U.S. patent application number 10/837951 was filed with the patent office on 2005-11-03 for exhaust after-treatment system for a lean burn internal combustion engine.
Invention is credited to Hoard, John W., McCabe, Robert W., Xu, Lifeng.
Application Number | 20050241296 10/837951 |
Document ID | / |
Family ID | 35185647 |
Filed Date | 2005-11-03 |
United States Patent
Application |
20050241296 |
Kind Code |
A1 |
McCabe, Robert W. ; et
al. |
November 3, 2005 |
Exhaust after-treatment system for a lean burn internal combustion
engine
Abstract
An exhaust gas after-treatment system having a NO.sub.x storage
material and a separate HC and CO oxidation section, such oxidation
section having an oxidation catalyst substantially free of the
NO.sub.x storage material.
Inventors: |
McCabe, Robert W.; (Lathrup
Village, MI) ; Xu, Lifeng; (Farmington Hills, MI)
; Hoard, John W.; (Livonia, MI) |
Correspondence
Address: |
RICHARD M. SHARKANSKY
PO BOX 557
MASHPEE
MA
02649
US
|
Family ID: |
35185647 |
Appl. No.: |
10/837951 |
Filed: |
May 3, 2004 |
Current U.S.
Class: |
60/274 ;
60/301 |
Current CPC
Class: |
F01N 2610/04 20130101;
F01N 3/0842 20130101; F01N 13/0097 20140603; F01N 2610/03 20130101;
F01N 2510/06 20130101; F01N 3/0814 20130101 |
Class at
Publication: |
060/274 ;
060/301 |
International
Class: |
F01N 003/00; F01N
003/10 |
Claims
What is claimed is:
1. An exhaust gas after-treatment system for an internal combustion
engine, comprising: a lean NO.sub.x trap, such lean NO.sub.x trap
comprising: an oxidation section having an oxidation material for
oxidizing hydrocarbons and carbon monoxide in the exhaust gas; and
a NO.sub.x storage section, such NO.sub.x storage section having a
NO.sub.x storing material for storing NO.sub.x in the exhaust gas;
wherein the oxidation material in the oxidation section is
physically separated from the NO.sub.x storing material in the
NO.sub.x storage section.
2. The exhaust gas after-treatment system recited in claim 1
wherein the hydrocarbon and carbon monoxide oxidation material
includes Pt.
3. The exhaust gas after-treatment system recited in claim 2
wherein the NO.sub.x storing material includes Ba, Cs, Na, K, or
Sr.
4. The exhaust gas after-treatment system recited in claim 2
wherein the physical separation between the two sections is
provided by coating the two sections on separate pieces of catalyst
material.
5. The exhaust gas after-treatment system recited in claim 2
wherein the physical separation between the two sections is
provided by zone-coating both sections on the same catalyst
body.
6. The exhaust gas after-treatment system recited in claim 1
wherein both the oxidation section and the NO.sub.x storing section
contain Pt, in various proportions, with the Pt providing a CO and
HC oxidation catalyst in the oxidation section and primarily as a
NO.sub.x oxidation catalyst in the NO.sub.x storing section second
section.
7. The exhaust gas after-treatment system recited in claim 1
wherein the ratio of the volume of the oxidation section to the
NO.sub.x storing section, ranges from {fraction (1/10)} to 1 and
more preferably from {fraction (1/10)} to 1/3.
8. An exhaust gas after-treatment system, comprising: a second
section having therein: a NO.sub.x oxidation component; a NO.sub.x
storage components; and a NO.sub.x reduction components; and a
first section for oxidizing hydrocarbons and carbon monoxide in the
exhaust gas. such first section being physically separate from the
second section, such first section being substantially free of the
NO.sub.x storage component and the NO.sub.x reduction
component.
9. The system recited in claim 9 wherein the first section is
upstream of the second section.
10. A method for treating exhaust gas produced by an internal
combustion engine, comprising: oxidizing hydrocarbons and carbon
monoxide in the exhaust gas; storing and reducing NO.sub.x in the
exhaust gas; wherein the oxidizing and storing/reducing functions
are performed as separate, sequential processes on the exhaust
gas.
