EXHAUST GAS TREATMENT SYSTEM AND METHODS FOR OPERATING THE SAME
An exhaust gas treatment system includes a selective catalytic reduction (SCR) catalyst and a dosing control responsive to exhaust gas operating conditions for controlling the dosing rate of a reductant such as aqueous urea into the exhaust stream. The dosing control is configured to reduce the dosing rate when either a sudden increase in the exhaust mass air flow is detected or when an exhaust gas temperature gradient is in an increasing state. The dosing control is also configured to shut-off dosing when a measured ammonia concentration level exceeds an ammonia slip trip level, provided that the exhaust gas temperature gradient is also in an increasing state.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. provisional application Ser. No. 61/108,172 filed Oct. 24, 2008 entitled “DIAGNOSTIC METHODS FOR SELECTIVE CATALYTIC REDUCTION (SCR) EXHAUST TREATMENT SYSTEM and EXHAUST GAS TREATMENT SYSTEM AND METHODS FOR OPERATING THE SAME” (attorney Docket No. DP-318283), the disclosure of which is hereby incorporated by reference in its entirety.
The present invention relates generally to an exhaust gas treatment system for use with an internal combustion engine where the exhaust treatment system is of the type using a selective catalytic reduction (SCR) catalyst and methods for operating the same.
BACKGROUND OF THE INVENTION
The relevant background includes the fields of exhaust gas treatment systems and diagnostics therefore. As to the former field of endeavor, there have been a variety of exhaust gas treatment systems developed in the art to minimize emission of undesirable constituent components of engine exhaust gas. It is known to reduce NOx emissions using a SCR catalyst, treatment device that includes a catalyst and a system that is operable to inject material such as ammonia (NH3) into the exhaust gas feedstream ahead of the catalyst. The SCR catalyst is constructed so as to promote the reduction of NOx by NH3 (or other reductant, such as aqueous urea which undergoes decomposition in the exhaust to produce NH3). NH3 or urea selectively combine with NOx to form N2 and H2O in the presence of the SCR catalyst, as described generally in U.S. Patent Publication 2007/0271908 entitled “ENGINE EXHAUST EMISSION CONTROL SYSTEM PROVIDING ON-BOARD AMMONIA GENERATION”. For diesel engines, for example, selective catalytic reduction (SCR) of NOx with ammonia is perhaps the most selective and active reaction for the removal of NOx in the presence of excess oxygen. The NH3 source must be periodically replenished and the injection of NH3 into the SCR catalyst requires precise control. Overinjection may cause a release of NH3 (“slip”) out of the tailpipe into the atmosphere, while underinjection may result in inadequate emissions reduction (i.e., inadequate NOx conversion to N2 and H2O).
These systems have been amply demonstrated in the stationary catalytic applications. For mobile applications where it is generally not possible (or at least not desirable) to use ammonia directly, urea-water solutions have been proven to be suitable sources of ammonia in the exhaust gas stream. This has made SCR possible for a wide range of vehicle applications.
Increasingly stringent demands for low tail pipe emissions of NOx have been placed on heavy duty diesel powered vehicles. Liquid urea dosing systems with selective catalytic NOx reduction (SCR) technologies have been developed in the art that provide potentially viable solutions for meeting current and future diesel NOx emission standards around the world. Ammonia emissions may also be set by regulation or simply as a matter of quality. For example, European emission standards (e.g., EU 6) for NH3 slip targets specify 10 ppm average and 30 ppm peak. However, the challenge described above remains, namely, that such treatment systems achieve maximum NOx reduction (i.e., at least meeting NOx emissions criteria) while at the same time maintaining acceptable NH3 emissions, particularly over the service life of the treatment system.
