JP2020051399A - Exhaust emission control system for internal combustion engine and exhaust emission control method for internal combustion engine - Google Patents

Abstract

To provide an exhaust emission control system for an internal combustion engine and an exhaust emission control method for an internal combustion engine capable of suppressing a quantity of ammonia flowing out from a selective reduction type catalyst device to a downstream side exhaust passage by improving estimation accuracy of an accumulation quantity of ammonia in the selective reduction type catalyst device.SOLUTION: When rich reduction control of an NOx occlusion reduction type catalyst device 4 is performed, an oxygen concentration λ of exhaust gas G passing through the NOx occlusion reduction type catalyst device 4 and an acquisition value D of NOx concentration sensor 12 relative to the exhaust gas G flowing into a selective reduction type catalyst device 6 on the downstream side of the NOx reduction type catalyst device 4 are acquired. Based on the acquired oxygen concentration λ and NOx concentration D, whether or not ammonia is produced in the NOx occlusion reduction type catalyst device 4 is determined. When a determination that ammonia is produced is made, an ammonia quantity Na generated in the NOx occlusion reduction type catalyst device 4 is calculated, and based on the calculated value Na, an accumulation quantity Ns of ammonia in the selective reduction type catalyst device 6 is corrected.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine and an exhaust gas purification method for an internal combustion engine.

  In recent years, the automobile industry has been required to reduce the amount of NOx contained in exhaust gas emitted to the atmosphere during city driving and highway driving due to the tightening of exhaust gas regulations. Conventional NOx reduction technologies include LNT (NOx storage-reduction type catalyst device) and urea SCR (urea-based selective reduction type catalyst device). However, since each device alone cannot cover the entire running state of a vehicle, LNT and It has been studied to mount the SCR in a vehicle with a combination of the SCRs (for example, see Patent Document 1).

JP-A-2017-36700

  By the way, the NOx storage reduction catalyst device accumulates NOx contained in the exhaust gas when the air-fuel ratio of the exhaust gas is a lean air-fuel ratio, and releases the accumulated NOx when the air-fuel ratio is a rich air-fuel ratio. This device purifies the exhausted NOx by reducing it to nitrogen with hydrocarbons or carbon monoxide contained in the exhaust gas. If the air-fuel ratio of the exhaust gas is smaller than 1 during the reduction reaction from NOx to nitrogen, ammonia may be generated as a by-product. Ammonia generated as a by-product flows out of the NOx storage reduction catalyst device toward the downstream selective reduction catalyst device.

  On the other hand, in the selective reduction catalyst device, a predetermined amount of ammonia is accumulated therein and used for purifying NOx contained in the exhaust gas. However, the accumulated amount of ammonia during the urea water supply control is an estimated value. There is a little different from the actual storage amount. Therefore, when the actual amount of accumulation is larger than the estimated value of the amount of accumulated ammonia, when the ammonia generated by the NOx storage reduction type catalyst device flows into the selective reduction type catalyst device, the inflowing ammonia is removed by the selective reduction type catalyst device. And ammonia may flow out of the selective reduction catalyst device into the exhaust passage on the downstream side.

  SUMMARY OF THE INVENTION It is an object of the present invention to improve the accuracy of estimating the amount of accumulated ammonia in a selective catalytic reduction device and to suppress the amount of ammonia flowing out of a selective reduction catalytic device into a downstream exhaust passage. It is an object of the present invention to provide a system and a method for purifying exhaust gas of an internal combustion engine.

  An exhaust gas purification system for an internal combustion engine according to the present invention for achieving the above object has an internal combustion engine including a NOx storage reduction catalyst device and a selective reduction catalyst device in an exhaust passage of an internal combustion engine in order from an upstream side. In an exhaust gas purification system for an engine, an oxygen concentration acquisition device for acquiring an oxygen concentration of exhaust gas is provided in the exhaust passage upstream of the selective reduction catalyst device, wherein the NOx storage reduction catalyst device and the selective reduction catalyst A NOx concentration acquisition device for acquiring NOx concentration of exhaust gas in the exhaust passage between the devices, and a control device electrically connected to the oxygen concentration acquisition device and the NOx concentration acquisition device; When the device is performing rich reduction control of the NOx storage reduction catalyst device, the obtained value of the oxygen concentration obtaining device and the obtained value of the NOx concentration obtaining device are obtained. When it is determined that ammonia is generated in the NOx storage reduction catalyst device based on the above, the amount of ammonia generated in the NOx storage reduction catalyst device is calculated, and the selected amount is determined based on the calculated value. The present invention is characterized in that control for correcting the amount of accumulated ammonia in the reduction catalyst device is performed.

