JP2007218177A - Exhaust gas purification system - Google Patents

Abstract

An exhaust gas purification system capable of further reducing NOx emissions mainly as compared with the prior art is provided.
An exhaust gas purification system according to the present invention includes a NOx adsorption / release material 2 capable of adsorbing and releasing NOx and a NOx reduction catalyst 1 capable of reducing NOx in an exhaust passage (downstream of an engine ENG) of an internal combustion engine. In the arranged exhaust gas purification system,
NOx discharged from the NOx reduction catalyst 1 is adsorbed by the NOx adsorption / release material 2, and NOx adsorbed by the NOx adsorption / release material 2 is released from the NOx adsorption / release material 2 under a predetermined condition, and the NOx is reduced by the NOx reduction catalyst. 1 (arrow R ₁₃ ).
Even under conditions where the NOx reduction catalyst 1 cannot purify NOx, NOx discharged from the NOx reduction catalyst 1 is once adsorbed to the NOx adsorption / release material 2 and then returned to the NOx reduction catalyst 1 again after being released. Purification efficiency is improved.
[Selection] Figure 1

Description

  The present invention relates to an exhaust gas purification system that purifies exhaust gas discharged from an internal combustion engine such as an automobile, and more particularly, to an exhaust gas purification system that mainly reduces emission of nitrogen oxides (NOx).

  Currently, three-way catalysts are widely used as catalysts for purifying nitrogen oxide (NOx), carbon monoxide (CO), and hydrocarbons (HC) in exhaust gas from automobiles and the like. The three-way catalyst is formed by supporting a noble metal on a porous oxide support and oxidizes and purifies HC and CO and reduces and purifies NOx. Since these reactions proceed most efficiently in an atmosphere in which an oxidizing component and a reducing component are present in approximately equivalent amounts, in a vehicle equipped with a three-way catalyst, the vicinity of the stoichiometric air-fuel ratio (Stoichi) (A / F = 14) The air-fuel ratio is controlled so that it is burned at about .6 ± 0.2).

  However, the three-way catalyst has a catalyst bed temperature below a temperature suitable for purifying NOx or the like after a cold start or after a certain period of idle time. It is thought that it is done. Moreover, since the load on the engine increases during acceleration, the amount of NOx and other emissions further increases. Therefore, for example, there is known an exhaust gas purification apparatus in which a NOx storage reduction catalyst capable of storing NOx and releasing / reducing together with a three-way catalyst is disposed.

Patent Document 1 discloses an exhaust mechanism that mainly includes an oxidation catalyst and a NOx absorber disposed downstream thereof. In this exhaust mechanism, NOx is absorbed by the NOx absorption device during idling, and NOx is released during acceleration from the traveling mode and / or idling to traveling. Therefore, although the NOx emission amount changes according to the traveling state, the absolute amount of NOx emission amount does not decrease.
Special table 2003-536012 gazette

  As described above, by combining a three-way catalyst with a NOx occlusion reduction catalyst or the like, NOx emissions are reduced to some extent as compared to using a three-way catalyst alone, but further reduction is required. In view of the above situation, an object of the present invention is to provide an exhaust gas purification system capable of further reducing the amount of NOx emission mainly compared to the conventional art.

The exhaust gas purification system of the present invention is an exhaust gas purification system in which a NOx adsorption / release material capable of adsorbing and releasing NOx and a NOx reduction catalyst capable of reducing NOx are disposed in an exhaust passage of an internal combustion engine.
NOx discharged from the NOx reduction catalyst is adsorbed by the NOx adsorption / release material, and NOx adsorbed by the NOx adsorption / release material is released from the NOx adsorption / release material under a predetermined condition to reduce the NOx to the NOx reduction. It is characterized by being delivered to the catalyst.

  With this configuration, even under conditions where NOx cannot be purified by the NOx reduction catalyst, NOx discharged from the NOx reduction catalyst is once adsorbed to the NOx adsorption / release material and then returned to the NOx reduction catalyst again after being released. Purification efficiency is improved.

  Furthermore, it is desirable to have temperature control means for adjusting the temperature of the incoming gas flowing into the NOx adsorption / release material.

