Abstract:
A system includes a controller programmed to determine an oxidation state of a three-way catalyst (TWC) assembly based on a first signal representative of a measured oxygen (O 2 ) storage of the TWC assembly received from a radio frequency (RF) probe disposed within the TWC assembly, to determine whether a temperature of a fluid flowing into an ammonia slip catalyst (ASC) assembly is within a desired temperature operating range based on a second signal representative of the temperature of the fluid adjacent an inlet of the ASC assembly, to determine whether a concentration of nitrogen oxides (NO X ) in the fluid exiting an outlet of the ASC assembly is within desired limits based on a third signal representative of the concentration of NO X  in the fluid, and to determine whether to perform diagnostics on a component of an exhaust aftertreatment system based at least on the first, second, and third signals.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to an exhaust aftertreatment system for an internal combustion engine and, more specifically, to monitoring the health of a mid-bed oxidant injection system of the exhaust aftertreatment system. 
         [0002]    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 (e.g., aftertreatment) systems that include one or more catalytic converters (e.g., three-way catalyst (TWC) assembly, ammonia slip catalyst (ASC) assembly, etc.) to reduce the emissions of nitrogen oxides (NO X ), hydrocarbons (HC), carbon monoxide (CO), and other emissions. However, if the health of one or more components of the exhaust treatment system is not closely monitored, over time the effectiveness of the catalysts at reducing emissions may decrease. 
       BRIEF DESCRIPTION 
       [0003]    Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
         [0004]    In accordance with a first embodiment, a system includes an exhaust aftertreatment system configured to treat emissions from a combustion engine. The exhaust aftertreatment system includes a TWC assembly having a first outlet, an ASC assembly configured to receive a fluid from the TWC assembly, wherein the ASC assembly has an inlet and a second outlet, and a fluid conduit disposed between the TWC assembly and the ASC assembly and configured to transfer the fluid from the TWC assembly to the ASC assembly. The exhaust aftertreatment system also includes an oxidant injection system coupled to the fluid conduit and configured to inject oxidant into the fluid conduit upstream of the inlet of the ASC assembly to provide sufficient oxidant in the fluid flowing into the inlet of the ASC assembly to enable catalytic activity in the ASC assembly. The exhaust aftertreatment system further includes a first radiofrequency (RF) probe disposed within the TWC assembly and configured to measure oxygen (O 2 ) storage of the TWC assembly, at least one temperature sensor disposed downstream of a location of oxidant injection into the fluid conduit by the oxidant injection system and upstream of the inlet of the ASC assembly, wherein the at least one temperature sensor is configured to measure a temperature of the fluid adjacent the inlet of the ASC assembly, and at least one NO X  sensor disposed downstream of the inlet of the ASC assembly and configured to measure a concentration of NO X  in the fluid exiting the outlet of the ASC assembly. The exhaust aftertreatment system still further includes a controller programmed to receive a first signal representative of a measured O 2  storage of the TWC assembly from the first RF probe, to receive a second signal representative of the temperature of the fluid adjacent the inlet of the ASC assembly, to receive a third signal representative of the concentration of NO X  in the fluid exiting the ASC assembly, and to determine whether to perform diagnostics on a component of the exhaust aftertreatment system based at least on the first, second, and third signals. 
         [0005]    In accordance with a second embodiment, a system includes a controller programmed to determine an operational state of an exhaust aftertreatment system coupled to a combustion engine, wherein the exhaust aftertreatment system comprises a TWC assembly fluidly coupled to an ASC assembly located downstream of the TWC assembly. The controller is also programmed to determine an oxidation state of the TWC assembly of the exhaust aftertreatment system based on a first signal representative of a measured O 2  storage of the TWC assembly received from a first RF probe disposed within the TWC assembly, to determine whether a temperature of a fluid flowing into the ASC assembly is within a desired temperature operating range based on a second signal representative of the temperature of the fluid adjacent an inlet of the ASC assembly received from a temperature sensor, to determine whether a concentration of NO X  in the fluid exiting an outlet of the ASC assembly is within desired limits based on a third signal representative of the concentration of NO X  in the fluid exiting the ASC assembly received from a NO X  sensor, and to determine whether to perform diagnostics on a component of the exhaust aftertreatment system based at least on the first, second, and third signals. 
