U.S. patent application number 11/605895 was filed with the patent office on 2008-05-29 for partial wall-flow filter and diesel exhaust system and method.
Invention is credited to Douglas Munroe Beall, Achim Karl-Erich Heibel, Pushkar Tandon.
Application Number | 20080120968 11/605895 |
Document ID | / |
Family ID | 39103767 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080120968 |
Kind Code |
A1 |
Beall; Douglas Munroe ; et
al. |
May 29, 2008 |
Partial wall-flow filter and diesel exhaust system and method
Abstract
A exhaust system and method for venting exhaust from an engine,
such as a diesel engine, through an exhaust line coupled to the
engine includes a first particulate filter disposed in the exhaust
line and "close-coupled" with the engine, and a second particulate
filter spaced a distance (d) from the first filter. The first
particulate filter is "close-coupled" so that it operates in a
passive regeneration mode to a greater extent than the second
particulate filter. The first particulate filter may be a partial
wall-flow filter including some plugged and some open channels.
Some of the plugged channels may be plugged adjacent to an inlet
end and others may be plugged adjacent to an outlet end. A partial
wall-flow filter is also described having some unplugged
flow-through channels and some plugged channels wherein some
plugged channels are located adjacent to the inlet end and some are
located adjacent to the outlet end.
Inventors: |
Beall; Douglas Munroe;
(Painted Post, NY) ; Heibel; Achim Karl-Erich;
(Corning, NY) ; Tandon; Pushkar; (Corning,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
39103767 |
Appl. No.: |
11/605895 |
Filed: |
November 29, 2006 |
Current U.S.
Class: |
60/295 ;
60/311 |
Current CPC
Class: |
F01N 3/035 20130101;
F01N 2340/04 20130101; F01N 3/0222 20130101; F01N 13/009 20140601;
F01N 3/0231 20130101; F01N 3/031 20130101; F01N 2330/06 20130101;
F01N 3/021 20130101; F01N 13/0093 20140601 |
Class at
Publication: |
60/295 ;
60/311 |
International
Class: |
F01N 3/022 20060101
F01N003/022; F01N 3/023 20060101 F01N003/023 |
Claims
1. An exhaust system for venting exhaust from an engine through an
exhaust line coupled to the engine, comprising: a first particulate
filter disposed in the exhaust line and close-coupled with the
engine, the first particulate filter operating in a first relative
extent in a passive regeneration mode; and a second particulate
filter positioned inline with, and spaced a distance from, the
first particulate filter and operating to a second relative extent
in a passive regeneration mode less substantially than the first
relative extent.
2. The diesel exhaust system of claim 1 wherein the first
particulate filter exhibits an initial filtration efficiency of
greater than 45%.
3. The diesel exhaust system of claim 2 wherein the initial
filtration efficiency is greater than 50%.
4. The diesel exhaust system of claim 2 wherein the initial
filtration efficiency is greater than 60%.
5. The diesel exhaust system of claim 2 wherein the initial
filtration efficiency is greater than 70%.
6. The diesel exhaust system of claim 1 wherein the first
particulate filter exhibits greater than 50% plugged channels as
compared to a total number of channels.
7. The diesel exhaust system of claim 6 further comprising greater
than 60% plugged channels as compared to the total number of
channels.
8. The diesel exhaust system of claim 1 wherein the first
particulate filter exhibits a total porosity of greater than
45%.
9. The diesel exhaust system of claim 8 further comprising total
porosity greater than 50%.
10. The diesel exhaust system of claim 8 further comprising total
porosity greater than 55%.
11. The diesel exhaust system of claim 8 further comprising total
porosity greater than 60%.
12. The diesel exhaust system of claim 8 further comprising total
porosity of 63% or greater.
13. The diesel exhaust system of claim 1 wherein the first
particulate filter exhibits: a total porosity of greater than 45%,
greater than 50% plugged channels as compared to a total number of
channels, and at least some unplugged channels.
14. The diesel exhaust system of claim 13 wherein the total
porosity is greater than 60%.
15. The diesel exhaust system of claim 1 wherein the first
particulate filter exhibits: a total porosity of greater than 60%,
greater than 60% of the channels are plugged channels as compared
to a total number of channels, and at least some unplugged
flow-through channels.
16. The diesel exhaust system of claim 1 wherein the first
particulate filter exhibits: a total porosity of greater than 60%,
greater than 50% of the channels are plugged channels as compared
to a total number of channels, at least some unplugged flow-through
channels, and some of the plugged channels are plugged adjacent to
an inlet end and some of the plugged channels are plugged adjacent
to an outlet end.
17. The diesel exhaust system of claim 1 wherein the first
particulate filter includes some unplugged flow-through channels
and some plugged channels wherein at least some of the plugged
channels are plugged adjacent to both an inlet end and an outlet
end.
18. The diesel exhaust system of claim 17 wherein a percentage of
plugged channels adjacent to the inlet end is less than a percent
of plugged channels adjacent to the outlet end.
