U.S. patent application number 12/327945 was filed with the patent office on 2010-04-29 for diagnostic methods for selective catalytic reduction (scr) exhaust treatment system.
Invention is credited to David D. Cabush, Andrew D. Herman, Ming-Cheng Wu.
Application Number | 20100101214 12/327945 |
Document ID | / |
Family ID | 41528591 |
Filed Date | 2010-04-29 |
United States Patent
Application |
20100101214 |
Kind Code |
A1 |
Herman; Andrew D. ; et
al. |
April 29, 2010 |
DIAGNOSTIC METHODS FOR SELECTIVE CATALYTIC REDUCTION (SCR) EXHAUST
TREATMENT SYSTEM
Abstract
In an internal combustion engine system having an exhaust
aftertreatment system including a selective catalytic reduction
(SCR) catalyst, diagnostic methods involve the intrusive
perturbation of a target surface coverage parameter theta to
determine the state of health of the SCR catalyst or an ammonia
concentration sensor. An adaptive learning block adapts the target
theta based on the use of NH.sub.3 sensing feedback from a
mid-brick positioned ammonia concentration sensor to pull in system
variation. A further diagnostic monitors the amount of adaptation
and when the adaptive learning excessively learns, the diagnostic
assumes that some system-level degradation must have occurred and
the diagnostic will notify the overall emissions control
monitor.
Inventors: |
Herman; Andrew D.; (Grand
Blanc, MI) ; Wu; Ming-Cheng; (Troy, MI) ;
Cabush; David D.; (Howell, 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: |
41528591 |
Appl. No.: |
12/327945 |
Filed: |
December 4, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61108172 |
Oct 24, 2008 |
|
|
|
Current U.S.
Class: |
60/277 |
Current CPC
Class: |
F01N 2900/1621 20130101;
F01N 3/208 20130101; Y02T 10/47 20130101; F01N 2900/0412 20130101;
F01N 2610/146 20130101; F01N 11/002 20130101; F01N 2560/021
20130101; F01N 2610/02 20130101; F01N 2900/1622 20130101; Y02T
10/40 20130101; F01N 9/00 20130101; F01N 2900/0402 20130101; F01N
2900/0411 20130101; F01N 2570/18 20130101; F01N 2900/1402 20130101;
F01N 11/00 20130101; Y02T 10/12 20130101; F01N 9/005 20130101; F01N
2900/0408 20130101; F01N 2900/14 20130101; F01N 2550/02 20130101;
Y02T 10/24 20130101 |
Class at
Publication: |
60/277 |
International
Class: |
F01N 11/00 20060101
F01N011/00 |
Claims
1. In an internal combustion engine having an exhaust treatment
system having a selective catalytic reduction (SCR) catalyst, a
method of performing a diagnostic on the exhaust treatment system,
comprising the steps of: introducing a reductant into an exhaust
gas stream in an amount based on at least a target surface coverage
parameter theta (.theta.); perturbing theta in accordance with a
diagnostic function; measuring an operating characteristic of the
treatment system; determining a state of health of a component of
the treatment system based on the diagnostic function and the
measured operating characteristic.
2. The method of claim 1 wherein said reductant is selected from
the group comprising ammonia (NH.sub.3) and urea, said measuring
step including the substep of measuring an ammonia concentration
level.
3. The method of claim 2 wherein said one component comprises the
SCR catalyst, said state of health including an ammonia storage
capability of the SCR catalyst, said step of determining the state
of health including the sub-steps of: comparing the measured
ammonia concentration level with a predetermined threshold;
determining a state of health fault based on an amount that the
measured ammonia concentration exceeds the predetermined
threshold.
4. The method of claim 3 further including the step of setting an
SCR catalyst fault.
5. The method of claim 3 wherein said diagnostic function comprises
one selected from the group (including a periodic function and a
non-periodic function) a periodic function, said step of
determining the state of health further includes the substep of
determining whether the measured ammonia concentration level
correlates to the diagnostic function.
6. The method of claim 3 further comprising the steps of: providing
an ammonia concentration sensor for measuring ammonia
concentration; and verifying proper operation of the ammonia
concentration sensor.
7. The method of claim 6 further comprising the step of:
positioning the ammonia concentration sensor in a sensing location
selected from the group comprising a mid-brick position of the SCR
catalyst and a rear-brick position of the SCR catalyst.
