U.S. patent application number 11/098367 was filed with the patent office on 2005-10-13 for bypass controlled regeneration of nox adsorbers.
Invention is credited to Ancimer, Richard, Harris, Jonathan M.S., Lebastard, Olivier, Nedelcu, Costi.
Application Number | 20050223699 11/098367 |
Document ID | / |
Family ID | 32070531 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050223699 |
Kind Code |
A1 |
Ancimer, Richard ; et
al. |
October 13, 2005 |
Bypass controlled regeneration of NOx adsorbers
Abstract
In a method and apparatus for regenerating a lean NOx adsorber,
the NOx adsorber treats exhaust gases created during the combustion
of gaseous fuels in general. A bypass line maintains a target
regeneration flow of exhaust gas through the NOx adsorber during
regeneration regardless of operating demands on the engine.
Closed-loop and open-loop control are employed. The closed-loop
control employs sensors that determine properties of the exhaust
gas during regeneration, and the controller uses those properties
to provide an efficient regeneration cycle. A regeneration map is
also provided that uses creation of in-cylinder regeneration
conditions for the exhaust gas in combination with in-line
regeneration conditions for the exhaust gas.
Inventors: |
Ancimer, Richard;
(Vancouver, CA) ; Lebastard, Olivier; (Burnaby,
CA) ; Harris, Jonathan M.S.; (Vancouver, CA) ;
Nedelcu, Costi; (North Vancouver, CA) |
Correspondence
Address: |
MCANDREWS HELD & MALLOY, LTD
500 WEST MADISON STREET
SUITE 3400
CHICAGO
IL
60661
|
Family ID: |
32070531 |
Appl. No.: |
11/098367 |
Filed: |
April 4, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11098367 |
Apr 4, 2005 |
|
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PCT/CA03/01467 |
Oct 2, 2003 |
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Current U.S.
Class: |
60/286 ;
60/287 |
Current CPC
Class: |
B01J 38/04 20130101;
F01N 13/011 20140603; B01D 2251/208 20130101; B01J 38/10 20130101;
F01N 11/002 20130101; F01N 2560/026 20130101; Y02T 10/40 20130101;
F01N 2250/02 20130101; F01N 3/0871 20130101; B01D 2251/202
20130101; F01N 3/0878 20130101; F01N 2610/04 20130101; F01N 2240/36
20130101; F01N 2410/04 20130101; F01N 13/0093 20140601; F01N 3/2033
20130101; B01D 53/92 20130101; F01N 3/0821 20130101; F01N 2610/03
20130101; Y02T 10/22 20130101; B01D 53/8612 20130101; F01N 3/0814
20130101; B01D 53/9454 20130101; B01D 53/9481 20130101; F01N 3/106
20130101; F02D 21/08 20130101; B01D 53/96 20130101; Y02T 10/47
20130101; F01N 9/00 20130101; F01N 11/007 20130101; Y02T 10/12
20130101; F01N 3/035 20130101; F01N 2240/30 20130101; Y02T 10/26
20130101; F01N 13/009 20140601 |
Class at
Publication: |
060/286 ;
060/287 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2002 |
CA |
2,406,386 |
Mar 14, 2003 |
CA |
2,422,188 |
Claims
What is claimed is:
1. A method of regenerating a lean NOx adsorber, said lean NOx
adsorber used to remove NOx from exhaust gas generated by
combustion of a fuel in a combustion chamber of an operating
internal combustion engine, said method comprising: (a) determining
a target regeneration flow of said exhaust gas through said lean
NOx adsorber; (b) directing a regeneration flow of said exhaust gas
through said lean NOx adsorber, said regeneration flow established
by one of: bypassing a bypass flow of said exhaust gas around said
lean NOx adsorber when said target regeneration flow is less than
exhaust gas flow from said engine, resulting in said regeneration
flow being substantially the same as said target regeneration flow,
and directing substantially all of said exhaust gas through said
lean NOx adsorber when said target regeneration flow is greater
than said exhaust gas flow from said engine, said exhaust gas flow
from said engine and said bypass flow established by reference to
at least one of: (1) engine speed, (2) engine load, (3) engine
intake manifold temperature, (4) intake air mass flow, (5) engine
inlet fuel flow, (6) engine intake manifold pressure, (7) measured
engine exhaust gas flow, (8) exhaust gas temperature, and (9)
exhaust gas pressure; (c) reacting, within said exhaust gas and
upstream of said lean NOx adsorber, a reductant to maintain a
lambda of said regeneration flow of less than 1 across said lean
NOx adsorber.
2. The method of claim 1 wherein said target regeneration flow is
determined based, at least in part, on an NOx concentration in said
exhaust gas.
3. The method of claim 1 wherein said reductant is hydrogen.
4. The method of claim 1 wherein said reductant is a
hydrocarbon.
5. The method of claim 4 wherein said hydrocarbon comprises
methane.
6. The method of claim 5 further comprising introducing hydrogen
into said regeneration flow by reforming said methane within said
exhaust gas upstream of said lean NOx adsorber.
7. The method of claim 1 wherein said fuel and said reductant are
interchangeable.
8. The method of claim 1 wherein said bypass flow is directed
through a second lean NOx adsorber.
9. A method of operating an internal combustion engine equipped
with an aftertreatment system for removing NOx from exhaust gas
generated by combustion of a fuel in at least one combustion
chamber of said engine, said method comprising: during normal
operation of said engine, directing all of said exhaust gas through
a lean NOx adsorber; periodically regenerating said lean NOx
adsorber by a regeneration cycle defined by a regeneration cycle
start time and a regeneration cycle end time, during said
regeneration cycle: (a) determining a target regeneration flow of
said exhaust gas through said lean NOx adsorber, (b) directing a
regeneration flow of said exhaust gas through said lean NOx
adsorber, said regeneration flow established by one of: bypassing a
bypass flow of said exhaust gas around said lean NOx adsorber when
said target regeneration flow is less than exhaust gas flow from
said engine, resulting in said regeneration flow being
substantially the same as said target regeneration flow, and
directing substantially all of said exhaust gas through said lean
NOx adsorber when said target regeneration flow is greater than
said exhaust gas flow from said engine, said exhaust gas flow from
said engine and said bypass flow determined by reference to at
least one of: (1) engine speed, (2) engine load, (3) engine intake
manifold temperature, (4) intake air mass flow, (5) engine inlet
fuel flow, (6) engine intake manifold pressure, (7) measured engine
exhaust gas flow, (8) exhaust gas temperature, (9) exhaust gas
pressure; (c) reacting, within the exhaust gas and upstream of the
lean NOx adsorber, a reductant to maintain a lambda of said
regeneration flow of less than 1 across the lean NOx adsorber.
10. The method of claim 9 wherein said reductant is hydrogen.
11. The method of claim 9 wherein said reductant is a
hydrocarbon.
12. The method of claim 11 wherein said hydrocarbon comprises
methane.
13. The method of claim 12 further comprising introducing hydrogen
into said regeneration flow by reforming said methane within said
exhaust gas upstream of said lean NOx adsorber.
14. The method of claim 9 wherein said fuel and said reductant are
interchangeable.
15. The method of claim 12 wherein reacting of said hydrocarbon
occurs within said exhaust gas prior to directing said bypass flow
around said lean NOx adsorber.
16. The method of claim 12 wherein reacting of said hydrocarbon
occurs within said regeneration flow.
17. The method of claim 12 wherein said hydrocarbon is directed
into said exhaust gas by at least one of a valve and an
injector.
18. The method of claim 9 wherein said regeneration cycle end time
is determined when said lambda of said regeneration flow downstream
of said lean NOx adsorber is below a pre-determined threshold
concentration.
19. The method of claim 9 wherein said regeneration cycle end time
is determined when a concentration of said reductant downstream of
said lean NOx adsorber is above a pre-determined threshold
concentration.
20. The method of claim 13 wherein said regeneration cycle end time
is determined when a concentration of at least one of CO or H.sub.2
downstream of said lean NOx adsorber is above a pre-determined
threshold concentration.
21. The method of claim 9 wherein said regeneration flow is
controlled by at least one valve.
22. The method of claim 9 wherein said regeneration flow is
controlled by a bypass valve in a bypass line and an exhaust valve
in an exhaust line.
23. The method of claim 22 wherein said bypass valve is a variable
control valve.
24. The method of claim 23 wherein said exhaust valve is a variable
control valve.
25. The method of claim 9 wherein said regeneration cycle start
time is determined when a NOx concentration within said exhaust gas
downstream of said lean NOx adsorber is in excess of a threshold
concentration as compared to a NOx concentration out of said
engine.
26. The method of claim 12 further comprising, when operating said
engine in a predefined low load, low speed mode, wherein said
lambda of said exhaust gas is less than one as a result of
combustion of said fuel within said combustion chamber.
27. The method of claim 26 wherein said engine is a direct
injection engine.
28. The method of claim 9 further comprising during said
regeneration cycle directing said exhaust gas downstream of said
lean NOx adsorber through a clean-up catalyst.
29. The method of claim 28 wherein said clean-up catalyst removes
NOx from said exhaust gas.
30. The method of claim 28 wherein said clean-up catalyst removes
reductant from said exhaust gas.
31. The method of claim 28 wherein said clean-up catalyst removes
hydrogen sulfide from said exhaust gas.
32. The method of claim 9 further comprising directing said exhaust
gas through a particulate filter upstream of said lean NOx
adsorber.
33. The method of claim 9 wherein said bypass flow is directed
through a second lean NOx adsorber.