  Further, an exhaust gas purification method for an internal combustion engine according to the present invention for achieving the above object is provided with a NOx storage reduction type catalyst device and a selective reduction type catalyst device in an exhaust passage of an internal combustion engine in order from the upstream side. In the exhaust gas purification method for an internal combustion engine, when the rich reduction control of the NOx storage reduction type catalyst device is performed, the oxygen concentration of the exhaust gas passing through the NOx storage reduction type catalyst device and the NOx storage reduction type The NOx concentration of the exhaust gas passing through the exhaust passage between the catalyst device and the selective catalytic reduction device is obtained, and ammonia is generated in the NOx storage reduction type catalytic device based on the obtained oxygen concentration and NOx concentration. It is determined whether or not ammonia has been generated, and when it is determined that ammonia has been generated, the amount of ammonia generated by the NOx storage reduction catalyst device is calculated. To, and correcting the accumulated amount of ammonia of the selective reduction catalyst device on the basis of this calculated value.

  The NOx concentration sensor (NOx concentration acquisition device) provided in the preceding stage of the selective catalytic reduction device detects ammonia in addition to NOx contained in exhaust gas due to the nature of this sensor. In the present invention, it is determined whether or not ammonia is generated in the NOx storage reduction type catalyst device during the rich reduction control of the NOx storage reduction type catalyst device by utilizing the property of this sensor.

  That is, according to the present invention, the air-fuel ratio of the exhaust gas passing through the NOx storage reduction type catalyst device may cause ammonia to be generated in the NOx storage reduction type catalyst device during the rich reduction control of the NOx storage reduction type catalyst device. When the measured value of the NOx concentration of the exhaust gas flowing into the selective reduction catalyst device is an air-fuel ratio and is estimated to be a value including erroneous detection of ammonia, the ammonia generated by the NOx storage reduction catalyst device is The amount is calculated, and the accumulated amount of ammonia in the selective catalytic reduction device is corrected based on the calculated value. As a result, the accuracy of estimating the amount of accumulated ammonia in the selective catalytic reduction device can be improved, and the amount of ammonia flowing from the selective catalytic reduction device to the downstream exhaust passage can be suppressed.

FIG. 1 is a diagram illustrating a configuration of an exhaust gas purification system for an internal combustion engine according to the present invention. FIG. 4 is a diagram illustrating a transition of a NOx concentration and a transition of an air-fuel ratio during rich reduction control. It is a figure which illustrates the control flow of the exhaust gas purification method of the internal combustion engine of the present invention.

  Hereinafter, an exhaust gas purification system for an internal combustion engine according to the present invention will be described based on an embodiment shown in the drawings. As shown in FIG. 1, an exhaust gas purification system 1 for an internal combustion engine according to the present invention includes a NOx storage reduction type catalyst device (LNT) 4 and a diesel particulate collection in an exhaust passage 3 of an engine (internal combustion engine) 2 in order from an upstream side. A filter device (CSF) 5 and a selective catalytic reduction device (SCR) 6 are provided. Further, a fuel injection device 7 is disposed in the exhaust passage 3 upstream of the NOx storage reduction type catalyst device 4. A urea water injection device 8 is disposed in the exhaust passage 3 between the diesel particulate filter device 5 and the selective catalytic reduction device 6 (the exhaust passage 3 upstream of the selective catalytic reduction device 6).

  The NOx storage-reduction catalyst device 4 carries a NOx storage material and a noble metal catalyst, and stores NOx contained in the exhaust gas G when the air-fuel ratio of the exhaust gas G is lean. The stored NOx is released by setting the air-fuel ratio of the exhaust gas G to a rich state, and the released NOx is reduced by a noble metal catalyst. The control of releasing the NOx stored in the NOx storage reduction type catalyst device 4 and reducing it by shifting the air-fuel ratio of the exhaust gas G to the rich state is referred to as rich reduction control. The transition of the air-fuel ratio of the exhaust gas G to the rich state is performed by post-injecting fuel in a cylinder (cylinder) of the engine 2 or by injecting the fuel F from the fuel injection device 7 into the exhaust passage 3. Further, the NOx storage reduction type catalyst device 4 also has a function of an oxidation catalyst device (DOC), and oxidizes carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gas G.

