Abstract:
A catalyst system for a marine engine includes a plurality of catalyst devices arranged in parallel with each other to accommodate a smaller overall size in a preferred dimension to allow more efficient packaging of the catalyst system.

Description:
CROSS REFERENCE TO CO-PENDING PATENT APPLICATION 
       [0001]    This patent application is a member of a family of co-pending and commonly owned patent applications which were all filed on ______, 2005. This family includes patent application (M09964) which was filed by White (Ser. No. ______), patent application (M09966) which was filed by White (Ser. No. ______), patent application (M09967) which was filed by Burk et al (Ser. No. ______), patent application (M09968) which was filed by White et al (Ser. No. ______), patent application (M09969) which was filed by White et al (Ser. No. ______), patent application (M09971) which was filed by White (Ser. No. ______), patent application (M09972) which was filed by White et al (Ser. No. ______), patent application (M09974) which was filed by White (Ser. No. ______), patent application (M09976) which was filed by White (Ser. No. ______). 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to a marine engine exhaust system and, more particularly, to an exhaust system that uses a plurality of catalyst devices arranged in parallel with each other between exhaust ports of the engine and a downstream exhaust conduit. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Those skilled in the art of internal combustion engines are aware of many types of catalyst systems that are available to improve exhaust emissions emitted by the engines. 
         [0004]    U.S. Pat. No. 4,848,082, which issued to Takahashi et al. on Jul. 18, 1989, describes an exhaust gas purifying device for a marine engine. A catalyst exhaust system for an outboard motor is described. A throttle control arrangement is incorporated for assuring rapid heating of the catalyst to its operating temperature. 
         [0005]    U.S. Pat. No. 4,900,282, which issued to Takahashi et al. on Feb. 13, 1990, describes an exhaust gas purifying device for a marine engine. A catalytic exhaust system for a marine outboard drive is described wherein the catalyzer material is supported by a heat conductive bracket and the bracket is cooled by a cooling jacket that is supplied with coolant from the engine cooling jacket. In one embodiment, the water jacket is cooled both internally and externally by delivering water from the cooling jacket into the exhaust system to impinge upon a wall of the cooling jacket. 
         [0006]    U.S. Pat. No. 5,133,185, which issued to Gilbreath et al. on Jul. 28, 1992, describes an anti-moisture device for engine exhaust systems. The device is intended to remove moisture droplets from an interior surface of a duct, characterized by an outer edge secured to the interior surface of the duct, an inner edge surrounding an opening, and a connecting wall between the outer edge and the inner edge. The inner edge of the anti-moisture device is positioned closer to a downstream end of the duct than the outer edge whereby the connecting wall is positioned at an angle relative to the interior surface of the duct. Moisture droplets traveling upstream will be caught between the connecting wall and the interior surface of the duct, on the downstream side of the device. 
         [0007]    U.S. Pat. No. 5,167,934, which issued to Wolf et al. on Dec. 1, 1992, describes a catalyzer installation for boat engines and a method for catalytic exhaust gas cleaning. The invention is intended for use in boat engines and the catalyzer is subdivided into a reduction part location upstream in the exhaust gas line and an oxidation part located coaxially downstream after it. An intermediate space is located between the reduction and oxidation parts. Both catalyzer parts are surrounded by a preferably cylindrical, water cooled casing and the casing has a downstream secondary air inlet to which a secondary air blower can be connected, the secondary air separating the very hot catalyzer from the double walled, water cooled casing and, in particular, flowing around the oxidation catalyzer part in counterflow for air preheating so that the air preheating in this manner is passed through the intermediate space into the oxidation part. 
         [0008]    U.S. Pat. No. 5,203,167, which issued to Lassanske et al. on Apr. 20, 1993, describes a marine propulsion device internal combustion engine and method is for making the same. The propulsion device comprises a driveshaft housing, a propeller shaft rotatably supported by the driveshaft housing, an internal combustion engine drivingly connected to the propeller shaft, the engine including a cylinder block defining a cylinder having an exhaust port, and defining an exhaust outlet, and an exhaust passage between the exhaust port and the exhaust outlet, an exhaust catalyst apparatus mounted on the cylinder block, the apparatus including a tongue extending into the cylinder block exhaust passage and dividing the cylinder block exhaust passage into an upstream portion communicating with the exhaust port and a downstream portion communicating with the exhaust outlet. The apparatus includes an exhaust passage communicating between the upstream portion and the downstream portion. The catalyst is located in the apparatus exhaust passage. 
