Method for controlling a selective catalytic reduction aftertreatment system in response to an ammonia slip condition

Abstract

Method comprising: Operating an internal combustion engine (14) to produce an exhaust gas stream (12) in an exhaust system including a selective catalytic reduction (SCR) catalyst (24) disposed in the exhaust gas stream (12), the exhaust system comprising: a reductant injection valve (20) connected to the exhaust system at a position upstream of the SCR catalyst (24); a first NOx sensor (16) upstream of the SCR catalyst (24); a second NOx sensor (30) downstream of the SCR catalyst (24); Determining a DeNOx target efficiency (102) for the SCR catalyst (24); Injecting a first amount of reductant into the exhaust stream (12) in response to the target DeNOx efficiency (102); Determining a current DeNOx efficiency (104) of the SCR catalyst (24) by removing NOx from the exhaust stream (12) as a function of a first result from the first NOx sensor (16) and a second result from the second NOx sensor (30); Determining that the current DeNOx efficiency (104) is less than the target DeNOx efficiency (102); Detecting an NH3 slip condition; in response to determining the NH3 slip condition, determining a preliminary DeNOx target efficiency (106) for the SCR catalyst (24); and Injecting a reduced amount of reductant in response to the preliminary target DeNOx efficiency (106) while increasing the preliminary target DeNOx efficiency (106) toward the target DeNOx efficiency (102) until at least one adjustment condition of the DeNOx efficiency is met, wherein the reduced amount of reductant is less than the first amount of reductant and wherein the preliminary target DeNOx efficiency (106) is determined as a function of the current DeNOx efficiency (104).

Description

STATE OF THE ART

[0001] The technical field of the present disclosure generally relates to the control of selective catalytic reduction (SCR) aftertreatment systems for internal combustion engines.

[0002] SCR systems present various control challenges for applications such as internal combustion engines, including mobile applications. SCR systems comprise a reductant catalyst and a reductant, such as urea or ammonia. An injector delivers the reductant to the exhaust stream at a position upstream of the reductant catalyst, and the reductant enters the gas phase of the exhaust stream as ammonia. Sometimes there is a delay between the introduction of the reductant and the availability of the reductant product. For example, injected reductant particles must evaporate in the exhaust stream, hydrolyze from urea to ammonia, and / or be thoroughly mixed in the exhaust stream for general availability throughout the reductant catalyst. Furthermore, the reductant catalyst may include some ammonia storage capacity.The storage capacity can complicate the control process, for example, by creating additional control objectives (e.g., a storage objective), by unexpectedly releasing ammonia (e.g., when a system condition causes a reduction in storage capacity), and / or by adsorbing some of the injected ammonia in an initial portion of the catalyst, thereby reducing the availability of ammonia in a rear portion of the catalyst during catalyst fill times.

[0003] The challenges identified with currently available SCR systems are compounded by the transient nature of mobile applications. Load and speeds vary during an operator-specified operating mode and are generally not predictable in advance by the SCR control system. Furthermore, feedback control systems have several disadvantages. For example, the ammonia concentration is difficult to determine in real time. Commercially available NOx sensors exhibit cross-sensitivity to ammonia, making it difficult to determine the amount of NOx present in the exhaust outlet from the SCR catalyst. Adding an NH3 sensor to the control system can improve control capability but increases the system's cost.

[0004] Ammonia is generally an undesirable constituent in final exhaust emissions, and the ammonia emitted, or "slipped," from the catalyst represents ineffectively used reductant, increasing operating costs. In systems employing an ammonia oxidation catalyst (AMOX) downstream of the SCR catalyst, ammonia slip can be converted to NOx under certain conditions. Therefore, operation with very low or zero ammonia concentration at the SCR catalyst outlet is desirable. NOx sensors that exhibit cross-sensitivity to ammonia and the conversion of NH3 slip to NOx in systems employing an AMOX catalyst complicate the ability to provide a reliable estimate of the occurrence and / or amount of ammonia slip.This reduces the ability of an SCR feedback control to deliver an optimal amount of ammonia to the exhaust system and potentially provides false indications of SCR and / or reductant injector failure conditions. Consequently, additional SCR control is required in response to ammonia slip conditions.

[0005] US 2010 / 0 107 609 A1 discloses a method for controlling a selective catalytic reduction catalyst in an exhaust line of an internal combustion engine, wherein the supply of a quantity of a gaseous ammonia reducing agent to the SCR catalyst uses a closed-loop SCR catalyst model coupled to an SCR-out NOx sensor that measures an SCR-out NOx emission value. WO 2013 / 010 625 A1 relates to a method for determining the quality of an ammonia-containing reducing agent solution used for nitrogen oxide reduction in an SCR exhaust gas purification system of an internal combustion engine. DE 10 2008 036 884 A1 relates to a method for operating an exhaust gas purification system with an SCR catalyst. SUMMARY

[0006] The invention is defined in the independent patent claims. Advantageous embodiments of the invention are defined in the subclaims.

