U.S. patent application number 15/983751 was filed with the patent office on 2019-11-21 for selective catalytic reduction device control.
The applicant listed for this patent is GM Global Technology Operations LLC. Invention is credited to Alberto Bemporad, Daniele Bernardini, Giulio Binetti, Maria Camuglia, Carlos Ildefonso Hoyos Velasco, Giuseppe Mazzara Bologna.
Application Number | 20190353071 15/983751 |
Document ID | / |
Family ID | 68419322 |
Filed Date | 2019-11-21 |
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United States Patent
Application |
20190353071 |
Kind Code |
A1 |
Camuglia; Maria ; et
al. |
November 21, 2019 |
SELECTIVE CATALYTIC REDUCTION DEVICE CONTROL
Abstract
Technical solutions described herein include an emissions
control system for treating exhaust gas in a motor vehicle
including an internal combustion engine. The emissions control
system includes a model-based controller to control reductant
injection into the exhaust gas. Controlling the reductant injection
includes determining an amount of NOx and an amount of NH3 at an
outlet of the first SCR device, and at an outlet of the second SCR
device. The controlling further includes computing an amount of
reductant to inject to maintain a first predetermined ratio between
the amount of NH3 and the amount of NOx at the outlet of the first
SCR device and to maintain a second predetermined ratio between the
amount of NH3 and the amount of NOx at the outlet of the second SCR
device. Further, the controlling includes sending a command for
receipt by the reductant injector to inject the computed amount of
reductant.
Inventors: |
Camuglia; Maria;
(Francavilla di Sicilia (ME), IT) ; Binetti; Giulio;
(Molfetta, IT) ; Hoyos Velasco; Carlos Ildefonso;
(Turin, IT) ; Mazzara Bologna; Giuseppe; (Nicosia
(EN), IT) ; Bemporad; Alberto; (Lucca, IT) ;
Bernardini; Daniele; (Milano, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM Global Technology Operations LLC |
Detroit |
MI |
US |
|
|
Family ID: |
68419322 |
Appl. No.: |
15/983751 |
Filed: |
May 18, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01N 3/208 20130101;
F01N 2900/0406 20130101; F01N 3/2066 20130101; F01N 2900/1821
20130101; F01N 2900/1402 20130101; F01N 2560/12 20130101; F01N
2900/1616 20130101; F01N 2560/026 20130101; F01N 2900/1622
20130101; F01N 2610/02 20130101; F01N 3/105 20130101; F01N 2610/146
20130101; F01N 13/0093 20140601 |
International
Class: |
F01N 3/20 20060101
F01N003/20 |
Claims
1. An emissions control system for treating exhaust gas from an
internal combustion engine in a motor vehicle, the emissions
control system comprising: a reductant injector; a first selective
catalytic reduction (SCR) device; a second SCR device; and a
model-based controller that is configured to control the reductant
injection into the exhaust gas, the controlling of the reductant
injection comprising: determining an amount of NOx and an amount of
NH3 at an outlet of the first SCR device; determining an amount of
NOx and an amount of NH3 at an outlet of the second SCR device;
computing an amount of reductant to inject to maintain a first
predetermined ratio between the amount of NH3 and the amount of NOx
at the outlet of the first SCR device and to maintain a second
predetermined ratio between the amount of NH3 and the amount of NOx
at the outlet of the second SCR device to ensure the optimal
operation of both selective catalytic reduction systems, the first
SCR device and the second SCR device; and sending a command for
receipt by the reductant injector to inject the computed amount of
reductant.
2. The emissions control system of claim 1, wherein determining the
amount of NH3 at the outlet of the first SCR device is based on
computing a first estimated NH3 storage level for the first SCR
device, and is further based on receiving a NOx measurement at an
inlet of the first SCR device.
3. The emissions control system of claim 2, wherein determining the
amount of NH3 at the outlet of the second SCR device is based on
computing a second estimated NH3 storage level for the second SCR
device, and the amount of NH3 at the outlet of the first SCR
device.
4. The emissions control system of claim 3, wherein determining the
amount of NH3 at the outlet of the second SCR device is further
based on receiving a NOx measurement at the outlet of the first SCR
device of the first SCR device.
5. The emissions control system of claim 1, wherein the first SCR
device is a SCR filter.
6. The emissions control system of claim 5, wherein the second SCR
device is an underfloor SCR device.
7. The emissions control system of claim 1, wherein computing the
amount of reductant comprises estimating the amount of NH3 and the
amount of NOx at the outlet of the second SCR device based on a
state observer that includes a combination of physical models for
the first SCR device and the second SCR device.
8. An exhaust system for treating exhaust gas emitted by an
internal combustion engine, configured to perform a selective
catalytic reduction (SCR) of exhaust gas, the exhaust system
comprising: at least a first SCR device; a controller configured to
control injection of a reductant into the exhaust gas, the
controlling of the reductant injection comprising: determining if
the exhaust system includes a second SCR device; in response to the
exhaust system including the first SCR device only, computing an
amount of reductant to inject based on a first model of the first
SCR device, the first model estimating a first NH3 storage level at
the first SCR device; in response to the exhaust system including
the second SCR device, computing the optimal amount of reductant to
inject based on a combination of the first model of the first SCR
device and a second model of the second SCR device, the combination
estimating the first NH3 storage level at the first SCR device and
a second NH3 storage level at the second SCR device; and sending a
command to a reductant injector to inject the amount of
reductant.
9. The exhaust system of claim 8, wherein the first model uses a
first NOx measurement from an inlet of the first SCR device and a
second NOx measurement from an outlet of the first SCR device.
10. The exhaust system of claim 9, wherein the second model uses
the second NOx measurement from the outlet of the first SCR device
and a third NOx measurement from an outlet of the second SCR
device.
11. The exhaust system of claim 9, wherein the first model uses a
first NH3 estimation from the outlet of the first SCR device and
the amount of reductant injected.
12. The exhaust system of claim 11, wherein the second model uses
the first NH3 estimation from an outlet of the first SCR device and
a second NH3 estimation from an outlet of the second SCR
device.
13. The exhaust system of claim 12, wherein, in response to the
exhaust system including the second SCR device, computing the
optimal amount of reductant comprises maintaining a first
predetermined trade-off between an amount of NH3 and an amount of
NOx at the outlet of the first SCR device and to maintain a second
predetermined trade-off between an amount of NH3 and an amount of
NOx at the outlet of the second SCR device.
