JP2008240640A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of further surely restraining emission of NOx to an external part included in exhaust gas, by securing adsorbing performance of an NH<SB>3</SB>adsorbent. <P>SOLUTION: This exhaust emission control device is provided with an NOx catalyst device 54 including an NOx adsorbent for adsorbing NOx included in the exhaust gas of an oxygen excessive atmosphere and the zeolite-based NH3 adsorbent for adsorbing NH<SB>3</SB>formed by converting this adsorbed NOx and purifying the NOx by reacting the exhausted NOx with the adsorbed NH<SB>3</SB>when the exhaust gas becomes the oxygen excessive atmosphere again, a catalyst regeneration control means 74 changing the air-furl ratio of the exhaust gas to the air-fuel ratio capable of regenerating at least the NH<SB>3</SB>adsorbent for regenerating the NOx catalyst device 54, and an NOx concentration detecting means 37 detecting the concentration of the NOx flowing out of the NOx catalyst device 54. The catalyst regeneration control means 74 is constituted so as to start regeneration of the NOx catalyst device 54 based on the concentration of the NOx detected by the NOx concentration detecting means 37. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification device for suppressing emission of NOx contained in engine exhaust gas to the outside.

As an exhaust gas purifying apparatus as described above, in an exhaust passage of an engine, provided with a NOx catalyst device having a suction capable NH ₃ adsorbing material and NH ₃ generated from adsorbable NOx adsorbent and the NOx of NOx, the NH It is known that the NOx is reduced and purified by NH ₃ adsorbed on the _three adsorbents. In this apparatus, in an oxygen deficient atmosphere to convert the NOx into the NH _3, allowed to adsorb NH _3, which is the converted to the NH ₃ adsorbent. Then, NOx is adsorbed on the NOx adsorbent in an oxygen-excess atmosphere, and the adsorbed NOx is reduced and purified with NH ₃ .

As an exhaust gas purifying apparatus using such a NOx catalyst device, for example, in Patent Document 1, the amount of NOx adsorbed on the NOx adsorbent is estimated, and if the estimated value becomes a predetermined value or more, the combustion is performed. An apparatus that additionally generates the NH ₃ by controlling the air-fuel ratio of the chamber to a rich state is disclosed. In this device, the amount of NOx discharged from the engine is estimated based on the engine speed and the accelerator opening, and the NOx amount reduced and purified by the NH _{3 is} determined based on the temperature in the NOx catalyst device, the engine speed, and Estimation is made based on the accelerator opening, and the amount of NOx adsorbed on the NOx adsorbent is estimated based on these estimated values.
Japanese Patent Laid-Open No. 2005-214098

Here, zeolite is often used as the NH ₃ adsorbent. However, since this zeolite has many active sites called acid sites in the crystal, not only the NH ₃ but also HC components in the fuel contained in the exhaust gas are adsorbed. If the adsorbed HC component is deposited, the zeolite is coked, and NH ₃ cannot be sufficiently adsorbed on the NH ₃ adsorbent, and the NOx may not be sufficiently reduced with NH ₃ .

However, in the apparatus disclosed in the Patent Document 1, no consideration for the reduction of the NH ₃ adsorption capacity of the NH ₃ adsorbent by the caulking or the like. Therefore, the estimation error of the NOx amount that is reduced and purified by NH _{3 and} thus the estimation error of the NOx amount adsorbed on the NOx adsorbent become large, and there is a possibility that the NH ₃ cannot be additionally generated appropriately. Further, in the apparatus, since the regeneration of the NH ₃ adsorbent is not performed, the adsorption performance of the NH ₃ adsorbent is still deteriorated and the NH ₃ cannot be sufficiently adsorbed, and as a result, NOx is sufficiently reduced. It may become impossible to purify.

The present invention has been made in view of the circumstances as described above, and is an exhaust gas that can secure the adsorption performance of the NH ₃ adsorbent and more reliably suppress the discharge of NOx contained in the exhaust gas to the outside. The purpose is to provide a purification device.

In order to solve the above-described problems, the present invention provides an exhaust gas purification apparatus for a lean burn engine, which is provided in an exhaust passage of an engine, and contains NOx contained in exhaust gas in an oxygen-excess atmosphere exhausted from the combustion chamber of the engine. contains a zeolitic NH ₃ adsorbent for adsorbing NH ₃ to NOx adsorbed on the NOx adsorbent and the NOx adsorbent that adsorbs is being converted in an oxygen deficient atmosphere, the exhaust gas again becomes oxygen-excess atmosphere The NOx catalyst device that purifies the NOx by reacting NOx discharged when the engine operating conditions are switched and the adsorbed NH _3, and the exhaust gas for regenerating the NOx catalyst device Catalyst regeneration control means for changing the air-fuel ratio of the engine to at least an air-fuel ratio capable of regenerating the NH ₃ adsorbent, and an exhaust passage of the engine. A NOx concentration detecting means provided downstream of the NOx catalyst device for detecting the concentration of NOx flowing out from the NOx catalyst device, wherein the catalyst regeneration control means is detected by the NOx concentration detecting means. An exhaust gas purification device is provided, wherein regeneration of the NOx catalyst device is started based on the concentration of the NOx (Claim 1).

