WO2015095332A1

WO2015095332A1 - Techniques for control of an scr aftertreatment system in response to nh3 slip conditions

Info

Publication number: WO2015095332A1
Application number: PCT/US2014/070868
Authority: WO
Inventors: Ming-Feng Hsieh; Michael Haas; Phanindra V. GARIMELLA
Original assignee: Cummins Inc.
Priority date: 2013-12-18
Filing date: 2014-12-17
Publication date: 2015-06-25

Abstract

A selective catalytic reduction (SCR) catalyst disposed in an exhaust gas stream of an internal combustion engine. A reductant introducer is operationally coupled to the exhaust gas stream at a position upstream of the SCR catalyst, and a NOx sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst. A system and method is disclosed for operating the system to determine an NH₃ slip condition and/or to decouple NOx-NH₃ from the output of the NOx sensor under NH₃ slip conditions to provide control of the reductant introduction amount.

Description

TECHNIQUES FOR CONTROL OF AN SCR AFTERTREATMENT SYSTEM IN RESPONSE TO NH₃ SLIP CONDITIONS

Cross-Reference to Related Application:

[0001] The present application claims the benefit of the filing date of U.S. Provisional Application No. 61/917,490 filed on December 18, 2013, which is incorporated herein by reference in its entirety.

BACKGROUND

[0002] The technical field of the present disclosure generally relates to control of selective catalytic reduction (SCR) aftertreatment systems for internal combustion engines.

[0003] SCR systems present several control challenges for internal

combustion engine applications, including for mobile applications. SCR systems include a reduction catalyst and a reductant, such as urea or ammonia. An injector provides the reductant to the exhaust stream at a position upstream of the reduction catalyst, and the reductant enters the gas phase of the exhaust stream as ammonia. A delay sometimes occurs between the introduction of the reductant and the availability of the reductant product. For example, injected particles of the reductant may need to evaporate into the exhaust stream, hydrolyze from urea to ammonia, and/or thoroughly mix into the exhaust stream for general availability across the reduction catalyst.

Additionally, the reductant catalyst may include some ammonia storage capacity. Storage capacity can complicate the controls process, for example by creating additional controls targets (e.g. a storage target), by releasing ammonia unexpectedly (e.g. when a system condition causes a decrease in storage capacity), and/or by adsorbing some of the injected ammonia in an early part of the catalyst thereby reducing the availability of ammonia at a rear portion of the catalyst during catalyst filling operating periods. [0004] The challenges presented by presently available SCR systems are exacerbated by the transient nature of mobile applications. The engine load and speed profile varies during operations in a manner that is determined by an operator and generally not known in advance to the SCR control system. Additionally, available feedback control systems suffer from several

drawbacks. For example, the concentration of ammonia is difficult to determine in real time. Commercially reasonable NOx sensors can suffer from cross-sensitivity with ammonia, complicating the determination of the amount of NOx present in the exhaust gas outlet from the SCR catalyst. Further, ammonia is generally an undesirable constituent of the final exhaust emissions, and ammonia that is emitted from or "slips" from the catalyst represents ineffectively utilized reductant that increases operating costs.

[0005] Therefore it is desirable to operate at a very low or zero ammonia concentration at the outlet of SCR catalyst outlet. However, NOx sensors that are cross-sensitive to ammonia hinder the ability to provide a reliable estimate of the amount of ammonia slip, reducing the effectiveness of feedback SCR control in providing an optimal amount of ammonia to the exhaust system and potentially creating false indications of an SCR and/or reductant injector fault conditions. As a result, further contributions in the detection, determination, and control of ammonia slip conditions in SCR systems are needed.

SUMMARY

[0006] One embodiment is a unique method for controlling an SCR

aftertreatment system using NOx sensor output at an outlet of, or downstream of, an outlet of an SCR catalyst that receives exhaust gas from an internal combustion engine. Other embodiments include unique methods, systems, and apparatus to determine an ammonia (NH3) slip condition, to decouple NOx and NH3 amounts from the NOx sensor output, to estimate SCR catalyst efficiency from the NOx variation upstream and downstream of the SCR catalyst under transient operating conditions, and/or to operate an SCR aftertreatment system in response to the estimate of SCR catalyst efficiency and/or to an estimate of an NH3 slip amount determined from NOx and NH3 amounts decoupled from a NOx sensor output.

[0007] This summary is provided to introduce a selection of concepts that are further described below in the illustrative embodiments. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a schematic diagram of a system for operating an SCR catalyst to reduce NOx emissions of an engine.

[0009] Fig. 2 is a schematic of one embodiment of a controller apparatus of the system of Fig. 1.

[0010] Figs. 3A-3D are a graphical illustrations associated with a procedure for detecting ammonia slip from an SCR catalyst.

[0011] Figs. 4A-4B are graphical illustrations associated with a procedure for decoupling NH3 and NO_x from a NOx sensor output downstream of an SCR catalyst.

[0012] Fig. 5 is a graphical illustration associated with a procedure for decoupling NH3 and NOx amounts from a NOx sensor output downstream of an SCR catalyst.

[0013] Fig. 6 is a flow diagram of one embodiment of a procedure for determining a present NH3 amount.

[0014] Fig. 7 is a flow diagram of another embodiment of a procedure for determining a present NH3 amount.

[0015] Fig. 8 is a flow diagram of another embodiment of a procedure for determining a present NH3 amount.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0016] For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, any alterations and further modifications in the illustrated embodiments, and any further applications of the principles of the invention as illustrated therein as would normally occur to one skilled in the art to which the invention relates are contemplated herein.

[0017] Referencing Fig. 1, a system 10 includes an exhaust gas stream 12 that flows from operation of an internal combustion engine 14, the exhaust gas stream 12 including an amount of NOx. The system 10 includes a first NOx sensor 16 to provide a first output corresponding to a measurement or other suitable indication of an engine-out NOx amount. The engine-out NOx amount may be determined by a signal indicative of the engine out NOx amount, or alternatively be determined virtually by a model in response to engine and exhaust operating parameters, or by a sensor positioned at a different location in the system 10. As used herein, and unless specified to the contrary, first NOx sensor 16 covers both arrangements, i.e. a physical NOx sensor or a virtual NOx sensor. The system 10 includes an upstream

aftertreatment component 18 that may be an oxidation catalyst, a particulate filter, or both. In certain embodiments, the system 10 does not include any oxidation catalyst and/or particulate filter.

