An exhaust gas purification device for an internal combustion engine

Abstract: An NO x occluding and reducing catalyst (7) is disposed in an exhaust gas passage (2) of an internal combustion engine (1) to absorb NO x in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, and to release and reduce the absorbed NO x when the air-fuel ratio of the exhaust gas is rich. A recovery operation is executed when an estimated amount of contaminants adsorbed or absorbed by the NO x occluding and reducing catalyst (7) has reached a predetermined value. At that time, the total amount of reducing substances in the exhaust gas flowing into the NO x occluding and reducing catalyst (7) is changed depending on the degree of recovery of the capability of absorbing, releasing and reducing NO x by reduction of the NO x occluding and reducing catalyst (7) or depending on the concentration of precursors of the contaminants in the fuel.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the invention relates to an exhaust gas purification device equipped with an NOx occluding and reducing catalyst for removing NOx components from the exhaust gas of an engine. 2. Description of the Related Art There has been known an exhaust gas purification device using an NOx occluding and reducing catalyst comprising at least one element selected from alkali metals such as pottasium K, sodium Na, lithium Li and cesium Cs, alkaline earth elements such as barium Ba and calcium Ca, and rare earth elements such as lanthanum La and yttrium Y, as well as a noble metal such as platinum Pt. The NOx occluding and reducing catalyst works to absorb and release NOx, i.e., absorbs NOx in the exhaust gas in the form of nitrates when the air-fuel ratio of the exhaust gas that flow in is lean, releases the absorbed NOx when the oxygen concentration decreases in the exhaust gas that flow in, and purifies the released NOx by reduction with reducing components in the exhaust gas. The action for absorbing and releasing NOx by the NOx occluding and reducing catalyst will be described later. However, when sulfur oxides (SOx) exist in the exhaust gas, the NOx occluding and reducing catalyst absorbs SOx in the exhaust gas based on the same mechanism as that of NOx absorption. However, SOx absorbed by the NOx occluding and reducing catalyst forms stable sulfates which are generally very little decomposed or released and, hence, tend to build up in the NOx occluding and reducing catalyst. When SOx is held in an increased amount by the NOx occluding and reducing catalyst, the NOx occluding and reducing catalyst exhibits a decreased NOx-absorbing capacity by an amount by which SOx is held. When SOx is held in increased amounts by the NOx occluding and reducing catalyst, therefore, it becomes no longer possible to remove NOx from the exhaust gas to a sufficient degree, i.e., a so-called SOx contamination of the NOx occluding and reducing catalyst occurs. It is also known that SOx absorbed by the NOx occluding and reducing catalyst can be released and reduced relying on the same mechanism for releasing and reducing NOx. However, since sulfates held in the NOx occluding and reducing catalyst are relatively stable, SOx absorbed by the NOx occluding and reducing catalyst cannot be released at a temperature (e.g., about 250°C) at which the operation is usually conducted to release and purify NOx by reduction (hereinafter referred to as "a regenerating operation" for the NOx occluding and reducing catalyst). In order to remove the SOx contamination, therefore, the SOx contamination-recovery operation must be regularly conducted by heating the NOx occluding and reducing catalyst at a temperature (e.g., 600°C or higher) which is higher than that when the NOx occluding and reducing catalyst is usually being generated, and by shifting the air-fuel ratio of the exhaust gas that is flowing in toward the rich side. An exhaust gas purification device which executes the SOx contamination-removing operation for the NOx occluding and reducing catalyst, has been taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-88518. In order to recover the SOx-contaminated NOx occluding and reducing catalyst according to the device of the above publication, the exhaust gas is put into a stoichiometric state or a rich state to lower the oxygen concentration when the temperature of the exhaust gas becomes high. Japanese Unexamined Patent Publication (Kokai) No. 8-61052 discloses another exhaust gas purification device for recovering the NOx occluding and reducing catalyst from the SOx contamination by regenerating the NOx occluding and reducing catalyst at a high temperature. In order to recover the NOx occluding and reducing catalyst arranged in the exhaust passage of an internal combustion engine from the SOx contamination according to the exhaust gas purification device of this publication, some of the cylinders of the engine are operated at a rich air-fuel ratio and the rest of the cylinders are operated at a lean air-fuel ratio to remove the SOx contamination when the amount of SOx held by the NOx occluding and reducing catalyst becomes greater than a predetermined value and the engine is operated in a particular load region (intermediate to high load region). By operating some cylinders of the engine at a rich air-fuel ratio and the rest of the cylinders at a lean air-fuel ratio, exhaust gas from the rich air-fuel ratio cylinders containing unburned fuel and exhaust gas from the lean air-fuel ratio cylinders containing large amounts of oxygen flow into the catalyst. Therefore, the unburned fuel burns on the catalyst, whereby the temperature of the catalyst rises and SOx held by the NOx occluding and reducing catalyst is released. According to the devices of the above-mentioned Japanese Unexamined Patent Publications (Kokai) Nos. 6-88518 and 8-610652, however, it is not allowed to frequently conduct the SOx contamination-recovery operation, and a state in which the purification efficiency of the NOx occluding and reducing catalyst is lowered due to the SOx contamination often continues for extended periods of time. According to the devices of the above-mentioned publications, the recovery operation from the SOx contamination is not executed unless the exhaust gas temperature of the engine becomes high or unless the engine is operated under a particular condition even if the amount of SOx held by the NOx occluding and reducing catalyst exceeds a predetermined value. By using the devices of the above-mentioned publications, therefore, the operation often continues for extended periods of time in a state where SOx are held in increased amounts by the NOx occluding and reducing catalyst depending upon the operating conditions of the engine, and NOx which has not been purified are released to the open air due to a drop in the purification efficiency of the NOx occluding and reducing catalyst. In conducting the above-mentioned recovery operation from the SOx contamination of the NOx occluding and reducing catalyst, SOx is still formed by sulfur contained in the fuel when the engine is in operation, and a difference occurs in the degree of removing the SOx contamination from the NOx occluding and reducing catalyst depending upon the SOx concentration in the exhaust gas during this period. When the exhaust gas is put into a predetermined stoichiometric state or a rich state, depending on the prior art, without taking the above-mentioned fact into consideration, then, the SOx contamination is not removed to a sufficient degree, or the exhaust gas becomes excessively rich to deteriorate the fuel efficiency and the exhaust gas emission. SUMMARY OF THE INVENTION In view of the problems in the related art as set forth above, the object of the present invention is to provide an exhaust gas