U.S. patent application number 12/363054 was filed with the patent office on 2010-08-05 for exhaust aftertreatment system.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Steven J. Schmieg, Thompson M. Sloane.
Application Number | 20100192545 12/363054 |
Document ID | / |
Family ID | 42396573 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100192545 |
Kind Code |
A1 |
Schmieg; Steven J. ; et
al. |
August 5, 2010 |
EXHAUST AFTERTREATMENT SYSTEM
Abstract
An exhaust aftertreatment system connected to an internal
combustion engine operative lean of stoichiometry includes first
and second selective catalytic reactor devices. A preferred ratio
of hydrocarbon:NOx to achieve a preferred concentration of ammonia
immediately downstream of the first selective catalytic reactor
device is determined. An ethanol-based reductant is dispensed
upstream of the first selective catalytic reactor device.
Inventors: |
Schmieg; Steven J.; (Troy,
MI) ; Sloane; Thompson M.; (Oxford, MI) |
Correspondence
Address: |
CICHOSZ & CICHOSZ, PLLC
129 E. COMMERCE
MILFORD
MI
48381
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
42396573 |
Appl. No.: |
12/363054 |
Filed: |
January 30, 2009 |
Current U.S.
Class: |
60/286 ; 60/297;
60/299 |
Current CPC
Class: |
B01D 2255/20723
20130101; Y02T 10/12 20130101; B01D 2255/20761 20130101; B01D
2257/406 20130101; F01N 3/208 20130101; B01D 2255/20707 20130101;
F01N 2610/03 20130101; B01D 2255/1025 20130101; B01D 53/9418
20130101; F01N 13/0093 20140601; B01D 2255/2092 20130101; F01N
2560/02 20130101; B01D 2255/1023 20130101; B01D 2255/104 20130101;
B01D 2251/2062 20130101; B01D 2255/1021 20130101; B01D 2255/20776
20130101; B01D 2251/21 20130101; B01D 53/9477 20130101; B01D
2255/20738 20130101; Y02T 10/24 20130101 |
Class at
Publication: |
60/286 ; 60/299;
60/297 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10 |
Goverment Interests
GOVERNMENT CONTRACT RIGHTS
[0001] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of DE-FC26-02NT41218 awarded by the U.S. Department of Energy.
Claims
1. Method for operating an internal combustion engine operative
lean of stoichiometry and providing an exhaust gas feedstream
fluidly connected to an exhaust aftertreatment system including a
first selective catalytic reactor device fluidly connected upstream
to an ammonia-selective catalytic reactor device, comprising:
monitoring operation of the internal combustion engine and the
exhaust gas feedstream upstream of the first selective catalytic
reactor device; determining a preferred ratio of hydrocarbon:NOx
for the exhaust gas feedstream upstream of the first selective
catalytic reactor device effective to achieve a preferred
concentration of ammonia immediately downstream of the first
selective catalytic reactor device based upon the operation of the
internal combustion engine and the exhaust gas feedstream upstream
of the first selective catalytic reactor device; and dispensing an
ethanol-based reductant upstream of the first selective catalytic
reactor device to achieve the preferred ratio of hydrocarbon:NOx in
the exhaust gas feedstream upstream of the first selective
catalytic reactor device.
2. The method of claim 1, wherein the preferred concentration of
ammonia corresponds to a preferred ratio of ammonia:NOx upstream of
the ammonia-selective catalytic reactor device.
3. The method of claim 2, wherein the preferred ratio of
ammonia:NOx upstream of the ammonia-selective catalytic reactor
device comprises substantially equal concentrations of NOx and
ammonia.
4. The method of claim 1, wherein monitoring operation of the
internal combustion engine and the exhaust gas feedstream upstream
of the first selective catalytic reactor device comprises
determining a concentration of NOx, a temperature, and a mass flow
of the exhaust gas feedstream upstream of the first selective
catalytic reactor device, the method further comprising:
determining a space velocity of the first selective catalytic
reactor device; and wherein dispensing an ethanol-based reductant
upstream of the first selective catalytic reactor device comprises
controlling a mass flow rate of the ethanol-based reductant
upstream of the first selective catalytic reactor device based on
the concentration of NOx and the temperature of the exhaust gas
feedstream upstream of the first selective catalytic reactor
device, and the space velocity of the first selective catalytic
reactor device.
5. The method of claim 4, wherein determining the preferred ratio
of hydrocarbon:NOx for the exhaust gas feedstream upstream of the
first selective catalytic reactor device comprises determining the
preferred ratio of hydrocarbon:NOx for the exhaust gas feedstream
upstream of the first selective catalytic reactor device effective
to achieve substantially equal concentrations of NOx and ammonia
upstream of the ammonia-selective catalytic reactor device
corresponding to the concentration of NOx, the temperature, and the
mass flow of the exhaust gas feedstream upstream of the first
selective catalytic reactor device.
6. Method for operating an internal combustion engine at a lean
air/fuel ratio, comprising: fluidly connecting an exhaust
aftertreatment system to an exhaust gas feedstream of the internal
combustion engine, the exhaust aftertreatment system including a
silver-alumina catalytic reactor device fluidly connected upstream
to an ammonia-selective catalytic reactor device; determining a
concentration of NOx in the exhaust gas feedstream upstream of the
silver-alumina catalytic reactor device; determining a preferred
ratio of hydrocarbon:NOx for the exhaust gas feedstream upstream of
the silver-alumina catalytic reactor device effective to achieve
substantially equal concentrations of NOx and ammonia in the
exhaust gas feedstream immediately downstream of the silver-alumina
catalytic reactor device; and dispensing a reductant upstream of
the silver-alumina catalytic reactor device to achieve the
preferred ratio for hydrocarbon:NOx in the exhaust gas feedstream
upstream of the silver-alumina catalytic reactor device.
