U.S. patent application number 12/327958 was filed with the patent office on 2010-04-29 for exhaust gas treatment system and methods for operating the same.
Invention is credited to Andrew D. Herman, Mark Shost, Ming-Cheng Wu.
Application Number | 20100101215 12/327958 |
Document ID | / |
Family ID | 41559520 |
Filed Date | 2010-04-29 |
United States Patent
Application |
20100101215 |
Kind Code |
A1 |
Wu; Ming-Cheng ; et
al. |
April 29, 2010 |
EXHAUST GAS TREATMENT SYSTEM AND METHODS FOR OPERATING THE SAME
Abstract
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.
Inventors: |
Wu; Ming-Cheng; (Troy,
MI) ; Herman; Andrew D.; (Grand Blanc, MI) ;
Shost; Mark; (Northville, MI) |
Correspondence
Address: |
DELPHI TECHNOLOGIES, INC;LEGAL STAFF - M/C 483-400-402
5725 DELPHI DRIVE, PO BOX 5052
TROY
MI
48007
US
|
Family ID: |
41559520 |
Appl. No.: |
12/327958 |
Filed: |
December 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61108172 |
Oct 24, 2008 |
|
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|
Current U.S.
Class: |
60/286 ;
60/299 |
Current CPC
Class: |
F01N 2610/02 20130101;
F01N 9/00 20130101; F01N 2900/0408 20130101; F01N 2570/18 20130101;
F01N 2610/146 20130101; F01N 2560/06 20130101; F01N 2900/14
20130101; F01N 2900/0402 20130101; F01N 2560/08 20130101; F01N
2900/1806 20130101; F01N 2900/0412 20130101; F01N 13/0093 20140601;
F01N 2560/026 20130101; Y02T 10/40 20130101; Y02T 10/47 20130101;
Y02T 10/24 20130101; Y02T 10/12 20130101; F01N 2560/021 20130101;
F01N 3/208 20130101; F01N 2560/14 20130101 |
Class at
Publication: |
60/286 ;
60/299 |
International
Class: |
F01N 9/00 20060101
F01N009/00 |
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 (NH.sub.3) 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 (.theta.) 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 (NH.sub.3) 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 (.theta.) based on
measured exhaust gas temperature and predetermined data; and
wherein said decreasing step includes adjusting the determined
theta (.theta.) parameter value downwards by a predetermined
amount.
10. The method of claim 9 further including the step of: increasing
the adjusted theta (.theta.) 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 (.theta.) parameter
value equals the theta (.theta.) 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.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] 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.
TECHNICAL FIELD
[0002] 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
[0003] 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 (NH.sub.3) into the exhaust gas feedstream ahead of the
catalyst. The SCR catalyst is constructed so as to promote the
reduction of NOx by NH.sub.3 (or other reductant, such as aqueous
urea which undergoes decomposition in the exhaust to produce
NH.sub.3). NH.sub.3 or urea selectively combine with NOx to form
N.sub.2 and H.sub.2O 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 NH.sub.3 source must be periodically
replenished and the injection of NH.sub.3 into the SCR catalyst
requires precise control. Overinjection may cause a release of
NH.sub.3 ("slip") out of the tailpipe into the atmosphere, while
underinjection may result in inadequate emissions reduction (i.e.,
inadequate NOx conversion to N.sub.2 and H.sub.2O).
[0004] 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.
[0005] 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 NH.sub.3 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 NH.sub.3
emissions, particularly over the service life of the treatment
system.
[0006] 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.
[0007] 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 NH.sub.3 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.
[0008] 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
[0009] 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.degree. 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.
[0010] 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., NH.sub.3 or aqueous urea) into the
exhaust gas stream in an amount based on predetermined surface
coverage parameter theta values ("target .theta..sub.NH3"). 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 NH.sub.3 desorption, resulting in
perhaps a high concentration NH.sub.3 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.degree.
