Abstract:
In an exhaust gas purifying system wherein an NOx occlusion reduction type catalyst ( 30 ) including a catalytic metal and an NOx occluding substance is installed in an exhaust passage ( 3 ) of an engine ( 10 ), a restore processing against the sulfur poisoning of the NOx occlusion reduction type catalyst ( 30 ) is executed, by performing a lax rich control for controlling the air/fuel ratio of the exhaust gas to the theoretical air/fuel ratio or slightly lower than the theoretical air/fuel ratio, when the NOx occlusion reduction type catalyst ( 30 ) is heated equal or superior to the sulfur purge temperature. NOx is purified up effectively, by excluding effects of the sulfur poisoning, suppressing the deterioration of fuel-efficiency, by the control method for exhaust gas purifying system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a control method of an exhaust gas purifying system to remove NOx (nitrogen oxides) in an exhaust gas of an internal combustion engine through the reduction using an NOx occlusion reduction type catalyst. More particularly, it relates to a control method of an exhaust gas purifying system to restore the state of the NOx occlusion reduction type catalyst deteriorated through a sulfur poisoning.  
         [0002]     Various studies and proposals have been made relating to an NOx catalyst for removing NOx by reducing it from an exhaust gas of an internal combustion engine such as diesel engine, kind of gasoline engines and various combustion equipments.  
         [0003]     Among them, there is an exhaust gas purifying system for an internal combustion engine wherein an NOx occlusion reduction type catalyst is arranged in the exhaust passage of an engine, as described in Laid-Open Japanese Patent Publication 2000-274279. In the exhaust gas purifying system, NOx is absorbed in the NOx occlusion reduction type catalyst when the air/fuel ratio of an inflowing exhaust gas is lean. Then, when the NOx absorption capacity gets close to its saturation point, the control for regenerating the catalyst is done. By this control, the oxygen concentration of the inflowing exhaust gas is lowered by setting its air/fuel ratio to the theoretical air/fuel ratio or to rich condition for discharging the absorbed Nox, and the discharged NOx is reduced by using an annexed precious catalytic metal.  
         [0004]     This NOx occlusion reduction type catalyst supports an NOx occluding substance (NOx absorbing material) made of an alkaline-earth metal such as Barium (Ba) and a precious catalytic metal such as platinum (Pt) on a catalyst support. Then, under a high oxygen concentration atmosphere, NO in the exhaust gas is oxidized by the catalytic activity of the platinum to change into NO 2 . This NO 2  diffuses in the catalyst with a form of NO 3 -, and absorbed by the NOx occluding substance with a form of nitrate.  
         [0005]     When the air/fuel ratio becomes rich and the oxygen concentration lowers, NO 3 - will be discharged from the NOx occluding substance with a form of NO 2 . This NO 2  is reduced to N 2 , under the catalyst activity of the platinum by reducers such as unburned HC, CO and H 2  contained in the exhaust gas. This reduction effect permits to prevent NOx from being discharged in the atmosphere.  
         [0006]     However, since a sulfur content contained in the fuel of the diesel engine is accumulated in the NOx occluding substance to stabilize as sulfate, the NOx occlusion reduction type catalyst has a problem of sulfur poisoning that the amount of NOx occlusion reduces.  
         [0007]     If the deterioration of NOx occluding substance progresses by the sulfur poisoning, the NOx purifying efficiency declines because the NOx absorbing capacity is lowered, even if the air/fuel ratio of the exhaust gas is in lean state and the oxygen concentration is high. In addition, because the NOx absorbing activity lowers immediately to the proximity of its limit, a frequent regeneration processing by a rich-burn is required. As a result, the fuel efficiency deteriorates.  
         [0008]     Consequently, a regeneration processing is required to restore the NOx absorbing capacity of the NOx occluding substance by releasing under the low oxygen concentration atmosphere NOx absorbed under the high oxygen concentration atmosphere through changing over from the lean state to the rich state. In addition, a catalyst deterioration restoring process to remove the sulfur content using sulfur purge is also required at a phase when the deterioration has progressed to some extent by monitoring the progress of the catalyst deterioration caused by sulfur poisoning.  
