U.S. patent application number 14/079475 was filed with the patent office on 2015-05-14 for method and system for nox sensor degradation.
This patent application is currently assigned to Ford Global Technologies, LLC. The applicant listed for this patent is Ford Global Technologies, LLC. Invention is credited to David John Kubinski, Devesh Upadhyay, Michiel J. Van Nieuwstadt, In Kwang Yoo.
Application Number | 20150128565 14/079475 |
Document ID | / |
Family ID | 52822422 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150128565 |
Kind Code |
A1 |
Upadhyay; Devesh ; et
al. |
May 14, 2015 |
METHOD AND SYSTEM FOR NOX SENSOR DEGRADATION
Abstract
Various systems and method for detecting exhaust NO.sub.x sensor
degradation are disclosed. In one example, degradation of the
NO.sub.x sensor is indicated responsive to reductant injection in
an exhaust passage under engine off conditions. For example,
degradation of the NO.sub.x sensor is indicated when an actual
NO.sub.x sensor output differs from an expected NO.sub.x sensor
output by more than a threshold amount.
Inventors: |
Upadhyay; Devesh; (Canton,
MI) ; Yoo; In Kwang; (Ann Arbor, MI) ; Van
Nieuwstadt; Michiel J.; (Ann Arbor, MI) ; Kubinski;
David John; (Canton, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ford Global Technologies, LLC |
Dearborn |
MI |
US |
|
|
Assignee: |
Ford Global Technologies,
LLC
Dearborn
MI
|
Family ID: |
52822422 |
Appl. No.: |
14/079475 |
Filed: |
November 13, 2013 |
Current U.S.
Class: |
60/274 ;
60/286 |
Current CPC
Class: |
F01N 3/18 20130101; Y02T
10/24 20130101; F01N 2900/0416 20130101; F01N 11/00 20130101; F01N
2560/026 20130101; Y02A 50/20 20180101; F01N 3/208 20130101; Y02A
50/2325 20180101; F01N 2560/021 20130101; F01N 3/206 20130101; F02D
2041/1469 20130101; Y02T 10/40 20130101; Y02T 10/12 20130101; F02D
41/1461 20130101; F01N 2900/1404 20130101; F01N 2550/05 20130101;
F02D 41/222 20130101; F02D 41/042 20130101; F01N 2610/146
20130101 |
Class at
Publication: |
60/274 ;
60/286 |
International
Class: |
F01N 3/18 20060101
F01N003/18 |
Claims
1. A method for NOx sensor degradation detection in an engine
exhaust having a reduction catalyst and a feedgas exhaust NO.sub.x
sensor upstream of the reduction catalyst, comprising: shutting
down an engine to rest; estimating an exhaust reductant level from
the sensor following engine shutdown to rest; and indicating
degradation of the feedgas exhaust NO.sub.x sensor by setting a
flag based on the exhaust reductant level estimated by the sensor
following engine shutdown to rest.
2. The method of claim 1, wherein indicating degradation based on
an exhaust reductant level estimated by the sensor includes
indicating degradation in response to a difference between the
estimated exhaust reductant level and an expected reductant level
being higher than a threshold.
3. The method of claim 2, wherein the expected reductant level is
based on reductant dosing by an exhaust reductant injector prior to
the engine shutdown.
4. The method of claim 2, wherein following engine shutdown to rest
includes following a vehicle shutdown, while an engine exhaust flow
is below a threshold flow.
5. The method of claim 4, wherein the expected reductant level is
based on an amount of reductant injected by an exhaust reductant
injector into an exhaust passage following the engine shutdown to
rest.
6. The method of claim 5, wherein the amount of reductant injected
is based on each of ambient temperature and exhaust temperature,
the amount increased as the ambient temperature increases and/or
the exhaust temperature increases.
7. The method of claim 5, wherein the amount of reductant is
injected as a single injection.
8. The method of claim 5, wherein the amount of reductant is
injected as an injection pulse train having a magnitude and
frequency based on a response time of the feedgas exhaust NOx
sensor and an exhaust gas temperature.
9. The method of claim 5, wherein each of the feedgas exhaust
NO.sub.x sensor and the reductant injector are coupled to the
exhaust passage downstream of an oxidation catalyst and upstream of
the reduction catalyst.
10. The method of claim 9, wherein the reductant is one of urea and
ammonia, and wherein the reduction catalyst is an SCR catalyst.
11. The method of claim 1, wherein indicating degradation includes
setting a diagnostic code.
12. A method for an engine, comprising: following a vehicle-off
condition, while an engine is at rest, injecting an amount of
reductant into an exhaust passage; estimating an amount of
reductant in the exhaust passage based on an output of a feedgas
exhaust NO.sub.x sensor; and indicating NO.sub.x sensor degradation
based on the injected amount relative to the estimated amount.
13. The method of claim 12, wherein the vehicle-off condition
includes an engine exhaust flow being below a threshold flow.
14. The method of claim 13, wherein the injected amount of
reductant is based on each of ambient temperature, exhaust
temperature, and a reductant load of an exhaust reduction catalyst
at the vehicle-off condition.
15. The method of claim 14, wherein the injecting includes
injecting the amount of reductant according to a pulse train having
a magnitude and a frequency, the pulse train selected based on a
response time of the NOx sensor and an evaporation time of the
injected reductant.
16. The method of claim 14, wherein the injecting includes
injecting the amount of reductant from a reductant injector as a
single injection, the reductant including urea or ammonia, the
exhaust reduction catalyst including an SCR catalyst.
17. An engine system, comprising: an engine including an intake and
an exhaust; a reductant injector configured to inject reductant
into the engine exhaust, upstream of an exhaust reduction catalyst;
a feedgas NO.sub.x sensor coupled to the engine exhaust downstream
of the reductant injector and upstream of the reduction catalyst;
and a controller configured with computer readable instructions
for: during a first engine shutdown to rest, operating in a first
mode to indicate reductant injector degradation based on an output
of the NO.sub.x sensor; and during a second engine shutdown to
rest, operating in a second mode to indicate NO.sub.x sensor
degradation based on the output of the NO.sub.x sensor.
18. The system of claim 17, wherein during the first engine
shutdown to rest, indicating reductant injector degradation based
on an output of the NO.sub.x sensor includes indicating degradation
responsive to the output being higher than a first threshold, the
first threshold based on exhaust gas flow during the first engine
shutdown to rest.
19. The system of claim 18, wherein the controller includes further
instructions for, during the second mode, injecting an amount of
reductant into the engine exhaust, the amount based on ambient
temperature and exhaust temperature.
20. The system of claim 19, wherein during the second engine
shutdown to rest, indicating NO.sub.x sensor degradation based on
an output of the NO.sub.x sensor includes indicating degradation
responsive to the output being lower than a second threshold, the
second threshold based on the amount of reductant injected into the
engine exhaust.
Description
TECHNICAL FIELD
[0001] The present application relates to methods for diagnosing a
NOx sensor coupled to an exhaust gas treatment system of an
internal combustion engine.