34. A method of operating a lean burn internal combustion engine
equipped with an aftertreatment system for removing NOx from
exhaust gas generated by combustion of a fuel in at least one
combustion chamber of said engine, said method comprising: (a)
during normal operation of said engine, directing all of said
exhaust gas, which results from combustion of a lean fuel mixture,
through a lean NOx adsorber; (b) periodically regenerating said
lean NOx adsorber using a predetermined regeneration strategy
selected from one of a high load strategy, a midrange load strategy
and a low load strategy, said regeneration strategy causing said
exhaust gas pass through said lean NOx adsorber: during said high
load strategy: reacting, upstream of said lean NOx adsorber, a
reductant to maintain a lambda of said exhaust gas of less than 1
across said NOx adsorber, during said low load strategy:
transitioning from lean burn in said normal operation to rich burn
of said fuel in said combustion chamber to generate said exhaust
gas wherein said lambda of said exhaust gas is less than 1;
transitioning from lean burn in said normal operation to rich burn
of said fuel, with said combustion chamber in a rich environment,
to generate said exhaust gas wherein said lambda is less than 1,
and reacting, upstream of said lean NOx adsorber, a reductant.
35. An aftertreatment system for removing NOx from exhaust gas
produced during combustion of a fuel within a combustion chamber of
an operating internal combustion engine, said aftertreatment system
comprising: (a) an exhaust line for directing said exhaust gas from
said engine, (b) a lean NOx adsorber disposed in said exhaust line
for removing said NOx, (c) a regeneration catalyst disposed in said
exhaust line upstream of said lean NOx adsorber, said catalyst
capable of promoting oxidizing or reforming of a reductant, (d) a
reductant line for delivering said reductant from a reductant store
to said exhaust line upstream of said catalyst, (e) a reductant
flow control disposed in said reductant line for controlling flow
of said reductant into said exhaust line, (f) a bypass line for
directing said exhaust gas around said lean NOx adsorber, (g) a
second lean NOx adsorber, (h) at least one bypass flow control
capable of controlling flow of said exhaust gas through said bypass
line, (i) a controller, (j) at least one sensor providing control
information to said controller, said controller capable of
adjusting said at least one valve in response to said control
information.
36. The aftertreatment system of claim 35 wherein said reductant is
hydrogen.
37. The aftertreatment system of claim 35 wherein said reductant is
a gaseous hydrocarbon, said catalyst capable of reducing said
gaseous hydrocarbon to provide hydrogen with said exhaust gas.
38. The aftertreatment system of claim 35 wherein said catalyst is
a reformer in series with an oxidation catalyst.
39. The aftertreatment system of claim 35 wherein said catalyst is
a oxidation catalyst.
40. The aftertreatment system of claim 35 wherein said catalyst
comprises an oxidation catalyst combined with a reformer.
41. The aftertreatment system of claim 35 further comprising a
second close coupled catalyst proximate to said engine for
oxidizing said reductant when said exhaust gas proximate to said
regeneration catalyst is at a temperature below a predetermined
threshold temperature, said predetermined threshold temperature
below which said catalyst is unable to efficiently promote reaction
of said reductant.
42. The aftertreatment system of claim 35 further comprising an
injector for injecting said reductant into said exhaust line.
43. The aftertreatment system of claim 35 wherein said by-pass flow
control is a valve.
44. The aftertreatment system of claim 35 wherein said reductant
store is a fuel system of said engine.
45. The aftertreatment system of claim 35 further comprising a
particulate filter disposed in said exhaust line downstream of and
proximate to said regeneration catalyst.
46. The aftertreatment system of claim 35 further comprising a
second lean NOx adsorber in said bypass line.
47. The aftertreatment system of claim 46 further comprising a
second regeneration catalyst disposed in said bypass line upstream
of said second lean NOx adsorber.
48. The aftertreatment system of claim 35 further comprising a
clean-up catalyst disposed downstream of said lean NOx adsorber,
said clean-up catalyst capable of removing, from said exhaust gas,
at least one of: (a) said NOx, (b) said reductant, and (c) hydrogen
sulfide.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of International
Application No. PCT/CA2003/001467, having an international filing
date of Oct. 2, 2003, entitled "Bypass Controlled Regeneration Of
NOx Adsorbers". International Application No. PCT/CA2003/001467
claimed priority benefits, in turn, from Canadian Patent
Application No. 2,406,386 filed Oct. 2, 2002, and Canadian Patent
Application No. 2,422,188 filed Mar. 14, 2003. International
Application No. PCT/CA2003/001467 is also hereby incorporated by
reference herein in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to methods and apparatus for
regenerating NOx absorbers used in association with internal
combustion engines.
BACKGROUND OF THE INVENTION
[0003] Emissions controls for internal combustion engines are
becoming increasingly important in transportation and energy
applications. One class of pollutants of concern is oxides of
nitrogen (NOx). NOx form during combustion in internal combustion
engines.
[0004] One effective NOx treatment system is a lean NOx adsorber
(LNA), also known as NOx trap. LNA systems need to be periodically
regenerated. That is, over time, a reductant is needed to treat NOx
traps to permit further NOx removal to take place. It is desirable
to provide an efficient means of regeneration.
[0005] As discussed in, by way of example, PCT/International
Publication No. WO 00/76637, there are a variety of reductants
available for NOx trap regeneration. By way of example, many
hydrocarbons, carbon monoxide (CO) and hydrogen can be used as
reductants.
[0006] Hydrogen is especially effective as a reductant (see U.S.
Pat. No. 5,953,911). Also, hydrogen is advantageous in regard to
the emissions generated when hydrogen is used as a reductant since
the products are water and nitrogen. Other carbon-based reductants
such as CO can also be useful, however, carbon-based reductants
result in production of the greenhouse gas carbon dioxide.
[0007] Hydrogen is difficult to store and is generally not readily
available. However, hydrocarbons are readily available since
internal combustion engines typically use hydrocarbons as fuel. As
hydrocarbons comprise hydrogen atoms, they provide a possible
source of hydrogen. A hydrocarbon fuel may be passed through a
reformer to yield hydrogen.
[0008] Further, while hydrogen is an excellent reductant, any
regeneration process that takes advantage of hydrogen runs the risk
of expelling hydrogen with exhaust gas when regeneration is
complete. This is undesirable due to the flammability of hydrogen.
Also, regeneration using hydrogen from a hydrocarbon source
consumes a potential fuel. Therefore, improving regeneration
efficiency not only reduces expulsion of untreated NOx, it also
helps to reduce consumption of hydrocarbons otherwise available as
a fuel.
[0009] NOx emissions can also be reduced by managing combustion.
NOx emissions can be reduced by using certain gaseous fuels in
place of heavy hydrocarbons.
[0010] Examples of such fuels include natural gas, methane and
propane. Even with gaseous fuel, however, NOx emissions are not
insignificant.
[0011] Developments in gaseous combustion processes have sought to
address NOx emissions problems. Spark ignited gaseous fuel engines,
wherein a premixed charge of air and gaseous fuel is ignited with a
spark within the combustion chamber, have resulted in further
reductions of NOx. Also, high pressure directly injected gaseous
fuel, ignited by an ignition source such as a small quantity of a
more readily auto-ignitable pilot fuel introduced within the engine
combustion chamber, yields an improvement over diesel-fuelled
engines by reducing the emissions levels of NOx depending on the
gaseous fuel chosen. However some NOx is still generated in such
engines and therefore, it is desirable to reduce this
pollutant.
[0012] This invention provides an efficient means of regenerating
NOx adsorbers.
SUMMARY OF THE INVENTION
[0013] The invention is directed to an efficient method and
apparatus for regenerating lean NOx adsorbers. A method is
disclosed providing a bypass strategy for regenerating lean NOx
adsorbers efficiently.
[0014] A method is disclosed for regenerating a lean NOx adsorber
efficiently by providing an easily recognizable marker indicating
the completion of a regeneration cycle. This allows for real time
monitoring of regeneration or a closed-loop regeneration method. A
preferred method of regenerating a lean NOx adsorber that is used
to remove NOx from exhaust gas generated by combustion of a fuel in
a combustion chamber of an operating internal combustion engine
comprises:
[0015] (a) determining a target regeneration flow of the exhaust
gas through the lean NOx adsorber,
[0016] (b) directing a regeneration flow of the exhaust gas through
the lean NOx adsorber, the regeneration flow established by one of
either:
[0017] bypassing a bypass flow of the exhaust gas around the lean
NOx adsorber when the target regeneration flow is less than the
flow of the exhaust gas from the engine, resulting in the
regeneration flow being substantially the same as the target
regeneration flow, or
[0018] directing substantially all of the exhaust gas through the
lean NOx adsorber when the target regeneration flow is greater than
the exhaust gas flow from the engine; the flow of the exhaust gas
from the engine and the bypass flow are determined by reference to
at least one of:
[0019] (1) engine speed,
[0020] (2) engine load,
[0021] (3) engine intake manifold temperature,
[0022] (4) intake air mass flow,
[0023] (5) engine inlet fuel flow,
[0024] (6) engine intake manifold pressure,
[0025] (7) measured engine exhaust gas flow,
[0026] (8) exhaust gas temperature,
[0027] (9) exhaust gas pressure;
[0028] (c) reacting, within the exhaust gas and upstream of the
lean NOx adsorber, the first quantity of the reductant to maintain
a lambda of the regeneration flow of less than one across the lean
NOx adsorber.
[0029] The method can be practiced with the reductant being
hydrogen. The method can also be practiced with the reductant being
a hydrocarbon, and in a preferred example, the hydrocarbon is
methane. A further aspect of the method can comprise reforming a
second quantity of the hydrocarbon within the exhaust gas upstream
of the lean NOx adsorber to introduce hydrogen into the
regeneration flow.
