Abstract:
An exhaust system for receiving exhaust gas of an engine is provided. The exhaust system comprises a cooler configured to cool at least a portion of exhaust gas, a pressure sensor configured to measure an exhaust pressure drop across the cooler, and a controller configured to determine exhaust flowrate as a function the measured pressure drop across the cooler. Also provided is a method for operating an engine. The method comprises the steps of combusting a fuel and air mixture, exhausting at least some of the combusted fuel and air mixture as exhaust gas to an exhaust system of an engine, cooling at least some of the combusted exhaust gas in a cooler, measuring pressure drop of the exhaust gas across the cooler, and determining a flowrate of the exhaust gas across the cooler as a function of pressure drop.

Description:
TECHNICAL FIELD  
       [0001]     The present disclosure relates generally to a flow sensor and, more particularly, to a flow sensor within the recirculated exhaust stream of an engine. Background  
         [0002]     Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of chemical compounds. The chemical compounds may be composed of gaseous compounds, which may include nitrous oxides (“NOx”), and solid particulate matter, which may include unburned carbon particulates called soot.  
         [0003]     Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of gaseous compounds emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. One method that has been implemented by engine manufacturers to comply with the regulation of these engine emissions is exhaust gas recirculation (“EGR”). EGR systems recirculate some of the exhaust gas byproducts into the intake air supply of the internal combustion engine. The exhaust gas directed to the engine cylinder reduces the concentration of oxygen within the cylinder and increases the specific heat of the air/fuel mixture, thereby lowering the local combustion temperature within the cylinder. The lowered local combustion temperature and reduced oxygen concentration can slow the chemical reaction of the combustion process and decrease the formation of NOx.  
         [0004]     Maintaining the proper ratio of EGR to intake air is important in lowering local combustion temperatures and, consequently, NOx formation. As such, a reliable and accurate EGR flow meter, in conjunction with other engine components, helps achieve stringent NOx emission limitations.  
         [0005]     Some engines with external EGR loops, such as the one disclosed in U.S. Pat. No. 6,786,210 (“&#39; 210”), have separate EGR meters for measuring EGR flow. &#39; 210 discloses a venturi measurement sensor for measuring flow disposed within the EGR passage. The presence of a separate airflow sensor within the airflow loop, as disclosed in &#39;210, unfortunately, results in extra hardware to the EGR loop. The extra hardware may lead to increased costs and pressure loss. Furthermore, many engines are constrained by tight space limitations, as there may be limited space available “under the hood” of an automobile. In some of these engines, the extra hardware may present problems in meeting these space limitations.  
         [0006]     The disclosed flow meter is directed to overcoming one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0007]     In one embodiment of the present disclosure, an exhaust system for receiving exhaust gas of an engine is provided. In this embodiment, the system comprises a cooler configured to cool at least a portion of exhaust gas, a pressure sensor configured to measure an exhaust pressure drop across the cooler, and a controller configured to determine exhaust flowrate as a function the measured pressure drop across the cooler.  
         [0008]     In another embodiment, a method for operating an engine is provided. In this embodiment, the method comprises the steps of combusting a fuel and air mixture, exhausting at least some of the combusted fuel and air mixture as exhaust gas to an exhaust system of an engine, cooling at least some of the combusted exhaust gas in a cooler, measuring pressure drop of the exhaust gas across the cooler, and determining a flowrate of the exhaust gas across the cooler as a function of pressure drop. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a diagrammatic illustration of an engine having an exhaust treatment system according to an exemplary embodiment of the present disclosure; and  
         [0010]      FIG. 2  is a cross-sectional view of a cooler shown in the embodiment of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  illustrates a power source  12  having an exemplary exhaust treatment system  10 . The power source  12  may include an internal or external combustion engine such as, for example, a diesel engine, a gasoline engine, a natural gas engine, or any other engine apparent to one skilled in the art. The power source  12  may, alternately, include another source of power known in the art.  
         [0012]     The exhaust treatment system  10  may be configured to direct exhaust gases out of the power source  12 , treat the gases, and introduce a portion of the treated gases into an intake  21  of the power source  12 . The exhaust treatment system  10  may include an energy extraction assembly  22 , a regeneration device  20 , a filter  16  (which may be catalyzed), a recirculation line  24  fluidly connected between the filter  16  and the exhaust system outlet  17 , and a flow cooler  26 . The exhaust treatment system  10  may further include a mixing valve  30 , a compression assembly  32 , and an aftercooler  34 .  
