Exhaust purification device of compression ignition type internal combustion engine Number:7,181,904 from the United States Patent and Trademark Office (PTO) owispatent An SO.sub.x trap catalyst (11) able to trap SO.sub.x contained in exhaust gas is arranged in an engine exhaust passage upstream of an NO.sub.x storing catalyst (12). When the SO.sub.x trap rate by the SO.sub.x trap catalyst (11) falls, the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst (11) is maintained lean and in that state the SO.sub.x trap catalyst (11) is raised in temperature. The SO.sub.x trapped at that time diffuses inside the SO.sub.x trap catalyst (11), whereby the SO.sub.x trap rate is restored.<H purification, compression, Exhaust, purific, 7181904.html, Patent, pto, 7181904

Title: Exhaust purification device of compression ignition type internal combustion engine

Abstract: An SO.sub.x trap catalyst (11) able to trap SO.sub.x contained in exhaust gas is arranged in an engine exhaust passage upstream of an NO.sub.x storing catalyst (12). When the SO.sub.x trap rate by the SO.sub.x trap catalyst (11) falls, the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst (11) is maintained lean and in that state the SO.sub.x trap catalyst (11) is raised in temperature. The SO.sub.x trapped at that time diffuses inside the SO.sub.x trap catalyst (11), whereby the SO.sub.x trap rate is restored.

Patent Number: 7,181,904 Issued on 02/27/2007 to Yoshida, et al.

Inventors: Yoshida; Kohei (Susono, JP), Hirota; Shinya (Susono, JP), Nakano; Yasuaki (Sunto-gun, JP)
Assignee: Toyota Jidosha Kabushiki Kaisha (Toyota, JP)

Appl. No.: 10/535,896

Filed: October 21, 2004

PCT Filed: October 21, 2004

PCT No.: PCT/JP2004/015954

371(c)(1),(2),(4) Date: May 23, 2005

PCT Pub. No.: WO20/05/040571

PCT Pub. Date: May 06, 2005

Foreign Application Priority Data


Oct 29, 2003 [JP]			2003-369173

Current U.S. Class: 60/285 ; 60/276; 60/295; 60/297; 60/300; 60/303

Current International Class: F01N 3/00 (20060101)

Field of Search: 60/274,276,277,285,295,297,301,303,300

References Cited [Referenced By]

U.S. Patent Documents


5472673	December 1995	Goto et al.
5473890	December 1995	Takeshima et al.
6145303	November 2000	Strehlau et al.
6318075	November 2001	Gunther et al.
6482377	November 2002	Bartley et al.
6758036	July 2004	Molinier
6938411	September 2005	Hoffmann et al.
7021049	April 2006	Berris et al.
2003/0115856	June 2003	Surnilla et al.
2005/0170954	August 2005	Yoshida et al.

Foreign Patent Documents


1 030 043	Aug., 2000	EP
A 06-173652	Jun., 1994	JP
A 06-272541	Sep., 1994	JP
A 07-217474	Aug., 1995	JP
A 2000-145436	May., 2000	JP
A 2001-152836	Jun., 2001	JP
A 2002-38930	Feb., 2002	JP

Primary Examiner: Nguyen; Tu M.
Attorney, Agent or Firm: Oliff & Berridge, PLC

Claims

The invention claimed is:

1. An exhaust purification device for a compression ignition type internal combustion engine having an SO.sub.x trap catalyst arranged in an engine exhaust passage and adapted for trapping SO.sub.x contained in exhaust gas and having an NO.sub.x storing catalyst arranged in the exhaust passage downstream of the SO.sub.x trap catalyst, the NO.sub.x storing catalyst storing NO.sub.x contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NO.sub.x when the air-fuel ratio of the exhaust gas flowing in becomes the stoichiometric air-fuel ratio or rich, wherein said SO.sub.x trap catalyst traps SO.sub.x contained in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst is lean, has the property that the trapped SO.sub.x gradually diffuses inside the SO.sub.x trap catalyst when the temperature of the SO.sub.x trap catalyst rises under a lean air-fuel ratio of the exhaust gas, and has the property of releasing the trapped SO.sub.x when the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst becomes rich if the temperature of the SO.sub.x trap catalyst is the SO.sub.x release temperature or more; said device is provided with air-fuel ratio control means continuing to maintain the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst lean, without allowing said exhaust gas to become rich, during engine operation and estimating means for estimating an SO.sub.x trap rate showing a ratio of SO.sub.x trapped and the SO.sub.x trap catalyst in the SO.sub.x contained in the exhaust gas; and said device makes the temperature of the SO.sub.x trap catalyst rise under a lean air-fuel ratio of the exhaust gas when the SO.sub.x trap rate falls below a predetermined rate so as to thereby restore the SO.sub.x trap rate.

2. An exhaust purification device as set forth in claim 1, wherein said SO.sub.x trap catalyst is comprised of a coated layer formed on a catalyst carrier and a precious metal catalyst maintained on the coated layer and the coated layer contains diffused in said coated layer an alkali metal, alkali earth metal, or rare earth metal.

3. An exhaust purification device as set forth in claim 1, wherein the amount of SO.sub.x trapped in said SO.sub.x trap catalyst is estimated, it is judged that the SO.sub.x trap rate has fallen below a predetermined rate when the amount of SO.sub.x trapped in the SO.sub.x trap catalyst exceeds a predetermined amount, and the SO.sub.x trap rate is restored at that time by raising the temperature of the SO.sub.x trap catalyst under a lean air-fuel ratio of the exhaust gas.

4. An exhaust purification device as set forth in claim 3, wherein said predetermined amount is increased along with an increase in the number of times of processing for restoring the SO.sub.x trap rate and the ratio of increase of the predetermined amount is reduced the greater the number of times of processing.

5. An exhaust purification device as set forth in claim 1, which arranges in an exhaust gas passage downstream of the SO.sub.x trap catalyst an SO.sub.x sensor able to detect a concentration of SO.sub.x in exhaust gas and calculates the SO.sub.x trap rate from an output signal of said SO.sub.x sensor.