         [0009]     In the sulfur purge, however, it is necessary to raise the catalyst temperature as high as around 600° C. to 700° C. and to keep a reducing atmosphere as well. Therefore, a temperature and a rich state of the exhaust gas is needed for the sulfur purge different from that of the exhaust gas during the regeneration process to restore the NOx absorbing capacity of the NOx occluding substance.  
         [0010]     Besides, it is necessary to raise the exhaust gas temperature to 600° C. or more for the sulfur purge. However, a lot of fuel is required to make the catalyst temperature around 600° C. to 700° C. for the sulfur purge by performing the controls such as intake throttling and post-injection to raise the exhaust gas temperature from the low temperature of the lean state of normal operation condition. And the fuel consumption deteriorates. As a result, it is particularly important to make the rich state optimal for the sulfur purge.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention has been devised to resolve the problems mentioned above, and has an object to provide a control method for an exhaust gas purifying system allowing to pulify NOx efficiently, by excluding activity of sulfur poisoning, all the way limiting the deterioration in the mileage, in an exhaust gas purifying system using an NOx occlusion reduction type catalyst for purifying NOx in the exhaust gas.  
         [0012]     The control method for the exhaust gas purifying system for achieving the object as mentioned above is a method in which NOx occlusion reduction type catalyst including a catalytic metal and an NOx occluding substance is installed in an exhaust passage of the engine, wherein a recovery process against the sulfur poisoning of the NOx occlusion reduction type catalyst is carried out by performing a lax rich control for controlling the air/fuel ratio of the exhaust gas at the theoretical air/fuel ratio or slightly lower, when said NOx occlusion reduction type catalyst is heated up to more than the sulfur purge temperature.  
         [0013]     This catalytic metal can be made of those having the catalytic function to reductive oxidations such as platinum. And, the NOx occluding substance can be made of any one or combination of alkali metals including potassium (K), sodium (Na), lithium (L), cesium (Cs) and so on, alkali alkaline-earth metals including barium (Ba), calcium (Ca), or precious earth metals such as lanthanum (La), yttrium (Y).  
         [0014]     In addition, the NOx occlusion reduction type catalyst also can comprise a reducer occluding substance made of zeolite or the like, which occludes HC and CO at low temperature and discharges at high temperature.  
         [0015]     Besides, the operation of a rich air/fuel ratio in the exhaust gas does not necessarily mean an operation of rich-burn in the cylinder bore, but it means an operation that a ratio of air quantity and fuel quantity in the exhaust gas flowed into the NOx occlusion reduction type catalyst is near the theoretical air/fuel ratio or the fuel quantity exceeds the fuel quantity of the theoretical air/fuel ratio to be rich-burn.  
         [0016]     According to the composition, it is possible to perform the sulfur purge efficiently preventing the temperature from falling, for the air/fuel ratio of the exhaust gas is controlled at the theoretical ratio or slightly lower than the theoretical air/fuel ratio, namely, the excess air factor is controlled at 1.0 or slightly lower than 1.0.  
         [0017]     In addition, in the control method for exhaust gas purifying system described above, an optimal environment for the sulfur purge is realized by a lax rich control setting the excess air factor of the exhaust gas to 0.95 to 1.0. In passing, the excess air factor of the exhaust gas in the regeneration control for restoring the NOx occluding capacity is normally at around 0.8 to 0.95 at the catalyst inlet and in the lax rich control, the oxygen concentration is controlled to be higher than in the rich control for regeneration.  
         [0018]     According to the control method for exhaust gas purifying system of the composition, the following effects can be brought about.  
