Abstract:
An exhaust system of a marine propulsion device is provided with a bifurcated cooling passage in its elbow structure in order to limit the effect of cold water being disposed in thermal communication with exhaust gas passing through the elbow structure. This thermal communication between a stream of cold water and exhaust gas passing through the elbow structure is minimized in order to reduce the likelihood that water vapor will condense out of the stream of exhaust gas as it passes through the elbow structure. An obstruction, or water dam, is used to bifurcate the coolant chamber within the elbow structure while allowing passage of coolant through the obstruction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to a marine propulsion system and, more particularly, to a propulsion system that circulates cooling water through water jackets of its exhaust elbows in a manner that reduces the affected surface area in thermal communication with cooling water from a body of water so that condensation of water vapor entrained in the exhaust gas stream is reduced. 
     2. Background 
     Those skilled in the art of marine propulsion systems are familiar with various techniques used to conduct exhaust gas away from an internal combustion engine and emit the exhaust gas at a location behind a watercraft. In some applications, the exhaust gas is directed to flow through a marine drive unit and through the central opening of a propeller. In other applications, exhaust gas can be directed to flow through outlets above the surface of a body of water in which the marine vessel is operated. Regardless of the particular routing system used to conduct the flow of exhaust gas to a location behind the watercraft, the vast majority of marine propulsion systems collect the exhaust from the cylinders of an engine, in a manifold structure of some type, and then direct the exhaust gas through an elbow that is connected to one or more exhaust pipes. Typically, in V-type engines, two manifolds and two elbows are used. 
     U.S. Pat. No. 3,696,620, which issued to Pace on Oct. 10, 1972, describes a marine engine water cooling device. Improved water jacketed manifolds and water cooling systems for marine engines are described. Water circulation through the manifold water jacket is provided through an apertured pipe which extends through the jacket. 
     U.S. Pat. No. 3,759,041, which issued to North et al. on Sep. 18, 1973, discloses an exhaust water separator for marine engines. Arcuate exhaust elbows are provided which force cooling water in the exhaust to the outside by centrifugal action. This water is removed and expelled through the transom. 
     U.S. Pat. No. 3,780,712, which issued to Pace on Dec. 25, 1973, describes marine engine cooling. Heated water which is circulated through an engine cooling system for cooling purpose is mixed in the improved engine exhaust manifold water jacket with raw, relatively cool water to controllably cool the manifold and avoid condensing water from the exhaust gases flowing through the exhaust manifold. 
     U.S. Pat. No. 4,573,318, which issued to Entringer et al. on Mar. 4, 1986, discloses an exhaust elbow for a marine propulsion system. The exhaust elbow has an intake exhaust passage extending upwardly from the engine and communicating through a bend with a discharge exhaust passage, and a water jacket having pockets around the exhaust passages for cooling the latter. A central channel extends longitudinally along the exterior of the exhaust passages to guide water there along to the end of the discharge exhaust passage to mix with exhaust thereat. The central channel has a pair of sidewalls extending longitudinally and laterally tapered away from each other at the outer end of the discharge exhaust passage to create an outward draw from the central channel to minimize break-up of longitudinally outward water flow and maintain the end tip of the discharge exhaust passage dry and prevent water ingestion and creeping back into the discharge exhaust passage due to pulsations of the engine. Dam and port structure is also provided enabling faster heating of the exhaust passage and in turn minimizing condensation within the elbow which may otherwise ingest back into the engine. 
     U.S. Pat. No. 4,845,945, which issued to Widmer et al. on Jul. 11, 1989, discloses an exhaust elbow trough. A water jacketed exhaust elbow for a marine propulsion system includes an intake exhaust passage communicating with a discharge exhaust passage, a water jacket around the exhaust passages, and a trough member extending longitudinally along the water channel along the exterior of the discharge exhaust passage to guide water therealong to mix with exhaust at the end of the discharge exhaust passage. The trough member extends beyond the end tip of the discharge exhaust passage and has a sharp edge providing a clean parting surface for the coolant water and preventing ingestion of water back into the discharge exhaust passage. 
     U.S. Pat. No. 4,866,934, which issued to Lindstedt on Sep. 19, 1989, discloses a marine drive exhaust system with shaped O-ring seals. The exhaust system is provided with resilient, shaped rubber O-ring seals between facing surfaces of the exhaust manifold and exhaust elbow, and the facing surfaces of the exhaust elbow and the exhaust pipe. Each of the shaped O-ring seals has an inner peripheral rib extending peripherally around the exhaust passage and generally conforming to the shape thereof and being spaced laterally between the exhaust passage and the peripheral water passage. Each of the shaped O-ring seals has an outer peripheral rib extending peripherally around the water passages and spaced laterally outward of the inner rib by a gap through which the water passages extend. 