C./second). The logic for this condition is that rapid temperature
increases can also lead to NH.sub.3 desorption, and thus NH.sub.3
slips.
[0011] In a second aspect of the invention, a method of NH.sub.3
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.,
NH.sub.3, 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
NH.sub.3 slip and warrant shutting-off dosing until the conditions
subside.
[0012] An exhaust gas treatment system is also presented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present invention will now be described by way of
example, with reference to the accompanying drawings:
[0014] FIG. 1 is a diagrammatic and block diagram showing an
exhaust treatment system in which the control methods of the
invention may be practiced.
[0015] FIG. 2 is a block diagram showing an overview of the dosing
control that includes an SCR model as well as improved control
features.
[0016] FIG. 3 is a signal flow mechanization schematic showing
inputs and outputs of the SCR model.
[0017] FIG. 4 is a simplified diagram showing typical target theta
(.theta.) values or curves as a function of temperature.
[0018] FIG. 5 is flowchart showing a method for controlling an
exhaust treatment system involving exhaust transient
compensation.
[0019] FIG. 6 is a timing diagram illustrating exhaust mass air
flow transient detection.
[0020] FIG. 7 is a timing diagram illustrating exhaust gas
temperature gradient state detection as well as forward temperature
estimation.
[0021] FIG. 8 is a flowchart of a method for controlling an exhaust
treatment system involving ammonia (NH.sub.3) slip control.
[0022] FIG. 9 is a timing diagram showing recognition of certain
exhaust conditions to activate NH.sub.3 slip control.
[0023] FIG. 10 is a timing diagram showing NOx removal and exhaust
temperature.
[0024] FIG. 11 is a timing diagram showing the respective outputs
of mid-brick positioned and post-SCR catalyst positioned ammonia
sensors.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] Referring now to the drawings wherein like reference
numerals are used to identify identical components in the various
views, FIG. 1 is a diagrammatic and block diagram showing an
exemplary diesel cycle internal combustion engine 10 whose
combustion exhaust gas 12 is fed to an exhaust gas treatment system
14. The exhaust gas is represented as a stream flowing through the
exhaust gas treatment system 14 and is shown as a series of arrows
designated 12.sub.EO (engine out), 12.sub.1, 12.sub.2, 12.sub.3 and
12.sub.TP (tail pipe). It should be understood that while the
invention will be described in connection with an automotive
vehicle (i.e., mobile) embodiment, the invention may find useful
application in stationary applications as well. In addition,
embodiments of the invention may be used in heavy-duty applications
(e.g., highway tractors, trucks and the like) as well as light-duty
applications (e.g., passenger cars). Moreover, embodiments of the
invention may find further useful application in various types of
internal combustion engines, such as compression-ignition (e.g.,
diesel) engines as well as spark-ignition engines.
[0026] 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.
[0027] FIG. 1 also shows an engine control unit (ECU) 16 configured
to control the operation of the engine 10. The ECU 16 may comprise
conventional apparatus known generally in the art for such purpose.
Generally, the ECU 16 may include at least one microprocessor or
other processing unit, associated memory devices such as read only
memory (ROM) and random access memory (RAM), a timing clock, input
devices for monitoring input from external analog and digital
devices and controlling output devices. The ECU 16 is operable to
monitor engine operating conditions and other inputs (e.g.,
operator inputs) using the plurality of sensors and input
mechanisms, and control engine operations with the plurality of
output systems and actuators, using pre-established algorithms and
calibrations that integrate information from monitored conditions
and inputs. It should be understood that many of the conventional
sensors employed in an engine system have been omitted for clarity.
The ECU 16 may be configured to calculate an exhaust mass air flow
(MAF) parameter 20 indicative of the mass air flow exiting engine
10.
[0028] 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.
[0029] 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 FIG.