  According to the exhaust gas purification system of the present invention, when the NOx reduction catalyst has not reached a temperature (activation temperature) suitable for reducing NOx (hereinafter, such a state is referred to as “catalyst inactive” and vice versa. When the state is abbreviated as “catalytic activity”), NOx contained in the exhaust gas from the internal combustion engine is discharged without being reduced by the NOx reduction catalyst. The NOx discharged from the NOx reduction catalyst is adsorbed by the NOx adsorption / release material. Further, the adsorbed NOx is released from the NOx adsorption / release material under a predetermined condition. In the present invention, this NOx is sent to the NOx reduction catalyst. When NOx is released from the NOx adsorption / release material, the NOx reduction catalyst is already in the catalytically active state. Therefore, the NOx sent to the NOx reduction catalyst is reduced and purified by the NOx reduction catalyst and discharged to the outside. In addition, when the internal combustion engine is left idle for a long time and the NOx reduction catalyst becomes inactive, NOx is not reduced but discharged from the NOx reduction catalyst. Even if NOx is discharged from the NOx reduction catalyst, the NOx adsorption / release material is in a state in which NOx can be adsorbed again. Therefore, NOx discharged without being reduced is adsorbed by the NOx adsorption / release material. That is, most NOx circulates through the NOx adsorption / release material and the NOx reduction catalyst until it is reduced. Therefore, the amount of NOx contained in the exhaust gas finally discharged to the outside is reduced.

  Moreover, NOx is adsorbed or released efficiently by adjusting the temperature of the exhaust gas flowing into the NOx adsorption / release material using the temperature control means. As a result, the amount of NOx contained in the exhaust gas finally discharged to the outside is further reduced.

  In order to describe the exhaust gas purification system of the present invention in more detail, the best mode for carrying out the present invention will be described below.

  The exhaust gas purification system of the present invention mainly has a NOx adsorption / release material capable of adsorbing / releasing NOx and a NOx reduction catalyst capable of reducing NOx. The NOx adsorption / release material and the NOx reduction catalyst may be provided in the exhaust passage of the internal combustion engine, for example, on the downstream side (exhaust system side) of the engine.

The NOx reduction catalyst is not particularly limited as long as at least NOx can be reduced. For example, a three-way catalyst that oxidizes and purifies HC and CO and reduces and purifies NOx may be used. The three-way catalyst may be a three-way catalyst that is generally used at present. Specifically, a porous oxide carrier such as alumina (Al ₂ O ₃ ), ceria (CeO ₂ ), zirconia (ZrO ₂ ), ceria-zirconia solid solution, platinum (Pt), rhodium (Rh), palladium ( It is preferable to carry a noble metal such as Pd). Further, such a NOx reduction catalyst may have a composition exhibiting catalytic activity at 250 ° C. or higher.

  Each of these components is preferably supported on a carrier substrate. For the carrier substrate, a conventionally used ceramic or metal honeycomb-like structure may be used. In order to carry each component on the carrier substrate, for example, a honeycomb structure is washed with a slurry containing a porous oxide and an added oxide, and then fired to form a coating layer. The noble metal may be supported on the layer by an adsorption support method or a water absorption support method. Alternatively, a catalyst powder in which a noble metal is previously supported on a porous oxide may be prepared, and an additive oxide may be mixed with the catalyst powder to form a coating layer.

  The NOx adsorption / release material is not particularly limited as long as it can adsorb NOx and release adsorbed NOx, and may be capable of adsorbing / releasing CO and the like in addition to NOx. A NOx adsorption / release material having the property of adsorbing NOx when the temperature of the NOx adsorption / release material is lower than a specific temperature and desorbing and releasing NOx above the specific temperature is preferable. Examples of the NOx adsorption / release material having such properties include various zeolites and ceria supporting transition metals such as platinum (Pt), rhodium (Rh), palladium (Pd), silver (Ag) and noble metals such as iron. Can be mentioned. For example, if the NOx adsorption / release material is ceria carrying Pd, NOx is adsorbed at 200 ° C. or lower, and NOx is desorbed when it exceeds 250 ° C. Further, the NOx adsorption / release material may have a performance of adsorbing NOx of exhaust gas in a lean state where the air-fuel ratio of inflowing exhaust gas is high, and releasing adsorbed NOx in a stoichiometric to rich state where the air-fuel ratio is low. Furthermore, the NOx adsorption / release material may be a NOx occlusion / reduction catalyst that occludes NOx in a lean state and releases NOx in a stoichiometric to rich state and reduces it to nitrogen.