[0007] One embodiment provides a unique method for controlling an SCR aftertreatment system in response to the detection of an NH3 slip condition by manipulating a DeNOx target efficiency of the SCR catalyst. The SCR aftertreatment system is operated so that the DeNOx target efficiency is achieved by the SCR catalyst. When an NH3 slip condition is detected at a current DeNOx efficiency, a modified DeNOx efficiency target is set that is reduced relative to the DeNOx target efficiency and is a function of the current DeNOx efficiency. The modified DeNOx target efficiency results in a reduced amount of reductant injected into the exhaust system. The modified DeNOx target efficiency is increased over time until one or more NH3 slip condition compensation conditions are met.

[0008] This summary is intended to demonstrate a selection of concepts that are further explained in the illustrative embodiments. This summary is not intended to identify important or essential functions of the claimed subject matter, nor should it be used as an aid to limiting the scope of the claimed subject matter. Additional embodiments, designs, objects, functions, advantages, aspects, and benefits will become apparent from the following description and figures. BRIEF DESCRIPTION OF THE ILLUSTRATIONS Fig. 1 is a schematic diagram of a system including an SCR catalyst for reducing NO produced by an internal combustion engine x -Emissions in an exhaust stream. Fig. Figure 2 is a graphical representation of a control process of the SCR aftertreatment system in response to an NH3 slip condition. Fig. 3 is a flowchart of a control process of the SCR aftertreatment system in response to an NH3 slip condition. Fig. 4 is a schematic diagram of an embodiment of a controller device that, in operation, controls the SCR aftertreatment system in response to an NH3 slip condition of the system of Fig. 1 controls. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0009] For the purpose of promoting a better understanding of the principles of the invention, reference will now be made to the embodiments shown in the figures and described using specific technical language. It should be understood, however, that no limitations on the scope of the invention are intended, and all variations and further modifications of the illustrated embodiments and any further applications of the principles of the invention presented herein that would normally occur to those skilled in the art and to which the invention pertains are contemplated.

[0010] Referring to Fig. 1, an internal combustion engine system 10 comprises an exhaust gas stream 12 generated by the operation of an internal combustion engine 14, wherein the exhaust gas stream 12 contains an amount of NO xThe internal combustion engine 14 includes a number of cylinders (not shown) that receive a fuel supply from a fuel source (not shown). Any number of cylinders and arrangement of cylinders, such as in-line or V-shaped, are contemplated. Combustion of fuel in the cylinders produces the exhaust stream 12, a portion of which may be recirculated to an intake system (not shown) via an exhaust gas recirculation (EGR) system. Embodiments without an EGR system are also contemplated.

[0011] The system 10 comprises a first NO x -Sensor 16 for providing a measurement result with regard to NO coming from the engine upstream of an SCR catalyst 24 x -amount. The NO coming from the machine xThe NOx amount may alternatively be determined virtually by a model in response to engine and exhaust operating parameters, or by a sensor or sensors mounted at a different location in the system 10. The NOx sensor 16 used herein refers to either an actual physical NOx sensor or a virtual NOx sensor, or both.

[0012] The system 10 includes an upstream aftertreatment system component 18, which may include an oxidation catalyst, a particulate filter, or both. In some embodiments, the system 10 does not include an oxidation catalyst and / or a particulate filter. The system 10 also includes a reductant injector 20 fluidly connected to a reductant source 22. The reductant injector 20 injects reductant, such as urea, NH3, or another NH3-producing component, into the exhaust stream 12. The system 10 includes an SCR catalyst 24 downstream of the reductant injector 20 to receive the injected reductant with the exhaust stream and a second NOx sensor 30 downstream of the SCR catalyst 24. In one embodiment, the NOx sensor 30 is a physical NOx sensor having a cross-sensitivity to ammonia.In another embodiment, the NOx sensor 30 is a virtual NOx sensor, a NOx sensor that does not have ammonia cross sensitivity, an ammonia sensor, a virtual ammonia sensor, or a combination thereof.

[0013] The system 10 may include an optional NH3 oxidation catalyst (AMOX) 26 provided to oxidize at least a portion of the NH3 slipped from the SCR catalyst 24 under at least some operating conditions. The AMOX catalyst 26 may exist as a discrete catalytic element in the same or different housing as the SCR catalyst 24 and may be integrated as a primer of a component (particularly a rear component) of the SCR catalyst 24. The SCR catalyst 24 may include one or more catalyst elements located in the same or different housing. Additional SCR catalyst elements may exist and are schematically integrated into the SCR catalyst 24. Additionally, certain embodiments contemplate that the AMOX catalyst 26 is removed entirely from the system 10 in view of the systems and methods disclosed herein to mitigate or eliminate NH3 slip.The NOx sensor 30 may be located at the outlet of the SCR catalyst 24, between the outlet of the SCR catalyst 24 and an inlet to the AMOX catalyst 26 if an AMOX catalyst is provided, or at the outlet of the AMOX catalyst 26 if an AMOX catalyst is provided.

[0014] The system 10 further includes a controller 28. The controller 28 may be a component of the processing subsystem and may include functional elements in terms of software and / or hardware. The controller 28 may represent a single device or multiple distributed devices. The controller 28 may include modules structured to functionally execute any operational sequences of any methods or processes described herein. In the embodiment shown, the controller 28 is directly or indirectly connected to the reductant injector to provide a reductant injection amount in the exhaust flow path. The controller 28 is also connected to the NOx sensors 16, 30 to receive sensor output signals and, in the case of a virtual sensor, the output of one or more signals indicative of a NOx amount and / or the presence of NOx and / or NH3 in the exhaust stream.

