Abstract:
An exhaust system for an internal combustion engine of an outboard motor is provided. The exhaust system provides dual exhaust passages, simplifying the engine&#39;s construction and reducing manufacturing costs, while optimizing engine performance. The exhaust system includes exhaust ports having equal lengths that avoid differential pressure losses across cylinders. The arrangement of the exhaust passages enables a cowling to have a tapered aerodynamic design.

Description:
RELATED APPLICATION 
   This application claims priority to Japanese Patent Application No. 2001-257031, filed on Aug. 27, 2001, the entire contents of which are hereby expressly incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to outboard motors for watercraft, and in particular, to an exhaust system for an outboard motor. 
   2. Description of the Related Art 
   Outboard motors containing internal combustion engines are commonly used for powering watercraft. A housing, which is mounted to a transom of the watercraft, typically encloses the engine. Rotation of a crankshaft of the internal combustion engine drives a driveshaft. The driveshaft drives a water propulsion device, such as a propeller. When the watercraft operates, the propeller is submerged beneath a water surface. Rotation of the propeller moves the watercraft across the water surface. 
   Many internal combustion engines in outboard motors include four cylinders and operate on the four-stroke combustion cycle. The four-stroke combustion cycle is well known to those of skill in the art, and therefore will not be explained in detail here. Four-stroke engines comprise a crankcase in which the crankshaft is housed, a cylinder block extending generally horizontally from the crankcase, and a cylinder head extending generally horizontally from the cylinder block. The cylinder block defines four cylinder bores that are generally arranged vertically. The cylinder head defines four exhaust ports, with one exhaust port being associated with each cylinder. Each exhaust port expels exhaust gases into a runner, which provides a fluid communication path between its associated cylinder and an exhaust passage. The exhaust passage discharges the exhaust gases to the atmosphere. 
   Some four-cylinder four-stroke engines include first and second exhaust passages extending generally vertically through the cylinder block. An engine having a dual exhaust passage configuration is simpler and less expensive to construct than an engine with four exhaust passages. The exhaust ports of two of the cylinders communicate with the first exhaust passage, and the exhaust ports of the other two cylinders communicate with the second exhaust passage. 
   In order to reduce interference of the exhaust gas flow from the cylinders, the cylinders are paired according to the firing order. For example, the cylinders are paired such that the timing of the opening of the exhaust valves of the two cylinders communicating with the first exhaust passage are not sequential. Similarly, the timing of the opening of the exhaust valves of the two cylinders communicating with the second exhaust passage are not sequential. For example, if the first and fourth cylinders communicate with the first exhaust passage, and the second and third cylinders communicate with the second exhaust passage, then a preferred firing order would be 1-2-4-3. 
   Non-sequential exhaust timing eliminates interference between exhaust gases traveling through the same exhaust passage. If the exhaust times of the two exhaust ports communicating with the same exhaust passage were sequential, then the exhaust gas pulse from one cylinder could interfere with the next exhaust pulse from the other cylinder. Providing a temporal gap between exhaust pulses entering the same exhaust passage reduces the interaction between exhaust pulses discharged from the cylinders. Thus, non-sequential exhaust timing allows each cylinder to exhaust under equal pressure, leading to better engine performance. 
   In engines having non-sequential exhaust timing, the exhaust passages are arranged side by side in a lateral (port to starboard) direction. This arrangement increases an overall width of the engine. A sturdy plastic cowling typically encloses the engine. As the width of the engine increases, so must the width of the cowling. Ideally, however, the width of the cowling decreases gradually toward the top. This design provides the cowling with a more pleasing appearance and with more favorable aerodynamic properties. A wider engine compromises these two desirable properties of the cowling. Some outboard motors provide a tapered cowling even for very wide, non-tapering engines by creating a large lateral gap between the engine and the cowling at a lower end of the engine. However, this solution only expands the overall width of the cowling and wastes material for the cowling. 
   Because the exhaust passages are arranged side by side in a lateral direction, one of the exhaust passages is spaced farther from the cylinders. Therefore, the exhaust runners leading to the farther exhaust passage are longer than the exhaust runners leading to the nearer exhaust passage. The cylinder head defines exhaust runners. A cylinder head having exhaust runners of unequal lengths is more complex, and therefore more expensive to manufacture, than a cylinder head having exhaust runners of equal lengths. 
