U.S. patent application number 12/741841 was filed with the patent office on 2011-04-07 for exhaust purifying device of internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Hiroto Imai, Kenji Katoh, Shigeki Miyashita, Naoto Miyoshi, Kenji Sakurai.
Application Number | 20110079001 12/741841 |
Document ID | / |
Family ID | 40521713 |
Filed Date | 2011-04-07 |
United States Patent
Application |
20110079001 |
Kind Code |
A1 |
Sakurai; Kenji ; et
al. |
April 7, 2011 |
EXHAUST PURIFYING DEVICE OF INTERNAL COMBUSTION ENGINE
Abstract
An upstream catalyst and a downstream catalyst are arranged in
series and are housed in a common casing disposed in an engine
exhaust passage. The upstream catalyst including a NOx
storage-reduction catalyst that adsorbs NOx contained in incoming
exhaust gas when the air-fuel ratio of the incoming exhaust gas is
lean, and releases and reduces the adsorbed NOx when the air-fuel
ratio of the incoming exhaust gas becomes rich, and the downstream
catalyst includes a three-way catalyst. The upstream catalyst has a
higher oxidizing capability than the downstream catalyst, and the
downstream catalyst has a higher reducing capability than the
upstream catalyst. The upstream catalyst has a multi-layer
structure including an upper layer and a lower layer, and is
prepared such that the upper layer has a higher oxidizing
capability than the lower layer, and the lower layer has a higher
reducing capability than the upper layer.
Inventors: |
Sakurai; Kenji;
(Shizuoka-ken, JP) ; Miyashita; Shigeki;
(Shizuoka-ken, JP) ; Katoh; Kenji; (Shizuoka-ken,
JP) ; Miyoshi; Naoto; (Aichi-ken, JP) ; Imai;
Hiroto; (Shizuoka-ken, JP) |
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
CATALER CORPORATION
Kakegawa-shi
JP
|
Family ID: |
40521713 |
Appl. No.: |
12/741841 |
Filed: |
November 6, 2008 |
PCT Filed: |
November 6, 2008 |
PCT NO: |
PCT/IB2008/002964 |
371 Date: |
October 15, 2010 |
Current U.S.
Class: |
60/295 ;
60/301 |
Current CPC
Class: |
B01D 2255/102 20130101;
F01N 2530/26 20130101; B01D 2255/1021 20130101; B01D 2255/204
20130101; B01J 23/464 20130101; F01N 13/0097 20140603; F01N 2370/02
20130101; B01D 2255/9022 20130101; F01N 3/0814 20130101; B01D
53/9422 20130101; B01D 2255/202 20130101; B01D 2255/2042 20130101;
B01J 23/63 20130101; B01J 37/0244 20130101; F01N 2510/0682
20130101; B01D 53/945 20130101; B01D 53/9477 20130101; B01J 37/0234
20130101; Y02T 10/12 20130101; F01N 3/101 20130101; Y02T 10/22
20130101; B01D 2255/91 20130101; F01N 3/0842 20130101 |
Class at
Publication: |
60/295 ;
60/301 |
International
Class: |
F01N 3/10 20060101
F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 7, 2007 |
JP |
2007-289927 |
Claims
1-5. (canceled)
6. An exhaust purifying device for an internal combustion engine,
comprising: an upstream catalyst and a downstream catalyst arranged
in series with each other and housed in a common casing disposed in
an engine exhaust passage, wherein: the upstream catalyst comprises
a NOx storage-reduction catalyst that absorbs NOx contained in
incoming exhaust gas when the air-fuel ratio of the incoming
exhaust gas is lean, and releases and reduces the absorbed NOx when
the air-fuel ratio of the incoming exhaust gas becomes rich, and
the downstream catalyst comprises one of a three-way catalyst and a
NOx storage-reduction catalyst; the upstream catalyst has a higher
oxidizing capability than the downstream catalyst, and the
downstream catalyst has a higher reducing capability than the
upstream catalyst; the upstream catalyst achieves a higher
oxidizing capability than the downstream catalyst and achieves a
lower reducing capability than the downstream catalyst by a
multi-layer structure including an upper layer and a lower layer,
each of the upper layer and the lower layer of the upstream
catalyst contains both of a noble-metal catalyst and a NOx
absorbent comprising at least one selected from alkali metals,
alkaline earths, and rare earths, the upper layer of the upstream
catalyst contains, as the noble-metal catalyst, at least one
selected from platinum (Pt), palladium (Pd), osmium (Os), and gold
(Au); the lower layer of the upstream catalyst contains, as the
noble-metal catalyst, at least one selected from rhodium (Rh),
iridium (Ir), and ruthenium (Ru), the downstream catalyst has a
multi-layer structure including an upper layer and a lower layer,
and the upper layer has a higher reducing capability than the lower
layer, and the lower layer has a higher oxidizing capability than
the upper layer: rhodium (Rh) is used as a noble-metal component of
the upper layer of the downstream catalyst; and platinum (Pt) is
used as a noble-metal component of the lower layer of the
downstream catalyst.