[0018] The system 10 further includes a reductant introducer 20 fluidly coupled to a reductant source 22. In one embodiment, the reductant

introducer 20 is a reductant injector that includes an actuator that is configured to operate reductant injector in response to one or more reductant introduction control commands to inject reductant, such as urea, NEU, or other NH.3 producing constituent, into exhaust gas stream 12. Other embodiments contemplate a reductant introducer 20 that is a tube, fogger, or any suitable reductant introduction device. The system 10 also includes an SCR catalyst 24 downstream of reductant introducer 20 and a second NOx sensor 30

downstream of SCR catalyst 24.

[0019] The system 10 may include an optional NH3 oxidation (AMOX) catalyst 26, provided to oxidize at least a portion of the slipping NH3 from the SCR catalyst 24 during at least some operating conditions. The AMOX catalyst 26 may be presented as a discrete catalytic element, in the same or a different housing from the SCR catalyst 24, and may be included as a washcoat on a portion (specifically a rear portion) of the SCR catalyst 24. The SCR catalyst 24 may include one or more catalyst elements located in the same or a different housing. Additional SCR catalyst elements may be present, and are schematically included with the SCR catalyst 24 herein. In addition, certain embodiments contemplate that the AMOX catalyst 26 can be completely removed or omitted from system 10 in view of the systems and techniques disclosed herein to mitigate or eliminate NH3 slip.

[0020] The system 10 further includes a controller 28. In certain

embodiments, the controller 28 forms a portion of a processing subsystem including one or more computing devices having memory, processing, and communication hardware. The controller 28 may be a single device or a distributed device, and the functions of the controller 28 may be performed by hardware or by instructions encoded on computer readable medium. The controller 28 may be included within, partially included within, or completely separated from an engine controller (not shown). The controller 28 is in communication with any sensor or actuator throughout the system 28, including through direct communication, communication over a datalink, and/or through communication with other controllers or portions of the processing subsystem that provide sensor and/or actuator information to the controller 28.

[0021] In certain embodiments, the controller 28 includes one or more modules structured to functionally execute the operations of the controller. The description herein including modules emphasizes the structural

independence of the aspects of the controller, and illustrates one grouping of operations and responsibilities of the controller. Other groupings that execute similar overall operations are understood within the scope of the present application. Modules may be implemented in hardware and/or as computer instructions on a non-transient computer readable storage medium, and modules may be distributed across various hardware or computer based components.

[0022] Example and non-limiting controller and/or module implementation elements include sensors providing any value determined herein, sensors providing any value that is a precursor to a value determined herein, datalink and/or network hardware including communication chips, oscillating crystals, communication links, cables, twisted pair wiring, coaxial wiring, shielded wiring, transmitters, receivers, and/or transceivers, logic circuits, hard-wired logic circuits, reconfigurable logic circuits in a particular non-transient state configured according to the module specification, any actuator including at least an electrical, hydraulic, or pneumatic actuator, a solenoid, an op-amp, analog control elements (springs, filters, integrators, adders, dividers, gain elements), and/or digital control elements.

[0023] The listing herein of specific implementation elements is not limiting, and any implementation element for any controller described herein that would be understood by one of skill in the art is contemplated herein. The controllers herein, once the operations are described, are capable of numerous hardware and/or computer based implementations, many of the specific implementations of which involve mechanical steps for one of skill in the art having the benefit of the disclosures herein and the understanding of the operations of the controllers provided by the present disclosure.

[0024] One of skill in the art, having the benefit of the disclosures herein, will recognize that the controllers, control systems and control methods disclosed herein are structured to perform operations that improve various technologies and provide improvements in various technological fields. Without limitation, example and non-limiting technology improvements include improvements in emissions performance, aftertreatment system performance, and improved durability of exhaust system components for internal combustion engines. Without limitation, example and non-limiting technological fields that are improved include the technological fields of internal combustion engines, exhaust aftertreatment systems, and related apparatuses and systems as well as vehicles including the same.

[0025] Certain operations described herein include operations to interpret or determine one or more parameters. Interpreting or determining, as utilized herein, includes receiving values by any method known in the art, including at least receiving values from a datalink or network communication, receiving an electronic signal (e.g. a voltage, frequency, current, or PWM signal) indicative of the value, receiving a software parameter indicative of the value, reading the value from a memory location on a non-transient computer readable storage medium, receiving the value as a run-time parameter by any means known in the art, and/or by receiving a value by which the interpreted or determined parameter can be calculated, and/or by referencing a default value that is interpreted or determined to be the parameter value.

[0026] An exemplary system 10 includes controller 28, such as controller apparatus 80 in Fig. 2, configured for execution of control algorithms to determine a reductant introduction command 86 providing a reductant introduction amount upstream of SCR catalyst 24, which is disposed in exhaust gas stream 12 produced by operation of internal combustion engine 14. The exhaust gas stream 12 passes through SCR catalyst 24, and reductant introducer 20 is operationally coupled to the exhaust gas stream 12 at a position upstream of the SCR catalyst 24 to provide the reductant introduction amount in response to the reductant introduction command 86. The system 10 also includes first NO_x sensor 16 at a position upstream of the SCR catalyst 24 and second NOx sensor 30 coupled to the exhaust gas stream 14 downstream of the SCR catalyst 24. First NOx sensor 16 is operable to provide one or more outputs 82 indicative of the engine-out NOx amount to controller apparatus 80, and second NOx sensor 30 is operable to provide one or more outputs 84 indicative of the SCR outlet NOx amount to controller apparatus 80.