In addition to the substantive emissions standards described above, vehicle-based engine and emission systems typically also require various self-monitoring diagnostics to ensure tailpipe emissions compliance. In this regards, U.S. federal and state on-board diagnostic regulations (e.g., OBDII) require that certain emission-related systems on the vehicle be monitored, and that a vehicle operator be notified if the system is not functioning in a predetermined manner. Automotive vehicle electronics therefore typically include a programmed diagnostic data manager or the like service configured to receive reports from diagnostic algorithms/circuits concerning the operational status of various components or systems and to set/reset various standardized diagnostic trouble codes (DTC) and/or otherwise generate an alert (e.g., MIL). The intent of such diagnostics is to inform the operator when performance of a component and/or system has degraded to a level where emissions performance may be affected and to provide information (e.g., via the DTC) to facilitate remediation.
Over the service life of the above-described exhaust treatment systems, various constituent components can wear, degrade or the like, possibly impairing overall performance. For example, degradation of either the SCR catalyst or the dosing system may impair the treatment system in meeting either or both of the NOx and NH3 emission standards. Open loop control does not appear to provide an adequate solution. It would be advantageous to provide diagnostic routines to detect any such degradation.
There is therefore a need for diagnostic methods that minimize or eliminate one or more of the problems set forth above.
SUMMARY OF THE INVENTION
The invention provides an advantage for exhaust gas treatment systems that use ammonia or other reductant (e.g., aqueous urea solution) injection in combination with an SCR catalyst for NOx removal from the engine exhaust gas. More specifically, the invention allows for an increased default dosing rate for normal operation, particularly at lower temperatures (e.g., below 300° C.), for maximal NOx conversion under certain driving conditions, without significant risk of high ammonia concentration slips when the temperature increases. This is because the control features of the invention are configured to recognize when possible ammonia slips are likely and reduce the dosing in advance.
In one aspect of the invention, a method of operating the exhaust treatment system is provided where predetermined dosing rates are dynamically reduced when certain exhaust transients are detected. The method involves a number of steps. The first step involves dosing reductant (e.g., NH3 or aqueous urea) into the exhaust gas stream in an amount based on predetermined surface coverage parameter theta values (“target θNH3”). In one embodiment, the target theta values are selected based on exhaust temperature. The next step involves decreasing the dosing when at least one of a plurality of transient compensation trigger conditions are satisfied. The amount of the dosing decrease is configured to mitigate or prevent the occurrence of an unacceptably high ammonia slip. The first trigger condition is when a rate of change of an exhaust mass air flow exceeds a first predetermined threshold. The logic for this condition is that a sudden increase in the exhaust mass air flow portends a near-term increase in the exhaust temperature. The near-term increase in exhaust temperature, in turn, can lead to rapid NH3 desorption, resulting in perhaps a high concentration NH3 slip. The second trigger condition is when an exhaust gas temperature gradient is in an increasing state. In one embodiment, this is satisfied when the gradient exceeds a predetermined level (e.g., 0.5-0.6° C./second). The logic for this condition is that rapid temperature increases can also lead to NH3 desorption, and thus NH3 slips.
In a second aspect of the invention, a method of NH3 slip control is provided. The slip control feature, in one embodiments, shuts-off dosing when certain exhaust conditions are detected, thereby mitigating an ammonia slip. The method includes a number of steps. The first step involves dosing reductant (e.g., NH3, aqueous urea) into the exhaust gas stream. Next, establishing an ammonia slip trip level based on the exhaust temperature. Finally, decreasing (perhaps significantly), and preferably, discontinuing, the dosing step when an ammonia concentration level, measured at the SCR catalyst (e.g., mid-brick position) exceeds the ammonia slip trip level, provided that the exhaust temperature gradient is in an increasing state. The combination of conditions indicate the risk of an unacceptably high NH3 slip and warrant shutting-off dosing until the conditions subside.
An exhaust gas treatment system is also presented.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described by way of example, with reference to the accompanying drawings:
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,
In the illustrative embodiment, the engine 10 may be a turbocharged diesel engine. In a constructed embodiment, the engine 10 comprised a conventional 6.6-liter, 8-cylinder turbocharged diesel engine commercially available under the DuraMax trade designation. It should be understood this is exemplary only.