7. The method of claim 6, wherein dispensing a reductant upstream
of the silver-alumina catalytic reactor device comprises dispensing
an ethanol-based reductant.
8. The method of claim 6, further comprising: monitoring the
internal combustion engine and the exhaust gas feedstream upstream
of the silver-alumina catalytic reactor device to determine the
concentration of NOx in the exhaust gas feedstream upstream of the
silver-alumina catalytic reactor device, a temperature of the
exhaust gas feedstream upstream of the silver-alumina catalytic
reactor device and a mass flow of the exhaust gas feedstream
upstream of the silver-alumina catalytic reactor device,
determining a space velocity of the silver-alumina catalytic
reactor device corresponding to the mass flow of the exhaust gas
feedstream upstream of the silver-alumina catalytic reactor device;
and wherein dispensing a reductant upstream of the silver-alumina
catalytic reactor device comprises controlling a mass flow rate of
an ethanol-based reductant upstream of the silver-alumina catalytic
reactor device based on the concentration of NOx and the
temperature of the exhaust gas feedstream upstream of the
silver-alumina catalytic reactor device, and the space velocity of
the silver-alumina catalytic reactor device.
9. Apparatus, comprising an internal combustion engine configured
to operate lean of stoichiometry and provide an exhaust gas
feedstream; an exhaust aftertreatment system fluidly connected to
the internal combustion engine to receive the exhaust gas
feedstream, including an ethanol-selective catalytic reactor device
fluidly connected upstream to an ammonia-selective catalytic
reactor device and a reductant dispensing system configured to
dispense an ethanol-based reductant upstream of the
ethanol-selective catalytic reactor device; a control system
signally and operatively connected to the internal combustion
engine and the exhaust aftertreatment system, the control system
configured to monitor the internal combustion engine and the
exhaust aftertreatment system and control the reductant dispensing
system to dispense the ethanol-based reductant upstream of the
ethanol-selective catalytic reactor device to achieve a preferred
ratio of ammonia:NOx downstream of the ethanol-selective catalytic
reactor device and upstream of the ammonia-selective catalytic
reactor device.
10. The apparatus of claim 9, wherein the ethanol-selective
catalytic reactor device comprises a flow-through substrate coated
with a silver-alumina catalyst.
11. The apparatus of claim 10, wherein the silver-alumina catalyst
comprises a 3 wt. % Ag.sub.2O catalyst supported on an alumina
washcoat.
12. The apparatus of claim 9, wherein the ammonia-selective
catalytic reactor device comprises a flow-through substrate coated
with a copper-zeolite catalyst.
13. The apparatus of claim 9, further comprising the reductant
dispensing system operative to dispense the ethanol reductant into
the exhaust gas feedstream at a HC1:NOx ratio that achieves
substantially equal concentrations of NOx and ammonia in the
exhaust gas feedstream downstream of the ethanol-selective
catalytic reactor device and upstream of the ammonia-selective
catalytic reactor device.
Description
TECHNICAL FIELD
[0002] This disclosure relates to exhaust aftertreatment
systems.
BACKGROUND
[0003] The statements in this section merely provide background
information related to the present disclosure and may not
constitute prior art.
[0004] It is known that operating an internal combustion engine at
an air/fuel ratio that is lean of stoichiometry can improve fuel
economy but can increase some exhaust emissions, including oxides
of nitrogen (NOx). Known aftertreatment systems for internal
combustion engines operating lean of stoichiometry can include an
oxidation catalytic converter followed by other exhaust
aftertreatment devices, including a lean-NOx reduction catalyst,
also referred to as a lean NOx adsorber and a selective catalytic
reduction (SCR) catalytic device. Known three-way
oxidation-reduction catalytic converters (TWC) function to reduce
engine-out hydrocarbons (HC), carbon monoxide (CO), and NOx
emissions during stoichiometric engine operation and HC and CO
emissions during lean operation. Known SCR devices include catalyst
material(s) that promotes the reaction of NOx with a reductant,
such as ammonia (NH.sub.3) or urea, to produce nitrogen and water.
The reductants may be dispensed into an exhaust gas feedstream
upstream of the SCR device, requiring dispensing systems, tanks and
control schemes. The tanks may require periodic refilling and can
freeze in cold climates requiring additional heaters and
insulation.
[0005] Known aftertreatment systems include dispensing a NOx
reductant, e.g., urea, upstream of an urea-SCR catalyst device to
reduce NOx to N.sub.2. Use of urea as a reductant necessitates a
urea distribution infrastructure and an on-vehicle monitoring
system for the urea, and can have problems in cold weather climates
due to a relatively high freezing point of the urea solution.
Systems using NOx storage catalysts can require large catalyst
volumes and large amounts of platinum-group metals, coupled with
use of low sulfur fuel to operate efficiently. Known NOx storage
systems require periodic catalyst regeneration that can include
injecting fuel into the exhaust gas feedstream to generate high
exhaust gas temperatures along with dispensing reductants to
regenerate the storage material of the catalyst.