   Furthermore, the cylinders all have equal volumes. Therefore, gases passing through exhaust runners of different lengths experience different pressure losses. As explained above, cylinders exhausting under unequal pressure compromise engine performance. Prior attempts at eliminating the problems caused by laterally side-by-side exhaust passages and exhaust runners of unequal lengths have been unsuccessful. 
   SUMMARY OF THE INVENTION 
   The preferred embodiments of the present exhaust system for outboard motor have several features, no single one of which is solely responsible for their desirable attributes. Without limiting the scope of this exhaust system for outboard motor as expressed by the claims that follow, its more prominent features are discussed below. After considering this discussion, and particularly after reading the section entitled “Detailed Description of the Preferred Embodiments,” one will understand how the features of the preferred embodiments provide advantages, which include optimized engine performance with a simplified design that is inexpensive to manufacture, and the ability to receive a cowling that is attractive and aerodynamic. 
   One aspect of the present invention includes the realization that an arrangement of dual exhaust passages can be altered such that the exhaust ports defined in the cylinder head can have the same length, while allowing the exhaust passages to remain in a side-by-side arrangement. As such, engine performance can be enhanced and the complexity of certain engine components can be reduced. 
   In accordance with one preferred embodiment of the present invention, a four-cycle, four-cylinder internal combustion engine for an outboard motor comprises a crankcase, a crankshaft rotatably supported by the crankcase about a vertically extending axis. The engine includes a first cylinder, a second cylinder, a third cylinder and a fourth cylinder, with their axes being substantially parallel and intersecting the axis of rotation. A cylinder block extends from the crankcase and defines the cylinders. A cylinder head is secured to the cylinder block opposite the crankcase. The cylinder head defines at least four exhaust ports, each exhaust port being in selective fluid communication with one of the cylinders. The exhaust ports provide fluid communication paths from combustion chambers in the cylinders to exhaust passages. The cylinder block has first and second exhaust passages formed therein, the exhaust passages extending generally vertically. The exhaust ports of the first and second cylinders each have a downstream end connected to the first exhaust passage at first inlets, and the exhaust ports of the third and fourth cylinders each have a downstream end connected to the second exhaust passage at second inlets. The engine is configured such that first and second cylinders do not perform exhaust strokes consecutively and such that the third and fourth cylinders do not perform exhaust strokes consecutively. The first and second inlets are located equidistant from the cylinders as measured in a direction perpendicular to both the axis of rotation and the cylinder axes. 
   In accordance with another aspect of the invention, an internal combustion engine comprises an engine body, the engine body defining at least first and second cylinder bores. A crankshaft is journaled for rotation at least partially within the engine body. The engine is configured such that the crankshaft is generally vertical and the cylinder bores are generally horizontal during operation. The engine body defines at least first and second combustion chambers therein. First and second exhaust ports extend from the first and second combustion chambers, respectively. A first exhaust passage extends downwardly from a first outlet of the first exhaust port. A second exhaust passage extends from a second outlet of the second exhaust port, generally parallel to the cylinder bores and around the outlet of the first exhaust port, then downwardly and generally parallel to the first exhaust passage. 
   In accordance with yet another aspect of the presnet invention, an exhaust system for an outboard motor includes a four-cylinder internal combustion engine. The exhaust system comprises a first exhaust passage extending generally vertically along a cylinder block and having an outlet end in a surrounding medium. A second exhaust passage extends along the cylinder block and has an outlet end in the surrounding medium. First exhaust ports provide first fluid communication paths from a first combustion chamber and a second combustion chamber to the first exhaust passage. Second exhaust ports provide second fluid communication paths from a third combustion chamber and a fourth combustion chamber to the first exhaust passage. The first and second exhaust ports are all equal in length. 