The software algorithms and calibrations which are executed in the ECU 16 may generally comprise conventional strategies known to those of ordinary skill in the art. Overall, in response to the various inputs, the ECU 16 develops the necessary outputs to control the throttle valve position, fueling (fuel injector opening, duration and closing), spark (ignition timing) and other aspects, all as known in the art.
In addition to the control of the engine 10, the ECU 16 is also typically configured to perform various diagnostics. For this purpose, the ECU 16 may be configured to include a diagnostic data manager or the like, a higher level service arranged to manage the reports received from various lower level diagnostic routines/circuits, and set or reset diagnostic trouble code(s)/service codes, as well as activate or extinguish various alerts, all as known generally in the art. For example only, such a diagnostic data manager may be pre-configured such that certain non-continuous monitoring diagnostics require that such diagnostic fail twice before a diagnostic trouble code (DTC) is set and a malfunction indicator lamp (MIL) is illuminated. As shown in
With continued reference to
The DOC 28 and the DPF 30 may comprise conventional components to perform their known functions.
The dosing subsystem 32 is responsive to an NH3 Request signal produced by a dosing control 80 and configured to deliver a NOx reducing agent at an injection node 68, which is introduced in the exhaust gas stream in accurate, controlled doses 70 (e.g., mass per unit time). The reducing agent (“reductant”) may be, in general, (1) NH3 gas or (2) a urea-water solution containing a predetermined known concentration of urea. The dosing unit 32 is shown in block form for clarity and may comprise a number of sub-parts, including but not limited to a fluid delivery mechanism, which may include an integral pump or other source of pressurized transport of the urea-water solution from the storage tank, a fluid regulation mechanism, such as an electronically controlled injector, nozzle or the like (at node 68), and a programmed dosing control unit. The dosing subsystem 32 may take various forms known in the art and may comprise commercially available components.
The SCR catalyst 38 is configured to provide a mechanism to promote a selective reduction reaction between NOx, on the one hand, and a reductant such as ammonia gas NH3 (or aqueous urea, which decomposes into ammonia, NH3) on the other hand. The result of such a selective reduction is, as described above in the Background, N2 and H2O. In general, the chemistry involved is well documented in the literature, well understood to those of ordinary skill in the art, and thus will not be elaborated upon in any greater detail. In one embodiment, the SCR catalyst 38 may comprise copper zeolite (Cu-zeolite) material, although other materials are known. See, for example, U.S. Pat. No. 6,576,587 entitled “HIGH SURFACE AREA LEAN NOx CATALYST” issued to Labarge et al., and U.S. Pat. No. 7,240,484 entitled “EXHAUST TREATMENT SYSTEMS AND METHODS FOR USING THE SAME” issued to Li et al., both owned by the common assignee of the present invention, and both hereby incorporated by reference in their entirety. In addition, as shown, the SCR catalyst 38 may be of multi-brick construction, including a plurality of individual bricks 381, 382 wherein each “brick” may be substantially disc-shaped. The “bricks” may be housed in a suitable enclosure, as known.
The NOx concentration sensor 40 is located upstream of the injection node 68. The NOx sensor 40 is so located so as to avoid possible interference in the NOx sensing function due to the presence of NH3 gas. The NOx sensor 40, however, may alternatively be located further upstream, between the DOC 28 and the DPF 30, or upstream of the DOC 28. In addition, the exhaust temperature is often referred to herein, and for such purpose, the temperature reading from the SCR inlet temperature sensor 44 (TIN) may be used.
