Patent Publication Number: US-9429059-B2

Title: Enhanced real-time ammonia slip detection

Description:
FIELD 
     The present application relates generally to ammonia slip detection in an exhaust gas treatment system included in an exhaust system of an internal combustion engine. 
     BACKGROUND AND SUMMARY 
     Diesel vehicles may be equipped with an exhaust gas treatment system which may include, for example, a urea based selective catalytic reduction (SCR) system and one or more exhaust gas sensors such as nitrogen oxide (NO x ) sensors, at least one of which may be disposed downstream of the SCR system. When the SCR system becomes loaded with urea to a point of saturation, which varies with temperature, the SCR system may begin to slip ammonia (NH 3 ). The NH 3  slip from the SCR system may be detected by the tailpipe NO x  sensor as NO x  resulting in an inaccurate NO x  output which is too high. As such, the efficiency of the SCR system may actually be higher than the efficiency determined based on the inaccurate NOx output. 
     US 2012/0085083 describes a method for estimating NOx conversion using a polynomial model that also allows for the NH 3  concentration at the downstream tailpipe NOx sensor to be estimated. As described therein, temporal sensor signatures of a feedgas NOx sensor located upstream of the SCR and a tailpipe NOx sensor located downstream of the SCR are quantified and fit using a polynomial model that enables estimation of NH 3  slip and NOx conversion efficiency. However, because the method uses a segment of each sensor signal for processing, a time lag exists between the acquisition of each NOx sensor output signal and the allocation of downstream NOx sensor output to NOx and NH 3 . When the time lag is combined with the polynomial fitting algorithm described, which may be prone to localized estimation errors, realization of a real-time NH 3  slip detection system by the methods described would be difficult to implement. 
     The inventors have recognized disadvantages with the approach above and herein disclose methods for the real-time control of ammonia slip in an engine exhaust system. Methods described use transient responses of a NOx sensor to identify the rates of change of a NOx signal. Then, a processor further uses the rates of change to determine how the downstream tailpipe NOx sensor is expected to change based on the flow upstream of the SCR, which allows allocation of a tailpipe NOx sensor in the manner described below with substantially no perceivable delays in processing. 
     In one particular example, the exhaust system includes two NOx sensors that continuously monitor the exhaust gas flow upstream and downstream of an SCR device. Then, when entry conditions of the engine system are met, for example when the SCR device is above a temperature threshold, the rate of change of the upstream feedgas NOx sensor is combined with a current tailpipe reading to estimate the rate of change of the tailpipe NOx sensor expected based on the feedgas signal slope. The expected tailpipe NOx signal is then compared with the actual NOx signal in order to allocate the NOx sensor output to NOx and NH 3 . 
     In another example, a method is provided that comprises allocating a NOx sensor output to each of NH 3  and NOx based on an upstream NOx rate of change and a downstream NOx rate of change relative to the SCR emission device, which thereby allows the amount of reductant delivered to the engine exhaust to be adjusted based on the relative sensor signals. Because the method uses transient responses of the upstream and downstream NOx sensors, in addition to the expected NOx signal, it is therefore possible to achieve a high level of NH 3  detection. In this way, it is possible to provide enhanced allocation of the NOx sensor output in order to determine the relative NOx and NH 3  levels in the exhaust system. 
     The present description may provide several advantages. In particular, the approach may allow for the real-time detection of NH 3  slip with a high level of detection sensitivity without high feedgas NOx interventions. Thus, NH 3  slip can be detected while a vehicle is in operation and corrective measures taken based on the current state of the exhaust system. Furthermore, because the detection sensitivity is increased, high levels of NOx are not required in order to determine the allocation of the tailpipe NOx sensor output to NOx and NH 3 . 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages described herein will be more fully understood by reading an example of an embodiment, referred to herein as the Detailed Description, when taken alone or with reference to the drawings, where: 
         FIG. 1  shows a schematic diagram of an engine including an exhaust system with an exhaust gas treatment system. 
         FIGS. 2  A-D shows graphs illustrating an ammonia slip condition. 
         FIG. 3  shows a flow chart illustrating a routine for detecting ammonia slip in an exhaust gas treatment system. 
         FIG. 4  shows a flow chart illustrating a routine for controlling operating parameters when an exhaust gas sensor output is allocated to nitrogen oxide. 
