U.S. patent application number 13/185531 was filed with the patent office on 2013-01-24 for exhaust aftertreatment system for compression-ignition engines.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS LLC. The applicant listed for this patent is Chang H. Kim, Wei Li, Gongshin Qi. Invention is credited to Chang H. Kim, Wei Li, Gongshin Qi.
Application Number | 20130019589 13/185531 |
Document ID | / |
Family ID | 47502354 |
Filed Date | 2013-01-24 |
United States Patent
Application |
20130019589 |
Kind Code |
A1 |
Kim; Chang H. ; et
al. |
January 24, 2013 |
EXHAUST AFTERTREATMENT SYSTEM FOR COMPRESSION-IGNITION ENGINES
Abstract
An internal combustion engine configured to operate in a
compression-ignition combustion mode includes an exhaust
aftertreatment system. The exhaust aftertreatment system includes a
catalyst device fluidly coupled upstream of an ammonia-selective
catalytic reduction device. The, catalyst device includes first,
second, and third elements fluidly coupled in series. The first
element includes a three-way catalytic element, the second element
includes a NOx adsorber, and the third element includes an
oxidation catalytic element.
Inventors: |
Kim; Chang H.; (Rochester,
MI) ; Qi; Gongshin; (Troy, MI) ; Li; Wei;
(Troy, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kim; Chang H.
Qi; Gongshin
Li; Wei |
Rochester
Troy
Troy |
MI
MI
MI |
US
US
US |
|
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS
LLC
DETROIT
MI
|
Family ID: |
47502354 |
Appl. No.: |
13/185531 |
Filed: |
July 19, 2011 |
Current U.S.
Class: |
60/297 |
Current CPC
Class: |
Y02T 10/12 20130101;
B01D 53/9495 20130101; B01D 53/9409 20130101; Y02T 10/22 20130101;
F01N 13/0093 20140601; F01N 2610/02 20130101; F01N 3/2066 20130101;
Y02T 10/24 20130101; B01D 53/9477 20130101; F01N 3/2803 20130101;
Y02C 20/10 20130101 |
Class at
Publication: |
60/297 |
International
Class: |
F01N 3/035 20060101
F01N003/035 |
Claims
1. An exhaust aftertreatment system for an internal combustion
engine configured to operate in a compression-ignition combustion
mode, comprising: a catalyst device fluidly coupled upstream of an
ammonia-selective catalytic reduction device, said catalyst device
comprising first, second, and third elements fluidly coupled in
series; said first element comprising a three-way catalytic
element; said second element comprising a NOx adsorber; and said
third element comprising an oxidation catalytic element.
2. The exhaust aftertreatment system of claim 1, wherein the
three-way catalytic element comprises a substrate coated with a
washcoat including Pd/Al2O3.
3. The exhaust aftertreatment system of claim 1, wherein the NOx
adsorber comprises a substrate coated with a washcoat including
LaMnO2 and BaO.
4. The exhaust aftertreatment system of claim 1, wherein the
oxidation catalytic element comprises a substrate coated with a
washcoat including Rh/CeO2 and Al2O3.
5. An exhaust aftertreatment system for a compression-ignition
internal combustion engine, the exhaust gas aftertreatment system
consisting essentially of: a device comprising first, second, and
third elements fluidly coupled in series, wherein the first element
comprises a three-way catalytic element, the second element
comprises a NOx adsorber, and the third element comprises an
oxidation catalytic element; an ammonia-selective catalytic
reduction device; and a particulate filter; the device located
fluidly upstream of the ammonia-selective catalytic reduction
device located fluidly upstream of the particulate filter.
6. The exhaust aftertreatment system of claim 5, wherein the
three-way catalytic element comprises a substrate coated with a
washcoat including Pd/Al2O3.
7. The exhaust aftertreatment system of claim 5, wherein the NOx
adsorber comprises a substrate coated with a washcoat including
LaMnO2 and BaO.
