Patent Publication Number: US-2018028975-A1

Title: Exhaust gas purification system for internal combustion engine, internal combustion engine, and exhaust gas purification method for internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to an exhaust gas purification system for an internal combustion engine, an internal combustion engine, and an exhaust gas purification method for an internal combustion engine and more particularly to an exhaust gas purification system for an internal combustion engine, an internal combustion engine and an exhaust gas purification method for an internal combustion engine which can execute a desulfurization process in an ensured fashion while preventing a lean NOx trap catalyst device from being thermally deteriorated or fused. 
     BACKGROUND ART 
     In general, to purify such purification target constituents as hydrocarbons (HC), carbon monoxides (CO), nitrogen oxides and suspended particulate matters (PM) which are contained in exhaust gas discharged from an internal combustion engine such as a diesel engine, an exhaust gas purification system is used which includes an exhaust gas purification apparatus having various of catalyst devices such as a diesel oxidation catalyst (DOC) device, a catalyzed soot filter (CSF, SCRF or the like) device, a selective catalytic reduction (SCR) device, a lean NOx trap catalyst (LNT) device and the like. 
     In an exhaust gas purification system in which a lean NOx trap (LNT) catalyst device is provided on an exhaust passage to purify oxides of nitrogen (NOx) contained in exhaust gas, when a lean state exists where excessive oxygen is contained in exhaust gas as in lean burning or in a diesel engine, NOx is temporarily occluded in the lean NOx trap catalyst device. Then, when an amount of occluded NOx is increased to such an extent that no more NOx can be occluded, increasing an amount of NOx slipping to a downstream side, a rich control is executed periodically in which the air-fuel ratio of exhaust gas is shifted to a rich side, so that NOx occluded in the lean NOx trap catalyst device is discharged and reduced. In this mechanism, the occluding and reducing operations are repeated to maintain the NOx purification factor by the lean NOx trap catalyst device. 
     It is known that this lean NOx trap catalyst device occludes sulfur (S), which is one of the constituents of fuel and engine oil and which is contained in exhaust gas, as it does NOx, thereby causing a sulfur poisoning problem that the NOx occlusion capacity is reduced. To cope with this problem, a desulfurization control (an S purge control) is executed periodically in which exhaust gas is heated to a high temperature of the order of 600° C. and the air-fuel ratio of exhaust gas is shifted to a rich atmosphere, so that the sulfur occluded in the lean NOx trap catalyst device is released in the high-temperature and rich atmosphere. 
     The desulfurization control in the lean NOx trap catalyst device is such that the desulfurization efficiency is increases as the temperature of exhaust gas increases and the excess air factor (λ) decreases, which causes fears that the lean NOx trap catalyst device is thermally deteriorated or fused. Consequently, normally, the desulfurization control is executed under such a desulfurizing condition that a best purification factor of NOx contained in exhaust gas can be maintained by balancing a suppression of the thermal deterioration or fusing and the recovery of sulfur poisoning of the lean NOx trap catalyst device. 
     In relation to this, as described in JP-A-2010-144557 of Japanese Patent Application, an exhaust gas purification system and an exhaust gas purification method for an internal combustion engine are proposed in which a desulfurization control of a NOx storage reduction catalyst and a PM regeneration process of a DPF or a DPF with a catalyst are carried out simultaneously and in parallel by maintaining a PM regeneration temperature at an appropriate temperature by suppressing a flow rate of exhaust gas that passes through the NOx storage reduction catalyst to flow into the DPF or the DPF with a catalyst. 
     However, since the desulfurization control of the lean NOx trap catalyst device needs a certain length of time, in a case where a state where an engine operates goes out of an operation zone where the desulfurization control can be executed or the engine is stopped in the midst of execution of the desulfurization control, the desulfurization control is suspended or interrupted. Normally, in this desulfurization control, it is necessary to increase the temperature of exhaust gas to a high temperature, and therefore, the desulfurization control is executed at the same time that the PM regeneration control is executed on many occasions. Thus, when the desulfurization control is suspended or interrupted, the desulfurization control is put off until the next PM regeneration control is executed. 
