JP6955449B2 - Exhaust gas purification device - Google Patents

Description

The present invention relates to an exhaust gas purifying device that purifies exhaust gas discharged from an engine.

A NOx storage reduction catalyst (LNT: Lean NOx Trap) that removes nitrogen oxides (NOx) from the exhaust gas may be provided in the exhaust passage through which the exhaust gas discharged from the engine passes. The NOx storage reduction catalyst has a property of adsorbing not only NOx but also the sulfur component contained in the exhaust gas, and the sulfur component remaining in the NOx storage reduction catalyst increases with time. When the residual sulfur component increases, the purification performance of the NOx storage reduction catalyst deteriorates. Therefore, in the exhaust gas purification device having a NOx storage reduction catalyst, a sulfur desorption treatment (S purge) for desorbing a sulfur component from the NOx storage reduction catalyst is performed in order to restore the purification performance of the NOx storage reduction catalyst. .. The sulfur desorption treatment is disclosed in, for example, Patent Document 1.

Japanese Unexamined Patent Publication No. 2013-53583

In the sulfur desorption treatment, it is necessary to make the atmosphere in the NOx storage reduction catalyst a rich atmosphere. However, even if the sulfur desorption treatment is started and the combustion in the engine is switched from lean combustion to rich combustion, the atmosphere inside the NOx storage reduction catalyst must be consumed after the oxygen stored in the NOx storage reduction catalyst is consumed. It doesn't create a rich atmosphere. Further, the NOx storage reduction catalyst on the upstream side of the exhaust passage has transitioned to a rich atmosphere, but the exhaust gas in rich combustion has not sufficiently reached the NOx storage reduction catalyst on the downstream side of the exhaust passage. It may be maintained in a lean atmosphere. In such a case, in the NOx storage reduction catalyst, reattachment occurs in which the sulfur component desorbed on the upstream side is reattached on the downstream side. If such reattachment occurs for a long time, the sulfur desorption treatment cannot be completed at an early stage, and the efficiency of the sulfur desorption treatment is not good.

An object of the present invention is to provide an exhaust gas purification device capable of efficiently performing a sulfur desorption treatment of a NOx storage reduction catalyst.

In order to solve the above problems, the exhaust gas purification device of the present invention is provided in the exhaust passage through which the exhaust gas discharged from the engine passes, and the NOx storage reduction catalyst containing the OSC material and the sulfur removal of the NOx storage reduction catalyst are provided. When the release treatment start condition is satisfied, the target air-fuel ratio, which is the air-fuel ratio on the upstream side of the NOx storage reduction catalyst in the exhaust passage, is controlled to the rich first air-fuel ratio, and then according to the establishment of the target air-fuel ratio switching condition. The air-fuel ratio control unit is provided for switching and controlling the target air-fuel ratio to the second air-fuel ratio between the theoretical air-fuel ratio and the first air-fuel ratio.

It is equipped with an air-fuel ratio sensor that detects the air-fuel ratio on the downstream side of the NOx storage reduction catalyst in the exhaust passage, and the air-fuel ratio control unit sets the target air-fuel ratio from the first air-fuel ratio based on the air-fuel ratio detected by the air-fuel ratio sensor. You may switch to the second air-fuel ratio.

A plurality of NOx storage reduction catalysts are provided in series in the exhaust passage, and the air-fuel ratio sensor may be provided on the downstream side of the NOx storage reduction catalyst on the most downstream side in the exhaust passage.

The air-fuel ratio control unit may control the time for maintaining the first air-fuel ratio according to the mileage of the vehicle to which the exhaust gas purification device is applied.

The air-fuel ratio control unit may control the time for maintaining the second air-fuel ratio according to the sulfur poisoning time of the NOx storage reduction catalyst.

The air-fuel ratio control unit may control the value of the second air-fuel ratio according to the temperature of the NOx storage reduction catalyst.

According to the present invention, the sulfur desorption treatment of the NOx storage reduction catalyst can be efficiently performed.

It is the schematic which shows the structure of the engine system including the exhaust gas purification device by this embodiment. It is a flowchart which shows the flow of the sulfur desorption processing in an exhaust gas purification apparatus. It is explanatory drawing explaining the operation of the sulfur desorption treatment of the comparative example. It is explanatory drawing explaining the operation of the sulfur desorption treatment in the exhaust gas purification apparatus by this embodiment. It is a figure which shows an example of the relationship between the mileage of a vehicle to which an exhaust gas purification device is applied, and the time of the first air-fuel ratio. It is a figure which shows an example of the relationship between the sulfur poisoning time and the time of the second air-fuel ratio. It is the schematic which shows the structure of the engine system including the exhaust gas purification device by another modification. It is a figure which shows an example of the relationship between the temperature of the NOx storage reduction catalyst of the 2nd stage, and the value of the 2nd air-fuel ratio.

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The dimensions, materials, other specific numerical values, etc. shown in the embodiment are merely examples for facilitating the understanding of the invention, and do not limit the present invention unless otherwise specified. In the present specification and drawings, elements having substantially the same function and configuration are designated by the same reference numerals to omit duplicate description, and elements not directly related to the present invention are not shown. do.

FIG. 1 is a schematic view showing the configuration of an engine system 100 including an exhaust gas purification device 200 according to the present embodiment. In FIG. 1, the signal flow is indicated by a broken line arrow. Hereinafter, the configurations and processes related to the present embodiment will be described in detail, and the configurations and processes unrelated to the present embodiment will be omitted.