         [0006]    In accordance with a third embodiment, a method for monitoring an operational state of an exhaust aftertreatment system coupled to a combustion engine is provided, wherein the exhaust aftertreatment system includes a TWC assembly fluidly coupled to an ASC assembly located downstream of the TWC assembly. The method includes receiving, at a controller, a first signal representative of a measured O 2  storage of the TWC assembly from a first RF probe disposed within the TWC assembly. The method also includes receiving, at the controller, a second signal representative of a temperature of a fluid flowing into an inlet of the ASC assembly from a temperature sensor. The method further includes receiving, at the controller, a third signal representative of a concentration of NO X  in the fluid exiting the ASC assembly from a NO X  sensor. The method still further includes determining, via the controller, whether to perform diagnostics on a component of the exhaust aftertreatment system based at least on the first, second, and third signals. The method yet further includes receiving, at the controller, a fourth signal representative of a concentration of ammonia (NH 3 ) in the fluid within the ASC assembly. The method even further includes, if the controller determines to perform diagnostics on the component, determining, via the controller, the component of the exhaust aftertreatment system to perform diagnostics on based on the fourth signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other features, aspects, and advantages of the present invention 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: 
           [0008]      FIG. 1  is a schematic diagram of an embodiment of an exhaust treatment (e.g., aftertreatment) system coupled to an engine; and 
           [0009]      FIG. 2  is a flow chart of an embodiment of a computer-implemented method for monitoring the health of a mid-bed oxidant injection system coupled to an engine. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    One or more specific embodiments of the present invention 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. 
         [0011]    When introducing elements of various embodiments of the present invention, 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. 
         [0012]    The present disclosure is directed to systems and methods for monitoring the health of an oxidant injection system (e.g., mid-bed air injection system) coupled to a series of catalyst assemblies or catalytic converters (e.g., TWC assembly, ASC assembly, etc.) coupled to a combustion engine (e.g., reciprocating internal combustion engine or gas turbine engine). In particular, embodiments of the present disclosure include an aftertreatment system (e.g., exhaust treatment) system configured to couple to the combustion engine and to treat emissions (e.g., in the engine exhaust) from the combustion engine (e.g., NO X , HC, CO, etc.). The aftertreatment system may include catalyst based systems, chemical injection systems, or other types. In particular, the aftertreatment system may include a TWC assembly fluidly coupled (e.g., via a fluid conduit) to an ASC assembly and an oxidant injection system configured to inject an oxidant (e.g., air, O 2 , oxygen-enriched air, or oxygen-reduced air) into a fluid (e.g., exhaust treated by TWC assembly) within the fluid conduit. The disclosed embodiments include determining an oxidative state (e.g., O 2  storage) of the TWC assembly via one or more RF probes. If the combustion engine is operated under rich enough conditions, the O 2  storage of the TWC assembly should be zero or near zero. If the O 2  storage of the TWC assembly is zero or near zero, disclosed embodiments include measuring a temperature of the fluid prior to flowing into the ASC assembly but downstream of a location of oxidant injection by the oxidant injection system and adjusting the temperature of the fluid (e.g., adjacent the ASC assembly) so that the temperature is within a desired temperature operating range (e.g., 400-510° C.) to maximize catalytic activity within the ASC assembly. If the temperature of the fluid adjacent the ASC assembly is within the desired temperature operating range, disclosed embodiments include determining whether a concentration of NO X  in the fluid (e.g., treated by both the TWC assembly and the ASC assembly) exiting the ASC assembly is within desired limits (e.g., within a desired range or below a desired threshold). If the concentration of NO X  in the fluid exiting the ASC assembly is not within the desired limits, disclosed embodiments include determining an NH 3  concentration in the fluid within the ASC assembly and deciding which component of the exhaust aftertreatment system to perform diagnostics on based on the determined NH 3  concentration in the fluid within the ASC assembly. Monitoring the health of the oxidant injection system and/or exhaust aftertreatment system, via the disclosed techniques, enables operation of the combustion engine to remain within emissions compliance for an extended period of time. In addition, maintenance to components of the exhaust aftertreatment system may be minimized Further, the disclosed embodiments provide an on-board diagnostics capability. 