     As this occurs, a sulfur accumulation amount becomes greater than that resulting when the desulfurization control is completed. Thus, even in the event that the next desulfurization control is executed, it is not possible to desulfurize sufficiently the accumulated sulfur. Then, since a NOx purification is executed with a NOx occlusion capacity reduced by an amount corresponding to the remaining amount of sulfur, there are caused fears that the NOx purification factor is reduced. 
     For example, in the event that a desulfurization control fails from the beginning where the desulfurization control is executed at Tx ° C. every X mg at which a highest purification factor can be maintained, sulfur accumulates to Y mg (&gt;X mg) until the next desulfurization control starts. Thus, even in the event the next desulfurization control is completed, all the accumulated sulfur cannot be desulfurized, whereby sulfur remains, reducing the NOx purification factor. 
       FIG. 5  shows schematically a relation between LNT catalyst temperature and NOx purification factor with a sulfur accumulation amount (a sulfur remaining amount) being X mg (S=S 1 ) and (Y=2×X) mg (S=S 2 ). As shown in  FIG. 5 , in S=S 2  where a sulfur accumulation amount is greater than S=S 1 , NOx is reduced remarkably in a range where the LNT catalyst temperature is low. On the other hand, NOx is reversed to be increased in a range where the LNT catalyst temperature is high. However, since the LNT catalyst temperature is high, the thermal deterioration of the catalyst is progressed. 
     PRIOR ART LITERATURE 
     Patent Literature 
     
         
         Patent Literature 1: JP-A-2010-144557 of Japanese Patent Application 
       
    
     SUMMARY OF THE INVENTION 
     According to the knowledge that the inventor of the invention applied for patent has obtained, when executing a desulfurization control on a lean NOx trap catalyst device provided in an exhaust gas system of an internal combustion engine, a sulfur desulfurization amount becomes great as a desulfurization temperature increases during the desulfurization control. Due to this, the desulfurization temperature is set so as to correspond to a sulfur accumulation amount by reflecting thereto the result of a decision made on whether or not the previous desulfurization control is suspended or interrupted by calculating a sulfur accumulation amount every time the desulfurization control is executed. By doing so, even in the event that the desulfurization control is suspended or interrupted to increase the sulfur accumulation amount, the desulfurization process can be executed in an ensured fashion, thereby making it possible to suppress a reduction in NOx purification factor that is caused by sulfur poisoning. Moreover, when the sulfur accumulation amount is small, since the desulfurization temperature can be reduced, the thermal deterioration of the catalyst can also be suppressed. 
     The present invention has been made in view of the situations described above, and an object thereof is to provide an exhaust gas purification system for an internal combustion engine, an internal combustion engine, and a exhaust gas purification method for an internal combustion engine which can execute a desulfurization control with superior robustness in which when a desulfurization control is executed on a lean NOx trap catalyst device which is provided in an exhaust gas system of an internal combustion engine, the desulfurization process can be executed in an ensured fashion while suppressing the lean NOx trap catalyst device from being thermally deteriorated or fused, whereby a high NOx purification factor can be maintained. 
     With a view to achieving the object, according to the invention, there is provided an exhaust gas purification system for an internal combustion engine including a lean NOx trap catalyst device which is provided on an exhaust passage of the internal combustion engine, characterized in that 
     a control unit for controlling the exhaust gas purification system sets a desulfurization temperature so as to correspond to a sulfur accumulation amount in the lean NOx trap catalyst device, the desulfurization temperature being a target temperature in executing a desulfurization control on the lean NOx trap catalyst device. 
     According to the configuration described above, when the desulfurization control is executed on the lean NOx trap catalyst device, the desulfurization control with superior robustness can be executed by calculating the sulfur accumulation amount in the lean NOx trap catalyst device in consideration of whether or not the previous desulfurization control is suspended due to the internal combustion engine being stopped and setting the desulfurization temperature according to the calculated sulfur accumulation amount, whereby the desulfurization process can be executed in an ensured fashion while suppressing the lean NOx trap catalyst device from being thermally deteriorated or fused, thereby making it possible to maintain a high NOx purification factor. 
