U.S. patent application number 11/029694 was filed with the patent office on 2005-08-11 for exhaust emission control device for an internal combustion engine.
This patent application is currently assigned to Mitsubishi Jidosha Kogyo Kabushiki Kaisha. Invention is credited to Okumura, Akihisa, Tamura, Yasuki, Tashiro, Keisuke.
Application Number | 20050172614 11/029694 |
Document ID | / |
Family ID | 34805317 |
Filed Date | 2005-08-11 |
United States Patent
Application |
20050172614 |
Kind Code |
A1 |
Tamura, Yasuki ; et
al. |
August 11, 2005 |
Exhaust emission control device for an internal combustion
engine
Abstract
A three-way catalyst (30) comprises a microporous catalyst
element (30a) having a micropore group whose average pore opening
size is smaller than molecular size of HC in a washcoat and a
macroporous catalyst element (30b) having a macropore group whose
average pore opening size is larger than the molecular size of HC
in a washcoat.
Inventors: |
Tamura, Yasuki;
(Nisshin-shi, JP) ; Tashiro, Keisuke;
(Okazaki-shi, JP) ; Okumura, Akihisa; (Himeji-shi,
JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Mitsubishi Jidosha Kogyo Kabushiki
Kaisha
2347 Commerical Drive
Tokyo
MI
48326
ICT CO., LTD.
Osaka
INTERNATIONAL CATALYST TECHNOLOGY, INC.
Auburns Hills
|
Family ID: |
34805317 |
Appl. No.: |
11/029694 |
Filed: |
January 6, 2005 |
Current U.S.
Class: |
60/285 |
Current CPC
Class: |
B01D 53/945 20130101;
Y02T 10/12 20130101; F01N 3/08 20130101; F01N 3/0835 20130101; Y02T
10/22 20130101; F01N 2510/00 20130101; F01N 13/009 20140601; Y02T
10/20 20130101; F01N 2430/06 20130101; F01N 3/0857 20130101 |
Class at
Publication: |
060/285 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 8, 2004 |
JP |
2004-002999 |
Claims
What is claimed is:
1. An exhaust emission control device for an internal combustion
engine, comprising: a three-way catalyst located in an exhaust
channel of an internal combustion engine, wherein: said three-way
catalyst comprises one or more catalyst elements and has two or
more pore groups different in average pore opening size in a
washcoat.
2. The exhaust emission control device for an internal combustion
engine according to claim 1, wherein: said three-way catalyst
comprises a microporous catalyst element having a micropore group
whose average pore opening size is smaller than prescribed size in
a washcoat and a macroporous catalyst element having a macropore
group whose average pore opening size is larger than said
prescribed size in a washcoat.
3. The exhaust emission control device for an internal combustion
engine according to claim 2, wherein: said three-way catalyst
comprises a microporous catalyst element having a micropore group
whose average pore opening size is smaller than molecular size of
HC in a washcoat and a macroporous catalyst element having a
macropore group whose average pore opening size is larger than the
molecular size of HC in a washcoat.
4. The exhaust emission control device for an internal combustion
engine according to claim 2, wherein: said microporous catalyst
element and said macroporous catalyst element are arranged in
series with each other, facing in a direction of an exhaust
flow.
5. The exhaust emission control device for an internal combustion
engine according to claim 4, wherein: said microporous catalyst
element is disposed on the upstream side of the exhaust flow, and
said macroporous catalyst element on the downstream side of the
exhaust flow.
6. The exhaust emission control device for an internal combustion
engine according to claim 2, wherein: said microporous catalyst
element and said macroporous catalyst element are arranged in
layers.
7. The exhaust emission control device for an internal combustion
engine according to claim 6, wherein: said microporous catalyst
element is disposed on a surface layer side, and said macroporous
catalyst element on an internal layer side.
8. The exhaust emission control device for an internal combustion
engine according to claim 1, further comprising: air-fuel ratio
modulating means for periodically modulating an air-fuel ratio of
an exhaust emission that flows into said three-way catalyst between
a lean air-fuel ratio and a rich air-fuel ratio.
Description
CROSS-REFERENCE TO THE RELATED ART
[0001] This application incorporates by reference the subject
matter of Application No. 2004-2999, filed in Japan on Jan. 8,
2004, on which a priority claim is based under 35 U.S.C
S119(a).
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an exhaust emission control
device for an internal combustion engine,. and more specifically to
a technology for raising purification efficiency of a three-way
catalyst.