         FIG. 5  shows a flow chart illustrating a routine for controlling operating parameters when an exhaust gas sensor output is allocated to ammonia. 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to methods and systems for detecting NH 3  slip from an SCR system based on transient NOx signals detected therein. In one example, a method comprising using information from two NOx sensors, a feedgas sensor located upstream of the SCR and a tailpipe sensor located downstream, to predict a tailpipe NOx slope responsive to the transient feedgas NOx signal is described. The method further comprises generating an envelope around the expected tailpipe NOx signal and allocating output from the NOx sensor to each of ammonia and nitrogen oxide in different amounts depending on changes in sensor output. For example, a transient tailpipe sensor output that falls outside of an expected envelope indicates an exhaust system that is slipping NH 3 , which is further quantified by ramping a counter positive toward an upper level that indicates NH 3  slip. Conversely, a transient tailpipe sensor output falling within the expected envelope indicates NOx slip, which is further quantified by ramping a counter negative toward a lower level that indicates NOx slip. In this manner, the exhaust gas sensor may be used to indicate both a reduced exhaust gas treatment system efficiency and an NH 3  slip condition. The method further comprises adjusting one or more operating parameters based on the allocation and change in sensor output. 
     Referring now to  FIG. 1 , a schematic diagram showing one cylinder of multi-cylinder engine  10 , which may be included in a propulsion system of an automobile, is illustrated. The engine  10  may be controlled at least partially by a control system including a controller  12  and by input from a vehicle operator  132  via an input device  130 . In this example, input device  130  includes an accelerator pedal and a pedal position sensor  134  for generating a proportional pedal position signal PP. A combustion chamber (or cylinder)  30  of the engine  10  may include combustion chamber walls  32  with a piston  36  positioned therein. The piston  36  may be coupled to a crankshaft  40  so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. The crankshaft  40  may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to the crankshaft  40  via a flywheel to enable a starting operation of the engine  10 . 
     The combustion chamber  30  may receive intake air from an intake manifold  44  via an intake passage  42  and may exhaust combustion gases via an exhaust passage  48 . The intake manifold  44  and the exhaust passage  48  can selectively communicate with the combustion chamber  30  via respective intake valve  52  and exhaust valve  54 . In some embodiments, the combustion chamber  30  may include two or more intake valves and/or two or more exhaust valves. 
     In the example depicted in  FIG. 1 , the intake valve  52  and exhaust valve  54  may be controlled by cam actuation via respective cam actuation systems  51  and  53 . The cam actuation systems  51  and  53  may each include one or more cams and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT), and/or variable valve lift (VVL) systems that may be operated by the controller  12  to vary valve operation. The position of the intake valve  52  and the exhaust valve  54  may be determined by position sensors  55  and  57 , respectively. In alternative embodiments, the intake valve  52  and/or exhaust valve  54  may be controlled by electric valve actuation. For example, the cylinder  30  may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT systems. 
     In some embodiments, each cylinder of the engine  10  may be configured with one or more fuel injectors for providing fuel thereto. As a non-limiting example, the cylinder  30  is shown including one fuel injector  66 . The fuel injector  66  is shown coupled directly to the cylinder  30  for injecting fuel directly therein in proportion to the pulse width of signal FPW received from the controller  12  via an electronic driver  68 . In this manner, the fuel injector  66  provides what is known as direct injection (hereafter also referred to as “DI”) of fuel into the combustion cylinder  30 . 
     It will be appreciated that in an alternate embodiment, the injector  66  may be a port injector providing fuel into the intake port upstream of the cylinder  30 . It will also be appreciated that the cylinder  30  may receive fuel from a plurality of injectors, such as a plurality of port injectors, a plurality of direct injectors, or a combination thereof. 
     In one example, the engine  10  is a diesel engine that combusts air and diesel fuel through compression ignition. In other non-limiting embodiments, the engine  10  may combust a different fuel including gasoline, biodiesel, or an alcohol containing fuel blend (e.g., gasoline and ethanol or gasoline and methanol) through compression ignition and/or spark ignition. 
     The intake passage  42  may include a throttle  62  having a throttle plate  64 . In this particular example, the position of the throttle plate  64  may be varied by the controller  12  via a signal provided to an electric motor or actuator included with the throttle  62 , a configuration that is commonly referred to as electronic throttle control (ETC). In this manner, the throttle  62  may be operated to vary the intake air provided to the combustion chamber  30  among other engine cylinders. The position of the throttle plate  64  may be provided to the controller  12  by throttle position signal TP. The intake passage  42  may include a mass air flow sensor  120  and a manifold air pressure sensor  122  for providing respective signals MAF and MAP to the controller  12 . 
