EP1657411B1

EP1657411B1 - Exhaust gas purifying apparatus of internal combustion engine

Info

Publication number: EP1657411B1
Application number: EP05024424A
Authority: EP
Inventors: Yasuaki Nakano; Shinya Hirota; Kohei Yoshida
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2004-11-10
Filing date: 2005-11-09
Publication date: 2007-05-16
Anticipated expiration: 2025-11-09
Also published as: JP2006138223A; DE602005001150T2; DE602005001150D1; EP1657411A1; JP4089683B2

Description

(Background of the Invention)

(Field of the Invention)

The present invention relates to an exhaust gas purifying apparatus of an internal combustion engine.

(Description of the Related Art)

To remove NOx contained in the exhaust gas discharged from an internal combustion engine, a catalyst such as an NOx storage reduction catalyst for removing NOx is provided in an exhaust passage. However, the exhaust gas contains SOx as well as NOx, and the NOx removing performance of the catalyst is sometimes deteriorated when SOx is taken into the catalyst.
To deal with this problem, in a technology that has been disclosed, an SOx absorbing agent is provided in the upstream and an NOx absorbing agent is provided in the downstream (see, for example, Japanese Patent Application Laid-Open No. 6-173652). According to this technology, when a lean air-fuel mixture is burned, SOx contained in the exhaust gas is absorbed by the SOx absorbing agent and only NOx is absorbed by the NOx absorbing agent disposed in the downstream of the SOx absorbing agent. On the other hand, as the oxygen concentration in the exhaust gas flowing into the SOx absorbing agent and the NOx absorbing agent is decreased, SOx is discharged from the SOx absorbing agent and NOx is discharged from the NOx absorbing agent. As a result, SOx becomes hard to be taken into the NOx absorbing agent.
When an SOx absorbing agent is provided in the upstream and an NOx removing catalyst such as an NOx absorbing agent is provided in the downstream in the exhaust passage of an internal combustion engine, SOx contained in the exhaust gas is taken into the SOx absorbing agent by an amount that does not exceed the absorption capacity of the SOx absorbing agent, and the SOx in excess of the capacity is taken into the NOx removing catalyst provided in the downstream. Moreover, since the SOx absorbing rate of the SOx absorbing agent decreases as it absorbs SOx in its interior, there is a possibility that unabsorbed SOx by the SOx absorbing agent is gradually taken into the NOx removing catalyst.

(Summary of the Invention)