         [0019]     When the sulfur purge is required, the intake gas is throttled to raise the temperature by the temperature raising control operation. And, when the catalyst temperature reaches the temperature capable of desulphurizing by the temperature raising control operation, the air/fuel ratio of the exhaust gas is controlled at the theoretical air/fuel ratio or slightly lower than the theoretical air/fuel ratio, preferably at 0.95 to 1.0 in terms of excess air factor by the lax rich control operation. Thereby, an efficient sulfur purge operation is available under the optimal low oxygen concentration atmosphere. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  shows a composition of an exhaust gas purifying system according to an embodiment of the present invention;  
         [0021]      FIG. 2  shows a composition of an engine system portion of the exhaust gas purifying system of  FIG. 1 ;  
         [0022]      FIG. 3 ( a ) shows a monolithic honeycomb structure of an NOx occlusion reduction type catalyst of an embodiment according to the present invention;  
         [0023]      FIG. 3 ( b ) shows a cell structure of  FIG. 3 ( a );  
         [0024]      FIG. 3 ( c ) shows a catalyst support structure of  FIG. 3 ( b );  
         [0025]      FIG. 4  shows a composition of a control system for exhaust gas purifying system on an embodiment according to the present invention;  
         [0026]      FIG. 5  is a flow chart of an operation control flow showing an exhaust gas purifying method of an embodiment according to the present invention;  
         [0027]      FIG. 6  is a more detailed flow chart of a sulfur purge control operation shown in  FIG. 5 ;  
         [0028]      FIG. 7  is a more detailed flow chart of a regeneration control operation shown in  FIG. 5 ; and  
         [0029]      FIG. 8  shows the excess air factor λ, the catalyst temperature and the optimal desulphurization environment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Hereafter, the control method for exhaust gas purifying system according to the present invention shall be described, referring to drawings.  
         [0031]     The exhaust gas purifying system according to the present invention is an exhaust gas purifying system  1  provided with an NOx occlusion reduction type catalyst (or DPF supporting the relevant catalyst)  30 . In the exhaust gas purifying system  1 , an air purifier  21 , a compressor  5   a  of a turbocharger  5 , intercooler  22  and a throttle butterfly (intake throttle valve)  23  are arranged in sequence from the upstream side in an exhaust gas duct  2  of an engine  10 . On the other hand, the upstream side exhaust gas temperature sensor  52 , a turbine  5   b  of the turbocharger  5 , an upstream side air/fuel ratio sensor (or oxygen concentration sensor)  53 , an NOx occlusion reduction type catalyst  30 , a downstream side exhaust gas temperature sensor  54 , a downstream side air/fuel ratio sensor (or oxygen concentration sensor)  55  and a silencer  61  are arranged in sequence from the upstream side in an exhaust passage  3 . Furthermore, an EGR passage  4  provided with an EGR cooler  41  and an EGR valve  42  is connected an exhaust manifold  12  to the intake gas passage  2  on the downstream side of the throttle butterfly  23 .  
         [0032]     Then, a common-rail injection system  6  which injects a fuel to the engine  10 , and an electronic control apparatus (electronic control box)  7  called an ECU (engine control unit) for controlling the whole engine are installed.  
         [0033]     Besides, any of variable capacity type turbo (VGS), turbo with waist gate or ordinary turbo may be used as the turbocharger  5 . In case of using a variable capacity type turbo (VGS) or a turbo with waist gate, both of the variable nozzle and the waist gate is also controlled by electronic control apparatus  7 .  
         [0034]     In the exhaust gas purifying system  1 , an air A is supercharged by the compressor  5   a  of the turbocharger  5  after passing through the air purifier  21 . Thereafter, the air A passes through the throttle butterfly  23  after cooled by the intercooler  22  to be supplied in the cylinder from the intake manifold  11  of the engine  10 . The flow of the intake air is adjusted by the throttle butterfly  23  which is controlled by the electronic control apparatus  7 .  
         [0035]     On the other hand, after an exhaust gas G is discharged from the exhaust manifold  12  to drive the turbine  5   b  of the turbocharger  5 , it passes through the NOx occlusion reduction type catalyst  30  to be purified into the exhaust gas Gc, and after passing through the silencer  61  ( FIG. 1 ), the exhaust gas Gc is discharged from a tail pipe  62  ( FIG. 1 ).  
         [0036]     Then, an EGR gas Ge which is a part of the exhaust gas G, passes through the EGR valve  42  after being cooled by the EGR cooler  41 , to enter the  2 , and circulates again. And for the EGR gas Ge, ON/OFF and the gas flow are adjusted by the EGR valve  42 .  