purification device for an internal combustion engine which is capable of completely removing the SOx contamination without deteriorating the exhaust gas emission at the time of conducting the recovery operation for removing the contamination from the NOx occluding and reducing catalyst. The object as set forth above is achieved by an exhaust gas purification device for an internal combustion engine according to claim 1. According to the present invention, there is provided an exhaust gas purification device for an internal combustion engine, comprising: an NOx occluding and reducing catalyst disposed in an exhaust gas passage of the internal combustion engine to absorb NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is larger than a stoichiometric air-fuel ratio and to release and reduce the absorbed NOx when the air-fuel ratio of the exhaust gas flowing in becomes smaller than the stoichiometric air-fuel ratio, said NOx occluding and reducing catalyst exhibiting a decrease in the capability of absorbing, releasing and reducing the NOx as it adsorbs or absorbs contaminants in the exhaust gas; a holding amount estimation means for estimating the amount of said contaminants held by being adsorbed or absorbed by said NOx occluding and reducing catalyst; a recovering means which maintains said NOx occluding and reducing catalyst within a predetermined high-temperature range for a predetermined recovering period when the holding amount of said contaminants estimated by said holding amount estimation means has reached a predetermined value, and controls the air-fuel ratio of the exhaust gas flowing into said NOx occluding and reducing catalyst to assume a recovering air-fuel ratio at which said contaminants can be released; and a changing means for changing the total amount of the reducing substances in the exhaust gas flowing into said NOx occluding and reducing catalyst (7) during recovery operation. According to a first aspect of the invention, a recovering degree evaluation means evaluates the degree of recovery of the capability of absorbing, releasing and reducing NOx by reduction of said NOx occluding and reducing catalyst accomplished by said recovering means and said changing means changes the total amount of the reducing substances during the recovering period of the next time depending upon the recovering degree evaluated by said recovering degree evaluation means. According to the first aspect of the invention, the total amount of the reducing substances flowing into the NOx occluding and reducing catalyst is adjusted during the recovering period depending upon the degree of recovery from the SOx contamination. Therefore, the total amount of the reducing substances in the exhaust gas is optimized during the recovering period, and the NOx occluding and reducing catalyst is completely recovered from the contaminated state without deteriorating the exhaust gas emission. According to a second aspect of the present invention, an estimation means estimates the concentration of precursors of said contaminants in the fuel and said changing means changes the total amount of the reducing substances during said recovering period depending upon the concentration of said precursors estimated by said estimation means. According to the second aspect of the invention, the changing means adjusts the total amount of the reducing substances flowing into the NOx occluding and reducing catalyst during the recovering period depending upon the concentration of the precursors estimated by the estimation means. Therefore, the total amount of the reducing substances is optimized during the recovering period depending upon the concentration of contaminants in the exhaust gas during the recovering period, the concentration of contaminants changing depending upon the concentration of precursors of the contaminants in the fuel. It is thus made possible to completely remove the contaminant without deteriorating the exhaust gas emission. The dependent claims set forth developments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the description as set forth hereinafter with reference to the accompanying drawings in which: Fig. 1 is a view schematically illustrating the constitution of an embodiment in which the present invention is applied to an internal combustion engine for an automobile; Fig. 2 is a flowchart explaining the operation for judging the contamination-recovery conditions for the NOx occluding and reducing catalyst; Fig. 3 is a flowchart explaining the operation for setting a flag in the flowchart of Fig. 2; Fig. 4 is a flowchart explaining a rich-spike operation of Fig. 2; Fig. 5 is a flowchart explaining the contamination-recovery operation of Fig. 2; Fig. 6 is a flowchart explaining the contamination-recovery operation different from that of Fig. 5; Fig. 7 is a flowchart explaining the operation for setting a target air-fuel ratio at the time of the recovery operation; and Fig. 8 is a map of target air-fuel ratios used for the operation of Fig. 7. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the exhaust gas purification device according to the present invention will be explained with reference to Figs. 1 through 8. Fig. 1 is a view schematically illustrating the constitution of an embodiment in which the present invention is applied to an internal combustion engine for an automobile. In Fig. 1, reference numeral 1 designates an internal combustion engine for an automobile. In this embodiment, the engine 1 is a four-cylinder gasoline engine having four cylinders #1 to #4 which are equipped with fuel injection valves 111 to 114 for directly injecting fuel into the cylinders. As will be described later, the internal combustion engine 1 of this embodiment is a lean burn engine that can be operated at a lean air-fuel ratio (i.e., an air-fuel ratio larger than a stoichiometric air-fuel ratio). In this embodiment, furthermore, the cylinders #1 to #4 are grouped into two groups of cylinders each including two cylinders in which the ignition timings do not take place consecutively (in the embodiment of Fig. 1, for example, the order of igniting the cylinders is 1-3-4-2. Therefore, the cylinders #1 and #4 form one group of cylinders, and the cylinders #2 and #3 form another group of cylinders). The exhaust port of each cylinder is connected to an exhaust manifold of each group of cylinders, and is connected to an exhaust passage of each group of cylinders. In Fig. 1, reference numeral 21a denotes an exhaust manifold for connecting exhaust ports of the group of the cylinders #1 and #4 to an independent exhaust passage 2a, and 21b denotes an exhaust manifold for connecting exhaust ports of the group of the cylinders #2 and #4 to an independent exhaust passage 2b. In this embodiment, start catalysts (hereinafter referred to as "SC") 5a and 5b comprising a three-way catalyst are arranged on the independent exhaust passages 2a and 2b. The independent exhaust passages 2a and 2b meet in a common exhaust passage 2 on the downstream side of the SC. An NOx occluding and reducing catalyst 7 that will be described later is arranged in the common exhaust passage 2. In Fig. 1, reference numerals 29a and 29b denote air-fuel sensors arranged on the upstream side of the start catalysts 5a and 5b of the independent exhaust passages 2a and 2b, and reference numeral 31 denotes an air-fuel sensor arranged at an outlet port of the NOx occluding and reducing catalyst in the exhaust passage 2. The air-fuel sensors 29a, 29b and 31 are so-called linear air-fuel sensors that produce voltage signals which change continuously, corresponding to the air-fuel ratio of an exhaust gas, over a wide range of air-fuel ratios. In Fig. 1, furthermore, reference numeral 30 denotes an electronic control unit (ECU) of the engine 1. In