The engine system 100 is provided with an ECU (Engine Control Unit) 110 and an engine 120. The ECU 110 is composed of a central processing unit (CPU), a ROM in which programs and the like are stored, and a microcomputer including a RAM as a work area and the like. The ECU 110 controls the entire engine 120 in an integrated manner.

The engine 120 is, for example, a gasoline engine that operates a piston and a crankshaft by burning an air-fuel mixture of gasoline and air as fuel. An exhaust manifold (not shown) is connected to the exhaust port of the engine 120, and an exhaust passage 130 is connected to the gathering portion of the exhaust manifold. Exhaust gas discharged from the engine 120 flows through the exhaust passage 130. Hereinafter, along the flow of exhaust gas in the exhaust passage 130, the one closer to the engine 120 is referred to as an upstream, and the one far from the engine 120 is referred to as a downstream.

The engine system 100 is provided with an exhaust gas purifying device 200 that purifies the exhaust gas discharged from the engine 120. Hereinafter, the specific configuration of the exhaust gas purification device 200 will be described in detail.

The exhaust gas purification device 200 includes a three-way catalyst (TWC) 210, two NOx storage reduction catalysts (LNT) 220A and 220B, and an air-fuel ratio sensor 230. The ECU 110 also functions as an air-fuel ratio control unit 310 that constitutes the exhaust gas purification device 200 by executing a program stored in the ROM.

The exhaust passage 130 is provided with a three-way catalyst 210, a NOx storage reduction catalyst 220A, a NOx storage reduction catalyst 220B, and an air-fuel ratio sensor 230 in this order from upstream to downstream. Hereinafter, the NOx storage reduction catalyst 220A on the upstream side may be referred to as a first-stage NOx storage reduction catalyst 220B, and the NOx storage reduction catalyst 220B on the downstream side may be referred to as a second-stage NOx storage reduction catalyst 220B.

The three-way catalyst 210 is composed of, for example, a carrier carrying platinum (Pt), palladium (Pd), and rhodium (Rh). The three-way catalyst 210 contains hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides contained in the exhaust gas discharged from the engine 120 when the engine 120 is performing stoichiometric (theoretical air-fuel ratio) combustion. Purifies (oxidation-reduction) (NOx).

The NOx storage reduction catalysts 220A and 220B are composed of carriers carrying, for example, platinum (Pt), palladium (Pd), rhodium (Rh), barium (Ba), and potassium (K). Here, when the engine 120 performs lean combustion, the atmosphere in the NOx storage reduction catalysts 220A and 220B becomes a lean atmosphere. Further, when the engine 120 is performing lean combustion, a large amount of NOx (nitrogen oxide) contained in the exhaust gas discharged from the engine 120 reaches the NOx storage reduction catalysts 220A and 220B. When the atmosphere inside the NOx storage reduction catalysts 220A and 220B is a lean atmosphere, the NOx storage reduction catalysts 220A and 220B temporarily store NOx (nitrogen oxide) that has reached the NOx storage reduction catalysts 220A and 220B. Lean indicates an air-fuel ratio in which the ratio of fuel to air is small, based on the stoichiometric air-fuel ratio.

Further, when the engine 120 performs rich combustion, the atmosphere in the NOx storage reduction catalysts 220A and 220B becomes a rich atmosphere. The NOx storage reduction catalysts 220A and 220B purify (reduce) the stored NOx when the atmosphere inside the NOx storage reduction catalysts 220A and 220B is rich. Rich indicates the air-fuel ratio in which the ratio of fuel to air is large, based on the stoichiometric air-fuel ratio.

Further, the NOx storage and reduction catalysts 220A and 220B are configured such that the carrier contains an OSC (Oxygen Storage Capacity: oxygen storage and release capacity) material such as ceria. Therefore, the NOx storage reduction catalysts 220A and 220B have OSC having the ability to occlude oxygen in a lean atmosphere and release oxygen in a rich atmosphere.

The air-fuel ratio sensor 230 detects the air-fuel ratio of the exhaust gas on the downstream side of the second-stage NOx storage reduction catalyst 220B. The signal indicating the air-fuel ratio detected by the air-fuel ratio sensor 230 is transmitted to the ECU 110. Hereinafter, the air-fuel ratio detected by the air-fuel ratio sensor 230 may be referred to as a detected air-fuel ratio.

The air-fuel ratio control unit 310 controls the air-fuel ratio of the air-fuel mixture burned by the engine 120. That is, the air-fuel ratio control unit 310 controls the amount of fuel and the amount of air supplied to the combustion chamber of the engine 120. The air-fuel ratio of the air-fuel mixture burned by the engine 120 is the same as the air-fuel ratio of the exhaust gas discharged from the exhaust port of the engine 120 and flowing into the three-way catalyst 210. Therefore, the air-fuel ratio control unit 310, as a result, controls the air-fuel ratio of the exhaust gas on the upstream side of the three-way catalyst 210 in the exhaust passage 130. Hereinafter, the air-fuel ratio of the exhaust gas on the upstream side of the three-way catalyst 210 in the exhaust passage 130 may be referred to as a target air-fuel ratio.

Here, the NOx storage reduction catalysts 220A and 220B have a property of adsorbing not only the storage of NOx but also the sulfur component contained in the exhaust gas. Further, the sulfur component remaining in the NOx storage reduction catalysts 220A and 220B increases with time. When the sulfur component remaining in the NOx storage reduction catalysts 220A and 220B increases, the range in which NOx can be stored in the NOx storage reduction catalysts 220A and 220B decreases depending on the sulfur component. As a result, the NOx purification performance of the NOx storage reduction catalysts 220A and 220B deteriorates.