According to the present invention, since the NOx concentration detecting means capable of detecting the NOx concentration is provided downstream of the NOx catalyst device, the change in the NOx purification performance by the NOx catalyst device can be grasped more accurately. Since the regeneration of the NOx catalyst device is started based on the NOx concentration directly detected by the NOx concentration detection means, the purification performance of the NOx catalyst device can be regenerated more reliably and efficiently. In particular, the at catalyst regeneration control means, so that by performing the regeneration of NH ₃ adsorbent holds the air-fuel ratio of the exhaust gas can be air-fuel ratio reproduction of at least the NH ₃ adsorbing material, the NH ₃ adsorbent to be able to adsorb the sufficient amount of NH ₃ for reducing NOx, the NH ₃ adsorbed in the NH ₃ adsorbing material can be more reliably reduced and purified NOx.

In the present invention, the catalyst regeneration control means includes means for regenerating the NH ₃ adsorbent by removing coking of the NH ₃ adsorbent, and at least a part of the regeneration period of the NOx catalyst device. It is preferable that the air-fuel ratio of the exhaust gas is made richer than that during normal operation and leaner than the stoichiometric air-fuel ratio during the period. ).

In this way, the air-fuel ratio of the exhaust gas is kept leaner than the stoichiometric air-fuel ratio, the NH ₃ adsorbent is exposed to an oxygen-excess atmosphere, and the air-fuel ratio is kept richer than during normal operation. in if the temperature of the NOx catalyst device Atsushi Nobori, the NH ₃ carbon component attached can skip easily shrink the adsorbent, it is possible to easily caulk removal of NH ₃ adsorbent . Then, if it is possible to carry out the coking removal of the thus NH ₃ adsorbent, it is possible to secure the NH ₃ adsorption capacity of the NH ₃ adsorbing material, it is possible to reduce and purify NOx more reliably.

  Further, in the present invention, the NOx adsorbent adsorbs the sulfur component contained in the exhaust gas in an oxygen-excess atmosphere having a high oxygen concentration, and discharges the sulfur component as the oxygen concentration decreases, The catalyst regeneration control means includes means for regenerating the NOx adsorbent by desorbing the sulfur component from the NOx adsorbent, and for desorbing the sulfur component from the NOx adsorbent. Preferably, the exhaust gas air-fuel ratio is set to a poisoning recovery air-fuel ratio richer than the stoichiometric air-fuel ratio during at least a part of the regeneration period of the catalyst device.

If it does in this way, since the poisoning by the sulfur component of the NOx adsorbent can be recovered and the NOx adsorbing performance of the NOx adsorbent can be ensured, the emission of NOx to the outside can be more reliably suppressed. It becomes possible. In particular, during the regeneration period of the NOx catalyst device, coking removal of the NH ₃ adsorbent is performed, and the temperature in the NOx catalyst device is maintained at a high temperature. If the air-fuel ratio is kept on the rich side, the fuel / fuel ratio is reduced more efficiently and the sulfur component of the NOx adsorbent is more efficiently consumed than when the air-fuel ratio is made rich for only removing sulfur at a different time regardless of coking removal. The poisoning can be recovered.

Here, when the regeneration of the NH ₃ adsorbent and the regeneration of the NOx adsorbent are performed during the regeneration period of the NOx catalyst device as described above, the air-fuel ratio of the exhaust gas is set to the coking removal empty space during the regeneration period. It may be configured to alternately hold the fuel ratio and the poisoning recovery air-fuel ratio a plurality of times, or after maintaining the air-fuel ratio of the exhaust gas at the coking removal air-fuel ratio for a predetermined period, You may comprise so that it may hold | maintain to an air fuel ratio (Claim 4, Claim 5).

  In the present invention, after the regeneration period ends, the NOx concentration detecting means has deterioration determining means for determining deterioration of the NOx adsorbent based on the NOx concentration flowing out from the NOx catalyst device. Preferred (claim 6).

  In this configuration, since the NOx adsorbent deterioration determination is performed after the poisoning of the NOx adsorbent due to the sulfur component is recovered, the deterioration state of the NOx adsorbent can be grasped more accurately. In addition, in this configuration, the deterioration determination is performed based on the NOx concentration directly detected by the NOx concentration detection means, and it is possible to perform a more accurate deterioration determination.

In the present invention, the NOx adsorbent is an oxide containing cerium, and the NOx catalyst device includes an alkali metal component or an alkaline earth metal component in addition to the NOx adsorbent and the NH ₃ adsorbent. It is preferable to provide at least one NOx occlusion material.

In this way, not only reduction and purification of NOx by NH ₃ adsorbed on the NH ₃ adsorbent, but also NOx can be purified by the NOx storage material, so the amount of NOx discharged to the outside is further reduced. It becomes possible to do.

As described above, according to the present invention, to ensure the NH ₃ adsorption capacity of the NH ₃ adsorbing material, the exhaust gas purifying device capable of reducing and purifying the more reliably NOx by NH ₃ adsorbed in the NH ₃ adsorbent Can be provided.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Here, the case where the exhaust gas purifying apparatus according to the present invention is mounted on a diesel engine will be described. FIG. 1 is a schematic configuration diagram of a diesel engine including an exhaust gas purifying apparatus 10 according to the present invention.

  A plurality of cylinders 12 (for example, four cylinders) are formed in the engine body 1 of this diesel engine, and a piston 13 connected to the crankshaft 3 via a connecting rod is fitted into each cylinder 12. A combustion chamber 14 is formed above the piston 13. The diesel engine is configured so that the air-fuel ratio in the combustion chamber 14 is leaner than the stoichiometric air-fuel ratio in a normal operation state.

