Patent Publication Number: US-9890723-B2

Title: Methods to adapt air-fuel (A/F) controls for catalyst aging

Description:
BACKGROUND 
     The subject matter disclosed herein relates to an exhaust treatment system for an internal combustion engine and, more specifically, to adapting controls based on catalyst performance. 
     Engines (e.g., internal combustion engines such as reciprocating engines or gas turbines) combust a mixture of fuel and air to generate combustion gases that apply a driving force to a component of the engine (e.g., to move a piston or drive a turbine). Subsequently, the combustion gases exit the engine as an exhaust, which may be subject to exhaust treatment systems that include one or more catalytic converters (e.g., three-way catalyst (TWC) assembly, selective catalytic reduction (SCR) assembly) to reduce the emissions of nitrogen oxides (NO X ), hydrocarbons (HC), carbon monoxide (CO), and other emissions. However, the effectiveness of the catalysts at reducing emissions may decrease over time, resulting in the engine falling out of emissions compliance. 
     BRIEF DESCRIPTION 
     Certain embodiments commensurate in scope with the originally claimed subject matter are summarized below. These embodiments are not intended to limit the scope of the claimed subject matter, but rather these embodiments are intended only to provide a brief summary of possible forms of the subject matter. Indeed, the subject matter may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
     In a first embodiment, a system includes an exhaust treatment system configured to treat emissions from a combustion engine via a catalyst, and a controller configured to obtain an operating parameter indicating catalyst performance, determine a deterioration factor indicating deterioration of the catalyst based at least in part on the operating parameter, determine an adaptation term configured to modify an air-fuel ratio command for the combustion engine to account for the deterioration factor of the catalyst, and generate a signal indicating the adaptation term. 
     In a second embodiment, an electronic control unit includes a processor operatively coupled to a memory, wherein the processor is programmed to execute instructions on the memory to obtain an operating parameter that indicates how well a catalyst is performing in treating emissions from a combustion engine, determine a deterioration factor that indicates how much the catalyst has deteriorated based at least in part on the operating parameter, determine an adaptation term configured to modify an air-fuel ratio command for the combustion engine to account for the deterioration factor of the catalyst, and generate a signal indicating the adaptation term. 
     In a third embodiment, one or more non-transitory computer-readable media encoding one or more processor-executable routines wherein the one or more routines, when executed by a processor of a controller, cause acts to be performed including obtaining an operating parameter that indicates a conversion performance of a catalyst in treating emissions from a combustion engine, determining a deterioration factor that indicates how much the catalyst has deteriorated based at least in part on the operating parameter, determining an adaptation term configured to modify an air-fuel ratio command for the combustion engine to account for deterioration factors of the catalyst, and generating a signal indicating of the adaptation term. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present subject matter will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a schematic diagram of an embodiment of a three-way catalyst (TWC) exhaust treatment (e.g., aftertreatment) system coupled to an engine; 
         FIG. 2  is a schematic diagram of an embodiment of the functional operation of a controller (e.g., an electronic control unit (ECU)) that controls the air-fuel command of the engine of  FIG. 1 ; 
         FIG. 3  is a flow diagram of an embodiment of a process performed by a processor of the controller of  FIG. 1 ; 
         FIG. 4  is a schematic diagram of an embodiment of a selective catalytic reduction (SCR) exhaust treatment system for a lean burn engine; 
         FIG. 5  is a schematic diagram of an embodiment of a function operation of a controller (e.g., an electronic control unit (ECU)) that controls the air-fuel command of the lean burn engine of  FIG. 4 ; and 
         FIG. 6  is a flow diagram of an embodiment of a process performed by a processor of the controllers of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present subject matter will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present subject matter, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     The present disclosure is directed to systems and methods for monitoring or estimating the deterioration of catalysts in catalytic converters and adjusting controls in response to the detected or estimated deterioration (e.g., deactivation of catalysts). The system and method discussed herein may be performed in three-way catalyst (TWC) and/or selective catalytic reduction (SCR) exhaust treatment systems. Exhaust treatment (e.g., aftertreatment) systems are configured to couple to combustion engines to treat emissions (e.g., in the engine exhaust) from the combustion engine. The exhaust treatment system may include a catalyst based system, such as a TWC system that utilizes a catalyst to convert harmful pollutants, such as NO X , HC, CO, to less toxic emissions. Unfortunately, subjecting the TWC to certain operating conditions over time often causes changes in the number and type of active sites reactions take place on. The loss of active sites on the surface of the catalysts can result in a loss of conversion performance (i.e., how well the catalyst is operating). As catalyst conversion performance decreases, the emissions of pollutants (e.g., NO X , HC, CO, etc.) from the engine can exceed emission compliance values (e.g., thresholds or requirements). By adapting the air-fuel ratio controls of the engine based on the catalyst performance, the engine can remain in emissions compliance for a longer duration of time than if the air-fuel ratio controls were not adapted based on catalyst performance. 
