U.S. patent application number 13/977225 was filed with the patent office on 2013-10-31 for exhaust purification system of internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is Daichi Imai, Shigeki Nakayama, Hiromasa Nishioka, Yuichi Sobue, Kou Sugawara. Invention is credited to Daichi Imai, Shigeki Nakayama, Hiromasa Nishioka, Yuichi Sobue, Kou Sugawara.
Application Number | 20130287638 13/977225 |
Document ID | / |
Family ID | 46757523 |
Filed Date | 2013-10-31 |
United States Patent
Application |
20130287638 |
Kind Code |
A1 |
Sugawara; Kou ; et
al. |
October 31, 2013 |
EXHAUST PURIFICATION SYSTEM OF INTERNAL COMBUSTION ENGINE
Abstract
This exhaust gas purification device for an internal combustion
engine is provided with a particulate filter disposed in an engine
exhaust system, and a catalytic device disposed downstream of the
particulate filter in the engine exhaust system. The catalytic
device holds sulfuric acid as a sulfate and releases the product as
SO.sub.2.
Inventors: |
Sugawara; Kou; (Susono-shi,
JP) ; Nakayama; Shigeki; (Gotemba-shi, JP) ;
Sobue; Yuichi; (Susono-shi, JP) ; Nishioka;
Hiromasa; (Susono-shi, JP) ; Imai; Daichi;
(Susono-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sugawara; Kou
Nakayama; Shigeki
Sobue; Yuichi
Nishioka; Hiromasa
Imai; Daichi |
Susono-shi
Gotemba-shi
Susono-shi
Susono-shi
Susono-shi |
|
JP
JP
JP
JP
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi
JP
|
Family ID: |
46757523 |
Appl. No.: |
13/977225 |
Filed: |
February 28, 2011 |
PCT Filed: |
February 28, 2011 |
PCT NO: |
PCT/JP2011/055161 |
371 Date: |
June 28, 2013 |
Current U.S.
Class: |
422/169 |
Current CPC
Class: |
F01N 13/009 20140601;
F01N 3/033 20130101; Y02T 10/20 20130101; F01N 3/106 20130101; F01N
3/0807 20130101; F01N 3/085 20130101; Y02T 10/12 20130101 |
Class at
Publication: |
422/169 |
International
Class: |
F01N 3/08 20060101
F01N003/08 |
Claims
1. An exhaust purification system of an internal combustion engine,
comprising a particulate filter which is arranged in an engine
exhaust system and a catalyst device which is arranged at a
downstream side of said particulate filter of the engine exhaust
system, said catalyst device holding sulfuric acid as a sulfuric
acid salt and releasing it as SO.sub.2, regeneration processing
being performed to burn off trapped particulate at said particulate
filter when an amount of SO.sub.2 adsorption of said particulate
filter reaches a set amount, and sulfuric acid which is released
from said particulate filter due to said regeneration processing
being held by said catalyst device.
2. The exhaust purification system of an internal combustion engine
according to claim 1 wherein the sulfuric acid salt which is held
at said catalyst device is released as SO.sub.2 by heat
decomposition.
3. The exhaust purification system of an internal combustion engine
according to claim 1 wherein the sulfuric acid salt which is held
at said catalyst device is released as SO.sub.2 by lowering the
oxygen concentration in said catalyst device.
4. The exhaust purification system of an internal combustion engine
according to claim 1, wherein said catalyst device is comprised of
a substrate on which zirconium oxide or cerium oxide is coated.
5. The exhaust purification system of an internal combustion engine
according to claim 2 wherein said catalyst device is comprised of a
substrate on which zirconium oxide or cerium oxide is coated.
6. The exhaust purification system of an internal combustion engine
according to claim 3 wherein said catalyst device is comprised of a
substrate on which zirconium oxide or cerium oxide is coated.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust purification
system of an internal combustion engine.