  A fuel injection valve 16 that directly injects fuel into the combustion chamber 14 is provided at the top of the combustion chamber 14 of each cylinder 12. The fuel injection valve 16 includes a needle valve and a solenoid (not shown). When a pulse signal is input, the fuel injection valve 16 is driven and opened for a time corresponding to the pulse width at the pulse input timing. It is comprised so that the quantity of fuel according to may be injected. During the regeneration period described later, the fuel injection valve 16 performs main injection for injecting fuel at least near the top dead center of the compression stroke and post-injection for injecting fuel in the expansion stroke after the main injection.

  The engine body 1 is provided with an engine speed sensor 30 that detects the rotational speed of the crankshaft 3, that is, the engine speed. The combustion chamber 14 of each cylinder 12 communicates with an intake passage 21 and an exhaust passage 22, and an intake valve 17 and an exhaust valve 18 are provided between the passages 21 and 22. An intake throttle 40 is provided in the intake passage 21. The intake throttle 40 controls the amount of intake air flowing into each cylinder 12 by adjusting the cross-sectional area of the intake passage 21 in accordance with the opening. Further, an EGR passage 23 for recirculating exhaust gas from the exhaust passage 22 is connected to the intake passage 21 downstream of the intake throttle 40. The EGR passage 23 is provided with an EGR valve 41 for adjusting the amount of exhaust gas recirculated to the intake passage 21 (hereinafter referred to as EGR gas). The EGR gas recirculated to the intake passage 21 through the EGR passage 23 merges with fresh air flowing in from the upstream of the intake passage 21 and flows into each cylinder 12.

  The exhaust passage 22 is provided with a DOC (oxidation catalyst) 50, a DPF (PM filter) 52, and an LNT (NOx catalyst device) 54. Back pressure sensors 33 and 34 are attached to the upstream and downstream portions of the DPF 52, specifically between the DOC 50 and the DPF 52 and between the DPF 52 and the LNT 54, respectively. Further, a temperature sensor 35 and a NOx sensor 36 are attached upstream of the LNT 54, specifically between the LNT 54 and the DPF 52. Further, a NOx sensor 37 is attached to the downstream portion of the LNT 54. The EGR passage 23 is connected to the upstream portion of the DOC 50, and the exhaust gas upstream of the DOC 50 is configured to flow into the EGR passage 23. The back pressure sensors 33 and 34 are for detecting pressures in the upstream and downstream portions of the DPF 52, and the NOx sensors 36 and 37 are for detecting NOx (nitrogen oxide) concentrations in the upstream and downstream portions of the LNT 54. Is to do.

  The DOC 50 is for promoting an oxidation reaction of exhaust gas, and CO (carbon monoxide) and HC (hydrocarbon) in the exhaust gas are oxidized and purified in the DOC 50.

  The DPF 52 is for collecting PM (particulate matter: particulate matter composed of soot contained in the exhaust gas) in the exhaust gas. This DPF 52 is a wall-through type made of ceramic such as SiC, for example, and when the PM passes through the cell wall from the inflow side cell to the outflow side cell of the DPF 52, the PM is collected in the inside. It is configured as follows. The DPF 52 carries a catalyst that promotes an oxidation reaction of exhaust gas, and the collected PM can be burned and removed by the oxidation reaction in the DPF 52.

The LNT 54 is for purifying NOx in the exhaust gas. As shown in FIG. 2, the LNT 54 includes a cordierite carrier base material 54a having a honeycomb shape, an NOx adsorption catalyst layer 54b, and an NH ₃ adsorption catalyst layer 54c. In FIG. 2, the NH ₃ adsorption catalyst layer 54c is disposed on the NOx adsorption catalyst layer 54b, that is, on the exhaust gas side.

The NOx adsorption catalyst layer 54b carries a noble metal such as platinum, a NOx adsorbent, a NOx occlusion material, and a support material such as alumina. The platinum acts as a catalyst and promotes various reactions described later. The NOx adsorbent is, for example, CeO ₂ (cerium oxide), or a ceria-based double oxide in which Ce (cerium) and Zr (zirconium) or a rare earth element are combined, and adsorbs NOx in exhaust gas. . Specifically, NO ₂ (nitrogen dioxide) generated by the reaction of NO (nitrogen monoxide) and oxygen in the exhaust gas by the action of the catalyst adsorbs NO in the exhaust gas.

The NOx storage material is, for example, barium, and stores NOx in the exhaust gas in the form of Ba (NO ₃ ) ₂ (barium nitrate).

The NH ₃ adsorption catalyst layer 54c carries zeolite as an NH ₃ adsorbent. When the exhaust gas flowing into the LNT 54 is brought into an oxygen-deficient atmosphere, that is, in a rich state, the NH ₃ adsorbent allows NO ₂ adsorbed on the NOx adsorbent to act as CO ₂ in the exhaust gas by the action of the catalyst. Alternatively, NH ₃ (ammonia) converted by reacting with hydrogen generated by the reaction of HC and water is adsorbed and held as NH ₄ + (ammonium ion).

When the LNT 54 is in a normal operation state, that is, when the air-fuel ratio in the combustion chamber 14 is leaner than the stoichiometric air-fuel ratio and the interior of the LNT 54 is in an oxygen-excess atmosphere, the NH ₃ adsorption is performed. The NH ₃ adsorbed on the material reduces and purifies NOx in the exhaust gas and NOx held on the NOx adsorbent to N ₂ (nitrogen). Further, when unburned fuel is supplied into the LNT 54 when the air-fuel ratio in the combustion chamber 14 is rich, the NOx occlusion material occludes it in the form of Ba (NO ₃ ) ₂ . NOx is reduced and purified to reduce the amount of NOx discharged to the outside.