     The disclosed embodiments include measuring or obtaining one or more operating parameters of a combustion engine that indicate the conversion performance of the catalysts. The operating parameters may include any actual or estimated aspects of the system performance suitable for indicating the conversion performance of the catalysts, such as time (e.g., engine run time, catalyst aging time, times at different engine temperatures, etc.), temperatures, flow rates, and/or emission measurements. The conversion performance may describe how well the catalyst is performing at converting pollutants to less harmful emissions. A control system may determine a deterioration factor that indicates how much the catalyst has deteriorated (e.g., over a period of time) based on the operating parameter. The control system may then adapt air-fuel controls of the combustion engine based on the conversion performance to account for deterioration of the catalyst, such as the loss of active sites on the catalyst due to aging, temperature, flow rate, and/or species inputs. 
     Turning now to the drawings and referring to  FIG. 1 , a schematic diagram of a TWC exhaust treatment (e.g., aftertreatment) system  6  coupled to an engine  12  is illustrated. As described in detail below, the disclosed exhaust treatment system  6  monitors operating parameters (e.g., oxidation state) of a catalyst assembly  14  of the exhaust treatment system  6 . The engine  12  may include an internal combustion engine such as a reciprocating engine (e.g., multi-stroke engine such as two-stroke engine, four-stroke engine, six-stroke engine, etc.) or a gas turbine engine. The engine  12  may operate on a variety fuels (e.g., natural gas, diesel, syngas, gasoline, blends of fuel (e.g., methane, propane, ethane, etc.), etc.). The engine  12  may be part of a power generation system that generates power ranging from 10 kW to 10 MW. In some embodiments, the engine  12  may operate at less than approximately 1800 revolutions per minute (RPM). In some embodiments, the engine  12  may operate at less than approximately 2000 RPM, 1900 RPM, 1700 RPM, 1600 RPM, 1500 RPM, 1400 RPM, 1300 RPM, 1200 RPM, 1000 RPM, or 900 RPM. In some embodiments, the engine  12  may operate between approximately 800-2000 RPM, 900-1800 RPM, or 1000-1600 RPM. In some embodiments, the engine  12  may operate at approximately 1800 RPM, 1500 RPM, 1200 RPM, 1000 RPM, or 900 RPM. Exemplary engines  12  may include General Electric Company&#39;s Jenbacher Engines (e.g., Jenbacher Type 2, Type 3, Type 4, Type 6 or J920 FleXtra) or Waukesha Engines (e.g., Waukesha VGF, VHP, APG, 275GL), for example. 
     During operation, the engine  12  receives air  8  (e.g., an oxidant) and fuel  10  that are used in a combustion process to apply a driving force to a component of the engine  12  (e.g., one or more pistons reciprocating in cylinders or one more turbines). The combustion gases  16  subsequently exit the engine  12  as an exhaust  16 , which includes a variety of emissions (e.g., NO X , HC, CO, or other pollutants). The exhaust treatment system  6  treats these emissions to generate milder emissions (carbon dioxide (CO 2 ), water, etc). As depicted, the exhaust treatment system  6  includes the catalytic converter or catalyst assembly  14 . The catalyst assembly  14  (e.g., TWC assembly) includes an inlet  18  to receive the exhaust  16  (e.g., fluid) from the engine  12  and an outlet  20  to discharge treated engine exhaust  22 . As shown in  FIG. 1 , the catalyst assembly  14  includes a TWC assembly. The TWC assembly, via its catalytic activity, reduces NO X  via multiple reactions. For example, NO X  may be reduced via CO to generate N 2  and CO 2 , NO X  may be reduced via H 2  to generate NH 3 , N 2 , and water, and NO X  may be reduced via a hydrocarbon (e.g., C 3 H 6 ) to generate N 2 , CO 2 , and water. The TWC assembly also oxidizes CO to CO 2 , and oxidizes unburnt HC to CO 2  and water. 