[0004] 2. Description of the Related Art
[0005] A three-way catalyst is commonly used as an exhaust gas
purifying catalyst for an internal combustion engine for a vehicle.
The three-way catalyst is designed to bring an exhaust air-fuel
ratio close to a theoretical air-fuel ratio (Stoichio) for
optimization of the oxidation of HC (hydrocarbons) and CO (carbon
monoxide) and the reduction of NOx to promote the purification of
exhaust gases.
[0006] An exhaust emission control device that has been lately
developed has a construction in which the catalyst is for example a
porous structure to trap NOx, Oxygen (O.sub.2), HC and CO in pores,
to thereby trap HC and CO in the pores and oxidize them using the
trapped NOx and O.sub.2 in a reducing atmosphere, and on the other
hand to thereby trap NOx and O.sub.2 in the pores and reduce NOx
using the trapped HC and CO in an oxidizing atmosphere.
[0007] Moreover, a technology for encouraging only reactions useful
for the purification of NOx by reducing the pores in size to
prevent HC, which serves as a reducing agent, from approaching the
oxidation catalyst in the porous structure has also been developed
(see Unexamined Japanese Patent Publication No. 2001-525241 as an
example).
[0008] It is noted that, generally in the three-way catalyst, the
oxidation-reduction reaction of CO and NOx is faster than that of
HC and NOx in reaction speed. This means that if it is possible to
separate HC and CO from each other and to preferentially cause the
oxidation-reduction reaction of CO and NOx, NOx can be improved in
its purifying performance.
[0009] In the reducing atmosphere, however, HC and CO are mixed in
the exhaust emission. Conventional technologies related to porous
structures, including the technology disclosed in the
above-mentioned publication, are not designed to trap HC and CO
separately from each other. This causes the problem that the
presence of HC having large molecular size hinders the
oxidation-reduction reaction of CO and NOx having small molecular
size, thereby decelerating the oxidation-reduction reaction of CO
and NOx which are fast in reaction speed. Such deceleration in the
oxidation-reduction reaction of CO and NOx generates the problem
that part of CO is reacted with O.sub.2, resulting in a shortage of
O.sub.2 for the oxidation of HC.
SUMMARY OF THE INVENTION
[0010] The present invention has been made to solve the above
problems, and an object thereof is to provide an exhaust emission
control device for an internal combustion engine designed to
actively separate HC and CO and preferentially produce
oxidation-reduction reaction of CO and NOx on a catalyst, to
thereby upgrade an exhaust gas purifying performance.
[0011] To achieve this object, in the exhaust emission control
device for an internal combustion engine according to the present
invention, there is provided a three-way catalyst in an exhaust
channel of an internal combustion engine. The three-way catalyst
comprises one or more catalyst elements and has two or more pore
groups different in average pore opening size in a washcoat.
[0012] A further scope of applicability of the present invention
will become apparent from the detailed description given
hereinafter. However, it should be understood that the detailed
description and specific example, while indicating preferred
embodiments of the invention, are given by way of illustration
only, since various changes and modifications within the spirit and
scope of the invention will become apparent to those skilled in the
art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The nature of this invention, as well as other objects and
advantages thereof, will be explained in the following with
reference to the accompanying drawings, in which like reference
characters designate the same or similar parts throughout the
figures and wherein:
[0014] FIG. 1 is a schematic view of a construction of an exhaust
emission control device for an internal combustion engine according
to an embodiment 1 of the present invention, which is installed in
a vehicle;
[0015] FIG. 2 shows a view (a) of a quarter portion of a unit cell
of a microporous catalyst element, an enlarged view (b) of
catalysts coating the quarter portion, and an enlarged view (c) of
one particle of a washcoat (W/C);
[0016] FIG. 3 shows a view (a) of a quarter portion of a unit cell
of a macroporous catalyst element; an enlarged view (b) of
catalysts coating the quarter portion; and an enlarged view (c) of
one particle of a washcoat (W/C);
[0017] FIG. 4 is a graph showing frequency distribution of pore
opening size in the microporous catalyst element (solid line) and
in the macroporous catalyst element (broken line), and average pore
opening size X and average pore opening size Y of the microporous
and macroporous catalyst elements, respectively;
[0018] FIG. 5 is a flowchart showing a control routine of O.sub.2
F/B control according to the first embodiment;
[0019] FIG. 6 is a view showing a three-way catalyst according to
another embodiment of the first embodiment;
[0020] FIG. 7 is a view showing a three-way catalyst according to a
second embodiment;
[0021] FIG. 8 is a view showing a three-way catalyst according to
another embodiment of the second embodiment;
[0022] FIG. 9 is a view showing a quarter portion of a unit cell of
a three-way catalyst according to a third embodiment;
[0023] FIG. 10 is a view showing a quarter portion of a unit cell
of a three-way catalyst according to a fourth embodiment; and
[0024] FIG. 11 is a flowchart showing a control routine of A/F
modulation control according to a fifth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] Embodiments of the present invention will be explained below
with reference to the attached drawings.