[0015] An exemplary method in which the controller 28 may be configured to execute control algorithms includes providing an SCR catalyst located in the exhaust stream generated by the internal combustion engine 14 and a reductant injection valve 20 operatively connected to the exhaust stream at a position upstream of the SCR catalyst 24. The method includes providing a first NO x -Sensor 16, which is a virtual sensor or is connected to the exhaust gas flow at a position upstream of the SCR catalyst 24 and a second NOx sensor 30, which is connected to the exhaust gas flow at a position downstream of the SCR catalyst 24.

[0016] The method may further include setting a target DeNOx efficiency, determining a current DeNOx efficiency of the SCR catalyst 24, determining an NH3 slip condition at which ammonia slip occurs downstream of the SCR catalyst 24 to the current DeNOx efficiency, determining a preliminary target DeNOx efficiency in response to the NH3 slip condition as a function of the current DeNOx efficiency, determining an amount of NH3 to inject into the exhaust stream in response to the preliminary target DeNOx efficiency, and injecting reductant in response to the amount of NH3 until a balance condition for DeNOx efficiency is met.

[0017] Referring to Fig. 2, a graphical representation of one embodiment of an NH3 slip compensation algorithm 100 is shown. The graphical representation includes a commanded NH3 amount along its horizontal axis and a feedback amount for DeNOx efficiency along its vertical axis. A DeNOx efficiency curve 110 is shown representing the DeNOx efficiency at various commanded NH3 amounts. In one embodiment, the commanded NH3 amount represents a ratio of ammonia to NOx, which includes an amount of NH3 commanded by controller 28. In feedforward control, a commanded NH3 amount is determined that achieves the target DeNOx efficiency 102.A feedback control then determines the deviation of the current DeNOx efficiency from the target DeNOx efficiency using, for example, NOx amounts from the NOx sensors 16, 30 and adjusts the feedforward control amount of NH3 to achieve the target DeNOx efficiency 102 at various NOx amounts exiting the engine over time.

[0018] However, under certain operating conditions, the DeNOx target efficiency 102 cannot be achieved through feedback control based on NOx efficiency feedback due, for example, to NH3 slip conditions and / or because NOx production is provided by an AMOX catalyst. If the DeNOx target amount 102 cannot be achieved, the commanded NH3 amount must be either increased or reduced to achieve the DeNOx target efficiency 102. Because the feedback determination of the DeNOx efficiency is affected by the cross-sensitivity of the NOx sensor 30 and by the NOx production from any AMOX catalyst 26, the DeNOx efficiency feedback control cannot be reliably used to determine the commanded NH3 amount during NH3 slip conditions.

[0019] The controller 28 is configured to determine the DeNOx target efficiency 102 for the SCR catalyst 24 and also a current DeNOx efficiency 104 for the SCR catalyst 24. The DeNOx target efficiency 102 is determined by the controller 28 in response to one or more output signals indicative of one or more operating conditions of the engine 14 and the aftertreatment system. The DeNOx target efficiency 102 may represent a fixed amount or vary in response to operating conditions. Any reasonable method for determining the current DeNOx efficiency 104 is contemplated. For example: current ηSCR=(CNOx,in−CNOx,out) / CNOx,in where C NOx,in is the amount of NOx upstream of the inlet of the SCR catalyst 24, which is measured by the actual or virtual output of the first NOx sensor 16 and C NOx,outis the amount of NOx downstream of the SCR catalyst 24, which is measured by the output of the second NOx sensor 30. In addition, the current η SCR the DeNOx efficiency of the SCR catalyst 24 by removing NOx, as determined from the outputs of the first NOx sensor 16 and the second NOx sensor 30. For example, in equation 1, the current η SCR 0.7 if the SCR catalyst removes 70% of the NOx coming from the engine.

[0020] The NOx amount indicated by the NOx sensor 30 is above the representative value of the current NOx amount at the outlet of the SCR catalyst 24 during NH3 slip conditions due, for example, to the cross-sensitivity of the NOx sensor 30 to NH3 and / or the conversion of NH3 to NOx by the AMOX catalyst 26, if an AMOX catalyst is provided. Therefore, the controller 28 is unable to determine from the DeNOx efficiency curve 110 whether the commanded NH3 amount providing the current DeNOx efficiency 104 should be increased or decreased to achieve the target DeNOx efficiency 102.

[0021] The algorithm 100 also includes determining a preliminary DeNOx target efficiency 106 in response to the current DeNOx efficiency 104 deviating from the DeNOx target efficiency 102 by more than a threshold amount and / or for more than a threshold time. The threshold deviation may be set as a predetermined fixed amount or as a variable amount in response to engine operating conditions. Since the current DeNOx efficiency 104 is less than the DeNOx target efficiency 102, a preliminary DeNOx target efficiency 106 may be set as a function of the current DeNOx efficiency 104. The corresponding preliminary commanded NH3 amount is selected that is less than the commanded NH3 amount of the DeNOx target efficiency 102, resulting in a reduced amount of NH3 being provided by the injector 20 in the exhaust stream.Furthermore, the feedback control of the commanded NH3 amount is suspended in response to the current DeNOx efficiency and DeNOx target efficiency.