  In addition, also in the NOx adsorption / release material, it is preferable that each component is supported on the carrier base material in the same manner as the NOx reduction catalyst described above.

  In the exhaust gas purification system of the present invention, NOx discharged from the NOx reduction catalyst is adsorbed by the NOx adsorption / release material. If the NOx reduction catalyst is catalytically active, NOx is reduced and purified by the NOx reduction catalyst. However, when the NOx reduction catalyst is in an inactive state, NOx contained in the exhaust gas flowing into the NOx reduction catalyst is discharged without being purified by the NOx reduction catalyst. This NOx is adsorbed by the NOx adsorption / release material.

  The NOx adsorbed on the NOx adsorption / release material is released from the NOx adsorption / release material under a predetermined condition. The predetermined condition under which NOx is released from the NOx adsorption / release material is that the temperature of the NOx adsorption / release material rises above a specific temperature. In addition, exhaust gas from an engine whose air-fuel ratio is controlled to be stoichiometric to rich is directly flowed into the NOx adsorption / release material, or part of exhaust gas purified by the NOx reduction catalyst in the vicinity of stoichiometry is not exhausted to the outside. The air-fuel ratio may be made to be in a stoichiometric or rich state by a method such as inflowing into the NOx adsorption / release material.

  In the exhaust gas purification system of the present invention, NOx released from the NOx adsorption / release material is sent to the NOx reduction catalyst. The NOx released from the NOx adsorption / release material may be sent directly to the NOx reduction catalyst, or may be sent to the upstream side (intake system side) of the engine and indirectly sent to the NOx reduction catalyst.

  If the NOx reduction catalyst is catalytically active, the delivered NOx is reduced and purified by the NOx reduction catalyst. The exhaust gas that has been purified to reduce the amount of NOx is exhausted to the outside in whole or in part. In addition, as already described, a part thereof may flow into the NOx adsorption / release material.

  On the other hand, if the NOx reduction catalyst is inactive, NOx is not reduced but discharged from the NOx reduction catalyst. As described above, NOx discharged from the NOx reduction catalyst is adsorbed by the NOx adsorption / release material. That is, NOx that is not purified circulates through the exhaust gas purification system of the present invention until it is purified. Thus, the amount of NOx contained in the exhaust gas discharged to the outside is reduced.

  As described above, the adsorption / release action of the NOx adsorption / release material depends on the temperature condition. Therefore, the exhaust gas purification system of the present invention may further include a temperature control means for adjusting the temperature of the entering gas of the exhaust gas flowing into the NOx adsorption / release material. The inlet gas temperature may be adjusted based on the temperature of the detected / estimated NOx adsorbing / releasing material by installing a temperature sensor or the like on the NOx adsorbing / releasing material or estimating from the operating state.

  There is no particular limitation on the arrangement of the NOx reduction catalyst, the NOx adsorption / release material, the pipe connecting these, and the valve body for opening and closing the pipe for implementing the exhaust gas purification system of the present invention described in detail above. Exhaust gas purification can be efficiently performed by performing optimal arrangement and control of the valve body in accordance with the type, operating condition, catalyst performance, and the like of the internal combustion engine. As will be described in detail in the section of the embodiment described later, as the exhaust gas purification system of the present invention, the exhaust gas purification system described in the system diagrams of FIGS. 1 to 4 can be used as an example of the exhaust gas path.