8. The method of claim 5 wherein said theta parameter is controlled
in accordance with a control strategy configured to increase NOx
conversion in the SCR catalyst and reduce NH.sub.3 emission from
the SCR catalyst, said diagnostic function being configured to
result in a detectable excess of NH.sub.3 emission from the SCR
catalyst.
9. The method of claim 2 wherein said determining a state of health
step includes the sub-step of: comparing an aspect of the measured
operating characteristic and the diagnostic function wherein the
aspect is selected from the group comprising (i) a signal
amplitude; (ii) a phase or time delay; and (iii) a frequency
difference.
10. The method of claim 2 wherein said one component comprises an
ammonia concentration sensor, said diagnostic function comprising a
periodic function, said step of determining the state of health
including the sub-steps of: comparing a correlation factor by
comparing the measured ammonia concentration level with periodic
function; determining a state of health fault based on the
correlation factor.
11. The method of claim 10 further including the step of setting an
ammonia concentration sensor fault.
12. The method of claim 11 further comprising the step of:
positioning the ammonia concentration sensor in a sensing location
selected from the group comprising a mid-brick position of the SCR
catalyst and a rear-brick position of the SCR catalyst.
13. In an internal combustion engine having an exhaust treatment
system having a selective catalytic reduction (SCR) catalyst, a
method of performing a diagnostic on the exhaust treatment system,
comprising the steps of: introducing a reductant into an exhaust
gas stream in an amount based on at least a surface coverage
parameter theta (0) selected so as to increase NOx conversion and
reduce NH.sub.3 emission from the SCR catalyst; adapting the theta
parameter based on a measured NH.sub.3 level emitted from the SCR
catalyst; generating an exhaust system fault when an adaptation
amount for the theta parameter exceeds a predetermined
threshold.
14. The method of claim 13 wherein said generating step includes
the sub-steps of: establishing respective upper and lower
adaptation limits; generating the fault when the adaptations of the
theta parameter exceeds one of the upper and lower adaptation
limits.
15. The method of claim 14 wherein said reductant comprises one of
ammonia and aqueous urea, said step adapting step includes the
sub-steps of: defining a base value for the theta parameter based
on an inlet temperature of the SCR catalyst; determining a
compensation factor based on a measured NH.sub.3 concentration
level emitted from the SCR catalyst; and determining the adapted
theta parameter value in accordance with the base value and the
compensation factor.
16. The method of claim 15 wherein said sub-step of determining a
compensation factor includes the sub-step of: determining when the
measured NH.sub.3 concentration level exceeds an upper bound and
increasing the compensation factor.
17. The method of claim 16 wherein said sub-step of determining a
compensation factor includes the sub-step of: determining when the
measured NH.sub.3 concentration level is less than a lower bound
and decreasing the compensation factor.
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 diagnostics and
more particularly to diagnostic methods for selective catalytic
reduction (SCR) based engine exhaust treatment systems.
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,
proposed future 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 has particular utility in an internal
combustion engine including an exhaust gas treatment system having
selective catalytic reduction (SCR) catalyst.
[0010] In one aspect of the invention, a method of performing a
diagnostic is provided. The method includes a number of steps. The
first step includes introducing a reductant (e.g., aqueous urea)
into an exhaust gas stream in an amount based on a target surface
coverage parameter theta. The next step involves perturbing the
target theta parameter of in accordance with a diagnostic function.
Next, measuring an operating characteristic of the exhaust gas
treatment system. Finally, determining a state of health of a
component of the treatment system based on an evaluation of the
diagnostic function and the measured operating characteristic.
[0011] In one embodiment, the component under diagnosis is the SCR
catalyst. Through perturbation of the target theta about a nominal
level, an expected level of excess NH.sub.3 is expected to be in
the exhaust stream (at the SCR catalyst). The NH.sub.3
concentration level is measured as the measured "operating
characteristic". A healthy SCR catalyst will exhibit a relatively
small magnitude perturbation in the NH.sub.3 sensed feedback.
However, for an SCR catalyst that has lost ammonia storage
capability, the NH.sub.3 sensed feedback exhibits a much larger
magnitude, indicating degraded SCR catalyst performance.
[0012] In another embodiment, the state of health of an NH.sub.3
sensor is determined. Likewise, if the measured NH3 sensing
feedback tracks with the target theta perturbation, then the
NH.sub.3 sensor is healthy. Otherwise, where the NH.sub.3
concentration does not track the target theta perturbation, the
NH.sub.3 sensor is unhealthy.
