JP4941111B2 - Exhaust gas purification device - Google Patents

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, provided with a NOx catalyst device having a NH ₃ and can adsorb an NH ₃ adsorbing material produced the NOx in the exhaust passage of the engine from adsorbable NOx adsorbent and NOx, the NH ₃ It is known that the NOx is reduced and purified by NH ₃ adsorbed on an adsorbent. Specifically, the NOx is converted to NH ₃ under an oxygen-deficient atmosphere, and the converted NH ₃ is adsorbed on 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 by the NH ₃ .

For example, Patent Document 1, the NOx adsorbent containing ceria having NOx adsorbing capacity, the apparatus having a NOx catalyst device and a NH ₃ adsorbing material containing a zeolite having a NH ₃ adsorbing capacity is disclosed Yes. In this apparatus, the amount of NOx adsorbed on the NOx adsorbent is estimated, and when the estimated value is equal to or greater than a predetermined value, the air-fuel ratio of the combustion chamber is controlled to a rich state to control the NOx to the NH ₃ Convert to.
Japanese Patent Laid-Open No. 2005-214098

  Here, the NOx adsorbent as described above has a characteristic that the adsorbed NOx is desorbed when the temperature of the exhaust gas exceeds a predetermined value. Therefore, depending on the engine operating conditions and the like, there is a problem that the temperature of the exhaust gas becomes equal to or higher than the predetermined temperature, NOx is desorbed from the NOx adsorbent, and the NOx is discharged to the outside.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exhaust gas purification device that can more reliably suppress NOx emission to the outside even when the exhaust gas temperature is high. .

In order to solve the above-mentioned problem, the present invention is an exhaust gas purification apparatus for a lean burn engine, and is provided in an exhaust passage of the engine to adsorb NOx contained in exhaust gas discharged from a combustion chamber of the engine. a NOx trap catalyst device containing a NH ₃ adsorbing material for adsorbing NH ₃ to NOx adsorbed on the NOx adsorbent and the NOx adsorbent is been converted in an oxygen deficient atmosphere, downstream of the NOx trap catalyst device provided, the NOx storage catalytic device containing a NOx-absorbing material for absorbing the NOx in the exhaust passage at a temperature higher than the temperature at which the NOx adsorbent adsorbs NOx, storage of NOx in the NOx storage catalytic converter An NOx storage state in the NOx storage catalyst device determined by the storage state determination means; Then, the enrichment control is performed to make the air-fuel ratio of the exhaust gas richer than the stoichiometric air-fuel ratio, and the NOx occluded in the NOx occlusion material included in the NOx occlusion catalyst device is released. In addition , an exhaust gas purification device is provided, characterized by comprising NOx release means for reducing and purifying the released NOx (claim 1).

  According to the present invention, the NOx occlusion catalyst containing the NOx occlusion material that occludes NOx at a temperature higher than the temperature at which the NOx adsorbent adsorbs NOx on the downstream side of the NOx adsorption catalyst device containing the NOx adsorbent. Since the apparatus is provided, even if NOx is desorbed from the NOx adsorbent due to a high exhaust gas temperature, the NOx occlusion material can occlude the desorbed NOx. This suppresses the emission of NOx to the outside over a wider exhaust gas temperature range and thus a wider operating range.

  In this configuration, the NOx stored in the NOx storage material is released and reduced and purified by controlling the air-fuel ratio of the exhaust gas to a rich side by the NOx release means, and the NOx storage of the NOx storage catalyst device. The ability is restored. This maintains the NOx storage capacity in the exhaust gas by the NOx storage catalyst device, and more reliably suppresses the release of NOx to the outside. In particular, in this configuration, the enrichment control is performed based on the NOx occlusion state of the NOx occlusion catalyst device determined by the NOx occlusion state judgment means, and the NOx occlusion capacity of the NOx occlusion catalyst device is recovered. It is done efficiently.

In the above configuration, the NOx release means preferably starts the enrichment control when the storage state determination means determines that the NOx storage catalyst device is saturated with respect to storage of NOx. (Claim 2 ).

