U.S. patent application number 11/660518 was filed with the patent office on 2007-12-06 for exhaust purification system of internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Takamitsu Asanuma, Kotaro Hayashi, Shunsuke Toshioka, Kohei Yoshida.
Application Number | 20070277513 11/660518 |
Document ID | / |
Family ID | 37771463 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070277513 |
Kind Code |
A1 |
Yoshida; Kohei ; et
al. |
December 6, 2007 |
Exhaust Purification System of Internal Combustion Engine
Abstract
An engine exhaust passage is provided with a fuel addition valve
(14) for adding fuel in the form of atomized drops, an oxidation
catalyst (11), and an NO.sub.x storage and reduction catalyst (12)
in that order. The oxidation catalyst (11) carries as a precious
metal not only platinum Pt, but also palladium Pd. A molar ratio of
the platinum Pt with respect to the sum of the platinum Pt and
palladium Pd carried on the oxidation catalyst (11) is set to
between about 50 percent to about 80 percent.
Inventors: |
Yoshida; Kohei; (Shizuoka,
JP) ; Hayashi; Kotaro; (Shizuoka, JP) ;
Asanuma; Takamitsu; (Shizuoka, JP) ; Toshioka;
Shunsuke; (Shizuoka, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
471-8571
|
Family ID: |
37771463 |
Appl. No.: |
11/660518 |
Filed: |
August 9, 2006 |
PCT Filed: |
August 9, 2006 |
PCT NO: |
PCT/JP06/16059 |
371 Date: |
February 20, 2007 |
Current U.S.
Class: |
60/299 |
Current CPC
Class: |
F01N 13/009 20140601;
F01N 3/0814 20130101; B01D 53/9422 20130101; F01N 3/2033 20130101;
B01D 2255/1023 20130101; Y02T 10/12 20130101; B01D 2255/904
20130101; F01N 3/0864 20130101; B01D 2251/208 20130101; F01N 3/103
20130101; F01N 3/0842 20130101; F02B 37/00 20130101; F01N 3/035
20130101; B01D 2255/1021 20130101; B01D 2255/915 20130101; Y02T
10/26 20130101 |
Class at
Publication: |
060/299 |
International
Class: |
F01N 3/10 20060101
F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 24, 2005 |
JP |
2005-242182 |
Claims
1. An internal combustion engine providing a fuel addition device
adding fuel in the form of atomized drops inside an engine exhaust
passage, providing an oxidation catalyst inside the engine exhaust
passage downstream of the fuel addition device, providing an
NO.sub.x storage and reduction catalyst storing the NO.sub.x
contained in the exhaust gas when the air-fuel ratio of the
inflowing exhaust gas is lean and releasing the stored NO.sub.x
when the air-fuel ratio of the inflowing exhaust gas becomes a
stoichiometric air-fuel ratio or rich inside the engine exhaust
passage downstream of the oxidation catalyst, adding fuel from the
fuel addition device when making the air-fuel ratio of the exhaust
gas flowing into the NO.sub.x storage and reduction catalyst rich
so as to make the NO.sub.x storage and reduction catalyst release
NO.sub.x, and making the fuel added at this time deposit in the
form of drops on the oxidation catalyst, wherein to make the fuel
deposited in the form of drops on the oxidation catalyst vaporize,
the oxidation catalyst carries as a precious metal not only
platinum Pt, but also palladium Pd and the molar ratio of the
platinum Pt with respect to the sum of the platinum Pt and
palladium Pd carried on the oxidation catalyst is set from about 50
percent to about 80 percent.
2. An exhaust purification system of internal combustion engine as
set forth in claim 1, wherein a molar ratio of platinum Pt with
respect to the sum of the platinum Pt and palladium Pd carried on
the oxidation catalyst is set to about 58 percent to about 75
percent.
3. An exhaust purification system of internal combustion engine as
set forth in claim 1, wherein a carrier of the oxidation catalyst
is comprised of a basic carrier including at least one element
selected from an alkali earth or rare earth.
4. An exhaust purification system of internal combustion engine as
set forth in claim 1, wherein the NO.sub.x storage and reduction
catalyst is carried on a particulate filter.
5. An internal combustion engine providing a fuel addition device
adding fuel in the form of atomized drops inside an engine exhaust
passage, providing an NO.sub.x storage and reduction catalyst
storing the NO.sub.x contained in the exhaust gas when the air-fuel
ratio of the inflowing exhaust gas is lean and releasing the stored
NO.sub.x when the air-fuel ratio of the inflowing exhaust gas
becomes a stoichiometric air-fuel ratio or rich inside the engine
exhaust passage downstream of the oxidation catalyst, adding fuel
from the fuel addition device when making the air-fuel ratio of the
exhaust gas flowing into the NO.sub.x storage and reduction
catalyst rich so as to make the NO.sub.x storage and reduction
catalyst release NO.sub.x, and making the fuel added at this time
deposit in the form of drops on the NO.sub.x storage and reduction
catalyst, wherein to make the fuel deposited in the form of drops
on the NO.sub.x storage and reduction catalyst vaporize, the
NO.sub.x storage and reduction catalyst carries as a precious metal
not only platinum Pt, but also palladium Pd and the molar ratio of
the platinum Pt with respect to the sum of the platinum Pt and
palladium Pd carried on the NO.sub.x storage and reduction catalyst
is set from about 50 percent to about 80 percent.
