U.S. patent application number 14/786341 was filed with the patent office on 2016-04-14 for catalytic converter.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is Yuki Aoki, Takahiko Fujiwara, Naohiro Hayashi, Ryosuke Kayanuma, Hiroyuki Matsubara, Yuji Yabuzaki. Invention is credited to Yuki Aoki, Takahiko Fujiwara, Naohiro Hayashi, Ryosuke Kayanuma, Hiroyuki Matsubara, Yuji Yabuzaki.
Application Number | 20160102591 14/786341 |
Document ID | / |
Family ID | 50685967 |
Filed Date | 2016-04-14 |
United States Patent
Application |
20160102591 |
Kind Code |
A1 |
Kayanuma; Ryosuke ; et
al. |
April 14, 2016 |
CATALYTIC CONVERTER
Abstract
A catalytic converter (10) includes i) an outer tube (1) that
includes a cylindrical portion (1a), an upstream-side cone portion
(1b), and a downstream-side cone portion (1c), and ii) a substrate
(2) having a cell structure that is arranged in the cylindrical
portion (1a) of the outer tube (1). The substrate (2) has a center
region (2a) where a cell density is relatively high and a
peripheral region (2b) where the cell density is relatively low, in
a cross-section that is orthogonal to a length direction of the
substrate (2). A projection portion when a connecting portion (5)
of the exhaust duct (4) and the downstream-side cone portion (1c)
is projected onto the substrate (2) is within the center region
(2a).
Inventors: |
Kayanuma; Ryosuke;
(Susono-shi, Shizuoka-ken, JP) ; Fujiwara; Takahiko;
(Suntou-gun, Shizuoka-ken, JP) ; Aoki; Yuki;
(Nisshin-shi, Aichi-ken, JP) ; Matsubara; Hiroyuki;
(Gifu-shi, Gifu-ken, JP) ; Hayashi; Naohiro;
(Kariya-shi, Aichi-ken, JP) ; Yabuzaki; Yuji;
(Kakegawa-shi, Shizuoka-ken, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kayanuma; Ryosuke
Fujiwara; Takahiko
Aoki; Yuki
Matsubara; Hiroyuki
Hayashi; Naohiro
Yabuzaki; Yuji |
Toyota-shi, Aichi-ken
Toyota-shi, Aichi-ken
Toyota-shi, Aichi-ken
Kariya-shi, Aichi-ken
Kariya-shi, Aichi-ken
Kakegawa-shi, Shizuoka-ken |
|
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi-ken
JP
|
Family ID: |
50685967 |
Appl. No.: |
14/786341 |
Filed: |
April 23, 2014 |
PCT Filed: |
April 23, 2014 |
PCT NO: |
PCT/IB2014/000596 |
371 Date: |
October 22, 2015 |
Current U.S.
Class: |
60/299 |
Current CPC
Class: |
F01N 3/24 20130101; F01N
3/20 20130101; F01N 2260/14 20130101; F01N 3/28 20130101; F01N
2330/60 20130101; F01N 3/2803 20130101; F01N 2340/00 20130101; F01N
2260/06 20130101; F01N 2330/48 20130101 |
International
Class: |
F01N 3/20 20060101
F01N003/20 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2013 |
JP |
2013-094303 |
Claims
1. A catalytic converter comprising: an outer tube that is
connected to an exhaust duct through which exhaust gas flows, the
exhaust duct including an upstream-side exhaust duct and a
downstream-side exhaust duct with respect to a direction of an
exhaust-gas flow, the outer tube including a cylindrical portion,
an upstream-side cone portion connected to the upstream-side
exhaust duct, and a downstream-side cone portion connected to the
downstream-side exhaust duct, the upstream-side cone portion
extending from an upstream-side end of the cylindrical portion in
such a manner that a cross-section diameter of the upstream-side
cone portion decreases toward the upstream-side exhaust duct, the
downstream-side cone portion extending from a downstream-side end
of the cylindrical portion in such a manner that a cross-section
diameter of the downstream-side cone portion decreases toward the
downstream-side exhaust duct; and a substrate having a cell
structure and being arranged inside of the cylindrical portion of
the outer tube, wherein a catalyst layer in which a precious metal
catalyst is carried on a carrier is formed on a cell wall surface
of the substrate, the substrate has a center region and a
peripheral region in a cross-section orthogonal to a length
direction of the substrate, a cell density of the center region is
higher than a cell density of the peripheral region, a projection
portion when a projection of a cross-section of a connecting
portion of the exhaust duct and the downstream-side cone portion on
the substrate falls within the center region, a downstream-side
surface of the substrate is directly opposite to an opening of the
downstream-side exhaust duct, and a diameter of the center region
is larger than a diameter of the connecting portion of the exhaust
duct and the downstream-side cone portion.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a catalytic converter that forms an
exhaust system for exhaust gas.