         [0013]     A flow of exhaust produced by the power source  12  may be directed from the power source  12  to components of the exhaust treatment system  10  by flow lines  15 . The flow lines  15  may include pipes, tubing, and/or other exhaust flow carrying means known in the art. The flow lines  15  may be made of alloys of steel, aluminum, and/or other materials known in the art. The flow lines  15  may be rigid or flexible, and may be capable of safely carrying high temperature exhaust flows, such as flows having temperatures in excess of 700 degrees Celsius (approximately 1,292 degrees Fahrenheit).  
         [0014]     The energy extraction assembly  22  may be configured to extract energy from, and reduce the pressure of, the exhaust gases produced by the power source  12 . The energy extraction assembly  22  may be fluidly connected to the power source  12  by one or more flow lines  15  and may reduce the pressure of the exhaust gases to any desired pressure. The energy extraction assembly  22  may include one or more turbines  14 , diffusers, or other energy extraction devices known in the art. In an exemplary embodiment wherein the energy extraction assembly  22  includes more than one turbine  14 , the multiple turbines  14  may be disposed in parallel or in series relationship. It is also understood that in an embodiment of the present disclosure, the energy extraction assembly  22  may, alternately, be omitted. In such an embodiment, the power source  12  may include, for example, a naturally aspirated engine. As will be described in greater detail below, a component of the energy extraction assembly  22  may be configured in certain embodiments to drive a component of the compression assembly  32 .  
         [0015]     In an exemplary embodiment, the regeneration device  20  may be fluidly connected to the energy extraction assembly  22  via flow line  15 , and may be configured to increase the temperature of an entire flow of exhaust produced by the power source  12  to a desired temperature. The desired temperature may be, for example, a regeneration temperature of the filter  16 . Accordingly, the regeneration device  20  may be configured to assist in regenerating the filter  16 . Alternatively, in another exemplary embodiment the regeneration device  20  may be configured to increase the temperature of only a portion of the entire flow of exhaust produced by the power source  12 . The regeneration device  20  may include, for example, a fuel injector and an igniter (not shown), heat coils (not shown), fuel sprayed on a catalytic surface (not shown), and/or other heat sources known in the art. Such heat sources may be disposed within the regeneration device  20  and may be configured to assist in increasing the temperature of the flow of exhaust through convection, combustion, and/or other methods. In an exemplary embodiment in which the regeneration device  20  includes a fuel injector and an igniter, it is understood that the regeneration device  20  may receive a supply of a combustible substance and a supply of oxygen to facilitate combustion within the regeneration device  20 . The combustible substance may be, for example, gasoline, diesel fuel, reformate, and/or any other combustible substance known in the art. The supply of oxygen may be provided in addition to the relatively low-pressure flow of exhaust gas directed to the regeneration device  20  through flow line  15 . In an exemplary embodiment, the supply of oxygen may be carried by a flow of gas directed to the regeneration device  20  from downstream of the compression assembly  32  via a supply line  40 . In such an embodiment, the flow of gas may include, for example, recirculated exhaust gas and ambient air. It is understood that, in an exemplary embodiment of the present disclosure, the supply line  40  may be fluidly connected to an outlet of the compression assembly  32 . In an exemplary embodiment, the regeneration device  20  may be dimensioned and/or otherwise configured to be housed within an engine compartment or other compartment of a work machine (not shown) to which the power source  12  is attached. In such an embodiment, the regeneration device  20  may be desirably calibrated in conjunction with, for example, the filter  16 , the energy extraction assembly  22 , and/or the power source  12 . Calibration of the regeneration device  20  may include, for example, among other things, adjusting the rate, angle, pressure, and/or atomization at which fuel is injected into the regeneration device  20 , adjusting the flow rate of the oxygen supplied, adjusting the intensity and/or firing pattern of the igniter, and adjusting the length, diameter, mounting angle, and/or other configurations of a housing of the regeneration device  20 . Such calibration may reduce the time required to regenerate the filter  16  and the amount of fuel or other combustible substances needed for regeneration. Either of these results may improve the overall efficiency of the exhaust treatment system  10 . It is understood that the efficiency of the exhaust treatment system  10  described herein may be measured by a variety of factors including, among other things, the amount of fuel used for regeneration, the length of the regeneration period, and the amount (parts per million) of pollutants released to the atmosphere.  