   In accordance with another aspect of the present inevntion, a method of improving the performance of a four-cylinder internal combustion engine comprises the steps of providing a first exhaust passage, providing a second exhaust passage, providing a first fluid path from a first cylinder to the first exhaust passage, providing a second fluid path from a second cylinder to the first exhaust passage, providing a third fluid path from a third cylinder to the second exhaust passage, and providing a fourth fluid path from a fourth cylinder to the second exhaust passage. All the fluid paths are of equal length. Additionally, the method includes providing an exhaust sequence in which exhaust times of the first and second cylinders are not consecutive, and in which exhaust times of the third and fourth cylinders are not consecutive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the exhaust system for outboard motor, illustrating its features, will now be discussed in detail. These embodiments depict the novel and non-obvious exhaust system for outboard motor shown in the accompanying drawings, which are for illustrative purposes only. These drawings include the following figures, in which like numerals indicate like parts: 
       FIG. 1  is a side elevational view of an outboard motor constructed in accordance with the present exhaust system for outboard motor, with certain features including an engine, driveshaft, and transmission shown in phantom; 
       FIG. 2  is a cross-sectional rear view of the outboard motor of  FIG. 1 , taken along line  2 — 2  in FIG.  3  and illustrating the cylinder block and exhaust passages; 
       FIG. 3  is a partial cross-sectional top view of the outboard motor of  FIG. 1 , taken along a horizontal plane passing through the first cylinder and extending generally fore to aft and illustrating the cylinder block, cylinder head and exhaust passages; 
       FIG. 4  is a partial cross-sectional view of the outboard motor of  FIG. 1 , taken along line  4 — 4  in FIG.  2  and illustrating the exhaust passages; 
       FIG. 5  is a partial cross-sectional top view of a modification of the outboard motor of  FIG. 1 , taken along a horizontal plane passing through the first cylinder and extending generally fore to aft and illustrating the cylinder block, cylinder head and exhaust passages; 
       FIG. 6  is a cross-sectional rear view of the outboard motor of  FIG. 5 , taken along line  6 — 6  in FIG.  5  and illustrating the cylinder block and exhaust passages; 
       FIG. 7  is a partial cross-sectional side view of the outboard motor of  FIG. 5 , taken along line  7 — 7  in FIG.  6  and illustrating the exhaust passages; 
       FIG. 8  is an enlarged cross-sectional rear view of a modification of the outboard motor of  FIG. 1 , taken along a vertical plane passing through the junction between the cylinder block and cylinder head and illustrating lower portions of the cylinder block and exhaust passages; and 
       FIG. 9  is an enlarged cross-sectional side view of the outboard motor of  FIG. 8 , taken along line  9 — 9  in FIG.  8  and illustrating lower portions of the exhaust passages. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 1 , the description below provides an overall configuration of an outboard motor. This description assists the reader&#39;s understanding of a preferred environment of use for the present exhaust system for outboard motor. However, as those of skill in the art will appreciate, the present exhaust system and associated components described below can be used in other vehicles, such as, for example, but without limitation, personal watercraft, jet boats, off-road vehicles, and other vehicles. Additionally, the outboard motor is described with reference to a coordinate system wherein a longitudinal axis extends from fore to aft and a lateral axis extends from port side to starboard side normal to the longitudinal axis. In addition, relative heights are expressed as elevations in reference to the under surface of the watercraft. In various figures, an arrow labeled “F” points along the longitudinal axis and indicates a forward direction of travel for the watercraft. 
     FIG. 1  illustrates a watercraft  10  comprising a hull  12  floating on a water surface  14 . The watercraft  10  includes an outboard motor  16 . A clamping bracket  18  secures the outboard motor  16  to the hull  12 . 
   A casing houses the components of the outboard motor  16 . The casing includes a lower portion  20 , which is submerged beneath the water surface  14 , an intermediate portion  22  of “driveshaft housing” extending generally vertically from the lower portion  20 , and an upper portion  24  extending generally vertically from the intermediate portion  22 . 
   The upper portion  24  comprises a cowling  26 , which is generally constructed of a sturdy plastic. The cowling  26  contains an internal combustion engine  28 , which generates power to propel the watercraft  10  across the water surface  14 . In the illustrated embodiment, the engine  28  includes four cylinders and operates on the four-stroke combustion cycle, however, engines operating under different principles (rotary, diesel, two-stroke, etc.) and having other sumbers of cylinders can be used. 
   The engine  28  turns a crankshaft  30 , which is housed in a crankcase  32 . The crankshaft  30  turns a vertically extending driveshaft  34 . The driveshaft  34 , having an axis of rotation  36 , extends from the upper portion  24 , through the intermediate portion  22  and into the lower portion  20 . A lower end of the driveshaft  34  is operably connected to a propeller shaft  38 , which rotates with the driveshaft  34 . The propeller shaft  38  extends generally parallel to the water surface  14 , and includes a propeller  40  mounted to an aft end thereof. The propeller  40  rotates with the propeller shaft  38 , generating force on the water. The reaction force of the water upon the propeller  40  propels the watercraft  10  across the water surface  14 . 
   The engine  28  comprises a cylinder block  42  defining the cylinders  44 ,  46 ,  48 ,  50  (FIG.  2 ). The cylinder block  42  is preferably constructed of die-cast aluminum. However, as those of skill in the art will appreciate, the cylinder block  42  may be constructed of a variety of other materials, such as iron. 