[0022] The preliminary DeNOx target efficiency 106 is increased over time, as represented by the adjustment curve 108, up to the DeNOx target efficiency amount 102. The increase in the preliminary DeNOx target efficiency 106 may be incremental in any reasonable manner until one or more DeNOx efficiency adjustment conditions are met. The DeNOx efficiency adjustment conditions may include, for example, the following: determining that the NH3 slip condition no longer exists, the preliminary DeNOx target efficiency 106 exceeds a fixed or dynamic threshold amount, the preliminary DeNOx target efficiency 106 reaches the DeNOx target efficiency 102, a deadline has expired, or a combination of two or more of these conditions.

[0023] Certain operations described herein include operations for interpreting one or more parameters. Interpretation, as used herein, includes receiving values ​​by any means known in the art, including receiving values ​​via a data link or network communication, receiving an electronic signal (e.g., a voltage, a frequency, a current, or a PWM signal) indicative of a value, receiving a software parameter indicative of a value, reading the value from a memory location onto a computer-readable medium, receiving the value as a runtime parameter by any means known in the art, and / or receiving a value from which the interpreted parameter can be calculated and / or by reference to a default value that is interpreted as a parameter value.

[0024] Referring to Fig.3, an exemplary process 200 includes providing the SCR catalyst 24 mounted in an exhaust system of an internal combustion engine 14 and a reductant injection valve 20 operatively connected to the exhaust system at a position upstream of the SCR catalyst 24. The process 200 further includes providing a first NOx sensor 16 upstream of the SCR catalyst 14 and a second NOx sensor 30 connected to the exhaust system at a position downstream of the SCR catalyst 24. The process 200 includes an operation 202 to operate the internal combustion engine 14 to generate an exhaust stream.

[0025] During operation of the internal combustion engine 14, process 200 includes a workflow 204 to determine a target DeNOx efficiency for the SCR catalyst 24 by removing NOx from the exhaust stream 12. Process 200 further includes a workflow 206 to inject reductant into the exhaust stream 12 by providing a reductant injection command to the reductant injector 20 that provides a commanded amount of NH3 to the exhaust stream upstream of the SCR catalyst 24 in response to the target DeNOx efficiency.

[0026] A workflow 208 determines the current DeNOx efficiency of the SCR catalyst 24. The current DeNOx efficiency may, for example, be used in feedback control of the commanded NH3 amount to obtain the target DeNOx efficiency. The process 200 further includes a condition 210 to determine whether an NH3 slip condition exists. The NH3 slip condition may be determined by any appropriate method and / or device, including an NH3 sensor located downstream of the SCR catalyst 24 and / or an NH3 slip detection algorithm programmed into the controller 28. In one embodiment of the present disclosure, no NH3 sensor downstream of the SCR catalyst is required or involved in NH3 slip detection, and the NH3 slip condition is determined from the output of one or more NOx sensors 16, 30.Non-limiting, exemplary detection methods for NH3 slip are disclosed in Provisional Patent Application No. 61 / 917,490, filed December 18, 2013, which is incorporated herein by reference for all purposes.

[0027] If condition 210 is negative, operation 200 proceeds to operation 212 to adjust the ammonia to NOx ratio to provide the commanded amount of NH3 that achieves the target DeNOx efficiency. Operation 212 may include providing feedback control in response to a deviation of the current DeNOx efficiency from a target DeNOx efficiency and determining the ammonia to NOx ratio in response to the deviation and to one or more operating conditions of the engine 14 and the aftertreatment system. Operation 200 then proceeds to operation 204 while the engine is operating.

[0028] If condition 210 is positive, operation 200 proceeds to operation 214 to reset the DeNOx target efficiency to a preliminary DeNOx target efficiency that is a function of the current DeNOx efficiency. In one embodiment, the preliminary DeNOx target efficiency is set to the current DeNOx efficiency. The commanded NH3 amount is then adjusted in response to the preliminary DeNOx target efficiency while suspending feedback control of the NH3 amount. Operation 200 proceeds to operation 216 to increase the preliminary DeNOx target efficiency over time until one or more DeNOx efficiency trade-off conditions are met.The compensation conditions for a DeNOx efficiency may include, for example, the determination that the NH3 slip condition no longer exists, the preliminary DeNOx target efficiency 106 exceeds a fixed or dynamic limit amount, the preliminary DeNOx target efficiency reaches the DeNOx target efficiency, a deadline has expired, or a combination of two or more of these conditions.

[0029] An exemplary system includes a processing subsystem having a controller 28 structured to functionally perform certain operations to control an SCR aftertreatment system. In some embodiments, the controller 28 includes one or more modules structured to functionally perform the operations of the controller 28. The description herein of modules emphasizes the structural independence of the aspects of the controller and illustrates a grouping of operations and responsibilities of the controller. Other groupings that perform similar general operations are contemplated within the scope of the present application. Modules may be implemented in hardware and / or via a computer-readable medium, and modules may be distributed across various hardware components.