1, the ECU 16 may be configured to set a corresponding diagnostic
trouble code (DTC) 24 and/or generate an operator alert, such an
illumination of a MIL 26. Although not shown, in one embodiment,
the ECU 16 may be configured so as to allow interrogation (e.g., by
a skilled technician) for retrieval of such set DTCs. Generally,
the process of storing diagnostic trouble codes and subsequent
interrogation and retrieval is well known to one skilled in the art
and will not be described in any further detailed.
[0030] With continued reference to FIG. 1, the exhaust gas
treatment system 14 may include a diesel oxidation catalyst (DOC)
28, a diesel particulate filter (DPF) 30, a dosing subsystem 32
including at least (i) a reductant (e.g., urea-water solution)
storage tank 34 and (ii) a dosing unit 36, and a selective
catalytic reduction (SCR) catalyst 38. In addition, FIG. 1 shows
various sensors disposed in and/or used by the treatment system 14.
These include a DOC inlet temperature sensor 39 configured to
generate a DOC inlet temperature signal 41 (T.sub.DOC-IN), a NOx
sensor 40 configured to generate a NOx signal 42 (NOx) indicative
of a sensed NOx concentration, a first exhaust gas temperature
sensor 44, located at the inlet of the SCR catalyst 38, configured
to generate a first temperature signal 46 (T.sub.IN), an optional
second exhaust gas temperature sensor 48 configured to generate a
second temperature signal 50 (T.sub.OUT), a first pressure sensor
52 configured to generate a first pressure signal 54 (P.sub.IN), a
second pressure sensor 56 configured to generate a second pressure
signal 58 (P.sub.OUT), and an ammonia (NH.sub.3) concentration
sensor 60 configured to generate an ammonia concentration signal 62
indicative of the sensed NH.sub.3 concentration. In many commercial
vehicles, a NOx sensor 64 is provided for generating a second NOx
signal 66 indicative of the NOx concentration exiting the tail
pipe. However, such is shown for completeness only.
[0031] The DOC 28 and the DPF 30 may comprise conventional
components to perform their known functions.
[0032] The dosing subsystem 32 is responsive to an NH.sub.3 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) NH.sub.3 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.
[0033] 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 NH.sub.3 (or aqueous
urea, which decomposes into ammonia, NH.sub.3) on the other hand.
The result of such a selective reduction is, as described above in
the Background, N.sub.2 and H.sub.2O. 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.
[0034] 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 NH.sub.3 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
(T.sub.IN) may be used.
[0035] The NH.sub.3 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
NH.sub.3 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
NH.sub.3 slip, where the downstream NOx conversion with this
detectable NH.sub.3 can be assumed in the presence of the rear
brick, even further reducing NH.sub.3 concentration levels at the
tail pipe to within acceptable levels. Alternatively, in certain
embodiments, the NH.sub.3 sensor 60 may be located at the outlet of
the SCR catalyst 38. The remainder of the sensors shown in FIG. 1
may comprise conventional components and be configured to perform
in a conventional manner known to those of ordinary skill in the
art.
[0036] The dosing control 80 is configured to generate the NH.sub.3
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.
[0037] FIG. 2 is a block diagram showing an overview of the dosing
control 80 of FIG. 1. The basic strategy is to control the dosing
rate (e.g., urea-water solution) so as to ensure that the there is
adequate ammonia stored in the SCR catalyst 38 to achieve (i) a
high NOx conversion rate (i.e., conversion of NOx into N.sub.2 and
H.sub.2O), with (ii) a low occurrence or no occurrence at all of
ammonia (NH.sub.3) slips exceeding predetermined maximum
thresholds.
[0038] Overall, the dosing control 80 is configured to generate an
NH.sub.3 Request, which is communicated to the dosing unit 36
(i.e., shown as the "NH.sub.3/Urea Dosing"). In the illustrative
embodiment, the NH.sub.3 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
(.theta..sub.NH3), which corresponds to the NH.sub.3 surface
storage fraction associated with the SCR catalyst 38. In other
words, the ammonia surface coverage parameter theta
(.theta..sub.NH3) indicates the amount of ammonia-NH.sub.3 stored
in the SCR catalyst 38. One aspect of the operation of the dosing
control 80 involves an SCR model 82.