this embodiment, the ECU 30 is a microcomputer of a known type having a RAM, a ROM and a CPU, and performs basic control operations such as an ignition timing control and a fuel injection control for the engine 1. In this embodiment, the ECU 30 further works as a holding amount estimation means for estimating the amount of contaminants such as SOx absorbed by the NOx occluding and reducing catalyst 7, works as a heating means for heating the NOx occluding and reducing catalyst 7, and works as a heating-conducting condition-setting means for setting the heating-conducting condition as will be described later, in addition to executing the above-mentioned basic control operations. The input ports of the ECU 30 receive signals from the air-fuel ratio sensors 29a and 29b representing the exhaust gas air-fuel ratios at the inlet ports of the start catalysts 5a and 5b, a signal from the air-fuel ratio sensor 31 representing an exhaust gas air-fuel ratio at the outlet port of the NOx occluding and reducing catalyst 7, a signal corresponding to the intake air pressure of the engine from an intake-air-pressure sensor 33 provided in the intake manifold (not shown), and a signal corresponding to the engine rotational speed from a rotational speed sensor 35 disposed near the crank shaft (not shown) of the engine. In this embodiment, furthermore, the input ports of the ECU 30 receive a signal representing the amount the accelerator pedal is depressed by a driver (degree of accelerator opening) from an accelerator opening-degree sensor 37 disposed near an accelerator pedal (not shown) of the engine 1 and a signal representing the temperature of the cooling water from a cooling water temperature sensor 39 disposed in a cooling water passage (not shown) of the engine. The output port of the ECU 30 is connected to the fuel injection valves 111 to 114 of the cylinders to control the amount of fuel injected into the cylinders and the timings for fuel injection. In this embodiment, the ECU 30 operates the engine 1 in the following five combustion modes depending upon the amount of fuel injection into the engine, i.e., depending upon the load exerted on the engine: 1 ○ A lean air-fuel ratio stratified charge combustion (one fuel injection in the compression stroke). 2 ○ A lean air-fuel ratio uniform mixture/stratified charge combustion (one fuel injection in the intake stroke and one fuel injection in the compression stroke). 3 ○ A lean air-fuel ratio uniform mixture combustion (one fuel injection in the intake stroke). 4 ○ A stoichiometric air-fuel ratio uniform mixture combustion (one fuel injection in the intake stroke). 5 ○ A rich air-fuel ratio uniform mixture combustion (one fuel injection in the intake stroke). That is, the lean air-fuel ratio stratified charge combustion 1 ○ is carried out in the light-load operation region of the engine 1. In this state, the fuel is injected into the cylinders only one time in the latter half of the compression stroke in each cylinder, and the injected fuel forms a layer of a combustible air-fuel ratio mixture near the spark plug in the cylinder. In this operation state the fuel is injected in a very small amount, and the air-fuel ratio in the cylinder as a whole is about 30 to about 20. As the load increases from the above-mentioned state 1 ○ to enter into the low-load operation region, there takes place the above-mentioned lean air-fuel ratio uniform mixture/stratified charge combustion 2 ○. The amount of fuel injected into the cylinder increases with an increase in the load exerted on the engine. In the above-mentioned stratified charge combustion 1 ○, the fuel is injected in the latter half of the compression stroke, whereby the injection time is limited and limitation is imposed on the amount of fuel for forming the stratified charge. In this load region, therefore, the fuel is injected in advance in the former half of the intake stroke in an amount to compensate for the shortage of the fuel injected in the latter half of the compression stroke, thereby to supply the fuel in a target amount into the cylinder. The fuel injected into the cylinder in the former half of the intake stroke forms a very lean and uniform mixture before being ignited. In the latter half of the compression stroke, the fuel is further injected into this very lean and uniform mixture in order to form the charge of a combustible air-fuel ratio mixture near the spark plug. At the time of ignition, this combustible air-fuel ratio mixture charge starts burning, and the flame propagates to the surrounding lean mixture charge, so that the combustion takes place stably. In this state, the amount of fuel injected in the intake stroke and in the compression stroke is larger than that of the mode 1 ○, but the air-fuel ratio as a whole is still lean (e.g., an air-fuel ratio of about 25 or smaller). When the load on the engine further increases, the engine combustion becomes the lean air-fuel ratio uniform mixture combustion 3 ○. In this state, the fuel is injected only one time in the former half of the intake stroke, and the amount of the injected fuel becomes larger than that of the mode 2 ○. The uniform mixture formed in the cylinder in this state assumes a lean air-fuel ratio which is relatively close to the stoichiometric air-fuel ratio. As the load on the engine further increases to enter into the high-load operation region of the engine, the amount of fuel becomes larger than that of the mode 3 ○, and the engine operation becomes the stoichiometric air-fuel ratio uniform mixture operation 4 ○. In this state, a uniform mixture of the stoichiometric air-fuel ratio is formed in the cylinder, and the engine output increases. When the load on the engine further increases to cause the full-load operation of the engine, the amount of fuel is further increased in excess of that of the mode 4 ○, and the engine operation becomes the rich air-fuel ratio uniform mixture operation 5 ○. In this state, the uniform mixture formed in the cylinder becomes a rich air-fuel ratio. In this embodiment, optimum operation modes 1 ○ to 5 ○ have been empirically set depending upon the degree of accelerator opening (amount of the accelerator pedal depressed by the driver) and the rotational speed of the engine, and a map using the degree of accelerator opening and the engine rotational speed is stored in the ROM of the ECU 30. When the engine 1 is in operation, the ECU 30 determines which one of the above-mentioned operation modes 1 ○ to 5 ○ is selected based on the degree of accelerator opening detected by the accelerator opening-degree sensor 37 and the rotational speed of the engine, and determines the amount of fuel injection, the timing for fuel injection and the number of times, depending on each of the modes. When any one of the modes 1 ○ to 3 ○ (lean air-fuel ratio combustion) is selected, the ECU 30 determines the amount of fuel injection from the degree of accelerator opening and the rotational speed of the engine based on the maps that have been prepared in advance for the modes 1 ○ to 3 ○. When the modes 4 ○ or 5 ○ (stoichiometric air-fuel ratio uniform mixture combustion or rich air-fuel ratio uniform mixture combustion) are selected, the ECU 30 sets the amount of fuel injection based on the intake air pressure detected by the intake air pressure sensor 33 and the rotational speed of the engine by using maps that have been prepared for the modes 4 ○ and 5 ○. When the mode 4 ○ (stoichiometric air-fuel ratio uniform mixture combustion) is selected, the ECU 30 corrects by feedback the amount of fuel injection calculated as described above relying on the outputs of the air-fuel sensors 29a and 29b, so that the air-fuel ratio in the exhaust gas of the engine is controlled at the stoichiometric air-fuel ratio. In