     In the exhaust gas purification system for the internal combustion engine, in a case where the control unit includes sulfur occlusion amount calculation means for calculating a sulfur occlusion amount by adding a sulfur amount that flows into to be occluded in the lean NOx trap catalyst device when the internal combustion engine operates normally, sulfur desulfurization amount calculation means for calculating a sulfur reduction amount by adding a sulfur amount that is desulfurized from the lean NOx trap catalyst device when the desulfurization control is executed, sulfur accumulation amount calculation means for calculating the sulfur accumulation amount by subtracting the sulfur reduction amount calculated by the sulfur desulfurization amount calculation means from the sulfur occlusion amount calculated by the sulfur occlusion amount calculation means, desulfurization temperature calculation means for calculating a desulfurization temperature, when the desulfurization control is executed, from the sulfur accumulation amount calculated by the sulfur accumulation amount calculation means based on a database indicating a relation between the sulfur accumulation amount and the desulfurization temperature which is the target temperature when the desulfurization control is executed, and desulfurization control execution means for controlling an exhaust gas temperature so as to increase so that the exhaust gas temperature reaches the desulfurization temperature calculated by the desulfurization temperature calculation means, it is possible to set the desulfurization temperature while calculating the amount of accumulation of sulfur in the lean NOx trap catalyst device. 
     When referred to herein, the normal operation of the internal combustion engine means an operation of the engine (including a stopped state) that is performed when the desulfurization control for recovering the sulfur poisoning of the lean NOx trap catalyst device and the NOx regeneration control for recovering the NOx occlusion capability of the lean NOx trap catalyst device are not executed. 
     With a view to achieving the object described above, according to the present invention, there is provided an internal combustion engine including the exhaust gas purification system described above, and with this internal combustion engine, it is possible to provide the same working effect as that provided by the exhaust gas purification system for an internal combustion engine described above. 
     With a view to achieving the object described above, according to the present invention, there is provided an exhaust gas purification method for an internal combustion engine including a lean NOx trap catalyst device which is provided on an exhaust passage of the internal combustion engine designed to achieve the object described above is a method characterized in that a desulfurization temperature is set so as to correspond to a sulfur accumulation amount in the lean NOx trap catalyst device, the desulfurization temperature being a target temperature in executing a desulfurization control on the lean NOx trap catalyst device. 
     The exhaust gas purification method for the internal combustion engine described above includes a sulfur occlusion amount calculation step of calculating a sulfur occlusion amount by adding a sulfur amount that flows into to be occluded in the lean NOx trap catalyst device when the internal combustion engine operates normally, a sulfur desulfurization amount calculation step of calculating a sulfur reduction amount by adding a sulfur amount that is desulfurized from the lean NOx trap catalyst device when the desulfurization control is executed, a sulfur accumulation amount calculation step of calculating the sulfur accumulation amount by subtracting the sulfur reduction amount calculated in the sulfur desulfurization amount calculation step from the sulfur occlusion amount calculated in the sulfur occlusion amount calculation step, a desulfurization temperature calculation step of calculating a desulfurization temperature, when the desulfurization control is executed, from the sulfur accumulation amount calculated in the sulfur accumulation amount calculation step based on a database indicating a relation between the sulfur accumulation amount and the desulfurization temperature which is the target temperature when the desulfurization control is executed, and a desulfurization control execution step of controlling an exhaust gas temperature so as to increase so that the exhaust gas temperature reaches the desulfurization temperature calculated in the desulfurization temperature calculation step. 
     According to this method, it is possible to provide the same working effect as that provided by the exhaust gas purification system for an internal combustion engine described above. 
     Advantageous Effect of the Invention 
     According to the exhaust gas purification system for an internal combustion engine, the internal combustion engine, and the exhaust gas purification method for an internal combustion engine of the present invention, when the desulfurization control is executed on the lean NOx trap catalyst device, the sulfur accumulation amount in the lean NOx trap catalyst device is calculated in consideration of whether or not the previous desulfurization control is suspended due to the internal combustion engine being stopped and the desulfurization temperature is set according to the calculated sulfur accumulation amount, and therefore, the desulfurization control with superior robustness can be executed in which the desulfurization process can be executed in an ensured fashion while suppressing the lean NOx trap catalyst device from being thermally deteriorated or fused, thereby making it possible to maintain a high NOx purification factor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing schematically the configuration of an internal combustion engine including an exhaust gas purification system for an internal combustion engine according to an embodiment of the present invention. 
         FIG. 2  is a drawing showing the configuration of a control unit. 
         FIG. 3  is a drawing showing control steps of an exhaust gas purification method for an internal combustion engine according to the embodiment of the present invention. 