8. The exhaust aftertreatment system of claim 5, wherein the
oxidation catalytic element comprises a substrate coated with a
washcoat including Rh/CeO2 and Al2O3.
9. An apparatus, including: said internal combustion engine
configured to operate in a compression-ignition combustion mode
fluidly coupled to a passive NH3-SCR exhaust aftertreatment system;
said passive NH3-SCR exhaust aftertreatment system comprising a
catalyst device fluidly coupled upstream of an ammonia-selective
catalytic reduction device fluidly coupled upstream of a
particulate filter; said catalyst device comprising first, second,
and third elements fluidly coupled in series; said first element
comprising a three-way catalytic element; said second element
comprising a NOx adsorber; and said third element comprising an
oxidation catalytic element.
10. The apparatus of claim 9, wherein the three-way catalytic
element comprises a substrate coated with a washcoat including
Pd/Al2O3.
11. The apparatus of claim 9, wherein the NOx adsorber comprises a
substrate coated with a washcoat including LaMnO2 and BaO.
12. The apparatus of claim 9, wherein the oxidation catalytic
element comprises a substrate coated with a washcoat including
Rh/CeO2 and Al2O3.
Description
TECHNICAL FIELD
[0001] This disclosure relates to compression-ignition internal
combustion engine emissions aftertreatment.
BACKGROUND
[0002] The statements in this section merely provide background
information related to the present disclosure. Accordingly, such
statements are not intended to constitute an admission of prior
art.
[0003] Compression-ignition internal combustion engines operate at
lean air/fuel ratios to achieve desirable fuel efficiencies. Lean
engine operation may produce oxides of nitrogen (NOx) when nitrogen
and oxygen molecules present in engine intake air disassociate in
the high temperatures of combustion. Rates of NOx production follow
known relationships in the combustion process, for example with
higher rates of NOx production being associated with higher
combustion temperatures and longer exposure of air molecules to the
higher temperatures.
[0004] NOx molecules may be reduced to elemental nitrogen and
oxygen in aftertreatment devices. Efficacy of known aftertreatment
devices is dependent upon operating conditions including operating
temperature, which is associated with exhaust gas flow temperatures
and engine air/fuel ratio. Aftertreatment devices include materials
prone to damage or degradation when exposed to elevated
temperatures and/or contaminants in the exhaust gas feedstream.
[0005] Aftertreatment systems include catalytic devices to generate
chemical reactions to treat exhaust gas constituents. Three-way
catalytic devices (TWC) oxidize and reduce exhaust gas
constituents. NOx adsorbers store NOx, which may be subsequently
desorbed and reduced under specific engine operating conditions.
One known strategy includes using a NOx adsorber to store NOx
emissions during lean operations and then purging and reducing the
stored NOx to nitrogen and water using a TWC during rich engine
operating conditions. Particulate filters (DPF) are able to remove
particulate matter in the exhaust gas feedstream, which may then be
periodically purged, e.g. during high temperature regeneration
events.
[0006] One known aftertreatment device is a selective catalytic
reduction device (SCR). The SCR device includes catalytic material
that promotes the reaction of NOx with a reductant such as ammonia
(NH3) or urea to produce nitrogen and water. Reductants, e.g. urea,
may be injected into an exhaust gas feedstream upstream of the SCR
device, which requires an injection system, storage tank and a
control scheme. Reductants, e.g. NH3, may be generated in an
exhaust gas feedstream upstream of the SCR device during specific
engine operating conditions.
[0007] Catalytic materials used in SCR devices include vanadium (V)
and tungsten (W) on titanium (Ti) and base metals including iron
(Fe) or copper (Cu) with a zeolite washcoat. Catalytic materials
including copper may perform effectively at lower temperatures but
have been shown to have poor thermal durability at higher
temperatures. Catalytic materials including iron may perform well
at higher temperatures but with decreasing reductant storage
efficiency at lower temperatures.