Therefore, in the exhaust gas purification device 200 according to the present embodiment, a sulfur desorption treatment for desorbing a sulfur component from the NOx storage reduction catalysts 220A and 220B is performed. In the sulfur desorption treatment, the sulfur component is desorbed as, for example, sulfur dioxide (SO ₂ ) or hydrogen sulfide (H ₂ S). The air-fuel ratio control unit 310 controls the target air-fuel ratio to a rich first air-fuel ratio according to the start of the sulfur desorption treatment, and then sets the target air-fuel ratio to the theoretical air-fuel ratio and the first air-fuel ratio based on the detected air-fuel ratio. It is controlled by switching to the second air-fuel ratio between the air-fuel ratio and the air-fuel ratio. Hereinafter, the sulfur desorption treatment will be specifically described.

FIG. 2 is a flowchart showing the flow of the sulfur desorption treatment in the exhaust gas purification device 200. The sulfur desorption process is executed as an interrupt process at predetermined time intervals.

When the predetermined interrupt timing is reached, the air-fuel ratio control unit 310 determines whether or not the start condition of the sulfur desorption process is satisfied (S110). The condition for starting the sulfur desorption treatment is, for example, whether or not the mileage of the vehicle has reached a predetermined distance. A predetermined distance as a judgment standard is set, for example, every 1000 km. In this case, the sulfur desorption treatment is performed every 1000 km. The setting of the predetermined distance is not limited to every 1000 km, and may be, for example, every 10,000 km.

If the start condition is not satisfied (NO in S110), the interrupt process ends and the sulfur desorption process is not performed.

When the start condition is satisfied (YES in S110), the air-fuel ratio control unit 310 starts the sulfur desorption treatment and controls the target air-fuel ratio to a rich first air-fuel ratio (S120). This first air-fuel ratio is a value that is more biased toward the rich side than the second air-fuel ratio described later. The first air-fuel ratio is determined by experiments and simulations. When the theoretical air-fuel ratio is set to "1" as a reference, the first air-fuel ratio is preferably, for example, "0.9" or less, and specifically, it is about "0.85". The value of the first air-fuel ratio is not limited to this example, and may be, for example, "0.8". Further, as the air-fuel ratio is smaller than the standard "1" indicating the stoichiometric air-fuel ratio, the degree of richness is large (that is, the ratio of fuel to air is larger).

Before the start of the sulfur desorption treatment, lean burning is performed in the engine 120 in order to suppress fuel consumption. Therefore, in response to the establishment of the start condition of the sulfur desorption treatment, the air-fuel ratio control unit 310 causes rich combustion by increasing the fuel supplied to the combustion chamber, and makes the target air-fuel ratio rich first. Transition to air-fuel ratio.

Further, when the start condition of the sulfur desorption treatment is satisfied, the ECU 110 performs a process of raising the temperature of the NOx storage reduction catalysts 220A and 220B to a predetermined temperature or higher. This is because it is necessary to raise the temperature of the NOx storage reduction catalysts 220A and 220B in order to release the sulfur component from the NOx storage reduction catalysts 220A and 220B. The predetermined temperature is a lower limit temperature at which the sulfur component can be released, and is higher than, for example, a temperature required for releasing NOx from the NOx storage reduction catalysts 220A and 220B. The ECU 110 raises the temperature of the exhaust gas by increasing the traveling load of the vehicle, and raises the temperatures of the NOx storage reduction catalysts 220A and 220B by the heat of the exhaust gas.

Next, the air-fuel ratio control unit 310 determines whether or not the switching condition, which is a condition for switching the target air-fuel ratio, is satisfied (S130). The switching condition in the present embodiment is whether or not the detected air-fuel ratio has transitioned from lean to rich beyond the stoichiometric air-fuel ratio.

When the switching condition is not satisfied (NO in S130), that is, when the detected air-fuel ratio does not transition richly, the air-fuel ratio control unit 310 maintains the control of the first air-fuel ratio, and the switching condition is satisfied. Wait until.

When the switching condition is satisfied (YES in S130), that is, when the detected air-fuel ratio transitions to rich, the air-fuel ratio control unit 310 switches and controls the target air-fuel ratio from the first air-fuel ratio to the second air-fuel ratio (S140). ). The second air-fuel ratio is a rich air-fuel ratio between the theoretical air-fuel ratio and the first air-fuel ratio. The second air-fuel ratio is determined by experiments and simulations. When the theoretical air-fuel ratio is set to "1" as a reference, the second air-fuel ratio is preferably, for example, between "1" and "0.9", and specifically, it is about "0.95". .. The value of the second air-fuel ratio is not limited to this example. Further, the second air-fuel ratio may be at the same level as the air-fuel ratio in the conventional sulfur desorption treatment.

As described above, the second air-fuel ratio after switching has a smaller degree of richness than the first air-fuel ratio before switching. Therefore, when the switching condition is satisfied, the air-fuel ratio control unit 310 reduces the fuel supplied to the combustion chamber as compared with the first air-fuel ratio, so that the ratio of fuel to air is higher than that of the first air-fuel ratio. To reduce. The second air-fuel ratio is still based on the theoretical air-fuel ratio, and the ratio of fuel to air is still large. In this way, the air-fuel ratio control unit 310 causes rich combustion having a degree of richness smaller than that of the first air-fuel ratio, and shifts the target air-fuel ratio to the second air-fuel ratio.

