U.S. patent application number 10/770393 was filed with the patent office on 2004-08-12 for exhaust gas purifying system for internal combustion engine.
This patent application is currently assigned to Isuzu Motors Ltd.. Invention is credited to Gabe, Masashi, Nagaoka, Daiji, Sakamoto, Takayuki.
Application Number | 20040154285 10/770393 |
Document ID | / |
Family ID | 32653019 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040154285 |
Kind Code |
A1 |
Nagaoka, Daiji ; et
al. |
August 12, 2004 |
Exhaust gas purifying system for internal combustion engine
Abstract
The exhaust gas purifying system (1) for the internal combustion
engine, comprising the first exhaust gas purifying device (31)
provided with the NOx occlusion reduction type catalyst arranged in
the exhaust gas passage (30) of the internal combustion engine
(10), and also comprising the respective oxygen concentration
sensors (34, 35) arranged at the upstream and downstream of the
first exhaust gas purifying device (31), and which controls to end
the rich control when a difference between the oxygen
concentrations detected by both of said oxygen concentration
sensors (34, 35) falls not higher than a predetermined judgment
value during the rich control for restoring the catalytic ability
of the first exhaust gas purifying device (31), is provided with
the second exhaust gas purifying device (32) for cleaning HC and CO
at the downstream of said first exhaust gas purifying device (31).
In such a manner, HC and CO can be prevented from being discharged
into the atmospheric air at the time of ending the rich
control.
Inventors: |
Nagaoka, Daiji;
(Fujisawa-shi, JP) ; Gabe, Masashi; (Fujisawa-shi,
JP) ; Sakamoto, Takayuki; (Fujisawa-shi, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
Isuzu Motors Ltd.
Tokyo
JP
|
Family ID: |
32653019 |
Appl. No.: |
10/770393 |
Filed: |
February 4, 2004 |
Current U.S.
Class: |
60/285 |
Current CPC
Class: |
F01N 3/0814 20130101;
F01N 11/007 20130101; Y02T 10/22 20130101; F01N 2430/06 20130101;
Y02A 50/20 20180101; Y02A 50/2344 20180101; F01N 3/035 20130101;
F01N 3/2006 20130101; Y02T 10/12 20130101; F01N 3/0885 20130101;
B01D 53/9431 20130101; F01N 13/0097 20140603; F01N 2550/03
20130101; F02D 41/027 20130101; Y02T 10/47 20130101; F01N 2570/14
20130101; F02D 41/0295 20130101; F02D 41/1454 20130101; Y02T 10/26
20130101; Y02T 10/40 20130101; F01N 3/0842 20130101 |
Class at
Publication: |
060/285 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2003 |
JP |
2003-031184 |
Claims
What is claimed is:
1. An exhaust gas purifying system for an internal combustion
engine, comprising a first exhaust gas purifying device composed of
a NOx occlusion reduction type catalyst arranged in the exhaust
passage of an internal combustion engine, and also comprising the
respective oxygen concentration sensors arranged at the upstream
and downstream of the first exhaust gas purifying device, and which
controls to end the rich control when a difference between the
oxygen concentrations detected by both of said oxygen concentration
sensors falls not higher than a predetermined judgment value during
the rich control for restoring the catalytic ability of the first
exhaust gas purging device, wherein a second exhaust gas purifying
device for purifying HC and CO is arranged at the downstream of
said first exhaust gas purifying device.
2. An exhaust gas purifying system as claimed in claim 1, wherein
said second exhaust gas purifying device is composed of any one of
the NOx occlusion reduction type catalyst, a DPF supporting thereon
the NOx occlusion reduction type catalyst, a three-way catalyst,
and an oxidation catalyst with an oxygen occlusion function, or
composed of a combination of some of these.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an exhaust gas purifying
system for an internal combustion engine, composed of a NOx
occlusion reduction type catalyst which reduces NOx (nitrogen
oxides) in the exhaust gas of the internal combustion engine. To be
more concrete, the invention relates to the technique for
preventing HC, CO from being exhausted into the atmospheric air
during rich-condition control for restoring the catalytic function
of the NOx occlusion reduction type catalyst.
[0002] Various kinds of researches and proposals have been made
concerning an NOx catalyst for reducing and purifying NOx in the
exhaust gas of the internal combustion engines such as diesel
engines and a part of gasoline engines, and various combustion
systems.