The NH3 sensor 60 may be located, in certain embodiments, at a mid-brick position, as shown in solid line (i.e., located anywhere downstream of the inlet of the SCR catalyst 38 and upstream of the outlet of the SCR catalyst 38). As illustrated, the NH3 sensor 60 may be located at approximately the center position. The mid-brick positioning is significant. The sensed ammonia concentration level in this arrangement, even during nominal operation, is at a small yet detectable level of mid-brick NH3 slip, where the downstream NOx conversion with this detectable NH3 can be assumed in the presence of the rear brick, even further reducing NH3 concentration levels at the tail pipe to within acceptable levels. Alternatively, in certain embodiments, the NH3 sensor 60 may be located at the outlet of the SCR catalyst 38. The remainder of the sensors shown in
The dosing control 80 is configured to generate the NH3 Request signal that is sent to the dosing unit 36, which represents the command for a specified amount (e.g., mass rate) of reductant to be delivered to the exhaust gas stream. The dosing control 80 includes a plurality of inputs and outputs, designated 18, for interface with various sensors, other control units, etc., as described herein. Although the dosing control 80 is shown as a separate block, it should be understood that depending on the particular arrangement, the functionality of the dosing control 80 may be implemented in a separate controller, incorporated into the ECU 16, or incorporated, in whole or in part, in other control units already existing in the system (e.g., the dosing unit). Further, the dosing control 80 may be configured to perform not only control functions described herein but perform the various diagnostics also described herein as well. For such purpose, the dosing control 80 may include conventional processing apparatus known in the art, capable of executing pre-programmed instructions stored in an associated memory, all performing in accordance with the functionality described herein. That is, it is contemplated that the control and diagnostic processes described herein will be programmed in a preferred embodiment, with the resulting software code being stored in the associated memory. Implementation of the invention, in software, in view of the foregoing enabling description, would require no more than routine application of programming skills by one of ordinary skill in the art. Such a control may further be of the type having both ROM, RAM, a combination of non-volatile and volatile (modifiable) memory so that the software can be stored and yet allow storage and processing of dynamically produced data and/or signals.
Overall, the dosing control 80 is configured to generate an NH3 Request, which is communicated to the dosing unit 36 (i.e., shown as the “NH3/Urea Dosing”). In the illustrative embodiment, the NH3 Request is indicative of the mass flow rate at which the dosing subsystem 32 is to introduce the urea-water solution into the exhaust gas stream. The control variable used in implementing the dosing control strategy is a so-called ammonia surface coverage parameter theta (θNH3), which corresponds to the NH3 surface storage fraction associated with the SCR catalyst 38. In other words, the ammonia surface coverage parameter theta (θNH3) indicates the amount of ammonia-NH3 stored in the SCR catalyst 38. One aspect of the operation of the dosing control 80 involves an SCR model 82.
The SCR model 82 may be configured to have access to a plurality of signals/parameters as needed to execute the predetermined calculations needed to model the catalyst 38. In the illustrative embodiment, this access to sensor outputs and other data sources may be implemented over a vehicle network (not shown), but which may be a controller area network (CAN) for certain vehicle embodiments. Alternatively, access to certain information may be direct to the extent that the dosing control 80 is integrated with the engine control function in the ECU 16. It should be understood that other variations are possible.
The SCR model 82 may comprise conventional models known in the art for modeling an SCR catalyst. In one embodiment, the SCR model 82 is responsive to a number of inputs, including: (i) predicted NO and NO2 levels 88; (ii) an inlet NOx amount, which may be derived from the NOx indicative signal 42 (best shown in
Referring again to
As shown in
The theta perturbation diagnostic block 100 is configured to perturb the target theta parameter in accordance with a small diagnostic function and to measure the resulting response to determine the state of health of one or more components of the exhaust treatment system 14. The adaptive learning diagnostic block 102 includes a diagnostic feature that monitors how much adaptation has been applied in adjusting the target theta parameter and generates an error when the level of adaptation exceeds predetermined upper and lower limits. The logic in operation is that at some level, the ability to adapt target theta values to overcome errors (e.g., reagent misdosing, reagent quality problems, SCR catalyst degradation) will reach its control limit for maintaining emissions. When this control limit is exceeded, the diagnostic generates an error. These features are described in greater detail in the co-pending patent application entitled “DIAGNOSTIC METHODS FOR SELECTIVE CATALYTIC REDUCTION (SCR) EXHAUST TREATMENT SYSTEM”, (Attorney Docket No. DP-318283), filed on even date herewith, owned by the common assignee of the present invention, the disclosure of which is hereby incorporated by reference in its entirety.