14. The exhaust system of claim 12, wherein the second SCR device
is an underfloor SCR device.
15. A computer-implemented method for controlling reductant
injection into an emissions control system that comprises a first
selective catalytic reduction (SCR) device, the method comprising:
determining if the emissions control system includes a second SCR
device; in response to the emissions control system including the
first SCR device only, computing an optimal amount of reductant to
inject based on a first model of the first SCR device, the first
model estimating a first NH3 storage level at the first SCR device;
in response to the emissions control system including the second
SCR device, computing the optimal amount of reductant to inject
based on a combination of the first model of the first SCR device
and a second model of the second SCR device, the combination
estimating the first NH3 storage level at the first SCR device and
a second NH3 storage level at the second SCR device; and sending a
command to a reductant injector to inject the amount of
reductant.
16. The method of claim 15, wherein the first model uses a first
NOx measurement from an inlet of the first SCR device and a second
NOx measurement from an outlet of the first SCR device.
17. The method of claim 16, wherein the second model uses the
second NOx measurement from the outlet of the first SCR device and
a third NOx measurement from an outlet of the second SCR
device.
18. The method of claim 15, wherein the first model uses a first
NH3 estimation from an outlet of the first SCR device and the
amount of reductant injected.
19. The method of claim 15, wherein, in response to the emissions
control system including the second SCR device, computing the
optimal amount of reductant comprises maintaining a first
predetermined ratio between an amount of NH3 and an amount of NOx
at a first outlet of the first SCR device and to maintain a second
predetermined ratio between an amount of NH3 and an amount of NOx
at an outlet of the second SCR device.
20. The method of claim 15, wherein the second SCR device is an
underfloor SCR device.
Description
INTRODUCTION
[0001] The present disclosure relates to exhaust systems for
internal combustion engines, and more particularly to exhaust
systems using selective catalytic reduction (SCR) units for
emissions control.
[0002] Exhaust gas emitted from an internal combustion engine,
particularly a diesel engine, is a heterogeneous mixture that
contains gaseous emissions such as carbon monoxide ("CO"), unburned
hydrocarbons ("HC") and oxides of nitrogen ("NO.sub.x") as well as
condensed phase materials (liquids and solids) that constitute
particulate matter ("PM"). Catalyst compositions, typically
disposed on catalyst supports or substrates, are provided in an
engine exhaust system as part of an aftertreatment system to
convert certain, or all of these exhaust constituents into
non-regulated exhaust gas components.
[0003] Exhaust gas treatment systems typically include selective
catalytic reduction (SCR) devices. An SCR device includes a
substrate having an SCR catalyst disposed thereon to reduce the
amount of NOx in the exhaust gas. The typical exhaust treatment
system also includes a reductant delivery system that injects a
reductant such as, for example, ammonia (NH3), urea
((NH.sub.2).sub.2 CO, etc.). The SCR device makes use of NH.sub.3
to reduce the NOx. For example, when the proper amount of NH3 is
injected to the SCR device under the proper thermal conditions, the
NH3 reacts with the NOx in the presence of the SCR catalyst to
reduce the NOx emissions. If the NH3 injection rate is too high,
then there is excess of ammonia in the exhaust and, ammonia (NH3)
can slip from the SCR. On the other hand, if there is too little
ammonia in the exhaust, SCR NOx conversion efficiency will be
decreased.
SUMMARY
[0004] According to one or more embodiments an emissions control
system for treating exhaust gas from an internal combustion engine
in a motor vehicle, includes a reductant injector. The emissions
control system also includes a first selective catalytic reduction
(SCR) device. The emissions control system also includes a second
SCR device. The emissions control system further includes a
controller to control the reductant injection into the exhaust gas.
The controlling of the reductant injection includes determining an
amount of NOx and an amount of NH3 at an outlet of the first SCR
device. The controlling of the reductant injection further includes
determining an amount of NOx and an amount of NH3 at an outlet of
the second SCR device. The controlling of the reductant injection
further includes computing an amount of reductant to inject to
maintain a first predetermined ratio between the amount of NH3 and
the amount of NOx at the outlet of the first SCR device and to
maintain a second predetermined ratio between the amount of NH3 and
the amount of NOx at the outlet of the second SCR device to ensure
the optimal operation of both selective catalytic reduction
systems, the first SCR device and the second SCR device. The
controlling of the reductant injection further includes sending a
command for receipt by the reductant injector to inject the
computed amount of reductant.
[0005] In one or more examples, determining the amount of NH3 at
the outlet of the first CR device is based on computing a first
estimated NH3 storage level for the first SCR device, and is
further based on receiving a NOx measurement at an inlet of the
first SCR device. In one or more examples, determining the amount
of NH3 at the outlet of the second SCR device is based on computing
a second estimated NH3 storage level for the second SCR device, and
the amount of NH3 at the outlet of the first SCR device.
Determining the amount of NH3 at the outlet of the second SCR
device is further based on receiving a NOx measurement at the
outlet of the first SCR device. In one or more examples, the first
SCR device is a SCR filter. In one or more examples, the second SCR
device is an underfloor SCR device. In one or more examples,
computing the amount of reductant includes estimating the amount of
NH3 and the amount of NOx at the outlet of the second SCR device
based on an operating model that includes a combination of the
first SCR device and the second SCR device.
[0006] According to one or more embodiments, an exhaust system for
treating exhaust gas emitted by an internal combustion engine,
performs a selective catalytic reduction (SCR) of exhaust gas. The
exhaust system includes a first SCR device, and a controller to
control reductant injection into the exhaust gas. The controlling
of the reductant injection includes determining if the exhaust
system includes a second SCR device. In response to the exhaust
system including the first SCR device only, the controlling of the
reductant injection includes computing an amount of reductant to
inject based on a first model of the first SCR device, the first
model estimating a first NH3 storage level at the first SCR device.
In response to the exhaust system including the second SCR device,
the controlling of the reductant injection includes computing the
optimal amount of reductant to inject based on a combination of the
first model of the first SCR device and a second model of the
second SCR device, the combination estimating the first NH3 storage
level at the first SCR device and a second NH3 storage level at the
second SCR device. The controlling of the reductant injection
further includes sending a command to a reductant injector to
inject the amount of reductant.