     Further, in the disclosed embodiments, an exhaust gas recirculation (EGR) system may route a desired portion of exhaust gas from the exhaust passage  48  to the intake passage  42  via an EGR passage  140 . The amount of EGR provided to the intake manifold  44  may be varied by a controller  12  via an EGR valve  142 . By introducing exhaust gas to the engine, the amount of available oxygen for combustion is decreased, thereby reducing combustion flame temperatures and reducing the formation of NO x  for example. As depicted, the EGR system further includes an EGR sensor  144  which may be arranged within the EGR passage  140  and may provide an indication of one or more of pressure, temperature, and concentration of the exhaust gas. Under some conditions, the EGR system may be used to regulate the temperature of the air and fuel mixture within the combustion chamber, thus providing a method of controlling the timing of ignition during some combustion modes. Further, during some conditions, a portion of combustion gases may be retained or trapped in the combustion chamber by controlling exhaust valve timing, such as by controlling a variable valve timing mechanism. 
     An exhaust system  128  includes an exhaust gas sensor  126  coupled to the exhaust passage  48  upstream of an exhaust gas treatment system  150 . The sensor  126  may be any suitable sensor for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a HEGO (heated EGO), a NO x , HC, or CO sensor. The exhaust gas treatment system  150  is shown arranged along the exhaust passage  48  downstream of the exhaust gas sensor  126 . 
     In the example shown in  FIG. 1 , the exhaust gas treatment system  150  is a urea based selective catalytic reduction (SCR) system. The SCR system includes at least an SCR catalyst  152 , a urea storage reservoir  154 , and a urea injector  156 , for example. In other embodiments, the exhaust gas treatment system  150  may additionally or alternatively include other components, such as a particulate filter, lean NO x  trap, three way catalyst, various other emission control devices, or combinations thereof. In the depicted example, the urea injector  156  provides urea from the urea storage reservoir  154 . However, various alternative approaches may be used, such as solid urea pellets that generate an ammonia vapor, which is then injected or metered to the SCR catalyst  152 . In still another example, a lean NO x  trap may be positioned upstream of SCR catalyst  152  to generate NH 3  for the SCR catalyst  152 , depending on the degree or richness of the air-fuel ratio fed to the lean NO x  trap. 
     The exhaust gas treatment system  150  further includes an exhaust gas sensor  158  positioned downstream of the SCR catalyst  152 . In the depicted embodiment, the exhaust gas sensor  158  may be a NO x  sensor, for example, for measuring an amount of post-SCR NO x . In some examples, an efficiency of the SCR system may be determined based on the exhaust gas sensor  158 , for example, and further based on the exhaust gas sensor  126  (when the sensor  126  measures NO x , for example) positioned upstream of the SCR system . In other examples, the exhaust gas sensor  158  may be any suitable sensor for determining an exhaust gas constituent concentration, such as a UEGO, EGO, HEGO, HC, CO sensor, etc. 
     The controller  12  is shown in  FIG. 1  as a microcomputer, including a microprocessor unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values shown as a read only memory chip  106  in this particular example, random access memory  108 , keep alive memory  110 , and a data bus. The controller  12  may be in communication with and, therefore, receive various signals from sensors coupled to the engine  10 , in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from the mass air flow sensor  120 ; engine coolant temperature (ECT) from a temperature sensor  112  coupled to a cooling sleeve  114 ; a profile ignition pickup signal (PIP) from a Hall effect sensor  118  (or other type) coupled to the crankshaft  40 ; throttle position (TP) from a throttle position sensor; absolute manifold pressure signal, MAP, from the sensor  122 ; and exhaust constituent concentration from the exhaust gas sensors  126  and  158 . Engine speed signal, RPM, may be generated by controller  12  from signal PIP. 
     The storage medium read-only memory  106  can be programmed with non-transitory, computer readable data representing instructions executable by the processor  102  for performing the methods described below as well as other variants that are anticipated but not specifically listed. 