  In this way, since the enrichment control is started when the NOx storage catalyst device is saturated, the NOx storage capability of the NOx storage catalyst device is more efficiently recovered.

In addition, it is preferable to have NOx release air-fuel ratio calculating means for calculating the NOx release air-fuel ratio in accordance with the NOx storage state in the NOx storage catalyst device determined by the storage state determination means (Claim 3 ).

  In this way, since the air-fuel ratio of the exhaust gas is controlled according to the NOx storage state in the NOx storage catalyst device, the NOx storage capability of the NOx storage catalyst device is more efficiently recovered.

In the above configuration, the NOx concentration detecting means is provided on the downstream side of the NOx occlusion catalyst device and detects the NOx concentration in the exhaust passage, and the occlusion state determining means is detected by the NOx concentration detecting means. It is preferable to determine the NOx occlusion state in the NOx occlusion catalyst device based on the concentration of the NOx thus obtained (Claim 4 ).

  In this way, the NOx occlusion state of the NOx occlusion catalyst device is more accurately determined, so that the NOx occlusion capacity of the NOx occlusion catalyst device can be recovered more efficiently.

In the above configuration, the exhaust gas temperature detection means is provided upstream of the NOx adsorption catalyst device and detects the temperature of the exhaust gas in the exhaust passage, and the NOx release means is the exhaust gas temperature detection means. When the detected temperature of the exhaust gas is equal to or higher than a predetermined value set in advance, it is preferable to allow the enrichment control to be performed (Claim 5 ).

  As described above, NOx is occluded in the NOx occlusion catalyst device when NOx is desorbed from the NOx adsorption catalyst device due to the high temperature of the exhaust gas. As described above, the temperature of the exhaust gas is a predetermined value. If the enrichment control is permitted at the above high temperature, the NOx storage capability of the NOx storage catalyst device can be efficiently recovered while suppressing excessive fuel consumption accompanying the enrichment.

Further, in the present invention, the NOx adsorption catalyst device and the NOx occlusion catalyst device have a particulate filter that collects particulate matter discharged from the engine and burns and removes the collected particulate matter. Is preferably provided downstream of the particulate filter (claim 6 ).

  According to this configuration, even when the exhaust gas temperature in the particulate filter and the exhaust gas temperature on the downstream side of the particulate filter rise during combustion removal of the particulate matter, and the NOx adsorbent desorbs NOx, Since the NOx occlusion material occludes the desorbed NOx, it is possible to suppress the discharge of the NOx to the outside while suppressing the discharge of the particulate matter to the outside by the combustion removal of the particulate matter.

In the configuration, the particles collected in the particulate filter are subjected to regeneration control so that the air-fuel ratio of the exhaust gas is richer than the air-fuel ratio during normal operation and leaner than the stoichiometric air-fuel ratio. Preferably, the NOx releasing means permits the enrichment control to be performed during the regeneration control by the regeneration control means (claim 7 ).

  In this way, the particulate matter collected by the particulate filter is burned and removed by the regeneration control means, and the particulate filter can be regenerated. Even if the temperature rises and the NOx occlusion amount of the NOx occlusion material increases, the enrichment control is performed during the regeneration control, so that the occlusion performance of the NOx occlusion material can be ensured. Become.

Here, an exhaust gas temperature detecting means for detecting the temperature of the exhaust gas in the exhaust passage is provided upstream of the NOx adsorption catalyst device, and the NOx releasing means is provided immediately after the regeneration control by the regeneration control means is completed. Even when the exhaust gas temperature detected by the exhaust gas temperature detection means is equal to or higher than a preset normal operating temperature, the NOx storage catalyst device is configured to store NOx by the storage state determination means. If it is determined that the saturation for may be configured to implement the rich control (claim 8).

  In this way, even after the regeneration control is finished, the enrichment control is executed when the temperature of the exhaust gas is high and the NOx storage catalyst device is in a saturated state. The NOx occlusion capacity of the NOx occlusion catalyst layer is recovered more reliably, and the release of NOx to the outside is more reliably suppressed.

