U.S. patent application number 12/988450 was filed with the patent office on 2011-05-12 for exhaust gas purification system for internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Mikio Inoue, Kenichi Tsujimoto.
Application Number | 20110107749 12/988450 |
Document ID | / |
Family ID | 41199100 |
Filed Date | 2011-05-12 |
United States Patent
Application |
20110107749 |
Kind Code |
A1 |
Tsujimoto; Kenichi ; et
al. |
May 12, 2011 |
EXHAUST GAS PURIFICATION SYSTEM FOR INTERNAL COMBUSTION ENGINE
Abstract
There is provided an exhaust gas purification system for an
internal combustion engine in which exhaust gas that is in a state
in which an even concentration distribution of reducing agent is
achieved is caused to flow into an exhaust gas purification
apparatus. The system includes a small-diameter catalyst 7 that is
provided in an exhaust pipe 3 upstream of an NOx catalyst 4 and
adapted in such a way that not the entire amount but a portion of
the exhaust gas flowing in the exhaust pipe 3 passes through it, a
fuel addition valve 5 that is provided upstream of the
small-diameter catalyst 7 and adds fuel to the exhaust gas flowing
into the small-diameter catalyst 7, and an isolation pipe 8 that
separates the cross section of the exhaust pipe 3 into a catalyst
side passage 8a through which catalyst outflowing exhaust gas is
caused to pass and a detour side passage 3c through which catalyst
detouring exhaust gas is caused to pass, over a region from the
downstream end of the small-diameter catalyst 7 to a predetermined
position between the small-diameter catalyst 7 and the NOx catalyst
4.
Inventors: |
Tsujimoto; Kenichi;
(Shizuoka-ken, JP) ; Inoue; Mikio; (Shizuoka-ken,
JP) |
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Aichi
JP
|
Family ID: |
41199100 |
Appl. No.: |
12/988450 |
Filed: |
April 10, 2009 |
PCT Filed: |
April 10, 2009 |
PCT NO: |
PCT/JP2009/057386 |
371 Date: |
January 7, 2011 |
Current U.S.
Class: |
60/297 |
Current CPC
Class: |
F01N 2240/30 20130101;
B01D 2251/208 20130101; F01N 3/2892 20130101; F01N 2610/03
20130101; B01D 53/9431 20130101; F01N 2240/20 20130101; F01N 3/206
20130101; F01N 3/0814 20130101; B01D 2258/012 20130101; F01N 13/009
20140601; F01N 2490/20 20130101; B01D 53/9477 20130101; F01N 13/08
20130101; F01N 3/0842 20130101; B01D 2255/91 20130101; F01N 3/0871
20130101 |
Class at
Publication: |
60/297 |
International
Class: |
F01N 3/035 20060101
F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2008 |
JP |
2008-107916 |
Apr 17, 2008 |
JP |
2008-108165 |
Claims
1. An exhaust gas purification system for an internal combustion
engine comprising: an exhaust gas purification apparatus provided
in an exhaust passage of an internal combustion engine; a pre-stage
catalyst that is provided in the exhaust passage upstream of said
exhaust gas purification apparatus and adapted in such a way that
not the entire amount but a portion of exhaust gas flowing in the
exhaust passage passes through it; a reducing agent addition unit
provided upstream of said pre-stage catalyst for adding reducing
agent to exhaust gas flowing into the pre-stage catalyst; and a
separation member that separates a cross section of the exhaust
passage into a catalyst side passage through which catalyst
outflowing exhaust gas flowing out of said pre-stage catalyst is
caused to pass and a detour side passage through which catalyst
detouring exhaust gas that has detoured around the pre-stage
catalyst is caused to pass, over a region from the downstream end
of said pre-stage catalyst to a predetermined position between the
pre-stage catalyst and said exhaust gas purification apparatus,
wherein an expanding portion in which the cross sectional area of
the exhaust passage becomes larger as compared to the portion of
the exhaust passage in which the separation member is disposed is
connected to the upstream end of said exhaust gas purification
apparatus, and the axis of the expanding portion is inclined
relative to the axis of said pre-stage catalyst.
2. (canceled)
3. An exhaust gas purification system for an internal combustion
engine according to claim 1, further comprising: a turbocharger
having a turbine disposed in the exhaust passage upstream of said
reducing agent addition unit and a compressor disposed in an intake
passage of the internal combustion engine; and a cylindrical member
disposed on a front end face of said pre-stage catalyst to guide a
portion of exhaust gas flowing out of said turbine to the front end
face of the pre-stage catalyst, wherein the side wall of said
cylindrical member is provided with an opening, and said reducing
agent addition unit adds liquid reducing agent to exhaust gas
introduced in said cylindrical member in such a way that a spray of
the reducing agent is ejected toward the center of the cylindrical
member through said opening of the cylindrical member.
4. (canceled)
5. An exhaust gas purification system for an internal combustion
engine according to claim 3, wherein said cylindrical member is
arranged to be offset from the axis of the exhaust passage toward
an inner surface of the exhaust passage.
6. An exhaust gas purification system for an internal combustion
engine according to claim 5, wherein a portion of said cylindrical
member is in contact with the inner surface of the exhaust
passage.
7. An exhaust gas purification system for an internal combustion
engine according to claim 1, wherein the cross sectional area of
the channel of said catalyst side passage is constant over a region
from the upstream end to the downstream end of said separation
member.
8. An exhaust gas purification system for an internal combustion
engine according to claim 1, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
9. An exhaust gas purification system for an internal combustion
engine according to claim 3, wherein the cross sectional area of
the channel of said catalyst side passage is constant over a region
from the upstream end to the downstream end of said separation
member.
10. An exhaust gas purification system for an internal combustion
engine according to claim 5, wherein the cross sectional area of
the channel of said catalyst side passage is constant over a region
from the upstream end to the downstream end of said separation
member.
11. An exhaust gas purification system for an internal combustion
engine according to claim 6, wherein the cross sectional area of
the channel of said catalyst side passage is constant over a region
from the upstream end to the downstream end of said separation
member.
12. An exhaust gas purification system for an internal combustion
engine according to claim 3, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
13. An exhaust gas purification system for an internal combustion
engine according to claim 5, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
14. An exhaust gas purification system for an internal combustion
engine according to claim 6, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
13. An exhaust gas purification system for an internal combustion
engine according to claim 7, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
14. An exhaust gas purification system for an internal combustion
engine according to claim 9, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
15. An exhaust gas purification system for an internal combustion
engine according to claim 10, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
16. An exhaust gas purification system for an internal combustion
engine according to claim 11, wherein said expanding portion is
provided in a region from a location between the downstream end of
said separation member and the upstream end of said exhaust gas
purification apparatus to the upstream end of said exhaust gas
purification apparatus, and the cross sectional area of the channel
of the exhaust passage is constant in a region from the downstream
end of said separation member to the upstream end of said expanding
portion.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas purification
system for an internal combustion engine.
BACKGROUND ART
[0002] When supplying reducing agent to an exhaust gas purification
apparatus provided in an exhaust passage of an internal combustion
engine, there is a known method in which reducing agent is added to
the exhaust gas through a reducing agent addition valve provided in
the exhaust passage upstream of the exhaust gas purification
apparatus.
[0003] In recent years, a pre-stage catalyst that is configured to
allow not the entire amount of the exhaust gas flowing in the
exhaust passage but only a portion thereof to pass through it is
provided between the reducing agent addition valve and the exhaust
gas purification apparatus in some cases. For example, Patent
document 1 discloses an arrangement in which a reforming catalyst
having a cross section smaller than the exhaust passage is provided
in the exhaust passage between a fuel addition valve that adds fuel
as a reducing agent to the exhaust gas and an NOx storage reduction
catalyst (which will be simply referred to as the "NOx catalyst"
hereinafter).
[0004] In the exhaust gas purification system having the above
configuration, fuel added to the exhaust gas through the fuel
addition valve is reformed in the reforming catalyst and thereafter
flows out of the reforming catalyst. This exhaust gas merges with
the exhaust gas that has not flown into (i.e. has detoured around)
the reforming catalyst in the downstream of the reforming catalyst
and then flows into the NOx catalyst. When performing NOx reduction
processing for the NOx catalyst, it is desirable to cause the
exhaust gas having an even distribution of fuel concentration to
flow into the NOx catalyst in order to reduce NOx evenly in the
entire region of the NOx catalyst.
PRIOR ART DOCUMENTS
Patent Documents
[0005] [Patent document 1]: Japanese patent Application Laid-Open
No. 2005-127257 [0006] [Patent document 2]: Japanese patent
Application Laid-Open No. 9-38467 [0007] [Patent document 3]:
Japanese patent Application Laid-Open No. 2005-127260 [0008]
[Patent document 4]: Japanese patent Application Laid-Open No.
2007-177672
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] However, when introducing the reducing agent supplied
through the reducing agent addition valve into the pre-stage
catalyst together with the exhaust gas, it is difficult to make the
concentration distribution of the reducing agent in the exhaust gas
uniform with respect to cross sectional directions of the catalyst
(i.e. directions perpendicular to the axial direction). In
consequence, the exhaust gas flowing out of the pre-stage catalyst
has an uneven distribution in terms of the concentration of
reducing agent when it merges with the exhaust gas that has
detoured around the pre-stage catalyst. If this is the case, it is
difficult to achieve an even concentration distribution of reducing
agent contained in the exhaust gas flowing into the exhaust gas
purification apparatus.