[0008] Known SCR devices preferably operate within an operating
temperature range of 150.degree. C. to 600.degree. C. The
temperature range may vary depending on the catalyst materials. An
operating temperature range may decrease during or after higher
engine load operation. Temperatures greater than 600.degree. C. may
cause reductants to breakthrough and degrade an SCR catalyst, while
the effectiveness of NOx processing decreases at temperatures lower
than 150.degree. C.
SUMMARY
[0009] An internal combustion engine configured to operate in a
compression-ignition combustion mode includes an exhaust
aftertreatment system. The exhaust aftertreatment system includes a
catalyst device fluidly coupled upstream of an ammonia-selective
catalytic reduction device. The, catalyst device includes first,
second, and third elements fluidly coupled in series. The first
element includes a three-way catalytic element, the second element
includes a NOx adsorber, and the third element includes an
oxidation catalytic element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] One or more embodiments will now be described, by way of
example, with reference to the accompanying drawings, in which:
[0011] FIG. 1 schematically illustrates a portion of a single
cylinder of a compression-ignition internal combustion engine and
an exhaust aftertreatment system in accordance with the
disclosure;
[0012] FIG. 2 graphically shows data associated with operating an
engine system equipped with an aftertreatment system including a
known catalytic converter upstream of an NH3-SCR catalyst in
accordance with the disclosure;
[0013] FIG. 3 graphically shows data illustrative of operating an
engine system equipped with an embodiment of the passive NH3-SCR
exhaust aftertreatment system in accordance with the
disclosure;
[0014] FIG. 4 graphically shows concentrations of engine-out H2 and
H2 downstream of a first catalyst element obtained from operating
an engine system equipped with an embodiment of the passive NH3-SCR
exhaust aftertreatment system described herein in accordance with
the disclosure; and
[0015] FIG. 5 graphically shows concentrations of engine-out H2 and
a corresponding H2 concentration downstream of a third catalyst
element obtained from operating an engine system equipped with an
embodiment of the passive NH3-SCR exhaust aftertreatment system
described herein in accordance with the disclosure.
DETAILED DESCRIPTION
[0016] Referring now to the drawings, wherein the showings are for
the purpose of illustrating certain exemplary embodiments only and
not for the purpose of limiting the same, FIG. 1 schematically
illustrates a portion of a single cylinder 12 of a
compression-ignition internal combustion engine 10 fluidly coupled
to a passive NH3-SCR exhaust aftertreatment system 40. The passive
NH3-SCR exhaust aftertreatment system 40 includes a first catalytic
converter 42 fluidly coupled to and upstream of an
ammonia-selective catalytic reduction (NH3-SCR) catalyst 44 fluidly
coupled to and upstream of a particulate filter 46. The internal
combustion engine 10 is configured to operate in a four-stroke
compression-ignition combustion cycle including repetitively
executed intake-compression-ignition-exhaust strokes, or any other
suitable combustion cycle. The internal combustion engine 10
preferably includes an intake manifold 14, combustion chamber 16,
intake and exhaust valves 17 and 15, respectively, an exhaust
manifold 18, and an EGR system 20 including an EGR valve 22. The
intake manifold 14 may include a mass airflow sensing device 24
that generates a signal output 71 corresponding to a mass flowrate
of engine intake air. The intake manifold 14 optionally includes a
throttle device 23 in one embodiment. An air/fuel ratio sensing
device 41 is configured to monitor an exhaust gas feedstream of the
internal combustion engine 10, and preferably generates signal
outputs including an air/fuel ratio signal 75 and an exhaust gas
feedstream temperature signal 73. A fuel injector 28 is configured
to directly inject a fuel pulse into the combustion chamber 16 in
response to a pulsewidth command 77. In one embodiment, one or more
pressure sensor(s) 30 is configured to monitor in-cylinder pressure
in one of, or preferably all of the cylinders of the engine 10
during each combustion cycle. A rotational position sensor 25 is
configured to monitor rotational position and speed of a crankshaft
of the engine 10. A single one of the cylinders 12 is depicted, but
it is appreciated that the engine 10 includes a plurality of
cylinders each having an associated combustion chamber 16, fuel
injector 28, and intake and exhaust valves 17 and 15. The
description of the engine 10 is illustrative, and the concepts
described herein are not limited thereto. Although the internal
combustion engine 10 is described as a compression-ignition
internal combustion engine, it is appreciated that the concepts
described herein may apply to other internal combustion engines
configured to operate lean of stoichiometry that may employ the
passive NH3-SCR exhaust aftertreatment system 40 described
herein.