[0003] 2. Description of Related Art
[0004] In various industries, various efforts to reduce the
environmental impact are being made on a worldwide scale. Among
these, on a daily basis in the automotive industry efforts are
being made to popularize so-called eco-cars such as hybrid vehicles
and electric vehicles, and of course, gasoline engine vehicles with
excellent fuel efficiency performance, and developments are being
made to further improve the performance of these vehicles.
[0005] A catalytic converter for purifying exhaust gas is typically
arranged in an exhaust system for exhaust gas that connects a
vehicle engine to a muffler.
[0006] The engine discharges toxic substances such as unburned HC
and VOC. In order to convert these toxic substances into allowable
substances, a catalyst layer formed by a precious metal catalyst
such as palladium or platinum is formed on a cell wall surface of a
substrate that includes multiple cells. More specifically, a
catalyst layer is formed in the length direction of the substrate,
i.e., along a direction in which the exhaust gas flows, on the cell
wall surface of the multiple cells. When the exhaust gas passes
through the catalytic converter provided with the substrate having
this kind of structure, CO is consequently converted to CO.sub.2,
NOx is converted to N.sub.2 and O.sub.2, and VOC is combusted to
produce CO.sub.2 and H.sub.2O.
[0007] In a typical catalytic converter, the cell density of the
substrate is uniform with a substrate provided with cells having a
honeycomb structure, for example. Because a flow rate distribution
of the exhaust gas in a sectional center region of the substrate is
higher than it is in a sectional peripheral region, the catalyst
layer of the entire substrate is unable to be sufficiently
utilized.
[0008] Therefore, taking this kind of exhaust gas flow rate
distribution into consideration, a difference in flow rate
distribution in a cross-section of the substrate is able to be
reduced as quickly as possible, by making the catalytic converter
have a higher cell density in the center region than in the
sectional peripheral region of the substrate, so exhaust gas
purification that effectively utilizes the catalyst layer of the
entire catalytic converter becomes possible.
[0009] Also, changing the cell density in the cross-section of the
substrate in this way enables pressure loss in the catalytic
converter to also be reduced, which also contributes to improved
exhaust gas purification performance.
[0010] The relationship between the catalytic converter and an
exhaust duct on the downstream side where purified exhaust gas
flows out from this catalytic converter is also extremely important
in improving the exhaust gas conversion efficiency of the entire
catalytic converter and reducing pressure loss. It is necessary to
comprehensively design a catalytic converter, including the exhaust
duct on the downstream side of the catalytic converter.
[0011] Japanese Patent Application Publication No. 2008-18370 (JP
2008-18370 A) focuses on the relationship between a catalytic
converter and an exhaust duct on the upstream side, and describes a
ceramic catalyst body in which an aperture ratio of a substrate
portion corresponding to a projection portion with respect to the
substrate of the exhaust duct on the upstream side is smaller than
the aperture ratio of a substrate portion corresponding to a
portion to the outside of this projection portion.
[0012] The ceramic catalyst body described in JP 2008-18370 A
focuses on the relationship between the exhaust duct on the
upstream side of the catalytic converter and the cell density of
the substrate that forms the catalytic converter. The exhaust gas
purification performance is also able to be increased by this
ceramic catalyst body However, the inventors have discovered that
the relationship between the substrate that forms the catalytic
converter and the exhaust duct on the downstream side of this
catalytic converter is even more important to the exhaust gas
purification performance than the relationship between the exhaust
duct on the upstream side of the catalytic converter and the
substrate that forms this catalytic converter is.
SUMMARY OF THE INVENTION
[0013] The invention thus provides a catalytic converter having
excellent exhaust gas purification performance, by specifying a
relationship between a substrate that forms the catalytic converter
and an exhaust duct on a downstream side of the catalytic
converter.