     U.S. Pat. No. 4,977,741, which issued to Lulloff et al. on Dec. 18, 1990, discloses a combination exhaust manifold and elbow for marine propulsion systems. A combination exhaust manifold and exhaust elbow for an internal combustion engine includes an exhaust cavity for receiving exhaust from the engine, an exhaust passage leaving from the exhaust cavity, and an exhaust discharge outlet. A first water jacket is provided around the exhaust cavity and a second water jacket is provided around the exhaust discharge passage. A dam is provided between the first and second water jackets, having a passage therein for allowing fluid communication between the first and second water jackets. A warm water inlet is provided in the first water jacket around the exhaust cavity for receiving cooling water which has been warmed by the engine, and which flow is controlled by a temperature sensitive thermostat. A cold water inlet is provided adjacent the discharge exhaust passage. The cold water inlet is disposed either upstream or downstream of the dam adjacent the exhaust passage, and allows cold bypass water to be discharged without the necessity of the cold water flowing through the entire assembly, so as to prevent moisture from condensing out of the exhaust in the exhaust cavity. 
     U.S. Pat. No. 4,991,546, which issued to Yoshimura on Feb. 12, 1991, describes a cooling device for a boat engine. A number of embodiments of cooling systems for internal combustion engines powering marine watercraft are described. The engine coolant jacket delivers its coolant to an exhaust manifold cooling jacket adjacent the inlet end of the exhaust manifold. Coolant is delivered from the exhaust manifold cooling jacket to a further cooling jacket around the inlet portion of an exhaust elbow. In one embodiment, a cooling jacket system is provided for the engine cooling jacket, exhaust manifold cooling jacket and the elbow cooling jacket. In another embodiment, the system discharges coolant back to the body of water in which the watercraft is operating through a further cooling jacket of the exhaust elbow that communicates with its discharge end. 
     U.S. Pat. No. 5,109,668, which issued to Lindstedt on May 5, 1992, discloses a marine exhaust manifold and elbow. An exhaust assembly includes a manifold portion, an elbow portion, a water jacket portion, and exhaust runner walls, providing a smooth continuous transition of exhaust gas flow from intake exhaust passages in the manifold portion to transfer exhaust passages in the elbow portion around a bend to a discharge exhaust passage, minimizing turbulent flow of exhaust through the manifold portion and elbow portion. 
     U.S. Pat. No. 5,644,914, which issued to Deavers et al. on Jul. 8, 1997, discloses an exhaust pressure pulsation control apparatus for a marine propulsion system. It has a front ring and a reflector disk located downstream of the front ring. There is a space between the front ring and the reflector disk that is sufficiently large so that the mixture of water and water cooled exhaust passing through the apparatus does not have a significant pressure drop. The apparatus attenuates pressure pulsations in the exhaust system, thereby significantly reducing water ingestion through the exhaust system into the engine. The apparatus does not create significant exhaust back pressure, and typically increases engine maximum power output. 
     U.S. Pat. No. 6,290,558, which issued to Erickson on Sep. 18, 2001, discloses an exhaust elbow with a water trap for a marine propulsion system. The water trap section defines a water collection cavity. Within the water trap section, a barrier extends downward into the water collection cavity to define first and second exhaust passages. When water begins to collect in the water collection cavity, the cross-sectional area of the exhaust passage is reduced and the velocity of exhaust gases passing through the exhaust passage is increased. The water collection cavity is shaped to be easily cleared when exhaust gas pressure increases as the engine speed increases. 
     U.S. Pat. No. 6,478,645, which issued to Allbright et al. on Nov. 12, 2002, describes a moisture migration inhibitor for wet marine exhaust. A moisture inhibitor system for wet exhaust as utilized in marine applications, such as boats and other watercraft, is described. The preferred embodiment contemplates an exhaust manifold having an inner exhaust passage which has situated therein a collection barrier or raised pocket situated to collect moisture migrating from the exhaust port, generally at the stern of the vessel. The collection pocket is heated by the exhaust stream and is formed to collect and retain the migrating moisture while simultaneously the heated walls of the collection barrier evaporate the collected moisture forming moisture vapor which moisture vapor is urged through the exhaust passage and the exhaust port, where it leaves the system. 