[0027] Controller apparatus 80 includes an N¾ slip detection module 88 structured to determine a presence or absence of an NH3 slip mode 94. The presence of NH₃ slip mode 94 indicates that NH3 is present in the exhaust gas flow downstream of SCR catalyst 24, which can negatively impact the reduction of NOx in exhaust gas stream 12 if the NH3 slip conditions are not accounted for in the amount of reductant that is injected into the exhaust gas stream 12. Controller apparatus 80 also includes a NOX-NH3 decoupling module 90 structured to determine a present NH3 amount 96 downstream of SCR catalyst in response to determining that NH3 slip mode 94 is present. The present NH3 amount 96, as discussed further below, can be determined from the outputs of first and second NOx sensors 16, 30 without

implementation of a separate NH3 sensor. Controller apparatus 80 further includes a reductant command module 92 structured to determine reductant introduction command 86 in response to the present NH3 amount 96.

[0028] The NH3 slip detection module 88 of controller apparatus 80 is structured to operate an NH3 slip detection algorithm to determine an NH3 slip condition from SCR catalyst 24 based on the outputs from first and second NOx sensors 16, 30. The NOX-NH3 decoupling module 90 of controller apparatus 80 is structured to operating a ΝΟχ-Ν¾ decoupling algorithm to determine the present NH3 amount from an output of at least the second NOx sensor 30 when SCR catalyst 24 is operating in an NH3 slip mode.

[0029] Referencing Figs. 3A-3D, an exemplary NH3 slip detection algorithm executable by NH3 slip detection module 88 is illustrated, with a perturbation or pulsing operation 102 that superposes a pulse of an excess reductant amount on a nominal reductant introduction command 100 provided to reductant introducer 20 by controller 28. The pulsing operation 102 may be performed, for example, at periodic intervals, in response to the occurrence of one or more predetermined operating conditions, in response to an NH3 slip detection request, or combinations of these. In addition, the pulse may be positive as shown in Fig. 3A, or a negative pulse. A negative pulse provides a reductant amount that is less than the commanded reductant introduction amount over the pulse period.

[0030] Figs. 3B-3D show possible responses of second NOx sensor 30 to pulsing operation 102. As shown in Fig. 3B, second NOx sensor 30 measures a corresponding pulse 104, indicating that a significant amount of NH3 slip occurs through SCR catalyst 24. In Fig. 3C no corresponding pulse is detected by output 106 of second NOx sensor 30 in response to pulsing operation 102, indicating little or no NH3 slip from SCR catalyst 24. In Fig. 3D, a pulse 108 in the output of second NOx sensor 30 is opposite in phase to pulsing operation 102, indicating there is no NH3 slip from SCR catalyst 24. The NH3 slip mode indication information can be used, for example, by controller 28, to evaluate the accuracy of an output of second NOx sensor 30 in measuring a NOx amount by providing an indication when cross-sensitivity of NOx sensor 30 to NH3 and NOx is in effect due to NH3 slip conditions. The indication of the presence or absence of an NH3 slip mode may also be used in subsequent control of the reductant introduction amount. In addition, the NH3 slip mode indication can be employed for control of the reductant introduction amounts to treat NOx emissions from engine 14 and on-board diagnostic operations. For example, when NH3 slip mode indications are not present, closed loop control of the reductant introduction amount can be used in response to outputs from second NOx sensor 30 since such outputs accurately indicate NOx slip amounts from SCR catalyst 24.

[0031] Referencing Figs. 4A-4B, another exemplary NH3 slip detection algorithm executable by NH3 slip detection module 88 is illustrated. In Figs. 4A-4B there is shown a first graphical representation 120 of the outputs of first NOx sensor 16 indicative of the engine out NOx amount and/or of the NOx amount at the inlet of SCR catalyst 24, designated as EONOx 122. There is also shown a second graphical representation of the outputs of second NOx sensor 30, which, when N¾ slip conditions are not present, are indicative of the system out NOx amount (also known as the tailpipe NOx amount, and/or the SCR outlet NOx amount) designated as SONOx 124 in Figs. 4A-4B.

Controller 28 is configured with a NH3 slip detection module 88 that

determines the presence and/or amount of N¾ amount at the tailpipe or outlet of SCR catalyst 24, designated as SONH3 126, and a corresponding prediction of an NH3 slip mode 94 as shown by an NH3 slip trigger 128.

[0032] In one embodiment, NH3 slip detection module 88 is structured to determine an NH3 slip mode 94 from SCR catalyst 24 is present by comparing pulse numbers of first NOx sensor 16 to pulse numbers of second NOx sensor 30 in a receding time period. A pulse as used herein is represented by a sign change of an output signal time derivative of the respective NOx sensor. An output signal from NOx sensor 30 including NH3 has much slower dynamics than a signal that measures NOx only. Accordingly, when NH3 slip is present, the number of sign changes in the output signal of second NOx sensor 30 will be substantially less than when NH3 slip conditions are not present. This relationship is shown in Fig. 4B, in which during NH3 slip conditions the number of pulses for EONOx 122 for first NOx sensor 16 is substantially greater than the number of pulses for SONOx 124 provided by second NOx sensor 30, indicating an NH3 slip mode 94 is present and an NH3 amount is represented in the outputs from second NOx sensor 30.

[0033] When NH3 is not present or is insubstantial, the number of sign changes in the output of second NOx sensor 30 is substantially the same as or greater than the number of sign changes of in the output of first NOx sensor 16. This relationship is shown in Fig. 4B in which the number of pulses for EONOx 122 for first NOx sensor 16 is the same or less than the number of pulses for SONOx 123 of second NOx sensor 30, indicating NH3 slip conditions are not present. Furthermore, in response to determining the presence of NH3 slip mode 94, NOx-N¾ decoupling module 90 can be structured with an algorithm to determine an estimate of the present N¾ amount 96 downstream of SCR catalyst 24 from the difference between the number of pulses from first NOx sensor 16 and the number of pulses from second NOx sensor 30. The present NH.3 amount 96 can be provided to, for example, reductant command module 92 to determine a reductant introduction command 86 that provides an amount of reductant that compensates for the NH3 amount downstream of the SCR catalyst 24 during NH3 slip conditions.