7. The exhaust purifying device according to claim 6, wherein: the
air-fuel ratio in the internal combustion engine is normally set to
a lean air-fuel ratio that is larger than a stoichiometric ratio;
and when NOx stored in the upstream NOx storage-reduction catalyst
is to be released and reduced, the air-fuel ratio of exhaust gas
flowing into the upstream NOx storage-reduction catalyst is
temporarily controlled to a rich air-fuel ratio that is smaller
than the stoichiometric ratio.
8. The exhaust purifying device according to claim 7, wherein the
air-fuel ratio in the internal combustion engine is temporarily
controlled to the stoichiometric ratio, depending on engine
operating conditions.
9. The exhaust purifying device according to claim 6, wherein: the
amount of NOx absorbed per unit time by the upstream NOx
storage-reduction catalyst is stored in advance in a memory as a
function of engine operating conditions; by integrating the NOx
amount, a total value of the amount of NOx stored in the upstream
NOx storage-reduction catalyst is calculated; and each time the
total value of the stored NOx amount exceeds an upper limit, a
rich-mode operation is temporarily performed in which an air-fuel
mixture having a rich air-fuel ratio is burned.
10. The exhaust purifying device according to claim 6, wherein the
air-fuel ratio of the incoming exhaust gas is enriched by supplying
a reductant or secondary fuel into an exhaust passage upstream of
the NOx storage-reduction catalyst.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to an exhaust purifying device of an
internal combustion engine.
[0003] 2. Description of the Related Art
[0004] In a known example of internal combustion engine (as
disclosed in, for example, Japanese Patent Application Publication
No. 11-44234 (JP-A-11-44234)), a NOx storage-reduction catalyst,
which adsorbs NOx contained in exhaust gas flowing into the
catalyst when the air-fuel ratio of the exhaust gas is lean, and
releases and reduces the adsorbed NOx when the air-fuel ratio of
the exhaust gas becomes rich, is disposed in an exhaust passage of
the engine (i.e., lean-burn engine) in which an air-fuel mixture
having a lean air-fuel ratio is normally burned. In this type of
engine, the air-fuel ratio of the exhaust gas flowing into the NOx
storage-reduction catalyst is temporarily switched to the rich side
when the NOx stored in the NOx storage-reduction catalyst is to be
released and reduced. In this internal combustion engine, NOx
contained in the exhaust gas is adsorbed by and stored in the NOx
storage-reduction catalyst. The amount of NOx stored in the NOx
storage-reduction catalyst gradually increases with the passage of
time. Thus, the air-fuel ratio of the exhaust gas flowing into the
NOx storage-reduction catalyst is temporarily switched to the rich
side before the NOx storage-reduction catalyst is saturated with
NOx, so that the NOx stored in the NOx storage-reduction catalyst
is released and reduced. In this case, the air-fuel ratio in the
internal combustion engine, for example, is controlled to be rich
(i.e., smaller than the stoichiometric ratio) so that the air-fuel
ratio of the exhaust gas flowing into the NOx storage-reduction
catalyst is switched to the rich side.
[0005] In another known example of internal combustion engine (as
disclosed in, for example, Japanese Patent Application Publication
No. 2006-291812 (JP-A-2006-291812)), an upstream catalyst and a
downstream catalyst are arranged in series with each other and are
housed in a common casing disposed in an engine exhaust passage,
and each of the upstream catalyst and the downstream catalyst has a
single-layer structure or a multi-layer structure.
[0006] Since fuel consumption increases with an increase in the
frequency at which the air-fuel ratio of the exhaust gas flowing
into the NOx storage-reduction catalyst is switched to the rich
side, it is preferable, in terms of reduction of the fuel
consumption, that the NOx storage-reduction catalyst has the
highest possible NOx adsorbing capability or storage capacity.
However, there is a limit to the space in which the NOx
storage-reduction catalyst is installed, and it is therefore
necessary to increase or enhance the NOx adsorbing capability of
the NOx storage-reduction catalyst while minimizing the dimensions
or capacity of the NOx storage-reduction catalyst.
[0007] Immediately after the air-fuel ratio of the exhaust gas
flowing into the NOx storage-reduction catalyst is switched to the
rich side, a large amount of NOx may be discharged from the NOx
storage-reduction catalyst without being reduced. In this case,
emissions of NOx need to be reduced.
[0008] To solve the above-described problems, an additional
catalyst may be disposed upstream or downstream of the NOx
storage-reduction catalyst, or the NOx storage-reduction catalyst
may have a multi-layer structure, i.e., may be constructed of two
or more layers, as disclosed in JP-A-2006-291812. However, the
state of the art does not provide satisfactory solutions to the
above problems.
