U.S. patent application number 13/496390 was filed with the patent office on 2012-07-19 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, Junichi Matsuo, Hiromasa Nishioka, Yoshihisa Tsukamoto, Kazuhiro Umemoto.
Application Number | 20120180462 13/496390 |
Document ID | / |
Family ID | 43856485 |
Filed Date | 2012-07-19 |
United States Patent
Application |
20120180462 |
Kind Code |
A1 |
Asanuma; Takamitsu ; et
al. |
July 19, 2012 |
EXHAUST PURIFICATION SYSTEM OF INTERNAL COMBUSTION ENGINE
Abstract
An exhaust purification system of an internal combustion engine
has a selective reduction type NO.sub.X catalyst device which can
hold ammonia and a storage reduction type NO.sub.X catalyst device
which is arranged at an upstream side of the selective reduction
type NO.sub.X catalyst device, which makes an air-fuel ratio of
exhaust gas which flows into the storage reduction type NO.sub.X
catalyst device change from a lean air-fuel ratio to a rich
air-fuel ratio for starting an ammonia generation period, and which
makes an air-fuel ratio of exhaust gas which flows into the storage
reduction type NO.sub.X catalyst device change from a rich air-fuel
ratio to a lean air-fuel ratio for ending the ammonia generation
period.
Inventors: |
Asanuma; Takamitsu;
(Mishima-shi, JP) ; Nishioka; Hiromasa;
(Susono-shi, JP) ; Tsukamoto; Yoshihisa;
(Susono-shi, JP) ; Matsuo; Junichi; (Susono-shi,
JP) ; Umemoto; Kazuhiro; (Susono-shi, JP) |
Assignee: |
Toyota Jidosha Kabushiki
Kaisha
Toyota-shi, Aichi
JP
|
Family ID: |
43856485 |
Appl. No.: |
13/496390 |
Filed: |
October 9, 2009 |
PCT Filed: |
October 9, 2009 |
PCT NO: |
PCT/JP2009/067954 |
371 Date: |
March 15, 2012 |
Current U.S.
Class: |
60/287 |
Current CPC
Class: |
F01N 13/02 20130101;
F01N 3/2073 20130101; F01N 3/0871 20130101; F01N 3/0842 20130101;
F01N 3/2066 20130101; Y02T 10/12 20130101; Y02T 10/24 20130101;
F01N 2900/1404 20130101; F01N 3/0814 20130101 |
Class at
Publication: |
60/287 |
International
Class: |
F01N 3/24 20060101
F01N003/24 |
Claims
1. An exhaust purification system of an internal combustion engine
which is provided with a selective reduction type NO.sub.X catalyst
device which can hold ammonia and a storage reduction type NO.sub.X
catalyst device which is arranged at an upstream side of said
selective reduction type NO.sub.X catalyst device, which makes an
air-fuel ratio of exhaust gas which flows into said storage
reduction type NO.sub.X catalyst device change from a lean air-fuel
ratio to a rich air-fuel ratio for starting an ammonia generation
period, and which makes an air-fuel ratio of exhaust gas which
flows into said storage reduction type NO.sub.X catalyst device
change from a rich air-fuel ratio to a lean air-fuel ratio for
ending said ammonia generation period, characterized in that an
amount of NO.sub.X which is held at said storage reduction type
NO.sub.X catalyst device for starting said ammonia generation
period when a temperature of said storage reduction type NO.sub.X
catalyst device is less than a set temperature is smaller than that
when a temperature of said storage reduction type NO.sub.X catalyst
device is said set temperature or more.
2. An exhaust purification system of an internal combustion engine
as set forth in claim 1, wherein an interval from when ending said
ammonia generation period to when next starting said ammonia
generation period when the temperature of said storage reduction
type NO.sub.X catalyst device is less than said set temperature is
made shorter than that when the temperature of said storage
reduction type NO.sub.X catalyst device is said set temperature or
more such that the amount of NO.sub.X which is held at said storage
reduction type NO.sub.X catalyst device for starting said ammonia
generation period when the temperature of said storage reduction
type NO.sub.X catalyst device is less than said set temperature is
smaller than that when the temperature of said storage reduction
type NO.sub.X catalyst device is said set temperature or more.
3. An exhaust purification system of an internal combustion engine
as set forth in claim 1, wherein when the temperature of said
storage reduction type NO.sub.X catalyst device is another set
temperature which is higher than said set temperature or more,
instead of said ammonia generation period, making the air-fuel
ratio of the exhaust gas which flows into said selective reduction
type NO.sub.X catalyst device change from a lean air-fuel ratio to
the stoichiometric air-fuel ratio or another rich air-fuel ratio
closer to the stoichiometric air-fuel ratio than said rich air-fuel
ratio and making said storage reduction type NO.sub.X catalyst
device purify the NO.sub.X which is disassociated from said storage
reduction type NO.sub.X catalyst device and the NO.sub.X in the
exhaust gas by reduction.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust purification
system of an internal combustion engine.