6. An exhaust purification device as set forth in claim 5, wherein it is judged that the SO.sub.x trap rate has fallen below a predetermined rate when the concentration of SO.sub.x in exhaust gas detected by the SO.sub.x sensor exceeds a predetermined concentration and at that time, the SO.sub.x trap rate is restored by raising the temperature of the SO.sub.x trap catalyst under a lean air-fuel ratio of the exhaust gas.

7. An exhaust purification device as set forth in claim 1, which a reducing agent supply device is arranged in an exhaust gas passage between the SO.sub.x trap catalyst and NO.sub.x storing catalyst and, when NO.sub.x should be released from the NO.sub.x storing catalyst, reducing agent is fed from the reducing agent supply device to the inside of the exhaust passage to temporarily make the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst rich.

8. An exhaust purification device as set forth in claim 7, which, when SO.sub.x is stored in the NO.sub.x storing catalyst, the temperature of the NO.sub.x storing catalyst is raised to the SO.sub.x release temperature and reducing agent is fed from the reducing agent supply device to the inside of the exhaust passage to make the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst rich.

9. An exhaust purification device as set forth in claim 1, wherein the NO.sub.x storing catalyst is carried on a particulate filter for trapping and oxidizing particulate matter contained in the exhaust gas.

10. An exhaust purification device as set forth in claim 9, which the temperature of the particulate filter is raised under a lean air-fuel ratio of the exhaust gas when the amount of particulate matter deposited on the particulate filter exceeds an allowable amount to remove the deposited particulate matter by oxidation.

Description

TECHNICAL FIELD

The present invention relates to an exhaust purification device of a compression ignition type internal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine having arranged in an engine exhaust passage an NO.sub.x storing catalyst which stores NO.sub.x contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the stored NO.sub.x when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. In this internal combustion engine, the NO.sub.x produced when burning fuel under a lean air-fuel ratio is stored in the NO.sub.x storing catalyst. On the other hand, when the NO.sub.x storing capability of the NO.sub.x storing catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich and thereby the NO.sub.x is released from the NO.sub.x storing catalyst and reduced.

However, the fuel and lubrication oil contain sulfur. Therefore, the exhaust gas contains SO.sub.x. This SO.sub.x is stored in the NO.sub.x storing catalyst along with the NO.sub.x. However, this SO.sub.x is not released from the NO.sub.x storing catalyst just by making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SO.sub.x stored in the NO.sub.x storing catalyst gradually increases. As a result, the amount of NO.sub.x which can be stored ends up gradually decreasing.

Known in the art therefore is an internal combustion engine having an SO.sub.x absorbent arranged in the engine exhaust passage upstream of the NO.sub.x storing catalyst so as to prevent SO.sub.x from being sent to the NO.sub.x storing catalyst (see Japanese Unexamined Patent Publication (Kokai) No. 2000-145436). In this internal combustion engine, the SO.sub.x contained in the exhaust gas is absorbed by the SO.sub.x absorbent. Therefore, the SO.sub.x is prevented from flowing into the NO.sub.x storing catalyst. As a result, the storing capability of the NO.sub.x can be prevented from dropping due to storage of SO.sub.x.

When using such an SO.sub.x absorbent, however, if the SO.sub.x absorption capability of the SO.sub.x absorbent ends up being saturated, the SO.sub.x will end up flowing into the NO.sub.x storing catalyst. With this SO.sub.x absorbent, however, if raising the temperature of the SO.sub.x absorbent and making the air-fuel ratio of the exhaust gas flowing into the SO.sub.x absorbent rich, it is possible to release the absorbed SO.sub.x from the SO.sub.x absorbent and therefore possible to restore the SO.sub.x absorbent. However, releasing SO.sub.x from the SO.sub.x absorbent in this way, the released SO.sub.x will end up being stored in the NO.sub.x storing catalyst. Therefore, in this internal combustion engine, a bypass passage bypassing the NO.sub.x storing catalyst is provided. When releasing SO.sub.x from the SO.sub.x absorbent, the released SO.sub.x is exhausted into the atmosphere through the bypass passage.

In the above SO.sub.x absorbent, by raising the temperature of the SO.sub.x absorbent and making the air-fuel ratio of the exhaust gas flowing into the SO.sub.x absorbent rich in this way, it is possible to release the SO.sub.x from the NO.sub.x absorbent. However, in this way, the SO.sub.x is only released from the SO.sub.x absorbent little by little. Therefore, for release of all of the absorbed SO.sub.x from the SO.sub.x absorbent, the air-fuel ratio must be made rich for a long time and therefore there is the problem that a large amount of fuel or a reducing agent becomes necessary. Further, the SO.sub.x released from the SO.sub.x absorbent is exhausted into the atmosphere. This is also not preferable.

In this way, when using an SO.sub.x absorbent, if the release of SO.sub.x is not controlled, limits end up arising in the SO.sub.x absorption capability. Therefore, when using an SO.sub.x absorbent, the release of SO.sub.x must be controlled. However, if the release of SO.sub.x is controlled, that is, so long as SO.sub.x is made to be released from the SO.sub.x absorbent, the above problem arises.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purification device of a compression ignition type internal combustion engine able to suppress the SO.sub.x release action from an SO.sub.x trap catalyst while maintaining the high NO.sub.x storing capability of an NO.sub.x storing catalyst.

According to the present invention, there is provided an exhaust purification device for a compression ignition type internal combustion engine having an SO.sub.x trap catalyst arranged in an engine exhaust passage and capable of trapping SO.sub.x contained in exhaust gas and having an NO.sub.x storing catalyst arranged in the exhaust passage downstream of the SO.sub.x trap catalyst, the NO.sub.x storing catalyst storing NO.sub.x contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NO.sub.x when the air-fuel ratio of the exhaust gas flowing in becomes the stoichiometric air-fuel ratio or rich, wherein the SO.sub.x trap catalyst traps SO.sub.x contained in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst is lean, has the property that the trapped SO.sub.x gradually diffuses inside the SO.sub.x trap catalyst when the temperature of the SO.sub.x trap catalyst rises under a lean air-fuel ratio of the exhaust gas, and has the property of releasing the trapped SO.sub.x when the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst becomes rich if the temperature of the SO.sub.x trap catalyst is the SO.sub.x release temperature or more; the device is provided with air-fuel ratio control means continuing to maintain the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst lean, without allowing it to become rich, during engine operation and estimating means for estimating an SO.sub.x trap rate showing a ratio of SO.sub.x trapped in the SO.sub.x trap catalyst and the SO.sub.x contained in the exhaust gas; and the device makes the temperature of the SO.sub.x trap catalyst rise under a lean air-fuel ratio of the exhaust gas when the SO.sub.x trap rate falls below a predetermined rate so as to thereby restore the SO.sub.x trap rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of a compression ignition type internal combustion engine;