BACKGROUND ART
[0002] To suppress release of particulate which is contained in
exhaust gas into the atmosphere, a particulate filter is arranged
in an engine exhaust system to trap the particulate. The
particulate which is trapped by the particulate filter increases
the exhaust resistance of the particulate filter, and therefore
before the exhaust resistance of the particulate filter rises to a
set value or more, regeneration processing for burning off the
trapped particulate is necessary.
[0003] The regeneration processing of a particulate filter feeds
additional fuel into the exhaust gas, burns the additional fuel at
the particulate filter or at the oxidation catalyst device at the
immediate upstream side of the particulate filter, and utilizes the
heat of combustion to raise the temperature of the particulate
filter to the combustion temperature of the particulate of about
600.degree. C.
[0004] Not only when making the additional fuel burn at the
particulate filter, but also when making the additional fuel burn
at the oxidation catalyst device at the immediate upstream side of
the particulate filter, to suppress release into the atmosphere of
additional fuel which slips through the oxidation catalyst device,
it is preferable to make the particulate filter carry a precious
metal catalyst using alumina as a carrier so as to enable the
additional fuel to be burned.
[0005] If providing an alumina carrier on a particulate filter in
this way, the SO.sub.2 in the exhaust gas is adsorbed at the
alumina carrier. The SO.sub.2 which is adsorbed at the alumina
carrier is released when the particulate filter is made high in
temperature at the time of regeneration processing. If the SO.sub.2
is oxidized by the precious metal catalyst of the particulate
filter and thus becomes SO.sub.3, this will react with the moisture
in the exhaust gas and become sulfuric acid. Sulfuric acid is low
in boiling point and immediately forms mist-like white fumes which
can be seen, and therefore if they are released as is into the
atmosphere, the vehicle will be seen as one which releases white
fumes and the commodity value of the vehicle will fall.
[0006] It has been proposed, in S poisoning reversal processing
which releases the Sa.sub.x, which is stored in the NO.sub.x
storage catalyst device, as SO.sub.2, to make the released SO.sub.2
react with water and liquefy as sulfuric acid and to hold this
sulfuric acid mist as iron sulfide at a mist catch plate and
thereby suppress release into the atmosphere (for example, see PLT
1).
CITATIONS LIST
Patent Literature
[0007] PLT 1: Japanese Patent Publication No. 2007-218108A [0008]
PLT 2: Japanese Patent Publication No. 2009-138667A
SUMMARY OF INVENTION
Technical Problem
[0009] If arranging a mist catch plate like the one disclosed in
PLT 1 at the downstream side of a particulate filter, it is
possible to sufficiently suppress release into the atmosphere of
sulfuric acid fumes which are generated at the time of regeneration
processing of a particulate filter.
[0010] However, such a mist catch plate has to be periodically
replaced. Each time, the vehicle has to be taken to a repair
factory.
[0011] Therefore, an object of the present invention is to provide
an exhaust purification system of an internal combustion engine
which does not require a mist catch plate, which requires periodic
replacement, and which suppresses release into the atmosphere of
sulfuric acid fumes which are generated at the time of regeneration
processing of a particulate filter.
Solution to Problem
[0012] The exhaust purification system of an internal combustion
engine according to the present invention as set forth in claim 1
is characterized by comprising a particulate filter which is
arranged in an engine exhaust system and a catalyst device which is
arranged at a downstream side of the particulate filter of the
engine exhaust system, the catalyst device holding sulfuric acid as
a sulfuric acid salt and releasing it as SO.sub.2.
[0013] The exhaust purification system of an internal combustion
engine according to the present invention as set forth in claim 2
provides the exhaust purification system of an internal combustion
engine as set forth in claim 1 characterized by making the sulfuric
acid salt which is held at the catalyst device be released as
SO.sub.2 by heat decomposition.
[0014] The exhaust purification system of an internal combustion
engine according to the present invention as set forth in claim 3
provides the exhaust purification system of an internal combustion
engine as set forth in claim 1 characterized by making the sulfuric
acid salt which is held at the catalyst device be released as
SO.sub.2 by lowering the oxygen concentration in the catalyst
device.