  As described above, according to the present invention, it is possible to provide an exhaust gas purification apparatus that can more reliably suppress the release of NOx to the outside under a wide range of operating conditions.

  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 to operate under a normal operating condition in which the air-fuel ratio in the combustion chamber 14 is leaner than the stoichiometric air-fuel ratio.

  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. The fuel injection valve 16 is configured to be able to perform main injection for injecting fuel 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 (diesel particulate filter; particulate filter) 52, a NOx adsorption catalyst device 54, and a NOx occlusion catalyst device 56. 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 NOx adsorption catalyst device 54, respectively. Further, an exhaust gas temperature sensor (exhaust gas temperature detection means) 35 and an air-fuel ratio sensor (A / F sensor) are disposed upstream of the NOx adsorption catalyst device 54, specifically between the NOx adsorption catalyst device 54 and the DPF 52. 36) is attached. Further, a NOx sensor (NOx concentration detecting means) 37 is attached to the downstream portion of the NOx storage catalyst device 56. 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 the pressure in the upstream and downstream portions of the DPF 52. The exhaust gas temperature sensor 35 is for detecting the temperature of the exhaust gas flowing into the NOx adsorption catalyst device 54 and the like, and the NOx sensor 37 is a NOx (nitrogen oxide) downstream of the NOx storage catalyst device 56. ) For detecting the concentration. The A / F sensor 36 is for detecting the air-fuel ratio of the exhaust gas flowing into the NOx adsorption catalyst device 54.

  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. Further, the DOC 50 plays a role of raising the temperature of the exhaust gas by the oxidation reaction and causing the high-temperature exhaust gas to flow into the DPF 52 provided downstream.

  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. Further, the DPF 52 carries a catalyst containing platinum or the like 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. Specifically, when the exhaust gas temperature is raised by an oxidation reaction in the DOC 50, the PM is burned by a catalytic action of platinum or the like.

The NOx adsorption catalyst device 54 is for purifying NOx in the exhaust gas. As shown in FIG. 2, the NOx adsorption catalyst device 54 includes a base material 54a, a NOx adsorption catalyst layer 54b, and an NH ₃ adsorption catalyst layer 54c. As the base material 54a, for example, a cordierite carrier base material having a honeycomb shape can be used.

The NOx adsorption catalyst layer 54b carries a noble metal such as platinum, a NOx adsorbent that adsorbs NOx in the exhaust gas, and a support material such as alumina. The platinum acts as a catalyst and promotes various reactions in the NOx adsorption catalyst device 54. 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, this NOx adsorbent adsorbs NO ₂ (nitrogen dioxide) produced by the reaction of NO (nitrogen monoxide) and oxygen in the exhaust gas by the action of the catalyst, and in the exhaust gas. NO is adsorbed.

The NH ₃ adsorption catalyst layer 54c carries zeolite as an NH ₃ adsorbent. This NH ₃ adsorbent is adsorbed on the NOx adsorbent by the action of the catalyst when the exhaust gas flowing into the NOx adsorption catalyst device 54 is made richer than an oxygen-deficient atmosphere, that is, the stoichiometric air-fuel ratio. NO ₂ adsorbs and holds NH ₃ (ammonia) converted by reacting with hydrogen produced by the reaction of CO or HC with water in the exhaust gas and NH ₄ + (ammonium ions).

In the NOx adsorption catalyst device 54 having the catalyst layers 54b and 54c, the normal operation state, that is, the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, and the inside of the NOx adsorption catalyst device 54 is oxygen if the excess atmosphere to reduce and purify NOx adsorbed on the NOx and the NOx adsorbent in the exhaust gas by NH ₃ that is adsorbed on the NH ₃ adsorbing material _{N 2} (nitrogen).

  Similar to the NOx adsorption catalyst device 54, the NOx storage catalyst device 56 is for purifying NOx in the exhaust gas. As shown in FIG. 3, the NOx storage catalyst device 56 includes a base material 56a and a NOx storage catalyst layer 56b. As the base material 56a, for example, a cordierite carrier base material in the form of a honeycomb can be used similarly to the base material 54a of the NOx adsorption catalyst device 54.