     In one example, the controller  12  may detect NH 3  slip based on output from the exhaust gas sensor  158 , as will be described in greater detail below with reference to  FIG. 2 . As an example, when the sensor  158  detects a threshold increase in NO x  output, the controller  12  adjusts the EGR valve  142  to reduce an amount of EGR such that NO x  emission from the engine  10  increases. Based on the change in sensor output during the period of reduced EGR, the sensor output is allocated to NO x  or NH 3 . For example, if the sensor output increases, the output is allocated to NO x , as increased NO x  from the engine is not reduced by the SCR system. On the other hand, if the sensor output does not change by more than a threshold amount, the output is allocated to NH 3  and NH 3  slip is indicated. Based on the change in output and the allocation, the controller  12  may adjust one or more engine operating parameters. As non-limiting examples, the controller  12  may adjust the amount of EGR and/or the delivery of reductant based on the change in output and the allocation. 
     As described above,  FIG. 1  shows one cylinder of a multi-cylinder engine, and each cylinder may similarly include its own set of intake/exhaust valves, fuel injector, spark plug, etc. 
     Turning to the plots shown in  FIGS. 2A-D , an example transient NOx signal depicts an ammonia slip condition is shown for the two sensor system of  FIG. 1 . Because NOx sensors produce output signals responsive to both NOx and NH 3 , a method to detect NH 3  slip may be useful for managing the output of the exhaust system and resources therein. For example, if an SCR system is loaded with urea to the point of saturation, which varies with temperature, it may start to slip NH 3 . The NH 3  slipping past the SCR may be read by the tailpipe NOx sensor as NOx, which confuses the SCR control and monitoring system into thinking the system has a lower efficiency than it really has since some of the signal is actually due to NH 3 . 
     In  FIGS. 2A-D , four temporal plots are shown that exemplify the method. The four plots are related and therefore use the same time axis, which for simplicity is shown along the bottom plot. Furthermore, although the data is shown schematically as a function of time in seconds, the unit of time is not limiting and other units of time are possible. From top to bottom, the four plots represent: NOx signals collected by NOx sensors in the exhaust gas system; derivative plots of the predicted and actual slopes of the tailpipe NOx sensor according to the method; a plot showing the difference between predicted and actual slopes of the tailpipe NOx sensor; and a counter with a threshold to indicate NH 3  slip. 
     In  FIG. 2A , an example feedgas signal  202  is shown dashed and an example tailpipe signal  204  is shown solid. When exhaust system  128  is in a state of NOx slip, for example, when the SCR is not saturated and NH 3  is not being released into the exhaust system, the tailpipe signal may generally be proportional to the feedgas signal. As such, the feedgas NOx signal and tailpipe NOx signal may be in phase and follow each other closely. Furthermore, when the NOx conversion efficiency is substantially zero, tailpipe signal  204  and feedgas signal  202  may be substantially identical. Conversely, for higher NOx conversion efficiencies, the shape of tailpipe signal  204  may resemble the shape of feedgas signal  202  but be a scaled down version of the feedgas signal. Alternatively, when exhaust system  128  is in a state of NH 3  slip, tailpipe signal  204  may have a somewhat flattened appearance or undulate at a lower frequency than feedgas signal  202 . Because of this, during NH 3  slip there is usually a period of time where the two signals are out of phase. Although the tailpipe signal can exceed the feedgas signal, particularly after an increase in temperature, this generally happens during transient or changing conditions, which allows NH 3  slip to be identified from the two signals by the method described herein. 
     The method relies on a transient response of the NOx sensors in order to allocate signal to NH 3  and NOx. Therefore, a central feature of the method is the rate of change of the NOx signal as a function of time, or d(NOx)/dt.  FIG. 2B  shows a derivative plot wherein the slope or rate of change of tailpipe signal  204  in  FIG. 2A  is plotted versus time. Transient NH 3  detection is built around a comparison of the actual tailpipe NOx slope detected to the predicted tailpipe NOx slope expected. As such,  FIG. 2B  includes actual slope  210  that represents the rate of change of the tailpipe signal  204  from  FIG. 2A . Actual slope  210  is shown in four parts labeled a-d for reasons that will be described in more detail below. The method further includes predicting a tailpipe NOx slope using the slope of the feedgas NOx (from feedgas signal  202  in  FIG. 2A ) and the current tailpipe NOx signal, or an instantaneous reading from the tailpipe sensor. The predicted slope  212  for tailpipe NOx sensor, for example, sensor  158  in  FIG. 1 , can be generated using a known relationship. Herein, the tailpipe NOx slope is predicted using:
 
( d TP NOx   /dt ) exp =(TP/FG)*( d FG NOx   /dt ) act ,
 
where (dTP NOx /dt) exp  is the expected or predicted rate of change of the tailpipe signal, TP is an instantaneous tailpipe reading, FG is an instantaneous feedgas reading, and (dFG NOx /dt) act  is the actual rate of change of the feedgas signal. Using this method, a comparison of the two slope signals based on the transient responses of the NOx sensors allows a high level of NH 3  detection sensitivity. For example, in some embodiments, the transient detection method can detect NH 3  levels as low as 25 ppm.