BACKGROUND AND SUMMARY
[0002] Vehicle systems may include an engine with an exhaust gas
treatment system coupled in an exhaust passage in order to control
regulated emissions. In some examples, the exhaust gas treatment
system may include a selective catalytic reduction (SCR) system in
which a reductant, such as urea or ammonia, is added to the exhaust
stream upstream of a reduction catalyst such that NO.sub.x may be
reduced by the catalyst. The SCR system may also include one or
more NO.sub.x sensors such as a feedgas NO.sub.x sensor coupled
upstream of the SCR catalyst and a tailpipe NO.sub.x sensor coupled
downstream of the SCR catalyst. Based on the output of the upstream
and downstream NO.sub.x sensors, a performance of the SCR catalyst
may be determined. In addition, dosing control of the reductant may
be adapted based on the output of the NO.sub.x sensors. Therefore,
to enable accurate dosing control as well as to enable monitoring
of the SCR system efficiency, the sensors may need to be
periodically diagnosed.
[0003] Thus, methods and systems for diagnosing of a feedgas
exhaust NO.sub.x sensor coupled in an exhaust passage upstream of
an exhaust SCR catalyst is provided. One example method comprises
indicating degradation of a feedgas exhaust NO.sub.x sensor based
on an exhaust reductant level estimated by the sensor following
engine shutdown to rest. In this way, NO.sub.x sensor health can be
correlated with the lingering presence of ammonia deposits after a
vehicle engine has been turned off.
[0004] For example, an engine system may be configured with an SCR
catalyst in the exhaust passage and a urea injector positioned
upstream of the SCR catalyst. A feedgas NO.sub.x sensor may be
coupled to the exhaust passage upstream of the SCR catalyst and
downstream of the urea injector. After an engine shutdown to rest,
a controller may operate a reductant injector to inject a defined
amount of reductant into the exhaust passage. The controller may
then monitor the response of the feedgas NO.sub.x sensor. If the
output of the NO.sub.x sensor does not match an output expected
based on the active injection of reductant, NO.sub.x sensor
degradation may be determined. Based on the deviation of the
estimated output from the expected output, dynamic characteristics
of the feedgas NO.sub.x sensor may be learned and updated so that
reductant dosing control can be adjusted during a subsequent engine
restart.
[0005] In this way, the health and performance characteristics of a
feedgas exhaust NO.sub.x sensor can be better identified. By
monitoring the output of an exhaust NO.sub.x sensor during engine
shutdown conditions, while reductant is injected upstream of the
sensor, correlations between the injection and the exhaust NO.sub.x
sensor output can be used to learn NO.sub.x sensor behavior.
Specifically, natural sublimation of ammonia injected in an exhaust
passage after an engine shutdown can be used to diagnose an exhaust
NO.sub.x sensor. By improving NO.sub.x sensor diagnostics,
emissions compliance is improved.
[0006] It should be understood that the summary above is provided
to introduce in simplified form a selection of concepts that are
further described in the detailed description. It is not meant to
identify key or essential features of the claimed subject matter,
the scope of which is defined uniquely by the claims that follow
the detailed description. Furthermore, the claimed subject matter
is not limited to implementations that solve any disadvantages
noted above or in any part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 shows a schematic diagram of an engine including an
exhaust system with an exhaust gas treatment system.
[0008] FIG. 2 shows urea deposit formation in an exhaust
passage.
[0009] FIG. 3 shows a flow chart illustrating a routine for
diagnosing a reductant injection system based on exhaust NO.sub.x
levels after an engine shutdown to rest.
[0010] FIG. 4 shows a flow chart illustrating a routine for
adjusting reductant dosing control during an engine restart
responsive to an indication of injector leakage.
[0011] FIG. 5 shows a flow chart illustrating a routine for
diagnosing a NO.sub.x sensor based on exhaust NO.sub.x levels after
an engine shutdown to rest.
[0012] FIG. 6 shows a graph illustrating an example of urea
injector degradation detection or an example of NO.sub.x sensor
degradation detection.
[0013] FIG. 7 shows a graph illustrating an example of feedgas
exhaust NOx sensor degradation detection.
DETAILED DESCRIPTION
[0014] The following description relates to methods and systems for
using an exhaust NO.sub.x sensor output generated after an engine
shutdown to rest to diagnose exhaust after-treatment system
components, such as those included in the engine system of FIG. 1.
For example, the method allows for detection of urea deposits in
the engine exhaust passage, as shown at FIG. 2. A controller may be
configured to perform a control routine, such as the routine of
FIG. 3, to identify reductant injector degradation based on the
output profile of an exhaust NO.sub.x sensor, estimated after the
engine has spun to rest, in relation to an expected output profile
based on engine conditions. The controller may then adjust
reductant dosing control during a subsequent engine start based on
an indication of injector leakage, as shown at FIG. 4. The
controller may also be configured to perform a control routine,
such as the routine of FIG. 5, to inject a known amount of
reductant into the exhaust passage after the engine has spun to
rest and identify NO.sub.x sensor degradation based on estimated
output profile of the exhaust NO.sub.x sensor in relation to an
expected output profile based on the injected reductant. Example
diagnostic operations are shown at FIGS. 6-7. In this way, exhaust
emissions are improved.
[0015] Referring now to FIG. 1, a schematic diagram showing one
cylinder of a multi-cylinder engine 10, which may be included in a
propulsion system of an automobile, is illustrated. The engine 10
may be controlled at least partially by a control system including
a controller 12 and by input from a vehicle operator 132 via an
input device 130. In this example, the input device 130 includes an
accelerator pedal and a pedal position sensor 134 for generating a
proportional pedal position signal PP. A combustion chamber (i.e.,
cylinder) 30 of the engine 10 may include combustion chamber walls
32 with a piston 36 positioned therein. The piston 36 may be
coupled to a crankshaft 40 so that reciprocating motion of the
piston is translated into rotational motion of the crankshaft. The
crankshaft 40 may be coupled to at least one drive wheel of a
vehicle via an intermediate transmission system. Further, a starter
motor may be coupled to the crankshaft 40 via a flywheel to enable
a starting operation of the engine 10.
[0016] The combustion chamber 30 may receive intake air from an
intake manifold 44 via an intake passage 42 and may exhaust
combustion gases via an exhaust passage 48. The intake manifold 44
and the exhaust passage 48 can selectively communicate with the
combustion chamber 30 via respective intake valve 52 and exhaust
valve 54. In some embodiments, the combustion chamber 30 may
include two or more intake valves and/or two or more exhaust
valves.
[0017] In the example depicted in FIG. 1, the intake valve 52 and
exhaust valve 54 may be controlled by cam actuation via respective
cam actuation systems 51 and 53. The cam actuation systems 51 and
53 may each include one or more cams and may utilize one or more of
cam profile switching (CPS), variable cam timing (VCT), variable
valve timing (VVT), and/or variable valve lift (VVL) systems that
may be operated by the controller 12 to vary valve operation. The
position of the intake valve 52 and the exhaust valve 54 may be
determined by position sensors 55 and 57, respectively. In
alternative embodiments, the intake valve 52 and/or exhaust valve
54 may be controlled by electric valve actuation. For example, the
cylinder 30 may alternatively include an intake valve controlled
via electric valve actuation and an exhaust valve controlled via
cam actuation including CPS and/or VCT systems.
[0018] In some embodiments, each cylinder of the engine 10 may be
configured with one or more fuel injectors for providing fuel
thereto. As a non-limiting example, the cylinder 30 is shown
including one fuel injector 66. The fuel injector 66 is shown
coupled directly to the cylinder 30 for injecting fuel directly
therein in proportion to the pulse width of signal FPW received
from the controller 12 via an electronic driver 68. In this manner,
the fuel injector 66 provides what is known as direct injection
(hereafter also referred to as "DI") of fuel into the combustion
cylinder 30.