         FIG. 4  is a drawing showing schematically a relation between sulfur accumulation amount and desulfurization temperature. 
         FIG. 5  is a drawing showing schematically a relation between lean NOx trap catalyst device temperature and NOx purification factor which corresponds to a difference in sulfur accumulation amount of a lean NOx trap catalyst device. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, referring to the drawings, an exhaust gas purification system for an internal combustion engine, an internal combustion engine and an exhaust gas purification method for an internal combustion engine according to the embodiment of the present invention will be described. An internal combustion engine according to the embodiment of the present invention includes an exhaust gas purification system for an internal combustion engine according to the embodiment of the present invention and can provide the same working effect as that provided by an exhaust gas purification system for an internal combustion engine which will be described later. 
     Firstly, referring to  FIG. 1 , an internal combustion engine (hereinafter, referred to as an engine)  10  and an exhaust gas purification system  20  for the internal combustion engine according to the embodiment of the present invention will be described. A fuel injector  11 , an intake valve  12  and an exhaust valve  13  are provided on this engine  10  so as to face a cylinder  10   a , and further, an intake passage  14  communicating with the intake valve  12 , an exhaust passage  15  communicating with the exhaust valve  13  and an EGR passage  16  are provided. 
     An air cleaner  17 , a compressor  18   b  of a turbocharger (a turbo-type super charging device)  18 , an inter-cooler  19   a , and an intake throttle valve  19   b  are provided sequentially in that order from an upstream side along the intake passage  14 . Additionally, a turbine  18   a  of the turbocharger  18  and an exhaust gas purification device  21  are provided sequentially in that order from an upstream side along the exhaust passage  15 . The EGR passage  16  is provided so as to connect a portion of the intake passage  14  which lies downstream of the compressor  18   b  and a portion of the exhaust passage  15  which lies upstream of the turbine  18   a , and an EGR cooler  16   a  and an EGR valve  16   b  are provided sequentially in that order from an upstream side along the EGR passage  16 . 
     Then, fresh air A introduced from the atmosphere is sent into the cylinder  10   a  as required by way of the intake valve  12  together with exhaust gas (EGR gas) Ge which flows into the intake passage  14  from the EGR passage  16 . Exhaust gas G generated in the cylinder  10   a  flows out into the exhaust passage  15  by way of the exhaust valve  13 , and part of the exhaust gas G flows into the EGR passage  16  as EGR gas Ge, while the remaining exhaust gas Ga (=G−Ge) flows into the exhaust gas purification device  21  by way of the turbine  18   a . After it has been purified, the exhaust gas Ga is discharged to the atmosphere by way of a muffler (not shown) and a tail pipe (not shown) as purified exhaust gas Gc. 
     In the configuration shown in  FIG. 1 , the exhaust gas purification device  21  of the exhaust gas purification system  20  includes catalyst devices such as a diesel oxidation catalyst (DOC) device  22 , a catalyzed soot filter (CSF) device  23 , a lean NOx trap catalyst (LNT) device  24 , a rear-stage diesel oxidation catalyst (DOC) device  25  and the like. There may be cases where the arranging order of the catalyzed soot filter device  23  and the lean NOx trap catalyst device  24  is reversed, so that the exhaust gas purification device  21  includes various of the catalyst devices in such a way that the diesel oxidation catalyst device  22 , the lean NOx trap catalyst device  24 , the catalyzed soot filter device  23  and the rear-stage diesel oxidation catalyst device  25  are arranged sequentially in that order. 
     A fuel injection device  26  for injecting unburned fuel into the exhaust passage  15  is provided on a portion of the exhaust passage  15  which lies upstream of the diesel oxidation catalyst device  22 , and unburned fuel is injected into the exhaust passage  15  when exhaust gas temperature increasing controls are executed which include a NOx regeneration control executed on the lean NOx trap catalyst device  24 , a sulfur purge control executed on the diesel oxidation catalyst device  22  and the lean NOx trap catalyst device  24  and a PM regeneration control executed on the catalyzed soot filter device  23 . By injecting unburned fuel in the way described above, hydrocarbons which are unburned fuel are oxidized by the diesel oxidation catalyst device  22  to thereby increase the temperature of exhaust gas Ga using heat generated by the oxidation of hydrocarbons. The temperature of the lean NOx trap catalyst device  24  is increased to a temperature zone where occluded NOx is released and reduced, the temperature of the catalyzed soot filter device  23  is increased to a temperature zone where PM can be burned or the temperatures of the diesel oxidation catalyst device  22  and the lean NOx trap catalyst device  24  are increased to a temperature zone where sulfur can be desulfurized by increasing the temperature of exhaust gas Ga or increasing the temperatures of the catalyst devices  22 ,  23 ,  24  through combustion of hydrocarbons therein. This allows the catalyst devices  22 ,  23 ,  24  to recover their exhaust purification capabilities. 