6. An exhaust purification system of internal combustion engine as
set forth in claim 5, wherein a molar ratio of platinum Pt with
respect to the sum of the platinum Pt and palladium Pd carried on
the NO.sub.x storage and reduction catalyst is set to about 58
percent to about 75 percent.
7. An exhaust purification system of internal combustion engine as
set forth in claim 5, wherein the NO.sub.x storage and reduction
catalyst is carried on a particulate filter.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust purification
system of an internal combustion engine.
BACKGROUND ART
[0002] Known in the art is an internal combustion engine provided
with an NO.sub.x storage and reduction catalyst storing the NO
contained in exhaust gas when the air-fuel ratio of the exhaust gas
flowing into the engine exhaust passage is lean and releasing the
stored NO.sub.x when the air-fuel ratio of the inflowing exhaust
gas becomes the stoichiometric air-fuel ratio or rich. This
NO.sub.x storage and reduction catalyst includes a precious metal
catalyst comprised of platinum Pt and an NO.sub.x absorbent. When
the air-fuel ratio is lean, the NO.sub.x contained in the exhaust
gas, that is, the NO, is oxidized on the platinum Pt to NO.sub.2,
then is stored in the form of nitrate ions NO.sub.3.sup.- in the
NO.sub.x absorbent.
[0003] On the other hand, when releasing and reducing the NO.sub.x
absorbed from the NO.sub.x absorbent, the air-fuel ratio of the
exhaust gas flowing into the NO.sub.x storage and reduction
catalyst is made rich. If the air-fuel ratio of the exhaust gas is
made rich, the oxygen concentration in the exhaust gas falls, so
the NO.sub.x absorbed in the NO.sub.x absorbent in the form of
nitrate ions NO.sub.3.sup.- appears in the form of NO.sub.2 on the
platinum Pt from inside the NO.sub.x absorbent. This NO.sub.2 is
reduced by the unburnt HC and CO contained in the exhaust gas.
[0004] Note that the air-fuel ratio of the exhaust gas is made rich
by supplying additional fuel into the combustion chamber or by
adding additional fuel into the engine exhaust passage. In this
case, when the added fuel is gaseous and flows into the NO.sub.x
storage and reduction catalyst, if the air-fuel ratio of the
exhaust gas is made rich, the NO.sub.x is released from the
NO.sub.x storage and reduction catalyst and reduced. However, when
the air-fuel ratio of the exhaust gas should be made rich,
additional fuel is added into the engine exhaust passage in the
form of atomized drops. The situation differs somewhat when this
added fuel deposits in the form of drops on the NO.sub.x storage
and reduction catalyst.
[0005] That is, if the fuel to be added when the air-fuel ratio of
the exhaust gas should be made rich deposits in the form of drops
on the NO.sub.x storage and reduction catalyst, the platinum Pt on
the NO.sub.x storage and reduction catalyst is covered by the fuel
drops. However, if the platinum Pt is covered by the fuel drops,
the oxygen in the exhaust gas cannot reach the surface of the
platinum Pt and as a result the oxidation reaction of the fuel
drops on the platinum Pt can no longer be performed well. If the
oxidation reaction of the fuel drops cannot be performed well, the
oxygen in the exhaust gas will not be sufficiently consumed and as
a result the oxygen concentration in the exhaust gas will not
sufficiently fall, so NO.sub.x will no longer be released well from
the NO.sub.x absorbent. Further, the fuel drops will insufficiently
vaporize, so the amount of the gaseous state unburned HC contained
in the exhaust gas will not be sufficient and therefore the
released NO.sub.x will not be able to be sufficiently reduced.
[0006] Therefore, the inventors took note of the oxygen storing
capability of palladium Pd in the process of their research and
discovered that by using as a precious metal not only platinum Pt,
but also palladium Pd, the large amount of oxygen stored on the
palladium Pd will promote the oxidation reaction of the fuel drops,
the heat of this oxidation reaction will promote the vaporization
of the fuel drops on the platinum Pt, and therefore the NO.sub.x
release action from the NO.sub.x absorbent will be promoted.
[0007] In this case, if increasing the amount of palladium Pd and
decreasing the amount of platinum Pt, the heat of the oxidation
reaction of the oxygen stored in the palladium Pd will promote the
vaporization of the fuel drops on the platinum Pt, but due to the
small amount of platinum Pt, the release action of NO.sub.x will be
weak and as a result an excellent NO.sub.x release action will not
be able to be obtained. As opposed to this, if reducing the amount
of palladium Pd and increasing the amount of platinum Pt, the
vaporization of the fuel drops on the platinum Pt will not be
sufficiently promoted due to the heat of the oxidation reaction of
the stored oxygen of the palladium Pd, so even if increasing the
amount of platinum Pt, only a weak NO.sub.x release action will be
obtained and, therefore, in this case as well, an excellent
NO.sub.x release action will not be obtained.
[0008] That is, the ratio of platinum Pt and palladium Pd giving an
excellent NO.sub.x release action exists in a ratio of suitable
ratios neither extremely large nor extremely small. Regarding this
point, a known diesel particulate filter (see Japanese Patent
Publication (A) No. 2003-205245) is designed to carry platinum Pt
and palladium Pd in amounts of 1 g each per liter volume of the
filter body. This becomes about 35.7 in terms of the molar ratio of
platinum Pt with respect to the sum of the platinum Pt and
palladium Pd. However, with this molar ratio, the amount of
palladium Pd is too great compared with the platinum Pt and as a
result an excellent NO.sub.x release action cannot be obtained.