the engine 1 of this embodiment as described above, the amount of fuel injection increases with an increase in the load on the engine, and the operation mode changes depending upon the amount of fuel injection. In the following description, therefore, the amount of fuel injection is commonly used in all of these modes as a parameter representing the load on the engine. Next, described below are the start catalysts 5a, 5b and the NOx occluding and reducing catalyst 7 according to this embodiment. The start catalysts (SC) 5a and 5b are constituted as a three-way catalyst by using a honeycomb-shaped substrate of cordierite or the like, forming a thin coating of alumina on the surface of the substrate, and carrying a noble metal catalyst component such as platinum Pt, palladium Pd or rhodium Rh on the alumina layer. The three-way catalyst highly efficiently removes the three components, i.e., HC, CO and NOx near the stoichiometric air-fuel ratio. The three-way catalyst exhibits a decreased ability for reducing NOx when the air-fuel ratio of the exhaust gas flowing in becomes higher than the stoichiometric air ratio. When the engine 1 is operating at a lean air-fuel ratio, therefore, the three-way catalyst is not capable of removing NOx in the exhaust gas to a sufficient degree. The start catalysts (SC) 5a and 5b are disposed in the exhaust passages 2a and 2b at positions close to the engine 1 and have a relatively small capacity to decrease their heat capacity, so that they can be heated to the activated temperature within a short period of time after the start of the engine. Next, described below is the NOx occluding and reducing catalyst 7 according to this embodiment. The NOx occluding and reducing catalyst 7 according to this embodiment uses alumina as a substrate to carry at least one component selected from the alkali metals such as potassium K, sodium Na, lithium Li and cesium Cs, alkaline earth elements such as barium Ba and calcium Ca, and rare earth elements such as lanthanum La, cerium Ce and yttrium Y, as well as a noble metal such as platinum Pt. The NOx occluding and reducing catalyst exhibits the action of absorbing and releasing NOx, i.e., absorbing NOx (NO 2 , NO) in the exhaust gas and holding them in the form of nitric acid ions NO 3- when the air-fuel ratio of the exhaust gas flowing in is lean, and releases the absorbed NOx when the air-fuel ratio of the exhaust gas flowing in becomes rich. The mechanism for absorbing and releasing NOx will be described next with reference to the case of using platinum Pt and barium Ba. The same mechanism, however, is created even when other noble metals, alkali metals, alkaline earth elements and rare earth elements are used. When the concentration of oxygen increases in the exhaust gas that are flowing in (i.e., when the air-fuel ratio of the exhaust gas becomes larger (leaner) than the stoichiometric air-fuel ratio), oxygen in the form of O 2 - or O 2- adheres onto platinum Pt, whereby NOx in the exhaust gas reacts with O 2 - or O 2- on platinum Pt and forms NO 2 . NO 2 in the exhaust gas and NO 2 thus formed are further oxidized on platinum Pt, are absorbed by the absorbing agent such as barium oxide BaO, in which they are bonded to barium oxide BaO, and are diffused in the form of nitric acid ions NO 3 - in the absorbing agent. In a lean atmosphere, therefore, NOx in the exhaust gas is absorbed in the form of nitrates by the NOx-absorbing agent. When concentration of oxygen greatly decreases in the exhaust gas (i.e., when the air-fuel ratio of the exhaust gas becomes smaller (richer) than the stoichiometric air-fuel ratio), the amount of NO 2 formed on platinum Pt decreases, and the reaction proceeds in the reverse direction. This causes nitric acid ions NO 3 - in the absorbing agent to be released in the form of NO 2 from the absorbing agent. In this case, if the reducing components such as CO and the like as well as components such as HC, CO 2 and the like are included in the exhaust gas, NO 2 is reduced by these components on platinum Pt. This embodiment uses the engine 1 capable of operating at a lean air-fuel ratio. When the engine 1 is operated at a lean air-fuel ratio, the NOx occluding and reducing catalyst absorbs NOx in the exhaust gas that flows in the catalyst. When the engine 1 is operated at a rich air-fuel ratio, the NOx occluding and reducing catalyst 7 releases and purities the absorbed NOx by reduction. According to this embodiment, when NOx is absorbed in increased amounts by the NOx occluding and reducing catalyst 7 during the operation at a lean air-fuel ratio, a rich-spike operation is carried out to switch the air-fuel ratio of the engine from a lean air-fuel ratio to a rich air-fuel ratio for a short period of time in order to release NOx from the NOx occluding and reducing catalyst and to purify NOx by reduction (to regenerate the NOx occluding and reducing catalyst). In this embodiment, the ECU 30 increases or decreases the value of an NOx counter in order to estimate the amount of NOx absorbed and held by the NOx occluding and reducing catalyst 7. The amount of NOx absorbed by the NOx occluding and reducing catalyst 7 per a unit time varies in proportion to the amount of NOx in the exhaust gas flowing into the NOx occluding and reducing catalyst per a unit time, i.e., varies in proportion to the amount of NOx generated by the engine 1 per a unit time. Further, the amount of NOx generated by the engine per a unit time is determined by the amount of fuel fed to the engine, air-fuel ratio, flow rate of the exhaust gas, etc. When the operating conditions of the engine are determined, therefore, the amount of NOx absorbed by the NOx occluding and reducing catalyst can be determined. According to this embodiment, the engine operating conditions (degree of accelerator opening, engine rotational speed, amount of the intake air, intake air pressure, air-fuel ratio, amount of feeding fuel, etc.) are changed to measure the amount of NOx generated by the engine per a unit time, and the amount of NOx absorbed by the NOx occluding and reducing catalyst 7 per a unit time is stored in the ROM of ECU 30 in the form of a numerical value map using, for example, the load on the engine (amount of fuel injection) and the engine rotational speed. The ECU 30 calculates the amount of NOx absorbed by the NOx occluding and reducing catalyst per a unit time after every predetermined period of time (after every unit time) by using the load on the engine (amount of fuel injection) and the engine rotational speed, and increases the NOx counter by the amount of NOx absorbed. Therefore, the value of the NOx counter always indicates the amount of NOx absorbed by the NOx occluding and reducing catalyst 7. When the value of the NOx counter reaches a predetermined value while the engine is in operation at a lean air-fuel ratio, the ECU 30 executes the rich-spike operation in which the operation air-fuel ratio of the engine is changed to a rich air-fuel ratio for a short period of time. Therefore, NOx absorbed by the NOx occluding and reducing catalyst is released and purified by reduction. The time for maintaining the exhaust gas air-fuel ratio at a rich air-fuel ratio during the rich-spike operation is experimentally determined depending upon the type and volume of the NOx occluding and reducing catalyst. The value of the NOx counter is reset to 0 after the NOx is released from the NOx occluding and reducing catalyst and is purified by reduction upon the execution of the rich-spike operation. Upon effecting the rich-spike operation depending upon the amount of NOx absorbed by the NOx occluding and reducing catalyst 7 as described above, the NOx occluding and reducing catalyst 7 is properly regenerated and is not saturated with NOx which the