Abstract:
A method of controlling emissions from an internal combustion engine including governing engine speed with respect to a constant speed, maintaining an air/fuel ratio of the engine, flowing exhaust from the engine through an exhaust system containing a catalyst, monitoring a variable with a feedback sensor located upstream of the catalyst, and controlling the air/fuel ratio of the engine as a function of the variable. In one application, the engine is configured for marine applications, including electric power generation and propulsion.

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 60/515,166, filed on Oct. 27, 2003, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to controlling emissions from internal combustion engines. 
     BACKGROUND 
     Reducing combustion engine exhaust emissions is a continual object of research and development, driven both by awareness of environmental effects and increased government regulation. Some of the most effective and cost-efficient emissions controls involve the use of downstream chemical catalysts that further oxygenate incompletely combusted compounds. Sometimes exhaust is directed sequentially through multiple catalyst beds. It is generally understood that higher catalyst temperatures provide more effective emissions control. Much exhaust catalysis development has been focused on developing catalytic converters for automotive applications, in which engine speed varies substantially with vehicle speed and gear selection. 
     In several other applications, such as in powering fixed-frequency electrical generators, engine speed is held as constant as possible during use, even while generator and engine loads fluctuate. Some engine-generator sets are designed for installation on-board moving vehicles, either on land or in water. 
     Marine generators are subjected to specific regulations, both for emissions and for safety concerns. For example, exposed engine surface temperatures (including exhaust system surface temperatures) must be kept low to avoid increased risk of fire hazard. Seawater is injected into many marine engine exhaust flows so as to cool exiting exhaust gases, and seawater is also frequently circulated through exhaust system components so as to maintain low surface temperatures. 
     Further improvements in exhaust emissions controls for constant and variable speed engine applications are desired, particularly improvements suitable for marine use. 
     SUMMARY 
     Many aspects of the invention feature methods of controlling emissions from an internal combustion engine. 
     In one aspect, the method includes governing engine speed with respect to a constant speed, maintaining an air/fuel ratio of the engine, flowing exhaust from the engine through an exhaust system containing a catalyst, monitoring a first variable with a feedback sensor located upstream of the catalyst, and controlling the air/fuel ratio of the engine as a function of the variable. 
     In some cases the first variable is oxygen and/or the feedback sensor is a narrow-band oxygen sensor. In some cases, the first variable is monitored with a MEMS device. In some embodiment, the method further includes monitoring a second variable with an exhaust sensor located downstream of the catalyst. In some embodiments, the second variable is carbon monoxide. In some other embodiments, the second variable is oxygen and/or the exhaust sensor is a wide-band oxygen sensor. 
     In a preferred embodiment, the air/fuel ratio is stoichiometric. In other embodiments, the air/fuel ratio is slightly lean. In some embodiments, the air/fuel ratio with is controlled with electronic fuel injection. In one embodiment, the electronic fuel injection is throttle-body fuel injection. In other embodiments, the electronic fuel injection is multi-point fuel injection. The the electronic fuel injection can be synchronized external fuel injection. Alternatively, the the electronic fuel injection can be nonsynchronized external fuel injection. In still other embodiments, the electronic fuel injection is direct fuel injection. 
     In one embodiment, the catalyst is configured to simultaneously reduce oxides of nitrogen, carbon monoxide and hydrocarbons. In some preferred embodiments, the catalyst is configured to reduce carbon monoxide to between about 9 parts per million and between about 30 parts per million. In one presently preferred embodiment, the catalyst is configured to reduce carbon monoxide to ambient levels. 
     In one embodiment, the engine is configured for marine applications and the exhaust system further comprises a water-jacketed manifold. In some cases, the engine is driving an electric generator. In one application, the generator is a multi-pole permanent magnet generator. In some embodiments, the generator is configured to operate at variable speeds. In some embodiments, the generator modulates between a high speed and a low speed having a ratio of 3 to 1. In other embodiments, the generator modulates between a high speed and a low speed having a ratio of 2 to 1. 
     In another aspect, the method includes driving an electric generator with the engine configured for marine applications, governing engine speed with respect to a selected constant speed, maintaining an air/fuel ratio of the engine, flowing exhaust from the engine through an exhaust system containing a catalyst, monitoring a first variable with a feedback sensor located upstream of the catalyst, the catalyst being configured to simultaneously reduce oxides of nitrogen, carbon monoxide and hydrocarbons, and controlling the air/fuel ratio of the engine as a function of the variable with electronic fuel injection. 