[0015] The exhaust purification system of an internal combustion
engine according to the present invention as set forth in claim 4
provides the exhaust purification system of an internal combustion
engine as set forth in any one of claims 1 to 3 characterized in
that the catalyst device is comprised of a substrate on which
zirconium oxide or cerium oxide is coated.
Advantageous Effects of Invention
[0016] According to the exhaust purification system of an internal
combustion engine according to the present invention as set forth
in claim 1, there are provided a particulate filter which is
arranged in an engine exhaust system and a catalyst device which is
arranged at a downstream side of the particulate filter of the
engine exhaust system, and the catalyst device is designed to hold
sulfuric acid as a sulfuric acid salt and release it as SO.sub.2.
Due to this, even if sulfuric acid fumes are generated at the time
of regeneration processing in the particulate filter, the sulfuric
acid fumes are held as a sulfuric acid salt in the downstream side
catalyst device and are released as SO.sub.2, and therefore there
is no need for a mist catch plate which requires periodic
replacement and it is possible to suppress the release of sulfuric
acid fumes into the atmosphere at the time of regeneration
processing of a particulate filter.
[0017] According to the exhaust purification system of an internal
combustion engine according to the present invention as set forth
in claim 2, based on the exhaust purification system of an internal
combustion engine as set forth in claim 1, the sulfuric acid salt
which was held at the catalyst device can be easily released as
SO.sub.2 by heat decomposition.
[0018] According to the exhaust purification system of an internal
combustion engine according to the present invention as set forth
in claim 3, based on the exhaust purification system of an internal
combustion engine as set forth in claim 1, the sulfuric acid salt
which was held at the catalyst device can be easily released as
SO.sub.2 by lowering the oxygen concentration in the catalyst
device.
[0019] According to the exhaust purification system of an internal
combustion engine according to the present invention as set forth
in claim 4, based on the exhaust purification system of an internal
combustion engine as set forth in any one of claims 1 to 3, the
catalyst device is comprised of a substrate on which zirconium
oxide or cerium oxide is coated, and a sulfuric acid salt which was
generated by reaction with the zirconium oxide or cerium oxide can
be easily released as SO.sub.2 by heat decomposition or by lowering
the oxygen concentration.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is a schematic view which shows an exhaust
purification system of an internal combustion engine according to
the present invention.
[0021] FIG. 2 is a first flowchart for suppressing the release of
fumes at the time of regeneration processing of a particulate
filter.
[0022] FIG. 3 is a map for estimating an amount of SO.sub.2
adsorption AS at a particulate filter per unit time.
[0023] FIG. 4 is a map for estimating an amount of SO.sub.2 release
BS from a catalyst device per unit time.
[0024] FIG. 5 is a second flowchart for suppressing the release of
fumes at the time of regeneration processing of a particulate
filter.
DESCRIPTION OF EMBODIMENTS
[0025] FIG. 1 is a schematic view which shows an exhaust
purification system of an internal combustion engine according to
the present embodiment. The internal combustion engine is for
example a diesel engine. In the figure, 1 is a particulate filter
for trapping particulate which is contained in exhaust gas. 2 is an
oxidation catalyst device which is arranged at an immediate
upstream side of the particulate filter 1, while 3 is a catalyst
device which is arranged at a downstream side of the particulate
filter 1.
[0026] The particulate filter 1 is a wall flow type which forms a
honeycomb structure which is for example formed from a porous
material such as cordierite and has a large number of axial
direction spaces which are formed by subdivision by partitions
which extend in the axial direction. In two adjoining axial
direction spaces, one is closed by a plug at an exhaust downstream
side, while the other is closed at the exhaust upstream side. In
this way, one of two adjoining axial direction spaces becomes an
inflow passage for exhaust gas and the other becomes an outflow
passage. The exhaust gas is designed to pass through the
partitions. The particulate in exhaust gas is extremely small
compared with the size of the pores of the partitions, but collide
with and are trapped by the exhaust upstream side surfaces of the
partitions and the pore surfaces in the partitions. In this way,
the partitions function as walls which trap the particulate.