BACKGROUND ART
[0002] In the exhaust system of a diesel engine or other such
internal combustion engine which carries out lean combustion, a
NO.sub.X catalyst device is arranged for purifying the NO.sub.X in
the exhaust gas. As such an NO.sub.X catalyst device, a selective
reduction type NO.sub.X catalyst device is known which uses ammonia
NH.sub.3 with its high reducing ability as reducing agent so to
selectively purify the NO.sub.X in the exhaust gas by
reduction.
[0003] The ammonia which is used as the reducing agent generally is
produced by hydrolysis of urea which is supplied to the catalyst
device. To eliminate the need for the supply of urea, it has been
proposed to arrange a three-way catalyst device at the upstream
side of the selective reduction type NO.sub.X catalyst device and
change the air-fuel ratio of the exhaust gas from the lean air-fuel
ratio at the time of lean combustion to a rich air-fuel ratio to
thereby make the three-way catalyst device sufficiently reduce the
NO.sub.X which is contained in the exhaust gas and produce ammonia
(refer to Japanese Unexamined Patent Publication No.
09-133032).
[0004] The ammonia which is produced in the three-way catalyst
device in this way flows together with the exhaust gas of the rich
air-fuel ratio which does not contain almost any NO.sub.X into the
selective reduction type NO.sub.X catalyst device which is
positioned at the downstream side of the three-way catalyst device
and is held at the selective reduction type NO.sub.X catalyst
device since the concentration of ammonia in the exhaust gas is
high. Next, if the air-fuel ratio of the exhaust gas is made the
lean air-fuel ratio at the time of lean combustion, exhaust gas of
the lean air-fuel ratio which includes a relatively large amount of
NO.sub.X flows into the three-way catalyst device. At this time,
the NO.sub.X flows into the selective reduction type NO.sub.X
catalyst device which is positioned at the downstream side of the
three-way catalyst device without being reduced much at all. In
this way, when the ammonia concentration is low, the selective
reduction type NO.sub.X catalyst device releases the held ammonia
and purifies the NO.sub.X by reduction.
[0005] However, in the three-way catalyst device, the NO.sub.X in
the exhaust gas which is contained in the exhaust gas of the rich
air-fuel ratio is sometimes also reduced to N.sub.2 or only
partially reduced to ammonia resulting in insufficient ammonia
being supplied to the selective reduction type NO.sub.X catalyst
device. Due to this, arranging a storage reduction type NO.sub.X
catalyst device at the upstream side of the selective reduction
type NO.sub.X catalyst device may be considered instead of a
three-way catalyst device.
[0006] A storage reduction type NO.sub.X catalyst device holds the
NO.sub.X in the exhaust gas well when the exhaust gas is the lean
air-fuel ratio. If making the air-fuel ratio of the exhaust gas the
stoichiometric air-fuel ratio or the rich air-fuel ratio, the held
NO.sub.X is disassociated and the thus disassociated NO.sub.X is
reduced. Due to this, if the exhaust gas is made the rich air-fuel
ratio, the storage reduction type NO.sub.X catalyst device will
release the held NO.sub.X when the exhaust gas is a lean air-fuel
ratio. If able to reduce to ammonia a part of the released NO.sub.X
in addition to a part of the NO.sub.X which is contained in the
exhaust gas, a sufficient amount of ammonia can be supplied to the
selective reduction type NO.sub.X catalyst device.
DISCLOSURE OF THE INVENTION
[0007] As explained earlier, in an exhaust purification system of
an internal combustion engine in which a storage reduction type
NO.sub.X catalyst device is arranged at an upstream side of a
selective reduction type NO.sub.X catalyst device, if making an
air-fuel ratio of the exhaust gas a rich air-fuel ratio so as to
start an ammonia generation period, it is possible to ensure the
presence of a relatively large amount of NO.sub.X inside the
storage reduction type NO.sub.X catalyst device. If the storage
reduction type NO.sub.X catalyst device has a sufficient reducing
ability, it is possible to produce a sufficient amount of ammonia
from the relatively large amount of NO.sub.X. However, if the
storage reduction type NO.sub.X catalyst device is low in
temperature and does not have a sufficient reducing ability, the
relatively large amount of NO.sub.X is reduced, due to insufficient
reduction, not to N.sub.2 and ammonia, but mainly dinitrogen
monoxide N.sub.2O and N.sub.2. Not only cannot a sufficient amount
of ammonia be supplied to the selective reduction type NO.sub.X
catalyst device, but also a relatively large amount of N.sub.2O for
which release into the atmosphere is undesirable ends up being
produced.