[0034] In certain embodiments, enable conditions are required to be met before NOX-NH3 decoupling module 90 executes the NOX-NH3 decoupling algorithm. For example, enablement of performance of the decoupling algorithm can be provided only when the number of pulses from first NOx sensor 16 over a predetermined time period exceeds a threshold number of pulses, such as during transient conditions. In other embodiment, other enable conditions are contemplated, including embodiments which do not include enablement conditions.

[0035] Referring to Fig. 4, another embodiment NOX-NH3 decoupling algorithm 200 operable by NOX-NH3 decoupling module 90 for determining a present NH₃ amount 96 and/or NH3 slip mode 94 is shown. In this

embodiment, NOX-NH3 decoupling module 90 is structured to execute algorithm 200 to decouple NOx and NH3 amounts from the output of second NOx sensor 30 by comparing the engine out NOx variation rate associated with outputs from first NOx sensor 16 with the system out NOx variation rate associated with outputs from second NOx sensor 30. It has been found that the ratio of the system out NOx variation rate to the engine out NOx variation rate has a linear relationship to the deNOx efficiency of SCR catalyst 24.

[0036] In one embodiment of the algorithm 200, the output of second NOx sensor 30 can be modeled by the following equation:

CNOxSen.SO- I1*CNOX,EO + CNH3,SO (Equation 1) where CNOxSen.so is the measurement of the system out (SO) NOx amount by second NOx sensor 30, and η is the efficiency of SCR catalyst 24 in removing NOx. For example, in Equation 1 η is 0.1 if SCR catalyst removes 90% of the engine out (EO) NOx. In addition, CNO_X.EO is the engine out NOx amount, and CNH3,SO is the system out N¾ amount. CNOX,EO can be determined from an output of a first NOx sensor 16 that is either a physical or virtual sensor.

[0037] In operation, the system out N¾ amount and SCR catalyst efficiency have very slow dynamics, thus the rate of change of CNH3,SO and η are close to zero in real time over most operating conditions. Thus, by taking the time derivative of Equation 1, an estimate of the SCR catalyst efficiency η, actual system out NOx amount CNO_X,SO, and the actual system out N¾ slip amount (present NH3 amount 96) CNH3,SO, can be determined from the following equations:

CNOxSen.so = ¾*CNOX,EO+¾*CNOX,EO+CNH3,SO = II*CNOX,EO (Equation 2) η = CNOxSen,so/CNOx,EO (Equation 3)

CNOX.SO = II*CNOX,EO (Equation 4)

CNH3,SO = II*CNOX,EO - CNOxSen.so (Equation 5)

[0038] In certain operating conditions, it can be assumed that:

CNOxSen,so = CNOX.SO + k*CNH3,so (Equation 6)

CNOX,EO = CNOxSen.EO (Equation 7)

¾SCR=(CNOX,EO-CNOX,SO)/CNOX,EO =(CN0xAmp,E0-CN0xAmp,S0)/CN0xAmp,E0 (Equation 8)

[0039] In Equations 6-8, k is a NOx sensor NH3 cross-sensitivity factor of the NOx sensor 30, CNOxSen.so is the output of second NOx sensor 30, CNOX,SO is the actual NOx amount at the outlet of or downstream of SCR catalyst 24, and CNH3,SO is the actual or present NH3 amount 96 at the outlet of or downstream of SCR catalyst 24. In addition, CNOX,EO is the actual engine out NOx amount, or the NOx amount at the inlet of SCR catalyst 24, which is assumed to be equal to the output of first NOx sensor 16, as represented by CNOxSen.Eo.

Furthermore, C]MOxAmp,Eo is the amplitude of the impulse measured by first NOx sensor 16 and CNOxAm_P,so is the amplitude of the impulse measured by second NOx sensor 30.

[0040] From Equations 6-8 and the assumptions discussed above, then:

CNOX,SO=CNOX,EO*(1-¾SCR) (Equation 9)

CNH3,so=(CNOxSen,so-CNOx,so)/k (Equation 10)

Therefore, an estimate of the actual system out NOx amount (CNOX,SO) can be determined from the output of NOx sensor 30 under N¾ slip conditions by determining the efficiency of SCR catalyst 24 from the ratio of the amplitude of the impulses of first and second NOx sensors 16, 30 under NOx variation conditions, such as during transients, and the NOx amount measured by first NOx sensor 16. In addition, an estimate of the system out present NH3 amount 96 (CNH3,SO) can be determined from the output of second NOx sensor 30, the estimate of the actual system out NOx amount, and the NH3 cross- sensitivity factor of NOx sensor 30.

[0041] The controller apparatus 80 includes reductant command module 92 structured to provide a reductant introduction command 86 in response to the system out present NH3 amount 96 and/or system out NOx amount determined above. The provided reductant introduction command 86 may be a reductant introduction amount that is found to minimize the NO_x

measurement of NOx sensor 30. Alternatively or additionally, the provided reductant introduction command 86 can include a reductant introduction amount that is offset for the N¾ amount to reduce or mitigate Ν¾ slip. The method and system further includes introducing an amount of the reductant in response to the reductant introduction command 86.

[0042] The controller 28 including controller apparatus 80 may be a part of a system including an SCR catalyst 24 with an SCR portion and a NOx sensor operationally coupled to an internal combustion engine exhaust at a position downstream of the SCR portion. The SCR portion includes any fraction of an SCR catalyst amount in the system, including the full SCR catalyst amount. The NOx sensor providing a NOx measurement, which may be an output value of the NOx sensor, a measurement of NOx in the exhaust stream, and/or an apparent measurement of NOx, for example combined with any apparent NOx due to cross-sensitivity to and presence of ammonia in the exhaust stream at the NOx sensor.