[0030] In a preferred method the fuel that is burned in the engine
is the same as, or is interchangeable with, the reductant. In a
further embodiment, the bypass flow is directed through a second
lean NOx adsorber.
[0031] A method is also provided of operating an internal
combustion engine equipped with an aftertreatment system for
removing NOx from exhaust gas generated by combustion of a fuel in
at least one combustion chamber of the engine. The method comprises
directing all of the exhaust gas through a lean NOx adsorber during
normal operation of the engine, and periodically regenerating the
lean NOx adsorber during a regeneration cycle defined by a
regeneration cycle start time and a regeneration cycle end time.
The regeneration cycle includes:
[0032] (a) determining a target regeneration flow of the exhaust
gas through the lean NOx adsorber,
[0033] (b) directing a regeneration flow of the exhaust gas through
the lean NOx adsorber, the regeneration flow established by one of
either:
[0034] bypassing a bypass flow of the exhaust gas around the lean
NOx adsorber when the target regeneration flow is less than the
flow of the exhaust gas from the engine, resulting in the
regeneration flow being substantially the same as the target
regeneration flow, and
[0035] directing substantially all of the exhaust gas through the
lean NOx adsorber when the target regeneration flow is greater than
the flow of the exhaust gas from the engine. the flow of the
exhaust gas from the engine and the bypass flow are determined by
reference to at least one of:
[0036] (1) engine speed,
[0037] (2) engine load,
[0038] (3) engine intake manifold temperature,
[0039] (4) intake air mass flow,
[0040] (5) engine inlet fuel flow,
[0041] (6) engine intake manifold pressure,
[0042] (7) measured engine exhaust gas flow,
[0043] (8) exhaust gas temperature,
[0044] (9) exhaust gas pressure,
[0045] (c) reacting, within the exhaust gas and upstream of the
lean NOx adsorber, a reductant to maintain a lambda of the
regeneration flow of less than one across the lean NOx
adsorber.
[0046] In a preferred method the fuel is the same as, or is
interchangeable with, the reductant.
[0047] A further aspect of this method comprises introducing
hydrogen into the regeneration flow by reforming a second quantity
of the hydrocarbon within the exhaust gas upstream of the lean NOx
adsorber. In a preferred example, the hydrocarbon is methane. In a
further embodiment, hydrogen is introduced into the regeneration
flow by reforming the methane within the exhaust gas upstream of
the lean NOx adsorber.
[0048] With regard to the introduction of a hydrocarbon comprising
methane into the aftertreatment system, in one embodiment of the
method, the hydrocarbon can be oxidized within the exhaust gas
prior to directing the bypass flow around the lean NOx adsorber.
However, in a preferred embodiment the hydrocarbon is oxidized
within the regeneration flow. In these embodiments, the first
quantity of the hydrocarbon can be directed into the exhaust gas by
at least one of a valve or an injector.
[0049] In another embodiment of the method of operating an internal
combustion engine equipped with an aftertreatment system, the
regeneration cycle end time is based on the lambda of the
regeneration flow downstream of the lean NOx adsorber being
representative of an oxygen potential below a pre-determined
threshold concentration. In a further embodiment, the regeneration
cycle is based on a concentration of the reductant downstream of
the lean NOx adsorber being above a pre-determined threshold
concentration.
[0050] In another embodiment of the introduction hydrogen into the
regeneration flow by reforming a second quantity of the hydrocarbon
within the exhaust gas upstream of the lean NOx adsorber, the
regeneration cycle end time is based on a concentration of at least
one of CO or H.sub.2 downstream of the lean NOx adsorber being
above a pre-determined threshold concentration.
[0051] In another embodiment of the method of operating an internal
combustion engine equipped with an aftertreatment system, the
regeneration flow is controlled by at least one valve. In a
particular embodiment, the regeneration flow is controlled by a
bypass valve in a bypass line and an exhaust valve in an exhaust
line. For greater control over the regeneration and bypass flows,
each one or both of the bypass valve and the exhaust valve can be a
variable control valve.
[0052] In another embodiment of the method of operating an internal
combustion engine equipped with an aftertreatment system, the
regeneration cycle start time is determined based on the
measurement of a NOx concentration within the exhaust gas
downstream of the lean NOx adsorber, with the start time occurring
when the measured NOx concentration is higher than a threshold
concentration, which is determined by reference to a NOx
concentration of the exhaust gas exiting from the engine.
[0053] In embodiments of the method that employ methane as the
hydrocarbon, the method can further comprise burning fuel in the
combustion chamber to generate the exhaust gas wherein the lambda
of the exhaust gas is less than one when operating in a predefined
low load, low speed mode.
[0054] In a preferred example the engine is a direct injection
engine.
[0055] In another embodiment of the method of operating an internal
combustion engine equipped with an aftertreatment system, exhaust
gas is directed downstream of the lean NOx adsorber through a
clean-up catalyst during the regeneration cycle. The clean up
catalyst can remove NOx or reductant. In a preferred example, the
clean up catalyst removes hydrogen sulfide from the exhaust
gas.
[0056] The method may further comprise directing the exhaust gas
through a particulate filter upstream of the lean NOx, adsorber, or
in another example, directing bypass flow through a second lean NOx
adsorber.
[0057] A further method is disclosed for operating an internal
combustion engine equipped with an aftertreatment system for
removing NOx from exhaust gas generated by combustion of a fuel in
at least one combustion chamber of the engine. The method
comprises:
[0058] (a) directing all of the exhaust gas through a lean NOx
adsorber during normal operation of the engine;.backslash.
[0059] (b) periodically regenerating the lean NOx adsorber using a
predetermined regeneration strategy selected from one of a high
load strategy, a midrange load strategy and a low load strategy,
the regeneration strategy causing the exhaust gas pass through the
lean NOx adsorber:
[0060] during the high load strategy: reacting, upstream of the
lean NOx adsorber, a reductant to maintain a lambda of the exhaust
gas of less than one across the NOx adsorber,
[0061] during the low load strategy: burning the fuel in the
combustion chamber generating the exhaust gas wherein the lambda of
the exhaust gas is less than one, and
[0062] during the midrange load strategy, causing the lambda of the
exhaust gas to be less than one across the NOx adsorber by: burning
the fuel with the combustion chamber in a rich environment, and
reacting a reductant upstream of the lean NOx adsorber.
[0063] An aftertreatment system is provided for removing NOx found
in exhaust gas produced during combustion of a fuel within a
combustion chamber of an operating internal combustion engine. This
system comprises:
[0064] (a) an exhaust line for directing the exhaust gas from the
engine,
[0065] (b) a lean NOx adsorber disposed in the exhaust line for
removing NOx,
[0066] (c) a regeneration catalyst disposed in the exhaust line
upstream of the lean NOx adsorber, with such regeneration catalyst
capable of oxidizing a reductant,
[0067] (d) a reductant line for delivering the reductant from a
reductant store to the exhaust line upstream of the regeneration
catalyst,
[0068] (e) a reductant flow control disposed in the reductant line
for controlling reductant flow into the exhaust line
[0069] (f) a bypass line for directing the exhaust gas around the
lean NOx, adsorber,
[0070] (g) at least one bypass flow control capable of controlling
flow of the exhaust gas through the bypass line,
[0071] (h) a controller, and
[0072] (i) at least one sensor providing control information to the
controller, the controller capable of adjusting the at least one
valve in response to the control information.
[0073] With this aftertreatment system, the reductant can be
hydrogen and in a preferred example, the reductant is a gaseous
hydrocarbon. In this embodiment the regeneration catalyst is
capable of reducing the gaseous hydrocarbon to provide hydrogen
with the exhaust gas.
[0074] In a preferred embodiment of the aftertreatment system, the
regeneration catalyst comprises a reformer in series with an
oxidation catalyst. The regeneration catalyst can also be an
oxidation catalyst. In another embodiment, the regeneration
catalyst comprises an oxidation catalyst combined with a
reformer.
[0075] The aftertreatment system can further comprise a second
close-coupled catalyst proximate to the engine for oxidizing the
reductant when the exhaust gas proximate to the regeneration
catalyst is at a temperature below a predetermined threshold
temperature. The predetermined threshold temperature is determined
as the temperature below which the reduction catalyst is unable to
efficiently oxidize the reductant.
[0076] The aftertreatment system can further comprise an injector
for injecting the reductant into the exhaust line.
[0077] The bypass valve can be a variable control valve for
improved modulation of flow through the by-pass line. In another
embodiment the reductant store can be a fuel system of the
engine.
[0078] A further embodiment introduces a second quantity of the
reductant such as hydrogen or a hydrocarbon through a second gas
line to deliver it to the oxidation catalyst.
[0079] The system may further comprise a particulate filter
disposed in the exhaust line downstream of and proximate to the
regeneration catalyst or a second lean NOx adsorber in the bypass
line.
[0080] A second regeneration catalyst may also be disposed in the
bypass line upstream of the second lean NOx adsorber.
[0081] The aftertreatment system can further comprise a clean-up
catalyst disposed downstream of the lean NOx adsorber. The clean-up
catalyst is capable of removing NOx or reductant, and in a
preferred example, hydrogen sulfide from the exhaust gas.
[0082] Further aspects of the invention and features of specific
embodiments of the invention are described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0083] In drawings which illustrate non-limiting embodiments of the
invention:
[0084] FIG. 1 shows a schematic of a NOx management system
according to one embodiment of the invention.
[0085] FIG. 2 shows a graphical representation of properties of the
exhaust gas plotted against time. Included are some system
properties over a regeneration cycle.
[0086] FIG. 3 shows an engine-operating map of torque against speed
with flow gradients over a regeneration cycle provided for use in a
closed-loop control strategy.