[0008] Therefore, an object of the present invention is to provide
an exhaust purification system of an internal combustion engine
which is provided with a selective reduction type NO.sub.X catalyst
device which can hold ammonia and a storage reduction type NO.sub.X
catalyst device which is arranged at an upstream side of the
selective reduction type NO.sub.X catalyst device, which makes an
air-fuel ratio of exhaust gas which flows into the storage
reduction type NO.sub.X catalyst device change from a lean air-fuel
ratio to a rich air-fuel ratio for starting an ammonia generation
period, and which makes an air-fuel ratio of exhaust gas which
flows into the storage reduction type NO.sub.X catalyst device
change from a rich air-fuel ratio to a lean air-fuel ratio for
ending the ammonia generation period, wherein even if the ammonia
generation period is started when the storage reduction type
NO.sub.X catalyst device is a low temperature, ammonia is easily
produced and N.sub.2O is difficult to produce.
[0009] An exhaust purification system of an internal combustion
engine as set forth in claim 1 according to the present invention
is provided, characterized in that the system is provided with a
selective reduction type NO.sub.X catalyst device which can hold
ammonia and a storage reduction type NO.sub.X catalyst device which
is arranged at an upstream side of the selective reduction type
NO.sub.X catalyst device, which makes an air-fuel ratio of exhaust
gas which flows into the storage reduction type NO.sub.X catalyst
device change from a lean air-fuel ratio to a rich air-fuel ratio
for starting an ammonia generation period, and which makes an
air-fuel ratio of exhaust gas which flows into the storage
reduction type NO.sub.X catalyst device change from a rich air-fuel
ratio to a lean air-fuel ratio for ending the ammonia generation
period, wherein an amount of NO.sub.X which is held at the storage
reduction type NO.sub.X catalyst device for starting the ammonia
generation period when the temperature of the storage reduction
type NO.sub.X catalyst device is less than a set temperature is
smaller than that when a temperature of the storage reduction type
NO.sub.X catalyst device is the set temperature or more.
[0010] An exhaust purification system of an internal combustion
engine as set forth in claim 2 according to the present invention
is provided as the exhaust purification system of an internal
combustion engine as set forth in claim 1 characterized in that an
interval from when ending the ammonia generation period to when
next starting the ammonia generation period when the temperature of
the storage reduction type NO.sub.X catalyst device is less than
the set temperature is made shorter than that when the temperature
of the storage reduction type NO.sub.X catalyst device is the set
temperature or more such that the amount of NO.sub.X which is held
at the storage reduction type NO.sub.X catalyst device for starting
the ammonia generation period when the temperature of the storage
reduction type NO.sub.X catalyst device is less than the set
temperature is smaller than that when the temperature of the
storage reduction type NO.sub.X catalyst device is the set
temperature or more.
[0011] An exhaust purification system of an internal combustion
engine as set forth in claim 3 according to the present invention
is provided as the exhaust purification system of an internal
combustion engine as set forth in claim 1 characterized in that
when the temperature of the storage reduction type NO.sub.X
catalyst device is another set temperature higher than the set
temperature or more, instead of the ammonia generation period,
making the air-fuel ratio of the exhaust gas which flows into the
selective reduction type NO.sub.X catalyst device change from the
lean air-fuel ratio to the stoichiometric air-fuel ratio or another
rich air-fuel ratio closer to the stoichiometric air-fuel ratio
than the above rich air-fuel ratio and making the storage reduction
type NO.sub.X catalyst device purify the NO.sub.X which is
disassociated from the storage reduction type NO.sub.X catalyst
device and the NO.sub.X in the exhaust gas by reduction.
[0012] According to the exhaust purification system of an internal
combustion engine as described in claim 1 according to the present
invention, an amount of NO.sub.X which is held at the storage
reduction type NO.sub.X catalyst device for starting the ammonia
generation period when the temperature of the storage reduction
type NO.sub.X catalyst device is less than the set temperature is
smaller than that when the temperature of the storage reduction
type NO.sub.X catalyst device is the set temperature or more. Due
to this, when the temperature of the storage reduction type
NO.sub.X catalyst device is less than the set temperature, even if
the ammonia generation period is started, only a small amount of
NO.sub.X is released from the storage reduction type NO.sub.X
catalyst device. Since there is not that large of an amount of
NO.sub.X present inside of the storage reduction type NO.sub.X
catalyst device, even if the reducing ability is low, the NO.sub.X
can be sufficiently reduced to N.sub.2 and ammonia. In this way,
even if the ammonia generation period is started when the storage
reduction type NO.sub.X catalyst device is a low temperature, it is
possible to make ammonia easier to produce and N.sub.2O harder to
produce. It is possible to supply a sufficient amount of ammonia to
the selective reduction type NO.sub.X catalyst device and to
sufficiently suppress the production of N.sub.2O for which release
into the atmosphere undesirable.
