Patent Publication Number: US-6662555-B1

Title: Catalyzer arrangement for engine

Description:
PRIORITY INFORMATION 
     This application is based on and claims priority to Japanese Patent Application No. 11-165,707, filed Jun. 11, 1999, the entire contents of which is hereby expressly incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a catalyzer arrangement for an engine, and more particularly to an improved catalyzer arrangement that is suitable to an engine of an outboard motor. 
     2. Description of Related Art 
     A typical outboard motor includes an engine for powering a propulsion device. A protective cowling surrounds the engine. The engine and protective cowling together define a power head of the outboard motor. A driveshaft housing depends from the power head and supports a driveshaft that extends from a crankshaft of the engine. A lower unit depends from the driveshaft housing and carries a propulsion device, such as, for example, a propeller that is driven by the driveshaft through a propulsion shaft. The engine is provided with an exhaust system that discharges exhaust gases from the motor. 
     A typical exhaust system generally comprises three exhaust passages. The first passage is disposed within the engine or on the engine and within the protective cowling. The first passage leads exhaust gases away from the engine. The second passage is disposed within the driveshaft housing and the lower unit and guides the exhaust gases to the third passage. The second passage also silences exhaust noise by passing the exhaust gases through at least one expansion chamber. The third passage is defined within a hollow hub of the propeller and terminates at a discharge port formed at the end of the hub. Normally, an idle exhaust passage with an idle discharge port is provided in the driveshaft housing above the water line of the body of water that surrounds the outboard motor. The majority of the exhaust gasses are discharged to the body of water through the discharge port of the propeller hub, while the idle exhaust gasses are discharged to the atmosphere through the idle discharge port. 
     It is quite important for environmental concerns to remove hydrocarbons and the like from exhaust gases. For at least this reason, the exhaust gases often are purified with a catalyzer that is disposed within the exhaust system. The catalyzer includes components that chemically react with the exhaust gases in a manner that renders certain of the exhaust gas constituents substantially environmentally harmless. The larger the catalyzer is, the greater its efficiency is; however, because the engine is surrounded by the protective cowling, space is at a premium and limited areas are available for positioning the catalyzer. If the engine has multiple cylinder bores, properly positioning the catalyzer becomes more difficult. Moreover, if a large single catalyzer or small multiple catalyzers are used to treat exhaust gases coming from the respective cylinder bores, finding adequate space within the cramped confines of the cowling becomes very difficult. 
     In one arrangement, such as that disclosed by U.S. Pat. No. 5,239,825, a catalyzer arrangement for a multiple cylinder engine features a single catalyzer that is disposed in the first exhaust passage and sideward of the engine. Although the arrangement is compact, the catalyzer is somewhat bulky. 
     U.S. Pat. No. 5,378,180 discloses another arrangement in which a catalyzer is disposed also in the first exhaust passage but rearward of an engine. This type of arrangement, however, requires a large amount of space rearward of the engine. It is undesirable to expand the motor rearward because such a construction would make handling of the motor more difficult. Additionally, if the engine operates on a four-stroke combustion principle, a voluminous valve system is disposed in this space and consumes a majority of the available area. 
     U.S. Pat. Nos. 5,174,112 and 5,280,708 disclose further arrangements of catalyzers. The catalyzers in these patents are disposed in the second exhaust passages that are positioned within the driveshaft housing. Although a relatively large capacity is available with the catalyzer in this arrangement, the catalyzer is likely positioned proximate the water line. As is known, catalyzers can be fouled or shattered by contact with water. Accordingly, positioning the catalyzers proximate the water line is disadvantageous due to the possibility of water back flow through the exhaust system. Thus, catalyzers preferably are positioned well above the water line or the exhaust system preferably includes a shelter that can protect the catalyzers from water contact. 
     A need therefore exists for an improved catalyzer arrangement that does not require a large space within an outboard motor for furnishing a catalyzer that has a relatively large volume, and that does not substantially increase the likelihood that the catalyzer will be contacted by water. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, an internal combustion engine is provided for an outboard motor. The outboard motor has a protective cowling surrounding the engine. The engine comprises a cylinder body in which a plurality of cylinder bores are spaced apart from each other. An exhaust manifold gathers exhaust gases from the respective cylinder bores. An exhaust passage is coupled to the manifold and extends, at least in part, within a space defined between a side surface of the cylinder body and the protective cowling. A catalyzer is disposed in the exhaust passage. 
     In accordance with another aspect of the present invention, an exhaust gas purifying system is provided for an internal combustion engine. The engine has a side surface. The purifying system comprises an exhaust passage disposed on the side surface of the engine for catalytic exhaust treatment. The exhaust passage communicates with the engine through an inlet opening and an outlet opening. At least one catalyzer is disposed between the inlet and outlet openings. 
     In accordance with a further aspect of the present invention, an exhaust gas purifying system is provided for an internal combustion engine. The engine has a side surface. The purifying system comprises an exhaust passage disposed on the side surface of the engine for catalytic exhaust treatment. The exhaust passage includes a vertical section extending generally vertically along the side surface of the cylinder body. A catalyzer is disposed within the vertical section. 