As described above, it has been determined that in pure ammonia storage control mode, different emission cycles (e.g., ESC, FTP) may call for different target theta values (target θNH3) in order to achieve the best NOx conversion within NH3 slip constraints. Therefore, due to the transient nature of such emission test cycles, the target theta curve (target θNH3) has to be set conservatively low in order to prevent NH3 slips. Ammonia slip is especially problematic when the temperature of the catalyst is increasing. The invention provides a robust control approach with a pair of improvements that overcome the above-described theta control limitations, each of which help mitigate a potential high NH3 slip in the event of an increase in the exhaust gas temperature.
The first of these improvements involve dosing reductions upon detection of certain exhaust transient conditions (“Transient Compensation”). One transient condition includes a sudden increase in the exhaust gas mass air flow, which portends a like increase in the exhaust gas temperature, which allows extra time for the dosing control to adjust NH3 dosing before possible NH3 slips can occur. Another transient condition includes an increasing exhaust temperature gradient.
The second of these improvements involve shutting-off dosing altogether when certain exhaust conditions are recognized by the dosing control (“NH3 slip control”). These improvements will each be described in turn.
Transient Compensation. The dynamics of NH3 adsorption and NOx conversion in the SCR catalyst are governed by several chemical reactions. Additionally, the desorption of NH3 into the exhaust is similarly governed. However, perhaps the most dominating effect on the availability of NH3 to desorb from the SCR catalyst into the exhaust stream is the local temperature in the SCR catalyst itself. This aspect of the invention provides a mechanism to estimate the potential increase in the exhaust gas temperature, which may lead to conditions favorable for excess ammonia to exit the SCR catalyst. Generally, to mitigate such an occurrence, a dosing rate reduction can be used to reduce the overall availability of ammonia, and thus minimize an excess amount of ammonia that may be stored in the catalyst prior to the anticipated rise in temperature.
In step 110, the method involves dosing a reductant (e.g., NH3 or aqueous urea) into the exhaust gas stream in an amount based on the target theta parameter (target θNH3). This is a baseline amount for purposes of the method. As a consequence of this feature, however, it should be pointed out that a more aggressive dosing may be employed, especially for lower temperatures to improve NOx conversion efficiency, since the risk of an uncontrolled NH3 slip that would conventionally be present is now reduced due to the detection and compensation aspects of the method. The method proceeds to step 112.
In step 112, the method involves decreasing the reductant dosing when at least one of multiple exhaust transient compensation trigger conditions are satisfied. The first trigger condition is satisfied when a rate of change of the mass air flow (i.e., exhaust gas mass air flow preferably) exceeds a predetermined threshold. A second trigger condition is satisfied when an exhaust gas temperature gradient is in an increasing state (i.e., in contrast to a decreasing state or to a steady-state).
The next step involves determining whether a rate of change of the monitored engine mass air flow exceeds a predetermined threshold. The numerical value for the predetermined threshold may vary depending on the overall dosing control configuration, the SCR catalyst characteristics, and the like. For example, the value of the threshold, which effectively corresponds to the severity of the mass air flow transient, may be determined empirically to determine at what severity the mass air flow transients cause unacceptable NH3 slips for the particular target theta curve in use. In one embodiment, and for exemplary purposes only, the predetermined threshold for the mass air flow rate of change was about 80 g/sec (i.e., Δ80 g/sec). As shown in
Once detected, this logic variable can be considered a state variable, and this state is then passed on (or is otherwise available) to the dosing control 80, which uses the boolean state of this logic variable to adjust (reduce) dosing. This is shown in
In one embodiment, the upper and lower thresholds 134, 136 may have values between about (0.50 to 0.60° C./sec) and (−0.5 to −0.60° C./sec), respectively. These values allow for a small amount of variation in the gradient and still be considered “steady-state”. The temperature gradient (trace 126), when compared against the thresholds, determine the appropriate gradient state for the state variable 128, namely, “increasing” (state variable 128 is equal to 3), “steady-state” (state variable 128 is equal to 1) or “decreasing” (state variable 128 is equal to 2). The trace of the state variable 128, particularly what value it assumes at any point in time, indicates directly the exhaust temperature gradient state. For example, during the time interval 138, the temperature gradient is in the “increasing” state since the value of the state variable 128 during that time is equal to three (3). The exhaust temperature gradient state is passed to the dosing control 80 (specifically the block 92 in
- TPRESENT is the current (measured) exhaust temperature;
- GRADIENT is the current (computed) exhaust temperature gradient;
—INTERVAL is the amount of time into the future at which the exhaust temperature estimate is to be made.