         [0009]    U.S. Pat. No. 5,212,949, which issued to Shiozawa on May 25, 1993, describes an exhaust gas cleaning system for a marine propulsion unit. It is intended for use with a watercraft engine. A plurality of horizontally positioned exhaust ports are located within an engine cylinder head. An exhaust manifold communicates with each of the exhaust ports at a first end and forms a gas collecting pipe at its second end. The second end of the gas collecting pipe is positioned above the exhaust ports. A generally horizontally positioned exhaust pipe extends from the second end of the gas collecting pipe and continues in a rearward direction. Means are provided for introducing coolant from the engine into the rearwardly extending portion of the exhaust pipe. 
         [0010]    U.S. Pat. No. 5,306,185, which issued to Lassanske et al. on Apr. 26, 1994, describes catalytic elements for marine propulsion devices. A marine propulsion device comprising a propulsion unit including a propeller shaft, a housing including an exhaust gas inlet and an exhaust gas outlet, a catalytic element supported in the housing for reorientation from a first orientation to a second orientation different from the first orientation, and structure for reorienting the element from the first orientation to the second orientation is described. 
         [0011]    U.S. Pat. No. 5,324,217, which issued to Mineo on Jun. 28, 1994, describes an exhaust system for a small boat. It includes a water trap device for precluding water entering the exhaust system if the watercraft becomes inverted from entering the engine through the exhaust system. Coolant from the engine is delivered to a cooling jacket that encircles the entire exhaust system and is introduced into the exhaust gases downstream of the water trap so that in the event of inversion and righting the engine coolant will also not enter the exhaust system. This also provides protection for catalyzers in the exhaust system. 
         [0012]    U.S. Pat. No. 5,408,827, which issued to Holtermann et al. on Apr. 25, 1995, describes a marine propulsion device with improved catalyst support arrangement. An internal combustion engine includes an exhaust port, an exhaust conduit communicating with the exhaust port and having an inner surface, the conduit including first and second conduit portions having respective ends, the first and second conduit portions being connected end to end, a catalyst which is located within the conduit and which includes catalytic material and a sleeve surrounding the catalytic material, wherein the sleeve has a length and an outer surface spaced from the inner surface of the conduit along substantially the entire length of the sleeve. The sleeve has a rigid, radially outwardly extending flange captured between the ends of the conduit portions, and a flexible gasket between to the flange and the end of one of the conduit portions. 
         [0013]    U.S. Pat. No. 5,425,232, which issued to Holtermann on Jun. 20, 1995, describes a marine propulsion device with means for supplying secondary air to a catalytic converter. The marine propulsion device comprises a combustion chamber, an exhaust passage, an air pump and a three-way catalytic converter. The air pump pumps air into the exhaust passage at or immediately upstream of the catalytic converter. By this construction the internal combustion engine can be run slightly rich, but the catalytic converter will see a close to stoichiometric mixture so that the pollutants in the exhaust stream can be oxidized or reduced appropriately since the catalytic converter will be able to operate as a three-way catalytic converter. 
         [0014]    U.S. Pat. No. 5,433,634, which issued to Nakayama et al. on Jul. 18, 1995, describes an exhaust treatment for an outboard motor. The exhaust gases are normally discharged to the atmosphere at a point below the level of the body of water in which the watercraft is operating. A catalyst bed is provided in the exhaust system and the catalyst bed is protected from damage by pumping water from the exhaust conduit in response to certain conditions. These conditions may be either rapid deceleration of the engine or watercraft, stopping of the engine, or any of the combination of sensed factors. The water is pumped by a water pump which is positioned either in the outboard drive or in the hull of an associated watercraft. The pumping of water is initiated for only a predetermined time and until the sensed condition no longer is existent. 
         [0015]    U.S. Pat. No. 6,053,785, which issued to Kato et al. on Apr. 25, 2000, describes an exhaust system and control for a marine propulsion engine. An outboard motor exhaust system and control for insuring good running and effective exhaust gas silencing and treatment is provided. The system includes a very compact exhaust system that includes an expansion chamber formed beneath the exhaust guide plate and to which the exhaust gases are delivered and removed at optimal locations. Furthermore, a feedback control employing a combustion condition sensor is employed along with a catalyst in the exhaust. Sensors are provided upstream and downstream of the catalyst to insure that it is operating at optimum conditions. 
         [0016]    U.S. Pat. No. 6,116,022, which issued to Woodward on Sep. 12, 2000, describes a catalytic reactor for marine applications. The reactor for an internal combustion engine has a cooling jacket surrounding multiple catalyst elements. A thermal barrier layer is formed between the catalyst elements and the cooling jacket to prevent overcooling of the catalyst elements. The thermal barrier layer can be formed from insulating elements such as fibrous material, a plurality of annular rings disposed around the catalyst elements, a corrugated layer, or can be formed by an empty space. 