[0006] Selective catalytic reduction of NOx using hydrocarbons
(HC-SCR) is a known method for reducing NOx emissions under
oxygen-rich conditions. Ion-exchanged base metal zeolite catalysts,
e.g., Cu-ZSM5, are not sufficiently active under many vehicle
operating conditions and are susceptible to degradation by sulfur
dioxide and water exposure. Catalysts employing platinum-group
metals, e.g., Pt/Al.sub.2O.sub.3, operate over a narrow temperature
window and are highly selective towards N.sub.2O production.
[0007] Catalytic devices using alumina-supported silver, e.g.,
Ag/Al.sub.2O.sub.3, can selectively reduce NOx under lean exhaust
conditions with various hydrocarbon species as reductants. It is
known to use reductants including light hydrocarbons (e.g.,
propene, propane) and heavier fuel-component hydrocarbons (e.g.,
octane, decane) on HC-SCR devices using Ag/Al.sub.2O.sub.3
catalysts to reduce NOx emissions. NOx reduction using lighter
hydrocarbons present in engine exhaust as the combustion products
are known to be effective at converting NOx emissions at higher
temperatures. NOx reduction using heavier hydrocarbons (e.g.,
octane, decane) and oxygenated hydrocarbons (e.g., ethanol) are
known to be effective at converting NOx emissions at lower
temperatures.
[0008] A known exhaust aftertreatment system for reducing NOx
emissions in lean exhaust gas feedstream includes using a HC-SCR
catalyst coupled to a NOx storage catalyst coupled to an
ammonia-SCR catalyst. In such systems the HC-SCR catalyst reduces
some fraction of the NOx emissions when a reductant (e.g., HC, CO,
or H.sub.2) is dispensed into the exhaust feedstream, thereby
decreasing the frequency of regenerating the NOx storage catalyst,
i.e., increasing periods of operation at lean air/fuel ratios. The
NOx storage catalyst adsorbs NOx molecules during lean engine
operation and produces ammonia and nitrogen during rich
regeneration, while the ammonia-SCR catalyst utilizes the ammonia
formed during the aforementioned regeneration to further reduce NOx
in the lean exhaust. Known catalyst materials used in SCR devices
have included vanadium (V) and tungsten (W) on titanium (Ti).
Mobile applications include base metals including iron (Fe) or
copper (Cu) with a zeolite washcoat as catalyst materials. Material
concerns for catalyst materials include temperature operating
ranges, thermal durability, and reductant storage efficiency. For
mobile applications, SCR devices have a preferred operating
temperature range of 200.degree. C. to 600.degree. C., and may vary
depending on the selected catalyst material(s). The operating
temperature range can increase during or after higher load
operations. Temperatures greater than 600.degree. C. may cause NOx
and reductants to breakthrough and degrade the SCR catalysts, and
effectiveness of NOx reduction can decrease at temperatures lower
than 200.degree. C.
SUMMARY
[0009] An internal combustion engine is operative lean of
stoichiometry and provides an exhaust gas feedstream fluidly
connected to an exhaust aftertreatment system which includes a
first selective catalytic reactor device fluidly connected upstream
to an ammonia-selective catalytic reactor device. A method for
operating the internal combustion engine includes monitoring
operation of the internal combustion engine and the exhaust gas
feedstream upstream of the first selective catalytic reactor
device. The method further includes determining a preferred ratio
of hydrocarbon:NOx for the exhaust gas feedstream upstream of the
first selective catalytic reactor device effective to achieve a
preferred concentration of ammonia immediately downstream of the
first selective catalytic reactor device based upon the operation
of the internal combustion engine and the exhaust gas feedstream
upstream of the first selective catalytic reactor device. An
ethanol-based reductant is dispensed upstream of the first
selective catalytic reactor device to achieve the preferred ratio
of hydrocarbon:NOx in the exhaust gas feedstream upstream of the
first selective catalytic reactor device.
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 is a schematic drawing of an engine and exhaust
aftertreatment system, in accordance with the present disclosure;
and
[0012] FIGS. 2-9 are data graphs, in accordance with the present
disclosure.
DETAILED DESCRIPTION
[0013] 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 an internal combustion engine 10, an aftertreatment
system 45, and an accompanying control system including a control
module 5 (CM) that have been constructed in accordance with an
embodiment of the disclosure. The engine 10 comprises a
multi-cylinder direct-injection four-stroke internal combustion
engine that is operative at a lean air/fuel ratio. The exemplary
engine 10 can comprise a compression-ignition engine, a
spark-ignition direction-injection engine, and other engine
configurations that operate using a combustion cycle that includes
lean operation.
[0014] The engine 10 is equipped with various sensing devices for
monitoring engine operation, including an exhaust gas sensor 42
configured to monitor the exhaust gas feedstream. The exhaust gas
sensor preferably comprises a device operative to generate an
electrical signal correlatable to air/fuel ratio of the exhaust gas
feedstream, from which oxygen content can be determined. The
exhaust gas sensor 42 can further comprise a device operative to
generate an electrical signal correlatable to a parametric state of
NOx concentration in the exhaust gas feedstream. Alternatively, a
virtual sensing device executed as an algorithm in the control
module 5 can be used as a substitute for the exhaust gas sensor,
wherein NOx concentration in the exhaust gas feedstream is
determined based upon engine operating conditions monitored using
other sensing devices (not shown). The engine 10 is preferably
equipped with a mass air flow sensor (not shown) to measure intake
air flow, and thus exhaust mass air flow. Alternatively or in
combination, an algorithm can be executed to determine mass air
flow through the engine 10 based upon engine rotational speed,
displacement, and volumetric efficiency.