For example, in FIGS. 1 and 2, the downstream of the NOx reduction catalyst (abbreviated as “catalyst 1”) is branched into two, and the exhaust gas discharged from the catalyst 1 is discharged from the discharge port (R ₁₁ and R ₂₁ : R _XX is a symbol in the figure) and a path (R ₁₂ and R ₂₂ ) that flows into the NOx adsorption / release material (abbreviated as “NOx absorption / release material 2”). Although having a path for transmitting the NOx released from the NOx-absorbing material 2 into the catalyst 1 (R ₁₃ and R _23), the route R ₁₃ and route R _23, the engine as shown in FIGS. 1 and 2 NOx may be sent to the upstream side (intake system side) of (ENG), or NOx may be sent to the downstream side of the engine (exhaust system side) and upstream of the catalyst 1. Further, as shown in FIG. 2, a cooler 3 may be provided as a temperature adjusting means before the NOx absorption / release material 2 in order to improve the adsorption efficiency of the NOx absorption / release material 2.

3 and 4, the first path (R ₃₀ → (catalyst 1) → R ₃₁ ) in which exhaust gas discharged from the engine (ENG) passes through the catalyst 1 and the NOx absorbent / release material 2 in this order is discharged. → (NOx-absorbing material 2) → R ₃₂ and R ₄₀ → (catalyst 1) → R ₄₁ → (NOx-absorbing material 2) → addition to R _42), exhaust gas discharged from the engine NOx absorbing material 2 The second path (R ₃₀ → R ₃₆ → (NOx absorption / release material 2) → R ₃₇ → (catalyst 1) → R ₃₈ → R ₃₂ and R ₄₀ → R ₄₆ that can pass through the catalyst 1 after passing through → (NOx absorption / release material 2) → R ₄₇ → (catalyst 1) → R ₄₈ → R ₄₂ ) The first path is used when NOx discharged from the catalyst 1 is adsorbed by the NOx adsorbing / releasing material 2. The second path is used when the catalyst 1 purifies NOx released from the NOx absorbing / releasing material 2. Moreover, as shown in FIG. 4, in order to improve the adsorption efficiency of the NOx absorbing / releasing material 2, a cooler 3 may be provided as a temperature adjusting means before the NOx absorbing / releasing material 2.

  The exhaust gas purification system of the present invention is not limited to the above embodiment. For example, as long as the effect of the exhaust gas purification system of the present invention is not impaired, another configuration may be added as necessary to add other functions.

  Hereinafter, the Example of the exhaust gas purification system of this invention is described concretely with a comparative example using FIGS.

[Example 1]
In this embodiment, as shown in the system diagram of FIG. 1, a three-way catalyst (catalyst 1) and a NOx adsorption / release material (NOx adsorption / release material 2) are arranged on the downstream side of the engine (ENG). The engine is connected to the catalyst 1 via a pipe 10. The downstream side of the catalyst 1 is branched in two directions of the pipe 11 or the pipe 12 in a flow path switching valve V ₁₁ (hereinafter abbreviated as “valve V _XX ”). The pipe 11 is connected to an exhaust side that discharges exhaust gas into the atmosphere. On the other hand, the pipe 12 is connected to the NOx absorbing / releasing material 2. Downstream of the NOx-absorbing material 2 is branched into two directions that communicates with the pipe 13 or the exhaust side in the valve V _12. The pipe 13 is connected to the upstream (intake system side) of the engine.

  The catalyst 1 was prepared by washing a slurry of a predetermined amount of alumina and silica-zirconia composite oxide onto a cordierite-based honeycomb carrier base having a capacity of 1.1 L, drying and firing, platinum (Pt) and rhodium ( An aqueous solution containing Rh) was used, and a three-way catalyst on which Pt and Rh were adsorbed and supported was used. The precious metal loading per liter of the carrier substrate was 1.5 g / L for Pt and 0.4 g / L for Rh. The three-way catalyst obtained by the above procedure was used as the catalyst 1 after enduring at 800 ° C. for 50 hours with an actual gas.

  The NOx adsorbing / releasing material 2 was formed by forming a washcoat layer made of ceria on a cordierite honeycomb carrier base material having a capacity of 0.9 L, and then adsorbing and supporting palladium (Pd). The amount of Pd supported per liter of the carrier substrate was 1.0 g / L.