[0003] One of them is the exhaust gas purifying system 1X where a
NOx occlusion reduction type catalyst 31X is arranged in the
exhaust gas passage 30 of the internal combustion engine 10 as
shown in FIG. 4. This exhaust gas purifying system 1X makes the NOx
occlusion reduction type catalyst 31X absorb NOx when an air/fuel
ratio of the inflow exhaust gas is lean. The regenerating operation
is performed, when the NOx occlusion ability is almost saturated.
In this regenerating operation, rich control for regenerating the
NOx occlusion ability is performed to decrease the oxygen
concentration of the inflow exhaust gas by making an air/fuel ratio
of the exhaust gas to the theoretical air/fuel ratio or rich. The
occluded NOx is discharged in the process. This discharged NOx is
reduced by the catalytic function of an attached noble metal
catalyst.
[0004] As shown in FIG. 5 to 7, this NOx occlusion reduction type
catalyst, a noble metal catalyst 31Xb and a NOx occluding material
(NOx occluding substance) 31Xc on a catalyst carrier 31Xa such as
alumina. The noble metal catalyst 31Xb is composed of platinum
(Pt), palladium (Pd), or the like. The NOx occluding material (NOx
occluding substance) 31Xc is composed of alkaline-earth metal such
as barium (Ba). As shown in FIG. 5, in a lean (high oxygen
concentration) atmosphere, NO in the exhaust gas is oxidized to
NO.sub.2 by the catalytic action of the noble metal catalyst 31Xb.
This NO.sub.2 is diffuised in the catalyst in the form of
NO.sub.3.sup.-, and occluded in the NOx occluding material 31Xc in
the form of a nitrate.
[0005] When the air/fuel ratio become rich and the oxygen
concentration is decreased as shown in FIG. 6, NO.sub.3.sup.- is
discharged from the NOx occluding material 31Xc in the form of
NO.sub.2. This NO.sub.2 is reduced to N.sub.2 with the reducer such
as the unburned HC, CO, and H.sub.2 contained in the exhaust gas by
the catalytic action of the noble metal catalyst 31Xb. This
catalytic action prevents NOx from being discharged into the
atmospheric air.
[0006] Moreover, with this NOx occlusion reduction type catalyst,
the sulfur contained in the fuel of a diesel engine is accumulated
in the NOx occluding material, to be stabilized as a sulfate.
Therefore, an occlusion quantity of NOx is reduced. The catalyst
deterioration by sulfur poisoning develops, as the purifying rate
of NOx is lowered and fuel cost is increased. For this reason, when
the catalyst deterioration has developed to some extent, sulfur
purge is carried to remove the sulfur.
[0007] By this sulfur purge, the sulfur (S) occluded in the NOx
occluding material as barium sulfate (Ba.sub.2PO.sub.4) is made
into sulfur dioxide (SO.sub.2) by bringing the catalyst into a high
temperature and oxygenless atmosphere, and also supplying carbon
oxide (CO) thereto. Thus, the NOx occluding ability is
restored.
[0008] Hereafter, rich control is performed for both of the rich
control for restoring the NOx occluding ability and the sulfur
purge control. Therefore, both of them shall be summarized and
called rich control hereafter.
[0009] In this rich control, as shown in FIG. 8, an excess air
factor .lambda. ext (dashed line B) at the catalyst exit is higher
than an excess air factor .lambda. ent (full line A) at the
catalyst entrance during the first term R1 of the rich control but
quickly decreases in the second term R2 to the same level as the
excess air factor .lambda. ent or lower.
[0010] Therefore, for example, according to the Japanese Patent
Laid-Open No. 10-121944 (121944/1998), an oxygen concentration
difference before and after the catalyst is monitored, and when
this oxygen concentration difference becomes small or the oxygen
concentration at the catalyst exit falls lower than the oxygen
concentration at the catalyst entrance, the discharge and reduction
of NO.sub.2 in the catalyst is judged as completed and the rich
control is ended at that point Re.
[0011] However, at the same time as the change of the oxygen
concentrations difference before and after the catalyst, an outflow
of the reducers such as HC and CO is caused. Therefore, there is a
problem that the reducers such as HC and CO throw out into the
atmospheric air to some extent, if the rich control is ended after
the changes in the oxygen concentrations are detected by the oxygen
concentration sensors. Namely, as shown in FIG. 9, the CO discharge
(full line E') is increased at the same time as the rich control is
ended.