[0043] The controller 28 including controller apparatus 80 may further include a reductant targeting module that determines a reductant introduction amount in response to the determination of the actual system out NOx amount and actual system out or present N¾ amount from the output of NOx sensor 30. Example and non-limiting reductant introduction amounts include an ammonia to NOx ratio (ANR) target, an ANR offset to account for NH3 slip, and/or an ANR corresponding to a NO_x minimum value at NOx sensor 30.

[0044] The system further includes reductant introducer 20 responsive to the reductant introduction command 86 to provide a corresponding reductant introduction amount. The reductant introducer response to the reductant introduction command 86 may be any type of response understood in the art. Example and non-limiting responses of the reductant introducer include targeting the reductant introduction amount as an introduction amount, progressing toward introducing the reductant introduction amount (e.g.

through a feedforward and/or feedback controller), and/or providing the reductant introduction amount into a controller accepting other competing or limiting values for introduction or injection (e.g. ammonia slip limits, SCR catalyst storage limits, current conversion efficiency limits, etc.).

[0045] An exemplary procedure includes providing a selective catalytic reduction (SCR) catalyst disposed in an exhaust gas stream of an internal combustion engine, and a reductant introducer operationally coupled to the exhaust gas stream at a position upstream of the SCR catalyst. The method further includes providing a NO_x sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst, and operating an N¾-NOx decoupling controller to determine a present NH3 amount represented in the output of the system out NOx sensor. The controller can be further configured to detect an NH3 slip condition to enable determining the NH3 amount. The method further includes providing a reductant introduction command in response to a NOx amount and the NH3 amount measured by the system out NOx sensor to minimize a subsequent NOx amount and the NH3 amount, and introducing an amount of the reductant in response to the reductant introduction command.

[0046] Referring to Fig. 6, an exemplary procedure 300 includes an operation 302 of passing the exhaust gas stream through an SCR catalyst, such as SCR catalyst 24 of system 10. Procedure 300 further includes an operation 304 for operating an ΝΗ3-ΝΟΧ decoupling algorithm, such as discussed above, to determine a present NH3 amount downstream of the SCR catalyst in response to outputs from NOx sensors upstream and downstream of the SCR catalyst. Procedure 300 further includes an operation 306 that includes determining a reductant introduction command in response to the present NH3 amount, and an operation 308 to inject a reductant amount into the exhaust gas stream in response to the reductant introduction command. [0047] In one embodiment of procedure 300, operation 304 is preceded by an operation to determine an N¾ slip mode is present by comparing a number of pulses from a plurality of first outputs of the first NOx sensor upstream of the SCR catalyst to a number of pulses from a plurality of second outputs of the second NOx sensor downstream of the SCR catalyst to determine the present N¾ amount. The number of pulses can be determined by a number of sign changes of an output signal time derivative in respective ones of the plurality of first and second outputs of the first and second NOx sensors. In one embodiment, the number of pulses from the first NOx sensor can be required to exceed a predetermined number of pulses before initiating operation 304. In a further embodiment, the present N¾ amount increases as a function of an amount that the number of pulses from the first NOx sensor exceeds the number of pulses from the second NOx sensor.

[0048] In another embodiment of procedure 300, operation 304 is preceded by an operation to determine an NH3 slip condition is present by superposing a pulsed reductant amount on the reductant introduction command and subsequently detecting the pulsed reductant amount with the second NOx sensor. The pulsed reductant amount can be a positive or negative amount. Furthermore, an NH3 slip condition can be determined to be absent by superposing a pulsed reductant amount on the reductant introduction command and subsequently failing to detect the pulsed reductant amount with the second NOx sensor. Failing to detect the pulsed reductant amount includes detecting a pulse with the second NOx sensor that is opposite of the pulsed reductant amount.

[0049] Referring to Fig. 7, an exemplary procedure 400 includes an operation 402 of passing the exhaust gas stream through an SCR catalyst, such as SCR catalyst 24 of system 10. Procedure 400 further includes an operation 404 for outputting a first output from a first NOx sensor upstream of the SCR catalyst and an operation 406 for outputting a second output from a second NOx sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst. The second NOx sensor is cross-sensitive to N¾. Procedure 400 includes an operation 408 to determine a present NH3 amount downstream of the SCR catalyst from the number of pulses associated with the first and second outputs of the first and second NOx sensors over a time period.

Procedure 400 includes an operation 410 to inject an amount of reductant into the exhaust gas stream at a position upstream of the SCR catalyst in response to the present NH3 amount.

[0050] Referring to Fig. 8, an exemplary procedure 500 includes an operation 502 of passing the exhaust gas stream through an SCR catalyst, such as SCR catalyst 24 of system 10. Procedure 500 further includes an operation 504 for outputting a first output from a first NOx sensor upstream of the SCR catalyst and an operation 506 for outputting a second output from a second NOx sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst. The second NOx sensor is cross-sensitive to NH3. Procedure 500 includes an operation 508 to determine a present NH3 amount downstream of the SCR catalyst from the variation rates in the outputs of the first and second NOx sensors. Procedure 500 includes an operation 510 o inject an amount of reductant into the exhaust gas stream at a position upstream of the SCR catalyst in response to the present NH3 amount.

[0051] In one embodiment of procedure 500, the first variation rate

corresponds to an amplitude of an impulse of the first output from the first NOx sensor and the second variation rate corresponds to an amplitude of an impulse of the second output from the second NOx sensor. Procedure 500 further includes an operation of determining a NOx conversion efficiency estimate of the SCR catalyst by a ratio of the difference between the

amplitudes of the first and second variation rates to the amplitude of the first variation rate. In a further embodiment, procedure 500 includes an operation to determine a NOx amount downstream of the SCR catalyst from the product of a NOx amount measured by the first output of the first NOx sensor and one minus the NOx conversion efficiency estimate of the SCR catalyst, and the present NH3 amount is determined by dividing a difference between a NOx amount measured by the second output of the second NOx sensor and the NOx amount downstream of the SCR catalyst with a NH3 cross-sensitivity coefficient of the second NOx sensor.