SUMMARY OF THE INVENTION
[0009] The invention provides an exhaust purifying device of an
internal combustion engine in which a NOx storage-reduction
catalyst exhibits a high NOx adsorbing capability and a high NOx
conversion efficiency.
[0010] According to one aspect of the invention, there is provided
an exhaust purifying device of an internal combustion engine
wherein an upstream catalyst and a downstream catalyst are arranged
in series with each other and are housed in a common casing
disposed in an engine exhaust passage, and wherein the upstream
catalyst comprises a NOx storage-reduction catalyst that adsorbs
NOx contained in incoming exhaust gas when the air-fuel ratio of
the incoming exhaust gas is lean, and releases and reduces the
adsorbed NOx when the air-fuel ratio of the incoming exhaust gas
becomes rich, and the downstream catalyst comprises one of a
three-way catalyst and a NOx storage-reduction catalyst. In the
exhaust purifying device, the upstream catalyst and the downstream
catalyst are prepared such that the upstream catalyst has a higher
oxidizing capability than the downstream catalyst, and such that
the downstream catalyst has a higher reducing capability than the
upstream catalyst, and the upstream catalyst has a multi-layer
structure including an upper layer and a lower layer, and is
prepared such that the upper layer has a higher oxidizing
capability than the lower layer, and such that the lower layer has
a higher reducing capability than the upper layer.
[0011] In the exhaust purifying device as described above, each of
the upper layer and the lower layer of the upstream catalyst may
contain a noble-metal catalyst comprising at least one selected
from platinum (Pt), palladium (Pd), osmium (Os), gold (Au), rhodium
(Rh), iridium (Ir), and ruthenium (Ru), and a NOx absorbent
comprising at least one selected from alkali metals, alkaline
earths, and rare earths.
[0012] In the exhaust purifying device as described above, the
upper layer of the upstream catalyst may contain, as the
noble-metal catalyst, at least one selected from platinum (Pt),
palladium (Pd), osmium (Os), and gold (Au), and the lower layer of
the upstream catalyst may contain, as the noble-metal catalyst, at
least one selected from rhodium (Rh), iridium (Ir), and ruthenium
(Ru).
[0013] Also, the downstream catalyst may have a multi-layer
structure including an upper layer and a lower layer, and may be
prepared such that the upper layer has a higher reducing capability
than the lower layer, and the lower layer has a higher oxidizing
capability than the upper layer.
[0014] Furthermore, rhodium (Rh) may be used as a noble-metal
component of the upper layer of the downstream catalyst, and
platinum (Pt) may be used as a noble-metal component of the lower
layer of the downstream catalyst.
[0015] The downstream catalyst may have a single-layer
structure.
[0016] Furthermore, the downstream catalyst may contain rhodium
(Rh) and platinum (Pt) as noble-metal components.
[0017] In the exhaust purifying device as described above, the
air-fuel ratio in the internal combustion engine may be normally
set to a lean air-fuel ratio that is larger than a stoichiometric
ratio, and, when NOx stored in the NOx storage-reduction catalyst
is to be released and reduced, the air-fuel ratio of exhaust gas
flowing into the NOx storage-reduction catalyst may be temporarily
controlled to a rich air-fuel ratio that is smaller than the
stoichiometric ratio.
[0018] Furthermore, the air-fuel ratio in the internal combustion
engine, may be temporarily controlled to the stoichiometric ratio,
depending on engine operating conditions.
[0019] With the above arrangements, the NOx adsorbing capability
and NOx conversion efficiency of the NOx storage-reduction catalyst
can be enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The foregoing and further features and advantages of the
invention will become apparent from the following description of
example embodiments with reference to the accompanying drawings,
wherein like numerals are used to represent like elements, and
wherein:
[0021] FIG. 1 is an overall view of an internal combustion
engine;
[0022] FIG. 2 is a cross-sectional view of a NOx storage-reduction
catalyst;
[0023] FIG. 3A and FIG. 3B are cross-sectional views of a surface
portion of a catalyst support;
[0024] FIG. 4 is an enlarged cross-sectional view of the NOx
storage-reduction catalyst;
[0025] FIG. 5A through FIG. 5C are views showing various examples
of NOx storage-reduction catalysts;
[0026] FIG. 6A and FIG. 6B are views showing various examples of
three-way catalysts;
[0027] FIG. 7 is a view useful for explaining a predetermined load
factor KLX;
[0028] FIG. 8 is a flowchart illustrating an engine operation
control routine;
[0029] FIG. 9A through FIG. 9C are view showing various
experimental results; and
[0030] FIG. 10 is a view useful for explaining a peak value of a
discharged NOx amount.
DETAILED DESCRIPTION OF EMBODIMENTS
[0031] FIG. 1 illustrates the case where the invention is applied
to a spark ignition type internal combustion engine. The invention
may also be applied to a compression ignition type internal
combustion engine.