Document US-A-5 472 673 discloses an exhaust gas purification device for an internal combustion engine which comprises a nitrogen oxide absorbent in the exhaust passage to absorb the oxides when an air-fuel ratio of exhaust gas flowing into the absorbent is lean, wherein the absorbent releases absorbed oxides when a concentration of oxygen in the exhaust gas is lowered. Further, a sulphur trap is placed in the exhaust passage upstream of the nitrogen oxide absorbent for trapping sulphur oxides contained in the exhaust gas.
To solve the above-described problem, according to the present invention, a solution of an SOx keeping agent is supplied to SOx keeping means having an ability of keeping SOx contained in exhaust gas. More specifically, according to the present invention, there is provided an exhaust gas purifying apparatus of an internal combustion engine comprising SOx keeping means provided in an exhaust passage of the internal combustion engine, the SOx keeping means including an SOx keeping agent supported thereon to have an SOx keeping ability of keeping in its interior SOx contained in exhaust gas, SOx keeping agent solution storage means that stores a solution of said SOx keeping agent, and SOx keeping agent supply means that supplies the solution of said SOx keeping agent stored in said SOx keeping agent solution storage means to said SOx keeping means.
The SOx keeping means mentioned above is used for purifying the exhaust gas of an internal combustion engine, and the SOx keeping agent having an SOx keeping ability or the ability of keeping in its interior SOx that has contained in the exhaust gas is supported thereon. The SOx keeping means prevents SOx from flowing to the downstream of it. However, the amount of the SOx keeping agent supported on the SOx keeping means is limited, and therefore, the SOx keeping ability is also limited. Accordingly, there is a risk that a relatively large amount of SOx can flow to the downstream depending on circumstances.
In view of this, a solution of the SOx keeping agent in the form of aqueous solution and preferably in the form of mist is supplied to the SOx keeping means by the SOx keeping agent supply means to increase the amount of the SOx keeping agent supported on the SOx keeping means. As a result, the SOx keeping ability of the SOx keeping means is enhanced, whereby it is possible to prevent SOx from flowing to the downstream as much as possible. Thus, it is possible to prevent SOx from being taken into, for example, a catalyst for removing NOx in the downstream as much as possible. The SOx keeping agent is supplied in the form of a solution in order that the SOx keeping agent is supported on the SOx keeping means more easily.
In the above-described exhaust gas purifying apparatus of the internal combustion engine, said SOx keeping agent solution storage means may store the solution in the form of fuel of said internal combustion engine in which said SOx keeping agent is dissolved, and said SOx keeping agent supply means may inject said solution through a fuel injection valve provided in an intake passage or a cylinder of said internal combustion engine. In other words, the SOx keeping agent may be supplied to the SOx keeping means by means of an existing fuel injection system of the internal combustion engine. In this case, the fuel tank of the internal combustion engine may also play a role of the SOx keeping agent solution storage means.
The exhaust gas purifying apparatus of the internal combustion engine described in the foregoing may be further provided with SOx keeping ability estimation means that estimates the degree of the SOx keeping ability of said SOx keeping means, and the solution of the SOx keeping agent may be supplied by said SOx keeping agent supply means when the degree of the SOx keeping ability estimated by said SOx keeping ability estimation means is lower than or equal to a predetermined degree. In other words, the solution is supplied by the SOx keeping agent supply means only when the SOx keeping ability of the SOx keeping means is deteriorated, whereby unnecessary supply of the SOx keeping agent can be prevented.
The SOx keeping ability estimation means may estimate the degree of the SOx keeping ability of said SOx keeping means based on a comparison between the amount of SOx flowing into the SOx keeping means and the amount of SOx flowing out of the SOx keeping means. Alternatively, the degree of the SOx keeping ability of said SOx keeping means may be estimated taking into account the amount of SOx discharged from the internal combustion engine, for example based-on a past running condition of the internal combustion engine.
In the case where the exhaust gas purifying apparatus of the internal combustion engine described in the foregoing is further provided with exhaust gas flow rate detection means that detects or estimates the flow rate of exhaust gas flowing into said SOx keeping means, the solution of the SOx keeping agent may be supplied by said SOx keeping agent supply means when the exhaust gas flow rate detected or estimated by said exhaust gas flow rate detection means is larger than or equal to a predetermined flow rate. In this case, the SOx keeping agent supplied by the SOx keeping agent supply means is diffused over a wide region of the SOx keeping means utilizing the flow of the exhaust gas flowing in the exhaust passage. In connection with this, the aforementioned predetermined flow rate is such a flow rate at which the solution of the SOx keeping agent can be diffused over a wide region of the SOx keeping means.
In the case where the exhaust gas purifying apparatus of the internal combustion engine described in the foregoing is further provided with temperature detection means that detects or estimates the temperature of said SOx keeping means, the solution of the SOx keeping agent may be supplied by said SOx keeping agent supply means when the temperature of said SOx keeping means detected or estimated by said temperature detection means falls within a predetermined temperature range. In this case, the SOx keeping agent supplied by the SOx keeping agent supply means is widely diffused in the SOx keeping means utilizing thermal energy that the SOx keeping means has. In connection with this, the aforementioned predetermined temperature range is such a temperature range in which the solution of the SOx keeping agent can be diffused over a wide region of the SOx keeping means.
The exhaust gas purifying apparatus of the internal combustion engine may be further provided with temperature raising means to raise the temperature of said SOx keeping means after the solution of the SOx keeping agent is supplied by said SOx keeping agent supply means. It is also possible to promote diffusion of the SOx keeping agent even by raising the temperature of the SOx keeping means after supplying the solution of the SOx keeping agent. The temperature of the SOx keeping means may be raised by raising the temperature of the exhaust gas flowing into the SOx keeping means, or by heating the SOx keeping means with a heater or the like.
In the case where the exhaust gas purifying apparatus of an internal combustion engine described in the foregoing is further provided with SOx keeping ability restoration means that restores the SOx keeping ability of the SOx keeping means by raising the temperature of said SOx keeping means and rendering the air-fuel ratio of exhaust gas flowing into said SOx keeping means lean, the solution of the SOx keeping agent may be supplied by said SOx keeping agent supply means when the SOx keeping ability of said SOx keeping means is not restored to a predetermined restoration condition by said SOx keeping ability restoration means
The solution of the SOx keeping agent is supplied when the predetermined restoration condition is not achieved, namely when the SOx keeping ability of the SOx keeping means is deteriorated and the SOx keeping ability cannot be restored by the SOx keeping ability restoration means to such a level that can sufficiently prevent SOx from flowing to the downstream. In such cases, a certain amount of SOx keeping agent that can compensate the deterioration of the SOx keeping ability is supplied..In this connection, the SOx keeping ability restoration means restores the SOx keeping ability of the SOx keeping means by causing the SOx keeping means to release SOx kept therein.
As per the above, according to the present invention, in an exhaust gas purifying apparatus in an internal combustion engine, it is possible to prevent SOx from being taken into an NOx removing catalyst provided in an exhaust passage as much as possible.