     In some embodiments, the method also includes monitoring a second variable downstream of the catalyst with an exhaust sensor downstream of the catalyst and providing a warning to an operator when the second variable reaches a threshold level. In some cases, the second variable is carbon monoxide. In other applications, the second variable is oxygen. 
     In some embodiments, the exhaust sensor is a wide-band oxygen sensor. In some embodiments, the generator is a permanent magnet generator. In some cases, the second variable is carbon monoxide. The other cases, the second variable is oxygen. 
     In a preferred embodiment, the air/fuel ratio is stoichiometric. In other embodiments, the air/fuel ratio is slightly lean. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a marine engine-generator set. 
         FIG. 2  is a schematic cross-section illustrating flow through the exhaust manifold and elbow of the engine-generator set of  FIG. 1 . 
         FIG. 3  illustrates an alternative second exhaust manifold construction and catalyst arrangement. 
         FIG. 4  is a perspective view of an engine exhaust manifold. 
         FIG. 5  is a partial cross-sectional view of the manifold of  FIG. 4 . 
         FIG. 6  shows a schematic view of a marine exhaust system according to the invention. 
         FIG. 7  is a detail view of a float valve and water level indicator contained within the marine exhaust system. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , an engine-generator set  10  includes an internal combustion engine  12  driving an electrical generator  14 . Engine  10  has an exhaust manifold  16  that receives and combines exhaust gasses from each cylinder of the engine and directs the combined exhaust gasses through a catalyst contained within the manifold, as is discussed in more detail below. Secured to the outlet of the manifold  16  is an exhaust elbow  18 . In a marine application, water, such as cold seawater, is supplied to manifold  16  through hose  30 . The water is directed through cooling passages in manifold  16  and elbow  18  to keep the outer surfaces of the exhaust system at or below a desired temperature, and is then injected into the exhaust stream in elbow  18 , downstream of the catalysts, to cool the exhaust. 
     In one embodiment, a variable is monitored with a feedback sensor  19  located upstream of the catalyst which provides a control signal to electronic controller  24 . In one embodiment, controller  24  provides controls the air fuel ratio of the engine  12  to correspond to a 1.0 stoichiometric ratio. In other embodiments, the air fuel ratio of the engine  12  is slightly lean. In one embodiment, the variable monitored by the feedback sensor  19  is oxygen and the feedback sensor  19  is a narrow-band oxygen sensor. 
     In one embodiment, an exhaust sensor  23  is mounted downstream of the catalyst. In one embodiment, the exhaust sensor  23  measures oxygen as a proxy for indirectly determining the level of carbon monoxide. In this application, a wide-band oxygen sensor can be used. In other applications, the exhaust sensor  23  directly measures carbon monoxide. The signal output from the exhaust sensor  23  can provide an anticipatory alarm apprising an operator when the catalyst  32  is functioning with reduced effectiveness. Accordingly, the exhaust sensor can inform the operator if the catalyst  32  has been damaged by seawater and requires replacement. The exhaust sensor  23  can be a MEMS device in some embodiments. 
     With continued reference to  FIG. 1  and in an alternative embodiment, air is delivered to manifold  16 , through a controllable dump valve  20 , from belt-driven air pump  22 . A fixed speed, electric air pump may also be employed. Valve  20  is controlled by an electronic controller  24  to moderate the flow of air into manifold  16  as a function of the load placed on engine  12 , such as by controllably dividing the output of the air pump between manifold  16  and exhaust elbow  18 . Controller  24  varies a signal to valve  20  as a function of engine load, or as a function of a sensible parameter that changes with engine load. In the illustrated embodiment, controller  24  senses an output voltage and/or current of generator  14 , such as at generator output  26 , and controls valve  20  accordingly. Controller  24  also senses engine speed, such as by receiving a signal from flywheel magnetic reluctance sensor  28 , and controls engine inputs (such as fuel and/or air flow) to maintain engine speed at or near a desired set point, so as to maintain the frequency of generator  14 . As an alternative to controlling a dump valve  20  splitting pump air flow between manifold  16  and either atmosphere or a lower point in the exhaust stream, a variable speed electric air pump  22   a  is employed in some instances, with controller  24  varying the operating speed of pump  22   a  as a function of engine load. In such cases, the entire output of pump  22   a  is preferably ported directly to manifold  16 . 
     Referring to now  FIG. 2 , a cylindrical catalyst  32  containing a catalyst bed is shown disposed within the exhaust manifold  16 . The catalyst  32  is wrapped in an insulating blanket  96 , such as a ⅛ inch (3.2 millimeter) thick sheet of cotton binding containing mica, for example, that helps reduce heat transfer from the catalyst into the housing and also helps to isolate the delicate catalyst bed from shocks and vibrations. In one embodiment, controlled air flow is injected either just forward of the catalyst at port  38   a , or at the far end of the manifold at port  38   b  so as to preheat the injected air flow. Single catalyst  32  may be of any preferred composition, such as a palladium-platinum catalyst, for example. In other embodiments, no air flow injection is required. 