19. The diesel exhaust system of claim 1, wherein the first
particulate filter comprises a ceramic honeycomb substrate
comprising a plurality of channels, wherein a first percentage of
the channels are plugged and a second percentage of the channels
are open and wherein the number of plugged channels divided by a
total number of channels, and expressed as a percentage, is greater
than 50%.
20. The diesel exhaust system of claim 19 wherein the percentage of
plugged channels to total channels is greater than 60%.
21. The diesel exhaust system of claim 1 wherein the first filter
includes an inlet end and outlet end, and the plugged channels are
positioned only adjacent the outlet end.
22. The diesel exhaust system of claim 1 wherein the honeycomb
substrate has an inlet end and an outlet end, and the plugged
channels include plugs adjacent to both the inlet end and the
outlet end.
23. The diesel exhaust system of claim 1 wherein the first filter
plugged channels and unplugged flow-through channels and a
hydraulic diameter of the plugged channels is different than a
hydraulic diameter of the unplugged flow-through channels.
24. The diesel exhaust system of claim 1 wherein the first
particulate filter is positioned downstream of a turbocharger
disposed in the exhaust line.
25. The diesel exhaust system of claim 1, further comprising a
diesel oxidation catalyst positioned between the first particulate
filter and the second particulate filter.
26. The diesel exhaust system of claim 1 wherein the second
particulate filter is positioned inline with, and spaced a distance
(d) of greater than or equal to 12 inches (30.5 cm) from the first
particulate filter.
27. The diesel exhaust system of claim 1 wherein the first
particulate filter includes a first inlet temperature (T.sub.1) and
the second particulate filter includes a second inlet temperature
(T.sub.2), wherein a ratio of inlet temperatures (T.sub.1/T.sub.2)
is greater than or equal to 1.1.
28. The diesel exhaust system of claim 1 wherein the first
particulate filter is positioned at a distance from the engine
wherein during operation, the inlet temperature (T.sub.1) is at a
temperature above 250.degree. C. for a some portion of the
operation.
29. A diesel exhaust system, comprising: a first particulate filter
close-coupled with the exhaust manifold, the first particulate
filter having a first percentage of plugged channels and a second
percentage of unplugged flow-through channels; and a second
particulate filter positioned inline with and spaced a distance (d)
of greater than or equal to 12 inches (30.5 cm) from the first
particulate filter and including only plugged channels such that
the first filter exhibits passive regeneration to a substantially
greater extent than the second filter.
30. The diesel exhaust system of claim 29 wherein the second
particulate filter is a wall flow filter including only plugged
channels.
31. The diesel exhaust system of claim 29 wherein the first
particulate filter includes plugged channels and unplugged
flow-through channels, and the plugged channels include some
channels that are plugged adjacent to an inlet end and other
channels that are plugged adjacent to an outlet end.
32. A method of operating a diesel exhaust system, comprising the
steps of: passing an exhaust gas through a first particulate filter
disposed in an exhaust line wherein the first particulate filter
includes a first inlet temperature (T.sub.1), the first particulate
filter having a first percentage of plugged channels and a second
percentage of unplugged flow-through channels wherein the exhaust
gas is first filtered; and passing the first filtered exhaust gas
through a second particulate filter positioned inline with and
spaced a distance from the first particulate filter wherein the
second particulate filter includes a second inlet temperature
(T.sub.2), wherein the first exhaust gas undergoes a second
filtering and a ratio of inlet temperatures (T.sub.1/T.sub.2) is
greater than or equal to 1.1.
33. A partial wall-flow filter, comprising: plugged channels and
unplugged, flow-through channels wherein the plugged channels
include some channels that are plugged adjacent to an inlet end and
other channels that are plugged adjacent to an outlet end.
34. The partial wall-flow filter of claim 33, further comprising a
percentage of plugged channels to total channels greater than
50%.
35. The partial wall-flow filter of claim 33, further comprising a
total porosity of greater than 45%.
36. The partial wall-flow filter of claim 33, further comprising a
total porosity of greater than 50%.
37. The partial wall-flow filter of claim 33, further comprising a
total porosity of greater than 60%.
38. The partial wall-flow filter of claim 33, further comprising a
total porosity of greater than 50%, and greater than 10% of the
plugged channels located adjacent to an inlet end and greater than
40% located adjacent to the outlet end.
39. The partial wall-flow filter of claim 33, further comprising
wherein a number of channels plugged adjacent to the inlet end is
less than a number of channels that are plugged adjacent to the
outlet end.
40. The partial wall-flow filter of claim 33, further comprising
wherein a number of channels plugged adjacent to the outlet end is
less than a number of channels that are plugged adjacent to the
inlet end.
41. The partial wall-flow filter of claim 33, further comprising an
oxidation catalyst residing on a porous wall of the partial
wall-flow filter.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to wall-flow filters
used to filter exhaust gases, and exhaust systems and methods
incorporating such filters.