[0027] The particulate which is trapped by the particulate filter 1
makes the exhaust resistance of the particulate filter 1 increase,
and therefore before the exhaust resistance of the particulate
filter 1 rises to a set value or more and thus the engine output
greatly falls, regeneration processing for burning off the trapped
particulate must be carried out.
[0028] The regeneration processing of the particulate filter 1 is
performed by feeding additional fuel to the cylinders from the fuel
injectors in the latter half of the expansion stroke or exhaust
stroke or feeding additional fuel by a fuel feed device (not shown)
which is arranged in the exhaust passage at the exhaust upstream
side of the oxidation catalyst device 2, making this additional
fuel burn off in the oxidation catalyst device 2, and utilizing
this heat of combustion to raise the particulate filter 1 to the
combustion temperature of the particulate, that is, about
600.degree. C.
[0029] The amount of additional fuel per unit time which flows into
the oxidation catalyst device 2 is controlled based on the exhaust
gas amount and the exhaust gas temperature which changes due to the
current engine operating state (engine load and engine speed). The
larger the exhaust gas amount, the greater the amount of additional
fuel per unit time, while the higher the exhaust gas temperature,
the smaller the amount of additional fuel per unit time. Due to
this, the temperature of the exhaust gas which flows into the
particulate filter 1 is controlled to about 600.degree. C. It is
also possible to measure the temperature of the exhaust gas which
flows into the particulate filter 1 and perform feedback control on
the amount of additional fuel per unit time which flows into the
oxidation catalyst device so that this measurement temperature
becomes about 600.degree. C. If in this way the temperature of the
exhaust gas which flows to the particulate filter 1 becomes about
600.degree. C., it is possible to make the particulate which is
trapped at the particulate filter 1 burn from the exhaust upstream
side of the particulate filter 1 toward the exhaust downstream side
and possible to easily make the trapped particulate completely
burn.
[0030] Further, even if the oxidation catalyst device 2 is omitted,
if the particulate filter 1 carries platinum or another precious
metal catalyst, it is possible to make the additional fuel which
was controlled in amount in the same way as above burn at the
particulate filter 1 and use this heat of combustion to raise the
particulate filter 1 to about 600.degree. C. to perform the
regeneration processing. In this case, the precious metal catalyst
is carried on the exhaust upstream side surfaces of the partitions
of the particulate filter 1 using alumina etc. as a carrier.
Further, even when an oxidation catalyst device 2 is provided, to
suppress the release into the atmosphere of the additional fuel
which slips through the oxidation catalyst device 2, it is
desirable to make the particulate filter 1 carry a precious metal
catalyst using alumina etc. as a carrier so as to make the
additional fuel burn.
[0031] If, in this way, the particulate filter 1 is provided with
an alumina carrier, the SO.sub.2 in the exhaust gas is adsorbed on
the alumina carrier. The SO.sub.2 which was adsorbed on the alumina
carrier is released if the particulate filter reaches a high
temperature such as 600.degree. C. at the time of the regeneration
processing. Substantially all of the released SO.sub.2 is oxidized
by the precious metal catalyst of the particulate filter 1 to
become SO.sub.3. If becoming SO.sub.3 in this way, this reacts with
the moisture in the exhaust gas to become sulfuric acid. Sulfuric
acid is low in boiling point and immediately forms mist-like fumes
which can be seen if left untreated and released into the
atmosphere, and therefore the commodity value of the vehicle
falls.
[0032] To suppress the release into the atmosphere of the sulfuric
acid fumes which are generated at the time of regeneration
processing of the particulate filter 1, an exhaust purification
system according to the present invention arranges a catalyst
device 3 at the immediate downstream side of the particulate filter
and controls the feed of additional fuel at the time of
regeneration processing by an electronic control unit (digital
computer) in accordance with the first flowchart which is shown in
FIG. 2.
[0033] The catalyst device 3 is comprised of a honeycomb-like
substrate on the surface of which zirconium oxide (ZrO.sub.2) or
cerium oxide (CeO.sub.2 or Ce.sub.2O.sub.3) or another catalyst
which can hold sulfuric acid as a sulfuric acid salt is coated. In
particular, a precious metal catalyst which has an oxidation
function is not carried.