     U.S. Pat. No. 6,582,263, which issued to Jaeger et al. on Jun. 24, 2003, discloses a marine exhaust elbow structure with enhanced water drain capability. The elbow is provided with a stainless steel tube within a water outlet opening to assure that a drain opening remains open even when the exhaust elbow is exposed to a corrosive environment. Since cast iron tends to expand in volume as a result of corrosion of its surface areas, water outlet openings intended to perform a draining function can be partially or fully closed as a result of corrosion. The insertion of a stainless steel tube in one or more water outlet openings of an exhaust elbow assures that an internal water cavity of the elbow can drain when the associated internal combustion engine is turned off, thereby minimizing the possibility of freeze damage to the exhaust components. 
     U.S. Pat. No. 6,652,337, which issued to Logan et al. on Nov. 25, 2003, discloses an exhaust system for a marine propulsion engine. A relationship between the exhaust passages and coolant passages of an exhaust manifold and exhaust elbow serves to maintain the joint of the exhaust passage at a higher temperature than would be possible with known exhaust manifolds and exhaust elbows. By providing a space between surfaces of a raised exhaust portion of the components and surfaces of the raised coolant portions of the exhaust system, leakage from the coolant conduit to the exhaust cavities is avoided. 
     U.S. Pat. No. 6,800,004, which issued to White et al. on Oct. 5, 2004, discloses a marine exhaust cooling system. It uses an orifice to distribute liquid coolant flow between two alternative and parallel paths. One coolant path passes through a generally horizontal portion of an exhaust elbow and the other coolant path passes through the orifice and directly to a vertical riser of the exhaust elbow. The ratio of flow between the two paths changes as a function of engine speed because of the operation of the orifice which provides increased resistance to flow as a function of increased pressure drop across the orifice. 
     U.S. Pat. No. 6,929,520, which issued to Hughes et al. on Aug. 16, 2005, discloses a cooling method for a marine propulsion system. It directs a portion of a recirculating stream of cooling water to a first portion of an exhaust manifold so that the cooling jacket of the exhaust manifold can be maintained in a filled condition. Water flows upwardly through the cooling jacket and exits through a port in the exhaust manifold back into a recirculating stream of cooling water that passes through a recirculation pump, the cooling passage of an engine, and a cavity of a thermostat housing. 
     U.S. Pat. No. 7,427,222, which issued to Auck et al. on Sep. 23, 2008, describes a reversion control device for a watercraft exhaust system. The device is a reversion control device including a housing for a stationary vane and a flapper. In one example, the housing includes an expansion chamber to house the stationary vane and the flapper. 
     The patents described above are hereby expressly incorporated by reference in the description of the present invention. 
     Marine propulsion systems normally draw water from a body of water in which the marine vessel is operating and direct the water to flow through various devices in order to remove heat from heat producing components. The temperature of the water drawn from the body of water can vary significantly, depending on the season of the year and the geographical region where the body of water is located. The exhaust gases produced by the engine contain water vapor. Exposing the stream of exhaust gas to low temperatures caused by the use of cold cooling water can result in condensation within the exhaust conduits of the system. The formation of condensation can be significantly disadvantageous, as described within numerous ones of the patents cited above. Among these disadvantages is the potential flow of condensed water back toward the exhaust ports of the engines&#39; cylinders. In addition, if the engine is provided with one or more catalyst devices, the condensed water can possibly flow toward and in contact with the catalyst components. It would therefore be significantly advantageous if the creation of condensed water vapor in the exhaust conduits could be reduced or eliminated. 
     SUMMARY OF THE INVENTION 
     A marine exhaust system made in accordance with a preferred embodiment of the present invention comprises a manifold structure, an elbow structure attached to the manifold structure, a manifold exhaust conduit formed within the manifold structure and configured to direct a flow of exhaust gas from an engine through a collection chamber of the manifold exhaust conduit to an outlet cavity of the manifold exhaust conduit, a manifold cooling jacket disposed around a substantial portion of the manifold exhaust conduit, an elbow exhaust conduit formed within the elbow structure and configured to direct a flow of exhaust gas from an inlet opening to an outlet opening, an elbow cooling jacket disposed around a substantial portion of the elbow exhaust conduit, and an obstruction disposed within the elbow cooling jacket. The elbow exhaust conduit is connected in fluid communication with the manifold exhaust conduit to direct the flow of exhaust gas from the outlet cavity to the inlet opening. The obstruction is configured to divide the elbow cooling jacket into an inlet portion which surrounds a substantial portion of an inlet section of the elbow exhaust conduit and an outlet portion which surrounds a substantial portion of an outlet section of the elbow exhaust conduit, wherein the obstruction has a passage formed therein to conduct fluid between the inlet portion and the outlet portion. 