         [0013]    Turning now to the drawings and referring to  FIG. 1 , a schematic diagram of an exhaust treatment system  10  coupled to an engine  12  is illustrated. As described in detail below, the disclosed exhaust treatment system  10  monitors the health (e.g., ability to reduce emissions) of the exhaust treatment system  10  and/or components of the system (e.g., oxidant injection system  40 , ASC assembly  24 , etc.). 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 operate as a rich-burn engine. 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. 
         [0014]    The engine  12  is coupled to an engine control unit (e.g., controller)  14  that controls and monitors the operations of the engine  12 . For example, the engine control unit  14  (in conjunction with or separately from an oxidant injection control unit  44 ) controls and monitors the operations of the engine  12 . For example, the engine control unit regulates or adjusts an oxidant-fuel ratio (e.g., air-fuel ratio) of the engine  12  (e.g., via one or more air-fuel ratio (AFR) regulators coupled to the engine  12  such as a fuel system, carburetor, fuel injector, fuel pass regulator, any system including one or more of these, or any combination thereof). The AFR is the mass ratio of air to fuel. The engine control unit  14  includes processing circuitry (e.g., processor  16 ) and memory circuitry (e.g., memory  18 ). The processor  16  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  16  may include one or more reduced instruction set (RISC) processors or complex instruction set (CISC) processors. The processor  16  may execute instructions to carry out the operation of the engine  12  and/or aftertreatment system  10 . 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) such as the memory  18 . In certain embodiments, the memory  18  may be wholly or partially removable from the ECU  14 . 
         [0015]    During operation, the engine  12  generates combustion gases  20  used 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  20  subsequently exit the engine  12  as an exhaust  20 , which includes a variety of emissions (e.g., NO X , HC, CO, etc.). The exhaust treatment system  10  treats these emissions to generate milder emissions (carbon dioxide (CO 2 ), water, etc). As depicted, the exhaust treatment system  10  includes catalytic converters or catalyst assemblies, such as the first catalyst assembly  22  (e.g., TWC assembly) and the second catalyst assembly  24  (e.g., ASC assembly). In certain embodiments, the first and second catalyst assemblies  22 ,  24  may be housed within a single housing. In embodiments that include the TWC assembly  22  and the ASC assembly  24 , 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 of less than 1.0 such as approximately 0.999, 0.998, 0.997, 0.996, 0.995, 0.994, 0.993, 0.980, 0.970, 0.960, 0.950, or any other value less than 1.0) to maximize the catalytic activity in both the TWC assembly  22  and the ASC assembly  24 . In certain embodiments, the engine  12  may be operated at a rich enough λ or AFR to enable the O 2  storage of the TWC assembly  22  be zero or near zero to enable monitoring of the operative state of other components (e.g., oxidant injection system  40 , ASC assembly  24 , etc.) of the exhaust treatment system  10 . The TWC assembly  22 , via its catalytic activity, reduces NO X  via multiple reactions. For example, NO may be reduced via CO to generate N 2  and CO 2 , NO may be reduced via H 2  to generate NH 3  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  22  also oxidizes CO to CO 2 , and oxidizes unburnt HC to CO 2  and water. A by-product of the reduction of NO in the TWC assembly  22  can be the emission of NH 3  (e.g., due to the reaction of NO and H 2 ). In certain embodiments, instead of the TWC assembly  22 , any catalytic converter that reduces NO may be utilized. The ASC assembly  24 , via its catalytic activity (e.g., at zeolite sites), selectively reduces the NH 3  to N 2 . In certain embodiments, the ASC assembly  24  also oxidizes CO to CO 2 . The ASC assembly  24  includes a catalyst operating window between upper and lower temperature thresholds, such as between approximately 400-510° C. The operating window represents a temperature where all of the NH 3  may be converted to N 2  and not oxidized to NO X . 