 
     To gauge how close actual slope  210  is to predicted slope  212  during operation, in other words, how the change in NOx signal detected corresponds to the change expected from feedgas signals and system efficiencies, the ammonia slip detection (ASD) method described includes generating an envelope around the predicted slope curve. The envelope defines a region around the predicted rate of change where the NOx output signal is likely to fall when the system is in NOx slip. Therefore,  FIG. 2B  shows two dash-dot lines that represent a positive envelope  214  offset from the predicted slope in the positive direction and negative envelope  216  offset from the predicted slope in the negative direction. When taken together, both the positive and negative envelopes define a region around the predicted rate of change curve that allows for signal discrimination and assessment of the NOx and NH 3  levels in the exhaust system. 
     Returning to actual slope  210  that is shown in four parts labeled a-d. The different regions of the curve signify time periods when entry conditions are met such that a comparison between the two slope curves can be expected to provide accurate determinations of the NOx and NH 3  levels in the exhaust system. For example, sensors  126  and  158  within exhaust system  128  are coupled to controller  12  that may include non-transitory, computer readable data representing instructions executable by processor  102  for enabling and disabling the method based on operating conditions of the engine. Therefore, curves  210 a and  210 c are shown as unbolded, dashed line segments to represent exemplary periods where entry conditions are not met and the method is disabled. Conversely, curves  210   b  and  210   d  are shown as bold, dashed line segments to represent exemplary periods where entry conditions are met and the method is enabled. When engaged, a controller processes the data by comparing actual slope  210  to predicted slope  212  and the surrounding envelope. Basically, when actual slope  210  falls within the envelope, a window counter ramps negative and is decremented toward zero to indicate the exhaust system is comprised of NOx whereas when actual slope  210  falls outside of the envelope, the window counter ramps positive and is incremented toward an upper level away from zero to indicate the presence of NH 3  slip in the exhaust system. 
     In some embodiments, during conditions of NH 3  slip the exhaust system may include a tailpipe NOx sensor signal comprised of lower frequency content relative to the feedgas signal. Because of this, an upper level tailpipe frequency may be indicative of NH 3  slip. Therefore, when actual slope  210  is greater than a frequency threshold, high frequency content may be indicated that is interpreted as a NOx signal. In response, the window counter may be decremented toward zero to indicate NOx slip regardless of whether the slope falls inside or outside of the envelope. For example, in some embodiments, a rate of change, (dTP NOx /dt) actual  (actual slope  210  in  FIG. 2B ), greater than a maximum allowed rate may be treated as a NOx response by the system. 
     In  FIG. 2C , difference plot  220  is shown that reflects the relative difference between actual slope  210  and predicted slope  212  from  FIG. 2B . For clarity, a horizontal line at y=0 that indicates no difference, is also shown. Therein, the fluctuations of the actual slope relative to the predicted slope may be more clearly observed. For example, at early times on the left a negative peak is observed that reflects a lower actual slope than was predicted by the method (e.g. actual slope  210  is less than predicted slope  212 ). Thereafter, following the contour of the difference plot, the actual slope fluctuates around the predicted slope based on conditions in the exhaust system. Although not shown, in some embodiments, other horizontal threshold lines may also be included to further indicate places where differences between the two plots are substantially large. 
