U.S. patent application number 12/984436 was filed with the patent office on 2011-08-04 for low temperature diesel particulate matter reduction system.
This patent application is currently assigned to DONALDSON COMPANY, INC.. Invention is credited to Julian A. Imes, Timothy L. Ricke, Todd R. Taubert, Wenzhong Zhang.
Application Number | 20110185709 12/984436 |
Document ID | / |
Family ID | 38456521 |
Filed Date | 2011-08-04 |
United States Patent
Application |
20110185709 |
Kind Code |
A1 |
Zhang; Wenzhong ; et
al. |
August 4, 2011 |
Low Temperature Diesel Particulate Matter Reduction System
Abstract
A system for treating diesel exhaust is disclosed. The system
includes a first filter including layers of filtration material
positioned between layers of corrugated metallic foil. The metallic
foil defines a honeycomb arrangement of longitudinal passageways
from an upstream end to a downstream end and also openings for
allowing exhaust to pass between adjacent longitudinal passageways
of the metallic foil. The filtration material is positioned such
that exhaust between the adjacent longitudinal passageways passes
through the filtration material. The metallic foil also includes
flow diverting structures to divert flow within the longitudinal
passageways through the openings. A second filter is positioned
downstream from the first filter. The second filter defines a
honeycomb arrangement of longitudinal passageways. The longitudinal
passages are selectively plugged adjacent upstream and downstream
ends to force flow radially through walls between the longitudinal
passages of the second filter.
Inventors: |
Zhang; Wenzhong; (Savage,
MN) ; Taubert; Todd R.; (St. Paul, MN) ;
Ricke; Timothy L.; (Savage, MN) ; Imes; Julian
A.; (Bloomington, MN) |
Assignee: |
DONALDSON COMPANY, INC.
Minneapolis
MN
|
Family ID: |
38456521 |
Appl. No.: |
12/984436 |
Filed: |
January 4, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11725578 |
Mar 19, 2007 |
7862640 |
|
|
12984436 |
|
|
|
|
60784621 |
Mar 21, 2006 |
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Current U.S.
Class: |
60/295 |
Current CPC
Class: |
B01D 2267/40 20130101;
B01D 46/527 20130101; F01N 2330/44 20130101; F01N 3/0226 20130101;
F01N 2330/06 20130101; F01N 2510/06 20130101; F01N 2330/60
20130101; F01N 3/2821 20130101; F01N 3/0231 20130101; F01N 3/035
20130101; B01D 46/247 20130101; F01N 2430/00 20130101; F01N 3/0222
20130101; B01D 2046/2492 20130101; F01N 2330/38 20130101; F01N
13/009 20140601; B01D 53/9431 20130101; F01N 2330/10 20130101; B01D
46/2451 20130101; F01N 2510/0682 20130101; B01D 2279/30 20130101;
F01N 13/0093 20140601; F01N 2330/02 20130101 |
Class at
Publication: |
60/295 |
International
Class: |
F01N 3/023 20060101
F01N003/023; F01N 3/022 20060101 F01N003/022 |
Claims
1. A system for treating diesel exhaust, the system comprising: a
first filter including a first substrate having layers of
filtration material positioned between layers of corrugated
metallic foil, the corrugated metallic foil defining a honeycomb
arrangement of longitudinal passageways that extend from an
upstream end to a downstream end of the first substrate, the
corrugated metallic foil defining openings for allowing exhaust to
pass between adjacent longitudinal passageways of the corrugated
metallic foil, the filtration material being positioned such that
exhaust passing between the adjacent longitudinal passageways
passes through the filtration material, the corrugated metallic
foil also including flow diverting structures positioned to divert
exhaust flow within the longitudinal passageways through the
openings; and a second filter positioned downstream from the first
filter, the second filter including a second substrate defining a
honeycomb arrangement of longitudinal passageways that extend from
an upstream end to a downstream end of the second substrate, the
longitudinal passages being selectively plugged adjacent upstream
and downstream ends of the second substrate such that exhaust flow
is forced to flow radially through walls between the longitudinal
passages of the second substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/725,578, filed Mar. 19, 2007, which claims
the benefit of U.S. Provisional Patent Application Ser. No.
60/784,621, filed Mar. 21, 2006, which applications are hereby
incorporated by reference in their entireties.
TECHNICAL FIELD
[0002] The present disclosure relates generally to diesel engine
exhaust systems. More particularly, the present disclosure relates
to systems and methods for controlling diesel engine exhaust
emissions.
BACKGROUND
[0003] Diesel engine exhaust contains particulate matter, the
emission of which is regulated for environmental and health
reasons. This particulate matter generally constitutes a soluble
organic fraction ("SOF") and a remaining portion of hard carbon.