DISCLOSURE OF THE INVENTION
[0009] The present invention provides a ratio of platinum Pt and
palladium Pd enabling an excellent NO.sub.x release action to be
secured even if the platinum Pt is covered by the fuel drops when
the fuel is added in the form of drops so as to release NO.sub.x
from the NO.sub.x storage and reduction catalyst.
[0010] That is, according to the present invention, there is
provided an internal combustion engine providing a fuel addition
device adding fuel in the form of atomized drops inside an engine
exhaust passage, providing an oxidation catalyst inside the engine
exhaust passage downstream of the fuel addition device, providing
an NO.sub.x storage and reduction catalyst storing the NO.sub.x
contained in the exhaust gas when the air-fuel ratio of the
inflowing exhaust gas is lean and releasing the stored NO.sub.x
when the air-fuel ratio of the inflowing exhaust gas becomes a
stoichiometric air-fuel ratio or rich inside the engine exhaust
passage downstream of the oxidation catalyst, adding fuel from the
fuel addition device when making the air-fuel ratio of the exhaust
gas flowing into the NO.sub.x storage and reduction catalyst rich
so as to make the NO.sub.x storage and reduction catalyst release
NO.sub.x, and making the fuel added at this time deposit in the
form of drops on the oxidation catalyst, wherein to make the fuel
deposited in the form of drops on the oxidation catalyst vaporize,
the oxidation catalyst carries as a precious metal not only
platinum Pt, but also palladium Pd and the molar ratio of the
platinum Pt with respect to the sum of the platinum Pt and
palladium Pd carried on the oxidation catalyst is set from about 50
percent to about 80 percent.
[0011] Further, according to the present invention, there is
provided an internal combustion engine providing a fuel addition
device adding fuel in the form of atomized drops inside an engine
exhaust passage, providing an NO.sub.x storage and reduction
catalyst storing the NO.sub.x contained in the exhaust gas when the
air-fuel ratio of the inflowing exhaust gas is lean and releasing
the stored NO.sub.x when the air-fuel ratio of the inflowing
exhaust gas becomes a stoichiometric air-fuel ratio or rich inside
the engine exhaust passage downstream of the oxidation catalyst,
adding fuel from the fuel addition device when making the air-fuel
ratio of the exhaust gas flowing into the NO.sub.x storage and
reduction catalyst rich so as to make the NO.sub.x storage and
reduction catalyst release NO.sub.x, and making the fuel added at
this time deposit in the form of drops on the NO.sub.x storage and
reduction catalyst, wherein to make the fuel deposited in the form
of drops on the NO.sub.x storage and reduction catalyst vaporize,
the NO.sub.x storage and reduction catalyst carries as a precious
metal not only platinum Pt, but also palladium Pd and the molar
ratio of the platinum Pt with respect to the sum of the platinum Pt
and palladium Pd carried on the NO.sub.x storage and reduction
catalyst is set from about 50 percent to about 80 percent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is an overall view of a combustion ignition type
internal combustion engine,
[0013] FIG. 2 is an overall view of another embodiment of a
combustion ignition type internal combustion engine,
[0014] FIG. 3 are views showing a particulate filter,
[0015] FIG. 4 are views for explaining an absorption/release action
of NO.sub.x,
[0016] FIG. 5 is a side cross-sectional view of an oxidation
catalyst,
[0017] FIG. 6 are cross-sectional views illustrating a surface part
of a substrate of an oxidation catalyst,
[0018] FIG. 7 is a view showing the relationship between an
oxidation speed and a platinum molar ratio,
[0019] FIG. 8 is a view showing the relationship between an
NO.sub.x purification rate and a temperature of the oxidation
catalyst,
[0020] FIG. 9 is a view showing the relationship between an
NO.sub.x purification rate and a platinum molar ratio,
[0021] FIG. 10 is a view of a comparative example,
[0022] FIG. 11 is a time chart of NO.sub.x release processing,
[0023] FIG. 12 is a view of a map of a per unit time NO.sub.x
storage amount,
[0024] FIG. 13 is a flowchart for exhaust purification processing,
and
[0025] FIG. 14 is a view of another embodiment.
BEST MODE FOR CARRYING OUT THE INVENTION
[0026] FIG. 1 is an overall view of a combustion ignition type
internal combustion engine.
[0027] Referring to FIG. 1, 1 indicates an engine body, 2 a
combustion chamber of a cylinder, 3 an electronic control type fuel
injector for injecting fuel into a combustion chamber 2, 4 an
intake manifold, and 5 an exhaust manifold. The intake manifold 4
is connected through an intake duct 6 to an outlet of a compressor
7a of the exhaust turbocharger 7, while an inlet of the compressor
7a is connected to an air cleaner 8. Inside the intake duct 6 is
provided a throttle valve 9 driven by a step motor. Further, around
the intake duct 6 is provided a cooling device for cooling the
intake air circulating inside the intake duct 6. In the embodiment
shown in FIG. 1, engine cooling water is led inside the cooling
device 10, where the engine cooling water cools the intake air.