[0034] The first flowchart of FIG. 2 will be explained below.
First, at step 101, the amount of SO.sub.2 adsorption AS which is
adsorbed per unit time at the particulate filter 1 in the current
engine operating state is estimated by, for example, using the map
which is shown in FIG. 3. The map shown in FIG. 3 is set with the
amount of SO.sub.2 adsorption AS to the particulate filter 1 per
unit time for each engine operating state (combination of engine
load L and engine speed NE) found experimentally. The SO.sub.2 in
the exhaust gas is mainly generated by oxidation of the sulfur
contained in the fuel. The greater the amount of fuel injection in
the engine operating state, the greater the amount of SO.sub.2
adsorption AS per unit time.
[0035] Next, at step 102, the amount of SO.sub.2 adsorption AS is
cumulatively added to calculate the current amount of SO.sub.2
adsorption A of the particulate filter 1. At step 103, it is judged
if the current amount of SO.sub.2 adsorption A of the particulate
filter 1 has reached a set amount A1.
[0036] When the judgment at step 103 is negative, the routine ends
as is, but when the current amount of SO.sub.2 adsorption A of the
particulate filter 1 has reached the set amount A1, the judgment as
step 103 is affirmative and, at step 104, regeneration processing
of the particulate filter 1 is performed.
[0037] At this time, the trapped amount of particulate of the
particulate filter 1 is prevented from increasing the exhaust
resistance of the particulate filter 1 to a set value or more. On
the other hand, when the particulate filter reaches a high
temperature (about 600.degree. C.) for the regeneration processing
and the set amount A1 of SO.sub.2 is totally released from the
particulate filter 1, the released SO.sub.2 becomes sulfuric acid
as explained above. The set amount A1 is set so that all of this
can be held at the catalyst device 3 as a sulfuric acid salt. That
is, the greater the amount of sulfuric acid which the catalyst
device 3 can hold, the greater the set amount A1 can be set.
However, when the amount of SO.sub.2 adsorption A of the
particulate filter 1 reaches the set amount A1, the trapped amount
of particulate of the particulate filter 1 has to be prevented from
increasing exhaust resistance of the particulate filter 1 to a set
value or more.
[0038] The regeneration processing of step 104, as explained above,
feeds additional fuel to maintain the particulate filter 1 at about
600.degree. C. At step 105, it is judged if the duration "t" of the
regeneration processing has reached the time t' required for
burning off all of the trapped particulate at the time of start of
regeneration processing (the greater the set amount A1 of the
SO.sub.2 adsorption amount A, the greater the amount of trapped
particulate at the time of start of regeneration becomes, and
therefore the required time t' becomes longer). The regeneration
processing is performed until this judgment becomes affirmative. If
the judgment of step 105 is affirmative and the regeneration
processing is completed, at step 106, temperature elevation control
of the catalyst device 3 is performed.
[0039] As explained above, during the regeneration processing for
burning off the trapped particulate of the particulate filter 1,
the set amount A1 of SO.sub.2 is released and becomes sulfuric
acid, but substantially all is held at the catalyst device 2 as a
sulfuric acid salt, ant therefore release of fumes of sulfuric acid
into the atmosphere can be sufficiently suppressed.
[0040] If the catalyst of the catalyst device 2 is zirconium oxide
(ZrO.sub.2), as the sulfuric acid salt, zirconium sulfate
Zr(SO.sub.4).sub.2 is formed. The temperature elevation control of
step 106 is designed to increase the amount of additional fuel per
unit time used for the regeneration processing of the particulate
filter 1 in the different engine operating states so that exhaust
gas of about 700.degree. C. flows into the catalyst device 3. Due
to this, the zirconium sulfate Zr(SO.sub.4).sub.2 is broken down by
heat as follows:
Zr(SO.sub.4).sub.2.fwdarw.ZrO.sub.2+2SO.sub.2+O.sub.2
[0041] The catalyst device 3 does not have an oxidation function.