   A cylinder head  52  extends from an aft facing substantially vertical face  54  ( FIG. 3 ) of the cylinder block  42 . The cylinder head  52  comprises a substantially vertical face  56  ( FIGS. 3 and 4 ) that is preferably secured to the cylinder block face  54  with fasteners  57  (FIG.  2 ). A sealing gasket (not shown) is preferably disposed between the abutting faces  54 ,  56 . Like the cylinder block  42 , the cylinder head  52  is preferably constructed of die-cast aluminum. However, as those of skill in the art will appreciate, the cylinder head  52  may be constructed of a variety of other materials, such as iron. 
   The cylinders  44 ,  46 ,  48 ,  50  are preferably arranged vertically, and are preferably equally spaced from one another in the vertical direction. For ease of reference, the cylinders  44 ,  46 ,  48 ,  50  are numbered first through fourth (FIG.  2 ). The uppermost cylinder will be referred to herein as the first cylinder  44 , the next uppermost cylinder will be referred to as the second cylinder  46 , the next uppermost cylinder will be referred to as the third cylinder  48 , and the lowermost cylinder will be referred to as the fourth cylinder  50 . A longitudinal axis  58  ( FIG. 2 ) of each cylinder  44 ,  46 ,  48 ,  50  extends in the direction of the longitudinal axis of the watercraft  10 . Each cylinder  44 ,  46 ,  48 ,  50  houses a piston  60  (FIG.  3 ), which is slidable within the cylinder  44 ,  46 ,  48 ,  50  along the cylinder axis  58 . 
   The pistons  60  reciprocate within their respective cylinders  44 ,  46 ,  48 ,  50  in response to combustion reactions in each cylinder  44 ,  46 ,  48 ,  50 . A piston rod  62  ( FIG. 3 ) connects each piston  60  to the crankshaft  30 , which is housed in the crankcase  32 . The crankcase  32  extends in the direction of the arrow F from a substantially vertical face  64  of the cylinder block  42  (FIG.  3 ). The reciprocating motion of the pistons  60  turns the crankshaft  30 , which turns the vertically extending driveshaft  34 . 
   A space defined between the cylinder head  52  and the piston  60  in each cylinder comprises a combustion chamber  66  (FIGS.  2  and  3 ). Each combustion chamber  66  includes an associated intake port  68  (FIG.  3 ), which is formed in the cylinder head  52 . An intake valve  70  selectively opens and closes each intake port  68 , enabling air-fuel charges  71  to enter the combustion chamber  66  during the intake stroke. 
   Each combustion chamber  66  also includes an associated exhaust valve seat  72  (FIG.  3 ), which is also formed in the cylinder head  52 . An exhaust valve  74  selectively opens and closes each vavle seat  72 , enabling the exhaust gases  76  to exit the combustion chamber  66  during the exhaust stroke. The opening and closing of the intake and exhaust valves  70 ,  74  is synchronized with rotation of the crankshaft  30 . 
   An exhaust port  78  extends from each of the valve seats  72 .  FIG. 3  illustrates only one exhaust valve  74  and valve seat  72  per cylinder. However, any number of exhaust valves  74  and seats  72  can be included for each combustion chamber  66 . 
   In the illustrated embodiment, the exhaust port  78  is a curved, substantially U-shaped, tubular passage ( FIG. 3 ) extending from the valve seat  72  to an exhaust passage  80 ,  82  (FIGS.  3  and  4 ). Each exhaust port  78  is preferably the same size, such that a gas path through each exhaust runner  78  is the same length. 
   A downstream end of each exhaust port  78  opens into one of the exhaust passages  80 ,  82  (FIGS.  3  and  4 ). The exhaust passages  80 ,  82  each comprise a tubular portion of the cylinder block  42  that is spaced from the cylinders  44 ,  46 ,  48 ,  50  in the lateral direction and positioned rearwardly of the axis of rotation  36 . 
   The exhaust passages  80 ,  82  extend generally vertically through the cylinder block  42 . A lower end of the first exhaust passage  80  opens into a first lower exhaust passage  84 , and a lower end of the second exhaust passage  82  opens into a second lower exhaust passage  86  (FIG.  2 ). The lower exhaust passages  84 ,  86  extend into the intermediate portions  20  of the casing (FIG.  1 ). Preferably, the exhaust passages  84 ,  86  terminate in an expansion chamber (not shown) before being expelled to the atmosphere. Preferably, the outboard motor  16  includes an above the water exhaust discharge (not shown) for idle speed operation and a below the water exhaust discharge (not shown) for higher speed operation. 