     In a preferred embodiment of the present invention, it further comprises a gasket disposed between the manifold structure and the elbow structure. In one embodiment of the present invention, the gasket has an opening formed through its thickness and configured to connect the manifold cooling jacket in fluid communication with the elbow cooling jacket. In an alternative embodiment of the present invention, the gasket is configured to prevent fluid communication between the manifold cooling jacket and the elbow cooling jacket. 
     In a particularly preferred embodiment of the present invention, the marine exhaust system further comprises a first port connected in fluid communication with the outlet portion to continuously conduct fluid into the outlet portion when the engine is operating. It can further comprise a second port connected in fluid communication with the inlet portion to conduct fluid into the inlet portion when the pressure of the fluid, pumped from a body of water, exceeds a predefined threshold magnitude. 
     In a particularly preferred embodiment of the present invention, the exhaust system further comprises a catalyzing component disposed within the outlet cavity of the manifold exhaust conduit and configured to conduct a substantial portion of the flow of exhaust gas through the catalyzing component. The catalyzing component can be retained in place by a portion of the catalyzing component being disposed between opposing surfaces of the manifold structure and the elbow structure. 
     In certain embodiments of the present invention, the manifold cooling jacket is connected in fluid communication with a cooling jacket of the engine within a closed cooling system, a first coolant being contained within the closed cooling system for recirculation through the manifold cooling jacket and the cooling jacket of the engine. The elbow cooling jacket, in this particular embodiment of the present invention, is isolated from the manifold cooling jacket and the cooling jacket of the engine. A second coolant is directed to flow through the elbow cooling jacket. In a preferred embodiment of the present invention, the second coolant is water drawn from a body of water. 
     In certain embodiments of the present invention, the manifold cooling jacket is connected in fluid communication with a cooling jacket of the engine and with the elbow cooling jacket within an open cooling system. A first coolant is directed to flow through the open cooling system for passage through the cooling jacket of the engine, the manifold cooling jacket and the elbow cooling jacket. The second coolant is water drawn from the body of water in this embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a section view of a manifold and elbow structure generally known to those skilled in the art; 
         FIG. 2  is an exploded isometric view of an exhaust and cooling system of a marine vessel; 
         FIG. 3  shows a section view of one preferred embodiment of the present invention illustrated in conjunction with a pressure responsive valve; 
         FIG. 4  is a section view of a portion of  FIG. 3 ; 
         FIG. 5  is a section view of another preferred embodiment of the present invention; 
         FIG. 6  is an exploded isometric view of a preferred embodiment of the present invention; 
         FIG. 7  is a partial section view of a preferred embodiment of the present invention showing an elbow structure; 
         FIG. 8  is a section view of a portion of  FIG. 7 ; and 
         FIG. 9  is a side view of a marine engine incorporating a preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the description of the preferred embodiment of the present invention, like components will be identified by like reference numerals. 
       FIG. 1  is a side section view of a known type of manifold  1  and an elbow  2  with a gasket  3  disposed between them. Exhaust gas, represented by solid line arrows in  FIG. 1 , is collected in the main chamber of the manifold  1  and directed into the elbow  2  which directs the flow of exhaust gas into an exhaust pipe  4  which is connected to the elbow  2  by an elastomeric tube  5  which is occasionally referred to as a “bellows”. Cooling water, represented by dashed line arrows in  FIG. 1 , is introduced into a water jacket  6  of the elbow  2  in order to control its temperature. The water, after flowing through the water jacket  6  of the elbow  2 , is mixed with the exhaust gas and directed away from the elbow. Those skilled in the art of marine propulsion systems and engine exhaust systems for watercraft are familiar with the basic structure illustrated in  FIG. 1 . Reference numeral  8  is used in  FIG. 1  to identify the internal surface of the exhaust gas conduit within the elbow  2 . Virtually all of this internal surface  8  is in thermal communication with the water flowing through the water jacket  6  of the elbow  2 . When the water, represented by dashed line arrows in  FIG. 1 , is cold this thermal communication with surface  8  can cause water vapor within the flow of exhaust gas, represented by solid line arrows in  FIG. 1 , to condense on the surface  8  of the walls of the elbow  2 . When the surface area of the walls within the exhaust gas passage is large and the water flowing through the water jacket  6  is cold, there exists a significant likelihood that liquid water will condense on surface  8  and some of that water can flow in a reverse direction toward the manifold  1 . In certain particularly deleterious circumstances, this water can flow downwardly into the exhaust ports of the cylinders of the engine. As will be described in greater detail below, various embodiments of the present invention are directed toward avoiding this disadvantageous situation. 