[0039] FIG. 3 is a signal flow mechanization schematic showing
inputs and outputs of the SCR model 82. The SCR model 82 is a
chemistry-based SCR model and is shown with a theta control block
84, and a "NO and NO.sub.2" predictor block 86. The SCR model 82 is
configured to model the physical SCR catalyst 38 and compute real
time values for the ammonia surface coverage parameter theta
(.theta..sub.NH3). The theta control block 84 is configured to
compare the computed theta (.theta..sub.NH3) against a target value
for theta ("Target .theta..sub.NH3"), which results in a theta
error. The theta control block 84 is configured to use a control
strategy (e.g., a proportional-integral (PI) control algorithm) to
adjust the requested NH.sub.3 dosing rate ("NH.sub.3 Request") to
reduce the theta error. The theta control block 84 also employs
closed-loop feedback, being responsive to ammonia sensing feedback
by way of the ammonia sensor 60. The theta control block 84 may use
NH.sub.3 feedback generally to adapt target theta values to account
for catalyst degradation, urea injection malfunction or dosing
fluid concentration variation that may be encountered during
real-world use. As will be described, the NH.sub.3 sensing feedback
is also used for various control and diagnostic improvements. The
predictor block 86 receives the DOC inlet temperature signal 41
(T.sub.DOC-IN), the NOx sensor signal 42 and the exhaust flow
signal 90 as inputs and is configured to produce data 88 indicative
of the respective NO and NO.sub.2 concentration levels (engine out)
produced by the engine 10. The predictor block 86 may comprise a
look-up table (LUT) containing NO and NO.sub.2 data experimentally
measured from the engine 10.
[0040] 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.
[0041] 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 NO.sub.2 levels 88; (ii) an inlet NOx amount,
which may be derived from the NOx indicative signal 42 (best shown
in FIG. 1); (iii) an exhaust mass air flow (MAF) amount 90, which
may be either a measured value or a value computed by the ECU 16
and shown as exhaust MAF parameter 20 in FIG. 1; (iv) an SCR inlet
temperature, which may be derived from the first temperature signal
46 (T.sub.IN); (v) an SCR inlet pressure, which may be derived from
the first pressure signal 54 (P.sub.IN); and (vi) the actual amount
of reductant (e.g., NH.sub.3, urea-water solution shown as
"NH.sub.3 Actual" in FIG. 2) introduced by the dosing subsystem 32.
The actual NH.sub.3 amount helps ensure that the model provides
accurate tracking of the reductant dosing. In one embodiment,
values for theta (.theta..sub.NH3) are updated at a frequency of 10
Hz, although it should be understood this rate is exemplary only.
There are a plurality of modeling approaches known in the art for
developing values for a surface coverage parameter theta
(.theta..sub.NH3), for example as seen by reference to the article
by M. Shost et. al, "Monitoring, Feedback and Control of Urea SCR
Dosing Systems for NOx Reduction: Utilizing an Embedded Model and
Ammonia Sensing", SAE Technical Paper Series 2008-01-1325.
[0042] Referring again to FIG. 2, the dosing control 80 includes
additional blocks. In particular, a target theta parameter (Target
.theta..sub.NH3) block 92 is shown, which is configured to provide
a value for the target theta parameter (Target .theta..sub.NH3)
preferably as function of temperature (e.g., exhaust gas
temperature, such as the SCR inlet temperature T.sub.IN). The
target .theta..sub.NH3, which is determined as a function of the
SCR catalyst inlet temperature T.sub.IN, is conventionally set-up
based on the following considerations: (1) desire to achieve a
maximum possible NOx conversion efficiency with acceptable NH.sub.3
slip levels (30 ppm peak, 10 ppm average) for a given emission test
cycle, and (2) recognition that limits must be set for the theta
values at low temperatures to prevent potential high NH.sub.3 slips
upon sudden temperature ramp up in off-cycle tests. In other words,
in a pure ammonia storage control mode (i.e., theta parameter
control), different emission cycles may call for different theta
values in order to achieve the best NOx conversion within the
confines of the applicable NH.sub.3 slip limits.