  The diesel particulate filter 5 collects particulate matter (PM) contained in the exhaust gas G. The collected PM is usually burned and removed by passing the high-temperature exhaust gas G through the diesel particulate filter device 5. Control for raising the temperature of the exhaust gas G and burning and removing the collected PM with the high-temperature exhaust gas G is referred to as forced PM regeneration control. The high-temperature exhaust gas G is post-injected by in-cylinder fuel injection of the engine 2 or fuel F is injected from the fuel injection device 7 to reduce carbon monoxide and hydrocarbons contained in the fuel F by NOx storage reduction. It is obtained by performing an oxidation treatment (exothermic reaction) in the catalytic device 4.

The selective reduction catalyst device 6 is a device for purifying NOx contained in the exhaust gas G by reducing it to nitrogen with a reducing agent. In the present embodiment, ammonia (NH ₃ ) is used as the reducing agent. The urea water U injected toward the exhaust gas G from the urea water injection device 8 is changed into ammonia by the heat of the exhaust gas G, and the ammonia flows into the selective reduction catalyst device 6 to selectively reduce the ammonia. When the exhaust gas G passes through the catalyst device 6, it reacts with NOx in the exhaust gas G to reduce NOx.

  Further, a temperature sensor 9 and a preceding stage oxygen concentration sensor 10 are provided in the exhaust passage 3 upstream of the NOx storage reduction catalyst device 4, and the exhaust passage 3 between the NOx storage reduction catalyst device 4 and the selective reduction catalyst device 6 is provided in the exhaust passage 3. A second stage oxygen concentration sensor 11 and a NOx concentration sensor (NOx concentration acquisition device) 12 are provided. The temperature sensor 9 is a sensor that acquires the temperature T of the exhaust gas G flowing into the NOx storage reduction type catalytic device 4. The NOx concentration sensor 12 is a sensor that acquires the NOx concentration of the exhaust gas G passing through the exhaust passage 3 between the NOx occlusion reduction type catalyst device 4 and the selective reduction type catalyst device 6.

  The oxygen concentration acquisition device of the present invention only needs to be able to acquire the oxygen concentration of the exhaust gas G passing through the NOx storage reduction catalyst device 4. What is necessary is just to comprise either of both of the latter oxygen concentration sensors 11. In the present embodiment, the latter oxygen concentration sensor 11 is an oxygen concentration acquisition device. When the oxygen concentration acquisition device is constituted by both the upstream oxygen concentration sensor 10 and the downstream oxygen concentration sensor 11, the acquired value of the oxygen concentration acquisition device is the acquired value of the upstream oxygen concentration sensor 10 and the acquired value of the downstream oxygen concentration sensor 11 It is preferable to use the average value of the obtained values.

  The control device 13 includes a CPU that performs various types of information processing, a program used to perform the various types of information processing, an internal storage device that can read and write information processing results, and software and hardware including various interfaces. Combination. The control device 13 is electrically connected to various sensors such as the above-described post-stage oxygen concentration sensor 11 and NOx concentration sensor 12 via signal lines.

  The control device 13 determines that it is necessary to perform the rich reduction control of the NOx storage reduction type catalyst device 4 when the NOx storage amount N of the NOx storage reduction type catalyst device 4 is equal to or more than a preset rich request threshold value N1. Then, by the fuel injection from the fuel injection device 7 or the like, the acquired value λ of the latter-stage oxygen concentration sensor 11 becomes lower than the preset oxygen concentration threshold λ1, that is, the air-fuel ratio of the exhaust gas G becomes rich. Control to transition. The set oxygen concentration threshold λ1 is set to, for example, 1.0, which is a stoichiometric air-fuel ratio.