     The engine  12  may operate as a rich-burn engine or a lean-burn engine depending on the mass ratio of air  8  to fuel  10  (AFR). In embodiments that include the TWC assembly, the engine  12  may be operated as a rich-burn engine (e.g., equivalence ratio (i.e., ratio of actual AFR to stoichiometric AFR), or lambda (λ) value oscillating around 1 (e.g., stoichiometric engine)) to maximize the catalytic activity in the TWC assembly. In other embodiments, the catalyst assembly  14  may include any other type of oxidation catalyst (e.g., two-way catalyst, hydrocarbon oxidation catalyst, diesel oxidation catalyst, etc.). In certain embodiments, the exhaust treatment system  6  may include one or more additional catalyst assemblies disposed upstream and/or downstream of the catalyst assembly  14  (e.g., an ASC assembly disposed between the engine  12  and the catalyst assembly). In certain embodiments, the exhaust treatment system  6  may include other components (e.g., an oxidant injection system that injects air  8  (e.g., an oxidant, O 2 , O 2 -enriched air, or O 2 -reduced air) into the exhaust  16 ). 
     The engine  12  and the exhaust treatment system  6  are coupled (e.g., communicatively) to a controller  24  (e.g., an engine control unit (ECU)) that controls and monitors the various operations of the engine  12 . The controller  24  may include multiple controllers in communication with each other (e.g., a respective controller for the engine  12  and the exhaust treatment system  6 ). The controller  24  includes processing circuitry (e.g., processor  26 ) and memory circuitry (e.g., memory  28 ). The processor  26  may include multiple microprocessors, one or more “general-purpose” microprocessors, one or more special-purpose microprocessors, and/or one or more application specific integrated circuits (ASICS), system-on-chip (SoC) device, or some other processor configuration. For example, the processor  26  may include one or more reduced instruction set (RISC) processors or complex instruction set (CISC) processors. The processor  26  may execute instructions to carry out the operation of the engine  12  and/or exhaust treatment system  6 . These instructions may be encoded in programs or code stored in a tangible non-transitory computer-readable medium (e.g., an optical disc, solid state device, chip, firmware, etc.) such as the memory  28 . In certain embodiments, the memory  28  may be wholly or partially removable from the controller  24 . 
     The memory  28  may store various tables (e.g., look-up tables (LUT)). The memory  28  may also store models (e.g., software models representing and/or simulating various aspects of the engine  12 , the exhaust treatment system  6 , and/or each of their components). For example, the memory  28  may store models used to estimate how flow rate, temperature, oxygen, or emissions correspond to catalyst performance. The models may be used to compare estimated values to measured values indicating the conversion performance of the catalyst. 
     The processor  26  of the controller  24  may be configured to execute instructions to control various aspects of the engine, such as the air-fuel ratio (AFR). That is, the processor  26  may be configured to control air  8  and fuel  10  quantities that enter the engine  12  during the combustion process to optimize the performance of the engine  12  (e.g., based on throttle, output, RPM, or any number of factors). Further, the controller  24  also controls and/or monitors the operations of the exhaust treatment system  6 , such as the AFR. In an embodiment, the processor  26  may control the air  8  and fuel  10  quantities based at least in part on an adaptation term (e.g., part, aspect, etc.) that accounts for changes (e.g., deterioration) in the conversion performance of the catalyst. 
       FIG. 2  is a schematic diagram of functional operations  40  for the controller  24  to control the AFR of the engine  12  of  FIG. 1 . All or some of the steps of the functional operations  40  described in  FIG. 2  may be executed by the controller  24  (e.g., utilizing the processor  26  to execute programs and access data stored on the memory  28 ). In addition, one or more of these steps may be performed simultaneously with other steps. 
     The operations  40  of  FIG. 2  may be performed to adapt an AFR command  38  to account for decreasing conversion performance of the catalyst. The conversion performance may deteriorate as the number of active sites that reactions take place on decrease. The loss of active sites (i.e., the loss of conversion performance) may occur due to aging, flow rate, temperature, and/or species inputs. The processor  26  may determine an adaptation term to account for the deterioration of the catalyst. The adaptation term is configured to modify the air-fuel ratio command  38  of the combustion engine  12  based at least in part on the conversion performance. 