(Brief Description of the Drawings)

Fig. 1 is a diagram showing the basic structure of an exhaust gas purifying apparatus of an internal combustion engine according to a first embodiment of the present invention.
Fig. 2 is a first flow chart of an SOx keeping agent supply control process executed in the exhaust gas purifying apparatus of an internal combustion engine according to the first embodiment of the present invention.
Fig. 3 is a second flow chart of an SOx keeping agent supply control process executed in the exhaust gas purifying apparatus of an internal combustion engine according to the first embodiment of the present invention.
Fig. 4 is a diagram showing the basic structure of an exhaust gas purifying apparatus of an internal combustion engine according to a second embodiment of the present invention.

(Description of the Preferred Embodiments)

An exhaust gas purifying apparatus of an internal combustion engine according to the present invention will be described with reference to the accompanying drawings.

(First Embodiment)

Fig. 1 is a diagram showing the basic structure of the exhaust gas purifying apparatus of an internal combustion engine according to the present invention. Exhaust gas is discharged from the internal combustion engine 1 to an exhaust passage 2. In the exhaust passage 2, there is provided an SOx trapping catalyst 3 in the upstream and an NOx catalyst 4 in the downstream. The SOx trapping catalyst 3 supports an SOx keeping agent having an SOx keeping ability, namely the ability of keeping SOx in its interior. The SOx keeping agent may be, for example, an alkaline metal or an alkaline earth metal such as barium or potassium. SOx contained in the exhaust gas is trapped by the SOx trapping catalyst 3 and kept therein within the limit of its capacity. The NOx catalyst 4 is what is called an NOx storage reduction catalyst, which mainly removes NOx contained in the exhaust gas.
A tank 7 containing an aqueous solution of the SOx keeping agent for the SOx trapping catalyst.3 is also provided. The tank 7 is connected with the exhaust passage 2 by means of a supply passage 5, in the upstream of the SOx trapping catalyst 3. In the supply passage 5, there is provided a pump 6 for pumping the aqueous solution of the SOx keeping agent from the tank 7 into the exhaust passage 2.
The internal combustion engine 1 is annexed with an electronic control unit 20 (which will be hereinafter referred to as "ECU") for controlling the internal combustion engine 1. The ECU 20 is a unit that has a CPU and a ROM, RAM or the like that stores various control routines and maps that will described later and controls the running state of the internal combustion engine 1 in accordance with running conditions of the internal combustion engine 1 and driver's demands. The pump 6 operates based on a control signal from the ECU 20.
An air flow meter 9 that detects the flow rate of intake air, provided in an air intake passage 8 of the internal combustion engine 1 is electrically connected with the ECU 20. A temperature sensor 10 that detects the temperature of the exhaust gas flowing through the exhaust passage upstream of the SOx trapping catalyst 3 is also electrically connected with the ECU 20. Thus, the ECU 20 receives various signals.
In the exhaust gas purifying apparatus of the internal combustion engine 1 having the above-described structure, SOx contained in the exhaust gas is trapped by the SOx trapping catalyst 3 and kept therein. Consequently, the amount of the SOx flowing into the NOx catalyst 4 is reduced, and a decrease in the NOx removing capacity of the NOx catalyst 4 due to poisoning by SOx can be prevented. However, the SOx keeping ability of the SOx trapping catalyst 3 is limited, and the SOx keeping ability is gradually deteriorated with increases in the amount of the SOx kept. As a result, the amount of the SOx flowing into the NOx catalyst 4 increases.
In view of the above problem, the aqueous solution of the SOx keeping agent in the tank 7 is supplied to the SOx trapping catalyst 3' by pumping using the pump 6 via the exhaust passage 2. By this operation, the SOx keeping ability of the SOx trapping catalyst 3 is restored to some extent, and it is possible to reduce the amount of the SOx flowing into the NOx catalyst 4. Fig. 2 is a flow chart of an SOx keeping agent supply control for supplying the SOx keeping agent to the SOx trapping catalyst 3. The control process shown in this flow chart is executed by the ECU 20.
Firstly in step S101, the SOx keeping ability of the SOx trapping catalyst 3 is estimated. Specifically, how much amount of SOx is kept in the SOx trapping catalyst 3 is estimated based on a running condition and/or a past running condition (i.e. history of running) of the internal combustion engine 1, and the current SOx keeping ability is estimated based on that estimation. The SOx trapping ability of the SOx trapping catalyst 3 generally decreases as the amount of the SOx kept therein increases. Therefore, it is possible to estimate the SOx keeping ability in relation to the amount of the SOx kept.
Alternatively, SOx sensors may be provided in the upstream and the downstream of the SOx trapping catalyst 3 to calculate the ratio of the SOx concentration in the exhaust gas flowing into the SOx trapping catalyst 3 and the SOx concentration in the exhaust gas flowing out of the SOx trapping catalyst 3 from detection values of these sensors, and the SOx keeping ability relative to the amount of SOx the SOx trapping catalyst 3 actually keeps may be determined based on that ratio. The ECU 20 that executes the process of step S101 constitutes the SOx keeping ability estimation means in the present invention. After completion of the process step S101, the process proceeds to step S102.
In step S102, a determination is made as to whether or not the SOx keeping ability of the SOx trapping catalyst 3 estimated in step S101 has been deteriorated or not. This determination is made by determining whether the SOx keeping ability of SOx trapping catalyst 3 estimated in step S101 is higher than a reference SOx keeping ability or not. If it is determined that the SOx keeping ability has been deteriorated, the process proceeds to step S103. If it is determined that the SOx keeping ability has not been deteriorated, this control process is terminated.