[0014] The catalytic converter according to the invention includes
an outer tube that is connected to an exhaust duct through which
exhaust gas flows. This outer tube includes a cylindrical portion,
an upstream-side cone portion that extends from one end of the
cylindrical portion in such a manner that a cross-section thereof
becomes smaller in diameter, and that is connected to the exhaust
duct on an upstream side with respect to an exhaust gas flow, and a
downstream-side cone portion that extends from the other end of the
cylindrical portion in such a manner that a cross-section thereof
becomes smaller in diameter, and that is connected to the exhaust
duct on a downstream side with respect to the exhaust gas flow. The
catalytic converter also includes a substrate having a cell
structure that is arranged inside of the cylindrical portion of the
outer tube. A catalyst layer in which a precious metal catalyst is
carried on a carrier is formed on a cell wall surface of the
substrate. The substrate is configured such that a cell density at
a center region is different than a cell density at a peripheral
region, in a cross-section that is orthogonal to a length direction
of the substrate, the cell density of the center region being
higher than the cell density of the peripheral region. The
substrate is such that a projection portion when a cross-section of
a connecting portion of the exhaust duct and the downstream-side
cone portion is projected onto the substrate falls within the
center region.
[0015] The catalytic converter of the invention includes a
substrate with a catalyst layer having a cell structure arranged in
a hollow interior of a metal outer tube formed by a cylindrical
portion that is between an upstream-side cone portion at one end
and a downstream-side cone portion at the other end, both of which
become smaller in diameter toward the outside. Providing the
substrate with the center region where the cell density is
relatively high and the peripheral region where the cell density is
relatively low enables a difference in the exhaust gas flow rate
distribution between the center region and the peripheral region to
be smaller than it is with a substrate in which the cell density is
uniform.
[0016] In addition, a projection portion formed when the connecting
portion of the exhaust duct on the downstream side and the
downstream-side cone portion of the outer tube in which the
sectional area is smaller than that of the substrate is projected
onto the substrate may fall within the center region. As a result,
the exhaust gas purification performance is able to be
increased.
[0017] The purified exhaust gas that flows out on the downstream
side from the catalytic converter passes through the
downstream-side cone portion of the outer tube and out into the
exhaust duct on the downstream side. Therefore, even if an attempt
is made to equalize the flow rate distribution of the entire
cross-section by making the cell density in the center region
different from that in the peripheral region in a cross-section of
the substrate, the flow rate of the exhaust gas at a substrate
portion corresponding to the projection portion when the
cross-section of the exhaust duct on the downstream side where the
exhaust gas flows out is projected onto the substrate is actually
faster than it is at another substrate portion.
[0018] Therefore, by placing this projection portion inside the
center region (i.e., the region where the cell density is high) of
the substrate, the exhaust gas is able to be made to effectively
flow through the center region where the cell density is high and
the catalyst quantity is large, so purification of this exhaust gas
is able to be promoted. As a result, the exhaust gas purification
performance of the overall catalytic converter is able to be
improved.
[0019] Here, the substrate having the cell structure may be made of
ceramic material such as cordierite or silicon carbine. A so-called
honeycomb structure that includes multiple cells with lattice
profiles that are square, hexagonal, or octagonal or the like, may
be applied to the substrate having the cell structure.
[0020] Also, a porous oxide is one possible example of the carrier
that forms the catalyst layer that is formed on the cell wall
surface of the substrate. A catalyst layer in which one or two or
more types of precious metal catalysts such as rhodium, palladium,
and platinum are carried on this carrier may be formed.
[0021] The catalytic converter of the invention has a cordierite
honeycomb carrier having excellent thermal shock resistance, but it
may alternatively be an electrically heated catalytic converter
(EHC: electrically heated converter). This electrically heated
catalytic converter is provided with a pair of electrodes that are
attached to a honeycomb catalyst, for example. The honeycomb
catalyst is heated by energizing the electrodes, which in turn
increases the activity of the honeycomb catalyst, such that the
exhaust gas that passes through this is purified. By applying the
electrically heated catalytic converter to an exhaust system for
exhaust gas that connects a vehicle engine to a muffler, the
catalyst can be activated by electric heating, thus enabling the
exhaust gas to be purified when it is cold, in addition to
purifying exhaust gas at normal temperature.