[0019] It will be appreciated that in an alternate embodiment, the
injector 66 may be a port injector providing fuel into the intake
port upstream of the cylinder 30. It will also be appreciated that
the cylinder 30 may receive fuel from a plurality of injectors,
such as a plurality of port injectors, a plurality of direct
injectors, or a combination thereof.
[0020] In one example, the engine 10 is a diesel engine that
combusts air and diesel fuel through compression ignition. In other
non-limiting embodiments, the engine 10 may combust a different
fuel including gasoline, biodiesel, or an alcohol containing fuel
blend (e.g., gasoline and ethanol or gasoline and methanol) through
compression ignition and/or spark ignition.
[0021] The intake passage 42 may include a throttle 62 having a
throttle plate 64. In this particular example, the position of the
throttle plate 64 may be varied by the controller 12 via a signal
provided to an electric motor or actuator included with the
throttle 62, a configuration that is commonly referred to as
electronic throttle control (ETC). In this manner, the throttle 62
may be operated to vary the intake air provided to the combustion
chamber 30 among other engine cylinders. The position of the
throttle plate 64 may be provided to the controller 12 by throttle
position signal TP. The intake passage 42 may include a mass air
flow sensor 120 and a manifold air pressure sensor 122 for
providing respective signals, MAF and MAP, to the controller
12.
[0022] Further, in the disclosed embodiments, an exhaust gas
recirculation (EGR) system may route a desired portion of exhaust
gas from the exhaust passage 48 to the intake passage 42 via an EGR
passage 140. The amount of EGR provided to the intake manifold 44
may be varied by a controller 12 via an EGR valve 142. By
introducing exhaust gas to the engine 10, the amount of available
oxygen for combustion is decreased, thereby reducing combustion
flame temperatures and reducing the formation of NO.sub.x for
example. As depicted, the EGR system further includes an EGR sensor
144 which may be arranged within the EGR passage 140 and may
provide an indication of one or more of pressure, temperature, and
concentration of the exhaust gas. Under some conditions, the EGR
system may be used to regulate the temperature of the air and fuel
mixture within the combustion chamber, thus providing a method of
controlling the timing of ignition during some combustion modes.
Further, during some conditions, a portion of combustion gases may
be retained or trapped in the combustion chamber by controlling
exhaust valve timing, such as by controlling a variable valve
timing mechanism.
[0023] An exhaust system 128 includes an exhaust gas sensor 126
coupled to the exhaust passage 48 upstream of an exhaust gas
treatment system 150. The sensor 126 may be any suitable sensor for
providing an indication of exhaust gas air/fuel ratio such as a
linear oxygen sensor or UEGO (universal or wide-range exhaust gas
oxygen), a two-state oxygen sensor or EGO, a HEGO (heated EGO), a
NO.sub.x, HC, or CO sensor. The exhaust gas treatment system 150 is
shown arranged along the exhaust passage 48 downstream of the
exhaust gas sensor 126.
[0024] In the example shown in FIG. 1, the exhaust gas treatment
system 150 is a urea based selective catalytic reduction (SCR)
system. The SCR system includes at least a reduction catalyst
(herein, SCR catalyst 152), a reductant storage tank (herein, urea
storage reservoir 154), and a reductant injector (herein, urea
injector 156), for example. In other embodiments, the exhaust gas
treatment system 150 may additionally or alternatively include
other components, such as a particulate filter, lean NO.sub.x trap,
three way catalyst, various other emission control devices, or
combinations thereof. For example, urea injector 156 may be
positioned upstream of reduction catalyst 152 and downstream of an
oxidation catalyst. In the depicted example, the urea injector 156
provides urea from the urea storage reservoir 154. However, various
alternative approaches may be used, such as solid urea pellets that
generate an ammonia vapor, which is then injected or metered to the
SCR catalyst 152. In still another example, a lean NO.sub.x trap
may be positioned upstream of the SCR catalyst 152 to generate
NH.sub.3 for the SCR catalyst 152, depending on the degree or
richness of the air-fuel ratio fed to the lean NO.sub.x trap.
[0025] The exhaust gas treatment system 150 further includes a
tailpipe exhaust gas sensor 158 positioned downstream of the SCR
catalyst 152. In the depicted embodiment, tailpipe exhaust gas
sensor 158 may be a NO.sub.x sensor, for example, for measuring an
amount of post-SCR NO.sub.x released via the tailpipe of exhaust
passage 48. Exhaust gas treatment system 150 may further include a
feedgas exhaust gas sensor 160 positioned upstream of the SCR
catalyst 152 and downstream of urea injector 156. In the depicted
embodiment, the feedgas exhaust gas sensor 160 may also be a
NO.sub.x sensor, for example, for measuring an amount of pre-SCR
NO.sub.x received in the exhaust passage for treatment at the SCR
catalyst.
[0026] In some examples, an efficiency of the SCR system may be
determined based on the output of one or more of tailpipe exhaust
gas sensor 158 and feedgas exhaust gas sensor 160. For example, the
SCR system efficiency may be determined by comparing NO.sub.x
levels upstream of the SCR catalyst (via sensor 160) with NO.sub.x
levels downstream of the SCR catalyst (via sensor 158). The
efficiency may be further based on the exhaust gas sensor 126 (when
the sensor 126 measures NO.sub.x, for example) positioned upstream
of the SCR system. In other examples, exhaust gas sensors 158, 160,
and 126 may be any suitable sensor for determining an exhaust gas
constituent concentration, such as a UEGO, EGO, HEGO, HC, CO
sensor, etc.
[0027] The controller 12 is shown in FIG. 1 as a microcomputer,
including a microprocessor unit 102, input/output ports 104, an
electronic storage medium for executable programs and calibration
values shown as a read only memory chip 106 in this particular
example, random access memory 108, keep alive memory 110, and a
data bus. The controller 12 may be in communication with and,
therefore, receive various signals from sensors coupled to the
engine 10, in addition to those signals previously discussed,
including measurement of inducted mass air flow (MAF) from the mass
air flow sensor 120; engine coolant temperature (ECT) from a
temperature sensor 112 coupled to a cooling sleeve 114; a profile
ignition pickup signal (PIP) from a Hall effect sensor 118 (or
other type) coupled to the crankshaft 40; throttle position (TP)
from a throttle position sensor; absolute manifold pressure signal,
MAP, from the sensor 122; and exhaust constituent concentration
from the exhaust gas sensors 126, 160, and 158. An engine speed
signal, RPM, may be generated by the controller 12 from signal
PIP.
[0028] The storage medium read-only memory 106 can be programmed
with non-transitory, computer readable data representing
instructions executable by the processor 102 for performing the
methods described below as well as other variants that are
anticipated but not specifically listed. Example methods are
described herein with reference to FIGS. 3-5.
[0029] As described above, FIG. 1 shows only one cylinder of a
multi-cylinder engine, and each cylinder may similarly include its
own set of intake/exhaust valves, fuel injector, spark plug,
etc.