In step S103, a determination is made as to whether or not the flow rate of the exhaust gas flowing through the exhaust passage is larger than or equal to a predetermined flow rate SVO. The flow rate of the exhaust gas is estimated by the ECU 20 based on a signal from the air flow meter 9. The ECU 20 that executes the process of estimating the exhaust gas flow rate constitutes the exhaust gas flow rate detection means in the present invention. The predetermined flow rate SV0 is the threshold value of the exhaust gas flow rates that allow the aqueous solution of the SOx keeping agent delivered by the pump 6 to be sufficiently diffused in the SOx trapping catalyst 3. In other words, the determination in step S103 is to be made in view of the fact that when the exhaust gas flow rate is smaller than SV0, the aqueous solution of the SOx keeping agent supplied tends to adhere only to the exhaust gas inlet side of the SOx trapping catalyst 3 and is hard to be diffused all over the SOx trapping catalyst 3. If it is determined in step S103 that the exhaust gas flow rate is larger than or equal to the predetermined flow rate SV0, the process proceeds to step S104, and if the exhaust gas flow rate is smaller than the predetermined flow rate SV0, this control process is terminated.
In step S104, a determination is made as to whether or not the catalyst temperature of the SOx trapping catalyst 3 is higher than or equal to a predetermined temperature T0. The catalyst temperature is estimated by the ECU 20 based on a signal from the temperature sensor 10. The ECU 20 that executes the process of estimating the catalyst temperature constitutes the temperature detection means in the present invention. The predetermined temperature T0 is a threshold value of the temperatures at which the aqueous solution of the SOx keeping agent delivered by the pump 6 is sufficiently diffused in the SOx trapping catalyst 3 by the thermal energy the SOx trapping catalyst 3 has. In other words, the determination in step S104 is to be made in view of the fact that when the catalyst temperature is lower than T0, the aqueous solution of the SOx keeping agent supplied is hard to be diffused all over the SOx trapping catalyst 3. If it is determined in step S104 that the catalyst temperature is higher than or equal to the predetermined temperature T0, the process proceeds to step S105. If the catalyst temperature is lower than the predetermined temperature T0, this control process is terminated.
In step S105, aqueous solution of the SOx keeping agent is pumped by the pump 6. By this process, the SOx keeping agent is supplied to the SOx trapping catalyst 3. As a result, the SOx keeping ability of the SOx trapping catalyst 3 is improved. After supply of the SOx keeping agent, a control process for raising the exhaust gas temperature of the internal combustion engine 1 may be executed in order to diffuse the SOx keeping agent in the SOx trapping catalyst 3 by increasing the temperature of the SOx trapping catalyst 3. The ECU 20 that executes the process of raising the exhaust gas temperature and the internal combustion engine 1 constitute the temperature raising means in the present invention. After completion of the process of step S105, this control process is terminated.
According to this control, when the SOx keeping ability of the SOx trapping catalyst 3 has been deteriorated, the SOx keeping ability is restored by supplying aqueous solution of the SOx keeping agent to the SOx trapping catalyst 3. Thus, it is possible to prevent the SOx from being taken into the NOx catalyst 4 as much as possible.
Fig. 3 is a flow chart of anther SOx keeping agent supply control process for supplying SOx keeping agent to the SOx trapping catalyst 3. The control process of this flow is executed by the ECU 20. In this SOx keeping agent.supply.control process, the process steps same as those in the SOx keeping agent supply control process shown in Fig. 2 are designated by the same reference signs and descriptions thereof will be omitted.
In the SOx keeping agent supply control process shown in Fig. 3, if it is determined in step S102 that the SOx keeping ability has been deteriorated, the process proceeds to step S201. In step S201, a process of restoring the SOx keeping ability of the SOx trapping catalyst 3 is effected. Specifically, the temperature of the exhaust gas flowing into the SOx trapping catalyst 3 is raised and the combustion condition of the internal combustion engine 1 is controlled in such a way as to render the air-fuel ratio of the exhaust gas lean against the stoichiometric air-fuel ratio. The ECU 20 that executes the SOx keeping ability restoring process and the internal combustion engine 1 constitute the SOx keeping ability restoration means in the present invention. By this process, the SOx kept in the SOx trapping catalyst 3 is released, so that the SOx keeping ability of the SOx trapping catalyst 3 is restored. After completion of the process of step S201, the process proceeds to step S202.
In step S202, a determination is made as to whether or not the SOx keeping ability of the SOx trapping catalyst 3 has been restored to a predetermined restored state. The predetermined restored state refers to a state in which the SOx keeping ability of the SOx trapping catalyst 3 has been restored to a level that is not determined as deteriorated in the determination made in step S102. If it is determined in step S202 that the SOx keeping ability of the SOx trapping catalyst 3 has not been restored to the predetermined restored state, the processes of step S103 and the subsequent steps are executed. If it is determined that the SOx keeping ability of the SOx trapping catalyst 3 has been restored to the predetermined restored state, this control process is terminated.
According to this control, supply of the SOx keeping agent to the SOx trapping catalyst 3 is performed only when the SOx keeping ability is not restored sufficiently, for example, when the SOx keeping ability has been deteriorated due to degradation of the SOx trapping catalyst 3 or other reasons. Therefore, it is possible to reduce consumption of the SOx keeping agent.

