Abstract:
One embodiment is a method including operating an SCR system at a plurality of commanded ammonia to NOx input ratios, providing a plurality of data indicating NOx output from the SCR system for the plurality of commanded ammonia to NOx input ratios, and evaluating the plurality of data to diagnose the SCR system. Additional embodiment are methods, systems, and apparatuses including SCR diagnostics. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.

Description:
BACKGROUND 
     Selective catalytic reduction (“SCR”) systems can be provided to reduce emissions of nitrogen oxides (“NOx”) in a number of applications. Present approaches to SCR diagnostics suffer from a variety of limitations and problems including those respecting diagnosis of error or malfunction of SCR systems providing reduced NOx emission levels. There is a need for the unique and inventive methods, systems and apparatuses of SCR diagnostics disclosed herein. 
     SUMMARY 
     One embodiment is a method including operating an SCR system at a plurality of commanded ammonia to NOx input ratios, providing a plurality of data indicating NOx output from the SCR system for the plurality of commanded ammonia to NOx input ratios, and evaluating the plurality of data to diagnose the SCR system. Additional embodiments are methods, systems, and apparatuses including SCR diagnostics. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic illustration of a vehicle including an exemplary SCR system. 
         FIG. 2  is a flowchart illustrating an exemplary control procedure for initiating an SCR diagnostic procedure. 
         FIG. 3  is a flowchart illustrating an exemplary SCR diagnostic procedure 
         FIG. 4  is a graph illustrating exemplary SCR diagnostic criteria. 
     
    
    
     DETAILED DESCRIPTION 
     For purposes of promoting an understanding of the principles of the invention, reference will now be made to the exemplary embodiments illustrated in the figures 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 created, and that the invention includes and protects such alterations and modifications to the illustrated embodiments, and such further applications of the principles of the invention illustrated therein as would occur to one skilled in the art to which the invention relates. 
     With reference to  FIG. 1  there is illustrated a vehicle  100  including an engine  110  which is configured to provide motive power to vehicle  100  and to output exhaust to an exhaust flow path  111 . Vehicle  100  is illustrated schematically and may be a car, truck, bus, boat, recreational vehicle, construction equipment or another type of vehicle. Other embodiments include an engine provided in other applications such as a generator set. The exhaust output by engine  110  includes NOx which is to be reduced using an exhaust aftertreatment system  120 . Exhaust aftertreatment system  120  includes oxidation catalyst  121  which is in fluid communication with exhaust flow path  111  and is operable to catalyze oxidation of one or more compounds in exhaust flowing through exhaust flow path  111 , for example, oxidation of NO to NO 2 . 
     Exhaust aftertreatment system  120  further includes diesel particulate filter  122  which is in fluid communication with exhaust flow path  111  and is operable to reduce the level of particulates in exhaust flowing through exhaust flow path  111 . In an exemplary embodiment diesel particulate filter  122  is a catalyzed soot filter. Other embodiments utilize other types of diesel particulate filters. 
     Exhaust aftertreatment system  120  also includes reductant injector  131  and SCR catalyst  133 . Reductant injector  131  is supplied with reductant from reservoir  180  and is operable to inject reductant into exhaust flow path  111 . In an exemplary embodiment the reductant is an aqueous solution of urea which decomposes to provide ammonia. Other embodiments utilize different reductants, for example, aqueous solutions of ammonia, anhydrous ammonia, or other reductants suitable for SCR. Reductant injected into exhaust flow path is provided to SCR catalyst  133  which is in flow communication with exhaust flow path  111  and is operable to catalyze the reduction of NOx. 
     Exhaust flow path  111  is illustrated schematically in  FIG. 1  and may be provided in a variety of physical configurations. In an exemplary embodiment exhaust flow path proceeds from the output of a turbocharger of engine  110  through a conduit to a can containing oxidation catalyst  121  and diesel particulate filter  122 , through a second conduit which includes a urea decomposition reactor, to a can containing SCR catalyst  133  and an Ammonia Oxidation AMOX catalyst which is operable to catalyze reaction of ammonia which slips past SCR catalyst  133 , and through another conduit which outlets to the ambient environment. Other embodiments, omit one or more of the foregoing elements, include additional elements, feature alternate elements, and/or feature different arrangements and configurations of elements. 