[0002] Diesel exhaust systems may include, for example, a diesel
particulate filters (DPFs) for removing particulates, such as soot
from diesel exhaust. Where multiple DPFs are used to remove
particulates, these DPFs are typically arranged in close proximity
to each other and housed within a common enclosure, such as taught
in US Pat. App. No. 2004/0161373. The most widely used DPFs are
wall-flow filter. The conventional wall-flow filter consists of a
ceramic honeycomb substrate having longitudinal, parallel cell
channels formed by a plurality of intersecting porous walls. The
ends of the cell channels are typically plugged with a ceramic
plugging cement to form a checkered pattern of plugs at the end
faces of the honeycomb substrate. The cell channels of the filter
typically have some ends plugged at an inlet end face of the
honeycomb substrate, referred to herein as "inlet channels."
Likewise, typically, the cell channels also have the remaining ends
plugged to form a checkered pattern of plugs at an outlet end face
of the honeycomb substrate, herein referred to as "outlet
channels." In use, exhaust gas containing entrained soot particles
enters into the inlet channels, flows through the porous walls
(i.e., the wall-flow) and into the outlet channels, and exits
through the outlet channels, with the porous walls retaining a
portion of the particles contained in the exhaust. Filtration
efficiencies greater than 90% have been realized with conventional
wall-flow filters.
[0003] Conventional wall-flow filters may be cleaned out to prevent
the filter from becoming blocked and to maintain a suitable
pressure drop across the filter below a prescribed limit. Increase
in pressure drop across the filter generally results in an increase
in backpressure against the engine which, if not controlled, may
lead to power loss. One known method for cleaning out the filter is
to remove the soot trapped in the filter by thermal regeneration
(hereinafter "regeneration"). The regeneration may be either
"passive" or "active" or a combination thereof. In "passive"
regeneration, the inlet temperature of the exhaust entering the
filter is sufficiently high to itself initiate combustion of the
soot trapped in the wall-flow filter. In "active" regeneration, the
temperature of the filter is relatively low and additional energy
input is required to raise the temperature of the exhaust (and the
filter) to a level that would cause combustion of the soot trapped
in the filter. Typically, the additional energy input is provided
by post injection of fuel into the exhaust in combination with a
diesel oxidation catalyst located upstream of the filter.
[0004] Diesel exhaust systems based on "active" regeneration have
become the industry standard because they desirably operate at
lower exhaust temperatures and assure suitable soot removal under
different engine duty cycles by implementing regeneration. On the
other hand, "active" regeneration comes with a fuel economy
penalty. Additionally, there is the possibility of large
temperature spikes during "active" regeneration which may be
detrimental to the filter. Accordingly, systems which fewer
regeneration event during operation are desired.
SUMMARY OF THE INVENTION
[0005] In view of the inefficiencies of the exhaust systems of the
prior art, diesel exhaust after treatment systems operating to a
larger extent in a passive regeneration mode may offer a
competitive advantage in terms of fuel economy. In one broad
aspect, the invention is an exhaust system adapted for venting
exhaust gas from an engine, such as a diesel engine, through an
exhaust line coupled to the engine. The exhaust system comprises a
first particulate filter disposed in the exhaust line and
"close-coupled," i.e., located in close proximity to the engine.
and a second particulate filter positioned inline with, and spaced
a distance (d) from, the first particulate filter. The spacing
between the first and second particulate filters is preferably such
that the difference between the inlet temperature of the first
filter (T.sub.1) and the inlet temperature of the second filter
(T.sub.2), namely T.sub.1-T.sub.2, is 20.degree. C. or greater. The
first filter is "close-coupled" to the engine so that it operates
at a temperature preferably sufficient to promote a significant
amount of "passive" regeneration whereas, the downstream second
filter operates in a cooler environment and, therefore, relies more
on active regeneration for soot removal. The relative extent if
passive regeneration undergone in the second filter may be
substantially less than in the first (close-coupled) filter. In one
implementation, the first particulate filter may have a first
percentage of plugged channels and a second percentage of unplugged
flow-through channels. According to additional embodiments, the
plugged channels may be plugged adjacent to both an inlet end and
an outlet end. In a preferred implementation, the second
particulate filter is positioned inline with, and spaced a distance
(d) of greater than or equal to 12 inches (30.5 cm) from, the first
particulate filter. Optionally, the spacing may be such that the
first particulate filter may include a first inlet temperature
(T.sub.1) and the second particulate filter includes a second inlet
temperature (T.sub.2), wherein a ratio of inlet temperatures
(T.sub.1/T.sub.2) is greater than or equal to 1.1, or even greater
than or equal to 1.15. According to further embodiments of the
invention, the first and second particulate filters may be mounted
into separate housings. A diesel oxidation catalyst may be included
in the system between the filters or the oxidation catalyst
function may be included in the first filter.
[0006] In another broad aspect, the invention is directed to an
exhaust system, such as a diesel exhaust system, comprising a first
particulate filter which is "close-coupled" to an engine, the first
particulate filter being a partial wall-flow filter having a first
percentage of plugged channels and a second percentage of unplugged
flow-through channels; and a second particulate filter positioned
inline with and spaced a distance (d) of greater than 12 inches
(30.5 cm) from the first particulate filter. The second filter may
include only plugged channels, and the first filter may be located
relative to the engine such that it exhibits passive regeneration
to a substantially greater extent than the second filter. The
second filter preferably may also be subjected to "active"
regeneration. The first and second filters are preferably housed in
separate, spaced housings.