The NOx occlusion catalyst layer 56b carries a noble metal such as platinum that acts as a catalyst, a NOx occlusion material that occludes NOx in the exhaust gas, and a support material such as alumina. As the NOx storage material, for example, alkali metals such as K (potassium) and Na (sodium), or alkaline earth metals such as Ba (barium), Mg (magnesium), and Sr (strontium) can be used. . This NOx occlusion material occludes and holds NOx in exhaust gas in the form of nitrate. In the NOx occlusion catalyst device 56 containing this NOx occlusion material, when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio and the inside of the NOx occlusion catalyst device 56 is in an oxygen-deficient atmosphere, NOx occluded in the NOx occlusion material is reduced and purified to N ₂ by unburned HC and CO contained in the exhaust gas.

Here, the NOx adsorbent included in the NOx adsorbing catalyst device 54 is a ceria-based double oxide and an NH ₃ adsorbent that adsorbs NOx or NH ₃ in the exhaust gas in an atmosphere of 300 ° C. or lower. In addition, the adsorbed NOx or NH ₃ is desorbed in an atmosphere of 300 ° C. or higher. On the other hand, barium or the like, which is a NOx storage material included in the NOx storage catalyst device 56, has a characteristic of storing the NOx in a high temperature state of 300 ° C to 600 ° C.

Therefore, when the temperature of the exhaust gas is 300 ° C. or lower, the NOx adsorption catalyst device 54 adsorbs NOx and the NH ₃ reduces and purifies the NOx, thereby suppressing the emission of NOx to the outside. . On the other hand, when the temperature of the exhaust gas is 300 ° C. or higher, NOx is released from the NOx adsorption catalyst device 54. The NOx storage catalyst device 56 is in the NOx and exhaust gas released from the NOx adsorption catalyst device 54. Occlusion of NOx suppresses the discharge of NOx to the outside.

  The exhaust gas temperature is determined when, for example, the exhaust gas temperature is increased by the oxidation reaction in the DOC being promoted by the DPF regeneration control unit 60 to be described later in order to burn the PM accumulated in the DPF 52, and the PM is When combustion is started, the temperature becomes 300 ° C. or higher.

  This 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 this embodiment includes a DPF regeneration control unit (regeneration control means) 60, an NH ₃ generation unit 72, an S regeneration control unit 74, and an occlusion state determination unit (occlusion) in addition to various control units (not shown). A state determining unit) 76, a NOx releasing unit (NOx releasing unit) 78, and a NOx releasing air-fuel ratio calculating unit (NOx releasing air-fuel ratio calculating unit) 80 are provided.

  The DPF regeneration control unit 60 is for executing regeneration control of the DPF 52 in accordance with the state of the DPF 52. The DPF regeneration control unit 60 calculates the differential pressure before and after the DPF 52 based on the outputs of the back pressure sensors 33 and 34. If the differential pressure is equal to or greater than a predetermined value, that is, the PM collected in the DPF 52. When the temperature exceeds a predetermined amount, 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), and the DPF regeneration in which the air-fuel ratio of the exhaust gas is richer than the air-fuel ratio during normal operation and leaner than the stoichiometric air-fuel ratio is performed. The air-fuel ratio (for example, 20) is set. Then, the unburned HC component is increased in the exhaust passage 22 and the exhaust gas temperature is increased by oxidizing and burning the HC component by the DOC 50, and the heated exhaust gas is caused to flow into the DPF 52. Thereby, the oxidation reaction of PM is started in the DPF 52 by the action of the catalyst provided in the DPF 52. At this time, the exhaust gas temperature is raised to 300 ° C. or higher as described above.

  The storage state determination unit 76 determines the storage state of NOx in the NOx storage catalyst device 56. The occlusion state determination unit 76 determines the occlusion state of NOx based on the NOx concentration detected by the NOx sensor 37. Specifically, if the NOx concentration detected by the NOx sensor 37 is equal to or higher than a preset saturation determination value (for example, 50 ppm), NOx is not stored in the NOx storage catalyst device 56 but discharged downstream. Therefore, it is determined that the NOx occlusion catalyst device 56 is saturated with respect to occlusion of NOx. That is, it is determined that it is necessary to release the stored NOx from the NOx storage catalyst device 56. On the other hand, if the NOx concentration detected by the NOx sensor 37 is equal to or lower than a preset enrichment end determination value (for example, 20 ppm), the NOx occlusion catalyst device 56 is not saturated with respect to NOx occlusion, and this It is determined that further NOx storage is possible in the NOx storage catalyst device 56.