The soluble organic fraction may be partially or wholly removed
through oxidation in an oxidation catalyst device such as a
catalytic converter; however, this typically results in a reduction
of only about 20 percent of total particulate emissions. Thus,
vehicles equipped with diesel engines may include diesel
particulate filters for more completely removing the particulate
matter from the exhaust stream, including the hard carbon portion.
Conventional wall flow type diesel particulate filters may have
particulate removal efficiencies of about 85 percent. However,
diesel particulate filters, particularly those that have relatively
high particulate filtration efficiency, are generally associated
with high back pressures because of the restriction to flow through
the filter. Further, with use, soot or other carbon-based
particulate matter accumulates on the diesel particulate filters
causing the buildup of additional undesirable back pressure in the
exhaust systems. Engines that have large particulate mass emission
rates may develop excessive back pressure levels in a relatively
short period of time. High back pressures decrease engine
efficiency and reduce engine performance. Therefore, it is desired
to have diesel particulate filtration systems that minimize back
pressure while capturing a high percentage of the particulate
matter in the exhaust.
[0004] Conventional wall flow diesel particulate filters (DPFs) are
high particulate removal efficiency filters that include a
porous-walled honeycomb substrate (i.e., monolith) with channels
that extend generally from an upstream end to a downstream end of
the substrate. Generally half the channels are plugged adjacent the
downstream end of the substrate and the other half of the channels
are plugged adjacent the upstream end of the substrate. This
plugged configuration forces exhaust flow to pass radially through
the porous walls defining the channels of the substrate in order to
exit the diesel particulate filter.
[0005] To prevent diesel particulate filters from becoming
excessively loaded with particulate matter, it is necessary to
regenerate the diesel particulate filters by burning off (i.e.,
oxidizing) the particulates that accumulate on the filters. It is
known to those of skill in the art that one method by which
particulate matter may be oxidized is to raise the temperature of
the exhaust gas sufficiently to allow the excess oxygen in the
exhaust gas to oxidize the particulate matter. Also well-known to
those of skill in the art is that particulate matter may be
oxidized at a lower temperature in the presence of sufficient
amounts of nitrogen dioxide (NO.sub.2).
[0006] Diesel exhaust inherently contains nitrogen oxides
(NO.sub.x), which consist primarily of nitric oxide (NO) and
nitrogen dioxide (NO.sub.2). Typically, the NO.sub.2 inherently
present in the exhaust stream is a relatively small percentage of
total NO.sub.x, such as in the range of 5 to 20 percent but usually
in the range of 5 to 10 percent. Although some regeneration of a
diesel particulate filter occurs at such levels, it is insufficient
to result in complete regeneration. The effectiveness of NO.sub.2
in regenerating a particulate filter depends in part on the ratio
of NO.sub.x to particulate matter in the exhaust stream. Generally,
the reaction of "2NO.sub.2+C=CO.sub.2+2NO" requires 8 times
NO.sub.2 per unit of C in mass.
[0007] To promote full regeneration, it is often necessary to
increase the quantity of NO.sub.2 in the exhaust stream. This is
particularly true where the NO.sub.x/particulate ratio is
relatively small. One method to produce sufficient quantities of
NO.sub.2 is to use an oxidation catalyst to oxidize a portion of
the NO present in the exhaust stream to NO.sub.2. For example, a
catalytic converter including a diesel oxidation catalyst can be
positioned upstream from the diesel particulate filter and/or the
diesel particulate filter itself can include a diesel oxidation
catalyst. However, these types of prior art arrangements may result
in excessive NO.sub.2 emissions.
SUMMARY
[0008] One aspect of the present disclosure relates to a system for
reducing particulate material emissions in diesel engine exhaust.
In one embodiment, the system is adapted to optimize the use of
NO.sub.2 to remove particulate matter (PM) from the exhaust stream
and to passively regenerate a diesel particulate filter that is a
part of the system.
[0009] Another aspect of the present disclosure relates to a diesel
particulate filtration system that at least one upstream filter to
optimize the NO.sub.2 to PM ratio at a downstream filter. In one
embodiment, the upstream filter is a catalyzed flow-through filter,
and the downstream filter is a catalyzed wall flow filter.
[0010] Examples representative of a variety of inventive aspects
are set forth in the description that follows. The inventive
aspects relate to individual features as well as combinations of
features. It is to be understood that both the forgoing general
description and the following detailed description merely provide
examples of how the inventive aspects may be put into practice, and
are not intended to limit the broad spirit and scope of the
inventive aspects.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 schematically illustrates an exhaust system having
features that are examples of inventive aspects in accordance with
the principles of the present disclosure.
[0012] FIG. 2 illustrates an example flow-through filter that can
be used as an upstream filter in the system of FIG. 1;.