         [0017]    U.S. Pat. No. 6,368,726, which issued to Holpp et al. on Apr. 9, 2002, describes a honeycomb body configuration. It includes a honeycomb body with a fluid inlet side and a fluid outlet side. The honeycomb body is formed of at least partially structured sheet metal layers which form channels through which a fluid can flow. The honeycomb body is surrounded by an inner tubular jacket and an outer tubular jacket provided concentrically in relation thereto. The inner tubular jacket is configured as a corrugated hose in at least one axial subregion thereof. The inner tubular jacket has at least one further axis subregion which lies smoothly against the honeycomb body. The corrugated subregion and the outer tubular jacket are connected at least in a longitudinal partial region of the corrugated subregion. 
         [0018]    U.S. Pat. No. 6,639,193, which issued to Schaper on Oct. 28, 2003, describes a method and apparatus for end-surface connection of a carrier matrix of a honeycomb body by a joining technique. It relates in particular to a catalyst carrier body. The matrix is disposed in a tubular jacket and is laminated and/or wound from at least partially structured sheet metal foils or layers. The end surface of the honeycomb body is at least partially heated with the aid of a surface inductor having induction coils. 
         [0019]    U.S. Pat. No. 6,660,235, which issued to Holpp et al. on Dec. 9, 2003, describes a catalyst carrier configuration for installation close to an engine. It includes a housing and at least one catalyst carrier body disposed in the housing. The catalyst carrier body has partition walls defining a plurality of passages for an exhaust gas. A flange surrounds the catalyst carrier body and extends radially outwards from the catalyst carrier body. The flange has a second that extends at least partially into an outer wall of the housing and can be disposed between a cylinder head and a manifold of an internal combustion engine. The catalyst carrier configuration can be mounted close to an internal combustion engine. A structural unit having at least two catalyst carrier configurations and an exhaust system are also provided. 
         [0020]    U.S. Pat. No. 6,740,178, which issued to Kurth et al. on May 25, 2004, describes a method for producing a centered honeycomb body. The method includes forming a honeycomb body by stacking and/or winding layers of steel sheet containing chromium and aluminum resulting in the honeycomb body having channels through which a fluid can flow. The honeycomb body is introduced into a tubular jacket. 
         [0021]    U.S. Pat. No. 6,799,422, which issued to Westerbeke et al. on Oct. 5, 2004, describes an emission control method. It is intended for use with a fixed speed internal combustion engine and includes injecting a controlled flow of air into the exhaust between a first catalyst bed adapted to reduce hydrocarbon and nitrogen oxide emissions and a second catalyst bed adapted to reduce carbon monoxide emissions. 
         [0022]    The patents described above are hereby expressly incorporated by reference in the description of the present invention. 
       SUMMARY OF THE INVENTION 
       [0023]    A marine engine exhaust system made in accordance with a preferred embodiment of the present invention comprises a plurality of exhaust ports configured to conduct exhaust gas from a plurality of cylinders of the engine, a first exhaust conduit disposed in fluid communication with the plurality of exhaust ports, a plurality of catalyst devices disposed in fluid communication with the first exhaust conduit, and a second exhaust conduit disposed in fluid communication with the plurality of catalyst devices. The catalyst devices are disposed in serial fluid communication between the first and second exhaust conduits and each of the plurality of catalyst devices is disposed in parallel with each other. The first exhaust conduit is disposed in serial fluid communication between the plurality of exhaust ports of the marine engine and the plurality of catalyst devices. The second exhaust conduit is disposed in serial fluid communication with the plurality of catalyst devices which, in a preferred embodiment, are aligned along a common plane. Each of the plurality of catalyst devices can be generally tubular with a catalyst material disposed within the generally tubular housing structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will be more fully and completely understood from a reading of the description of the preferred embodiment in conjunction with the drawings, in which: 
           [0025]      FIG. 1  is a side view of a marine engine which is partially sectioned to show internal portions of the exhaust system; 
           [0026]      FIG. 2  is an isometric partially sectioned view of the port and starboard exhaust components; 
           [0027]      FIG. 3  is a section view of the port exhaust system of a marine engine; 
           [0028]      FIG. 4  is a partially sectioned isometric view of the device shown in  FIG. 3 ; 
           [0029]      FIG. 5  is an exploded isometric view of the port side exhaust system of the present invention; 
           [0030]      FIG. 6  is a section view of the port side exhaust system of a marine engine; and 
           [0031]      FIG. 7  is an alternative exhaust system using an oblong catalyst device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    Throughout the description of the preferred embodiment of the present invention, like components will be identified by like reference numerals. In the following description of various embodiments of the present invention, certain configurations will be described and illustrated as having three catalyst devices used together as a system. It should be clearly understood that the catalyst devices can alternatively be combined together in systems comprising less than or greater than this number. In addition, it should also be clearly understood that certain embodiments of the present invention can comprise a single catalyst device. All of these alternative configurations are described below in relation to an exemplary engine arrangement. In addition, it should be understood that a catalyst system made in accordance with a preferred embodiment of the present invention, when used in conjunction with a V-type engine, would typically be provided at both sides, or cylinder banks, of the engine. 