  The diesel engine is provided with an ECU 2 as a control unit for controlling the operation of the engine. The ECU 2 includes a computer having a CPU, a ROM, a RAM, and the like, and controls the operation of each actuator and the like by executing a program stored in the ROM (or RAM) in advance by the CPU.

  The ECU 2 in the present embodiment is provided with a DPF control unit 60 and an LNT control unit 70 in addition to various control units (not shown).

  The DPF control unit 60 is for executing regeneration control of the DPF 52 in accordance with the state of the DPF 52. The DPF control unit 60 calculates a differential pressure before and after the DPF based on the outputs of the back pressure sensors 33 and 34, and when the differential pressure is a predetermined value or more, that is, the PM collected in the DPF 52 is a predetermined value. When the temperature exceeds the value, the temperature in the DPF 52 is increased. Then, the PM in the DPF 52 is burned and removed by this temperature rise, and the PM collecting ability of the DPF 52 is regenerated. Specifically, fuel is injected during the expansion stroke of the engine (so-called post-injection), the unburned HC component is increased in the exhaust passage 22, and this HC component is oxidized and burned by the DOC 50, thereby reducing the exhaust gas temperature. The exhaust gas whose temperature has been increased is caused to flow into the DPF 52, and further, an oxidation reaction of PM is caused in the DPF 52 by the action of the catalyst provided in the DPF 52.

The LNT control unit 70 controls NOx adsorption, occlusion, NH ₃ generation, coking removal of each catalyst material such as NH ₃ adsorbent, sulfur component removal, etc. in the LNT 54, An NH ₃ generation unit 72, a regeneration control unit (catalyst regeneration control unit) 74, and a deterioration determination unit (degradation determination unit) 78 are included.

The NH ₃ generator 72 is for generating NH ₃ from NOx contained in the exhaust gas. As described above, NH ₃ is generated by the reaction of hydrogen generated by the reaction between CO or HC in the exhaust gas and water, so that the NH ₃ generator 72 generates the exhaust gas to generate the hydrogen. In order to supply the CO or HC, the air-fuel ratio of the exhaust gas is kept richer than the stoichiometric air-fuel ratio.

Thus, if the air-fuel ratio of the exhaust gas is kept richer than the stoichiometric air-fuel ratio, the amount of CO and HC in the exhaust gas is secured, so that hydrogen is generated by the reaction of these with water, and this hydrogen NH ₃ is generated from the NOx. The NH ₃ thus generated is adsorbed on the NH ₃ adsorbent.

The reproduction control unit 74, in order to reliably suppress the discharge of the NOx external, NH ₃ adsorption performance of the NH ₃ adsorbing material, the NOx occlusion performance of the NOx adsorption performance and the NOx occlusion material of the NOx adsorbent in order to ensure that these NH ₃ adsorbent, NOx adsorbent, is intended to regenerate the NOx-absorbing material. The reproduction control unit 74 includes a NH ₃ reproduction control unit 75 for reproducing the NH ₃ adsorbing material and a S regeneration controller 76 to regenerate the NOx adsorbent and the NOx storage material.

In the regeneration control unit 74, first, the timing for starting the regeneration control by the NH ₃ regeneration control unit 75 and the S regeneration control unit 76, that is, the start time of the regeneration period is determined. Specifically, from the values of the NOx sensors 36 and 37 provided in the upstream and downstream portions of the NOx, the NOx purification rate in the LNT 54 (the detected value NOx_1 of the upstream NOx sensor 36 relative to the detected value NOx_1 of the upstream NOx sensor 36). A ratio (NOx_1−NOx_2) / NOx_1) of a difference between the detected value NOx_1 and the detected value NOx_2 of the downstream NOx sensor 37 is calculated, and whether or not this NOx purification rate is equal to or higher than a preset regeneration start purification rate. judge. When it is determined that the NOx purification rate is less than the regeneration start purification rate, NOx is not sufficiently purified in the LNT 54, and regeneration of the NH ₃ adsorbent, NOx adsorbent, and NOx occlusion material is performed. As necessary, the regeneration control by the NH ₃ regeneration control unit 75 and the S regeneration control unit 76 is started. Here, the regeneration start purification rate is a value set in advance based on the values of the engine speed, the actual injection amount, and the exhaust gas temperature, and is stored in the regeneration control unit 74 as a regeneration start purification rate map. . Then, based on the detection value of the engine speed sensor 30, the injection amount at the fuel injection valve 16, and the detection value of the temperature sensor 35, a value suitable for the operating condition is extracted from the regeneration start purification rate map.

The NH ₃ reproduction control unit 75, to ensure the NH ₃ adsorption capacity of the NH ₃ adsorbing material is for reproducing the NH ₃ adsorbent. Here, the zeolite used as the NH ₃ adsorbent is a substance in which a large number of active sites called acid sites are present in the crystal, and can adsorb NH ₃ as described above. The HC component of the unburned fuel contained is also adsorbed. Therefore, if this NH ₃ adsorbent is exposed to exhaust gas for a long time, it will be caulked by the HC component and its adsorption performance will be reduced. Therefore, in the NH ₃ reproduction control unit 75 performs a coking removal of NH ₃ adsorbent to ensure the NH ₃ adsorption capacity of the NH ₃ adsorbent play this NH ₃ adsorbent.