[0030] The controller 28 may be part of a system 300 including an SCR portion and first and second NOx sensors 16, 30 that are operatively or virtually connected to an exhaust stream of an internal combustion engine at respective positions upstream and downstream of the SCR portion. The SCR portion comprises any fraction of an SCR catalyst load in the system, including the total SCR catalyst load. The NOx sensors provide an output indicative of 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 ammonia and the presence of ammonia in the exhaust stream at the NOx sensor.

[0031] Controller 28 includes a DeNOx efficiency target module 302 configured to determine a DeNOx target efficiency 304 in response to, for example, operating conditions 306. The DeNOx target efficiency 302 may be a fixed or dynamic value that varies in response to system conditions 306. The system conditions 306 may be, for example, an engine speed, a temperature of one or more exhaust system components, an ammonia storage capacity of the SCR catalyst 24, an exhaust flow rate, or any other condition considered in determining a DeNOx target efficiency.

[0032] Controller 28 also includes a DeNOx efficiency feedback module 308 that determines a current DeNOx efficiency 310 in response to, for example, a first NOx sensor measurement 312 and a second NOx sensor measurement 314. Controller 28 further includes a DeNOx efficiency tracking module 316 that receives the target DeNOx efficiency 304 and the current DeNOx efficiency 310 to determine a commanded NH3 amount, such as the ammonia to NOx ratio amount 318, to achieve the target DeNOx efficiency 304 via controlling or regulating the ammonia to NOx ratio amount 318. The DeNOx efficiency tracking module 316 may further determine a reductant injection command 320 that provides the commanded amount of NH3 to the exhaust stream for DeNOx efficiency control to achieve the target DeNOx efficiency 304.

[0033] The controller 28 further includes an NH3 slip detection module 322 that receives system conditions 306 and determines an NH3 slip condition 324 at the outlet of the SCR catalyst 24, at the inlet of the AMOX catalyst 26, at the exhaust tailpipe, and / or NOx production by an AMOX catalyst 26. In response to the NH3 slip condition 326, either an NH3 slip detection module 322 or an NH3 compensation module 328 suspends feedback control upon achievement of the DeNOx target efficiency by the DeNOx efficiency tracking module 316.

[0034] In response to the NH3 slip condition, the NH3 compensation module 328 determines a preliminary DeNOx target efficiency 330 that is a function of the current DeNOx efficiency 310 and outputs a reductant injection command 332 for NH3 slip compensation. The reductant injection command 332 provides a commanded amount of NH3 corresponding to the preliminary DeNOx target efficiency 330. The NH3 compensation module 328 further determines a preliminary DeNOx target efficiency adjustment 334 that increases the preliminary DeNOx target efficiency 330 toward the DeNOx target efficiency 304 in either a stepwise, incremental, continuous, linear, or functional manner. During operation of the NH3 compensation module 328, the DeNOx efficiency adjustment conditions 336 are monitored.When one or more DeNOx efficiency adjustment conditions 336 are met, the DeNOx efficiency tracking module 316 again assumes control for determining the reductant injection amount, as explained above. As also explained above, the DeNOx efficiency adjustment conditions 336 may include one or more of: a fixed or dynamic limit amount of increase to the preliminary DeNOx target efficiency, the preliminary DeNOx target efficiency reaching the DeNOx target efficiency, a deadline has expired, or another condition indicating that the preliminary DeNOx efficiency adjustment should be terminated.

[0035] The system further includes a reductant injector 20 that is responsive to the issuance of reductant injection commands 320, 332 by the controller 28. The reductant injector's response to the reductant injection commands may be any type of response known in the art. Examples and non-limiting responses of the reductant injector to the reductant injection commands 320, 332 include setting a target reductant injection amount, advancing to inject the reductant amount (e.g., via a feedforward and / or feedback controller), and / or providing the reductant injection amount to a controller, while accepting other competing or limiting values ​​for the injection (e.g., ammonia slip limits, SCR catalyst storage limits, current conversion efficiency limits, etc.).

[0036] According to one aspect, a method comprises operating an internal combustion engine to generate an exhaust stream in an exhaust system including a selective catalytic reduction (SCR) catalyst disposed in the exhaust stream. The exhaust system includes a reductant injection valve connected to the exhaust system at a position upstream of the SCR catalyst, a first NOx sensor upstream of the SCR catalyst, and a second NOx sensor connected to the exhaust system at a position downstream of the SCR catalyst.The method further comprises determining a target DeNOx efficiency for the SCR catalyst; injecting an amount of reductant into the exhaust stream in response to the target DeNOx efficiency; determining a current DeNOx efficiency of the SCR catalyst by removing NOx from the exhaust stream as a function of a first result from the first NOx sensor and a second result from the second NOx sensor, wherein the current DeNOx efficiency is less than the target DeNOx efficiency; determining an NH3 slip condition; in response to determining the NH3 slip condition, determining a preliminary target DeNOx efficiency for the SCR catalyst as a function of the current DeNOx efficiency; injecting an amount of reductant in response to the preliminary target DeNOx efficiency until at least one adjustment condition of the DeNOx efficiency is met.