  The NOx release air-fuel ratio calculation unit 80 calculates the NOx release air-fuel ratio. The NOx release air-fuel ratio is a value of the air-fuel ratio of exhaust gas required for releasing NOx stored in the NOx storage catalyst device 56 and reducing and purifying the released NOx. The NOx release air-fuel ratio depends on the amount of NOx stored in the NOx storage catalyst device 56. Therefore, in this embodiment, an optimal NOx release air-fuel ratio with respect to the NOx occlusion amount is obtained in advance, a NOx release air-fuel ratio map is created, and this NOx release air-fuel ratio map is stored in the NOx release air-fuel ratio calculation unit 80. Keep it. Then, an appropriate NOx release air-fuel ratio is extracted from the NOx release air-fuel ratio map in accordance with an estimated value of the NOx occlusion amount described later. The air-fuel ratio of the exhaust gas is controlled to this NOx releasing air-fuel ratio by a NOx releasing unit 78 described later.

  The NOx occlusion amount can be calculated based on the difference between the NOx amount discharged from the NOx occlusion catalyst device 56 and the NOx occlusion capacity of the NOx occlusion catalyst device 56. Here, the amount of NOx discharged from the NOx occlusion catalyst device 56 can be calculated from the NOx concentration and the exhaust flow rate detected by the NOx sensor 37. Further, the exhaust flow rate and the NOx storable amount are values that are known in advance for each operating condition. Therefore, in this embodiment, an exhaust flow rate map and a NOx storable amount map based on the engine speed and the injection amount are created in advance, and these maps are provided in the NOx release air-fuel ratio calculation unit 80. The exhaust flow rate is extracted from the exhaust flow map based on the engine speed and the injection amount detected by the engine speed sensor 30, and the NOx storable amount is extracted from the NOx storable amount map. And the NOx occlusion amount of the NOx occlusion catalyst device 56 are estimated based on the NOx concentration detected by the NOx sensor 37. The NOx emission air-fuel ratio extracted from the NOx emission air-fuel ratio map based on this NOx occlusion amount is a value around 14. Here, the NOx storable amount may be corrected by the temperature of the exhaust gas.

  The NOx releasing unit 78 performs riching control for controlling the air-fuel ratio of the exhaust gas to the NOx releasing air-fuel ratio that is richer than the stoichiometric air-fuel ratio, and the NOx stored in the NOx storage catalyst device 56 is transferred to the NOx. It is discharged from the storage catalyst device 56.

  In the NOx release unit 78, when the storage state determination unit 76 determines that the NOx storage catalyst device 56 is saturated with respect to storage of NOx, the enrichment control is started. That is, when the NOx concentration detected by the NOx sensor 37 becomes equal to or higher than the saturation determination value, the air-fuel ratio of the exhaust gas is controlled to the NOx release air-fuel ratio. 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 Then, the amount of injection supplied into each cylinder 12 is increased by performing post injection in each cylinder 12. Thus, when the air-fuel ratio of the exhaust gas becomes the NOx release air-fuel ratio smaller than the stoichiometric air-fuel ratio, the inside of the NOx occlusion catalyst device 56 becomes an oxygen-deficient atmosphere, NOx is released from the NOx occlusion material, and NOx The NOx storage capacity of the storage catalyst device 56 is recovered. Here, the released NOx is reduced and purified by unburned HC and CO, and is not discharged outside.

  The NOx release unit 78 continues the enrichment control until the storage state determination unit 76 determines that the NOx storage amount of the NOx storage catalyst device 56 is sufficiently small. That is, the air-fuel ratio of the exhaust gas is maintained at the NOx release air-fuel ratio until the NOx concentration detected by the NOx sensor 37 becomes equal to or less than the enrichment end determination value. As a result, the NOx storage capacity of the NOx storage catalyst device 56 is reliably recovered.