[0007] In one or more examples, the first model uses a first NOx
measurement from an inlet of the first SCR device and a second NOx
measurement from an outlet of the first SCR device. Further, the
second model uses the second NOx measurement from the outlet of the
first SCR device and a third NOx measurement from an outlet of the
second SCR device. Further yet, in one or more examples, the first
model uses a first NH3 estimation from the outlet of the first SCR
device and the amount of reductant injected. In one or more
examples, the second model uses the first NH3 estimation from an
outlet of the first SCR device and a second NH3 estimation from an
outlet of the second SCR device. In response to the exhaust system
including the second SCR device, computing the optimal amount of
reductant includes maintaining a first predetermined trade-off
between an amount of NH3 and an amount of NOx at the outlet of the
first SCR device and to maintain a second predetermined trade-off
between an amount of NH3 and an amount of NOx at the outlet of the
second SCR device.
[0008] According to one or more embodiments a computer-implemented
method for controlling reductant injection into an emissions
control system that includes a first selective catalytic reduction
(SCR) device includes determining if the emissions control system
includes a second SCR device. In response to the exhaust system
including the first SCR device only, the controlling of the
reductant injection includes computing an amount of reductant to
inject based on a first model of the first SCR device, the first
model estimating a first NH3 storage level at the first SCR device.
In response to the exhaust system including the second SCR device,
the controlling of the reductant injection includes computing the
optimal amount of reductant to inject based on a combination of the
first model of the first SCR device and a second model of the
second SCR device, the combination estimating the first NH3 storage
level at the first SCR device and a second NH3 storage level at the
second SCR device. The controlling of the reductant injection
further includes sending a command to a reductant injector to
inject the amount of reductant.
[0009] In one or more examples, the first model uses a first NOx
measurement from an inlet of the first SCR device and a second NOx
measurement from an outlet of the first SCR device. Further, the
second model uses the second NOx measurement from the outlet of the
first SCR device and a third NOx measurement from an outlet of the
second SCR device. Further yet, in one or more examples, the first
model uses a first NH3 estimation from the outlet of the first SCR
device and the amount of reductant injected. In one or more
examples, the second model uses the first NH3 estimation from an
outlet of the first SCR device and a second NH3 estimation from an
outlet of the second SCR device. In response to the exhaust system
including the second SCR device, computing the optimal amount of
reductant includes maintaining a first predetermined trade-off
between an amount of NH3 and an amount of NOx at the outlet of the
first SCR device and to maintain a second predetermined trade-off
between an amount of NH3 and an amount of NOx at the outlet of the
second SCR device.
[0010] The above features and advantages, and other features and
advantages of the disclosure are readily apparent from the
following detailed description when taken in connection with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Other features, advantages and details appear, by way of
example only, in the following detailed description, the detailed
description referring to the drawings in which:
[0012] FIG. 1 is a generalized illustration of an engine and an
associated exhaust aftertreatment system that is configured to
treat the exhaust flow produced by the engine;
[0013] FIG. 2 depicts a block diagram of the reductant injection
control system according to one or more embodiments; and
[0014] FIG. 3 depicts a flowchart of an example method for
determining an amount of reductant to inject into the exhaust
system according to one or more embodiments.
DETAILED DESCRIPTION
[0015] The following description is merely exemplary in nature and
is not intended to limit the present disclosure, its application or
uses. It should be understood that throughout the drawings,
corresponding reference numerals indicate like or corresponding
parts and features. As used herein, the term module refers to
processing circuitry that may include an application specific
integrated circuit (ASIC), an electronic circuit, a processor
(shared, dedicated, or group) and memory module that executes one
or more software or firmware programs, a combinational logic
circuit, and/or other suitable components that provide the
described functionality.
[0016] In general, referring to the configuration shown in FIG. 1,
a schematic diagram depicts an embodiment of an internal combustion
engine 12, a control system 84, and an exhaust gas treatment system
10, in accordance with the one or more embodiments. In the
description herein, the engine 12 is described as a diesel engine,
however, the engine 12 may be a gasoline engine in one or more
examples. The exemplary diesel engine 12 and control system 84
comprises a four-cycle internal combustion diesel engine 12 and
electronic engine control module (ECM) 238 that may be configured
to accomplish the emission control of exhaust gas flow 16 at
tailpipe 19, in accordance with control methods and strategies
described herein. The engine may include a known
compression-ignition engine having an operating regime that is
primarily lean of stoichiometry. Alternatively, diesel engine 12
may include an engine configured to employ any one of a number of
engine configurations and associated engine control strategies, and
which also includes those having an operational regime (or regimes)
that is lean of stoichiometry, e.g., homogeneous-charge
compression-ignition engines.
[0017] Diesel engine 12 may be any diesel engine configuration or
application, including various vehicular applications (e.g.,
automotive, marine and the like), as well as various non-vehicular
applications (e.g., pumps, generators and the like). During
operation, diesel engine 12 generates an exhaust gas feedstream or
flow represented by arrows 16 containing regulated and unregulated
emission constituents, generally including constituent gases and
particulate matter. Exhaust gas treatment system 10 acts to convert
regulated constituents, such as, for example, various hydrocarbons
(HC), carbon monoxide (CO), nitrides of oxygen (NOx) and
particulate matter (PM), to unregulated constituents, such as, for
example, nitrogen (N2) and water (H2O).
[0018] The exhaust gas treatment system 10 contains piping, joints,
and other suitable flow passage and connection features that,
together, define a contained passage configured to receive the
exhaust flow 16 from the engine 12 and discharge a treated exhaust
flow 16 from a tailpipe 19. The exhaust gas treatment system 10
includes, as shown, a selective catalytic reduction device (SCR) 24
and an under-floor ammonia-SCR device (uSCR) 25. The exhaust gas
treatment system 10 may further include a Diesel Oxidation Catalyst
device (DOC) 22. Downstream from the DOC 22, the two SCR devices
are connected in series (serial positioning)--the SCR 24 and the
uSCR 25 respectively.
