting the rotating speed of the crankshaft  41  detects whether the crankshaft  41  has rotated in reverse. The pressure inside the exhaust chamber  76  is detected by a pressure sensor not shown. 
     At the upper end of the exhaust chamber  76 , as shown in  FIG. 7 , an oxygen sensor  103  is provided to detect the amount of oxygen in the exhaust gas. The oxygen sensor  103  is located at the upper end of the upstream exhaust gas chamber  93 , and transmits detection data indicative of the amount of oxygen in the exhaust gas flowing in the upstream exhaust gas chamber  93  to an ECU (Electronic Control Unit, not shown) of the engine  2 . The ECU controls the fuel injection amount of the injector  54  and the ignition timing of the ignition plug (not shown), etc., based on the speed of the engine  2 , the opening degree of the throttle valve  69 , and the amount of oxygen in the exhaust gas detected by the oxygen sensor  103 , etc. 
     The exhaust gases in the cylinders of the engine  2  respectively flow into the exhaust chamber  76  through the first to third exhaust pipes  73  to  75  and join together inside the exhaust chamber  76 , and are then discharged to the upstream side end of the main exhaust passage  77 . The exhaust gas led into the main exhaust passage  77  is discharged into water through the insides of the lower casing  35  and the propeller  36  from the inside of the upper casing  34 . 
       FIG. 10  is a view for describing in detail the flow channel structure of the supersonic nozzle  87 . The cross-section area at the upstream end of the narrowing portion  111  (right end in  FIG. 10 ) is referred to as “upstream cross-section area A 1 ,” and the cross-section area at the expanding portion  112  (left end in  FIG. 10 ) of the downstream portion is referred to as “downstream side cross-section area A 3 .” In addition, the cross-section area of the throat portion  113  is referred to as “throat cross-section area A 2 .” These satisfy the relationship of A 1 &gt;A 2 &lt;A 3 . In other words, the upstream cross-section area A 1  and the downstream cross-section area A 3  are larger than the throat cross-section area A 2 . 
     The narrowing portion  111 , the expanding portion  112 , and the throat portion  113  are formed such that their sectional shapes (shapes as viewed from the upstream side of the exhaust passage  11 ) preferably are circular or substantially circular. 
     In this preferred embodiment, the narrowing portion  111  and the expanding portion  112  preferably have a tapered pipe shape whose rate of change in cross-section area is fixed, that is, whose cross-section area changes linearly. However, the supersonic nozzle  87  to be used in the exhaust device  72  according to a preferred embodiment is not limited to this shape, and may be shaped so that the rate of change in cross-section area gradually changes. 
     The supersonic nozzle  87  is preferably formed so as to satisfy the conditions shown in the following mathematical formulas (1) and (2). Accordingly, when the flow rate of the exhaust gas flowing into the throat portion  113  reaches mach 1 (sonic speed), in the expanding portion  112 , the exhaust gas can be accelerated to a higher speed. 
     
       
         
           
             
               
                 
                   
                     
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                             2 
                           
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                               M 
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                           2 
                         