Specifically, post-injection is performed in each cylinder 12 in the expansion stroke, and the air-fuel ratio of the exhaust gas is maintained at a coking removal air-fuel ratio lower than that during normal operation and the temperature of the exhaust gas is raised. The NH ₃ adsorbent is exposed to a high temperature and oxygen-excess atmosphere to burn and remove the HC component. In the present embodiment, the post injection is performed so that the air-fuel ratio of the exhaust gas is about 20 and the temperature in the LNT 54 is 650 ° C to 750 ° C. Here, if the post injection is performed in the expansion stroke as described above, the temperature of the exhaust gas discharged from the combustion chamber 14 can be efficiently increased by the afterburning of the fuel injected by the post injection.

  The S regeneration control unit 76 is for regenerating the NOx adsorbent and the NOx occlusion material in order to ensure the NOx adsorption performance of the NOx adsorbent and the NOx occlusion material NOx occlusion performance. Here, the ceria-based oxide used as the NOx adsorbent adsorbs the NOx and the sulfur component contained in the fuel and engine oil in the exhaust gas in an oxygen-excess atmosphere. Have. Therefore, if the exhaust gas is continuously supplied to the NOx adsorbent made of this ceria-based oxide, the sulfur component is adsorbed and the NOx adsorption performance is lowered. On the other hand, the ceria-based oxide has a characteristic of desorbing a sulfur component adsorbed with a decrease in oxygen concentration.

  Further, barium used as the NOx occlusion material has a characteristic that occludes the NOx as barium nitrate and occludes the sulfur component as barium sulfate in an oxygen-excess atmosphere. Therefore, similarly to the NOx adsorbent, if the exhaust gas is continuously supplied to the NOx adsorbent, the sulfur component is occluded and the NOx occlusion performance is deteriorated. On the other hand, this NOx occlusion material also has a characteristic of desorbing the sulfur component occluded as the oxygen concentration decreases and the supplied HC or CO increases.

  Therefore, the S regeneration control unit 76 exposes the NOx adsorbent and the NOx occlusion material to an oxygen-deficient atmosphere, thereby desorbing the sulfur component from the NOx adsorbent and the NOx occlusion material, and the NOx adsorbent and NOx. Recycle the storage material.

  Specifically, the opening degree of the EGR valve 41 is increased and the opening degree of the intake throttle 40 is reduced to decrease the amount of fresh air flowing into the combustion chamber 14 while increasing the amount of EGR gas flowing into the combustion chamber 14. Let Further, by performing post-injection in each cylinder 12, the amount of injection supplied into each cylinder 12 is increased, and the air-fuel ratio of the exhaust gas is lowered to a poisoning recovery air-fuel ratio that is richer than the stoichiometric air-fuel ratio. In this embodiment, control is performed so that the air-fuel ratio of the exhaust gas becomes 14, for example.

Here, in order to desorb the sulfur component from the NOx adsorbent, it has been found that the temperature of the catalyst is preferably a predetermined value or higher (for example, 650 ° C. to 750 ° C.). Therefore, in the exhaust gas purification apparatus 10, immediately after the exhaust gas temperature is sufficiently raised in the NH ₃ regeneration control unit 75, the sulfur component is efficiently removed by performing regeneration control in the S regeneration control unit 76. It is configured to be detached. That is, in the regeneration control unit 74 in the present embodiment, the NH ₃ regeneration control unit 75 performs the NH ₃ adsorbent coking removal control (NH ₃ adsorption material regeneration control) and the S regeneration control unit 76 performs the NOx adsorption material. And S regeneration control of NOx occlusion material (regeneration control of NOx adsorbent and NOx occlusion material) are continuously performed.

Specifically, at the regeneration control unit 74, when the regeneration period starts, as shown in FIG. 5, the NH3 regeneration control unit 75 first empties the exhaust gas for t1 time (for example, 1 min to 10 min). Coking removal control is performed to maintain the fuel ratio (A / F) at about 20, and the NH ₃ adsorbent is removed from the coke while raising the temperature in the LNT 54. Thereafter, for t2 time (for example, 1 min to 10 min), the S regeneration control unit 76 performs S regeneration control for reducing the air-fuel ratio to about 14, and desorbs sulfur components from the NOx adsorbent and NOx occlusion material. Let Subsequently, after returning to the normal operation state, it is determined whether or not the NOx purification rate of the LNT 54 calculated from the values of the NOx sensors 36 and 37 is equal to or higher than a preset regeneration end purification rate, and this NOx purification rate is determined to be the end of regeneration. When the purification rate is lower than the regeneration completion purification rate, the coking removal control and the S regeneration control are alternately performed to regenerate the NH ₃ adsorbent, the NOx adsorbent, and the NOx occlusion material. . The regeneration end purification rate is the NOx purification rate that the LNT 54 is considered to have been sufficiently regenerated, and is based on the values of the engine speed, the actual injection amount, and the exhaust gas temperature in the same manner as the regeneration start purification rate. It is preset and stored in the regeneration control unit 74 as a regeneration start purification rate map.