[0032] Referring to FIG. 1, the spark ignition type internal
combustion engine includes an engine body 1, cylinder block 2,
cylinder head 3, piston 4, combustion chamber 5, intake valve 6,
intake port 7, exhaust valve 8, exhaust port 9, and a spark plug
10. The intake port 7 of each cylinder is connected to a surge tank
12 via a corresponding intake branch pipe 11. The surge tank 12 is
connected to an air cleaner 14 via an intake duct 13. An air flow
meter 15 and a throttle valve 17 adapted to be driven by a step
motor 16 are disposed in the intake duct 13. A fuel injection valve
18 is mounted to the intake port 7 of each cylinder. The fuel
injection valve 18 for each cylinder is connected to a common rail
19, and the common rail 19 is connected to a fuel tank 21 via a
fuel pump 20 capable of controlling the amount of fuel delivered
therefrom. A fuel pressure sensor 22 is mounted to the common rail
19, and the amount of fuel delivered from the fuel pump 20 is
controlled so that the fuel pressure in the common rail 19 becomes
equal to a target pressure.
[0033] On the other hand, the exhaust port 9 of each cylinder is
connected to a casing 25 via an exhaust manifold 23 and an exhaust
pipe 24, and the casing 25 is connected to an exhaust pipe 26. An
air-fuel ratio sensor 27 is mounted in the exhaust pipe 24, and a
catalyst 28 is housed in the casing 25.
[0034] An electronic control unit 30 consists of a digital
computer, and includes ROM (read-only memory) 32, RAM (random
access memory) 33, CPU (microprocessor) 33, input port 35, and
output port 36, which are connected to each other via a
bidirectional bus 31. A load sensor 40 that produces an output
voltage proportional to the amount of depression of an accelerator
pedal 39 is connected to the accelerator pedal 39. The input port
35 receives output voltages of the air flow meter 15, fuel pressure
sensor 22, air-fuel ratio sensor 27 and the load sensor 40, via
corresponding A/D converters 37. A crank angle sensor 41 produces
an output pulse each time the crankshaft rotates, for example,
30.degree., and the output pulse is transmitted to the input port
35. The CPU 34 calculates the engine speed Ne, based on the output
pulses received from the crank angle sensor 41. On the other hand,
the output port 36 is connected to the spark plug 10, step motor
16, fuel injection valve 18, and the fuel pump 20, via
corresponding driving circuits 38.
[0035] The catalyst 28 includes an upstream catalyst 28U and a
downstream catalyst 28D which are arranged in series with each
other in the casing 25. In one embodiment of the invention, the
upstream catalyst 28U consists of a NOx storage-reduction catalyst,
and the downstream catalyst 28D consists of a three-way catalyst.
However, the downstream catalyst 28D may consist of a NOx
storage-reduction catalyst. In this embodiment of the invention,
the capacity of the upstream catalyst 28U is made equal to or
larger than that of the downstream catalyst 28D. However, the
capacity of the upstream catalyst 28U may be made smaller than that
of the downstream catalyst 28D.
[0036] FIG. 2 illustrates the structure of the upstream catalyst,
or NOx storage-reduction catalyst 28U. In the embodiment shown in
FIG. 2, the NOx storage-reduction catalyst 28U has a honeycomb
structure, and includes a plurality of exhaust gas channels 51 that
are separated from each other by thin partition walls 50. A
catalyst support 55 made of, for example, alumina is loaded on the
opposite surfaces of each partition wall, or substrate 50. FIG. 3A
and FIG. 3B schematically illustrate a cross-section of a surface
portion of the catalyst support 55. As shown in FIG. 3A and FIG.
3B, a noble-metal catalyst 56 is supported, while being scattered,
on the surface of the catalyst support 55, and a layer of a NOx
absorbent 57 is formed on the surface of the catalyst support
55.
[0037] At least one selected from platinum (Pt), palladium (Pd),
osmium (Os), gold (Au), rhodium (Rh), iridium (Ir), and ruthenium
(Ru) is used as the noble-metal catalyst 56. As a component that
constitutes the NOx absorbent 57, at least one selected from alkali
metals, such as potassium (K), sodium (Na), and cesium (Cs),
alkaline earths, such as barium (Ba) and calcium (Ca), and rare
earths, such as lanthanum (La) and yttrium (Y), is used.
[0038] Where the ratio of air and fuel (hydrocarbon) supplied into
the engine intake passage, combustion chamber 5 and the exhaust
passage upstream of the NOx storage-reduction catalyst 28U is
referred to as "air-fuel ratio of exhaust gas", the NOx absorbent
57 performs NOx absorbing and releasing functions to absorb NOx
when the air-fuel ratio of exhaust gas is lean, and release the
absorbed NOx when the concentration of oxygen in the exhaust gas is
reduced.