[0013] In another aspect of the invention, a diagnostic method is
provided that determines when NH.sub.3 sensing feedback-based
adaptive learning for adjusting the target theta values excessively
learns. When this condition is detected, a fault or error is
generated by the diagnostic.
[0014] A system is also presented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention will now be described by way of
example, with reference to the accompanying drawings:
[0016] FIG. 1 is a diagrammatic and block diagram showing an
exhaust treatment system in which the diagnostic methods of the
invention may be practiced.
[0017] FIG. 2 is a block diagram showing an overview of the dosing
control that includes an SCR model as well as improvements in
diagnostic methods.
[0018] FIG. 3 is a signal flow mechanization schematic showing
inputs and outputs of the SCR model.
[0019] FIG. 4 is a simplified diagram showing typical target theta
(.theta.) values or curves as a function of temperature.
[0020] FIG. 5 is a flowchart showing a method of using theta
perturbation for diagnostics.
[0021] FIG. 6 is a combination chart showing a first embodiment of
the method of FIG. 5 involving theta perturbation for determining
an SCR catalyst state of health.
[0022] FIG. 7 is a combination chart showing a second embodiment of
the method of FIG. 5 involving theta perturbation for determining
an NH.sub.3 sensor state of health.
[0023] FIG. 8 is a flowchart showing a diagnostic method for
generating a fault when theta adaptation exceeds control
authority.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] The DOC 28 and the DPF 30 may comprise conventional
components to perform their known functions.
[0031] 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.
[0032] 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.
[0033] 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 (TIN)
may be used.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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 (TIN); (v) an SCR inlet pressure, which may be derived from the
first pressure signal 54 (PIN); 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.
[0041] 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 TIN). 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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 mis-dosing, 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.
[0047] The transient compensation block 104 is configured generally
to reduce NH.sub.3 dosing when specified exhaust transients are
detected, such as sudden increases in exhaust mass air flow or when
an exhaust temperature gradient is in an "increasing" state. The
NH.sub.3 slip control block 106 is configured to selectively
shut-off NH.sub.3 dosing when the measured NH.sub.3 slip level
(mid-brick sensor) exceeds a predetermined trip level at a time
when certain other exhaust conditions are satisfied (e.g.,
temperature gradient is in the "increasing" state). These features
are described in greater detail in co-pending patent application
entitled "EXHAUST GAS TREATMENT SYSTEM AND METHODS OF OPERATING THE
SAME", (Attorney Docket No. DP-318318), 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.
[0048] Theta Perturbation Diagnostics. FIG. 5 is a flowchart
showing a diagnostic method involving theta perturbation to
determine a state of health of one or more different components of
the exhaust treatment system 14. As described in the Background,
one challenge for SCR-based exhaust treatment system developers is
to incorporate sufficient robustness in the control scheme to
maintain performance throughout the service life of the exhaust
treatment system. However, the performance of various components
can degrade over time and with usage, even beyond that correctable
by a robust control scheme. Accordingly, it would be desirable to
know the state of health of one or more individual components in
order to take appropriate diagnostic or control action. For
example, such diagnostic, upon determining a degradation of a
component, may be configured to issue an alert to the operator
(e.g., such as turning on a MIL 26--FIG. 1) indicating that a
component has degraded in performance to the point where it is not
correctable by the control system and hence may have emissions
implications. Additionally, the diagnostic may further be
configured to set a diagnostic trouble code (e.g., DTC 24--FIG. 1)
to facilitate troubleshooting by a technician. As an overview, FIG.
5 shows a flowchart describing the general diagnostic method, while
FIGS. 6 and 7 will be used to describe particular applications of
this general method to determine the state of health of an SCR
catalyst (FIG. 6) and an ammonia concentration sensor (FIG. 7). The
general method includes steps 110, 112, 114 and 116.
[0049] The method begins in step 110. Step 110 involves introducing
a reductant (e.g., ammonia gas or urea-water solution, as described
above) into the exhaust stream in an amount based on the target
ammonia surface coverage parameter theta (target .theta..sub.NH3).