     A first temperature sensor  31  for detecting a temperature of exhaust gas Ga which flows into the diesel oxidation catalyst device  22  is disposed at a portion of the exhaust passage  15  which lies at an upstream side (an inlet side) of the diesel oxidation catalyst device  22 , and a second temperature sensor  32  for detecting a temperature of exhaust gas Ga which flows into the lean NOx trap catalyst device  24  is disposed at a portion of the exhaust passage  15  which lies at an upstream side of the lean NOx trap catalyst device  24 . Additionally, a third temperature sensor  33  for detecting a temperature of exhaust gas Ga which flows out of the diesel oxidation catalyst device  22  into the catalyzed soot filter device  23  at a portion of the exhaust passage  15  which lies between the diesel oxidation catalyst device  22  and the catalyzed soot filter device  23 . 
     When executing a desulfurization control on the lean NOx trap catalyst device  24  by controlling the temperature of exhaust gas Ga so as to increase, as a detection temperature which constitutes a control target which is controlled so as to become a desulfurization temperature which constitutes a control target in controlling the temperature of exhaust gas Ga to increase, it is normal to use a temperature which is detected by the second temperature sensor  32 . However, a temperature sensor (not shown) may be provided downstream of the lean NOx trap catalyst device  24 , so that a temperature which is detected by this temperature sensor may be used as the detection temperature. Alternatively, a temperature which takes an average value between the temperature detected by this sensor and the temperature detected by the second temperature sensor  32  may be used as the detection temperature. 
     Further, a λ sensor  34  for measuring an air excess air factor λ or an oxygen concentration of exhaust gas Ga or an oxygen concentration sensor (not show) is disposed downstream of the exhaust gas purification device  21 . This λ sensor or the oxygen concentration sensor may be disposed upstream of the exhaust gas purification device  21  or on an exhaust manifold. 
     A NOx concentration sensor  35  for detecting a NOx concentration D of exhaust gas Ga which flows into the lean NOx trap catalyst device  24  is provided at a portion of the exhaust passage  15  which lies upstream of the lean NOx trap catalyst device  24 . In a case where a NOx concentration D of exhaust gas Ga which flows into the lean NOx trap catalyst device  24  can be estimated by anyone of the conventional methods, this NOx concentration sensor  35  does not necessarily have to be disposed. 
     Additionally, a control unit  40  is provided for controlling the exhaust gas purification system  20  for an internal combustion engine of the present invention. Normally, the control unit  40  is incorporated in an engine control unit (ECU) for controlling the overall operating state of the engine  10 . However, the control unit  40  may be provided independently. 
     In the exhaust gas purification system  20  for an internal combustion engine according to the embodiment of the present invention, the control unit  40  for controlling the exhaust gas purification system  20  includes, as shown in  FIG. 2 , sulfur occlusion amount calculation means  41 , sulfur desulfurization amount calculation means  42 , sulfur accumulation amount calculation means  43 , desulfurization temperature calculation means  44 , and desulfurization control execution means  45 . 
     The sulfur occlusion amount calculation means  41  is a means for calculating a sulfur occlusion amount ΣSo by adding a sulfur amount So which flows into to be occluded in the lean NOx trap catalyst device  24  when the engine  10  is operating normally. When referred to herein, the normal operation of the engine  10  means an operation of the engine  10  (including a stopped state) which is performed when a desulfurization control for recovering the sulfur poisoning of the lean NOx trap catalyst device  24  and a NOx regeneration control for recovering the NOx occlusion capability of the lean NOx trap catalyst device are not executed. 