         [0033]      FIG. 1  shows a marine engine  10  within the structure of a marine vessel  12 . Although not shown in  FIG. 1 , the crankshaft of the engine  10  is supported for rotation about a horizontal axis and attached in torque transmitting relation with a driveshaft that extends through the transom  14  to provide motive power to a marine propulsion drive (not shown in  FIG. 1 ). The marine engine  10  has a plurality of exhaust ports  20  configured to conduct exhaust gas from a plurality of cylinders within the structure of the engine. A first exhaust conduit  22  is disposed in fluid communication with the plurality of exhaust ports  20 . The first exhaust conduit  22  performs the function of an exhaust manifold which receives the exhaust gas from the plurality of exhaust ports  20  and directs it away from the engine  10 . A plurality of catalyst devices  23 - 25  is disposed in fluid communication with the first exhaust conduit  22 . The plurality of exhaust conduits, as will be described in greater detail below, are configured and arranged in cooperation with the first exhaust conduit  22  to assure that all of the exhaust gas passes through the plurality of catalyst devices  23 - 25 . A second exhaust conduit  28  is disposed in fluid communication with the plurality of catalyst devices  23 - 25 . The catalyst devices are disposed in serial fluid communication between the first and second exhaust conduits,  22  and  28 . Each one of the plurality of catalyst devices  23 - 25  is disposed in parallel fluid communication with each other. 
         [0034]    With continued reference to  FIG. 1 , the first exhaust conduit  22 , or exhaust manifold, is disposed in serial fluid communication between the plurality of exhaust ports  20  and the plurality of catalyst devices  23 - 25 . The second exhaust conduit  28  is disposed in serial fluid communication with the plurality of catalyst devices  23 - 25 . The catalyst devices  23 - 25  are aligned along a common plane. The common plane is generally vertical and extends in a direction which is generally parallel with a crankshaft of the engine  10 . 
         [0035]    Each of the catalyst devices  23 - 25 , in a particularly preferred embodiment of the present invention, comprises a cylindrical housing. The housing can alternatively be generally tubular and non-cylindrical. A catalyst material is disposed within the generally tubular housing structure of each of the catalyst devices  23 - 25 . A central housing  30 , or catalyst housing structure, is provided to retain the catalyst devices  23 - 25  in their proper positions relative to both the first and second exhaust conduits,  22  and  28 . The path of the exhaust gas E is represented by the arrows in  FIG. 1 . The exhaust gas travels from the plurality of exhaust ports  20 , through the first exhaust conduit  22 , through the catalyst devices  23 - 25 , and through the second exhaust conduit  28 . From there, as is generally understood by those skilled in the art, the exhaust gas is directed away from the engine  10  either through the transom  14  and to an underwater outlet or through exhaust pipes above and to the rear of the engine  10 . 
         [0036]    With continued reference to  FIG. 1 , it should be understood that one of the advantages of a preferred embodiment of the present invention is that the use of three catalyst devices  23 - 25  reduces the overall required size of the components associated with the exhaust system. In other words, three catalyst devices  23 - 25  of a lesser diameter can be aligned as shown in  FIG. 1  in a space that requires less overall width of the engine structure than would be needed if a single circular catalyst device was used. It should be understood that, when the marine engine  10  is a V-type engine, two exhaust systems are provided, one on the port side of the engine as shown in  FIG. 1  and a similarly configured exhaust system on the starboard side of the engine. 
         [0037]      FIG. 2  is an isometric and partially sectioned view of two exhaust structures used in conjunction with one embodiment of the present invention. A port exhaust structure  40  and a starboard exhaust structure  42  are shown in  FIG. 2 . The port exhaust structure  40  is sectioned to illustrate various internal characteristics. The exhaust manifold, or first exhaust conduit  22 , directs the exhaust gas E from exhaust ports of the engine, as described above, through a plenum region  44 . The central housing structure  30 , or catalyst housing structure, has a plurality of generally tubular cavities  43 - 45  formed therein. Each of the tubular cavities is shaped to receive one of the catalyst devices  23 - 25  which are described above in conjunction with  FIG. 1 . Those catalyst devices are not shown in  FIG. 2 . Each of the tubular cavities  43 - 45  is sized to define a space between an inner surface, such as surfaces  47 - 49 , of the cavities  43 - 45 , respectively, and an outer surface of the generally tubular structure of the catalyst devices  23 - 25 . This generally annular space thus defined by the sizes of the catalyst devices  23 - 25  and the generally tubular cavities  43 - 45  provides an important thermally insulative function between the catalyst devices and the catalyst housing structure  30 . As shown in  FIG. 2 , cooling passages  50  are provided to limit the temperature of the first exhaust conduit  22 , the catalyst housing structure  30 , and the second exhaust conduit  28 . However, many types of catalyst devices operate more efficiently and effectively at raised temperatures. Therefore, it can become counterproductive if the catalyst devices  23 - 25  receive a cooling effect as a result of the water passing through the cooling passages  50 . By providing a space between the catalyst devices  23 - 25  and there respective tubular cavities  43 - 45 , this cooling effect is reduced. As a result of this insulating space, the catalyst devices  23 - 25  operate at higher temperatures because of the temperature of the exhaust gas E passing through them. 