FIG. 2 is an overview of another embodiment of a compression ignition type internal combustion engine;

FIG. 3 is an overview of still another embodiment of a compression ignition type internal combustion engine;

FIGS. 4A and 4B are views of the structure of a particulate filter;

FIG. 5 is a sectional view of a surface part of catalyst carrier of an NO.sub.x storing catalyst;

FIG. 6 is a sectional view of a surface part of a catalyst carrier of an SO.sub.x trap catalyst;

FIG. 7 is a view of an SO.sub.x trap rate;

FIG. 8 is a view for explaining temperature raising control;

FIG. 9 is a view showing an injection timing;

FIGS. 10A to 10C are views of the relationship between a stored SO.sub.x amount .SIGMA.SOX1 and a stored SO.sub.x amount SO(n) for temperature raising control etc.;

FIG. 11 is a time chart of changes in the stored SO.sub.x amount .SIGMA.SOX1 etc.;

FIG. 12 is a flow chart for working a first embodiment of SO.sub.x stabilization processing;

FIG. 13 is a flow chart for working a second embodiment of SO.sub.x stabilization processing;

FIG. 14 is a time chart of SO.sub.x stabilization processing;

FIG. 15 is a time chart of temperature raising control of a particulate filter;

FIG. 16 is a time chart of SO.sub.x release control;

FIGS. 17A and 17B are views of a stored NO.sub.x amount NOXA map etc.; and

FIG. 18 is a flow chart for execution of processing for the NO.sub.x storing catalyst.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows an overview of a compression ignition type internal combustion engine.

Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electronically controlled fuel injector for injecting fuel into each combustion chamber 2, 4 an intake manifold, and 5 an exhaust manifold. The intake manifold 4 is connected through an intake duct 6 to an outlet of a compressor 7a of an exhaust turbocharger 7. The inlet of the compressor 7a is connected to an air cleaner 8. Inside the intake duct 6 is arranged a throttle valve 9 driven by a step motor. Further, around the intake duct 6 is arranged a cooling device (intercooler) 10 for cooling the intake air flowing through the inside of the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 10. The engine cooling water cools the intake air. On the other hand, the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7b of the exhaust turbocharger 7, while the outlet of the exhaust turbine 7b is connected to an inlet of an SO.sub.x trap catalyst 11. Further, the outlet of the SO.sub.x trap catalyst 11 is connected through an exhaust pipe 13 to an NOx storing catalyst 12. The exhaust pipe 13 is provided with a reducing agent supply valve 14 for supplying a reducing agent comprised of for example hydrocarbons into the exhaust gas flowing through the inside of the exhaust pipe 13.

The exhaust manifold 5 and the intake manifold 4 are interconnected through an exhaust gas recirculation (hereinafter referred to as an "EGR") passage 15. The EGR passage 15 is provided with an electronically controlled EGR control valve 16. Further, around the EGR passage 15 is arranged a cooling device 17 for cooling the EGR gas flowing through the inside of the EGR passage 15. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 17. The engine cooling water cools the EGR gas. On the other hand, each fuel injector 3 is connected through a fuel feed tube 18 to a common rail 19. This common rail 19 is supplied with fuel from an electronically controlled variable discharge fuel pump 20. The fuel supplied into the common rail 19 is supplied through each fuel feed tube 18 to the fuel injector 3.

An electronic control unit 30 is comprised of a digital computer provided with a read only memory (ROM) 32, a random access memory (RAM) 33, a microprocessor (CPU) 34, an input port 35, and an output port 36 all connected to each other by a bidirectional bus 31. The SO.sub.x trap catalyst 11 is provided with a temperature sensor 21 for detecting the temperature of the SO.sub.x trap catalyst 11. The NO.sub.x storing catalyst 12 is provided with a temperature sensor 22 for detecting the temperature of the NO.sub.x storing catalyst 12. The output signals of the temperature sensors 21 and 22 are input through corresponding AD converters 37 to the input port 35. Further, the NO.sub.x storing catalyst 12 is provided with a differential pressure sensor 23 for detecting the differential pressure before and after the NO.sub.x storing catalyst 12. The output signal of the differential pressure sensor 23 is input through the corresponding AD converter 37 to the input port 35.

An accelerator pedal 40 has a load sensor 41 generating an output voltage proportional to the amount of depression L of the accelerator pedal 40 connected to it. The output voltage of the load sensor 41 is input through a corresponding AD converter 37 to the input port 35. Further, the input port 35 has a crank angle sensor 42 generating an output pulse each time the crankshaft turns for example by 15 degrees connected to it. On the other hand, the output port 36 is connected through corresponding drive circuits 38 to the fuel injectors 3, throttle valve 9 step motor, reducing agent supply valve 14, EGR control valve 16, and fuel pump 20.

FIG. 2 shows another embodiment of a compression ignition type internal combustion engine. In this embodiment, in addition to the reducing agent supply valve 14 attached to the exhaust pipe 13, a manifold tube 5a of for example the #1 cylinder of the exhaust manifold 5 is provided with a hydrocarbon supply valve 24 for supplying for example hydrocarbons.

On the other hand, FIG. 3 shows still another embodiment of a compression ignition type internal combustion engine. In this embodiment, the exhaust pipe 13 is provided with an SO.sub.x sensor 25 for detecting the concentration of SO.sub.x in the exhaust gas flowing out from the SO.sub.x trap catalyst 11.