Next, the air-fuel ratio control unit 310 determines whether or not the end condition of the sulfur desorption treatment is satisfied (S150). The termination condition in the present embodiment is whether or not a predetermined time has elapsed based on the time when the detected air-fuel ratio reaches the second air-fuel ratio. This predetermined time is determined by experiments and simulations, and is set to a time during which the sulfur component is sufficiently released from the NOx storage reduction catalysts 220A and 220B. This predetermined time is preferably set, for example, between 30 seconds and 600 seconds, and more preferably about 140 seconds. The predetermined time is not limited to this example, and may exceed, for example, 600 seconds.

When the end condition is not satisfied (NO in S150), that is, when the predetermined time has not elapsed, the air-fuel ratio control unit 310 maintains the control of the second air-fuel ratio until the end condition is satisfied (predetermined). Wait (until time elapses).

When the end condition is satisfied (YES in S150), that is, when the predetermined time has elapsed, the air-fuel ratio control unit 310 ends the sulfur desorption treatment.

When the end condition is satisfied, the ECU 110 ends the process of raising the temperature of the NOx storage reduction catalysts 220A and 220B.

FIG. 3 is an explanatory diagram illustrating the operation of the sulfur desorption treatment of the comparative example. In FIG. 3, the air-fuel ratio (target air-fuel ratio) of the exhaust gas on the upstream side of the three-way catalyst 210 is shown by a solid line, and the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B in the second stage (in the detected air-fuel ratio). The equivalent air-fuel ratio) is indicated by a single point chain line.

As shown in FIG. 3, it is assumed that the sulfur desorption treatment is started at time T11. Before the sulfur desorption treatment is started, the target air-fuel ratio and the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B in the second stage (the air-fuel ratio corresponding to the detected air-fuel ratio) are lean. At this time, since the NOx storage reduction catalysts 220A and 220B have an OSC, they store oxygen contained in the exhaust gas.

When the sulfur desorption treatment is started at time T11, the target air-fuel ratio transitions from lean to rich. The degree of richness is, for example, about "0.95", which is about the same as the second air-fuel ratio. As a result, the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B in the second stage gradually decreases and reaches the theoretical air-fuel ratio at time T12.

After that, as soon as the atmosphere in the NOx storage reduction catalysts 220A and 220B is changed from the stoichiometric air-fuel ratio to rich, the NOx storage reduction catalysts 220A and 220B release the stored oxygen. Specifically, the rich atmosphere is filled with exhaust gas containing more carbon monoxide (CO) and hydrocarbons (HC) constituting the fuel than the stoichiometric air-fuel ratio. Then, in a rich atmosphere, the oxygen stored in the NOx storage reduction catalysts 220A and 220B reacts with these carbon monoxide (CO) and hydrocarbon (HC) to form carbon dioxide (CO ₂ ) and water (H ₂ O). Is released from the NOx storage reduction catalysts 220A and 220B.

That is, in the exhaust gas in the NOx storage reduction catalysts 220A and 220B, the amount of hydrocarbon (HC), that is, fuel is reduced, and _{the amount of carbon dioxide (CO 2} ), that is, air is increased. Therefore, the atmosphere in the NOx storage reduction catalysts 220A and 220B returns to the stoichiometric air-fuel ratio. As a result, the atmosphere in the NOx storage reduction catalysts 220A and 220B is maintained at the stoichiometric air-fuel ratio until the NOx storage reduction catalysts 220A and 220B release all the stored oxygen. As a result, the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B is maintained at the stoichiometric air-fuel ratio until time T13.

After the NOx storage reduction catalysts 220A and 220B released all the stored oxygen, the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B gradually decreased from the stoichiometric air-fuel ratio, and the target air at time T14. Reach the same level of richness as the fuel ratio. When the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B becomes rich, sulfur is desorbed from the NOx storage reduction catalysts 220A and 220B. The description of the temperatures of the NOx storage reduction catalysts 220A and 220B will be omitted.

As described above, in the sulfur desorption treatment of the comparative example, the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B is maintained at the stoichiometric air-fuel ratio for a long time, and the atmosphere in the NOx storage reduction catalysts 220A and 220B is changed. It took time to get a rich atmosphere. Further, when a plurality of NOx storage reduction catalysts 220A and 220B are provided, it takes time for the atmosphere in the second stage NOx storage reduction catalyst 220B to become a rich atmosphere.

In addition, the desorbed sulfur component may reattach (occlude) to the NOx storage / reduction catalysts 220A and 220B again on the NOx storage / reduction catalysts 220A and 220B containing the OSC material. Here, after switching from lean combustion to rich combustion, the upstream side of the exhaust passage 130 (for example, NOx storage reduction catalyst 220A) transitioned to a rich atmosphere, but the downstream side of the exhaust passage 130 (for example, NOx storage). In the reduction catalyst 220B), the exhaust gas due to rich combustion does not reach sufficiently, and the lean atmosphere may be maintained. In such a case, since the NOx storage reduction catalyst 220A on the upstream side has a rich atmosphere, the sulfur component is desorbed after the oxygen stored in the OSC material is released. On the other hand, since the NOx storage reduction catalyst 220B on the downstream side has a lean atmosphere, oxygen is occluded in the OSC material and the sulfur component desorbed on the upstream side is attached. Such reattachment occurs until the NOx storage reduction catalyst 220B on the downstream side transitions to a rich atmosphere.

In this way, when the sulfur component is reattached, the reattached sulfur component is also detached in duplicate. In the sulfur desorption treatment of the comparative example, the efficiency of the sulfur desorption treatment was low because the reattachment took a long time.