[0037] In one embodiment, the reductant amount is determined by the ammonia to NOx ratio upstream of the SCR catalyst that achieves the preliminary DeNOx target efficiency. In another embodiment, the current DeNOx efficiency of the SCR catalyst is determined by: current η SCR = (C NOx,in - C NOx,out ) / C NOx,in ; where current η SCR the current DeNOx efficiency of the SCR catalyst, C NOx,in the NOx measurement indicated by the first NOx sensor and C NOx,outis the NOx measurement indicated by the second NOx sensor. In another embodiment, determining the NH3 slip condition comprises determining NH3 presence in the exhaust stream downstream of the SCR catalyst. In another embodiment, the exhaust system includes an ammonia oxidation catalyst downstream of the SCR catalyst, and determining the NH3 slip condition comprises determining NOx production from the AMOX catalyst.

[0038] In another embodiment, the method comprises adjusting the preliminary DeNOx target efficiency to the DeNOx target efficiency until at least one DeNOx efficiency adjustment condition is met. In the refinement of this embodiment, the at least one DeNOx efficiency adjustment comprises: the NH3 slip condition no longer exists; a period of time has elapsed since the NH3 slip condition was determined; an increase in the threshold range of the preliminary DeNOx target efficiency; and the preliminary DeNOx target efficiency equals the DeNOx target efficiency.

[0039] In another embodiment, determining the preliminary DeNOx target efficiency for the SCR catalyst comprises equating the preliminary DeNOx target efficiency with the current DeNOx efficiency. In another embodiment, the first NOx sensor is a virtual NOx sensor.

[0040] In another aspect, a system is disclosed that includes an internal combustion engine that, during operation, generates an exhaust gas stream in an exhaust system. The exhaust system includes an SCR catalyst disposed in the exhaust stream, a reductant injector operatively connected to the exhaust stream at a position upstream of the SCR catalyst, a first NOx sensor upstream of the SCR catalyst, and a second NOx sensor connected to the exhaust system at a position downstream of the SCR catalyst. The system also includes a controller connected to the first NOx sensor, the second NOx sensor, and the internal combustion engine.The controller is configured to determine a target DeNOx efficiency in response to one or more system operating conditions, a current DeNOx efficiency from a first reading of the first NOx sensor and a second reading of the second NOx sensor, and a reductant injection command that provides a feedback-controlled amount of NH3 in response at least in part to a deviation of the current DeNOx efficiency from the target DeNOx efficiency. The controller is further configured, in response to a determination of an NH3 slip condition, to determine a preliminary DeNOx target efficiency as a function of the current DeNOx efficiency and to provide a modified reductant injection command to provide a reduced amount of NH3 in response to the preliminary DeNOx target efficiency.

[0041] In one embodiment, the controller is configured to determine the feedback-controlled NH3 amount and the reduced NH3 amount as a function of an ammonia-to-NOx ratio upstream of the SCR catalyst. In another embodiment, the controller is configured to determine an NH3 slip condition in response to NH3 presence in the exhaust stream downstream of the SCR catalyst. In another embodiment, the preliminary DeNOx target efficiency for the SCR catalyst is set to the current DeNOx efficiency.

[0042] In another embodiment, the system includes at least one oxidation catalyst and at least one particulate filter upstream of the SCR catalyst and an ammonia oxidation catalyst downstream of the SCR catalyst. In another embodiment, the controller is configured, in response to the detection of an NH3 slip condition, to suspend the feedback-controlled NH3 amount via a reductant injection command until a DeNOx efficiency adjustment condition is met. In a refinement of this embodiment, the DeNOx efficiency adjustment condition includes at least one of: the NH3 slip condition no longer exists; a period of time has elapsed since the detection of the NH3 slip condition; an increase in the boundary range of the preliminary DeNOx target efficiency; and the preliminary DeNOx target efficiency equals the DeNOx target efficiency.

[0043] According to another aspect, an apparatus includes a controller connected to a first NOx sensor upstream of the SCR catalyst in an exhaust system and a second NOx sensor downstream of the SCR catalyst. The exhaust system receives an exhaust gas flow generated by operation of the internal combustion engine. The controller includes a DeNOx target efficiency module configured to determine a DeNOx target efficiency for injecting an amount of NH3 to achieve the DeNOx target efficiency. The controller also includes a DeNOx efficiency feedback module configured to determine a current DeNOx efficiency in response to NOx amounts from the first NOx sensor and the second NOx sensor, and an NH3 slip detection module configured to detect an NH3 slip condition. In response to the NH3 slip condition, the controller suspends the injection of the NH3 amount to achieve the DeNOx target efficiency.The controller further includes an NH3 compensation module configured to determine a preliminary DeNOx target efficiency in response to the NH3 slip condition for injecting a reduced amount of NH3, wherein the preliminary DeNOx target efficiency is a function of the current DeNOx efficiency.