  Further, the NOx release unit 78 performs the enrichment control only during a period from when the regeneration control by the DPF regeneration control unit 60 is executed until the temperature of the exhaust gas falls below a preset normal operation permission temperature. Allowed. This period is a period in which the temperature of the exhaust gas is raised by execution of regeneration control by the DPF regeneration control unit 60, NOx is released from the NOx adsorption catalyst device 54, and the NOx occlusion catalyst device 56 is This corresponds to a period during which the released NOx is occluded. Therefore, by configuring the enrichment control to be performed only during this period, the enrichment control can be performed efficiently. The normal operation permission temperature is set to 300 ° C., for example.

  When the regeneration control is completed, the air-fuel ratio of the exhaust gas returns to the normal lean state, and the platinum contained in the NOx adsorption catalyst device 54 and the NOx storage catalyst device 56 is exposed to a high temperature and lean atmosphere. It will be. However, if the enrichment control is performed even after the regeneration control by the DPF regeneration control unit 60 is completed as described above, the time during which the platinum is exposed to the high temperature and lean atmosphere is shortened. There is also an effect that platinum is suppressed from being oxidized by oxygen contained in the exhaust gas.

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.

When the air-fuel ratio of the exhaust gas is maintained on the richer side than the stoichiometric air-fuel ratio in this way, the amount of CO and HC in the exhaust gas is ensured, and hydrogen is generated by the reaction of these with water. NH ₃ is generated from the NOx. The NH ₃ thus generated is adsorbed on the NH ₃ adsorbent.

  The S regeneration control unit 74 receives the NOx adsorbent contained in the NOx adsorption catalyst device 54 and the sulfur component adsorbed on the NOx occlusion material contained in the NOx occlusion catalyst device 56 from the NOx adsorbent and the NOx occlusion material, respectively. The NOx adsorption capacity of the NOx adsorbent and the NOx occlusion capacity of the NOx occlusion material are recovered by desorption. The S regeneration control unit 74 utilizes the characteristic that the NOx adsorbent and the NOx occlusion material desorb the adsorbed sulfur component as the oxygen concentration decreases and the supplied HC or CO increases. Thus, the air-fuel ratio of the exhaust gas flowing into the NOx adsorption catalyst device 54 and the NOx storage catalyst device 56 is controlled to a value (for example, 14) smaller than the theoretical air-fuel ratio. Then, the sulfur component is desorbed from the NOx adsorbent and the NOx occlusion material.

  Next, the enrichment control in the NOx releasing unit 78 will be described with reference to the flowchart of FIG.

  First, in step S1, it is determined whether or not the DPF 52 is being regenerated. That is, the DPF regeneration control unit 60 performs the combustion removal control on the PM in the DPF 52 to determine whether or not the exhaust gas is heated. If this determination is NO, the process is terminated as it is. On the other hand, if this determination is YES, it is determined in step S2 whether or not the exhaust gas temperature Ts detected by the exhaust gas temperature sensor 35 is equal to or higher than the normal operation permission temperature T1. If this determination is NO, the process is terminated as it is. On the other hand, if this determination is YES, it is determined in step S3 whether or not S regeneration control is stopped. That is, the S regeneration control unit 74 determines whether the air-fuel ratio of exhaust gas is being enriched. When this determination is NO, that is, when the S regeneration control is being executed, the air-fuel ratio of the exhaust gas is enriched by the S regeneration control unit 74, and the enrichment control by the NOx releasing unit 78 is performed. Since there is no need, the processing is terminated as it is.

Further, even when the S regeneration control is not executed, if the air-fuel ratio of the exhaust gas is enriched, it is not necessary to perform the enrichment control in the same manner. It is determined whether or not the air-fuel ratio O _{1 of the} exhaust gas detected by the A / F sensor 36 is equal to or greater than a preset value O ₀ (for example, 14.7). If this determination is NO, the process is terminated, and the process proceeds to step S5 only when this determination is YES.

