JP7206756B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Description

The present invention relates to an exhaust gas purification system for internal combustion engines.

In the automobile industry, due to the tightening of exhaust gas regulations in recent years, there is a demand for reducing the amount of NOx contained in the exhaust gas emitted into the atmosphere during city driving and highway driving. Conventional technologies for reducing the amount of NOx include LNT (NOx storage reduction catalyst) and urea SCR (urea selective reduction catalyst). Mounting on automobiles having a configuration in which SCRs are combined is under consideration (see Patent Document 1, for example).

Japanese Patent Application Laid-Open No. 2017-36700

By the way, the NOx storage reduction type 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 released NOx by reducing it to nitrogen with hydrocarbons and carbon monoxide contained in the exhaust gas. If the air-fuel ratio of the exhaust gas is less than 1 during the reduction reaction of NOx to nitrogen, ammonia may be produced as a by-product. Ammonia produced as a by-product flows out from 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 inside and used to purify NOx contained in the exhaust gas. Therefore, it is slightly different from the actual accumulated amount. Therefore, when the actual accumulated amount of ammonia is larger than the estimated accumulated amount of ammonia and the ammonia generated in the NOx storage reduction catalyst device flows into the selective reduction catalyst device, the ammonia that has flowed into the selective reduction catalyst device is Therefore, there is a risk that the ammonia may flow out from the selective reduction catalyst device into the exhaust passage on the downstream side.

An object of the present invention is to improve the accuracy of estimating the accumulated amount of ammonia in a selective reduction catalyst device, and to suppress the amount of ammonia flowing out of the selective reduction catalyst device into the exhaust passage on the downstream side. An object of the present invention is to provide a system and a method for purifying an exhaust gas of an internal combustion engine.

In order to achieve the above objects, an exhaust gas purification system for an internal combustion engine according to the present invention comprises an internal combustion engine exhaust gas passage comprising a NOx storage reduction catalyst device and a selective reduction catalyst device in this order from the upstream side in the exhaust passage of the internal combustion engine. In an exhaust gas purification system for an engine, an oxygen concentration acquiring device for acquiring an oxygen concentration of exhaust gas is provided in the exhaust passage upstream of the selective catalytic reduction device, the NOx storage reduction catalytic device and the selective catalytic reduction catalyst. a NOx concentration acquiring device for acquiring the NOx concentration of the exhaust gas in the exhaust passage between the devices, and a control device electrically connected to the oxygen concentration acquiring device and the NOx concentration acquiring device, When the device is performing rich reduction control of the NOx storage reduction type catalyst device, the NOx storage reduction type catalyst device is controlled based on the acquired value of the oxygen concentration acquisition device and the acquired value of the NOx concentration acquisition device. When it is determined that ammonia is generated in the NOx storage reduction catalyst device, the amount of ammonia generated in the NOx storage reduction catalyst device is calculated, and the accumulated amount of ammonia in the selective reduction catalyst device is corrected based on this calculated value. and the control device determines that the value acquired by the oxygen concentration acquisition device is below a preset oxygen concentration threshold and the value acquired by the NOx concentration acquisition device is preset. control is performed to determine that ammonia is being generated in the NOx storage reduction type catalyst device when the NOx concentration exceeds the set NOx concentration threshold .
Further, the exhaust gas purification system for an internal combustion engine of the present invention comprises a NOx storage reduction catalyst device and a selective reduction catalyst device in this order from the upstream side in an exhaust passage of the internal combustion engine. in
An oxygen concentration acquiring device for acquiring an oxygen concentration of exhaust gas is provided in the exhaust passage upstream of the selective catalytic reduction device, and the exhaust passage between the NOx storage reduction catalytic device and the selective catalytic reduction catalytic device is provided with an oxygen concentration acquisition device for acquiring an oxygen concentration of exhaust gas. a NOx concentration acquisition device for acquiring the NOx concentration of exhaust gas, and a control device electrically connected to the oxygen concentration acquisition device and the NOx concentration acquisition device,
When the control device is performing rich reduction control of the NOx storage reduction type catalyst device, the NOx storage reduction type When it is determined that ammonia is being generated in the catalyst device, the amount of ammonia generated in the NOx storage reduction catalyst device is calculated, and based on this calculated value, the accumulated amount of ammonia in the selective reduction catalyst device is calculated. An engine outlet NOx concentration acquisition device is configured to perform correction control and acquires the NOx concentration of exhaust gas discharged from the internal combustion engine, and the engine outlet NOx concentration acquisition device is attached to the control device. While electrically connecting, the control device controls the NOx concentration based on the difference between the acquired value of the engine outlet NOx concentration acquiring device and the acquired value of the NOx concentration acquiring device, or the NOx concentration at the start of the rich reduction control. It is characterized in that control is performed to calculate the amount of ammonia generated in the NOx storage reduction type catalyst device based on the NOx storage amount of the NOx storage reduction type catalyst device.