[0028] On the other hand, the exhaust manifold 5 is connected to an
inlet of an exhaust turbine 7b of the exhaust turbocharger 7, while
an outlet of the exhaust turbine 7b is connected to an inlet of an
oxidation catalyst 11. Further, the outlet of the oxidation
catalyst 11 is connected through an exhaust pipe 13 to an NO.sub.x
storage and reduction catalyst 12. The exhaust manifold 5 mounts a
fuel addition valve 14 for adding mist-like, that is, particulates
of, fuel, in the form of drops into the exhaust gas. In the
embodiment according to the present invention, this fuel is
comprised of diesel oil.
[0029] The exhaust manifold 5 and the intake manifold 4 are
connected to each other through an exhaust gas recirculation
(hereinafter referred to as the "EGR") passage 15. Inside the EGR
passage 15 is arranged an electronic control type EGR control valve
16. Further, around the EGR passage 15 is arranged a cooling device
17 for cooling the EGR gas flowing through the inside of the EGR
passage 15. In the embodiment shown in FIG. 1, the engine cooling
water is led to the inside of the cooling device 17 where the
engine cooling water then cools the EGR gas. On the other hand,
each fuel injector 3 is connected through a fuel tube 18 to a
common rail 19. This common rail 19 is fed with fuel from an
electronic control type variable discharge fuel pump 20. The fuel
fed into the common rail 19 is supplied through each fuel tube 18
to each fuel injector 3.
[0030] An electronic control unit 30 is comprised of a digital
computer provided with a ROM (read only memory) 32, RAM (random
access memory) 33, CPU (microprocessor) 34, input port 35, and
output port 36 connected to each other by a bi-directional bus 31.
The NO.sub.x storage and reduction catalyst 12 is provided with a
differential pressure sensor 21 for detecting the pressure
difference before and after the NO.sub.x storage and reduction
catalyst 12. The output signal of this differential pressure sensor
21 is input through a corresponding AD converter 37 to an input
port 35. Further, an acceleration pedal 40 has connected to it a
load sensor 41 generating an output voltage proportional to the
amount of depression L of the acceleration pedal 40. The output
voltage of the load sensor 41 is input through a corresponding AD
converter 37 to the input port 35. Further, the input port 35 has
connected to it a crank angle sensor 42 generating an output pulse
each time a crankshaft for example rotates by 15.degree.. On the
other hand, the output port 36 is connected through a corresponding
drive circuit 38 to the fuel injector 3, throttle valve 9 drive
step motor, fuel addition valve 14, EGR control valve 16, and a
fuel pump 20.
[0031] FIG. 2 shows another embodiment of a combustion ignition
type internal combustion engine. In this embodiment, the engine
exhaust passage is not provided with any oxidation catalyst. The
outlet of the exhaust turbine 7b is connected to the inlet of the
NO.sub.x storage and reduction catalyst 12.
[0032] First, explaining the NO.sub.x storage and reduction
catalysts 12 shown in FIG. 1 and FIG. 2, these NO.sub.x storage and
reduction catalysts 12 are carried on three-dimensional net
structure monolith carriers or pellet-shaped carriers or are
carried on a particulate filter forming a honeycomb structure. In
this way, the NO.sub.x storage and reduction catalyst 12 can be
carried on various carriers, but below, the case of carrying the
NO.sub.x storage and reduction catalyst 12 on a particulate filter
will be explained.
[0033] FIGS. 3(A) and (B) show the structure of a particulate
filter 12a carrying an NO.sub.x storage and reduction catalyst 12.
Note that FIG. 3(A) shows the front view of the particulate filter
12a, while FIG. 3(B) shows the side cross-sectional view of the
particulate filter 12a. As shown in FIGS. 3(A) and (B), the
particulate filter 12a forms a honeycomb structure provided with a
plurality of exhaust passages 60, 61 extending in parallel. These
exhaust passages are comprised of exhaust gas inflow passages 60
with downstream ends sealed by plugs 62 and exhaust gas outflow
passages 61 with upstream ends sealed by plugs 63. Note that in
FIG. 3(A), the hatched parts show the plugs 63. Therefore, the
exhaust gas inflow passages 60 and exhaust gas outflow passages 61
are alternately arranged across thin partition walls 64. In other
words, the exhaust gas inflow passages 60 and exhaust gas outflow
passages 61 are arranged so that each exhaust gas inflow passage 60
is surrounded by four exhaust gas outflow passages 61 and each
exhaust gas outflow passage 61 is surrounded by four exhaust gas
inflow passages 60.
[0034] The particulate filter 12a is for example formed from a
porous material such as cordierite. Therefore, the exhaust gas
flowing into the exhaust gas inflow passage 60, as shown by the
arrows in FIG. 3(B), flows out into the adjoining exhaust gas
outflow passages 61 through the surrounding partition walls 64.
[0035] When carrying the NO.sub.x storage and reduction catalyst 12
on the particulate filter 12a in this way, the partition walls of
the exhaust gas inflow passages 60 and the exhaust gas outflow
passages 61, that is, the two surfaces of the partition walls 64
and the walls inside the holes in the partition walls 64, carry a
catalyst carrier comprised of for example alumina. FIGS. 4(A) and
(B) schematically show cross-sections of the surface part of this
catalyst carrier 45. As shown in FIGS. 4(A) and (B), the surface of
the catalyst carrier 45 carries a precious metal catalyst 46
dispersed on them. Further, the surface of the catalyst carrier 45
is formed with a layer of an NO.sub.x absorbent 47.
