Abstract:
In one aspect, the present disclosure is directed to a method for treating exhaust from an engine. The method may comprise generating a first signal indicative of an engine load and generating a second signal indicative of a pollutant level. The method may further comprise recirculating exhaust based on the first signal and injecting a reductant based on the first signal and the second signal.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to exhaust aftertreatment and, more particularly, to a system for controlling exhaust aftertreatment. 
       BACKGROUND 
       [0002]    Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants may be composed of gaseous compounds such as, for example, the oxides of nitrogen (NOx), unburned hydrocarbons, and particulate matter. Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOx and particulate matter emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. Two strategies that may be used to ensure compliance with the regulations are exhaust gas recirculation (EGR) systems and selective catalytic reduction (SCR) systems. 
         [0003]    EGR systems are used for controlling emissions of undesirable pollutant gases and particulates during operation of an internal combustion engine. Such systems have proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, trucks, and other on-road machines. EGR systems generally recirculate exhaust gas into an intake air supply of the internal combustion engine. The exhaust gas reintroduced to the engine cylinder reduces the concentration of oxygen in the cylinder, which lowers the maximum combustion temperature, slows the chemical reaction of the combustion process, and decreases the formation of nitrous oxides (NO x ). Furthermore, the exhaust gas typically contains unburned hydrocarbons which are burned after reintroduction into the engine cylinder further reducing the emission of undesirable pollutants from the internal combustion engine. 
         [0004]    SCR is a process where gaseous or liquid reductant (most commonly urea) is added to the exhaust gas stream of an engine and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H 2 O and N 2 . One system for selective catalytic reduction is described in U.S. Pat. No. 6,470,676 (the &#39;676 patent), issued to Dolling et al. Specifically, the &#39;676 patent describes a method for catalytic conversion of NOx. A reducing agent is added to the exhaust gas from an engine as a function of the NOx concentration and the operating condition of the engine. If the operating conditions of the engine indicate that less reducing agent is needed, an increased amount of reducing agent is temporarily added to the exhaust which is then stored on the catalyst. When the operating conditions of the engine indicate that more reducing agent is needed, the excess reducing agent that has been stored on the catalyst may be utilized to convert the added NOx until more reducing agent can be added to the system. 
         [0005]    While EGR systems may be effective at reducing undesirable pollutants and particulates, they may cause increased cylinder pressure and fuel consumption at high loads, and may increase exhaust temperature requiring relatively large heat exchangers to cool the exhaust before reintroduction into the engine. Furthermore, while prior art systems may be effective at reducing NOx emissions, they may not be the most effective choice across a wide range of engine loads. At low loads, the temperature of the exhaust gas is low and the efficiency of the SCR catalyst is reduced. Furthermore the extensive use of a reducing agent would require a large amounts of storage or alternatively a reduced operating time. 
         [0006]    The disclosed exhaust aftertreatment system is directed to improving prior art systems. 
       SUMMARY 
       [0007]    In one aspect, the present disclosure is directed to a method for treating exhaust from an engine. The method may comprise generating a first signal indicative of an engine load and generating a second signal indicative of a pollutant level. The method may further comprise recirculating exhaust based on the first signal and injecting a reductant based on the first signal and the second signal. 
         [0008]    In another aspect, the present disclosure is directed to an exhaust aftertreatment system for an engine. The system may comprise an exhaust gas recirculation system and a selective catalytic reduction system having a reductant injector. The system may further comprise a valve configured to direct the flow of exhaust and a sensor configured to generate a signal indicative of an engine load. The system may also include a controller, in communication with the valve and the sensor, configured to affect the direction of the flow of the exhaust based on the signal and to affect injection of a reductant based on the signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagrammatic illustration of an exemplary disclosed power system; 
           [0010]      FIG. 2  is a diagrammatic illustration of a control system that may be used with the power system of  FIG. 1 ; 
           [0011]      FIG. 3  is a flow diagram illustrating an exemplary disclosed method of operating the control system of  FIG. 2 ; and 
           [0012]      FIG. 4  is a diagrammatic illustration of an alternative exemplary power system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates an exemplary power system  10 . Power system  10  is described herein with respect to a diesel-fuel, internal combustion engine  12  for exemplary purposes only. However, it is contemplated that engine  12  may embody any other type of internal combustion engine, such as, for example, a gasoline or gaseous fuel-powered engine. Engine  12  may include an engine block  14  at least partially defining a plurality of cylinders  16 . Each cylinder  16  may be associated with a fuel injector, a cylinder liner, at least one air intake port  22  and corresponding intake valve (not shown), at least one exhaust port  24  and corresponding exhaust valve (not shown), a combustion chamber, and a reciprocating piston assembly moveable within each cylinder  16 . It is contemplated that engine  12  may include any number of cylinders  16  and that cylinders  16  may be disposed in an “in-line” configuration, a “V” configuration, or any other conventional configuration. A crankshaft  20  of engine  12  may be rotatably disposed within engine block  14 . 