[0015] The control system includes the control module 5 that is
signally connected to a plurality of sensing devices operative to
monitor the engine 10, the exhaust gas feedstream, and the exhaust
aftertreatment system 45. The control module 5 is operatively
connected to actuators of the engine 10 and the exhaust
aftertreatment system 45, many of which are not shown. The control
system executes control schemes, preferably comprising a set of
control algorithms in the control module 5, to control the engine
10 and the exhaust aftertreatment system 45. In operation the
control system monitors operation of the internal combustion engine
10 and the exhaust aftertreatment system 45 and controls a
reductant dispensing device 55 as described herein.
[0016] The control module 5 preferably comprises a general-purpose
digital computer including a microprocessor or central processing
unit, storage mediums comprising non-volatile memory including read
only memory and electrically programmable read only memory, random
access memory, a high speed clock, analog to digital conversion
circuitry and digital to analog circuitry, and input/output
circuitry and devices, and appropriate signal conditioning and
buffer circuitry. The control module 5 executes the control
algorithms to control operation of the engine 10. The control
algorithms comprise resident program instructions and calibrations
stored in the non-volatile memory which are executed to provide the
desired functions. The algorithms are executed during preset loop
cycles such that each algorithm is executed at least once each loop
cycle. Algorithms are executed by the central processing unit to
monitor inputs from the aforementioned sensing devices and execute
control routines and diagnostic routines to control and monitor
operation of the engine 10, the aftertreatment system 45, and the
actuators, including using preset calibrations. Loop cycles are
executed at regular intervals, for example each 3.125, 6.25, 12.5,
25 and 100 milliseconds during ongoing engine and vehicle
operation. Alternatively, algorithms may be executed in response to
occurrence of an event. The engine 10 is controlled to operate at a
preferred air-fuel ratio to achieve performance parameters related
to operator requests, fuel consumption, emissions, and
driveability, with the intake air flow controlled to achieve the
preferred air-fuel ratio.
[0017] The exhaust aftertreatment system 45 is fluidly coupled to
an exhaust manifold of the engine 10 to manage and treat the
exhaust gas feedstream. The exhaust aftertreatment system 45
comprises a plurality of aftertreatment devices fluidly connected
in series. Preferably, the exhaust aftertreatment system 45
includes a first selective catalytic reactor device fluidly
connected in series with a second selective catalytic reactor
device. In one embodiment, shown in FIG. 1, there are first,
second, third and fourth aftertreatment devices 50, 60, 70 and 80.
In the embodiment shown in FIG. 1, the first aftertreatment device
50 comprises an oxidation catalyst, the second aftertreatment
device 60 comprises the first selective catalytic reactor device,
the third aftertreatment device 70 comprises the second selective
catalytic reactor device, and the fourth aftertreatment device 80
comprises a catalyzed particulate filter, although the concepts
described herein are not so limited. The first, second, third and
fourth aftertreatment devices 50, 60, 70 and 80 can be assembled
into individual structures that are fluidly connected and assembled
in an engine compartment and a vehicle underbody with one or more
sensing devices placed therebetween. Alternatively, the first and
second aftertreatment devices 50 and 60 can be assembled into a
first structure and the third and fourth aftertreatment devices 70
and 80 can be placed into a second structure. One having ordinary
skill in the art can conceive of other assembly configurations.
[0018] A first sensing device 52 is upstream of the second
aftertreatment device 60, preferably operative to monitor
temperature of the exhaust gas feedstream downstream of the first
aftertreatment device 50. The first sensing device 52 generates a
signal correlatable to temperature of the exhaust gas feedstream
entering the second aftertreatment device 60, i.e., entering the
first selective catalytic reactor device.
[0019] A second sensing device 62 is upstream of the third
aftertreatment device 70, preferably operative to monitor
constituent elements of the exhaust gas feedstream, e.g., NOx
concentration. The second sensing device 62 generates a signal
correlatable to NOx concentration of the exhaust gas feedstream, or
another parameter (e.g., NH.sub.3). Alternatively, the second
sensing device 62 can comprise a virtual sensing device executed as
an algorithm in the control module 5 and used as a substitute for
the exhaust gas sensor, wherein NOx concentration in the exhaust
gas feedstream is determined based upon engine operating conditions
monitored using other sensing devices (not shown).
[0020] There is a third sensing device 72 downstream of the third
aftertreatment device 70, preferably operative to monitor
constituent elements of the exhaust gas feedstream, e.g., NOx
concentration. The third sensing device 72 generates a signal
correlatable to NOx concentration or another exhaust gas
constituent in the exhaust gas feedstream. Each of the first,
second, and third sensing devices 52, 62, and 72 are signally
connected to the control module 5.