The NOx concentration sensor (NOx concentration acquisition device) provided at the front stage of the selective reduction catalyst device detects ammonia as well as NOx contained in the exhaust gas due to the nature of this sensor. In the present invention, the property of this sensor is utilized to determine whether or not ammonia is being generated in the NOx storage reduction type catalyst device during rich reduction control of the NOx storage reduction type catalyst device.

That is, according to the present invention, during the rich reduction control of the NOx storage reduction type catalyst device, 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. is the air-fuel ratio, and the measured value of the NOx concentration of the exhaust gas flowing into the selective reduction catalyst device is estimated to be a value including erroneous detection of ammonia, the amount of ammonia generated in the NOx storage reduction catalyst device The amount of ammonia is calculated, and the accumulated amount of ammonia in the selective reduction catalyst device is corrected based on this calculated value. As a result, it is possible to improve the accuracy of estimating the accumulated amount of ammonia in the selective reduction catalyst device, and to suppress the amount of ammonia flowing out from the selective reduction catalyst device into the exhaust passage on the downstream side.

1 is a diagram illustrating the configuration of an exhaust gas purification system for an internal combustion engine according to the present invention; FIG. FIG. 5 is a diagram illustrating changes in NOx concentration and changes in air-fuel ratio during rich reduction control; 1 is a diagram illustrating a control flow of a method for purifying exhaust gas of an internal combustion engine according to the present invention; FIG.

Hereinafter, an exhaust gas purification system for an internal combustion engine according to the present invention will be described based on embodiments 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 catalyst device (LNT) 4 and a diesel particulate collector installed in an exhaust passage 3 of an engine (internal combustion engine) 2 in this order from the upstream side. A filter device (CSF) 5 and a selective catalytic reduction device (SCR) 6 are arranged. A fuel injection device 7 is arranged in the exhaust passage 3 on the upstream side of the NOx storage reduction type catalyst device 4 . A urea water injection device 8 is arranged in the exhaust passage 3 between the diesel particulate filter device 5 and the selective catalytic reduction device 6 (the exhaust passage 3 on the upstream side 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 with this NOx storage material when the air-fuel ratio of the exhaust gas G is lean. The stored NOx is released by making the air-fuel ratio of the exhaust gas G rich, and the released NOx is reduced by the noble metal catalyst. The control for releasing the NOx stored in the NOx storage reduction type catalyst device 4 for reduction treatment by shifting the air-fuel ratio of the exhaust gas G to a rich state is called rich reduction control. The transition to the rich state of the air-fuel ratio of the exhaust gas G is performed by post-injecting the fuel in the cylinder of the engine 2 or by injecting the fuel F from the fuel injection device 7 into the exhaust passage 3 . 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 filtering device 5 collects particulate matter (PM) contained in the exhaust gas G. The collected PM is generally removed by combustion by passing the high-temperature exhaust gas G through the diesel particulate filter device 5 . A control in which the temperature of the exhaust gas G is raised and the trapped PM is removed by combustion with the high temperature exhaust gas G is called forced PM regeneration control. The high-temperature exhaust gas G is post-injected in the in-cylinder fuel injection of the engine 2, or the fuel F is injected from the fuel injection device 7, and the carbon monoxide and hydrocarbons contained in this fuel F are absorbed and reduced by NOx absorption. It is obtained by oxidation treatment (exothermic reaction) in the type catalyst device 4 .