    
    
     Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of this invention will now be described with reference to the drawings of a couple of preferred embodiments which are intended to illustrate and not to limit the invention. 
     FIG. 1 is a side elevational view of an outboard motor that employs an exhaust unit configured in accordance with a preferred arrangement of the present invention. An associated watercraft on which the motor is mounted is partially shown in section. 
     FIG. 2 is a top plan view of the outboard motor. A top cowling is sectioned along the line  2 — 2  of FIG.  1 . 
     FIG. 3 is a partial, cross-sectional view showing a section through an engine of the motor taken along the line  3 — 3  of FIG.  4 . The rear/starboard side quarter generally is illustrated. 
     FIG. 4 is a partial, cross-sectional view of the engine taken along the line  4 — 4  of FIG.  3 . The starboard side half generally is illustrated. 
     FIG. 5 is a partial, side view of the engine looking in the direction of the Arrow  5  of FIG.  4 . Some inner and outer components of the engine are omitted to simplify the drawing. 
     FIG. 6 is a partial sectional, bottom plan view of the engine looking in the direction of the Arrow  6  of FIG.  4 . Some inner and outer components of the engine are omitted to simplify the drawing. 
     FIG. 7 is a partial, cross-sectional view of the engine taken along the line  7 — 7  of FIG.  5 . 
     FIG. 8 is a top plan view of an outboard motor that employs an exhaust unit configured in accordance with another arrangement of the present invention. A top cowling is sectioned along a line similar to the line  2 — 2  of FIG.  1 . 
     FIG. 9 is a side elevational view of the motor of FIG. 8. A protective cowling is sectioned along a generally vertical center plane. To simplify the drawing, an exhaust manifold is omitted. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     With reference to FIGS. 1 to  7 , an outboard motor  30  employs an exhaust unit  32  configured in accordance with a preferred embodiment of the present invention. The exhaust unit  32  is provided around an internal combustion engine  34  of the motor  30 . Although the present invention is shown in the context of the illustrated outboard motor, various aspects and features of the present invention also can be employed with engines for other types of marine outboard drive units (e.g., a stern drive unit) and land vehicles, as well as with stationary engines used in other types of devices, e.g., generators. 
     In the illustrated arrangement, the outboard motor  30  comprises a drive unit  36  and a bracket assembly  38 . The bracket assembly  38  supports the drive unit  36  on a transom  40  of an associated watercraft  42  so as to place a marine propulsion device in a submerged position with the watercraft  42  resting on the surface of a body of water. The bracket assembly  38  comprises a swivel bracket  44 , a clamping bracket  46 , a steering shaft and a pivot pin  48 . 
     The steering shaft extends through the swivel bracket  44  and is affixed to the drive unit  36  with an upper mount assembly  50  and a lower mount assembly  52 . The steering shaft is pivotally jounaled for steering movement about a generally vertically extending steering axis within the swivel bracket  44 . A steering handle  54  extends upwardly and forwardly from the steering shaft and can be used to pivot the drive unit  36  about the steering axis for steering the watercraft. 
     The clamping bracket  46  includes a pair of bracket arms spaced apart from each other and affixed to the watercraft transom  40 . The pivot pin  48  completes a hinge coupling between the swivel bracket  44  and the clamping bracket  46 . The pivot pin  48  extends through the bracket arms so that the clamping bracket  46  supports the swivel bracket  44  for pivotal movement about a generally horizontally extending tilt axis defined through the pivot pin  48 . Although not shown, a hydraulic tilt system can be provided between the swivel bracket  44  and the clamping bracket  46  to tilt up and down and also to enable trim adjustment of the drive unit  36 . 
     As used through this description, the terms “front,” forward” and “forwardly” mean at or to the side where the clamping bracket  46  is located, and the terms “rear,” “rearward,” “rearwardly” and “reverse” mean at or to the opposite side of the front side, unless indicated otherwise or otherwise readily apparent from the context of use. 
     Because the construction of the bracket assembly  38  is well known in the art, a further description of the assembly is not believed to be necessary to permit those skilled in the art to practice the present invention. 
     The drive unit  36  includes a power head  58 , a driveshaft housing  60  and a lower unit  62 . The power head  58  is disposed atop of the drive unit  36  and includes the aforenoted engine  34  and a protective cowling assembly  66 . The protective cowling assembly  66  includes a top cowling member  68  and a bottom cowling member  70 . 
     The cowling assembly  66  generally completely encloses the engine  34 . That is, the cowling assembly  66  defines a generally closed cavity  72  in which the engine  34  is contained. The top cowling member  68  is detachably affixed to the bottom cowling member  70  so that the operator can access the engine  34  for maintenance or for other purposes. 