     To account for aging of the catalyst, a clock  42  may be utilized to provide an amount of time  42  (e.g., how long the catalyst has been operating) based on clock cycles for the processor  26  to access. The processor  26  may utilize one or more look up tables (LUT) stored the in memory  28 , such as an omega parameter LUT  44  and/or a NO X , CO, or post catalyst emission LUT  46 . The omega parameter LUT  44  may include one or more catalysts, such as a platinum group metal (PGM), ceria, or any other suitable catalyst and an omega parameter that indicates how the catalyst ages over time. As such, the omega parameter LUT  44  may provide a deterioration factor  48  (e.g., Omega_PGM, Omega_Ceria, etc.) that indicates how much the catalyst has deteriorated (e.g., due to aging) based at least in part on one or more operating parameters, such as the time (e.g., from clock  42 ) and/or a type of the catalyst (e.g., PGM, Ceria, etc.), as different catalysts may age at different rates. The processor  26  may adjust the deterioration factor  48  linearly and/or exponentially, as the deterioration of some types of catalysts may vary linearly and/or exponentially based on time. The deterioration factor  48  may also be based on precious metal loading of the catalyst. 
     The deterioration factor  48  may further account for other causes of losses in conversion performance in the catalyst, such as flow rate, temperature, and species inputs. The various sensors  34  coupled to the system  6  may detect operating parameters that may be suitable for establishing deterioration factors. For example, temperature may be detected via the sensors  34 . As high temperatures may cause a decrease in conversion performance, the processor  26  may utilize the model based estimator  50  to determine a deterioration factor  48  that accounts for temperature. For example, the model may have temperatures that correspond to different rates of aging of the catalyst. As a further example, the model based estimator  50  may use a measured oxygen value (e.g., how much oxygen is missing from the expected amounts of O 2  storage  60 ) when determining deterioration factors. The model may be general to any type of catalyst or specific to certain catalysts. As mentioned above, the processor  26  may account for changes in catalyst conversion performance by varying the deterioration factor  48  linearly and/or exponentially proportional to precious metal loading. 
     The controller  24  may use a model stored in the memory to estimate the emissions of certain species (e.g., NO X  and NH 3 ). The processor  26  may utilize a NO X , CO, or post catalyst emission LUT  46  to determine the deterioration factors  52  based at least in part on pre or post catalyst emission values. That is, based on the amounts of various emissions, the processor  26  may determine a deterioration factor  52  for how well the catalyst is performing. The deterioration factor  52  based on emissions may then be compared to the O 2  storage data. In the embodiment shown in  FIG. 2 , the species control  58  can modify the O 2  storage set-point given the oxygen storage control  60 . Alternatively and/or additionally, the O 2  storage control  60  set-point may be used to modify the species control  62 . 
     The processor  26  may determine the adaptation term  64  to modify the air-fuel ratio command  38  provided to the combustion engine to account for the one or more deterioration factors  48 ,  52  of the catalyst. The controller  24  may then regulate or adjust the air-fuel ratio of the engine  12  based on the air-fuel ratio command  38 . Additionally and/or alternatively, the controller  24  may control one or more other engine operating parameters, such as spark timing. By modifying the air-fuel ratio command  38 , the controller  24  can allow the engine  12  to remain in emissions compliance for an extended duration of time longer as the catalyst ages, where the extended duration of time is longer than a duration of time had the air-fuel ratio not been modified. By extending the duration of remaining in compliance, the controller  24  reduces maintenance and further improving operation of the engine. The adaptation term  64  may include a linear or weighted combination of oxygen storage control  60  estimates and/or species concentration estimates. As explained above, the oxygen storage estimates and/or species concentration estimates utilize maps (e.g., LUT  44 ,  46 ) of deterioration factors to analyze the operating parameters. 
     The processor  26  may execute instructions (e.g., code) stored on the memory  28  to carry out the operation of the engine  12  and/or exhaust treatment system  6  in accordance with the processes described herein.  FIG. 3  is a flow diagram of an embodiment of a process  64  performed by one or more of the processors  26  of the controller  24 . This process  64  may be applied to TWC catalyst systems. The process  64  may begin by obtaining signals indicating catalyst performance (block  66 ). Catalyst performance indications may be obtained via the sensors  34 ,  36  and/or the clock  42 . The process  64  may then continue by determining (block  68 ) a deterioration factor indicating how much the catalyst has deteriorated based on the catalyst performance. Next, one or more of the processors  26  may determine an adaptation term to modify the air-fuel ratio of the engine to account for the deterioration factor (block  70 ). Then, the processor  26  may generate a signal that is based on the adaptation term that modifies the air-fuel ratio. For example, the processor  26  may generate a signal indicating the adaptation term (block  72 ). The processor  26  may then modify the air-fuel ratio of the combustion engine based on the adaptation term (block  74 ). By modifying the air-fuel ratio command or the oxidant injection based on an adaptation term that accounts for aging of a catalyst, the controller can enable the combustion engine to remain in emissions compliance for an extended duration of time as the catalyst ages. 