[0044] In one embodiment, the NH3 compensation module is configured to determine a reductant injection command in response to the preliminary DeNOx target efficiency, wherein the reductant injection command provides the reduced amount of NH3 that is less than the amount of NH3 provided in response to the DeNOx target efficiency. In another embodiment, the controller includes a DeNOx efficiency tracking module configured to determine a reductant injection command in response to a deviation of the current DeNOx efficiency from the DeNOx target efficiency. In another embodiment, the controller is configured to suspend injection of the amount of NH3 to achieve the DeNOx target efficiency until a DeNOx efficiency adjustment condition is met.The DeNOx efficiency adjustment condition includes at least one of the following: the NH3 slip condition no longer exists; a period of time has elapsed since the NH3 slip condition was detected; an increase in the boundary range of the preliminary DeNOx target efficiency; and the preliminary DeNOx target efficiency equals the DeNOx target efficiency.

[0045] Although the invention has been illustrated in detail in the drawings and described above, this is considered to be illustrative and not restrictive, and it is to be understood that only certain exemplary embodiments have been shown and described. Those skilled in the art will recognize that many modifications to the exemplary embodiments are possible without substantially departing from the invention. Accordingly, it is intended that all such modifications be included within the scope of this disclosure, which is defined by the following claims.

[0046] In the claims, it is intended that the use of words such as "a," "an," "at least one," or "at least a part" herein does not imply that the claim is limited to only one subject matter, unless specifically stated otherwise. When terms such as "at least one part" or "a part" are used, the subject matter may include a part and / or be the entire subject matter, unless specifically stated otherwise.