         [0016]     As shown in  FIG. 1 , the filter  16  may be connected downstream of the regeneration device  20 . The filter  16  may have a housing  25  including an inlet  23  and an outlet  31 . In an exemplary embodiment, the regeneration device  20  may be disposed outside of the housing  25  and may be fluidly connected to the inlet  23  of the housing  25 . In another exemplary embodiment, the regeneration device  20  may be disposed within the housing  25  of the filter  16 . The filter  16  may be any type of filter known in the art capable of extracting matter from a flow of gas. In an embodiment of the present disclosure, the filter  16  may be, for example, a particulate matter filter positioned to extract particulates from an exhaust flow of the power source  12 . The filter  16  may include, for example, a ceramic substrate, a metallic mesh, foam, or any other material known in the art. These materials may form, for example, a honeycomb structure within the housing  25  of the filter  16  to facilitate the removal of particulates. The particulates may be, for example, soot.  
         [0017]     Although the above disclosure goes into great detail to explain an engine with an aftertreatment system, the reader should appreciate that the flow sensor of the present disclosure may be applied to several different applications, including engines without an aftertreatment system.  
         [0018]     In an exemplary embodiment of the present disclosure, a portion of the exhaust produced by the combustion process may leak past piston rings within a crankcase (not shown) of the power source  12 . This portion of the exhaust may build up within the crankcase over time, thereby increasing the pressure within the crankcase. In such an embodiment, a ventilation line  42  may be fluidly connected to the crankcase of the power source  12 . The ventilation line  42  may comprise piping, tubing, and/or other exhaust flow carrying means known in the art and may be structurally similar to the flow lines  15  described above. The ventilation line  42  may be configured to direct, for example, the portion of exhaust gas from the crankcase to a port  46  of the flow line  15 . The port  46  may be located in the flow line  15  anywhere upstream of the filter  16 . For example, the ventilation line  42  may assist in directing the portion of exhaust gas from the crankcase to a port  46  disposed upstream of the regeneration device  20 . The ventilation line  42  may include, for example, a check valve  44  and/or any other valve assembly known in the art. The check valve  44  may be configured to assist in controllably regulating a flow of fluid through the ventilation line  42 .  
         [0019]     The exhaust treatment system  10  may further include a catalyst (not shown) disposed upstream or downstream of the filter  16 . The catalyst may contain catalyst materials for catalyzing hydrocarbons, oxides of sulfur, and/or oxides of nitrogen, for example, contained in a flow. Such catalyst materials may include, for example, aluminum, platinum, palladium, rhodium, barium, cerium, and/or alkali metals, alkaline-earth metals, rare-earth metals, or combinations thereof. The catalyst materials may be situated within the catalyst so as to maximize the surface area available for the collection of, for example, hydrocarbons. The catalyst may include, for example, a ceramic substrate, a metallic mesh, foam, or any other porous material known in the art, and the catalyst materials may be located on, for example, a substrate of the catalyst.  
         [0020]     As mentioned above, filter  16  of the exhaust treatment system  10  may include catalyst materials for catalyzing hydrocarbons, oxides of sulfur, and/or oxides of nitrogen contained in a flow. Such catalyst materials may include, for example, aluminum, platinum, palladium, rhodium, barium, cerium, and/or alkali metals, alkaline-earth metals, rare-earth metals, or combinations thereof. The catalyst materials may be situated within the filter  16  so as to maximize the surface area available for the collection of, for example, hydrocarbons.  
         [0021]     The catalyst materials may be located on a substrate of the filter  16 . The catalyst materials may be added to the filter  16  by any conventional means such as, for example, coating or spraying, and the substrate of the filter  16  may be partially or completely coated with the materials.  
         [0022]     It is also understood that the catalyst materials described above may be capable of oxidizing hydrocarbons in certain conditions. Thus, in the embodiment shown in  FIG. 1 , all or a portion of the hydrocarbons contained within the exhaust flow may be permitted to travel back to the power source  12  without being oxidized by the catalyst materials. It is further understood that the presence of these catalyst materials may improve the overall emissions characteristics of the exhaust treatment system  10  by removing hydrocarbons from the treated exhaust flow.  
         [0023]     Referring again to  FIG. 1 , the exhaust treatment system  10  may further include a recirculation line  24  fluidly connected downstream of the filter  16 . The recirculation line  24  may be disposed between the filter  16  and the exhaust system outlet  17  and may be configured to assist in directing a portion of the exhaust flow from the filter  16  to the inlet  21  of the power source  12 . The recirculation line  24  may comprise piping, tubing, and/or other exhaust flow carrying means known in the art and may be structurally similar to the flow lines  15  described above.  