The selective reduction catalyst device 6 is a device that purifies NOx contained in the exhaust gas G by reducing it to nitrogen with a reducing agent. In this embodiment, ammonia (NH ₃ ) is used as this 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 this ammonia flows into the selective reduction catalyst device 6, whereby the ammonia is selectively reduced. It is adsorbed by the type catalyst device 6 and reacts with NOx in the exhaust gas G to reduce the NOx when the exhaust gas G passes.

In addition, a temperature sensor 9 and a pre-stage oxygen concentration sensor 10 are provided in the exhaust passage 3 on the upstream side of the NOx storage reduction type catalyst device 4, and in the exhaust passage 3 between the NOx storage reduction type catalyst device 4 and the selective reduction type catalyst device 6 A post-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 catalyst 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 storage reduction catalyst device 4 and the selective reduction catalyst device 6 .

As the oxygen concentration acquisition device of the present invention, it is sufficient to acquire the oxygen concentration of the exhaust gas G passing through the NOx storage reduction type catalyst device 4. It may be composed of either both of the post-stage oxygen concentration sensors 11 . In this embodiment, the post-stage oxygen concentration sensor 11 is used as an oxygen concentration acquisition device. When the oxygen concentration acquisition device is composed of both the front oxygen concentration sensor 10 and the rear oxygen concentration sensor 11, the values acquired by the oxygen concentration acquisition device are the values acquired by the front oxygen concentration sensor 10 and the values acquired by the rear oxygen concentration sensor 11. is preferably the average value of the acquired values.

The control device 13 consists of a CPU that performs various types of information processing, a program used to perform the various types of information processing, an internal storage device capable of reading and writing information processing results, and various interfaces. It's a combination. The control device 13 is electrically connected to various sensors such as the post-stage oxygen concentration sensor 11 and the NOx concentration sensor 12 via signal lines.

The control device 13 determines that it is necessary to perform 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 greater than a preset rich request threshold value N1. Then, fuel injection from the fuel injection device 7 or the like is performed so that the acquired value λ of the post-stage oxygen concentration sensor 11 falls below the preset set oxygen concentration threshold value λ1, that is, the air-fuel ratio of the exhaust gas G becomes rich. Control to migrate. This set oxygen concentration threshold λ1 is set to 1.0, which is the stoichiometric air-fuel ratio, for example.

In the present invention, when the rich reduction control of the NOx storage reduction type catalyst device 4 is performed, the control device 13 detects whether or not ammonia is being generated in the NOx storage reduction type catalyst device 4 by acquiring 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 post-stage oxygen concentration sensor 11 is below a preset set oxygen concentration threshold value λ1 (=1.0) (determination A), and the NOx concentration sensor 12 obtained value D is greater than or equal to a preset concentration threshold value D1 (determination B), the control device 13 determines that the NOx storage reduction catalyst device 4 is generating ammonia. At times other than this, it is determined that ammonia is not generated in the NOx storage reduction type catalyst device 4 . The set concentration threshold value D1 is set to a value (for example, 500 ppm) that is larger than the NOx concentration of the exhaust gas G emitted from the engine 2 during rich reduction control and that allows determination that the NOx concentration sensor 12 has erroneously detected ammonia. be.

As for the determination B described above, the following determination may be made instead of the determination by comparing the acquired value D of the NOx concentration sensor 12 and the set concentration threshold value D1. That is, the NOx concentration of the exhaust gas G emitted from the engine 2 is calculated from the operating state of the engine 2 by the engine outlet NOx concentration acquisition device incorporated in the control device 13, and the calculated value D2 and the NOx concentration sensor 12 are acquired. Compare the value D. When determination A is established and the acquired value D of the NOx concentration sensor 12 is equal to or greater than the calculated value D2, the control device 13 determines that the NOx storage reduction catalyst device 4 is producing ammonia.

When the air-fuel ratio of the exhaust gas G passing through the NOx storage reduction type catalyst device 4 becomes rich, ammonia is produced as a by-product of the NOx reduction reaction during the rich reduction control. In addition, since the NOx released from the NOx storage reduction type catalyst device 4 during rich reduction control is reduced on the spot, the NOx concentration of the exhaust gas G flowing into the selective reduction type catalyst device 6 is reduced to the level at which rich reduction control starts. It gets smaller as time goes on. However, the NOx concentration sensor 12 erroneously detects not only NOx but also ammonia due to the nature of the sensor. Therefore, if ammonia is produced as a by-product, the value obtained by the NOx concentration sensor 12 will be different from that of the rich reduction control. It does not decrease as time elapses from the start, and it rises sharply during the execution period of the rich reduction control.