[0022] As described above, according to the catalytic converter of
the invention, exhaust gas purification is able to be promoted,
thus enabling the exhaust gas purification performance of the
entire catalytic converter to be improved, by passing exhaust gas
having a high flow rate through a center region where the cell
density is high and the catalyst quantity is large, while
effectively utilizing the catalyst in the peripheral region of the
substrate to purify exhaust gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Features, advantages, and technical and industrial
significance of exemplary embodiments of the invention will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0024] FIG. 1 is a view showing a frame format of one example
embodiment of a catalytic converter according to the invention,
together with exhaust ducts on an upstream side and a downstream
side;
[0025] FIG. 2 is a perspective view of a substrate that forms the
catalytic converter;
[0026] FIG. 3A is a view from a Y direction in FIG. 1, of a
connecting portion of an exhaust duct on a downstream side and a
downstream-side cone portion projected onto a cross-section of the
substrate;
[0027] FIG. 3B is a view of another example embodiment of the
projection view in FIG. 3A;
[0028] FIG. 3C is a view of yet another example embodiment of the
projection view in FIG. 3A;
[0029] FIG. 4 is a view showing analysis results related to a
pressure loss ratio when the number of substrate cells and the
diameter of the downstream-side exhaust duct are changed;
[0030] FIG. 5 is a view of test results identifying a relationship
between a difference in a diameter d2 of a center region of the
substrate and a diameter d3 of the connecting portion, and the
pressure loss in the catalytic converter;
[0031] FIG. 6 is a view of test results identifying a relationship
between the difference in the diameter d2 of the center region of
the substrate and the diameter d3 of the connecting portion, and a
NOx purification amount of the catalytic converter; and
[0032] FIG. 7 is a view of test results identifying a relationship
between the difference in the diameter d2 of the center region of
the substrate and the diameter d3 of the connecting portion, and a
ratio of the NOx purification amount to the pressure loss (i.e., NO
purification amount/pressure loss).
DETAILED DESCRIPTION OF EMBODIMENTS
[0033] Example embodiments of a catalytic converter of the
invention will now be described with reference to the accompanying
drawings.
[0034] (Example Embodiments of the Catalytic Converter)
[0035] FIG. 1 is a view showing a frame format of one example
embodiment of the catalytic converter according to the invention,
together with exhaust ducts on an upstream side and a downstream
side. FIG. 2 is a perspective view of a substrate that forms the
catalytic converter. Also, FIG. 3A is a view from a Y direction in
FIG. 1, of a connecting portion of a downstream-side cone portion
and the downstream-side exhaust duct projected on a cross-section
of the substrate
[0036] First, an overview of an exhaust system for exhaust gas, in
which a catalytic converter 10 of the invention is interposed will
be described. The exhaust system for exhaust gas to which the
catalytic converter of the invention is applied includes an engine,
a catalytic converter, a three-way catalytic converter, a sub
muffler, and a main muffler, all of which are connected together by
an exhaust duct. Exhaust gas produced by the engine flows through
each portion via the exhaust duct, so as to be discharged. In FIG.
1, the catalytic converter 10, an exhaust duct 3 on an upstream
side of the catalytic converter 10 (hereinafter also referred to as
the "upstream-side exhaust duct 3"), and an exhaust duct 4 on a
downstream side of the catalytic converter 10 (hereinafter also
referred to as the "downstream-side exhaust duct 4")are shown.
[0037] The catalytic converter 10 includes a metal outer tube 1,
and a substrate 2 arranged inside the outer tube 1. The outer tube
1 is formed, for example, by a cylindrical portion 1a having a
uniform cross-section, an upstream-side cone portion 1b that
extends from one end of the cylindrical portion 1a such that the
cross-section narrows in diameter, and that connects the exhaust
duct 3 on the upstream side with respect to the exhaust gas flow,
and a downstream-side cone portion 1c that extends from the other
end of the cylindrical portion 1a such that the cross-section
narrows in diameter, and that connects the exhaust duct 3 on the
downstream side with respect to the exhaust gas flow.
[0038] The substrate 2 arranged inside the outer tube 1 is formed
by a cylindrical member having multiple cells, with a catalyst
layer, not shown, formed on a cell wall surface. Possible examples
of material used to form the substrate 2 include ceramic material
such as silicon carbine or cordierite made of a composite oxide of
silicon dioxide, aluminum oxide, and magnesium oxide, and material
other than ceramic material such as metal material.