     The sulfur desulfurization amount calculation means  42  is a means for calculating a sulfur reduction amount ΣSd by adding a sulfur amount Sd which is released from the lean NOx trap catalyst device  24  when a desulfurization control is executed. The sulfur accumulation amount calculation means  43  is a means for calculating a sulfur accumulation amount Sa by subtracting the sulfur reduction amount ΣSd calculated by the sulfur desulfurization amount calculation means  42  from the sulfur occlusion amount ΣSo calculated by the sulfur occlusion amount calculation means  41 . 
     Then, the desulfurization temperature calculation means  44 , which constitutes a gist of the present invention, is a means for calculating a desulfurization temperature Tt when a desulfurization control is executed from the sulfur accumulation amount Sa calculated by the sulfur accumulation amount calculation means  43  based on a database indicating a relation between the sulfur accumulation amount Sa and the desulfurization temperature Tt which constitutes a target temperature when a desulfurization control is executed. 
     A relation between the sulfur accumulation amount Sa and the desulfurization temperature Tt is set in advance based on the results of experiments and is stored in the control unit  40  in the form of a controlling data map. The relation between the sulfur accumulation amount Sa and the desulfurization temperature Tt is, for example, something like a relation shown in  FIG. 4 , in which the desulfurization temperature is low when the sulfur accumulation amount Sa is small, whereas the desulfurization temperature Tt becomes high when the sulfur accumulation amount Sa is great. 
     Since there is a limit to a temperature to which the temperature is increased from the viewpoint of the heat resistance of the lean NOx trap catalyst device, an upper limit value Ttmax (for example, 650° C. to 700° C.) determined in consideration of the heat resistance is provided on the desulfurization temperature Tt used when a desulfurization control is executed. 
     Then, the desulfurization control execution means  45  is a means for controlling the temperature of exhaust gas so as to increase so that the temperature of exhaust gas Ga reaches the desulfurization temperature Tt calculated by the desulfurization temperature calculation means  44 . The well-known method is used as this desulfurization control. 
     By the configuration that has been described heretofore, in the present invention, the control unit  40  sets a desulfurization temperature Tt which constitutes a target temperature when a desulfurization control is executed on the lean NOx trap catalyst device  24  so as to correspond to the sulfur accumulation amount Sa accumulated in the lean NOx trap catalyst device  24 . 
     Next, an exhaust gas purification method S 40  for an internal combustion engine according to the embodiment of the present invention includes, as shown in  FIG. 2 , a sulfur occlusion amount calculation step S 41 , a sulfur desulfurization amount calculation step S 42 , a sulfur accumulation amount calculation step S 43 , a desulfurization temperature calculation step S 44  and a desulfurization control execution step S 45 . 
     The sulfur occlusion amount calculation step S 41  is a step of calculating a sulfur occlusion amount ΣSo by adding a sulfur amount So which flows into to be occluded in the lean NOx trap catalyst device  24  when the engine  10  is operating normally. The sulfur desulfurization amount calculation step S 42  is a step of calculating a sulfur reduction amount ΣSd by adding a sulfur amount Sd which is released from the lean NOx trap catalyst device  24  when a desulfurization control is executed. The sulfur accumulation amount calculation step S 43  is a step of calculating a sulfur accumulation amount Sa by subtracting the sulfur reduction amount ΣSd calculated in the sulfur desulfurization amount calculation step S 42  from the sulfur occlusion amount ΣSo calculated in the sulfur occlusion amount calculation step S 41 . 
     The desulfurization temperature calculation step S 44  is a step of calculating a desulfurization temperature Tt when a desulfurization control is executed from the sulfur accumulation amount Sa calculated in the sulfur accumulation amount calculation step S 43  based on a database indicating a relation between the sulfur accumulation amount Sa and the desulfurization temperature Tt which constitutes a target temperature when a desulfurization control is executed. The desulfurization control execution step S 45  is a step of controlling the temperature of exhaust gas so as to increase so that the temperature of exhaust gas Ga reaches the desulfurization temperature Tt calculated in the desulfurization temperature calculation step S 44 . The well-known method is used as this desulfurization control. 
     By the configuration of the method described above, the exhaust gas purification method S 40  for an internal combustion engine of the present invention constitutes a method for setting the desulfurization temperature Tt which becomes the target temperature in executing the desulfurization control on the lean NOx trap catalyst device  24  so as to correspond to the sulfur accumulation amount Sa accumulated in the lean NOx trap catalyst device  24 . 