The SO.sub.2 which is formed at this time is not oxidized to
SO.sub.3 which is easily oxidized to sulfuric acid. The SO.sub.2
which is released from the particulate filter 1 can be released
into the atmosphere not as sulfuric acid but finally as SO.sub.2.
In this way, the zirconium oxide (ZrO.sub.2) reacts with sulfuric
acid to generate a sulfuric acid salt at the temperature of the
regeneration processing of the particulate filter 1, that is, about
600.degree. C., or less. If becoming about 700.degree. C. or more,
the sulfuric acid salt is broken down by heat and SO.sub.2 is
released.
[0042] Further, if the catalyst of the catalyst device 2 is cerium
oxide (CeO.sub.2), as the sulfuric acid salt, cerium sulfate
Ce(SO.sub.4).sub.2 is formed. At this time, due to the temperature
elevation control of step 106, the cerium sulfate
Ce(SO.sub.4).sub.2 is broken down by heat as follows:
Ce(SO.sub.4).sub.2.fwdarw.CeO.sub.2+2SO.sub.2+O.sub.2
[0043] Due to this, in the same way, the SO.sub.2 which is released
from the particulate filter 1 can be released into the atmosphere
not as sulfuric acid but finally as SO.sub.2. In this way, the
cerium oxide (CeO.sub.2) reacts with sulfuric acid to generate a
sulfuric acid salt at the temperature of the regeneration
processing of the particulate filter 1, that is, about 600.degree.
C., or less. If becoming about 700.degree. C. or more, the sulfuric
acid salt is broken down by heat and SO.sub.2 is released.
[0044] At step 107, BS is the amount of SO.sub.2 release which is
released per unit time from the catalyst device 3 in the current
engine operating state and changes along with the engine operating
state. It is estimated by using the map which is shown in FIG. 4
etc. The map which is shown in FIG. 4 is set with the amount of
SO.sub.2 release BS from the catalyst device 3 per unit time for
each engine operating state (combination of engine load L and
engine speed NE) found experimentally. Here, it is assumed that the
catalyst device 3 is held at 700.degree. C. The greater the exhaust
gas amount, the greater the amount of SO.sub.2 release BS per unit
time. Further, the lower the oxygen concentration in the exhaust
gas, the greater the amount of SO.sub.2 release per unit time. The
oxygen concentration in the exhaust gas related to the amount of
SO.sub.2 release BS is the oxygen concentration in the exhaust gas
which flows into the catalyst device 3. In the map which is shown
in FIG. 4, consideration is given so that compared with the oxygen
concentration in the exhaust gas of the combustion air-fuel ratio
of the current engine operating state, the concentration becomes
further lower, since additional fuel is burned at the oxidation
catalyst device 2 and particulate filter 1.
[0045] At step 107, the previous SO.sub.2 holding amount A of the
catalyst device 3 is reduced by the amount of SO.sub.2 release BS
which is released per unit time from the catalyst device 3 in the
current engine operating state so as to calculate the current
SO.sub.2 holding amount A of the catalyst device 3. Here, the
initial SO.sub.2 holding amount A of the catalyst device 3 can be a
set amount A1 comprised of the SO.sub.2 holding amount at the time
of start of regeneration processing of the particulate filter 1
assuming that all of the SO.sub.2 which is released from the
particulate filter 1 is held as a sulfuric acid salt in the
catalyst device 3.
[0046] At step 108, it is judged if the current SO.sub.2 holding
amount A of the catalyst device 3 has become 0. The temperature
elevation control of step 106 is performed until this judgment
becomes affirmative. On the other hand, when the judgment at step
108 is affirmative and all of the sulfuric acid salt of the
catalyst device 3 is broken down by heat and all of the SO.sub.2 is
released, at step 109, A is reset to 0 and the routine is ended. In
preparation for the next regeneration processing of the particulate
filter 1, the catalyst device 3 is set to be able to hold the set
amount A1 of SO.sub.2 as a sulfuric acid salt.