[0036] In this embodiment according to the present invention,
platinum Pt is used as the precious metal catalyst 46, while as the
ingredient forming the NO.sub.x absorbent 47, at least one element
for example selected from potassium K, sodium Na, cesium Cs, or
another such alkali metal, barium Ba, calcium Ca, or another such
alkali earth, and lanthanum La, yttrium Y, or another such rare
earth is used.
[0037] If referring to the ratio of the air and fuel (hydrocarbons)
supplied into the engine intake passage, combustion chamber 2, and
exhaust passage upstream of the NO.sub.x storage and reduction
catalyst 12 as the "air-fuel ratio of the exhaust gas", the
NO.sub.x absorbent 47 performs the NO.sub.x absorption/release
action of absorbing the NO.sub.x when the air-fuel ratio of the
exhaust gas is lean and releasing the absorbed NO.sub.x when the
oxygen concentration in the exhaust gas falls.
[0038] That is, explaining the case of use of barium Ba as an
ingredient forming the NO.sub.x absorbent 47 as an example, when
the air-fuel ratio of the exhaust gas is lean, that is, when the
exhaust gas has a high oxygen concentration, the NO contained in
the exhaust gas, as shown in FIG. 4(A), is oxidized on the platinum
Pt 46 to become NO.sub.2, then is absorbed in the NO.sub.x
absorbent 47 and bonds with the barium oxide BaO while dispersing
in the form of nitrate ions NO.sub.3.sup.- into the NO.sub.x
absorbent 47. In this way, NO.sub.x is absorbed in the NO.sub.x
absorbent 47. So long as the oxygen concentration in the exhaust
gas is high, NO.sub.2 is formed on the surface of the platinum Pt
46. So long as the NO.sub.x absorption capability of the NO.sub.x
absorbent 47 is not saturated, the NO.sub.2 is absorbed in the
NO.sub.x absorbent 47 and nitrate ions NO.sub.3.sup.- are
generated.
[0039] As opposed to this, if the air-fuel ratio of the exhaust gas
is made rich or the stoichiometric air-fuel ratio, the oxygen
concentration in the exhaust gas falls, so the reaction proceeds in
the opposite direction (NO.sub.3.sup.-.fwdarw.NO.sub.2) and
therefore, as shown in FIG. 4(B), the nitrate ions NO.sub.3.sup.-
in the NO.sub.x absorbent 47 are released in the form of NO.sub.2
from the NO.sub.x absorbent 47. Next, the released NO.sub.x is
reduced by the unburned HC and CO contained in the exhaust gas.
[0040] When the air-fuel ratio of the exhaust gas is lean in this
way, that is, when combustion occurs under a lean air-fuel ratio,
the NO.sub.x in the exhaust gas is absorbed in the NO.sub.x
absorbent 47. However, if combustion is performed continuously
under a lean air-fuel ratio, the NO.sub.x absorption capability of
the NO.sub.x absorbent 47 ends up becoming saturated. Therefore,
NO.sub.x ends up no longer being able to be absorbed by the
NO.sub.x absorbent 47. Therefore, in the embodiment according to
the present invention, fuel is added from the fuel addition valve
14 before the absorption capability of the NO.sub.x absorbent 47
becomes saturated so as to make the air-fuel ratio of the exhaust
gas temporarily rich and thereby make the NO.sub.x absorbent 47
release the NO.sub.x.
[0041] Next, the oxidation catalyst 11 arranged at the upstream
side of the NO.sub.x storage and reduction catalyst 12 in FIG. 1
will be explained.
[0042] FIG. 5 shows a side cross-sectional view of the oxidation
catalyst 11. As shown in FIG. 5, the oxidation catalyst 11 forms a
honeycomb structure which is provided with a plurality of exhaust
gas passages 65 extending straight. The substrate of this oxidation
catalyst 11 is comprised of alumina, zirconia, or another complex
oxide. FIGS. 6(A) and (B) show cross-sections of the surface part
of the substrate of the oxidation catalyst 11. As shown in FIGS.
6(A) and (B), the surface of the substrate 50 carries the platinum
Pt shown by 51 and the palladium Pd shown by 52 dispersed on
it.
[0043] Platinum Pt has the property of trapping oxygen on its
surface, but the amount of oxygen which can be trapped is small. As
opposed to this, palladium Pd has the ability to trap a far larger
amount of oxygen than platinum Pt. Therefore, when the air-fuel
ratio of the exhaust gas is lean, as shown in FIG. 6(A), a far
greater amount of oxygen is trapped and stored on the palladium Pd
compared with the platinum Pt. On the other hand, if viewed from
the point of the oxidizing ability, platinum Pt has a much stronger
oxidizing ability, while the oxidizing ability of palladium Pd is
weak. In this way, platinum Pt and palladium Pd differ considerably
in properties.
[0044] Further, as explained above, if adding fuel from the fuel
addition valve 14 to make the air-fuel ratio of the exhaust gas
rich, the NO.sub.x absorbent 47 releases NO.sub.x and the released
NO.sub.x is reduced by the unburned HC and CO contained in the
exhaust gas. In this case, if the added fuel is liquid in state,
even if stoichiometrically the air-fuel ratio of the exhaust gas
becomes rich, the oxygen concentration in the exhaust gas will not
fall, so the NO.sub.x absorbent 47 will not release NO.sub.x.