         [0038]      FIG. 3  is a sectioned view of the port exhaust device  40 . With particular reference to catalyst device  23  in  FIG. 3 , it can be seen that the outer surface  54  of the generally tubular member  56  is smaller than the inner surface  60  of the associated tubular cavity which is described above in conjunction with  FIG. 2  and identified by reference numeral  43 . This difference in size between the outer surface  54  and the inner surface  60  defines the generally annular space  70  surrounding the catalyst device  23 . As a result, heat is not efficiently communicated away from the catalyst device  23  toward the inner surface  60  of the tubular cavity which is cooled by the water passages  50 . The catalyst devices  23 - 25  therefore operate at higher temperatures than would be possible if their tubular structures were in direct thermal contact with the inner walls of their associated tubular cavities. 
         [0039]      FIG. 4  is a partially sectioned view of the port exhaust device  40 . The view of  FIG. 4  is a section taken along a plane that is generally horizontal and intersects the catalyst housing structure  30  and the three catalyst devices  23 - 25 . This plane of intersection is illustrated in  FIG. 3  and identified by dashed line  66 . 
         [0040]    In  FIG. 4 , the space between the outer surface  54  of the tubular catalyst device and the inner cylindrical surface  60  of the tubular cavity is identified by reference numeral  70 . This space is generally annular and circular in cross-section except in the region directly between adjacent catalyst devices  23 - 25 . In that region the space, identified by reference numeral  72 , is larger because of the geometry of the components and the fact that adjacent tubular cavities, identified by reference numerals  43 - 45  in  FIG. 2 , are not isolated from each other. Also shown in  FIG. 4  is a water jacket  50  surrounding the wall  76  that defines the generally tubular cavities  43 - 45 . 
         [0041]      FIG. 5  is an isometric view of the port exhaust device  40  with the catalyst housing structure  30  separated to expose the three catalyst devices  23 - 25 . The exploded view of  FIG. 5  also shows the catalyst devices  23 - 25  spaced apart from the first exhaust conduit  22  or exhaust manifold. Several characteristics of the preferred embodiment of the present invention can be seen in the exploded isometric view of  FIG. 5 . Each of the catalyst devices  23 - 25  has a tubular portion, which is generally cylindrical in the embodiment shown in  FIG. 5 , and a rim  80  which is configured to lie in a plane which is generally perpendicular to a central axis of the tubular portion of the catalyst device. These rims  80  are configured to support the associated catalyst device on an upper surface  84  of the first exhaust conduit  22 , or exhaust manifold. In other words, the outer diameter of the rim  80  is greater than the inner diameter  86  of an associated opening formed in the upper surface  84  of the exhaust manifold or first exhaust conduit  22 . These relative sizes of the openings  86  and rims  80  prevent the catalyst devices  23 - 25  from passing into the associated opening  86 . When the catalyst housing structure  30  is attached to the exhaust manifold  22 , the rims  80  of the catalyst devices  23 - 25  are captured between opposed flange surfaces. Under the rims  80  is the surface identified reference numeral  84  and above the rims  80  is the lower surface of the catalyst housing structure  30 . As a result of the space  70  described above in conjunction with  FIG. 4 , the catalyst devices  23 - 25  are generally in non-contact association with the catalyst housing structure  30 . They are supported by their relationship of the rims  80  with the upper surface  84  of the first exhaust conduit  22  and a lower surface of the catalyst housing structure  30 . 
         [0042]    With continued reference to  FIG. 5 , a gasket  90  is disposed on the flange surface  84  of the first exhaust conduit  22 . It has a central opening  92  formed therethrough. The central opening  92  formed through the gasket  90  is sized to allow the rims  80  to rest on the surface  84  within the size of the opening  92 . In other words, when the catalyst housing structure  30  is attached to the exhaust manifold  22 , the rims  80  are in contact with surface  84  and the lower surface of the catalyst housing structure  30 , but the gasket  90  is not disposed between the rims  80  and either surface. 