FIG. 4 is an explanatory diagram illustrating the operation of the sulfur desorption treatment in the exhaust gas purification device 200 according to the present embodiment. In FIG. 4, the target air-fuel ratio, that is, the air-fuel ratio of the exhaust gas on the upstream side of the three-way catalyst 210 is shown by a solid line, and the detected air-fuel ratio, that is, the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B is one point. Shown by a chain line.

As shown in FIG. 4, it is assumed that the sulfur desorption treatment is started at time T21. Before the sulfur desorption treatment is started, the target air-fuel ratio and the detected air-fuel ratio are lean.

When the sulfur desorption treatment is started at time T21, the target air-fuel ratio shifts from lean to the first air-fuel ratio. As a result, the detected air-fuel ratio gradually decreases and reaches the theoretical air-fuel ratio. At time T21, the process of raising the temperature of the NOx storage reduction catalysts 220A and 220B to a predetermined temperature or higher is also started.

As described above, the first air-fuel ratio has a higher degree of richness than the second air-fuel ratio. As a result, when the atmosphere in the NOx storage reduction catalysts 220A and 220B transitions from the stoichiometric air-fuel ratio to rich, a large amount of oxygen is released (reduced) from the NOx storage reduction catalysts 220A and 220B. That is, the oxygen stored in the NOx storage reduction catalysts 220A and 220B is consumed at an early stage. Therefore, in the present embodiment, the time required for the NOx occlusion reduction catalysts 220A and 220B to release all the stored oxygen can be shortened as compared with the sulfur desorption treatment of the comparative example (see FIG. 3). .. As a result, the atmosphere in the NOx storage reduction catalysts 220A and 220B, in other words, the detected air-fuel ratio, transitions to rich without being maintained at the stoichiometric air-fuel ratio. Here, the detected air-fuel ratio is richly transitioned from the stoichiometric air-fuel ratio at time T22. When the detected air-fuel ratio is rich, the atmosphere inside the NOx storage reduction catalysts 220A and 220B is rich, so that the sulfur component is released from the NOx storage reduction catalysts 220A and 220B.

When the detected air-fuel ratio exceeds the theoretical air-fuel ratio and transitions to a rich state at time T22, the target air-fuel ratio transitions to the second air-fuel ratio. At this time, the detected air-fuel ratio gradually decreases and reaches the second air-fuel ratio at time T23.

At the time T23 when the detected air-fuel ratio reaches the second air-fuel ratio, the air-fuel ratio control unit 310 starts counting the predetermined time under the end condition. Then, at the time T24 when the predetermined time has elapsed, the sulfur desorption treatment is completed. After the sulfur desorption treatment is completed, the air-fuel ratio control unit 310 switches, for example, the target air-fuel ratio to a normal lean.

As described above, the exhaust gas purification device 200 according to the present embodiment controls the target air-fuel ratio in two stages of the first air-fuel ratio and the second air-fuel ratio. The first air-fuel ratio has a higher degree of richness than the second air-fuel ratio. Therefore, in the exhaust gas purification device 200, the oxygen of the OSC material in the NOx storage reduction catalysts 220A and 220B can be consumed at an early stage. As a result, in the exhaust gas purification device 200, the atmosphere in the NOx storage reduction catalysts 220A and 220B can be changed to rich at an early stage. Therefore, in the exhaust gas purification device 200, the time during which the sulfur component is redeposited can be shortened. Further, in the exhaust gas purification device 200, the sulfur components of the NOx storage reduction catalysts 220A and 220B can be desorbed at an early stage from the start of the sulfur desorption treatment.

Therefore, according to the exhaust gas purification device 200 according to the present embodiment, it is possible to efficiently perform the sulfur desorption treatment of the NOx storage reduction catalysts 220A and 220B.

Further, in the exhaust gas purification device 200 according to the present embodiment, the target air-fuel ratio is changed from the first air-fuel ratio to the second air-fuel ratio according to the transition of the air-fuel ratio of the exhaust gas on the downstream side of the NOx storage reduction catalyst 220B to rich. Can be switched to. That is, in the present embodiment, the period of the first air-fuel ratio with a large degree of richness is short. Therefore, the exhaust gas purification device 200 according to the present embodiment can suppress fuel consumption and prevent a decrease in fuel consumption as compared with the mode in which the first air-fuel ratio is maintained.

Further, in the exhaust gas purification device 200 according to the present embodiment, the air-fuel ratio sensor 230 is provided on the downstream side of the NOx storage reduction catalyst 220B, and the air-fuel ratio is from the first air to the second air according to the detection result of the air-fuel ratio sensor 230. Switching to the fuel ratio is performed. Therefore, in the exhaust gas purification device 200 according to the present embodiment, it is possible to reliably determine that the atmosphere in the NOx storage reduction catalysts 220A and 220B has become rich, and it is possible to accurately switch the target air-fuel ratio. It will be possible.

A modified example of this embodiment is shown below. FIG. 5 is a diagram showing an example of the relationship between the mileage of the vehicle to which the exhaust gas purification device 200 is applied and the time of the first air-fuel ratio. In the modified example shown in FIG. 5, the air-fuel ratio control unit 310 controls the time for maintaining the first air-fuel ratio according to the mileage of the vehicle.

For example, the ECU 110 stores a first air-fuel ratio time table, which is a table in which the mileage and the time of the first air-fuel ratio are related as shown in FIG. Further, the air-fuel ratio control unit 310 acquires a detection value (mileage) of the distance sensor that detects the mileage of the vehicle after the condition for starting the sulfur desorption process is satisfied. The air-fuel ratio control unit 310 derives the time for maintaining the first air-fuel ratio based on the obtained mileage and the first air-fuel ratio time table. Then, the air-fuel ratio control unit 310 switches the target air-fuel ratio to the first air-fuel ratio, and maintains the target air-fuel ratio at the first air-fuel ratio until the derived time elapses.