[0012] This reason is as follows. As shown in FIG. 6, in the first
term R1 of the rich control, O.sub.2 is supplied into the exhaust
gas by the oxidation-reduction reaction of NO.sub.2 discharged from
the occluding material 31Xc. However, as shown in FIG. 7, in the
second term R2 of the rich control, the supply of O.sub.2 into the
exhaust gas is also ended at the same time as the discharge and
reduction of NO.sub.2 are ended. Therefore, CO is also no longer
oxidized and flows out into the atmosphere.
[0013] In order to prevent this reducer from flowing out into the
atmospheric air, for example, the exhaust gas purifying system for
the internal combustion engine disclosed by the Japanese Patent NO.
2658753 is provided with a secondary air-feeding device in the
exhaust gas passage of the internal combustion engine. When the
air/fuel ratio is changed from lean to rich, the secondary air is
fed into the exhaust gas passage of the engine from the secondary
air-feeding device so as to oxidize the excessive unburned
components exhausted from the NOx occluding material.
[0014] However, in order to arrange the secondary air-feeding
device, an electric air pump, a secondary air-feeding pipe for
feeding the secondary air from the secondary air-feeding device
into the exhaust gas passage of the engine, an electromagnetic
secondary air valve for controlling the supply of the secondary
air, or the like are necessary. Therefore, the structure of such
exhaust gas treating system is too complex and costly.
SUMMARY OF THE INVENTION
[0015] The present invention has been made to resolved the
above-mentioned problems, and the purpose thereof is to provide an
exhaust gas purifying system for an internal combustion engine
capable of preventing HC and CO from being discharged into the
atmospheric air at the time of ending the rich control, by
arranging a secondary exhaust gas purifying device composed of an
oxidation catalyst or the like with an oxygen occluding function
for purifying HC and CO at the downstream of an NOx occlusion
reduction type catalyst, in an exhaust gas purifying system using a
NOx occlusion reduction type catalyst for purifying NOx in the
exhaust gas.
[0016] The exhaust gas purifying system for an internal combustion
engine to achieve the purposes as the above is the exhaust gas
purifying system which comprises a first exhaust gas purifying
device comprising an NOx occlusion reduction type catalyst in the
exhaust gas passage of the internal combustion engine, and also
comprises oxygen concentration detection sensors at the upstream
and downstream of the first exhaust gas purifying device,
respectively, and controls to end the rich control when a
difference between the oxygen concentrations detected by both of
the oxygen concentration sensors falls not higher than a
predetermined judgment value during the rich control for restoring
the catalytic ability of the first exhaust gas purifying system,
and further comprises a second exhaust gas purifying device for
purifying HC and CO at the downstream of the first exhaust gas
purifying device.
[0017] Further, in the above-mentioned exhaust gas purifying system
for the internal combustion engine, the second exhaust gas
purifying device is composed of any one of the NOx occlusion
reduction type catalyst, a DPF (diesel particular filter)
supporting the NOx occlusion reduction type catalyst, a three-way
catalyst, and an oxidation catalyst with oxygen occluding function,
or a combination of some of these.
[0018] Moreover, the rich control for restoring the catalytic
function of this first exhaust gas purifying device includes the
rich control for restoring the NOx occluding ability and the rich
control for purging sulfur for letting the catalyst restore from
deterioration caused by sulfur poisoning. And also the expressions
of the oxygen concentration sensors and the oxygen concentrations
include the indications to be related with the oxygen
concentrations such as an excess air factor sensor and an excess
air factor even though the indication modes are different.
[0019] According to the exhaust gas purifying system related to the
present invention, the second exhaust gas purifying device for
purifying HC and CO is arranged at the downstream of the first
exhaust gas purifying device, therefore, the reducers such as HC
and CO, which have been generated at the time of ending the rich
control with the conventional technology, can be prevented from
flowing out into the atmospheric air.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows a construction of the exhaust gas purifying
system in an embodiment according to the present invention.
[0021] FIG. 2 shows an example of a control flow chart of the
exhaust gas purifying system in an embodiment according to the
present invention.
[0022] FIG. 3 shows the excess air factors and carbon monoxide
concentration of the exhaust gas in an embodiment and an
example
[0023] FIG. 4 shows a construction of the exhaust gas purifying
system according to the conventional technology.