[0019] The SCR 24 and the uSCR 25 operate cooperatively to decrease
NOx emissions, present in the exhaust gas 16 at engine out, to
acceptable concentration levels. In general terms, the gaseous
emissions originally contained in the exhaust gas 16, are treated
to limit the quantity of regulated constituents delivered to the
atmosphere. A urea injector 236 is positioned upstream of the SCR
24 to inject an amount of urea solution (e.g., AdBlue, DEF) into
the exhaust flow 16.
[0020] The diesel engine 12 is fluidly coupled to an outlet
manifold 15 that collects the combustion exhaust products
discharged from each cylinder in the engine 12 and consolidates
them into the exhaust flow 16 that is delivered to the exhaust gas
treatment system 10.
[0021] The DOC 22 is mounted to the exhaust manifold with an inlet
that fluidly communicates directly with the exhaust manifold to
receive the exhaust flow 16. The exhaust flow 16 exits the DOC 22
and flows downstream towards the SCR 24, for a first selective
catalytic reduction and subsequently to the uSCR 25 for a second
selective catalytic reduction.
[0022] The DOC 22 may include a combination of platinum (Pt),
palladium (Pd), and rhodium (Rh) dispersed as fine particles on a
high-surface area base metal oxide such as .gamma.-alumina
(.gamma.-Al.sub.2O.sub.3) or a cerium/zirconium oxide
(CeO.sub.2--ZrO.sub.2). In one or more examples, the base metal
oxide is also present in the SCR 24 anywhere from about 70 g/L to
about 150 g/L of available flow volume over the SCR 24. In further
examples, the Pt/Pd/Rh loading on the base metal oxide ranges from
about 1 to about 7 g/L of available flow volume over the SCR
24.
[0023] In one or more examples, the SCR 24 includes one or more
components that utilize a reductant 246 and a catalyst to transform
NO and NO.sub.2 in the exhaust gas 16.
[0024] The SCR catalyst composition for the SCR 24 and the uSCR 25
is generally a porous and high surface area material, which can
operate efficiently to convert NO.sub.x constituents in the exhaust
gas 16 in the presence of a reductant 246, such as ammonia. For
example, the catalyst composition can contain a zeolite impregnated
with one or more base metal components such as iron (Fe), cobalt
(Co), copper (Cu), vanadium (V), sodium (Na), barium (Ba), titanium
(Ti), tungsten (W), and combinations thereof. In a particular
embodiment, the catalyst composition can contain a zeolite
impregnated with one or more of copper, iron, or vanadium. In some
embodiments the zeolite can be a .beta.-type zeolite, a Y-type
zeolite, a ZM5 zeolite, or any other crystalline zeolite structure
such as a Chabazite or a USY (ultra-stable Y-type) zeolite. In a
particular embodiment, the zeolite comprises Chabazite. In a
particular embodiment, the zeolite comprises SSZ. Suitable SCR
catalyst compositions can have high thermal structural stability,
particularly when used in tandem with known particulate filter (PF)
devices or when incorporated into SCR devices, which are
regenerated via high temperature exhaust soot burning
techniques.
[0025] The SCR catalyst composition for the SCR 24 and the uSCR 25
can optionally further include one or more base metal oxides as
promoters to further decrease the SO.sub.3 formation and to extend
catalyst life. The one or more base metal oxides can include
WO.sub.3, Al.sub.2O.sub.3, and MoO.sub.3, in some embodiments. In
one embodiment, WO.sub.3, Al.sub.2O.sub.3, and MoO.sub.3 can be
used in combination with V.sub.2O.sub.5.
[0026] The uSCR 25 is positioned downstream from the SCR 24 in the
under-floor position. In one or more examples, the distance between
the SCR 24 and the uSCR 25 ranges from about 3 ft. to about 10 ft.
The inlet of the uSCR 25 fluidly communicates with the outlet of
the SCR 24 to receive the exhaust flow 16. The outlet of the uSCR
25 communicates the exhaust flow 16 downstream towards the tailpipe
opening 19 that emits the exhaust flow to atmosphere.
[0027] The uSCR 25 may include fine particles of (1) a base metal
ion-substituted zeolite and/or a base metal ion-substituted
silicoaluminophosphate and (2) an oxygen storage material. Zeolites
and silicoaluminophosphates are open-framework, microporous, and
ammonia absorbent polymorphic molecular sieve materials that are
preferably ion-substituted with Cu or Fe. The base metal
ion-substituted particles are present in the uSCR 25, in total,
anywhere from about 120 g/L to about 180 g/L of available flow
volume over the uSCR 25, in one or more examples. The oxygen
storage material is a metal oxide or a mixed metal oxide that
exhibits oxygen storage and release capacity. In one or more
examples, the oxygen storage material is present in the uSCR
catalyst 25 anywhere from about 5 g/L to about 50 g/L of available
flow volume over the uSCR 25. Any suitable distribution of the
particulate materials may be employed. The fine particles of the
base metal ion-substituted zeolite/silicoaluminophosphate and the
oxygen storage material may, for example, be uniformly mixed within
a single washcoat layer or, alternatively, relegated to separate
and discrete contacting washcoat layers or zones. The oxygen
storage material may also be concentrated near the inlet or the
outlet of the uSCR 25 or in some other non-uniform
distribution.
[0028] The base metal ion-substituted zeolites that may be used to
prepare the uSCR 25 include a Cu or Fe substituted .beta.-type
zeolite, Y-type zeolite, ZSM-5 zeolite, Chabazite zeolite, or USY
(ultra-stable Y-type) zeolite. Further, the base metal
ion-substituted silicoaluminophosphates (SAPO) that may be used to
prepare the uSCR 25 include a Cu or Fe substituted SAPO-5, SAPO-34,
or SAPO-44. Some specific metal oxides or mixed metal oxides that
may be included in the uSCR 25 as the oxygen storage material are
cerium-containing and praseodymium-containing metal oxides or mixed
metal oxides such as CeO.sub.2, Pr.sub.6O.sub.11,
CeO.sub.2--ZrO.sub.2, CuO--CeO.sub.2, FeO.sub.x--CeO.sub.2
(1.0.ltoreq.X.ltoreq.1.5), MnO.sub.x--CeO.sub.2
(1.0.ltoreq.X.ltoreq.3.5), and Pr.sub.6O.sub.11--CeO.sub.2. Each of
these materials, without being bound by theory, are believed to
have crystal lattice structures that can accommodate
non-stoichiometric unit cell quantities of oxygen (both higher and
lower) without decomposing. This property equates to an ability to
reversibly store and release oxygen in response to the partial
pressure of oxygen in the exhaust flow 16 and/or equilibrium shifts
that accompany the localized consumption of oxygen during NOx
reduction.