[0007] In another broad aspect, the invention is directed to a
method of operating an exhaust system, such as a diesel exhaust
system, comprising the steps of passing an exhaust gas through a
first particulate filter disposed in an exhaust line wherein the
first particulate filter includes a first inlet temperature
(T.sub.1), the first particulate filter having a first percentage
of plugged channels and a second percentage of unplugged
flow-through channels wherein the exhaust gas is first filtered;
and passing the first filtered exhaust gas through a second
particulate filter positioned inline with and spaced from the first
particulate filter wherein the second particulate filter includes a
second inlet temperature (T.sub.2), wherein the first exhaust gas
undergoes a second filtering, and a ratio of inlet temperatures
(T.sub.1/T.sub.2) is greater than or equal to 1.1, or even greater
than or equal to 1.15.
[0008] In additional embodiments, the invention is a partial
wall-flow filter, adapted for use in a diesel exhaust system,
comprising plugged channels and unplugged flow-through channels
wherein the plugged channels include some channels that are plugged
adjacent to an inlet end and other channels that are plugged
adjacent to an outlet end.
[0009] Other features and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The accompanying drawings, described below, illustrate
typical embodiments of the invention and are not to be considered
limiting of the scope of the invention, for the invention may admit
to other equally effective embodiments. The figures are not
necessarily to scale, and certain features and certain view of the
figures may be shown exaggerated in scale or in schematic in the
interest of clarity and conciseness.
[0011] FIG. 1A and 1B are schematic diagrams of diesel exhaust
systems according to embodiments of the invention.
[0012] FIGS. 2A and 2B are perspective views of a partial wall-flow
filter for use in the exhaust systems of FIGS. 1A, 1B.
[0013] FIGS. 3A-3D are examples of plugging patterns which may be
employed for the partial wall-flow filter according to embodiments
of the invention.
[0014] FIG. 4A shows a partial plugging pattern applied at both
ends of the partial wall-flow filter of FIGS. 2A, 2B.
[0015] FIG. 4B and 4C illustrate an alternative partial plugging
pattern applied at both ends of the partial wall-flow filter.
[0016] FIGS. 5 and 6 illustrate alternative partial plugging
patterns applied at both end faces of the partial wall-flow filter
according to embodiment of the invention.
[0017] FIGS. 7-10 illustrate performance plots for various system
configurations including the partial wall-flow filter according to
embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The invention will now be described in detail with reference
to a few preferred embodiments, as illustrated in the accompanying
drawings. In describing the preferred embodiments, numerous
specific details are set forth in order to provide a thorough
understanding of the invention. However, it will be apparent to one
skilled in the art that the invention may be practiced without some
or all of these specific details. In other instances, well-known
features and/or process steps have not been described in detail so
as not to unnecessarily obscure the invention. In addition, like or
identical reference numerals are used to identify common or similar
elements.
[0019] FIG. 1A depicts an exhaust system 100, such as a diesel
exhaust system, for venting exhaust from an exhaust manifold 105 of
a diesel engine 107. The exhaust system 100, as shown, includes an
exhaust line 102 with inlet end 101 and outlet end 103. The inlet
end 101 is coupled to the diesel engine 107 through an exhaust
manifold 105. The inlet end 101 may include a connection device
104, which may take on any suitable form. For example, the
connection device 104 may be a flange that can be coupled to a
similar flange on a connection portion 109 of the exhaust manifold
105. Although the exhaust line 102 is shown as being generally
straight, in practice it may take on other profiles and may include
straight and curved sections and/or sections of differing
diameter.
[0020] The exhaust system 100 includes a first particulate filter
106 disposed adjacent to the inlet end 101 of the exhaust line 102
so as to be in a "close-coupled" position with respect to the
engine 107 and, of course, also the exhaust manifold 105. In this
"close-coupled" position, the first particulate filter 106 may take
advantage of the higher incident exhaust temperatures to effect a
substantially greater extent of "passive" regeneration of the
captured soot, as compared to the downstream second filter. The
term "close-coupled" as used herein, means the filter is in a
location in the exhaust stream in close proximity to the engine
107, and, in particular, in close proximity to the combustion
chambers of the engine, as measured along the exhaust stream. For
example, "close-coupled" would be considered in close proximity of
the engine 107, measured along the exhaust line, such that the
temperature for at least some portion of the operating cycle
exceeds 250.degree. C. Preferably, for at least 50% of the
operation, the inlet temperature (T.sub.1) of the first filter
exceeds 200.degree. C. In one example shown in FIG. 1A, a
turbocharger 111 is positioned in the exhaust line 102 and the
first particulate filter 106 is positioned upstream of the
turbocharger 111 such that the hot gases directly impinge upon the
first filter 106. In a more preferred embodiment, the first filter
106 is located directly downstream of the turbocharger 111 (See
FIG. 1B). In the close-coupled position, the first filter 106 may
experience temperature conditions of 250.degree. C. or greater for
a substantial amount, greater than 10%, or even greater than 20%,
of the operating cycle. These conditions promote a substantial
amount of "passive" regeneration. To avoid unwanted damage to the
filter, the inlet temperature T.sub.1 should preferably not exceed
about 400.degree. C.