[0030] FIG. 2 shows a detailed embodiment of an exhaust gas
after-treatment system 200, such as the exhaust gas treatment
system 150 described above with reference to FIG. 1. As shown, the
exhaust gas treatment system 200 includes a first catalyst 202,
such as a diesel oxidation catalyst, and a second catalyst 204,
such as an SCR catalyst, arranged along an exhaust passage 206. In
the example of FIG. 2, the second (reduction) catalyst 204 is
positioned downstream of the first (oxidation) catalyst 202. The
exhaust reductant injector 208 injects a reductant, such as urea or
ammonia, into the exhaust stream for reaction with NO.sub.x in the
second catalyst 204 responsive to signals received from a
controller 210.
[0031] In the example depicted in FIG. 2, the exhaust reductant
injector 208 is supplied with reductant from a reductant storage
reservoir 212. The reductant storage reservoir 212 may be a
reservoir suitable for holding the reductant throughout a range of
temperatures, for example. The reductant is pumped from the
reductant storage reservoir 212 via a pump 214. The pump 214 pumps
reductant from the reductant storage reservoir 212 and delivers the
reductant to the exhaust passage 206 at a higher pressure. As
shown, a reductant passage 216 fluidically couples the pump 214 and
the reductant injector 208. In some embodiments, reductant that
enters the exhaust passage 206 may be mixed into the exhaust stream
via a mixer 218.
[0032] The exhaust gas treatment system 200 further includes a
feedgas NO.sub.x sensor 220 disposed downstream of the first
catalyst 202, downstream of the reductant injector 208, and
upstream of the second catalyst 204. Thus, feedgas exhaust NOx
sensor may provide an estimate of NOx levels in exhaust gas
entering the SCR catalyst. The exhaust gas treatment system 200
further includes a tailpipe NO.sub.x sensor 222 disposed downstream
of the second catalyst 204. Thus, tailpipe exhaust NOx sensor may
provide an estimate of NOx levels in exhaust gas leaving the SCR
catalyst. The feedgas NO.sub.x sensor 220 and tailpipe NO.sub.x
sensor 222 may be used to determine an amount of NO.sub.x in the
exhaust passage 206, for example, such that reductant dosing may be
controlled, at least in part, based on the amount of NO.sub.x in
the exhaust passage 206. As described in greater detail below with
reference to FIG. 5, feedgas NO.sub.x sensor 220 degradation may be
determined under engine off conditions based on an amount of
reductant injected to the exhaust passage 206 after the engine has
been shut down to rest. For example, actual output of the feedgas
NO.sub.x sensor 220 may be compared to an expected output of the
feedgas NO.sub.x sensor 220, the expected output based on an amount
of reductant injected by the exhaust reductant injector 208 into
the exhaust passage 206 following an engine shutdown to rest. If
there is a discrepancy between the expected valve and the actual
output, NOx sensor degradation may be determined.
[0033] Further, as described in detail with reference to FIG. 3,
the feedgas NO.sub.x sensor may be utilized to determine reductant
injector 208 degradation. For example, over time the reductant
injector 208 may begin to leak, such that a greater than desired
amount of reductant enters the exhaust passage 206. As a result, a
reductant deposit 224 may form in the exhaust passage 206, for
example. Depending on an ambient temperature and an exhaust
temperature, which may be measured by temperature sensors 226, 228,
and 230, for example, the reductant deposit 224 may sublimate such
that reductant vapor may be sensed by the feedgas NO.sub.x sensor
220 when NO.sub.x is not present in the exhaust passage 206 (e.g.,
during engine off conditions). Thus, during conditions when the
engine is shutdown, the feedgas NOx sensor may be used to estimate
reductant levels (e.g., ammonia levels) in the exhaust passage,
downstream of the reductant injector and upstream of the SCR
catalyst, and infer if reductant deposits have formed. The feedgas
NOx sensor output may also be used to estimate a size of the
reductant deposit. Based on the size of the deposits (e.g., if they
are larger than a threshold size, or larger than an expected size),
injector leakage may be identified. As described in greater detail
below with reference to FIG. 3, the feedgas NO.sub.x sensor 220 may
provide an indication of reductant injector 208 degradation. For
example, actual output of the feedgas NO.sub.x sensor 220 is
compared to an expected output of the feedgas NO.sub.x sensor 220,
the expected output based on an estimated total amount of unreacted
reductant stored in the second catalyst 204 and in the exhaust
passage 206 during the engine shutdown, an ambient temperature,
exhaust flow conditions and exhaust temperature at the engine
shutdown.
[0034] Thus, the exhaust gas treatment system 200 includes the
feedgas NO.sub.x sensor 220 which may be utilized to determine an
amount of NO.sub.x present in the exhaust passage 206 during
engine-on conditions and an amount of reductant present in the
exhaust passage 206 during engine-off conditions. As will be
described below, based on output of the feedgas NO.sub.x sensor 220
under the various conditions, feedgas NO.sub.x sensor 220
degradation as well as reductant injector 208 degradation may be
determined.
[0035] As elaborated with reference to FIGS. 3-5, during conditions
when the engine is shutdown to rest and the vehicle is off (e.g.,
during an engine key-off and/or vehicle key-off event, or engine
stop event in a keyless system with a stop/start button), the
feedgas exhaust NO.sub.x sensor can be used to estimate reductant
levels in the exhaust volume between the injector and the SCR
catalyst. The NO.sub.x sensor output can then be used to diagnose
the continued presence of reductant deposits as may occur in the
presence of injector leakage. For example, urea injector leakage
may be diagnosed based on the detection of excess ammonia in the
defined space (between the reductant injector and the SCR
catalyst). In addition, urea may be actively injected and an output
of the feedgas NO.sub.x sensor may be monitored to determine the
dynamic characteristics of the NOx sensor. In this way, an output
of the feedgas NO.sub.x sensor generated after an engine key-off
condition can be advantageously used to identify injector
degradation as well as NO.sub.x sensor degradation.
[0036] Now turning to FIG. 3, an example routine 300 is shown for
diagnosing a reductant injection system based on exhaust NO.sub.x
levels after an engine is shutdown to rest. Specifically, the
routine determines an expected exhaust reductant level and an
actual exhaust reductant level during engine off condition
following engine shutdown. Based on a difference between the
expected and actual reductant levels, exhaust reductant injector
degradation may be indicated. For example, if the actual reductant
level is greater than the expected level, exhaust reductant
injector degradation such as injector leakage may be indicated.
[0037] At 302, it is determined if the engine is off and has spun
down to rest, such that the engine is not spinning and combustion
is not occurring in any cylinders of the engine. As an example, it
may be determined that the engine is off if the ignition key is in
the engine-off position or if the stop button has been pressed in a
keyless system. As other examples, it may be determined that the
engine is off if the vehicle in which the engine is positioned is
in a vehicle off condition and/or if an exhaust gas flow level is
below a threshold flow. If it is determined that the engine is not
off, the routine 300 ends.
[0038] On the other hand, if it is determined that the engine is
off, the routine continues to 304 where an SCR catalyst performance
is latched. For example, the control system may determine a health
or status of the SCR catalyst based on output from the exhaust
NO.sub.x sensors. The catalyst efficiency value may be used as a
reference for the determination of the validity of the corrective
control action to be taken on the next key-on cycle (as elaborated
at FIG. 4 at step 408).