         [0016]    The TWC assembly  22  includes an inlet  26  to receive the exhaust  20  from the engine  12  and an outlet  28  to discharge a fluid  30  (e.g., treated engine exhaust). The ASC assembly  24  includes an inlet  32  to receive the fluid  30  (e.g., including the treated engine exhaust and/or injected oxidant (e.g., air, O 2 , oxygen-enriched air, or oxygen-reduced air) and an outlet  34  to discharge an additionally treated fluid  36 . A fluid conduit  38  is disposed between the TWC assembly  22  and the ASC assembly  24 . Specifically, the fluid conduit  38  is coupled to the outlet  28  of the TWC assembly  22  and the inlet  32  of the ASC assembly  24 , thus, coupling both assemblies  22 ,  24  to enable fluid communication between them. The fluid conduit  38  enables the flow or transfer of the fluid  30  from the TWC assembly  22  to the ASC assembly  24 . 
         [0017]    As depicted, an oxidant injection system  40  (e.g., mid-bed air injection system) is coupled to the fluid conduit  38 . The oxidant injection system  40  injects oxidant (e.g., air, O 2 , oxygen-enriched air, or oxygen-reduced air) into the fluid  30  within the fluid conduit  38  at a point or location  42  downstream of the outlet  28  of the TWC assembly  22  and upstream of the inlet  32  of the ASC assembly  24 . In certain embodiments, oxidant injection via the oxidant injection system  40  may be actively driven via a pump or injector. In other embodiments, oxidant injection via the oxidant injection system  40  may occur via passive entrainment. The oxidant injection system  40  injects sufficient oxidant in the fluid  30  to enable the catalytic activity in the ASC assembly  24 . In addition, the injection of oxidant, via the oxidant injection system  40 , regulates a temperature of the fluid  30  entering the ASC assembly  24 . 
         [0018]    An oxidant injection control unit  44  (e.g., processor-based controller) controls an amount of oxidant injected by the oxidant injection system  40  into the fluid  30  prior to flowing into the inlet  32  of the ASC assembly  24 . The oxidant injection control unit  44  includes processing circuitry (e.g., processor  46 ) and memory circuitry (e.g., memory  48 ). The processor  46  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  46  may include one or more reduced instruction set (RISC) processors or complex instruction set (CISC) processors. The processor  46  may execute instructions to control the amount of oxidant injected by the oxidant injection system  40 . 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) such as the memory  48 . In certain embodiments, the memory  48  may be wholly or partially removable from the oxidant injection control unit  44 . The memory  48  may store various LUTs. The memory  28  may store a LUT listing corresponding voltages (e.g., similar to voltage readings received from the RF probes  52 ) to O 2  storage values representing O 2  storage ratio in the TWC assembly  22 . The memory  28  may also store a LUT listing corresponding voltages (e.g., similar to voltage readings received from the RF probes  58 ) to NH 3  concentration values. Also, the memory  28  may store a number of thresholds or ranges. For example, the memory  28  may store thresholds and/or ranges for various emissions or constituents (e.g., NO X , NH 3 , etc.) representing desired limits. The memory  28  may also store a temperature operating range (e.g., catalyst operating window) for the ASC catalyst assembly  24 . As depicted, the oxidant injection control unit  44  is coupled to the engine control unit  14 . In certain embodiments, the engine control unit  14  and/or the oxidant injection control unit  44  may form a single control unit. In certain embodiments, the engine control unit  14  and the oxidant injection control unit  44  may perform some or all of the same functions with regard to the engine  12  and/or the oxidant injection system  40 . 