     The systems and methods may be applied to selective catalytic reduction (SCR) exhaust treatment systems for lean-burn engine exhaust treatment controls.  FIG. 4  is schematic diagram of an embodiment of an SCR exhaust treatment system  78  that uses an SCR catalyst assembly  80  for a lean-burn engine  82 . Similar to the TWC system described with respect to  FIG. 1 , the lean-burn engine  82  is coupled to a controller (e.g., engine control unit)  84  that controls and monitors the operations of the engine  82 . The engine controller  84  includes processing circuitry (e.g., processor  86 ) and memory circuitry (e.g., memory  88 ). The processor  86  may execute instructions (e.g., stored in the memory  88 ) to carry out the operation of the engine  82 . The processor  86  of the controller  84  may operate similar to the controller  24  and may be configured to generate one or more commands to control the engine  82 , such as a reductant injection (e.g., anhydrous ammonia, aqueous ammonia, or urea) command  90  that controls the reductant that injected the engine  82 . 
     The SCR exhaust treatment system  78  may convert pollutants, such as NO X  emissions, from the exhaust  92  of the engine  82 . Further, the SCR exhaust treatment system  78  may include a reductant injection system  94  that injects a reductant, such as NH 3  or urea, into the exhaust  92  and received by the SCR catalyst assembly  80 . The lean-burn engine  82  may generate exhaust  92  having NO X  or other undesirable pollutants which are output at various temperatures and flow rates. Sensors  96  are coupled to or downstream of the engine  82  and are configured to measure the temperature and flow rates of the various exhaust  92  parameters. For example, the sensors  96  may include one or more pre-SCR ammonia (NH 3 ) sensors  98  and/or one or more pre-SCR nitrogen oxides (NO x ) sensors  100  configured to measure the concentrations of the the reductant and/or the pollutants, respectively, in the exhaust  92 . Further, one or more post-SCR NH 3  sensors  102  and/or NO X  sensors  106  may be disposed downstream of the catalyst assembly to measure a concentration or an amount of pollutants and/or reductants in the treated engine exhaust  101 . Even further, one or more RF probes or sensors  108  may be disposed within or coupled to the catalyst assembly  80  to measure reductant storage of the catalyst assembly  80 . In certain embodiments, the NH 3  storage measurement from the RF probes  108  may take the form of a voltage reading. In certain embodiments, the voltage reading may be converted to an NH 3  storage value, θ (e.g., utilizing a LUT). 
     Signals from the sensors  96 ,  102 ,  108  may be used by a reduction injection controller  112 . The reduction injection controller  112  may include a processor  114  and/or a memory  116 . The processor  114  and the memory  116  may be used to execute instructions related to controlling the reductant injected into the exhaust  92 . Additionally, signals may be sent to the engine controller  84  for the engine controller  84  to control operating parameters of the engine based on reductant measurements. 
     The controllers  84  and/or  112  may modify the reductant injection based on catalytic performance to allow the lean-burn engine  82  to remain in compliance for an extended period of time. That is, the controllers  84  and/or  112  enable the engine to be in emissions compliance for a period of time longer than if the controllers  84  and/or  112  did not modify the reductant injection based on the catalytic performance.  FIG. 5  is a schematic diagram of function operations for the controllers  84  and/or  112  to adapt the reductant injection to the exhaust of the engine  82  based on the catalytic performance. All or some of the steps of the functional operations  118  described in  FIG. 5  may be executed by the controllers  84  and/or  112  (e.g., utilizing the processor  86  to execute programs and access data stored on the memory  28 ). In addition, one or more of these steps may be performed simultaneously with other steps. 