However, in the invention of the present application, even if the
added fuel is liquid in state, the NO.sub.x absorbent 47 can be
made to release the NO.sub.x well.
[0045] That is, part of the fuel added from the fuel addition valve
14 becomes a gas, but the majority of the fuel flows in the form of
drops through the exhaust passage together with the exhaust gas,
then the fuel drops deposit on the oxidation catalyst 11. As a
result, as shown in FIG. 6(B), the platinum Pt and palladium Pd are
covered by the fuel drops 53. If the platinum Pt is covered by the
fuel drops 53, the oxygen included in the exhaust gas is blocked by
the deposited fuel drops 53 and cannot reach the surface of the
platinum Pt. Therefore, if taking note of only the platinum Pt, no
matter how strong the oxidizing ability of the platinum Pt, the
oxidation reaction of the fuel drops 53 will not proceed that much
and therefore the fuel drops 53 will not vaporize that much.
[0046] As opposed to this, the palladium Pd stores a large amount
of oxygen on its surface, so if the palladium Pd is covered by the
fuel drops 53, the fuel drops 53 will be oxidized by the larger
amount of oxygen on the palladium Pd. At this time, a large amount
of heat of oxidation reaction is produced. This heat of oxidation
reaction causes the fuel drops 53 covering the palladium Pd of
course and also the fuel drops 53 covering the platinum Pt to be
vaporized. If the fuel drops 53 covering the platinum Pt are
vaporized, the oxygen in the exhaust gas can reach the surface of
the platinum Pt and as a result the oxidation reaction of the
unburned HC and CO on the platinum Pt becomes brisk. As a result,
the oxygen concentration in the exhaust gas falls, so the NO.sub.x
absorbent 47 releases NO.sub.x and the vaporized unburned HC and CO
reduce the released NO.sub.x.
[0047] By having the oxidation catalyst 11 carry not only platinum
Pt, but also palladium Pd in this way, the NO.sub.x absorbent 47
can be made to release and reduce the NO.sub.x. However, when the
sum of the platinum Pt and palladium Pd is fixed, if increasing the
amount of palladium Pd and decreasing the amount of platinum Pt,
the vaporization of the fuel drops 53 on the platinum Pt is
promoted by the heat of oxidation reaction of the stored oxygen of
the palladium Pd, but since the amount of platinum Pt is small, the
unburned HC and CO are not sufficiently oxidized and, as a result,
an excellent NO.sub.x release action cannot be obtained.
[0048] As opposed to this, if reducing the amount of the palladium
Pd and increasing the amount of the platinum Pt, the vaporization
of the fuel drops 53 on the platinum Pt by the heat of oxidation
reaction of the stored oxygen of the palladium Pd is not
sufficiently promoted, so even if increasing the amount of platinum
Pt, sufficient amounts of unburned HC and CO are not oxidized,
therefore, in this case as well, an excellent NO.sub.x release
action cannot be obtained. That is, the ratio of platinum Pt and
palladium Pd giving an excellent NO.sub.x release action exists in
a ratio of suitable ratios neither extremely large nor extremely
small
[0049] FIG. 7 shows experimental results showing the relationship
between the oxidation speed showing the amount of oxidation per
unit time and the ratio of the number of moles of platinum Pt with
respect to the sum of the number of moles of the platinum Pt and
the number of moles of the palladium Pd (hereinafter referred to as
the "platinum molar ratio"). In FIG. 7, the higher the oxidation
speed, the better the action of release of the NO.sub.x from the
NO.sub.x absorbent 47, therefore, as shown in FIG. 7, the NO.sub.x
release action becomes the best when the platinum molar ratio is
about 66 percent.
[0050] FIG. 8 shows experimental results showing the relationship
between the NO.sub.x purification rate after the end of the
NO.sub.x release action and the temperature Tc of the oxidation
catalyst 11. Note that in FIG. 8, the black dots show the case
where the oxidation catalyst 11 carries only platinum Pt, that is,
the case where the platinum molar ratio is 100 percent, while the
white dots show the case where the platinum molar ratio is 66
percent. Both when the platinum molar ratio is 100 percent and is
66 percent, as the temperature Tc of the oxidation catalyst 11
becomes higher, the NO.sub.x purification rate becomes higher, but
no matter what the temperature Tc, it is learned that the NO.sub.x
purification rate is higher when the platinum molar ratio is 66
percent than when the platinum molar ratio is 100 percent.
[0051] FIG. 9 shows the relationship between the NO.sub.x
purification rate and platinum molar ratio when the temperature Tc
of the oxidation catalyst 11 is about 350.degree. C. in FIG. 8. The
pattern of change of the NO.sub.x purification rate shown in FIG. 9
has the same trend as the pattern of change of the oxidation speed
shown in FIG. 7. As shown in FIG. 9, the NO.sub.x purification rate
becomes highest when the platinum molar ratio is about 66 percent.
Therefore, the amounts of the platinum Pt and palladium Pd to be
carried on the oxidation catalyst 11 can be said to be most
preferably determined so that the platinum molar ratio becomes
about 66 percent.