     Vehicle  100  includes a controller  112  that is generally operable to control and manage operational aspects of vehicle  100  including engine  110 , exhaust aftertreatment system  120 . Controller  112  includes memory as well as a number of inputs and outputs for interfacing with various sensors and systems of vehicle  100 . Controller  112  can be an electronic circuit comprised of one or more components, including digital circuitry, analog circuitry, or both. Controller  112  may be a software and/or firmware programmable type; a hardwired, dedicated state machine; or a combination of these. In one embodiment, Controller  112  is of a programmable microcontroller solid-state integrated circuit type that includes memory and one or more central processing units. The memory of controller  112  can be comprised of one or more components and can be of any volatile or nonvolatile type, including the solid-state variety, the optical media variety, the magnetic variety, a combination of these, or other types of memory. Controller  112  can include signal conditioners, signal format converters (such as analog-to-digital and digital-to-analog converters), limiters, clamps, filters, and the like as needed to perform various control and regulation operations described herein. Controller  112 , in an exemplary embodiment, may be a type of controller sometimes referred to as an electronic or engine control module (ECM), electronic or engine control unit (ECU) or the like, that is directed to the regulation and control of engine operation. Alternatively, Controller  112  may be dedicated to control of just the operations described herein or to a subset of controlled aspects of vehicle  100 . In any case, controller  112  preferably includes one or more control algorithms defined by operating logic in the form of software instructions, hardware instructions, firmware instructions, dedicated hardware, or the like. These algorithms will be described in greater detail hereinafter, for controlling operation of various aspects of vehicle  100 . 
     Controller  112  is in operative interconnection with various elements of vehicle  100  as illustrated in  FIG. 1  with dashed lines extending between controller  112  and various elements of vehicle  100 . These operative interconnections may be implemented in a variety of forms, for example, through input/output interfaces coupled via wiring harnesses. In other instances all or a portion of the operative interconnection between controller  112  and an element of vehicle  100  may be virtual, for example, a virtual input indicative of an operating parameter may be provided by a model implemented by controller  112  or by another controller which models an operating parameter based upon other information. 
     Controller  112  is in operative communication with exhaust flow sensor  141  which provides controller  112  with information indicative of exhaust flow rate. In an exemplary embodiment exhaust flow sensor  141  is a virtual sensor which uses a model to determine the exhaust space velocity through SCR catalyst  133  based upon operating conditions of engine  110 , for example, information from an intake air flow sensor, fueling information and the volume of SCR catalyst  133 . In other embodiments exhaust flow sensor  141  is another type of sensor, for example, a mass flow rate sensor which is in fluid communication with exhaust flow path  111 . 
     Controller  112  is in operative communication with temperature sensor  142  which provides controller  112  with information indicative of the temperature of SCR catalyst  133 . In an exemplary embodiment exhaust flow sensor  141  determines information indicative of the temperature of SCR catalyst  133  based on a weighted average of information from temperature sensors located at the input outlet of SCR catalyst  133 . In other embodiments, information from temperature sensors in other locations or is utilized to determine information indicative of the temperature of SCR catalyst  133 . In other embodiments exhaust flow sensor  141  may be positioned on or in close proximity to SCR catalyst  133  to determine information indicative of the temperature of SCR catalyst  133 . 
     Controller  112  is in operative communication with NOx sensor  143  which provides controller  112  with information indicative of the level of NOx output from SCR catalyst  133 . In an exemplary embodiment NOx sensor  143  is a physical sensor which is in fluid communication with exhaust flow path  111 . Other embodiments may provide information indicative of the level of NOx output from SCR catalyst  133  using a greater number of sensors, or different types of sensors. 
     Controller  112  in also in operative communication with a virtual NOx sensor which provides controller  112  with information indicative of the level of NOx input to SCR catalyst  133  using a model based upon operating conditions of engine  110 , for example, engine load, engine fueling, exhaust temperature and/or other parameters. In other embodiments NOx sensor  141  is a physical NOx sensor which is in fluid communication with exhaust flow path  111  and is located upstream from SCR catalyst  133 . 
