Patent Publication Number: US-6338660-B1

Title: Exhaust system for an outboard motor

Description:
FIELD OF THE INVENTION 
     The present invention relates to an exhaust system for an outboard motor with enhanced engine performance by preventing exhaust interference from occurring between a plurality of cylinders. 
     BACKGROUND OF THE INVENTION 
     In a multi-cylinder four-stroke engine which is mounted in a vehicle, such as an automobile or a motorcycle, a plurality of exhaust pipes coming from each cylinder, each having a different respective exhaust timing, are joined together to make a group exhaust. By eliminating both positive pressure waves and negative pressure waves, which are generated by the pulsation of the exhaust gases, the charging efficiency of the intake fuel-air mixture and the engine performance, such as output characteristics and torque characteristics, are improved. In four cylinder engines, for example, four exhaust pipes from each cylinder are joined into two, and subsequently into one; thereby forming a 4-2-1 type group exhaust system that is often used. In addition, it is desirable that the differences in lengths between the exhaust pipes extending from each cylinder are small. 
     However, in an ordinary four-stroke engine for an outboard motor, the engine is mounted vertically so that the crankshaft is oriented in a vertical direction, as shown in FIG.  11 . Exhaust passages  101 , which extend vertically, are provided along the side of the cylinder block  100 . In a four-cylinder engine, for example, four exhaust ports (not shown) from four cylinders  102 A- 102 D communicate with the exhaust passage  101 . Exhaust gases discharged from each cylinder  102 A- 102 D are collected inside the exhaust passage  101 , flow downward and are subsequently discharged into water. 
     In this type of outboard motor, group exhaust does not occur. All exhaust gases discharged by each cylinder  102 A- 102 D are collected inside one exhaust passage  101 . As a result, it has the disadvantage that exhaust interference occurs among those cylinders which have the same exhaust timing, which results in decreased output or worsened torque characteristics. 
     In order to prevent such exhaust interference from occurring, and to apply the 4-2-1 group exhaust system discussed above, one easy method involves providing a separate exhaust manifold to the cylinder block, as disclosed in Japanese Laid-Open Patent Publication No. Hei-9-49425. 
     However, if, as described above, a separate exhaust manifold is mounted to the cylinder block, the number of components will increase, which will cause productivity and assembling efficiency to worsen. Additionally, in the case of an outboard motor, the engine is covered by an engine cover, and air inside the engine cover is used as intake air for the engine. However, the temperature within the engine cover is increased by the heat emitted from the exhaust manifold. Due to this, the density of the intake air is decreased, which results in the possibility of decreased performance or decreased durability of components. To prevent this, it is necessary to form a water jacket around the exhaust manifold, and thereby the structure of the manifold becomes complex and leads to larger manufacturing costs. Further, the possibility of problems, such as water leaks, increase accordingly. 
     The four-stroke engine for an outboard motor of the present invention was invented to solve the problems described above. Having a simple structure, good productivity and design characteristics, the exhaust system of the present invention makes group exhaust possible, prevents exhaust interference from occurring, and increases the cooling efficiency of the exhaust passages. 
     SUMMARY OF THE INVENTION 
     In order to solve the problems described above, the present invention provides an exhaust system for an outboard motor, which, as described in claim 1, is equipped with a four-stroke engine having a cylinder block comprising a plurality of cylinders which are arranged vertically so that the crankshaft is vertically oriented. The exhaust system includes a plurality of separate exhaust passages that extend vertically and are formed integrally with the cylinder block. Each of the exhaust ports from each cylinder which have different respective exhaust timings communicates with one of the plurality of exhaust passages. 
     By constructing the exhaust system for an outboard motor as described above, as a plurality of independent exhaust passages are integrally formed along with the cylinder block, it becomes possible to discharge exhaust gases in groups, while making the exhaust passages of a simple structure without increasing the number of components for the engine. This results in enhanced engine performance. Further, as the exhaust passages are easily formed along with the cylinder block by die-casting, the manufacturing efficiencies of the engine (cylinder block) and the exhaust system are extremely good. 