       FIG. 2  is an exploded isometric representation of a marine engine and certain selected peripheral components. Also shown in  FIG. 2  are port  512  and starboard  514  manifolds which are attached to port  516  and starboard  518  elbows, respectively. A heat exchanger is identified by reference numeral  520  in  FIG. 2  along with a recirculation pump  522 , a thermostat housing  524 , a distribution housing  526 , a water pump  527 , and a representation of a portion of a poppet valve  528  which, as will be described in greater detail below, selectively directs water to the elbows,  516  and  518 , in a preferred embodiment of the present invention.  FIG. 2  also shows numerous fittings, clamps, and hoses which will not be described in detail herein. 
       FIG. 3  shows a section view of a manifold structure  20  and an elbow structure  30 . A manifold exhaust conduit  22  is formed within the manifold structure  20  and configured to direct a flow of exhaust gas  23  from an engine  510 , such as that illustrated in  FIG. 2 , through a collection chamber  24  of the manifold exhaust conduit to an outlet cavity  26 . A manifold cooling jacket  28  is disposed around a substantial portion of the manifold exhaust conduit  22 . An elbow exhaust conduit  32  is formed within the elbow structure  30  and configured to direct a flow of exhaust gas  23  from an inlet opening  34  to an outlet opening  36 . The elbow exhaust conduit  32  is connected in fluid communication with the manifold exhaust conduit  22  in order to direct the flow of exhaust gas  23  from the outer cavity  26  to the inlet opening  34 . An elbow cooling jacket  37  is disposed around a substantial portion of the elbow exhaust conduit  32 . An obstruction  38  is disposed within the elbow cooling jacket  37 . The obstruction  38  is configured to divide the elbow cooling jacket  37  into an inlet portion  50  which surrounds a substantial portion of an inlet section  52  of the elbow exhaust conduit  32  and an outlet portion  60  which surrounds a substantial portion of an outlet section  62  of the elbow exhaust conduit  32 . The obstruction  38  has a passage  66  formed therein to conduct fluid between the inlet portion  50  and the outlet portion  60 . The passage  66  will be described below in conjunction with  FIG. 4 . 
     With continued reference to  FIG. 3 , it should be understood that the manifold  20  and elbow  30  can be used in either a closed cooling system or an open cooling system. The representation shown in  FIG. 3  is intended for use in a closed cooling system. In a marine propulsion system that incorporates a closed cooling system, a coolant is recirculated through the block and head of the engine and heat is removed from that coolant through the use of a heat exchanger such as the one identified by reference numeral  520  in  FIG. 2 . In  FIG. 3 , arrows  70  represent the path that a coolant would take in a system of this type. Typically, an ethylene glycol mixture is recirculated through the cooling passages of the engine and other components, such as the manifold  20  in  FIG. 3 , and then passed through a heat exchanger to remove heat from that coolant. The coolant is introduced through fitting  72  and exits from the elbow cooling jacket  28  through fitting  74 . In conjunction with a closed cooling system, the gasket  78  blocks passage of the coolant from the manifold cooling jacket  28  to the elbow cooling jacket  37 . 
     With continued reference to  FIG. 3 , a pressure responsive valve  80  is shown in the lower left portion of the figure. The isometric representation of the pressure responsive valve  80  illustrates an inlet conduit  82 , an outlet conduit  84 , and a pressure relief conduit  86 . Arrow  91  represents water flowing from a heat exchanger, like heat exchanger  520  in  FIG. 2 , after it is pumped from a body of water by a pump, like that identified by reference numeral  527  in  FIG. 2 . The components shown in  FIG. 3  are associated with a closed cooling system. However, it should be understood that various embodiments of the present invention can also be used in association with open cooling systems. Naturally, open cooling systems do not use a heat exchanger. Instead, water is pumped from a body of water and circulated through the cooling jackets of the engine block, heads, and exhaust manifolds. 