In the present invention, the control device 13 determines whether or not the NOx storage reduction catalyst device 4 is in a state where ammonia is generated based on the acquired value λ of the post-stage oxygen concentration sensor 11, and actually determines whether the NOx storage reduction catalyst device 4 is in a state where ammonia is generated. A value D acquired by the NOx concentration sensor 12 is used to determine whether or not ammonia is flowing out from the device 4 . That is, through double determination based on the acquired value λ of the post-stage oxygen concentration sensor 11 and determination based on the acquired value D of the NOx concentration sensor 12, ammonia is generated in the NOx storage reduction catalyst device 4 during rich reduction control. Confirm that

Then, when it is determined that ammonia is being generated in the NOx storage reduction type catalyst device 4 during rich reduction control, the control device 13 calculates the ammonia amount Na generated in the NOx storage reduction type 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 catalyst device 6 .

The ammonia amount Na generated in the NOx storage reduction type catalyst device 4 is calculated, for example, by multiplying the NOx storage amount of the NOx storage reduction type catalyst device 4 at the start of the rich reduction control by a correction coefficient determined by experiments or the like. . Alternatively, based on the difference between the NOx concentration of the exhaust gas G emitted from the engine 2 and the acquired value of the NOx concentration sensor 12 (the sum of the NOx concentration and the ammonia concentration of the exhaust gas flowing into the selective reduction catalyst device 6) D Then, the ammonia amount Na generated by 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 acquisition device, the NOx amount in this map is used as the engine cooling water temperature or during the transient operation of the engine 2. NOx concentration of the exhaust gas G emitted from the engine 2 is calculated by correcting with the oxygen concentration in the intake air, the boost pressure, and the like.

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

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

An example of the control flow of the exhaust gas purification method for 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 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 oxygen concentration λ between the NOx storage reduction type catalyst device 4 and the selective reduction type catalyst device 6 Acquisition value D of NOx concentration sensor 12 for acquiring NOx concentration of exhaust gas G passing through exhaust passage 3 is acquired. After performing step S10, the process proceeds to step S20.

In step S20, it is determined whether or not ammonia is being generated in the NOx storage reduction catalyst device 4 based on the two acquired values λ and D acquired in step S10. Since the determination method has been described above, the description is omitted here. In this determination, if it is determined that ammonia is not generated in the NOx storage reduction type catalyst device 4 (NO), the process proceeds to RETURN and ends this control flow. On the other hand, if it is determined that ammonia is being 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 ammonia generated) Na generated by the NOx storage reduction type catalyst device 4 is calculated. Since the calculation method has been described above, the explanation is omitted here. After performing step S30, the process proceeds to step S40. In step S40, the accumulated amount Ns of ammonia in the selective catalytic reduction catalytic converter 6 is corrected based on the amount Na of ammonia produced calculated in step S30. Since the correction method has been described above, the description is omitted here. After executing the control in step S40, the process proceeds to RETURN and terminates this control flow.

As described above, according to the present invention, during the rich reduction control of the NOx storage reduction type catalyst device 4, the air-fuel ratio λ of the exhaust gas G passing through the NOx storage reduction type catalyst device 4 becomes The NOx concentration sensor 12 actually measures the NOx concentration of the exhaust gas G passing through the exhaust passage 3 between the NOx storage reduction catalyst device 4 and the selective reduction catalyst device 6 at an air-fuel ratio that may be generated. When D is estimated to be a value including erroneous detection of ammonia, the amount Na of ammonia generated in the NOx storage reduction catalyst device 4 is calculated, and based on this calculated value, ammonia in the selective reduction catalyst device 6 is corrected. As a result, the accuracy of estimating the accumulated amount Ns of ammonia in the selective catalytic reduction catalyst device 6 can be improved, and the amount of ammonia flowing out from the selective catalytic reduction catalytic device 6 into 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 based on 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 and selective reduction are performed. The exhaust gas G passing through the exhaust passage 3 between the type catalyst devices 6 is subjected to a double determination based on the acquired value D of the NOx concentration sensor 12, so that the ammonia generation in the NOx storage reduction type catalyst device 4 is reduced. Detection accuracy can be improved.