     Also, in the present invention relating to an exhaust system for an outboard motor the exhaust port from the cylinder disposed in the lowest position out of the plurality of the cylinders communicates with the most distantly disposed exhaust passage out of the plurality of exhaust passages. With this arrangement, it becomes possible to form the exhaust port from the cylinder disposed at the lowest position longer than the exhaust port from the cylinder disposed at higher positions. Thus, the differences in lengths of the exhaust passages, including the exhaust port, are decreased. As a result, exhaust interference is more effectively prevented from occurring. 
     Further, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are joined together at a point which is farther downstream than the point where the exhaust port from the lowest cylinder communicates with one of the exhaust passages. With this arrangement, all of the cylinder exhaust ports communicate with one of the exhaust passages, and then the exhaust passages are joined together. Therefore, effective group exhaust becomes possible. Further, the exhaust passage that is formed from components below the cylinder block is made into one simple structure. 
     Still further, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are arranged to be disposed in a line in the width direction of the outboard motor. With this arrangement, by only changing the length of each exhaust port, it becomes possible to select a combination of groups of exhaust passages, as well as to make a simply designed exhaust system. 
     Moreover, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are arranged to be disposed in a line in the front to rear direction of the outboard motor. By this arrangement, because a plurality of exhaust passages do not protrude over the side face of the cylinder block, it becomes possible to provide a plurality of exhaust passages to make effective use of space, as well as to make a simple exhaust system design. 
     Furthermore, in the present invention relating to an exhaust system for an outboard motor a water jacket is formed within the bulkhead that partitions the plurality of exhaust passages. By including this water jacket, it becomes possible to increase the cooling efficiency within the exhaust passages. 
     Still further, in the present invention relating to an exhaust system for an outboard motor the water jacket formed within the bulkhead communicates with a water jacket that is provided around the exhaust passages. By this arrangement, the ability of cooling water to circulate within the water jacket in the bulkhead improves. The cooling efficiency of the exhaust passages is also greatly improved. 
     The present invention also provides an exhaust system for an outboard motor is equipped with a four-stroke engine having a cylinder block that includes a plurality of cylinders and a plurality of separate exhaust passages. The cylinders are arranged vertically so that a crankshaft is vertically oriented and the cylinders have different respective exhaust timings. Each cylinder has an exhaust port. Each exhaust passage extends in the vertical direction and is formed integrally with the cylinder block. Each exhaust port from each cylinder communicates with at least one of the plurality of exhaust passages. 
     In one embodiment of the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are comprised of an inner exhaust passage and an outer exhaust passage which are arranged in a line in the width direction of the outboard motor. The exhaust port from the cylinder disposed in the lowest position out of the plurality of cylinders communicates with the outer exhaust passage. 
     In a second embodiment of the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are comprised of a front exhaust passage and a rear exhaust passage which are arranged in a line in the front to rear direction of the outboard motor. The structure further includes a joint passage, such that the exhaust port from the cylinder disposed in the lowest position out of the plurality of cylinders communicates with the front exhaust passage through the joint passage. 
     Also, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are joined to each other at a point below the exhaust port connection point of the lowest cylinder. 
     Further, in the present invention relating to an exhaust system for an outboard motor a water jacket is formed within a bulkhead that partitions the plurality of exhaust passages. The water jacket formed within the bulkhead communicates with a water jacket that is provided around the plurality of exhaust passages. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a left side view of an outboard motor provided with an exhaust system of the prevent invention. 