11. An exhaust gas after-treatment system for an internal
combustion engine, comprising: a lean NO.sub.x trap, such lean
NO.sub.x trap comprising: an oxidation section having an oxidation
material for oxidizing hydrocarbons and carbon monoxide in the
exhaust gas; and a NO.sub.x storage section, such NO.sub.x storage
section having a NO.sub.x storing material for storing NO.sub.x in
the exhaust gas and noble metal components for both oxidizing NO
during NO.sub.x storage and reducing released NO.sub.x during trap
regeneration; and wherein the oxidation material in the oxidation
section is physically separated from the NO.sub.x storing material
and noble metal components in the NO.sub.x storage section.
12. The exhaust gas after-treatment system recited in claim 11
wherein the hydrocarbon and carbon monoxide oxidation material
includes Pt and/or other oxidation catalyst material.
13. The exhaust gas after-treatment system recited in claim 12
wherein the NO.sub.x storing material stores and releases NO.sub.x
in an operating temperature range of diesel exhaust gases.
14. The exhaust gas after-treatment system recited in claim 11
wherein both the oxidation section and the NO.sub.x storing section
contain Pt, in various proportions, such that the Pt is utilized
primarily as a CO and HC oxidation catalyst in the oxidation
section and primarily as a NO.sub.x oxidation catalyst in the
NO.sub.x storing section.
15. The exhaust gas after-treatment system recited in claim 11
wherein the ratio of the volume of the oxidation section relative
to the storing section ranges from {fraction (1/10)} to 1 and more
preferably from {fraction (1/10)} to 1/3.
16. An exhaust gas after-treatment system, comprising: a second
section having therein: one or more NO.sub.x oxidation components;
one or more NO.sub.x storage components; and one or more NO.sub.x
reduction components; and a first section for oxidizing
hydrocarbons and carbon monoxide in the exhaust gas. such first
section being physically separate from the second section, such
first section being substantially free of the NO.sub.x storage
component(s) and the NO.sub.x reduction component(s).
17. The system recited in claim 16 wherein the first section is
upstream of the second section.
Description
TECHNICAL FIELD
[0001] This invention relates to exhaust after-treatment systems
and more particularly to exhaust after-treatment systems for lean
burn internal combustion engines.
BACKGROUND AND SUMMARY
[0002] As is known in the art, precious metal three-way catalysts
are generally used as a means for removing pollutants from the
exhaust gas of an internal combustion engine. These three-way
catalysts remove CO, HC, and NO.sub.x simultaneously from engine
exhaust gases under stoichiometric conditions. However, under lean
fuel conditions, which are desired for optimal fuel efficiency, the
three-way catalyst is ineffective for the removal of NO.sub.x.
Accordingly, to achieve NO.sub.x control under fuel lean
conditions, exhaust after-treatment systems have included a lean
NO.sub.x trap (LNT).
[0003] An LNT has 3 essential components:
[0004] 1) a NO.sub.x storage medium (also called compound or
component). Prototypically, this is barium. Barium never exists by
itself; it will always be present in the form of a compound in the
trap, e.g., barium carbonate. Other storage components are those of
the alkali metal group (especially potassium and cesium) and other
alkaline earth elements besides Ba (e.g., strontium and
magnesium).
[0005] 2) a NO oxidation component. NO.sub.x is present in engine
exhaust gases as a mixture of NO and NO.sub.2. It is stored as a
nitrate species (NO.sub.3). To convert to the nitrate form, both
the NO and NO.sub.2 must be oxidized (i.e. reacted with oxygen from
the exhaust gas). Platinum is the prototypical metal for doing
that, but other metals have oxidation capability.
[0006] 3) a reducing component. Regeneration of the trap involves
driving the exhaust gas to rich conditions (i.e. excess of
reductant species such as carbon monoxide, hydrogen, and
hydrocarbons) and reacting the adsorbed nitrate back to nitrogen.
This is similar to the way NO.sub.x is treated in a three-way
catalyst. Rhodium is the prototypical element for NO.sub.x
reduction and it is used in most LNTs for the purpose of
regenerating the trap.
[0007] Those are the three main components. Additionally, a high
surface support phase is used such as alumina over which all the
components are dispersed to create finally divided, small particles
of all the active components. Various stabilizers and so-called
oxygen storage materials are often added as well.
[0008] An additional function of the Pt in the LNT is to combust
reductants such as CO, H.sub.2, and HC to release heat needed to
raise the operating temperature of the LNT to the high temperature
levels required for removal of stored sulfur.