[0026] Firstly, a first embodiment will be described.
[0027] FIG. 1 is a schematic view of a construction of an exhaust
emission control for an internal combustion engine according to the
present invention, which is installed in a vehicle. Hereinafter the
explanation about the construction of the exhaust emission control
device will be provided.
[0028] As illustrated in FIG. 1, in a cylinder head 2 of an engine
body (such as a gasoline engine, and hereinafter simply referred to
as engine) 1 that is an internal combustion engine, there is
disposed an ignition plug 4 in each cylinder. Connected to the
ignition plug 4 is an ignition coil 8 for outputting high
voltage.
[0029] In the cylinder 2, an intake port is formed for each
cylinder, and one end of an intake manifold 10 is connected to the
cylinder 2 so as to communicate with each intake port. An
electromagnetic fuel injection valve 6 is attached to the intake
manifold 10, and a fuel supply device, not shown, having a fuel
tank is connected to the fuel injection valve 6 through a fuel pipe
7.
[0030] An electromagnetic throttle valve 14 for adjusting an intake
air amount is disposed upstream from the fuel injection valve 6 of
the intake manifold 10 together with a throttle position sensor
(TPS) 16 for detecting the opening of the throttle valve 14.
Interposed in the upstream side of the throttle valve 14 is an air
flow sensor 18 for measuring the intake air amount.
[0031] Also in the cylinder head 2, an exhaust port is formed for
each cylinder, and one end of an exhaust manifold 12 is connected
to the cylinder head 2 so as to communicate with the exhaust
port.
[0032] An exhaust pipe (exhaust channel) 20 is connected to the
other end of the exhaust manifold 12. In the exhaust pipe 20, a
monolith-type three-way catalyst 30 having a catalyst support
honeycombed in section is interposed as an exhaust gas purifying
catalyst device.
[0033] The three-way catalyst 30 includes any one of copper (Cu),
cobalt (Co), silver (Ag), platinum (Pt), rhodium (Rh) and palladium
(Pd) as active metal for a washcoat of a support surface.
[0034] The three-way catalyst 30 not only has the active metal but
includes a great number of pores formed in a washcoat.
Specifically, the three-way catalyst 30 comprises a microporous
catalyst element 30a having a micropore group whose average pore
opening size is smaller than molecular size (prescribed size) of HC
and a macroporous catalyst element 30b having a macropore group
whose average pore opening size is larger than the molecular size
of HC, in which the microporous catalyst element 30a is disposed on
the upstream side of the exhaust flow, and the macroporous catalyst
element 30b is disposed on the downstream side of the exhaust flow
in series with the microporous catalyst element 30a.
[0035] FIG. 2 shows a view (a) of a quarter portion of a unit cell
of a microporous catalyst element 30a together with an enlarged
view (b) of catalysts coating the quarter portion and an enlarged
view (c) of one particle of a washcoat (W/C). As illustrated in
FIG. 2, in the microporous catalyst element 30a, a large number of
micropores S of smaller opening size than the molecular size of HC
are formed in the washcoat.
[0036] FIG. 3 shows a view (a) of a quarter portion of a unit cell
of a macroporous catalyst element 30b together with an enlarged
view (b) of catalysts coating the quarter portion and an enlarged
view (c) of one particle of a washcoat (W/C). As illustrated in
FIG. 3, in the macroporous catalyst element 30b, a great number of
macropores L of larger opening size than the molecular size of HC
are formed in the washcoat.
[0037] FIG. 4 shows frequency distribution of pore opening size in
the microporous catalyst element 30a (solid line) and in the
macroporous catalyst element 30b (broken line), and average pore
opening size X and average pore opening size Y of the microporous
and macroporous catalyst elements, respectively. In this way, the
microporous catalyst element 30a and the macroporous catalyst
element 30b are differentiated in the average pore opening size.