Here, the OSC material contained in the NOx storage reduction catalysts 220A and 220B deteriorates as the mileage of the vehicle increases. As the OSC material deteriorates, its ability to occlude and release oxygen decreases. That is, the mileage of the vehicle corresponds to the oxygen storage and release capacity of the OSC material, and the longer the mileage of the vehicle, the lower the oxygen storage and release capacity. Further, the time of the first air-fuel ratio corresponds to the time when oxygen is released from the NOx storage reduction catalysts 220A and 220B, and the longer the time of the first air-fuel ratio, the larger the amount of oxygen released. Can be done.

As shown in FIG. 5, the first air-fuel ratio time table is set so that the time of the first air-fuel ratio becomes shorter as the mileage of the vehicle increases. As a result, the time of the first air-fuel ratio in the sulfur desorption treatment performed when the mileage of the vehicle is long is compared with the time of the first air-fuel ratio in the sulfur desorption treatment performed when the mileage of the vehicle is short. It gets shorter. That is, when the mileage of the vehicle is short, a large amount of oxygen is occluded in the OSC material, so that a large amount of stored oxygen can be released by lengthening the time of the first air-fuel ratio. Further, when the mileage of the vehicle is long, only a small amount of oxygen is occluded in the OSC material due to deterioration of the OSC material. Therefore, by shortening the time of the first air-fuel ratio, the oxygen release is completed. It is prevented that the state of 1 air-fuel ratio is excessively continued.

Therefore, according to this aspect, the time required for the release of oxygen from the OSC material can be set to an appropriate length according to the amount of oxygen occluded in the OSC material, and the sulfur desorption treatment can be performed more efficiently. Can be done.

Further, according to this aspect, since the state of the first air-fuel ratio for rich combustion is not continuously continued, it is possible to prevent the fuel from being consumed excessively and to suppress the increase in fuel consumption. It will be possible.

The mode of deriving the time of the first air-fuel ratio from the mileage of the vehicle is not limited to the mode using the first air-fuel ratio time table. For example, the time of the second air-fuel ratio may be derived by using the relational expression in which the mileage of the vehicle and the time of the first air-fuel ratio are related.

FIG. 6 is a diagram showing an example of the relationship between the sulfur poisoning time and the time of the second air-fuel ratio. In the modified example shown in FIG. 6, the air-fuel ratio control unit 310 controls the time for maintaining the second air-fuel ratio according to the sulfur poisoning time of the NOx storage reduction catalysts 220A and 220B. It should be noted that the sulfur component being occluded by the NOx storage reduction catalysts 220A, 220B and the like is called sulfur poisoning, and the time during which the sulfur is poisoned is called the sulfur poisoning time.

For example, the ECU 110 stores a second air-fuel ratio time table, which is a table in which the sulfur poisoning time and the time of the second air-fuel ratio are related as shown in FIG. Further, the air-fuel ratio control unit 310 measures the cumulative time obtained by accumulating the elapsed time when the engine 120 is operating, starting from the time of the previous sulfur desorption treatment. This cumulative time corresponds to the sulfur poisoning time. After the condition for switching to the second air-fuel ratio is satisfied, the air-fuel ratio control unit 310 derives the time for maintaining the second air-fuel ratio based on the sulfur poisoning time obtained by timekeeping and the second air-fuel ratio time table. do. Then, after switching the target air-fuel ratio to the second air-fuel ratio, the air-fuel ratio control unit 310 maintains the target air-fuel ratio at the second air-fuel ratio until the derived time elapses.

Here, the sulfur poisoning time corresponds to the amount of sulfur components accumulated in the NOx storage reduction catalysts 220A and 220B, and the longer the sulfur poisoning time, the larger the amount of accumulated sulfur components. .. Further, the time of the second air-fuel ratio corresponds to the time when the sulfur component is desorbed from the NOx storage reduction catalyst, and the longer the time of the second air-fuel ratio, the larger the amount of the sulfur component desorbed can be. can.

As shown in FIG. 6, the second air-fuel ratio time table is set so that the time of the second air-fuel ratio becomes longer as the sulfur poisoning time becomes longer. As a result, the time of the second air-fuel ratio in the sulfur desorption treatment performed when the sulfur poisoning time is long is compared with the time of the second air-fuel ratio in the sulfur desorption treatment performed when the sulfur poisoning time is short. become longer. That is, when the amount of accumulated sulfur component is large, a large amount of sulfur component is desorbed by lengthening the time of the second air-fuel ratio. Further, when the accumulated amount of the sulfur component is small, the time of the second air-fuel ratio is shortened, so that the sulfur desorption treatment is prevented from being excessively continued after the desorption of the sulfur component is completed.

Therefore, according to this aspect, the time required for the sulfur desorption treatment can be set to an appropriate length according to the accumulated amount of the sulfur component, and the sulfur desorption treatment can be performed more efficiently.

Further, according to this aspect, since the state of the second air-fuel ratio for rich combustion is not continuously continued, it is possible to prevent the fuel from being consumed excessively and to suppress the increase in fuel consumption. It will be possible.

The mode of deriving the time of the second air-fuel ratio from the sulfur poisoning time is not limited to the mode using the second air-fuel ratio time table. For example, the time of the second air-fuel ratio may be derived by using the relational expression in which the sulfur poisoning time and the time of the second air-fuel ratio are related.