[0024] FIG. 5 diagrammatically illustrates the configuration of the
NOx occlusion reduction type catalyst and the purifying mechanism
in the lean state of the lean control (the occlusion of NOx).
[0025] FIG. 6 diagrammatically illustrates the configuration of the
NOx occlusion reduction type catalyst and the purifying mechanism
in the first term state of the rich control (R1: discharging and
reducing).
[0026] FIG. 7 diagrammatically illustrates the configuration of the
NOx occlusion reduction type catalyst and the purifying mechanism
in the second term state of the rich control (R2: after the
discharge of the NOx).
[0027] FIG. 8 is a chart showing the variations of the excess air
factor .lambda. ent at the catalyst entrance and the excess air
factor .lambda. ext at the catalyst exit during the rich control in
the exhaust gas purifying system according to the conventional
technology.
[0028] FIG. 9 shows an outflow situation of carbon monoxide at the
time of ending the rich control in the exhaust gas purifying system
according to the conventional technology.
[0029] FIG. 10 shows the relationships of the purifying factors of
NOx, HC, and CO to oxygen concentrations with the NOx reduction
type catalyst.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] In the following, the exhaust gas purifying system for an
internal combustion engine in the embodiment according to the
present invention will be explained referring to the drawings.
[0031] As shown in FIG. 1, this exhaust gas purifying system 1 is
comprised of the first exhaust gas purifying device 31 for
purifying NOx, and the second exhaust gas purifying device 32
having a function of purifying the reducers such as HC and CO in
the exhaust gas passage of the engine 10 from the upstream
side.
[0032] This exhaust gas purifying system 1 is provided with an air
cleaner 21, an air flow meter 22, a MAF (mass air flow) sensor 23,
and an intake throttle valve 24 from the upstream side in the
intake passage 20 of the engine 10.
[0033] Moreover, in the exhaust gas passage 30, the first exhaust
gas purifying device 31 and the second exhaust gas purifying device
32 are arranged. This first exhaust gas purifying device 31 is
composed of the NOx occlusion reduction type catalyst. Moreover,
the second exhaust gas purifying device 32 is composed of any one
of an NOx occlusion reduction type catalyst, a DPF supporting the
NOx occlusion reduction type catalyst, an oxidation catalyst with
an oxygen occluding function, and a three-way catalyst, or a
combination of some of these.
[0034] Further, in the exhaust gas passage 30, an exhaust gas
temperature sensor 33, a catalyst entrance exhaust gas
concentration sensor 34, a catalyst exit exhaust gas concentration
sensor 35, a catalyst entrance temperature sensor 36, and a
catalyst exit temperature sensor 37 are arranged. These exhaust gas
concentration sensors 34, 35 are the sensors for measuring an
oxygen concentration (or an excess air factor) and a NOx
concentration.
[0035] Moreover, an EGR passage 40 is arranged, and in this EGR
passage 40, an EGR cooler 41 and an EGR valve 42 are arranged.
[0036] The engine 10 with the exhaust gas purifying system 1 is
provided with a common-rail injection system 50 for performing fuel
injection, and an electronic control device (electronic control
box) 60 called an ECU (engine control unit) to carry out the whole
control for the engine 10.
[0037] In this exhaust gas purifying system 1, the air A, passing
through the air cleaner 21, the air flow meter 22, the MAF sensor
23, and the intake throttle valve 24, is fed into the cylinders
through the intake manifold 20a of the engine 10. The flow rate of
the air A is regulated by the control of the intake throttle valve
24 which is controlled by the electronic control device 50.
[0038] Moreover, the exhaust gas G, coming out of the an exhaust
manifold 30a and sequentially passing through the first exhaust gas
purifying device 31 and the second exhaust gas purifying device 32
in the exhaust gas passage 30, is exhausted from a tail pipe (not
illustrated) through a muffler (not illustrated). This exhaust gas
G is cleaned by the first exhaust gas purifying device 31 and the
second exhaust gas purifying device 32 into cleaned exhaust gas
Gc.
[0039] After passing through the EGR passage 40 and being cooled
through the EGR cooler 41, the EGR gas Ge, a part of the exhaust
gas G is re-circulated into the intake manifold 20a through the EGR
valve 42. The EGR valve 42 regulates the flow rate of the EGR gas
Ge and makes the EGR operation on or off.