[0017] The exhaust manifold 18 channels the exhaust gas feedstream
of the internal combustion engine 10 to the passive NH3-SCR exhaust
aftertreatment system 40. A second sensing device 45 is configured
to monitor the exhaust gas feedstream downstream of the NH3-SCR
catalyst 44, and may include, e.g., a NOx sensor, a NH3 sensor, or
another suitable sensor. The second sensing device 45 generates a
signal 81 readable by the control module 50 for purposes of control
and diagnostics.
[0018] The passive NH3-SCR exhaust aftertreatment system 40
includes the first catalytic converter 42 fluidly coupled to and
upstream of the aforementioned NH3-SCR catalyst 44. The first
catalytic converter 42 includes first, second and third catalyst
elements 51, 53, and 55, respectively. The first, second and third
catalyst elements 51, 53, and 55 are arranged in series with the
first catalyst element 51 fluidly coupled to the exhaust manifold
18 and configured to treat engine-out exhaust gas. Exhaust gas
treated in the first catalyst element 51 passes to the second
catalyst element 53 and then passes to the third catalyst element
55, as is appreciated. Each of the first, second and third catalyst
elements 51, 53, and 55 includes a ceramic or metallic substrate
element that is coated as described herein.
[0019] The first catalyst element 51 is preferably a catalytic
element that includes a substrate element coated with a washcoat
that is capable of oxidizing HC and CO molecules and reducing NOx
molecules in response to engine operating conditions including
air/fuel ratio. The catalytically active materials include Pd/Al2O3
in one embodiment. Alternatively, the first catalyst element 51 may
be another suitable three-way catalytic element including a
substrate element coated with a catalytically active washcoat that
oxidizes HC and CO molecules and reduces NOx molecules in response
to engine operating conditions.
[0020] The second catalyst element 53 is preferably a NOx adsorber
that includes a substrate element coated with a washcoat that is
capable of adsorbing and desorbing NOx molecules. The substrate
element is coated with a washcoat including LaMnO2 and BaO in one
embodiment. As such, there are preferably no platinum-group metals
(e.g., platinum, palladium, and rhodium) used in the NOx adsorber.
Alternatively, the second catalyst element 53 may be any other NOx
adsorber element including a substrate element coated with a
suitable washcoat that is capable of adsorbing and desorbing NOx
molecules.
[0021] The third catalyst element 55 is preferably a catalytic
element including a substrate element coated with a washcoat
containing one or more catalytically active materials for oxidizing
hydrocarbons in the exhaust gas feedstream. The catalytically
active materials include Rh/CeO2 and Al2O3 in one embodiment.
Alternatively, the third catalyst element 55 may be another
suitable oxidation catalytic element or three-way catalytic
electric including a substrate element coated with a catalytically
active washcoat. This may include a three-way catalyst device that
is capable of oxidizing HC and CO molecules and reducing NOx
molecules in response to engine operating conditions including
air/fuel ratio.
[0022] Ammonia (NH3) may be passively generated in the first
catalytic converter 42 using a system wherein engine operation is
periodically modulated to generate an exhaust gas feedstream that
includes nitric oxide (NO), carbon monoxide (CO), and hydrogen
(H2). The exhaust gas feedstream produces NH3 in the first
catalytic converter 42 under specific operating conditions. It is
appreciated that the amount of NH3 that is produced in either or
both the first, three-way catalytic element 51 and the third
oxidation catalytic element 55 is limited by the engine-out NOx
level. Additional H2 is available, which may be used to produce
NH3. The chemical equation expressing this relationship as
follows.