[0009] Thus, the LNT includes material to oxidize the CO and HC and
material to store NO.sub.x. Presently, however, the performance of
NO.sub.x trap technology is limited in several respects. NO.sub.x
trap performance is affected by the relatively narrow operating
temperature window of current trap formulations. At temperatures
outside this window, the system may not operate efficiently and
NO.sub.x emissions can increase.
[0010] Both three-way catalysts and lean NO.sub.x traps (LNT) are
generally inefficient at ambient temperatures and must reach high
temperatures before they are activated. Typically, contact with
high-temperature exhaust gases from the engine elevates the
temperature of the catalyst or LNT. The temperature at which a
catalytic converter can convert 50% of CO, HC, or NO.sub.x is
referred to as the "light-off" temperature of the converter.
[0011] During start up of the engine, the amount of CO and HC in
the exhaust gas is typically higher than during normal engine
operation. While a large portion of the total emissions generated
by the engine is generated within the first few minutes after start
up, the catalysts are relatively ineffective because they will not
have reached the "light-off" temperature. In other words, the
catalysts are the least effective during the time they are needed
the most.
[0012] As noted above, in order to achieve NO.sub.x control in lean
burn engines, exhaust after-treatment systems have included an
additional NO.sub.x storage device often referred to as a lean
NO.sub.x trap (LNT). Presently, however, the performance of
NO.sub.x trap technology is limited in several respects. NO.sub.x
trap performance is affected by the operating temperature and
requires a relatively narrow temperature-operating window of the
exhaust gases. At temperatures outside this window, the system will
not operate efficiently and NO.sub.x emissions will increase.
Exposure to high temperature will also result in permanent
degradation of the NO.sub.x trap capacity.
[0013] The LNT is purged periodically to release and convert the
oxides of nitrogen (NO.sub.x) stored in the trap during the
preceding lean operation. To accomplish the purge, the engine has
to be operated at an air-to-fuel ratio that is rich of
stoichiometry. As a result of the rich operation, substantial
amounts of feedgas carbon monoxide (CO) and hydrocarbons (HC) are
generated to convert the stored NO.sub.x. Typically, the purge mode
is activated on the basis of estimated trap loading. That is, when
the estimated mass of NO.sub.x stored in the trap exceeds a
predetermined threshold, a transition to the purge mode is
initiated. The rich operation continues for several seconds until
the trap is emptied of the stored NO.sub.x, whereupon the purge
mode is terminated and the normal lean operation is resumed. The
end of the purge is usually initiated by a transition in the
reading of the HEGO sensor located downstream of the trap, or based
on the model prediction of the LNT states. Since the engine is
operated rich of stoichiometry during the purge operation, the fuel
economy advantage of the lean operation is lost.
[0014] In addition to normal trap regeneration, the LNT may also be
subjected to a much higher temperature regeneration process for the
removal of stored sulfur (typically temperatures in excess of 600
degrees Celsius). Furthermore, if the LNT is contained in an
exhaust system that also contains a diesel particulate filter
(DPF), the LNT may also be subjected to temperatures in excess of
500 degrees Celsius during regeneration of the DPF (i.e. removal of
accumulated carbonaceous (i.e. soot) material via combustion with
oxygen in the exhaust gas). Both of these processes can result in
permanent, gradual deterioration in NO.sub.x trap performance--more
so even than normal trap regeneration to remove stored
NO.sub.x.
[0015] More particularly, as noted above, a LNT has both functions
of oxidation of HC and CO, etc. and storage/reduction of NO.sub.x.
In a conventional LNT, as shown in FIG. 1, an oxidation material
(namely platinum, Pt) used to oxidize the HC and CO is included
along with additional components such as rhodium (Rh), used for
NO.sub.x reduction, and barium (Ba) used to store the NO.sub.x. The
inventors have discovered that the exposure of the lean NO.sub.x
trap (LNT) to temperatures in the range of 600 to 700 degrees
Celsius, especially under the oxidizing conditions required for DPF
regeneration, can cause the deterioration of the LNT especially its
"light off" function, and largely reduces its low temperature
NO.sub.x reduction efficiency. The inventors speculate that it is
one or more of the major components of the LNT (i.e., such as
rhodium (Rh) and barium (Ba)) that interacts with the Pt in a
deleterious way following the high temperature operation of the LNT
required for de-sulfurization and/or DPF regeneration (if such DPF
is serially connected in the system). For example, it is known that
Rh and Pt can form alloys, and it may turn out that the high
temperature conditions required for LNT desulfurization and/or DPF
regeneration causes the Pt and Rh to alloy in the LNT in such a way
that the oxidation activity of the Pt is adversely affected.