     FIG. 2 is a left side view of the engine, the engine holder and the oil pan. 
     FIG. 3 is a rear view of the engine, the engine holder and the oil pan. 
     FIG. 4 is a rear view of a cylinder block showing one embodiment of the present invention. 
     FIG. 5 is a horizontal sectional view of the cylinder block and the cylinder head along line V—V in FIG.  4 . 
     FIG. 6 is a horizontal sectional view of the cylinder block and the cylinder head along line VI—VI in FIG.  4 . 
     FIG. 7 is a left side view of the cylinder block, as viewed from the direction of arrow VII in FIG.  4 . 
     FIG. 8 is a rear view of a cylinder block showing another embodiment of the present invention. 
     FIG. 9 is a horizontal sectional view of the cylinder block along line IX—IX in FIG.  8 . 
     FIG. 10 is a left side view of the cylinder block, as viewed from the direction of arrow X in FIG.  8 . 
     FIG. 11 is a rear view of a cylinder block depicting prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 1 is a left side view of an outboard motor incorporated with an exhaust system of the present invention. As shown in FIG. 1, the left side represents the front side (the hull side) of the outboard motor, while the right side represents the rear of the motor. FIGS. 2 through 7 show examples of one embodiment of an exhaust system of the present invention. 
     In the uppermost portion of the outboard motor  1 , an in-line four-cylinder, four-stroke engine  2  is mounted. The engine  2  is mounted vertically so that a provided crankshaft  3  is oriented in the vertical direction. As shown in FIGS. 1 and 2, the engine  2  comprises, in sequence from the front side, a crankcase  4 , a cylinder block  5 , a cylinder head  6  and a head cover  7 . The crankshaft  3  is supported by being sandwiched between the crankcase  4  and the cylinder block  5 , while a pair of right and left camshafts  8  within the cylinder head  6  are supported vertically. 
     As shown in FIGS. 1,  4 ,  5  and  6 , in one embodiment, four cylinders (cylinder bores)  10 A- 10 D extend horizontally in the front-rear direction. The four cylinders  10 A- 10 D are formed vertically in a single line within the cylinder block  5 . Referring to FIG. 1, a piston  11  is inserted so as to slide freely within each cylinder  10 A- 10 D and is connected to a crank pin  3   a  of the crankshaft  3  by a con rod  12 . Reciprocating movement of the piston  11  within the cylinders  10 A- 10 D rotates the crankshaft  3 . As shown in FIG. 4, around the cylinders  10 A- 10 D, a plurality of water jackets  13  for circulating cooling water are formed. Around the outside of the cylinders  10 A- 10 D, a total of ten head fastening screw holes  14  are formed. 
     As shown in FIGS. 5 and 6, four combustion chambers  15  are located in the cylinder head  6 . The combustion chambers  15  are formed in concave and integrate with the cylinders  10 A- 10 D of the cylinder block  5 . A spark plug mounting hole  16  is formed at the center of each combustion chamber  15 . Four intake ports  18  are formed at a side of each combustion chamber  15  (for example, at the right side, viewed from the front). Referring to FIGS. 4 through 6, four exhaust ports  19 A- 19 D are formed at another side of the combustion chamber  15  (for example, at the left side, viewed from the front). Each intake port  18  opens at the right side of the cylinder head  6 . Each exhaust port  19 A- 19 D extends toward the left side of the cylinder head  6 , curves toward the front side, and opens where the cylinder head  6  joins to the cylinder block  5 . 
     As shown in FIG. 1, an engine holder  21 , an oil pan  22 , a drive housing  23  and a gear housing  24  are disposed and fixed on the bottom face of the engine  2 , in downward order. A drive shaft  25  extends vertically through and is supported by these components, namely, engine holder  21 , oil pan  22 , drive housing  23  and gear housing  24 . The drive shaft  25  is slightly offset toward the rear side away from the axis line of the crankshaft  3 , and extends downward vertically. A driven gear  26  is located at the upper end of the drive shaft  25 , and the drive shaft  25  and driven gear  26  turn together. A drive gear  27  is provided at the lower end of the crankshaft  3 . The driven gear  26  engages with the drive gear  27  and turns together therewith. Thus, by the engagement between the drive gear  27  and the driven gear  26 , rotation of the crankshaft  3  is transmitted to the drive shaft  25 . 