     The water flows from the heat exchanger or from a thermostat bypass passage to the inlet conduit  82  of the pressure responsive valve  80  and then from the outlet  84  to a fitting  85  which directs the water flow into the outlet portion  60  of the elbow cooling jacket  37 . In association with a closed cooling system, water can be directed from the pressure relief conduit  86  into fitting  87  which conducts that water into the inlet portion  50  of the elbow cooling jacket  37 . The passage  66  formed in the obstruction  38  allows water to flow between the inlet and outlet portions,  50  and  60 , of the elbow cooling jacket  37 . The direction of flow through the passage  66 , in open cooling systems, depends on the immediately preceding operation history of the marine propulsion system, the temperature of the elbow  30 , and the relative pressures of the fluid within the inlet and outlet portions,  50  and  60 , of the elbow cooling jacket  37 . 
       FIG. 4  is a section view taken through the portion of the elbow  30  in  FIG. 3  at the obstruction  38 . The passage  66  is shown formed through the obstruction  38  at an upper region of the exhaust elbow. 
     With continued reference to  FIGS. 3 and 4 , two oxygen sensors,  101  and  102 , are shown in  FIG. 3 . In addition, it can be observed that the passage  66  is located at an upper portion of the elbow cooling jacket  37 . This has the additional beneficial effect of inducing any condensation formed within the elbow exhaust conduit  32  to flow downwardly toward the distal end of the elbow exhaust conduit and be discharged out of the outlet opening  36  either under the influence of gravity or with the passage of exhaust gas  23  through the elbow structure  30 . It can also be observed that the elbow cooling jacket  37  is effectively divided into two sections on both sides of dashed line  100  in  FIG. 3 . Dashed line  100  is generally coincident with the obstruction  38  which serves as a dam within the elbow cooling jacket  37 . The internal wall surface of the elbow exhaust conduit  32  is, essentially, divided into the inlet section  52  and the outlet section  62 . The inlet section  52  is influenced by the temperature of the water within the inlet portion  50  of the elbow cooling jacket  37  and the outlet section  62  is influenced by the temperature of the water within the outlet portion  60 . Since water flows virtually continuously from the outlet conduit  84  of the pressure responsive valve  80  to fitting  85 , the temperature of the water within the outlet portion  60  will typically be much colder than the water within the inlet portion  50 . This is particularly true on cold days when the body of water in which the marine vessel is operated is particularly cold. If the inlet portion  50  is generally empty when operation of the engine begins, it will be filled by water that first passes into the outlet portion  60  and then through the passage  66 . If the inlet portion  50  is filled with water when the engine operation begins, that water is not likely to be as cold as water freshly drawn from the body of water. The inlet portion  50 , once filled with water, becomes stagnant due to this portion of the circuit not flowing. The inlet portion  50  will continue to pick up heat from the exhaust gas. This portion of the circuit will not flow until the pressure responsive valve opens. Therefore, the wall temperature within the elbow exhaust conduit  32  will not be as cold within the inlet section  52  as it is in the outlet section  62 . In addition, only a portion of the elbow exhaust conduit surface, to the left of dashed line  100 , is exposed to this cold water. 
     When the engine begins to operate at elevated speeds, the pressure of the cooling water increases significantly. The pressure responsive valve  80  then conducts an increased flow from the pressure relief conduit  86  into fitting  87  and the inlet portion  50  of the elbow cooling jacket. When this occurs, the flow through fitting  87  exceeds the flow through fitting  85  and the flow of water through the passage  66  is from the inlet portion  50  to the outlet portion  60  and then out through the distal end of the elbow  30 . 
       FIG. 5  is a section view of a manifold structure  20  and elbow structure  30  made in accordance with a preferred embodiment of the present invention, but with an added feature that raises the outlet of the elbow structure  30  to a location higher than results from the use of the structure shown in  FIG. 3 . The structure shown in  FIG. 5  is also an example of the present invention used in association with an open cooling system. Water flows through fitting  72  and into the manifold cooling jacket  28  as represented by arrows  70 . This water is directed through the opening  79  formed through the gasket  78  and into the inlet portion  50  of the elbow cooling jacket. The water continues to flow through the inlet portion  50  until it reaches the passage  66  formed in the obstruction  38 . The passage  66  is specifically shown in  FIGS. 4 and 8  and its location is illustrated in  FIGS. 3 and 5  and will also be described in conjunction with  FIG. 7  below. The water then flows into the outlet portion  60  of the elbow cooling jacket. Water can also flow through fitting  85  into the outlet portion  60  as represented by arrows  340 . These two flows of water,  70  and  340 , combine at a point downstream of the obstruction  38  in an open cooling system which incorporates a preferred embodiment of the present invention.  FIG. 7 , which will be described in greater detail below, also shows the elbow structure  30  illustrated in  FIG. 5 .  FIG. 8  is a section view of a portion of  FIG. 7  taken through the obstruction  38 . 