         [0019]    The exhaust treatment system  10  includes a plurality of transducers or sensors  50  disposed throughout the system  10  to measure systems parameters (e.g., O 2  storage in the TWC assembly  22 , NH 3  concentration in the fluid  30  within the ASC assembly  24 , emissions concentration (e.g., NO X ) in exhaust gases treated by the exhaust treatment system  10 ) and to provide feedback (e.g., via signals representative of the system parameters) to the oxidant injection control unit  44  and/or the engine control unit  14 . For example, one or more RF probes  52  may be disposed within and/or coupled to the TWC assembly  22  to measure the oxidative state (e.g., O 2  storage) of the TWC assembly  22 . In certain embodiments, the O 2  storage measurement from the RF probes  52  may take the form of a voltage reading. In certain embodiments, the voltage reading may be converted to an O 2  storage value by the control units  14 ,  44  (e.g., utilizing a LUT stored in memory  18 ,  48 ). The sensors  50  also include one temperature sensors or transducers  54  disposed downstream of the location  42  of oxidant injection in the fluid conduit  38  and upstream of the inlet  32  of the ASC assembly  24 . The sensors  50  further include one or more RF probes  56  disposed within and/or coupled to the ASC assembly  24  to measure a concentration of NH 3  within the fluid  30  within the ASC assembly  24 . In certain embodiments, the NH 3  concentration measurement from the RF probes  56  may take the form of a voltage reading. In certain embodiments, the voltage reading may be converted to an NH 3  concentration value by the control units  14 ,  44  (e.g., utilizing a LUT stored in memory  18 ,  48 ). The sensors  50  still further include one or more NO X  sensors  58  disposed adjacent or proximal (e.g., downstream of) the outlet  34  of the ASC assembly  24 . The one or more NO X  sensors  58  measure a concentration of NO X  (e.g., in ppm) in the fluid  36 . 
         [0020]    Based at least on feedback from the sensors  50 , the oxidant injection control unit  44  and/or the engine control unit  14  monitor the state or health (e.g., ability to reduce emissions) of the exhaust treatment system  10  and/or components of the system  10  (e.g., oxidant injection system  40 , ASC assembly  24 , etc.). For example, the control units  14 ,  44  may run the engine  12  at a rich λ or AFR and utilize the measured O 2  storage (e.g., via RF probe  52 ) of the TWC assembly  22  to determine the oxidative state of the TWC assembly  22 . If the combustion engine is operated under rich enough conditions, the O 2  storage of the TWC assembly  22  should be zero or near zero. If the O 2  storage of the TWC assembly  22  is not near zero, the control units  14 ,  44  may adjust the operation of the engine  12  to run at a slightly richer λ (e.g., by decreasing the λ by 0.001, 0.002, etc.) or the AFR. If the O 2  storage of the TWC assembly  22  is zero or near zero, the control units  14 ,  44  may measure a temperature of the fluid  30  (e.g., via temperature sensor  54 ) prior to flowing into the ASC assembly  24  but downstream of the location  42  of oxidant injection by the oxidant injection system  40 . The control units  14 ,  44  may then determine if the temperature of the fluid  30  adjacent the inlet  32  of the ASC assembly  24  is within a desired temperature operating range or catalyst operating window (e.g., 400-510° C.) to maximize catalytic activity within the ASC assembly  24 . If the temperature of the fluid  30  adjacent the inlet  32  of the ASC assembly  24  is not within the desired temperature operating range, the control units  14 ,  44  may direct the oxidant injection system  40  to inject oxidant or additional oxidant into the fluid  30  to bring the temperature of the fluid  30  to within the desired temperature operating range. If the temperature of the fluid  30  adjacent the ASC assembly  24  is within the desired temperature operating range, the control units  14 ,  44  may measure a concentration of NO X  within the fluid  36  exiting the ASC assembly  24 . The control units  14 ,  44  may then determine whether the concentration of NO X  (e.g., via NO X  sensor  58 ) in the fluid  36  exiting the ASC assembly  24  is within desired limits (e.g., within a desired range or below a desired threshold). If the concentration of NO X  in the fluid  36  exiting the ASC assembly  24  is within the desired limits, the control units  14 ,  44  may determine the exhaust treatment system is operating at an acceptable state (e.g., complying with desired emissions limits) and that diagnostics do not need to perform on any component of the exhaust treatment system  10 . 