     As shown in FIG. 1, a propeller shaft  29  is provided horizontally (in the front to rear direction) and is supported within the gear housing  24 . A screw propeller  30  is disposed at the rear end of the propeller shaft  29  and turns together therewith. A bevel gear mechanism  31  and a clutch shifter  32  are disposed at a point where the propeller shaft  29  and the drive shaft  25  cross each other. Rotation of the drive shaft  25  is transmitted to the propeller shaft  29  via the bevel gear mechanism  31 . The screw propeller  30  is thus driven to turn and generate propulsion. As the fixed rotation direction of the drive shaft  25  is switched between forward and reverse by the clutch shifter  32  and transmitted to the propeller shaft  29 , the navigating direction of the outboard motor  1  (the hull of a boat) is selected between forward and reverse. 
     As shown in FIG. 1, a drive sprocket  33  is provided under the driven gear  26  to turn together therewith. A driven sprocket  34  is provided under the camshaft  8  so as to turn together therewith. A timing chain  35  is mounted around the driven sprocket  33  and the drive sprocket  34 . Accordingly, the camshaft  8  turns with the drive shaft  25 . When the camshaft  8  turns, a valve operating mechanism (not shown) provided within the cylinder head  6  is driven whereby intake valves and exhaust valves (not shown) open and close the intake ports  18  and exhaust ports  19 A- 19 D at a prescribed timing. 
     As shown in FIG. 1, the engine  2 , the engine holder  21  and the oil pan  22  are covered with an engine cover  36 , which is made of a synthetic resin to protect these components from water. The engine cover  36  can be separated into upper and lower parts at the border of sealing material  37 . Thus, the upper part of the engine cover  36  may be attached and removed to allow for inspections and maintenance services, on the engine  2 . 
     As shown in FIG. 1, a clamp bracket  40  is provided on the front portion of the main body of the outboard motor  1  and is fixed to the transom of a boat (not shown). The clamp bracket  40  is provided with a swivel bracket  42  via a tilt shaft  41 . A steering shaft  43  is supported vertically within the swivel bracket  42  so as to turn freely. At the upper end and the lower end of the steering shaft  43 , an upper bracket  44  and a lower bracket  45  are provided, which turn together therewith. 
     As shown in FIG. 1, a pair of right and left upper mount units  46  are disposed in the vicinity of the front edge of the engine holder  21 . The mount units  46  are coupled with the upper bracket  44 . A pair of right and left lower mount units  47  are disposed at the right and left sides on the drive housing  23 . The mount units  47  are coupled with the lower bracket  45 . Accordingly, the main body of the outboard motor  1  can freely turn right and left (for steering) around the steering shaft  43  in relation to the clamp bracket  40 , as well as be tilted upward around the tilt shaft  41 . 
     As shown in FIGS. 2 and 3, an intake system  49  is located at the right side of the engine  2 , viewed from the rear. An exhaust system  50  is disposed at the left side of the engine  2 , viewed from the rear. A fuel pump  51  is disposed on the back side of the engine  2 . Also, as shown in FIG. 1, an oil pump  52  is disposed on the bottom surface of the cylinder head  6  and is driven by the camshaft  8 . 
     An intake manifold  53  of the intake system  49  is divided into four branches that communicate with the four intake ports  18  of the cylinder head  6 . As shown in FIG. 1, at the upper end of the crankshaft  3 , a flywheel  54  projects over the upper surface of the engine  2  and is provided to turn integrally. The flywheel  54  is covered by a flywheel cover  55 . A generator  56  is provided within the flywheel  54 . 
     As shown in FIGS. 4 through 6, the exhaust system  50  generally comprises two independent exhaust passages  58 ,  59  that are integrally formed together with the cylinder block  5  at the left side thereof (viewed from the rear). The exhaust passages  58 ,  59  communicate with the exhaust ports  19 A- 19 D that extend from each combustion chamber  15  on each cylinder  10 A- 10 D in the cylinder head  6 . 
     In one embodiment, as shown in FIGS. 4 through 6, the two exhaust passages  58 ,  59  are formed to extend in substantially vertical directions in a line in the width direction, i.e., from left to right in the outboard motor  1  (viewed from the rear). The exhaust passages  58 ,  59  are formed to be opened in a groove configuration on the side of the head connection face  60  (the joined surface of cylinder head  6 ) of the cylinder block  5 . As shown in FIGS. 3 and 4, the inner exhaust,passage  59  is formed to extend downward directly outside (the left side, as viewed from the rear) of the cylinders  10 B,  10 C and  10 D, while the outer exhaust passage  58  is formed so that it extends first diagonally left and downward, and then extends downward in the vertical direction along the outside of the inner exhaust passage  59 . 