[0029] When the diesel engine 12 is operating, the exhaust gas
treatment system 10 removes the various regulated emissions from
the exhaust flow 16 while limiting the amount of ammonia that slips
into the exhaust flow 16. The exhaust flow 16 passes, first,
through the close-coupled SCR 24 and, second, through the
under-floor uSCR 25. The combined catalytic activity of the SCR 24
and the uSCR 25 are able to continuously treat the exhaust flow 16
across a robust variety of engine operating conditions. The initial
NOx reduction process takes place at SCR 24 where the NOx exiting
DOC 22 reacts with NH3 stored in the SCR 24. Any NOx that escapes
past the SCR 24 is reduced at the uSCR 25 with the NH3 stored in
the uSCR 25 further reducing levels of NOx concentration in the
treated exhaust flow 16. The NH3 stored in SCR 24 and uSCR 25 comes
from the urea injector 236, while the NH3 stored by uSCR 25 comes
from SCRF 24, when the NH3 is captured by the uSCR 25. The exhaust
gas treatment system 10 further includes a reductant injector
system 84 that controls an amount of reductant injected directly
into the SCR 24 and indirectly into the uSCR 25.
[0030] The air/fuel mixture supplied to the engine 12 is constantly
adjusted by an electronic fuel injection system (not shown) to
achieve a predetermined air to fuel mass ratio, for instance air to
fuel ratio may range from 15 to 50, or 15 to 80 on other diesel
engine applications. The combustion of the air/fuel mixture in the
cylinders of the engine 12 provides the exhaust flow 16 with a
relatively large amount of nitrogen (e.g. >70 vol. %.), a small
amount of oxygen, and unwanted gaseous emissions comprised of
carbon monoxide, HC's, and NOx. The amount of oxygen present is
generally less than about 2.0 vol. %. The amount of carbon
monoxide, HC's and NOx present is typically about 0.8 vol. % or
less, about 800 ppm or less, and about 1500 ppm or less,
respectively. The NO.sub.x constituency of the exhaust flow 16
generally includes a large molar proportion of NO (greater than 90
mol %). It should be noted that above values are examples and that
in one or more embodiments, the values may be different than those
listed above. It is understood that the above values are exemplary
and that in one or more examples, the engine 12 can operate with
the above measurements being different than those described
herein.
[0031] The instantaneous air to fuel mass ratio of the air/fuel
mixture, however, may oscillate between 15 to 80 according to the
engine calibrations and operating conditions. These oscillations
cause the chemical composition of the exhaust flow 16 to vary
within particular limits.
[0032] The SCR 24 receives the exhaust flow 16 mixed with the NH3
246 injected by the urea injector 236, and stores the NH3. The
NO.sub.x gas present in the exhaust gas 16 reacts with the stored
NH3. In so doing, the SCR 24 reduces the NO.sub.x contained in the
exhaust gas 16 to N.sub.2 and H.sub.2O. In some operative
conditions the SCR 24 may slip NH3. This feeds the NH3 to the uSCR
25, to drive a supplemental catalytic NO.sub.x reduction reaction
when NO.sub.x escapes from the first SCR 24. The NH3 stored in SCR
24 and uSCR 25 comes from the urea injector 236; the NH3 stored by
uSCR 25 coming from SCRF outlet, when the NH3 slips or escapes from
SCR 24.
[0033] The uSCR 25 receives the exhaust flow 16 from the SCR 24.
The uSCR 25 contributes to a further reduction of the NO.sub.x in
the exhaust flow 16 by continuously storing the NH3 ammonia slipped
from the SCR 24 and making it react with the NO.sub.x downstream of
the SCR 24. The interaction of the two reduction systems SCR 24 and
uSCR 25 leads to a substantial reduction of NOx emissions if a
suitable amount of reductant 246 (urea NH3) has been injected by
the urea injector 236. Any number of events may slightly diminish
the NO.sub.x conversion efficiency of the SCR 24 and permit
NO.sub.x to reach the uSCR 25 by way of the exhaust flow 16. The
NO.sub.x that passes through (i.e. slips) the SCR 24 is reduced by
the ammonia stored at the uSCR 25. The ability of the uSCR 25 to
accommodate variances in the chemical composition of the exhaust
flow 16 and out-of-phase concentration spikes in NO.sub.x and
ammonia helps limit the escape of these two substances to
atmosphere.
[0034] The oxygen storage material included in the uSCR 25 provides
a reserve oxygen supply that enhances the catalytic reduction
reaction between ammonia and NO.sub.x. The oxygen storage material
absorbs oxygen from the low-oxygen content exhaust flow 16 when
NO.sub.x is not present. The accumulated oxygen is then extracted
to supplement the sparingly available oxygen in the exhaust flow
16. This influx of reserve oxygen achieves NO.sub.x reduction
efficiency gains in several ways. First, the extra oxygen improves
the overall reaction kinetics of the NO.sub.x reduction reactions
(both NO and NO.sub.2) since oxygen scarcity can have a
rate-limiting effect. Second, the extra oxygen promotes the
oxidation of NO to NO.sub.2. This oxidation reaction decreases the
NO to NO.sub.2 molar ratio of the NO.sub.x in the uSCR 25. Such an
adjustment is desirable since the overall reduction of NO.sub.x
generally proceeds more efficiently when the NO/NO.sub.2 molar
ratio is decreased from that originally produced by the engine 12
to, preferably, about 1.0 (equimolar).
[0035] FIG. 2 depicts a block diagram of the reductant injection
control system 84 according to one or more embodiments. It should
be noted that FIG. 2 depicts a simplified view of the exhaust
system and does not depict one or more components, such as the DOC
22. It should be further noted that in one or more embodiments the
reductant injection control system 84 may include additional
components than those depicted, and that the depicted block diagram
is to describe the technical solutions herein. The SCR 24 and the
uSCR 25 receive a reductant 246, such as at variable dosing rates.
Reductant 246 can be supplied from a reductant supply source 234.