FIG. 7 is a schematic view showing the configuration of the engine system 102 including the exhaust gas purification device 202 according to another modification. The exhaust gas purification device 202 is different from the exhaust gas purification device 200 in that the air-fuel ratio sensor 230 is not provided and the temperature sensor 240 is provided. The temperature sensor 240 is provided on the downstream side of the NOx storage reduction catalyst 220B in the second stage in the exhaust passage 130. The temperature sensor 240 detects the temperature of the NOx storage reduction catalyst 220B in the second stage. The air-fuel ratio control unit 310 acquires the temperature detection result by the temperature sensor 240.

FIG. 8 is a diagram showing an example of the relationship between the temperature of the NOx storage reduction catalyst 220B in the second stage and the value of the second air-fuel ratio. In the modified examples shown in FIGS. 7 and 8, the air-fuel ratio control unit 310 responds to the temperature (temperature detected by the temperature sensor 240) of the second-stage NOx storage reduction catalyst 220B located on the most downstream side in the exhaust passage 130. The value of the second air-fuel ratio is controlled.

For example, the ECU 110 stores a second air-fuel ratio value table, which is a table in which the temperature of the second-stage NOx storage reduction catalyst 220B and the value of the second air-fuel ratio are associated with each other as shown in FIG. Further, the air-fuel ratio control unit 310 acquires the detection value (temperature) of the temperature sensor that detects the temperature of the NOx storage reduction catalyst 220B in the second stage after the condition for switching to the second air-fuel ratio is satisfied. The air-fuel ratio control unit 310 derives the value of the second air-fuel ratio based on the temperature of the obtained second-stage NOx storage reduction catalyst 220B and the second air-fuel ratio value table. Then, the air-fuel ratio control unit 310 controls the target air-fuel ratio to the second air-fuel ratio.

Here, the temperature of the NOx storage reduction catalyst 220B in the second stage corresponds to the amount of the sulfur component to be desorbed, and the higher the temperature, the larger the amount of the sulfur component to be desorbed. Further, the value of the second air-fuel ratio corresponds to the amount of the sulfur component to be desorbed, and the richer the atmosphere of the NOx storage reduction catalyst 220B, the larger the amount of the sulfur component to be desorbed can be.

As shown in FIG. 8, in the second air-fuel ratio table, the value of the second air-fuel ratio approaches the theoretical air-fuel ratio as the temperature of the NOx storage reduction catalyst 220B in the second stage increases, and the second stage The degree of richness of the second air-fuel ratio is set so as to approach the first air-fuel ratio as the temperature of the NOx storage reduction catalyst 220B decreases. As a result, when the temperature of the second stage NOx storage reduction catalyst 220B is relatively high, the degree of richness of the second air-fuel ratio becomes small, and the temperature of the second stage NOx storage reduction catalyst 220B is relatively low. Sometimes the degree of richness increases. That is, when the temperature of the NOx storage reduction catalyst 220B in the second stage is relatively high, the amount of sulfur components desorbed based on the temperature of the NOx storage reduction catalyst 220B increases, so even if the degree of richness is reduced. A predetermined amount of sulfur component can be desorbed. Further, when the degree of richness is large, the amount of sulfur components desorbed based on the degree of richness increases, so that a predetermined amount of sulfur components are desorbed even if the temperature of the NOx storage reduction catalyst 220B is relatively low. be able to.

Therefore, according to this aspect, even if the temperature of the NOx storage reduction catalyst 220B is different, a predetermined amount of sulfur component can be desorbed, and the sulfur desorption treatment can be performed more efficiently.

Since the temperature of the exhaust gas decreases toward the downstream side of the exhaust passage 130, the temperature of the NOx storage reduction catalyst 220B in the second stage is lower than the temperature of the NOx storage reduction catalyst 220A on the upstream side. Therefore, by controlling the value of the second air-fuel ratio based on the temperature of the NOx storage reduction catalyst 220B on the most downstream side, the sulfur component desorbed from the NOx storage reduction catalysts 220A and 220B provided in the exhaust passage 130 The amount can be controlled.

Further, the temperature sensor 240 was provided on the downstream side of the NOx storage reduction catalyst 220B in the second stage in the exhaust passage 130. However, as shown by the broken line in FIG. 7, the temperature sensor 240 is provided on the downstream side of the first-stage NOx storage-reduction catalyst 220A in the exhaust passage 130, and detects the temperature of the first-stage NOx storage-reduction catalyst 220A. You may. Then, the air-fuel ratio control unit 310 may control the value of the second air-fuel ratio according to the temperature of the first-stage NOx storage reduction catalyst 220A detected by the temperature sensor 240. In this case, the air-fuel ratio control unit 310 uses, for example, a table in which the temperature of the first-stage NOx storage-reduction catalyst 220A and the value of the second air-fuel ratio are associated with each other, and the first-stage NOx storage-reduction catalyst by the temperature sensor 240. The value of the second air-fuel ratio may be derived based on the temperature of 220A.

Further, the mode of deriving the value of the second air-fuel ratio from the temperature detected by the temperature sensor 240 is not limited to the mode using the second air-fuel ratio value table. For example, the value of the second air-fuel ratio may be derived using a relational expression in which the temperature detected by the temperature sensor 240 and the value of the second air-fuel ratio are associated with each other.

Further, in the exhaust gas purification device 202, the air-fuel ratio sensor 230 was not provided. However, in the exhaust gas purification device 202, an air-fuel ratio sensor 230 may be provided in addition to the temperature sensor 240.

Although the preferred embodiment of the present invention has been described above with reference to the accompanying drawings, it goes without saying that the present invention is not limited to such an embodiment. It is clear that a person skilled in the art can come up with various modifications or modifications within the scope of the claims, and it is understood that these also naturally belong to the technical scope of the present invention. Will be done.