First, explaining the NO.sub.x storing catalyst 12 shown in FIG. 1 to FIG. 3, the NO.sub.x storing catalyst 12 is carried on a three-dimensional mesh structure monolith carrier or pellet carriers or is carried on a honeycomb structure particulate filter. In this way, the NO.sub.x storing catalyst 12 can be carried on various types of carriers, but below, the explanation will be made of the case of carrying the NO.sub.x storing catalyst 12 on a particulate filter.

FIGS. 4A and 4B show the structure of the particulate filter 12a carrying the NO.sub.x storing catalyst 12. Note that FIG. 4A is a front view of the particulate filter 12a, while FIG. 4B is a side sectional view of the particulate filter 12a. As shown in FIGS. 4A and 4B, the particulate filter 12a forms a honeycomb structure and is provided with a plurality of exhaust passages 60 and 61 extending in parallel with each other. These exhaust passages are comprised by exhaust gas inflow passages 60 with downstream ends sealed by plugs 62 and exhaust gas outflow passages 61 with upstream ends sealed by plugs 63. Note that the hatched portions in FIG. 4A show plugs 63. Therefore, the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61 are arranged alternately through thin wall partitions 64. In other words, the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61 are arranged so that each exhaust gas inflow passage 60 is surrounded by four exhaust gas outflow passages 61, and each exhaust gas outflow passage 61 is surrounded by four exhaust gas inflow passages 60.

The particulate filter 12a is formed from a porous material such as for example cordierite. Therefore, the exhaust gas flowing into the exhaust gas inflow passages 60 flows out into the adjoining exhaust gas outflow passages 61 through the surrounding partitions 64 as shown by the arrows in FIG. 4B.

When the NO.sub.x storing catalyst 12 is carried on the particulate filter 12a in this way, the peripheral walls of the exhaust gas inflow passages 60 and exhaust gas outflow passages 61, that is, the surfaces of the two sides of the partitions 64 and inside walls of the fine holes of the partitions 64 carry a catalyst carrier comprised of alumina. FIG. 5 schematically shows the cross-section of the surface part of this catalyst carrier 45. As shown in FIG. 5, the catalyst carrier 45 carries a precious metal catalyst 46 diffused on its surface. Further, the catalyst carrier 45 is formed with a layer of an NO.sub.x absorbent 47 on its surface.

In this embodiment of the present invention, platinum Pt is used as the precious metal catalyst 46. As the ingredient forming the NO.sub.x absorbent 47, for example, at least one element selected from potassium K, sodium Na, cesium Cs, or another alkali metal, barium Ba, calcium Ca, or another alkali earth, lanthanum La, yttrium Y, or another rare earth may be used.

If the ratio of the air and fuel (hydrocarbons) supplied to the engine intake passage, combustion chambers 2, and exhaust passage upstream of the NO.sub.x storing catalyst 12 is referred to as the "air-fuel ratio of the exhaust gas", the NO.sub.x absorbent 47 performs an NO.sub.x absorption and release action of storing the NO.sub.x when the air-fuel ratio of the exhaust gas is lean and releasing the stored NO.sub.x when the oxygen concentration in the exhaust gas falls.

That is, if explaining this taking as an example the case of using barium Ba as the ingredient forming the NO.sub.x absorbent 47, when the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas is oxidized on the platinum Pt 46 such as shown in FIG. 5 to become NO.sub.2, then is absorbed in the NO.sub.x absorbent 47 and diffuses in the NO.sub.x absorbent 47 in the form of nitric acid ions NO.sub.3.sup.- while bonding with the barium oxide BaO. In this way, the NO.sub.x is absorbed in the NO.sub.x absorbent 47. So long as the oxygen concentration in the exhaust gas is high, NO.sub.2 is produced on the surface of the platinum Pt 46. So long as the NO.sub.x absorbing capability of the NO.sub.x absorbent 47 is not saturated, the NO.sub.2 is absorbed in the NO.sub.x absorbent 47 and nitric acid ions NO.sub.3.sup.- are produced.

As opposed to this, by supplying a reducing agent from the reducing agent supply valve 14 to make the air-fuel ratio of the exhaust gas rich or the stoichiometric air-fuel ratio, since the oxygen concentration in the exhaust gas falls, the reaction proceeds in the reverse direction (NO.sub.3.sup.-.fwdarw.NO.sub.2) and therefore the nitric acid ions NO.sub.3.sup.- in the NO.sub.x absorbent 47 are released from the NO.sub.x absorbent 47 in the form of NO.sub.2. Next, the released NO.sub.x is reduced by the unburned hydrocarbons or CO included in the exhaust gas.

In this way, when the air-fuel ratio of the exhaust gas is lean, that is, when burning fuel under a lean air-fuel ratio, the NO.sub.x in the exhaust gas is absorbed in the NO.sub.x absorbent 47. However, if continuing to burn fuel under a lean air-fuel ratio, during that time the NO.sub.x absorbing capability of the NO.sub.x absorbent 47 will end up becoming saturated and therefore NO.sub.x will end up no longer being able to be absorbed by the NO.sub.x absorbent 47. Therefore, in this embodiment according to the present invention, before the absorbing capability of the NO.sub.x absorbent 47 becomes saturated, a reducing agent is supplied from the reducing agent supply valve 14 so as to temporarily make the air-fuel ratio of the exhaust gas rich and thereby release the NO.sub.x from the NO.sub.x absorbent 47.

However, exhaust gas contains SO.sub.x, that is, SO.sub.2. When this SO.sub.2 flows into the NO.sub.x storing catalyst 12, this SO.sub.2 is oxidized at the platinum Pt 46 and becomes SO.sub.3. Next, this SO.sub.2 is absorbed in the NO.sub.x absorbent 47 and bonds with the barium oxide BaO while diffusing in the NO.sub.x absorbent 47 in the form of sulfate ions SO.sub.4.sup.2- to produce the stable sulfate BaSO.sub.4. However, the NO.sub.x absorbent 47 has a strong basicity, so this sulfate BaSO.sub.4 is stable and hard to decompose. If just making the air-fuel ratio of the exhaust gas rich, the sulfate BaSO.sub.4 will remain without being decomposed. Therefore, in the NO.sub.x absorbent 47, the sulfate BaSO.sub.4 will increase along with the elapse of time and therefore the amount of NO.sub.x which the NO.sub.x absorbent 47 can absorb will fall along with the elapse of time.