Therefore, the three-way catalyst 30 is capable of trapping CO,
O.sub.2, NOx and H.sub.2 having smaller molecular size than HC in
the micropores S in the microporous catalyst element 30a located on
the upstream side of the exhaust flow and is also capable of
trapping HC having large molecular size in the macropores L in the
macroporous catalyst element 30b located on the downstream side of
the exhaust flow.
[0038] For example, the pore opening size is controlled by an
impregnation method, a CVD (Chemical Vapor Deposition) method, or
the like.
[0039] The microporous catalyst element 30a is for example zeolite
3A, Ca-mordenite, or the like, and about 3 to about 3.8 angstroms
in diameter. On the other hand, the macroporous catalyst element
30b is for example zeolite 5A, ZSM-5, .beta., or the like, and
about 5 to about 6 angstroms in diameter. Additionally, the
macroporous catalyst element 30b may be an ordinary catalyst (such
as one composed mainly of Al.sub.2O.sub.3 and the like), instead of
being the above-described one.
[0040] Substances, on which control of effective pore diameters is
introduced, include zeolite, SAPO (silicoaluminophosphate), and
ALPO (aluminophosphate), but are not limited to these. Any other
substances may be utilized as long as the substances are different
in pore diameter. Moreover, the substances may be of any other size
and shapes on condition that the substances can sieve HC, CO, NOx,
H.sub.2, etc.
[0041] On the upstream side of the three-way catalyst 30 of the
exhaust pipe 20, there is disposed an air-fuel ratio sensor 22 for
detecting an exhaust air-fuel ratio (exhaust A/F), based on the
concentration of oxygen in the exhaust emission. Utilized as the
air-fuel ratio sensor 22 is an O.sub.2 sensor, but a linear A/F
sensor (LAFS) or the like may be employed instead.
[0042] An ECU (electrical control unit) 40 comprises an
input/output device, memories (ROM, RAM, nonvolatile RAM, etc.), a
central processing unit (CPU), a time counter, and the like. The
ECU 40 performs comprehensive control of the exhaust emission
control device including the engine 1.
[0043] Connected to an input side of the ECU 40 are various
sensors, such as a crank angle sensor 42 for detecting a crank
angle of the engine 1, in addition to the TPS 16, the air flow
sensor 18 and the air-fuel ratio sensor 22. Detection information
from these sensors is inputted to the ECU 40. Based on the crank
angle information from the crank angle sensor 42, engine revolution
speed Ne is detected.
[0044] Connected to an output side of the ECU 40 are various output
devices, including the fuel injection valve 6, the ignition coil 8,
the throttle valve 14 and the like. A fuel injection amount, fuel
injection timing, ignition timing, and the like, which are
calculated based on the detection information from the various
sensors, are outputted to the various output devices.
[0045] Specifically, the air-fuel ratio is set to a proper target
air-fuel ratio (target A/F), based on the detection information
from the various sensors, and fuel of an amount adjusted according
to the target A/F is injected from the fuel injection valve 6 with
the right timing. Furthermore, the throttle valve 14 is adjusted to
have proper opening, and spark ignition is carried out by the
ignition plug 4 with the right timing.
[0046] More specifically, O.sub.2 feedback (O.sub.2 F/B) control is
performed so that the exhaust A/F becomes the target A/F (for
example, Stoichio), based on the information from the air-fuel
ratio sensor 22. In response thereto, the fuel injection amount
fluctuates, and practically, the exhaust A/F periodically
fluctuates between a rich air-fuel ratio (rich A/F) side and a lean
air-fuel ratio (lean A/F) side with the target A/F therebetween
(air-fuel ratio modulating means).
[0047] Hereinafter, operation of the exhaust emission control
device according to the present invention, which is thus
constructed, will be described.
[0048] FIG. 5 shows a control routine of the O.sub.2 F/B control in
a flowchart, and explanations will be provided with reference to
the flowchart.
[0049] First, Step S10 judges whether the exhaust A/F is the lean
A/F or the rich A/F at present, based on the information from the
O.sub.2 sensor that is the air-fuel ratio sensor 22. If it is
judged that the exhaust A/F is the lean A/F, rich operation is
carried out in Step S12. To be concrete, the fuel injection amount
is compensated to increase.
[0050] If the rich operation is carried out in the above manner,
the exhaust A/F turns to the rich A/F, and a good deal of CO, in
addition to HC, is contained in the exhaust emission. As a result,
the reducing atmosphere is produced in the three-way catalyst
30.