  A temperature suitable for reducing NOx by the catalyst 1 was an activation temperature Tα = 300 ° C. or higher. The temperature at which NOx was easily desorbed by the NOx absorbing / releasing material 2 was higher than Tβ = 350 ° C.

Further, the catalyst 1 and the NOx adsorbing / releasing material 2 have temperature sensors T ₁₁ and T ₁₂ for measuring their internal temperatures, respectively. The exhaust gas purification system of this embodiment operates by controlling the valves V ₁₁ and V ₁₂ to open and close according to the temperatures detected by the sensors T ₁₁ and T ₁₂ . FIG. 6 is a flowchart illustrating control of the exhaust gas purification system according to the first embodiment. Below, the flowchart of FIG. 6 is demonstrated with the systematic diagram of FIG.

In this embodiment, whether or not the catalyst 1 is catalytically active is determined based on the temperature of the catalyst 1. That is, if it is determined in step 100 that the temperature of the sensor T ₁₁ is lower than the activation temperature Tα of the catalyst 1 (T ₁₁ <Tα), the catalyst 1 is inactive, so in step 111A, the valve V ₁₁ causes all exhaust gas that has passed through the catalyst 1 to flow into the NOx adsorbent 2. Therefore, in step 112A to step 114A, NOx released from the catalyst 1 is adsorbed by the NOx adsorbent / release material 2. The exhaust gas from the NOx-absorbing material 2 is discharged to the outside from the discharge port by the valve V _12. (See FIG. 5 <1-A> above.)

On the other hand, if it is determined in step 100 that the temperature of the sensor T ₁₁ is equal to or higher than the activation temperature Tα of the catalyst 1, the catalyst 1 is catalytically active and NOx can be reduced. At this time, in step 120, it is determined whether or not the NOx absorbent / release material 2 is adsorbing NOx. If NOx adsorbing / releasing material 2 is adsorbing NOx and the amount of NOx desorbed in step 121B is equal to or smaller than the capacity (t2) that NOx adsorbing / releasing material 2 can be adsorbed, desorption time is determined in step 122B. with adding counter, valve V ₁₁ and a valve V ₁₂ is controlled at step 123B. That is, the exhaust gas NOx discharged from the catalyst 1 is cleansed, a portion flows through the pipe 11 (bypass side) by the valve V _11, and is discharged to the outside from the exhaust system. Further, the exhaust gas flowing through the pipe 12 by the valve V ₁₁ is, NOx adsorbed on the NOx absorption and release material 2 with flowing into the NOx-absorbing material 2 is released is released. Exhaust gas containing NOx released from the NOx-absorbing material 2 is through a pipe 13 by the valve V ₁₂ is delivered to the upstream side of the engine (intake system side). (See FIG. 5 <1-B> above)

In step 100, the catalyst 1 is catalytically active and the NOx adsorbing material 2 is not adsorbing NOx in step 120, or the NOx adsorbed on the NOx adsorbing material 2 in step 121B is all desorbed. if state discharged, the exhaust gas NOx is purified to be discharged from the catalyst 1 and the valve V ₁₁ at step 126C, the total amount flows through the pipe 11 (bypass side) by the valve V _11, and out of the exhaust system Is done. (Refer to FIG. 5 <1-C> above.)

[Example 2]
In the present embodiment, as shown in the system diagram of FIG. 2, a pipe 24 branched from the valve V ₂₁ is provided between the valve V ₂₁ (corresponding to the valve V ₁₁ of the first embodiment) and the NOx absorbing / releasing material 2. the condenser 3 is arranged, also, except that a sensor T ₂₃ between the catalyst 1 and the valve V ₂₁ is a view similar to the exhaust gas purifying system of example 1. FIG. 8 is a flowchart showing the control of the exhaust gas purification system of the second embodiment. Below, the flowchart of FIG. 8 is demonstrated with the systematic diagram of FIG.