         [0024]     A flow cooler  26  is also provided. The flow cooler  26  may be fluidly connected to the filter  16  via the recirculation line  24  and may be configured to cool the portion of the exhaust flow passing through the recirculation line  24 . The flow cooler  26  may include a liquid-to-air heat exchanger, an air-to-air heat exchanger, or any other type of heat exchanger known in the art for cooling an exhaust flow. The cooler  26  may be a cross-flow, counter-flow, or parallel-flow heat exchanger, as well. In an exemplary embodiment, flow cooler  26  is a parallel-flow heat exchanger that uses jacket water from the power source&#39;s  12  cooling system as a cooling medium.  
         [0025]     Referring to  FIG. 2 , positioned near cooler  26  may be temperature sensors  103 ,  104  and pressure sensors  101 ,  102  used for determining exhaust gas flowrate through recirculation line  24 . Sensors  101 ,  102 ,  103 ,  104  generate pressure and temperature signals upstream and downstream of cooler  26  and sends them to controller  110 . Controller  110  uses some or all of these sensors  101 ,  102 ,  103 ,  104  to determine an approximate flowrate through line  24 . Controller  110  may calculate the flowrate, may refer to a map, or may use any other means known in the art for determining flowrate. Generally, flowrate through line  24  can be determined based upon the pressure drop across cooler  26 . Furthermore, temperature signals  103 ,  104  may also be used to provide a more accurate flowrate signal, as the flowrate is a function of both the pressure drop across the cooler and temperature of the fluid to be cooled.  
         [0026]     In some embodiments, the system may also include temperature sensors  105 ,  106 . These sensors  105 ,  106  provide for temperature measurement of the cooling fluid in line  107 . Over time, fouling of the heat transfer surfaces within cooler  26  may result in an increased pressure drop across cooler  26 , for the same flowrate of fluid through line  24 . This pressure drop may be the result of combustion byproducts clogging the fluid pipes through cooler  26 . Accordingly, it may be necessary to compensate for this fouling-induced pressure drop by determining the effectiveness of the heat transfer surfaces within cooler  26 . This can be accomplished by measuring the temperature drop of the cooling fluid in cooling line  107  across cooler  26 . Over time, as the heat transfer surfaces become fouled—and less effective—the temperature drop across cooling line  107  will decrease. By monitoring the temperature drop in line  107 , the level of fouling across cooler  26  can be approximated, thus providing for pressure drop compensation as a result of fouled heat transfer surfaces.  
         [0027]     Referring back to  FIG. 1 , the mixing valve  30  may be fluidly connected to the flow cooler  26  via the recirculation line  24  and may be configured to assist in regulating the flow of exhaust through the recirculation line  24 . It is understood that in an exemplary embodiment, a check valve (not shown) may be fluidly connected upstream of the flow cooler  26  to further assist in regulating the flow of exhaust through the recirculation line  24 . The mixing valve  30  may be a spool valve, a shutter valve, a butterfly valve, a gate valve, a shuttle valve, a ball valve, a globe valve, or a check valve, for example. The mixing valve  30  may be actuated manually, electrically, hydraulically, pneumatically, or in any other manner known in the art. The mixing valve  30  may be in communication with a controller  110  and may be selectively actuated in response to one or more predetermined conditions.  
         [0028]     The mixing valve  30  may also be fluidly connected to an ambient air intake  29  of the exhaust treatment system  10 . Thus, the mixing valve  30  may be configured to control the amount of exhaust flow entering a flow line  27  relative to the amount of ambient airflow entering the flow line  27 . For example, as the amount of exhaust flow passing through the mixing valve  30  is desirably increased, the amount of ambient air flow passing through the mixing valve  30  may be proportionally decreased and vise versa.  
         [0029]     The flow line  27  downstream of the mixing valve  30  may direct the ambient air/exhaust flow mixture to the compression assembly  32 . The compression assembly  32  may include a compressor  13  configured to increase the pressure of a flow of gas a desired pressure. The compressor  13  may include a fixed geometry type compressor, a variable geometry type compressor, or any other type of compressor known in the art. In the exemplary embodiment shown in  FIG. 1 , the compression assembly  32  may include more than one compressor  13  and the multiple compressors  13  may be disposed in parallel or in series relationship. A compressor  13  of the compression assembly  32  may be connected to a turbine  14  of the energy extraction assembly  22  and the turbine  14  may be configured to drive the compressor  13 . In particular, as hot exhaust gases exit the power source  12  and expand against the blades (not shown) of the turbine  14 , components of the turbine  14  may rotate and drive the connected compressor  13 . Alternatively, in an embodiment in which the turbine  14  is omitted, the compressor  13  may be driven by, for example, the power source  12 , or by any other drive known in the art. It is also understood that in a non-pressurized air induction system, the compression assembly  32  may be omitted.  