[0013] FIG. 3 illustrates an enlarged, exploded view of a portion
of the filter of FIG. 2;
[0014] FIG. 4 illustrates a further enlarged, exploded view of a
portion of the filter of FIG. 2;
[0015] FIG. 5 is a schematic representation showing the operation
of the filter of FIG. 2;
[0016] FIG. 6 is a cut-away view of an example wall flow filter
that can be used as the downstream filter in the system of FIG.
1;
[0017] FIG. 6A illustrates the wall flow filter of FIG. 6 coated
with a catalyst using a zone-coating technique;
[0018] FIG. 7 is an enlarged portion of FIG. 6;
[0019] FIG. 8 is a graph showing a FTP transient cycle;
[0020] FIG. 9 is a temperature profile graph for a diesel engine
for different torque cycling;
[0021] FIG. 10 is a graph that plots NO.sub.2 generation for 3 test
systems; and
[0022] FIG. 11 is a graph that plots particulate accumulation on
the downstream filter of the 3 test systems.
DETAILED DESCRIPTION
[0023] At relatively low temperatures (e.g., 200 to 350.degree.
C.), NO.sub.2 molecules are typically more active for combusting
soot than O.sub.2. NO.sub.2 reacts with soot according to the
following reaction: 2NO.sub.2+C=CO.sub.2+2NO. This reaction
requires 8 times more NO.sub.2 per unit of C in mass. The
NO.sub.2/PM ratio is a significant factor to boost this
reaction.
[0024] One way to increase the NO.sub.2/PM ratio at a filter is to
decrease the PM on the filter rather than increase the
concentration of NO.sub.2 at the filter. To achieve this goal, a
combination of an upstream filter and a downstream filter can be
used. The upstream filter can have a lower filtration efficiency
than the filtration efficiency of the downstream filter. In one
embodiment, the upstream filter includes a flow-through filter
(FTF), and the downstream filter includes a wall flow filter. The
system preferably optimizes the NO.sub.2/PM ratio on both filters
such that an optimum amount of NO.sub.2 is generated. Preferably,
the system allows for the effective passive regeneration of the
downstream filters at relatively low temperatures thereby
preventing plugging of the downstream filter, and also minimizes
the concentration of NO.sub.2 that exits the tailpipe.
[0025] Flow-through filters partially intercept solid PM particles
in exhaust. Some flow-through filters may exhibit a filtration
efficiency of 50% or less. As discussed above, in accordance with
the disclosure, while the first staged filter may be an FTF, the
downstream filter may be a wall-flow filter. The wall-flow filter
may have a filtration efficiency of at least 75% or higher. Both
filters may be catalyzed to remove and oxidize HC, CO, and PM.
Because of the flow-through nature, a portion of PM is intercepted
in the first filter and the rest of the PM passes to the downstream
high efficiency filter. The catalyst on the FTF may be chosen to
just oxidize a selected portion of NO coming from engine exhaust to
NO.sub.2. Then, a portion of the NO.sub.2 can be used to oxidize
captured PM, transferring the used NO.sub.2 back to NO, which can
be reused by catalyst inside the filter downstream before being
released.
[0026] The second filter may be catalyzed in such a way that the
NO.sub.2 being left over from the first filter and NO.sub.2 being
generated at the front section may be consumed by the captured soot
at the middle and rear section of the second filter. The
configuration of the system, including the design of the first
filter to achieve a desired filtration efficiency and oxidation
ability, allows the tailpipe NO.sub.2/NO.sub.x ratio to be reduced
to levels to meet California Air Resource Board Regulation.
[0027] Prior art systems have used a straight channel catalytic
converter positioned upstream from a wall flow filter to increase
the concentration of NO.sub.2 at the wall flow filter. The present
disclosure teaches using a flow-through filter upstream of the wall
flow filter instead of a straight channel catalytic converter.
Flow-through filters provide a number of advantages over catalytic
converters. For example, flow-through filters provide higher
residence times to allow locally generated NO.sub.2 to react with a
larger portion of PM (including both soluble organic fractions and
hard carbon constituents) coming from engine. This decreases the PM
portion that enters the down stream filter and increases
NO.sub.2/PM ratio inside the downstream filter. By optimizing the
NO.sub.2/PM ratio, the downstream filter is boosted to work
efficiently at lower temperatures. In contrast, catalytic converter
systems typically use a heavily catalyzed catalytic converter
upstream of a catalyzed DPF. Such a catalytic converter can consume
soluble fraction of particulate matters, but does not affect the
concentration of hard carbon soot in the exhaust. Thus, multistage
filtration with a catalyzed flow through pre-filter followed by a
catalyzed DPF is a better solution with maximized soot-NO.sub.2
residence time and minimized NO.sub.2 emissions at the
tailpipe.