     With reference to FIG. 2, in the illustrated arrangement, an air intake opening  74  is defined by the top cowling member  68  and a cover member  76  at a forward portion of the cowling member  68 . The top cowling member  68  has an air intake duct  77  that is covered by the cover member  76  to define a passage through which ambient air is introduced into the cavity  72 . The illustrated top cowling member  68  includes an opening  78  at a rear portion of the top cowling member  68  and another air intake duct internally of the opening  78 . Thus, ambient air also can be introduced into the cavity  72  through the opening  78  and the intake duct. 
     The bottom cowling  70  has an opening at its bottom portion through which an exhaust guide  80  extends. The exhaust guide  80  preferably is affixed atop the driveshaft housing  60 . The bottom cowling  70  and the exhaust guide  80 , thus, generally form a tray. The engine  34  is placed on this tray and is affixed to the exhaust guide  80 . The exhaust guide  80  also has an exhaust passage therein through which a burnt charge, i.e., exhaust gases, are discharged as described later. 
     The engine  34  in the illustrated arrangement operates on a four-stroke combustion principle and powers a propulsion device. The engine  34  has a cylinder body  82 . The cylinder body  82  defines four cylinder bores  84 , which are spaced apart from each other generally vertically and which extend generally horizontally along a major axis  85  of the engine  34 . As illustrated, the major axis  85  preferably extends between the front and rear ends of the engine  34 . That is, the engine  34  is an L4 (in-line four cylinder) type. This type of engine, however, is merely exemplary of a type on which various aspects and features of the present invention can be used. Engines having other numbers of cylinders, having other cylinder arrangements, and operating on other combustion principles (e.g., crankcase compression two-stroke or rotary) are all practicable. 
     A piston  86  can reciprocate in each cylinder bore  84 . A cylinder head member  88  is affixed to one end of the cylinder body  82  to define combustion chambers  89  with the pistons  86  and the cylinder bores  84 . A cylinder head cover member  90  is affixed to and covers the cylinder head member  88 . The cylinder head member  88  and cylinder head cover member  90  together form a cylinder head assembly  94 . 
     The other end of the cylinder body  82  is closed with a crankcase member  98  defining a crankcase chamber with the cylinder bores  84 . A crankshaft  100  extends generally vertically through the crankcase chamber. The crankshaft  100  is rotatably connected to the pistons  86  by connecting rods  101  and is driven by the reciprocal movement of the pistons  86 . In the illustrated arrangement, the crankcase member  98  is located at the most forward position, then the cylinder body  82  and the cylinder head assembly  94  extend rearwardly from the crankcase member  98  one after another. 
     The engine  34  includes an air induction system. The air induction system is arranged to supply air charges to the combustion chambers  89  and comprises a plenum chamber  102 , main air delivery conduits  104  and intake ports. The intake ports are defined through the cylinder head member  88  and can be opened or closed by intake valves. When each intake port is opened, the corresponding air delivery conduit  104  communicates with the respective combustion chamber  89 . 
     The plenum chamber  102  functions as an intake silencer and/or a coordinator of air charges. A plenum chamber member  106  defines the plenum chamber  102  and is mounted on the port side of the illustrated crankcase member  98 . The plenum chamber member  106  has an air inlet opening  108  that opens to the cavity  72 . The air delivery conduits  104  extend rearwardly from the plenum chamber member  106  along the cylinder body  82  on the port side and then bend toward the intake ports. Air passes into the plenum chamber  102  through the inlet opening  108  from the cavity  72  and is supplied to the combustion chambers  89  through the delivery conduits  104  and the intake ports. 
     The main air delivery conduits  104  include one or more throttle bodies  112 . The respective throttle bodies  112  feature butterfly-type throttle valves mounted therein for pivotal movement about axes of valve shafts that extend generally vertically. The valve shafts are linked together to form a single valve shaft that passes through the entire throttle body  112 . The throttle valves are operable by the operator through a suitable throttle cable and a linkage mechanism so that the valves allow a proper air flow rate to pass through the respective delivery conduits  104  in response to engine demands. When the operator operates the throttle cable, the linkage mechanism moves the valve shaft to open the throttle valves. Conversely, when the throttle cable is released, the linkage mechanism moves the valve shaft to close the throttle valves. 
     The air induction system further includes an idle air supply unit  114 . The idle air supply unit  114  bypasses the throttle valves. An upstream bypass conduit  116  couples the unit  114  with the plenum chamber member  106 , while a downstream bypass conduit  118  couples the unit  114  with one of the delivery conduits  104 . The idle air supply unit  114  contains a valve member pivotally disposed therein. When the throttle valves in the throttle bodies  112  are almost closed, such as at idle, the valve member in the idle air supply unit  114  is operated to supply sufficient air to the combustion chambers  89  under control of an ECU (Engine Control Unit), which is an electrically operable control device. 