NO.sub.x+H.sub.2/CONH.sub.3+CO.sub.2 [1]
[0023] The NH3-SCR catalyst 44 includes one or more substrate
elements preferably fabricated from cordierite material with a
multiplicity of flowthrough passageways that are preferably coated
with a zeolite washcoat and catalytic material including, e.g., a
catalytically active base metal. The catalytically active materials
store NH3, and release stored NH3 for reacting with NOx molecules
in the exhaust gas feedstream. It is appreciated that the storage
capacity of a NH3-SCR catalyst, i.e., the mass amount of NH3 that
may be stored on a NH3-SCR catalyst, correlates to an inlet
temperature of the NH3-SCR catalyst. When the inlet temperature
increases above a threshold temperature, the storage capacity
decreases.
[0024] The particulate filter 46 is fluidly coupled downstream of
the NH3-SCR catalyst 44, and includes a ceramic filter element
configured to trap particulate matter. In one embodiment, the
ceramic filter element is a wall-flow filtering element. In one
embodiment, the ceramic filter element is coated with a washcoat
including suitable catalytically active materials. Particulate
filters 46 may include other suitable features for trapping and
oxidizing particulate matter produced during combustion.
[0025] A control module 50 is signally connected to engine sensors
and operatively connected to engine actuators to execute control
schemes to control operation of the engine 10 to form cylinder
charges in response to an operator command. The sensors include,
e.g., the air/fuel ratio sensing device 41, the mass airflow
sensing device 24, and the pressure sensor(s) 30. The actuators
include, e.g., the fuel injector 28, the throttle device 23, and
the EGR valve 22. The control module 50 operates the fuel injector
28 by commanding a pulsewidth 77 to deliver a fuel pulse to the
combustion chamber 16. The pulsewidth 77 is an elapsed time period
during which the fuel injector 28 is opened and delivering the fuel
pulse. The delivered fuel pulse interacts with intake air and any
internally retained and externally recirculated exhaust gases to
form a cylinder charge in the combustion chamber 16 in response an
operator torque request. It is appreciated that the control module
50 may command multiple fuel injection events using multiple
pulsewidths 77 to cause the fuel injector 28 to deliver the fuel
pulse to the combustion chamber 16 during each cylinder event.
[0026] The control module 50 operates the EGR valve 22 by
commanding an EGR valve opening command 78 to cause the EGR valve
22 to effect a preferred EGR flowrate to achieve a preferred EGR
fraction in the cylinder charge. It is appreciated that age,
calibration, contamination and other factors may affect operation
of the EGR system 20, thus causing variations in in-cylinder
air/fuel ratio of the cylinder charge.
[0027] The control module 50 may operate the throttle device 23 by
commanding a throttle valve opening command 76 to command a
preferred fresh air mass flowrate for the cylinder charge. In one
embodiment, the control module 50 operates a turbocharger device to
command a preferred boost pressure associated with the cylinder
charge.
[0028] Control module, module, control, controller, control unit,
processor and similar terms mean any one or various combinations of
one or more of Application Specific Integrated Circuit(s) (ASIC),
electronic circuit(s), central processing unit(s) (preferably
microprocessor(s)) and associated memory and storage (read only,
programmable read only, random access, hard drive, etc.) executing
one or more software or firmware programs or routines,
combinational logic circuit(s), input/output circuit(s) and
devices, appropriate signal conditioning and buffer circuitry, and
other components to provide the described functionality. Software,
firmware, programs, instructions, routines, code, algorithms and
similar terms mean any controller executable instruction sets
including calibrations and look-up tables. The control module has a
set of control routines executed to provide the desired functions.
Routines are executed, such as by a central processing unit, and
are operable to monitor inputs from sensing devices and other
networked control modules, and execute control and diagnostic
routines to control operation of actuators. Routines may be
executed at regular intervals, for example each 3.125, 6.25, 12.5,
25 and 100 milliseconds during ongoing engine and vehicle
operation. Alternatively, routines may be executed in response to
occurrence of an event.