[0087] FIG. 4 shows a graph of flow versus temperature of the
exhaust gas out of the engine and through the NOx catalyst during
regeneration.
[0088] FIG. 5 shows an alternate embodiment of the subject
invention with a two-bed by-pass aftertreatment system.
[0089] FIG. 6 shows an alternate embodiment of a control strategy
for the subject invention representing an engine operating map of
torque against speed with flow gradients over a regeneration cycle
provided for use in a closed-loop control strategy utilizing both
in-line and in-cylinder regeneration.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0090] A method of regenerating a NOx adsorber that is used to
treat exhaust gases created during combustion in the combustion
chamber is disclosed. A hydrocarbon, preferably methane, is
introduced into the exhaust line wherein the hydrocarbon is
oxidized and reformed within the exhaust line to generate hydrogen,
which is used to regenerate the NOx absorber. CO, as well as
hydrogen, is generated during reformation of methane resulting in a
regeneration mixture that includes both hydrogen and CO. The
aftertreatment system is capable of directing an amount of exhaust
gas to by-pass the NOx adsorber during regeneration for the
purposes of reducing regeneration flow, hydrocarbon consumption,
emissions, and regeneration time. Specific markers, indicative of
the properties of the exhaust gas, can be used to identify
completion of regeneration.
[0091] FIG. 1 is a schematic showing an aftertreatment system
according to a preferred embodiment of the invention. An exhaust
line 22 carries exhaust gases flowing in the direction of arrow 20
from an engine block 11 to an outlet in the direction of arrow 31.
Components of a NOx aftertreatment system can be disposed in
exhaust line 22 such that exhaust gases are carried to NOx adsorber
46 as indicated by arrow 56. Regeneration catalyst 42 is disposed
in exhaust line 22 upstream of NOx adsorber 46.
[0092] By-pass line 12 is capable of carrying a portion of the
exhaust gases around adsorber 46 as may be desirable while absorber
46 is being regenerated. The exhaust gases can be directed through
by-pass line 12 as indicated by arrow 18 by opening by-pass valve
14. By-pass valve 14 can be disposed anywhere along by-pass line
12. In the embodiment shown, by-pass line 12 branches off from
exhaust line 22 at a junction 16 and rejoins exhaust line 22 at a
junction 48 downstream from NOx adsorber 46.
[0093] Valves 13 and 14 are provided to help control the flow of
exhaust gases through line 22 and by-pass line 12 during
regeneration.
[0094] Although not preferred if operating of the subject method
and apparatus is possible without it (see discussion below), FIG. 1
also shows a close coupled catalyst 74 in line 22 physically
proximate to engine block 11. A hydrocarbon, preferably methane
gas, can be introduced just prior to catalyst 42 and/or catalyst
74.
[0095] Hydrocarbon valves 28 and 29 are disposed in respective main
line 26 and close couple line 27, each of which branches off from
store line 34. Store line 34 is connected to store 36 from which
methane is allowed to flow as indicated by arrow 50. Flow direction
51 and 52 along lines 26 and 27 are also provided.
[0096] Lambda sensor 71 is used to measure lambda. Lambda, is
defined herein as a measure of the oxygen potential of the exhaust
gas. A lambda sensor measures this potential. Generally, a lambda
value above 1 denotes a high oxygen potential and a lambda value
below 1 denotes a low oxygen potential. A rich exhaust gas
environment is an environment with a lambda value below 1 while a
lean exhaust gas environment is an environment with a lambda value
above 1. Lambda sensor 71 measures lambda in the exhaust gas after
adsorber 46 and also near engine block 11 as shown by the
intersection point of feed lines 61 and 63 with exhaust line 22.
NOx sensor 72 is used to measure NOx levels after adsorber 46 and
near engine block 11 as shown by the intersection point of feed
lines 62 and 64 with exhaust line 22.
[0097] Temperature sensor 73 is also used to measure temperatures
before and after catalyst 42 as show by the intersection point of
feed lines 65 and 66 with exhaust line 22.
[0098] Finally, each of sensors 71, 72 and 73 feed information to
controller 70 through respective feed lines 67, 68 and 69. Line 60
provides engine data to controller 70.
[0099] Controller 70 drives valves 13 and 14, through feed lines 75
and 76, and valves 28 and 29, through feed lines 77 and 78.
[0100] FIG. 2 provides a graph demonstrating a sample set of
conditions within the exhaust gas at a typical mid-range engine
speed and load plotted against time. Line 500 is lambda of the
exhaust gas measured at point C (refer to FIG. 1). Line 502 is the
temperature of the exhaust gas measured in degrees Celsius at point
A (refer to FIG. 1). Line 504 is the flow of the exhaust gas at
point B measured in kg/hr. Line 506 is the NOx concentration of the
exhaust gas measured downstream of adsorber 46 at point D (refer to
FIG. 1), in ppm. Line 508 is lambda of the exhaust gas at point B
(refer to FIG. 1) and line 508 overlaps line 500 except for the
dashed line indicated by reference number 508 between the start and
end of the regeneration cycle. Line 510 represents a lambda value
of 1, above which the exhaust gas has high oxygen potential and
below which the exhaust gas has a low oxygen potential. Line S
provides the approximate start time of a regeneration cycle. Line O
provides the earliest end to a regeneration cycle. Line F provides
the end of the regeneration cycle in the example shown.
[0101] FIG. 3 provides an engine map of torque versus speed. Lines
900 through 906 provide gradient lines that demonstrate the
boundaries at which 100%, 50%, 35% and 20% of the total exhaust gas
flow is directed through the NOx adsorber during regeneration. Line
908 defines the boundary of the engine-operating map.
[0102] FIG. 4 provides a graph of flow or space velocity of the
exhaust gas plotted against temperature. Line 800 provides an
example of exhaust gas properties out of engine block 11 over all
operating conditions of the engine. Line 802 provides target
properties of the exhaust gas through NOx adsorber 46 during
regeneration.
[0103] Referring to FIG. 5, an alternate embodiment of the subject
invention is provided wherein by-pass system 968 is a two bed
system. Exhaust line 972 directs exhaust gas in direction 974 away
from engine block 970. Reformers 976, 978 are disposed in lines
977, 979 wherein exhaust gas can be directed according to arrows
984 or 986. Adsorbers 980 and 982 are also disposed in lines 977,
979. The resulting exhaust gas is directed from system 968 in
direction 988. Valves 990, 992 are disposed in each of lines 977,
979. Valves 994, 996 are also provided in store lines 995, 997 to
control flow of gas from store 993 in direction 998.
[0104] FIG. 6 provides an alternate torque versus speed map for
controlling the operation of the subject aftertreatment system.
Line 950 defines transition region 952 for regeneration wherein a
combination of in-cylinder and in-line regeneration is done.
[0105] Lines 954, 956, 958 and 960 provide the same gradient lines,
as found in FIG. 3, that demonstrate the boundaries at which 100%,
50%, 35% and 20% of the total exhaust gas flow is directed through
the NOx adsorber during regeneration. Line 962 defines the boundary
of the engine-operating map.
[0106] In the NOx aftertreatment systems of FIG. 1, exhaust gas is
generated by combustion events within one or more combustion
chambers disposed upstream of engine exhaust line 22 in engine
block 11. Exhaust gas results from the combustion of fuel (lean
burn combustion when a NOx adsorber is used for aftertreatment
purposes) such as natural gas or a mixed fuel that includes natural
gas or methane. The fuel is, in general, either directly injected
into the combustion chamber or pre-mixed with a quantity of air to
create a fumigated charge or is a combination of the two wherein a
premixed charge and directly injected charge drive the engine. In
each case, spark ignition, hot surface ignition or compression
ignition are utilized to initiate the combustion process within the
combustion chamber.
[0107] During normal operation of the engine valve 14 is closed and
exhaust gas flows along exhaust line 22. The exhaust gas also
passes through NOx adsorber 46, which removes NOx. By way of
example, during normal operation, NOx adsorber is under lean
operating conditions, that is, with an excess of oxygen available
in the exhaust gas, NOx is driven to (NO.sub.3).sub.2 by way of the
following reactions:
NO+1/2O.sub.2(Pt).fwdarw.NO.sub.2 (1)
XO+2NO.sub.2+1/2O.sub.2.fwdarw.X(NO.sub.3).sub.2 (2)
[0108] where X is a washcoat, as is well known to those skilled in
this technology.
[0109] Eventually NOx adsorber 46 will become less effective at
removing NOx as X(NO.sub.3).sub.2 uses up adsorbing sites in
adsorber 46. NOx slip is used to express a percentage increase in
NOx emissions above a base concentration of NOx. NOx slip can be
used to determine when an unacceptable level of NOx is being
expelled. When this unacceptable level is reached, adsorber 46 is
regenerated. Upon regeneration the NOx adsorber returns to removing
NOx from the exhaust gas. Controller 70 determines when NOx
adsorber 46 needs regenerating. This can be done through an open
loop control, based on selected parameters from the engine map, or
closed loop control, based, in part, on direct readings of the NOx
concentration within the treated exhaust gas. By way of example,
one such open loop control uses a calibration of the aftertreatment
system over a range of engine operating conditions to estimate the
time at which adsorber 46 needs regeneration. That is, the
controller monitors such variables as the engine load and speed,
determining from a look-up table, the time for regeneration. With
this method, the system is calibrated such that the engine
operating conditions, which are indicative of NOx production, are
used to estimate when regeneration for the NOx adsorber is
desirable. Conditions such as torque, speed, intake air mass flow,
the fuel flow into the engine, intake manifold temperature, intake
manifold pressure, as well as others, can be used for open loop
control.