[0010] The present invention has been made in view of the above
situations, and an object thereof is to provide a technology that
enables causing exhaust gas that is in a state in which an even
concentration distribution of reducing agent is achieved to flow
into an exhaust gas purification apparatus in an exhaust gas
purification system for an internal combustion engine in which the
reducing agent is supplied to a pre-stage catalyst that is
configured in such a way that not the entire amount of the exhaust
gas flowing in the exhaust passage but a portion thereof passes
through it.
Means for Solving the Problem
[0011] An exhaust gas purification system for an internal
combustion engine according to the present invention that achieves
the above object is characterized by comprising: an exhaust gas
purification apparatus provided in an exhaust passage of an
internal combustion engine; a pre-stage catalyst that is provided
in the exhaust passage upstream of said exhaust gas purification
apparatus and adapted in such a way that not the entire amount but
a portion of exhaust gas flowing in the exhaust passage passes
through it; reducing agent addition means provided upstream of said
pre-stage catalyst for adding reducing agent to exhaust gas flowing
into the pre-stage catalyst; and a separation member that separates
a cross section of the exhaust passage into a catalyst side passage
through which catalyst outflowing exhaust gas flowing out of said
pre-stage catalyst is caused to pass and a detour side passage
through which catalyst detouring exhaust gas that has detoured
around the pre-stage catalyst is caused to pass, over a region from
the downstream end of said pre-stage catalyst to a predetermined
position between the pre-stage catalyst and said exhaust gas
purification apparatus.
[0012] In the exhaust gas purification system according to the
present invention, the reducing agent added to the exhaust gas by
the reducing agent addition means is supplied to the exhaust gas
purification apparatus after passing through the pre-stage
catalyst. In the above-descried arrangement, the catalyst
outflowing exhaust gas that is isolated in the catalyst side
passage, among the catalyst side passage and the detour side
passage separated by the separation member, from the catalyst
detouring exhaust gas is firstly stirred in the catalyst side
passage. Consequently, if there is an unevenness in the
concentration distribution of the reducing agent in the exhaust gas
flowing into the pre-stage catalyst, the exhaust gas is mixed in
the catalyst side passage. In other words, mixing of the reducing
agent contained in the catalyst outflowing exhaust gas is promoted
in the catalyst side passage. Thereby, the concentration
distribution of the reducing agent in the catalyst outflowing
exhaust gas can be made uniform (or even) before the catalyst
outflowing exhaust gas and the catalyst detouring exhaust gas are
merged.
[0013] The catalyst outflowing exhaust gas flowing out of the
catalyst side passage separated by the separation member into the
exhaust passage is mixed with the catalyst detouring exhaust gas
that has passed through the detour side passage in the downstream
of the separation member. At that time, the catalyst outflowing
exhaust gas to be mixed with the catalyst detouring exhaust gas has
an even concentration distribution of the reducing agent. In
consequence, after the merging of the catalyst outflowing exhaust
gas and the catalyst detouring exhaust gas also, better uniformity
in the concentration distribution of the reducing agent in the
cross section of the exhaust passage can be achieved.
[0014] As above, in the above-described arrangement, the reducing
agent added to the exhaust gas is gradually dispersed from a
narrower area to a wider area in the cross section of the exhaust
passage until the reducing agent is supplied to the exhaust gas
purification apparatus. In consequence, exhaust gas having a
uniform distribution of the concentration of the reducing agent can
be caused to flow into the exhaust gas purification apparatus.
[0015] The separation member separates the cross section of the
exhaust passage into the catalyst side passage and the detour side
passage over the region from the downstream end of the pre-stage
catalyst to a predetermined position between the pre-stage catalyst
and the exhaust gas purification apparatus. The predetermined
position mentioned here is a location downstream of the pre-stage
catalyst and upstream of the exhaust gas purification apparatus in
the exhaust passage. The predetermined position may be determined
in advance by, for example, an experiment in such a way that better
uniformity in the distribution of the concentration of the reducing
agent contained in the exhaust gas flowing into the exhaust gas
purification apparatus is achieved.
[0016] It is preferred that an expanding portion in which the cross
sectional area of the exhaust passage becomes larger as compared to
the portion of the exhaust passage in which the separating member
is disposed be provided in at least a portion of the exhaust
passage between the separating member and the exhaust gas
purification apparatus. In the expanding portion, with an increase
in the area of the channel of the exhaust gas, flows of the exhaust
gas directed from the center to the outer circumference of the
exhaust passage are created. Consequently, mixing of the reducing
agent contained in the exhaust gas is further promoted. Thereby,
better uniformity in the distribution of the reducing agent
concentration in the exhaust gas flowing into the exhaust gas
purification apparatus can be achieved.
[0017] The system may further comprise a turbocharger having a
turbine disposed in the exhaust passage upstream of said reducing
agent addition means and a compressor disposed in an intake passage
of the internal combustion engine, and a cylindrical member
disposed on a front end face of said pre-stage catalyst to guide a
portion of the exhaust gas flowing out of said turbine to the front
end face of the pre-stage catalyst, and said reducing agent
addition means may add the reducing agent to the exhaust gas
introduced in said cylindrical member.
[0018] Since the exhaust gas flowing into the turbine of the
turbocharger causes a turbine wheel provided in the interior of the
turbine to rotate, a swirling flow is created in the exhaust gas
flowing out of the turbine. Therefore, the exhaust gas flowing out
of the turbine flows downstream helically while swirling along the
inner surface of the exhaust passage.
[0019] The cylindrical member guides a portion of the exhaust gas
flowing out of the turbine to the front end face of the pre-stage
catalyst. Since a swirling flow has been created in the exhaust gas
flowing out of the turbine, the exhaust gas introduced in the
cylindrical member flows toward the front end face of the pre-stage
catalyst while continuously swirling along the inner surface of the
cylindrical member.
[0020] In this arrangement, since the reducing agent is added to
the exhaust gas that has been introduced into the cylindrical
member and in which the swirling flow has been created as described
above, the reducing agent can be favorably agitated by the swirling
flow. Consequently, the reducing agent can be dispersed
appropriately in the exhaust gas introduced in the cylindrical
member. Furthermore, since the reducing agent that has been
dispersed and diffused sufficiently in the exhaust gas can be
supplied to the pre-stage catalyst, the utilization efficiency of
the pre-stage catalyst can be enhanced. The utilization efficiency
mentioned herein may be represented by, for example, the ratio of
the quantity of the reducing agent that reacts on the catalyst and
the quantity of the reducing agent added by the reducing agent
addition means.
[0021] An opening may be provided in the side wall of said
cylindrical member, and said reducing agent addition means may add
the reducing agent to the exhaust gas introduced in said
cylindrical member through said opening. This enables reliable
addition of the reducing agent to the exhaust gas that flows toward
the front end face of the pre-stage catalyst while swirling along
the inner surface of the cylindrical member.
[0022] Now, the position at which the cylindrical member is
disposed with respect to a cross sectional direction of the exhaust
passage and the strength of the swirling flow created in the
exhaust gas flowing in the cylindrical member will be discussed. In
this specification, the cross sectional direction of the exhaust
passage refers to a direction perpendicular to the axis (i.e. axial
center line) of the exhaust passage. The "strength" of the swirling
flow is a notion that represents the momentum of the swirling flow,
and the higher strength means the larger momentum of swirling of
the exhaust gas. The exhaust gas flowing in the portion of the
exhaust passage between the turbine and the cylindrical member
swirls along the inner surface of the exhaust passage about the
axis of the exhaust passage. In consequence, the strength of the
swirling flow increases from the axis of the exhaust passage toward
the inner surface of the exhaust passage.
[0023] Therefore, it is preferred that the cylindrical member be
arranged to be offset from the axis of the exhaust passage toward
the inner surface of the exhaust passage. With this arrangement, a
swirling flow having higher strength can be created in the
cylindrical member. In other words, the exhaust gas introduced in
the cylindrical member can be caused to swirl along the inner
surface of the cylindrical member with larger momentum. Thereby,
the reducing agent added by the reducing agent addition means can
be agitated more drastically, and therefore the reducing agent
added to the exhaust gas can be dispersed more excellently.
[0024] It is preferred that a portion of the cylindrical member is
externally in contact with the inner surface of the exhaust
passage. The strength of the swirling flow created in the exhaust
gas flowing in the portion of the exhaust passage between the
turbine and the cylindrical member becomes highest in the vicinity
of the inner surface of the exhaust passage. Therefore, the exhaust
gas that flows in the region in which the strength of the swirling
flow is the highest can be introduced into the cylindrical member.
Consequently, the strength of the swirling flow created in the
cylindrical member can be made as high as possible, and therefore
better dispersion of the reducing agent added by the reducing agent
addition means can be achieved.
[0025] The means for solving the problem in the present invention
may be adopted in combination, where feasible.