[0039] In the case where platinum (Pt) is used as the noble-metal
catalyst 56, and barium (Ba) is used as a component that
constitutes the NOx absorbent 57, by way of example, when the
air-fuel ratio of exhaust gas is lean, namely, when the
concentration of oxygen in the exhaust gas is high, NOx contained
in the exhaust gas is oxidized into NO.sub.2 on the platinum (Pt)
56 as shown in FIG. 3A, and NO.sub.2 is then absorbed into the NOx
absorbent 57, to be dispersed in the form of nitrate ions NO.sub.3
in the NOx absorbent 57 while combining with barium carbonate
(BaCO.sub.3). In this manner, NOx is absorbed into the NOx
absorbent 57. NO.sub.2 is produced on the surface of the platinum
(Pt) 56 as long as the concentration of oxygen in the exhaust gas
is sufficiently high, and NO.sub.2 is absorbed into the NOx
absorbent 57 to form nitrate ions NO.sub.3 as long as the NOx
absorbing capability of the NOx absorbent 57 is not saturated.
[0040] If the air-fuel ratio of the exhaust gas turns rich, on the
other hand, the concentration of oxygen in the exhaust gas is
reduced, and the reaction proceeds in the reverse direction
(NO.sub.3.fwdarw.NO.sub.2), so that nitrate ions NO.sub.3 in the
NOx absorbent 57 are released in the form of NO.sub.2 from the NOx
absorbent 57, as shown in FIG. 3B. Then, the released NOx is
reduced by unburned HC and CO contained in the exhaust gas.
[0041] In this embodiment of the invention, the NOx
storage-reduction catalyst 28U has a multi-layer structure
including an upper layer 28UU and a lower layer 28UL, as shown in
FIG. 4. Namely, the lower layer 28UL and the upper layer 28UU are
successively laminated on the substrate 50. In this case, each of
the upper layer 28UU and the lower layer 28UL provides a NOx
storage-reduction catalyst, namely, includes the above-described
noble-metal catalyst 56 and NOx absorbent 57. An additional layer
may be provided between the upper layer 28UU and the lower layer
28UL, or between the lower layer 28UL and the catalyst support
55.
[0042] At least one selected from noble metals having a high
oxidizing capability, such as platinum (Pt), palladium (Pd), osmium
(Os), and gold (Au), is used as the noble-metal catalyst 56 of the
upper layer 28UU. On the other hand, at least one selected from
noble metals having a high reducing capability, such as rhodium
(Rh), iridium (Ir), and ruthenium (Ru), is used as the noble-metal
catalyst 56 of the lower layer 28UL. In this case, a noble metal
having a high reducing capability is not contained in the upper
layer 28UU.
[0043] FIG. 5A through FIG. 5C show various examples of the
noble-metal catalysts 56 of the upper layer 28UU and the lower
layer 28UL. As the noble-metal catalyst 56 of the upper layer 28UU,
platinum (Pt) is used in the example of FIG. 5A, and palladium (Pd)
is used in the example of FIG. 5B, while platinum (Pt) and
palladium (Pd) are used in the example of FIG. 5C. On the other
hand, rhodium (Rh) is used as the noble-metal catalyst 56 of the
lower layer 28UL in all of the examples of FIG. 5A-FIG. 5C.
[0044] If the noble-metal catalysts 56 of the upper layer 28UU and
lower layer 29UL are selected in the above manners, the oxidizing
capability of the upper layer 28UU is made higher than that of the
lower layer 28UL, and the reducing capability of the lower layer
28UL is made higher than that of the upper layer 28UU.
[0045] In the meantime, the downstream catalyst, or three-way
catalyst 28D also has a honeycomb structure, like the NOx
storage-reduction catalyst 28U, and includes a plurality of exhaust
gas channels that are separated from each other by thin partition
walls. A catalyst support made of, for example, alumina is loaded
on the opposite surfaces of each partition wall, and a catalyst
component including a noble-metal component is supported on the
surface of the catalyst support.
[0046] In one embodiment of the invention, the three-way catalyst
28D has a multi-layer structure including an upper layer 28DU and a
lower layer 28DL. In this case, each of the upper layer 28DU and
the lower layer 28DL provides a three-way catalyst.
[0047] In the three-way catalyst 28D, at least one selected from
noble metals having a high reducing capability is used as a
noble-metal component of the upper layer 28DU, and at least one
selected from noble metals having a high oxidizing capability is
used as a noble-metal component of the lower layer 28DL. In the
example shown in FIG. GA, rhodium (Rh) is used as the noble-metal
component of the upper layer 28DU, and platinum (Pt) is used as the
noble-metal component of the lower layer 28DL.