         [0014]    Power system  10  may be used with a machine. The machine may embody a mobile or stationary machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, or any other industry known in the art. For example, the machine may be an earth moving machine such as an off-highway haul truck, a wheel loader, a motor grader, or any other suitable earth moving machine. The machine may alternatively embody an on-highway vocational truck, a passenger vehicle, or any other operation-performing machine. 
         [0015]    Power system  10  may include an air induction system  30 . Air induction system  30  may be associated with power system  10  and may include components that condition and introduce compressed air into cylinder  16  by way of intake port  22  and the intake valve. For example, air induction system  30  may include an air filter  32 , a compressor  34  connected to draw inlet air through air filter  32 , and an air cooler  36  located downstream of compressor  34 . It is contemplated that air induction system  30  may include different or additional components such as, for example, inlet bypass components, a throttle valve, and other components known in the art. 
         [0016]    Air filter  32  may be configured to remove or trap debris from air flowing into power system  10 . For example, air filter  32  may include a full-flow filter, a self-cleaning filter, a centrifuge filter, an electro-static precipitator, or any other type of air filtering device known in the art. It is contemplated that more than one air filter  32  may be included within air induction system  30  and disposed in a series or parallel arrangement. Air filter  32  may be connected to inlet port  22 . 
         [0017]    Compressor  34  may be located downstream of air filter  32  and configured to compress the air flowing into power system  10 . Compressor  34  may embody a fixed geometry type compressor, a variable geometry type compressor, or any other type of compressor known in the art. It is contemplated that more than one compressor  34  may be included within air induction system  30  and disposed in parallel or in series relationship. Air cooler  36  may be configured to cool air within air induction system  30  upstream of cylinders  16  and 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 air. 
         [0018]    Power system  10  may further include an exhaust gas recirculation system (EGR)  50 . EGR  50  may include components that condition and direct exhaust from cylinder  16  by way of exhaust port  24  and the exhaust valve. For example, EGR  50  may include a turbine  40  driven by the exiting exhaust, a particulate filter  42 , a valve  44  and an exhaust outlet  66  configured to direct treated exhaust to the atmosphere, a flow meter  52 , an exhaust cooler  54 , and a valve  56  configured to selectively pass or restrict the flow of exhaust through EGR  50 . It is contemplated that EGR  50  may include different or additional components than described above such as, for example, exhaust bypass components, an exhaust braking system, and other components known in the art. 
         [0019]    Turbine  40  may be located to receive exhaust leaving power system  10  via exhaust port  24 . Turbine  40  may be connected to compressor  34  of air induction system  30  by way of a common shaft to form a turbocharger. As the hot exhaust gases exiting power system  10  move through turbine  40  and act upon turbine  40 , i.e. expand against vanes (not shown), turbine  40  may rotate and drive compressor  34  to pressurize inlet air. It is contemplated that more than one turbine  40  may be included within EGR  50  and disposed in parallel or in series relationship. 
         [0020]    Particulate filter  42  may be disposed downstream of turbine  40  to remove particulates from the exhaust flow directed from power system  10 . It is contemplated that particulate filter  42  may include electrically conductive or non-conductive coarse mesh elements. It is also contemplated that particulate filter  42  may include a catalyst for reducing an ignition temperature of the particulate matter trapped by particulate filter  42 , a regeneration system that may regenerate the particulate matter trapped by particulate filter  42 , or both a catalyst and a regeneration system. The catalyst may support the reduction of HC, CO, and/or particulate matter, and may include, for example, a base metal oxide, a molten salt, and/or a precious metal. The regeneration system may include, among other things, a fuel-powered burner, an electrically-resistive heater, an engine control strategy, or any other means for regenerating known in the art. It is contemplated that particulate filter  42  may be selectively omitted. 