Effects of the Invention
[0026] According to the present invention, in an exhaust gas
purification system for an internal combustion engine in which the
reducing agent is supplied to a pre-stage catalyst that is
configured in such a way that not the entire amount of the exhaust
gas flowing in the exhaust passage but a portion thereof passes
through it, it is possible to supply exhaust gas that is in a state
in which an even concentration distribution of reducing agent is
achieved to an exhaust gas purification apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a diagram showing the general structure of an
engine and its exhaust system according to embodiment 1.
[0028] FIG. 2 is a partial enlarged view of FIG. 1.
[0029] FIG. 3 includes conceptual diagrams illustrating the first
to third mixing promotion sections, where conceptual diagram (a)
illustrates the first mixing promotion section, conceptual diagram
(b) illustrates the second mixing promotion section, and conceptual
diagram (c) illustrates the third mixing promotion section.
[0030] FIG. 4 is a partial enlarged view of the exhaust system of
an engine according to a modification of embodiment 1.
[0031] FIG. 5 is a partial enlarged view of an exhaust system of an
engine according to embodiment 2.
[0032] FIG. 6 is a partial enlarged view of an exhaust system of an
engine according to a modification of embodiment 2.
[0033] FIG. 7 is a diagram showing the general structure of an
engine and its intake and exhaust system according to embodiment
3.
[0034] FIG. 8 is a partial enlarged view of an exhaust system of an
engine according to embodiment 3.
[0035] FIG. 9 is a cross sectional view as seen in the direction of
arrows A-A in FIG. 8.
[0036] FIG. 10 is a diagram showing a first modification of the
exhaust system shown in FIG. 8.
[0037] FIG. 11 is a diagram showing a second modification of the
exhaust system shown in FIG. 8.
[0038] FIG. 12 is a diagram showing a third modification of the
exhaust system shown in FIG. 8.
[0039] FIG. 13 is a partial enlarged view of an exhaust system of
an engine according to embodiment 4.
[0040] FIG. 14 is a cross sectional view as seen in the direction
of arrows B-B in FIG. 13.
MODES FOR CARRYING OUT THE INVENTION
[0041] In the following, embodiments for carrying out the present
invention will be described in detail by way of example. The
dimensions, materials, shapes and relative arrangements etc. of the
components that will be described in connection with the
embodiments are not intended to limit the technical scope of the
present invention only to them, unless particularly stated.
Embodiment 1
[0042] Embodiment 1 for carrying out the present invention will be
described. FIG. 1 is a diagram showing the general construction of
an engine 1 and its exhaust system according to this embodiment.
FIG. 2 is an enlarged view of a portion of FIG. 1. The engine 1
shown in FIG. 1 is a four-cycle diesel engine. An exhaust pipe 3
having a substantially circular cross section is connected to the
engine 1. In this embodiment, the exhaust pipe 3 corresponds to the
exhaust passage according to the present invention.
[0043] The exhaust pipe 3 is connected to a muffler that is not
shown in the drawings. An NOx storage reduction catalyst 4 (which
will be simply referred to as the "NOx catalyst" hereinafter) is
provided in the middle of the exhaust pipe 3. The NOx catalyst 4
has the functions of storing NOx when the air-fuel ratio of the
inflowing exhaust gas is oxygen-rich (i.e. lean), releasing stored
NOx when the oxygen concentration in the inflowing exhaust gas
decreases, and reducing NOx under a condition in which a reducing
component (e.g. fuel or the like) is present. In this embodiment,
the NOx catalyst 4 corresponds to the exhaust gas purification
apparatus according to the present invention.
[0044] A fuel addition valve 5 that adds fuel to the exhaust gas by
ejecting a spray of liquid fuel (light oil) through an injection
port (not shown) is provided in the exhaust pipe 3 upstream of the
NOx catalyst 4. The fuel added to the exhaust gas through the fuel
addition valve 5 is used for processes of recovering the exhaust
gas purifying capability of the NOx catalyst 4, such as an NOx
reduction process for reducing NOx stored in the NOx catalyst 4 and
an SOx poisoning recovery process for recovery from SOx poisoning.
The fuel added through the fuel addition valve 5 is also used to
raise the temperature of the NOx catalyst 4.
[0045] Furthermore, a small-diameter catalyst 7 having an outer
diameter smaller than the inner diameter of the exhaust pipe 3 and
having a cylindrical shape is provided in the exhaust pipe 3
between the fuel addition valve 5 and the NOx catalyst 4. The
small-diameter catalyst 7 in this embodiment has the function of
partially oxidizing unburned components of the fuel (hydrocarbon:
HC) in the exhaust gas to produce H.sub.2 and CO. When the
aforementioned NOx reduction process and SOx poisoning recovery
process etc. are performed, liquid fuel added through the fuel
addition valve 5 is reformed and thereafter supplied to the NOx
catalyst 4. Thereby, the reactivity of fuel in the NOx catalyst 4
can be enhanced.
[0046] The small-diameter catalyst 7 will be described in detail
with reference to FIG. 2. The portion of the exhaust pipe 3 in
which the small-diameter catalyst 7 is provided has a double-tube
structure including an inner pipe 3a. The inner pipe 3a has an axis
coaxial with the exhaust pipe 3 (as shown by a chain double-dashed
line in FIG. 2). The small-diameter catalyst 7 is housed in the
inner pipe 3a in such a way as to be in contact with the inner
circumferential surface thereof. The small-diameter catalyst 7 is
disposed substantially at the center of the exhaust pipe 3 with
respect to the cross sectional directions, and the axis of the
small-diameter catalyst 7 coincides with the axis of the exhaust
pipe 3 and the inner pipe 3a. The cross sectional directions of the
exhaust pipe 3 referred to herein is directions perpendicular to
the axis of the exhaust pipe 3. The small-diameter catalyst 7 in
this embodiment is what is called a metal catalyst, in which a
number of cells made of a metal (which is, in this embodiment, a
stainless steel) extending in the axial direction of the
small-diameter catalyst 7 are provided in a honeycomb
configuration. In consequence, the exhaust gas flowing into the
small-diameter catalyst 7 flows in a passage (which will be
hereinafter referred to as the "catalyst interior passage") 7a that
is partitioned by a number of the cells.
[0047] Referring to the structure around the small-diameter
catalyst 7 configured as above, a detour passage 3b through which
the exhaust gas not flowing into the small-diameter catalyst 7
passes is defined between the outer circumferential surface of the
inner pipe 3a and the inner circumferential surface of the exhaust
passage 3 (outside the small-diameter catalyst 7 accordingly).
Therefore, a portion of the exhaust gas discharged from the engine
1 passes through the catalyst interior passage 7a, and the other
portion of the exhaust gas passes through the detour passage 3b to
detour around the small-diameter catalyst 7. In other words, the
small-diameter catalyst 7 is configured in such a way that not all
but a portion of the exhaust gas flowing in the exhaust pipe 3
passes through it. In this embodiment, the small-diameter catalyst
7 corresponds to the pre-stage catalyst according to the present
invention.
[0048] Now, relationship between the upstream end face 7b of the
small-diameter catalyst 7 and a spray of fuel ejected from the
injection port of the fuel addition valve 5 will be described. A
substantially conical spray (illustrated by cross hatching in FIG.
2) is ejected from the injection port of the fuel addition valve 5.
The injection port of the fuel addition valve 5 is opposed to the
upstream end face 7b of the small-diameter catalyst 7 so that the
spray of fuel ejected from the fuel injection port is introduced
into the small-diameter catalyst 7 without interruption. In this
embodiment, the fuel addition valve 5 corresponds to the reducing
agent addition means according to the present invention.
[0049] An ECU (Electronic Control Unit) 10 that controls the
operation state of the engine 1 in accordance with operation
conditions of the engine 1 and requests made by the driver is
annexed to the engine 1. The ECU 10 includes a CPU that executes
various computation processes pertaining to control of the engine
1, a ROM in which programs and data needed for the control are
stored, a RAM in which results of computations by the CPU are
stored temporarily, and input/output ports for inputting/outputting
signals to/from the outside etc. The fuel addition valve 5 is
connected with the ECU 10 via electrical wiring and controlled by
the ECU 10. Specifically, when the aforementioned NOx reduction
process, SOx poisoning recovery process, and temperature raising
process etc. are performed, the addition of fuel through the fuel
addition valve 5 is performed by the ECU 10.
[0050] Here, a control relating to the addition of fuel through the
fuel addition valve 5 will be described taking the case of the
execution of NOx reduction process for the NOx catalyst 4 as an
example. Since the NOx storage capacity of the NOx catalyst 4 is
limited, the NOx reduction process of reducing NOx stored in the
NOx catalyst 4 to remove it is executed at appropriate timing
before the NOx storage amount reaches the storage capacity. As
described before, in the NOx reduction process, it is necessary to
supply an NOx reducing component while decreasing the oxygen
concentration in the exhaust gas flowing into the NOx catalyst 4 to
the stoichiometric air-fuel ratio or to a rich air-fuel ratio. In
this embodiment, reduction and removal of NOx stored in the NOx
catalyst 4 is performed by making the air-fuel ratio of the exhaust
gas flowing into the NOx catalyst 4 rich in a spike manner (in a
short time) at a relatively short cycle time by controlling the
addition of fuel through the fuel addition valve 5.