     The engine  34  communicates with an exhaust system that includes the exhaust unit  32 . The exhaust system is arranged to discharge exhaust gases from the combustion chambers  89  to a location outside of the outboard motor  30 . Exhaust ports  120  are defined in the cylinder head member  88  and can be opened and closed by exhaust valves  122 . The cylinder body  82  defines an internal exhaust manifold  124  downstream of the exhaust ports  120 . When the exhaust ports  120  are opened, the combustion chambers  89  communicate with the exhaust manifold  124 . The exhaust manifold  124  thus combines the exhaust gases flowing from each combustion chamber and guides the exhaust flow to the exhaust unit  32 . 
     As seen in FIGS. 2 and 3, two camshafts  128  extend generally vertically to actuate the intake valves and exhaust valves  122 . The camshafts  128  have cam lobes  130  thereon to push the intake valves and exhaust valves  122  at certain timings such that the intake ports and the exhaust ports  120  are opened and closed. The camshafts  128  are journaled on the cylinder head assembly  94  and are driven by the crankshaft  100 . The respective camshafts  128  have sprockets  132  thereon, while the crankshaft  100  also has a sprocket  134  thereon. A timing belt or chain  136  is wound around the sprockets  132 ,  134 . Thus, when the crankshaft  100  rotates, the camshafts  128  also rotate. A tensioner  138  is also provided to adjust the tension of the belt or chain  136  by pushing it inwardly so as to keep the opening and closing timing of the intake and exhaust valves accurately and reduce the likelihood that the chain will jump from a sprocket. The tensioner  138  includes, for example, a gas cylinder containing compressed gases therein to produce the tensioning force. 
     In the illustrated embodiment, the engine  34  has a fuel injection system, although any other conventional fuel supply and charge forming systems can be applied. The fuel injection system includes four fuel injectors  140  which have injection nozzles directed to the respective intake ports. The fuel injectors  140  are supported by a fuel rail that is affixed to the cylinder head member  88 . The fuel injection system further includes a vapor separator, several fuel pumps, a pressure regulator, a fuel supply tank, a fuel filter and several fuel conduits connecting those components. Generally the fuel supply tank is disposed on a hull of the watercraft  42  and the other components are placed on the outboard motor  30 . One of the fuel pumps is a high pressure pump  142  mounted on the cylinder head cover member  90 . An amount of each fuel injection and injection timing are controlled by the ECU. 
     The engine  34  further has a firing system. Four spark plugs are exposed into the respective combustion chambers  89  and fire an air/fuel charge at a proper timing. This firing timing also is controlled by the ECU. The air/fuel charge is formed with an air charge supplied by the main air delivery conduits  104  or idle air supply unit  114  and a fuel charge sprayed by the fuel injectors  140 . The burnt charge, as described above, is discharged through the exhaust system. 
     A flywheel assembly  146  is affixed atop the crankshaft  100 . The flywheel assembly  146  includes a generator to supply electric power to the firing system, to the ECU and to other electrical equipment via a battery usually disposed in the hull of the watercraft  42 . A starter motor  148  is mounted on the cylinder body  82  in adjacent to the flywheel assembly  146 . A gear of the starter motor  148  can mesh with a ring gear provided on a periphery of the flywheel assembly  146  through a one-way clutch. The starter motor  148  rotates the crankshaft  100  via the flywheel assembly  146  when the operator operates a main switch. Because the starter gear and the ring gear are coupled by the one-way clutch, the crankshaft  100  and the starter motor  148  are disengaged immediately after the engine  34  starts. A protector  150  covers the flywheel assembly  146 , starter motor  148 , sprockets  132  and the belt  136 . 
     The engine  34  has also a lubrication system. A lubricant reservoir depends from the exhaust guide  80  within the driveshaft housing  60 . A lubricant pump is driven by the driveshaft to supply lubricant to various engine components through appropriate galleries. The lubricant then drains to the lubricant reservoir through a variety of return passages. Some of the engine components that are lubricated in this manner include the pistons  86  that reciprocate within the cylinder bores  84 . The pistons  86  need the lubrication such that they do not seize on surfaces of the cylinder bores  84  during operation. Piston rings are provided on the pistons  86  to isolate the combustion chambers  89  and the crankcase chambers. At least one piston ring can remove the lubricant from the surfaces of the cylinder bores  84  and can direct the lubricant back toward the crankcase chambers. 
     Unburned charges containing a small amount of the exhaust gas may leak from the combustion chamber, passed the piston rings and into the crankcase chamber as blow-by gas because of the huge pressure generated within the combustion chambers. The engine  34  has a ventilation system that returns the blow-by gases, which also may contain entrained lubricant, to the induction system for combustion in the combustion chambers  89 . 
     The ventilation system comprises an inner blow-by gas conduit, an oil separator or breather  154  and an outer blow-by gas conduit  156 . The inner conduit is formed within the crankcase member  98 , the cylinder body  82  and the cylinder head assembly  94  and connects the crankcase chamber with the oil separator  154 . The oil separator  154  is mounted on the cylinder head cover member  90  and has a labyrinth structure therein to separate the oil component from the blow-by gases. The outer blow-by gas conduit  156  couples the oil separator  154  to the plenum chamber member  106  to supply the blow-by gases to the induction system. 