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                               4 
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     The mathematical formula (1) shows the relationship between the shape of the exhaust pipe and the mach number in a primary flow with viscous friction. The mathematical formula (2) defines Λ in the mathematical formula (1). 
     In these mathematical formulas, M denotes a mach number, A denotes the flow channel cross-section area at an arbitrary cross-section of the exhaust pipe, D denotes a pipe corresponding diameter at the arbitrary cross-section, γ denotes a specific heat ratio, x denotes the distance (position) in the flow direction, and f denotes a friction coefficient. 
       FIG. 11  is a view showing the relationship of the mach number with the ratio (P/P 0 ) of the total pressure (P 0 ) of the upstream of the inlet of the supersonic nozzle  87  and the static pressure (P) of the downstream of the throat portion  113 . As is understood from this figure, the mach number reaches “1” under a condition that the pressure ratio (P/P 0 ) is smaller than the critical pressure ratio of about 0.528. In other words, according to a rise in the upstream total pressure (P 0 ) to make the ratio (P/P 0 ) smaller than the critical pressure ratio of about 0.528, the flow rate of the exhaust gas is accelerated when passing through the narrowing portion  111  and the flow rate of the exhaust gas flowing into the throat portion  74  reaches the sonic speed. 
     When the flow rate of the exhaust gas flowing into the throat portion  113  thus reaches the sonic speed, a shock wave  114  is generated inside the supersonic nozzle  87 . This shock wave  114  is accelerated when passing through the expanding portion  112  of the supersonic nozzle  87 . When the shock wave  114  is thus generated, inside the throat portion  113 , a shock wave  114  and an expansion wave  115  composed of a pressure wave advancing opposite to the shock wave  114  are generated. 
     While the shock wave  114  is accelerated in the expanding portion  112  of the supersonic nozzle  87 , the expansion wave  115  advances opposite to the shock wave  114 . Accordingly, an excessive negative pressure is generated between the shock wave  114  and the expansion wave  115 . As a result, the temperature of the exhaust gas between the shock wave  114  and the expansion wave  115  lowers. 
     The shock wave  114  and the expansion wave  115  propagate inside the first exhaust pipe  73  positioned on the upstream side of the first catalyst  78 . Therefore, a negative pressure is generated inside the first exhaust pipe  73 , and the exhaust gas is easily discharged from the exhaust port  7  into the exhaust passage  11  in the exhaust stroke of the engine  2 . In addition, the temperature of the exhaust gas in the first exhaust pipe  73  lowers. 
     In the exhaust device  72  according to this preferred embodiment, as shown in  FIG. 12A  to  FIG. 12C , a shock wave  114  propagating in the exhaust passage  11  and the first to fourth upstream portions  73   a  to  73   d  are used during the exhaust stroke. Accordingly, a condition for making the flow rate of the exhaust gas reach the sonic speed in the supersonic nozzle  87 , that is, the condition that the pressure ratio (P/P 0 ) becomes smaller than the critical pressure ratio of about 0.528 is easily satisfied. 
     In the cylinder #A, when the exhaust valve  6  opens during the exhaust stroke, a combustion gas with a high pressure jets out into the exhaust port  7  from the combustion chamber. The flow rate of the combustion gas (exhaust gas  116 ) jetting out into the exhaust port  7  increases according to the opening degree of the exhaust valve  6 , and reaches the sonic speed before the exhaust valve  6  becomes its full-open state. When the flow rate of the exhaust gas  116  thus reaches the sonic speed, a supersonic shock wave  114  is generated inside the exhaust port  7 . At this time, as shown in  FIG. 12A , the shock wave  114  advances from the inside of the exhaust port  7  into the first exhaust pipe  73 , that is, inside the upstream portion  17  of the exhaust passage  11 , and further propagates at a high speed toward the downstream. On the other hand, the exhaust gas  116  advances to the downstream side at a relatively low speed inside the upstream portions  17  behind the shock wave  114 . 
     The shock wave  114  advancing in the first exhaust pipe  73  is branched into the downstream portion  19  and the upstream portion  17  on the cylinder #B side (hereinafter, referred to as “branched passage  117 ”) as shown in  FIG. 12B  when it passes through the collecting portion  18 , and advances inside the downstream portion  19  and the branched passage  117  independently. The shock wave  114  advancing in the downstream portion  19  passes through the supersonic nozzle  87  and then attenuates and disappears. On the other hand, the shock wave  114  advancing inside the branched passage  117  is reflected by the exhaust valve  6  (closed state) of the cylinder #B, and reverses inside the branched passage  117 , and returns to the collecting portion  18 . 
     The branched passage  117  is designed such that the timing at which the shock wave  114  returns to the collecting portion  18  from the branched passage  117  and the timing at which the exhaust gas  116  with a high pressure discharged from the cylinder #A and advancing behind the shock wave inside the upstream portion  17  reaches the collecting portion  18  coincide with each other. Accordingly, as shown in  FIG. 12C , the shock wave  114  which has propagated inside the exhaust passage  11  on the cylinder #A side from the branched passage  117  and the exhaust gas  116  with a high pressure inside this exhaust passage  11  collide with each other. In other words, the cross-section area and length of the branched passage  117  are dimensionally set such that the shock wave  114  and the exhaust gas  116  thus collide with each other. 