The dosing control may be configured to control a reduction in the NH3 dosing by using the state of the temperature gradient, the forward exhaust temperature estimate or the combination of both.
In a first embodiment, the dosing control may be configured to apply a multiplier (e.g., less than one) to the entire target theta parameter curve/table when the state of the temperature gradient is “increasing”. This scaling downward will reduce the NH3 dosing. In a further variation, the multiplier value may vary with respect to the current exhaust temperature, as shown in exemplary fashion in Table 1 below. The values in Table 1 may be implemented in a look-up table (LUT) or the like, as known in the art. It should be further understood that the particular values contained in the table are calibratable, meaning that such values can be adjusted for any particular application to suit the specific configuration, SCR catalyst characteristics, the tradeoffs between the desired levels of NOx conversion versus the severity of NH3 slips, and the like. Moreover, in one variation, an advantage to using a LUT like that in Table 1 for defining the amount of dosing reduction is that the same LUT can be also be used for dosing reduction for detected exhaust mass air flow transients, as described above. It should be understood that implementation variations, such as but not limited to the number of table entries (i.e. the granularity of the table), whether interpolation should be used, or like considerations are within the spirit and scope of the invention.
In a second embodiment, the dosing control may be configured to use the forward (look-ahead) estimated exhaust temperature, calculated for a predetermined time in the future (e.g., sixty seconds). In particular, the dosing control may be configured so that when the gradient state is “increasing (value of three), the forward temperature estimate is used to select the target theta (target θNH3)- Given that the target theta values (i.e., see the curves in
In sum, it bears emphasizing that the dosing reduction that is implemented when transient conditions are detected are not dependent on NH3 sensor feedback, but rather are prospective in nature. In other words, the benefit of transient compensation is to allow the adjustment of the NH3 dosing rate in the event of a likely increase in the exhaust temperature. In ammonia storage control mode, transient compensation features enables the setup of higher target theta values especially at low temperatures for improved NOx conversion efficiency while at the same time reducing the risk of an unacceptably high NH3 slip.
NH3 Slip Control on Recognition of Certain Exhaust Conditions. This aspect of the invention addresses the NH3 slip risk during SCR catalyst operation while maximizing NOx conversion efficiency by using a mid-brick positioned ammonia sensor to provide feedback for detecting the slip risk. As described above, the ammonia sensor being located at a mid-brick position of the SCR catalyst provides greater sensitivity to NH3 dosing variation because of reduced NH3 storage capacity of the front (i.e., forward or upstream) brick (e.g., see
In step 142, the method involves dosing NH3 (i.e., a reductant, generally, such as urea-water solution of
In step 144, an NH3 slip trip level is established (e.g., 50 ppm). In one embodiment, the slip trip level may be adjustable and selected based on the exhaust temperature (i.e., the SCR inlet temperature (TIN)).
In step 146, the method is configured to decrease, or, in a preferred embodiment, entirely shut off dosing when certain exhaust conditions for activating the slip trip mode have been satisfied. The first condition to be satisfied is when the exhaust gas temperature gradient is in an “increasing” state. The method for making this determination has been described above. Note,
When the slip trip mode is active, as per method step 146, the dosing control is configured to preferably shut-off NH3 dosing. As shown in
While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.