[0021] The exhaust aftertreatment system 45 includes the reductant
dispensing device 55 having an dispensing mechanism and a nozzle
(not shown) that are fluidly connected to a reductant supply system
(Reductant Supply) 57 that preferably contains an ethanol-based
reductant. In one embodiment, the reductant supply system can
include a reservoir (not shown) that stores reductant materials for
dispensing into the exhaust gas feedstream via the reductant
dispensing device 55. In one embodiment, supply of the
ethanol-based reductant can originate with a fuel tank (not shown)
that contains ethanol-based fuel for powering the internal
combustion engine 10. The nozzle of the reductant dispensing device
55 is inserted into the exhaust system 45 upstream of the second
aftertreatment device 60, i.e., upstream of the first selective
catalytic reactor device. The reductant dispensing device 55 is
controlled by the control module 5 to dispense a mass flowrate of
the ethanol-based reductant into the exhaust gas feedstream.
Alternatively, in a system wherein the first selective catalytic
reactor device is directly fluidly connected to the engine 10
without an intervening oxidation catalytic device, the reductant
dispensing device 55 can be omitted and the ethanol-based reductant
can be dispensed by controlling engine fuel injectors to inject
fuel into the combustion chambers during the exhaust stroke of the
engine cycle, none of which is shown.
[0022] Each of the exhaust aftertreatment devices 50, 60, 70, and
80 comprises a device which employs technologies having various
capabilities for treating the constituent elements of the exhaust
gas feedstream, including oxidation, selective catalytic reduction,
reductant dosing, and particulate filtering. Design features for
each of the aftertreatment devices 50, 60, 70, and 80, e.g.,
volume, space velocity, cell density, washcoat density, and metal
loading can be determined for specific applications and are
determinable by a person having ordinary skill in the art. The
aftertreatment devices 50, 60, 70, and 80 are fluidly connected in
series using known pipes and connectors.
[0023] The first aftertreatment device 50 preferably comprises an
oxidation catalytic device, preferably comprising a cordierite
substrate including a multiplicity of flow-through channels coated
with an alumina-based washcoat containing one or more
platinum-group metals, e.g., platinum or palladium. In one
embodiment (not shown) the first aftertreatment device 50 may be
omitted.
[0024] The second aftertreatment device 60 comprises the first,
ethanol-selective catalytic reactor device in one embodiment,
preferably a cordierite substrate including a multiplicity of
flow-through channels coated with a washcoat. The preferred
washcoat uses silver-alumina (Ag--Al) as the catalytic material and
comprises 3 wt. % Ag.sub.2O supported on alumina in one
embodiment.
[0025] The third aftertreatment device 70 comprises the second,
ammonia-selective catalytic reactor device in one embodiment,
preferably a cordierite substrate including a multiplicity of
flow-through channels coated with a washcoat. The preferred
washcoat comprises a zeolite-based washcoat containing one or more
metals, e.g., iron (Fe), copper (Cu), vanadium (V), tungsten (W)
and titanium (Ti).
[0026] The fourth aftertreatment device 80 preferably comprises an
ammonia slip catalytic device, preferably a cordierite substrate
including a multiplicity of flow-through channels coated with an
alumina-based washcoat containing one or more platinum-group
metals, e.g., Pt, Pd, and Rh, operative to oxidize ammonia and
other exhaust gas feedstream constituents. The fourth
aftertreatment device 80 can comprise a second oxidation catalyst
combined with a particulate filter. It is understood the fourth
aftertreatment device 80 can include, singly or in combination,
other exhaust aftertreatment devices, including catalyzed or
uncatalyzed particulate filters, air pumps, external heating
devices, sulfur traps, phosphorous traps, selective reduction
devices, and others, according to specifications and operating
characteristics of a specific application.
[0027] The preferred reductant comprises an ethanol-based
reductant, which includes partially oxidized hydrocarbons, e.g.,
alcohols. The ethanol-based reductant can reduce NOx emissions in
the presence of Ag/Al.sub.2O.sub.3 at low exhaust gas feedstream
temperatures. The ethanol-SCR can also convert some of the NOx
emissions into ammonia. The ethanol-based reductant can be obtained
from on-board fuels, e.g., ethanol-containing fuels such as E85
which is an ethanol/gasoline mixture containing 70-85% ethanol.
[0028] The control system preferentially operates the internal
combustion engine 10 at an air/fuel ratio that is lean of
stoichiometry while monitoring operation of the internal combustion
engine and the exhaust gas feedstream. The monitored parameters of
the exhaust gas feedstream preferably comprise an inlet temperature
to the first selective catalytic reactor device, a mass flow rate
of the exhaust gas feedstream, and NOx and ammonia and oxygen
concentrations in the exhaust gas feedstream. The parameters are
preferably used by the control system to calculate an optimum
HC.sub.1/NO.sub.x ratio for the NO.sub.x reduction under specific
operating conditions. The HC.sub.1/NO.sub.x ratio is defined as an
amount of dispensed fuel on a C.sub.1 basis divided by the inlet
NO.sub.x concentration. By way of example, a reductant comprising 1
part per million (ppm) evaporated diesel fuel has approximately 14
carbon atoms. Thus a HC.sub.1:NO.sub.x ratio of 10:1 with 100 ppm
inlet NO.sub.x in the exhaust feedstream requires dispensing of
10.times.100/14=71 ppm diesel fuel. By way of example, a reductant
comprising 1 ppm evaporated ethanol has 2 carbon atoms. Thus a
HC.sub.1:NO.sub.x ratio of 10 with 100 ppm inlet NO.sub.x in the
exhaust feedstream requires dispensing of 10.times.100/2=500 ppm
ethanol. A preferred ratio of HC.sub.1:NO.sub.x in the exhaust gas
feedstream can be determined that achieves a preferred
concentration of ammonia downstream of the first selective
catalytic reactor device, i.e., the second aftertreatment device
60. The preferred ratio of HC1:NOx is determined based upon the
operation of the internal combustion engine 10 and the exhaust gas
feedstream. The reductant dispensing device 55 is controlled by the
control module 5 to dispense a mass flowrate of the ethanol-based
reductant into the exhaust gas feedstream upstream of the first
selective catalytic reactor device that corresponds to the
preferred ratio of HC.sub.1:NO.sub.x in the exhaust gas feedstream.