[0028] In certain embodiments, the combination of the FTF and the
DPF may lead to a filtration efficiency of higher than 92% and
NO.sub.2/NO.sub.x ratio on a CAT 3126 engine over FTP cycle to 28%
which may exhibit a 20% increase of NO.sub.2/NO.sub.x percentage
across the device from the engine out NO.sub.2/NO.sub.x level. Such
a device may improve PM filtration efficiency and reduce the
system-out NO.sub.2 to meet CARB NO.sub.2 rule. The primary PM
reduction from the FTF can increase the NO.sub.x/PM ratio inside
the downstream DPF, hence the captured soot oxidized at a
relatively lower temperature, leading to lower application
criteria.
[0029] FIG. 1 illustrates an exhaust system 20 that is in
accordance with inventive aspects of the present disclosure. The
system includes an engine 22 (e.g., a diesel engine) and an exhaust
conduit 24 for conveying exhaust gas away from the engine 22. A
first diesel particulate reduction device 26 is positioned in the
exhaust stream. Downstream from the first diesel particulate
reduction device 26 is a second diesel particulate reduction device
28. It will be appreciated that the first diesel particulate
reduction device 26 and the second diesel particulate reduction
device 28 function together to treat the exhaust gas that passes
through the conduit 24. It will also be appreciated that the first
diesel particulate reduction device 26 and the second diesel
particulate reduction device 28 may be separated by any distance,
including being positioned in close proximity or even in direct
contact.
[0030] The first diesel particulate device 26 is preferably a
flow-through filter. Flow-through filters are filters that
typically have moderate particulate mass reduction efficiencies.
For purposes of this specification, particulate mass reduction
efficiency is determined by subtracting the particulate mass that
enters the filter from the particulate mass that exits the filter,
and by dividing the difference by the particulate mass that enters
the filter. The test duration and engine cycling during testing are
preferably determined by the Federal Test Procedure (FTP)
heavy-duty transient cycle that is currently used for emission
testing of heavy-duty on-road engines in the United States (see CFR
Title 40, Part 86.1333). A typical flow-through filter has a
particulate mass reduction efficiency of 50 percent or less.
[0031] Certain flow-through filters do not require all of the
exhaust gas traveling through the filter to pass through a filter
media having a pore size sufficiently small to trap particulate
material. One embodiment of a flow-through filter includes a
plurality of flow-through channels that extend longitudinally from
the entrance end to the exit end of the flow-through filter. The
flow-through filter also includes filter media that is positioned
between at least some of the flow-through channels. The filter
further includes flow diversion structures that generate turbulence
in the flow-through channels. The flow diversion structures also
function to divert at least some exhaust flow from one flow-through
channel to another flow-through channel. As the exhaust flow is
diverted from one flow-through channel to another, the diverted
flow passes through the filter media causing some particulate
material to be trapped within the filter media. This
flow-through-type filter yields moderate filtration efficiencies,
typically up to 50% per filter, with relatively low back
pressure.
[0032] A catalyst coating (e.g., a precious metal coating) can be
provided on the flow-through channels of the flow-through filter to
promote the oxidation of the soluble organic fraction (SOF) of the
particulate matter to gaseous components and to promote the
oxidation of a portion of the nitric oxide (NO) within the exhaust
gas to nitrogen dioxide (NO.sub.2). Furthermore, the filter media
of the flow-through filter captures a portion of the hard carbon
particulate matter and a portion of the non-oxidized SOF present in
the exhaust. A portion of the net NO.sub.2 present, comprising the
combination of the NO.sub.2 generated by the oxidation catalyst and
the NO.sub.2 inherently present in diesel exhaust, reacts with the
particulate matter trapped on the filter media, according to the
reaction NO.sub.2+C=CO (or CO.sub.2)+NO. In doing so, the solid
particulate matter is converted to a gas, which flows out of the
particulate reduction device. To enhance to combustion of carbon at
the filter media, the filter media can also be coated with a
catalyst (e.g., a precious metal such as platinum).
[0033] The first diesel particulate reduction device 26 can also be
referred to as an upstream diesel particulate reduction device 26.
An example upstream diesel particulate reduction device 26 is shown
at FIGS. 2-5. The device 26 includes a canister 27 housing a
substrate (e.g., a honeycomb body) constructed from multiple layers
of filtration material 30 sandwiched between layers of corrugated
metallic foil 32. The corrugated metallic foil 32 defines elongated
passageways 34 (i.e., channels) that are generally parallel to a
net flow direction 7 of exhaust gases through the particulate
reduction device. The metallic foil 32 preferably includes
structures that generate turbulence for ensuring that mixing occurs
within the substrate. In the depicted embodiment, the structures
include openings 33 and flow diverting surfaces 35 (i.e., mixing
surfaces, deflecting surfaces, mixing shovels, flow diversion
structures or like terms). The flow diverting surfaces 35 cause
some flow to be diverted within the passageways 34 from the net
flow direction 7 to transverse directions 9 and radial directions
11. At least some of the diverted flow travels through the openings
33 between adjacent passageways 34 and through the filtration
material 30. As the exhaust flow travels through the filtration
material 30, at least some particulate material of the exhaust
stream is captured by the filtration material 30. As shown at FIG.