     The engine  34  further has a cooling system that provides coolant to various engine portions, for example, the cylinder body  82  and the cylinder head assembly  94 , and also to the exhaust system. In the illustrated arrangement, water is used as the coolant and is introduced from the body of water surrounding the outboard motor  30 . The water is delivered through cooling water jackets  160 . After passing through the cylinder head  94  and the cylinder body  82 , the water is discharged through a discharge conduit  162  and a water drain jacket that is formed in the exhaust unit  32 . A thermostat  164  is provided at the most upstream portion of the discharge conduit  162 . If the temperature of the water is lower than a preset temperature, the thermostat  164  will close such that water cannot flow passed the thermostat to the discharge conduit  162 . In this manner, the engine  64  can warm up properly. 
     With reference again to FIG. 1, the driveshaft housing  60  depends from the power head  58  and supports a driveshaft which is driven by the crankshaft  100  of the engine  34 . The driveshaft extends generally vertically through the exhaust guide  80  and the driveshaft housing  60 . The driveshaft housing  60  also includes several internal passages which form portions of the exhaust system. An idle exhaust passage branches from the internal passages and opens to the atmosphere above the body of water. In the illustrated arrangement, an apron  166  covers an upper portion of the driveshaft housing  60 . More particularly, the idle exhaust passage extends through an outer surface of the driveshaft housing  60  and the apron  166 . 
     The lower unit  62  depends from the driveshaft housing  60  and supports a propulsion shaft that is driven by the driveshaft. The propulsion shaft extends generally horizontally through the lower unit  62 . In the illustrated embodiment, the propulsion device includes a propeller  168  that is affixed to an outer end of the propulsion shaft and is driven thereby. The propulsion device, however, can take the form of a dual, a counter-rotating system, a hydrodynamic jet, or any other suitable propulsion device. 
     A transmission is provided between the driveshaft and the propulsion shaft. The transmission couples the two shafts, which lie generally normal to each other, (i.e., at a 90° shaft angle) with a bevel gear combination or the like. The transmission has a switchover or clutch mechanism to shift rotational directions of the propeller  168  among forward, neutral or reverse positions. The switchover mechanism is actuated by the operator through a shift linkage including a shift cam, a shift rod and a shift cable. 
     The lower unit  62  also includes an internal passage that forms a discharge section of the exhaust system. At engine speed above idle, the majority of the exhaust gases are discharged toward the body of water through the internal passage and a hub of the propeller  168 . At engine idle, the exhaust gases preferably are discharged only through the aforenoted idle exhaust passage. Because the exhaust pressure under this condition is smaller than the pressure that can overcome the pressure generated by the body of water. 
     Additionally, the driveshaft housing  60  has a water pump that is driven by the driveshaft and supplies cooling water to the aforenoted cooling system. Water is introduced through a water inlet (not shown) which opens at the lower unit  62 . The water inlet is connected to the water pump through an inlet passage and the water pump is connected to the water jackets of the engine portions and the exhaust system including the water jacket  160 . 
     With primarily reference to FIGS. 3 to  7 , the exhaust system will now be described in greater detail. As best seen in FIGS. 3 and 4, in the illustrated arrangement, the exhaust system comprises the exhaust unit  32 , which is disposed generally adjacent to the cylinder body  82  and the cylinder head assembly  94 , and which is positioned in a space defined between a side surface of the cylinder body  82  and the protective cowling assembly  66 . An inner member or exhaust passage member  180  of the exhaust unit  32  comprises a pan-like shape and defines a supplement exhaust passage  181 . As best seen in FIG. 3, the inner member  180  generally is rectangular in section. The inner member  180  is affixed to the cylinder body  82  via a supporting plate or spacer assembly  182 . That is, the spacer assembly  182  is interposed between the inner member  180  and the cylinder body  82  to couple the two components. The spacer assembly  182  is affixed to the cylinder body  82  by bolts  183  (see FIG. 5) via a sealing member. In addition, the inner member  180  and the spacer assembly  182  are affixed to the cylinder body  82  by bolts  184 . Bolt holes  186  are provided for accommodating the bolts  184  as seen in FIG.  5 . 
     The spacer assembly  182  includes two spacer pieces  188  that have generally the same configuration. The cylinder body  82  also comprises two openings  200 ,  202  that are vertically spaced from each other. Similarly, the spacer assembly  182  comprises two openings  204 ,  206  that are vertically spaced and that correspond to the openings  200 ,  202 . The lower opening  202  of the cylinder body  82  comprises a port of a discharge passage  208  that is formed below the exhaust manifold  124 . The supplemental passage  181  communicates with the exhaust manifold  124  through the upper openings  200 ,  204  and also with the discharge passage  208  through the lower openings  202 ,  206 . In the illustrated arrangement, both the exhaust manifold  124  and the discharge passage  208  are formed within the cylinder body  82 . However, the two are separated from each other by a partition or rib  212 . The exhaust manifold  124  communicates with the respective exhaust ports  120 . FIG. 4 shows a unified portion  210  positioned downstream of the respective exhaust ports  120 . The discharge passage  208 , in turn, communicates with the exhaust passage in the exhaust guide  80 . 