[0043] FIG. 4 is a diagram showing exemplary target theta
.theta..sub.NH3 curves determined for both the Euro Stationary
Cycle (ESC) and the Federal Test Procedure (FTP) emission cycles
using Cu-zeolite catalysts. As a practical matter, however, only
one curve can be used in real world situations. The values from one
of the target theta curves may be stored in a look-up table (LUT)
or the like for run-time use by the theta control block 84 of the
dosing control 80. Such values may take the form of (temperature,
theta value) data pairs.
[0044] As shown in FIG. 2, the theta control 84 further includes a
comparator 94 (e.g., a summer, or equivalent) configured to
generate the theta error signal described above, indicative of the
difference between the target theta (Target .theta..sub.NH3) and
the computed theta (.theta..sub.NH3) from the SCR model. A PI
control 96 is configured to produce an output signal configured to
reduce the magnitude of the theta error. A high level control block
98 is responsive to various inputs to produce the NH.sub.3 Request
signal, which is communicated to the dosing subsystem 32.
[0045] FIG. 2 also shows, in block form, a number of additional
control and diagnostic features. These additional control and
diagnostic features may be arranged to work together in some
embodiments to achieve maximum NOx conversion while maintaining
acceptable NH.sub.3 slip levels under various driving conditions
(i.e., in vehicle applications). The dosing control 80 thus
includes a number of functional blocks to implement these features:
a theta perturbation diagnostic block 100, an adaptive learning
diagnostic block 102, a transient compensation control block 104
and an NH.sub.3 slip control block 106.
[0046] 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.
[0047] 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
.theta..sub.NH3) in order to achieve the best NOx conversion within
NH.sub.3 slip constraints. Therefore, due to the transient nature
of such emission test cycles, the target theta curve (target
.theta..sub.NH3) has to be set conservatively low in order to
prevent NH.sub.3 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 NH.sub.3 slip in the event of
an increase in the exhaust gas temperature.
[0048] 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 NH.sub.3 dosing before possible NH.sub.3 slips
can occur. Another transient condition includes an increasing
exhaust temperature gradient.
[0049] The second of these improvements involve shutting-off dosing
altogether when certain exhaust conditions are recognized by the
dosing control ("NH.sub.3 slip control"). These improvements will
each be described in turn.
[0050] Transient Compensation. The dynamics of NH.sub.3 adsorption
and NOx conversion in the SCR catalyst are governed by several
chemical reactions. Additionally, the desorption of NH.sub.3 into
the exhaust is similarly governed. However, perhaps the most
dominating effect on the availability of NH.sub.3 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.
[0051] FIG. 5 is a flowchart showing the basic steps involved in
carrying out a method for exhaust transient detection and dosing
rate compensation. The method begins in step 110.
[0052] In step 110, the method involves dosing a reductant (e.g.,
NH.sub.3 or aqueous urea) into the exhaust gas stream in an amount
based on the target theta parameter (target .theta..sub.NH3). 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 NH.sub.3 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.
[0053] 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).
[0054] FIG. 6 is a timing diagram for an ESC driving cycle showing
an exhaust mass air flow trace 114 and a trace 116 of a logic
variable. The state of the logic variable indicates whether the
mass air flow transient condition has been satisfied (i.e., logical
"1") or not satisfied (i.e., logical "0"). Determining whether the
mass air flow condition has been satisfied may first involve
monitoring the engine air flow conditions. Preferably, the method
involves monitoring the engine exhaust mass air flow. In many
conventional engine control units, the engine exhaust mass air flow
parameter is computed and would be available.