         [0043]    With reference to  FIGS. 3 and 5 , it can be seen that the rims  80  provide a seal at the bottom of the spaces identified by reference numerals  70  and  72  and described above in conjunction with  FIG. 4 . This seal at the bottom of these spaces inhibits a liquid, such as water, from flowing downward out of the spaces  70  and  72 . As such, the seal cooperates with the space to form a reservoir that captures water which may flow along the walls of the catalyst housing structure  30  under the effect of gravity. This water can result from condensation formed on the inner walls of the catalyst housing structure  30 . If that condensation occurs, the seal provided by the rims  80  at the bottom portions of the catalyst devices  23 - 25  inhibits the flow of that water into the exhaust manifold  22  and, eventually, into is the exhaust ports of the engine. When the catalyst devices  23 - 25  reach elevated temperatures, as a result of their direct exposure to the exhaust gas E, the increased temperature will boil the captured water within the reservoir of the spaces  70  and  72  and that resulting water vapor will pass upwardly through the catalyst housing structure  30  and out of the second exhaust conduit  28  with the exhaust gas. 
         [0044]      FIG. 6  is a section view of the exhaust manifold  22 , or first exhaust conduit, and the catalyst housing structure  30  attached to it.  FIG. 6  also shows the surface  100  of the exhaust manifold which can be rigidly attached to a surface of the engine through which exhaust gas is conducted through its exhaust ports. Reference numeral  102  identifies a gasket between surface  100  of the exhaust manifold  22  and the corresponding surface of the engine surrounding the exhaust ports. The exhaust gas E flows from the exhaust ports of the engine, through the exhaust manifold  22  and through the catalyst devices  23 - 25  as described above in conjunction with  FIGS. 1-5 . The space  70  is shown in  FIG. 6  surrounding the outer surface  54  of the generally tubular portion of the catalyst device and the inner surface  60  of the generally tubular cavity formed within the catalyst housing structure  30 . 
         [0045]    Throughout the description of the exhaust system with reference to  FIGS. 1-6 , the catalyst devices  23 - 25  have been illustrated and described as being generally cylindrical in cross-section. However, it should be understood that this generally cylindrical shape is not necessary in all embodiments of the present invention. As an example, an oblong-shaped catalyst device can also be used.  FIG. 7  shows an oblong-shaped catalyst device  123  disposed within a catalyst housing structure  130 . The oblong nature of the catalyst device can be seen from its major axis  132  and the arrow  134  which represents half of its minor axis illustrated in the section view of the catalyst device  123  in  FIG. 7 . The structure shown in  FIG. 7  directs exhaust gas through four pipes  141 - 144  which conduct is the exhaust gas to a plenum region  144  where the exhaust gas from each pipe is free to combine with gas from other pipes within the plenum  144  of the catalyst housing structure  130 . The exhaust gas passes through the plenum  144  and then through the catalyst device  123 . After flowing through the catalyst device  123 , the exhaust gas E flows into and through the second exhaust conduit  28 . 
         [0046]    The exhaust system described above in conjunction with  FIGS. 1-7  exhibits numerous advantageous characteristics which improve the operation of a marine engine. These characteristics will be described in greater detail below in conjunction with the specific figures that best illustrate those characteristics. 
         [0047]      FIG. 7  illustrates the oblong catalyst device  123  and  FIG. 1  shows the relative positions of the exhaust ports  20  on the marine engine  10 . It should be understood that the generally cylindrical exhaust devices  23 - 25  could be replaced within the catalyst housing structure  30  by an appropriately shaped oblong catalyst device such as that which is identified by reference numeral  123  in  FIG. 7 . 
         [0048]    With reference to  FIGS. 1 and 7 , one embodiment of the present invention comprises a plurality of exhaust ports  20 , a first exhaust conduit,  22  or  122 , an oblong catalyst device  123  and a second exhaust conduit,  28  or  128 . The first exhaust conduit  122  is disposed in fluid communication with the plurality of exhaust ports by the exhaust pipes  141 - 144 . The oblong catalyst device  123  is disposed in fluid communication with the first exhaust conduit,  22  or  122 . The oblong catalyst  123  has a major axis  132 , a minor axis, half of which is represented by arrow  134 , and a central axis which extends through the catalyst device in a direction generally parallel to the arrows representing the flow of exhaust gas E. The oblong catalyst device is configured to conduct the exhaust gas E in a direction generally perpendicular to the major and minor axes,  132  and  134 , and generally parallel to the central axis. The second exhaust conduit,  28  or  128 , is disposed in fluid communication with the oblong catalyst device  123 . The oblong catalyst device is disposed in serial fluid communication between the first and second exhaust conduits,  122  and  128 . 