[0052] As is evident from the figures and text presented above, a variety of aspects according to the present disclosure are contemplated. According to one aspect, a method includes passing an exhaust gas stream from operation of an internal combustion engine through a SCR catalyst disposed in the exhaust gas stream. A reductant introducer is operationally coupled to the exhaust gas stream at a position upstream of the SCR catalyst, a first NOx sensor is coupled to the exhaust gas stream at a position upstream of the SCR catalyst, and a second NOx sensor is coupled to the exhaust gas stream at a position downstream of the SCR catalyst. The second NOx sensor is cross-sensitive to NH3. The method further includes operating a NOx-N¾ decoupling algorithm to determine a present NH3 amount downstream of the SCR catalyst in response to a first output from the first NOx sensor and a second output from the second NOx sensor; determining a reductant introduction command in response to the present NH3 amount; and introducing an amount of the reductant into the exhaust gas stream upstream of the SCR catalyst with the reductant introducer in response to the reductant introduction command.

[0053] In one embodiment of the method, operating the NOX-NH3 decoupling algorithm includes comparing a number of pulses from a plurality of first outputs of the first NOx sensor to a number of pulses from a plurality of second outputs of the second NOx sensor over a time period to determine the present NH₃ amount. In a refinement of this embodiment, the number of pulses is determined by a number of sign changes of an output signal time derivative in respective ones of the plurality of first and second outputs of the first and second NOx sensors. In a further refinement, the present NH3 amount increases as a function of an amount that the number of pulses from the first NOx sensor exceeds the number of pulses from the second NOx sensor. In another refinement, the method includes determining the number of pulses from the first NOx sensor exceeds a predetermined number of pulses before operating the NOx-N¾ decoupling algorithm.

[0054] In another embodiment, the method includes determining an N¾ slip condition is present by superposing a pulsed reductant amount on the reductant introduction command and subsequently detecting the pulsed reductant amount with the second NOx sensor. In a refinement of this embodiment, the pulsed reductant amount is a positive amount. In another refinement, the pulsed reductant amount is a negative amount.

[0055] In another embodiment, the method includes determining an N¾ slip condition is absent by superposing a pulsed reductant amount on the reductant introduction command and subsequently failing to detect the pulsed reductant amount with the second NOx sensor. In a refinement of this embodiment, failing to detect the pulsed reductant amount includes detecting a pulse with the second NOx sensor that is opposite of the pulsed reductant amount.

[0056] In another embodiment of the method, operating the NOx-N¾ decoupling algorithm includes determining a first variation rate of the first output from the first NOx sensor and determining a second variation rate of the second output from the second NOx sensor to determine the present NH3 amount. In a refinement of this embodiment, the first variation rate corresponds to an amplitude of an impulse of the first output from the first NOx sensor and the second variation rate corresponds to an amplitude of an impulse of the second output from the second NOx sensor. In a further refinement, the method includes determining a NOx conversion efficiency estimate of the SCR catalyst by a ratio of the difference between the amplitudes of the first and second variation rates to the amplitude of the first variation rate. In still a further refinement, the method includes determining a NOx amount downstream of the SCR catalyst from the product of a NOx amount measured by the first output of the first NOx sensor and one minus the NOx conversion efficiency estimate of the SCR catalyst. In yet a further refinement, the present NH3 amount is determined by dividing a difference between a NOx amount measured by the second output of the second NOx sensor and the NOx amount downstream of the SCR catalyst with a NH3 cross- sensitivity coefficient of the second NOx sensor.

[0057] According to another aspect, a method includes passing an exhaust gas stream through a SCR catalyst; outputting a first output from a first NOx sensor upstream of the SCR catalyst; outputting a second output from a second NOx sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst where the second NOx sensor is cross-sensitive to NH3;

determining a present NH3 amount downstream of the SCR catalyst in response to a first variation rate of the first output from the first NOx sensor and a second variation rate of the second output from the second NOx sensor; and introducing an amount of reductant into the exhaust gas stream at a position upstream of the SCR catalyst in response to the present NH3 amount.

[0058] In one embodiment, the first variation rate corresponds to an

amplitude of an impulse of the first output from the first NOx sensor and the second variation rate corresponds to an amplitude of an impulse of the second output from the second NOx sensor. In a refinement of this embodiment, the method includes determining a NOx conversion efficiency estimate of the SCR catalyst by a ratio of the difference between the amplitudes of the first and second variation rates to the amplitude of the first variation rate. In a further refinement, the method includes determining a NOx amount downstream of the SCR catalyst from the product of a NOx amount measured by the first output of the first NOx sensor and one minus the NOx conversion efficiency estimate of the SCR catalyst. In a further refinement, the present NH3 amount is determined by dividing a difference between a NOx amount measured by the second output of the second NOx sensor and the NOx amount downstream of the SCR catalyst with a NH3 cross-sensitivity coefficient of the second NOx sensor. [0059] According to another aspect, a method includes passing an exhaust gas stream through a SCR catalyst; outputting a first output from a first NOx sensor upstream of the SCR catalyst; outputting a second output from a second NOx sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst where the second NOx sensor is cross-sensitive to N¾;

determining a present NH3 amount downstream of the SCR catalyst by comparing a number of pulses from a plurality of first outputs of the first NOx sensor to a number of pulses from a plurality of second outputs of the second NOx sensor over a time period; and introducing an amount of reductant into the exhaust gas stream at a position upstream of the SCR catalyst in response to the present NH3 amount.

[0060] In one embodiment, the number of pulses is determined by a number of sign changes of an output signal time derivative in respective ones of the plurality of first and second outputs of the first and second NOx sensors. In refinement of this embodiment, the present NH3 amount increases as a function of an amount that the number of pulses from the first NOx sensor exceeds the number of pulses from the second NOx sensor.