         [0030]     The aftercooler  34  may be fluidly connected to the power source  12  via the flow line  27  and may be configured to cool a flow of gas passing through the flow line  27 . In an exemplary embodiment, this flow of gas may be the ambient air/exhaust flow mixture discussed above. The aftercooler  34  may include a liquid-to-air heat exchanger, an air-to air heat exchanger, or any other type of flow cooler or heat exchanger known in the art. In an exemplary embodiment of the present disclosure, the aftercooler  34  may be omitted if desired.  
         [0031]     The exhaust treatment system  10  may further include a condensate drain  38  fluidly connected to the aftercooler  34 . The condensate drain  38  may be configured to collect a fluid, such as, for example, water or other condensate formed at the aftercooler  34 . It is understood that such fluids may consist of, for example, condensed water vapor contained in recycled exhaust gas and/or ambient air. In such an exemplary embodiment, the condensate drain  38  may include a removably attachable fluid tank (not shown) capable of safely storing the condensed fluid. The fluid tank may be configured to be removed, safely emptied, and reconnected to the condensate drain  38 . In another exemplary embodiment, the condensate drain  38  may be configured to direct the condensed fluid to a fluid container (not shown) and/or other component or location on the work machine. Alternatively, the condensate drain  38  may be configured to direct the fluid to the atmosphere or to the surface by which the work machine is supported.  
       INDUSTRIAL APPLICABILITY  
       [0032]     The exhaust treatment system  10  of the present disclosure may be used with any combustion-type device such as, for example, an engine or any other device known in the art where the recirculation of exhaust into an inlet of the device is desired. The exhaust treatment system  10  may be useful in reducing the amount of regulated exhaust emissions discharged to the environment and reducing or substantially eliminating the amount of sulfate produced during treatment of the exhaust gas. The exhaust treatment system  10  may also be capable of purging the portions of the exhaust gas captured by components of the system through a regeneration process.  
         [0033]     As discussed above, the combustion process may produce a complex mixture of chemical compounds. These chemical compounds may exist in solid, liquid, and/or gaseous form. In general, the solid and liquid pollutants may fall into the three categories of soot, soluble organic fraction, and sulfates. The soot produced during combustion may include carbonaceous materials, and the soluble organic fraction may include unburned hydrocarbons that are deposited on or otherwise chemically combined with the soot. The sulfates produced in the combustion process may be formed from sulfur molecules contained within the fuel and may be released in the form of SO 2 . This SO 2  may react with oxygen molecules contained within the exhaust flow to form SO 3 . As explained above, SO 2  may also be converted into SO 3  in the presence of, for example, platinum, palladium, and/or other rare earth metals used as catalyst materials in conventional catalysts. It is understood that the combustion process may also produce small amounts of SO 3 .  
         [0034]     In a conventional exhaust treatment system, a portion of the SO 3  produced may be released to the atmosphere through an outlet of the exhaust system. The exhaust treatment system  10  of the present disclosure, however, may substantially reduce the formation of sulfates by minimizing the amount of platinum, palladium, and/or other precious earth metals used. The operation of the exhaust treatment system  10  will now be explained in detail. Unless otherwise noted, the exhaust treatment system  10  of  FIG. 1  will be referred to for the duration of the disclosure.  
         [0035]     The power source  12  may combust a mixture of fuel, recirculated exhaust gas, and ambient air to produce mechanical work and an exhaust flow containing the gaseous compounds discussed above. The exhaust flow may be directed, via flow line  15 , from the power source  12  through the energy extraction assembly  22 . The hot exhaust flow may expand on the blades of the turbines  14  of the energy extraction assembly  22 , and this expansion may reduce the pressure of the exhaust flow while assisting in rotating the turbine blades.  
         [0036]     The reduced pressure exhaust flow may pass through the regeneration device  20  to the filter  16 . The regeneration device  20  may be deactivated during the normal operation of the power source  12 . As the exhaust flow passes through the filter  16 , a portion of the particulate matter entrained with the exhaust flow may be captured by the substrate, mesh, and/or other structures within the filter  16 .  