   The strokes of the pistons  60  are performed in sequence. A phase difference between sequential pistons  60  is preferably 90°, as measured in terms of an angle of the crankshaft  30 . However, the phase difference would be different in engines having other numbers of cylinders and other cylnder configurations (e.g., opposed, V, and W configurations). 
   A preferred firing order for the cylinders  44 ,  46 ,  48 ,  50  is the first cylinder  44 , followed by the third cylinder  48 , followed by the fourth cylinder  50 , followed by the second cylinder  46 . Thus, the exhaust strokes of the first cylinder  44  and the fourth cylinder  50 , which both communicate with the first exhaust passage  80 , are not consecutive, i.e., another cylinder (the third cylinder  48 ) performs the exhaust stroke between the exhaust strokes of cylinders  44  and  50 . Similarly, the exhaust strokes of the second cylinder  46  and the third cylinder  48 , which both communicate with the second exhaust passage  82 , are not consecutive. 
   Non-consecutive exhaust timing avoids the negative engine performance consequences described above. Those of skill in the art will appreciate that non-consecutive exhaust timing is not crucial to proper functioning of the present exhaust system. Those of skill in the art will further appreciate that the cylinders  44 ,  46 ,  48 ,  50  could have a different firing order, whether the order achieved non-consecutive exhaust timing or not. 
   The downstream ends of the exhaust ports  78  of the first and fourth cylinders  44 ,  50  open into the first exhaust passage  80  at inlets  88  (FIGS.  2 - 4 ). The downstream ends of the exhaust ports  78  of the second and third cylinders  46 ,  48  open into the second exhaust passage  82  at inlets  88 . 
   As shown in  FIG. 2 , all four exhaust inlets  88  are located equidistant from their associated cylinder  44 ,  46 ,  48 ,  50  in the lateral direction. Thus, all the exhaust runners  78  have the same length, and each provides the same pressure loss to the exhaust gases  76  as the gases  76  pass through the ports  78 . Thus, each cylinder exhausts gases  76  at more uniform pressure, with more uniform fluid flow characteristics, reducing differences across cylinders that impede optimum engine performance. 
   The present exhaust system facilitates optimum engine performance even with the relatively simple dual exhaust passage configuration. The equal-length exhaust ports  78  further simplify the cylinder head  52 . If the cylinder head  52  is die-cast, the cores used to form the exhaust vale seats  72  can be equal sized, which simplifies the process of making the die. The simplified engine manufacturing process advantageously lowers the cost of manufacturing the engine  28 . 
   An upper end of the first exhaust passage  80  is located relatively close to the cylinders  44 ,  46 ,  48 ,  50 , as measured in the lateral direction (FIG.  2 ). The first exhaust passage  80  extends straight down through the cylinder block  42  in a direction parallel to the axis of rotation  36  (FIG.  2 ). 
   The upper portion of the second exhaust passage  82  is located the same distance from the cylinders  44 ,  46 ,  48 ,  50  as the first exhaust passage  80 , as measured in the lateral direction and at substantially the same height as a midportion of the first exhaust passage  80  (FIG.  2 ). The upper portion of the second exhaust passage  82  extends straight down through the cylinder block  42  in a direction parallel to the first exhaust passage  80 . 
   At approximately the height of a lower portion of the third cylinder  48  (FIGS.  2  and  4 ), a lower portion of the second exhaust passage  82  preferably extends away from the cylinders  44 ,  46 ,  48 ,  50  in the lateral direction (FIG.  2 ). The lower portion subsequently curves toward the first exhaust passage  80  in the longitudinal direction (FIG.  4 ), and then downward ( FIGS. 2 and 4 ) in a direction parallel to the axis of rotation  36 . An outlet  90  of the second exhaust passage  82  (FIGS.  2  and  4 ), which is located approximately at the height of the fourth cylinder  50 , extends in a direction parallel to an outlet  90  of the first exhaust passage  80 , and is spaced therefrom in the lateral direction (FIG.  2 ). 