[0110] A closed loop control for determining the commencement of a
regeneration cycle could also be used. By way of example, one such
control monitors NOx levels within exhaust line 22 downstream of
adsorber 46 with sensor 72 through line 64 and near the engine
through line 62. Controller 70 can commence regeneration once the
ratio of NOx at point C to NOx out of block 11 exceeds a
predetermined threshold NOx slip level.
[0111] During the regeneration cycle, controller 70 needs to
provide H.sub.2 and/or CO to NOx adsorber 46 and do so in a rich
exhaust gas environment (oxygen depleted environment). The
controller can control exhaust gas flow and the introduction of
methane to provide a regeneration strategy that will help reduce
hydrocarbons used for regeneration, and reduce the time required
for regeneration. One hydrocarbon that can be used is methane.
[0112] During regeneration, the following provides a set of
reactions found across catalyst 42:
CH.sub.4+2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O (3)
CH.sub.4+1/2O.sub.2.fwdarw.CO+2H.sub.2 (4)
CH.sub.4+H.sub.2O.fwdarw.CO+3H.sub.2 (5)
CO+H.sub.2OCO.sub.2+H.sub.2 (6)
[0113] where reaction (6) can be held in equilibrium depending on
exhaust gas temperature. Note also, that equation (3) may occur but
is not preferred. The CO and H.sub.2 generated according to
equations (4) through (6) are then used for regeneration as
follows:
X(NO.sub.3).sub.2.fwdarw.XO+2NO+{fraction (3/2)}O.sub.2 (7)
X(NO.sub.3).sub.2.fwdarw.XO+2NO.sub.2+1/2O.sub.2 (8)
NO+H.sub.2.fwdarw.H.sub.2O+1/2N.sub.2 (9)
NO.sub.2+2H.sub.2.fwdarw.1/2N.sub.2+2H.sub.2O (10)
NO+CO(Rh).fwdarw.1/2N.sub.2+CO.sub.2 (11)
NO.sub.2+2CO.fwdarw.1/2N.sub.2+2CO.sub.2 (12)
[0114] where X is provided in a washcoat. A lambda less than 1,
which denotes a low oxygen potential in the exhaust gas, favors
reactions (7) through (12); this is not the case, in general, when
lambda is above 1.
[0115] Controller 70 determines a regeneration strategy based,
generally:
[0116] on the exhaust gas flow,
[0117] the exhaust gas temperature,
[0118] desired exhaust gas flow chosen considering the reactive
capacity of catalyst 42 at a given exhaust gas temperature,
[0119] lambda of the exhaust gas from the engine, adsorber 46
and/or catalyst 42 throughout a regeneration cycle, and
[0120] the type of adsorber 46 and catalyst 42.
[0121] The catalyst 42 is chosen to suit the engine used and the
operating conditions contemplated for the engine. The regeneration
strategy for a given regeneration cycle can be done by an open loop
or closed loop strategy. The regeneration strategy can control the
quantity and rate of introducing the methane into the
aftertreatment system from store 36 and the quantity of by-pass
flow by controlling valves 13 and 14. The goal during regeneration
is to efficiently provide an exhaust gas environment wherein lambda
is below one, thereby promoting reactions (7) through (12). This is
realized when reductants are provided to the NOx adsorber that
remove oxygen released according to the reactions above. However,
oxidation through the NOx adsorber with other reductants such as
methane or other hydrocarbons can also provide the necessary rich
exhaust gas environment. It is also desirable to provide conditions
where all hydrocarbons are used efficiently to reduce NOx, and
minimize the time required for regeneration.
[0122] In an open loop strategy, the controller is preferably
calibrated to direct flow of exhaust gas through the adsorber and
methane into the exhaust gas based on the engine speed and load
just prior to and during regeneration. Engine intake manifold
temperature, intake air mass flow, the fuel flow into the engine or
intake manifold pressure can also be used as indicators for
controlling regeneration. A constant regeneration cycle time can
also be used in certain static operating conditions when load and
speed remain relatively constant over extended periods of time.
Such an open loop strategy employs an engine calibration that
considers one or more engine operating conditions, each of which is
indicative of at least one of exhaust gas temperature, flow and
lambda value. The controller is calibrated to direct a desired flow
of exhaust gas through the NOx adsorber based on the
characteristics of catalyst 42 and adsorber 46.
[0123] The flow through NOx adsorber 46 during regeneration is
referred to herein as the regeneration flow. A look-up table is
used to determine whether the exhaust gas flow exceeds the desired
regeneration flow and, if so, directs excess exhaust gas around
adsorber 46 via by-pass line 12. This is referred to as the by-pass
flow if exhaust gas is by-passed during regeneration. The desired
by-pass flow is achieved by adjusting valves 13 and 14 to match the
target regeneration flow though adsorber 46.
[0124] Referring to the engine map of FIG. 3, the percentage flow
of exhaust gas through NOx adsorber 46 is provided based on the
torque and speed of the engine and this percentage is reduced as
speed and torque increases, as is seen in contour lines 900 through
906. Operating conditions falling above or to the right of line 900
show a reduction in the proportion of the total exhaust gas that
flows through adsorber 46. Below and to the left of line 900,
controller 70 does not open valve 14 allowing all exhaust gas to
flow through adsorber 46.
[0125] The look-up table for this open loop control also provides a
target methane concentration upstream of catalyst 42. The engine
operating conditions provide information about the exhaust gas
temperature and lambda of the exhaust gas from block 11. The lambda
of the exhaust gas, determined based on the engine operating
conditions, and the flow of the exhaust gas determines in part the
amount of methane required to generate a sufficiently rich exhaust
gas environment to support efficient regeneration.
[0126] A closed loop strategy could also be used. In a closed loop
system lambda may be measured out of engine block 11 by sensor 71
and the temperature may be measured prior to catalyst 42 by sensor
73 through line 66 and after catalyst 42 through line 65. The load
and speed of the engine may be used by the controller to infer the
exhaust gas flow based on look-up tables or a flow meter within the
exhaust line may also be used for complete closed loop control. The
look-up table along with sensor information are used to determine
the flow of methane to be introduced into exhaust line 22 and how
much flow of exhaust gas, if any, to direct through valve 14 and
line 12 during regeneration. When exhaust gas flow is too high for
catalyst 42 to allow complete oxidation and reformation of methane
or too high to regenerate catalyst 46 efficiently, some flow is
directed into by-pass line 12 until the desired flow is met.
[0127] If temperature prior to and after catalyst 42 is too high or
too low, the methane quantity can be increased or reduced according
to those temperature readings. For example, if the temperature
falls below a predetermined temperature set during calibration and
based, in part, on the catalyst chosen, methane could be reduced to
ensure that the exhaust gas temperature is elevated to an
acceptable level to support the reformation reaction (5) set out
above (assuming the inlet mixture to catalyst 42 originally had
excess fuel). Further, if the post-catalyst temperature is too
high, the methane quantity can be shut-off to avoid overheating the
catalyst and damaging it during regeneration. Such a strategy can
employ a series of cycles whereby the methane flow through valve 28
is opened and closed a few times through one regeneration cycle to
ensure that adsorber 46 is regenerated while protecting catalyst
42.
[0128] Likewise, lambda sensor 71 can allow the controller to
adjust the quantity of methane introduced through valve 28 to
ensure that the exhaust gas was rich enough to approach target
regeneration efficiency across adsorber 46 according to reactions
(7) through (12) set out above.
[0129] Also, a lambda sensor could be provided after catalyst 42
and before adsorber 46 rather than, or in addition to, sensor 71
provided. This would monitor the oxygen potential out of catalyst
42 to provide for efficient regeneration through adsorber 46. That
is, if the flow of methane through line 26 is unknown, then the
lambda sensor could be used to close loop control the flow of
methane to help provide for a target lambda in the regeneration
flow prior to regeneration of NO, adsorber 46.
[0130] An optional close-coupled catalyst 74 is also available to
increase exhaust gas temperatures when desired. The proximity of
catalyst 74 to block 11, helps ensure that exhaust gas is not too
cool to oxidize methane within the exhaust gas environment.
Therefore, when the controller detects an exhaust gas temperature
below a threshold amount, valve 29 will provide methane upstream of
catalyst 74, heating and oxidizing the exhaust gas well upstream of
adsorber 46. Catalyst 74 can also be used to produce CO and
hydrogen for use in regeneration as was done with catalyst 42.
[0131] As noted above, these closed loop strategies are preferred
but they are not necessary. The open loop strategy discussed above
utilizing a calibration of the system that provides a target
methane injection rate and quantity over a regeneration cycle that
is based on the engine operating parameters such as load and speed,
could eliminate dynamic monitoring and the added complexity in
hardware and software for the system. However, the trade-off is
that such a strategy is more likely to regenerate incompletely or
to regenerate with a higher methane penalty.
[0132] The controller can determine completion of a regeneration
cycle by reference to a closed or open loop control. In a closed
loop control, the controller can use readings from lambda sensor 71
downstream of adsorber 46 to determine when the oxygen potential
within line 22 downstream of adsorber 46 is decreasing. Referring
to reactions (7) through (12), once most nitrogen has been released
from adsorber 46, oxygen potential begins to decrease as oxygen is
no longer being released from adsorber 46. Other sensors may be
appropriate for closed loop monitoring of regeneration cycle
completion, including a CO or H.sub.2 sensor that detects increases
in CO or H.sub.2 downstream of adsorber 46. These increases would
occur when oxygen is no longer released from the adsorber causing
H.sub.2 and CO to pass through the adsorber unreacted.