         [0037]     A portion of the exhaust flow may be extracted downstream of the filter  16  and upstream of the exhaust system outlet  17 . The extracted portion of the exhaust flow may enter the recirculation line  24  and may be recirculated back to the power source  12 . The remainder of the exhaust flow may exit the exhaust system outlet  17 . The catalyst materials contained within the catalyst may assist in oxidizing the hydrocarbons and soluble organic fraction carried by the flow.  
         [0038]     In the exemplary embodiment illustrated in  FIG. 2 , the filter  36  may contain small amounts of catalyst materials such as platinum. The catalyst materials may be disposed on a substrate of the filter  36  and may substantially oxidize the hydrocarbons and soluble organic fraction contained within the exhaust flow. Such a configuration may result in the production of substantially less sulfate in the recirculated filtered exhaust flow than conventional exhaust treatment systems containing a separate catalyst upstream of a filter.  
         [0039]     Referring again to  FIG. 1 , the recirculated portion of the exhaust flow may pass through the flow cooler  26 . The flow cooler  26  may reduce the temperature of the portion of the exhaust flow before the portion enters the flow line  27 . The mixing valve  30  may be configured to regulate the ratio of recirculated exhaust flow to ambient inlet air passing through flow line  27 . As described above, the calculated flow rate from recirculation line  24  may be used to establish the desired ratio.  
         [0040]     The mixing valve  30  may permit the ambient air/exhaust flow mixture to pass to the compression assembly  32  where the compressors  13  may increase the pressure of the flow, thereby increasing the temperature of the flow. The compressed flow may pass through the flow line  27  to the aftercooler  34 , which may reduce the temperature of the flow before the flow enters the inlet  21  of the power source  12 .  
         [0041]     Over time, soot produced by the combustion process may collect in the filter  16  and may begin to impair the ability of the filter  16  to store particulates or may result in an undesirable increase in pressure drop across the filter  16 , which may lead to higher exhaust temperatures and increased fuel consumption. Pressure sensors  101 ,  102 , temperature sensors  103 ,  104 ,  105 ,  106 , and other sensors (not shown) sense parameters of the power source  12  and/or the exhaust treatment system  10 . Such parameters may include, for example, engine speed, engine temperature, and particulate matter content. Controller  110  may use the information sent from the sensors in conjunction with an algorithm or other pre-set criteria to determine whether the filter  16  has become saturated and is in need of regeneration. Once this saturation point has been reached, the controller  110  may send appropriate signals to components of the exhaust treatment system  10  to begin the regeneration process. A preset algorithm stored in the controller  110  may assist in this determination and may use the sensed parameters as inputs. Alternatively, regeneration may commence according to a set schedule based on fuel consumption, hours of operation, and/or other variables.  
         [0042]     The signals sent by the controller  110  may alter the position of the mixing valve  30  to desirably alter the ratio of the ambient air/exhaust flow mixture. Some of these signals may be the exhaust gas flow pressure drop across the cooler  26 , the temperature drop of the cooling medium in line  107  across cooler  26 , and the temperature drop of the exhaust gas across cooler  26 . As discussed earlier, measuring the pressure drop of the exhaust gas across cooler  26  along with measuring the temperature drop of both the cooling medium and exhaust gas across cooler  26  enables controller  110  to compensate for fouling of the heat transfer surfaces of cooler  26  in determining an accurate flowrate. This fouling-compensated flowrate measurement can then be used to send control signals to mixing valve  30 , for example.  
         [0043]     These signals may also activate the regeneration device  20 . Upon activation, oxygen and a combustible substance, such as, for example, fuel may be directed to the regeneration device  20 . The regeneration device  20  may ignite the fuel and may increase the temperature of the exhaust flow passing to the filter  16  to a desired temperature for regeneration. This temperature may be in excess of 700 degrees Celsius (approximately 1,292 degrees Fahrenheit) in some applications, depending on the type and size of the filter  16 . At these temperatures, soot contained within the filter  16  may be burned away to restore the storage capabilities of the filter  16 .  
         [0044]     Other embodiments of the disclosed exhaust treatment system  10  will be apparent to those skilled in the art from consideration of the specification. For example, the system  10  may include additional filters such as, for example, a sulfur trap disposed upstream of the filter  16 . The sulfur trap may be useful in capturing sulfur molecules carried by the exhaust flow. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.