[0029] It is appreciated that the engine-out NOx in the exhaust gas
feedstream of a compression-ignition engine is substantially lower
than that for a spark-ignition engine. During ongoing engine
operation, NOx emissions generated during cold-start and lean
air/fuel engine operation is stored on the second NOx adsorber
catalyst element 53, represented as follows.
NOx+MOM(NO3).sub.2 [2]
[0030] During engine operation under other conditions, the stored
NOx is used to produce additional NH3 that passes through to the
NH3-SCR catalyst 44 and stored for NOx reduction, represented as
follows.
NOx+H2/CONH3+CO2 [3]
[0031] The configuration of the passive NH3-SCR exhaust
aftertreatment system 40 including the first catalytic converter 42
as described herein enables use of a passive NH3-SCR exhaust
aftertreatment system in a compression-ignition engine system. The
first catalytic converter 42 produces NH3, thus minimizing any fuel
penalty associated with rich events required to produce NH3.
Absorbing NOx in the first catalytic converter 42 reduces reliance
upon the NH3-SCR catalyst 44 to achieve NOx reduction during lean
operation.
[0032] An exemplary control scheme for managing an exhaust gas
feedstream from the engine 10 coupled to the passive NH3-SCR
exhaust aftertreatment system 40 includes a process for
repetitively cycling between a lean air/fuel ratio and a
stoichiometric or rich air/fuel ratio, depending upon operating
conditions. Preferably the exhaust gas feedstream and/or selected
elements of the passive NH3-SCR exhaust aftertreatment system 40
are monitored to detect or otherwise determine NOx breakthrough and
NH3 breakthrough downstream of the NH3-SCR catalyst 44, e.g., using
the second sensing device 45. Monitoring the NH3-SCR catalyst 44
preferably includes monitoring temperature of the exhaust gas
feedstream proximal to the NH3-SCR catalyst 44 to ensure the
temperature of the NH3-SCR catalyst 44 is within a preferred
temperature range. In one embodiment, the preferred temperature
range is 150.degree. C. to 450.degree. C. When the temperature of
the NH3-SCR catalyst 44 is maintained within the preferred
temperature range, engine operation may be modulated to produce
nitric oxide (NO), carbon monoxide (CO), and hydrogen (H2) to
produce NH3 in the first catalytic converter 42 that may be
transferred downstream to the NH3-SCR catalyst 44 for storage and
subsequent NOx reduction.
[0033] The modulated engine operation includes operating the engine
10 rich or at stoichiometry while meeting the operator torque
request and without changing engine output power. One exemplary
method for operating the exemplary engine 10 rich of stoichiometry
may include executing multiple fuel injection pulses during a
combustion cycle including injecting a first fuel pulse into the
combustion chamber 16 during each compression stroke. The mass of
fuel injected during the first fuel pulse is determined based upon
an amount sufficient to operate the engine 10 to meet the operator
torque request and other load demands. Subsequent fuel pulses may
be injected into the combustion chamber 16 during other strokes of
the combustion cycle thereby generating an exhaust gas feedstream
including nitric oxide (NO), carbon monoxide (CO), and hydrogen
(H2) to produce NH3 in the first catalytic converter 42. In one
embodiment, the subsequent fuel pulses are executed late in a power
stroke or early in an exhaust stroke of the combustion cycle
thereby minimizing likelihood of combustion in the combustion
chamber 16.