For example, in the above embodiment, two NOx storage reduction catalysts 220A and 220B are provided in the exhaust passage 130. However, the number of NOx storage reduction catalysts provided in the exhaust passage 130 is not limited to two. For example, one NOx storage reduction catalyst may be provided in the exhaust passage 130, or three or more NOx storage reduction catalysts may be provided in the exhaust passage 130.

Further, even when there is only one NOx storage reduction catalyst, reattachment of the sulfur component may occur. For example, when the upstream portion of the NOx storage reduction catalyst has a rich atmosphere and the downstream portion thereof has a lean atmosphere, the sulfur component desorbed in the upstream portion of the NOx storage reduction catalyst is the NOx storage reduction catalyst. It is reattached in the downstream part. In the above embodiment, even in such a case, the sulfur desorption treatment can be efficiently performed.

Further, the three-way catalyst 210 may be composed of an OSC material. In the above embodiment, oxygen in the OSC material of the three-way catalyst 210 can be consumed at an early stage, and the sulfur desorption treatment can be efficiently performed.

Further, in the above embodiment, the mileage of the vehicle is set as a start condition of the sulfur desorption treatment. However, the starting condition of the sulfur desorption treatment is not limited to the mileage. For example, the start condition of the sulfur desorption treatment may be whether or not a speed equal to or higher than a predetermined speed continues for a predetermined time or longer (that is, whether or not high-speed operation is performed). When the high-speed operation is performed, the temperature of the exhaust gas is high, so that the temperatures of the NOx storage reduction catalysts 220A and 220B are high, and the process of raising the temperature of the NOx storage reduction catalysts 220A and 220B is omitted. Because it can be done. Further, the sulfur desorption treatment may be started when both the mileage and the high-speed operation are satisfied.

Further, in the above embodiment, the first air-fuel ratio is switched to the second air-fuel ratio according to the transition of the detected air-fuel ratio to rich. However, the switching condition for switching from the first air-fuel ratio to the second air-fuel ratio is not limited to this embodiment. For example, the switching condition may be whether or not a predetermined time has elapsed from the start of the sulfur desorption treatment (switching from lean to the first air-fuel ratio). The time required for the transition from lean to rich may be derived by experiments or simulations, and the derived time may be set as a predetermined time. According to the mode of switching according to the time, the air-fuel ratio sensor 230 can be omitted.

Further, in the above embodiment, the sulfur desorption treatment is completed according to the elapse of a predetermined time with respect to the time when the detected air-fuel ratio reaches the second air-fuel ratio. However, the conditions for terminating the sulfur desorption treatment are not limited to this embodiment. For example, the air-fuel ratio control unit 310 may end the sulfur desorption treatment according to the elapse of a predetermined time with respect to the time when the first air-fuel ratio is switched to the second air-fuel ratio.

Further, in the above embodiment, the three-way catalyst 210 is provided between the engine 120 in the exhaust passage 130 and the NOx storage reduction catalyst 220A. However, the three-way catalyst 210 may not be provided in the exhaust passage 130. In this case, the target air-fuel ratio is the air-fuel ratio of the exhaust gas on the upstream side of the NOx storage reduction catalyst 220A in the exhaust passage 130.

INDUSTRIAL APPLICABILITY The present invention can be used for an exhaust gas purifying device that purifies exhaust gas discharged from an engine.

120 Engine 130 Exhaust passage 200, 202 Exhaust gas purification device 220A, 220B NOx storage reduction catalyst 230 Air-fuel ratio sensor 310 Air-fuel ratio control unit

Claims (6)

A NOx storage reduction catalyst containing OSC material, which is provided in the exhaust path through which the exhaust gas discharged from the engine passes,
When the conditions for starting the sulfur desorption treatment of the NOx storage reduction catalyst are satisfied, the target air-fuel ratio, which is the air-fuel ratio on the upstream side of the NOx storage-reduction catalyst in the exhaust passage, is controlled to a rich first air-fuel ratio. An air-fuel ratio control unit that switches and controls the target air-fuel ratio to a second air-fuel ratio between the theoretical air-fuel ratio and the first air-fuel ratio according to the establishment of the target air-fuel ratio switching condition.
Exhaust gas purification device equipped with. The air-fuel ratio sensor for detecting the air-fuel ratio on the downstream side of the NOx storage reduction catalyst in the exhaust passage is provided.
The exhaust gas purification device according to claim 1, wherein the air-fuel ratio control unit switches the target air-fuel ratio from the first air-fuel ratio to the second air-fuel ratio based on the air-fuel ratio detected by the air-fuel ratio sensor. A plurality of the NOx storage reduction catalysts are provided in series in the exhaust passage.
The exhaust gas purification device according to claim 2, wherein the air-fuel ratio sensor is provided on the downstream side of the NOx storage reduction catalyst on the most downstream side in the exhaust passage. The exhaust gas purification device according to claim 1, wherein the air-fuel ratio control unit controls the time for maintaining the first air-fuel ratio according to the mileage of the vehicle to which the exhaust gas purification device is applied. The exhaust gas purification device according to any one of claims 1 to 4, wherein the air-fuel ratio control unit controls the time for maintaining the second air-fuel ratio according to the sulfur poisoning time of the NOx storage reduction catalyst. .. The exhaust gas purification device according to any one of claims 1 to 5, wherein the air-fuel ratio control unit controls the value of the second air-fuel ratio according to the temperature of the NOx storage reduction catalyst.

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