In this case, however, as explained at the start, if raising the temperature of the NO.sub.x storing catalyst 11 to the SO.sub.x release temperature of 600.degree. C. or more and in that state making the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 11 rich, SO.sub.x will be released from the NO.sub.x absorbent 47. However, in this case, SO.sub.x will only be released from the NO.sub.x absorbent 47 a little at a time. Therefore, for release of all of the absorbed SO.sub.x from the SO.sub.x absorbent 47, the air-fuel ratio must be made rich for a long time and therefore there is the problem that a large amount of fuel or a reducing agent becomes necessary. Further, the SO.sub.x released from the SO.sub.x absorbent 47 is exhausted into the atmosphere. This is also not preferable.

Therefore, in the present invention, an SO.sub.x trap catalyst is arranged upstream of the NO.sub.x storing catalyst 12 and this SO.sub.x trap catalyst 11 is used to trap the SO.sub.x contained in the exhaust gas and thereby prevent SO.sub.x from flowing into the NO.sub.x storing catalyst 12. Next, this SO.sub.x trap catalyst 11 will be explained.

The SO.sub.x trap catalyst 11 is comprised of for example a honeycomb structure monolithic catalyst and has a large number of exhaust gas through holes extending straight in the axial direction of the SO.sub.x trap catalyst 11. When forming the SO.sub.x trap catalyst 11 from a honeycomb structure monolithic catalyst in this way, the inside peripheral walls of the exhaust gas through holes carry a catalyst carrier comprised of for example alumina. FIG. 6 schematically shows the cross-section of the surface part of this catalyst carrier 50. As shown in FIG. 6, the catalyst carrier 50 is formed with a coated layer 51 on its surface. Further, the coated layer 51 carries a precious metal catalyst 52 diffused on its surface.

In this embodiment of the present invention, platinum Pt is used as the precious metal catalyst 52. As the ingredient forming the coated layer 51, for example, at least one element selected from potassium K, sodium Na, cesium Cs, or another alkali metal, barium Ba, calcium Ca, or another alkali earth, lanthanum La, yttrium Y, or another rare earth may be used. That is, the coated layer 51 of the SO.sub.x trap catalyst 11 exhibits a strong basicity.

Therefore, the SO.sub.x contained in the exhaust gas, that is, the SO.sub.2, is oxidized at the platinum 52 as shown in FIG. 6, then is trapped in the coated layer 51. That is, the SO.sub.2 diffuses in the coated layer 51 in the form of sulfate ions SO.sub.4.sup.2- to form a sulfate. Note that as explained above, the coated layer 51 exhibits a strong basicity. Therefore, part of the SO.sub.2 contained in the exhaust gas is directly trapped in the coated layer 51 as shown in FIG. 5.

The concentration in the coated layer 51 in FIG. 6 shows the concentration of trapped SO.sub.x. As will be understood from FIG. 6, the concentration of SO.sub.x in the coated layer 51 is highest near the surface of the coated layer 51 and gradually becomes lower the further deeper. If the SO.sub.x concentration near the surface of the coated layer 51 becomes high, the basicity of the surface of the coated layer 51 becomes weaker and the SO.sub.x trap capability becomes weaker. Therefore, if the percentage of the SO.sub.x trapped by the SO.sub.x trap catalyst 11 in the SO.sub.x contained in the exhaust gas is referred to as the "SO.sub.x trap rate", the SO.sub.x trap rate falls along with the basicity of the surface of the coated layer 51 becoming weaker.

FIG. 7 shows the changes in the SO.sub.x trap rate along with time. As shown in FIG. 7, the SO.sub.x trap rate is first close to 100 percent, but rapidly falls along with the elapse of time. Therefore, in the present invention, as shown in FIG. 8, when the SO.sub.x trap rate falls below a predetermined rate, temperature raising control is performed to raise the temperature of the SO.sub.x trap catalyst 11 under a lean air-fuel ratio of the exhaust gas and thereby restore the SO.sub.x trap rate.

That is, if raising the temperature of the SO.sub.x trap catalyst 11 under a lean air-fuel ratio of the exhaust gas, the SO.sub.x concentrated near the surface of the coated layer 51 diffuses toward the deep part of the coated layer 51 so that the SO.sub.x concentration in the coated layer 51 becomes even. That is, the sulfate produced in the coated layer 51 changes from an unstable state concentrated near the surface of the coated layer 51 to a stable state evenly diffused across the entirety of the inside of the coated layer 51. If the SO.sub.x present near the surface of the coated layer 51 diffuses toward the deep part of the coated layer 51, the concentration of SO.sub.x near the surface of the coated layer 51 will fall. Therefore, when the temperature raising control of the SO.sub.x trap catalyst 11 is completed, the SO.sub.x trap rate will be restored as shown in FIG. 8.

When performing temperature raising control of the SO.sub.x trap catalyst 11, if making the temperature of the SO.sub.x trap catalyst 11 substantially 450.degree. C. or so, the SO.sub.x present near the surface of the coated layer 51 can be made to diffuse in the coated layer 51. If raising the temperature of the SO.sub.x trap catalyst 11 to about 600.degree. C., the concentration of SO.sub.x in the coated layer 51 can be made considerably even. Therefore, at the time of temperature raising control of the SO.sub.x trap catalyst 11, it is preferable to raise the temperature of the SO.sub.x trap catalyst 11 to about 600.degree. C. under a lean air-fuel ratio of the exhaust gas.

Note that when raising the temperature of the SO.sub.x trap catalyst 11, if making the air-fuel ratio of the exhaust gas rich, SO.sub.x will end up being released from the SO.sub.x trap catalyst 11. Therefore, when raising the temperature of the SO.sub.x trap catalyst 11, the air-fuel ratio of the exhaust gas may not be made rich. Further, when the concentration of SO.sub.x near the surface of the coated layer 51 becomes high, even if not raising the temperature of the SO.sub.x trap catalyst 11, if making the air-fuel ratio of the exhaust gas rich, SO.sub.x will end up being released from the SO.sub.x trap catalyst 11. Therefore, in the present invention, when the temperature of the SO.sub.x trap catalyst 11 is the SO.sub.x release temperature or more, the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 11 is prevented from being made rich.