         [0037]     Now, the NOx occlusion reduction type catalyst  30  shall be described.  
         [0038]      FIG. 3  shows the wall surface structure of the NOx occlusion reduction type catalyst  30 . The NOx occlusion reduction type catalyst  30  is made of a monolithic honeycomb  30 M shaped by a support body  31  such as γ-alumina. A catalytic metal  32  and an NOx occluding substance  33  are supported on the surface in a cell  30 S of this monolithic honeycomb  30 M.  
         [0039]     This catalytic metal  32  is made of platinum (Pt) and the like, having an oxidation activity in a temperature range higher than the starting temperature of the activity. The starting temperature of the activity for platinum is at about 150° C. to 200° C.  
         [0040]     Besides, the NOx occluding substance  33  is made of alkali metals such as potassium (K), sodium (Na), lithium (L), cesium (Cs) and so on, alkali alkaline-earth metals such as barium (Ba), calcium (Ca) and precious earth metals such as lanthanum (La), yttrium (Y). The NOx occlusion reduction type catalyst  33  occludes NOx when the oxygen concentration in the gas is high, and releases NOx when the oxygen concentration in the gas is low.  
         [0041]     The regeneration control method in the exhaust gas purifying system  1  is performed by a control means comprising means as shown in  FIG. 4 . This control means C 1  is composed of a normal control operation means C 10 , a sulfur purge start judgment means C 20 , a regeneration control start judgment means C 30 , a sulfur purge control operation means C 40 , and a regeneration control operation means C 50  and so on.  
         [0042]     This normal control operation means C 10  is a control means for performing the normal lean-burn operation. The sulfur purge start judgment means C 20  is a means for judging whether the sulfur purge operation is performed, and judges to start the sulfur purge operation when the sulfur poisoning reaches the limit. Moreover, the regeneration control start judgment means C 30  is a means to judge whether the NOx absorbing capacity of the NOx occlusion reduction type catalyst  30  has saturated enough to start the regeneration control.  
         [0043]     Then, the sulfur purge control operation means C 40  is a means for purging sulfur from the NOx occluding substance poisoned by sulfur, and is composed of a temperature raising control operation means C 41  and a lax rich control operation means C 42 .  
         [0044]     Moreover, the regeneration control operation means C 50  is a means for regenerating the catalyst by rich-burn. This regeneration control operation means C 50  make the NOx occluding substance  30  discharge NOx by generating an exhaust gas with an excess air factor λ is 0.8 to 0.95 and almost zero of oxygen concentration at the catalyst inlet and at the same time, reduces and purifies the released NOx to restore the NOx absorbing capacity, and regenerates the catalyst.  
         [0045]     The control method of the exhaust gas purifying system  1  is carried out according to the operation control flow as illustrated in from  FIG. 5  to  FIG. 7 .  
         [0046]     The operation control flow of the  FIG. 5  is shown as being carried out in parallel with the other control flows of the engine during the operation of the engine  10 . In this flow, if the operation of the engine is stopped with the key of the engine turned OFF, an interruption in the middle of the execution occurs, resulting the execution of this operation control flow will be suspended and terminated. The portion of the suspension and the termination of the operation control flow caused by turning the key OFF of the engine is illustrated by the dotted line.  
         [0047]     The outline of the control flow shall be described as follows.  
         [0048]     In the step S 10 , a normal lean-burn operation is carried out by the normal control operation means C 10 . In the step S 20 , the necessity of a sulfur purge control operation by the NOx occlusion reduction type catalyst  30  is judged by the sulfur purge start judgment means C 20 .  
         [0049]     Then, when the sulfur purge control operation is judged to be needed in the step S 20 , a sulfur purge for restoring the deterioration of the catalyst due to a sulfur poisoning is carried out by the sulfur purge control operation means C 40 ,by intermediate of the sulfur purge control operation means C 40  in the step S 40 , before returning to the step S 10 .  