[0133] An open loop control could also be used relying on the
calibration of the system wherein regeneration time is
pre-determined based on engine operating conditions such as speed,
load, intake air mass flow, the fuel flow into the engine, intake
manifold temperature and pressure.
[0134] Referring again to FIG. 3, as either or both speed and load
of the engine at the commencement of and during a regeneration
cycle increase, controller 70 commands valve 14 to open when load
and speed fall above line 900. Valves 13 and 14 can be variable
control valves providing for a wide range of operating conditions
as the engine operating parameters continue to generate exhaust gas
flows above a target flow through catalyst 42 or adsorber 46
determined, in part, by the properties of catalyst 42. Therefore,
when controller 70 determines a desired exhaust gas flow, valves 13
and 14 can be adjusted to maintain this pre-determined flow of
exhaust gas through line 22 during regeneration.
[0135] To simplify the system, an alternative to variable flow
control valves used for valve 13 and 14 are two position valves
(or, for that matter, other multiple position valves). Here, the
controller can elect from one of three possible settings. Valve 13
can be fully open or partially open. Valve 14 can be closed or
fully open. Therefore, controller 70 can select a position for each
valve according to the engine operating parameters in order to
match exhaust flow through line 22 to a pre-determined target
value. That is, at low speed and load, valve 13 is open fully and
valve 14 is closed. At higher loads and speeds, valve 13 is fully
opened and valve 14 is fully opened. At still higher speeds and
loads, valve 13 is partially closed and valve 14 is opened.
[0136] Other valve configurations can be used as well. More
flexibility for the controller to manage flow through line 22
during regeneration helps the controller to meet a target
pre-determined flow rate for each operating condition. One
trade-off is that such flexibility may result in a more expensive
system that requires more expensive valves and more complicated
software to control those valves.
[0137] As would be understood by a person skilled in the
technology, valves 13 and 14 can be any flow control mechanism and
need not be limited to valves.
[0138] Referring to FIG. 4, flow and temperature over the range of
engine operating conditions are provided. Area 800 shows typical
exhaust gas flow and temperature conditions expelled from block 11.
Area 802 provides the controller a desired operating range for
temperature and flow of exhaust gas through regeneration catalyst
42 during regeneration--which provide the exhaust gas conditions
which then allow the conditions necessary for regeneration of NOx
adsorber 46. Therefore, when the flow out of block 11 is above area
802, flow through bypass line 12 can be used to bring the exhaust
flow through adsorber 46 to within area 802 and below the upper
limit flow or upper flow of the range. Ideally, regeneration flow
is targeted to a desired flow within this range, however, depending
on such things as exhaust gas flow, valve reaction times in the
system, pressure and temperature changes, a different regeneration
flow within the range defined by 802 is all that can be maintained.
When the temperature falls below area 802 (to the left of area 802)
at catalyst 42, additional heat can be generated through the
operation of the engine as described below or using close coupled
catalyst 74, proximate to block 11, as described above and
below.
[0139] Referring to FIG. 2, selected properties of the system are
plotted over the course of a partial adsorbing cycle and an entire
regeneration cycle. Referring to FIG. 1, lambda at points B and C
(lines 508 and 500, respectively), temperature and space velocity
at point A (lines 502 and 504, respectively) and NOx at point D
(line 506) are all shown, plotted against time. The example
provided is representative of operation of the aftertreatment
system when an engine is running at a typical midrange speed and
load.
[0140] Referring to line 506, NOx concentrations increase gradually
until the controller determines that the level has exceeded a
pre-determined threshold--this could be done by monitoring the
engine operating parameters or measuring the NOx concentration.
Regeneration then commences with opening of valve 14. Commencement
of regeneration is shown at time S. Opening valve 14 drops the
space velocity or flow through line 22, line 504, at time S.
Methane is also directed into line 22 causing lambda to drop to a
level below 1 between catalyst 42 and adsorber 46 (line 508).
Lambda following the NOx adsorber also drops after regeneration is
complete (line 500), but during regeneration of the adsorber, it is
maintained near a lambda value of 1 as the rich mixture entering
the adsorber releases oxygen from the oxides of nitrogen resulting
in a leaner mixture expelled from adsorber 46 than that entering
adsorber 46. Eventually, however, no further oxygen is released
from adsorber 46 and lambda falls until the lambda out of adsorber
46 is the same as lambda into adsorber 46 (line 508). Once a
threshold lambda out of adsorber 46 is detected, valve 14 is closed
along with valve 28. Immediately, the flow begins to rise, line
504, as all exhaust gas is again routed through exhaust line 22.
Soon, lambda begins to rise resulting in a lean exhaust gas
environment, lines 500 and 508.
[0141] Note, that the regeneration cycle is complete at time F in
FIG. 2. This is, in practice, a delayed end of the regeneration
cycle. Preferably, the regeneration cycle would be completed
sometime between time O and time F when lambda after the NOx
adsorber (line 500) drops below a pre-determined threshold amount
and before it matches lambda upstream of the NOx adsorber (line
508).
[0142] During the regeneration cycle, the NOx levels out of line 22
increase substantially, as the engine is continuing to operate
without NOx treatment of the exhaust gas routed through by-pass
line 12, line 504. Once regeneration is complete, however, NOx
quickly falls as all exhaust gas is routed through recently
regenerated adsorber 46. Therefore, as well as limiting fuel
consumption (consumption of methane), short regeneration times also
limit the amount of NOx emitted during regeneration through by-pass
line 12. The longer the period of time needed for regeneration, the
more cumulative exhaust gas flows through by-pass line 12. The
target cycle is based on generating as much reductant per unit
methane injected over the shortest time period. This is a function
of variables such as the temperature of the exhaust gas, flow of
exhaust gas, catalyst specifications, and lambda of exhaust gas,
since a higher lambda requires more methane to burn off the oxygen
present but more oxygen is available to generate CO. Preferably,
regeneration cycles should be kept to less than 5% of operating
time of the engine. Also, as noted above, a greater flow of exhaust
gas routed through by-pass line 12, results in higher NOx emissions
since by-pass line 12 does not generally include a separate NOx
adsorber.
[0143] FIG. 5, shows an alternative embodiment of the
aftertreatment system. Here a second NOx adsorber and catalyst is
disposed in the by-pass line to treat NOx through that line during
regeneration. That is either one of line 977 or line 979 act as the
bypass lines during regeneration of the either one of adsorber 980
or adsorber 982. Valves 994, 996, 990 and 992 all work to control
which of the adsorber is being regenerated. For example, for
regeneration of adsorber 980, bypass line is line 979. Here, a
regeneration cycle is begun when exhaust gas is directed according
to a control strategy (see FIG. 3), through reformer 976. Excess
exhaust gas is bypassed through adsorber 982 during regeneration in
the same manner as exhaust gas was also bypassed through line 12
referring to FIG. 1. Here, however, the excess exhaust gas is
treated by NOx adsorber 982.
[0144] In the example, the reductant source, methane, is directed
from store 993 through valve 994 to line 977. At the same time,
valves 990 and 992 are opened according to the desired split of
exhaust gas through each valve for the purposes of regeneration.
Eventually, NOx adsorber 982, as well, would need to be
regenerated. In which case valve 994 would close and 996 would open
and the acting bypass line would be 977.
[0145] This system in general could also be extended to a multi-bed
system with 3 or 4 or 5 or more beds with one "off-line" at any one
time. The benefit here is improved NOx conversion for the same
catalyst volume.
[0146] Note also, for multi-bed systems--2 or more beds--there is
no need for two physically distinct adsorbers. Parts of a single
physical adsorber could act as an isolated adsorber as well.
[0147] Also, while two reformers are shown in FIG. 5, one could be
used. In fact, one reformer could be used for other multiple bed
designs. Here the reformer would include lines that routed exhaust
around the reformer to each adsorber as well as lines that routed
exhaust gas from the reformer through to each adsorber. Valves
would control the design of such a system.
[0148] In general, while these multi-bed systems provide better
conversion, they also tend to add cost and complexity to the system
both in terms of the architecture and in terms of the control
mechanisms.
[0149] An alternative method of operating the aftertreatment system
that can help to reduce regeneration time employs an additional
exhaust line that routes exhaust gas around catalyst 42 and through
adsorber 46 during regular operation. A valve disposed in this
additional exhaust line could be used such that valve 13, closed
during regular operation, would be opened just prior to
commencement of regeneration, while maintaining the catalyst bypass
open. This would allow a flow of exhaust gas through line 22,
lighting off catalyst 42 and warming the line prior to a
regeneration cycle. When a valve used to bypass catalyst 42 is
closed at the beginning of a regeneration cycle, there can be less
time needed to heat line 22 and less time before regeneration can
commence. Alternatively, in such an embodiment with an additional
exhaust line around catalyst 42, the flow rate within reformer line
22 can be set to ensure a certain amount of exhaust gas is always
flowing through line 22 eliminating the need for valve 13 by
employing a valve to regulate flow through catalyst 42 by
controlling flow through the additional exhaust line.
[0150] Catalyst 42 is generically describe as a bed that promotes
reactions (3) through (5) to provide a desired exhaust gas with
elevated concentrations of H.sub.2 and/or CO and minimal amounts of
oxygen. To varying extents, reactions (3) through (6), a
combination of exothermic and endothermic reactions, drive the
process across this catalyst. This catalyst can be a reformer that
oxidizes methane and promotes reaction (5) to provide H.sub.2 and
CO. It can also be a partial oxidation catalyst, which partially
oxidizes methane and reforms methane to provide H.sub.2 and CO, see
reaction (4). Catalyst 42 can also be a back-to-back oxidation
catalyst and reformer sharing a common boundary surface. This
catalyst would first oxidize methane until little oxygen remains
within the exhaust gas and then, use excess methane to generate
H.sub.2 and CO within the reformer. These two catalysts, the
oxidation catalyst and reformer, can also be disposed in line 22 in
series and need not share a common boundary surface. Also, a
combination reformer and oxidation catalyst could be used that
integrates the reformer and oxidation catalyst together in a mixed
catalyst. Each option has balancing cost and efficiency
considerations that weigh in any decision as to which catalyst to
use depending on the aftertreatment system sought.