[0055] 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
NH.sub.3 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., .DELTA.80 g/sec). As shown in FIG. 6, the mass air flow
transients along edges 118, 120 and 122, corresponding roughly to
times t.sub.1, t.sub.2 and t.sub.3, exceed the threshold and thus
cause the logic variable (trace 116) to transition from a logic "0"
to a logic "1". In an alternative embodiment, to combat false
triggering due to noise, the method may additionally involve a
confirmatory check of the mass air flow level, which is taken at a
predetermined amount of time (e.g., 5-10 seconds) after the initial
detection, to ensure that the large mass air flow transient was
indeed true and not the result of noise. In the illustrated
embodiment, once the mass air flow transient condition has been
detected, the method is configured to sustain or hold this logic
state for a predetermined amount of time, which is shown as time
interval 124. In one embodiment, the time interval 124 may be in a
range of between about 20 and 40 seconds. The time interval 124 is
also selectable and may be determined empirically for any
particular exhaust treatment system.
[0056] 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
FIG. 3 where the output of the transient compensation block 104 is
provided to the target theta determining block 92. The dosing
control 80, through block 92, may be configured to reduce the
dosing by adjusting downward the target theta values, for example,
by a predetermined amount when the state of the logic variable
(trace 116) is a Boolean "1" (e.g., up to a maximum of 25%
reduction using the target theta as a baseline). For example, the
de-rated target theta values in Table 1 below, albeit in the
context for the temperature gradient transient, may also be
applicable to the exhaust mass air flow transient compensation.
[0057] FIG. 7 is a timing diagram for an ESC driving cycle showing
an exhaust gas temperature gradient (trace 126) as well as a state
or logic variable (trace 128). The value of the logic variable
indicates the what state the exhaust temperature gradient is in
(i.e., decreasing, steady-state, or increasing). FIG. 7 also shows
a measured exhaust gas temperature (trace 130--as measured at the
SCR inlet by temperature sensor 44 (T.sub.IN)) as well as a forward
exhaust gas temperature estimate as shown in trace 132. The exhaust
temperature gradient (trace 126) may be computed at predetermined
intervals based on the current (measured) SCR inlet temperature, as
compared to the SCR inlet temperature that was measured a
predetermined time ago (e.g., every 15 seconds). From this
calculation, an updated exhaust temperature gradient value is
produced. The time-series of these computed gradient values
collectively define the trace 126. Finally, to determine the state
of the temperature gradient (i.e., decreasing, steady-state,
increasing) at any particular point in time, the value of the
gradient is compared against upper and lower gradient thresholds,
which are shown in FIG. 7 by reference numerals 134, 136.
[0058] In one embodiment, the upper and lower thresholds 134, 136
may have values between about (0.50 to 0.60.degree. C./sec) and
(-0.5 to -0.60.degree. 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 FIG. 3), which may
use it in determining whether to reduce the NH.sub.3 dosing rate,
and if so, by how much (more on this below). Otherwise, if the
gradient state is either "decreasing" or "steady-state", the dosing
control will generally not make any dosing reductions.
[0059] FIG. 7 also shows the variations in time of a so-called
forward exhaust temperature estimate, which is reproduced as trace
132. For comparison, the measured (actual) exhaust temperature is
also plotted, and is shown as trace 130. The forward temperature
estimate may also be used in the dosing control (more on this below
as well). To provide the forward temperature estimate, a forward
exhaust temperature estimator block (not shown) may be provided,
which may be responsive to a number of available parameter values
to calculate or estimate the exhaust temperature value at a future
time (forward-looking), as set forth in equation (1) below:
T.sub.FWD.sub.--.sub.EST=T.sub.PRESENT+GRADIENT*t.sub.FWD.sub.--.sub.INT-
ERVAL. (1)
[0060] where T.sub.FWD.sub.--.sub.EST is the forward estimated
exhaust temperature at a future time; [0061] T.sub.PRESENT is the
current (measured) exhaust temperature; [0062] GRADIENT is the
current (computed) exhaust temperature gradient; [0063]
t.sub.FWD.sub.--.sub.INTERVAL is the amount of time into the future
at which the exhaust temperature estimate is to be made.