[0048] If the noble-metal components of the upper layer 28DU and
the lower layer 28DL are selected as described above, the reducing
capability of the upper layer 28DU is made higher than that of the
lower layer 28DL, and the oxidizing capability of the lower layer
28DL is made higher than that of the upper layer 28DU.
[0049] Alternatively, the three-way catalyst 28D may have a
single-layer structure. In this case, at least a noble metal having
a high reducing capability is used as a noble-metal component of
the three-way catalyst 28D. In addition, a metal having a high
oxidizing capability may be used or may not be used. In the example
shown in FIG. 6B, rhodium (Rh) and platinum (Pt) are used as
noble-metal components of the three-way catalyst 28D.
[0050] If the noble-metal catalysts 56 of the upstream catalyst or
NOx storage-reduction catalyst 28U and the noble-metal component(s)
of the downstream catalyst or three-way catalyst 28D are selected
as described above, the oxidizing capability of the NOx
storage-reduction catalyst 28U is made higher than that of the
three-way catalyst 28D, and the reducing capability of the
three-way catalyst 28D is made higher than that of the NOx
storage-reduction catalyst 28U.
[0051] In this embodiment of the invention, the upstream catalyst
or NOx storage-reduction catalyst 28U and the downstream catalyst
or three-way catalyst 28D are independently supported on the
respective substrates, and these substrates are coupled in series
with each other, thereby to form the catalyst 28. The NOx
storage-reduction catalyst 28U may be supported on an upstream
portion of a common substrate, and the three-way catalyst 28D may
be supported on a downstream portion of the substrate.
[0052] The NOx storage-reduction catalyst 28U having a multi-layer
structure is manufactured, for example, in the following manner.
Here, the manufacturing method will be explained with regard to the
case where rhodium (Rh) is used as the noble-metal catalyst 56 of
the lower layer 28UL, and platinum (Pt) is used as the noble-metal
catalyst 56 of the upper layer 28UU. Initially, a slurry is
prepared in which support powder that forms the catalyst support of
the lower layer 28UL and rhodium powder are dispersed, and the
slurry is applied onto a substrate. In this case, zirconium (Zr),
alumina (Al.sub.2O.sub.3), ceria (CeO.sub.2),
ZrO.sub.2-Al.sub.2O.sub.3, ZrO.sub.2-Al.sub.2O.sub.3-TiO.sub.2, for
example, may be used as the catalyst support of the lower layer
28UL. The rhodium powder is formed from PM powder, and is dispersed
in the form of nitrate or acetate in the slurry. The viscosity of
the slurry is preferably around 30%, for example, and the amount of
coating preferably ranges from 50 g/L to 200 g/L. Then, drying
(200.degree. C., 2 hours) and firing (400.degree. C., 4 hours) are
conducted, so that the lower layer 28UL is formed.
[0053] Subsequently, a slurry is prepared in which support powder
that forms the catalyst support of the upper layer 28UU and
platinum powder are dispersed, and the slurry is applied onto the
lower layer 28UL. In this case, zirconium (Zr), alumina
(Al.sub.2O.sub.3), ceria (CeO.sub.2), Al.sub.2O.sub.3-CeO.sub.2,
ZrO.sub.2-Al.sub.2O.sub.3, or ZrO.sub.2-Al.sub.2O.sub.3-TiO.sub.2,
for example, may be used as the catalyst support of the upper layer
28UU. The platinum powder is dispersed in the form of nitrate or
acetate, such as tetrachroloplatinum or dinitroplatinum, in the
slurry. The viscosity of the slurry is preferably around 30%, for
example, and the amount of coating preferably ranges from 50 g/L to
200 g/L. Then, drying (200.degree. C., 2 hours) and firing
(400.degree. C., 4 hours) are conducted, so that the upper layer
28UU is formed. In another method, a catalyst support may be first
formed on the lower layer 28UL, and the catalyst support may be
impregnated with an aqueous solution of tetrachroloplatinum or
dinitroplatinum.
[0054] The three-way catalyst 28D having a multi-layer structure
may also be manufactured in a manner similar to the NOx
storage-reduction catalyst 28U.
[0055] In the embodiment of the invention, when the engine operates
at a low load with the engine load factor KL being smaller than a
predetermined or preset load factor KLX as shown in FIG. 7, a
lean-mode operation is performed in which an air-fuel mixture
having a lean air-fuel ratio is burned. When the engine operates at
a high load with the engine load factor KL being larger than the
predetermined load factor KLX, a stoichiometric-ratio operation is
performed in which an air-fuel mixture having the stoichiometric
ratio is burned. Here, the engine load factor KL represents the
proportion of the engine load to the full load. In this case, it
may also be said that an internal combustion engine that normally
operates in a lean mode (i.e., a lean-burn engine) is temporarily
switched to a stoichiometric-ratio operation, depending on the
engine operating conditions.