     As shown in FIG. 4, the lower portion of the inner exhaust passage  59  curves toward the outside and is joined with the outer exhaust passage  58  near the bottom end area to form an exhaust junction  61 . Thus, the lowermost ends of both exhaust passages  58 ,  59  open at the bottom surface of the cylinder block  5  to form an exhaust port  62 . 
     As shown in FIG. 4, on the outside external wall  63  of the exhaust passage  58  and on the bulkhead  64  that partitions the exhaust passages  58 ,  59 , a total of eight head fastening screw holes  65  are formed. Using these head fastening screw holes  65 , along with the ten head fastening screw holes  14  formed around the cylinders  10 A- 10 D, as described above, the cylinder head  6  is fastened using two types (large and small) of fastening bolts. As a result, the openings of the exhaust passages  58 ,  59  are enclosed by the cylinder head  6  in an airtight manner. 
     As shown in FIGS. 4 through 6, a water jacket  68  is formed inside the external wall  63  and inside the bulkhead  64 . In the rear view, in FIG. 4, this water jacket  68  looks as if it is intercepted by a plurality of the head fastening screw holes  65 . However, as shown in FIGS. 5 and 6, the water jacket  68  communicates with a water jacket  69  provided around (for example, the front side and the left side) the exhaust passages  58 ,  59 . Therefore, cooling water that flows inside the water jacket  69  also flows inside the water jacket  68 . In addition, around the exhaust ports  19 A- 19 D, a water jacket  70  is formed, which also connects to the water jacket  68 . 
     As shown in FIGS. 6 and 7, in the bulkhead  63 , an inspection hole  71 , which communicates with the water jacket  69 , is formed. As shown in FIGS. 2 and 3, this inspection hole  71  is enclosed by an exhaust cover  72  in an airtight manner. 
     As shown in FIGS. 4 through 6, each exhaust port  19 A- 19 D that extends from each combustion chamber  15  of each cylinder  10 A- 10 D communicates with one of the plurality of exhaust passages  58 ,  59 . Thus, of the cylinders  10 A- 10 D, the exhaust ports  19 A- 19 D from the cylinders  10 A- 10 D which have different exhaust timings are joined together into either the exhaust passage  58  or the exhaust passage  59 . For example, the ignition sequence of this engine  2  is  10 A→ 10 C→ 10 D→ 10 B. As shown in FIG. 4, the exhaust port  19 A and the exhaust port  19 D from the cylinder  10 A and the cylinder  10 D, respectively, communicate with the outer exhaust passage  58 , while the exhaust port  19 B and the exhaust port  19 C from the cylinder  10 B and the cylinder  10 C, respectively, communicate with the inner exhaust passage  59 . 
     At the same time, the exhaust port of the cylinder in the lowest position among the cylinders  10 A- 10 D communicates with the most distantly disposed exhaust passage of the plurality of exhaust passages  58 ,  59 . In the present invention, as shown in FIGS. 4 and 6, the exhaust port  19 D from the lowest positioned cylinder  10 D communicates with the outer exhaust passage  58 , crossing over the inner exhaust passage  59 . Because the upper portion of the outer exhaust passage  58  is adjacent to the cylinder  10 A, the exhaust port  19 A from the cylinder  10 A communicates with the exhaust passage  58  by a short distance, without crossing over the inner exhaust passage  59 . 
     Further, the position where the exhaust port  19 D from the lowest cylinder  10 D communicates with the outer exhaust passage  58  is located further upstream, or higher, than the exhaust junction  61  where the inner exhaust passage  59  communicates with the outer exhaust passage  58 . An exhaust passage (not shown) formed within the engine holder  21  is connected to the exhaust port  62 . Furthermore, exhaust passages (not shown) formed inside the oil pan  22  and the drive housing  23  are connected sequentially. 
     As shown in FIG. 4, in the exhaust system  50  structure as described above, when the engine  2  starts, as indicated by arrows with broken lines, exhaust gases a, d are discharged from the cylinder  10 A and the cylinder  10 D, respectively, through the exhaust ports  19 A and  19 D, respectively, and are gathered in the outer exhaust passage  58 . Similarly, exhaust gases b, c are discharged from the cylinder  10 B and the cylinder  10 C, respectively, through the exhaust ports  19 B and  19 C, respectively, and are gathered in the inner exhaust passage  59 . Thus, the gases a, b, c, d are discharged in groups. The exhaust gases a, d that flow in the outer exhaust passage  58  and the exhaust gases b, c that flow in the inner exhaust passage  59  are joined at the exhaust junction  61 , and are finally discharged outside (generally into water) through the exhaust passage (not shown) formed in the engine holder  21 , the oil pan  22 , and the drive housing  23 . 