[0040] As shown in FIG. 5-FIG. 7, the NOx occlusion reduction type
catalyst in the first exhaust gas purifying device 31 is composed
of a catalytic metal 31Xb and a NOx occluding material (NOx
occluding substance) 31Xc which are carried on the surface of a
carrier body 31Xa formed of monolithic honeycomb cells made of
.gamma.-alumina or the like.
[0041] This catalytic metal (noble metal catalyst) 31Xb can be
composed of platinum (Pt), Palladium (Pd), or the like having
oxidation activity in the temperature area higher than the activity
start temperature. Moreover, the NOx occluding material (NOx
absorbent) 31Xc can be composed of any one of an alkaline metal, an
alkaline earth metal, and a rare earth metal, or a combination of
them. The alkaline metals include potassium (K), sodium (Na),
lithium (Li), cesium (Cs), or the like. The alkaline earth metals
include barium (Ba), calcium (Ca), or the like. Moreover, the rare
earth metals include lanthanum (La), yttrium (Y), or the like. This
NOx occluding material 31Xc occludes NOx when the oxygen
concentration in the gas is high, and discharges NOx when the
oxygen concentration in the gas is low.
[0042] As shown in FIG. 5, NO in the exhaust gas is oxidized to
NO.sub.2 by the catalytic action of the catalytic metal 31Xb in a
high oxygen concentration atmosphere in which the exhaust gas is of
the lean state (lean combustion). This NO.sub.2 diffuses in the
catalyst in the form of NO.sub.3.sup.-, and is absorbed in the NOx
occluding material 31Xc in the form of a nitrate (Ba
(NO.sub.3).sub.2). Namely, NO.sub.2 is selectively occluded in the
NOx occluding material 31Xc by changing from barium carbonate
(BaCO.sub.3) into barium nitrate (Ba (NO.sub.3).sub.2).
[0043] As shown in FIG. 6, NO.sub.3.sup.- changes into NO.sub.2
when the exhaust gas is brought into the rich state and the oxygen
concentration decreases, and NO.sub.2 is discharged from the NOx
occluding material 31Xc. Namely, NO.sub.2 is discharged by changing
from barium nitrate (Ba (NO.sub.3).sub.2) into barium carbonate
(BaCO.sub.3). This discharged NO.sub.2 is reduced to N.sub.2 with
the reducers such as unburned HC and CO contained in the exhaust
gas by the catalytic action of the catalytic metal 31Xb. This
catalytic action is able to prevent NOx from being discharged into
the atmospheric air.
[0044] To make the exhaust gas rich-condition as described above
does not always mean only rich combustion in the cylinder bores,
but also means that a ratio of an air quantity to a fuel quantity
(including the portion burned in the cylinder bores) which have
been supplied into the exhaust gas flowing into the first exhaust
gas purifying device 31 with the NOx occlusion reduction type
catalyst has only to be close to the theoretical air-fuel ratio or
to be in a rich state in which the fuel quantity is more than that
at the theoretical air-fuel ratio.
[0045] Moreover, the NOx occlusion reduction type catalyst carried
in the second exhaust gas purifying device 32 is a NOx occlusion
reduction type catalyst similar to the catalyst in the first
exhaust gas purifying device 31. Moreover, the DPF (diesel
particulate filter) supporting the NOx occlusion reduction type
catalyst is made to support a NOx occlusion reduction type catalyst
similar to the catalyst in the first exhaust gas purifying device
31 on the DPF.
[0046] The three-way catalyst is the one used conventionally, and
the oxidation catalyst with the oxygen occluding function is a
conventional oxidation catalyst added with CeO.sub.2 (cerium oxide)
or the like as an OSC agent (oxygen occluding material, oxygen
absorbent).
[0047] This exhaust gas purifying system 1 is controlled by the
method according to the control flow as shown in FIG. 2 as an
example.
[0048] This control flow in FIG. 2 is shown repeatedly called and
executed together with another control flow for the engine 10
during operating the engine 10.
[0049] When this control flow starts, it is judged whether or not
the rich control for restoring the NOx occluding ability or purging
sulfur is required at the step S11. If it is not required (NO),
normal lean mode operation that is normal lean-burn operation (lean
combustion) is performed for a predetermined time (a time related
to a judgment interval of rich control) at the step S14, and then
the control flow returns.
[0050] Moreover, if the rich control is required (YES) at the step
S11, the rich control is performed at the step S12.