(Second Embodiment)

In the following, the second embodiment of the exhaust gas purifying apparatus of an internal combustion engine according to the present invention will be described. Fig. 4 is a diagram schematically showing the exhaust gas purifying apparatus of the internal combustion engine according to the second embodiment. The elements same as those in the exhaust gas purifying apparatus of the internal combustion engine according to the first embodiment will be designated by the same reference numerals, and descriptions thereof will be omitted.
A feature that characterizes the exhaust gas purifying apparatus of the internal combustion engine 1 according to this embodiment resides in that the SOx keeping agent is dissolved in the fuel of the internal combustion engine 1 and it is supplied through a fuel injection valve 11 together with the fuel. The fuel injection valve 11 with which the internal combustion engine 1 is equipped is connected with a pressure accumulation chamber 12, which in turn is connected with a fuel tank 14 via a fuel supply passage 13. The SOx keeping agent to be supplied to the SOx trapping catalyst 3 is dissolved in the fuel contained in the fuel tank 14.
Therefore, SOx keeping agent is injected together with the fuel injected through the fuel injection valve 11, so that the SOx keeping agent contained in the exhaust gas is continuously supplied to the SOx trapping catalyst 3 provided in the exhaust passage 2. Thus, the SOx keeping ability of the SOx trapping catalyst 3 is maintained, and it is possible to prevent the SOx from being taken into the NOx catalyst 4 as much as possible.
An exhaust gas purifying apparatus of an internal combustion engine has SOx keeping means 3 provided in an exhaust passage, the SOx keeping means including an SOx keeping agent and having an SOx keeping ability of keeping in its interior SOx contained in exhaust gas, SOx keeping agent solution storage means 7 that stores a solution of said SOx keeping agent and SOx keeping agent supply means 6 that supplies the solution of said SOx keeping agent stored in said SOx keeping agent solution storage means to said SOx keeping means 3. By this structure of the exhaust gas purifying apparatus of the internal combustion engine, SOx is prevented from taken into an NOx removing catalyst provided in an exhaust passage as much as possible.