[0056] Thus, when the engine operates in a lean mode, the air-fuel
ratio of exhaust gas flowing into the NOx storage-reduction
catalyst 28U becomes lean, and NOx contained in the exhaust gas is
adsorbed by and stored in the NOx storage-reduction catalyst 28U.
If the lean-mode operation continues to be performed, however, the
NOx storage-reduction catalyst 28U adsorbs NOx to the full NOx
adsorbing capability (namely, the NOx storage-reduction catalyst
28U is saturated with NOx adsorbed thereon), whereby the NOx
storage-reduction catalyst 28U becomes unable to adsorb NOx any
more. In the embodiment of the invention, therefore, the air-fuel
ratio of the exhaust gas is temporarily made rich before the NOx
storage-reduction catalyst 28U reaches the full NOx adsorbing
capability (i.e., before the NOx storage reduction catalyst 28U is
saturated with NOx), so that NOx is released from the NOx
storage-reduction catalyst 28U, and reduced by HC, CO in the
exhaust gas, into N.sub.2, or the like.
[0057] Namely, in the embodiment of the invention, the amount of
NOx adsorbed per unit time by the NOx storage-reduction catalyst
28U is stored in advance in the ROM 32, in the form of a map as a
function of engine operating conditions, such as the engine load
factor KL and the engine speed Ne. By integrating the NOx amount, a
total value SN of the amount of NOx stored in the NOx
storage-reduction catalyst 28U is calculated. Then, each time the
total value SN of the stored NOx amount exceeds the upper limit
MAX, a rich-mode operation is temporarily performed in which an
air-fuel mixture having a rich air-fuel ratio is burned. As a
result, NOx is released from the NOx storage-reduction catalyst
28U, and is reduced.
[0058] FIG. 8 illustrates a routine for implementing engine
operation control according to the embodiment of the invention.
This routine is executed as an interrupt at predetermined time
intervals.
[0059] Referring to FIG. 8, it is initially determined in step 100
whether the engine load factor KL is larger than the predetermined
load factor KLX (FIG. 7). If KL.ltoreq.KLX, the control proceeds to
step 101 in which a lean-mode operation is performed. In the
following step 102, the total value SN of the stored NOx amount is
calculated. In the following step 103, it is determined whether the
total value SN of the stored NOx amount is larger than the upper
limit MAX. If SN.ltoreq.MAX, the current cycle of the routine of
FIG. 8 ends, and the lean-mode operation is continued. If
SN>MAX, on the other hand, the control proceeds to step 104, and
a rich-mode operation is performed, for example, for a given period
of time. In the following step 105, the total value SN of the
stored NOx amount is cleared. If it is determined in step 100 that
the engine load factor KL is larger than the predetermined load
factor KLX, the control proceeds to step 106 in which a
stoichiometric-ratio operation is performed.
[0060] According to the embodiment of the invention, the NOx
adsorbing capability of the catalyst 28 or the NOx
storage-reduction catalyst 28U can be enhanced.
[0061] FIG. 9A shows experimental results on the NOx storage
capacity ST of the catalyst 28. In Comparative Example Ca shown in
FIG. 9A, the catalyst 28 consists solely of a NOx storage-reduction
catalyst having a single-layer structure, and platinum (Pt) and
rhodium (Rh) are used as a noble-metal catalyst. In Example Ea1,
the catalyst 28 consists solely of a NOx storage-reduction catalyst
having a double-layer structure, and platinum (Pt) is used as a
noble-metal catalyst of the upper layer while rhodium (Rh) is used
as a noble-metal catalyst of the lower layer. In Example Ea2, the
catalyst 28 consists solely of a NOx storage-reduction catalyst
having a double-layer structure, and platinum (Pt) and palladium
(Pd) are used as a noble-metal catalyst of the upper layer while
rhodium (Rh) is used as a noble-metal catalyst of the lower
layer.
[0062] As is understood from FIG. 9A, the NOx storage capacity ST
of the catalyst 28 is relatively large in Examples Ea1, Ea2, and is
larger in Example Ea2 than in Example Ea1. This may be because the
NOx storage-reduction catalyst has a multi-layer structure, namely,
consists of two layers. Accordingly, the frequency at which the
air-fuel ratio of exhaust gas flowing into the catalyst 28 is
switched to the rich side (on which the air-fuel ratio is smaller
than the stoichiometric ratio) can be reduced, and fuel consumption
(i.e., the amount of fuel consumed) can be reduced.
[0063] When the air-fuel ratio A/F of the exhaust gas flowing into
the catalyst 28 is switched to the rich side as shown in FIG. 10,
the amount EXN of NOx discharged from the catalyst 28 per unit time
rapidly increases, reaches its peak value PKN, and then decreases.
In the embodiment of the invention, the peak value PKN of the
discharged NOx amount EXN can be reduced.