[0151] As noted briefly above, referring again to FIG. 1, an
additional catalyst, close coupled catalyst 74, is shown positioned
near engine block 11. Some systems need such a catalyst disposed
close to the engine to ensure that the exhaust gas is hot enough to
support oxidation of methane. That is, there are some
aftertreatment system designs that would benefit from employing a
close coupled catalyst near the engine block so that the exhaust
gas temperature under low load and/or speed or idle conditions can
be prevented from falling below a threshold limit at which stable
oxidation of methane in catalyst 42 would be compromised.
Therefore, under such conditions, there are advantages in having
close-coupled catalyst 74 near engine block 11 with line 27 feeding
methane upstream of such catalyst. This catalyst would then either
oxidize the methane provided from store 36 to heat the exhaust gas
to a temperature suitable to allow catalyst 42 to light off
satisfactorily. Alternatively, catalyst 74 can provide the rich
exhaust gas environment along with H.sub.2 and CO needed to
regenerate adsorber 46. It would be desirable here, however, to
operate this way only when valve 14 is closed in order to prevent
CO and H.sub.2 from escaping through the by-pass line, since this
would be inefficient.
[0152] An additional method of operating the regeneration cycle
under low load conditions is to burn a fuel rich combustible
mixture, preferably comprising methane, in the combustion chamber
within engine block 11. Alternatively, a method wherein fuel burned
lean with an injection of fuel late in the cycle can be used. Fuel
can oxidize in the combustion chamber, or in exhaust or over the
catalyst. In each case, this will generate an excess of CO and some
H.sub.2 while creating a rich exhaust gas environment. With this
method, no methane needs to be provided to catalyst 42 when the
necessary reductants are present within a rich exhaust gas
environment. Preferably, such a strategy would be limited to
conditions when flow and temperature are low which is typically
associated with light load and low speed conditions or idle
conditions when full flow through adsorber 46 is desirable.
[0153] Use of in-cylinder techniques to generate the necessary
conditions and reductants to regenerate the NOx adsorber can have
drawbacks.
[0154] For example, a spark-ignited engine running under lean
conditions has an engine out NOx that is relatively low when
compared to that resulting from operation under stoichiometric
conditions. In general, the peak in NOx production occurs at
conditions slightly lean of stoichiometric where engine out NOx can
be an order of magnitude larger than that produced near the lean
limit. Thus, transitioning from lean to rich operation to create
the conditions conducive to NOx adsorber regeneration also results
in a substantial increase in engine out NOx emissions. While
operating under rich conditions, the NOx adsorber reduces both the
engine out and adsorber released NOx to nitrogen. However, when
transitioning back to lean operation, the substantial amounts of
NO, created during operation just lean of stoichiometric are
captured by the NOx adsorber.
[0155] Under light load/low speed conditions, for example, idle,
the mass flow rate of NOx is relatively low and the storage
capacity of the NOx adsorber is relatively high. Adsorption phases
in excess of 1000 seconds can be realized. Under this condition,
the influence of the NOx adsorbed during the transition from rich
to lean operation has relatively little impact. For example, if the
transition from lean to rich takes five seconds to accomplish, and
the NOx produced is an order of magnitude larger than that of lean
operation, the adsorption phase would be reduced by 5% to 950
seconds in the example provided. However, as the mass flow rate of
the engine out NOx increases with speed and load, the relative
impact of the additional NOx produced during the transition starts
to have an impact on the system operation. For example, at an
engine operating condition at a higher speed and load relative to
idle, the engine out NOx flow rate doubles. Under these conditions,
the fill time of the NOx adsorber, based on lean engine out NOx
emissions and temperatures, is reduced to 500 seconds. When using
in-cylinder regeneration, the 5-second transition now reduces the
fill time by 10% to 450 seconds. Continuing along this line, at
some point, the in-cylinder technique is not an effective means to
regenerate the NOx adsorber.
[0156] Similar results would be expected for a fumigated or
port-injected lean burn engine. However, the port-injected lean
burn engine is expected to be more tolerant to in-cylinder
regeneration because the transition time from lean to rich is
expected to be shorter.
[0157] Retarding the spark timing would help the situation,
possibly extending the region where the in-cylinder regeneration
could be used. Similarly, if increasing EGR rates were available to
reduce the in-cylinder oxygen concentration, the concern considered
above would still exist, but be ameliorated.
[0158] This issue is not expected to arise for engines using the
direct injection of gaseous fuels. For direct injection engines,
the use of an injector provides more flexibility in transition to
rich operation. For example a combination of increasing the EGR
rates, retarding of the gas injection timing, multiple injections
and throttling of the engine is available. These conditions would
not lead to significant increases in the engine out NOx levels.
Rather, engine out NOx levels would decrease (with a potential
increase in particulate matter emissions).
[0159] Where in-cylinder regeneration is not effective, one can
resort to the use of the in-line regeneration technique. However,
under full flow conditions at relatively low exhaust gas
temperatures the in-line regeneration method may not be efficient.
The fuel penalty and hydrocarbon slip can be significantly higher
than that associated with the in-cylinder regeneration. As
mentioned above, the in-line regeneration efficiency can be
improved with the use of a close-couple catalyst, but may still not
be satisfactory.
[0160] Therefore, the system needs special management consideration
when the speed load region where efficient in-cylinder regeneration
is possible does not overlap with the region where efficient
in-line regeneration can be realized.
[0161] Therefore, referring to FIG. 6, the region where effective
in-cylinder regeneration can be realized is below line 950 in FIG.
6. The region where effective in-line regeneration can be realized
is above line 950. The region where neither in-cylinder nor in-line
regeneration is efficient is defined as area 952 bounded by line
950 in FIG. 6. Within area 952, the use of a combination of
in-cylinder and in-line regeneration allows this region of the
engine map to be managed. One embodiment of the combined method
provides for the engine an enrichment of the combustion environment
during regeneration to reduce the oxygen concentration in the
exhaust. At the same time, the spark timing may or may not be
retarded to increase exhaust gas temperature (and reduce engine out
NOx emissions). A quantity of reductant is injected upstream of the
regeneration catalyst to react with the species in the exhaust gas
to reduce the oxygen potential further and create conditions
conducive to regenerating the NOx adsorber. The benefits associated
with this technique include:
[0162] the NOx spike associated with the rich to lean transition of
the engine is avoided, and
[0163] the fuel penalty and hydrocarbon slip are reduced.
[0164] A second possible embodiment of the method involves using
the in-cylinder transition from lean to rich operation to
regenerate the NOx adsorber catalyst. During the time when the
transition back to lean operation is desired, a quantity of
reductant is injected upstream of the regeneration catalyst. The
injected reductant is used to maintain a rich atmosphere over the
NOx adsorber during the rich to lean transition such that the high
engine out NOx emissions are reduced over the NOx adsorber. This
embodiment has similar benefits to that outlined for the first
embodiment.
[0165] As noted above, the first region below line 950, region 951,
and represented by low load and, in general, low speed,
demonstrates the operating range wherein in-cylinder regeneration
is desired. As exhaust gas is cooler when the engine is operating
under these conditions, in-cylinder generation of a rich
environment with excess CO and some H.sub.2 is, generally, more
efficient than would be the case if catalyst 42 (or close coupled
catalyst 74) were needed to generate the desirable condition
in-line for regeneration.
[0166] The region outside of region 952 with high speed or load and
bounded by line 954 shows a range of torque and speed at which full
flow of exhaust gas though the adsorber is preferred. Beyond this
region to high speeds or torque or both as represented by lines
956, 958, 960, bypass flow is accommodated as demonstrated and
discussed above in relation to FIG. 3.
[0167] Note when this method of control of the aftertreatment
system is used for a high-pressure direct injection engine the
range of region 951 tends to be larger, pushing line 950 higher on
the load/speed plot as compared to a spark ignited engine that uses
fumigated fuels to operate. That is, the effective expansion ratio
tends to be larger resulting in cooler exhaust gas in general being
expelled from the engine than is the case for a spark-ignited
engine.
[0168] Note also, in-cylinder regeneration as demonstrated in FIG.
6 need not rely on a by-pass flow during regeneration. That is, the
high load region bounded by line 954 could be the extent of
operation of the regeneration control map. Here three regions would
exist for regeneration of a lean NOx adsorber. Low load strategy
represented by region 951, midrange load strategy represented by
region 952 and high load strategy bounded by line 954 would
represent the entire regeneration control map. Each region would
use in-cylinder and inline regeneration strategies as taught above
with no need for by-pass.
[0169] As noted above, the regeneration cycle is dependant on the
exhaust gas temperature. It is important that the exhaust gas
introduced into catalyst 42 have a temperature above a minimum
temperature to ensure that the catalyst is "lit-off" initially. An
additional way of controlling the regeneration process from the
combustion chamber is to choose a combustion strategy or combustion
timing that ensures either relatively late heat release, as might
be the case with spark ignited engines, or a delayed or second
direct injection of fuel into the combustion chamber late in the
power stroke when regeneration is required. This can also reduce
NOx levels with associated benefits during regeneration as a
quantity of exhaust gas can be directed through the by-pass line
without NOx treatment. A reduced NOx level has benefits here. Other
strategies are well known to persons skilled in the art.