[0047] FIG. 5 is a second flowchart for controlling the release of
fumes at the time of regeneration processing of the particulate
filter 1. Only the differences from the first flowchart will be
explained below. In the present flowchart, at step 206, to make the
sulfuric acid salt which is held at the catalyst device 3 release
SO.sub.2, not temperature elevation control, but rich control is
performed. The rich control feeds additional fuel to the cylinders
or the exhaust passage at the upstream side of the oxidation
catalyst device 2 (by method similar to regeneration processing of
particulate filter 1) so that the air-fuel ratio of the exhaust gas
reaches the desired rich air-fuel ratio (or stoichiometric air-fuel
ratio) with respect to the combustion air-fuel ratio of the current
engine operating state, and sufficiently lowers the oxygen
concentration in the exhaust gas after passing through the
oxidation catalyst device 2 and particulate filter 1.
[0048] If, in this way, the oxygen concentration in the catalyst
device 3 falls, the zirconium sulfate Zr(SO.sub.4).sub.2 or cerium
sulfate Ce(SO.sub.4).sub.2 of the catalyst device 3 tries to
release oxygen, is easily broken down in the same way as the above
heat decomposition, and releases SO.sub.2.
Zr(SO.sub.4).sub.2.fwdarw.ZrO.sub.2+2SO.sub.2+O.sub.2
Ce(SO.sub.4).sub.2.fwdarw.CeO.sub.2+2SO.sub.2+O.sub.2
[0049] At step 207, BS' is the amount of SO.sub.2 release which is
released per unit time from the catalyst device 3 in the current
engine operating state. It changes along with the engine operating
state and is estimated by, for example, using a map. The map is set
with the amount of SO.sub.2 release BS' from the catalyst device 3
per unit time for each engine operating state (combination of
engine load L and engine speed NE) found experimentally. Here, it
is assumed that the air-fuel ratio of the exhaust gas flowing into
the catalyst device 3 is a desired rich air-fuel ratio by the
additional fuel. The greater the exhaust gas amount, the greater
the amount of SO.sub.2 release BS' per unit time, while the higher
the exhaust gas temperature, the greater the amount of SO.sub.2
release BS' per unit time. The exhaust gas temperature related to
the amount of SO.sub.2 release BS' is the temperature of the
exhaust gas which flows into the catalyst device 3. In the
above-mentioned map, consideration is given so that compared with
the temperature of the exhaust gas which is exhausted from the
cylinders in the current engine operating state, the temperature
becomes further higher, since additional fuel is burned at the
oxidation catalyst device 2 and particulate filter 1.
[0050] At step 207, the previous SO.sub.2 holding amount A of the
catalyst device 3 is reduced by the amount of SO.sub.2 release BS'
which is released per unit time from the catalyst device 3 in the
current engine operating state so as to calculate the current
SO.sub.2 holding amount A of the catalyst device 3. Here, the
initial SO.sub.2 holding amount A of the catalyst device 3, in the
same way as the first flowchart, can be a set amount A1 comprised
of the SO.sub.2 holding amount at the time of start of regeneration
processing of the particulate filter 1 assuming that all of the
SO.sub.2 which is released from the particulate filter 1 is held as
a sulfuric acid salt in the catalyst device 3.
[0051] At step 208, it is judged if the current SO.sub.2 holding
amount A of the catalyst device 3 has become 0. The rich control of
step 206 is performed until this judgment becomes affirmative. On
the other hand, if the judgment of step 208 is affirmative and all
of the sulfuric acid salts of the catalyst device 3 is broken down
and all of the SO.sub.2 is released, at step 209, A is reset to 0
and the routine is ended. In preparation for the next regeneration
processing of the particulate filter 1, the catalyst device 3 is
set to be able to hold the set amount A1 of SO.sub.2 as a sulfuric
acid salt.
REFERENCE SIGNS LIST
[0052] 1 . . . particulate filter [0053] 2 . . . oxidation catalyst
device [0054] 3 . . . catalyst device
* * * * *