In one or more examples, the reductant 246 is injected into exhaust
gas conduit 14 at a location upstream of the SCR 24 using a urea
injector 236. The reductant 246 can be in the form of a gas, a
liquid, or an aqueous solution, such as an aqueous urea solution.
In one or more examples, the reductant 246 can be mixed with air in
the injector 236 to aid in the dispersion of the injected spray.
The SCR 24 and the uSCR 25 utilize the reductant 246 to reduce the
NOx in exhaust 16.
[0036] The reductant injection control system 84 further includes
the control module 238 operably connected, via a number of sensors,
to monitor the engine 12, FIG. 1, and the SCR devices 24 and 25. As
used herein, the term module refers to an application specific
integrated circuit (ASIC), an electronic circuit, a processor
(shared, dedicated, or group) and memory that executes one or more
software or firmware programs, a combinational logic circuit,
and/or other suitable components that provide the described
functionality. For example, the control module 238 can execute a
SCR chemical model, as described below. The control module 238 can
be operably connected to the engine 12, the SCR 24, the urea
injector 236, the uSCR 25, and/or one or more sensors.
[0037] The sensors can include a first NO.sub.x sensor 242, a
second NO.sub.x sensor 243, and a third NOx sensor 244, each of
which are in fluid communication with the exhaust gas conduit 14.
The NO.sub.x sensors 242, 243, 244 detect a NO.sub.x level
proximate their location within exhaust gas conduit, and generate a
NOx signal, which corresponds to the NOx level. A NOx level can
comprise a concentration, a mass flow rate, or a volumetric flow
rate, in some embodiments. A NOx signal generated by a NOx sensor
can be interpreted by the control module 238, for example. The
control module 238 can additionally be in communication with one or
more temperature sensors, such as temperature sensor 32, FIG. 1. In
one or more examples, the first NOx sensor 242 may be disposed
downstream of the engine 12, at DOC inlet or at DOC outlet, to
measure the NOx concentration upstream of SCR 24 so as to detect
NOx level at the inlet of the SCR 24. In the last case, since a NOx
sensor is cross-sensitive to ammonia NH3, the NOx sensor 242 is
disposed before the urea injector 236; the second NOx sensor 243 is
disposed downstream of the SCR 24 and upstream of the uSCR 25 to
detect NOx level at the inlet of the uSCRF 25 (or the outlet of the
SCR 24); and the third NOx sensor 244 is disposed downstream of the
uSCR 25 to detect NOx level at the outlet of the uSCR 25. In one or
more examples, the first NOx sensor 242 is located upstream of the
DOC 22, the second NOx sensor 243 is at the outlet of SCR 24, and
the third NOx sensor 244 is at the outlet of uSCR 25. It should be
noted that the positions of the sensors depicted in FIG. 2 are
illustrative, and that in one or more embodiments, the sensors may
be in different positions than those depicted. Further, in one or
more embodiments, a different number of sensors may be used than
those depicted herein.
[0038] The reductant 246 can be any compound capable of decomposing
or reacting in the presence of exhaust gas 16 and/or heat to form
ammonia. When the urea is injected into the hot exhaust gas 16, the
water evaporates and the urea thermally decomposes into NH.sub.3
and CO.sub.2. The NH.sub.3 molecules then are stored in SCR 24 or
uSCR 25 components to perform the NOx reduction.
[0039] Equations (1)-(5) provide exemplary chemical reactions for
NO.sub.x reduction involving ammonia.
6NO+4NH.sub.3.fwdarw.5N.sub.2+6H.sub.2O (1)
4NO+4NH.sub.3+O.sub.2.fwdarw.4N.sub.2+6H.sub.2O (2)
6NO.sub.2+8NH.sub.3.fwdarw.7N.sub.2+12H.sub.2O (3)
2NO.sub.2+4NH.sub.3+O.sub.2.fwdarw.3N.sub.2+6H.sub.2O (4)
NO+NO.sub.2+2NH.sub.3.fwdarw.2N.sub.2+3H.sub.2O (5)
[0040] It should be appreciated that Equations (1)-(5) are merely
illustrative, and are not meant to confine the SCR 24 and the uSCR
25 to a particular NOx reduction mechanism or mechanisms, nor
preclude the operation of other mechanisms. The SCR 24 and the uSCR
25 can be configured to perform any one of the above NOx reduction
reactions, combinations of the above NOx reduction reactions, and
other NO.sub.x reduction reactions.
[0041] The reductant 246 can be diluted with water in various
implementations. In implementations where the reductant 246 is
diluted with water, heat (e.g., from the exhaust) evaporates the
water, and ammonia is supplied to the SCR 24 and the uSCR 25.
Non-ammonia reductants can be used as a full or partial alternative
to ammonia as desired. Reaction (6) below provides an exemplary
general chemical reaction of ammonia production via evaporation and
urea decomposition.
(NH.sub.2).sub.2CO+H.sub.2O.fwdarw.2NH.sub.3+CO.sub.2 (6)
[0042] It should be appreciated that Equation (6) is merely
illustrative, and is not meant to confine the urea or other
reductant 246 decomposition to a particular single mechanism, nor
preclude the operation of other mechanisms.
[0043] Modeling and optimizing the operation of the two components,
SCR 24 and uSCR 25, is a technical challenge addressed by the
technical solutions described herein. In addition, the technical
solutions described herein facilitate controlling the operation of
both, the SCR 24 and the uSCR 25, and the resulting NOx from the
exhaust gas treatment system 10, with only one reductant (urea)
injector 236 located upstream of SCR 24. The technical solutions
accordingly facilitate a systematic and modular control approach to
manage, in a flexible way, both the SCR 24, and the uSCR 25
architectures with a single controller module 238. The technical
solutions described herein facilitate such flexibility by using a
Model Predictive Control (MPC) approach that considers the uSCR 25
and optimizes overall performance of the exhaust gas treatment
system 10, while acting only on the single unique urea injector
236.
[0044] The controller module 238 extends the MPC beyond just the
SCR 24 to optimally determine the urea injection (constrained by
the presence of only one single urea injector), while optimizing
the trade-off between NOx and NH3 chemical species both at the SCR
24 and uSCR 25 outlets.