  In the present invention, when performing the rich reduction control of the NOx occlusion reduction type catalyst device 4, the control device 13 determines whether or not ammonia is generated in the NOx occlusion reduction type catalyst device 4 by the acquisition of the post-stage oxygen concentration sensor 11. The determination is made based on the value λ and the acquired value D of the NOx concentration sensor 12. More specifically, as shown in FIG. 2, the acquired value λ of the subsequent oxygen concentration sensor 11 falls below a preset oxygen concentration threshold λ1 (= 1.0) (determination A), and the NOx concentration sensor When the acquired value D obtained in Step 12 becomes equal to or greater than the preset concentration threshold value D1 (determination B), the control device 13 determines that ammonia is generated in the NOx storage reduction catalyst device 4. In other cases, it is determined that ammonia has not been generated in the NOx storage reduction catalyst device 4. The set concentration threshold D1 is set to a value (for example, 500 ppm) that is larger than the NOx concentration of the exhaust gas G discharged from the engine 2 during the rich reduction control and that can be determined that the NOx concentration sensor 12 has erroneously detected ammonia. You.

  As for the above-mentioned determination B, the following determination may be made instead of the determination based on the comparison between the acquired value D of the NOx concentration sensor 12 and the set concentration threshold D1. That is, the NOx concentration of the exhaust gas G discharged from the engine 2 is calculated from the operating state of the engine 2 by the engine outlet NOx concentration obtaining device incorporated in the control device 13, and the calculated value D 2 and the NOx concentration sensor 12 are obtained. Compare the value D. When the determination A is satisfied and the acquired value D of the NOx concentration sensor 12 is equal to or more than the calculated value D2, the control device 13 determines that ammonia is generated in the NOx storage reduction catalyst device 4.

  When the air-fuel ratio of the exhaust gas G passing through the NOx storage reduction type catalyst device 4 becomes rich, ammonia is generated as a by-product of the NOx reduction reaction during the rich reduction control. Further, since the NOx released from the NOx occlusion reduction type catalytic device 4 during the rich reduction control is reduced in situ, the NOx concentration of the exhaust gas G flowing into the selective reduction type catalytic device 6 is determined by the start of the rich reduction control. It becomes smaller as time passes from time. However, the NOx concentration sensor 12 erroneously detects ammonia in addition to NOx due to the nature of the sensor. Therefore, if ammonia is generated as a by-product, the acquired value of the NOx concentration sensor 12 will be the value of the rich reduction control. It does not decrease as time elapses from the start, and rises rapidly during the execution period of the rich reduction control.

  In the present invention, the control device 13 determines whether or not ammonia is generated in the NOx storage reduction catalyst device 4 based on the obtained value λ of the post-stage oxygen concentration sensor 11, and actually determines the NOx storage reduction catalyst. Whether or not ammonia flows out of the device 4 is determined based on the acquired value D of the NOx concentration sensor 12. That is, the ammonia is generated in the NOx storage reduction type catalyst device 4 during the rich reduction control through the double determination of the determination based on the obtained value λ of the latter oxygen concentration sensor 11 and the determination based on the obtained value D of the NOx concentration sensor 12. Make sure that

  Then, when it is determined that ammonia is generated in the NOx storage reduction catalyst device 4 during the rich reduction control, the control device 13 calculates the amount of ammonia Na generated in the NOx storage reduction catalyst device 4, Based on this calculated value Na, control is performed to correct the accumulated amount Ns of ammonia in the selective catalytic reduction device 6.

  The amount of ammonia Na generated in the NOx storage reduction catalyst device 4 is calculated, for example, by multiplying the NOx storage amount of the NOx storage reduction catalyst device 4 at the start of the rich reduction control by a correction coefficient determined by an experiment or the like. . Alternatively, it is based on the difference between the NOx concentration of the exhaust gas G discharged from the engine 2 and the value D obtained by the NOx concentration sensor 12 (the sum of the NOx concentration and the ammonia concentration of the exhaust gas flowing into the selective catalytic reduction device 6) D. Then, the amount Na of ammonia generated in the NOx storage reduction type catalyst device 4 is calculated. More specifically, based on the NOx amount map corresponding to the operating state of the engine 2 stored in the engine outlet NOx concentration acquiring device, the NOx amount in this map is used to determine the engine cooling water temperature and the engine 2 during transient operation of the engine 2. The NOx concentration of the exhaust gas G discharged from the engine 2 is calculated by making corrections based on the intake oxygen concentration in the intake air and the boost pressure.