Furthermore, the ammonia amount Na generated by the NOx storage reduction type catalyst device 4 is obtained by obtaining the NOx amount of the exhaust gas G discharged from the engine 2 and the NOx concentration sensor 12 for the exhaust gas G flowing into the selective reduction type catalyst device 6. If calculated based on the difference between the values (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.

Alternatively, a method of calculating based on the NOx storage amount of the NOx storage reduction type catalyst device 4 at the start of the rich reduction control may be used. The NOx storage amount N of the NOx storage reduction 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 acquired value D of the NOx concentration sensor 12 by the controller 13 over time. value. The control device 13 calculates and stores the NOx storage amount N of the NOx storage reduction type catalyst device 4 at the time when it is determined that the rich reduction control of the NOx storage reduction type catalyst device 4 needs to be performed.

1 Exhaust gas purification system for internal combustion engine 2 Engine (internal combustion engine)
3 exhaust passage 4 NOx storage reduction catalyst device 5 diesel particulate filter device 6 selective reduction 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 (2)

An exhaust gas purification system for an internal combustion engine, comprising a NOx storage reduction catalyst device and a selective reduction catalyst device in this order from the upstream side in an exhaust passage of the internal combustion engine,
An oxygen concentration acquiring device for acquiring an oxygen concentration of exhaust gas is provided in the exhaust passage upstream of the selective catalytic reduction device, and the exhaust passage between the NOx storage reduction catalytic device and the selective catalytic reduction catalytic device is provided with an oxygen concentration acquisition device for acquiring an oxygen concentration of exhaust gas. a NOx concentration acquisition device for acquiring the NOx concentration of exhaust gas, and a control device electrically connected to the oxygen concentration acquisition device and the NOx concentration acquisition device,
The control device
When rich reduction control is being performed on the NOx storage reduction type catalyst device,
When determining that ammonia is generated in the NOx storage reduction catalyst device based on the acquired value of the oxygen concentration acquisition device and the acquired value of the NOx concentration acquisition device,
The amount of ammonia generated in the NOx storage reduction catalyst device is calculated, and based on the calculated value, control is performed to correct the accumulated amount of ammonia in the selective reduction catalyst device , and
The control device
When the acquired value of the oxygen concentration acquisition device is lower than a preset set oxygen concentration threshold and the acquired value of the NOx concentration acquisition device is equal to or higher than a preset set NOx concentration threshold, the NOx storage reduction type An exhaust gas purification system for an internal combustion engine, characterized in that it is configured to perform control to determine that ammonia is being generated in a catalytic device . An exhaust gas purification system for an internal combustion engine, comprising a NOx storage reduction catalyst device and a selective reduction catalyst device in this order from the upstream side in an exhaust passage of the internal combustion engine,
An oxygen concentration acquiring device for acquiring an oxygen concentration of exhaust gas is provided in the exhaust passage upstream of the selective catalytic reduction device, and the exhaust passage between the NOx storage reduction catalytic device and the selective catalytic reduction catalytic device is provided with an oxygen concentration acquisition device for acquiring an oxygen concentration of exhaust gas. a NOx concentration acquisition device for acquiring the NOx concentration of exhaust gas, and a control device electrically connected to the oxygen concentration acquisition device and the NOx concentration acquisition device,
The control device
When rich reduction control is being performed on the NOx storage reduction type catalyst device,
When determining that ammonia is generated in the NOx storage reduction catalyst device based on the acquired value of the oxygen concentration acquisition device and the acquired value of the NOx concentration acquisition device,
The amount of ammonia generated in the NOx storage reduction catalyst device is calculated, and based on the calculated value, control is performed to correct the accumulated amount of ammonia in the selective reduction catalyst device, and
An engine outlet NOx concentration acquiring device for acquiring the NOx concentration of exhaust gas discharged from the internal combustion engine is provided, 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 acquiring device and the acquired value of the NOx concentration acquiring device, or based on the NOx storage amount of the NOx storage reduction type catalyst device at the start of the rich reduction control 1. An exhaust gas purification system for an internal combustion engine, characterized in that it is configured to perform control for calculating the amount of ammonia generated by said NOx storage reduction type catalyst device.

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