     With continued reference to  FIG. 2  and in one embodiment, catalyst  32  is configured and dimensioned for fitting within a marine exhaust manifold  16 . In one presently preferred embodiment, the catalyst  32  has a diameter of 3.66 inch (9.30 cm) and a length of 6.0 inch (15.24 cm). The catalyst  32  can include a round ceramic having a diameter of 3.0 inch (7.62 cm) and a length of 6.0 inch (15.24 cm) and a 400-cells per inch with 95-grams per cubic foot of a 3-to-1 ratio of platinum to rhodium. The catalyst  32  can also include a specialized wash coat designed to be the most effective at a 1.0 stoichiometric air fuel ratio. The catalyst  32  is configured to simultaneously reduce oxides of nitrogen, carbon monoxide and hydrocarbons. In one preferred embodiment, the catalyst  32  is configured to reduce carbon monoxides levels to below 50 part per million, preferably to below 35 parts per million, and most preferably to below ambient levels, i.e., 9 part per million. 
     Other catalyst configuration are contemplated within the exhaust manifold  16 . For example as illustrated in  FIG. 3 , the catalyst  32  in an alternative embodiment can include a first catalyst  33  and second catalyst  36  contained within a second bore of the manifold, parallel to and offset from the first bore. The manifold can be equipped with a removable cover  44  through which the air is injected, enabling loading of both of the catalysts into their respective bores. As in the first illustrated embodiment, after flowing through both catalyst beds the exhaust flow is combined with cooling water in elbow  18   a.    
     The exhaust is combined and directed through a first catalyst bed  32 , through a space  34 , and then through a second catalyst bed  36 . The air is injected into the manifold in space  34 , through air inlet  38 . Cooling water flows around both catalyst beds, through appropriate channels cast into manifold  16   a  and elbow  18 , and is then injected into the exhaust flow. In marine applications where the cooling seawater can have a high salt content, the water injection outlets  40  in elbow  18  are preferably at least about six inches (15 centimeters) below the lowest edge of the catalysts or the upper edge of any internal elbow baffles  42  positioned to avoid salt water splash on the hot catalysts. Also, it is preferred that for such marine applications manifold  16   a  and elbow  18  be cast of a corrosion-resistant material, such as an aluminum-magnesium alloy. It will be apparent from  FIG. 2  that the connection between manifold  16   a  and elbow  18  can be readily positioned between the two catalyst beds, such that second catalyst  36  is carried within elbow  18 . 
     The construction of the catalyst  32  according to this embodiment can include a first catalyst bed  33  which preferably includes a catalyst such as one containing rhodium as the precious metal, selected to reduce hydrocarbon and NO x  emissions. For example, one preferred catalyst bed is in the form of a cylinder 3.0 inches (76 millimeters) in diameter and 2.6 inches (6.7 centimeters) long. The ceramic substrate has a cross-sectional area of about 7 square inches (45 square centimeters) and has about 400 cells per square inch (62 per square centimeter), and is washed with 6.1 grams per cubic foot (0.06 grams per cubic centimeter) of rhodium. Such a catalyst bed is available from ASEC/Delphi Exhaust and Engine Management of Flint, Mich. Catalysis efficiency within first catalysis bed  33  may be accomplished by various methods known in the art, either in carbureted or fuel-injected systems with oxygen sensors, to remove as much of the overall emissions components as possible. 
     The second catalyst bed  36  contains a catalyst selected to further reduce CO emissions. In one arrangement, second catalyst bed  36  contains a three to one ratio of palladium and platinum, carried on a honey-combed substrate of ceramic or metal. The active precious metals are washed onto the substrate and then heated to set the metals onto the surface as known in the art. An example of a preferred second catalyst bed is a metal substrate in the form of a cylinder of 5.0 inch (12.7 centimeter) diameter and 6.3 inch (16 centimeter) length, with 19.6 square inches (126 square centimeters) of cross-sectional area, washed with 40 grams per cubic foot (0.4 grams per cubic centimeter) each of palladium and platinum. Such a catalyst is available from Miratech of Tulsa, Okla., for example. Second catalyst  36  will tend to run hotter, such as perhaps about 400 degrees Fahrenheit (220 degrees Celsius) hotter than the rhodium catalyst. Preferably, the temperature of the combined air and exhaust entering the second catalyst is about 1000 degrees Fahrenheit (540 degrees Celsius). 
       FIGS. 4 and 5  show another example of a catalyst exhaust manifold  16   b . The catalyst  32  is loaded as a cylinder from the large end of the manifold, with the NO x  catalyst loaded into bore  46  ( FIG. 5 ) and the CO catalyst loaded into bore  48  ( FIG. 5 ). In this example, coolant enters the manifold at inlet  50  and leaves the manifold at outlet  52 , without joining the exhaust stream. The cooling channels  54  cast into the manifold are partially shown in  FIG. 5 , providing a closed flow path between inlet  50  and outlet  52 . 
     Various control techniques may be employed to vary air injection rate for good CO reduction. In one embodiment, the air injection rate is varied as a function of approximate engine load. In one test using a Westerbeke 4-cylinder, 1.5 liter gasoline engine and the palladium-platinum second catalyst bed described above, the lowest CO emissions were provided by varying the rate of air flow into the manifold ahead of the second catalyst (at 100 liter per minute graduations) according to the following table: 
     