     During operation controller  112  uses the information indicative of the level of NOx provided to SCR catalyst  133  along with information from sensors  141  and  142  to determine the amount or rate of reductant to be injected by reductant injector  131 . Controller  112  is in operative communication with reductant injector  131  and can command reductant injector  131  to inject selected amount of reductant or to inject reductant at a selected rate. In an exemplary embodiment controller  112  commands reductant injection that is determined to maximize the catalytic reduction of NOx by SCR catalyst  133 , to maximize ammonia storage by SCR catalyst  133 , and to minimize the slip of ammonia past SCR catalyst  133 . In other embodiments controller  112  commands reductant injection to differently balance these parameters or to account for additional or different parameters. 
     Controller  112  is in operative communication with a malfunction indicator  191  which is provided in an operator compartment  190  of vehicle  100 . Malfunction indicator  191  can be a malfunction indicator light, or another type of display operable to provide information to an operator of vehicle  100 . Controller  112  is operable to command malfunction indicator  191  to display one or more indications based upon the diagnostics described herein, and may also store one or more error codes in a memory based upon the diagnostics described herein. 
       FIG. 2  is a flowchart illustrating an exemplary control procedure  200  for initiating an SCR diagnostic procedure. Procedure  200  may be implemented by controller  112  of vehicle  100  described above in connection with  FIG. 1  or in another controller. Procedure  200  is directed to initiating a diagnostic procedure for an SCR system. 
     At operation  210  a value indicative of SCR catalyst temperature is stored as variable TC. The value of variable TC may be based upon information from a sensor such as sensor  142  or another sensor adapted to provide information indicative of the temperature of an SCR catalyst. Variable TC is provided to operation  225 , conditional  250 , and conditional  260 . 
     Operation  225  determines a value indicative of the average SCR catalyst temperature over a time period and stores this value as variable TCA. In one embodiment variable TCA stores a value indicative of the average SCR catalyst temperature over the preceding minute. In other embodiments variable TCA stores a value indicative of the average SCR catalyst temperature over different time periods. Variable TCA is provided to conditional  230  and conditional  240 . 
     Conditional  230  tests whether variable TCA is less than MAX_TCA which is a calibratable maximum average catalyst temperature below which SCR diagnostics may be initiated. In one embodiment MAX_TCA is set to 550° C. In other embodiments MAX_TCA is set to a value between 500° C. and 600° C. In further embodiments MAX_TCA is set to another temperature determined not to be conducive to parasitic oxidation of ammonia by oxygen in an SCR system. Conditional  230  outputs true if TCA is less than MAX_TCA and, if not, outputs false. The output of conditional  230  is provided to operator  280 . 
     Conditional  240  tests whether variable TCA is greater than MIN_TCA which is a calibratable minimum catalyst average temperature above which SCR diagnostics may be initiated. In one embodiment MIN_TCA is set to 350° C. In other embodiments MIN_TCA is set to a value between 300° C. and 400° C. In further embodiments MIN_TCA is set to another temperature determined not to be conducive to ammonia storage by an SCR catalyst. Conditional  240  outputs true if TCA is greater than MIN_TCA and, if not, outputs false. The output of conditional  240  is provided to operator  280 . 
     Conditional  250  tests whether variable TC is less than MAX_TC which is a calibratable maximum catalyst temperature below which SCR diagnostics may be initiated. In one embodiment MAX_TC is set to 550° C. In other embodiments MAX_TC is set to a value between 500° C. and 600° C. In further embodiments MAX_TC is set to another temperature determined not to be conducive to parasitic oxidation of ammonia by oxygen in an SCR system. Conditional  250  outputs true if TC is less than MAX_TC and, if not, outputs false. The output of conditional  250  is provided to operator  280 . 