     In this manner, the exhaust gases a, d discharged from the cylinder  10 A and the cylinder  10 D, respectively, which have different exhaust timings, are collected in the outer exhaust passage  58 , and the exhaust gases b, c discharged from the cylinder  10 B and the cylinder  10 C, respectively, are collected in the inner exhaust passage  59 . Thus, the gases a, b, c, d are discharged in groups. This arrangement prevents exhaust interference from occurring between, for example, the exhaust gas from the cylinder  10 A and the exhaust gas from the cylinder  10 C, or between the exhaust gas from the cylinder  10 B and the exhaust gas from the cylinder  10 D. As a result, the charging efficiency of the intake fuel-air mixture is increased, which results in greatly increased output characteristics and torque characteristics. 
     In this exhaust system  50 , because the two exhaust passages  58 ,  59  are integrally formed together with the cylinder block  5 , it is not necessary to include separate components, such as an exhaust manifold, to construct the exhaust passages  58 ,  59 . Therefore, it becomes possible to discharge exhaust gases in groups by providing exhaust passages  58 ,  59  that have simple structure, without increasing the number of components for the engine  2 . This results in enhanced engine performance. Moreover, because the exhaust passages  58 ,  59  are formed easily together with the cylinder block  5  by die-casting, the manufacturing efficiencies of the engine  2  (cylinder block  5 ) and the exhaust system  50  are extremely good. 
     In addition, because the cylinder  10 D is the lowest cylinder, the exhaust discharge distance from the cylinder  10 D to the exhaust junction  61  (or the exhaust opening  62 ) is apt to be shorter than the exhaust discharge distance for the other cylinders  10 A- 10 C. However, as the exhaust port  19 D crosses over the inner exhaust passage  59  and communicates with the more distantly disposed outer exhaust passage  58 , the exhaust port  19 D may be made long enough to assure sufficient distance for the exhaust discharge distance from the cylinder  10 D. Owing to this arrangement, the differences in the lengths of the exhaust discharge distances between the cylinders  10 A- 10 D may be decreased, which results in a more effective prevention of exhaust interference. 
     Also, since the two exhaust passages  58 ,  59  are disposed in a line in the width direction of the outboard motor  1 , it becomes possible to freely select the group exhaust combination by just changing the length of each exhaust port  19 A- 19 D from each cylinder  10 A- 10 D. Therefore, the designs of the exhaust system  50  and the engine  2  are extremely simple. 
     Still further, the exhaust junction  61  of the two exhaust passages  58 ,  59  is located further downstream than the joining point of the exhaust port  19 D from the lowest cylinder  10 D. Therefore, all of the exhaust ports  19 A- 19 D communicate with either the exhaust passage  58  or the exhaust passage  59 , and then the exhaust passage  58  and the exhaust passage  59  are joined together. With this arrangement, an effective 4-2-1 group exhaust format is provided, and any other exhaust passage connected downstream of the exhaust passages  58 ,  59  may be formed into one unit. Accordingly, the internal configuration of the engine holder  21  and the oil pan  22 , etc., can be made relatively simple. 
     Furthermore, as shown in FIGS. 4 through 6, the water jacket  68 , which is formed in the external wall  63  outside the exhaust passage  58  inside the bulkhead  64  and which partitions the exhaust passage  58  from the exhaust passage  59 , communicates with the water jacket  69  that is provided around the exhaust passages  58 ,  59 . Thus, the circulation of the cooling water inside the water jacket  68  is favorable, and, as a result, the exhaust passages  58 ,  59  are cooled effectively. 
     FIGS. 8 through 10 illustrate another embodiment of the exhaust system of the present invention. The exhaust system  75  is comprised of, generally in the same manner as the exhaust system  50  of the first embodiment, two separate exhaust passages  77 ,  78  formed integrally on the left side of the cylinder block  76 , four exhaust ports  79 A- 79 D extending from combustion chambers (not shown), four cylinders  80 A- 80 D, and a joint passage  82 , which will be described later. 