[0013] According to the exhaust purification system of an internal
combustion engine as described in claim 2 according to the present
invention, in the exhaust purification system of an internal
combustion engine as set forth in claim 1, an interval from when
ending the ammonia generation period to when next starting the
ammonia generation period, for example, the time interval or
running distance interval, when the temperature of the storage
reduction type NO.sub.X catalyst device is less than the set
temperature is shorter than that when the temperature of the
storage reduction type NO.sub.X catalyst device is the set
temperature or more such that the amount of NO.sub.X which is held
at the storage reduction type NO.sub.X catalyst device for starting
the ammonia generation period when the temperature of the storage
reduction type NO.sub.X catalyst device is less than the set
temperature is smaller than that when the temperature of the
storage reduction type NO.sub.X catalyst device is the set
temperature or more. Due to this simple control, even if the
ammonia generation period is started when the storage reduction
type NO.sub.X catalyst device is a low temperature, it is possible
to make ammonia easier to produce and N.sub.2O harder to
produce.
[0014] According to the exhaust purification system of an internal
combustion engine as described in claim 3 according to the present
invention, in the exhaust purification system of an internal
combustion engine as set forth in claim 1, when the temperature of
the storage reduction type NO.sub.X catalyst device is another set
temperature higher than the set temperature or more, instead of the
ammonia generation period, the system makes the air-fuel ratio of
the exhaust gas which flows into the selective reduction type
NO.sub.X catalyst device change from the lean air-fuel ratio to the
stoichiometric air-fuel ratio or another rich air-fuel ratio close
to the stoichiometric air-fuel ratio. Here, if starting the ammonia
generation period when the temperature of the storage reduction
type NO.sub.X catalyst device is another set temperature higher
than the set temperature or more and the reducing ability of the
storage reduction type NO.sub.X catalyst device is extremely high,
a large amount of ammonia would be produced and a relatively large
amount of ammonia would pass straight through the selective
reduction type NO.sub.X catalyst device. Due to this, at this time,
instead of the ammonia generation period, the air-fuel ratio of the
exhaust gas which flows into the selective reduction type NO.sub.X
catalyst device is made to change from the lean air-fuel ratio to
the stoichiometric air-fuel ratio or another rich air-fuel ratio
close to the stoichiometric air-fuel ratio so that the storage
reduction type NO.sub.X catalyst device purifies by reduction the
NO.sub.X which is disassociated from the storage reduction type
NO.sub.X catalyst device and the NO.sub.X in the exhaust gas
without almost any of the NO.sub.X being made ammonia. In this way,
in addition to the effects of the exhaust purification system of
the internal combustion engine as set forth in claim 1, when the
storage reduction type NO.sub.X catalyst device is a high
temperature and the reducing ability is extremely high, the ammonia
generation period is not started and the storage reduction type
NO.sub.X catalyst device is made to purify NO.sub.X by reduction so
as to prevent a relatively large amount of ammonia from passing
straight through the selective reduction type NO.sub.X catalyst
device.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is a schematic view which shows an embodiment of an
exhaust purification system of an internal combustion engine
according to the present invention.
[0016] FIG. 2 is a flow chart for producing ammonia in a storage
reduction type NO.sub.X catalyst device, which is carried out in
the exhaust purification system according to the present
invention.
DESCRIPTION OF EMBODIMENTS
[0017] FIG. 1 is a schematic view which shows an exhaust
purification system of an internal combustion engine according to
the present invention. In the figure, 1 indicates an exhaust
passage of a diesel engine or a direct-fuel-injection type spark
ignition internal combustion engine carrying out lean combustion.
The exhaust gas in the internal combustion engine carrying out lean
combustion contains a relatively large amount of NO.sub.X, so a
selective reduction type NO.sub.X catalyst device 2 is arranged in
the exhaust passage 1 for purifying the NO.sub.X. This selective
reduction type NO.sub.X catalyst device 2 has an ammonia holding
ability by which it holds (or adsorbs) the ammonia when the
concentration of the ammonia NH.sub.3 in the exhaust gas is high
and disassociates (or releases) the ammonia when the concentration
of ammonia NH.sub.3 in the exhaust gas becomes low and is, for
example, formed as a zeolite-based denitration catalyst device
comprised of a carrier on the surface of which copper zeolite,
platinum-copper zeolite, or iron zeolite is carried or is formed
including zeolite, silica, silica alumina, titania, or other solid
acid and carrying iron Fe or copper Cu or other transition metal or
platinum Pt or palladium Pd or other precious metal.