5, the diverting surfaces 35 do not completely block/plug the
passageways 34. This assists in keeping the pressure drop across
the device 26 relatively low.
[0034] In one embodiment, the filtration material 30 is a
woven-type material constructed from metallic fibers (e.g., a
metallic fabric or fleece) which capture particles both by
impingement and by blocking their flow. The particle-blocking
properties of the filtration material 30 are determined in part by
the diameter of the metallic fibers used to construct the fleece.
For example, metallic fibers of 20 to 28 microns (millionths of a
meter) and 35 to 45 microns have been found to work acceptably. As
the exhaust gases flow out of the foil 32 and into the filtration
material 30, significant internal turbulence is induced. Of course,
types of filtration material other than metallic fleece could also
be used
[0035] In one embodiment, the device 26 has a diameter of about
10.5 inches and a length of about 3 inches, with 200 cpsi. In
certain embodiments, the residence time of the device 26 can be at
least 10% or 15% greater than the residence time of a standard
straight channel flow-through catalytic converter having the same
space velocity.
[0036] The space velocity (i.e., the volumetric flow rate of the
exhaust gas divided by the volume of the particulate reduction
device) of the upstream particulate removal device 26 is greater
than the space velocity of the downstream particulate removal
device 28. In certain embodiments, the space velocity of the
upstream particulate reduction device is equal to at least 2, 3 or
4 times the space velocity of the downstream particulate reduction
device for a given volumetric flow rate. In other embodiments, the
space velocity of the upstream particulate reduction device is
equal to 2-6 or 3-5 times the space velocity of the downstream
particulate reduction device for a given volumetric flow rate. In
still other embodiments, the device 26 can have a particulate mass
reduction efficiency of 15-50 percent or 20-50 percent.
[0037] In a preferred embodiment, the first diesel particulate
reduction device 26 is manufactured by Emitec Gmbh and sold under
the name "PM Kat." The device 26 may, however, comprise any
flow-through-type construction known to those of skill in the art,
such as wire mesh, metallic or ceramic foam. Further details
relating to the constructions of the Emitec filters suitable for
use as upstream filters can be found at U.S. Patent Application
Publication Numbers US 2005/0232830, US 2005/0274012 and US
2005/0229590, which are hereby incorporated by reference in their
entireties.
[0038] The upstream diesel particulate reduction device 26 also
contains a catalyst coating adapted to promote the oxidation of
hydrocarbons and the conversion of NO to NO.sub.2. Exemplary
catalyst coatings include precious metals such as platinum,
palladium and rhodium, and other types of components such as
alumina, cerium oxide, base metal oxides (e.g., lanthanum,
vanadium, etc,) or zeolites. A preferred catalyst for the first
particulate reduction device 26 is platinum with a loading level
greater than 50 grams/cubic foot of substrate. In other embodiments
the platinum loading level is in the range of 50-100 grams/cubic
foot of substrate. In a preferred embodiment, the platinum loading
is about 70 grams/cubic foot.
[0039] In a preferred embodiment, the catalyst coating is available
from Intercat, Inc.
[0040] In one embodiment, the device 26 may exhibit a 27% PM
reduction efficiency. In one embodiment, the NO.sub.2/NO.sub.x
ratio at the out end of the device 26 on a CAT 3126 engine over FTP
cycle is around 32%.
[0041] The second diesel particulate reduction device 28, also
called the downstream diesel particulate reduction device 28, can
have a variety of known configurations. As shown at FIG. 6, the
device 28 is depicted as a wall-flow filter having a substrate 50
housed within an outer casing 52. In certain embodiments, the
substrate 50 can have a ceramic (e.g., a foamed ceramic) monolith
construction. A mat layer 54 can be mounted between the substrate
50 and the casing 52. Ends 56 of the casing can be bent radially
inwardly to assist in retaining the substrate 50 within the casing
52. End gaskets 58 can be used to seal the ends of the device 28 to
prevent flow from passing through the mat to by-pass the substrate
50.
[0042] In one embodiment, the device 28 has a diameter of about
10.5 inches and a length of about 12 inches, with 200 cpsi/12
mil.
[0043] Referring to FIG. 6, the substrate 50 includes walls 60
defining a honeycomb arrangement of longitudinal passages 62 (i.e.,
channels) that extend from a downstream end 63 to an upstream end
64 of the substrate 50. The passages 62 are selectively plugged
adjacent the upstream and downstream ends 63, 64 such that exhaust
flow is forced to flow radially through the walls 60 between the
passages 62 in order to pass through the device 28. As shown at
FIG. 7, this radial wall flow is represented by arrows 66.