     Similar to the TWC system described above, these instructions may be encoded in programs or code stored in a tangible non-transitory computer-readable medium (e.g., an optical disc, solid state device, chip, firmware, etc.) such as the memory  88 . In certain embodiments, the memory  88  may be wholly or partially removable from the controller  24 . The memory  88  may store various tables (e.g., look-up tables (LUT)). The memory  88  may also store models (e.g., software models representing and/or simulating various aspects of the engine  82 , the exhaust treatment system  78 , and/or each of their components). For example, the memory  88  may store models used to estimate how flow rate, temperature, ammonia, or emissions correspond to catalyst performance. The models may be used to compare estimated values to measured values indicating the conversion performance of the catalyst. 
     As catalysts deteriorate over time, the catalysts in the SCR catalyst assembly  80  may not convert the pollutants as efficiently and/or reduce pollutants enough for the engine  82  to stay in compliance and/or to minimize maintenance. That is, the conversion performance may deteriorate as the number of active sites that reactions take place on decrease. The loss of active sites (i.e., the loss of conversion performance) may occur due to aging, flow rate, temperature, and/or species inputs. 
     The controllers  84  and/or  112  may utilize one or more engine operating parameters (e.g. actual operating parameters measured by the sensors  96 ,  102 ,  108  and/or estimated operating parameters), such as the measured NH 3  and/or NO X  concentrations (e.g., received from the NH 3  sensors  98 ,  104  and/or the NO X  sensors  100 ,  106 ) upstream and downstream of the catalyst assembly  80 . 
     The controllers  84  and/or  112  may utilize the signals from the sensors  96 ,  102 ,  108  to determine a deterioration factor  122  indicating the deterioration (e.g., due to aging) of the catalyst based on one or more operating parameters of catalytic performance, such as a type of catalyst, as different catalysts may age at different rates. Alternatively and/or additionally, the processors  86  and/or  114  may receive signals from a clock  120  that can be used to estimate the deterioration of the catalyst. For example, the processors  86  and/or  114  may utilize one or more LUTs  124 ,  126  with times from the clock  120  (e.g., based on clock cycles of the processor  86 ) associated with aging of the catalyst to determine the deterioration factor  122 . As time measured by the clock  120  progresses, the LUT may provide increasing deterioration of the catalyst. The processors  86  and/or  114  may adjust the deterioration factor  122  linearly and/or exponentially, as the deterioration of some types of catalysts may vary linearly and/or exponentially based on time. 
     The deterioration factor  122  may further account for other causes of losses in conversion performance in the catalyst and be used by a model based estimator  128  to determine an NH 3  storage control estimation signal  132 . The controller  84  may input the operating parameters and/or the deterioration factor  122  into the model based estimator  128  (e.g., software-based model) to generate an estimate of the NH 3  storage control  140  state of the catalyst assembly  80  and/or estimates of emissions controls  148  for emissions (e.g., NO X ) exiting the catalyst assembly  80 . For example, the measured NH 3  concentration upstream and downstream of the catalyst assembly  80  may be utilized in the model to generate the estimated NH 3  storage control  140  of the catalyst assembly  80  and/or the estimated NH 3  emissions exiting the catalyst assembly  80 . In other embodiments, the measured NO X  concentration upstream and downstream of the catalyst assembly  80  may be utilized in the model to generate the estimated emissions control for the catalyst assembly  80 . The controller  84  may compare an estimated NH 3  storage to a measured NH 3  storage (e.g., based on feedback from the RF probes  108 ) for the catalyst assembly  80 . For example, the model based estimator  128  may determine an estimation signal  132  that accounts for aging of the catalyst (e.g., via the deterioration factor  122  from the LUT  124  and/or the LUT  126 ), temperature, flow rate, and/or species inputs. As explained above with respect to the TWC catalyst assembly, the model  128  may have temperatures that correspond to different rates of aging of the catalyst. 
     To account for aging of the catalyst, the clock  120  may be utilized to provide an amount of time (e.g., how long the catalyst has been operating) based on clock cycles for the processor  86  and/or  114  to access. The processor  86  and/or  114  may utilize the LUT  124  and/or the LUT  126  stored in the memory  88  and/or  116 , such as an omega parameter LUT  124  and/or a NH 3 , NO X , and/or post catalyst emission LUT  126 . The omega parameter LUT  124  may provide a deterioration factor  122  (e.g., Omega_Vanadium, Omega_zeolite, etc.) that indicates how much the catalyst has deteriorated (e.g., due to aging) based at least in part on one or more operating parameters, such as the time (e.g., from clock  120 ) and/or a type of the catalyst (e.g., vanadium, zeolite, etc.), as different catalysts may age at different rates. In other words, the processor  86  and/or  114  may utilize the LUT  124  and/or post catalyst emission LUT  126  to provide omega values based on time and/or a type of catalytic components. For example, the omega values may be associated with active site density of vanadium, zeolites, and/or precious metals. For instance, as vanadium often does not withstand temperatures as high as zeolite, the processor  86  and/or  114  may determine that vanadium omega values increase faster than zeolite omega values at higher temperatures than optimal temperatures for vanadium over a similar duration of time. Further, the processor  86  and/or  114  may adjust the deterioration factor  122  linearly and/or exponentially, as the deterioration of some types of catalysts may vary linearly and/or exponentially based on time. The deterioration factor  122  may also be based on precious metal loading of the catalyst. 