         [0049]    The configuration of a preferred embodiment of the present invention, described above in conjunction with  FIGS. 1-7 , promotes a generally uniform flow of exhaust gas through the catalyst module, whether the catalyst module comprises a plurality of catalyst devices  23 - 25  or whether it comprises a single larger catalyst device  123 . With reference to  FIGS. 2 and 6 , the exhaust gas E flowing into the exhaust manifold  22 , or first exhaust conduit, flows from regions of relatively smaller cross-sectional area (e.g. the exhaust ports of the engine) to a plenum area  44  having a greater cross-sectional area. As a result of this increase in cross-sectional area along the path of the exhaust gas E, the velocity of the exhaust gas decreases. This allows the exhaust to more uniformly seek areas of lower pressure along the inlet surfaces of the catalyst devices  23 - 25 . In other words, without the plenum area  44 , the exhaust gas stream would be more subject to the influences of gas stream velocity and momentum that could urge the exhaust to flow through limited portions of the inlet area of the catalyst devices. However, when a plenum  44  is provided, the velocity of the gas stream slows and allows the exhaust to more uniformly seek lower pressure areas along the inlet surfaces of the catalyst devices. Rather than directing the exhaust gas stream with a restrictive conduit, the expanded area  44  of the plenum decreases the velocity of the flow and encourages a more uniform distribution of the exhaust gas through the plurality of exhaust devices or, alternatively, through all of the areas of the inlet of a single catalyst device. 
         [0050]    With reference to  FIGS. 1 and 3 , a non-catalytic porous member  150  is disposed within the second exhaust conduit  128 . This non-catalyst porous member can be made of the same material used to begin the manufacturing process is associated with the catalyst devices  23 - 25 . That manufacturing process is described in U.S. Pat. Nos. 6,368,726 and 6,639,193. U.S. Pat. Nos. 6,660,235 and 6,740,178 also describe the manufacturing process associated with creating a catalyst device. The internal portions of the catalyst devices described in those patents comprise a support structure which is porous. The support structure is also provided with a catalytic material to manufacture a catalyst device. The non-catalytic porous member  150  comprises the internal support structure, but without the catalytic material being included. Its purpose is not to serve as a catalyst device. Its purpose is to serve as a structure which inhibits the flow of liquid water in a reverse direction through the second exhaust conduit  28 . Exhaust gas freely passes through the porous non-catalytic member as it flows away from the engine  10 . As a result, the non-catalytic porous member is heated to approximately the temperature of the exhaust gas stream. If water attempts to migrate in a reverse direction through the non-catalytic porous member  150 , it will be rapidly evaporated and the resulting vapor will be carried away from the engine  10  by the exhaust stream. This embodiment of the present invention provides an exhaust conduit  128  disposed in serial fluid communication downstream from a plurality of exhaust ports  20  as described above. The non-catalytic porous member can comprise a metallic mesh material and it can be configured to direct the exhaust gas through the metallic mesh material. In a preferred embodiment, the non-catalyst porous member comprises a metallic catalytic substrate, but without a catalytic coating. 
         [0051]    The embodiment of the present invention described above in conjunction with  FIGS. 1-6  comprises a catalyst device (e.g. devices  23 - 25 ) that comprises a first catalyst material disposed within a first housing structure, such as the tubular or cylindrical structure illustrated in  FIG. 5 . The tubular portion of the catalyst devices has a central axis. The rim portion  80  extends from an end of the generally tubular portion and is disposed in a plane which is generally perpendicular to the central axis of the tubular portion. A gasket  90  is disposable between the exhaust manifold  22  and the catalyst housing structure  30 . The gasket  90  is disposed between the two flange surfaces which include the upper surface  84  of the exhaust manifold  22  and the lower surface of the catalyst housing structure  30 . The gasket has an opening  92  which is formed through its thickness. The opening  92  is configured to receive the rim portion  80  of a catalyst device. The size of the opening  92  is selected to allow the rim portions  80  to be captured between the upper surface  84  of the exhaust manifold  22  and the lower surface of the catalyst housing  30  without the gasket  90  being compressed between the rim portion  80  and either of the two flange surfaces. 
         [0052]    As described above in conjunction with  FIGS. 3 ,  4  and  6 , the outer surface  54  of each tubular catalyst device  23 - 25  is shaped to be received within an associated tubular cavity  43 - 45  with a space  70  therebetween. The space  70  is a generally annular space defined by the difference in size between the outer surface  54  of the catalyst devices  23 - 25  and the inner surface  60  of the associated tubular cavity. This space  70  provides an effective thermal insulation between the catalyst devices  23 - 25  and the catalyst housing structure  30 . The presence of the rim  80  at the bottom portion of each catalyst device  23 - 25  provides a seal which prevents liquid from flowing downward and out of the space  70  if liquid is trapped therein. As a result, a reservoir is defined which holds the liquid until the temperature becomes sufficiently high to boil the liquid and allow the water vapor to escape with the gas stream. 