[0051] In the arrangement shown in FIG. 1, the exhaust pipe 3 is
curved at a location upstream of the small-diameter catalyst 7.
Therefore, there occurs a local difference in the flow rate of the
exhaust gas along radial directions (cross sectional directions) of
the exhaust pipe 3 in the vicinity of the upstream end face 7b of
the small-diameter catalyst 7. In addition, since the exhaust gas
ceaselessly strikes the end of the cells when the exhaust gas
introduced into the catalyst interior passage 7a, flow of the
exhaust gas is apt to be disturbed at the upstream end face 7b of
the small-diameter catalyst 7. In consequence, it is highly likely
that the fuel concentration in the exhaust gas flowing into the
small-diameter catalyst 7 becomes uneven along radial directions of
the small-diameter catalyst 7.
[0052] In conventional systems, the exhaust gas flowing out of the
catalyst interior passage 7a in the small-diameter catalyst 7
(which exhaust gas will be hereinafter referred to as the "catalyst
outflowing exhaust gas") and the exhaust gas having passed through
the detour passage 3b (which exhaust gas will be hereinafter
referred to as the "catalyst detouring exhaust gas") are caused to
merge immediately at the downstream end face 7c of the
small-diameter catalyst 7. However, since the catalyst interior
passage 7a provided in the small-diameter catalyst 7 is partitioned
by a number of the cells into narrow and long spaces, the
above-mentioned unevenness in the distribution of the fuel
concentration in the exhaust gas is maintained also in the catalyst
outflowing exhaust gas. If the catalyst outflowing exhaust gas in
which the fuel concentration is uneven is mixed with the catalyst
detouring exhaust gas, it is difficult to make the distribution of
the fuel concentration in the exhaust gas uniform by the time when
the mixed exhaust gas reaches the NOx catalyst 4. Therefore, it has
been difficult to reduce NOx evenly in the entire NOx catalyst
4.
[0053] In view of the above, in the exhaust gas purification system
according to the embodiment, the following configuration is adopted
in order to eliminate the above-described disadvantage. As shown in
FIG. 2, an isolation pipe 8 having an outer shape substantially the
same as the inner pipe 3a (i.e. cylindrical shape) is provided on
the downstream end face 7c of the small-diameter catalyst 7. The
isolation pipe 8 is arranged coaxially with the above-described
exhaust pipe 3, the inner pipe 3a, and the small-diameter catalyst
7. In FIG. 2, the upstream end of the isolation pipe 8 is in
contact with and joined to the downstream end of the inner pipe 3a.
The isolation pipe 8 is made of a material that has a relatively
high thermal conductivity.
[0054] Inside the isolation pipe 8, a catalyst side passage 8a
through which only the catalyst outflowing exhaust gas passes is
formed, while a detour side passage 3c through which only the
catalyst detouring exhaust gas that has passed through the detour
passage 3b passes is formed outside the isolation pipe 8 (i.e.
between the outer circumferential surface of the isolation pipe 8
and the inner circumferential surface of the exhaust pipe 3). The
isolation pipe 8 partitions or separates the cross section of the
exhaust pipe 3 into the catalyst side passage 8a and the detour
side passage 3c over a section ranging from the downstream end face
7c of the small-diameter catalyst 7 to a position X located between
the small-diameter catalyst 7 and the NOx catalyst 4. In this
embodiment, the position X is designed to be located downstream of
the small-diameter catalyst 7 (specifically, the downstream end
face 7c of the small-diameter catalyst 7) and upstream of the NOx
catalyst 4 in the exhaust pipe 3, as illustrated. In this
embodiment, the position X and the isolation pipe 8 correspond to
the predetermined position and the separation member.
[0055] As shown in FIG. 2, the exhaust pipe 3 in this embodiment
has a tapered portion 3d with which the inner diameter thereof
gradually increases over a region from a position (midpoint)
between the downstream end of the isolation pipe 8 (i.e. position
X) and the upstream end face 4a of the NOx catalyst 4 to the
upstream end face 4a of the NOx catalyst 4. Therefore, the cross
sectional area of the tapered portion 3d is larger than the cross
sectional area of the portion of the exhaust passage 3 in which the
isolation pipe 8 is provided. In this embodiment, the tapered
portion 3d corresponds to the expanding portion.
[0056] In this embodiment, unevenness in the fuel concentration
distribution present in the catalyst outflowing exhaust gas is
eliminated by promoting the mixing of fuel in the exhaust gas in
the later-described first to third mixing promotion sections that
are created in order from upstream in the exhaust pipe 3 by the
above-described structure. FIG. 3 shows conceptual diagrams for
illustrating the first to third mixing promotion sections. FIG.
3(a) is a conceptual diagram for illustrating the first mixing
promotion section, FIG. 3(b) is a conceptual diagram for
illustrating the second mixing promotion section, and FIG. 3(c) is
a conceptual diagram for illustrating the third mixing promotion
section. In the diagrams, the portions corresponding to the
respective mixing promotion sections are indicated by hatching.
[0057] As shown in the diagrams, the first mixing promotion section
is formed in the catalyst side passage 8a (i.e. the region inside
the isolation pipe 8), and the second mixing promotion section is
formed in the interior of the exhaust pipe 3 in the section from
the downstream end of the isolation pipe 8 to the upstream end of
the tapered portion 3d. The third mixing promotion portion is
formed in the tapered portion 3d of the exhaust pipe 3. The areas
occupied respectively by the first to third mixing promotion
sections in the cross section of the exhaust pipe 3 increase
stepwise in the mentioned order. In these diagrams, the occupied
areas in the cross section of the exhaust pipe 3 increases in order
of the first to third mixing promotion portions from the central
portion in the cross section of the exhaust pipe 3 toward the inner
circumferential surface thereof.
[0058] The state of mixture of fuel in the first mixing promotion
section will be described with reference to FIG. 3(a). In the first
mixing promotion section, the catalyst outflowing exhaust gas is
agitated while it is isolated from the catalyst detouring exhaust
gas, so that mixing of fuel in the catalyst outflowing exhaust gas
is promoted. The arrows in the diagram show, in a simulated manner,
flows of the exhaust gas passing through the catalyst interior
passage 7a formed in the small-diameter catalyst 7 and flows of the
catalyst outflowing exhaust gas having been introduced from the
catalyst interior passage 7a to the catalyst side passage 8a.
[0059] As shown by the broken arrows in the diagram, the catalyst
interior passage 7a is partitioned finely by a number of the cells
along the axial direction of the small-diameter catalyst 7 (which
coincides with the axial direction of the exhaust pipe 3). In
consequence, the exhaust gas passing through the catalyst interior
passage 7a is allowed to flow only along the aforementioned axial
direction. However, after flowing into the catalyst side passage 8a
(or the first mixing promotion section), this exhaust gas is
allowed to move in radial directions of the exhaust pipe 3 as
illustrated by the solid arrows in the diagram. Thus, portions of
the exhaust gas having passed through separate catalyst interior
passages 7a are mixed with each other in the first mixing promotion
section, and mixing of fuel contained in the exhaust gas is also
promoted. Consequently, unevenness in the fuel concentration
distribution in the catalyst outflowing exhaust gas is firstly
eliminated before the catalyst outflowing exhaust gas and the
catalyst detouring exhaust gas are mixed together. In other words,
the fuel concentration distribution in the catalyst outflowing
exhaust gas can be made uniform.
[0060] Next, the state of mixture of fuel in the second mixing
promotion section will be described with reference to FIG. 3(b).
The catalyst outflowing exhaust gas flowing out of the catalyst
side passage 8a (or the first mixing promotion section) merges with
the catalyst detouring exhaust gas having passed through the detour
passage 3b and the detour side passage 3c in the downstream of the
isolation pipe 8 in the exhaust pipe 3. In the second mixing
promotion section, the catalyst outflowing exhaust gas having a
fuel concentration distribution having been improved (or made
uniform) in the first mixing promotion section is mixed with the
catalyst detouring exhaust gas.
[0061] In comparison of the flow speed and temperature between the
catalyst outflowing exhaust gas and the catalyst detouring exhaust
gas, the catalyst outflowing exhaust gas has a lower flow speed and
higher temperature than the catalyst detouring exhaust gas.
Therefore, there is a significant difference in the flow speed and
temperature of the exhaust gas between the central portion and the
outer peripheral portion in the cross section of the exhaust pipe
3. This leads to the creation of a vortex flow in the vicinity of
the interface on which the catalyst outflowing exhaust gas and the
catalyst detouring exhaust gas meet with each other, thereby
promoting the mixing of the catalyst outflowing exhaust gas and the
catalyst detouring exhaust gas. In this embodiment, the catalyst
outflowing exhaust gas that has been made uniform in terms of the
fuel concentration distribution in the first mixing promotion
section can be mixed with the catalyst detouring exhaust gas,
unlike with prior art systems in which the catalyst outflowing
exhaust gas having uneven fuel concentration is mixed with the
catalyst detouring exhaust gas. Therefore, the fuel concentration
in the exhaust gas produced as the mixture of these exhaust gases
(which will be hereinafter referred to as the "mixed exhaust gas")
can also be made more uniform.