Thus there can be real-time and simultaneous NOx reduction over the
first and second selective catalytic reactor devices, i.e., the
second and third aftertreatment devices 60 and 70. The NOx
reduction can occur under lean exhaust conditions without operating
with a rich exhaust gas feedstream to generate the ammonia and
without the use of platinum-group metals. Preferably, there is
sufficient NOx reduction in the first selective catalytic reactor
device, i.e., the second aftertreatment device 60, such that there
are similar amounts of NOx and ammonia exiting the first selective
catalytic reactor device comprising the silver-alumina catalytic
reactor device. The preferred ammonia:NOx ratio exiting the first
selective catalytic reactor device is preferably near a
stoichiometric ammonia:NOx ratio to achieve NOx reduction with
minimal or no ammonia slip.
[0029] FIGS. 2-9 illustrate data graphs that depict results
indicative of operating an embodiment of the engine and
aftertreatment system constructed in accordance with the system
depicted and described with reference to FIG. 1. The embodiment of
the aftertreatment system 45 included a first selective catalytic
reactor device comprising an ethanol-selective catalytic reactor
device using silver alumina as the catalytic material and
comprising 3 wt. % Ag.sub.2O supported on alumina. The second
selective catalytic reactor device comprised an ammonia-selective
catalytic reactor device using Cu-zeolite as the catalytic
material. The catalytic material was supported on a 400 cell per
square inch cordierite monolith substrate. The selective catalytic
reactor devices were hydro-thermally aged employing air and 10%
water at 650.degree. C. for 16 hours prior to testing. The data
graphs comprise results achieved by selectively dispensing the
ethanol-based reductant upstream of the first selective catalytic
reactor device and by selectively dispensing a simulated diesel
fuel reductant upstream of the first selective catalytic reactor
device. The results described in the data graphs were developed
using a laboratory reactor to flow a simulated exhaust gas
feedstream through the first and second selective catalytic reactor
devices. The aftertreatment system was instrumented with
appropriate sensors, including a magneto-pneumatic exhaust gas
analyzer to determine O.sub.2 concentration in the exhaust, a
Fourier transform infrared spectrometer to determine NOx and
ammonia concentration levels entering and exiting the first and
second selective catalytic reactor devices, and a flow meter to
determine exhaust flow rate translatable to catalyst space velocity
(SV). Space velocity represents a rate of feed of exhaust gas, in
volume, per unit volume of one of the first and second selective
catalytic reactor devices, and has a unit of inverse hour
(h.sup.-1).
[0030] Baseline laboratory conditions included the following
standard gases in the simulated exhaust feedstream: 6% O.sub.2, 5%
CO.sub.2, 5% H.sub.2O, 750 ppm CO, 250 ppm H.sub.2, and 400 ppm NO.
Ethanol and a simulated diesel fuel mixture consisting of a
volumetric mixture of n-dodecane (67 vol. %, long-chain alkane) and
m-xylene (33 vol. %, aromatic) were used as the NOx reductants. The
effect of space velocity and concentration effects of O.sub.2, NOx,
and HC were evaluated as a function of catalyst inlet
temperature.
[0031] Table I shows numerical values of catalyst inlet
temperature, NOx concentration, and space velocity that correspond
to labeled low and high states for the experimental results
described with reference to FIGS. 2-9 that were conducted with
ethanol as the reductant. The numerical values are
approximations.
TABLE-US-00001 TABLE I Low High Temperature, .degree. C.
300-375.degree. C. 375-450.degree. C. Inlet NOx, ppm 100-250 ppm
250-400 ppm Space Velocity, 1/h 5,000-12,000 1/h 12,000-20,000
1/h
[0032] The preferred inlet concentration of ethanol to the first
selective catalytic reactor device, expressed as the catalyst inlet
HC.sub.1:NOx ratio needed to achieve equal (i.e. stoichiometric)
concentrations of NOx and ammonia at the exit of the first
selective catalytic reactor device was then determined using a
known model at combinations of low, medium, and high values of
these variables. An exemplary operating strategy for controlling
the inlet concentration of ethanol-based reductant to the first
selective catalytic reactor device was constructed. This comprises
a preferred range of HC.sub.1:NOx concentration for low and high
ranges of the variables. For example, the range of low temperature
operation is defined to be between 300 and 375.degree. C., and the
range of high temperature operation is defined to be between 375
and 450.degree. C. for the embodiment. This exemplary operating
strategy is shown in Tables IIA and IIB.