[0018] In such a selective reduction type NO.sub.X catalyst device
2, the NO.sub.X which is contained in the exhaust gas of the lean
air-fuel ratio at the time of lean combustion is reduced by the
ammonia NH.sub.3 which is released from the selective reduction
type NO.sub.X catalyst device 2 (for example,
4NH.sub.3+4NO+O.sub.2.fwdarw.4N.sub.2+6H.sub.2O and
8NH.sub.3+6NO.sub.2.fwdarw.7N.sub.2+12H.sub.2O). In this way, if
the selective reduction type NO.sub.X catalyst device 2 is made to
adsorb a sufficient amount of ammonia, it is possible to purify the
NO.sub.X in the exhaust gas well by reduction.
[0019] In the present exhaust purification system, to produce the
ammonia to be supplied to the selective reduction type NO.sub.X
catalyst device 2, a storage reduction type NO.sub.X catalyst
device 3 is arranged at the upstream side of the selective
reduction type NO.sub.X catalyst device 2. The storage reduction
type NO.sub.X catalyst device 3 is formed by a carrier on which an
NO.sub.X holding agent and platinum Pt or another such precious
metal catalyst are carried. The NO.sub.X holding agent is at least
one agent which is selected from potassium K, sodium Na, lithium
Li, cesium Cs, or other such alkali metal, barium Ba, calcium Ca,
or other such alkali earth, and lanthanum La, yttrium Y, or other
such rare earth.
[0020] The storage reduction type NO.sub.X catalyst device 3 holds
the NO.sub.X in the exhaust gas well, that is, absorbs it as
nitrates well or adsorbs it well as NO.sub.2 when the exhaust gas
is a lean air-fuel ratio, that is, the concentration of oxygen in
the exhaust gas is high. On the other hand, if making the air-fuel
ratio of the exhaust gas the stoichiometric air-fuel ratio or the
rich air-fuel ratio, that is, if making the concentration of oxygen
in the exhaust gas low, the held NO.sub.X is disassociated, that
is, the absorbed NO.sub.X is released and, further, the adsorbed
NO.sub.2 is disassociated. The disassociated NO.sub.X is reduced by
the reducing substances in the exhaust gas.
[0021] In the reduction of NO.sub.X, if the air-fuel ratio of the
exhaust gas is the desired rich air-fuel ratio, when the
temperature of the storage reduction type NO.sub.X catalyst device
3 is high, NO.sub.X is sufficiently reduced and mainly ammonia
NH.sub.3 and N.sub.2 are produced (for example,
5H.sub.2+2NO.fwdarw.2NH.sub.3+2H.sub.2O,
7H.sub.2+2NO.sub.2.fwdarw.2NH.sub.3+4H.sub.2O,
2CO+2N.fwdarw.N.sub.2+2CO.sub.2,
2H.sub.2+2NO.fwdarw.2N.sub.2+2H.sub.2O,
4CO+2NO.sub.2.fwdarw.N.sub.2+4CO.sub.2,
4H.sub.2+2NO.sub.2.fwdarw.N.sub.2+4H.sub.2O) . In this way,
according to the storage reduction type NO.sub.X catalyst device 3,
not only part of the NO.sub.X in the exhaust gas, but also part of
the disassociated NO.sub.X can be reduced to ammonia. A sufficient
amount of ammonia NH.sub.3 can be produced and supplied to the
selective reduction type NO.sub.X catalyst device 2.
[0022] However, if the storage reduction type NO.sub.X catalyst
device is a low temperature, the NO.sub.X is reduced, due to
insufficient reduction, not to N.sub.2 and ammonia, but mainly
dinitrogen monoxide N.sub.2O (for example,
2NO+N.sub.2.fwdarw.2N.sub.2O and
2NO.sub.2+3N.sub.2.fwdarw.4N.sub.2O) and N.sub.2. Not only cannot a
sufficient amount of ammonia be supplied to the selective reduction
type NO.sub.X catalyst device, but a relatively large amount of
N.sub.2O for which release into the atmosphere is undesirable ends
up being produced.
[0023] To alleviate this problem, the exhaust purification system
according to the present invention produces ammonia in the storage
reduction type NO.sub.X catalyst device 3 according to the flow
chart which is shown in FIG. 2.
[0024] First, at step 101, it is judged if a current temperature T
of the storage reduction type NO.sub.X catalyst device 3 is less
than a first set temperature Ti (for example, 300.degree. C.).