[0062] The material of which the isolation pipe 8 in this
embodiment is made has a high thermal conductivity as described
above. Therefore, the isolation pipe 8 is heated to a high
temperature by the heat of reaction of fuel generated in the
small-diameter catalyst 7 and the heat given by the catalyst
outflowing exhaust gas etc. In consequence, heat is discharged from
the outer circumferential surface of the isolation pipe 8 thus
heated to a high temperature to the catalyst detouring exhaust gas
that passes through the detour side passage 3c and has a
temperature lower than isolation pipe 8. Consequently, in the
catalyst detouring exhaust gas passing through the detour side
passage 3c, the exhaust gas flowing in the region closer to the
isolation pipe 8 has a higher temperature, and the exhaust gas
flowing in the region closer to the inner circumferential surface
of the exhaust pipe 3 has a lower temperature. Thus, there is
created a local temperature difference in the catalyst detouring
exhaust gas passing through the detour side passage 3c, so that
turbulence is generated in the catalyst detouring exhaust gas. The
turbulence of the exhaust gas continues to exist even after the
exhaust gas reaches the second mixing promotion section. In
consequence, mixing of the catalyst outflowing exhaust gas and the
catalyst detouring exhaust gas in the second mixing promotion
section can be promoted more favorably.
[0063] Next, the state of mixture of fuel in the third mixing
promotion section will be described with reference to FIG. 3(c).
Since the third mixing promotion section is provided in the tapered
portion 3d of the exhaust pipe 3, the area of the flow channel of
the mixed exhaust gas is larger than that in the second mixing
promotion section. Consequently, flows of the mixed exhaust gas
diverging in radial directions of the exhaust pipe 3 are created in
the third mixing promotion section as indicated by arrows in FIG.
3(c). This disperses fuel contained in the mixed exhaust gas from
the central portion toward the outer peripheral portions in the
exhaust pipe 3. Thus, in the third mixing promotion section, the
distribution of the fuel concentration in the mixed exhaust gas can
be made uniform over the entire cross section of the tapered
portion 3d.
[0064] As described above, according to the structure of this
embodiment, even if unevenness in the fuel concentration
distribution is created in the exhaust gas flowing into the
small-diameter catalyst 7 as the addition of fuel through the fuel
addition valve 5 is performed, mixing of fuel can be achieved
gradually in the first to third mixing promotion sections.
Consequently, the exhaust gas having a uniform fuel concentration
distribution can be introduced into the NOx catalyst 4. Thus, NOx
and SOx can be reduced evenly in the entire region of the NOx
catalyst 4 in the NOx reduction process and the SOx reduction
process. In addition, when a control for raising the temperature of
the NOx catalyst 4 is performed, the temperature of the NOx
catalyst 4 can be raised evenly over the entire region thereof.
[0065] Although in this embodiment an exemplary structure in which
the first to third mixing promotion sections are arranged in order
from upstream in the exhaust pipe 3 has been described as a mode
for carrying out the present invention, the structure may be
modified in the following way. That is, the tapered portion 3d need
not be necessarily provided in the exhaust pipe 3 having the
above-described structure. A uniform fuel concentration
distribution in the mixed exhaust gas can be achieved by making the
fuel concentration distribution in the catalyst outflowing exhaust
gas uniform while isolating the catalyst outflowing exhaust gas and
the catalyst detouring exhaust gas from each other in the first
mixing promotion section and mixing the catalyst outflowing exhaust
gas and the catalyst detouring exhaust gas in the second mixing
promotion section.
[0066] The position X at which the downstream end of the isolation
pipe 8 in the embodiment is located may be set to any appropriate
position between the downstream end face 7c of the small-diameter
catalyst 7 and the upstream end face 4a of the NOx catalyst 4 in
the exhaust pipe 3. Changing this position X causes a change in the
proportions of the lengths over which the first to third mixing
promotion sections respectively extend along the longitudinal
direction (or the axial direction) of the exhaust pipe 3. More
specifically, changing the position X to a more downstream position
in the exhaust pipe 3 increases the length over which the first
mixing promotion section extends and decreases the length over
which the second mixing promotion section and the third mixing
promotion section extend. Conversely, changing the position X to a
more upstream position in the exhaust pipe 3 decreases the length
over which the first mixing promotion section extends and increases
the length over which the second mixing promotion section and the
third mixing promotion section extend. Therefore, it is preferred
that the position X or the position at which downstream end of the
isolation pipe 8 is located be determined appropriately by, for
example, an experiment in such a way that better uniformity in the
concentration distribution of fuel contained in the exhaust gas
flowing into the NOx catalyst, i.e. the mixed exhaust gas, is
achieved.
[0067] In the exhaust gas purification system according to the
embodiment, various modifications can be made to the shapes and
relative positional relationships of components such as the
small-diameter catalyst 7, the isolation pipe 8, and the exhaust
pipe 3 without departing from the essential scope of the present
invention. For example, the isolation pipe 8 may have a shape
different from a cylindrical shape. Although in this embodiment the
isolation pipe 8 completely separates the cross section of the
exhaust pipe 3 into the catalyst side passage 8a and the detour
side passage 3c, other configurations may be adopted on condition
that a major part of the catalyst outflowing exhaust gas in the
catalyst side passage 8a can be isolated from the catalyst
detouring exhaust gas.
[0068] Although in the exemplary configuration shown in FIGS. 1 to
3 the axes of the exhaust pipe 3 (including the tapered portion
3d), the small-diameter catalyst 7, and the isolation pipe 8 are
coaxial, this is not essential. For example, the axes of the
small-diameter catalyst 7 and the isolation pipe 8 may be arranged
obliquely to the axis of the exhaust pipe 3 as shown in FIG. 4.
This enables better mixing of the catalyst outflowing exhaust gas
flowing out of the first mixing promotion section (catalyst side
passage 8a) and the catalyst detouring exhaust gas having passed
through the detour side passage 3c. Various modifications other
than the configuration shown in FIG. 4 can be made to the
embodiment without departing from the essential scope of the
present invention, where feasible.
Embodiment 2
[0069] Next, embodiment 2 for carrying out the present invention
will be described. In the basic configuration of the engine 1 and
other hardware to which this embodiment is applied, counterparts of
portions shown in FIGS. 1 to 3 are denoted by the same symbols to
eliminate description. FIG. 5 is an enlarged partial view of the
exhaust system of the engine 1 according to this embodiment. In the
exhaust system of the engine 1 according to this embodiment, the
axis L1 of the tapered portion 3d is inclined relative to the axis
L2 of the small-diameter catalyst 7. Although the axis of the
portion of the exhaust pipe 3 located upstream of the tapered
portion 3d and the axis of the isolation pipe 8 are coaxial with
the axis L2 of the small-diameter portion 7, this is not essential
to this embodiment.
[0070] The above-described arrangement is designed in such a way
that the mixed exhaust gas flowing in the tapered portion 3d of the
exhaust pipe 3 or the above-described third mixing promotion
section impinges on the inner wall surface of the tapered portion
3d. The mixed exhaust gas impinging on the inner wall surface of
the tapered portion 3d loses momentum of forward movement in the
direction of the axis L1, and a vortex flow illustrated by an arrow
in the drawing is created. This further promotes mixing of fuel in
the mixed exhaust gas in the third mixing promotion section. In
consequence, better uniformity in the fuel concentration
distribution in the mixed exhaust gas flowing into the NOx catalyst
4 can be achieved.
(Modification)
[0071] Next, a modification of the mode according to this
embodiment will be described. FIG. 6 is an enlarged partial view of
the exhaust system of the engine 1 according to the modification.
In FIG. 6, what is different from the configuration shown in FIG. 5
is that the second mixing promotion section and the third mixing
promotion section are combined. Specifically, the tapered portion
3d in this modification is configured to expand downstream from the
position at which the downstream end of the isolation pipe 8 is
located (position X) to the upstream end face 4a of the NOx
catalyst 4. This can make the exhaust system of the engine 1 more
compact while achieving operations and effects same as those of the
configuration shown in FIG. 5. In other words, mixing of the added
fuel contained in the mixed exhaust gas can be achieved in a more
compact space.
[0072] Although in the above-described embodiment, a case in which
the small-diameter catalyst 7 functions as a fuel reforming
catalyst has been described by way of example, it may be other type
of catalyst (e.g. oxidation catalyst). This also applies to the
embodiments described in the following.
Embodiment 3
[0073] Embodiment 3 for carrying out the present invention will be
described. FIG. 7 is a diagram showing the general configuration of
the engine 1 and its intake and exhaust system according to this
embodiment. FIG. 8 is an enlarged partial view of the exhaust
system of the engine according to the embodiment. In this
embodiment, counterparts of components in embodiments 1 and 2 are
denoted by the same symbols to eliminate detailed description
thereof. The engine 1 is connected with an exhaust pipe 3 and an
intake pipe 2 having a substantially circular cross section. In the
intake pipe 2, a compressor housing 6a of a turbocharger 6 is
provided. A throttle valve 24 is provided in the intake pipe 2
downstream of the compressor housing 6a. In this embodiment, the
intake pipe 2 and the exhaust pipe 3 correspond to the intake
passage and the exhaust passage according to the present
invention.