In the present invention, basically, it is assumed that the SO.sub.x trap catalyst 11 will be used as it is without replacement from the time of purchase of the vehicle to its end of life. In recent years, the amount of sulfur contained in fuel in particularly has been reduced. Therefore, if the capacity of the SO.sub.x trap catalyst 11 is made larger to a certain extent, the SO.sub.x trap catalyst 11 can be used as it is until the end of life of the vehicle without replacement. For example, if the potential running distance of a vehicle is 500,000 km, the capacity of the SO.sub.x trap catalyst 11 is made a capacity enabling SO.sub.x to continue to be trapped by a high SO.sub.x trap rate without temperature raising control until about 250,000 km. In this case, the initial temperature raising control is performed at about 250,000 km.

Next, the method of raising the temperature of the SO.sub.x trap catalyst 11 will be explained with reference to FIG. 9.

One of the effective methods for raising the temperature of the SO.sub.x trap catalyst 11 is the method of retarding the fuel injection timing until after compression top dead center. That is, normally, the main fuel Q.sub.m is, in FIG. 9, injected near compression top dead center as shown in (I). In this case, as shown in (II) of FIG. 9, if the injection timing of the main fuel Q.sub.m is retarded, the after burning period will become longer and therefore the exhaust gas temperature will rise. If the exhaust gas temperature rises, the temperature of the SO.sub.x trap catalyst 11 will rise along with it.

Further, to raise the temperature of the SO.sub.x trap catalyst 11, as shown in (III) of FIG. 9, it is also possible to inject supplementary fuel Q.sub.v near intake top dead center in addition to the main fuel Q.sub.m. If additionally injecting supplementary fuel Q.sub.v in this way, the fuel which is burned is increased by exactly the amount of the supplementary fuel Q.sub.v, so the exhaust gas temperature rises and therefore the temperature of the SO.sub.x trap catalyst 11 rises.

On the other hand, if injecting supplementary fuel Q.sub.v near intake top dead center in this way, aldehydes, ketones, peroxides, carbon monoxide, and other intermediate products are produced from the supplementary fuel Q.sub.v due to the heat of compression during the compression stroke. These intermediate products accelerate the reaction of the main fuel Q.sub.m. Therefore, in this case, as shown by (III) of FIG. 9, even if the injection timing of the main fuel Q.sub.m is greatly retarded, good combustion will be obtained without misfires. That is, it is possible to greatly retard the injection timing of the main fuel Q.sub.m, so the exhaust gas temperature will become considerably high and therefore the temperature of the SO.sub.x trap catalyst 11 can be quickly raised.

Further, to raise the temperature of the SO.sub.x trap catalyst 11, as shown in (IV) of FIG. 9, it is also possible to inject supplementary fuel Q.sub.p during the expansion stroke or exhaust stroke in addition to the main fuel Q.sub.m. That is, in this case, the major part of the supplementary fuel Q.sub.p is not burned and is exhausted into the exhaust passage in the form of unburned hydrocarbons. The unburned hydrocarbons are oxidized by the excess oxygen on the SO.sub.x trap catalyst 11. The heat of oxidation reaction arising at that time raises the temperature of the SO.sub.x trap catalyst 11.

On the other hand, in the internal combustion engine shown in FIG. 2, it is also possible to supply hydrocarbons from the hydrocarbon supply valve 24 and use the heat of oxidation reaction of the hydrocarbons to raise the temperature of the SO.sub.x trap catalyst 11. Further, it is possible to perform any of the injection control routines shown from (II) to (IV) of FIG. 9 and supply hydrocarbons from the hydrocarbon supply valve 24. Note that no matter which method is used to raise the temperature, the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 11 is not made rich, but is maintained lean.

Next, a first embodiment of the SO.sub.x stabilization processing at the SO.sub.x trap catalyst 11 will be explained while referring to FIG. 10A to FIG. 12.

In the first embodiment, the amount of SO.sub.x trapped by the SO.sub.x trap catalyst 11 is estimated. When the amount of SO.sub.x trapped by the SO.sub.x trap catalyst 11 exceeds a predetermined amount, it is judged that the SO.sub.x trap rate has fallen lower than the predetermined rate. At this time, to restore the SO.sub.x trap rate, the temperature of the SO.sub.x trap catalyst 11 is raised under a lean air-fuel ratio of the exhaust gas in temperature raising control.

That is, fuel contains a certain ratio of sulfur. Therefore, the amount of SO.sub.x contained in the exhaust gas, that is, the amount of SO.sub.x trapped by the SO.sub.x trap catalyst 11, is proportional to the fuel injection amount. The fuel injection amount is a function of the required torque and the engine speed. Therefore, the amount of SO.sub.x trapped by the SO.sub.x trap catalyst 11 also becomes a function of the required torque and the engine speed. In this embodiment of the present invention, the SO.sub.x amount SOXA trapped per unit time in the SO.sub.x trap catalyst 11 is stored as a function of the required torque TQ and the engine speed N in the form of a map as shown in FIG. 10A in advance in the ROM 32.

Further, lubrication oil contains a certain ratio of sulfur. The amount of lubrication oil burned in a combustion chamber 2, that is, the amount of SO.sub.x contained in the exhaust gas and trapped in the SO.sub.x trap catalyst 11, becomes a function of the required torque and the engine speed. In this embodiment of the present invention, the amount SOXB of SO.sub.x contained in the lubrication oil and trapped per unit time in the SO.sub.x trap catalyst 11 is stored as a function of the required torque TQ and the engine speed N in the form of a map as shown in FIG. 10B in advance in the ROM 32. By cumulatively adding the SO.sub.x amount SOXA and the SO.sub.x amount SOXB, the SO.sub.x amount .SIGMA.SOX1 trapped in the SO.sub.x trap catalyst 11 is calculated.