[0045] FIG. 3 depicts a flowchart of an example method for
determining an amount of reductant 246 to inject into the exhaust
gas treatment system 10 according to one or more embodiments. The
method 300 is implemented by the controller module 238. In one or
more examples, the controller module 238 executes one or more
computer executable instructions that are stored on a computer
readable storage device to implement the method. Alternatively, or
in addition, the implementation includes the controller module 238
operating according to one or more application specific integrated
circuits or field programmable gate array configurations.
[0046] The method 300 includes determining if the uSCR 25 is
present and is to be supplied reductant by the reductant injection,
at 305. The determination may be made based on a predetermined flag
that is indicative if the uSCR 25 is included in the exhaust gas
treatment system 10. If the uSCR 25 is not to be controlled, the
method 300 includes reading a first set of input signals, at 310.
The first set of input signals includes, measurement signals from
the first NOx sensor 242 and the second NOx sensor 243. The first
set of input signals can further include temperature measurement of
the SCR 24, and a gas mass flow rate, denoted by F, in the SCR
24.
[0047] The method further includes estimating an NH3 and NOx output
of the SCR 24, at 320. The estimation includes using an SCR state
observer model of the operation of the SCR 24. The SCR state
observer may include a model based prediction and correction stage.
The estimation includes computing an estimated NH3 storage level of
the SCR 24, and further computing an estimate of NOx and an
estimate of NH3 at the outlet of the SCR 24. The SCR state
estimation uses an SCR physics model given as follows:
{ x ( k + 1 ) = x ( k ) + T s ( 1 M NH 3 ( u ( k ) - y 2 ( k ) ) -
1 M NOx ( C Nox , in ( k ) - y 1 ( k ) ) - a 1 ( k ) x ( k ) ) y 1
( k ) = F ( k ) C NOx , in ( k ) F ( k ) + a 2 ( k ) x ( k ) y 2 (
k ) = F ( k ) ( u ( k ) + a 4 ( k ) x ( k ) ) F ( k ) + a 5 - a 3 x
( k ) ( 7 ) ##EQU00001##
[0048] Here, x(k) is an estimated NH3 storage level at the SCR 24
at time interval k, T.sub.s is the sampling or scheduling time at
which the aftertreatment control module is iterated in control
module 238, u(k) is the amount of reductant injected, y.sub.1 is
the concentration of NO.sub.x at the second NO.sub.x sensor 243,
and y.sub.2 is the concentration of NH.sub.3 at the outlet of the
SCR 24. Further, the estimation uses the concentration of NO.sub.x
(C.sub.NOx,in) at the SCR inlet from the first NOx sensors 242.
F(k) represents the exhaust flow measurement (it may be an
estimate) in the SCR 24 at the k-th time instant. Further, the
estimation uses multiple pre-calibrated temperature dependent
reaction functions a.sub.1-a.sub.5. In the above equation M.sub.NOx
and M.sub.NH3 represent the molar mass of NOx and the molar mass of
NH3, respectively.
[0049] Referring to the flowchart of FIG. 3 again, the method 300
includes optimizing the amount of reductant (u(k)) that is injected
by the reductant injector 236, into the exhaust gas 16, at 330. The
controller computes the amount of reductant u(k) to be injected
into the exhaust gas, to optimally operate the reduction systems
SCR 24 in order to maintain the NOx and NH3 emissions at the outlet
of the SCR 24 as low as possible:
u ( k ) = f ( NH 3 in - k , .DELTA. NH 3 in - k , NH 3 k , NOx k ,
wi u ) ( 8 ) ##EQU00002##
[0050] Here, wi terms represent weight calibrations, with w.sub.u
being a weight calibration for the amount of urea to be injected
NH.sub.3in, w.sub.du being a weight calibration for ensuring a low
variation of the injection pattern of reductant. Further,
w.sub.NOx,SCRF and W.sub.NH3,SCRF are weight calibrations based on
a tradeoff between NOx and NH.sub.3 at the outlet of the SCR 24.
The control module 238 determines the optimal amount of reductant
246 to be injected so as to minimize a cost function that expresses
the system performance, for example a combination of NOx and NH3
concentrations at SCR 24 outlets, NOx reduction efficiency, urea
injection efforts, among other factors, at 340 and 350.
[0051] Referring to the flowchart of FIG. 3 again if it is
determined that the uSCR 25 is to be controlled (i.e. the uSCR 25
is part of the exhaust gas treatment system 10; at 305) the method
300 includes receiving a second input set, at 360; else, only the
SCRF 24 is used, at 340. The second input set includes the NOx
sensor readings from the first NOx sensor 242, the second NOx
sensor 243, and the third NOx sensor 244, the third reading being
from the uSCR outlet. The second input set further includes the SCR
24 temperature and an uSCR 25 temperature measured by the one or
more temperature sensors at the respective devices. Further, the
second input set includes the gas mass flow rates through the SCR
24 and through the uSCR 25.
[0052] Further, the method includes a model based observer to
estimate one or more values for the SCR 24 and the uSCR 25, at 370.
The estimation includes computing an NH3 storage level at the SCR
24 and at the uSCR 25. Further, the estimation includes computing
NOx and NH3 at the SCR outlet and at the uSCR outlet. The
estimations can be based on the following physic-based equation for
the uSCR:
x uSCR ( k + 1 ) = x uSCR ( k ) + T s ( 1 M NH 3 ( C NH 3 , in ,
uSCR ( k ) - y 2 , uSCR ( k ) ) - 1 M NOx ( C NOx , in , uSCR ( k )
- y 1 , uSCR ( k ) ) - a 1 , uSCR ( k ) x uSCR ( k ) ) ( 9 ) y 1 ,
uSCR ( k ) = F uSCR ( k ) C NOx , in , uSCR ( k ) F uSCR ( k ) + a
2 , uSCR x uSCR ( k ) ( 10 ) y 2 , uSCR ( k ) = F uSCR ( k ) ( C NH
3 , in , uSCR ( k ) + a 4 , uSCR ( k ) x uSCR ( k ) ) F uSCR ( k )
+ a 5 , uSCR - a 3 , uSCR x uSCR ( k ) ( 11 ) ##EQU00003##
[0053] Here, the C.sub.NH3,in,uSCR (k) is the y.sub.2(k) from
equation (7) and C.sub.NOx,in,uSCR(k) is the y.sub.1(k) from the
equation (7). y.sub.1,uSCR(k) and y.sub.2,uSCR(k) represent the
concentration of NOx and NH3 concentration at the uSCR outlet,
respectively.