[0021] The exhaust system 100 of the invention further includes a
second particulate filter 108 positioned in the exhaust line 102,
and spaced a distance (d) from the first particulate filter 106. In
the examples shown in FIG. 1A, 1B, the second particulate filter
106 may be positioned downstream of a turbocharger 111. Additional
particulate filters may be positioned in the exhaust line 102,
downstream of the second particulate filter 106 to meet desired
filtration and backpressure requirements. The second particulate
filter 106 may be preceded by an upstream diesel oxidation catalyst
(DOC) 114, which may incorporate any known active catalytic species
for purifying exhaust, such as catalytic species for oxidizing
carbon monoxide, hydrocarbons, and soluble organic fraction of
particulates, as is known in the art. If included, the DOC 114 may
be located between the first 106 and second 108 filters, or more
preferably between the first filter 106 and the turbocharger 111.
The exhaust system 100 may further include devices such as
diffusion and expansion cones 110, 112 at the inlet and outlet ends
of the particulate filters 106, 108 to aid in achieving desired
exhaust flow distribution in the particulate filters, and/or size
and weight reductions in the exhaust line 102.
[0022] During normal operation of the engine, such as a diesel
engine, exhaust from the engine 107 and exhaust manifold 105 passes
sequentially through the first particulate filter 106, turbocharger
111 (if present), oxidation catalyst 114 (if present), and second
particulate filter 108, as indicated by arrow 116 in FIG. 1A.
Particulates in the exhaust are trapped inside the first and second
particulate filters 106, 108 as the exhaust passes through them. In
particular, part of the soot is trapped in the first filter, while
some of the remaining soot is trapped in the second filter. The
engine operating conditions and location of the first filter 106
relative to the engine 107 may be set such that the inlet
temperature T.sub.1 of the exhaust at the first filter 106 is
sufficient to itself initiate combustion of soot trapped in the
first filter 106, i.e., promote "passive" regeneration. Contrarily,
the second filter 108 is spaced a distance (d) from the first
filter 106 such that its inlet temperature T.sub.2 is low in
comparison to the inlet temperature T.sub.1 of the first filter
106. In particular, the distance (d) is spaced generally such that
a ratio of inlet temperatures (T1/T2) is greater than or equal to
1.1, or even greater than or equal to 1.15. Preferably, the spacing
of the filters is such that the temperature difference,
T.sub.1-T.sub.2, is 20.degree. C. or more, or even the difference
is 25.degree. C. or more.
[0023] According to embodiments of the invention, the first
particulate filter 106 has a relatively low pressure drop in
comparison to the second particulate filter 108. In one example,
the first particulate filter 106 is small enough to fit into the
available space near the exhaust manifold 105, between the exhaust
manifold 105 and the turbocharger 111, or just downstream of the
turbocharger. According to additional aspects of the invention, the
physical space (volume) needed to house the first particulate
filter 106 may be relatively smaller than the space (volume) to
house the second filter 108, because the second particulate filter
108 provides the additional volume needed to meet filtration
requirements. In a preferred implementation, the second particulate
filter 108 may be a conventional wall-flow filter, for example.
However, a conventional wall-flow filter would typically not be
suitable for use as the first particulate filter 106 because of the
size and pressure drop requirements for a filter in a
"close-coupled" position. In particular, the it is desirable that
the first filter exhibit low pressure drop. Because of the low
pressure drop requirement, the first particulate filter 106 may
have a lower filtration efficiency than the second particulate
filter 108. As an example, the first particulate filter 106 may
have an initial filtration efficiency of less than about 80%.
However, in certain configurations described herein, it is possible
to achieve higher filtration efficiency in the first filter, such
as no less than 40%, or even no less than 50%, or even no less than
60% or 70% or more, while the second particulate filter 108
preferably has filtration efficiency greater than about 80%, or
even 90% or greater. In certain embodiments, the first filter
exhibits initial filtration efficiency of greater than 40% but less
than 80%, and the second filter exhibits initial filtration
efficiency of greater than 90%. The first particulate filter 106
may be any suitable filter exhibiting one or more of the
aforementioned characteristics. For example, the first particulate
filter 106 may be a ceramic foam-type filter. Alternatively, the
first particulate filter 106 may be a partial wall-flow filter. The
partial wall flow filter being named because of having a
combination of plugged and unplugged flow-through channels. In the
unplugged flow-through channels, flow is straight through the
channel, i.e., not through the wall. Thus, the "partial" indicates
that only a part of the flow is through the wall. Partial wall-flow
filters according to the invention exhibiting high porosity,
greater than 45% and which have a combination of plugged and
unplugged channels have been discovered to be most effective.