     Turning to  FIG. 2D , a plot of the window counter that is used for indicating NH 3  slip is shown. As described briefly above, when the ASD system is enabled by controller  12 , the window counter increments toward an upper level that indicates NH 3  slip when actual slope  210  falls outside of the envelope, and decrements toward a lower level that indicates NOx slip when actual slope  210  falls within the envelope. Therefore, window counter  230  is shown increasing when actual slope  210   b  falls outside of the envelope. In  FIG. 2D , two thresholds are shown. First threshold  236  indicates NH 3  slip in the exhaust system. As such, when the window counter exceeds first threshold  236 , an NH 3  flag is set to indicate that NH 3  is slipping from the SCR. For simplicity, in this example method, the NH 3  flag is a binary flag. Therefore, when window counter  230  is greater than first threshold  236 , an NH 3  flag is set to 1. Alternatively, when window counter  230  falls below first threshold  236 , the NH 3  flag is reset to 0. In the example signal processing application shown, the detection system is enabled in the two regions identified at  210   b  and  210   d .During these periods, the counter is active and the controller uses the state of the system to identify whether NH 3  slip is occurring or not. In some embodiments, the relative magnitude of window counter  230  compared to first threshold  236  may be used to indicate when exhaust system  128  is slipping NH 3  while in other embodiments, the instantaneous location of window counter  230  relative to an upper level (indicating NH 3 ) and a lower level (e.g. 0 indicating NOx) may be used to indicate a probability or degree of NH 3  slip in the exhaust system. In still other embodiments, a second threshold  234  may be included that is lower than first threshold  236 . When second threshold  234  is present, the NH 3  flag may be reset to 0 when window counter  230  falls below second threshold  234  instead of first threshold  236 , as was described above. Different thresholds allow for hysteresis in the system so the NH 3  flag is not reset to indicate NOx if window counter  230  falls briefly below first threshold  236 . Rather, NOx is indicated when window counter  230  falls below the lower threshold that is set to indicate a higher degree of NOx slip in the exhaust system. 
     Because the ammonia slip detection system is under the control of controller  12 , instructions for disabling the detection system may be included in the programmable software stored by the control system. Although the detection system can be disabled based on many conceivable operating conditions, and many combinations of variables are possible, in one embodiment, the programmable instructions may implement the following conditions to disable the detection system: a low SCR temperature, high feedgas NOx levels indicating a saturated feedgas sensor output, high tailpipe NOx levels indicating a saturated tailpipe sensor output, low feedgas or tailpipe NOx levels below a detection threshold, high or low rates of change in the NOx conversion efficiency, low torque output by the engine system, low injection pulses of urea from a storage reservoir, a calibrateable delay after a feedgas sensor or tailpipe sensor becomes activate, a high rate of change of space velocity, a low exhaust flow, a minimum/maximum actual or predicted slope indicating a deadzone in the detected signal, and a low rate of change of feedgas NOx that identifies feedgas inflection points. In response to detection of one or more of these conditions by controller  12 , the ASD method may be disabled so no processing of the signal occurs in the manner described herein. For example, actual slope  210   c  refers to a slope signal acquired during a period when the detection system is disabled. As another example, line  232  is a binary line indicating the disable state of the system. Therefore, when line  232  is substantially on the x-axis, the ASD system is enabled and controller  12  may monitor the exhaust conditions in the manner already described. Conversely, when line  232  is above the x-axis, the ASD system may be disabled so no signal processing occurs. As such, further processing of the tailpipe NOx signal is substantially prohibited since the information obtained may not reliably express NOx and NH 3  levels within the exhaust system. During periods where the detection system is disabled, the control system may still monitor conditions within the exhaust system and further have the flexibility to activate the detection system, which in some instances, may involve overriding the disabling software or conditions identified therein. 
     Turning to the method for processing NOx signals by the control system, in  FIG. 3 , a flow chart illustrating example method  300  for the detection of ammonia slip in an exhaust gas treatment system is shown. Therein, the set of programmable decisions a controller may utilize when allocating a NOx sensor signal to either NOx or NH 3 , or a combination thereof, is described. 
     At  302 , method  300  includes determining the engine operating conditions. The operating conditions may include both engine operating conditions (e.g., engine speed, engine load, amount of EGR, air fuel ratio, etc.) and exhaust gas treatment system conditions (e.g., exhaust temperature, SCR catalyst temperature, amount of urea injection, etc.). 