     A plurality of monolithic catalyzers  216  are provided in the supplemental passage  181 . In the illustrated arrangement, the exhaust unit  32  contains two catalyzers  216  which preferably are cylindrical in shape. The bodies of the catalyzers  216  are enclosed in metal cases which also have cylindrical shapes. As best seen in FIG. 5, the catalyzers  216  preferably are disposed in parallel to each other relative to the exhaust flow in the passage  181 . 
     The catalyzer  216  causes a chemical reaction that renders certain of the exhaust gas constituents harmless. The catalyzer  216  has a carrier member that carries, for example, a three-way catalyst element. The three-way catalyst element can oxidize CO and HC and reduce NOx contained in the exhaust gases. Thus, the gases are purified when passing through the catalyzer  216 . It should be noted, however, any conventional catalyzers can be used depending upon the application and the desired effects. 
     The catalyzers  216  are interposed between a pair of brackets  217  that are vertically spaced from each other. As best seen in FIGS. 3,  5  and  6 , each bracket  217  is configured generally as a dual ring construction and both end portions  218  of the catalyzers  216  slightly protrude from through-holes of the brackets  217 . The brackets  217  are united with the metal cases of the catalyzers with bolts  219  and affixed to the spacer assembly  182  by a plurality of bolts  220 . 
     In the illustrated embodiment, a guide member  228  is additionally affixed to the spacer assembly  182  by bolts  230  and a sealing member is interposed therebetween. A skirt portion  231  (see FIG. 5) of the lower bracket  217  overlaps with the upper portion of the guide member  228 . The guide member  228  preferably has a half-dome shape to lead the exhaust gases toward the lower opening  202   
     An air fuel ratio sensor or oxygen (O 2 ) sensor  232  is affixed to a top portion of the inner member  180  so that a sensor element thereof is exposed to the supplemental passage  181 . The oxygen sensor  232  sends a signal to the ECU. The ECU controls the fuel injection system, firing system or the like based upon signals sent thereto by sensors such as the oxygen sensor  232 . An exhaust temperature sensor  234  is provided for indirectly monitoring the temperature of the catalyzers  216 . The temperature sensor  234  is placed in the discharge passage  208 . Preferably, the sensor  234  is affixed to the spacer assembly  182  in one arrangement. The ECU also can use the output this sensor  234  in its control of the aforenoted systems. 
     As best seen in FIG. 7, the exhaust unit  32  additionally comprises a water supply jacket  240  and a water drain jacket  242 . An outer member  244  overlies and is affixed to the inner member  180  by bolts  245 . The supply jacket  240  comprises an internal supply passage  246  defined in the inner member  180  and an external supply passage  248  defined between the inner member  180  and the outer member  244 . The drain jacket  242 , in turn, comprises an internal drain passage  250  defined in the inner member  180  and an external drain passage  252  defined between the inner member  180  and the outer member  244 . The respective external passages  248 ,  252  are divided by a partition  254  of the outer member  244  and extend along the exhaust passage  181  next to each other. Any ratio for allotting areas for the respective external passages  248 ,  252  can be selected. Preferably, however, the area for the supply passage  248  is greater than the area for the drain passage  252  because the supply water is cooler than the drain water. 
     The internal supply passage  246  also connects to a water supply jacket  258  defined in the cylinder body  82  and is coupled to the aforenoted water pump. Hatched portions of the jacket  258  in FIG. 6 indicate that these portions extend to deeper passages defined in the cylinder body  82 . The other hatched portions in FIGS. 4 and 6 indicate similar constructions. The internal drain passage  250 , in turn, connects to a water drain jacket  260 , which also is defined in the cylinder body  82  and which couples to a water drain from the outboard motor  30 . As seen in FIG. 7, the spacer assembly  182  has a branch passage  262  defined between the respective spacer pieces  188 . The branch passage  262  connects to both the internal supply passage  246  and the water supply jacket  258  so that the supply water can be directed to the supply passage  246  and to the branch passage  262  from the supply jacket  258 . 
     The external supply passage  248  connects to a water supply inlet  264  that is in communication with water supply jackets defined in the cylinder head assembly  94 . For instance, as seen in FIG. 4, the supply inlet  264  and the external supply passage  248  are coupled to the water jacket  160 . The external drain passage  252  is connected to a water drain outlet  266 , which is coupled to the water discharge conduit  162 . A discharge opening  268  of the discharge conduit  162  is positioned at the drain outlet  266 . Thus, all of the water that has passed through the thermostat  164  can be directed to the external drain passage  252 . 