Here, the current temperature T of the storage reduction type
NO.sub.X catalyst device 3 may be measured by a temperature sensor,
but may also be estimated. For example, the current engine
operating state (engine speed, fuel injection amount, combustion
air-fuel ratio, etc.) may be used as the basis to estimate the
temperature T of the storage reduction type NO.sub.X catalyst
device 3. Further, the temperature of the exhaust gas (measured or
estimated) which flows into the storage reduction type NO.sub.X
catalyst device 3 may be used as the basis to estimate the
temperature T of the storage reduction type NO.sub.X catalyst
device 3.
[0025] When the judgment at step 101 is negative, at step 102, it
is judged that the temperature (T) of the storage reduction type
NO.sub.X catalyst device 3 is a second set temperature (for example
400.degree. C.) or more. When this judgment is negative, that is,
the temperature (T) of the storage reduction type NO.sub.X catalyst
device 3 is the first set temperature (T1) or more and less than
the second set temperature (T2), at step 103, it is judged if the
amount (A) of NO.sub.X which is held at the storage reduction type
NO.sub.X catalyst device 3 has reached a first set amount (A1).
[0026] Here, the amount (A) of the NO.sub.X which is held at the
storage reduction type NO.sub.X catalyst device 3 can be estimated,
for example, by setting in advance the amount of NO.sub.X which is
contained in exhaust gas per unit time for each engine operating
state, assuming that a given set rate of it is held at the storage
reduction type NO.sub.X catalyst device 3 per unit time, and
cumulatively adding the held amount per unit time.
[0027] When the judgment at step 103 is negative, the routine is
ended as is. At this time, the NO.sub.X which was not held at the
storage reduction type NO.sub.X catalyst device 3 is purified by
reduction at the selective reduction type NO.sub.X catalyst device
2 by using the disassociated ammonia. If the judgment at step 103
is positive, at step 104, to start the ammonia generation period at
the storage reduction type NO.sub.X catalyst device 3, the air-fuel
ratio (AF) of the exhaust gas which flows into the storage
reduction type NO.sub.X catalyst device 3 is changed from the lean
air-fuel ratio (AFL) at the time of lean combustion to the first
rich air-fuel ratio (AFR1). For this reason, for example, it is
possible to supply additional fuel to the exhaust passage 1 at the
upstream side of the storage reduction type NO.sub.X catalyst
device 3 or supply additional fuel from a fuel injector in the
exhaust stroke or expansion stroke into a cylinder.
[0028] Next, at step 105, it is judged if the elapsed time (t) from
when the ammonia generation period was started has reached the
first set time (t1). Up to when this judgment is positive, the
air-fuel ratio (AF) of the exhaust gas which flows into the storage
reduction type NO.sub.X catalyst device 3 is made the first rich
air-fuel ratio (AFR1). For example, the first set time (t1) is made
the time until all of the first set amount A1 of NO.sub.X which is
held at the storage reduction type NO.sub.X catalyst device 3 is
disassociated by the exhaust gas of the first rich air-fuel ratio
(AFR1).
[0029] When the judgment at step 105 is positive, that is, when the
ammonia generation period reaches the first set time t1, at step
106, the air-fuel ratio (AF) of the exhaust gas which flows into
the storage reduction type NO.sub.X catalyst device 3 is changed to
the lean air-fuel ratio (AFL) at the time of lean combustion. That
is, the supply of additional fuel into the exhaust passage 1 or
cylinder is stopped. In this way, the ammonia generation period is
ended. From this time, cumulative addition of the NO.sub.X amount
which is held at the storage reduction type NO.sub.X catalyst
device 3 is started based on the current engine operating state.
Further, the NO.sub.X which was not held at the storage reduction
type NO.sub.X catalyst device 3 is purified by reduction using the
disassociated ammonia at the selective reduction type NO.sub.X
catalyst device 2.
[0030] In this way, when the temperature (T) of the storage
reduction type NO.sub.X catalyst device 3 is relatively high and
the storage reduction type NO.sub.X catalyst device 3 has a
sufficient reducing ability, in the ammonia generation period, as
explained earlier, the NO.sub.X in the exhaust gas and the NO.sub.X
which is disassociated from the storage reduction type NO.sub.X
catalyst device 3 are mainly reduced to ammonia and nitrogen, and a
sufficient amount of ammonia NH.sub.3 is produced and supplied to
the selective reduction type NO.sub.X catalyst device 2 to be held
at the selective reduction type NO.sub.X catalyst device 2.
[0031] However, when the temperature (T) of the storage reduction
type NO.sub.X catalyst device 3 is relatively low and the reducing
ability of the storage reduction type NO.sub.X catalyst device 3
falls, if the ammonia generation period is started in the same way
as above, the NO.sub.X in the exhaust gas and the NO.sub.X which
was disassociated from the storage reduction type NO.sub.X catalyst
device 3 end up being reduced to mainly nitrogen and dinitrogen
monoxide.