     Due to the collision between the shock wave  114  and the exhaust gas  116 , the total pressure (P 0 ) of the upstream of the inlet of the supersonic nozzle  87  becomes higher. Accordingly, it becomes easy to satisfy the condition that the ratio (P/P 0 ) of the total pressure (P 0 ) of the upstream of the inlet of the supersonic nozzle  87  and the downstream static pressure (P) of the throat portion  113  of the supersonic nozzle  87  becomes smaller than the critical pressure ratio of about 0.528. By satisfying this condition, the exhaust gas  116  flows into the throat portion  113  at a speed over the sonic speed. 
     Thus, the flow rate of the exhaust gas  116  flowing into the throat portion  113  reaches the sonic speed, and accordingly, the shock wave is regenerated inside the supersonic nozzle  87 . Accordingly, a negative pressure is generated inside the first exhaust pipe  73 , and as a result, the temperature of the exhaust gas inside the first exhaust pipe  73  lowers. 
     When the cylinder #B is at the time of the exhaust stroke, conversely, the upstream portion  17  including the inside of the exhaust port  7  of the cylinder #A substantially functions as the branched passage  117 . In other words, the shock wave  114  generated in the exhaust port  7  of the cylinder #B propagates in the upstream portion  17  (branched passage  117 ) on the cylinder #A side toward the cylinder #A, and is reflected by the exhaust valve  6  in a closed state. The reflected shock wave  114  propagates in the upstream portion  17  (branched passage  117 ) on the cylinder #A side toward the collecting portion  18 , and in this collecting portion  18 , collides with the exhaust gas  116  from the cylinder #B. Accordingly, the total pressure (P 0 ) of the upstream of the inlet of the supersonic nozzle  87  rises, and the exhaust gas  116  flowing into the throat portion  113  easily reaches the sonic speed. Accordingly, a shock wave  114  can be caused in the supersonic nozzle  87 , as a result, a negative pressure can be generated inside the first exhaust pipe  73 . Thus, the temperature of the exhaust gas generated from the cylinder #B can be lowered. 
     The first to fourth cylinder upstream portions  73   a  to  73   d  in the first exhaust pipe  73  preferably have the same predetermined pipe length so as to return a shock wave  114  generated in the exhaust port  7  of another cylinder whose ignition timing is different by  360  degrees to the first or second collecting portions  73   e  or  73   f  (the collecting portion  18  of the exhaust passage  11 ) at an optimum timing. For example, during the exhaust stroke of the first cylinder # 1 , a shock wave  114  generated in the first cylinder upstream portion  73   a  propagates from the first collecting portion  73   e  to the fourth cylinder upstream portion  73   d,  and is reflected by the fourth cylinder upstream portion  73   d  and returns to the first collecting portion  73   e . The time necessary for this is equal to the time necessary for the shock wave  114  generated in the fourth cylinder upstream portion  73   d  to be reflected by the first cylinder upstream portion  73   a  and return to the first collecting portion  73   e . This applies to the second cylinder upstream portion  73   b  and the third cylinder upstream portion  73   c.    
     Thus, the time necessary for the shock wave  114  to return to the first or second collecting portion  73   e  or  73   f  is the same among all cylinders. Accordingly, in the supersonic nozzles  87  respectively provided at the first to fourth downstream portions  73   g  to  73   j,  the speed of the exhaust gases  116  can equally increase. As a result, the pressure in all the exhaust passages  11  can be substantially uniformly lowered. 
       FIG. 13  to  FIG. 16  show the results of simulation of the effect of the exhaust device  72  of this preferred embodiment. As a result of verification through simulation, it was discovered that the exhaust gas pressure and the exhaust gas temperature significantly lowered inside the supersonic nozzle  87 . 
       FIG. 13  is a graph showing the relationship between the crank angle and the flow rate of the exhaust gas in the first exhaust pipe  73 . In this figure, the solid line shows changes in flow rate of the exhaust gas before the throat portion, and the dashed line shows changes in flow rate of the exhaust gas after the throat portion, and the chain line shows changes in flow rate of the exhaust gas at the rear end of the supersonic nozzle. As is understood from this figure, the shock wave is accelerated from about 559 m/s to about 1450 m/s, for example. 
       FIG. 14  to  FIG. 16  are graphs showing the relationship among the crank angle, the exhaust gas temperature, and the exhaust gas pressure inside the first exhaust pipe  73 .  FIG. 14  shows changes in exhaust gas temperature and exhaust gas pressure before the throat portion,  FIG. 15  shows changes in exhaust gas temperature and exhaust gas pressure after the throat portion, and  FIG. 16  shows changes in exhaust gas temperature and exhaust gas pressure at the rear end of the supersonic nozzle. In  FIG. 14  to  FIG. 16 , the period during which the pressure of the exhaust gas is lower than the atmospheric pressure (the inside of the exhaust pipe  53  becomes a negative pressure) is shown by hatching. As is understood from  FIG. 14  to  FIG. 16 , the temperature of the exhaust gas suddenly lowers inside the supersonic nozzle  87  during the exhaust stroke. 