[0054] The method 300 further includes optimizing the amount of
reductant (u(k)) that is injected by the reductant injector 236, at
380. The optimization includes linearizing a combination of the SCR
model and the uSCR model from the equations (7) and (9). The
linearized model of the combination of the SCR 24 and the uSCR 25
can be expressed as follows:
x ( k + 1 ) = A ( p ( k ) ) x ( k ) + B u ( p ( k ) ) u ( k )
##EQU00004## y ( k ) = C ( p ( k ) ) x ( k ) , where :
##EQU00004.2## x ( k ) = [ .THETA. .theta. ( k ) u ( k - 1 )
.THETA. UF .theta. uSCR ( k ) ] ##EQU00004.3## u ( k ) = C NH 3 ,
in ( k ) ##EQU00004.4## A ( p ( k ) ) = [ A 11 ( p ( k ) ) 0 0 0 0
0 A 21 ( p ( k ) ) 0 A 22 ( p ( k ) ) ] ##EQU00004.5## B u ( p ( k
) ) = [ B 1 ( p ( k ) ) 1 B 2 ( p ( k ) ) ] ; ##EQU00004.6## C ( p
( k ) ) = [ C 1 ( p ( k ) ) 0 0 C 2 ( p ( k ) ) D 2 ( p ( k ) ) 0 C
3 ( p ( k ) ) 0 0 C m ( p ( k ) ) D m ( p ( k ) ) 0 C 12 _ 1 ( p (
k ) ) 0 C 12 _ 2 ( p ( k ) ) C 22 _ 1 ( p ( k ) ) D 22 ( p ( k ) )
C 22 _ 2 ( p ( k ) ) 0 0 C 32 _ 2 ( p ( k ) ) C m 2 _ 1 ( p ( k ) )
D m 2 ( p ( k ) ) C m 2 _ 2 ( p ( k ) ) ] ; and ##EQU00004.7## y (
k ) = [ y 1 ( k ) y 2 ( k ) y 3 ( k ) y 4 ( k ) y 1 , uSCR ( k ) y
2 , uSCR ( k ) y 3 , uSCR ( k ) y 4 , uSCR ( k ) ] .
##EQU00004.8##
[0055] Here, .THETA. is the NH3 storage capacity of the SCR 24,
.theta.(k) is the NH.sub.3 storage level at the SCR 24 at time k,
.THETA..sub.UF is the NH.sub.3 storage capacity of the uSCR 25,
.theta..sub.uSCR(k) is the NH.sub.3 storage level at the uSCR 25 at
time k.
[0056] The method 300 includes optimizing the amount of reductant
(u(k)) that is injected into the exhaust gas 16 by the reductant
injector 236, at 390. The optimization includes solving at real
time a numerical optimization problem to determine the optimal
amount of urea to be injected so as to minimize a cost function
that expresses the system performance. For example, the cost
function can include a combination of NOx and NH3 concentrations at
SCR outlet and/or uSCR outlet, NOx reduction efficiency, urea
injection efforts, and other such parameters described herein.
u ( k ) = arg min NH 3 in f ( NH 3 in , NH 3 k , NOx k , NOx UF , k
, NH 3 UF , k , w i , .di-elect cons. , .rho. .di-elect cons. ) (
12 ) ##EQU00005##
[0057] Here, in addition to the terms from equation (8), the w
terms include, w.sub.NOx,uSCR and W.sub.NH3,uSCR, which are weight
calibrations for NOx measurement at the outlet of the uSCR 25 and
an estimated for NH.sub.3 at the outlet of the uSCR 25,
respectively. The controller module 238 accordingly is responsible
for computing an optimal amount of reductant to inject to maintain
a first predetermined ratio between the amount of NH3 and the
amount of NOx at the outlet of SCR 24 and to maintain a second
predetermined ratio between the amount of NH3 and the amount of NOx
at the outlet of the uSCR 25.
[0058] The optimization can be solved by using linear and nonlinear
programming techniques to determine the amount of reductant 246 to
be injected by computing the minimal u(k) per equation (12). The
method 300 hence, includes determining the optimal level of
reductant 246 to be injected into the exhaust gas treatment system
10, at 350. In one or more examples, the optimal level of the
reductant 246 is the minimum u(k) that is computed by optimizing
the expressions in equation (12) (or in case without the uSCR the
equation (8)).
[0059] The controller module 238 instructs the injector 236 to
inject the corresponding amount of reductant 246 according to the
computed u(k) value. The injector 236 injects the commanded amount
of reductant 246 into the exhaust gas treatment system 10 in
response.
[0060] The technical solutions described herein facilitate
improvements to emissions control systems used with internal
combustion engines, such as those used in vehicles. For example,
the technical solutions provide a control strategy that optimizes
the overall performance of the exhaust gas treatment system
composed of an SCR 24 and a uSCR 25 to maintain tailpipe NOx
emissions within a predetermined range, and by using only a single
reductant (urea) injector 236, at 340. Further, the technical
solutions facilitate the controller module 238 to operate based on
a calibration value that indicates whether the controller module
238 computes an amount of reductant for only the SCR 24 or a
combination of both the SCR 24 and the uSCR 25. The system
automatically handles the selected configuration without any manual
intervention.
[0061] The technical solutions described herein accordingly
optimize the performance of the entire exhaust gas treatment system
10 including the two SCR devices, the SCR 24 and the uSCR 25, using
a single reductant injector 236. The single reductant injector is
controlled to inject a computed amount of reductant that may
directly be injected at a first SCR device, such as the SCR 24 and
indirectly at the second SCR device, such as the uSCR 25. The
reductant amount is computed using a physics-based model and the
amount of reductant is computed in real time by solving a numerical
programming problem in the ECM processor. Accordingly, the
technical solutions described herein provide a systematic and
modular control approach to manage, in a flexible way, both, SCR
and/or SCR+uSCR architectures with a single optimal controller and
injector system.
[0062] While the above disclosure has been described with reference
to exemplary embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from its scope.
In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the disclosure without
departing from the essential scope thereof. Therefore, it is
intended that the present disclosure not be limited to the
particular embodiments disclosed, but will include all embodiments
falling within the scope thereof.
* * * * *