[0044] An example diesel particulate reduction device is a
wall-flow filter having a monolith ceramic substrate including a
"honey-comb" configuration of plugged passages as described in U.S.
Pat. No. 4,851,015 that is hereby incorporated by reference in its
entirety. Example materials for manufacturing the substrate 50
include cordierite, mullite, alumina, SiC, refractory metal oxides,
or other materials conventionally used as catalyzed substrates. In
a preferred embodiment, the device 28 includes a diesel particulate
filter sold by Engelhard Corporation under the name "DPX
Filter."
[0045] In certain embodiments, the substrate 50 can be coated a
catalyst. Exemplary catalysts include precious metals such as
platinum, palladium and rhodium, and other types of components such
as base metal oxides or rare earth metal oxides. In certain
embodiments, the substrate 50 has a platinum loading of 30-80 grams
per cubic foot. In a preferred embodiment, the substrate 50 has a
platinum loading of about 50 grams per cubic foot. In another
embodiment of the diesel particulate reduction device 28, the
substrate 50 may have a precious metal loading of about 25 grams
per cubic foot, wherein the filter is coated substantially
uniformly throughout its length. In one embodiment of the diesel
particulate device 28, the substrate 50 may have a precious metal
loading between about 5 and 35 grams per cubic foot, wherein the
filter is coated substantially uniformly throughout its length.
[0046] The upstream particulate reduction device 26 preferably has
a higher precious metal loading than the downstream particulate
reduction device 28. In certain embodiments, the precious metal
loading of the upstream device 26 is at least 10 percent, 20
percent, 30 percent or 40 percent higher than the precious metal
loading of the downstream device 28. In other embodiments, the
precious metal loading of the upstream device 26 is in the range of
10-80 percent, 20-60 percent or 30-50 percent higher than the
precious metal loading of the downstream device 28.
[0047] In certain embodiments, catalyst coating of the substrate 50
may be banded with first 2 inches being coated at 48 g/ft3 and the
last 10 inches being coated at 2 g/ft3 for a filter having a length
of 12 inches. In this embodiment, the downstream particulate
reduction device exhibited a filtration efficiency higher than 85%.
In one example operation of the system, the NO.sub.2/NO.sub.x ratio
out of the filter on a CAT 3126 engine over FTP cycle was
essentially the same as the ratio out of the engine at 8%.
[0048] As illustrated in FIG. 6A, in certain embodiments, the
downstream device 28 may be zone-coated with a catalyst, wherein
the substrate 50 is coated at the ends with a wash coat including a
catalyst and is not coated with a wash coat at the middle of the
substrate 50. As illustrated in FIG. 6A, first and third zones 71,
73, respectively, are located at the ends of the substrate 50 and
are coated with a wash coat. A second, middle zone 72 is positioned
between the first and the third zones 71, 73, respectively, and is
not coated with a wash coat.
[0049] The sizes of the wash coated zones may vary in different
embodiments of the filter. For example, in certain embodiments, the
first coated zone 71 of the filter 28 may be between about 1/6 and
1/3 of the length of the filter 28 and the third coated zone 73 may
be between about 1/6 and 1/3 of the length of the filter 28.
[0050] The precious metal loading values may also vary in different
embodiments of the filter. In certain embodiments, the precious
metal loading of the first zone 71 may be between about 25 and 50
grams/cubic foot and the precious metal loading of the third zone
73 may be between about 5 and 50 grams/cubic foot. In certain
embodiments, the overall precious metal loading of the filter 28
may be between about 5 and 35 grams/cubic foot.
[0051] In one embodiment, for a filter that is 12 inches in length
and 10.5 inches in diameter, the first and the last 3 inches of the
device 28 may be wash-coated with a catalyst at a precious metal
loading of about 50 grams/cubic foot and the middle 6 inches may be
left uncoated. In such an embodiment, the overall precious metal
loading of the filter 28 would be around 25 grams/cubic foot.
[0052] In another embodiment, the filter 28 may be zone-coated, but
with different levels of loading on the coated portions. For
example, for a filter that is 12 inches in length and 10.5 inches
in diameter, the first (inlet) 3 inches may be wash coated with 50
grams/cubic foot of precious metal loading and the last (outlet) 3
inches may be coated with 10 grams/cubic foot of precious metal
loading and the middle 6 inches may be left uncoated. In such an
embodiment, the overall precious metal loading of the filter 28
would be around 15 grams/cubic foot.
[0053] Further details of zone-coating of catalysts can be found at
U.S. Provisional Patent Application Ser. No. 60/835,953, entitled
"CRACK RESISTANT SUBSTRATE FOR AN EXHAUST TREATMENT DEVICE", the
entire disclosure of which is hereby incorporated by reference.