[0052] Note that even if the NO.sub.x purification rate becomes
lower than the maximum NO.sub.x purification rate, if the degree of
drop is about 5 percent, it still can be said that the rate is
about the maximum NO.sub.x purification rate. The range of the
platinum molar ratio where the NO.sub.x purification rate can be
said to be about the maximum NO.sub.x purification rate in this way
becomes between about 58 percent to about 75 percent as shown by X
in FIG. 9. Therefore, the platinum molar ratio can be said to be
preferably set between about 58 percent to about 75 percent.
[0053] Note that NO.sub.x purification rate can be practically used
even if about 10 percent lower than the maximum NO.sub.x
purification rate. The range of the platinum molar ratio where the
NO.sub.x purification rate becomes 10 percent lower than the
maximum NO.sub.x purification rate becomes, as shown by Y in FIG.
9, between about 50 percent to about 80 percent. Therefore, when
viewed practically, the platinum molar ratio should be set between
about 50 percent to 80 percent.
[0054] In the embodiment shown in FIG. 2, the catalyst carrier 45
of the NO.sub.x storage and reduction catalyst 12 (FIG. 4) carries
not only platinum Pt but also palladium Pd. The NO.sub.x
purification rate in this case also becomes about the same as the
NO.sub.x purification rate shown in FIG. 9. Therefore, in this case
as well, the NO.sub.x purification rate becomes highest when the
platinum molar ratio is about 66 percent. Therefore, the amounts of
the platinum Pt and palladium Pd to be carried on the NO.sub.x
storage and reduction catalyst 12 can be said to be most preferably
determined so that the platinum molar ratio becomes about 66
percent.
[0055] Further, in the embodiment shown in FIG. 2 as well, the
range of the platinum molar ratio where the NO.sub.x purification
rate can be said to be about the maximum NO.sub.x purification rate
is between about 58 percent to about 75 percent. Therefore, in this
embodiment as well, it can be said that the platinum molar ratio is
preferably set to about 58 percent to about 75 percent.
[0056] Further, in the embodiment shown in FIG. 2 as well, the
range of the platinum molar ratio where the NO.sub.x purification
rate becomes 10 percent lower than the maximum NO.sub.x
purification rate, that is, the range of the platinum molar ratio
which can be used when viewed practically, becomes about 50 percent
to about 80 percent, therefore, in this embodiment as well, when
viewed from a practical perspective, the platinum molar ratio may
be set between about 50 percent to 80 percent.
[0057] FIG. 10 shows the NO.sub.x concentration in the exhaust gas
flowing out from the NO.sub.x storage and reduction catalyst 12 in
the case where for example, in a gasoline engine, the air-fuel
ratio in the combustion chamber is made rich, that is, the air-fuel
ratio of the exhaust gas itself forming the gas state is made rich,
for the case where the oxidation catalyst 11 carries only platinum
Pt and the case where it carries platinum Pt and palladium Pd. As
will be understood from FIG. 10, the NO.sub.x concentration when
the air-fuel ratio of the exhaust gas should be made rich is lower
when the oxidation catalyst 11 carries only platinum Pt, but
becomes higher when the oxidation catalyst 11 carries platinum Pt
and palladium Pd.
[0058] That is, if the air-fuel ratio of the exhaust gas is made
rich, the oxygen concentration in the exhaust gas falls, so the
NO.sub.x absorbent 47 releases NO.sub.x. However, if the oxidation
catalyst 11 carries palladium Pd, the unburned HC and CO contained
in the exhaust gas are oxidized by the large amount of oxygen
stored on the palladium Pd and the released NO.sub.x can no longer
be reduced by the unburned HC and CO. As a result, when the
oxidation catalyst 11 carries platinum Pt and palladium Pd, the
NO.sub.x concentration becomes higher.
[0059] That is, when the NO.sub.x absorbent 47 should release
NO.sub.x, when the air-fuel ratio of the exhaust gas is made rich
in the gas state, if the oxidation catalyst 11 is made to carry
palladium Pd, a large amount of NO.sub.x is exhausted into the
atmosphere and therefore the NO.sub.x purification rate ends up
falling. However, even if using the same oxidation catalyst 11
carrying palladium Pd, when the NO.sub.x absorbent 47 should
release NO.sub.x, a high NO.sub.x purification rate is obtained
opposite to the case where fuel is added to the exhaust gas in the
form of drops. If viewed from the point of the purification of
NO.sub.x in this way, the palladium Pd acts effectively when the
fuel is added in the form of drops.
[0060] Next, the NO.sub.x release control will be explained with
reference to FIG. 11.
[0061] FIG. 11 shows the changes in the NO.sub.x amount .SIGMA.NOX
stored in the NO.sub.x storage and reduction catalyst 12 and the
timing of adding fuel for release of NO.sub.x to make the air-fuel
ratio A/F of the exhaust gas rich. The NO.sub.x amount exhausted
from the engine per unit time changes in accordance with the engine
operating state. Therefore, the NO.sub.x amount stored in the
NO.sub.x storage and reduction catalyst 12 per unit time also
changes in accordance with the engine operating state. In the
embodiment according to the present invention, the NO.sub.x amount
NOXA stored in NO.sub.x storage and reduction catalyst 12 per unit
time is stored as a function of the required torque TQ and engine
rotational speed N in the form of the map shown in FIG. 12 in
advance in the ROM 32. By cumulatively adding this NO.sub.x amount
NOXA, the NO.sub.x amount .SIGMA.NOX stored in the NO.sub.x storage
and reduction catalyst 12 is calculated.