[0032] In the present flow chart, when the judgment of step 101 is
positive, at step 107, it is judged if the NO.sub.X amount (A)
which is being held at the storage reduction type NO.sub.X catalyst
device 3 has reached a second set amount (A2). The second set
amount (A2) is an amount smaller than the first set amount
(A1).
[0033] When the judgment at step 107 is negative, the routine is
ended as it is. The NO.sub.X which was not held at the storage
reduction type NO.sub.X catalyst device 3 is purified by reduction
using the disassociated ammonia at the selective reduction type
NO.sub.X catalyst device 2. On the other hand, if the judgment of
step 107 is positive, at step 108, to start the ammonia generation
period at the storage reduction type NO.sub.X catalyst device 3, in
the same way as above, the air-fuel ratio (AF) of the exhaust gas
which flows into the storage reduction type NO.sub.X catalyst
device 3 is changed from the lean air-fuel ratio (AFL) at the time
of lean combustion to the first rich air-fuel ratio (AFR1).
[0034] Next, at step 109, it is judged if the elapsed time (t) from
when the ammonia generation period was started has reached a second
set time (t2). Until this judgment is positive, the air-fuel ratio
(AF) of the exhaust gas which flows into the storage reduction type
NO.sub.X catalyst device 3 is made the first rich air-fuel ratio
(AFR1). For example, the second set time (t2) is made the time
until all of the second set amount (A2) of NO.sub.X which is held
at the storage reduction type NO.sub.X catalyst device 3 is
disassociated by the exhaust gas of the first rich air-fuel ratio
(AFR1).
[0035] When the judgment at step 109 is positive, that is, when the
ammonia generation period reaches the second set time (t2), at step
110, the air-fuel ratio (AF) of the exhaust gas which flows into
the storage reduction type NO.sub.X catalyst device 3 is changed to
the lean air-fuel ratio (AFL) at the time of lean combustion. In
this way, the ammonia generation period is ended. The cumulative
addition of the NO.sub.X amount which is held at the storage
reduction type NO.sub.X catalyst device 3 is started based on the
current engine operating state from this time. Further, the
NO.sub.X which was not held at the storage reduction type NO.sub.X
catalyst device 3 is purified by reduction using the disassociated
ammonia at the selective reduction type NO.sub.X catalyst device
2.
[0036] In this way, when the temperature (T) of the storage
reduction type NO.sub.X catalyst device 3 is relatively low and the
storage reduction type NO.sub.X catalyst device 3 does not have a
sufficient reducing ability, the ammonia generation period is
started when the amount of NO.sub.X which is held at the storage
reduction type NO.sub.X catalyst device 3 is small, the amount of
NO.sub.X which is disassociated from the storage reduction type
NO.sub.X catalyst device 3 in the ammonia generation period is made
smaller, and the NO.sub.X which is disassociated from the storage
reduction type NO.sub.X catalyst device 3 and the NO.sub.X in the
exhaust gas can be reduced to mainly ammonia and nitrogen.
[0037] Further, when the temperature (T) of the storage reduction
type NO.sub.X catalyst device 3 is extremely high and the reducing
ability of the storage reduction type NO.sub.X catalyst device 3 is
extremely high, if starting the ammonia generation period such as
at step 103, a large amount of ammonia would be produced and a
relatively large amount of ammonia would end up passing straight
through the selective reduction type NO.sub.X catalyst device.
[0038] Due to this, when the judgment at step 102 is positive, at
step 111, it is judged if the amount (A) of the NO.sub.X which is
held at the storage reduction type NO.sub.X catalyst device 3 has
reached the first set amount (A1). When the judgment at step 111 is
negative, the routine is ended as it is When the judgment at step
111 is positive, at step 112, the air-fuel ratio (AF) of the
exhaust gas which flows into the storage reduction type NO.sub.X
catalyst device 3 is changed from the lean air-fuel ratio (AFL) at
the time of lean combustion to a second rich air-fuel ratio (AFR2).
The second rich air-fuel ratio (AFR2) is a rich air-fuel ratio
closer to the stoichiometric air-fuel ratio than the first rich
air-fuel ratio (AFR1). Further, the air-fuel ratio (AF) of the
exhaust gas which flows into the storage reduction type NO.sub.X
catalyst device 3 may also be made the stoichiometric air-fuel
ratio instead of the second rich air-fuel ratio (AFR2).
[0039] In this way, if making the air-fuel ratio of the exhaust gas
the stoichiometric air-fuel ratio or a rich air-fuel ratio (AFR2)
near the stoichiometric air-fuel ratio, the reducing ability at the
storage reduction type NO.sub.X catalyst device 3 falls, the
NO.sub.X which is disassociated from the storage reduction type
NO.sub.X catalyst device 3 and the NO.sub.X in the exhaust gas are
purified by reduction mainly to nitrogen, and almost no ammonia is
produced or the amount of production of ammonia can be sufficiently
decreased to enable the selective reduction type NO.sub.X catalyst
device 2 to hold it.