     In the exhaust gas purification system  20  for an internal combustion engine and the exhaust gas purification method S 40  for an internal combustion engine described above, in executing the desulfurization control on the lean NOx trap catalyst device  24 , the sulfur accumulation amount Sa is calculated at the start of the desulfurization control, and the desulfurization temperature Tt which is determined according to the sulfur accumulation amount Sa is used as the target temperature. Then, the rich control including the exhaust gas temperature increasing control in which the detection temperature of exhaust gas becomes this desulfurization temperature Tt is executed for a preset length of time to execute the desulfurization process. 
     Due to this, when the previous desulfurization control is suspended or interrupted because the operating state of the engine  10  changes, since the desulfurization control is not completed, the sulfur accumulation amount Sa increases. In addition to the reason described above, also when the desulfurization is suppressed as a result of the exhaust gas temperature being lowered due to the operating state of the engine  10  in the previous desulfurization control, the sulfur reduction amount resulting from the release of sulfur is reduced, increasing the sulfur accumulation amount Sa. In this case, since the desulfurization temperature Tt is set high for the next desulfurization control, the desulfurization is promoted, and this increases the sulfur reduction amount resulting from the release of sulfur and reduces the sulfur accumulation amount Sa. 
     On the other hand, in a case where the desulfurization is promoted as a result of the exhaust gas temperature being increased due to the operating state of the engine in the previous desulfurization control, the sulfur reduction amount resulting from the release of sulfur is increased, reducing the sulfur accumulation amount Sa. In this case, since the desulfurization temperature Tt is set low for the next desulfurization control, the desulfurization is promoted less, and this decreases the sulfur reduction amount resulting from the release of sulfur. However, since the sulfur accumulation amount Sa is small originally, it is possible to allow the sulfur accumulation amount Sa after desulfurization to stay at a target value. Then, in this desulfurization control, since the desulfurization temperature Tt is low, it is possible to reduce the thermal deterioration of the catalyst of the lean NOx trap catalyst device  24 . 
     According to the exhaust gas purification system  20  for an internal combustion engine, the internal combustion engine  10 , and the exhaust gas purification system S 40  for an internal combustion engine described above, when the desulfurization control is executed on the lean NOx trap catalyst device  24 , the sulfur accumulation amount Sa in the lean NOx trap catalyst device  24  is calculated in consideration of the degree of desulfurization, that is, whether or not the previous desulfurization control is suspended or interrupted and the desulfurization temperature Tt is set according to the calculated sulfur accumulation amount Sa. Thus, the desulfurization control with superior robustness can be executed in which the desulfurization process can be executed in an ensured fashion while suppressing the lean NOx trap catalyst device  24  from being thermally deteriorated or fused, thereby making it possible to maintain a high NOx purification factor. 
     DESCRIPTION OF REFERENCE NUMERALS AND CHARACTERS 
     
         
           10  Engine (Internal combustion engine) 
           11  Fuel injection device 
           15  Exhaust passage 
           20  Exhaust gas purification system 
           21  Exhaust gas purification device 
           22  Diesel oxidation catalyst (DOC) device 
           23  Catalyzed soot filter device 
           24  Lean NOx trap (LNT) catalyst device 
           25  Rear-stage diesel oxidation catalyst (DOC) device 
           26  Fuel injection device 
           31  First temperature sensor 
           32  Second temperature sensor 
           33  Third temperature sensor 
           34  λ sensor 
           35  NOx concentration sensor 
           40  Control unit 
           41  Sulfur occlusion amount calculation means 
           42  Sulfur desulfurization amount calculation means 
           43  Sulfur accumulation amount calculation means 
           44  Desulfurization temperature calculation means 
           45  Desulfurization control execution means 
         S 41  Sulfur occlusion amount calculation step 
         S 42  Sulfur desulfurization amount calculation step 
         S 43  Sulfur accumulation amount calculation step 
         S 44  Desulfurization temperature calculation step 
         S 45  Desulfurization control execution step 
         A Fresh air 
         G Generated exhaust gas 
         Ga Exhaust gas passing through exhaust gas purification device 
         Gc Purified exhaust gas 
         Ge EGR gas