catalyst has absorbed. It is known that sulfur oxides (SOx) contained in the exhaust gas is also absorbed in the NOx occluding and reducing catalyst and forms sulfates (e.g., BaSO 4 ) by quite the same mechanism as that for absorbing NOx under lean conditions. Like NOx, the sulfates are released in the form of SO 2 from the NOx occluding and reducing catalyst when the air-fuel ratio of the exhaust gas becomes rich. Generally, however, the sulfates remain stable. To release SOx from the NOx occluding and reducing catalyst, however, the NOx occluding and reducing catalyst must be heated at a temperature higher than that of when NOx are to be released. Therefore, SOx are not released to a sufficient degree by the normal regenerating operation of the NOx occluding and reducing catalyst; i.e., SOx are gradually accumulated in the absorbing agent, and the amount of the absorbing agent that participates in absorbing NOx decreases. As SOx accumulate, therefore, the capacity of the NOx occluding and reducing catalyst for absorbing NOx decreases, giving rise to the occurrence of a so-called SOx contamination of the NOx occluding and reducing catalyst. In order to recover from the SOx contamination, the NOx occluding and reducing catalyst must be maintained at a contamination-recovery temperature (e.g., 600°C or higher) which is higher than a temperature (e.g., 250°C or higher) at which the NOx occluding and reducing catalyst is usually regenerated to release NOx and, besides, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst must be maintained at an air-fuel ratio smaller than the stoichiometric air-fuel ratio (a rich air-fuel ratio). Therefore, when the temperature of the NOx occluding and reducing catalyst is lower than the above-mentioned contamination-recovery temperature, the NOx occluding and reducing catalyst 7 must be heated to a temperature higher than the contamination-recovery temperature in order to perform SOx contamination-recovery operation. According to this embodiment as will be described later, the contamination of the NOx occluding and reducing catalyst 7 is removed by conducting two operations, i.e., a temperature-raising operation for heating the NOx occluding and reducing catalyst 7 up to the contamination-recovery temperature and a recovery operation for releasing SOx from the NOx occluding and reducing catalyst by maintaining the NOx occluding and reducing catalyst 7, after the temperature thereof is raised, in an atmosphere of a rich air-fuel ratio. On the other hand, the temperature of the NOx occluding and reducing catalyst 7 greatly changes depending on the operating conditions of the engine (such as the exhaust gas temperature). In some operating conditions of the engine, the temperature of the NOx occluding and reducing catalyst 7 may be much lower than the above-mentioned contamination-recovery temperature. When the temperature-raising operation is carried out to heat the NOx occluding and reducing catalyst 7, energy is consumed in an increased amount to deteriorate the fuel efficiency of the engine. When the temperature of the NOx occluding and reducing catalyst 7 is much lower than the contamination-recovery temperature, therefore, the contamination-recovery operation is not executed despite the holding amount of SOx has reached a predetermined value but, instead, it must wait until the temperature of the NOx occluding and reducing catalyst rises to near the contamination-recovery temperature depending upon a change in the engine operating conditions, so that the fuel efficiency will not be deteriorated. Here, however, if the temperature range in which the temperature-raising operation for the NOx occluding and reducing catalyst 7 is carried out is fixed, the contamination-recovery operation will not be executed even if holding amount of SOx is increased when the engine is operated for extended periods of time under the operating conditions in which the catalyst temperature lies outside the above-mentioned temperature range. Therefore, the amount of SOx held by the NOx occluding and reducing catalyst continues to increase. If the holding amount of SOx continues to increase, the NOx-absorbing capacity of the NOx occluding and reducing catalyst 7 decreases, and the catalyst is saturated with NOx after it has absorbed NOx in only small amounts. Therefore, unpurified NOx is released into the open air when the engine is in operation. According to this embodiment, therefore, the range of the catalyst temperature in which the temperature-raising operation is conducted is expanded with an increase in the amount of SOx held by the NOx occluding and reducing catalyst 7, in order to prevent the NOx occluding and reducing catalyst from being saturated. Thus, the range of the catalyst temperature in which the temperature-raising operation is conducted is expanded with an increase in the amount of SOx held by the NOx occluding and reducing catalyst, and the contamination-recovery operation is executed even at a low catalyst temperature. Therefore, the contamination-recovery operation is executed when the holding amount of SOx increases to some extent despite the engine being continuously operated under such operating conditions that the catalyst temperature remains low, and the NOx occluding and reducing catalyst recovers its absorbing capacity. Thus, the NOx occluding and reducing catalyst is not saturated, and unpurified NOx is prevented from being released. Fig. 2 is a flowchart explaining the operation for judging the contamination-recovery conditions for the NOx occluding and reducing catalyst 7 according to this embodiment. This operation is conducted as a routine executed by the ECU 30 after every predetermined period of time. In this operation, the ECU 30 judges at a step 2001 whether a flag F1 that will be described later in detail is 1 or not. Under normal condition, this judgement is negative and the routine proceeds to a step 2002 where it is judged whether a flag F2 that will be described later in detail is 1 or not. Under normal condition, this judgement is negative and the routine proceeds to a step 2003 where it is judged whether a flag F3 that will be described later in detail is 1 or not. Under normal condition, this judgement is negative and the routine proceeds to a step 2004 to execute the lean operation. Thus, the lean operation is executed under the normal condition, and NOx are contained in relatively large amounts in the exhaust gas. However, the exhaust gas are acquiring a lean state and have a large oxygen concentration. As described above, therefore, the NOx occluding and reducing catalyst 7 absorbs NOx in the exhaust gas. The flag F1 is set when the engine in the present operating condition must produce a large output to cope with acceleration or a large load exerted on the engine. When the judgement at the step 2001 is affirmative, therefore, the routine proceeds to a step 2005 to execute a stoichiometric air-fuel ratio operation. Due to the lean air-fuel ratio operation, the amount of NOx absorbed by the NOx occluding and reducing catalyst 7 gradually increases. As described above, since the amount of NOx that can be held by the NOx occluding and reducing catalyst 7 is finite, NOx must be released from the NOx occluding and reducing catalyst 7 and must be purified by reduction before the amount of NOx occluded exceeds the maximum NOx holding capacity. In this case, the flag F2 is set and the routine proceeds to a step 2006 where a rich-spike operation is executed. Fig. 3 is a flowchart explaining the operation for setting the flag F2. This operation is conducted as a routine executed by the ECU 30 after every predetermined period of time. At a step 2101, first, it is judged whether a flag F4 which will be described later in detail is 1 or not. At