     The pressure of the exhaust gas, for example, as shown in  FIG. 15 , becomes negative not only during the exhaust stroke but for a long period. The period during which the negative pressure is thus generated and the temperature of the exhaust gas is lowered is 520 degrees of the crank angle of 720 degrees of the 4-cycle stroke, and this corresponds to about 72% of the whole stroke period. 
     Technical effects in the outboard motor  1  of this preferred embodiment will be illustrated hereinafter. 
     Exhaust gases discharged from the cylinders of the engine  2  into the upstream portions  73   a  to  73   d  of the first exhaust pipe  73  pass through the collecting portion  73   e  or the collecting portion  73   f  and are distributed into the two downstream portions  73   g  and  73   h  or the two downstream portions  73   i  and  73   j,  and flow into the respective first catalysts  78 . For example, the exhaust gas which has flowed into the first cylinder upstream portion  73   a  passes through the first collecting portion  73   e  and is distributed into the first downstream portion  73   g  and the second downstream portion  73   h . This exhaust gas flows into the first catalyst  78  connected to the first downstream portion  73   g  and also flows into the first catalyst  78  connected to the second downstream portion  73   h . Therefore, the substantial catalyst cross-section area per cylinder is large. 
     Therefore, according to this preferred embodiment, as compared to an exhaust device in which the total amount of the exhaust gas discharged from one cylinder flows into one cylinder, the exhaust resistance is greatly reduced and minimized. 
     Further, in the exhaust device  72  of this preferred embodiment, exhaust gases do not flow concurrently into the first to fourth downstream portions  73   g  to  73   j  from the plurality of cylinders. Therefore, the influence exerted by pressure of the exhaust gas in other cylinders, that is, exhaust interference can be minimized as much as possible. 
     Thus, as compared to the exhaust device  72  configured such that exhaust gases of all cylinders pass through one catalyst, the exhaust resistance can be reduced, and in addition, the exhaust interference can be minimized. As a result, the pressure in the exhaust passage  11  can be lowered, so that the amount of exhaust gas remaining in the cylinders  48  due to the internal EGR can be reduced. 
     As a result, the temperature of intake air suctioned into the cylinders  48  in the intake stroke of the engine  2  becomes relatively low, so that an occurrence of abnormal combustion such as the above-described self-ignition and knocking can be reliably prevented. 
     Further, in this preferred embodiment, exhaust gas outlets  8  of cylinders are provided on the side portion of the cylinder head  3 . The exhaust passage  11  inside the first exhaust pipe  73  connected to these exhaust gas outlets  8  extends from the exhaust gas outlets  8  to the vicinity of the crankshaft  41  in a plan view. Therefore, the exhaust passage  11  connected to the exhaust gas outlets  8  of the cylinders can be long. Accordingly, the pressure inside the exhaust passage  11  can be further lowered for the first and second reasons described below. 
     (1) First Reason 
     When the exhaust valve  6  opens during engine operation, an exhaust gas with a high pressure inside the cylinder  48  passes through the gap between the valve body of the exhaust valve  6  and the valve seat on the cylinder head  3  side and jets out to the exhaust port  7 . It is known that the flow rate of the exhaust gas flowing in the gap reaches the sonic speed even when the number of rotations of the engine is approximately 2000 rpm. When the flow rate of the exhaust gas flowing in the gap reaches the sonic speed, and the exhaust gas flows into the exhaust port  7  with a relatively large passage cross-section area, a shock wave  114  is generated inside the exhaust port  7  (see  FIG. 12A ). This shock wave  114  advances toward the downstream side inside the exhaust passage  11 . 
     Inside the exhaust passage  11  in which a shock wave  114  has been thus generated, an expansion wave  115  which advances opposite to the shock wave  114  is also generated. The pressure between the expansion wave  115  and the shock wave  114  becomes negative. According to this preferred embodiment, the exhaust passage  11  connected to the exhaust gas outlet  8  can be sufficiently long, so that the time during which the shock wave  114  and the expansion wave  115  are allowed to exist in the exhaust passage  11  becomes longer. As a result, a high negative pressure is generated inside the exhaust passage  11 . 
     (2) Second Reason 
     The pressure inside the exhaust passage  11  gradually rises from the upstream side toward the downstream side according to discharge of the exhaust gas from the exhaust port  7 . The exhaust passages  11  of the plurality of cylinders are collected in the exhaust chamber  7  at the downstream side, so that the pressure is also transmitted to the exhaust passages  11  of other cylinders via exhaust chamber  76 . According to this preferred embodiment, the lengths of the exhaust passages  11  connected to the exhaust gas outlets  8  can be long, so that the time during which the pressure reaches the exhaust ports  7  of other cylinders can be lengthened. 