In this embodiment, the pipes 20 to 23 correspond to the pipes 10 to 13 of the first embodiment, the sensors T ₂₁ and T ₂₂ are sensors T ₁₁ and T ₁₂ , and the valves V ₂₁ and V ₂₂ are valves V ₁₁ and V _22. Each corresponds to ₁₂ . In FIG. 8, steps 220, 221C to 223C (corresponding to FIG. 7 <2-C>) and 226D to 228D (corresponding to FIG. 7 <2-D>) are the same as steps 120, 121B of the first embodiment. It is the same as 123B and 126C-128C.

If it is determined in step 200 that the temperature of the sensor T ₂₁ is lower than the activation temperature Tα of the catalyst 1 (T ₂₁ <Tα), the catalyst 1 is inactive. Next, when the temperature of the sensor T ₂₃ is determined to be equal to or less than T [beta in step 210, since the gas temperature entering into the NOx-absorbing material 2 tends NOx is adsorbed in the following T [beta, in step 211A, the valve V ₂₁ Thus, all exhaust gas that has passed through the catalyst 1 is caused to flow into the NOx absorption / release material 2. Therefore, in step 212A to step 214A, NOx released from the catalyst 1 is adsorbed by the NOx adsorbing / releasing material 2. (Refer to Fig. 7 <2-A>.)

On the other hand, if in step 210 the temperature of the sensor T ₂₃ is determined to be high (T _23> Tβ) than T [beta, since NOx easy desorbed when the gas temperature entering into the NOx-absorbing material 2 is more than T [beta, step All the exhaust gas that has passed through the sensor T ₂₃ by the valve V ₂₁ at 211B to flow into the cooler 3. The exhaust gas that has passed through the cooler 3 flows into the NOx absorption / release material 2 after the temperature of the incoming gas is adjusted to Tβ or lower. In step 212A to step 214A, NOx adsorption is performed by the NOx absorbent material 2. (Refer to FIG. 7 <2-B> above.)

  Note that step 210 and step 211A can be omitted, and when NOx is adsorbed by the NOx absorbing / releasing material 2, the temperature of the entering gas may always be passed through the cooler 3 to adjust the temperature.

[Example 3]
In this embodiment, as shown in the system diagram of FIG. 3, the three-way catalyst (catalyst 1) and the NOx adsorption / release material (NOx adsorption / release material 2) are arranged on the downstream side of the engine (ENG). The engine is connected to the catalyst 1 through a pipe 30. The pipe 30 is branched to the pipe 36 at the valve V ₃₁ . The catalyst 1 is connected to the NOx absorption / release material 2 through a pipe 31 (37). Downstream of the NOx-absorbing material 2 is communicated with the exhaust side, the pipe 36 between the NOx-absorbing material 2 and the outlet is connected via a valve V _34. Further, the pipe 31 (37), the process branches to pipes 33 in the valve V _33. The pipe 33 communicates with a pipe 38 branched at a valve V ₃₂ disposed between the valve V ₃₁ and the catalyst 1 on the pipe 30, and the other end of the pipe 38 is between the valve V ₃₄ and the exhaust side. Communicate with

Further, the catalyst 1 and the NOx adsorbing / releasing material 2 have temperature sensors T ₃₁ and T ₃₂ for measuring their internal temperatures, respectively. The valves V _{31 to} V ₃₄ are controlled to open and close according to the temperatures detected by the sensors T ₃₁ and T ₃₂ . FIG. 10 is a flowchart illustrating control of the exhaust gas purification system according to the third embodiment. Below, the flowchart of FIG. 10 is demonstrated with the systematic diagram of FIG.

In this embodiment, whether or not the catalyst 1 is catalytically active is determined based on the temperature of the catalyst 1. That is, if it is determined in step 300 that the temperature of the sensor T ₃₁ is lower than the activation temperature Tα of the catalyst 1 (T ₃₁ <Tα), the catalyst 1 is inactive, so in step 311A the valve V All exhaust gas that has passed through the catalyst 1 by the valve ₃₁ and the valve V _{32 is} caused to flow into the NOx absorption / release material 2 by the valve V ₃₃ . Therefore, in step 312A to step 314A, NOx released from the catalyst 1 is adsorbed by the NOx adsorbing / releasing material 2. The exhaust gas from the NOx-absorbing material 2 is discharged to the outside from the exhaust system by the valve V _34. (Refer to Fig. 9 <3-A>.)