[0039] At 306, exhaust NO.sub.x sensors are kept enabled
intrusively. For example, the feedgas NO.sub.x sensor (positioned
between the oxidation catalyst and the SCR catalyst) and the
tailpipe NO.sub.x sensor (positioned downstream of the SCR
catalyst) remain enabled after the engine is shut down such that
they continue to output signals indicating NO.sub.x levels in the
exhaust passage. As such, the feedgas NOx sensor is maintained
enabled even though no further exhaust NOx is expected after the
engine is shutdown to rest.
[0040] At 308, it is determined if the tailpipe exhaust NO.sub.x
level, or tailpipe exhaust flow has stabilized. For example, the
system may wait until the signal from the tailpipe NO.sub.x sensor
has stabilized or equilibrated before the routine proceeds.
Alternatively, the system may wait until the tailpipe exhaust
levels have stabilized and the exhaust flow rate is below a
threshold flow rate. If it is determined that the tailpipe exhaust
NO.sub.x level or exhaust flow is not stabilized, the routine 300
moves to 324 where the system waits for the tailpipe NO.sub.x level
or exhaust flow to stabilize.
[0041] Once the tailpipe NO.sub.x level/exhaust flow has stabilized
or if it is determined that the tailpipe NO.sub.x/flow is stable at
308, the routine 300 continues to 310 where it is determined if the
amount or level (e.g., concentration) of feedgas NO.sub.x is
greater than the amount or level (e.g., concentration) of tailpipe
NO.sub.x. For example, the amount of feedgas NO.sub.x and the
amount of tailpipe NO.sub.x may be determined based on signals from
the respective sensors. As such, after engine shutdown, once the
tailpipe exhaust flow has stabilized, the output of the feedgas NOx
sensor is expected to stabilize and equilibrate with the output of
the tailpipe NOx sensor. Also after engine shutdown, when no
further exhaust NOx is generated, the feedgas NOx sensor may sense
vaporized reductant (e.g., ammonia) remaining in the exhaust
passage, in the region between the injector and the SCR catalyst.
Therefore, under engine shutdown conditions, once exhaust flow gas
stabilized, the feedgas NOx sensor output being higher than the
tailpipe NOx sensor output may be indicative of the presence of
ammonia deposits in the exhaust passage. If the feedgas NOx amount
is less than the amount of tailpipe NO.sub.x amount, the routine
moves to 326 and waits for the feedgas NOx sensor signal to
stabilize. Once the levels have stabilized, the routine may move
forward to 312 to check reductant injector degradation based on the
feedgas NOx sensor output. In alternate examples, if after a
predetermined duration has elapsed, the feedgas NOx sensor signal
continues to not show values that are higher than the tailpipe NOx
sensor signal, the controller may indicate that the reductant
injection system is in good health (not degraded) and may move
directly to step 328 of routine 300.
[0042] On the other hand, if it is determined that the amount of
feedgas NO.sub.x is greater than the amount of tailpipe NO.sub.x,
the routine directly proceeds to 312 to diagnose the reductant
injector. Specifically, at 312, the expected (e.g., residual)
exhaust reductant level is determined. In some examples, the
expected reductant level may be an expected ammonia level. For
example, based on exhaust flow and temperature conditions,
injection conditions, ambient conditions, catalyst conditions, and
amount of unreacted reductant stored in the exhaust reduction
catalyst, an amount of unreacted exhaust reductant that is expected
to remain (or linger) in the exhaust passage between the reductant
injector and the SCR catalyst after the engine shutdown is
determined. This includes determining an expected size of a
reductant deposit in the exhaust passage, a rate of reductant
sublimation from the deposit, and a corresponding feedgas NOx
sensor output. In one example, the controller may determine an
expected feedgas NOx sensor output profile for a duration since the
engine shutdown based on the expected size of the ammonia deposit
and a rate of natural sublimation of the ammonia deposit (based on
the exhaust temperature in the exhaust passage and the ambient
temperature at the engine shutdown).
[0043] Once the expected exhaust reductant level is determined, the
routine 300 continues to 314 where an actual exhaust reductant
level is estimated based on feedgas exhaust NO.sub.x sensor output
and profile. For example, the actual exhaust reductant level is
determined based on a signal output from the feedgas exhaust
NO.sub.x sensor. As such, during engine-off conditions when the
exhaust gas flow is substantially zero and NO.sub.x is not present
in the exhaust passage, the NO.sub.x sensor may act as a reductant
(e.g., ammonia) sensor, as the NO.sub.x sensor may be
cross-sensitive to gas phase ammonia which sublimates from urea
deposits in the exhaust passage. In one example, the actual exhaust
reductant level may be estimated via the feedgas NO.sub.x sensor
for a duration since the vehicle-off condition to determine an
amount of reductant and a rate of reductant sublimation.
[0044] At 316 of routine 300, it is determined if the actual
exhaust reductant level (determined at 314) is greater than the
expected exhaust reductant level (determined at 312). If it is
determined that the actual exhaust reductant level is less than the
expected exhaust reductant level, the routine moves to 328 where no
reductant injector leakage is indicated (e.g., reductant injector
leakage is not diagnosed).
[0045] On the other hand, if it is determined that the actual
exhaust reductant level is greater than the expected exhaust
reductant level, the routine continues to 318 where reductant
injector degradation is indicated and a diagnostic code is set.
Specifically, based on the higher than expected reductant level,
the controller infers that a larger than expected reductant deposit
is present in the exhaust passage, between the reductant injector
and the SCR catalyst, due to reductant injector leakage. As an
example, the indication of reductant injector degradation may be an
indication of reductant injector leakage.
[0046] In one example, the expected exhaust reductant level may be
a threshold level, for example. The indication of exhaust reductant
injector degradation may be made in response to an output of the
feedgas exhaust NO.sub.x sensor being higher than the threshold
level. Further still, the indication of exhaust reductant injector
degradation may be made in response to the output of the feedgas
exhaust NOx sensor being higher than the threshold level for longer
than a threshold duration, each of the threshold and threshold
duration based on the total amount of unreacted reductant (e.g.,
SCR catalyst ammonia loading at engine shutdown), the ambient
temperature, and the exhaust temperature at the engine shutdown, as
described above. Thus, if there is more reductant in the exhaust
passage, as sensed by the feedgas NOx sensor, and/or if the
reductant in the exhaust passage continues to linger for a longer
than expected duration, the controller may determine that a larger
than expected ammonia deposit has formed in the exhaust passage due
to reductant injector leakage.
[0047] At 320, a size of the reductant deposit is estimated based
on the feedgas NO.sub.x sensor output. Since the output of the
feedgas NO.sub.x sensor corresponds to an amount of reductant in
the exhaust passage while NO.sub.x is not present in the exhaust
passage (e.g., during engine-off conditions), the size of an
exhaust passage reductant deposit may be determined based on a
reductant level output by the feedgas NO.sub.x sensor during the
engine-off conditions.
[0048] At 322, reductant dosing control is adjusted during the next
engine-on condition based on an indication of degradation, which is
described in detail with reference to FIG. 4 below. For example, in
response to the indication of degradation, reductant dosing may be
reduced during a subsequent engine restart from engine rest.