     In other words, it is assumed that, when an exhaust valve  6  of the first cylinder (for example, the first cylinder # 1 ) which is ignited first between two cylinders whose ignition timings are close to each other is open, the exhaust gas is discharged to the exhaust passage  11  of the second cylinder (for example, the third cylinder # 3 ) which is ignited second. The exhaust passages  11  are long, so that the time until the pressure of the exhaust gas of the second cylinder is transmitted to the exhaust port  7  of the second cylinder via the exhaust chamber  76  and the exhaust passage  11  is long. Therefore, when the exhaust valve  6  is open in the first cylinder, exhaust interference can be prevented and minimized by the exhaust gas of the second cylinder. Therefore, the discharge of the exhaust gas in the first cylinder can be prevented from being obstructed by the exhaust interference. 
     Therefore, according to this preferred embodiment, the exhaust pressure in the exhaust passage  11  can be further lowered during the exhaust stroke, so that the exhaust gas can be smoothly discharged into the exhaust passage from inside the cylinder  48 . Therefore, the amount of exhaust gas remaining in the cylinder  48  due to internal EGR can be further reduced. 
     As a result, during the intake stroke, the temperature of intake suctioned into the cylinder  48  can be prevented from being raised by the heat of the exhaust gas. Therefore, the temperature of the intake suctioned into the cylinder  48  becomes relatively low, so that an occurrence of abnormal combustion such as self-ignition and knocking can be reliably prevented. 
     As shown in  FIG. 4 , the exhaust passage  11  of the exhaust device  72  through the vicinity of the outside of the crank case  42  from the exhaust gas outlet  8  of the cylinder head  3  and bypasses the engine  2  in a plan view, and is connected to the exhaust chamber  76  positioned on the opposite side in the width direction of the outboard motor  31 . The exhaust passage  11  is connected to the main exhaust passage  77  via the exhaust chamber  76 . 
     Therefore, the exhaust passage  11  can be formed as long as possible around the engine  1 , so that a higher negative pressure can be caused inside the exhaust passage  11 . 
     In the exhaust device  72 , a supersonic nozzle  87  is provided in the first exhaust pipe  73 . This supersonic nozzle  87  accelerates the exhaust gas and the flow rate of the exhaust gas reaches the sonic speed, and accordingly, a negative pressure is generated inside the first exhaust pipe  73  as described above. As a result, the temperature of the exhaust gas in the first exhaust pipe  73  lowers. 
     Therefore, according to this preferred embodiment, due to the negative pressure, the exhaust gas in the cylinder  48  is efficiently discharged to the exhaust passage  11 . As a result, the amount of exhaust gas remaining in the cylinder  48  due to internal EGR can be further reduced. Therefore, according to the preferred embodiment, during operation of the engine  2 , it becomes much more difficult for self-ignition and knocking to occur. 
     In this preferred embodiment, by lowering the temperature of the exhaust gas by the operation of the supersonic nozzle  87 , the exhaust gas at a relatively low temperature can be made to flow into the first catalyst  78 . Therefore, an occurrence of the above-described sintering phenomenon can be more reliably prevented. 
     Further, the first exhaust pipe  73  of this preferred embodiment is preferably formed by integrally molding by casting the plurality of tubular portions (first to fourth upstream portions  73   a  to  73   d,  the first and second collecting portions  73   e  and  73   f,  and the first to fourth downstream portions  73   g  to  73   j ). Therefore, as compared to the case where the first exhaust pipe  73  is formed by welding a plurality of pipes, the manufacturing cost of the first exhaust pipe  73  can be reduced. 
     Further, the four upstream portions  73   a  to  73   d  and the four downstream portions  73   g  to  73   h  are connected to each other by the collecting portions  73   e  and  73   f,  so that the rigidity of the first exhaust pipe  73  increases. Further, the second exhaust pipe  74  and the third exhaust pipe  75  are also integrally formed preferably by casting such that the four tubular portions  74   a  and  75   a  are integrated, respectively. Therefore, the areas of the connecting portions between the exhaust pipe assembly formed of the first, second, and third exhaust pipes  73 ,  74 ,  75  and the cylinder head  3 , and between the exhaust pipe assembly and the exhaust chamber  76  can be formed to be wide. Therefore, the sealing performance of the connecting portions can be improved. 
     Further, in this preferred embodiment, two pairs of catalysts each pair of which includes the first catalyst  78  and the second catalyst  79  are connected to one cylinder. The exhaust gas discharged from each cylinder is purified by these two first and second catalysts  78  and  79 . 
     When the first and second catalysts  78  and  79  are made of a ternary catalyst, if the flow rate of the exhaust gas passing through the catalysts  78  and  79  is high, redox reaction hardly occurs in the catalysts  78  and  79 , and the exhaust gas purifying efficiency of the catalysts  78  and  79  lowers. To solve this problem, the space velocity S/V value when the exhaust gas passes through the catalysts  78  and  79  is preferably lowered. The S/V value can be obtained from the following mathematical formula (3). 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       / 
                       V 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     value 
                   