[0074] The exhaust pipe 3 is connected to a muffler that is not
shown in the drawings, and a turbine housing 6b of the turbocharger
6 is provided in the middle of the exhaust pipe 3. An NOx catalyst
4 is provided in the exhaust pipe 3 downstream of the turbine
housing 6b. The engine 1 is provided with an in-cylinder fuel
injection valve 9 that supplies fuel to be burned in the engine 1
into a cylinder.
[0075] The ECU 10 is connected, by electrical wiring, with a crank
position sensor 21 that outputs an electrical signal indicative of
the crank angle of the engine 1, an accelerator opening degree
sensor 22 that outputs an electrical signal indicative of the
opening degree of the accelerator, and an air flow meter (not
shown) that outputs an electrical signal indicative of the mass of
the intake air flowing in the intake pipe 2 etc, and the output
signals of them are input to the ECU 10. The ECU 10 can sense the
engine speed of the engine 1, the engine load, and the intake air
quantity etc. based on the output signals from the aforementioned
sensors input to it. The ECU 10 is connected, by electrical wiring,
with a throttle valve 24 and an in-cylinder fuel injection valve 9
etc, and the opening degree and the open/close status of them etc.
are controlled by the ECU 10. In this embodiment, a small-diameter
catalyst 7 is provided in the exhaust pipe 3 downstream of the
turbine housing 6b and upstream of the NOx catalyst 4.
[0076] The configuration of the small-diameter catalyst 7 and the
portion around it will be specifically described with reference to
FIG. 8. The chain double-dashed line in FIG. 8 indicates the axis
(axial center line) of the exhaust pipe 3. Here, the direction of
flow of the exhaust gas from the upstream to the down stream of the
exhaust pipe 3 that is parallel to the axis of the exhaust pipe 3
will be referred to as the "exhaust gas mainstream direction"
(indicated by arrow Y in FIG. 8). In this embodiment also, the
portion of the exhaust pipe 3 in which the small-diameter catalyst
7 is provided has a double-tube structure including an inner pipe
3a. The inner pipe 3a has an axis coaxial with the exhaust pipe 3
and has a cylindrical shape. The inner pipe 3 having the
above-described shape is disposed at the center of the exhaust pipe
3 in the cross section perpendicular to the axis of the exhaust
pipe 3.
[0077] In the inner pipe 3a, a small-diameter catalyst 7 having a
cylindrical shape is accommodated in such a way that the outer
circumferential surface thereof is in contact with the inner
circumferential surface of the inner pipe 3a. The axis of the
small-diameter catalyst 7 is coaxial with the axis of the exhaust
pipe 3 and the inner pipe 3a. The configuration of the portion
downstream of the small-diameter catalyst 7 is the same as that in
embodiment 1. Specifically, an isolation pipe 8 is disposed on the
downstream end face 7c of the small-diameter catalyst 7, and the
isolation pipe 8 separates the cross section of the exhaust pipe 3
into a catalyst side passage 8a and a detour side passage 3c.
[0078] On the front end face (upstream end face 7b) of the
small-diameter catalyst 7 is provided an introduction pipe 28 that
introduces a portion of the exhaust gas flowing out of the turbine
housing 6b to the upstream end face 7b of the small-diameter
catalyst 7. The introduction pipe 28 is a cylindrical member having
a cross sectional shape same as that of the inner pipe 3a, which is
arranged coaxially with the exhaust pipe 3, the inner pipe 3a, and
the small-diameter catalyst 7. The introduction pipe 28 has a
"circumferential wall 28a", an "upstream end edge 28b" or an edge
that defines an upstream end opening, a "downstream end edge 28c"
or an edge that defines a downstream end opening, and a
"circumferential wall opening 28d" or an opening provided on the
circumferential wall 28a.
[0079] The direction normal to a virtual plane determined by three
different points on the upstream end edge 28b (downstream end edge
28c) coincides with the exhaust gas mainstream direction. The
downstream end edge 28c abuts on and is joined to the upstream end
edge of the inner pipe 3a. The circumferential wall opening 28d is
configured to be circular. In this embodiment, the introduction
pipe 28 corresponds to the cylindrical member according to the
present invention. The circumferential wall 28a in this embodiment
corresponds to the side wall of the cylindrical member according to
the present invention, and the circumferential wall opening 28d
corresponds to the opening provided on the side wall of the
cylindrical member according to the present invention.
[0080] In the exhaust gas purification system having the
above-described configuration, the outer circumferential surface of
the introduction pipe 28 and the outer circumferential surface of
the inner pipe 3a are configured to be continuous, and the outer
diameter of them is smaller than the inner diameter of the exhaust
pipe 3. Thus, a detour passage 3b through which the exhaust gas
detours around the introduction pipe 28 and the small-diameter
catalyst 7 is formed between the outer circumferential surface of
the introduction pipe 28 and the inner pipe 3a and the inner
circumferential surface of the exhaust pipe 3. A portion of the
exhaust gas flowing out of the turbine housing 6b flows into the
introduction pipe 28 through the upstream end edge 28b, and the
other part of the exhaust gas passes through the detour passage
3b.
[0081] Next, a fuel addition apparatus 30 in this embodiment will
be described. Details of the construction of the fuel addition
apparatus 30 will be described later. Its general function is to
add fuel serving as a reducing agent to the exhaust gas introduced
into the introduction pipe 28 or the exhaust gas flowing in the
introduction pipe 28. Since the exhaust gas flowing in the
introduction pipe 28 is introduced to the upstream end face 7b of
the small-diameter catalyst 7, the fuel added by the fuel addition
apparatus 30 is supplied to the same catalyst 7. The fuel supplied
to the small-diameter catalyst 7 is partly oxidized in the
small-diameter catalyst 7, so that H.sub.2 and CO that function as
good reducing components are generated. The fuel having been
reformed in the small-diameter catalyst 7 is supplied to the NOx
catalyst 4 disposed downstream and used in the process of
recovering the exhaust gas purification capability of the NOx
catalyst 4. Examples of the process of recovering the exhaust gas
purification capability include an NOx reduction process for
reducing NOx stored in the NOx catalyst 4 and an SOx poisoning
recovery control for recovery from poisoning by sulfur oxide
(SOx).
[0082] Details of the construction of the fuel addition apparatus
30 is now described. The fuel addition apparatus 30 has a fuel
addition valve 5 (corresponding to the reducing agent addition
means), a fuel intake pipe 33, a fuel pump 34, and a fuel supply
pipe 35. A fuel injection port 5a is provided at the end of the
fuel addition valve 5. The axis of the fuel addition valve 5 is
perpendicular to the axis of the exhaust pipe 3. The fuel addition
valve 5 is connected with the ECU 10 through electrical wiring, and
opening/closing of the fuel addition valve 5 and the opening period
thereof are controlled by the ECU 10.
[0083] One end of the fuel intake pipe 33 is connected to the fuel
tank 36 of the engine 1, and the other end thereof is connected to
the fuel pump 34. One end of the fuel supply pipe 35 is connected
to the fuel pump 34, and the other end thereof is connected to the
fuel addition valve 5. The fuel pump 34 is a mechanical pump that
is mechanically actuated. The fuel pump 34 operates utilizing a
driving force of the power output shaft (crankshaft) of the engine
1 that is not shown in the drawings. Alternatively, the fuel pump
34 may be an electrical supply pump that operates utilizing a
driving force of a motor (not shown). In the latter case, the fuel
discharge pressure is controlled by adjusting the electrical power
supplied to the fuel pump 34.
[0084] In the fuel addition apparatus 30 having the above-described
construction, the fuel pump 34 sucks fuel stored in the fuel tank
36 through the fuel intake pipe 33 and discharges it to the fuel
supply pipe 35. The fuel discharged to the fuel supply pipe 35 is
supplied to the fuel addition valve 5. As the fuel addition valve 5
is opened by a control signal from the ECU 10, fuel is injected
into the exhaust gas through the injection port 5 of the fuel
addition valve 5.
[0085] Next, the method of supplying fuel injected through the fuel
addition valve 5 into the introduction pipe 28 will be described
with reference to the relationship between the injection port 5a of
the fuel addition valve 5 and the circumferential wall opening 28d
provided in the circumferential wall 28a of the introduction pipe
28. The injection port 5a is disposed at a position facing the
interior of the detour passage 3b provided outside the
circumferential wall 28a of the introduction pipe 28, and fuel is
ejected from the injection port 5a toward the circumferential wall
opening 28d.
[0086] The opening position and opening diameter etc. of the
circumferential wall opening 28d are determined in such a way that
a spray of fuel ejected from the injection port 5a is introduced
into the introduction pipe 28 without falling on the
circumferential wall 28a. In this embodiment, the apex angle of the
cone subtended by the edge of the circumferential wall opening 28d
at the injection port 5a is designed to be larger than the angle of
the spray ejected from the injection port 5a. With this design, the
fuel ejected from the injection port 5a of the fuel addition valve
5 can be added to the exhaust gas introduced into the introduction
pipe 28 appropriately through the circumferential wall opening
28d.