   The illustrated arrangement of the exhaust passages  80 ,  82  prevents the exhaust passages  80 ,  82  from interfering with each other. The upper extent of the second exhaust passage  82  is located below the inlet  88  associated with the first cylinder  44  (FIG.  2 ), and the lower portion of the second exhaust passage  82  is routed around the inlet  88  associated with the fourth cylinder  50  (FIG.  2 ). This arrangement creates greater freedom to arrange the lower portions of the exhaust passages  80 ,  82 , and facilitates easy connection of the outlets  90  to the first and second lower exhaust passages  84 ,  86  (FIG.  2 ). 
   With reference to  FIG. 2 , because the upper portions of the first and second exhaust passages  80 ,  82  are located in close proximity to the cylinders  44 ,  46 ,  48 ,  50  as measured in the lateral direction, the width of the upper portion of the engine  28  is less than the width of the lower portion of the engine  28 . Thus, a width of the cowling  26  that covers the engine  28  can have an inwardly tapered shape, which creates the aesthetic and aerodynamic benefits described above without wasting cowling  26  material. 
   In the illustrated embodiment, the first and second exhaust passages  80 ,  82  are entirely independent from one another. Thus, the exhaust gases  76  from the first and fourth cylinders  44 ,  50  do not interfere with the exhaust gases  76  from the second and third cylinders  46 ,  48 . This arrangement eliminates the undesirable effects on engine performance of exhaust back pressure, which are described above. 
   Those of skill in the art will appreciate that the relationship of the cylinders  44 ,  46 ,  48 ,  50  to the exhaust passages  80 ,  82  could be altered. For example, the first cylinder  44  and second cylinder  46  could be connected to the first exhaust passage  80 , and the third cylinder  48  and fourth cylinder  50  could be connected to the second exhaust passage  82 . In this arrangement, the second exhaust passage  82  would extend only as high as the third cylinder  48 . Also in this arrangement, a preferred firing order for the cylinders  44 ,  46 ,  48 ,  50  would be the first cylinder  44 , followed by the third cylinder  48 , followed by the second cylinder  46 , followed by the fourth cylinder  50 . 
     FIGS. 5-9  illustrate further preferred embodiments of the present exhaust system for outboard motor. In these figures reference numbers that are identical to reference numbers in  FIGS. 1-4  indicate features that are substantially identical to the same features in the embodiment of  FIGS. 1-4 . Such features will not be described again below. Rather, the description below focuses on the differences between the further embodiments and the embodiment of  FIGS. 1-4 . Those of skill in the art will appreciate that the features of the further embodiments may be combined with the features of the embodiment of  FIGS. 1-4 . 
   In the embodiment of  FIGS. 5-7 , the second exhaust passage  82  extends back toward the cylinders  44 ,  46 ,  48 ,  50  in the lateral direction ( FIG. 6 ) at a position below the inlet  88  of the fouth cylinder  50 . The outlet  90  of the second exhaust passage  82  connects to the first exhaust passage  80 . The combined outlets  90  of the first and second exhaust passages  80 ,  82  open into a lower exhaust passage  84 , which extends into the intermediate portion  22 . 
   This embodiment requires only one lower exhaust passage to transfer exhaust gases  76  from the first and second exhaust passages  80 ,  82  to intermediate portion  22 . Thus, this embodiment is well adapted for simplified exhaust arrangements including only one lower exhaust passage. 
   In the embodiment of  FIGS. 8 and 9 , the outlet  90  of the second exhaust passage  82  extends back toward the cylinders  44 ,  46 ,  48 ,  50  in the lateral direction ( FIG. 8 ) at a position below the inlet  88  of the cylinder  50 . The passage  82  extends forward into the cylinder block  42  and then extends straight downward in the direction of the axis of rotation  36 , and does not connect with outlet  90  of the first exhaust passage  80 . This arrangement reduces a lateral width of the engine  28  at the height of the outlets  90 . Thus, the cowling  26  at this height can be made more narrow. This arrangement is advantageous for outboard motors requiring more narrow cowlings. 
   Scope of the Invention 
   The above presents a description of the best mode contemplated for carrying out the present exhaust system for outboard motor, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains to make and use this exhaust system for outboard motor. This exhaust system for outboard motor is, however, susceptible to modifications and alternate constructions from that discussed above that are fully equivalent. Consequently, this exhaust system for outboard motor is not limited to the particular embodiments disclosed. On the contrary, this exhaust system for outboard motor covers all modifications and alternate constructions coming within the spirit and scope of the exhaust system for outboard motor as generally expressed by the following claims, which particularly point out and distinctly claim the subject matter of the exhaust system for outboard motor.