first, this judgement is negative, and the routine proceeds to a step 2102 where a target NOx occluding amount st is set to s1. This amount may be, for example, 70% of the NOx occluding capacity of the NOx occluding and reducing catalyst 7. Then, the amount of NOx absorbed by the NOx occluding and reducing catalyst 7 per a unit time is integrated during the lean air-fuel ratio operation, and the amount of NOx s held by the catalyst is calculated. The NOx holding amount is integrated relying on the amount of NOx generated by the internal combustion engine per a unit time based on the engine operating conditions as described above. Then, at a step 2105, it is judged whether the NOx holding amount s has reached the target value st or not. When this judgement is negative, the routine proceeds to a step 2106 where the flag F2 ends remaining at 0. When the judgement at the step 2105 is affirmative, the routine proceeds to a step 2107 where the flag F2 is set to 1. At a step 2108, the NOx occluding amount s is set to 0, and the routine ends. Fig. 4 is a flowchart illustrating the rich-spike operation of the step 2006 of Fig. 2. In Fig. 4, first, the rich-spike is executed at a step 2201. The rich-spike operation is executed by the same method as the one described above, and is not described here in detail. Upon executing the rich-spike operation, Nox is released from the NOx occluding and reducing catalyst 7, and is purified by reduction with HC and CO contained in the exhaust gas. Next, a timer is actuated at a step 2202, and it is judged at a step 2203 whether an average output value A1 of the air-fuel ratio sensors 29a and 29b on the upstream side is nearly in agreement with the output A2 of the air-fuel ratio sensor 31 on the downstream side or not. When this judgement is negative, the rich-spike operation is continued at the step 2201. As described above, when the NOx are being reduced by the rich-spike, the exhaust gas flowing out of the NOx occluding and reducing catalyst assume a stoichiometric air-fuel ratio which is different from the rich air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst 7. Therefore, the average output Al of the air-fuel sensors 29a and 29b on the upstream side is different from the output A2 of the air-fuel ratio sensor 31 on the downstream side. On the other hand, when NOx is all released from the NOx occluding and reducing catalyst 7 and is all reduced, the air-fuel ratio of the exhaust gas flowing out of the NOx occluding and reducing catalyst 7 becomes nearly equal to the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst 7, whereby the average output Al of the air-fuel ratio sensors 29a and 29b on the upstream side becomes nearly in agreement with the output A2 of the air-fuel sensor 31 on the downstream side, and the routine proceeds to a step 2204 where the rich-spike is interrupted and the timer stops. In this embodiment, the air-fuel ratio sensors are provided on both the upstream side and the downstream side of the NOx occluding and reducing catalyst 7 as shown in Fig. 1. It is, however, also allowable to provide the air-fuel ratio sensor on the downstream side of only the NOx occluding and reducing catalyst and to so judge that NOx is all released from the NOx occluding and reducing catalyst and is all reduced at a moment when the output of the air-fuel ratio sensor has changed from the stoichiometric air-fuel ratio to a rich air-fuel ratio. Next, at the step 2205, it is judged whether the flag F4 that will be described later in detail is 1 or not. At first, this judgement is negative and the routine proceeds to a step 2206 where it is judged whether the time T counted by the timer is shorter than a first predetermined period of time T1 or not. The first predetermined period of time T1 is for releasing and reducing NOx in an amount of 70% of the NOx occluding capacity of the NOx occluding and reducing catalyst 7 at an air-fuel ratio of the exhaust gas during rich-spike operation. At present, where the target NOx occluding amount is 70% of the NOx occluding capacity, therefore, if the NOx occluding and reducing catalyst 7 is occluding NOx in the target amount, the judgement at the step 2206 is negative, an the routine proceeds to a step 2207 where the flag F3 is set to 0. When the judgement is affirmative at the step 2206, on the other hand, it means that the NOx occluding and reducing catalyst 7 is occluding NOx only in an amount that is smaller than 70% of the NOx occluding capacity. This means that the SOx contamination has exceeded 30% of the NOx occluding capacity, since the above-mentioned integration operation is to relatively correctly calculate the amount of NOx that will be occluded by the NOx occluding and reducing catalyst 7. The SOx contamination mildly builds up compared with the frequency for executing the rich-spike. When the judgement at the step 2206 is affirmative, therefore, the SOx contamination is nearly 30% of the NOx occluding capacity. At this moment, the routine proceeds to a step 2208 where the flag F3 is set to 1. When the flag F3 is set, the judgement at the step 2003 becomes affirmative in the above-mentioned operation of Fig. 2, and the routine proceeds to a step 2007 to execute the SOx contamination-recovery operation. After having passed the step 2207 or 2208 in the operation of Fig. 4, the flag F2 is set to 0 at a step 2212, and the flag F4 is set to 0 at a step 2213 to end the routine. Prior to describing the steps 2209 and 2210 of Fig. 22, described below is the operation for removing the SOx contamination with reference to Fig. 23. At a step 2301, first, it is judged whether the processing can be executed or not. To remove the contamination due to SOx as described above, the NOx occluding and reducing catalyst 7 must have been heated at a temperature not lower than, for example, 600°C. Based on the present operating conditions of the engine, therefore, the judgement at step 2301 becomes affirmative when the temperature of the NOx occluding and reducing catalyst 7 lies within a predetermined temperature range which is not lower than 600°C, and the routine proceeds to a step 2302. The SOx contamination builds up relatively slowly. During this period, therefore, the amount of SOx occluded in the NOx occluding and reducing catalyst does not increase greatly. When the temperature of the NOx occluding and reducing catalyst 7 is low as in the above-mentioned embodiment, the cylinders #1 and #4, for example, are operated at a rich air-fuel ratio and the cylinders #2 and #3 are operated at a lean air-fuel ratio; i.e., the exhaust gas of a rich state and the exhaust gas of a lean state are alternatingly emitted from the cylinders, and HC and CO are burned on the NOx occluding and reducing catalyst 7, so that the temperature of the NOx occluding and reducing catalyst 7 lies within a predetermined temperature range. At a step 2302, it is judged whether a flag F5 that will be described later in detail is 1 or not. This judgement is, first, negated and the routine proceeds to a step 2304. At the step 2304, a target air-fuel ratio AFt of exhaust gas during the recovery operation is shifted toward the lean side by an amount dAF. The target air-fuel ratio AFt at first lies on the rich side beyond the stoichiometric air-fuel ratio by dAF. At a moment when the routine passes through the step 2304, first, the target air-fuel ratio AFt is the stoichiometric air-fuel ratio. Then, at a step 2305, the operation is so conducted that the exhaust gas acquires the target air-fuel ratio AFt. Next, at a step 2306, it is judged whether a predetermined period of time has passed or not. When this judgement is negative, the processing of step 2305 is continued. That is, the operation in which the exhaust gas acquires the target air-fuel ratio