Partial wall-flow filters having total porosities of 50% and more
exhibit excellent filtration efficiency and low pressure drop.
[0024] FIGS. 2A and 2B depict an exemplary partial wall-flow filter
200 for use as the first particulate filter (106 in FIG. 1) which
is located in close-coupled relationship to the engine 107. The
partial wall-flow filter 200 of the invention includes a porous
honeycomb substrate 202 having, for example, a generally
cylindrical shape. Optionally, the transverse cross-section of the
honeycomb substrate 202 may be circular, elliptical, square or may
have other shape. The honeycomb substrate 202 has opposite ends
204, 206 and interior porous walls 208 extending between the ends
204, 206. The interior porous walls 208 define generally parallel
flow channels 210, which also extend between the ends 204, 206. The
channels 210 may have a square cross-section or other type of
cross-section, e.g., triangle, circle, octagon, rectangle, hexagon
or combinations thereof. The honeycomb substrate 202 is preferably
made of a porous ceramic material, such as cordierite, aluminum
titanate, or silicon carbide. The interior porous walls 208 of the
honeycomb substrate 202 may include thereon an active catalytic
species useful for passive regeneration of particulates accumulated
in the porous walls.
[0025] For diesel exhaust systems, the porous walls 208 may
incorporate pores having mean diameters in the range of 1 to 60
.mu.m, more typically in the range of 10 to 50 .mu.m, and the
honeycomb substrate 202 may have a cell density between
approximately 10 and 400 cells/in.sup.2 (1.5 and 62
cells/cm.sup.2), more typically between approximately 100 and 320
cells/in.sup.2 (15.5 and 49.6 cells/cm.sup.2). The thickness of the
porous walls 208 may range from approximately 0.002 in. to 0.060
in. (0.05 mm to 1.5 mm), more typically between approximately 0.010
in. and 0.030 in. (0.25 mm and 0.76 mm), and the total porosity of
the walls may be greater than 45%, or even greater than 50%, or
even greater than 55%, or even greater than 60%.
[0026] Plugs 212 may be inserted at, for example, an end face of
some of the channels 210, while the remaining channels 210 remain
open (unplugged). This differs from the conventional wall-flow
filter where all the cell channels are end-plugged. The unplugged,
flow-through channels 210a, which are open at both ends 204, 206
and are unplugged along their length are preferably evenly
distributed among the plugged channels 210b, or vice versa. Plugs
may be included only at one of the ends 204, 206, or at both of the
ends 204, 206. Optionally, the plugs may be included spaced in from
the ends. In a partial wall-flow filter with plugs on only one
side, partial filtration occurs by passage of exhaust through some
of the walls, while some flow is straight through the filter. When
the plugs are positioned adjacent to the outlet end of the filter,
a pressure differential between plugged and unplugged, flow-through
channels results in transfer of exhaust from plugged channels to
unplugged, flow-through channels, and soot may be accumulated in
the plugged channels. When plugs are positioned adjacent to the
inlet end of the filter, exhaust enters the unplugged, flow-through
channels and a pressure differential between the unplugged,
flow-through channels and adjacent plugged channels forces some
exhaust through the wall to exit through the outlet side of the
plugged channels. Soot accumulation in this case occurs on the
walls of the unplugged, flow-through channels. In one example,
plugs are positioned only adjacent to the outlet end face of the
filter. Filters with combinations of only outlet end plugs and
unplugged, flow-through channels where the porosity is greater than
45%, greater than 50%, greater than 60% have been found to be
particularly effective as a first filter promoting high soot
capture in the first filter and exhibiting low pressure drops.
Optionally, plugs may be include only adjacent to the inlet
face.
[0027] In another example, plugs are positioned adjacent to both
ends of the first filter. Thus, in this embodiment, a partial
wall-flow filter is provided, comprising plugged channels and
unplugged, flow-through channels wherein the plugged channels
include some channels that are plugged adjacent to an inlet end,
and other channels that are plugged adjacent to an outlet end.
Preferred embodiments include relatively more plugs formed adjacent
the outlet end than the inlet end. Embodiments including this
configuration and high porosity, greater than 45%, have relatively
minimal pressure drop as a function of soot loading. For example,
FIG. 8 illustrates that configurations with 25% of the plugs
located adjacent to the inlet end and 25% of the plugs located
adjacent to the outlet end and porosity greater than 60% exhibits a
pressure drop of less than 0.5 kPa from 0 to 2 g/l soot loading.
Partial flow filters with high porosity, greater than 60%, with
about 50% rear plugs also exhibit low pressure drop change as a
function of soot loading. That being said, in some instances, it
may be desirable to include a larger percentage of plugs adjacent
to the inlet end that the outlet end.
[0028] This partial flow embodiment is demonstrated, for example,
in FIGS. 2A and 2B, where the unplugged, flow-through channels are
designated 210a, the plugged channels having plugs located at the
end 204 are designated 210c, and the plugged channels having plugs
at the end 206 are designated 210b.