[0016] In accordance with the present invention, an exhaust gas
after-treatment system is provided having a NO.sub.x storage
material in a NO.sub.x storage section and an HC and CO oxidation
catalyst in a separate HC and CO oxidation section, such oxidation
section being substantially free of the NO.sub.x storage
material.
[0017] In one embodiment the oxidation section is substantially
free of Rh.
[0018] With such an arrangement, the HC and CO oxidation catalyst
is physically separated from the NO.sub.x storage material. Thus,
the oxidation catalyst used in the oxidation section will not
become adversely affected by any alloying or other types of
interactions with components contained in the NO.sub.x storing
section.
[0019] In one embodiment, the oxidation catalyst is Pt for
generating heat required to "light off". Thus, while it is known
that Pt is an effective NO.sub.x oxidation catalyst, the negative
effects described above of using the Pt completely in conjunction
with the NO.sub.x storage material such as Ba and reducing
components such as Rh are avoided by separating part of the Pt out
into a separate oxidation (combustion) catalyst preceding the
NO.sub.x storage section.
[0020] In one embodiment, an exhaust gas after-treatment system is
provided. The system includes in one section thereof, a NO.sub.x
oxidation component, a NO.sub.x storage component, and a NO.sub.x
reduction component, and, in a separate section thereof, a
catalytic HC and CO combustion section substantially free of the
NO.sub.x storage component and the NO.sub.x reduction
component.
[0021] In accordance with another feature of the invention, a
method is provided for treating exhaust gas produced by an internal
combustion engine. The method includes oxidizing hydrocarbons and
carbon monoxide present in the exhaust gas and storing NO.sub.x in
the exhaust gas; wherein the oxidizing and NO.sub.x storing are
performed as separate, sequential processes on the exhaust gas.
[0022] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a diagram of an after-treatment system coupled to
the exhaust of an internal combustion engine, such after-treatment
system having a Lean NO.sub.x Trap (LNT) according to the prior
art;
[0024] FIG. 2 is a diagram of an after-treatment system coupled to
the exhaust of an internal combustion engine, such after-treatment
system providing NO.sub.x storage and HC and CO oxidation according
to the invention;
[0025] FIG. 3 is a diagram of an after-treatment system coupled to
the exhaust of an internal combustion engine, such after-treatment
system providing NO.sub.x storage and HC and CO oxidation according
to another embodiment of the invention;
[0026] FIG. 4 are curves showing NO.sub.x conversion percentage as
function of LNT temperature with and without deterioration by a
de-SO.sub.x treatment of the trap at 600 degrees Celsius for 16
hours; and
[0027] FIG. 5 are curves showing the effect of an HC and CO
oxidation section separate from a NO.sub.x storage section
according to the invention with the prior art, each of three curves
therein showing the functional relationship between NO.sub.x
conversion percent as a function of temperature, one of the curves
being associated with an exhaust gas after-treatment system having
an HC and CO oxidation section separate from a NO.sub.x storage
section according to the invention, another one of the curves being
associated with a LNT according to the prior art, and the third one
of the curves being associated with an LNT which has not been
deteriorated;
[0028] FIGS. 6 and 7 are curves showing the inlet and the catalyst
middle temperatures for the two tests showed in FIG. 5 at 200
degrees Celsius, which are the deteriorated LNT (1" long) and the
same LNT (1" long) plus a 1/8" thick diesel oxygen catalyst (DOC)
mounted in front of the it.