         [0053]    With reference to  FIG. 5 , a concentricity spacer  160  is provided for each of the catalyst devices  23 - 25 . The purpose of the concentricity spacer is to maintain the outer cylindrical surface of the catalyst devices in a concentric relationship with the inner cylindrical surface of the associated tubular cavity  43 - 45 . The concentricity spacer  160 , in a preferred embodiment of the present invention, comprises a relatively thin sheet of material that is embossed with raised portions which maintain the concentricity of the catalyst device and its associated tubular cavity while allowing fluid to flow in a vertical direction past the concentricity spacer. 
         [0054]    With reference to  FIGS. 3 and 4 , two oxygen sensors are illustrated. An upstream oxygen sensor  170  is disposed in fluid communication with the exhaust gas passing through the exhaust manifold  22 . A downstream oxygen sensor  175  is disposed in the upper portion of the catalyst housing structure  30 . 
         [0055]    As shown in  FIG. 4 , the upstream oxygen sensor  170  is disposed within the overall exhaust structure and, more specifically, within the exhaust manifold  22 . It is therefore disposed downstream from the plurality of exhaust ports  20  (shown in  FIG. 1 ) and upstream from the catalyst devices  23 - 25 . The oxygen sensor  170  is configured to remain at or below the temperature of the exhaust gas E when the exhaust gas is flowing from the plurality of exhaust ports  20  as shown in  FIG. 1 . In other words, the oxygen sensor  170  located upstream from the catalyst devices  23 - 25  is unheated other than the effect it experiences from the hot exhaust gas flowing over it. The unheated nature of the upstream oxygen sensor  170  provides a significant advantage because it is less susceptible to damage in the event that liquid, such as water condensate, flows in a reverse direction from the second exhaust conduit  28  toward the plurality of exhaust ports of the engine  10 . If the upstream oxygen sensor  170  is heated, as most oxygen sensors now are, it could be severely damaged if water flows in contact with it. The use of an unheated oxygen sensor  170  therefore provides a significant benefit in an exhaust system of a marine engine. 
         [0056]      FIG. 3  illustrates an advantage provided by the system described herein in conjunction with a reverse flow of liquid, as represented by arrows W, from the second exhaust conduit  28  toward the catalyst devices  23 - 25 . The downstream oxygen sensor  175  is disposed within the catalyst housing structure  30  and in fluid communication with the plurality of exhaust ports  20  described above in conjunction with  FIG. 1 . It is also disposed in fluid communication with the second exhaust conduit  28 . The second exhaust conduit  28  is connected to a first portion of the catalyst housing structure  30  which, as illustrated in  FIG. 3 , is at the upper right portion of this device. The oxygen sensor  175  is connected to a second portion of the catalyst housing structure  30  which is located at the upper left portion as shown in  FIG. 3 . In a preferred embodiment of the present invention, the first and second portions are disposed at opposite sides of the catalyst housing structure  30  as shown. 
         [0057]    With continued reference to  FIG. 3 , the first exhaust conduit  22 , the catalyst housing structure  30  and the catalyst device  23 - 25  are configured and arranged to cooperatively define a reversion liquid trajectory path W for water that flows in a direction from the second exhaust conduit  28  toward the first exhaust conduit  22 . This reversion liquid trajectory path is governed by the positions of the second exhaust conduit  28  and the catalyst devices  23 - 25  in conjunction with the resulting inertia of the water droplets as they flow under the effect of differential pressure that can result from the opening of exhaust valves of the engine. 
         [0058]    The causes for water reversion are well known to those skilled in the art of marine propulsion systems. As water droplets are caused to flow in a reverse direction, as indicated by arrows W, the trajectory of those water droplets is governed by the magnitude of the differential pressure between the second exhaust conduit  28  and the first exhaust conduit  22  in conjunction with the size of the various droplets, the shape of the internal cavity of the catalyst housing structure  30 , and the positions of the upper portions of the catalyst devices  23 - 25 . The location of the downstream oxygen sensor  175  is selected, in a preferred embodiment of the present invention, to be away from this reversion liquid trajectory path illustrated by arrows W in  FIG. 3 . As such, the water droplets are less likely to strike the downstream oxygen sensor  175 . This advantageous location of the downstream oxygen sensor  175  therefore avoids damage that would otherwise occur to it if the hot sensor  175  is struck by water droplets flowing in a reverse direction from the second exhaust conduit  28  toward the catalyst devices  23 - 25 . 
         [0059]    Although the present invention has been described in particular detail and illustrated to show various embodiments, it should be understood that alternative embodiments are also within its scope.