[0029] In the partial wall-flow filter 200, soot accumulates on the
porous walls 208 as exhaust passes through the filter. This
accumulation of soot decreases the permeability of the walls 208
and reduces exhaust flow to channels adjacent to the unplugged,
flow-through channels 210a. Thus, the ability of the partial
wall-flow DPF 200 to capture soot decreases as soot is accumulated
in the filter. One advantage of a filter which decreases in
filtration efficiency is that a maximum soot load can be
established for the filter and overloading of soot in the filter is
less likely to occur in a partial wall-flow filter. In conventional
wall-flow filters, filtration efficiency generally increases as
soot load accumulation on the porous walls increases, making the
filter more susceptible to soot overload. Soot overload is
undesirable because maximum temperatures encountered in the filter
during regeneration are directly proportional to soot load. The
partial wall-flow filter 200 has a built-in protection against high
temperature excursions resulting from soot overload.
[0030] Various examples of partial plugging patterns will now be
described. However, these examples should not be construed as
limiting the invention as otherwise described herein.
[0031] FIG. 3A shows a partial plugging pattern 300 wherein the
number of plugged channels 302 is greater than the number of
unplugged, flow-through channels 304. Also, the unplugged,
flow-through channels 304 may be evenly distributed among the
plugged channels 302. As an example, the ratio of the number of
plugged channels 302 to total number of channels, expressed as a
percentage, may constitute greater than 50%, or even greater than
60%, or even 75%. Plugged channels for this configuration are
preferably located at the outlet end. Having a higher percentage of
plugs at the outlet end coupled with high porosity, greater than
45%, or even greater than 50% appears to provide initial filtration
efficiency greater than 40%, or even 50%, or even 60% or more (See
FIG. 7).
[0032] FIG. 3B shows an alternative plugging pattern 306 wherein
the number of unplugged, flow-through channels 308 is greater than
the number of plugged channels 310. Again, the plugged channels 310
may be evenly distributed among the unplugged, flow-through
channels 308. As an example, the number of unplugged, flow-through
channels 308 may constitute greater than 50% of the total number of
channels, or even greater than 60%, or even 75% or more of the
total number of channels.
[0033] FIG. 3C shows a partial plugging pattern 312 for the first
filter including unplugged, flow-through channels 314 and plugged
channels 316, wherein the hydraulic diameters of the plugged and
unplugged, flow-through channels 314, 316 are different. In
particular, the hydraulic diameter of the plugged channels 316 are
larger than the hydraulic diameter of the unplugged, flow-through
channels 314. Again, the plugged channels are preferably located
adjacent to the outlet end. In particular, an area ratio of the
plugged area to open area of the filter is preferably greater than
1.2.
[0034] FIG. 3D illustrates a partial plugging pattern 318 including
unplugged, flow-through channels 320 and plugged channels 322,
wherein the hydraulic diameter of the unplugged, flow-through
channels 320 is larger than the hydraulic diameter of the plugged
channels 322. In FIGS. 3C and 3D, the unplugged, flow-through
channels are evenly distributed among the plugged channels, and
vice versa.
[0035] The partial plugging patterns described above and variations
thereof can be applied to one or both end faces of the honeycomb
substrate (202 in FIG. 2A, 2B). For example, FIG. 4A shows a
partial top view of the honeycomb substrate 202 if the partial
plugging pattern of FIG. 3B is applied to both end faces of the
honeycomb substrate 202. The unplugged, flow-through channels are
designated 210a. The plugged channels on one of the end faces of
the honeycomb substrate 202 are designated 210b. The plugged
channels on the other of the end faces of the honeycomb substrate
202 (shown double hatched) are designated 210c. In this example,
the plugged channels 210b, 210c account for about 50% of the total
channels in the honeycomb substrate 202 and the unplugged,
flow-through channels 210a are evenly distributed within the
honeycomb substrate 202. FIG. 4A am 4B illustrates another partial
plugging configuration wherein the outlet end is shown in FIG. 4B
and the inlet end is shown in FIG. 4C. This embodiment illustrates
a configuration with more plugged outlet channels than inlet
channels. In particular, the channels plugged at the inlet end
consist of about 25% of the total number of cell channels, whereas,
at the outlet end, the plugged channels account for about 50% of
the total number of channels. Channels plugged adjacent to the end
shown are doubly hatched, whereas cells plugged adjacent the other
end are singly hatched. Flow through channels are un-hatched.
[0036] FIGS. 5 and 6 illustrate additional embodiments of plug
patterns that may be employed in the partial wall-flow filter. In
these embodiments, the inlet end plugged cells are shown single
hatched, the outlet end plugged cells are shown double hatched, and
the flow-though cells are shown un-hatched. In these examples,
about 50% of the cell channels are plugged and some of the plugged
channels are located adjacent the inlet end and some are located
adjacent the outlet end. Other combinations of plugged and
unplugged cells in the partial wall-flow filter may be employed.