[0087] In order to promote the reduction reaction of NOx and SOx in
the aforementioned process of recovering the exhaust gas
purification capability of the NOx catalyst 4, it is preferred that
the fuel reforming efficiency (or utilization efficiency) of the
small-diameter catalyst 7 be enhanced. The fuel reforming
efficiency can be expressed by the ratio of the quantity of fuel
reformed in the small-diameter catalyst 7 and the quantity of fuel
supplied to the same catalyst. In this connection, the fuel
reforming efficiency can be enhanced by sufficiently dispersing
fuel in the exhaust gas when fuel is supplied to the small-diameter
catalyst 7.
[0088] In view of this, in this embodiment, a swirling flow is
created in the exhaust gas introduced in the introduction pipe 28,
so that fuel is agitated by the swirling flow, thereby promoting
dispersion of fuel. Specifically, as the exhaust gas discharged
from the engine 1 flows into the turbine housing 6b, the exhaust
gas causes the turbine wheel (not shown) having blades provided in
the turbine housing 6b to rotate. In consequence, a swirling flow
is created in the exhaust gas flowing out of the turbine housing
6b. The swirling flow mentioned here refers to a flow of exhaust
gas swirling in the circumferential direction of the exhaust pipe
3.
[0089] The axis of rotation of the swirling flow created in the
exhaust gas as described above coincides with the axis of the
exhaust pipe 3. Specifically, the exhaust gas flowing in the
exhaust pipe 3 downstream of the turbine housing 6b flows toward
the introduction pipe 28 located downstream while swirling along
the inner circumferential surface of the exhaust pipe 3 about the
axis of the exhaust pipe 3. In other words, the exhaust gas flowing
out of the turbine housing 6b flows in the exhaust gas mainstream
direction in a helical manner along the inner circumferential
surface of the exhaust pipe 3.
[0090] FIG. 9 is a cross sectional view as seen in the direction of
arrows A-A in FIG. 8. The cross hatching in FIG. 9 schematically
represents a spray of fuel ejected from the fuel addition valve 5.
In this embodiment, since the introduction pipe 28 and the exhaust
pipe are coaxial, the exhaust gas in which a swirling flow is
created is introduced into the introduction pipe 28 without
interruption. Therefore, the swirling flow that has been created in
the exhaust pipe 3 upstream of the introduction pipe 28 continues
to exist also in the interior of the intake pipe 28. Consequently,
the exhaust gas introduced into the introduction pipe 28 flows
toward the upstream end face 7b of the small-diameter catalyst 7
while swirling along the inner circumferential surface of the
circumferential wall 28a.
[0091] The rotational axis (which may also be called helical axis)
of the swirling flow created in the exhaust gas in the interior of
the introduction pipe 28 coincides with the axis of the
introduction pipe 28, and the direction of swirling of the exhaust
gas coincides with the circumferential direction of the
introduction pipe 28 as indicated by broken arrows a, b in the
drawing. On the other hand, a spray of fuel ejected from the
injection port 5a of the fuel addition valve 5 and introduced into
the introduction pipe 28 through the circumferential wall opening
28d travels toward the center of the introduction pipe 28 with
respect to the radial directions. Consequently, the spray of fuel
impinges transversely on the swirling flow of the exhaust gas
created in the introduction pipe 28. Therefore, the swirling flow
can appropriately agitate the fuel added to the exhaust gas flowing
in the introduction pipe 28, thereby promoting dispersion and
diffusion of the fuel in the exhaust gas. Thus, the fuel reforming
efficiency in the small-diameter catalyst 7 can be enhanced.
[0092] Furthermore, the fuel flowing out of the small-diameter
catalyst 7 is mixed in the exhaust gas gradually in the first to
third mixing promotion sections that have been already described.
To sum up, before the exhaust gas flows into the small-diameter
catalyst 7, dispersion and diffusion of fuel in the exhaust gas is
promoted in the introduction pipe 28, and after the exhaust gas
flows out of the small-diameter catalyst 7, mixing of fuel in the
exhaust gas is promoted by the mixing promotion sections. In
consequence, the exhaust gas having a uniform fuel concentration
distribution can flow into the NOx catalyst 4. Thus, NOx and SOx
can be reduced evenly in the entire region of the NOx catalyst 4 in
the NOx reduction process and the SOx poisoning recovery
process.
[0093] The rotational axis of the swirling flow created in the
introduction pipe 28 coincides with the axis of the introduction
pipe 28. Therefore, the strength of the swirling flow increases
from the axis of the introduction pipe 28 toward the inner
circumferential surface of the circumferential wall 28a. Here, the
"strength" of the swirling flow is a notion that represents the
momentum of the swirling flow, and the higher strength means the
larger momentum of swirling of the exhaust gas. Therefore, the
strength of the swirling flow indicated by broken arrow b created
in a portion closer to the inner circumferential surface of the
circumferential wall 28a is greater than the strength of the
swirling flow indicated by broken arrow a created in a portion
distant from the inner circumferential surface of the
circumferential wall 28a.
[0094] In this embodiment, fuel is introduced into the introduction
pipe 28 through the circumferential wall opening 28d provided in
the circumferential wall 28a. As described above, the strength of
the swirling flow created in the introduction pipe 28 is greatest
in the neighborhood of the inner circumference of the
circumferential wall 28a. Therefore, in the structure according to
the embodiment, fuel can be added to a portion in which the
strength of the swirling flow is very high, and therefore fuel can
be dispersed in the exhaust gas more appropriately.
[0095] In this embodiment, since the circumferential wall opening
28d opens in the circumferential wall 28a, the circumferential wall
opening 28d does not abut on the upstream end edge 28b. In other
words, the circumferential wall opening 28d is provided in a
portion between the upstream end edge 28b and the downstream end
edge 28c. This eliminates the possibility that a large quantity of
fuel flows out through from the upstream end edge 28b of the
introduction pipe 28, even if fuel supplied into the introduction
pipe 28 through the circumferential wall opening 28d evaporates and
expands. Therefore, the fuel injected through the fuel addition
valve 5 can be supplied appropriately to the small-diameter
catalyst 7.
[0096] In this embodiment, the fuel ejected from the injection port
5a of the fuel addition valve 5 is introduced into the introduction
pipe 28 after traversing the detour passage 3b. The momentum of the
spray of fuel or the penetration force of the spray generally
decreases with an increase in the distance from the injection port
5a. Therefore, if the flow speed of the exhaust gas flowing in the
introduction pipe 28 is excessively high, it may be difficult to
introduce fuel into the introduction pipe 28 through the
circumferential wall opening 28d in some cases.
[0097] According to the arrangement in this embodiment, the flow
speed of the exhaust gas flowing in the introduction pipe 28 can be
made lower than the flow speed of the exhaust gas flowing in the
detour passage 3b. This is because the introduction pipe 28 is
disposed on the upstream end face 7b of the small-diameter catalyst
7, and the flow speed of the exhaust gas flowing in the
introduction pipe 28 is limited by the flow speed of the exhaust
gas passing through the small-diameter catalyst 7. In consequence,
it is easy to introduce fuel from the detour passage 3b into the
interior of the introduction pipe 28 through the circumferential
wall opening 28d, and the addition of fuel to the exhaust gas
introduced in the introduction pipe 28 can be achieved readily.
[0098] Although in this embodiment the exhaust pipe 3 and the
introduction pipe 28 are configured to have cylindrical shapes, and
the small-diameter catalyst 7 is configured to have a cylindrical
column-like shape, the shapes of them are not limited to those in
the above-described embodiment. In order to maintain the swirling
flow created in the exhaust gas flowing in the exhaust pipe 3 and
the introduction pipe 28, it is preferred that they have circular
internal shapes as is the case with this embodiment. However, the
internal shapes of them may be modified appropriately to an extent
that allows the swirling flow of the exhaust gas to be maintained.
The circumferential wall opening 28d that opens in the
circumferential wall 28a of the introduction pipe 28 may have a
shape other than circular shapes. Although in the arrangement
according to this embodiment the isolation pipe 8 is provided on
the downstream end face 7c of the small-diameter catalyst 7, the
isolation pipe 8 may be eliminated. This is because fuel added to
the exhaust gas flowing in the introduction pipe 28 can be agitated
by the swirling flow of the exhaust gas, and dispersion and
diffusion of fuel in the exhaust gas can be promoted even if the
isolation pipe 8 is not provided downstream of the small-diameter
catalyst 7.
(Modification)
[0099] Next, modifications of this embodiment will be described.
FIG. 10 is a diagram showing a first modification of the exhaust
system shown in FIG. 8. The fuel addition apparatus 30 in this
modification has a fuel introduction pipe 29 that interconnects the
fuel addition valve 5 and the introduction pipe 28, and fuel is
supplied from the fuel addition valve 5 into the introduction pipe
28 through the fuel introduction pipe 29. The fuel introduction
pipe 29 in this modification has a cylindrical shape. One end of it
is connected to the outer circumferential side surface of the fuel
addition valve 5, and the other end of it is connected to the edge
of the circumferential wall opening 28d of the circumferential wall
28a. The diameter of the fuel introduction pipe 29 increases from
the injection port 5a toward the circumferential wall opening 28d
in a flaring manner so that a spray of fuel ejected from the
injection port 5a does not fall on the inner circumferential
surface of the fuel introduction pipe 29.