[0029] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0030] Referring now to the drawing and initially to FIG. 2, a
block diagram of an exhaust gas after-treatment system 10 coupled
to an internal combustion engine 12, here a diesel engine. The
exhaust gas after-treatment system 10 has two separate sections 14,
16. The first section 14 is used to combust reductants such as CO,
H.sub.2, and HC and is substantially free of the NO.sub.x storage
component and the NO.sub.x reduction component. Here, the first
section 14 contains platinum, for example, as the active combustion
component. The second section 16 provides NO.sub.x storage and
includes: a NO.sub.x oxidation component, here for example,
platinum, Pt; a NO.sub.x storage component, here for example,
barium, Ba, and a NO.sub.x reduction component, here for example,
rhodium, Rh. The first section 14 is upstream of the second section
16.
[0031] In FIG. 2, the second section 16 is in a separate housing
from the first section 14. The first and second sections 14, 16 are
then physically attached by any convenient means, such as welding
the two sections together. Note that as drawn, the exhaust gas
after-treatment system 10 is comprised of cylindrical flow-through
devices. Such devices are nominally monolithic honeycomb type
structure catalysts containing the active components dispersed on
either ceramic or metallic type substrates of various cell
densities, wall thicknesses, length, shape (e.g., round, oval, or
racetrack). Furthermore, sections 14 and 16 can either be separated
from one another as shown in the diagram or butted against one
another. In FIG. 3, the first and second sections 14, 16 are
contained on the same substrate body via a process known as
zone-coating wherein two different catalyst washcoat formulations
are coated on different regions of the substrate body. In both
embodiments, the first section 14 is used to combust reductants
such as CO, H.sub.2, and HC and is substantially free of the
NO.sub.x storage component and the NO.sub.x reduction component and
the second section 16 provides NO.sub.x storage and includes: a
NO.sub.x oxidation component; a NO.sub.x storage component; and a
NO.sub.x reduction component.
[0032] It is noted that, in FIGS. 2 and 3, the exhaust gases from
the engine 12 pass sequentially, i.e., serially, through the first
section 14 and the second section 16. Thus, a method is provided
for treating exhaust gases from an internal combustion engine. The
method includes oxidizing hydrocarbons and carbon monoxide in the
exhaust gas and storing NO.sub.x in the exhaust gas; wherein the
oxidizing and storing are performed as separate, sequential
processes on the exhaust gas after-treatment device.
[0033] Both the oxidation section and the NO.sub.x storing section
contain Pt, in various proportions, with the Pt providing a CO and
HC oxidation catalyst in the oxidation section and primarily as a
NO.sub.x oxidation catalyst in the NO.sub.x storing section second
section. The ratio of the volume of the oxidation section to the
NO.sub.x storing section, ranges from {fraction (1/10)} to 1 and
more preferably from {fraction (1/10)} to 1/3.
[0034] With the exhaust gas after-treatment system of either FIG. 2
or FIG. 3 the NO.sub.x reduction efficiency is improved over the
system of FIG. 1 at low temperature. More particularly, the
inventors have observed that frequent de-sulfurization of the
diesel lean NO.sub.x trap (LNT) at 600 to 700 degrees Celsius can
cause the deterioration of the LNT especially its light off
function, and largely reduces its low temperature NO.sub.x
reduction efficiency as shown in FIG. 4, which contains two
NO.sub.x conversion vs. catalyst inlet temperature curves tested
over core (1" diameter with 1" length) samples at 30,000 s.v./hr
(Note that s.v. refers to space velocity, a term commonly used to
characterize the amount of gas flow through the catalyst body in
relation to the volume of the catalyst body; e.g. cubic feet of gas
flow per hour divided by the cubic feet of volume of the catalyst
body based on external dimensions. The space velocity therefore
carries the units of inverse time, e.g., 1/hr. With regard to space
velocity, it is also a convenient measure for matching
laboratory-scale experiments as reported here to larger scale
applications such as would be practiced on a vehicle. Hence, the 1"
diameter by 1"length laboratory samples used in the laboratory at
relatively low gas flow rates may translate into a 6" diameter by
6" length catalyst unit on a vehicle at much higher flow rates. The
exact dimensions could be adjusted to yield the same s.v. in both
cases, however, and those skilled in the art will recognize that
the s.v. can vary between about 5000/hr to 50,000/hr under
conditions experienced in automotive diesel exhaust). The diesel
oxidation catalyst formulation is much more stable in the
de-sulfurization temperature range (600 to 700 degrees Celsius)
than the LNT.