       FIG. 6  is an exploded isometric view of a manifold structure  20 , an elbow structure  30 , a gasket  78 , and a catalyzing component  110 . In certain embodiments of the present invention, the catalyzing component  110  comprises a generally cylindrical tube which has a rim  112 . Inside the tube, a catalyzing metal is contained and configured to allow exhaust gas to pass through the central portion of the tube and in contact with the surface area of the catalyzing material. As described above, in conjunction with  FIGS. 3 and 5 , the manifold exhaust conduit  22  comprises an outlet cavity  26 . The outlet cavity  26  is shown in  FIGS. 3 ,  5  and  6 . The outlet cavity is shaped to receive the catalyzing component  110 . The rim  112 , in a preferred embodiment of the present invention, is shaped to be confined between opposing faces of the manifold  20  and the elbow  30 . These two opposing faces also confine the gasket  78 . In open cooling systems, the gasket  78  is provided with the openings  79  which are aligned with appropriate portions of the cooling jackets of both the manifold  20  and elbow  30 . The relationship between the catalyzing component  110 , and its rim  112 , with the outlet cavity  26  of the manifold  20  and the inlet opening of the elbow  30  directs the exhaust flow  23  through the cylindrical tube of the catalyzing component  110  and in contact with the surface of the catalyst material contained within the cylindrical housing. 
       FIG. 7  is a section view of the elbow  30  that is used in an application, such as that illustrated in  FIG. 5 , where a dimensional requirement necessitates the use of an elbow which raises the exhaust outlet to a point higher than the type of application illustrated in  FIGS. 3 and 6 . The basic concepts of the present invention are similar, but certain dimensions are changed for the purpose of adapting those concepts to an exhaust system with different dimensional requirements. The elbow shown in  FIG. 7  is generally the same as that shown in  FIG. 5 , but with certain components removed for clarity and the section view taken at a slightly different position so that the structure in  FIG. 7  can be further sectioned to show the illustration of  FIG. 8 . 
     With continued reference to  FIGS. 7 and 8 , opening  200  is the location where fitting  85 , as shown in  FIG. 5 , would be connected to direct the flow of bypass water either from the thermostat of an engine in an open cooling system or from the heat exchanger in a closed cooling system. That water flows into the outlet portion  60  of the water jacket and this water is then directed to flow in thermal communication with the outlet section  62  of the elbow exhaust conduit  32 . Coolant flowing into the inlet portion  50  flows in thermal communication with the inlet section  52  of the elbow exhaust conduit  32 . The obstruction  38  is generally aligned with dashed line  100  which is intended to show the plane in which the obstruction  38  is located. The passage  66  permits fluid communication between the inlet portion  50  and the outlet portion  60 . 
       FIG. 8  is a section view of a portion of the elbow  30  illustrated in  FIG. 7 . The section view is taken along dashed line  100  and within the water dam provided by the obstruction  38 . The passage  66  is shown in  FIG. 8  at the upper portion of the elbow cooling jacket  37 . With reference to  FIGS. 3 ,  4 ,  5 ,  7  and  8 , it can be seen that the basic principles of the preferred embodiment of the present invention are similar in both of these embodiments, whether the elbow provides a negligible rise, as in  FIG. 3 , or a significant rise as in  FIGS. 5 and 7 . 
       FIG. 9  is a side view of an engine  10  incorporating a manifold structure  20  and an elbow structure  30  made in accordance with a preferred embodiment of the present invention. The oxygen sensors,  101  and  102 , are provided to allow comparison with the section view shown in  FIG. 5 . An elastomeric cylinder  305  connects the outlet end of the elbow structure  30  to an exhaust pipe  304 . An additional elastomeric cylinder  310  connects the exhaust pipe  304  to another exhaust conduit  312  which is sometimes referred to as a “bullhorn”. The flange  316  of the bullhorn  312  is attached to another component (not shown in  FIG. 9 ) which directs exhaust gas through the plane defined by dashed line  320  and through a transom of a marine vessel to be discharged. A conduit  324  directs the flow of bypass water to a fitting  85  which directs the bypass water into the outlet portion  60  which is described above in conjunction with  FIGS. 5 and 7 . A pressure relief fitting  87  directs the flow of water into the inlet portion  50  of the elbow cooling jacket as described above in conjunction with  FIG. 3 . Since  FIG. 9  shows the starboard side of the engine  10 , it should be understood that another manifold structure  20  and elbow structure  30  is attached to the port side of the engine. 