[0051] As mentioned below, right before the reducing atmosphere is
caused in the three-way catalyst 30, NOx and O.sub.2 having smaller
molecular size than HC are trapped in the micropores S of the
microporous catalyst element 30a. Therefore, when the reducing
atmosphere is created in the three-way catalyst 30, the NOx and the
O.sub.2 are released to produce the oxidation-reduction reaction
with CO and HC contained in the exhaust emission. Since the
oxidation-reduction reaction of CO and NOx is faster than that of
HC and NOx in reaction speed, the released NOx is reacted with CO
by priority, whereas the released O.sub.2 is well reacted with
HC.
[0052] Once NOx and O.sub.2 are sufficiently released, CO and
H.sub.2 having smaller molecular size than HC are well trapped in
the micropores S of the microporous catalyst element 30a located on
the upstream side of the exhaust flow, while HC having large
molecular size is successfully trapped in the macropores L of the
macroporous catalyst element 30b located on the downstream side of
the exhaust flow. That is to say, in the three-way catalyst 30, CO
and HC are actively separated and trapped in the microporous
catalyst element 30a and the macroporous catalyst element 30b,
respectively.
[0053] Thereafter if Step S10 judges that the exhaust A/F is the
rich A/F, the lean operation is then provided in Step S14. To be
concrete, the fuel injection amount is compensated to decrease.
[0054] Once the lean operation is implemented, the exhaust A/F
turns to the lean A/F, and a great deal of NOx is contained in the
exhaust emission together with O.sub.2. The oxidizing atmosphere is
therefore produced in the three-way catalyst 30.
[0055] When the oxidizing atmosphere is generated in the three-way
catalyst 30, the CO, H.sub.2 and HC trapped as described are
released to cause the oxidation-reduction reaction with O.sub.2 and
NOx contained in the exhaust emission. In this case, as stated
above, CO and HC are trapped in the microporous catalyst element
30a and the macroporous catalyst element 30b, respectively, so as
to be separated from each other, and CO and H.sub.2 are released in
the microporous catalyst element 30a located on the upstream side
of the exhaust flow. As a result, the released CO is preferentially
and reliably reacted with NOx contained in the exhaust emission in
the microporous catalyst element 30a located on the upstream side
of the exhaust flow due in part to the fact that the
oxidation-reduction reaction of CO and NOx is faster than that of
HC and NOx in reaction speed. By using CO for reaction with NOx,
the released HC is fully reacted with O.sub.2 contained in the
exhaust emission in the macroporous catalyst element 30b located on
the downstream side of the exhaust flow.
[0056] In other words, in the exhaust emission control device
according to the first embodiment, the exhaust A/F is modulated
between the lean A/F and the rich A/F due to the O.sub.2 F/B
control, to thereby satisfactorily create the oxidizing atmosphere
and the reducing atmosphere. Therefore, CO and HC are repeatedly
trapped well in the three-way catalyst 30 so as to be separated
from each other in the reducing atmosphere. In the oxidizing
atmosphere, on the other hand, the released CO and H.sub.2 are
preferentially and surely reacted with NOx contained in the exhaust
emission without being hampered by the released HC. As a result,
the purifying performance of NOx is upgraded. Furthermore, due to
the use of CO and H.sub.2 for reaction with NOx, the released HC is
well reacted with O.sub.2 contained in the exhaust emission, which
enhances the purifying performance of HC. Consequently, the exhaust
gas purifying performance of the three-way catalyst 30 is improved
as a whole and is maintained in a high level.
[0057] The above explanation has been provided taking as an example
the three-way catalyst 30 in which the microporous catalyst element
30a and the macroporous catalyst element 30b are completely coupled
and integrated with each other in a direction of the exhaust flow
as illustrated in FIG. 1. The microporous catalyst element 30a and
the macroporous catalyst element 30b, however, do not have to be
coupled to each other but may be located away from each other in
the direction of the exhaust flow as another embodiment, as
illustrated in FIG. 6.
[0058] Hereinafter a second embodiment will be described.
[0059] The second embodiment is different from the first only in
that a three-way catalyst 301 is employed in place of the three-way
catalyst 30.
[0060] As illustrated in FIG. 7, the three-way catalyst 301
comprises a macroporous catalyst element 301a having pores whose
average pore opening size is larger than the molecular size of HC
and a microporous catalyst element 301b having pores whose average
pore opening size is smaller than the molecular size of HC. The
macroporous catalyst element 301a is disposed on the upstream side
of the exhaust flow, and the microporous catalyst element 301b on
the downstream side. In other words, in the three-way catalyst 301,
the microporous and macroporous catalyst elements are reversely
positioned, compared to the three-way catalyst 30.