         [0050]     On the other hand, when the sulfur purge control operation is judged to be unnecessary in the step S 20 , the necessity of a regeneration control operation for regenerating the NOx occlusion reduction type catalyst  30  is judged by the regeneration control start judgment means C 30  in the step S 30 . In the case where the regeneration control operation is judged to be necessary in the step S 30 , the catalyst is regenerated by the regeneration control operation by the regeneration control operation means C 50  in the step S 50 , before returning to the step S 10 . In case where the regeneration control operation is judged to be unneeded in the step S 30 , it returns to the step S 10 .  
         [0051]     The operation control flow shown in  FIG. 5  to  FIG. 7  shall be described in detail as follows.  
         [0052]     When the operation control flow starts, the NOx occlusion quantity NOxS and the sulfur accumulated quantity SOxS accumulated in the NOx occlusion reduction type catalyst  30  during the previous engine operation is read in from the memory, by the data loaded in the step S 20 .  
         [0053]     Then, the normal control operation is carried out in the step S 10 , before proceeding to the step S 20 .  
         [0054]     In the normal control operation in the step S 10 , the lean-burn control operation is carried out for a pre-assigned time (for instance, time corresponding to the time interval for judging whether or not to carry out the catalyst regeneration control) by a lean-burn control operation means C 11  of the normal control operation means C 10 .  
         [0055]     This normal control operation is a lean-burn control operation discharging an exhaust gas of the normal lean air/fuel ratio (lean-burn operation of a gasoline engine, normal combustion operation of a diesel engine) and the exhaust gas component or the exhaust temperature to be discharged turns into that of the exhaust gas of the normal diesel engine. Consequently, NOx in the exhaust gas is occluded by the NOx occlusive substance  33  because of the high oxygen concentration in the exhaust gas.  
         [0056]     At the same time as the normal control operation, the NOx occlusion quantity NOXS occluded by the NOx occlusion reduction type catalyst  30  is calculated by an NOx occlusion quantity calculation means C 12 . The calculation of the NOx occlusion quantity is carried out using the NOx discharge map stored in a control apparatus in advance. This NOx discharge map expresses the relationship between the engine operation state and the NOx discharging quantity NOxS with a form of map data based on results such as preliminary measured values.  
         [0057]     The calculation of the NOx occlusion quantity may be calculated from both measurement values of an inlet side NOx sensor and an exit side NOx sensor by installing an NOx sensor (not shown) in front and rear of the NOx occlusion reduction type catalyst  30 . Otherwise, a method to evaluate how the NOx absorbing capacity gets to the saturation from both measurement values of the inlet side NOx sensor and the exit side NOx sensor is useful.  
         [0058]     Moreover, the sulfur accumulated quantity SOxS stored in the NOx occlusion reduction type catalyst  30  is calculated by a sulfur accumulated quantity computation means C 13 . The calculation of the sulfur accumulated quantity SOxS is performed by calculating the sulfur deposition quantity from the fuel consumption quantity and the sulfur concentration in the fuel during the normal operation control operation for a predetermined period of time, adding the deposition quantity to the sulfur accumulated quantity SOxS so far, the calculated value is served as a new sulfur accumulated quantity SOxS, or by other ways.  
         [0059]     In the step S 20 , the start period of the sulfur purge control operation is judged. The judgment is decided comprehensively from the load and the revolution condition, the exhaust temperature, the water temperature and so on, and the judgment is carried out as follows.  
         [0060]     This determination is performed by whether the sulfur accumulated quantity SOxS reaches the sulfur storage limit value SOxMAX where the sulfur accumulated quantity SOxS becomes the saturated state as set in advance. Namely, the state which reached the sulfur storage limit value SOxMAX means such states as the NOx purifying capability deteriorates to cause troubles, if the sulfur poisoning progresses furthermore, or the frequency of the regeneration processing operation for a catalyst brings problematically low mileage. In case of the state as mentioned above, the sulfur purge shall be performed forcibly to restore the catalyst from the deterioration.  
         [0061]     In the step S 20 , when it is judged to start the sulfur purge control operation, the sulfur purge control operation of the step S 40  is performed, but if it is judged not to start, it goes to the step S 30 .  