On the other hand, the temperature of the sensor T ₃₁ in step 300, when it is determined that the activation temperature Tα more catalysts 1, catalyst 1 is a catalyst activity, a reduction state capable of NOx. At this time, in step 320, it is determined whether or not the NOx adsorbing / releasing material 2 is adsorbing NOx. If NOx adsorbing / releasing material 2 is adsorbing NOx and the amount of NOx desorbed in step 321B is equal to or less than the capacity (t2) that NOx adsorbing / releasing material 2 can adsorb, desorption time is determined in step 322B. While adding the counter, each valve is controlled in step 323B. That is, the exhaust gas from the engine, the piping 30, the valve V _31, NOx adsorbed by the NOx absorption and release material 2 with through a pipe 36 to flow directly into NOx-absorbing material 2 and the valve V ₃₄ is released is released. Exhaust gas containing NOx released from the NOx-absorbing material 2, through a pipe 37 flows into the catalyst 1, and is then discharged to the outside from the discharge port through a pipe 38 by the valve V _32. (See FIG. 9 <3-B> above)

In step 300, the catalyst 1 is catalytically active and the NOx adsorbing material 2 is not adsorbing NOx in step 320, or the NOx adsorbed on the NOx adsorbing material 2 in step 321B is all desorbed. If so, the exhaust gas from which the NOx discharged from the catalyst 1 by the valve V ₃₁ and the valve V ₃₂ has been purified in step 326C is entirely discharged through the pipe 33 by the valve V ₃₃ and discharged out of the exhaust system. Is done. (Refer to FIG. 5 <1-C> above.)

[Example 4]
In this embodiment, as shown in the system diagram of Figure 4, it established a valve V ₄₅ between the valve V ₄₃ (corresponding to the valve V ₃₃ of Example 3) and NOx-absorbing material 2, the valve V ₄₅ The exhaust gas purification system is the same as that of the third embodiment except that the cooler 3 is arranged via the branching pipe 45 and the sensor T ₄₃ is provided between the valve V ₄₃ and the valve V ₄₅ . FIG. 12 is a flowchart illustrating the control of the exhaust gas purification system according to the fourth embodiment. Below, the flowchart of FIG. 12 is demonstrated with the systematic diagram of FIG. 12, steps 420, 421C to 423C (corresponding to FIG. 11 <4-C>) and 426D to 428D (corresponding to FIG. 11 <4-D>) are the same as steps 320, 311B of Example 3. It is the same as 313B and 321C-323C.

If it is determined in step 400 that the temperature of the sensor T ₄₁ is lower than the activation temperature Tα of the catalyst 1 (T ₄₁ <Tα), the catalyst 1 is inactive. Next, when the temperature of the sensor T ₄₃ is determined to be equal to or less than T [beta in step 410, since the gas temperature entering into the NOx-absorbing material 2 tends NOx is adsorbed in the following T [beta, in step 411A, the valve V ₄₁ All exhaust gas that has passed through the catalyst 1 by the valve V _{42 is} caused to flow into the NOx adsorbing / releasing material 2 by the valve V ₄₃ and the valve V ₄₅ . Therefore, in step 412A to step 414A, the NOx released from the catalyst 1 is adsorbed by the NOx adsorbing / releasing material 2. Then, the exhaust gas from the NOx-absorbing material 2 is discharged to the outside from the exhaust system by the valve V _44. (See FIG. 11 <4-A> above)

On the other hand, if it is determined in step 410 that the temperature of the sensor T ₄₃ is higher than Tβ (T ₄₃ > Tβ), NOx tends to be desorbed if the temperature of the gas entering the NOx absorbent material 2 exceeds Tβ. All the exhaust gas that has passed through the sensor T ₄₃ by the valve V ₄₅ is flowed into the cooler 3 in 411B. The exhaust gas that has passed through the cooler 3 flows into the NOx absorption / release material 2 after the temperature of the incoming gas is adjusted to Tβ or lower. Then, in step 412A to step 414A, NOx adsorption is performed by the NOx absorbing / releasing material 2. (See FIG. 11 <4-B> above.)