Further, in this embodiment of the present invention, as shown in FIG. 10C, the relationship between the SO.sub.x amount .SIGMA.SOX1 and the predetermined SO.sub.x amount SO(n) when raising the temperature of the SO.sub.x trap catalyst 11 is stored in advance. When the SO.sub.x amount .SIGMA.SOX1 exceeds the predetermined SO(n) (n=1, 2, 3, . . . ), temperature raising control of the SO.sub.x trap catalyst 11 is performed. Note that in FIG. 10C, n shows the number of times of the temperature raising processing. As will be understood from FIG. 10C, as the number of times n of temperature raising processing for restoring the SO.sub.x trap rate increases, the predetermined amount SO(n) is increased. The rate of increase of the predetermined amount SO(n) is reduced the greater the number of times n of processing. That is, the rate of increase of SO(d) to SO(2) is reduced from the rate of increase of SO(2) to SO(1).

That is, as shown by the time chart of FIG. 11, the amount .SIGMA.SOX1 of SO.sub.x trapped by the SO.sub.x trap catalyst 11 continues to increase along with the elapse of time until the allowable value MAX. Note that in FIG. 11, the time when .SIGMA.SOX1=MAX is the time of the running distance of about 500,000 km.

On the other hand, in FIG. 11, the concentration of SO.sub.x shows the concentration of SO.sub.x near the surface of the SO.sub.x trap catalyst 11. As will be understood from FIG. 11, when the concentration of SO.sub.x near the surface of the SO.sub.x trap catalyst 11 exceeds the allowable value SOZ, the temperature T of the SO.sub.x trap catalyst 11 is raised under a lean air-fuel ratio A/F of the exhaust gas in the temperature raising control. When the temperature raising control is performed, the concentration of SO.sub.x near the surface of the SO.sub.x trap catalyst 11 falls, but the amount of reduction of the SO.sub.x concentration becomes smaller with each temperature raising control routine. Therefore, the period from when one temperature raising control routine is performed to the next temperature raising control is performed becomes shorter with each temperature raising control routine.

Note that as shown in FIG. 11, the trapped SO.sub.x amount .SIGMA.SOX1 reaching SO(1), S0(2), . . . means the concentration of SO.sub.x near the surface of the SO.sub.x trap catalyst 11 reaching the allowable value SOZ.

FIG. 12 shows the routine for working the first embodiment of the SO.sub.x stabilization processing.

Referring to FIG. 12, first, at step 100, the amounts SOXA and SOXB of the SO.sub.x trapped per unit time are read from FIG. 10A and FIG. 10B. Next, at step 101, the sum of these SOXA and SOXB is added to the SO.sub.x amount .SIGMA.SOX1. Next, at step 102, it is judged if the SO.sub.x amount .SIGMA.SOX1 has reached the predetermined amount SO(n) (n=1, 2, 3, . . . ) shown in FIG. 10C. When the SO.sub.x amount .SIGMA.SOX1 has reached the predetermined amount SO(n), the routine proceeds to step 103, where temperature raising control is performed.

FIG. 13 and FIG. 14 show a second embodiment of the SO.sub.x stabilization processing. In this embodiment, as shown in FIG. 3, the SO.sub.x sensor 25 is arranged downstream of the SO.sub.x trap catalyst 11. This SO.sub.x sensor 25 detects the concentration of SO.sub.x in the exhaust gas flowing out from the SO.sub.x trap catalyst 11. That is, in the second embodiment, as shown in FIG. 14, when the concentration of SO.sub.x in the exhaust gas detected by the SO.sub.x sensor 25 exceeds the predetermined concentration SOY, it is judged that the SO.sub.x trap rate has fallen below the predetermined rate. At that time, to restore the SO.sub.x trap rate, the temperature T of the SO.sub.x trap catalyst 11 is raised under a lean air-fuel ratio A/F of the exhaust gas in the temperature raising control.

FIG. 13 shows the routine for working the second embodiment.

Referring to FIG. 13, first, at step 100, the output signal of the SO.sub.x sensor 25, for example, the output voltage V, is read. Next, at step 111, it is judged whether the power voltage V of the SO.sub.x sensor 25 has exceeded a setting VX, that is, if the concentration of SO.sub.x in the exhaust gas has exceeded the predetermined concentration SOY. When V>VX, that is, when the concentration of SO.sub.x in the exhaust gas has exceeded the predetermined concentration SOY, the routine proceeds to step 112, where the temperature raising control is performed.

Next, the processing for the NO.sub.x storing catalyst 12 will be explained while referring to FIG. 15 to FIG. 18.

In this embodiment of the present invention, the amount NOXA of NO.sub.x stored per unit time in the NO.sub.x storing catalyst 12 is stored as a function of the required torque TQ and the engine speed N in the form of the map shown in FIG. 17A in advance in the ROM 32. By cumulatively adding this NO.sub.x amount NOXA, the amount .SIGMA.NOX of NO.sub.x stored in the NO.sub.x storing catalyst 11 is calculated. In this embodiment of the present invention, as shown in FIG. 15, each time the NO.sub.x amount .SIGMA.NOX reaches the allowable value NX, the air-fuel ratio A/F of the exhaust gas flowing into the NO.sub.x storing catalyst 12 is made temporarily rich, whereby NO.sub.x is released from the NO.sub.x storing catalyst 12.

Note that when making the air-fuel ratio A/F of the exhaust gas flowing into the NO.sub.x storing catalyst 12 rich, the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 11 must be maintained lean. Therefore, in this embodiment of the present invention, a reducing agent supply device, for example, as shown in FIG. 1 to FIG. 3, a reducing agent supply valve 14, is arranged in the exhaust passage between the SO.sub.x trap catalyst 11 and the NO.sub.x storing catalyst 12. By supplying reducing agent into the exhaust passage from the reducing agent supply valve 14 when NO.sub.x should be released from the NO.sub.x storing catalyst 12, the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 is temporarily made rich.

On the other hand, the particulate matter contained in the exhaust gas is trapped on the particulate filter 12a carrying the NO.sub.x storing catalyst 12 and successively oxidized. However, if the amount of the particulate matter trapped becomes greater than the amount of the particulate matter oxidized, the particulate matter will gradually deposit on the particulate filter 12a. In this case, if the deposition of particulate matter increases, it is necessary to remove the deposited particulate matter. In this case, if raising the temperature of the particulate filter 12a under an excess of air to about 600.degree. C., the deposited particulate matter is oxidized and removed.