[0062] On the other hand, in FIG. 11, MAX indicates the maximum
NO.sub.x storage amount which the NO.sub.x storage and reduction
catalyst 12 can store, while NX indicates the allowable value of
the NO.sub.x amount which the NO.sub.x storage and reduction
catalyst 12 can be made to store. Therefore, as shown in FIG. 11,
if the NO.sub.x amount .SIGMA.NOX reaches the allowable value NX,
fuel is added, whereby the air-fuel ratio A/F of the exhaust gas
flowing into the NO.sub.x storage and reduction catalyst 12 is
temporarily made rich and therefore the NO.sub.x storage and
reduction catalyst 12 releases NO.sub.x.
[0063] On the other hand, the particulate matter contained in the
exhaust gas is trapped and successively oxidized on the particulate
filter 12a carrying the NO.sub.x storage and reduction catalyst 12.
However, if the amount of the particulate matter deposited becomes
greater than the particulate matter oxidized, the particulate
matter will gradually build up on the particulate filter 12a. In
this case, if the deposited amount of the particulate matter
increases, a drop in the engine output will end up being caused.
Therefore, when the deposited amount of the particulate matter
increases, the deposited particulate matter must be removed. In
this case, if raising the temperature of the particulate filter 12a
under an excess of air to about 600.degree. C., the deposited
particulate matter is removed by oxidation.
[0064] Therefore, in the embodiment according to the present
invention, when the amount of the particulate matter deposited on
the particulate filter 12a exceeds the allowable amount, the
temperature of the particulate filter 12a is raised under a lean
air-fuel ratio of the exhaust gas, whereby the deposited
particulate matter is removed by oxidation. Specifically speaking,
in the embodiment according to the present invention, when the
differential pressure .DELTA.P before and after the particulate
filter 12a detected by the differential pressure sensor 21 exceeds
the allowable value PX, it is judged that the amount of the
deposited particulate matter has exceeded the allowable amount. At
this time, the air-fuel ratio of the exhaust gas flowing into the
particulate filter 12a is maintained lean and fuel is added from
the fuel addition valve 14 to raise the temperature of the
particulate filter 12a by the heat of oxidation reaction of this
added fuel.
[0065] FIG. 13 shows an exhaust purification processing
routine.
[0066] Referring to FIG. 13, first, at step 100, the NO.sub.x
amount NOXA stored per unit time is calculated from the map shown
in FIG. 12. Next, at step 101, this NOXA is added to the NO.sub.x
amount .SIGMA.NOX stored in the NO.sub.x storage and reduction
catalyst 12. Next, at step 102, whether the stored NO.sub.x amount
.SIGMA.NOX has exceeded the allowable value NX is judged. When
.SIGMA.NOX>NX, the routine proceeds to step 103, where
processing for addition of fuel from the fuel addition valve 14 is
performed. Next, at step 104, the differential pressure sensor 21
detects the differential pressure .DELTA.P before and after the
particulate filter 12a. Next, at step 105, whether the differential
pressure .DELTA.P has exceeded the allowable value PX is judged.
When .DELTA.P>PX, the routine proceeds to step 106, where
temperature elevation control of the particulate filter 12a is
performed.
[0067] FIGS. 14(A) and (B) show another embodiment.
[0068] If the carrier carrying the palladium Pd is made to contain
an element of a basicity able to give electrons to the palladium
Pd, a large number of electrons will gather on the surface of the
palladium Pd. If a large number of electrons gather on the surface
of the palladium Pd in this way, a large amount of the oxygen
contained in the exhaust gas will be adsorbed on the surface of the
palladium Pd in search of electrons, therefore the oxygen storage
amount of the palladium Pd will increase. Therefore, in the
embodiment shown in FIGS. 14(A) and (B), to increase the oxygen
storage amount of the palladium Pd, the carrier carrying the
palladium Pd may be made to contain an element of a basicity able
to give electrons to the palladium Pd, for example, an alkali earth
or rare earth element.
[0069] That is, FIG. 14(A), like FIG. 6(A), shows the cross-section
of the surface part of the substrate 50 of the oxidation catalyst
11. In this embodiment, as shown in FIG. 14(A), the substrate 50 is
made to include the rare earth element lanthanum La. In this
embodiment, this lanthanum La functions as a thermal stabilizer of
the substrate 50. On the other hand, FIG. 14(B) shows an embodiment
in the case where the oxidation catalyst 11 is comprised of an
aggregate of the particulate carrier. In this embodiment, the
particulate carrier is comprised of a basic carrier 70 including an
alkali earth or rare earth element and an acidic carrier 71
including tungsten or titanium or another element with a high
electric cathodicity. In addition to platinum Pt, palladium Pd is
selectively carried on the basic carrier 70.
[0070] As explained above, according to the present invention, by
setting the molar ratio of the platinum Pt with respect to the sum
of the platinum Pt and palladium Pd carried on the oxidation
catalyst or NO.sub.x storage and reduction catalyst between about
50 percent to about 80 percent, it is possible to secure an
excellent NO.sub.x release action from the NO.sub.x storage and
reduction catalyst.
* * * * *