         [0062]     In the sulfur purge control operation of the step S 40 , the temperature raising control operation, the lax rich control operation, the lax lean control operation and the sulfur purge control termination processing are performed as shown in  FIG. 6 .  
         [0063]     The temperature raising control operation in the step S 41  is an operation to heat up the catalyst to make an optimal desulphurization environment. In this operation, the main injection is retarded and a post injection is performed during the fuel injection control together with the intake throttling. Reducers such as HC and CO are supplied into the exhaust gas by these injection controls. Then, the temperature is raised by heat generated by the oxidation reaction of these reducers reacting with catalysts or by other means. The temperature raising control is performed until the catalyst temperature reaches to the temperature higher than the temperature capable of desulphurizing. Then, it shifts to the lax rich control operation of the following step S 42 .  
         [0064]     The lax rich control operation of the step S 42  is a control to maintain the optimal desulphurization environment by setting the air/fuel ratio of the exhaust gas to the theoretical air/fuel ratio or slightly lower than the theoretical air/fuel ratio, performing the EGR or intake throttling together with the increase of the post injection quantity. As shown in  FIG. 8 , it is known from the test results that the optimal desulphurization environment can be obtained, when the excess air factor λ in the downstream side of the catalyst is control to stay at a range where a three-way-catalyst functions. Therefore, the excess air factor λ, in particular, is controlled at the range 0.95 to 1.0.  
         [0065]     In the sulfur purge, if the excess air factor λ is set below 0.95 to be deep rich, the oxygen concentration decreases substantially to zero. As a result, oxygen required for the oxidation reaction of reducers becomes insufficient, and the quantity of oxidation of HC and CO decreases. The diminution of heat generated by this oxidation reaction lowers catalyst temperature to the point where the sulfur purge can not be performed satisfactorily, and results in the reducers to be discharged into the open air without being used.  
         [0066]     On the contrary, if the intake quantity increases to raise the excess air factor λ to 1.0 or higher, the exhaust gas temperature lowers and, at the same time, heat quantity of the catalyst taken out by the increased exhaust gas increases. Then the catalyst is cooled. Consequently, it is unable to execute the sulfur purge satisfactorily because of the lowered catalyst temperature.  
         [0067]     In this lax rich control operation, the air/fuel ratio in the exhaust gas is set to the range of three-way-catalyst functions, by performing the intake quantity restriction by the valve throttling and the adjustment of the EGR quantity, together with the supply of a large quantity of reducers such as HC and CO required for the temperature raising by EGR and post-injection. Then, the large part of the catalyst is brought into a rich atmosphere by supplying such quantity of oxygen that can be consumed out by the oxidation reaction of these reduces, and the sulfur is separated in an optimal desulphurization atmosphere of a high temperature rich.  
         [0068]     This lax rich control operation for a sulfur purge is sustained until the time of period calculated beforehand for the lax rich operation passes and the desulphurization is performed satisfactorily. The lax rich operation time is calculated based on the sulfur accumulated quantity calculated beforehand and the desulfurized quantity per unit time which is obtained by collating the exhaust gas quantity and the exhaust temperature at the beginning of the sulfur purge operation with the desulfurized quantity map data preliminarily input.  
         [0069]     However, if the catalyst temperature exceeds the upper limit value where the catalyst deterioration appears, or the engine load gets out of a range where the smokeless rich operation is possible, or the catalyst temperature lowers to be under a range where the desulphurization is possible, the lax lean control operation of the step S 42  is suspended and the sulfur purge control termination processing of the step S 43  are performed.  
         [0070]     In place of terminating the operation by the time of the calculated lax rich operation period, calculating the quantity of the sulfur discharge through the sulfur purge by collating the exhaust gas quantity and the exhaust temperature with the sulfur discharge map data preliminarily input, and subtracting from the sulfur accumulated quantity SOxS until it becomes below zero, the sulfur purge operation control may be repeated.  
         [0071]     Then, if the lax rich control operation for sulfur purge of the step S 42  is terminated, it goes to the sulfur purge control termination processing of the step S 43 .  
         [0072]     In the sulfur purge control termination processing of the step S 43 , the lean duration time TleanS, the sulfur accumulated quantity SOxS and the like are reset, before returning to the step S 10 .  