[0033] Tables IIA and IIB show preferred ranges of HC.sub.1/NOx to
achieve equal (i.e. stoichiometric) NOx and ammonia concentrations
after the first selective catalytic reactor device for combinations
of low and high ranges of exhaust gas temperature, inlet NOx
concentration, and space velocity of the first selective catalytic
reactor device for the embodiment, as described with reference to
Table I.
TABLE-US-00002 TABLE IIA Temperature Low High High High Inlet NOx
Low Low High Low Space Velocity Low Low Low High HC.sub.1:NOx
5.0-9.2 5.0-14.7 4.2-7.6 5.0-12.7
TABLE-US-00003 TABLE IIB Temperature Low Low Low High Inlet NOx Low
High High High Space Velocity High Low High High HC.sub.1:NOx
6.5-10.7 4.5-6.6 4.2-6.7 5.0-6.9
[0034] For example, when the inlet temperature of the first
selective catalytic reactor device is in the range of 300 to
375.degree. C., the inlet NOx concentration is in the range 100 to
250 ppm, and the space velocity is in the range 5000 to 12,000
h.sup.-1, the range of HC.sub.1:NOx needed to achieve equal (i.e.
stoichiometric) concentrations of NOx and ammonia after the first
selective catalytic reactor device is 5.0:1 to 9.2:1. These high
and low ranges can be further subdivided as needed to determine the
preferred HC.sub.1:NOx over as narrow a variable range as required
for effective control and operation of an embodiment of the engine
10 and aftertreatment system 45 that is constructed to operate in
accordance with the system described herein.
[0035] FIG. 2 graphically illustrates test results showing NOx
conversion (%) across the first selective catalytic reactor device,
i.e., the Ag--Al catalyst, plotted as a function of average
catalyst temperature (C) wherein SV of the first selective
catalytic reactor device was 12,750 h.sup.-1 with an exhaust gas
feedstream comprising 10% oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm
CO, 250 ppm H.sub.2, and 100 ppm NO. A reductant flow rate of 431
ppm of the ethanol generates an HC.sub.1:NOx of approximately 8.6
for light-duty vehicle application test conditions. Additionally,
an exhaust gas feedstream is shown wherein SV was 25,500 h.sup.-1
comprising 6% oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm
H.sub.2, 400 ppm NO. A reductant flow rate of 1724 ppm of the
ethanol provides a range of HC.sub.1:NOx of approximately 8.6 for
heavy-duty vehicle application test conditions.
[0036] FIG. 3 graphically illustrates test results showing NOx
conversion (%) across the first selective catalytic reactor device,
plotted as a function of average catalyst temperature (C) wherein
SV was 12,750 h.sup.-1 with an exhaust gas feedstream comprising
10% oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm H.sub.2,
and 100 ppm NO. A reductant flow rate of 79 ppm of the simulated
diesel fuel provides a HC.sub.1:NOx ratio of approximately 8.4 for
light-duty vehicle application test conditions. Additionally, an
exhaust gas feedstream wherein SV was 25,500 h.sup.-1 comprising 6%
oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm H.sub.2, and
400 ppm NO. A reductant flow rate of 316 ppm of the simulated
diesel fuel reductant provides a HC.sub.1:NOx ratio of
approximately 8.4 for heavy-duty vehicle application test
conditions.
[0037] FIG. 4 graphically illustrates test results showing the
production of CH.sub.3CHO, NH.sub.3, HCN, N.sub.2O, and N.sub.2
{N.sub.2(calc)=[.sup.inNO-.sup.out(NO+NO.sub.2+NH.sub.3+HCN+2.times.N.sub-
.2O)]/2} (ppm) across the first selective catalytic reactor device,
plotted as a function of average catalyst temperature (C) wherein
SV was 25,500 h.sup.-1 with an exhaust gas feedstream comprising 6%
oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm H.sub.2, and
400 ppm NO. A reductant flow rate of 1724 ppm of the ethanol
reductant provides a HC.sub.1:NOx ratio of approximately 8.6.
[0038] FIG. 5 graphically illustrates test results showing the
production of CH.sub.3CHO, NH.sub.3, HCN, N.sub.2O, and N.sub.2
{N.sub.2(calc)=[.sup.inNO-.sup.out(NO++NO.sub.2+NH.sub.3+HCN+2.times.N.su-
b.2O)]/2} (ppm) across the first selective catalytic reactor
device, plotted as a function of average catalyst temperature (C)
wherein SV was 25,500 h.sup.-l with an exhaust gas feedstream
comprising 6% oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm
H.sub.2, and 400 ppm NO. A reductant flow rate of 316 ppm of
simulated diesel fuel reductant provides a HC.sub.1:NOx ratio of
approximately 8.4.
[0039] FIG. 6 graphically illustrates test results showing the
production of CH.sub.3CHO, NH.sub.3, HCN, N.sub.2O, and N.sub.2
{N.sub.2(calc)=[.sup.inNO-.sup.out(NO+NO.sub.2+NH.sub.3+HCN+2.times.N.sub-
.2O)]/2} (ppm) across the first selective catalytic reactor device,
plotted as a function of average catalyst temperature (C) wherein
SV was 12,750 h.sup.1 with an exhaust gas feedstream comprising 10%
oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm H.sub.2, and
100 ppm NO. A reductant flow rate of 431 ppm of the ethanol
reductant provides a HC.sub.1:NOx ratio of approximately 8.6.