[0039] Also, a possible example of a carrier that forms the
catalyst layer formed on the cell wall surface of the substrate 2
is an oxide having at least one of CeO.sub.2, ZrO.sub.2, and
Al.sub.2O.sub.3 as the main component, which is a porous oxide.
Possible examples include an oxide formed from one of ceria
(CeO.sub.2), zirconia (ZrO.sub.2), and alumina (Al.sub.2O.sub.3),
and a composite oxide formed from two or more of these (i.e., ceria
(CeO.sub.2), zirconia (ZrO.sub.2), and alumina (Al.sub.2O.sub.3))
(a so-called CeO.sub.2--ZrO.sub.2 compound that is CZ material, or
an Al.sub.2O.sub.3--CeO.sub.2--ZrO.sub.2 ternary composite oxide
(ACZ material) into which Al.sub.2O.sub.3 has been introduced as a
diffusion barrier). Also, the entire catalyst layer is formed by
one or two or more of Pd, Pt, and Rh that are precious metal
catalysts being carried on these carriers.
[0040] The substrate 2 is formed by a honeycomb structure that
includes multiple cells with lattice profiles that are square,
hexagonal, or octagonal or the like, such that exhaust gas flows
through the inside of each of the cells (in direction X1).
[0041] Moreover, the substrate 2 is has two regions, i.e., a center
region 2a where the cell density is relatively high, and a
peripheral region 2b where the cell density is relatively low.
[0042] By providing the center region 2a and the peripheral region
2b having different cell densities in this way, exhaust gas that
has flowed through from the upstream-side exhaust duct 3
effectively flows into the peripheral region 2b where the cell
density is relatively low which facilitates the flow. As a result,
the difference in the flow rate distribution between the center
region 2a and the peripheral region 2b decreases, so exhaust gas
purification that effectively utilizes the entire catalyst layer of
the catalytic converter 10 can be performed.
[0043] As shown in the drawing, when the diameter of the substrate
2 is d1, the diameter of the center region that forms the substrate
2 is d2, and the diameter of the connecting portion 5 of the
downstream-side exhaust duct 4 and the downstream-side cone portion
1c is d3, the catalytic converter 10 illustrated has a relationship
of d1>d2.gtoreq.d3. Further, the catalytic converter 10 and the
downstream-side exhaust duct 4 are formed such that a projection
portion formed by projecting the connecting portion 5 onto a
cross-section of the substrate 2 is within the center region 2a, as
shown in FIG. 3A.
[0044] The mode in which the projection portion formed by
projecting the connecting portion 5 onto a cross-section of the
substrate 2 exists in the center region 2a includes both a mode in
which a true circle cross section of the connecting portion 5 and a
true circle cross section of the center region 2a have the same
circle center as shown in FIG. 3A, a mode in which the circle
centers of both of the true circle cross sections are offset from
one another as shown in FIG. 3B. Moreover, when a substrate 2A
having an oblong sectional shape is applied as shown in FIG. 3C, in
a center region 2a' having an oblong sectional shape and a
peripheral region 2b' around this center region 2a', the true
circle cross section of the connecting portion 5 is in the center
region 2a' having this oblong shape. In a case in which the
substrate and the center region thereof are polygonal and a case in
which the downstream-side exhaust duct is oblong or polygonal (and
therefore a case in which the projection portion of the connecting
portion is also oblong or polygonal) as well, the structure of the
invention is satisfied as long as the projection portion of the
connecting portion is in the center region of the substrate.
[0045] [Analysis related to pressure loss ratio when the number of
substrate cells and the diameter of the downstream-side exhaust
duct are changed, and results thereof]
[0046] The inventors used a computer to simulate a catalytic
converter with three different numbers of substrate cells and
downstream-side exhaust duct diameters as parameters, an performed
an analysis to obtain the pressure loss ratio in each of these
catalytic converters. Thermo-fluid analysis (software: STAR-CD by
IDAJ Co., LTD.) was used in the analysis.
[0047] The downstream-side exhaust duct diameter was set to 55 mm
as a condition when analyzing the relationship between the number
of substrate cells and the pressure loss ratio. Meanwhile, the
number of substrate cells was set to 600 as a condition when
analyzing the relationship between the downstream-side exhaust duct
diameter and the pressure loss ratio. The analysis results are
shown in FIG. 4.
[0048] Upon verifying the degree to which each of the factors
contributes to pressure loss in the catalytic converter, the
inventors found that an increase in the downstream-side exhaust
duct diameter contributes significantly to a decrease in pressure
loss, as is also evident from FIG. 4.