[0049] Thus, the feedgas NO.sub.x sensor disposed in the exhaust
passage upstream of the SCR catalyst may be used to detect exhaust
reductant injector degradation. Under conditions in which the
engine is off and a NO.sub.x level in the exhaust passage is
substantially zero, the feedgas NO.sub.x sensor may be used to
measure a level of reductant (e.g., ammonia from urea deposits) in
the exhaust passage. Based on the signal output by the feedgas
NO.sub.x sensor, reductant injector degradation may be indicated
and reductant dosing may be adjusted during subsequent engine
restart from rest, as described below with reference to FIG. 4.
[0050] FIG. 4 shows a flow chart illustrating an example routine
400 for adjusting reductant dosing control during an engine restart
responsive to an indication of injector leakage. Specifically, the
routine adjusts an amount of reductant injected to an SCR system
based on an indication of exhaust reductant injector leakage
determined via the routine described above with reference to FIG.
3. For example, the reductant injector may be controlled to inject
less reductant to the SCR system when reductant injector leakage is
indicated.
[0051] At 402 of the routine 400, it is determined if the engine is
on, such that the engine is spinning and combustion may be
occurring in any or all of the cylinders of the engine. For
example, it may be confirmed that the engine has been started from
rest. As another example, it may be determined that the engine is
on if the key is in the engine-on position or if the start button
has been pressed in a keyless system. As another example, it may be
determined that the engine is on if an exhaust gas flow level is
above a threshold flow. If it is determined that the engine is off,
the routine 400 ends.
[0052] On the other hand, if it is determined that the engine is
on, the routine 400 proceeds to 404 where it is determined if a
reductant injector degradation flag has been set. As an example,
the reductant injector degradation flag may be set when the
diagnostic code is set at 318 of routine 300. The reductant
injector degradation flag provides an indication that the exhaust
reductant injector is degraded, for example, and reductant dosing
should be adjusted accordingly.
[0053] If it is determined that the reductant injector degradation
flag has not been set, the routine moves to 420 where a reductant
(e.g., urea) dosing control is adapted based on engine operating
conditions. For example, the reductant may be injected to the SCR
system based on a current exhaust NO.sub.x level, ambient
temperature, exhaust temperature, and/or the like. In one example,
the amount of reductant injected to the exhaust passage is based on
an estimated exhaust NO.sub.x level relative to a target exhaust
NO.sub.x level which is based on engine operating conditions.
[0054] If, on the other hand, if it is determined that the
reductant injector degradation flag has been set, the routine 400
continues to 406 where reductant (e.g., urea) dosing control for
reduced injection based on leak indication is adapted. For example,
in order to reduce a size of reductant deposits in the exhaust
passage, the amount of reductant injected to the SCR system may be
reduced by an amount corresponding to the estimated size of the
reductant deposit determined at 320 of routine 300. Also since a
leaky injector was detected, the urea quantity demanded during
regular engine operation may be adaptively reduced to account for
the leaky injector. Herein, the dosing control compensates for the
presence of extra reductant lingering in the exhaust passage in the
form of reductant deposits. In this way, by adapting the reductant
dosing control based on the indication of reductant injector
leakage, the target exhaust NO.sub.x level may be maintained, for
example.
[0055] At 408 it is determined if there is a drop in SCR catalyst
performance. A drop in SCR catalyst performance may be indicated
based on an increase in exhaust NO.sub.x levels as sensed by the
tailpipe NO.sub.x sensor and/or a change in other parameter
determined at 304 of routine 300.
[0056] If it is determined that there is no drop in SCR catalyst
performance, the routine 400 proceeds to 410 where leak adaptation
of reductant dosing control is maintained. In an alternate example,
the routine proceeds to 410 if it is determined that there is an
improvement in the SCR catalyst performance relative to the latched
value learned earlier (specifically, at step 304 of routine 300).
For example, reductant dosing continues to be modified (e.g.,
decreased) as described at 406 due to the indication of exhaust
reductant injector leakage. Next, at 412, it is determined if the
engine is off (i.e., the engine is not spinning and combustion is
not occurring in any cylinders of the engine). As described above,
it may be determined that the engine is off if the key is in the
engine-off position or if the stop button has been pressed in a
keyless system. If the engine is still on, the routine 400 returns
to 410 and leak adaptation of reductant dosing control is
maintained. Thus, leak adaptation of reductant dosing control is
maintained while the engine is running if there continues to be no
drop in SCR catalyst performance.
[0057] Turning back to 408, if it is determined that there is a
drop in SCR catalyst performance, the routine moves to 422 where
reductant dosing control is resumed without leak adaptation. For
example, the drop in SCR catalyst performance may be due to an
insufficient amount of reductant, thereby resulting in an increase
of exhaust NO.sub.x detected at the tailpipe NO.sub.x sensor. As
such, reductant dosing may be returned to an amount corresponding
to a target exhaust NO.sub.x level without any adjustment for
reductant injector leakage. Next, at 424, it is determined if the
engine is off, as described above with reference to 412. If the
engine is still on, the routine 400 returns to 412 and reductant
dosing control without leak adaptation is maintained.
[0058] If it is determined that the engine is on (e.g., the engine
is spinning and combustion is carried out in one or more cylinders
of the engine) at either 412 or 424, the routine 400 moves to 414
where the reductant injection leak detection routine 300 described
above with reference to FIG. 3 is performed again.
[0059] At 416, it is determined if a leak is detected.
Specifically, it is determined if a leak was identified on the
second iteration of the reductant injector leak detection. For
example, as described above, it may be determined that the
reductant injector is leaking if an actual exhaust NO.sub.x level
is greater than a threshold level based on an expected exhaust
NO.sub.x level during the engine shutdown conditions following 412.
If a reductant injector leak was detected on a first iteration of
the leak detection routine (at 300, and as indicated by the flag at
404), and if no leak is detected on the (second) iteration of the
leak detection routine (performed at 414), the routine moves to 426
where the system initiates or waits for the SCR catalyst monitor.
In one example, this may be an independently performed catalyst
performance monitoring routine within the Aftertreatment management
and OBD system. Herein, it may be determined that the injector
leakage and deposit formation indicated at 404 was transient. In
addition, it may be determined that the injector leakage and
reductant deposit formation was possibly due to high bandwidth
changes in operating conditions and/or other transient disturbance
factors that may have resulted in excessive urea injection leading
to deposits in the exhaust system at or around key-off
[0060] On the other hand, if a leak is detected on each of the
first and subsequent iteration of the injector leak diagnostic
routine, the routine 400 continues to 418 where alternate leakage
detection monitors are initiated, if available. The alternate
leakage detection monitors may determine if reductant leakage is
occurring via a method other than the method described with
reference to FIG. 3. If alternate leakage detection monitors are
not initiated, a leakage diagnostic code may be set. For example,
in the absence of any alternate, independent injector leakage
detection monitoring routines the determination of leakage made
under routines 300-400 may be considered adequate to set a leakage
flag.
[0061] Thus, reductant dosing control may be adjusted based on the
indication of exhaust reductant injector leakage. By adjusting the
amount of reductant injected to the exhaust passage to compensate
for reductant injector leakage, the exhaust passage may receive an
amount of reductant closer to a desired amount of desired
reductant. As such, the target NO.sub.x level in the exhaust
passage may be maintained and the formation of reductant deposits
may be reduced.