This basic control approach has already been described above in
connection with FIGS. 2-4. In addition, it has been described above
that the dosing control 80 utilizes a calculated theta parameter
(feedforward control--.theta..sub.NH3) in conjunction with
closed-loop control via ammonia concentration level as feedback
from ammonia sensor 60, preferably located at a mid-brick position
of the catalyst 38. The method proceeds to step 112.
[0050] In step 112, the diagnostic method involves perturbing the
target theta parameter in accordance with a known, predetermined
diagnostic function. Preferably, this is performed during
steady-state engine operating conditions so any observed variations
in the sensed NH.sub.3 concentration level signal can be safely
attributed to the perturbation. In this regard, applying a known,
intrusive theta perturbation about the nominal target theta
parameter value can be expected to result in a predictable
response, which response can be measured and later evaluated to
determine the state of health. The method then proceeds to step
114.
[0051] In step 114, the diagnostic method involves measuring an
operating characteristic, preferably of a component of or
associated with the exhaust treatment system. In the particular SCR
catalyst and NH.sub.3 sensor embodiments to be described below,
this step of the method involves measuring the NH.sub.3
concentration level sensor output. It should be understood,
however, that other sensor outputs may be measured or other
operating characteristics can form the basis for determining the
state of health. The method then proceeds to step 116.
[0052] In step 116, the diagnostic method involves determining the
state of health of the component based on an evaluation of both (i)
the original diagnostic function which formed the basis for the
theta perturbation, and (ii) the measured operation characteristic
that results from the theta perturbation (or that is the result of
the theta perturbation). The evaluation may involve an assessment
of the (i) the signal amplitude or magnitude of the measured,
resultant operating characteristic in view of perturbing diagnostic
function, as compared to expected levels; (ii) the relative phasing
(or delay time) of the measured, resultant operating characteristic
compared to expected phasing; as well as (iii) frequency of
perturbation switch between the measured, resultant operating
characteristic relative to the perturbing diagnostic function. In
addition, it should be understood that while the illustrative
embodiments use a periodic perturbing function (e.g., triangle
wave), other functions are possible, for example only, use of a
step function.
[0053] Diagnostic for SCR Catalyst State of Health. FIG. 6 is a
combination chart showing responses of an ammonia concentration
sensor to theta perturbation where the responses indicate,
respectively, a healthy and an unhealthy SCR catalyst. The X-axis
shows time (seconds), while the Y-axis at left shows ammonia
concentration (ppm) and the Y-axis at right shows the theta
parameter value. The SCR catalyst can degrade over time and with
usage, particularly its ammonia storage capability. To determine
whether the SCR catalyst may have become degraded in its ammonia
storage capability, the theta perturbation block 100 (best shown in
FIG. 3) is configured to perturb the target theta parameter (target
.theta..sub.NH3) such than an excess of ammonia would be expected
to be present and available to be sensed by the ammonia
concentration sensor 60 (located at the mid-brick position). One
predicate condition before using the NH.sub.3 sensor to check the
SCR catalyst is to first confirm proper operation of the NH.sub.3
sensor itself. This predicate check may be performed using
conventional methods, or may be performed using the theta
perturbation method described below in connection with FIG. 7.
Accordingly, upon confirmation that the ammonia sensor is
functioning properly, if the magnitude of the measured ammonia
concentration exceeds an expected level, then this finding implies
that the SCR catalyst has lost some of its ammonia storage
capability. Furthermore, a reduction in NOx conversion efficiency
would be expected. As described above, appropriate diagnostic
action may be taken, such as issuing operator alerts or setting
diagnostic trouble codes. In addition, the diminished ammonia
storage capability may be communicated to the dosing control 80,
which may in turn be configured to use this information in its
theta control strategy.
[0054] FIG. 6 shows the results of a simulation conducted to show
how SCR catalyst degradation may be detected. In particular, the
simulation makes use of a six inch SCR catalyst, which for purposes
of the simulation was considered to provide the nominal amount
(i.e., 100%) of ammonia storage. To simulate a loss in ammonia
storage capability, a three inch SCR catalyst was used for
comparison. The three-inch SCR catalyst can be taken as indicative
of the degraded performance for a six-inch SCR catalyst. In FIG. 6,
the target ammonia surface coverage parameter theta (target
.theta..sub.NH3), was perturbed in accordance with a predetermined,
known diagnostic function, and is shown by reference numeral 118.