[0061] According to another aspect, a system includes an internal combustion engine and an exhaust system for receiving an exhaust gas stream produced by operation of the internal combustion engine. The system also includes a SCR catalyst disposed in the exhaust system and a reductant introducer coupled to the exhaust system at a position upstream of the SCR catalyst. The system further includes a first NOx sensor upstream of the SCR catalyst for providing a first output indicative of a first NOx amount in the exhaust gas stream upstream of the SCR catalyst, a second NOx sensor downstream of the SCR catalyst for providing a second output indicative of a second NOx amount in the exhaust gas stream downstream of the SCR catalyst, and a controller for receiving the first and second outputs. The controller is structured to determine a present NH3 amount downstream of the SCR catalyst in response to the first and second outputs. The controller is further structured to determine a reductant introduction amount for introduction by the reductant introducer in response to the present N¾ amount.

[0062] In one embodiment, the controller is structured to compare a number of pulses from a plurality of first outputs from the first NOx sensor to a number of pulses from a plurality of second outputs of the second NOx sensor over a time period to determine the present N¾ amount. In a refinement of this embodiment, the number of pulses is determined in response to a number of sign changes of an output signal time derivative in the plurality of outputs of the respective one of the first and second NOx sensors. In a further refinement, the present NH3 amount increases as a function of an amount that the number of pulses from the first NOx sensor exceeds the number of pulses from the second NOx sensor.

[0063] In another embodiment, the controller is further structured to determine an NH3 slip condition is present in response to the second output from the second NOx sensor indicating a pulsed reductant amount that is superposed on the reductant introduction amount. In a refinement of this embodiment, the pulsed reductant amount is one of a positive amount of reductant and a negative amount of reductant.

[0064] In another embodiment, the controller is further structured to determine an NH3 slip condition is absent in response to the second output from the second NOx sensor failing to indicate a pulsed reductant amount that is superposed on the reductant introduction amount. In yet another

embodiment, the controller is structured to determine an NH3 slip condition is absent in response to detecting a pulse with the second NOx sensor that is opposite of the pulsed reductant amount.

[0065] In a further embodiment, the controller is structured to determine a NOx conversion efficiency estimate of the SCR catalyst by a ratio

corresponding to a difference between a first amplitude of an impulse of the first output of the first NOx sensor and a second amplitude of an impulse of the second output of the second NOx sensor to the first amplitude. The controller is also structured to determine a NOx amount downstream of the SCR catalyst from the product of a NOx amount measured by the first output of the first NOx sensor and one minus the NOx conversion efficiency estimate of the SCR catalyst, and to determine the present NH3 amount by dividing a difference between a NOx amount measured by the second output of the second NOx sensor and the NOx amount downstream of the SCR catalyst with a NH3 cross-sensitivity coefficient of the second NOx sensor.

[0066] In a refinement of this embodiment, a first variation rate of the first output from the first NOx sensor corresponds to the first amplitude of the impulse of the first NOx sensor, and a second variation rate of the second output from the second NOx sensor corresponds to the second amplitude of the impulse of the second NOx sensor.

[0067] While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described. Those skilled in the art will appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.

[0068] In reading the claims, it is intended that when words such as "a," "an," "at least one," or "at least one portion" are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language "at least a portion" and/or "a portion" is used the item can include a portion and/or the entire item unless specifically stated to the contrary.

Claims

WHAT IS CLAIMED IS:

1. A method, comprising:

passing an exhaust gas stream from operation of an internal combustion engine through a selective catalytic reduction (SCR) catalyst disposed in the exhaust gas stream, wherein a reductant introducer is operationally coupled to the exhaust gas stream at a position upstream of the SCR catalyst, a first NOx sensor is coupled to the exhaust gas stream at a position upstream of the SCR catalyst, and a second NOx sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst, wherein the second NOx sensor is cross-sensitive to Nl¾;

operating a NOx-NHs decoupling algorithm to determine a present N¾ amount downstream of the SCR catalyst in response to a first output from the first NOx sensor and a second output from the second NOx sensor;

determining a reductant introduction command in response to the present N¾ amount; and

introducing an amount of the reductant into the exhaust gas stream upstream of the SCR catalyst with the reductant introducer in response to the reductant introduction command.

2. The method of claim 1 , wherein operating the NOx-NHs decoupling algorithm comprises comparing a number of pulses from a plurality of first outputs of the first NOx sensor to a number of pulses from a plurality of second outputs of the second NOx sensor over a time period to determine the present NHa amount.

3. The method of claim 2, wherein the number of pulses is determined by a number of sign changes of an output signal time derivative in respective ones of the plurality of first and second outputs of the first and second NOx

4. The method of claim 3, wherein the present N¾ amount increases as a function of an amount that the number of pulses from the first NOx sensor exceeds the number of pulses from the second NOx sensor.

5. The method of claim 2, further comprising determining the number of pulses from the first NOx sensor exceeds a predetermined number of pulses before operating the NOx-N¾ decoupling algorithm.

6. The method of claim 1, further comprising determining an Nl¾ slip condition is present by superposing a pulsed reductant amount on the reductant introduction command and subsequently detecting the pulsed reductant amount with the second NOx sensor,

7. The method of claim 6, wherein the pulsed reductant amount is a positive amount.

8. The method of claim 6, wherein the pulsed reductant amount is a negative amount.

9. The method of claim 1, further comprising determining an N¾ slip condition is absent by superposing a pulsed reductant amount on the reductant introduction command and subsequently failing to detect the pulsed reductant amount with the second NOx sensor.

10. The method of claim 9, wherein failing to detect the pulsed reductant amount includes detecting a pulse with the second NOx sensor that is opposite of the pulsed reductant amount.

11. The method of claim 1, wherein operating the NOx-N¾ decoupling algorithm includes determining a first variation rate of the first output from the first NOx sensor and determining a second variation rate of the second output from the second NOx sensor to determine the present NH,¾ amount.

12. The me hod of claim 11, wherein the first variation rate corresponds to an amplitude of an impulse of the first output from the first NOx sensor and the second variation rate corresponds to an amplitude of an impulse of the second output from the second NOx sensor.