[0062] In one example, the engine system is configured to operate
in two different modes. During a first mode in which the engine is
running and exhaust flow is above a threshold flow, a level of
NO.sub.x in the exhaust passage (e.g., exhaust NO.sub.x) may be
estimated based on output of one or more of the feedgas NO.sub.x
sensor and the tailpipe NO.sub.x sensor. During a second mode in
which the engine is off and exhaust flow is below the threshold
flow, an amount of exhaust ammonia may be estimated based on the
output of the feedgas NO.sub.x sensor. Further, during the first
mode, an amount of urea injected into the exhaust passage may be
adjusted based on the estimated exhaust NO.sub.x level relative to
a target NO.sub.x level. During the second mode, urea injector
degradation may be indicated based on the estimated exhaust ammonia
level relative to an expected ammonia level.
[0063] Continuing to FIG. 5, a flow chart illustrating a routine
500 for diagnosing a NO.sub.x sensor based on exhaust NO.sub.x
levels after an engine shutdown to rest is shown. Specifically, the
routine controls the injection of reductant into an exhaust passage
once exhaust flow through the exhaust passage has stabilized after
engine shutdown. Based on actual output from a feedgas exhaust
NO.sub.x sensor compared to expected output from the feedgas
NO.sub.x sensor, degradation of the feedgas NO.sub.x sensor may be
indicated.
[0064] At 502, it is determined if the engine is off. As described
above, when the engine is off, the engine is not spinning and
combustion is not occurring in any cylinders of the engine. As an
example, it may be determined that the engine is off if the key is
in the engine-off position or if the stop button has been pressed
in a keyless system. As other examples, it may be determined that
the engine is off following a vehicle shutdown, after an engine
shutdown to rest, and/or if an exhaust gas flow level is below a
threshold flow. If it is determined that the engine is on (e.g.,
spinning, combusting, and not off), the routine 500 ends.
[0065] On the other hand, if it is determined that the engine is
off, the routine continues to 504 where exhaust NO.sub.x sensors
are kept enabled intrusively. For example, the feedgas NO.sub.x
sensor and the tailpipe NO.sub.x sensor remain on and continue to
output exhaust NO.sub.x levels after the engine is turned off.
[0066] At 506, it is determined if the tailpipe NO.sub.x level or
tailpipe exhaust flow has stabilized. For example, the system may
wait until the signal from the tailpipe NO.sub.x sensor has
equilibrated or fallen below a threshold level before the routine
proceeds. If it is determined that the tailpipe NO.sub.x or exhaust
flow has not stabilized, the routine 500 moves to 526 where the
system waits for the tailpipe NO.sub.x or exhaust flow to
stabilize.
[0067] Once the tailpipe NO.sub.x/flow has stabilized, or if it
determined that the tailpipe NO.sub.x/flow has stabilized at 506,
the routine moves to 508 where reductant is injected into the
exhaust passage. Specifically, because the NO.sub.x sensor may
measure ammonia in the absence of NO.sub.x (e.g., during engine off
conditions), reductant may be injected into the exhaust passage
such that the feedgas NO.sub.x sensor may measure a corresponding
amount of injected reductant and output a corresponding sensor
output. Based on the output of the sensor, feedgas NO.sub.x sensor
degradation may be determined. Further, an amount of reductant
injected to the exhaust passage may be based on each of ambient
temperature and exhaust temperature. For example, the amount of
reductant injected may be increased as the ambient temperature
increases and/or the exhaust temperature increases. The amount of
reductant injected may be further based on a reductant load of an
exhaust reduction catalyst (e.g., the SCR catalyst) at the
vehicle-off condition.
[0068] In some examples, reductant may be injected as an active
single injection of a predefined amount at 510 (based on the
various factors described above). In other examples, the reductant
may be injected via an injection pulse train with predefined
characteristics at 512. As an example, the injection pulse train
may have pulse train features including a magnitude and frequency
designed to inject a similar total amount of reductant (e.g., urea)
as for the single active injection amount adjusted as a function of
exhaust temperature (at 510). The pulse train features may be
further based on the response time of the feedgas exhaust NOx
sensor and the exhaust temperature (at the time of the routine).
For example, the frequency (or period) of the pulse may be chosen
to reflect an expected 10-90% response time of a healthy NOx sensor
plus the urea to ammonia evaporation time of the injected reductant
at the given exhaust temperature. In still other examples, the
reductant injection may be a pre-existing reductant deposit at 514.
Further, in some examples, the reductant injection may be a
combination of a single injection, an injection pulse train, and/or
a pre-existing reductant deposit.
[0069] At 516, an expected exhaust NO.sub.x sensor output profile
based on the engine exhaust conditions is determined. In one
example, the expected NO.sub.x sensor output profile may be based
on reductant dosing by the exhaust reductant injector prior to the
engine shutdown. In another example, the expected NO.sub.x sensor
output profile may be based on an amount of reductant actively
injected by the exhaust reductant injector into the exhaust passage
following the engine shutdown to rest (at 508). The expected NOx
sensor profile may include an expected NOx sensor output over time,
a peak output, an expected peak width, etc.
[0070] Once the expected exhaust NO.sub.x sensor output profile is
determined, the routine 500 proceeds to 518 where the actual
exhaust NO.sub.x sensor output profile based on feedgas exhaust
NO.sub.x sensor output is estimated. For example, the actual
exhaust NO.sub.x sensor output profile is determined based on a
signal output from the feedgas exhaust NO.sub.x sensor and
corresponds to a reductant level in the exhaust passage. During
engine off conditions when the exhaust gas flow is substantially
zero and NO.sub.x is not present in the exhaust passage, the
NO.sub.x sensor may act as a reductant sensor, as the NO.sub.x
sensor may be cross-sensitive to reductant injected into the
exhaust passage.
[0071] At 520, feedgas exhaust NO.sub.x sensor dynamic
characteristics are updated based on the estimated profile. At 522,
it is determined if the actual exhaust NO.sub.x sensor output
profile (determined at 518) is different than the expected exhaust
NO.sub.x sensor output profile (determined at 516). In one example,
it may be determined if a difference between the actual NO.sub.x
sensor output profile and the expected NO.sub.x sensor output
profile is higher than a threshold. If it is determined that the
actual profile is substantially similar to the expected profile,
the routine 500 moves to 528 where no NOx sensor degradation is
indicated, and the routine ends.
[0072] On the other hand, if it is determined that the actual
profile is different from the expected profile (e.g., greater than
the expected profile by more than a threshold difference, or
smaller than the expected profile by more than a threshold
difference), the routine 500 continues to 524 where NO.sub.x sensor
degradation is indicated and a diagnostic code is set. In this
manner, the system may be informed that the NO.sub.x sensor is not
outputting a correct indication of exhaust NO.sub.x during
subsequent engine operating conditions, for example. Additionally,
the learned NOx sensor output profile may be used as an input to an
alternate dedicated NOx sensor diagnostics routine.
[0073] In some embodiments, based on a difference between the
expected NOx sensor output and the estimated output, a nature of
the degradation may also be indicated. For example, the controller
may indicate that a stuck feedgas exhaust NOx sensor condition if
the feedgas exhaust NOx sensor does not show any increase in output
signal in response to the intrusive urea injection methods.
[0074] As another example, the dynamic response time of the feedgas
exhaust NOx sensor (such as a 10-90% response or the s3% response)
may be established during the rise phase of the signal. If the
signal saturates during the rise phase, then the response time may
be ascertained during the decay phase. Alternately the reductant
injection (e.g., urea) pulse train may be used to determine the
same information from the frequency response of the NOx sensor
signal in response to the urea pulse.