     With reference to  FIGS. 2-9 , it can be seen that a marine exhaust system made in accordance with one or more preferred embodiments of the present invention comprises a manifold structure  20  and an elbow structure  30  which is attached to the manifold structure. It also comprises a manifold exhaust conduit  22  formed within the manifold structure  20  and configured to direct a flow of exhaust gas  23  from an engine  10  through a collection chamber  24  to an outlet cavity  26 . It further comprises a manifold cooling jacket  28  which is disposed around a substantial portion of the manifold exhaust conduit  22 . An elbow exhaust conduit  32  is formed within the elbow structure  30  and configured to direct a flow of exhaust gas  23  from an inlet opening  34  to an outlet opening  36 . The elbow exhaust conduit  32  is connected in fluid communication with the manifold exhaust conduit  22  to direct the flow of exhaust gas  23  from the outlet cavity  26  to the inlet opening  34 . An elbow cooling jacket  37  is disposed around a substantial portion of the elbow exhaust conduit  32  and an obstruction  38  is disposed within the elbow cooling jacket  37 . The obstruction  38  is configured to divide the elbow cooling jacket  37  into an inlet portion  50  which surrounds a portion of an inlet section  52  of the elbow exhaust conduit  32  and an outlet portion  60  which surrounds a substantial portion of an outlet section  62  of the elbow exhaust conduit  32 . The obstruction  38  has a passage  66  formed therein to conduct fluid between the inlet portion  50  and the outlet portion  60 . A gasket  78  is disposed between the manifold structure  20  and the elbow structure  30  in a preferred embodiment of the present invention and the gasket  78  has an opening  79  formed through its thickness and configured to connect the manifold cooling jacket  28  in fluid communication with the elbow cooling jacket  37  in one embodiment of the present invention when it is used in association with an open cooling system. When used in association with a closed cooling system, the gasket  78  is configured to prevent fluid communication between the manifold cooling jacket  28  and the elbow cooling jacket  37 . Particularly preferred embodiments of the present invention further comprise a first port  400  connected in fluid communication with the outlet portion  60  to continuously conduct fluid into the outlet portion when the engine  10  is operating. In certain embodiments of the present invention, it can further comprise a second port  402  connected in fluid communication with the inlet portion  50  to conduct fluid into the inlet portion when the pressure of the fluid, pumped from a body of water, exceeds a predefined threshold magnitude as determined by the pressure sensitive valve  80 . A catalyzing component  110  is disposed within the outlet cavity  26  and configured to conduct a substantial portion of the flow of exhaust gas through the catalyzing component  110 . The catalyzing component is retained in place by a portion, such as rim  112 , of the catalyzing component being disposed between opposing surfaces of the manifold structure  20  and the elbow structure  30 . When used in a closed cooling system, the manifold cooling jacket  28  is connected in fluid communication with a cooling jacket of the engine  10 . A first coolant is contained within the closed cooling system for recirculation by a recirculating pump  522  through the manifold cooling jacket  28  and the cooling jacket of the engine  10 . The elbow cooling jacket  37  is isolated from the manifold cooling jacket  28  and the cooling jacket of the engine  10 . A second coolant is directed to flow through the elbow cooling jacket  37 . In a typical application of the present invention, the second coolant is water drawn from a body of water. When used in an open cooling system, the manifold cooling jacket  28  is connected in fluid communication with a cooling jacket of the engine  10  and with the elbow cooling jacket  37 . A first coolant is directed to flow through the open cooling system for passage through the cooling jacket of the engine  10 , the manifold cooling jacket  28  and the elbow cooling jacket  37 . The second coolant is water drawn from a body of water. In order to facilitate comparison of the various illustrations and to recognize certain similarities in the location of particular components, a stainless steel tube  470  is identified in  FIGS. 1 ,  3 ,  5  and  7 . 
     Although the present invention has been described in particular detail and illustrated to show different preferred embodiments, it should be understood that alternative embodiments are also within its scope.