  Then, the control device 13 corrects the accumulated amount Ns of ammonia in the selective reduction catalyst device 6 based on the calculated value Na of the amount of ammonia generated in the NOx storage reduction catalyst device 4 calculated as described above. Control. More specifically, by adding the calculated value Na of the amount of ammonia generated by the NOx storage reduction type catalyst device 4 to the accumulated amount Ns of ammonia in the selective reduction type catalyst device 6 before correction, the corrected selective reduction after correction is performed. The amount of accumulation Nsc of ammonia in the catalytic converter 6 is calculated.

  The accumulated amount Ns of ammonia in the selective reduction catalyst device 6 before the correction is determined by the amount of urea water injected from the urea water injection device 8 and the temperature of the exhaust gas G flowing into the selective reduction catalyst device 6 (by the temperature sensor 9 or the like). The detection efficiency is calculated based on the ammonia accumulation efficiency set for each detection.

  An example of a control flow of the method for purifying exhaust gas of an internal combustion engine of the present invention will be described based on the control flow of FIG. The control flow shown in FIG. 3 is a control flow that is periodically executed during the rich reduction control of the NOx storage reduction type catalyst device 4.

  When the control flow shown in FIG. 3 starts, in step S10, the oxygen concentration λ of the exhaust gas G passing through the NOx storage reduction type catalyst device 4 and the difference between the NOx storage reduction type catalyst device 4 and the selective reduction type catalyst device 6 are determined. The acquisition value D of the NOx concentration sensor 12 that acquires the NOx concentration of the exhaust gas G passing through the exhaust passage 3 of FIG. After performing step S10, the process proceeds to step S20.

  In step S20, it is determined based on the two obtained values λ and D obtained in step S10 whether or not ammonia is generated in the NOx storage reduction catalyst device 4. Since the determination method has been described above, the description is omitted here. If it is determined in this determination that ammonia has not been generated in the NOx storage reduction catalyst device 4 (NO), the process proceeds to return and ends the present control flow. On the other hand, if it is determined in this determination that ammonia has been generated in the NOx storage reduction catalyst device 4 (YES), the process proceeds to step S30.

  In step S30, the amount of ammonia (the amount of generated ammonia) Na generated by the NOx storage reduction catalyst device 4 is calculated. Since the calculation method has been described above, the description is omitted here. After performing step S30, the process proceeds to step S40. In step S40, the amount of accumulation Ns of ammonia in the selective catalytic reduction device 6 is corrected based on the amount of ammonia Na calculated in step S30. Since the correction method has been described above, the description is omitted here. After performing the control in step S40, the process proceeds to return and ends the control flow.

  As described above, according to the present invention, the air-fuel ratio λ of the exhaust gas G passing through the NOx occlusion reduction type catalytic device 4 is reduced by the NOx occlusion reduction type catalytic device 4 during the rich reduction control of the NOx occlusion reduction type catalytic device 4. The measured value of the NOx concentration of the exhaust gas G passing through the exhaust passage 3 between the NOx occlusion reduction type catalytic device 4 and the selective reduction type catalytic device 6, which is an air-fuel ratio that is likely to be generated, measured by the NOx concentration sensor 12. When D is estimated to be a value including erroneous detection of ammonia, the amount Na of ammonia generated by the NOx storage reduction type catalyst device 4 is calculated, and the ammonia of the selective reduction type catalyst device 6 is calculated based on the calculated value. Is corrected. As a result, the accuracy of estimating the accumulated amount Ns of ammonia in the selective catalytic reduction device 6 can be improved, and the amount of ammonia flowing from the selective catalytic reduction device 6 to the exhaust passage 3 on the downstream side can be suppressed.

  Further, whether or not ammonia is generated in the NOx storage reduction type catalyst device 4 is determined by the oxygen concentration λ of the exhaust gas G passing through the NOx storage reduction type catalyst device 4 and the NOx storage reduction type catalyst device 4 is selectively reduced. Of the generation of ammonia in the NOx storage-reduction type catalyst device 4 by performing the double determination of the determination based on the obtained value D of the NOx concentration sensor 12 for the exhaust gas G passing through the exhaust passage 3 between the catalyst devices 6. Detection accuracy can be improved.