[0170] As natural gas is, overwhelmingly, methane with a few
additional heavier hydrocarbons, C2 and C3 hydrocarbons in general,
the methane store 36 can be the fuel storage tanks if the engine is
fueled by natural gas. That is, methane store 36 can be a natural
gas source such as the engine fuel tanks.
[0171] Also, valves 28 and 29 can be injectors that would directly
inject methane into exhaust line 22. An injector as the reductant
flow control would provide greater control over the timing and
quantity of methane and, therefore, greater control over the
regeneration cycle.
[0172] A metal substrate for carrying the catalyst is generally
preferred, rather than, for example, a ceramic substrate, if the
metal substrate improves thermal response to catalyst 42. As noted
above, the quicker the thermal response the quicker the
regeneration process can be completed, thereby reducing the amount
of untreated exhaust gas allowed to flow through by-pass line
12.
[0173] An additional embodiment of the aftertreatment system can
include a valve for introducing methane downstream from an
oxidation catalyst and upstream of a reformer with catalyst 42
comprising an oxidation catalyst and reformer in series but not
sharing a common interface. Flow of methane through such a
downstream valve can be controlled in response to the quantity of
methane needed within the exhaust gas entering a reformer. After
the exhaust gas has passed through an oxidation catalyst its
properties are changed. There will be less oxygen within the gas
and less methane. This is because oxidation of methane occurs
within the catalyst. This consumes oxygen. As methane serves to
provide the source for H.sub.2 and CO, which are preferred
components in the regeneration process (see reactions (4) and (5)
above), the quantity of methane needed within the reformer is
determined by the amount present within the exhaust stream upstream
of the reformer. The amount of methane preferred is determined by
that present in the gases which are exiting the oxidation catalyst
and the H.sub.2 and CO concentrations preferred in light of this
initial quantity of methane present, which is the methane not
oxidized within oxidation catalyst.
[0174] Once forced through the oxidation catalyst, the exhaust gas,
supplemented with methane via a downstream valve, is forced through
the reformer. The reformer utilizes the high temperature of exhaust
gas heated in the oxidation catalyst and the combustion chamber to
drive reformation of methane within the reformer in line 22 to
provide H.sub.2 and CO downstream from catalyst 42. This stream is
directed into NOx adsorber 46 when H.sub.2 and CO regenerate NOx
adsorber 46.
[0175] An oxidation catalyst can be a component of catalyst 42, and
can be any oxidization catalyst suitable for oxidizing the exhaust
gas to reduce the oxygen content. By way of example, a suitable
oxidation catalyst can promote the following reactions:
C.sub.xH.sub.y+(x+(y/4))O.sub.2(Pt).fwdarw.xCO.sub.2+y/2H.sub.2O
C.sub.xH.sub.y+(x+(y/4))O.sub.2(Pd).fwdarw.xCO.sub.2+y/2H.sub.2O
C.sub.xH.sub.y+(x/2)O.sub.2(Pd).fwdarw.xCO+y/2H.sub.2
CO+1/2O.sub.2.fwdarw.CO.sub.2
[0176] By way of example only, for the operating conditions known
for this application, a suitable washcoat formulation comprises
Al.sub.2O.sub.3. Other suitable washcoat formulations may also be
used, as would be understood by a person skilled in the art.
[0177] A reformer can be a component of catalyst 42, and reformers
suitable for this application are well known. The reformer is
preferably suitable to convert methane with water to CO and
H.sub.2. By way of example, the reformer can be a precious
metal-based catalyst with washcoat materials including
Al.sub.2O.sub.3.
[0178] NOx adsorber 46 typically adsorbs and stores of NOx in the
catalyst washcoat while operating under lean conditions and
NO.sub.2 can be released and reduced to N.sub.2 under rich
operating conditions when a regeneration mixture, that includes
hydrogen and rich exhaust gas, is passed through the adsorber. As
noted above, the following shows typical operation of the NOx
adsorber under lean conditions:
NO+1/2O.sub.2(Pt).fwdarw.NO.sub.2
XO+2NO.sub.2+1/2O.sub.2.fwdarw.X(NO.sub.3).sub.2
[0179] and under rich conditions:
X(NO.sub.3).sub.2.fwdarw.XO+2NO+{fraction (3/2)}O.sub.2
X(NO.sub.3).sub.2.fwdarw.XO+2NO.sub.2+1/2O.sub.2
NO+CO(Rh).fwdarw.1/2N.sub.2+CO.sub.2
2NO.sub.2+4H.sub.2.fwdarw.N.sub.2+4H.sub.2O
[0180] where X is provided in the washcoat and is typically an
alkali (for example, K, Na, Li, Ce), an alkaline earth (for
example, Ba, Ca, Sr, Mg) or a rare earth (for example, La, Yt).
[0181] An inline external heater can be used to help light off
catalyst 42 and promote reformation and oxidation of exhaust gas
during regeneration. While, it is preferable that the majority of
heat is provided by the exhaust gas, things such as, by way of
example and not limited to:
[0182] transient response,
[0183] efficiency considerations,
[0184] combustion strategies that utilize a quick heat release,
[0185] valve timing, or
[0186] cylinder design that takes advantage of a large expansion
ratio,
[0187] can release exhaust gas that could benefit from such a
heater in order to initiate oxidation prior to or during
regeneration.
[0188] A heat exchanger could direct a quantity of heat from the
outlet of catalyst 42 or heat from gases unused after regeneration
out of adsorber 46 back through to a point along line 22 upstream
of catalyst 42. This could be used to help reduce the load on such
heater after it initially lights the catalyst off.
[0189] Note that for reforming, as noted above, steam is required
in order to generate H.sub.2 and CO for regeneration. This need
tends to be met as exhaust gas has sufficient quantities of water.
However, if water levels are low, a partial oxidation catalyst
(POX) catalyst can be employed to reform the gas without the need
for supplemental water: see reaction (4). Other reformers could be
used as understood by a person skilled in the art.
[0190] Steam is made more available by oxidizing methane as
compared to other hydrocarbons. This is an additional advantage in
light of the above.
[0191] A further advantage can be realized if a fuel is used that
combines methane and hydrogen as two major components. By way of
example, natural gas with 10 to 50% hydrogen might be appropriate
as an engine fuel and appropriate for regeneration. Such a fuel
could then be utilized in the embodiments discussed wherein the
hydrogen introduced with the fuel prior to the oxidation catalyst
could help to light off those catalysts and help to provide an
exhaust gas environment with a lambda less than 1. Further, by
providing a quantity of hydrogen into the exhaust stream, the
burden on catalyst 42 is reduced. Less reforming is required for
regeneration due to the presence of hydrogen in the injected
fuel.
[0192] The method taught above for bypassing exhaust gas can also
be used if hydrogen is injected into the exhaust gas. Here, the
regeneration strategy is driven by a target regeneration flow
through the NOx adsorber that would efficiently regenerate while
limiting the associated fuel penalty and release of untreated NOx
and NOx slip during regeneration. This is that much more beneficial
if the engine is fueled by hydrogen, with the fuel providing a
ready source of reductant, but this method would be useful, as
well, if an external reformer can be used. Further, use of a
two-bed aftertreatment system, as discussed above and demonstrated
in FIG. 5, would be useful if hydrogen can be directly injected
into the exhaust gas upstream of a NOx adsorber during a
regeneration cycle after determining a target regeneration
flow.
[0193] A further embodiment of the invention includes an
aftertreatment system that has an in-line particulate filter.
Referring to FIG. 1, a particulate filter may be disposed in line
22 near and downstream of reformer 42. The particulate filter could
then benefit from the excess heat generated in line 22 during
regeneration that, upon reintroduction of oxygen to this line after
completion of a regeneration cycle, would generate an exotherm
across the particulate filter that could burn off soot collected
here extending or eliminating the regeneration cycles for the
particulate filter. Note, however, one issue which arises is that
the effectiveness of the particulate filter in reducing particulate
matter emission is lowered because part of the flow is diverted
around the particulate filter during the regeneration event.
[0194] In a further embodiment of the invention, a clean-up
catalyst is used in the aftertreatment system. Referring to FIG. 1,
a clean-up catalyst (not shown) may be provided in line 22 beyond
junction 48. The catalyst could be selected to reduce NOx (lean NOx
catalyst) from bypass line 12 (resulting during a regeneration
cycle) or selected to remove reductant (CO or H.sub.2) or other
hydrocarbons that pass through the aftertreatment system during
regeneration or selected to remove hydrogen sulfide the might
result during any desulfurization process of the adsorbers.
[0195] Whenever flow is referred to in this disclosure, it is the
mass or molar flow, rate of the gas in question.
[0196] Exhaust gas recirculation (EGR) can also be utilized to help
reduce NOx emissions during regeneration when a by-pass line is
opened. Increased EGR rates during regeneration can reduce NOx
generated in the combustion chamber resulting in less NOx flowing
through by-pass line 12 and into the atmosphere. Further, increases
in EGR can also be used to reduce the concentration in oxygen in
the exhaust gas during regeneration, reducing, in turn the burden
on the oxidation catalyst to reduce oxygen during a regeneration
cycle as well as reduce the amount of methane needed to burn off
oxygen.
[0197] While methane is the preferred source for hydrogen, as would
be understood by a person skilled in the art, other lighter
hydrocarbons, generally, gaseous hydrocarbons, could be used
including but not limited to other gaseous hydrocarbons such as
ethane, propane and butane.
[0198] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art without
departing from the scope of the present disclosure, particularly in
light of the foregoing teachings.
* * * * *