     The two exhaust passages  77 ,  78 , which extend in the vertical direction, are formed on the left side (viewed from the rear) of each cylinder  80 A- 80 D in the cylinder block  76  in a front to rear direction in the outboard motor  1 . The upper end of the exhaust passage  77  opens in a head connection face  81  of the cylinder block  76  at a place adjacent to the left side of the cylinder  80 A. From there, the exhaust passage  77  extends a short distance toward the front and then turns at a 90° angle and extends downward. Thus, the exhaust passage  77  is formed generally in an upside down “L” shape. The lower end of the exhaust passage  77  opens at the bottom surface of the cylinder block  76 . 
     The other exhaust passage  78  is formed in a groove configuration opened on the side of the head connection face  81  and extends downward along the outside (the left side, viewed from the rear) of the cylinders  80 B,  80 C and  80 D. The lower end of the exhaust passage  78  opens in the bottom face of the cylinder block  76 . The lower ends of the plurality of exhaust passages  77 ,  78  open in the bottom face of the cylinder block  76  separately or after being joined integrally into one. Generally, because the exhaust passage  77  is positioned in front of the other exhaust passage  78 , the exhaust passages may be called the front exhaust passage  77  and the rear exhaust passage  78 . 
     Preferably, the joint passage  82  is formed at the lowest, outside point of the exhaust passages  77 ,  78 . The joint passage  82  extends in the front to rear direction (the axial direction of the cylinder  80 D) along the outside of the exhaust passages  77 ,  78 . One end of the joint passage  82  opens in the head connection face  81 , and the other end communicates with the lower end of the exhaust passage  77 . 
     As shown in FIGS. 8 and 9, within the external wall  83  forming the exhaust passages  77 ,  78  and the bulkheads  84 ,  85 , water jackets  86 ,  87  and  88  are formed. In the external wall  83 , as shown in FIG. 10, an inspection hole  89  is formed that communicates with the water jacket  86 . The inspection hole  89  is enclosed by an exhaust cover (not shown). 
     Because the cylinder head is joined to the cylinder block  76  at the head connection face  81 , the exhaust passages  77 ,  78  are closed by the cylinder head at the open side in an airtight manner. As shown in FIG. 8, the exhaust ports  79 A and  79 D from the cylinder  80 A and the cylinder  80 D, respectively, which have different exhaust timings, communicate with the exhaust passage  77 , while the exhaust ports  79 B and  79 C from the cylinder  80 B and the cylinder  80 C, respectively, communicate with the exhaust passage  78 . In particular, the exhaust port  79 D from the cylinder  80 D initially communicates with the joint passage  82  and then joins with the exhaust passage  77  through the joint passage  82 . 
     When the engine starts, as shown with arrows with broken lines in FIGS. 8 and 10, exhaust gases e, h are discharged from the cylinder  80 A and the cylinder  80 D, respectively, through the exhaust ports  79 A and  79 D, respectively, and are gathered into the exhaust passage  77  and the joint passage  82 . Similarly, exhaust gases f, g are discharged from the cylinder  80 B and the cylinder  80 C, respectively, through the exhaust ports  79 B and  79 C, respectively, and are gathered into the exhaust passage  78 . The gases e, f, g, h are thus exhausted in respective groups. 
     In the exhaust system  75 , by forming the exhaust passages  77 ,  78  integrally together with the cylinder block  76 , it becomes possible to discharge exhaust gases in groups using a simple structure, without increasing the number of components for the engine  2 . This results in improved performance by preventing exhaust interference from occurring. Further, as the exhaust port  79 D from the lowest cylinder  80 D communicates with the exhaust passage  77 , which is located further away than the exhaust passage  78 , through the joint passage  82 , the differences in lengths between the exhaust distance from the cylinder  80 D and the exhaust distance from the cylinders  80 A- 80 C are decreased. This results in more effective prevention of exhaust interference. 
     Furthermore, as the exhaust passages  77 ,  78  are formed in a double layer in the front to rear direction in the outboard motor  1  (at the left side of the cylinder block  76 ), the exhaust passages  77 ,  78  do not greatly project out from the left side surface (viewed from the rear) of the cylinder block  76 . Therefore, compared to the exhaust system  50  of the first embodiment, the exhaust passages  77 ,  78  may be arranged effectively in terms of space, and the designs of the exhaust system  75  and its surroundings may be easily designed. 