       
         
               
               
             
               
               
             
           
               
                   
               
               
                 Engine Load (Percent Full Load) 
                 Air Flow Rate (liters per minute) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 100 
                 500 
               
               
                 75 
                 500 
               
               
                 50 
                 500 
               
               
                 25 
                 400 
               
               
                 10 
                 300 
               
               
                 0 
                 300 
               
               
                   
               
             
          
         
       
     
     Of course, optimal air flow rates will be different for different applications. The air flow controller can be configured to interpolate between adjacent entries in the load-air correlation table to provide finer control sensitivity. 
     There are various ways to determine approximate engine load, such that a table like that shown above can be used to determine an optimal air injection rate. For example, if substantially all of the engine load is provided by an electrical generator (as shown in  FIG. 1 ), monitoring the electrical output of the generator can provide a good estimate of engine load. Current can be monitored as a most direct measure of electrical load, such as by providing a current transformer about the output of the generator. In some cases in which generator voltage is known to predictably decrease a measurable amount with load, voltage may alternately be monitored. In most cases, however, current monitoring is preferred for systems with proper generator voltage regulation. Other options include measuring engine output driveshaft torque (or some measurable parameter that varies predictably with torque), or measuring the pressure within the manifold, such as upstream of the catalyst beds, or exhaust backpressure below the catalysts and above a muffler or other exhaust restriction. Because the engine speed is substantially fixed in the primary embodiments, other parameters may also be found to vary predictably with engine load, such as throttle position and fuel flow rate, for example. 
     As an alternative to controlling the air injection rate as a function of load, the air injection rate can be controlled as a function of other measured parameters that signify catalysis efficiency. For example, a CO sensor may be provided downstream of the catalyst as described above. 
     With renewed reference to  FIG. 2  an in one embodiment, an exhaust pressure sensor  62  can be placed in the manifold  16 , to measure exhaust manifold pressure, or downstream of the catalyst  32  to measure exhaust backpressure developed upstream of a muffler or other exhaust restriction (not shown). If the air pump delivering air to inlet  38  is not a fixed displacement pump, changes in exhaust backpressure with engine load can cause a significant fluctuation in the injected air rate. This fluctuation will tend to work against the desired variation of air flow rate with engine load, as backpressure, which rises with engine load, will cause a reduction in air injection rate that should be accounted for in the control of the pump or valve. It will be understood that sensors  62  are shown in optional and alternative locations, and are not necessary in some embodiments, such as when air flow rate is controlled as a function of generator current or some other primary control parameter. 
     Referring now to  FIG. 6 , an exhaust system  60  for the engine  12  mounted in a boat  67  is shown. The exhaust manifold  16  directs exhaust gases through the catalyst  32  and exhaust elbow  18  and past a water injected exhaust elbow  65 . To reduce the operating temperature of the exhaust components, cooling seawater is injected at the inlet to the exhaust elbow  70 . The exhaust gases and cooling water then pass through an exhaust valve and water level indicator  75  (discussed in more detail below). The exhaust gasses and cooling water enter a water lift marine muffler  80  before proceeding to a high point at the U-bend  85  and out of the boat through the through-hull fitting  90  above the water line  97 . In one embodiment, the muffler  80  includes a drain  97 . 
     In marine applications, it is desirable to prevent cooling seawater from contacting the catalyst  32  disposed within the exhaust manifold  16 . It is also desirable to prevent cooling seawater from reaching the engine  12 , which can results in catastrophic failure. Referring to  FIG. 7 , an exhaust valve and water level indicator  75  are shown and disposed within the marine exhaust manifold  16  between the water injected exhaust elbow  65  and the water lift muffler  80  ( FIG. 6 ). The valve/indicator  75  can include a float valve  105 , such as a ball valve and a water level indicator  110  combined in a housing  115 . The ball valve  105  translates along the housing  115  between ball valve guides  120   a ,  120   b  and is supported by ball valve supports  130   a ,  130   b  when the ball valve is disposed in an open position  135  (shown in phantom). When the ball valve  105  ascends upward to the closed position (as shown) the surface of the ball valve  105  contacts the housing  115  along valve sealing areas  140   a ,  140   b  thereby closing the valve. The rising water level within the housing  115  floats the water level indicator  110  upward to an alarm level which provides a signal  145  to warn an operator that the muffler  80  is overfilled. 
     A number of embodiments of the invention have been described. For example, the engine  12  as described above can be used for propulsion in marine applications. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.