[0051] This rich control brings the exhaust gas into the rich state
in which the oxygen concentration is almost zero, by adjusting an
injection quantity of multi-step injection and injection timing in
the control of the fuel injection into the cylinders, EGR, the
intake throttle valve, or the like. Namely, the multi-step
injection is performed in the fuel injection control. At the same
time, an excess air factor .lambda. detected from the catalyst exit
exhaust gas concentration sensor 35 is monitored and controlled by
feedback so as to be .lambda.t as a target. Then, the EGR gas
quantity and the intake throttle quantity are controlled by
feedback while monitoring the output of the MAF sensor 23 for
measuring the intake air quantity.
[0052] In this rich control, the controlled range of the rich
atmosphere is controlled on the basis of an oxygen concentration.
The oxygen concentration at the catalyst entrance needs to be
controlled so as to be a concentration (for example 1%) or lower in
which NO.sub.2 can be discharged from the NOx occluding material
31Xc. And also the oxygen concentration at the catalyst exit needs
to be controlled so as to be a concentration (for example 1%) or
higher in which HC and CO can be oxidized and are not caused to
flow out into the atmospheric air. p After having performed this
rich control for a predetermined control time (a time related to an
interval for judging to end the rich control), it is judged at the
step S13 whether or not the difference (.DELTA. .lambda.=.lambda.
ext-.lambda. ent) between the excess air factor .lambda. ext at the
catalyst exit and the excess air factor .lambda. ent at the
catalyst entrance is a predetermined judgment value .DELTA.
.lambda.th or lower. Namely, it is judged whether or not the
difference between the oxygen concentration at the catalyst exit
and the oxygen concentration at the catalyst entrance is a
predetermined concentration difference or smaller, namely, is not
higher than the predetermined judgment value.
[0053] If it is judged at the step S13 that the excess air factor
difference .DELTA. .lambda. is larger than the predetermined
judgment value .DELTA. .lambda.th, the control flow returns to the
step S12 and the rich control is continued until the excess air
factor difference .DELTA. .lambda. becomes the predetermined
judgment value .DELTA. .lambda.th or smaller. When the excess air
factor difference .DELTA. .lambda. becomes the predetermined
judgment value .DELTA. .lambda.th or smaller, the rich control is
ended to be operated in the normal lean mode at the step S14, and
then returns to the start.
[0054] This control flow is repeatedly called for until the engine
key is switched off.
[0055] Namely, this control flow of starting, executing the steps
S11-S14 and returning to the start is repeatedly performed.
[0056] Moreover, the above-mentioned flow is simplified for an easy
understanding. Therefore, the descriptions of the data entry and
data processing parts for judging the necessity for the rich
control are omitted. And the descriptions of the difference in the
rich control contents at the step S12 based on the difference
between the rich control for restoring the NOx occluding ability
and that for purging sulfur are omitted. Moreover, the descriptions
of the process for writing the quantity of NOx, the quantity of
sulfur and the lean-driving duration at the ending of engine
operation into the memory, is also omitted.
[0057] Next, the states of the exhaust gas during the first and
second terms of the rich control and at the time of ending it in
the exhaust gas purifying system 1 of the above-mentioned
configuration will be explained below.
[0058] During the first term (initial stage) of the rich control,
the discharge and reduction of NO.sub.2 at the NOx occluding
material 31Xc are not performed at the part Z1 in FIG. 10, unless
the oxygen concentration in the atmosphere is at a certain level
(about 1%) or lower. When the oxygen concentration is low, namely
during Z1, NOx is reduced. Moreover, as shown in FIG. 3, during the
first term stage R1 of this rich control, the high oxygen
concentration is measured at the catalyst exit, namely, the excess
air factor .lambda. ext is high. According to above-mentioned
matter, it is presumed that the discharge and reduction of NO.sub.2
are started in the front part of the catalyst since the oxygen
concentration at the catalyst entrance falls, and that the oxygen
concentration in the rear part of the catalyst is increased due to
the discharge and reduction of NO.sub.2 at the front part. And it
is presumed that the oxidation-reduction function of CO and HC is
stronger than the discharge function of NO.sub.2 at the part Z2 in
FIG. 10. Practically, the outflows (slips) of HC, CO, and NOx from
the catalyst exit are not caused. When the oxygen concentration is
high, namely during Z2, the HC and CO are oxidized.