     Conditional  260  tests whether variable TC is greater than MIN_TC which is a calibratable minimum catalyst temperature above which SCR diagnostics may be initiated. In one embodiment MIN_TC is set to 350° C. In other embodiments MIN_TC is set to a value between 300° C. and 400° C. In further embodiments MIN_TC is set to another temperature determined not to be conducive to ammonia storage by an SCR catalyst. Conditional  260  outputs true if TC is greater than MIN_TC and, if not, outputs false. The output of conditional  260  is provided to operator  280 . 
     At operation  211  a value indicative of exhaust flow is stored as variable EX. The value of variable EX may be based upon information from a sensor such as sensor  141  or another sensor adapted to provide information indicative of exhaust flow such as exhaust mass flow rate, exhaust space velocity through an SCR catalyst, or another characteristic of exhaust flow. Variable EX is provided to conditional  270 . Conditional  270  tests whether variable EX is less than MAX_EX which is a calibratable maximum exhaust flow value below which SCR diagnostics may be initiated. Conditional  270  outputs true if EX is less than MAX_EX and, if not, outputs false. The output of conditional  270  is provided to operation  280 . 
     The outputs of conditionals  230 ,  240 ,  250 ,  260  and  270  are provided to inputs of operation  280 . Operation  280  is a logical AND which outputs true when all of its inputs are true and, if not, outputs false. The output of operation  280  is provided to operation  290  where it is stored as variable INIT_DIAG which can be used to determine whether to initiate diagnosis of an SCR system. When variable INIT_DIAG is true, diagnosis of an SCR system may be initiated, if not, diagnosis of an SCR system may not be initiated. 
       FIG. 3  is a flowchart illustrating an exemplary SCR diagnostic procedure  300 . Procedure  300  may be implemented by controller  112  of vehicle  100  described above in connection with  FIG. 1  or in another controller. Procedure  300  begins at conditional  310  which tests whether one or more conditions for initiating diagnosis are satisfied. In an exemplary embodiment conditional  310  tests whether variable INIT_DIAG of procedure  200  described above in connection with  FIG. 2  is true. In other embodiments conditional  310  tests whether other conditions for initiating diagnosis are satisfied. If the conditions for initiating diagnosis are satisfied procedure  300  proceeds to operation  320 . If the conditions for initiating diagnosis are not satisfied procedure  300  waits a predetermined time and retests one or more conditions for initiating diagnosis are satisfied. 
     From conditional  310  procedure  300  proceeds to operation  320 . Operation  320  controls an SCR system to operate at a first commanded ammonia to NOx input ratio, i.e., a ratio of ammonia to NOx which is commanded to be provided to an SCR catalyst, and provides information indicative of the NOx output of the SCR system at the first commanded ammonia to NOx ratio. 
     From operation  320  procedure  300  proceeds to operation  330 . Operation  330  controls an SCR system to operate at a second commanded ammonia to NOx input ratio and provides information indicative of the NOx output of the SCR system at the second commanded ammonia to NOx ratio. 
     From operation  330  procedure  300  proceeds to operation  340 . Operation  340  controls an SCR system to operate at a third commanded ammonia to NOx input ratio and provides information indicative of the NOx output of the SCR system at the third commanded ammonia to NOx ratio. 
     From operation  340  procedure  300  proceeds to operation  350 . Operation  350  controls an SCR system to operate at a fourth commanded ammonia to NOx input ratio and provides information indicative of the NOx output of the SCR system at the fourth commanded ammonia to NOx ratio. 
     In one embodiment operations  320 ,  330 ,  340 , and  350  control an SCR system to operate at first, second, third and fourth commanded ammonia to NOx input ratios by commanding a reductant injector to inject first, second, third, and fourth amounts of reductant. The first, second, third, and fourth amounts of reductant may be selected based upon the ratio of reductant to ammonia and the NOx input to the SCR system. The ratio of reductant to ammonia may account for the concentration of the reductant utilized, the type of reductant utilized, and/or the degree of reductant decomposition. The amount of NOx may be determined using a virtual NOx sensor, for example, as described above in connection with  FIG. 1 . Additional embodiments include greater or fewer operations similar to operations  320 ,  330 ,  340 , and  350  which control an SCR system to operate at a plurality of commanded ammonia to NOx input ratios, for example, by commanding a reductant injector to inject a plurality of amounts of reductant. Additional embodiments include control an SCR system to operate at first, second, third and fourth commanded ammonia to NOx input ratios by commanding a reductant injector to inject first, second, third, and fourth amounts of reductant and controlling operation of an engine to provide one or more amounts of NOx. 