Claims

[1] Method comprising: Operating an internal combustion engine (14) to produce an exhaust gas stream (12) in an exhaust system including a selective catalytic reduction (SCR) catalyst (24) disposed in the exhaust gas stream (12), the exhaust system comprising: a reductant injection valve (20) connected to the exhaust system at a position upstream of the SCR catalyst (24); a first NOx sensor (16) upstream of the SCR catalyst (24); a second NOx sensor (30) downstream of the SCR catalyst (24); Determining a DeNOx target efficiency (102) for the SCR catalyst (24); Injecting a first amount of reductant into the exhaust stream (12) in response to the target DeNOx efficiency (102); Determining a current DeNOx efficiency (104) of the SCR catalyst (24) by removing NOx from the exhaust stream (12) as a function of a first result from the first NOx sensor (16) and a second result from the second NOx sensor (30); Determining that the current DeNOx efficiency (104) is less than the target DeNOx efficiency (102); Detecting an NH3 slip condition; in response to determining the NH3 slip condition, determining a preliminary DeNOx target efficiency (106) for the SCR catalyst (24); and Injecting a reduced amount of reductant in response to the preliminary target DeNOx efficiency (106) while increasing the preliminary target DeNOx efficiency (106) toward the target DeNOx efficiency (102) until at least one adjustment condition of the DeNOx efficiency is met, wherein the reduced amount of reductant is less than the first amount of reductant and wherein the preliminary target DeNOx efficiency (106) is determined as a function of the current DeNOx efficiency (104). [2] The method of claim 1, wherein the reduced amount of reductant is determined by a ratio of ammonia to NOx upstream of the SCR catalyst (24) that achieves the preliminary DeNOx target efficiency (106). [3] The method of claim 1, wherein the current DeNOx efficiency (104) of the SCR catalyst (24) is determined by: current ηSCR=(CNOx,in−CNOx,out) / CNOx,in; where current η SCRis the current DeNOx efficiency (104) of the SCR catalyst, C NOX,in the NOx measurement indicated by the first NOx sensor (16) and C NOx,out the NOx measurement indicated by the second NOx sensor (30). [4] The method of claim 1, wherein determining the NH3 slip condition comprises determining an NH3 presence in the exhaust stream (12) downstream of the SCR catalyst (24). [5] The method of claim 1, wherein the exhaust system comprises an ammonia oxidation catalyst (26) downstream of the SCR catalyst (24), and determining the NH3 slip condition comprises determining that the AMOX catalyst (26) is converting NH3 to NOx. [6] The method of claim 5, wherein the at least one adjustment of the DeNOx efficiency comprises at least one of the following: the NH3 slip state no longer exists; a period of time has elapsed since the NH3 slip condition was detected; an increase in the limit of the preliminary DeNOx target efficiency (106); and the preliminary DeNOx target efficiency (106) corresponds to the DeNOx target efficiency (102). [7] The method of claim 1, wherein determining the preliminary DeNOx target efficiency (106) for the SCR catalyst (24) comprises setting the preliminary DeNOx target efficiency (106) equivalent to the current DeNOx efficiency (104). [8] The method of claim 1, wherein the first NOx sensor (16) is a virtual NOx sensor. [9] System (10) comprising: an internal combustion engine (14) which, during operation, generates an exhaust gas stream (12) in an exhaust system, the exhaust system comprising: a catalyst (24) for selective catalytic reduction (SCR) arranged in the exhaust gas stream (12); a reductant injection valve (20) operatively connected to the exhaust gas stream (12) at a position upstream of the SCR catalyst (24); a first NOx sensor (16) upstream of the SCR catalyst (24); a second NOx sensor (16) downstream of the SCR catalyst (24); a controller (28) connected to the first NOx sensor (16), the second NOx sensor (30) and the internal combustion engine (14), the controller (28) being configured to determine: a target DeNOx efficiency (102) in response to one or more operating conditions of the system and a current DeNOx efficiency (104) from a first result of the first NOx sensor (16) and from a second result of the second NOx sensor (30); a reductant injection command providing a feedback-controlled amount of NH3 in response to at least in part a deviation of the current DeNOx efficiency (104) from the target DeNOx efficiency (102); wherein the controller (28) is further configured to determine, in response to a detection of an NH3 slip condition: a preliminary DeNOx target efficiency (106) and a modified reductant injection command to provide a reduced amount of NH3 in response to the preliminary target DeNOx efficiency (106) while increasing the preliminary target DeNOx efficiency (106) toward the target DeNOx efficiency (102), wherein the preliminary target DeNOx efficiency (106) is determined as a function of the current DeNOx efficiency (104). [10] The system (10) of claim 9, wherein the controller (28) is configured to determine the feedback controlled NH3 amount and the reduced NH3 amount as a function of an ammonia to NOx ratio upstream of the SCR catalyst (24). [11] The system (10) of claim 9, wherein the controller (28) is configured to determine the NH3 slip condition in response to NH3 presence in the exhaust stream (12) downstream of the SCR catalyst (24). [12] The system (10) of claim 9, wherein the preliminary DeNOx target efficiency (106) for the SCR catalyst (24) is set as the current DeNOx efficiency (104). [13] The system (10) of claim 9, further comprising at least one of an oxidation catalyst and a particulate filter upstream of the SCR catalyst (24) and an ammonia oxidation catalyst (26) downstream of the SCR catalyst (24). [14] The system (10) of claim 9, wherein the controller (28) is configured, in response to a detection of an NH3 slip condition, to suspend providing the feedback-controlled NH3 amount via reductant injection command until a DeNOx efficiency adjustment condition is met. [15] The system (10) of claim 14, wherein the DeNOx efficiency adjustment condition comprises at least one of the following: the NH3 slip state no longer exists; a period of time has elapsed since the NH3 slip condition was detected; an increase in the limit of the preliminary DeNOx target efficiency (106) and the preliminary DeNOx target efficiency (106) corresponds to the DeNOx target efficiency (102). [16] The system (10) of claim 9, wherein the first NOx sensor (16) is one of a physical component of a gas sensor and a virtual component of a gas sensor, and the second NOx sensor (30) is one of a physical component of a gas sensor or a virtual component of a gas sensor. [17] Device comprising: a controller (28) connected to a first NOx sensor (16) upstream of a selective catalytic reduction (SCR) catalyst (24) in an exhaust system and to a second NOx sensor (30) downstream of the SCR catalyst (24), the exhaust system receiving an exhaust gas stream (12) generated by the operation of an internal combustion engine (14), the controller (28) comprising: a DeNOx target efficiency module configured to determine a DeNOx target efficiency (102) for injecting an amount of NH3 to achieve the DeNOx target efficiency (102); a DeNOx efficiency feedback module configured to determine a current DeNOx efficiency (104) in response to NOx amounts from the first NOx sensor (16) and the second NOx sensor (30); an NH3 slip detection module configured to detect an NH3 slip condition; wherein, in response to the NH3 slip condition, the controller (28) suspends the injection of the NH3 amount to achieve the DeNOx target efficiency (102); and further comprising an NH3 compensation module configured to determine a preliminary DeNOx target efficiency (106) in response to the NH3 slip condition for injecting a reduced amount of NH3 and to increase the preliminary DeNOx target efficiency (106) toward the DeNOx target efficiency (102) during injection of the reduced amount of NH3, wherein the preliminary DeNOx target efficiency (106) is determined as a function of the current DeNOx efficiency (104). [18] The apparatus of claim 17, wherein the NH3 compensation module is configured to determine the preliminary DeNOx target efficiency (106) as a function of the current DeNOx efficiency (104) and a reductant injection command responsive to the preliminary DeNOx target efficiency (106), the reductant injection command providing the reduced NH3 amount that is less than the NH3 amount provided in response to the DeNOx target efficiency (102). [19] The apparatus of claim 17, further comprising a DeNOx efficiency tracking module configured to determine a reductant injection command in response to a deviation of the current DeNOx efficiency (104) from the target DeNOx efficiency (102). [20] The apparatus of claim 17, wherein the controller (28) is configured to suspend injection of the amount of NH3 to achieve the target DeNOx efficiency (102) until a DeNOx efficiency adjustment condition is met, wherein the DeNOx efficiency adjustment condition comprises at least one of the following: the NH3 slip state no longer exists; a period of time has elapsed since the NH3 slip condition was detected; an increase in the limit range of the preliminary DeNOx target efficiency (106) and the preliminary DeNOx target efficiency (106) corresponds to the DeNOx target efficiency (102). [21] The apparatus of claim 17, wherein the first NOx sensor (16) is one of a material component of a gas sensor and a virtual component of a gas sensor, and wherein the second NOx sensor (30) is one of a material component of a gas sensor and a virtual component of a gas sensor.

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