     At  304 , method  300  includes determining a predicted rate of change of the tailpipe NOx sensor and generating an envelope based on the expected slope. As described above, the rate of change of the tailpipe NOx sensor may be predicted using the feedgas NOx sensor signal output and a current measure of the tailpipe NOx sensor signal output. Then, based on the rate of change of the tailpipe NOx sensor predicted, the method may further generate the envelope to define a region wherein the signal may be expected to fall when the exhaust system is operating in conditions of NOx slip. Although many methods can be conceived of to generate an envelope, in some embodiments, the envelope is simply a percentage of the predicted slope that is offset from the predicted slope in the positive and negative directions. For example, a controller that defines a region within 5% of a predicted slope of 10.0 may generate a positive envelope with a value of 10.5 and a negative envelope with a value of 9.5. Alternatively, if the predicted slope is smaller, e.g. 1.0, the positive envelope may have a value of 1.05 and the negative envelope may have a value of 0.95. In this way, the envelope will define a region surrounding the predicted curve that is within 5% of the curve. Returning to the envelope of  FIG. 2B , the size of the region defined by the envelope clearly deviates as the magnitude of the slope of the predicted curve fluctuates around zero. At  306 , method  300  includes determining the actual rate of change of the tailpipe NOx sensor. 
     Although method  300  may monitor NOx sensors frequently, or even continuously, controller  12  may also enable or disable the system in the manner already described with respect to  FIG. 2B . As such, at  308  method  300  includes determining whether the entry conditions have been met. If controller  12  determines that the entry conditions allow for accurate measurements to be made by the detection system, for instance because the temperature of the SCR is above a threshold, then the ASD system may be activated. Therefore, at  310 , the activated system includes enabling the window counter in order to compare the actual slope to the predicted slope, as indicated at  312 . Alternatively, if controller  12  determines that an accurate measurement by the NOx system is not possible based on conditions detected in the engine system, at  314 , the control system may disable the counter so no further signal processing occurs after signal acquisition. In some embodiments, when the ASD system is disabled, the counter may be reset by ramping the counter negative to indicate NOx slip by the system. In other embodiments, the counter may not be ramped in the manner described above, but simply hold a value until the detection system is reactivated. 
     Returning to  312 , wherein controller  12  has determined that the entry conditions have been met and the detection system is activated to allow adjustment of a counter based on a comparison between the actual and predicted NOx rates, once the comparison is made, at  318  the controller may be programmed to determine whether the actual slope falls within the envelope. Then, based on a location of the actual slope relative to the envelope, a positive or negative score may be assigned based on the relative differences. As described in more detail above with respect to  FIG. 2D , at  320  the counter is ramped positive toward an upper level that indicates NH 3  slip when the actual slope falls outside of the envelope whereas at  316  the counter is ramped negative toward a lower level (e.g. zero) indicating NOx slip when the measured rate falls within the envelope. 
     After ramping the counter based on the relative location of the actual slope compared to the envelope surrounding the predicted rate of change, at  322  method  300  further compares the counter to a threshold to determine whether the tailpipe NOx sensor is to be allocated to NOx or NH 3 . In one embodiment, sensor allocation includes allocating a first portion of the NOx sensor output to NOx and a second, remaining portion of the NOx sensor output to ammonia. Then, based on the allocation, delivering reductant to the engine exhaust based on each of the first and second portions. For example, reductant can be increased responsive to increased NOx but decreased responsive to increased NH 3 . Therefore, the amount of reductant injected generally depends on the relative amounts indicated by the first and the second portions. 
     If the counter is above the first threshold, for example first threshold  236  in  FIG. 2D , at  324  controller  12  may allocate at least some of the tailpipe output signal to NH 3  slip and set a flag to indicate such at  326 . Alternatively, if controller  12  determines that the counter falls below the first threshold, at  328  it may allocate at least some of the tailpipe output signal to NOx slip and set a flag to indicate such at  330 . In some embodiments, the current status of the sensor allocation may correspond to a probability of NH 3  slip while in other embodiments, NH 3  slip may be indicated by a binary flag. In this manner, controller  12  can detect ammonia slip within the exhaust system and allocate the NOx sensor output to either one or both of NOx and NH 3  while communicating the current status to a driver and adjusting one or more operating parameters based on the sensor output. 
     Continuing to  FIG. 4 , a routine for adjusting system operation based on the allocation of sensor output to NO x  is shown. Specifically, the routine determines an exhaust NO x  concentration downstream of the SCR catalyst and adjusts one or more operating parameters based on the sensor output. 