[0054] It should be noted that similar zone-coating techniques for
catalysts may be used in the upstream diesel particulate reduction
device 26 as well.
[0055] The diesel particulate reduction device 28 preferably has a
particulate mass reduction efficiency greater than 75%. More
preferably, the diesel particulate reduction device 28 has a
particulate mass reduction efficiency greater than 85%. Most
preferably, the diesel particulate reduction device 28 has a
particulate mass reduction efficiency equal to or greater than 90%.
It is preferred for the particulate reduction device 28 to have a
higher particulate mass reduction efficiency than the particulate
reduction device 26. In certain embodiments, the particulate mass
reduction efficiency of the device 28 is at least 50, 100, 200,
300, 400 or 500 percent higher than the particulate mass reduction
efficiency of the device 26. In other embodiments, the particulate
mass reduction efficiency of the device 28 is at least 50-600 or
100-500 or 200-500 percent higher than the particulate mass
reduction efficiency of the device 26.
[0056] Preferably, to ensure regeneration without excessive
NO.sub.2 emissions, the ratio of the mass of NO.sub.2 to the mass
of particulate matter in the exhaust stream between the upstream
device 26 and the downstream device 28 is preferably between 8 and
14. More preferably, this ratio is between 8 and 12. In certain
embodiments, it is desirable for the concentration of NO.sub.2
between the devices 26, 28 to be in the range of 50-700 parts per
million. In other embodiments, the ratio of NO.sub.2 to total
NO.sub.x between the devices 26, 28 is in the range of 20-55
percent or in the range of 30-50 percent. The ratio of NO.sub.2 to
NO.sub.x can be determined by measuring the total amount of
NO.sub.2 and the total amount of NO.sub.x in the exhaust stream
between the upstream and downstream filters, and the dividing the
total NO.sub.2 by the total NO.sub.x to obtain a flow weighted
average over a given test period. An example test period and engine
cycling protocol during testing are set for by the FTP heavy-duty
transient cycle that is currently used for emission testing of
heavy-duty on-road engines in the United States.
[0057] In operation of the system, a first portion of the
particulate matter contained in the diesel exhaust is deposited on
the first diesel particulate reduction device 26 in an amount that
is a function of the particle capture efficiency of the first
diesel particulate reduction device 26. The exhaust gas exits the
first diesel particulate reduction device 26 containing a residual
portion of particulate matter, defined as the amount of particulate
matter not deposited on the first diesel particulate reduction
device 26. The exhaust gas thereafter enters the second diesel
particulate reduction device 28, where a portion of the particulate
matter present in the exhaust gas is deposited on the second diesel
particulate reduction device 28 according to the particle capture
efficiency of the second diesel particulate reduction device
28.
[0058] Simultaneously, as the exhaust gases travel through the
first diesel particulate reduction device 26, the SOF portion of
particulate matter is oxidized by contact with the oxidation
catalyst coating. Furthermore, the NO present within the exhaust
stream is converted to NO.sub.2 by the oxidation catalyst coating
within the first diesel particulate reduction device 26. A portion
of this NO.sub.2, along with the NO.sub.2 inherently present in the
exhaust gas, reacts with the particulate matter trapped on the
first diesel particulate reduction device 26. By the reaction of
NO.sub.2+=NO+CO or CO.sub.2, a portion of the particulate matter is
oxidized and converted from a solid carbon form to carbon monoxide
or carbon dioxide gas, which thereby exits the particulate
reduction device. There is insufficient mass of soot, however,
trapped on the first diesel particulate reduction device 26 to
completely consume the NO.sub.2 present in the exhaust stream.
[0059] Consequently, the exhaust gas exiting the first diesel
particulate reduction device 26 contains a residual portion of
NO.sub.2. This exhaust gas then enters the second diesel
particulate reduction device 28 and the NO.sub.2 in the exhaust
stream reacts with soot on the device 28, converting a portion of
the NO.sub.2 into NO and regenerating the device 28. In this way,
particulate matter is captured and the particulate reduction
devices are regenerated while minimizing NO.sub.2 emissions.
[0060] Moreover, the preferred design of the particulate reduction
devices create significant internal, three-dimensional, turbulent
flow patterns by virtue of the highly tortuous, twisted flow
vectors that result from flow impacting into the filtration
material 30 and being channeled into and out of the openings in the
corrugated foil 32. Other flow-through filter designs such as wire
mesh or ceramic or metallic foams produce similar favorable
internal turbulence. This internal local turbulence increases the
interaction of the exhaust gas with the catalytic coating on the
filtration substrate material, thereby promoting the conversion of
NO to NO.sub.2. Furthermore, this turbulence increases the
interaction of the NO.sub.2 with the particulate matter trapped on
the surfaces of the diesel particulate reduction device. In doing
so, the design of the diesel particulate reduction device promotes
the consumption of NO.sub.2 and the regeneration of the particulate
filter.