[0061] Operation of the exhaust emission control device, in which
the macroporous catalyst element 301a is disposed on the upstream
side of the exhaust flow, and the microporous catalyst element 301b
on the downstream side as mentioned above, will be described
below.
[0062] When the rich operation is carried out in the O.sub.2 F/B
control, and the reducing atmosphere is generated in the three-way
catalyst 301, the trapped NOx and O.sub.2 are released to produce
the oxidation-reduction reaction with CO and HC contained in the
exhaust emission as seen in the above embodiment. Since the
oxidation-reduction reaction of CO and NOx is faster than that of
HC and NOx in reaction speed, the released NOx is reacted with CO
by priority, whereas the released O.sub.2 is well reacted with
HC.
[0063] Once NOx and O.sub.2 are sufficiently released, HC of large
molecular size is satisfactorily trapped in macropores L of the
macroporous catalyst element 301a located on the upstream side of
the exhaust flow, while CO and H.sub.2 having smaller molecular
size than HC are satisfactorily trapped in micropores S of the
microporous catalyst element 301b located on the downstream side of
the exhaust flow. Similarly to the above embodiment, in the
three-way catalyst 301, HC and CO are actively separated from each
other and trapped in the macroporous catalyst element 301a and the
microporous catalyst element 301b, respectively.
[0064] On the other hand, when the lean operation is carried out,
and the oxidizing atmosphere is created in the three-way catalyst
301, the trapped HC, CO and H.sub.2 are released to cause the
oxidation-reduction reaction with O.sub.2 and NOx contained in the
exhaust emission. In this case, HC and CO are separately trapped in
the macroporous catalyst element 301a and the microporous catalyst
element 301b, respectively. Therefore, in the macroporous catalyst
element 301a located on the upstream side of the exhaust flow, the
released HC is well reacted with O.sub.2 contained in the exhaust
emission. At the same time in the microporous catalyst element 301b
located on the downstream side of the exhaust flow, the released CO
and H.sub.2 are well reacted with NOx contained in the exhaust
emission without being hindered by the released HC.
[0065] This improves not only the purifying performance of NOx but
that of HC, thereby upgrading the exhaust gas purifying performance
of the three-way catalyst 301 as a whole.
[0066] In this case, too, the macroporous catalyst element 301a and
the microporous catalyst element 301b do not have to be coupled and
integrated with each other. On the contrary, the macroporous
catalyst element 301a and the microporous catalyst element 301b may
be located away from each other in the direction of the exhaust
flow as another embodiment, as illustrated in FIG. 8.
[0067] Next, a third embodiment will be described.
[0068] The third embodiment differs from the first simply in that a
three-way catalyst 302 is employed in place of the three-way
catalyst 30.
[0069] FIG. 9 shows a quarter portion of a unit cell of the
three-way catalyst 302. The three-way catalyst 302 is formed in
layers of a microporous catalyst element 302a having pores whose
average pore opening size is smaller than the molecular size of HC
and a macroporous catalyst element 302b having pores whose average
pore opening size is larger than the molecular size of HC. The
microporous catalyst element 302a is disposed on a surface layer
side, whereas the macroporous catalyst element 302b on an internal
layer side.
[0070] Hereinafter, operation of the exhaust emission control
device, in which the microporous catalyst element 302a is disposed
on the surface layer side, and the macroporous catalyst element
302b on the internal layer side, will be described.
[0071] When the rich operation is performed in the O.sub.2 F/B
control, and the reducing atmosphere is produced in the three-way
catalyst 302, the trapped NOx and O.sub.2 are released to cause the
oxidation-reduction reaction with CO and HC contained in the
exhaust emission as seen in the above embodiments. In this
procedure, since the oxidation-reduction reaction of CO and NOx is
faster than that of HC and NOx in reaction speed, the released NOx
is preferentially reacted with CO, while the released O.sub.2 is
well reacted with HC.
[0072] Once NOx and O.sub.2 are sufficiently released, CO and
H.sub.2 having smaller molecular size than HC are satisfactorily
trapped in micropores S of the microporous catalyst element 302a
located on the surface layer side, and HC of large molecular size
passes through gaps in the microporous catalyst element 302a to be
successfully trapped in macropores L of the macroporous catalyst
element 302b located on the internal layer side. Like the above
embodiments, in the three-way catalyst 302, CO and HC are actively
separated from each other and trapped in the microporous catalyst
element 302a and the macroporous catalyst element 302b,
respectively.