[0040] FIG. 7 graphically illustrates test results showing the
breakthrough of NOx (comprising NO+NO.sub.2) and NH.sub.3 (ppm)
across the first selective catalytic reactor device, plotted as a
function of average catalyst temperature (C) wherein SV was 25,500
h.sup.-1 with an exhaust gas feedstream comprising 6% oxygen, 5%
H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm H.sub.2, and 400 ppm NO.
Reductant flow rates of variable amounts of the ethanol-based
reductant provide HC.sub.1:NOx ratios of 2.2, 4.3, and 8.6. The
hash-marked area (M) indicates the desired concentrations of NOx
and NH.sub.3 (ppm) entering the downstream ammonia-SCR catalyst,
which is obtainable with a HC.sub.1:NOx ratio between 2.2 and
4.3.
[0041] FIG. 8 graphically illustrates test results showing NOx
conversion (%) across the first selective catalytic reactor device
(HC SCR only) plotted as a function of average catalyst temperature
(C) wherein SV was 25,500 h.sup.-1 with an exhaust gas feedstream
comprising 6% oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm
H.sub.2 and 400 ppm NO. A reductant flow rate of 1724 ppm of the
ethanol reductant provides a HC.sub.1:NOx ratio of approximately
8.6. Additionally, test results show NOx conversion (%) across the
first and second selective catalytic reactor devices
(HC+NH.sub.3SCR) plotted as a function of average catalyst
temperature (C) wherein SV was 25,500 h.sup.-1 across the first
selective catalytic reactor device and 60,000 h.sup.-1 across the
second selective catalytic reactor device with an exhaust gas
feedstream comprising 6% oxygen, 5% H.sub.2O, 5% CO.sub.2, 750 ppm
CO, 250 ppm H.sub.2 and 400 ppm NO. Reductant flow rates of
variable amounts of the ethanol-based reductant providing
equivalent NOx conversion levels to that observed across the first
selective catalytic reactor device are shown.
[0042] FIG. 9 graphically shows test results showing the production
of CH.sub.3CHO and NH.sub.3 (ppm) across the first selective
catalytic reactor device (HC SCR only) plotted as a function of
average catalyst temperature (C) wherein SV was 25,500 h.sup.-1
with an exhaust gas feedstream comprising 6% oxygen, 5% H.sub.2O,
5% CO.sub.2, 750 ppm CO, 250 ppm H.sub.2 and 400 ppm NO. A
reductant flow rate of 1724 ppm of the ethanol reductant provides a
HC.sub.1:NOx ratio of approximately 8.6. Additionally, test results
show the production of CH.sub.3CHO and NH.sub.3 (ppm) across the
first and selective catalytic reactor devices (HC+NH.sub.3SCR)
plotted as a function of average catalyst temperature (C) wherein
SV was 25,500 h.sup.-1 across the first selective catalytic reactor
device and 60,000 h.sup.-1 across the second selective catalytic
reactor device with an exhaust gas feedstream comprising 6% oxygen,
5% H.sub.2O, 5% CO.sub.2, 750 ppm CO, 250 ppm H.sub.2 and 400 ppm
NO. Reductant flow rates of variable amounts of the ethanol-based
reductant provide equivalent NOx conversion levels to that observed
across the first selective catalytic reactor device are shown.
[0043] Results of the data presented with reference to FIGS. 2-9
indicate that NOx conversion for the exemplary first selective
catalytic reactor device is affected by the type of reductant and
the conditions under which the catalyst is tested. Under operating
conditions for a light-duty vehicle application (i.e., lower SV,
lower NO concentration, higher O.sub.2 concentration) good low
temperature (250-400.degree. C.) NOx conversion performance is
observed for both diesel fuel and ethanol as reductants. However,
under operating conditions for a heavy-duty vehicle application
(i.e., higher SV, higher NO concentration, lower O.sub.2
concentration) only the ethanol-based reductant shows a wide
temperature window for NOx conversion efficiency. However,
preferred catalyst performance limits the production of undesired
reaction by-products other than the desired product, nitrogen
(N.sub.2). Under heavy-duty test conditions using an ethanol-based
reductant (See FIG. 4) and diesel as a reductant (See FIG. 5), the
production of ammonia (NH.sub.3) and acetaldehyde (CH.sub.3CHO) is
significant, particularly when ethanol is used as the reductant.
While lesser amounts of ammonia are formed when diesel fuel is used
as the reductant, as shown in FIG. 5, and when ethanol is used as
the reductant under operating conditions for a light-duty vehicle
application, as shown in FIG. 6, the control scheme described
herein still apply.
[0044] A further embodiment can include dispensing a mass of
hydrogen into the exhaust gas feedstream upstream of the first
selective catalytic reactor device. A further embodiment can
include controlling oxygen in the exhaust gas feedstream upstream
of the first selective catalytic reactor device.
[0045] Operating embodiments of the engine 10 and exhaust
aftertreatment system 45 configured as described according to the
control schemes as described can result in improved selectivity
toward the desired reaction product, i.e., nitrogen. Results
related to operating the engine and exhaust aftertreatment system
provide design considerations for determining a preferred volume of
the first, silver-alumina catalytic reactor device based upon
maximum engine airflow and corresponding space velocity related to
the maximum engine airflow. Usage of the ethanol-based reductant
can be minimized for a selected engine and exhaust aftertreatment
system.
[0046] 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.
* * * * *