The selected diagnostic function may be a periodic signal, which
oscillates above and below the nominal value for theta (target
.theta..sub.NH3) for the operating condition shown. In the
illustrative embodiment, the diagnostic function is a triangle
waveform, although many other types of waveforms may be used. The
response of the ammonia concentration sensor, for the well
performing catalyst, namely the six-inch SCR catalyst, is shown as
trace 120. As shown, the ammonia concentration sensor output signal
indicates a low ammonia concentration, having a magnitude
designated by reference numeral 122. The response of the ammonia
sensor for the three-inch SCR catalyst ("degraded") is shown as
trace 124, which has a significantly greater magnitude (designated
126) than that of the normal SCR catalyst (trace 120). The
increased magnitude can be attributed to a loss of ammonia storage
capability in the "degraded" SCR catalyst, which may in real-world
applications impact NOx emissions performance (tail pipe). To
quantify this evaluation, a maximum expected magnitude threshold
may be established, which may be no less than the magnitude 122,
for example. From this, a "good"/"bad" evaluation may be made.
Alternatively, a more specific measure of the degradation can be
obtained by comparing the measured magnitude with the expected
magnitude and arriving at a ratio, percentage or other more
descriptive measure of storage capability (or loss thereof, e.g.,
90% of full capability).
[0055] Diagnostic For Ammonia Concentration Sensor State of Health.
FIG. 7 is a combination chart showing respective responses of a
healthy and an unhealthy NH.sub.3 concentration sensor. The X-axis
shows time (seconds), while the Y-axis at left shows ammonia
concentration (ppm) and the Y-axis at right shows the theta
parameter value. In this embodiment, the theta perturbation varies
the amount of ammonia provided to the SCR catalyst, and the
valuation involves determining whether the ammonia sensor can
detect the resulting variations in ammonia concentration. The
performance of the ammonia sensor to properly detect changes in
NH.sub.3 concentration will allow distinguishing a properly
functioning ammonia sensor from an improperly functioning one, for
either control purposes or for diagnostic purposes. Again,
preferably, the diagnostic is performed under substantially
steady-state engine operating conditions (i.e., constant exhaust
flow and temperature) so that any variation in the sensor response
can be properly attributed to just the theta perturbation.
[0056] FIG. 7 shows a trace 128 of the target ammonia surface
coverage theta parameter (target .theta..sub.NH3) as varied by the
theta perturbation block 100 about a nominal value in accordance
with a diagnostic function (e.g., triangle wave in the illustrative
embodiment). The diagnostic function is configured and applied so
that an excess of NH.sub.3 is expected to be present in the SCR
catalyst for detection by the ammonia concentration sensor 60
(located at a mid-brick position). Trace 130 illustrates the
response of a properly functioning ammonia concentration sensor
while trace 132 shows the response of a malfunctioning (or degraded
performance) ammonia concentration sensor. When the ammonia sensor
does not measure the predicted concentration level of ammonia in
the exhaust gas stream, based on the perturbed target theta
parameter (target .theta..sub.NH3), then the diagnostic determines
that an ammonia sensor performance issue exists. Note, the measured
operating characteristic here may include a magnitude, or may be a
relative phase between the theta perturbation and the response, or
may be a tracking characteristic. As shown in FIG. 7, a point 134,
on the perturbed target theta parameter trace 128, which is a local
maximum on the trace, results in a corresponding local maximum in
the output of a properly functioning NH.sub.3 sensor, for example
as shown at point 1342. Likewise, a point 136, on the perturbed
target theta parameter trace 128, which is a local minimum on the
trace, results in a corresponding local minimum in the output of a
properly functioning NH.sub.3 sensor, for example as shown at point
1362. Note, that this correspondence in the phasing in absent in
output of the degraded sensor, as shown in trace 132.
[0057] Diagnostic--Target Theta Adaptation Exceeds Control
Authority. FIG. 8 is a flowchart showing a further diagnostic
method to detect when adaptation of the target theta map (i.e.,
values) exceeds the control authority limits for such adaptation.