  Further, the ammonia amount Na generated by the NOx storage reduction type catalyst device 4 is obtained by the NOx amount sensor 12 for the NOx amount of the exhaust gas G discharged from the engine 2 and the exhaust gas G flowing into the selective reduction type catalyst device 6. When the calculation is performed based on the difference between the value (the amount of NOx and the amount of ammonia) D, the calculation accuracy of the amount of ammonia Na generated by the NOx storage reduction catalyst device 4 can be improved.

  Further, a method of calculating based on the NOx storage amount of the NOx storage reduction catalyst device 4 at the start of the rich reduction control may be used. The NOx storage amount N of the NOx storage reduction type catalyst device 4 is obtained by integrating the difference between the NOx amount of the exhaust gas G discharged from the engine 2 and the obtained value D of the NOx concentration sensor 12 by the control device 13 over time. Value. The control device 13 calculates and stores the NOx storage amount N of the NOz storage reduction catalyst device 4 when it is determined that the rich reduction control of the NOx storage reduction catalyst device 4 needs to be performed.

1 internal combustion engine exhaust gas purification system 2 engine (internal combustion engine)
3 Exhaust passage 4 NOx storage reduction type catalyst device 5 Diesel particulate collection filter device 6 Selective reduction type catalyst device 7 Fuel injection device 8 Urea water injection device 9 Temperature sensor 10 Front stage oxygen concentration sensor 11 Rear stage oxygen concentration sensor 12 NOx concentration sensor 13 Control device

Claims (4)

In an exhaust gas purification system for an internal combustion engine configured to include a NOx storage reduction type catalyst device and a selective reduction type catalyst device in order from the upstream side in an exhaust passage of the internal combustion engine,
An oxygen concentration acquisition device for acquiring an oxygen concentration of exhaust gas in the exhaust passage upstream of the selective catalytic reduction device; and an oxygen concentration acquisition device for acquiring an oxygen concentration of the exhaust gas in the exhaust passage between the NOx storage reduction catalytic device and the selective catalytic reduction device. A NOx concentration acquisition device that acquires the NOx concentration of the exhaust gas, and a control device that is electrically connected to the oxygen concentration acquisition device and the NOx concentration acquisition device,
The control device,
When performing the rich reduction control of the NOx storage reduction type catalyst device,
When determining that ammonia is generated in the NOx storage reduction catalyst device based on the obtained value of the oxygen concentration obtaining device and the obtained value of the NOx concentration obtaining device,
The amount of ammonia generated by the NOx storage reduction catalyst device is calculated, and control is performed to correct the amount of accumulated ammonia in the selective reduction catalyst device based on the calculated value. An exhaust gas purification system for an internal combustion engine. The control device,
When the acquired value of the oxygen concentration acquiring device is lower than a preset set oxygen concentration threshold, and the acquired value of the NOx concentration acquiring device is equal to or greater than a preset NOx concentration threshold, the NOx storage reduction type The exhaust gas purification system for an internal combustion engine according to claim 1, wherein control is performed to determine that ammonia is generated in the catalyst device. An engine outlet NOx concentration acquiring device for acquiring a NOx concentration of exhaust gas exhausted from the internal combustion engine, and the engine outlet NOx concentration acquiring device is electrically connected to the control device;
The control device,
Based on the difference between the acquired value of the engine outlet NOx concentration acquisition device and the acquired value of the NOx concentration acquisition device, or based on the NOx storage amount of the NOx storage reduction catalyst device at the start of the rich reduction control. 3. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein control is performed to calculate an amount of ammonia generated by said NOx storage reduction type catalyst device. In an exhaust gas purification method for an internal combustion engine configured to include a NOx storage reduction catalyst device and a selective reduction catalyst device in order from the upstream side in an exhaust passage of the internal combustion engine,
When performing the rich reduction control of the NOx storage reduction type catalyst device,
Acquiring the oxygen concentration of the exhaust gas passing through the NOx storage reduction catalyst device and the NOx concentration of the exhaust gas passing through the exhaust passage between the NOx storage reduction catalyst device and the selective reduction catalyst device;
Based on the obtained oxygen concentration and NOx concentration, it is determined whether or not ammonia has been generated in the NOx storage reduction catalyst device,
When it is determined that ammonia has been generated, the amount of ammonia generated by the NOx storage reduction catalyst device is calculated, and the amount of accumulated ammonia in the selective reduction catalyst device is corrected based on the calculated value. An exhaust gas purifying method for an internal combustion engine, comprising:

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