         [0021]    If the concentration of NO X  in the fluid  36  exiting the ASC assembly  24  is not within the desired limits, the control units  14 ,  44  may measure a concentration of NH 3  (e.g., via RF probe  56 ) within the fluid  30  within the ASC assembly  24 . The control units  14 ,  44  may then compare the concentration of NO X  in the fluid  36  exiting the ASC assembly  24  to a reference value to determine which component of the exhaust treatment system  10  to perform diagnostics on. If the concentration of NH 3  in the fluid  36  within the ASC assembly  24  equals the reference value, the control units  14 ,  44  may determine to perform diagnostics on the NO X  sensor  58 . If the concentration of NH 3  in the fluid  36  within the ASC assembly  24  does not equal the reference value, the control units  14 ,  44  may determine to perform diagnostics on the ASC assembly  24 . In certain embodiments, the control units  14 ,  44  may provide a user perceptible indication (e.g., textual, visual, audible, etc.) of any system parameter not within a desired limit or range, an indication that diagnostics were performed on a specific component of the exhaust treatment system  10 , and/or the results of the diagnostics. These series of analyses enable the determination of the state of the exhaust treatment system  10  and/or oxidant injection system  40 . Monitoring the health of the oxidant injection system  40  and/or exhaust treatment system  10 , via the disclosed techniques, enables operation of the engine  12  to remain within emissions compliance for an extended period of time. In addition, maintenance to components of the exhaust treatment system  10  may be minimized Further, the disclosed embodiments provide an on-board diagnostics capability. 
         [0022]      FIG. 2  is a flow chart of an embodiment of a computer-implemented method  60  for monitoring the health of a mid-bed oxidant injection system (e.g., oxidant injection system  40 ) coupled to the engine  12 . All or some of the steps of the method  60  may be executed by the control unit  14  and/or  44  (e.g., utilizing the processor  16  and/or  46  to execute programs and access data stored on the memory  18  and/or  48 ). In addition, one or more of these steps may be performed simultaneously with other steps. The method  60  includes operating or running the engine  12  at a rich λ (e.g., λ value of less than 1.0 such as approximately 0.999, 0.998, 0.997, 0.996, 0.995, 0.994, 0.993, 0.980, 0.970, 0.960, 0.950, or any other value less than 1.0) or AFR (block  62 ). The method  60  also includes measuring O 2  loading of the TWC assembly  22  via the one or more RF probes  52  (block  64 ) to determine the oxidative state of the TWC assembly  22 . Upon measuring the O 2  loading, the method  60  includes determining whether the measured O 2  storage is approximately zero (block  66 ). If the measured O 2  storage is not approximately zero, the method  60  further includes operating or running the engine  12  at a slightly richer λ (e.g., decreasing λ by 0.001, 0.002, etc.) or AFR (block  68 ) followed by measuring the O 2  loading of the TWC assembly  22  again (block  64 ). This sequence of steps (blocks  64 - 68 ) may be repeated until the measured O 2  storage of the TWC assembly  22  is approximately zero. 