AFt is continued for a predetermined period of time and at a step 2307, the flag F4 is set to 1 to end the routine. The operation in which the exhaust gas acquires the target air-fuel ratio AFt may be so conducted that the combustion air-fuel ratios in the cylinders acquire the target air-fuel ratio AFt, or may be so conducted that the combustion air-fuel ratios in the two cylinders of consecutive ignition timings as a whole assume the target air-fuel ratio AFt, or may be so conducted that the secondary fuel injection is effected, i.e., the fuel is directly fed into the engine exhaust system or the fuel is injected into the cylinders in the exhaust stroke to increase the amount of unburned fuel in the exhaust gas, so that the exhaust gas acquires the target air-fuel ratio AFt. Thus, the Sox contamination-recovery operation ends and at this moment only, the flag F4 is set. In the operation of Fig. 3 for setting the flag F2, therefore, the judgement at the step 2101 is affirmed, and the routine proceeds to the step 2103. At this moment only, the target NOx occluding amount st is set to s2. This amount s2 is equal to the NOx occluding capacity of the NOx occluding and reducing catalyst 7. At the step 2105, therefore; the flag F2 is not set until it is so calculated that NOx are occluded in an amount equal to 100% of the NOx occluding capacity only one time after the SOx contamination-recovery operation. After the flag F2 is set, the above-mentioned operation of Fig. 4 is executed. This time, however, the flag F4 is 1, the judgement at the step 2205 is affirmative, and the routine proceeds to the step 2209. At the step 2209, it is judged whether the time T counted by the timer is shorter than a second predetermined period of time T2. The second predetermined period of time T2 is for releasing and reducing NOx in an amount equal to 100% of the NOx occluding capacity of the NOx occluding and reducing catalyst 7 at an air-fuel ratio of the exhaust gas during rich-spike operation. If NOx in an amount equal to 100% of the NOx occluding capacity has been occluded in the NOx occluding and reducing catalyst, i.e., if the contamination due to SOx has been completely removed by the recovery operation, the judgement at the step 2209 is negative, and the routine proceeds to a step 2210 where the flag F5 is set to 0. When the judgement at the step 2209 is affirmative, on the other hand, it means that NOx has not been occluded by the NOx occluding and reducing catalyst 7 in an amount equal to 100% of the NOx occluding capacity, and the NOx occluding and reducing catalyst 7 has not completely recovered from the SOx contamination. In this case, the flag F5 is set to 1 at a step 2211. In this embodiment as described above, a general deterioration detection method is used for evaluating the degree of recovery after the recovery operation. When the flag F5 is set, the judgement at the step 2302 is affirmed in the operation of Fig. 5 after the SOx contamination-recovery operation of the next time, and the target air-fuel ratio AFt of exhaust gas is shifted, at step 2303, toward the rich side by dAF, and the recovery operation is conducted for a predetermined period of time. At a moment when the SOx contamination of the NOx occluding and reducing catalyst 7 has reached 30% of the NOx occluding capacity, the recovery operation is conducted in which the exhaust gas acquires a stoichiometric air-fuel ratio or a rich air-fuel ratio for only a predetermined period of time as described above. When the NOx occluding and reducing catalyst is completely recovered, however, the air-fuel ratio of the exhaust gas during the recovery operation is gradually shifted toward the lean side. When the recovery is not sufficient, the air-fuel ratio of the exhaust gas is gradually shifted toward the rich side. The time T counted by the timer is inversely proportional to the non-recovering degree from the SOx contamination. The amount dAF for shifting the air-fuel ratio toward the rich side may be increased with a decrease in the counted time T. To recover the NOx occluding and reducing catalyst from the SOx contamination, stable sulfates must be decomposed. For this purpose, the NOx occluding and reducing catalyst is heated to a temperature as high as 600°C or more, and the oxygen concentration in the exhaust gas is decreased. In removing the SOx contamination, SOx is easily released when the temperature of the NOx occluding and reducing catalyst is high or when the oxygen concentration is low in the exhaust gas. Furthermore, the larger the amount of reducing substances in the exhaust gas, the larger the amount of oxygen consumed by the reducing substances on platinum Pt of the NOx occluding and reducing catalyst, and SOx is released more easily. It is not allowed to elevate the temperature of the NOx occluding and reducing catalyst to an excess degree since the catalyst may be mechanically and functionally damaged. In this embodiment, the NOx occluding and reducing catalyst is heated to a predetermined temperature range (e.g., from 700°C to 800°C). When the air-fuel ratio of the exhaust gas is set to become excessively rich during the recovery operation, the oxygen concentration decreases since a large amount of reducing substances is included in the exhaust gas. Though this guarantees the recovery from the SOx contamination this causes a decrease in the fuel efficiency, and deterioration of the exhaust gas emission. In order to completely recover the NOx occluding and reducing catalyst that is contaminated with SOx to a degree of 30% of the NOx occluding capacity by heating it within a predetermined temperature range for a predetermined period of time without deteriorating the exhaust gas emission as done in this embodiment, it can be contrived to set the air-fuel ratio of the exhaust gas to a predetermined air-fuel ratio. However, easiness for releasing SOx from the NOx occluding and reducing catalyst 7 varies depending upon the SOx concentration in the exhaust gas during the recovery operation. That is, when the sulfur concentration in the fuel which is a precursor of SOx is high, the SOx concentration becomes high in the exhaust gas, and SOx are less released from the NOx occluding and reducing catalyst 7 correspondingly. To accomplish the complete recovery, therefore, the air-fuel ratio of the exhaust gas must be set to be more rich than the predetermined air-fuel ratio. When the sulfur concentration in the fuel is low, on the other hand, the SOx concentration in the exhaust gas becomes low, and SOx is easily released from the NOx occluding and reducing catalyst 7, correspondingly. To improve the exhaust gas emission, therefore, the air-fuel ratio of the exhaust gas must be set to be more lean than the predetermined air-fuel ratio. In this embodiment as described above, the degree of recovery from SOx contamination is evaluated based on the NOx occluding capacity after the recovery operation. When the catalyst has been completely recovered, the air-fuel ratio of the exhaust gas is shifted toward the lean side in the recovery operation of the next time. When the catalyst has not been completely recovered, the air-fuel ratio of the exhaust gas is shifted toward the rich side in the recovery operation of the next time. In the recovery operation, therefore, the air-fuel ratio of the exhaust gas is set to an optimum air-fuel ratio for guaranteeing a complete recovery depending upon the sulfur concentration in the fuel without deteriorating the exhaust gas emission. The optimized air-fuel ratio of the exhaust gas is a value corresponding to the sulfur concentration in the fuel and the SOx concentration in the exhaust gas. Upon monitoring the air-fuel ratio, therefore, it is possible to estimate the amount of sulfates formed by SOx and