     The water supply jacket  240  of the exhaust unit  32 , in other words, forms a portion of the water supply plumbing that originates at the cylinder body  82  and that extends to the cylinder head assembly  94 , while the water drain jacket  242  forms a portion of the water drain plumbing that originates at the cylinder head assembly  94  and that extends to the cylinder body  82 . In addition, the branch passage  262 , which is defined within the spacer assembly  182 , forms another portion of the water supply plumbing. 
     In the illustrated embodiment, the spacer assembly  182  includes the branch passage  262 , which acts as a supply passage. However, the spacer assembly  182  can be provided with another branch passage that acts as a drain passage. Alternatively, the branch passage  262  itself can be the drain passage instead of being the supply passage so long as the exhaust unit  32  includes the supply jacket  240 . 
     When the engine  34  operates, exhaust gases are produced in the combustion chambers  89 . The gases as directed through the exhaust ports  120  when the exhaust valves  122  are opened by the cam mechanism. The exhaust gases merge in the exhaust manifold  124  and are directed to the supplemental passage  181  in the exhaust unit  32 , as indicated by the arrows  270  of FIGS. 3 to  5 . The exhaust gases then flow into the catalyzers  216  to be purified, as indicated by the arrows  272  of FIGS. 4 and 5. After passing through the catalyzers  216 , the gases are directed through the guide member  228  and into the discharge passage  208 , as indicated by the arrows  274  of FIGS. 4 and 5. The gases, then, are directed into the exhaust passage in the exhaust guide  80 , as indicated by the arrow  276  of FIG.  4 . From the exhaust passage in the exhaust guide  80 , the gases are discharged to the body of water through the hub of the propeller  168  or to the atmosphere as described above. 
     Cooling water moves into the water supply jacket  258  in the cylinder body  82  from the aforenoted water pump. A certain part of the water is diverted to the branch passage  262  of the spacer assembly  182  and is passed to the water supply jackets of the cylinder head assembly  94 , as indicated by the arrow  278  of FIG. 7. A major portion of the water, however, flows into the water supply jacket  240  of the exhaust unit  32  and is directed to the supply inlet  264 , as indicated by the arrows  280  of FIG.  7 . The water then circulates in the water jackets, which include the jacket  160  formed within the cylinder body  82  and the cylinder head assembly  94 , to take heat therefrom. The water, after the circulation, returns to the drain outlet  266  through the discharge conduit  162  and flows through the water drain jacket  242  of the exhaust unit  32 , as indicated by the arrows  282  of FIG.  7 . The water then is directed to certain discharge passages defined in the outboard motor  30  such that the water is discharged to the body of water. While flowing through both the supply jacket  240  and the drain jacket  242 , the water absorbs heat accumulated in the catalyzers  216  as well as in the exhaust passage  181 . 
     As described above, the exhaust unit  32  in the illustrated embodiment is provided downstream of the exhaust manifold and includes the exhaust passage  181  that extends within a space defined between a side surface of the cylinder body  82  and the protective cowling assembly  66 . This arrangement does not require any large space that must be specially created for the exhaust unit  32 . Nevertheless, the exhaust unit  32  can hold two catalyzers  216  that have relatively large volumes. Additionally, because the catalyzers  216  are generally confined solely in the exhaust unit  32  and because the catalyzers  216  are not exposed to the body of water in the illustrated arrangement, no particular protection for the catalyzers is necessary to guard against water that may damage the catalyzers  216 . 
     In the illustrated arrangement, the inner and outer members  180 ,  244  and the guide member  228  can be easily detached from one another and from the spacer assembly  182 . After removing all of the members  180 ,  244 ,  228 , the catalyzers  216  can be removed from the brackets  217 . Thus, this illustrated construction eases maintenance and exchange of the catalyzers  216 . 
     With reference to FIGS. 8 and 9, another exhaust unit  290  configured in accordance with another arrangement having certain features and aspects of the present invention will be described. The same members and components that have already been described in connection with the first arrangement will be assigned the same reference numerals and will not be described again unless a particular need for such a repeated description exists. 
     As seen in FIG. 8, an exhaust manifold  292  extends generally horizontally toward the top cowling member  68  on the starboard side of the exhaust ports  120  and then turns generally forwardly. The exhaust unit  290  is coupled to the exhaust manifold  292  by bolts  293  and extends forwardly along the cylinder body  82  and the crankcase member  98 . The exhaust manifold  292  and the exhaust unit  290  together define an exhaust passage  294  therein. As seen FIG. 9, the exhaust unit  290  then turns downwardly at an angle to extend generally horizontally and rearwardly. The exhaust manifold  292 , an upper horizontal portion  295 , a vertical portion  296  and a major part of an angled portion  298  are disposed within the protective cowling assembly  66 . The rest part of the angled portion  298  and a lower horizontal portion  300  extend out of the cowling assembly  66  and are coupled to the exhaust passage in the exhaust guide  80  at a coupling portion  302 . This coupling portion  302  is affixed to the exhaust guide  80  by bolts  304 . 