                   = 
                   
                     
                       
                         
                           
                             displacement 
                             × 
                             engine 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             speed 
                             × 
                           
                         
                       
                       
                         
                           
                             coefficient 
                             × 
                             inhalation 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             efficiency 
                           
                         
                       
                     
                     
                       2 
                       × 
                       catalyst 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       volume 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In the mathematical formula (3), the coefficient is 1 in the case of a 4-cycle engine, and is 2 in the case of a 2-cycle engine. The inhalation efficiency can be set to, for example, 0.75. The catalyst volume can be obtained from the following mathematical formula (4).
 
Catalyst volume (cm 3 ) =(Catalyst outer diameter) 2 ×π×(foil length)×¼×10 −3    (4)
 
     In this preferred embodiment, four catalysts (two first and second catalysts  78  and  79 ) are preferably provided for one cylinder, for example, so that the catalyst volume is increased and the S/V value can be reduced. Therefore, the flow rate of the exhaust gas passing through the first and second catalysts  78  and  79  can be lowered, and the exhaust gas purifying efficiency can be increased. 
     In this preferred embodiment, the first catalyst  78  and the second catalyst  79  preferably are separately arranged on the upstream side and the downstream side, so that the outer diameters of the catalysts can be formed to be small when the catalyst volume is increased. Therefore, the catalyst volume can be increased while preventing the outboard motor  31  from increasing in size. 
     Further, the first catalyst  78  is arranged ahead of the crank case  42  (on the opposite side of the cylinder body  43  with respect to the crank case  42 ). Therefore, the distance between the exhaust gas outlet  8  of the cylinder head  3  and the first catalyst  78  can be set as long as possible. Therefore, the first catalyst  78  can be arranged at a position at which the temperature thereof does not excessively rise. 
     On the other hand, in the exhaust device  72  of this preferred embodiment, the inside of the exhaust chamber  76  is partitioned by the partition  95  into the upstream exhaust gas chamber  93  and the downstream exhaust gas chamber  94 . The communicating hole  97  is formed in the partition  95 , and the on-off valve  98  which opens and closes this communicating hole  97  is provided. Therefore, by closing the on-off valve  98 , the exhaust passage  11  can be closed inside the exhaust chamber  76 . Accordingly, the exhaust passage  11  on the upstream side of the exhaust chamber  76  can be shut off from the main exhaust passage  77  on the downstream side. 
     Therefore, in the outboard motor  31  of this preferred embodiment, when water flows back inside the main exhaust passage  77 , the water can be prevented by the exhaust chamber  76  from reaching the engine  2  through the first to third exhaust pipes  73  to  75  and the catalysts  78  and  79 . 
     The reversed flow of water inside the main exhaust passage  77  occurs in rare cases when the shift position is switched to “reverse” by the forward-reverse switching mechanism  46  to brake the hull when the hull moves ahead. In other words, while the hull moves ahead at high speed, when the forward-reverse switching mechanism  46  of the outboard motor  31  is switched to the reverse side, the propeller  36  is subjected to a strong force due to water. If this force exceeds the driving force of the engine  2 , the drive shaft  45  (engine  2 ) is rotated in reverse. 
     When the engine  2  is thus rotated in reverse, the piston lowers while the exhaust valve  6  is open, and the exhaust gas in the exhaust passage  11  is suctioned into the cylinder  48 . As the time during which the engine  2  rotates in reverse becomes longer, the amount of the exhaust gas suctioned into the engine  2  becomes larger, and the negative pressure inside the exhaust passage  11  becomes higher. Due to this negative pressure, water rises inside the main exhaust passage  77 . When the outboard motor  31  is used at sea, seawater enters the exhaust passage. 
     When seawater enters the inside of the exhaust passage  11  and comes into contact with the catalysts  78  and  79 , the seawater poisons and deteriorates the catalysts  78  and  79  due to constituents of seawater such as Na, Mg, and Cl, etc. If water is poured on the catalysts  78  and  79  at a high temperature, sudden shrinkage may occur and crack the catalysts  78  and  79 . Further, if water goes upstream in the exhaust passage  11  and is suctioned into the engine  2 , a so-called water hammer phenomenon may occur and break the engine  2 . 
     In the outboard motor  31  of this preferred embodiment, as described above, when water goes upstream inside the exhaust passage  11  (when the engine  2  rotates in reverse or the inside of the exhaust chamber  76  becomes an excessively low pressure), the on-off valve  98  in the exhaust chamber  76  closes. Accordingly, the water going upstream can be stopped by the exhaust chamber  76 , so that water can be reliably prevented from being suctioned into the engine  2  through the first to third exhaust pipes  73  to  75 . 