[0073] When the lean operation is carried out, and the oxidizing
atmosphere is created in the three-way catalyst 302, the trapped
CO, H.sub.2 and HC are released to produce the oxidation-reduction
reaction with O.sub.2 and NOx contained in the exhaust emission. In
this case, CO and HC are separately trapped in the microporous
catalyst element 302a and the macroporous catalyst element 302b,
respectively, as mentioned above. Also CO and H.sub.2 are released
in the microporous catalyst element 302a located on the surface
layer side. Therefore, the released CO and H.sub.2 are
preferentially and surely reacted with NOx contained in the exhaust
emission in the microporous catalyst element 302a located on the
surface layer side, due in part to the fact that the
oxidation-reduction reaction of CO and NOx is faster than that of
HC and NOx in reaction speed. By using CO for reaction with NOx in
this manner, the released HC is fully reacted with O.sub.2
contained in the exhaust emission in the macroporous catalyst
element 302b located on the internal layer side.
[0074] This upgrades not only the purifying performance of NOx but
that of HC, thereby enhancing the exhaust gas purifying performance
of the three-way catalyst 302 as a whole.
[0075] Furthermore, if the microporous catalyst element 302a and
the macroporous catalyst element 302b are formed in layers, at the
time of cold start of the engine 1, the microporous catalyst
element 302a and the macroporous catalyst element 302b are raised
in temperature substantially at the same time and are successfully
activated.
[0076] A fourth embodiment will be described below.
[0077] The fourth embodiment differs from the third only in that a
three-way catalyst 303 is employed in place of the three-way
catalyst 302.
[0078] FIG. 10 shows a quarter portion of a unit cell of the
three-way catalyst 303. The three-way catalyst 303 is formed in
layers of a macroporous catalyst element 303a having pores whose
average pore opening size is larger than the molecular size of HC
and a microporous catalyst element 303b having pores whose average
pore opening size is smaller than the molecular size of HC. The
macroporous catalyst element 303a is disposed on the surface layer
side, and the microporous catalyst element 303b on the internal
layer side. In other words, in the three-way catalyst 303, the
microporous and macroporous catalyst elements are reversely
positioned, compared to the three-way catalyst 302.
[0079] Hereinafter, operation of the exhaust emission control
device, in which the macroporous catalyst element 303a is disposed
on the surface layer side, and the microporous catalyst element
303b on the internal layer side, will be described.
[0080] When the rich operation is implemented in the O.sub.2 F/B
control, and the reducing atmosphere is generated in the three-way
catalyst 303, the trapped NOx and O.sub.2 are released to cause the
oxidation-reduction reaction with CO and HC contained in the
exhaust emission as seen in the above embodiments. Since the
oxidation-reduction reaction of CO and NOx is faster than that of
HC and NOx in reaction speed, the released NOx is reacted with CO
by priority, whereas the released O.sub.2 is well reacted with
HC.
[0081] Once NOx and O.sub.2 are sufficiently released, HC of large
molecular size is satisfactorily trapped in macropores L of the
macroporous catalyst element 303a located on the surface layer
side. On the other hand, CO having smaller molecular size than HC
is successfully trapped in micropores S of the microporous catalyst
element 303b located on the internal layer side. Like the above
embodiments, in the three-way catalyst 303, HC and CO are actively
separated from each other and trapped in the macroporous catalyst
element 303a and the microporous catalyst element 303b,
respectively.
[0082] When the lean operation is performed, and the oxidizing
atmosphere is created in the three-way catalyst 303, the trapped HC
and CO are released to produce the oxidation-reduction reaction
with O.sub.2 and NOx contained in the exhaust emission. In this
case, as mentioned above, HC and CO are trapped separately in the
macroporous catalyst element 303a and the microporous catalyst
element 303b, respectively. Therefore, the released HC is well
reacted with O.sub.2 contained in the exhaust emission in the
macroporous catalyst element 303a located on the surface layer
side, and the released CO is relatively well reacted with NOx
contained in the exhaust emission without being severely hindered
by the released HC in the microporous catalyst element 303b located
on the internal layer side.
[0083] Consequently, not only the purifying performance of NOx but
that of HC is upgraded, which enhances the exhaust gas purifying
performance of the three-way catalyst 303 as a whole.