[0049] As described above, it is evident that when reducing the
pressure loss in the catalytic converter, the cross-section
diameter of the downstream-side exhaust duct that is connected to
the outer tube that forms the catalytic converter has a greater
effect than the constituent elements that directly make up the
catalytic converter. The relationship between the constituent
elements of the catalytic converter and the downstream-side exhaust
duct is specified based on this verification result.
[0050] Because the purified exhaust gas that has flowed out to the
downstream side from the catalytic converter 10 passes through the
downstream-side cone portion 1c of the outer tube 1, and then flows
out to the downstream-side exhaust duct 4, the cell density in the
center region 2a is made different from the cell density in the
peripheral region 2b in the substrate 2 in an attempt to equalize
the flow rate distribution of the entire cross-section. In this
case, the flow rate of the exhaust gas at a substrate portion
corresponding to a projection portion when a cross section of the
downstream-side exhaust duct 4 where the exhaust gas flows out is
projected onto the substrate 2 is actually faster than it is at
another substrate portion. Therefore, by providing this projection
portion in the center region 2a of the substrate 2 as shown in FIG.
3A, the exhaust gas can be made to effectively flow through the
center region 2a where the cell density is high and there are a
large amount catalysts, so purification can be promoted. As a
result, the exhaust gas purification performance of the overall
catalytic converter 10 is able to be improved.
[0051] [Test to identify the relationship between the difference in
the diameter d2 of the center region of the substrate and the
diameter d3 of the connecting portion, and the pressure loss of the
catalytic converter, the relationship between that difference and
the NOx purification amount of the catalytic converter, the
relationship between that difference and the NOx purification
amount/the pressure loss, as well as the results of these]
[0052] The inventors conducted a test to identify the relationship
between the difference in the diameter d2 of the center region of
the substrate and the diameter d3 of the connecting portion of the
downstream-side exhaust duct and the downstream-side cone portion,
and the pressure loss of the catalytic converter, the relationship
between that difference and the NOx purification amount of the
catalytic converter, the relationship between that difference and
the NOx purification amount/the pressure loss.
[0053] The test conditions were such that the diameter dl
(sectional diameter=103 mm) of the substrate and the diameter d2 of
the center region were set, and the diameter d3 of the
downstream-side exhaust duct was changed to four dimensions, i.e.,
41.2 mm, 52.7 mm, 55 mm, and 60.5 mm. The ratio of the area of the
center region to the total sectional area of the substrate was 25%.
Also, when measuring the pressure loss, a 2.5 liter gasoline engine
was used, and the intake air amount Ga was 100 g/s. Further, when
measuring the NOx purification amount as well, a 2.5 liter gasoline
engine was used, and the intake air amount Ga was 20 g/s.
[0054] The test results are shown in FIGS. 5 to 7. Here, FIG. 5 is
a view of test results related to the pressure loss. FIG. 6 is a
view of test results related to the NOx purification amount. FIG. 7
is a view of test results related to the NO purification amount/the
pressure loss). In each of the drawings, an approximate curve is
created based on the plotted test results.
[0055] From FIG. 5 it is evident that the pressure loss increases
as the diameter of the downstream-side exhaust duct decreases.
[0056] Also, from FIG. 6 it is evident that the amount of change in
the conversion efficiency decreases as the diameter of the
downstream-side exhaust duct becomes smaller than the diameter of
the center region.
[0057] In this way, the pressure loss increases as the diameter of
the downstream-side exhaust duct becomes smaller, while the amount
of change in the conversion efficiency decreases as the diameter of
the downstream-side exhaust duct decreases. This is thought to be
because exhaust gas having a fast flow rate flows into the center
region where the cell density is high, so exhaust gas purification
is efficiently performed in the center region where the catalyst
quantity is large and the exhaust gas purification performance is
high.
[0058] Based on the results in FIGS. 5 and 6, it is evident that a
high exhaust gas conversion efficiency is able to be maintained in
a range in which the difference is equal to or greater than zero,
i.e., in a region where the diameter of the center region is larger
than the diameter of the downstream-side exhaust duct.
[0059] While the invention has been described in detail with
reference to the foregoing example embodiments, the invention is
not limited to the example embodiments, but may also include design
modifications and the like without departing from the scope of the
invention.
* * * * *