[0100] In this modification, the injection port 5a of the fuel
addition valve 5 is injected into the fuel introduction pipe 29 and
added to the exhaust gas flowing in the introduction pipe 28
through the fuel introduction pipe 29. Thus, since fuel is not
injected from the fuel addition valve 5 into the detour passage 3b,
fuel can surely be introduced into the introduction pipe 28 without
affected by the exhaust gas passing through the detour passage
3b.
[0101] FIG. 11 is a diagram showing a second modification of the
exhaust system shown in FIG. 8. This modification is different from
the arrangement shown in FIG. 8 in that the circumferential wall
28a of the introduction pipe 28 is not provided with the
circumferential wall opening 28d. The fuel addition valve 5 is
configured to have a substantially L-shape, and the injection port
5a is located in the interior of the introduction pipe 28. With
this arrangement also, fuel can be added to the exhaust gas
introduced in the introduction pipe 28. Therefore, swirling flow of
the exhaust gas created in the introduction pipe 28 can
appropriately agitate fuel, thereby favorably promoting dispersion
and diffusion of the fuel in the exhaust gas.
[0102] FIG. 12 is a diagram showing a third modification of the
exhaust system shown in FIG. 8. Arrow Y in the drawing indicates
the "exhaust gas mainstream direction" that has been described in
connection with FIG. 8. Arrow Z in the drawing indicates the
direction in which fuel is ejected from the injection port 5a of
the fuel addition valve 5. This direction of ejection is the
direction in which the center of the spray of fuel is oriented. As
shown in the drawing, in this modification the axis of the fuel
addition valve 5 is inclined relative to the axis of the exhaust
pipe 3, and the fuel addition valve 5 is arranged in such a way
that the direction of ejection of fuel and the exhaust gas
mainstream direction form an acute angle. With this arrangement,
the fuel addition valve 5 injects fuel into the detour passage 3b
toward the downstream direction with respect to the axial direction
of the exhaust pipe 3.
[0103] The swirling flow of the exhaust gas created in the
introduction pipe 28 travels in the exhaust gas mainstream
direction with its rotation axis coinciding with the axis of the
introduction pipe 28. With the above arrangement, fuel supplied
into the introduction pipe 28 is supplied through the
circumferential wall opening 28d toward the downstream direction
with respect to the axial direction of the introduction pipe 28.
This enables the fuel supplied into the introduction pipe 28 to be
favorably caught in the swirling flow of the exhaust gas and can
accordingly achieve appropriate dispersion of fuel in the
introduction pipe 28.
Embodiment 4
[0104] Next, embodiment 4 for carrying out the present invention
will be described. In this embodiment, counterparts of components
in the embodiment 3 are denoted by the same symbols to eliminate
detailed description thereof. Here, what is different from
embodiment 3 will be mainly described.
[0105] The exhaust gas flowing in the portion of the exhaust pipe 3
between the turbine housing 6b and the introduction pipe 28 is
swirling along the inner circumferential surface of the exhaust
pipe 3, and the center of the swirling flow coincides with the axis
of the exhaust pipe 3. The variation in the strength of the
swirling flow with the change in the radial position in the exhaust
pipe 3 is that the strength of the swirling flow increases from the
axis of the exhaust pipe 3 toward the inner circumferential surface
of the exhaust pipe 3. In view of this, in this embodiment the
position at which the introduction pipe 28 is disposed is offset
from the axis of the exhaust pipe 3 toward the inner
circumferential surface of the exhaust pipe 3, in order to create
swirling flow having higher strength. The aforementioned position
at which the introduction pipe 28 is disposed refers to the
position in the cross section perpendicular to the axis of the
exhaust pipe 3.
[0106] FIG. 13 is an enlarged partial view of the exhaust system of
the engine according to this embodiment. FIG. 14 is a cross
sectional view as seen in the direction of arrows B-B in FIG. 13.
In FIG. 13, the alternate long and short dashed line represents the
axis of the introduction pipe 28 and the inner pipe 3a, and the
chain double-dashed line represents the axis of the exhaust pipe 3.
In this embodiment, the axis of the introduction pipe 28 and the
axis of the inner pipe 3a are coaxial and offset from the axis of
the exhaust pipe 3. In this embodiment, the introduction pipe 28
and the inner pipe 3a are externally in contact with the inner
circumferential surface of the exhaust pipe 3 so that the injection
port 5a of the fuel addition valve 5 faces the interior of the
introduction pipe 28 through the circumferential wall opening
28d.
[0107] The broken arrow in FIG. 14 schematically illustrates a
swirling flow created in the introduction pipe 28, and the
alternate long and short dashed arrow schematically illustrates a
swirling flow created in the detour passage 3b. As described above,
at locations upstream of the upstream end opening of the
introduction pipe 28, the strength of the swirling flow created in
the exhaust gas becomes highest in the vicinity of the inner
circumferential surface of the exhaust pipe 3. In this embodiment,
since the upstream end opening of the introduction pipe 28 opens in
the vicinity of the inner circumferential surface of the exhaust
pipe 3, the strength of the swirling flow created in the exhaust
gas introduced in the introduction pipe 28 can be made as high as
possible. In consequence, dispersion and diffusion of fuel in the
exhaust gas flowing in the introduction pipe 28 can be promoted as
much as possible, and the fuel reforming efficiency of the
small-diameter catalyst 7 can be enhanced.
[0108] In the above-described arrangement according to this
embodiment, since the injection port 5a of the fuel addition valve
5 is arranged to face the interior of the introduction pipe 28,
fuel can be added directly to the exhaust gas in the introduction
pipe 28. Therefore, fuel injected through the fuel addition valve 5
can be supplied to the small-diameter catalyst 7 reliably.
[0109] In this embodiment, the portion of contact of the
introduction pipe 28 and the inner circumferential surface of the
exhaust pipe 3 may be made of the same member integrally, as a
matter of course. In this embodiment, the introduction pipe 28 is
arranged to be externally in contact with the inner circumferential
surface of the exhaust pipe 3 in order that a swirling flow having
as high strength as possible is created in the introduction pipe
28. However, the degree of offset of the axis of the introduction
pipe 28 from the axis of the exhaust pipe 3 may be changed fittly.
By offsetting the axis of the introduction pipe 28 from the axis of
the exhaust pipe 3, an advantageous effect that the strength of the
swirling flow can be made higher than that in the case where there
is no offset between them is achieved.
[0110] Although in embodiments 3 and 4 the introduction pipe 28 and
the inner pipe 3a are separate members, the invention is not
limited to this, and various changes can be made to the embodiments
without departing from the essential scope of the present
invention. For example, the introduction pipe 28 and the inner pipe
3a may be constituted by the same member. The downstream end edge
28c of the introduction pipe 28 and the inner pipe 3a need not be
necessarily joined, but they may be arranged close to each other.
If this is the case, it is preferred that the introduction pipe 28
be disposed in such a way that the projection of the downstream end
edge 28c along the axial direction of the introduction pipe 28 onto
the upstream end face 7b of the small-diameter catalyst 7 is
contained within the area of the upstream end face 7b. This can
prevent the fuel supplied into the introduction pipe 28 from
slipping into the detour passage 3b through the gap between the
introduction pipe 28 and the inner pipe 3a (or the upstream end
face 7b of the small-diameter catalyst 7). In other words, the fuel
introduced into the introduction pipe 28 and added to the exhaust
gas can surely be supplied to the small-diameter catalyst 7.
[0111] To agitate the fuel added by the fuel addition apparatus 30
by the swirling flow created in the introduction pipe 28, it is
necessary to maintain the strength of the swirling flow that is
created as the exhaust gas flows out of the turbine housing 6b at a
level not lower than a certain level until the swirling flow is
introduced into the introduction pipe 28. If the axial direction of
the exhaust pipe 3 changes drastically between the turbine housing
6b and the introduction pipe 3, the momentum of swirling of the
exhaust gas may decrease. Therefore, it is preferred that the
portion of the exhaust pipe 3 between the turbine housing 6b and
the introduction pipe 28 do not have a bent portion. If the exhaust
pipe 3 is provided with a bent portion, it is preferred the angle
of change in the axial direction of the exhaust pipe 3 between
before and after (i.e. between upstream and downstream of) the bent
portion be moderate. Allowable values of the above-mentioned angle
may be determined in advance by an experiment, and an appropriate
angle (e.g. 45 degrees or so) may be selected.
DESCRIPTION OF THE REFERENCE NUMERALS AND SYMBOLS
[0112] 1: engine [0113] 3: exhaust pipe [0114] 3a: inner pipe
[0115] 3b: detour passage [0116] 3c: detour side passage [0117] 3d:
tapered portion [0118] 4: NOx storage reduction catalyst [0119] 5:
fuel addition valve [0120] 6: turbocharger [0121] 6b: turbine
housing [0122] 7: small-diameter catalyst [0123] 7a: catalyst
interior passage [0124] 8: isolation pipe [0125] 8a: catalyst side
passage [0126] 10: ECU [0127] 28: introduction pipe [0128] 28a:
circumferential wall [0129] 28b: upstream end edge [0130] 28c:
downstream end edge [0131] 28d: circumferential wall opening
* * * * *