1. In an internal combustion engine producing an exhaust gas stream to an exhaust treatment system having a selective catalytic reduction (SCR) catalyst, a method of reductant slip control, comprising the steps of:
- dosing reductant into the exhaust gas stream;
- establishing a reductant slip trip level based on an exhaust gas temperature;
- decreasing the reductant dosing when an exhaust gas temperature gradient is in an increasing state and a reductant concentration level measured at the SCR catalyst exceeds the reductant slip trip level.
2. The method of claim 1 wherein said step of discontinuing is further performed when an exhaust gas temperature exceeds a predetermined threshold.
3. The method of claim 2 wherein said reductant is selected from the group comprising ammonia (NH3) and urea, said reductant concentration level being an ammonia concentration level, said dosing step including the sub-step of mixing the reductant with the exhaust gas upstream of the SCR catalyst.
4. The method of claim 3 wherein the SCR catalyst is multi-brick in construction, said method further comprising the step of:
- disposing an ammonia gas concentration sensor at a mid-brick position of the SCR catalyst.
5. The method of claim 4 wherein the mid-brick position is located at the substantial center of the SCR catalyst.
6. The method of claim 1 where said decreasing step includes the sub-step of discontinuing reductant dosing.
7. In an internal combustion engine producing a stream of an exhaust gas to an exhaust treatment system having a selective catalytic reduction (SCR) catalyst, a method of operating the treatment system, comprising the steps of:
- dosing reductant into the exhaust gas stream in an amount based on at least a reductant surface coverage parameter theta (θ) of the SCR;
- decreasing the reductant dosing when one of a plurality of transient compensation trigger conditions are satisfied, wherein the trigger conditions include a first condition when a rate of change in a mass air flow (MAF) level exceeds a first predetermined threshold and a second condition when an exhaust gas temperature gradient is in an increasing state.
8. The method of claim 7 wherein said reductant is selected from the group comprising ammonia (NH3) and urea, said reductant concentration level being an ammonia concentration level, said dosing step including the sub-step of mixing the reductant with the exhaust gas upstream of the SCR catalyst.
9. The method of claim 7 wherein said dosing step includes the sub-steps of:
- measuring an exhaust gas temperature;
- determining a value for the surface coverage parameter theta (θ) based on measured exhaust gas temperature and predetermined data;
- and wherein said decreasing step includes adjusting the determined theta (θ) parameter value downwards by a predetermined amount.
10. The method of claim 9 further including the step of:
- increasing the adjusted theta (θ) parameter value when none of the transient compensation triggers conditions are satisfied.
11. The method of claim 10 further including the step of:
- repeating said increasing step until the adjusted theta (θ) parameter value equals the theta (θ) parameter value determined based on the measured exhaust gas temperature and the predetermined data.
12. The method of claim 7 further including the steps of:
- detecting the first condition at a first time; and
- sustaining the first condition for a predetermined time after the first time.
13. The method of claim 12 wherein said detection step includes the sub-steps of:
- determining, at an initial time, that a rate of change of the MAF level exceeds the first predetermined threshold; and
- deeming the first condition detected when the rate of change of the MAF level continues to exceed the first predetermined threshold as assessed at a confirmation time interval after the initial time.
14. The method of claim 7 further including the steps of:
- providing an exhaust gas temperature gradient signal;
- establishing predetermined upper and lower state limits;
- determining the state of the exhaust gas temperature gradient as (i) increasing when the exhaust gas temperature gradient signal is greater than the upper state limit; (ii) steady state when the exhaust gas temperature gradient signal is between the upper and lower state limits; and (iii) decreasing when the exhaust gas temperature gradient signal is lower than the lower state limit; and
- deeming the second condition as detected when the exhaust gas temperature gradient is in the increasing state.
International Classification: F01N 9/00 (20060101);