[0084] In this case, too, the macroporous catalyst element 303a and
the microporous catalyst element 303b are formed in layers, so that
at the time of cold start of the engine 1, the macroporous catalyst
element 303a and the microporous catalyst element 303b are raised
in temperature substantially at the same time and are successfully
activated.
[0085] A fifth embodiment will be described below.
[0086] The fifth embodiment is different from the first simply in
that A/F modulation (air-fuel ratio modulating means) is forcibly
implemented instead of performing the O.sub.2 F/B control.
[0087] FIG. 11 shows a control routine of A/F modulation control in
a flowchart. Explanations will be provided with reference to the
flowchart.
[0088] First, Step S20 judges whether a time counter has counted
predetermined time t1. The predetermined time t1 is set equal to or
less than the amount of time that is expected to be taken before a
trapping amount of CO in the microporous catalyst element 30a of
the three-way catalyst 30 reaches a saturated state, or a
breakthrough point, based on for example a preliminary experiment
or the like. That is to say, Step S20 judges whether the trapping
amount of CO is at a stage immediately before reaching the
breakthrough point.
[0089] If a result of the judgement in Step S20 is "NO", which
means that the prescribed time t1 has not yet elapsed, the trapping
of CO is considered to be quite possible. The procedure then
advances to Step S22, and the rich operation is carried out or
continued. On the contrary, if the result of the judgement is
"YES", which means that the prescribed time t1 has elapsed, the
procedure proceeds to Step S24.
[0090] In Step S24, it is judged whether the time counter has
counted prescribed time t2. Prescribed time t2-t1 is set equal to
or less than time that is expected to be taken before a trapping
amount of NOx in the microporous catalyst element 30a of the
tree-way catalyst 30 reaches a saturated state, or a breakthrough
point, based on for example a preliminary experiment or the like.
In short, Step S24 judges whether the trapping amount of NOx is at
a stage immediately before reaching the breakthrough point.
[0091] If a result of the judgement of Step S24 is "NO", which
means that the prescribed time t2 has not yet elapsed, NOx is
considered to be quite trappable. Subsequently the procedure
advances to Step S26, and the lean operation is implemented or
continued. On the contrary, the result of the judgement of Step S24
is "YES", which means that the prescribed time t2 has elapsed, the
procedure proceeds to Step S28, and the time counter is reset to
zero. Thereafter the rich operation and the lean operation are
repeatedly implemented.
[0092] Basically in the fifth embodiment, the exhaust A/F is
efficiently modulated between the rich A/F and the lean A/F within
a range in which the trapping amounts of CO and NOx in the
three-way catalyst 30 do not reach the respective breakthrough
points.
[0093] As a consequence, in the exhaust emission control device
according to the fifth embodiment, the exhaust A/F is efficiently
modulated between the lean A/F and the rich A/F due to the A/F
modulation control, to thereby satisfactorily create the oxidizing
and reducing atmospheres. Therefore, CO and HC repeatedly and
satisfactorily continues to be trapped in the three-way catalyst 30
while being separated from each other in the reducing atmosphere.
In the oxidizing atmosphere, the released CO is surely reacted with
NOx contained in the exhaust emission by priority without being
hampered by the released HC. Accordingly, the purifying performance
of NOx is improved. By using CO for reaction with NOx, the released
HC is fully reacted with O.sub.2 contained in the exhaust emission,
resulting in the enhancement of the purifying performance of HC.
Consequently, the exhaust gas purifying performance of the
three-way catalyst 30 is upgraded as a whole and is constantly
maintained in a high level.
[0094] Although the three-way catalyst 30 of the first embodiment
used in the above explanation, the three-way catalyst is not
limited to this, and the fifth embodiment is applicable if using
any one of the three-way catalysts 301, 302 and 303 of the second,
third and fourth embodiments.
[0095] In the above-described embodiments, the three-way catalyst
is provided with the microporous and macroporous catalyst elements
that are different in average pore opening size, to thereby
separate CO and HC, that is, two components contained in the
exhaust emission. It is also possible, however, to further vary the
average pore opening size according to components to be trapped and
dispose three or more catalyst elements (pore group), to thereby
separate three or more components contained in the exhaust
emission. Furthermore, components to be separated, which are
contained in the exhaust emission, are not limited to CO and HC. On
the contrary, the components to be separated may be selected as
needed.
[0096] Although in the above embodiments, the gasoline engine is
employed as the engine 1, the engine 1 may be a diesel engine.
[0097] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are intended to be included within the scope of the
following claims.
* * * * *