Therefore, in this embodiment of the present invention, when the amount of the particulate matter deposited on the particulate filter 12a exceeds the allowable amount, the temperature of the particulate filter 12a is raised under a lean air-fuel ratio of the exhaust gas and thereby the deposited particulate matter is removed by oxidation. Specifically speaking, in this embodiment of the present invention, when the differential pressure .DELTA.P before and after the particulate filter 12a detected by the differential pressure sensor 23 exceeds the allowable value PX as shown in FIG. 15, it is judged that the amount of deposited particulate matter has exceeded the allowable amount. At that time, the air-fuel ratio of the exhaust gas flowing into the particulate filter 12a is maintained lean and the temperature T of the particulate filter 12a is raised in the temperature raising control. Note that if the temperature T of the particulate filter 12a becomes high, the amount of NO.sub.x .SIGMA.NOX trapped falls due to release of NO.sub.x from the NO.sub.x storing catalyst 12.

When the particulate filter 12a should be raised in temperature, the SO.sub.x trap catalyst 11 does not have to be raised in temperature. Therefore, when raising the temperature of the particulate filter 12a, reducing agent is supplied from the reducing agent supply valve 14 in the range at which the air-fuel ratio of the exhaust gas can be maintained lean and the heat of the oxidation reaction of the reducing agent is used to raise the temperature T of the particulate filter 12a.

On the other hand, when the SO.sub.x trap rate by the SO.sub.x trap catalyst 11 is 100 percent, no SO.sub.x at all flows into the NO.sub.x storing catalyst 12. Therefore, in this case, there is absolutely no danger of SO.sub.x being stored in the NO.sub.x storing catalyst 12. As opposed to this, when the SO.sub.x trap rate is not 100 percent, even if the SO.sub.x trap rate is close to 100 percent, SO.sub.x is stored in the NO.sub.x storing catalyst. However, in this case, the amount of SO.sub.x stored in the NO.sub.x storing catalyst 12 per unit time is extremely small. This being said, if a long time passes, a large amount of SO.sub.x will be stored in the NO.sub.x storing catalyst 12. If a large amount of SO.sub.x is stored, the stored SO.sub.x has to be made to be released.

As explained above, to get the SO.sub.x released from the NO.sub.x storing catalyst 12, it is necessary to make the temperature of the NO.sub.x storing catalyst 12 rise to the SO.sub.x release temperature and make the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 rich. Therefore, in this embodiment of the present invention, as shown in FIG. 16, when the amount .SIGMA.SOX2 of SO.sub.x stored in the NO.sub.x storing catalyst 12 reaches the allowable value SOX2, the temperature T of the NO.sub.x storing catalyst 12 is made to rise to the NO.sub.x release temperature TX and the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 is made rich. Note that the amount SOXZ of SO.sub.x stored in the NO.sub.x storing catalyst 12 per unit time is stored as a function of the required torque TQ and the engine speed N in the form of a map as shown in FIG. 17B in advance in the ROM 32. By cumulatively adding this amount SOXZ of SO.sub.x, the stored SO.sub.x amount .SIGMA.SOX2 is calculated.

When releasing SO.sub.x from the NO.sub.x storing catalyst 12, if making the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 11 rich, the SO.sub.x trapped in the SO.sub.x trap catalyst 11 will be released from the SO.sub.x trap catalyst 11 and the released SO.sub.x will end up being stored in the NO.sub.x storing catalyst 12. Therefore, when releasing SO.sub.x from the NO.sub.x storing catalyst 12, it is not possible to make the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 11 rich. Accordingly, in this embodiment according to the present invention, when SO.sub.x should be released from the NO.sub.x storing catalyst 12, first the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 11 and the NO.sub.x storing catalyst 12 is maintained lean and a reducing agent is supplied from the reducing agent supply valve 14 to raise the temperature T of the NO.sub.x storing catalyst 12 up to the NO.sub.x release temperature TX, then the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 11 is maintained lean and the amount of supply of reducing agent from the reducing agent supply valve 14 is increased to make the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 rich. Note that in this case, it is also possible to alternately switch the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 between rich and lean.

FIG. 18 shows the processing routine for the NO.sub.x storing catalyst 12.

Referring to FIG. 18, first, at step 120, the amount NOXA of NO.sub.x absorbed per unit time is calculated from the map shown in FIG. 17A. Next, at step 121, this NOXA is added to the NO.sub.x amount .SIGMA.NOX stored in the NO.sub.x storing catalyst 12. Next, at step 122, it is judged if the stored NO.sub.x amount .SIGMA.NOX has exceeded the allowable value NX. When .SIGMA.NOX>NX, the routine proceeds to step 123, where rich processing is performed to switch the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 by the reducing agent supplied from the reducing agent supply valve 14, temporarily from lean to rich and .SIGMA.NOX is cleared.

Next, at step 124, the differential pressure sensor 23 is used to detect the differential pressure .DELTA.P before and after the particulate filter 12a. Next, at step 125, it is judged if the differential pressure .DELTA.P has exceeded the allowable value PX. When .DELTA.P>PX, the routine proceeds to step 126, where temperature raising control of the particulate filter 12a is performed. This temperature raising control is performed by maintaining the air-fuel ratio of the exhaust gas flowing into the particulate filter 12a lean and supplying reducing agent from the reducing agent supply valve 14.

Next, at step 127, the SO.sub.x amount SOXZ stored per unit time is calculated from the map shown in FIG. 17B. Next, at step 128, this SOXZ is added to the SO.sub.x amount .SIGMA.SOX2 stored in the NO.sub.x storing catalyst 12. Next at step 129, it is judged if the stored SO.sub.x amount .SIGMA.SOX2 has exceeded the allowable value SX2. When .SIGMA.SOX2>SX2, the routine proceeds to step 130, where the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 is maintained lean and reducing agent is supplied from the reducing agent supply valve 14 to raise the temperature T of the NO.sub.x storing catalyst 12 to the SO.sub.x release temperature TX in temperature raising control. Next, at step 131, the reducing agent supplied from the reducing agent supply valve 14 is used to maintain the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storing catalyst 12 rich in the rich processing and the .SIGMA.SOX2 is cleared.

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