         [0073]     Then, in case where it is judged that the sulfur purge control is unnecessary in the step S 20 , it proceeds to the step S 30 , to judge whether the regeneration control is necessary or not. This judgment depends on whether or not the NOx occluded quantity NOxS has attained the NOx occlusion limit value NOxMAX set preliminarily, or, whether or not the time (lean duration time) TleasS of the normal control operation has attained the lean operation limit time TleanMAX.  
         [0074]     In case where it is judged that the regeneration control operation is unnecessary in the step S 30 , it returns to the step S 10 . In case where it is judged that the regeneration control operation is necessary, it proceeds to the regeneration control operation of the step S 50 .  
         [0075]     In the regeneration control operation in the step S 50 , the calculation of rich operation time, a rich control operation and a rich regeneration termination processing are performed.  
         [0076]     In the calculation of rich operation time of step S 51 , the rich operation time Trich is calculated from the NOx occluded quantity NOxS, engine speed and load during the rich control operation. In case where the engine is in an acceleration state, the compensation is made to multiply the rich operation time Trich by the rich frequency factor calculated from the catalyst temperature and the engine speed. Moreover, a substantial rich operation time TrichMAX is given by calculating the early start time and the termination delay time of the rich control operation considering the driving of an EGR valve  42  and a throttle butterfly  23 .  
         [0077]     In the rich control operation of the step S 52 , the opening of the EGR valve  42 , the opening of the throttle butterfly  23 , the fuel injection quantity, the fuel injection timing, the fuel injection pattern and the like are adjusted and controlled by referring to the map data and feeding back the output value of a λ sensor  52  to maintain the catalyst temperature within a predetermined temperature range in which the catalyst is active, and what is more, the excess air factor λ is controlled to maintain a rich state (rich-burn state) of 0.8 to 0.95 at the catalyst inlet.  
         [0078]     Then, if an exhaust gas of the rich state in which the oxygen concentration is zero and HC and CO concentrations are low, flows in the NOx occlusion reduction type catalyst  30 , the occluded NO 2  is discharged from the catalyst  30  of a high temperature to regenerate the catalyst  30 . At the same time, the discharged NO 2  is reduced by HC and CO in the exhaust gas to be purified, producing N 2 , H 2 O and CO 2 . Moreover, HC and CO in the exhaust gas are also consumed as reducer of NO 2 , and not discharged outside.  
         [0079]     This rich control operation proceeds with counting the operation time TrichS and terminates when the operation time TrichS exceeds the rich operation time TrichMAX calculated in the step S 51 . In the regeneration termination processing of the following step S 53 , the data such as the NOx occluded quantity NOxS, the lean operation duration time TleanS, the rich operation duration time TrichS are reset to be zeros.  
         [0080]     The regeneration control operation of the step S 50  is performed by the series operations of steps from S 51  to S 53  and it returns to the step S 10 .  
         [0081]     Then, the steps of the control flow from S 10  to S 50  are repeated until the engine key is turned OFF, and it proceeds to the control flow termination processing of the step S 60  by an interruption of a termination command such as engine key OFF. In the control flow termination processing, the data such as NOx occluded quantity NOxS and the sulfur accumulated quantity SOxS accumulated in the NOx occlusion reduction type catalyst  30  and the lean operation duration time TleanS in the current engine operation are written in a memory. Thereafter, the control flow is suspended (STOP) and terminated (END).  
         [0082]     According to the control method for the exhaust gas purifying system  1  composed as described above, when the sulfur purge is necessary, the exhaust temperature is increased to heat the catalyst temperature up equal or superior to the sulfur purge temperature by performing the temperature raise operation, namely the intake restriction for example. At the same time, a lax rich condition is obtained, more specifically, the excess air factor λ is set to a range of 0.95 to 1.0 by the rich control operation. By the exhaust temperature raising operation and by the lax rich condition, the state of the optimal temperature and low oxygen concentration for the sulfur purge is obtained. The sulfur purge can be performed efficiently while preventing the catalyst temperature from lowering.