[0040] By purifying NO.sub.X by reduction in this way instead of
starting the ammonia generation period, a relatively large amount
of ammonia is prevented from passing straight through the selective
reduction type NO.sub.X catalyst device.
[0041] Next, at step 113, it is judged if the elapsed time (t) from
the start of purification of NO.sub.X by reduction to make the
air-fuel ratio of the exhaust gas the second rich air-fuel ratio
(AFR2) has reached a third set time (t3). Until this judgment is
positive, the air-fuel ratio (AF) of the exhaust gas which flows
into the storage reduction type NO.sub.X catalyst device 3 is made
the second rich air-fuel ratio (AFR2). For example, the third set
time (t3) is made the time until all of the first set amount (A1)
of NO.sub.X which is held at the storage reduction type NO.sub.X
catalyst device 3 is disassociated by the exhaust gas of the second
rich air-fuel ratio (AFR2).
[0042] When the judgment at step 113 is positive, at step 114, the
air-fuel ratio (AF) of the exhaust gas which flows into the storage
reduction type NO.sub.X catalyst device 3 is changed to the lean
air-fuel ratio (AFL) at the time of lean combustion. In this way,
the purification of NO.sub.X by reduction is ended. From this time,
cumulative addition of the amount of NO.sub.X which is held at the
storage reduction type NO.sub.X catalyst device 3 is started based
on the current engine operating state.
[0043] When the temperature (T) of the storage reduction type
NO.sub.X catalyst device 3 is extremely high, the NO.sub.X holding
ability of the storage reduction type NO.sub.X catalyst device 3 is
also high and almost all of the NO.sub.X in the exhaust gas is held
at the storage reduction type NO.sub.X catalyst device 3. In this
way, at this time, almost all of the NO.sub.X in the exhaust gas is
purified by reduction as explained above after being held at the
storage reduction type NO.sub.X catalyst device 3. Therefore, the
selective reduction type NO.sub.X catalyst device 2 may not almost
purify NO.sub.X by reduction.
[0044] In the ammonia generation period (step 104 and 108) and the
purification of NO.sub.X by reduction (step 112) at the present
flow chart, the air-fuel ratio of the exhaust gas may be
continuously made the first rich air-fuel ratio (AFR1) or the
second rich air-fuel ratio (AFR2), but it is also possible to make
it so that the first rich air-fuel ratio (AFR1) or the second rich
air-fuel ratio (AFR2) and the lean air-fuel ratio are repeated. In
this case, the set times (t1), (t2), and (t3) until all of the
NO.sub.X which is held at the storage reduction type NO.sub.X
catalyst device 3 is disassociated are made longer compared to when
continuously making the air-fuel ratio of the exhaust gas the first
rich air-fuel ratio (AFR1) or the second rich air-fuel ratio
(AFR2).
[0045] In the present flow chart, when the temperature (T) of the
storage reduction type NO catalyst device 3 is the second set
temperature T2 or more, instead of starting the ammonia generation
period, the NO.sub.X is purified by reduction (steps 111 to 114),
but when the temperature (T) of the storage reduction type NO.sub.X
catalyst device 3 becomes the first set temperature T1 or more, it
is also possible to not start the ammonia generation period, but
purify the NO.sub.X by reduction (steps 111 to 114).
[0046] Further, in the present flow chart, the amount (A) of the
NO.sub.X which is held at the storage reduction type NO.sub.X
catalyst device 3 is estimated so as to start the ammonia
generation period or the purification of NO.sub.X by reduction, but
it is also possible to start the ammonia generation period or
purification of NO.sub.X by reduction every set time or set running
distance. In this case, if the set time or the set running distance
from the end of the ammonia generation period to the start of the
next ammonia generation period when the temperature of the storage
reduction type NO.sub.X catalyst device is less than the first set
temperature (T1) is made shorter than that when the temperature of
the storage reduction type NO.sub.X catalyst device is the first
set temperature (T1) or more, it is possible to start the ammonia
generation period when the amount of NO.sub.X which is held at the
storage reduction type NO.sub.X catalyst device is small when the
temperature of the NO.sub.X catalyst device is less than the first
set temperature as opposed to when the temperature of the NO.sub.X
catalyst device is the first set temperature or more.
LIST OF REFERENCE NUMERALS
[0047] 1 exhaust passage [0048] 2 selective reduction type NO.sub.X
catalyst device [0049] 3 storage reduction type NO.sub.X catalyst
device
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