[0034] The modulated engine operation is discontinued and the
engine 10 is controlled to operate lean of stoichiometry when the
NH3-SCR catalyst 44 has stored a sufficient amount of NH3,
preferably before reaching a saturation point. NH3 production may
instead be discontinued after a predetermined threshold of NH3
molecules are generated or when engine operating conditions are not
conducive to NH3 production, e.g., during vehicle decelerations,
engine idling, or engine stops. NH3 saturation may be estimated
based upon a predetermined elapsed time period of operation in the
modulated mode, or by monitoring the exhaust gas feedstream
downstream of the NH3-SCR catalyst 44 to detect NH3 breakthrough,
or determined after executing a predetermined number of cylinder
events. NH3 breakthrough may be detected by monitoring signal
output of an NH3 sensor configured to monitor the exhaust gas
feedstream downstream of the NH3-SCR catalyst 44. In one
embodiment, NH3 saturation may be estimated using a model according
to methods sufficient to accurately estimate operation of the
combustion cycle, aftertreatment processes, conversions, and
monitored operating conditions including intake mass airflow,
air/fuel ratio, engine speed, and temperatures and aging states of
the first catalytic converter 42 and the NH3-SCR catalyst 44. Such
a model may be calibrated according to test results corresponding
to a particular hardware application.
[0035] After determining the NH3-SCR catalyst 44 is saturated with
NH3, or deciding based upon other considerations including
operating conditions, the modulated engine operation is
discontinued and the engine operation transitions to lean engine
operation, which may result in increased NOx emissions into the
exhaust gas flow. The first catalytic converter 42 may reduce a
portion of the NOx emissions. NH3 stored on the NH3-SCR catalyst 44
reacts with NOx thereby reducing NOx emissions and producing
nitrogen and water. The NH3 stored in the NH3-SCR catalyst 44 is
depleted as NH3 molecules react with NOx molecules. When the NH3 on
the NH3-SCR catalyst 44 is depleted, NOx emissions may pass through
the NH3-SCR catalyst 44. Therefore, the lean engine operation may
be discontinued and the control system may revert to the modulated
engine operation after detecting NOx breakthrough downstream from
the NH3-SCR catalyst 44. NH3 depletion and any associated NOx
breakthrough may be detected using the second sensing device 45.
Alternatively, NH3 depletion on the NH3-SCR catalyst 44 may be
estimated using an executable model.
[0036] FIG. 2 graphically shows data associated with operating an
engine system equipped with an aftertreatment system including a
known catalytic converter upstream of an NH3-SCR catalyst. The
known catalytic converter includes a first catalytic element
employing palladium as a catalyst and a second catalytic element
employing palladium and rhodium as catalysts. The x-axis depicts
elapsed time (202) and the y-axis depicts units of concentrations
of the respective gases (204) and engine operation in the form of
air/fuel ratio (207). The engine air/fuel ratio (207) includes
operation at lean air/fuel ratio (206) with periodic rich air/fuel
ratio excursions (208). When operating at the lean air/fuel ratio
(206), the exhaust gas feedstream includes the conditions of 550
ppm NO, 700 ppm HC, and 10% O2. When operating at the rich air/fuel
ratio (208), the exhaust gas feedstream includes the conditions of
550 ppm NO, 700 ppm HC, and 0.5% O2, with 1.5% CO and 0.5% H2.
Plotted data indicates concentrations of NO (212), N20 (214), NH3
(216), NO2 (218), C3H6 (220), and C4H8 (222). The data indicates
that for a system with the known catalytic converter including the
first catalytic element employing palladium and the second
catalytic element employing palladium and rhodium, the known
catalytic converter generates NH3 at a maximum concentration of 550
ppm.
[0037] FIG. 3 graphically shows data illustrative of operating an
engine system equipped with an embodiment of the passive NH3-SCR
exhaust aftertreatment system 40 described herein. The passive
NH3-SCR exhaust aftertreatment system 40 includes a first catalytic
converter upstream of an NH3-SCR catalyst, including the previously
described first catalytic converter 42 including the aforementioned
first, second and third catalyst elements 51, 53, and 55,
respectively. The first catalyst element 51 employs palladium. The
second catalyst element 53 is a NOx adsorber, as previously
described. The third catalyst element 55 employs palladium and
rhodium. The x-axis depicts elapsed time (202) and the y-axis
depicts units of concentrations of the respective gases (204) and
engine operation in the form of air/fuel ratio (207). The engine
air/fuel ratio (207) includes operation at a lean air/fuel ratio
(206) with periodic rich air/fuel ratio excursions (208). When
operating at the lean air/fuel ratio (206), the exhaust gas
feedstream includes the conditions of 550 ppm NO, 700 ppm HC, and
10% O2. When operating at the rich air/fuel ratio (208), the
exhaust gas feedstream includes the conditions of 550 ppm NO, 700
ppm HC, and 0.5% O2, with 1.5% CO and 0.5% H2. Thus, the engine
operating conditions are analogous to those described with
reference to FIG. 2. Plotted data indicates concentrations of NO
(212), N20 (214), NH3 (216), NO2 (218), C3H6 (220), and C4H8 (222).