         [0021]    EGR  50  may also include flow meter  52  and exhaust cooler  54 . Flow meter  52  may be configured to measure exhaust flow and may embody, for example, a thermal mass flow meter, a laminar flow element, a mass compensated positive displacement roots meter, or any other suitable device configured to measure gaseous flows. Exhaust cooler  54  may be disposed downstream of particulate filter  42  and configured to cool the portion of exhaust flowing through EGR  50 . Exhaust cooler  54  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. It is contemplated that exhaust cooler  54  may be selectively omitted. 
         [0022]    Power system  10  may also include a selective catalytic reduction system (SCR)  60 . SCR  60  may include components to condition and direct exhaust from cylinder  16  by way of exhaust port  24  and the exhaust valve. SCR  60  may include a reductant supply  64 , an injector  62  connected to the reductant supply  64 , and a catalyst  66 . The reductant may be drawn from reductant supply  64 , and sprayed by injector  62  onto catalyst  66 . Reductant supply  64  may be fluidly connected to injector  62 . The reductant contained in reductant supply  64  may be gaseous, liquid, or solid, and may be any reductant known in the art, such as, for example, urea, ammonia, or a hydrocarbon reductant. Injector  62  may inject reductant from reductant supply  64  into selective catalytic reduction system  60  to reduce the concentration of a constituent therein. For example, to reduce the concentration of an oxide of nitrogen (NOx) by reacting with the NOx in the exhaust and catalyst  66  to form H 2 O and N 2 . 
         [0023]    Catalyst  66  may be disposed in SCR  60  such that the exhaust stream flows through catalyst  66  in a substantially equally distributed manner and causes the constituent to contact and react with the reductant. Catalyst  66  may be made from a variety of materials. For example, catalyst  66  may include a support material and a metal promoter dispersed within the catalyst support material. The support material may include at least one of alumina, zeolite, aluminophosphates, hexaluminates, aluminosilicates, zirconates, titanosilicates, and titanates, and the metal promoter may include silver (Ag). Combinations of these materials may be used, and the support material may be chosen, based on the type of fuel used, the reductant used, the air to fuel-vapor ratio desired, and/or for conformity with environmental standards. One of ordinary skill in the art will recognize that numerous other catalyst compositions, including catalyst compositions usable with a hydrocarbon reductant, may be used without departing from the scope of this disclosure. Further, multiple catalytic devices may also be included with SCR  60 . 
         [0024]    As illustrated in  FIG. 2 , power system  10  may include control system  70  configured to determine operational characteristics of power system  10  and to control the flow of the exhaust. Specifically, control system  70  may regulate flow of exhaust through EGR  50  and may control the injection of reductant from the injector  62  into the SCR  60 . In particular, control system  70  may include a controller  72  configured to receive signals generated by air-to-fuel ratio sensor  74 , a cylinder pressure sensor  76 , a fuel consumption sensor  78 , and a pollutant sensor  46 , and in response, affect the operation of valve  44 , valve  56 , and injector  62 . 
         [0025]    Controller  72  may include a single microprocessor or multiple microprocessors that include a manner for controlling an operation of power system  10 . Numerous commercially available microprocessors can be configured to perform the functions of controller  72 . It should be appreciated that controller  72  could readily embody a general microprocessor capable of controlling numerous functions of power system  10 . Controller  72  may include a memory, a secondary storage device, a processor, and other components for running an application. Various other circuits may be associated with controller  72  such as power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, and other types of circuitry. 
         [0026]    One or more electronic maps relating to engine load may be stored within the memory of controller  72 . Each of these maps may be in the form of tables, graphs, and/or equations and include a compilation of data collected from lab and/or field operation of power system  10 . The maps may relate an engine load to a parameter such as, for example, an air-to-fuel ratio, a cylinder pressure, or a fuel consumption. The maps may relate a portion of exhaust to recirculate through EGR  50  to an engine load. The maps may relate an amount of reductant to be injected to one of a pollutant level and an engine load. Controller  72  may access these maps and affect the operation of power system  10  accordingly. 
         [0027]      FIG. 3  shows a flow-diagram illustrating a method of controlling EGR  50  and SCR  60 .  FIG. 3  will be discussed in greater detail below. 
         [0028]      FIG. 4  illustrates an alternative power system  10 . Similar to power system  10  of  FIG. 1 , power system  10  of  FIG. 4  may include EGR  50  and SCR  60 . However, in contrast to power system  10  of  FIG. 1 , EGR  50  of  FIG. 4  may direct exhaust from upstream of turbine  40  to downstream of compressor  34  and air cooler  36 . 