[0064] The dosing control may be configured to control a reduction
in the NH.sub.3 dosing by using the state of the temperature
gradient, the forward exhaust temperature estimate or the
combination of both.
[0065] 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
NH.sub.3 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
NH.sub.3 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.
TABLE-US-00001 TABLE 1 Theta Multiplier To Reduce Reductant Dosing
TEMPERATURE (.degree. C.) MULTIPLIER 200 1 225 1 250 1 275 0.83 300
0.75 350 0.75 400 0.75 450 0.83 500 1 550 1 600 1
[0066] 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 .theta..sub.NH3)- Given that the target theta values (i.e.,
see the curves in FIG. 4) are inversely proportional to
temperature, the step of selecting the target theta (target
.theta..sub.NH3) based on the expected increase in exhaust
temperature will result in a reduced value for the target theta
(target .theta..sub.NH3). This will immediately begin reducing the
NH.sub.3 dosing rate by virtue of the lower value target theta. As
with the LUT referred to in the dosing control approach above, the
target theta curve may likewise be calibratable, and this approach
likewise has the benefit that it may also be used when exhaust mass
air flow transients have been detected. Additionally, this second
embodiment has been observed in some circumstances to provide for a
smoother transition to the correct (i.e., stable) target theta
value at the end of transient compensation. This is believed to be
due to the fact that as the temperature gradient gets smaller, the
future temperature and current (measured) temperature tend to
merge.
[0067] Referring to FIGS. 6 and 7, both transient compensation
features, although independent of each other, may be used
simultaneously, and in many instances, both features may be active
at the same time (i.e., both can indicate to the dosing control
that reduction in the dosing rate should be considered). For
example, in FIG. 6, the time interval 138 (i.e., gradient state is
"increasing") has been superimposed and in timed relationship with
the MAF transient "detect and hold" interval 124. Note, in some
instances, there will be some measure of overlap. In addition, time
interval 140 is the composite time interval over which the dosing
control is making adjustments (reductions) based on the detection
of exhaust transient conditions.
[0068] In sum, it bears emphasizing that the dosing reduction that
is implemented when transient conditions are detected are not
dependent on NH.sub.3 sensor feedback, but rather are prospective
in nature. In other words, the benefit of transient compensation is
to allow the adjustment of the NH.sub.3 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 NH.sub.3 slip.
[0069] NH.sub.3 Slip Control on Recognition of Certain Exhaust
Conditions. This aspect of the invention addresses the NH.sub.3
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
NH.sub.3 dosing variation because of reduced NH.sub.3 storage
capacity of the front (i.e., forward or upstream) brick (e.g., see
FIG. 11 below). Therefore, the more rapid response to dosing
errors, by virtue of the mid-brick detection, can be utilized to
control ammonia slip with the help of the back (i.e., rear or
downstream) brick. This feature of the invention utilizes this
quick response in combination with the recognition of certain
exhaust conditions to activate a slip trip mode, which calls for,
in one embodiment, complete dosing shut-off. This feature provides
even greater flexibility in ammonia slip control while maximizing
NOx removal.
[0070] FIG. 8 is a flowchart of a method of NH.sub.3 slip control
while FIG. 9 is a timing diagram showing the first 550 seconds of
an exemplary ESC driving cycle, illustrating the relevant signals.
In particular, FIG. 9 shows the temperature at the SCR inlet
increase from less than 250.degree. C. to 400.degree. C. between
about time t.sub.290.sub.--.sub.seconds and time
t.sub.450.sub.--.sub.seconds, while the engine out NOx (trace 160)
starts to decrease after time t.sub.370.sub.--.sub.seconds. Since
the engine out NOx drops significantly, the ammonia stored on the
SCR catalyst at low temperatures would otherwise slip without the
slip trip intervention of this feature of the invention. The method
for begins in step 142.