  The LNT degradation determination unit (degradation determination means) 78 performs the degradation determination of the LNT 54. The LNT deterioration determination means 78 calculates the NOx purification rate in the LNT 54 from the detected values of the NOx sensors 36 and 37 when the regeneration control by the regeneration control unit 74 ends and returns to normal operation, and this NOx If the purification rate is less than the preset deterioration determination purification rate, it is determined that the LNT 54 has deteriorated. The deterioration determination purification rate may be the same value as the regeneration start purification rate, or may be a value set in advance for each operation condition. Here, when the NOx occlusion material is not provided in the LNT 54, the LNT deterioration determination means 78 determines the deterioration state of only the NOx adsorbent.

  Next, the point of the reproduction control of the LNT 54 in the LNT control unit 70 will be described based on the flowcharts of FIGS.

  First, in step S1, it is determined whether or not the DPF 52 is being regenerated. That is, in order to regenerate the DPF 52 by burning and removing the PM in the DPF 52, the DPF controller 60 determines whether PM combustion acceleration control by post injection is being performed. If this determination is YES, the process returns to step S1 without performing the regeneration control of the LNT 54. On the other hand, when this determination is NO, the engine speed detected by the engine speed sensor 30, the injection amount injected by the fuel injection valve 16, and the exhaust gas upstream of the LNT 54 detected by the temperature sensor 35. Based on the temperature, the regeneration start purification rate is extracted from the regeneration start purification rate map, the regeneration end purification rate is extracted from the regeneration end purification rate map, and the deterioration determination purification rate is extracted from the deterioration determination purification rate map. (Step S2). Next, the value of the NOx purification rate of the LNT 54 is calculated from the detected value NOx_1 of the upstream NOx sensor 36 of the LNT 54 and the detected value NOx_2 of the downstream NOx sensor 37 as NOx purification rate = (NOx_1−NOx_2) / NOx_1. (Step S3).

  Next, the value of the NOx purification rate is compared with the regeneration start purification rate (step S4). Here, when the value of the NOx purification rate is equal to or higher than the regeneration start purification rate (YES in step S4), the LNT 54 has sufficient NOx purification performance, and regeneration of each adsorbent and occlusion material is unnecessary. Then, the process returns to step S1. On the other hand, when the value of the NOx purification rate is less than the regeneration start purification rate (NO in step S4), NOx is not sufficiently reduced and purified by the LNT 54, and the adsorption performance of each adsorbent and the occlusion of the occlusion material. Reproduction control of the LNT 54 is started on the assumption that the performance has deteriorated.

First, the NH ₃ regeneration control unit 75 performs coking removal control for t1 time. That is, post injection is performed so that the air-fuel ratio (A / F) of the exhaust gas becomes 20, and the NH ₃ adsorbent is removed by coking while raising the temperature in the LNT 54 (step S5). Next, the S regeneration control unit 76 performs S regeneration control for t2 hours. That is, post injection is performed so that the air-fuel ratio (A / F) of the exhaust gas becomes 14, and the opening degree of the intake throttle 40 and the EGR valve 41 is adjusted to remove the sulfur component from the NOx adsorbent and the NOx occlusion material. Desorb (step S6).

Thereafter, the normal operation is resumed (step S7). Then, the NOx purification rate is calculated again (step S8), and the value of the NOx purification rate is compared with the regeneration end purification rate (step S9). Here, if the value of the NOx purification rate than the playback end purification rate (NO in step S9) is, NH ₃ adsorption performance of the NH ₃ adsorbent, the NOx occlusion of the NOx adsorption performance and NOx-absorbing material of the NOx adsorbent Since the performance is recovered and the NOx purification performance of the LNT 54 is recovered, the process proceeds to step S10. On the other hand, when the value of the NOx purification rate is less than the regeneration end purification rate (YES in step S9), the step until the value of the NOx purification rate becomes equal to or higher than the regeneration end purification rate (until NO in step S9). S5, 6, 7, and 8 are repeated.

  In step S10, the NOx purification rate calculated in step S8 is compared with the deterioration determination purification rate. Here, if the NOx purification rate is equal to or higher than the deterioration determination purification rate (YES in step S10), the process is terminated, and if the NOx purification rate is less than the deterioration determination purification rate (NO in step S10), the LNT 54 is deteriorated. It determines with having carried out (step S11), and a process is complete | finished.

As described above, according to the present exhaust gas purification apparatus 10, since the NOx sensors 36 and 37 that can directly detect the NOx concentration are provided upstream and downstream of the LNT, the change in the NOx purification performance by the LNT54 is changed. It can be grasped more accurately. Since the regeneration start time of the LNT 54 is determined based on the NOx concentration directly detected by the NOx sensors 36 and 37, the purification performance of the LNT 54 can be efficiently recovered. In particular, during playback of the LNT54, since playback is being performed in the NH ₃ adsorbing material holds the air-fuel ratio of the exhaust gas can be air-fuel ratio reproduction of at least the NH ₃ adsorbing material, to the NH ₃ adsorbent can be adsorbed sufficient amount of NH ₃ for reducing NOx, it is possible to reduce and purify NOx by NH ₃ adsorbed in the NH ₃ adsorbing material.

Further, in the present embodiment, the NH ₃ is performed caulking removed as regeneration control of the NH ₃ adsorbent by the reproducing control unit 75, it is possible to recover the lowering of NH ₃ adsorption performance of the NH ₃ adsorbing material due to coking it can. The NH ₃ regeneration control unit 75 maintains the air-fuel ratio of the exhaust gas on the richer side than during normal operation, raises the temperature in the LNT 54 and exposes the NH ₃ adsorbent to a high temperature atmosphere. the hold the air-fuel ratio to the lean side from the stoichiometric air-fuel ratio, skip this NH ₃ adsorbent by controlling so that exposure to an oxygen-rich atmosphere, the carbon component adhered to the NH ₃ adsorbing material easily baked It is possible to remove the coking of the NH ₃ adsorbent efficiently.