For example, greater than 50%, or even greater than 60%, or even
75% or greater of the channels may be plugged, as compared to the
total number of channels. The total porosity of the walls of the
first filter may be greater than 45%, or even greater than 50%,
greater than 55%, or even greater than or 60%. Preferred
combinations excellent initial filtration efficiency and low
pressure drop include greater than 50% plugged channels (as
compared to the total number of channels), and total porosity of
greater than 45%. Examples exhibiting relatively higher porosity,
of greater than 60%, and which include greater than 50% (or even
60%) plugged channels exhibit relatively high ratios of filtration
efficiency to pressure drop in both the clean and soot loaded
states (see FIG. 10). Examples where the first filter is a partial
flow filter having greater than 60% total porosity, greater than
50% plugged channels, some plugged channels adjacent to the inlet
end and some adjacent to the outlet end and a relatively higher
percentage of plugged channels at the outlet end achieve the best
combination of initial filtration efficiency and low pressure drop
(see the 25% Inlet, 50% Outlet 200/12/63% embodiment.
[0037] FIG. 7-10 are graphical plots illustrating experimental test
examples that were investigated on the system and partial flow
filter of the invention. As can be seen from the FIGS. 7-10, when
the partial flow filter exhibits higher porosity and rear plugging,
the combination of relatively high initial filtration efficiency
(greater than 50%) can be achieved in the first filter while also
exhibiting low pressure drop across the first filter. Filters
having porosity of greater than 45%, or even 50%, or even 55%, or
even 60% or higher are desired. In particular, the examples tested
are outlined in Table 1 below. As can be seen, filters having 50%
and 75% plugged channels (as compared to the total number of
channels) and well as high (50%) and very high (63%) porosities.
Several cell densities and wall thicknesses were also tested, as
were inlet plugged, outlet plugged, and combinations of inlet and
outlet plugged. In each case, the remainder of the channels not
being deemed plugged were unplugged. In each case, the partial flow
filter was matched with a 200/12 cell configuration, 50% porosity,
16 .mu.m mean pore size filter with 50% of the channels
conventionally plugged in a checkerboard pattern on each end.
Certain examples of partial wall flow filters having porosity
greater than 50%, greater than 10% of the plugged channels located
adjacent the inlet end, and greater than 40% of the plugged
channels located adjacent the outlet end exhibit excellent
filtration efficiency and low pressure drop.
TABLE-US-00001 TABLE 1 Experimental Tests Cell Clean Ex. % Plugged
Porosity Density T Wall % FE .DELTA.P FE/PD No. Inlet/Outlet (%)
(Cells/in.sup.2) (mil) Plugged Init % (kPa) (%/kPa) 1 50%/0% 50 200
12 50 15.8 0.74 21.4 2 0%/50% 50 200 12 50 41.7 0.92 45.3 3 25%/25%
50 200 12 50 27.8 0.72 38.6 4 50%/0% 63 200 12 50% 45 0.68 66.2 5
0%/50% 63 200 12 50% 61.1 0.87 70.2 6 50%/25% 50 200 12 75% 50 0.92
54.3 7 25%/50% 50 200 12 75% 48 1.09 44.0 8 0%/50% 63 300 8 50%
45.5 0.95 47.9 9 25%/50% 63 200 12 75% 73.7 0.91 81.0
[0038] FIG. 7 compares various test examples in a clean state,
i.e., without soot loading (see points located inside the lower
dotted outline) and in a soot loaded state (within the upper dotted
outline) at 2 g/l soot loading of artificial soot. It was
discovered the relatively higher porosity embodiments which combine
greater than 50% plugged channels and more plugs adjacent the
outlet end achieve excellent filtration efficiency coupled with low
pressure drop. This allows relatively more soot to be captured
within the first partial flow filter where such soot can be
subjected to a higher relative extent of passive regeneration.
[0039] FIG. 8 illustrates additional examples of the partial
wall-flow filter and shows the pressure drop for various
embodiments as a function of grams per liter of soot loading. It
should be recognized that certain embodiments exhibit little
pressure drop increase as a function of soot loading. For example,
the 50% rear plugged embodiment including greater than 60% porosity
exhibits low pressure drop at even high soot loading rates (at 2
g/l and above).
[0040] FIGS. 9 and 10 illustrates that certain exemplary
embodiments exhibit both high filtration efficiency is the first
filter as well as low pressure drop. In particular, FIG. 10 plots
the ratio of Filtration efficiency divided by pressure drop (in
%/kPa) for various exemplary embodiments. In particular, several
embodiment allow the capture of relatively lager amounts of soot in
the first filter without a significant increase in pressure drop.
For example, those embodiments exhibiting 60% or greater porosity
and 50% or greater plugged channels and, more specifically, a
larger amount of outlet plugged channels that inlet plugged
channels, exhibit excellent filtration efficiency combined with low
pressure drop. This a large portion of the soot may be subjected to
passive regeneration in the first filter, thus requiring a lesser
number of active regenerations intervals in the second filter.
[0041] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
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