         [0023]    Having the measured O 2  storage of the TWC assembly  22  at approximately zero enables the analysis of the state of rest of the exhaust treatment system  10 , in particular, those components associated with determining the health or state of the oxidant injection system  40  (e.g., oxidant injection system  40 , ASC assembly  24 , NO X  sensor  58 , etc.). If the measured O 2  storage of the TWC assembly  22  is at approximately zero, the method  60  includes measuring the temperature of the fluid  30  downstream of the location  42  of oxidant injection in the fluid conduit  38  and upstream of the inlet  32  of the ASC assembly  24  via the one or more temperature sensors  54  (block  70 ). The method  60  also includes determining whether the temperature of the fluid  30  flowing into the ASC assembly  24 , T ASC , is within a desired temperature operating range (e.g., a catalyst operating window) that promotes maximum catalytic activity within the ASC assembly  24  (block  72 ). The temperature operating range or catalyst operating window may include an upper temperature threshold, T UL  (e.g., 400° C.), and a lower temperature threshold, T LL  (e.g., 510° C.). The temperature operating range may be 400 to 510° C., 400 to 455° C., 455 to 510° C., 400 to 430° C., 480 to 510° C., 440 to 480° C., and all subranges therebetween. If T ASC  is not between T UL  and T LL , the method  60  includes adding (i.e., injecting) oxidant or additional oxidant into the fluid conduit  38  at the location  42  to adjust T ASC  so that it falls between T UL  and T LL  (block  74 ). This sequence of steps (blocks  70 - 74 ) may be repeated until T ASC  is between T UL  and T LL . 
         [0024]    If T ASC  is between T UL  and T LL , the method  60  includes measuring the concentration of NO X  in the fluid  36  exiting the ASC assembly  24 , NO X, ASC, OUT , via the one or more NO X  sensors  58  (block  76 ). The method  60  also includes determining whether the NO X, ASC, OUT  falls within desired limits (e.g., below a desired NO X  threshold and/or within a desired NO X  range) (block  78 ). In certain embodiments, these limits may be empirically determined and stored in the memory  18 ,  48 . This step (block  78 ) determines whether to perform diagnostics on a component of the exhaust treatment system. If the NO X, ASC, OUT  falls within the desired limits, the method  60  includes looping thru the method  60  from the beginning (block  80 ). 
         [0025]    If the NO X, ASC, OUT  falls without the desired limits, the method  60  includes measuring a concentration of NH 3  within the fluid  30  within the ASC assembly  24 , NH 3 ASC , via the one or more RF probes  56  (block  82 ). The method  60  also includes determining whether NH 3 ASC  equals a reference value (block  84 ). This step (block  84 ) determines which component of the exhaust treatment system  10  to perform diagnostics on. If NH 3 ASC  does not equal the reference value, the method  60  includes performing diagnostics on the ASC assembly  24  (block  86 ). If NH 3 ASC  does equal the reference value, the method  60  includes performing diagnostics on the NO X  sensor  58  (block  88 ). In certain embodiments, the method  60  may provide a user perceptible indication (e.g., textual, visual, audible, etc.) of any system parameter not within a desired limit or range, an indication that diagnostics were performed on a specific component of the exhaust treatment system  10 , and/or the results of the diagnostics. 
         [0026]    Technical effects of the disclosed embodiments include providing (e.g., computer implemented) systems and methods monitoring the health of the oxidant injection system  40  (e.g., mid-bed air injection system) coupled to a series of catalyst assemblies or catalytic converters (e.g., TWC assembly  22  and ASC assembly  24 ) coupled to the engine  12 . In particular, embodiments include analyzing a plurality of system parameters (e.g., O 2  loading of the TWC assembly  22 , T ASC , and NO X, ASC, OUT ) acquired from a plurality of sensors (e.g., RF probe  52 , temperature sensor  54 , and NO X  sensor  58 ) to determine whether or not to perform diagnostics on a component of the exhaust treatment system  10 . In addition, embodiments include analyzing NH 3 ASC  within the ASC assembly  24  to determine which component of the exhaust treatment system  10  to perform diagnostics on. Monitoring the health of the oxidant injection system  40  and/or exhaust treatment system  10 , via the disclosed embodiments, enables operation of the engine  12  to remain within emissions compliance for an extended period of time. In addition, maintenance to components of the exhaust treatment system  10  may be minimized Further, the disclosed embodiments provide an on-board diagnostics capability. 
         [0027]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention 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 languages of the claims.