[0064] FIG. 9B shows experimental results on the peak value PKN of
the discharged NOx amount of the catalyst 28. In Comparative
Example Cb1 shown in FIG. 9B, the catalyst 28 consists solely of a
NOx storage-reduction catalyst having a single-layer structure, and
platinum (Pt) and rhodium (Rh) are used as a noble-metal catalyst.
In Comparative Example Cb2, the catalyst 28 consists solely of a
NOx storage-reduction catalyst having a double-layer structure, and
platinum (Pt) is used as a noble-metal component of the upper layer
while rhodium (Rh) is used as a noble-metal component of the lower
layer. In Example Eb, the catalyst 28 consists of an upstream
catalyst and a downstream catalyst. The upstream catalyst consists
of a NOx storage-reduction catalyst having a double-layer
structure, and platinum (Pt) is used as a noble-metal catalyst of
the upper layer while rhodium (Rh) is used as a noble-metal
catalyst of the lower layer. The downstream catalyst consists of a
three-way catalyst having a single-layer structure, and platinum
(Pt) and rhodium (Rh) are used as noble-metal components.
[0065] As is understood from FIG. 9B, the peak value PKN of the
discharged NOx amount is larger in Comparative Example Cb2 than
that of Comparative Example Cb1. In Example Eb, however, the peak
value PKN of the discharged NOx amount can be significantly
reduced. This may be because NOx released from the upstream
catalyst or NOx storage-reduction catalyst is reduced by the
downstream catalyst. Accordingly, the NOx conversion efficiency can
be held at a high level during lean-mode operation while assuring a
large NOx storage capacity.
[0066] Furthermore, according to the embodiment of the invention,
the NOx conversion efficiency EFFS of the catalyst 28 can be held
at a high level when the air-fuel ratio of exhaust gas flowing into
the catalyst 28 is substantially equal to the stoichiometric ratio,
for example, during high-load operation.
[0067] FIG. 9C shows experimental results on the NOx conversion
efficiency EFFS of the catalyst 28 when the air-fuel ratio of the
incoming exhaust gas is substantially equal to the stoichiometric
ratio. In Comparative Example Cc1 shown in FIG. 9C, the catalyst 28
consists solely of a NOx storage-reduction catalyst having a
single-layer structure, and platinum (Pt) and rhodium (Rh) are used
as a noble-metal catalyst. In Comparative Example Cc2, the catalyst
28 consists solely of a three-way catalyst having a double-layer
structure, and rhodium (Rh) is used as a noble-metal catalyst of
the upper layer while platinum (Pt) is used as a noble-metal
catalyst of the lower layer. In Example Ec, the catalyst 28
consists of an upstream catalyst and a downstream catalyst. The
upstream catalyst consists of a NOx storage-reduction catalyst
having a double-layer structure, and platinum (Pt) is used as a
noble-metal catalyst of the upper layer while rhodium (Rh) is used
as a noble-metal catalyst of the lower layer. The downstream
catalyst consists of a three-way catalyst having a single-layer
structure, and platinum (Pt) and rhodium (Rh) are used as
noble-metal components. Where INN represents the amount of NOx
flowing into the catalyst 28 per unit time, and EXN represents the
amount of NOx flowing out of the catalyst 28, the NOx conversion
efficiency EFFS of the catalyst 28 may be expressed by the
following equation:
EFFS=(INN-EXN)/INN
As is understood from FIG. 9C, the NOx conversion efficiency EFFS
of Example Ec is higher than that of Comparative Example Cc1, and
is substantially equal to that of Comparative Example of Cc2.
[0068] In the embodiment of the invention as described above, a
rich-mode operation (i.e., operating the engine at a rich air-fuel
ratio) is performed so as to make the air-fuel ratio of exhaust gas
flowing into the NOx storage-reduction catalyst 28U rich. However,
in an internal combustion engine provided with fuel injection
valves through which fuel is directly injected into combustion
chambers, the air-fuel ratio of the incoming exhaust gas may be
made rich by injecting fuel into the combustion chamber during the
expansion stroke or exhaust stroke. It is also possible to make the
air-fuel ratio of the incoming exhaust gas rich by supplying a
reductant or secondary fuel into an exhaust passage upstream of the
NOx storage-reduction catalyst 28U.
[0069] In the embodiment of the invention as described above, a
lean-mode operation is performed when the engine operates at a low
load, and a stoichiometric-ratio operation is performed when the
engine operates at a high load. However, a stoichiometric-ratio
operation may also be performed during acceleration.
[0070] While the invention has been described with reference to
example embodiments thereof, it is to be understood that the
invention is not limited to the described embodiments or
constructions. To the contrary, the invention is intended to cover
various modifications and equivalent arrangements. In addition,
while the various elements of the example embodiments are shown in
various combinations and configurations, other combinations and
configurations, including more, less or only a single element, are
also within the spirit and scope of the invention.
* * * * *