[0035] Referring specifically to the embodiment shown in FIG. 3,
the zone coating of an oxidation formulation (i.e., the first
section 14) in a small area of the inlet of the monolith body or
attachment of a small piece of diesel oxidation catalyst in front
of the second section 16 (FIG. 2), helps to maintain the
"light-off" property of the aged LNT. The rich condition in the
diesel LNT vehicle operation is unique from gasoline (TWC, or LNT)
or diesel SCR with about 1% oxygen in rich condition (gasoline
exhaust contains much lower levels of oxygen for an equivalent
degree of richness). Consequently, much more reaction heat, or
exothermic temperature rise, can be generated in the diesel case.
With good "light-off" function, a LNT catalyst temperature can be
raised an additional 30 to 80 degrees Celsius utilizing the
embodiments of FIGS. 2 and 3, which can make quite a big impact on
the low temperature NO.sub.x reduction efficiency.
[0036] FIG. 5 are curves showing the effect of an HC and CO
oxidation section separate from a NO.sub.x storage section
according to the invention compared with the prior art, each of
three curves therein showing the functional relationship between
NO.sub.x conversion percent as a function of temperature, curve 20
is associated with an exhaust gas after-treatment system having an
HC and CO oxidation section separate from a NO.sub.x storage
section according to the invention, curve 22 is associated with a
LNT according to the prior art, and curve 24 is associated with an
LNT according to the prior art which has not been deteriorated;
[0037] Here, a one eighth inch long diesel oxidation catalyst,
first section 14 (1" diameter) is attached in front of a one inch
long aged second section 16 (i.e., the same piece as shown in FIG.
4 deteriorated by the de-sulfurization) Addition of the small
section of diesel oxidation catalyst improved the NO.sub.x
reduction from 10% to 70% with the same inlet temperature of 200
degrees Celsius. This specific diesel oxidation catalyst was aged
in a much more severe condition (670 degrees Celsius for 64 hrs)
than the LNT catalyst and also, it has the same Pt loading per unit
volume as the LNT.
[0038] Since the exhaust temperature of a light duty diesel vehicle
is usually in the range of 150 to 250 degrees Celsius, improving
the low temperature NO.sub.x reduction efficiency will have a large
impact on the overall vehicle NO.sub.x reduction efficiency.
[0039] The inventors have concluded that the main reason that the
NO.sub.x reduction efficiency of the LNT is improved by a 1/8
volume of diesel oxidation catalyst with same precious metal
loading per unit volume in front of it, is the "light-off" function
of the diesel oxidation catalyst, which raised the LNT operation
temperature with the same catalyst inlet temperature by burning the
CO, HC and H.sub.2 in the rich condition during the lean/rich cycle
since there is about 1% oxygen in the diesel LNT rich condition.
FIGS. 6 and 7 show the inlet (curve 30) and the catalyst middle
temperatures (curve 32) for the two tests shown in FIG. 5 at 200
degrees Celsius, which are the deteriorated LNT (1" long) and the
same LNT (1" long) plus a 1/8" thick diesel oxidation catalyst
(DOC) attached in front of it. Obviously, the 1/8" DOC helped raise
the LNT middle temperature by about 35 degrees Celsius, thus
resulting in much higher NO.sub.x conversion.
[0040] The zone coating of a DOC formulation at the inlet of a
catalyst will function similarly as attaching a same volume of DOC
catalyst in front of the catalyst.
[0041] A number of embodiments of the invention have been
described. It should be noted that the hydrocarbon and carbon
monoxide oxidation material might includes Pt and/or other
oxidation catalyst material. Further, the NO.sub.x storing material
might include Ba, or Cs, Na, K, Sr, and/or any other similar
material for storing and releasing NO.sub.x in operating
temperature range of diesel exhaust gases. Still further, it should
be noted that both the oxidation section and the NO.sub.x storing
section contain Pt, in various proportions, such that the Pt is
utilized primarily as a CO and HC oxidation catalyst in the
oxidation section and primarily as a NO.sub.x oxidation catalyst in
the NO.sub.x storing section. Also, the oxidation section might
include one or more CO and HC oxidation components with the
oxidation section being substantially free of the NO.sub.x storage
component(s) and the NO.sub.x reduction component(s). Thus, it will
be understood that various modifications may be made without
departing from the spirit and scope of the invention. Accordingly,
other embodiments are within the scope of the following claims.
* * * * *