[0061] A number of tests were preformed to provide comparative data
between an example system in accordance with the principles of the
present disclosure and other systems. The systems tested included
system A, system B and system C.
[0062] System A is an example of system in accordance with the
principles of the present disclosure. System A included an Emitec
PM Kat flow-through filter positioned upstream from a wall flow
filter. The Emitec filter had a platinum loading of about 70 grams
per cubic foot while the wall flow filter had a platinum loading of
about 50 grams per cubic foot. The Emitec filter had a diameter of
10 1/2 inches and a length of about 3 inches, and the wall flow
filter had a diameter of 10 1/2 inches and a length of about 12
inches. The Emitec filter had a particulate mass reduction
efficiency less than 50 percent, while the wall flow filter had a
particulate mass reduction efficiency greater than 85 percent.
[0063] System B was manufactured by Johnson Matthey, Inc. and sold
under the name CCRT. The system included a catalytic converter
positioned upstream from a catalyzed wall/flow filter.
[0064] System C had the same configuration as System A, except the
Emitec filter was loaded with a low NO.sub.2 producing catalyst
sold by Engelhard. The catalyst includes constituents that inhibit
the production of NO.sub.2, but allow the oxidation of
hydrocarbons. The platinum loading for the low NO.sub.2 producing
catalyst was also 70 grams per cubic foot.
[0065] Systems A, B and C were tested using a caterpillar 3126
diesel engine having 210 horsepower at 2200 rotations per minute.
During testing, the engine was cycled according to the parameters
set forth under standard FTP heavy-duty transient cycling. FIG. 8
shows the FTP transient cycle as a plot of the engine torque and
speed over a 20 minute time period. The cycle is divided into 4
phases including the New York Non Freeway (NYNF) phase, the Los
Angeles Non Freeway (LANF) phase, the Los Angeles Freeway (LAF)
phase and the New York Non Freeway (NYNF) phase.
[0066] FIG. 9 shows temperature profiles for exhaust generated from
the caterpillar 3126 diesel engine during FTP transient cycling.
The cycling was done at different torque levels. For example, the
100 percent line represents testing done at torque level that match
the standard FTP transient cycle shown at FIG. 8. The other
profiles were generated by using the same transient cycling, but
with the torques lowered a certain percentage relative to the
standard torque levels during the federal transient testing
protocol. For example, the 85 percent line represents a torque
level of 85 percent of the standard torque level specified by the
standard FTP transient cycle shown at FIG. 8. Similar plot lines
are provided for 77%, 70%, 55%, 48% and 40% of the torque levels
set by the standard FTP transient cycle. As shown at the graph of
FIG. 9, at 48% torque, the exhaust temperature is above 220.degree.
C. for about 37% of the testing duration.
[0067] During testing, the percentage of NO.sub.2 relative to the
total NO.sub.x emitted from the engine was measured for each of the
systems between the upstream and downstream exhaust treatment
devices. FIG. 10 shows the percentage of NO.sub.2 relative to total
NO.sub.x between the aftertreatment devices for each of the
systems. The NO.sub.2 to NO.sub.x percentage is shown across the
range of torque levels at which the federal transient testing
protocol was conducted. The graph of FIG. 10 shows that System A
generated moderate levels of NO.sub.2 (e.g., generally less than
50%), System B generated relatively high levels of NO.sub.2 (e.g.,
generally greater than 60%) and System C generated relatively low
levels of NO.sub.2 (e.g., generally less than 10%).
[0068] FIG. 11 is a graph showing the weight gain on the downstream
filters of each of the systems when subjected to federal transient
testing protocol cycles at torque levels of 48% for an extended
duration. Once again, during testing, a CAT 3126 diesel engine
having a 210 horsepower at 2200 RPM was used. During testing,
particulate material was emitted from the engine at an average rate
of 3.67 grams per hour. As shown by the graph of FIG. 11, System A
experienced relatively low weight gain during the testing period.
Thus, it can be concluded that a relatively large amount of passive
regeneration occurred within the system during the testing period.
This regeneration occurred despite the fact that at the 48% torque
level, the temperatures of the exhaust gas were relatively low
(e.g., the temperature exceeded 220.degree. C. less than 40% of the
time and almost never exceeded 300.degree. C. In contrast, both
Systems B and C experienced substantial weight gain. This indicates
that the combination of optimized upstream filtration and NO.sub.2
production has significant advantages over systems that merely
provide increased NO.sub.2 without upstream filtration, or systems
that have upstream filtration without generating NO.sub.2.
[0069] It will be appreciated that the specific dimensions
disclosed herein are examples applicable for certain embodiments in
accordance with the principles of the disclosure, but that other
embodiments in accordance with this disclosure may or may not
include such dimensions.
* * * * *