[0071] In step 142, the method involves dosing NH.sub.3 (i.e., a
reductant, generally, such as urea-water solution of FIG. 1) into
the exhaust stream in an amount based on target theta (target
.theta..sub.NH3). The target theta parameter has been described in
detail above. Generally speaking, this feature may remain inactive
until a predetermined, minimum temperature has been reached, which
may range between about 250 to 300.degree. C. may further be
between about 275 to 300.degree. C. and may be about 300.degree. C.
It should be understood that the compromises of a fixed target
theta curve may manifest themselves more acutely as the catalyst
temperature increases rapidly, particularly the low end of the
overall target theta table where ammonia storage is high but where
there is more risk of NH.sub.3 slip. Therefore, it should be
understood that while the NH.sub.3 slip control feature will be
most useful in those certain temperature ranges, the invention is
not so limited. In addition, it bears emphasizing that the ammonia
concentration sensor 60 is preferably located for this method at a
mid-brick position. The sensor 60 produces an ammonia concentration
signal 62 that is indicative of the ammonia concentration level.
FIG. 9 shows the ammonia concentration level as trace 148. The
method proceeds to step 144.
[0072] In step 144, an NH.sub.3 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 (T.sub.IN)). FIG. 9 shows an exemplary slip
trip level as trace 150. The method proceeds to step 146.
[0073] 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, FIG. 9
shows the exhaust temperature as trace 154 and the corresponding
state of the exhaust temperature gradient as trace 156. Trace
(state variable) 156 may take values of either "1" (steady-state),
"2" (decreasing) or "3" (increasing). The second condition to be
satisfied is when the NH.sub.3 concentration level (trace 148)
exceeds the NH.sub.3 slip trip level 150. FIG. 9 shows the NH.sub.3
concentration level (trace 148) exceeding the slip trip level
(trace 150) during the time interval 158. Accordingly, since the
first condition and the second condition are both satisfied during
time interval 158, the slip trip mode is activated, which is
represented by trace 152 (logic "1"). Note, the slip trip mode 152
can assume a logic value of "0" (inactive) or "1" (active).
[0074] When the slip trip mode is active, as per method step 146,
the dosing control is configured to preferably shut-off NH.sub.3
dosing. As shown in FIG. 3, the output of the NH.sub.3 slip control
block 106, namely the state variable (trace 152) indicating whether
the slip trip mode is active or inactive, is provided directly to
the high-level control 98. The dosing shut-off imposed by
high-level control 98 continues for as long as the requisite
conditions remain satisfied. This feature prevents potentially high
slips of ammonia, especially when the target ammonia storage
(coverage) parameter theta (target .theta..sub.NH3) is set to
relatively high values especially for low temperatures. The
significant benefit is that it allows for high ammonia storage
(i.e., high target .theta..sub.NH3) at low temperatures (e.g.,
below 300.degree. C.) for maximal NOx conversion under certain
driving conditions while at the same time having significantly less
risk of unacceptably high NH.sub.3 slips when the exhaust
temperature increases.
[0075] FIGS. 10 and 11 show the data for an entire ESC driving
cycle, where NOx conversion is about 90.6%, the peak NH.sub.3 slip
was about 35 ppm and the average NH.sub.3 slip was about 9 ppm.
[0076] FIG. 10 shows the pre-SCR catalyst inlet temperature (TIN)
as trace 162, a pre-SCR catalyst NOx concentration level as trace
164 and a post-SCR catalyst NOx concentration level as trace 166,
all for the ESC driving cycle. Note that the NOx conversion (i.e.,
removal), as represented by the difference between traces 164 and
166, was about 90.6%.
[0077] FIG. 11 shows the post-SCR catalyst NH.sub.3 concentration
level as trace 168 and the mid-brick NH.sub.3 concentration level
as trace 170, all for the ESC driving cycle. Note that the peak
NH.sub.3 slip was about 34 ppm while the average NH.sub.3 slip was
about 9 ppm.
[0078] 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.
* * * * *