While developers of SCR-based exhaust treatment systems face
challenges in configuring systems that meet both NOx emission
standards as well as NH.sub.3 slip targets, such challenges are
heightened when one considers the additional requirement of in-use
compliance over the service life of the system. The adaptive
learning block 102 (FIG. 3) is generally configured to account for
and insert compensation into the target theta values for catalyst
degradation, urea injection malfunction or dosing fluid
concentration variation that may be encountered during real-world
use. The adaptive learning block 102 is responsive to the ammonia
concentration signal 62 produced by sensor 60, preferably located
at a mid-brick position. The adaptive learning block 102 is
configured to use the NH.sub.3 sensing feedback to adapt (or
adjust) the target theta parameter (Target .theta..sub.NH3),
thereby "pulling in" system variation that may occur in the field.
The ability of the adaptation block 102 to adjust for dosing errors
and the like enables some level of control authority to bring the
system to near nominal levels. At some point, however, the ability
to adapt target theta to overcome errors due to reagent mis-dosing
into the exhaust gas stream, the quality of the reagent being
introduced (e.g., the aqueous urea concentration level) and the
state of the SCR catalyst, will reach its control limit for
maintaining emissions desired performance. Accordingly, when the
adaptive learning block 102 excessively learns (i.e., adapts the
target theta parameter beyond a high or low threshold, the
diagnostic assumes that some system-level degradation must have
occurred, and the diagnostic may be configured to notify the
overall emissions control monitor.
[0058] FIG. 8 is a flowchart of the logic of the this diagnostic in
the context of a simplified adaptation scheme. The diagnostic is
configured for detecting and reporting excessive adaptation. The
method begins in step 138, where the most recent, measured ammonia
(NH.sub.3) concentration level is provided by the mid-brick
positioned sensor, and which will be used as feedback in the
adaptation. The method then proceeds to step 140.
[0059] In step 140, the method is configured to determine whether
the ammonia concentration level sensed by the ammonia sensor 60
(mid-brick) exceeds predetermined bounds. Predetermined bounds may
be a defined window extending higher ("upper bound") and lower
("lower bound") than the expected ammonia concentration, to account
for small variations that are deemed not significant enough to
require adaptation of the target theta parameter (Target
.theta..sub.NH3). If the answer in this decision step is "NO" then
the method branches to step 142 ("Keep theta values"). Step 142
means that the dosing control 80 will follow the existing target
.theta..sub.NH3 values (e.g., as shown in curve form in FIG. 4),
but as modified by any previous adaptations. Otherwise, if the
answer is "YES" (i.e., the measured NH.sub.3 concentration is out
of bounds-outside the window defined above), then the method
branches to step 144. Following step 144 means that adaptation may
be possible.
[0060] In step 144, the method is configured to determine whether
the measured NH.sub.3 concentration is lower than the lower bound
described above. If the answer is "YES", then the method branches
to step 146.
[0061] In step 146, the adaptation method is configured to increase
the then-existing Target .theta..sub.NH3 values by a predetermined
step (e.g., a compensation factor>1). The method then proceeds
to step 148.
[0062] In step 148, the method determines whether the compensation
factor (as increased) is still within an upper limit (i.e., does
not exceed the upper limit). If the answer is "YES", then the
adaptation has not exceeded its control authority and the method
returns to the beginning. However, if the answer in decision block
148 is "NO" then the method branches to step 150 ("Generate an
error"). In this situation, the net, accumulated positive-going
adjustment to the target theta due to the adaptation logic has
exceeded the control authority limit. As alluded to above, the
control authority limits (both upper and lower) may be selected
such that if exceeded, the logic can infer than there has been a
compromise in one or more of the components of the exhaust
treatment system 14. These compromises in performance may be due to
problems in the dosing delivery, quality issues with the urea-water
solution, or perhaps a decrease in the ammonia storage capability
of the SCR catalyst. In step 150, the diagnostic may set a
diagnostic trouble code, may activate an alert to an operator, or
take such other action may be appropriate.
[0063] Otherwise, in step 144, if the measured ammonia
concentration exceeds the upper bounds, thereby requiring
adaptation, then the method branches to step 152.
[0064] In step 152, the method is configured to decrease the
then-existing target .theta..sub.NH3 values by a predetermined step
(e.g., a compensation factor<1). The method then proceeds to
step 154.
[0065] In step 154, the method determines whether the compensation
factor (as decreased) is still within the lower limit (i.e., is
still higher than the lower limit). If the answer is "YES", then
the adaptation has not exceeded its control authority and the
method returns to the beginning. However, if the answer in decision
block 154 is "NO" then the method branches to step 150 ("Generate
an error"; See the description of step 150 above).
[0066] 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.
* * * * *