The regeneration control unit 74 is provided with an S regeneration control unit 76, and the S regeneration control unit 76 maintains the air-fuel ratio of the exhaust gas at a poisoning recovery air-fuel ratio richer than the stoichiometric air-fuel ratio during the regeneration period. If comprised in this way, since the poisoning by the sulfur component of the NOx adsorbing material can be recovered and the NOx adsorbing performance of the NOx adsorbing material can be secured, it is possible to suppress the emission of NOx to the outside. In particular, during the regeneration period, the NH ₃ adsorbent is removed by coking, and the exhaust gas air-fuel ratio is maintained on the richer side than during normal operation. Compared with the case where the air-fuel ratio is made rich, the poisoning due to the sulfur component of the NOx adsorbent can be efficiently recovered with less fuel consumption. In addition, since the temperature in the LNT 54 is kept high by the coking removal control, the sulfur component can be efficiently desorbed from the NOx adsorbent.

  In addition, if the deterioration determining means 78 for determining the deterioration state of the NOx adsorbent based on the NOx concentration detected by the NOx sensors 36 and 37 after the end of the regeneration period is provided, the NOx adsorbent Since the deterioration determination of the NOx adsorbent is performed after the poisoning by the sulfur component is recovered, the deterioration state of the NOx adsorbent can be grasped more accurately. In addition, in this configuration, the deterioration determination is performed based on the NOx concentration directly detected by the NOx sensors 36 and 37, so that a more accurate deterioration determination can be performed.

Further, if the LNT 54 is configured to include a NOx occlusion material made of at least one of an alkali metal component such as barium or an alkaline earth metal component in addition to the NOx adsorbent and the NH ₃ adsorbent, the NH ₃ Since not only reduction and purification of NOx by NH ₃ adsorbed on the adsorbent, but also NOx can be purified by the NOx occlusion material, the amount of NOx discharged to the outside can be further reduced.

  Here, in the above embodiment, in the regeneration control of the LNT 54, it is shown that the coking removal control is performed for a predetermined period during the regeneration period, and then the S regeneration control is performed for a predetermined period. It may be executed repeatedly for a set number of times. For example, as shown in FIG. 6, the coking removal in the t11 period and the S regeneration control in the t12 period in a predetermined reproduction period may be performed a preset number of times.

Further, the value of the air-fuel ratio for performing the coking removal of the NH ₃ adsorbent in the NH ₃ regeneration control unit 75 is not limited to 20 as described above. Similarly, the value of the air-fuel ratio for desorbing the sulfur component from the NOx adsorbent and the NOx occlusion material in the S regeneration control unit 76 is not limited to 14 as described above. Further, when the air-fuel ratio is set to around 14 in the S regeneration control unit 76, the air-fuel ratio (A / F) is not kept constant at 14 as shown in FIG. 5, but as shown in FIG. In addition, the air-fuel ratio may be controlled to increase or decrease in small increments near the air-fuel ratio (A / F) = 14. In this case, it is possible to effectively promote the desorption of the sulfur component from the NOx storage material.

Further, the method for maintaining the air-fuel ratio of the exhaust gas flowing into the LNT 54 at each predetermined value in the NH ₃ regeneration control unit 75 and the S regeneration control unit 76 is not limited to the above.

  In the above-described embodiment, the NOx sensors 36 and 37 are mounted on the upstream and downstream sides of the LNT 54, and various determinations are made based on the NOx purification rate calculated based on the detection values of these sensors. Alternatively, the NOx sensor may be attached only to the downstream side. In this case, the various determinations may be performed based on the absolute value of the NOx concentration detected by the NOx sensor attached on the downstream side instead of the NOx purification rate. Further, the NOx concentration value on the upstream side of the LNT 54 may be estimated from operating conditions and the like. Further, the regeneration start purification rate, the regeneration end purification rate, the deterioration determination purification rate, and the like may be set to constant values without being provided for each operation condition as in the embodiment.

  Further, in the above embodiment, the case where the regeneration control is executed immediately when the DPF regeneration is not being performed and the NOx purification rate is less than the regeneration start purification rate has been described, but the operation condition further satisfies a predetermined condition. The reproduction control may be executed only when it is. For example, the regeneration control may be executed only when the exhaust gas temperature upstream of the LNT 54 detected by the temperature sensor 35 is equal to or higher than a predetermined value.

  Further, the arrangement of the LNT 54, the DPF 52, and the DOC 50 is not limited to FIG. 1, and the LNT 54 may be arranged upstream of the DPF 52 as shown in FIG. Further, as shown in FIG. 9, the DOC 50 may be excluded.

The material of the NOx storage material is not limited to the above, and strontium or the like may be used. Further, this NOx occlusion material may be omitted. Similarly, the materials of the NH ₃ adsorbent and the NOx adsorbent are not limited to the above. Further, the arrangement of the NH ₃ adsorption catalyst layer 54c and the NOx adsorption catalyst layer 54b in the LNT 54 is not limited to the above, and the NOx adsorption catalyst layer 54b may be arranged on the NH ₃ adsorption catalyst layer 54c.