The data indicates that for a system employing an embodiment of the
passive NH3-SCR exhaust aftertreatment system 40, the first
catalytic converter 42 generates NH3 at a concentration approaching
1200 ppm during the rich air/fuel ratio excursion (208). Thus, the
system employing an embodiment of the passive NH3-SCR exhaust
aftertreatment system 40 including a NOx adsorber as a second
catalyst element 53 produces a substantially greater amount of NH3
that an analogous engine system that is equipped with an
aftertreatment system including a known catalytic converter that
does not employ a NOx adsorber.
[0038] FIGS. 4 and 5 graphically show data from measurements in an
exhaust gas feedstream obtained from operating an engine system
equipped with an embodiment of the passive NH3-SCR exhaust
aftertreatment system 40 described herein. The data demonstrates
operation of an exemplary system, and is meant to be illustrative
of the concepts described herein. The engine system is a
compression-ignition engine system. The passive NH3-SCR exhaust
aftertreatment system 40 includes a first catalytic converter
upstream of an NH3-SCR catalyst, which includes the previously
described first catalytic converter 42 including the aforementioned
first, second and third catalyst elements 51, 53, and 55,
respectively. The first catalyst element 51 employs palladium. The
second catalyst element 53 is a NOx adsorber, as previously
described. The third catalyst element 55 employs palladium and
rhodium. The x-axis depicts elapsed time (202). The y-axis includes
concentrations of the respective gases (204), engine operation in
the form of air/fuel ratio (207), and catalyst temperature (209).
The engine air/fuel ratio (207) includes operation at lean air/fuel
ratio (206) with periodic air/fuel ratio excursions (208).
[0039] FIG. 4 shows concentrations of engine-out H2 (213) and H2
downstream of the first catalyst element 51 (215). FIG. 5 shows
concentrations of engine-out H2 (213) and a corresponding H2
concentration downstream of the third catalyst element 55
(217).
[0040] The engine operating conditions of air/fuel ratio (207) and
catalyst temperature (209) include the periodic air/fuel ratio
excursions, with specific magnitudes of the air/fuel ratio
excursions (208) including 15.0:1, 14.9:1, 14.8:1, 14.7:1, 14.6:1,
14.5:1, 14.4:1, 14.3:1, 14.2:1, 14.1:1, as shown.
[0041] As indicated, the concentration of engine-out H2 (213)
increases as the air/fuel ratio becomes increasingly rich, whereas
the H2 concentration downstream of the first catalyst element 51
(215) is negligible. However, the corresponding H2 concentration
downstream of the third catalyst element 55 (217) increases when
the air/fuel ratio becomes rich of stoichiometry, and reaches a
peak concentration of about 1400 ppm when the air/fuel ratio
excursion is at 14.4:1.
[0042] The increased H2 in the exhaust gas feedstream upstream of
the upstream of the NH3-SCR catalyst 44 reacts with NOx gases to
form NH3, which may be stored on the NH3-SCR catalyst 44 and used
during lean operation for NOx reduction.
[0043] The disclosure has described certain preferred embodiments
and modifications thereto. Further modifications and alterations
may occur to others upon reading and understanding the
specification. Therefore, it is intended that the disclosure not be
limited to the particular embodiment(s) disclosed as the best mode
contemplated for carrying out this disclosure, but that the
disclosure will include all embodiments falling within the scope of
the appended claims.
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