  Note that step 410 and step 411A can be omitted, and when NOx is adsorbed by the NOx absorbing / releasing material 2, the temperature of the entering gas may always be passed through the cooler 3 to adjust the temperature.

[Comparative Example 1]
The exhaust gas purification system is the same as that of the first embodiment except that the NOx absorbing / releasing material 2 is not used. That is, all the exhaust gas from the engine was caused to flow into the catalyst 1, and the gas discharged from the catalyst 1 was discharged outside from the discharge port.

[Evaluation]
About the exhaust gas purification system of Examples 1-4 and the comparative example 1, the purification performance evaluation test using an evaluation system was done. As an evaluation system, an evaluation system simulating an actual vehicle with an engine with a displacement of 2.4 L, each exhaust gas purification system, and a low inertia dynamo was used. In this test, the average purification rate was calculated from the exhaust gas when the vehicle was driven for 10 minutes at a speed of 60 km / h, held in an idle state for 10 minutes, and accelerated at a speed of 60 km / h. The results are shown in FIG.

  In Comparative Example 1 using only the three-way catalyst, the average purification rate was 70%, whereas in Examples 1 to 4, the NOx discharged from the three-way catalyst 1 and adsorbed on the NOx absorbent / release material 2 was reduced. By returning to the three-way catalyst 1 again, the purification rate was improved. Furthermore, by adjusting the temperature of the gas entering the NOx absorption / release material 2 with the cooler 3, the average purification rate exceeded 90%, and the purification efficiency was further improved.

1 is a system diagram of an exhaust gas purification system of Example 1. FIG. It is a systematic diagram of the exhaust gas purification system of Example 2. It is a systematic diagram of the exhaust gas purification system of Example 3. It is a systematic diagram of the exhaust gas purification system of Example 4. It is a systematic diagram of the exhaust gas purification system of Example 1, Comprising: It is a systematic diagram which shows the flow path of exhaust gas. It is a flowchart which shows control of the exhaust gas purification system of Example 1. FIG. It is a systematic diagram of the exhaust gas purification system of Example 2, Comprising: It is a systematic diagram which shows the flow path of exhaust gas. It is a flowchart which shows control of the exhaust gas purification system of Example 2. It is a systematic diagram of the exhaust gas purification system of Example 3, Comprising: It is a systematic diagram which shows the flow path of exhaust gas. 6 is a flowchart illustrating control of an exhaust gas purification system according to a third embodiment. It is a systematic diagram of the exhaust gas purification system of Example 4, Comprising: It is a systematic diagram which shows the flow path of exhaust gas. 6 is a flowchart illustrating control of an exhaust gas purification system according to a fourth embodiment. It is a graph which shows the result of the evaluation test using the evaluation system which simulated the real vehicle, Comprising: The average purification rate of the exhaust gas purification system of an Example and a comparative example is shown.

Explanation of symbols

1: Three-way catalyst (NOx reduction catalyst)
2: NOx absorption / release material (NOx adsorption / release material)
3: Cooler (temperature control means)
V: Valve (valve)
T: Temperature sensor ENG: Engine

Claims (3)

In an exhaust gas purification system in which a NOx adsorption / release material capable of adsorbing and releasing NOx and a NOx reduction catalyst capable of reducing NOx are disposed in an exhaust passage of an internal combustion engine,
NOx discharged from the NOx reduction catalyst is adsorbed by the NOx adsorption / release material, and NOx adsorbed by the NOx adsorption / release material is released from the NOx adsorption / release material under a predetermined condition to reduce the NOx to the NOx reduction. An exhaust gas purification system characterized by being sent to a catalyst.   The exhaust gas purification system according to claim 1, wherein the NOx reduction catalyst is a three-way catalyst that oxidizes and purifies HC and CO and reduces NOx.   The exhaust gas purification system according to claim 1, further comprising a temperature control unit that adjusts a temperature of an incoming gas flowing into the NOx adsorption / release material.

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