[0059] Moreover, during the second term R2 of the rich control R,
the discharge and reduction of NO.sub.2 are performed in the whole
area of the catalyst, and when the reaction starts to end, the
oxygen concentration in the rear side of the catalyst starts to
fall, and falls to the same level as that in the front side of the
catalyst or lower. Namely, as shown in FIG. 3, the excess air
factor .lambda. ext starts to fall and approaches to the excess air
factor .lambda. ent. Since the oxygen concentration in the rear
part of the catalyst is also lowered in this state during the part
Z1 in FIG. 10, CO and HC begin to flow out to the downstream side
of the catalyst without being oxidized.
[0060] As described above, it is presumed that the discharge and
reduction of NO.sub.2, and the oxidation of HC and CO are
time-sequentially progressing from the front part to the rear part
of the catalyst according to the change of the oxygen concentration
inside of the NOx occlusion reduction type catalyst in the first
exhaust gas purifying device.
[0061] As the rich control progresses, the discharge and reduction
of NO.sub.2 proceed in the catalyst, and the rich control
approaches to an end, the oxygen concentration at the catalyst exit
falls to the same level as that at the catalyst entrance or lower.
Namely, the difference between the outputs of the oxygen
concentration sensors approaches to a predetermined judgment value
and exceeds this predetermined judgment value. Namely, the
difference .DELTA. .lambda. between the excess air factors exceeds
the predetermined judgment value .DELTA. .lambda. ent. The rich
control is ended at this point Re.
[0062] However, in the present invention, the second exhaust gas
purifying device 32 is located at the downstream of the first
exhaust gas purifying device 31. Therefore, at this ending stage of
the rich control R, the reducers such as HC and CO flowing out of
the first exhaust gas purifying device 31 can be purified by the
second exhaust gas purifying device 32. Therefore, these reducers
can be prevented from flowing out into the atmospheric air.
[0063] Namely, in the case that the second exhaust gas purifying
device 32 is provided with the NOx occlusion reduction type
catalyst or the DPF with the NOx occlusion reduction type catalyst,
the discharge and reduction of NO.sub.2 is in progress in this
second exhaust gas purifying device 32 and the oxygen is high in
the concentration. The reducers such as HC and CO flowing out of
the first exhaust gas purifying device 31 at the upstream are
consumed for reducing NO.sub.2. Namely, since HC and CO are
oxidized by this reduction, the reducers such as HC and CO can be
prevented from flowing out into the atmospheric air.
[0064] Moreover, in the case that the second exhaust gas purifying
device 32 is provided with the three-way catalyst or the oxidation
catalyst having the oxygen occluding function, HC and CO are
reduced by O.sub.2 discharged from these catalysts, therefore, the
reducers such as HC and CO can be prevented from flowing out into
the atmospheric air in the similar way.
[0065] Therefore, the exhaust gas purifying system 1 of the
configuration described above performs the rich control when it is
required, and purifies HC and CO which are not oxidized through at
the time of ending the rich control on the basis of the difference
between the oxygen concentrations before and after the first
exhaust gas purifying device 31, by the second exhaust gas
purifying device 32 at the downstream side. Accordingly HC and CO
can be prevented from being discharged into the atmospheric
air.
[0066] An embodiment of cleaning the exhaust gas by this exhaust
gas purifying system 1 is shown in FIG. 3. In this embodiment, the
second exhaust gas purifying device 32 is provided with the NOx
occlusion reduction type catalyst. FIG. 3 illustrates the relation
ship between the excess air factor .lambda. ent (full line A) at
the entrance of the first exhaust gas purifying device 31 and the
excess air factor .lambda. ext (dot-dash line B) at the exit of the
first exhaust gas purifying device 31. And FIG. 3 illustrates the
CO concentration (dot-dashed line C) at the downstream of the
second exhaust gas purifying device 32 at that moment. Moreover,
FIG. 3 also illustrates the excess air factor .lambda. ext (dashed
line B') at the exit of the NOx occlusion reduction type catalyst
31 and the CO concentration (dashed line C') at the downstream of
the NOx occlusion reduction type catalyst 31X, in the case that the
second exhaust gas purifying device 32 is not arranged (the example
for comparison).
[0067] Comparing the CO concentration (dot-dashed line C) of this
embodiment with the CO concentration (dashed line C') of the
example for comparison, the outflow of CO at the time of ending the
rich control has disappeared in the embodiment. So, the effect of
the present invention is apparent.
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