     A similar process may be performed by the processor  86  and/or  114  based on post catalyst NH 3  and/or NO X  emissions. For example, the processor  86  and/or  114  may determine the deterioration factor  122  that indicates how much the catalyst has deteriorated based on NH 3 , NO X , and/or other post catalyst emissions. The LUT  126  may include different deterioration rates based on quantities of NH 3  and/or NO X  detected. Further, the deterioration rates may vary depending on the type of post catalyst emissions, similar to the types of catalysts described above. 
     The processor  86  may adapt the NH 3  storage  140  of the catalyst assembly  80  based on the estimation signal  132  to account for the deterioration of the catalyst. The estimated variables are then utilized to add an adaptation term  142  to modify the reductant injection command  90  (e.g., a urea injection command, NH3 injection command, etc.) of the combustion engine  82  based at least in part on the conversion performance. 
     As shown in  FIG. 5 , the clock  120  may be used with pre-SCR NH 3  and/or NO X , post-catalyst NH 3  and/or NO X , or both. That is, the deterioration factor  122  may be generated from a pre-SCR LUT  124 , a post-catalyst LUT  126 , or both. In  FIG. 5 , the estimation signal  132  accounts for the deterioration factor  122  using in both pre-SCR (e.g., reference number  144 ) and post-catalyst (e.g., reference number  146 ) values. Further, post catalyst emissions controls  148  may also account for the deterioration factor  122  and the reductant injection command  90  may be based at least in part on the adaptation terms  142  (e.g., a linear or weighted combination of the one or more adaptation terms  142 ). 
     The processor  86  and/or  114  may execute instructions (e.g., code) stored in the memory  88  and/or  116  to carry out the operation of the exhaust treatment system  78  in accordance with the processes described herein.  FIG. 6  is a flow diagram of an embodiment of a process  160  performed by one or more of the processors  86  and/or  114  of the engine controller  84  and/or the reductant injection controller  112 . This process  160  may be applied to SCR catalysts for lean burn engines. The process  160  may begin by the processor  86  obtaining signals indicating catalyst performance (block  162 ). Catalyst performance indications may be obtained via the sensors  96  and/or the clock  120 . The process  160  may then continue by determining (block  164 ) a deterioration factor indicating how much the catalyst has deteriorated based on the catalyst performance. Next, one or more of the processors  86  and/or  114  may determine an adaptation term to modify a reductant injection command for the engine  82  to account for deterioration of the catalyst (block  166 ). Then, the processor  86  and/or  114  may generate a signal that is based on the adaptation term that modifies the reductant injection command. For example, the processor  86  and/or  114  may generate a signal indicating the reductant injection command (block  166 ). The processor  86  and/or  114  may then modify injection of a reductant based on the reductant injection command (block  170 ). By modifying the reductant injection based on an adaptation term that accounts for aging of a catalyst, the controller can enable the combustion engine  82  to remain in emissions compliance for an extended duration of time as the catalyst ages. 
     Technical effects of the present embodiments relate to controlling an air/fuel ratio or reductant injection of an engine. In certain embodiments, the engine may include one or more operating parameters that are used to indicate catalyst performance. A controller may receive the operating parameters which may be used to determine a deterioration factor of the catalyst. In an embodiment, the deterioration factor indicates aging of the catalyst. The controller may determine an adaptation term to modify an air-fuel ratio or a reductant injection to account for the aging of the catalyst. By changing the air-fuel ratio or the reductant injection, the engine can remain in emissions compliance for an extended duration and the lifetime of the engine can be improved. 
     This written description uses examples to disclose the subject matter, including the best mode, and also to enable any person skilled in the art to practice the subject matter, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.