     Therefore, it can be reliably prevented that the catalysts  78  and  79  are contacted and deteriorated by seawater, and that the catalysts  78  and  79  at a high temperature are suddenly cooled and broken by water. Of course, an occurrence of the water hammer phenomenon can also be prevented. 
     Third Preferred Embodiment 
     Next, a third preferred embodiment of the present invention will be described with reference to  FIG. 17 ,  FIG. 18 ,  FIG. 19A  and  FIG. 19B . In these figures, members identical or equivalent to those described in  FIG. 1  to  FIG. 18  will be represented with the same reference numerals, and detailed description thereof will be omitted as appropriate. 
       FIG. 17  is a side view of an outboard motor  31 , and mainly shows a configuration of a first exhaust pipe  73 . In addition,  FIG. 18  is a plan view of the outboard motor  31 . In this preferred embodiment, to the first collecting portion  73   e  and the second collecting portion  73   f  of the first exhaust pipe  73 , a secondary air introducing pipe  121  is connected. From this secondary air introducing pipe  121 , secondary air is respectively introduced into the first collecting portion  73   e  and the second collecting portion  73   f . “Secondary air” means air which has not passed through the insides of the cylinders  48  of the engine  2 . 
     The secondary air introducing pipe  121  preferably includes a first secondary air introducing pipe  122  and a second secondary air introducing pipe  123 . The first secondary air introducing pipe  122  extends in the up-down direction, and the lower end thereof is connected to the first collecting portion  73   e . The second secondary air introducing pipe  123  extends in the up-down direction, and the lower end thereof is connected to the second collecting portion  73   f.    
     The first secondary air introducing pipe  122  is connected to the intake duct  64  via a first reed valve  124  and a first communicating pipe  125 . The second secondary air introducing pipe  123  is connected to the intake duct  64  via a second reed valve  126  and a second communicating pipe  127 . The first and second reed valves  124  and  126  prevent the exhaust gas from flowing into the intake system from the secondary air introducing pipe  121 . In other words, in the first and second reed valves  124  and  126 , valve bodies  124   a  and  126   a  (see  FIG. 19B ) opens when a negative pressure is generated inside the first exhaust pipe  73 , and only secondary air is made flow to the secondary air introducing pipe  121  side from the intake dust  64  side. 
     The first and second secondary air introducing pipes  122  and  123  preferably include lower portions  122   a  and  123   a  preferably formed by casting integrally with the first exhaust pipe  73 , and upper portions  122   b  and  123   b  attached to the upper ends of the lower portions. The upper portions  122   b  and  123   b  are bent so as to extend across the engine  2  in the width direction of the outboard motor  31  (direction crossing the direction in which the crank case  42  and the cylinder body  43  are lined up in a plan view) above the engine  2  as shown in  FIG. 18 . On the upper portion  122   b  of the first secondary air introducing pipe  122 , as shown in  FIGS. 19A and 19B , a valve housing  124   b  of the first reed valve  124  is preferably formed integrally. On the upper portion  123   b  of the second secondary air introducing pipe  123 , a valve housing  126   b  of the second reed valve  126  is preferably integrally provided. 
       FIG. 19A  is a cross-sectional view of the reed valves  124  and  126 , and  FIG. 19B  is a longitudinal sectional view of reed valves  124  and  126 . The first and second reed valves  124  and  126  include, valve housings  124   b  and  126   b,  reed valve main bodies  124   c  and  126   c  inserted in the valve housings  124   b  and  126   b,  and covers  124   d  and  126   d  which form air passages in cooperation with the valve housings  124   b  and  126   b . The cover  124   d  of the first reed valve  124  is connected to the first communicating pipe  125 , and the cover  126   d  of the second reed valve  126  is connected to the second communicating pipe  127 . 
     In the exhaust device  72  of this preferred embodiment, when the inside of the first exhaust pipe  73  becomes a negative pressure, air is suctioned into the first exhaust pipe  73  through the secondary air introducing pipe  121 . 
     Generally, the outboard motor  31  preferably is mostly used for a long time in an operating state in which the output of the engine becomes maximum. In such an operating state, when the air-fuel ratio during engine operation (hereinafter, this air-fuel ratio will be referred to as “combustion air-fuel ratio”) is set to a theoretical air-fuel ratio to improve the exhaust gas purifying efficiency of the catalysts, the combustion temperature becomes excessively high. Accordingly, problems such as melting of the piston and deterioration of the valve seat may occur. 
     According to the exhaust device  72  of this preferred embodiment, when the inside of the first exhaust pipe  73  becomes negative pressure, secondary air is suctioned into the first exhaust pipe  73  from the secondary air introducing pipe  121 . Accordingly, while high purifying efficiency of the catalysts  78  and  79  is maintained, the combustion temperature can be lowered by setting the combustion air-fuel ratio to the richer side than the theoretical air-fuel ratio. In other words, the exhaust gas and the secondary air flow to the catalysts  78  and  79 , so that even if oxygen in the exhaust gas is short due to the richer combustion air-fuel ratio, supplemental oxygen can be supplied by the secondary air. Therefore, even when the combustion air-fuel ratio is set to the richer side than the theoretical air-fuel ratio, toxic components in the exhaust gas can be sufficiently purified by the catalysts  78  and  79 . 