     The upper horizontal portion  295  has a generally rectangular shape that is relatively slim in a transverse direction but voluminous in a longitudinal direction. A first monolithic catalyzer  308  is disposed in this upper horizontal portion  295 . Because of the configuration of the horizontal portion  295 , the catalyzer  308  is configured as a rectangular shape that is thin in the transverse direction but thick in the longitudinal direction. The rest of the horizontal portion  295  is reduced in volume. The catalyzer  308  can be detached from an opening  309  that is so formed that the catalyzer  308  can pass through. The vertical portion  296 , in turn, has a generally cylindrical shape. A second monolithic catalyzer  310  is disposed in this vertical portion  296 . The second catalyzer  308  has also a cylindrical shape complying with the cylindrical configuration of the vertical portion  296 . The rest of the vertical portion  296  as well as the angled portion  298  and the lower horizontal portion  300  are narrowed to be generally the same dimension as the reduced part of the upper horizontal portion  295 . In the illustrated embodiment, the vertical portion  296  is actually separately formed from the horizontal portion  295  and both portions  295 ,  296  are mated together at a juncture  312  that preferably is disposed immediately above the second catalyzer  310 . The first catalyzer  308  and the second catalyzer  310  in this arrangement are disposed in series with each other. 
     The exhaust unit  290  and the manifold  292  in this embodiment have a cooling water jacket  314  that generally surrounds the exhaust passage  294 . The cooling water preferably is supplied from the water jacket  316  of the cylinder head member  88 . Additionally, the exhaust unit  290  has a special water supply. The water supply comprises a water delivery pipe  320  that communicates to one of the water passages coming from the water pump. The delivery pipe  320  is coupled to the exhaust unit  290  with a coupler  322 . Through this delivery pipe  320 , fresh water can be supplied to the water jacket  314  in addition to the water that has circulated within the cylinder body  82  and the cylinder head assembly  94 . The coupling portion  302  also has a water jacket  324  around its exhaust passage. The water jacket  324  is coupled to the internal water passage defined in the exhaust passage that communicates to the water discharge passage. 
     Exhaust gases move to the exhaust passage  294 , which is defined in both the exhaust manifold  292  and the exhaust unit  290 , from the exhaust ports  316  that are defined in the cylinder head member  88 . The exhaust gases then flow into the first catalyzer  308  as indicated by the arrows  328  of FIGS. 8 and 9. The first catalyzer  308  purifies the gases. The exhaust gases then are directed to the second catalyzer  310  to be cleaned thereby as indicated by the arrows  330  of FIGS. 8 and 9. After being cleaned, they are directed toward the exhaust guide  80  and flow into it through the coupling portion  302  as indicated by the arrow  332  of FIG.  9 . 
     Cooling water is supplied to the water jacket  314  from both the water jacket  316  and the water delivery pipe  320 . The water absorbs heat from the first and second catalyzers  308 ,  310  and from the exhaust gases passing through the exhaust manifold  292  and the exhaust unit  290 . 
     In the illustrated arrangement, the exhaust unit  290  extends forwardly and connects to the exhaust guide  80 . However, other arrangements are practicable. For instance, the unit  290  can extend rearwardly and can connect directly to the internal passages of the exhaust system formed within the driveshaft housing  60 . 
     Any number, size, configuration and position of the catalyzers can be selected in accordance with certain of the features and aspects of the present invention. For example, three rectangular shaped catalyzers that are longer than the illustrated catalyzers  216  can be placed at higher or lower positions and can be disposed in parallel or in series. 
     The spacer assembly or supporting plate can be omitted and the exhaust unit can be mounted directly on a portion of the engine. However, the spacer assembly can contribute to increasing the number of alternative constructions of the exhaust unit without changing the particular engine configuration. 
     The water jackets of the exhaust unit are optional in some applications. If the water jackets are not provided in the exhaust unit, the water jackets of the cylinder body can be coupled directly with the water jackets of the cylinder head assembly. Conversely, the exhaust unit can be more perfectly surrounded by water jackets. Whether one increases or decreases the surface area of water jackets depends upon the particular thermal characteristics of the chosen exhaust unit. 
     In the illustrated arrangements, the exhaust units contain monolithic catalyzers of a single type. However, the respective catalyzers can be different relative to each other. For instance, it is practicable that one catalyzer has a three-way catalyst element and the other one has a catalyst element that works specifically on oxide of nitrogen (NOx). The arrangement featuring different catalyzers is particularly effective when the catalyzers are placed in series because the differing location along the exhaust system results in different exhaust gas temperatures, which can be varied to suit the particular catalyst elements being used. Moreover, either one of the inner and outer members or both of them can have fins thereon to expedite the cooling effect. 
     Although the present invention has been described in terms of certain preferred arrangements, other arrangements apparent to those of ordinary skill in the art also are within the scope of this invention. Thus, various changes and modifications may be made without departing from the spirit and scope of the invention. Moreover, not all of the features aspects and advantages are necessarily required to practice the present invention. Accordingly, the scope of the present invention is intended to be defined only by the claims that follow.