     In the first embodiment and the second embodiment described above, it is assumed that the engine  2  is an in-line four-cylinder engine. However, it is possible to apply the exhaust system of the present invention to, for example, V-type engines or horizontal opposed-cylinder-type, etc., or any multi-cylinder engine having other arrangements of cylinders. Therefore, a wide range of variations may exist in terms of the configuration and the number of exhaust passages provided in the cylinder block, or in the configuration of communicating the exhaust ports with the exhaust passages or grouping thereof. 
     According to the exhaust system for an outboard motor of the present invention, as described above, in the outboard motor equipped with a four-stroke engine having a cylinder block comprising a plurality of cylinders arranged vertically so that a crankshaft is vertically oriented, because a plurality of separate exhaust passages that extend vertically are formed integrally and each of the exhaust passages communicates with at least one of the exhaust ports from a plurality of cylinders having different respective exhaust timings, it becomes possible to provide an exhaust system with simple structure and with enhanced engine performance that provides group exhaust from a plurality of cylinders and prevents exhaust interference from occurring, while featuring a simple design and good manufacturing characteristics for the engine and the exhaust system. 
     Also, in the exhaust system for an outboard motor relating to the present invention, because the exhaust port from the cylinder located in the lowest position communicates with the most distantly disposed exhaust passage out of the plurality of exhaust passages, it becomes possible to decrease the differences in the lengths of the exhaust discharge distances between the cylinders, including the lengths of the exhaust ports, which results in more effective prevention of exhaust interference from occurring. 
     Further, in the exhaust system for an outboard motor relating to the present invention, because the plurality of exhaust passages are joined together at a point further downstream from a point where the exhaust port from the lowest cylinder is joined, it becomes possible to carry out effective group exhaust, as well as to simplify the structure of the exhaust system. 
     Moreover, in the exhaust system for an outboard motor relating to the present invention, because, in one embodiment, the plurality of exhaust passages are arranged in a line in the width direction of the outboard motor, it becomes possible to freely select the combination of group exhaust, as well as to simplify the design of the exhaust structure and engine. 
     Furthermore, in the exhaust system for an outboard motor relating to the present invention, because, in a second embodiment, the plurality of exhaust passages are arranged in a line in the longitudinal (front to rear) direction of the outboard motor, it becomes possible to provide a plurality of exhaust passages in a spatially beneficial manner and to make the design of the exhaust system easy. 
     Still further, in the exhaust system for an outboard motor relating to the present invention, because a water jacket is formed inside the bulkhead that partitions the plurality of exhaust passages, the cooling efficiency of the exhaust passages is enhanced. 
     Still further, in the exhaust system for an outboard motor relating to the present invention, because the water jacket formed inside the bulkhead communicates with a water jacket that is provided around the exhaust passages, the circulation of the cooling water within the water jackets may be enhanced to effectively cool the exhaust passages. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 
     
       
         
           
               
             
               
                   
               
               
                 Description of Reference Numerals 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 1 
                 Outboard motor 
               
               
                   
                 2 
                 Engine 
               
               
                   
                 3 
                 Crankshaft 
               
               
                   
                 5, 76 
                 Cylinder block 
               
               
                   
                 50, 75 
                 Exhaust system 
               
               
                   
                 58, 59, 77, 78 
                 Exhaust passages 
               
               
                   
                 10A-10D, 80A-80D 
                 Cylinders 
               
               
                   
                 19A-19D, 79A-79D 
                 Exhaust ports 
               
               
                   
                 64, 84, 85 
                 Bulkhead 
               
               
                   
                 68, 69, 86, 87, 88 
                 Water jackets