     At  402 , operating conditions are determined. As described above, the operating conditions may include engine operating conditions (e.g., engine speed, engine load, amount of EGR, air fuel ratio, etc.) and exhaust gas treatment system conditions (e.g., exhaust temperature, SCR catalyst temperature, amount of urea injection, etc.). 
     Once the operating conditions are determined, the routine proceeds to  404  and the exhaust NO x  concentration downstream of the SCR catalyst is determined based on the exhaust gas sensor output. 
     At  406 , one or more operating parameters are adjusted based on the NO x  concentration. As non-limiting examples, the operating parameters may include amount of EGR and amount of urea injection, or dosing level, wherein the urea dosing level may be adjusted until an actual NOx efficiency matches a predicted NOx efficiency. For example, the amount of EGR may be increased by an amount corresponding to the change in NO x  amount above the threshold amount. By increasing the amount of EGR, less NO x  may be emitted by the engine resulting in a reduced amount of NO x  passing through the SCR catalyst. As another example, the amount of urea injection may be increased by an amount corresponding to the change in NO x  amount above the threshold amount and a temperature of the SCR catalyst. The amount of urea injection may be increased by changing the pulsewidth or duration of the urea injection, for example. By increasing the amount of urea injected to the SCR catalyst, a greater amount of NO x  may be reduced by the catalyst, thereby reducing the amount of NO x  which passes through the catalyst. In other examples, a combination of amount of EGR and amount of urea injection may be adjusted. 
     In  FIG. 5 , a routine for adjusting system operation based on the allocation of sensor output to NH 3  is shown. Specifically, the routine determines an exhaust NH 3  concentration downstream of the SCR catalyst and adjusts one or more operating parameters based on the sensor output. 
     At  502 , operating conditions are determined. As described above, the operating conditions may include engine operating conditions (e.g., engine speed, engine load, amount of EGR, air fuel ratio, etc.) and exhaust gas treatment system conditions (e.g., exhaust temperature, SCR catalyst temperature, amount of urea injection, etc.). 
     Once the operating parameters are determined, the routine continues to  504  and the exhaust NH 3  concentration downstream of the SCR catalyst is determined based on the exhaust sensor output. 
     At  506 , one or more operating parameters are adjusted based on the NH 3  concentration. As non-limiting examples, the operating parameters may include amount of urea injection and amount of EGR. For example, the amount of urea injection may be reduced such that an amount of excess NH 3  which slips from the SCR catalyst is reduced. As described above, the amount of urea injection may be increased by changing the pulsewidth or duration of the urea injection. As another example, the amount of EGR may be reduced. For example, by reducing the amount of EGR, a greater amount of NO x  may be emitted from the engine. The increased NO x  may be reduced by the excess NH 3  in the SCR catalyst, thereby reducing the amount of NO x  which passes through the SCR catalyst. 
     With regard to urea dosing, in one embodiment, the exhaust system may be an adaptive SCR system that achieves the proper adaptive value by adjusting the urea dosing level until the actual NOx efficiency matches the predicted NOx efficiency. For example, as tailpipe NOx levels increase, the calculated NOx efficiency decreases. If the efficiency drops too low, the adaptive system responds by increasing urea dosing to achieve the predicted NOx efficiency. Conversely, as NH 3  levels increase, the calculated efficiency also decreases since NH 3  looks like NOx to a NOx sensor. As such, the adaptive system responds by decreasing urea dosing to achieve the predicted efficiency. Because the adaptive correction is different for NOx versus NH 3  slip, the control system may depend on allocation of a NOx sensor output to NOx and NH 3  by the methods described herein. 
     The amount the operating parameters are adjusted may be further based on a temperature of the SCR catalyst, as the point of urea saturation of the catalyst varies with temperature. For example, when the temperature of the catalyst is a relatively higher temperature, the amount of EGR may be reduced less and/or the amount of urea injection may be reduced by a smaller amount. In contrast, when the temperature of the catalyst is a relatively lower temperature, the amount of EGR may be increased more and/or the amount of urea injection may be reduced by a larger amount. 
     In other examples, only the amount of EGR may be decreased or only the amount of urea injected to the SCR catalyst may be increased. In still other examples, one or more other operating parameters may be additionally or alternatively adjusted. As such, one or more operating parameters are adjusted in order to reduce the NH 3  slip. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system. 
     This concludes the description. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the description. For example, I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas, gasoline, diesel, or alternative fuel configurations could use the present description to advantage. 
     The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.