to let the driver know also the degree of the possibility of corrosion in the engine exhaust system caused by sulfates. Fig. 6 is a flowchart explaining the SOx contamination-recovery operation different from that of Fig. 5. Described below is only a difference from the flowchart of Fig. 5. in this operation, when it is judged at a step 2402 that the flag F5 is 1, i.e., when the recovery from the SOx contamination is not sufficient, the target recovery operation time t is extended by dt. When the flag F5 is 0, i.e., when the SOx contamination is completely removed, the target recovery operation time t is shortened by dt. Thereafter, at steps 2405 and 2406, the air-fuel ratio of the exhaust gas is set to be a stoichiometric air-fuel ratio or a predetermined rich air-fuel ratio and, at a step 2403 or 2404, the recovery operation is conducted for only a target recovery operation time t that is changed. During the recovery operation as described above, the air-fuel ratio of the exhaust gas is set to be a predetermined air-fuel ratio and the target recovery operation time is changed, in order to accomplish an optimum recovery operation time that guarantees a complete recovery depending upon the sulfur concentration in the fuel without deteriorating the exhaust gas emission. In the above-mentioned operations of Figs. 5 and 6, either the air-fuel ratio of the exhaust gas or the recovering time is fixed and the other one is optimized depending upon the sulfur concentration in the fuel during the recovery operation. This, in other words, is to optimize the total amount of reducing substances flowing into the NOx occluding and reducing catalyst and to optimize the total amount of oxygen in compliance therewith during the period of recovery operation. For this purpose, therefore, both the air-fuel ratio of the exhaust gas and the recovering time may be changed depending upon the sulfur concentration in the fuel. In the operation of Fig. 5, furthermore, the target air-fuel ratio of the exhaust gas during the recovery operation is, first, set to be the stoichiometric air-fuel ratio as described earlier and is then gradually shifted toward the rich side to accomplish an optimum air-fuel ratio depending upon the sulfur concentration in the fuel. When the sulfur concentration of the fuel is very low, the target air-fuel ratio changes near the stoichiometric air-fuel ratio, and the target air-fuel ratio readily becomes an optimum air-fuel ratio. When the sulfur concentration in the fuel is relatively high and the optimum air-fuel ratio is rich to a high degree, the recovery from the SOx contamination is not sufficient in the recovery operation of before the target air-fuel ratio becomes the optimum air-fuel ratio and, hence, the recovery operation must be conducted relatively frequently. The recovery operation is not desirable since it deteriorates the fuel efficiency compared to that of during the normal lean operation. In order to solve this problem, the sulfur concentration in the fuel may be detected as shown in a flowchart of Fig. 7, and the target air-fuel ratio AFt may be set depending upon the sulfur concentration. That is, in the operation of Fig. 7, the sulfur concentration in the fuel is directly detected by the sulfur concentration sensor at a step 2501, and a first target air-fuel ratio AFt in the recovery operation is set at a step 2502 based on the sulfur concentration found from a map shown in Fig. 8. It is therefore allowed to readily set the target air-fuel ratio AFt to the optimum air-fuel ratio in the operation of Fig. 5, preventing the recovery operation from being frequently executed. In the map shown in Fig. 8, the rich degree of the target air-fuel ratio AFt is set to increase with an increase in the sulfur concentration. If the map is correctly formed by experiment, the target air-fuel ratio AFt needs not be changed depending upon the degree of recovery from the SOx contamination in the operation of Fig. 5, and can be used as the air-fuel ratio of the exhaust gas during the recovery operation. It is, of course, possible to set a target recovery operation time from a map depending upon the sulfur concentration in the fuel relying on the same idea.

Claims: An exhaust gas purification device for an internal combustion engine comprising: an NO x occluding and reducing catalyst (7) disposed in an exhaust gas passage (2) of the internal combustion engine (1) to absorb NO x in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is larger than a stoichiometric air-fuel ratio and to release and reduce the absorbed NO x when the air-fuel ratio of the exhaust gas flowing in becomes smaller than the stoichiometric air-fuel ratio, said NO x occluding and reducing catalyst (7) exhibiting a decrease in the capability of absorbing, releasing and reducing the NO x as it adsorbs or absorbs contaminants in the exhaust gas; a holding amount estimation means (2206) for estimating the amount of said contaminants adsorbed or absorbed by said NO x occluding and reducing catalyst (7); a recovering means (2007) which maintains said NO x occluding and reducing catalyst (7) within a predetermined high-temperature range for a predetermined recovering period when the holding amount of said contaminants estimated by said holding amount estimation means (2206) has reached a predetermined value, and controls the air-fuel ratio (AFt) of the exhaust gas flowing into said NO x occluding and reducing catalyst (7) to assume a recovering air-fuel ratio at which said contaminants can be released; and a changing means (2302; 2402; 2502) for changing the total amount of the reducing substances in the exhaust gas flowing into said NO x occluding and reducing catalyst (7) during recovery operation, wherein a) a recovering degree evaluation means (2209) evaluates the degree of recovery of the capability of absorbing, releasing and reducing NO x by reduction of said NO x occluding and reducing catalyst (7) accomplished by said recovering means (2007) and said changing means (2302; 2402) changes the total amount of the reducing substances during the recovering period of the next time depending upon the recovering degree evaluated by said recovering degree evaluation means (2209), or b) an estimation means (2501) estimates the concentration (C) of precursors of said contaminants in the fuel and said changing means (2502) changes the total amount of the reducing substances during said recovering period depending upon the concentration (C) of said precursors estimated by said estimation means (2501). An exhaust gas purification device for an internal combustion engine according to claim 1, wherein said changing means (2302; 2402; 2502) changes the total amount of said reducing substances flowing into said NO x occluding and reducing catalyst (7) during recovery operation by changing said recovering air-fuel ratio (AFt). An exhaust gas purification device for an internal combustion engine according to claim 1, wherein said changing means (2302; 2402; 2502) changes the total amount of said reducing substances flowing into said NO x occluding and reducing catalyst (7) during recovery operation by changing the length (t) of said recovering period. An exhaust gas purification device for an internal combustion engine according to any preceding claim, wherein said recovering degree evaluation means (2209) includes a means for detecting the NO x absorbing capacity of said NO x occluding and reducing catalyst (7), and evaluates said recovering degree based on the NO x absorbing capacity of said NO x occluding and reducing catalyst (7) at the time when said recovering period has elapsed.

Inventors: SUZUKI NAOTO, SUZUKI, NAOTO
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1999
Main classification: F01N3
IPC codes: F01N7, F01N3, F02D41