13. The method of claim 12, further comprising determining a NOx conversion efficiency estimate of the SCR. cataly st by a ratio of the difference between the amplitudes of the first and second variation rates to the

amplitude of the first variation rate,

14. The method of claim 3, further comprising determining a NOx amount downstream of the SCR catalyst from the product of a NOx amount measured by the first output of the first NOx sensor and one minus the NOx conversion efficiency estimate of the SCR catalyst.

15. The method of claim 14, wherein the present N¾ amount is

determined by dividing a difference between a NOx amount measured by the second output of the second NOx sensor and the NOx amount downstream of the SCR catalyst with a N¾ cross-sensitivity coefficient of the second NOx sensor.

16. A method, comprising:

passing an exhaust gas stream through a selective catalytic reduction (SCR) catalyst;

outputting a first output from a first NOx sensor upstream of the SCR catalyst;

outputting a second output from a second NOx sensor coupled to the exhaust gas stream at a position downstream of the SCR catalyst, wherein the second NOx sensor is cross-sensitive to N¾;

determining a present NH.3 amount downstream of the SCR catalyst in response to a first variation rate of the first output from the first NOx sensor and a second variation rate of the second output from the second NOx sensor; and

introducing an amount of reductant into the exhaust gas stream at a position upstream of the SCR catalyst in response to the present N¾ amount.

17. The method of claim 16, wherein the first variation rate corresponds to an amplitude of an impulse of the first output from the first NOx sensor and the second variation rate corresponds to an amplitude of an impulse of the second output from the second NOx sensor.

18. The method of claim 17, further comprising determining a NOx conversion efficiency estimate of the SCR catalyst by a ratio of the difference between the amplitudes of the first and second variation rates to the

amplitude of the first variation rate.

19. The method of claim 18, further comprising determining a NO amount downstream of the SCR catalyst from the product of a NOx amount measured by the first output of the first NOx sensor and one minus the NOx conversion efficiency estimate of the SCR catalyst.

20. The method of claim 19, wherein the present N¾ amount is determined by dividing a difference between a NOx amount measured by the second output of the second NOx sensor and the NOx amount downstream of the SCR catalyst with a N¾ cross-sensitivity coefficient of the second NOx sensor.

21. A method, comprising:

outputting a first output from a first NOx sensor upstream of the SCR catalyst:

determining a present N¾ amount downstream of the SCR catalyst by comparing a number of pulses from a plurality of first outputs of the first NOx sensor to a number of pulses from a plurality of second outputs of the second NOx sensor over a time period: and

introducing an amount of reductant into the exhaust gas stream at a position upstream of the SCR catalyst in response to the present NH,¾ amount.

22, The method of claim 21, wherein the number of pulses is

determined by a number of sign changes of an output signal time derivative in respective ones of the plurality of first and second outputs of the first and second NOx sensors.

23, The method of claim 22, wherein the present N3¾ amount increases as a functio of an amount that the number of pulses from the first NOx sensor exceeds the number of pulses from the second NOx sensor.

24. A system, comprising:

an internal combustion engine;

an exhaust system for receiving an exhaust gas stream produced by operation of the interna] combustion eng ne;

a selective catalytic reduction (SCR) catalyst disposed in the exhaust system;

a reductant introducer coupled to the exhaust system at a position upstream of the SCR catalyst;

a first NOx sensor upstream of the SCR catalyst for providing a first output indicative of a first NOx amount in the exhaust gas stream upstream of the SCR catalyst;

a second NOx sensor downstream of the SCR catalyst for providing a. second output indicative of a second NOx amount in the exhaust gas stream downstream of the SCR catalyst; and

a controller for receiving the first and second outputs, wherein the controller is structured to determine a present Nf¾ amount downstream of the SCR catalyst in response to the first and second outputs, wherein the

controller is further structured to determine a reductant introduction amount for introduction by the reductant introducer in response to the present NI¾ amount.

25. The system of claim 24, wherein the controller is structured to compare a number of pulses from a plurality of first outputs from the first NOx sensor to a num ber of pulses from a plurality of second outputs of the second NOx sensor over a time period to determine the present NHs amount.

26. The system of claim 25, wherein the number of pulses is determined in response to a number of sign changes of an output signal time derivative in the plurality of outputs of the respective one of the first and second NOx sensors.

27. The system of claim 26, wherein the present NHs amount increases as a function of an amount that the number of pulses from the first NOx sensor exceeds the number of pulses from the second NOx sensor.

28. The system of claim 24, wherein the controller is further structured to determine an NHs slip condition is present in response to the second output from the second NOx sensor indicating a pulsed reductant amount that is superposed on the reductant introduction amount.

29. The system of claim 28, wherein the pulsed reductant amount is one of a positive amount of reductant and a negative a nun ml of reductant.

30. The system of claim 24, wherein the controller is further structured to determine an NHs slip condition is absent in response to the second output from the second NOx sensor failing to indicate a pulsed reductant amount that is superposed on the reductant introduction amount.

31. The system of claim 24, wherein the controller is structured to determine an NHs slip condition is absent in response to detecting a pulse with the second NOx sensor that is opposite of the pulsed reductant amount.

32. The system of claim 24, wherein the controller structured to:

determine a NOx conversion efficiency estimate of the SCR catalyst by a ratio corresponding to a difference between a first amplitude of an impulse of the first output of the first NOx sensor and a second amplitude of an impulse of the second output of the second NOx sensor to the first amplitude;

determine a NOx amount downstream of the SCR catalyst from the product of a NOx amount measured by the first output of the first NOx sensor and one minus the NOx conversion efficiency estimate of the SCR catalyst: and determine the present N¾ amount by dividing a difference between a NOx amount measured by the second output of the second NOx sensor and the NOx amount downstream of the SCR catalyst with a N¾ cross-sensitivity coefficient of the second NOx sensor.

33. The system of claim 32. wherein:

a first variation rate of the firs output from the first NOx sensor corresponds to the first amplitude of the impulse of the first NOx sensor; and a second variation rate of the second output from the second NOx sensor corresponds to the second amplitude of the impulse of the second NOx sensor.