Claims

An exhaust gas purifying apparatus of an internal combustion engine comprising
SOx keeping means (3) provided in an exhaust passage (2) of an internal combustion engine (1),
characterized by
SOx keeping agent solution storage means (7, 14) that stores a solution of said SOx keeping agent; and
SOx keeping agent supply means (6) that supplies the solution of said SOx keeping agent stored in said SOx keeping agent solution storage means (7, 14) to said SOx keeping means (3) to be supported on the SOx keeping means (3) to achieve an SOx keeping ability of keeping in its interior SOx contained in exhaust gas.
An exhaust gas purifying apparatus of an internal combustion engine according to claim 1,
characterized in that:
said SOx keeping agent solution storage means (14) stores the solution in the form of fuel of said internal combustion engine (1) in which said SOx keeping agent is dissolved; and

said SOx keeping agent supply means (6) injects said solution through a fuel injection valve (11) provided in an intake passage (8) or a cylinder of said internal combustion engine (1).
An exhaust gas purifying apparatus of an internal combustion engine according to claim 1 or 2, further characterized by SOx keeping ability estimation means (20) that estimates the degree of the SOx keeping ability of said SOx keeping means (3), wherein the solution of the SOx keeping agent is supplied by said SOx keeping agent supply means (6) when the degree of the SOx keeping ability estimated by said SOx keeping ability estimation means (20) is lower than or equal to a predetermined degree.
An exhaust gas purifying apparatus of an internal combustion engine according to claim 3, characterized in that said SOx keeping ability estimation means (20) estimates the degree of the SOx keeping ability of said SOx keeping means (3) based on either a result of comparison between the amount of SOx flowing into said SOx keeping means (3) and the amount of SOx flowing out of said SOx keeping means (3) or a past running condition of said internal combustion engine (1).
An exhaust gas purifying apparatus of an internal combustion engine according to claim 1 or 2, further characterized by exhaust gas flow rate detection means (9) that detects or estimates the flow rate of exhaust gas flowing into said SOx keeping means (3), wherein the solution of the SOx keeping agent is supplied by said SOx keeping agent supply means (6) when the exhaust gas flow rate detected or estimated by said exhaust gas flow rate detection means (9) is larger than or equal to a predetermined flow rate:
An exhaust gas purifying apparatus of an internal combustion engine according to claim 1 or 2, further characterized by temperature detection means (10) that detects or estimates the temperature of said SOx keeping means (3), wherein the solution of the SOx keeping agent is supplied by said SOx keeping agent supply means (6) when the temperature of said SOx keeping means (3) detected or estimated by said temperature detection means (10) falls within a predetermined temperature range.
An exhaust gas purifying apparatus of an internal combustion engine according to any one of claims 1 to 6, further characterized by temperature raising means (20, 1) that raises the temperature of said SOx keeping means (3) after the solution of the SOx keeping agent is supplied by said SOx keeping agent supply means (6).
An exhaust gas purifying apparatus of an internal combustion engine according to any one of claims 1 to 7, further characterized by SOx keeping ability restoration means (20, 1) that restores the SOx keeping ability of the SOx keeping means (3) by raising the temperature of said SOx keeping means (3) and rendering the air-fuel ratio of inflowing exhaust gas lean, wherein the solution of the SOx keeping agent is supplied by said SOx keeping agent supply means (6) when the SOx keeping ability of said SOx keeping means (3) is not restored to a predetermined restored state by said SOx keeping ability restoration means (20, 1).