  The exhaust gas purification apparatus 10 can be applied not only to a diesel engine but also to various engines that perform lean burn combustion.

1 is a schematic configuration diagram of an engine including an exhaust gas purifying apparatus according to an embodiment of the present invention. It is a schematic sectional drawing which expands and shows the internal structure of the NOx catalyst apparatus in the exhaust gas purification apparatus shown in FIG. 2 is a schematic flowchart of regeneration control of a NOx catalyst device in the exhaust gas purification device shown in FIG. FIG. 4 is a schematic flowchart of deterioration determination of the NOx catalyst device performed subsequent to FIG. 3. FIG. 2 is a time chart showing changes in the air-fuel ratio during regeneration control of the NOx catalyst device in the exhaust gas purification device shown in FIG. 1. 6 is a time chart showing a modification of the air-fuel ratio changing method shown in FIG. 5. 6 is a time chart showing a modification of the air-fuel ratio changing method shown in FIG. 5. FIG. 1 is a schematic configuration diagram showing a modified example of the arrangement of the NOx catalyst device of the exhaust gas purification device. FIG. 1 is a schematic configuration diagram showing a modified example of the arrangement of the NOx catalyst device of the exhaust gas purification device.

Explanation of symbols

1 Engine body 2 ECU
16 Fuel injection valve 22 Exhaust passage 23 EGR passage 36 NOx sensor (NOx concentration detection means)
37 NOx sensor (NOx concentration detection means)
40 Intake throttle 41 EGR valve 54 LNT (NOx catalyst device)
54a Cordierite carrier base material 54b NOx adsorption catalyst layer 54c NH ₃ adsorption catalyst layer 70 LNT control unit 72 NH ₃ generation unit 74 regeneration control unit (catalyst regeneration control means)
75 NH ₃ regeneration control unit 76 S regeneration control unit 78 Degradation determination means A / F Air-fuel ratio

Claims (7)

An exhaust gas purification device for a lean burn engine,
A NOx adsorbent provided in the exhaust passage of the engine and adsorbs NOx contained in exhaust gas in an oxygen-excess atmosphere exhausted from the combustion chamber of the engine, and NOx adsorbed on the NOx adsorbent is converted in an oxygen-deficient atmosphere. And a zeolite-based NH ₃ adsorbent that adsorbs NH _3, and NOx discharged when the engine operating conditions are switched so that the exhaust gas becomes an oxygen-excess atmosphere again, and the adsorbed NH a NOx catalyst device for purifying the NOx ₃ and reacted,
Catalyst regeneration control means for changing the air-fuel ratio of the exhaust gas to an air-fuel ratio capable of regenerating at least the NH ₃ adsorbent in order to regenerate the NOx catalyst device;
NOx concentration detecting means provided downstream of the NOx catalyst device in the exhaust passage of the engine and detecting the concentration of NOx flowing out from the NOx catalyst device;
The exhaust gas purification device, wherein the catalyst regeneration control means starts regeneration of the NOx catalyst device based on the NOx concentration detected by the NOx concentration detection means. The exhaust gas purification apparatus according to claim 1,
The catalyst regeneration control means includes means for regenerating the NH ₃ adsorbent by removing coking removal of the NH ₃ adsorbent, and the exhaust gas is regenerated during at least a part of the regeneration period of the NOx catalyst device. An exhaust gas purifying apparatus, characterized in that the air-fuel ratio is made richer than that during normal operation and leaner than the stoichiometric air-fuel ratio. The exhaust gas purification apparatus according to claim 2,
The NOx adsorbent adsorbs the sulfur component contained in the exhaust gas in an oxygen-excess atmosphere with a high oxygen concentration, and discharges the sulfur component as the oxygen concentration decreases,
The catalyst regeneration control means includes means for regenerating the NOx adsorbent by desorbing the sulfur component from the NOx adsorbent, and for desorbing the sulfur component from the NOx adsorbent. An exhaust gas purification apparatus configured to make the air-fuel ratio of the exhaust gas a poison recovery air-fuel ratio richer than the stoichiometric air-fuel ratio during at least a part of the regeneration period of the catalyst device. In the exhaust gas purification apparatus according to claim 3,
The catalyst regeneration control means alternately maintains the air-fuel ratio of the exhaust gas at the coking removal air-fuel ratio and the poisoning recovery air-fuel ratio a plurality of times during the regeneration period of the NOx catalyst device. Exhaust gas purification device. In the exhaust gas purification apparatus according to claim 3,
The catalyst regeneration control means maintains the air-fuel ratio of the exhaust gas at the coking removal air-fuel ratio for a predetermined period during the regeneration period of the NOx catalyst device, and then changes the air-fuel ratio of the exhaust gas to the poisoning recovery air-fuel ratio. An exhaust gas purifying apparatus characterized by holding. In the exhaust gas purifying apparatus according to any one of claims 1 to 5,
After the end of the regeneration period of the NOx catalyst device, there is provided a deterioration determination means for determining the deterioration of the NOx adsorbent based on the concentration of NOx flowing out from the NOx catalyst device detected by the NOx concentration detection means. Exhaust gas purification device. In the exhaust gas purification apparatus according to any one of claims 1 to 6,
The NOx adsorbent is an oxide containing cerium,
The NOx catalyst device includes an NOx occlusion material comprising at least one of an alkali metal component or an alkaline earth metal component in addition to the NOx adsorbent and the NH ₃ adsorbent.

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