       INDUSTRIAL APPLICABILITY 
       [0029]    The disclosed exhaust aftertreatment system may be applicable to any power system having an exhaust gas recirculation system (EGR) and a selective catalytic reduction system (SCR), and may include the performance of the EGR and SCR. The disclosed system selectively operates the EGR at low and medium loads and the SCR at medium and high loads to optimize each system and to improve NOx reduction over either system alone. 
         [0030]    Atmospheric air may be drawn into air induction system  30  via air filter  32  and may be directed through compressor  34  where it may be pressurized to a predetermined level before entering the combustion chamber of engine  12 . Fuel may be mixed with the pressurized air before or after entering the combustion chamber of engine  12 . The fuel and air mixture may be ignited by engine  12  to produce mechanical work and an exhaust flow containing gaseous compounds. The exhaust flow may be a fluid that may also contain solid particulate matter and pollutants such as, for example, carbon, sulfur, and NOx. The exhaust flow may be directed from engine  12  to turbine  40  where the expansion of hot exhaust gases may cause turbine  40  to rotate, thereby rotating connected compressor  34  to compress the inlet air. After exiting turbine  40  the exhaust may flow through particulate filter  42 . 
         [0031]    Control system  70  may regulate the flow of exhaust and the quantity of reductant to inject based on signals received from sensors  46 ,  74 ,  76 , and  78 , valves  44  and  56 , and injector  62 . Control system  70  may affect the operation of power system  10 , more particularly, valve  44 , valve  56 , and injector  62 , to regulate the flow of exhaust through SCR  60  and EGR  50 . The exhaust may flow through SCR  60  based on the positions of valves  44  and  56 . Exhaust that may flow through SCR  60  may pass through catalyst  66  and injector  62  may inject urea, stored in reductant supply  64 , such that the reductant reacts with the NOx in the presence of catalyst  66  to form H 2 O and N 2 . The exhaust may then exit power system  10  via outlet  68 . The exhaust may flow through EGR  50  based on the positions of valves  44  and  56 . Exhaust that may flow through EGR  50  may pass though flow meter  52  and exhaust cooler  54  before being reintroduced into air induction system  30 . 
         [0032]      FIG. 3  is a flow diagram illustrating an exemplary disclosed method for operating an exhaust aftertreatment control system. Controller  72  may determine the load of power system  10  based on a signal indicative of at least one of an air-to fuel ratio received from sensor  74 , a cylinder pressure received from sensor  76 , or a fuel consumption received from sensor  78  (Step  80 ). Power system  10  may operate at loads at or below a first value indicative of a given engine load. Power system  10  may operate at loads at or above a second value indicative of a second given engine load. Power system  10  may operate at loads between the first value and the second value. It is contemplated that the first value may equal the second value. 
         [0033]    Controller  72  may then determine whether to recirculate exhaust through EGR  50  based on a determination that power system  10  may be operating below the second value indicative of a given engine load (Step  82 ). Controller  72  may then cause the positions of valves  44  and  56  to change such that at least a portion of the exhaust flows through EGR  50 : Controller  72  may access an electronic map to determine the portion of exhaust to recirculate through EGR  50 . It is further contemplated that controller  72  may recirculate exhaust through EGR  50  only when the load of power system  10  is below the second value or, alternatively, at any load. 
         [0034]    Controller  72  may then determine whether to inject reductant into SCR  60  based on a determination that power system  10  may be operating above the first value indicative of a given engine load (Step  84 ). Controller  72  may then cause reductant to be drawn from reductant supply  64  and be injected by injector  62  onto catalyst  66  or into the exhaust upstream of catalyst  66 . Controller  72  may access an electronic map to determine an amount of reductant to inject into SCR  60 . Controller  72  may inject reductant into SCR  60  only when the load of power system  10  is above the first value or, alternatively, at any load. 
         [0035]    Several advantages may be associated with the currently disclosed aftertreatment system. By operating the EGR at low and medium loads, the power system may benefit from the EGR without having excessive cylinder pressures, air-fuel-ratios, or fuel consumption associated with EGR at high loads. Furthermore the exhaust being recirculated through the EGR may be cooler and may require smaller heat exchangers to cool the exhaust before reintroduction into the engine. By operating the SCR at medium and high loads, the power system may benefit from pollutant reduction with minimal or no use of the EGR. Furthermore, by selectively employing the SCR, less reductant storage may be required. 
         [0036]    It will be apparent to those skilled in the art that various modifications and variations can be made to the aftertreatment system of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.