     From operation  350  procedure  300  proceeds to operation  360  which determines a diagnostic parameter based upon the information indicative of the NOx output of the SCR system at the plurality of commanded ammonia to NOx input ratios. In an exemplary embodiment operation  360  calculates the slope of a line fitting the information indicative of the NOx outputs of the SCR system at the plurality of commanded ammonia to NOx input ratios. In another embodiment operation  360  calculates the slope and an intercept of a line fit to the information indicative of the NOx outputs of the SCR system at the plurality of commanded ammonia to NOx input ratios, for example, by performing a least squares fit or another linear regression on the information. 
     From operation  360  procedure  300  proceeds to conditional  370  which evaluates the diagnostic parameter to diagnose the SCR system. In an exemplary embodiment conditional  370  evaluates whether the slope is above a maximum slope limit, whether the slope is below a maximum slope limit, whether the intercept is above a maximum intercept limit, and whether the intercept is below a minimum intercept limit. 
     From conditional  370  procedure  300  proceeds to operation  380  where an indication of malfunction may be provided to an operator and a error code may be stored based upon the evaluation performed by conditional  370 . In certain embodiments, if conditional  370  determines that the slope is above the maximum slope limit, operation  380  outputs a command to provide an indication and set an error code indicating that the reductant injector is providing greater than the commanded amount of reductant, or a malfunction of a NOx sensor. In certain embodiments, if conditional  370  determines that the slope is below the minimum slope limit, operation  380  outputs a command to provide an indication and set an error code indicating that the SCR catalyst function has degraded, that the reductant injector is providing less than the commanded amount of reductant, or a malfunction of a NOx sensor. In certain embodiments, if the intercept is above a maximum intercept limit, or below a minimum intercept limit, the diagnosis is determined to be unreliable and ignored. If the evaluated data is within prescribed limits no indication of malfunction is provided, no error code is stored and procedure  300  may return to conditional  310  or terminate. 
       FIG. 4  is a graph  400  illustrating exemplary SCR diagnostic criteria. The horizontal axis of graph  400  indicates the ratio of ammonia to NOx provided to an SCR catalyst. The vertical axis of graph  400  indicates the percentage of NOx converted by the SCR catalyst. Data points  410 - 421  illustrate the relationship between percent NOx conversion and ammonia to NOx ratio. Over a range  440  of ammonia to NOx ratios the relationship between percent NOx conversion and ammonia to NOx input ratio is linear and falls along line  430  which has a slope of  100  and an intercept of zero. For ammonia to NOx ratios outside range  440 , the relationship between percent NOx conversion and ammonia to NOx input ratio departs from line  430  as indicated by data points  417 - 421  or may result unnecessarily low NOx conversion for example, that indicated at data point  410 . In the illustrated embodiment, range  440  extends from an ammonia to NOx ratio of 0.2 to an ammonia to NOx ratio of 0.7. Other embodiments include other ranges falling between about 0.7 and 0. Other embodiments include other ranges falling between about 0.8 and 0. Further embodiments include other ranges determined to provide a substantially linear relationship between NOx conversion and an ammonia to NOX input ratio. A linear or substantially linear relationship between percent NOx conversion and ammonia to NOx input ratio can be used to establish diagnostic criteria including maximum permissible slope  432 , minimum permissible slope  431 , a maximum intercept value and/or a minimum intercept value which can be utilized in a diagnostic procedure such as procedure  300  described above in connection with  FIG. 3 . 
     While exemplary embodiments of the invention have 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 the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. It should be understood that while the use of words such as preferable, preferably, preferred or more preferred utilized in the description above indicate that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the invention, the scope being defined by the claims that follow. 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.