[0075] Thus, exhaust NO.sub.x sensor degradation may be determined
during engine off conditions. By injecting a known amount of
reductant into the exhaust passage upstream of the SCR catalyst, an
expected output of the feedgas NO.sub.x sensor may be determined.
When the actual output of the feedgas NO.sub.x sensor differs from
the expected output by more than a threshold amount, feedgas
NO.sub.x sensor degradation is indicated, and NOx sensor
characteristics may be dynamically learned and updated. In this
way, reliability of a feedgas exhaust NOx sensor output can be
improved.
[0076] In one example embodiment, the engine system may be operated
such that exhaust reductant injector degradation and feedgas
NO.sub.x sensor degradation may be indicated. For example, during a
first engine shutdown to rest, the system may be operated in a
first mode to indicate reductant injector degradation based on an
output of the NO.sub.x sensor. During a second engine shutdown to
rest, the system may be operated in a second mode to indicate
feedgas NO.sub.x sensor degradation based on the output of the
NO.sub.x sensor. Further, during the first engine shutdown to rest,
the system may be operated to indicate reductant injector
degradation based on an output of the NO.sub.x sensor responsive to
the output being higher than a first threshold. The first threshold
may be based on exhaust gas flow during the first engine shutdown
to rest, for example. During the second engine shutdown to rest,
the system may be operated to indicate feedgas NO.sub.x sensor
degradation based on an output of the NO.sub.x sensor responsive to
the output being lower than a second threshold. The second
threshold may be based on the amount of reductant injected into the
exhaust passage, for example.
[0077] FIG. 6 shows a graph illustrating an example of urea
injector degradation detection. Map 600 depicts the output of a
feedgas NOx sensor at curve 602 (solid line) and the output of a
tailpipe NOx sensor at curve 604 (dashed line). The curve 602 shows
the feedgas NO.sub.x signal continues to ramp up for a duration
(e.g., approximately 20 seconds in the example of FIG. 6) after the
engine is turned off, while the tailpipe NO.sub.x signal indicated
by the curve 604 remains substantially zero and stable. Because the
engine is off and there is no exhaust flow through the exhaust
passage (e.g., NO.sub.x is not present in the exhaust passage), the
indication of increased feedgas NO.sub.x may be due to a source of
excess reductant detected by the feedgas NO.sub.x sensor which is
positioned between the oxidation catalyst and the SCR catalyst, for
example. As one example, the reductant injector may be leaking or
injecting too much reductant into the exhaust passage during engine
operation and the increased feedgas NO.sub.x signal may be an
indication of reductant injector degradation. For example, if the
reductant injector leaks, the increased feedgas NO.sub.x signal may
be due to sublimation of reductant deposits resulting from excess
reductant in the exhaust passage between the oxidation catalyst and
the SCR catalyst. As such, the increased feedgas NO.sub.x signal
after engine shutdown is indicative of reductant injector
degradation.
[0078] FIG. 7 shows an example of NO.sub.x sensor degradation
detection. Maps 700 and 710 depict reductant dosing at curves 702
and 706 (solid lines), and corresponding feedgas NOx sensor outputs
at curves 704 and 708 (dashed lines). In the depicted example, the
increased feedgas NO.sub.x signal may be due to an intentionally
created reductant deposit formed from reductant injection after the
engine is turned off. In such an example, feedgas NO.sub.x sensor
degradation may be indicated if the feedgas NO.sub.x signal fails
to correspond to an expected feedgas NO.sub.x signal corresponding
to the amount of reductant injected to the exhaust passage. As
described above with reference to FIG. 5, the reductant may be
injected via a single injection of a predefined amount or via an
injection pulse train. At map 700, curve 702 shows a single
injection of reductant while curve 704 shows the corresponding
feedgas NO.sub.x signal. Herein, the feedgas NOx signal corresponds
to an amount less than the injected amount and NO.sub.x sensor
degradation may be indicated. At map 710, curve 706 shows a
reductant injection pulse train, while the curve 708 shows the
corresponding feedgas NO.sub.x signal output responsive to the
reductant injection pulse train. As depicted, the NO.sub.x signal
corresponds to a higher level of reductant than what is injected to
the exhaust passage. Thus, feed gas NO.sub.x sensor degradation may
be indicated.
[0079] Thus, the feedgas NO.sub.x sensor positioned in the exhaust
passage between the oxidation catalyst and the SCR catalyst may be
utilized to indicate reductant injector degradation after engine
shutdown (FIG. 6) or the feedgas NO.sub.x sensor may be diagnosed
after engine shutdown based on reductant injection (FIG. 7).
[0080] In this way, the output of a feedgas exhaust NOx sensor can
be advantageously used during engine-off conditions to estimate an
amount of exhaust reductant present in the exhaust passage. Based
on the estimated exhaust reductant level, each of a reductant
injector and the feedgas exhaust NOx sensor can be diagnosed. By
correlating the detection of elevated reductant levels by the
feedgas exhaust NOx sensor during engine off conditions with
reductant injector leakage, the health of the reductant injection
system can be diagnosed using existing engine components. Likewise,
by correlating variations between the output of the feedgas exhaust
NOx sensor and a known amount of reductant injection, the health
and dynamic characteristics of the exhaust NOx sensor can be
reliably assessed. By using the natural sublimation of ammonia in
an exhaust passage after an engine shutdown to diagnose the exhaust
NO.sub.x sensor and the reductant injector, diagnostics can be
completed using fewer components. Overall, exhaust emissions are
improved.
[0081] Note that the example control and estimation routines
included herein can be used with various engine and/or vehicle
system configurations. The control methods and routines disclosed
herein may be stored as executable instructions in non-transitory
memory. The specific routines described herein may represent one or
more of any number of processing strategies such as event-driven,
interrupt-driven, multi-tasking, multi-threading, and the like. As
such, various actions, operations, and/or functions illustrated may
be performed in the sequence illustrated, in parallel, or in some
cases omitted. Likewise, the order of processing is not necessarily
required to achieve the features and advantages of the example
embodiments described herein, but is provided for ease of
illustration and description. One or more of the illustrated
actions, operations and/or functions may be repeatedly performed
depending on the particular strategy being used. Further, the
described actions, operations and/or functions may graphically
represent code to be programmed into non-transitory memory of the
computer readable storage medium in the engine control system.
[0082] It will be appreciated that the configurations and routines
disclosed herein are exemplary in nature, and that these specific
embodiments are not to be considered in a limiting sense, because
numerous variations are possible. For example, the above technology
can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine
types. The subject matter of the present disclosure includes all
novel and non-obvious combinations and sub-combinations of the
various systems and configurations, and other features, functions,
and/or properties disclosed herein.
[0083] The following claims particularly point out certain
combinations and sub-combinations regarded as novel and
non-obvious. These claims may refer to "an" element or "a first"
element or the equivalent thereof. Such claims should be understood
to include incorporation of one or more such elements, neither
requiring nor excluding two or more such elements. Other
combinations and sub-combinations of the disclosed features,
functions, elements, and/or properties may be claimed through
amendment of the present claims or through presentation of new
claims in this or a related application. Such claims, whether
broader, narrower, equal, or different in scope to the original
claims, also are regarded as included within the subject matter of
the present disclosure.
* * * * *