     Therefore, while the state in which a clean exhaust gas is discharged is maintained, the combustion temperature can be lowered by setting the combustion air-fuel ratio to the richer side than the theoretical air-fuel ratio. As a result, the inside of the exhaust passage  11  becomes a negative pressure and the amount of exhaust gas remaining inside the cylinder  48  of the engine  2  is reduced, and in addition, by making richer the combustion air-fuel ratio, the combustion temperature can be lowered. Accordingly, an occurrence of abnormal combustion such as self-ignition and knocking in the engine  2  can be more reliably prevented. 
     By setting the combustion air-fuel ratio to the richer side than the theoretical air-fuel ratio, the members inside the combustion chamber can be cooled by the vaporization heat of the fuel. Therefore, according to this preferred embodiment, the inner surface of the combustion chamber is cooled by the fuel, so that problems caused by excessive temperature rise inside the combustion chamber, that is, melting of the piston and deterioration of the valve seat, can be prevented. 
     The temperature of the secondary air is substantially the temperature of the atmosphere, and is much lower than the temperature of the exhaust gas. Therefore, according to this preferred embodiment, the temperature of the exhaust gas can be lowered by the large amount of secondary air at the relatively low temperature introduced into the exhaust passage  11 . Accordingly, at the catalysts  78  and  79 , an occurrence of the sintering phenomenon described above can be more reliably prevented. 
     Further, in this preferred embodiment, the upstream portion of the secondary air introducing passage is connected to the intake duct  64  via the reed valves  124  and  126 . Therefore, intake noise caused by suctioning of the air into the secondary air introducing passage and seating noise caused when the valve bodies  124   a  and  126   a  of the reed valves  124  and  126  seat on the valve seats can be reduced by the intake duct  64 . Therefore, noise caused from the secondary air introducing passage can be reduced. 
     The intake duct  64  to which the secondary air introducing passage is connected preferably has a U shape (see  FIG. 6 ) extending from the upper end to the lower end of the engine  2  in a side view. Therefore, even if water entering inside the engine cover  37  is suctioned into the intake duct  64  together with intake air, the water can be prevented from entering so as not to cause problems. The intake air flowing into the air suction port  70  of the intake duct  64  lowers inside the upstream side vertical portion  68  and collides with the bottom or the wall around the bottom of the upstream side horizontal portion  67  and changes its direction, and rises inside the downstream side vertical extending portion  66 . Accordingly, entry of water can be prevented. 
     In other words, even when the outboard motor  31  is used at sea and seawater is suctioned into the intake duct  64  together with intake air, seawater adheres to the bottom or the wall around the bottom of the upstream side horizontal portion  67 , and accordingly, the amount of seawater contained in the intake air can be reduced. By thus reducing the amount of seawater contained in the intake air, although this intake air is introduced into the exhaust passage  11  as secondary air, the catalysts  78  and  79  can be protected from corrosion caused by the salt content in seawater. 
     Further, in this preferred embodiment, the secondary air introducing pipe  121  is connected to the first exhaust pipe  73  including the supersonic nozzle  87 . A negative pressure can be generated inside the exhaust passage  11  by the operation of the supersonic nozzles  87 , so that more secondary air can be suctioned into the exhaust passage  11 . 
     A negative pressure is continuously generated inside the exhaust passage  11  by the supersonic nozzle  87  even when the speed of the engine  2  becomes higher than the speed of the maximum output. In a general engine, in the high-speed operation range, the pressure of the exhaust gas becomes relatively high, so that the secondary air suction amount is greatly reduced. However, according to this preferred embodiment, a high negative pressure is generated inside the exhaust passage  11  even in this high-speed operation range, so that secondary air can be sufficiently introduced into the exhaust passage  11 . As a result, without using an air pump for forcibly blowing secondary air into the exhaust passage  11 , a sufficient supply amount of secondary air as much as the supply amount in the case where such an air pump is adopted can be actively introduced into the exhaust passage  11 . 
     Preferred embodiments of the present invention are described in detail above, and these are merely detailed examples used for making clear the technical content of the present invention, and the present invention should not be construed as being limited to these detailed examples, and the spirit and scope of the present invention are limited only by the appended claims. 
     The present application corresponds to Japanese Patent Application No. 2008-188428 filed on Jul. 22, 2008 to the Japan Patent Office, and whole disclosure of this 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.