  In step S5, the occlusion state determination unit 76 determines whether the NOx concentration Vs detected by the NOx sensor 37 is equal to or higher than the saturation determination value V1.

  If this determination is YES, that is, if the NOx concentration Vs is equal to or higher than the saturation determination value V1 and the NOx storage catalyst device 56 is found to be saturated with respect to storage of NOx, The NOx release air-fuel ratio calculation unit 80 estimates the NOx storage amount of the NOx storage catalyst device 56 and calculates the NOx release air-fuel ratio AF2 (step S6). Next, the NOx releasing section 78 reduces the air-fuel ratio of the exhaust gas to the NOx releasing air-fuel ratio AF2 (step S7). At this time, unburned HC or the like flows into the NOx occlusion catalyst device 56 and NOx occluded in the NOx occlusion material is released. The released NOx is reduced and purified by the HC or the like.

  Next, the occlusion state determination unit 76 determines whether or not the NOx concentration detected by the NOx sensor 37 is equal to or less than the enrichment end determination value V2 (step S8). If this determination is YES, it is determined that the NOx occluded from the NOx occlusion catalyst device 56 has been sufficiently released, and the process ends. Then, the process returns to the regeneration control of the DPF 52 by the DPF regeneration control unit 60, and the air-fuel ratio of the exhaust gas is returned to the DPF regeneration air-fuel ratio AF1 for the regeneration control (step S9). On the other hand, if the determination is NO, the process returns to step S7 again. That is, the air-fuel ratio of the exhaust gas is held at the NOx release air-fuel ratio AF2 until the NOx concentration becomes equal to or less than the enrichment end determination value V2. Further, if the regeneration control of the DPF 52 is continued by the DPF regeneration control unit 60 after the air-fuel ratio of the exhaust gas is returned to the DPF regeneration air-fuel ratio AF1 (YES in Step S10), the process returns to Step S2. Steps S2 to S10 are repeated.

  Here, in the present embodiment, as described above, after the regeneration control by the DPF regeneration control unit 60 is completed, the enrichment control is performed until the exhaust gas temperature Ts drops below the normal operation permission temperature T1. Is running.

  That is, if it is determined that regeneration control of DPF 52 has ended (NO in step S10), whether or not the exhaust gas temperature Ts detected by the exhaust gas temperature sensor 35 in step S11 is equal to or higher than the normal operation permission temperature T1. Determine if. If this determination is NO, the temperature of the exhaust gas is sufficiently lowered, and NOx is not released from the NOx adsorption catalyst device 54, and normal operation is resumed (step S15). On the other hand, if the determination is YES, that is, if the temperature of the exhaust gas is not sufficiently lowered, the process proceeds to step S12.

  In step S12, the occlusion state determination unit 76 determines whether the NOx concentration Vs detected by the NOx sensor 37 is equal to or higher than the saturation determination value V1. If this determination is YES, the NOx release unit 78 reduces the air-fuel ratio of the exhaust gas to the NOx release air-fuel ratio AF2 (step S13), and the NOx concentration Vs becomes equal to or less than the enrichment end determination value V2. This air-fuel ratio is maintained until it becomes (step S14). On the other hand, when the determination in step S12 is NO, and when the NOx releasing section 78 holds the air / fuel ratio of the exhaust gas at the NOx releasing air / fuel ratio AF2, the NOx concentration Vs becomes the enrichment end determination value V2. When it becomes below, it returns to normal driving | operation (step S15).

  Thereafter, it is determined again whether the exhaust gas temperature Ts is equal to or higher than the normal operation permission temperature T1 (step S16). If this determination is NO, that is, if the temperature of the exhaust gas has not yet been sufficiently lowered, the process returns to step S12 again, and step S12 to step until the exhaust gas temperature Ts becomes lower than the normal operation permission temperature T1. Repeat S16.

  Changes in the air-fuel ratio (A / F) of the exhaust gas when the enrichment control is performed as described above, the NOx concentration detected by the NOx sensor 37, and the exhaust gas temperature detected by the exhaust gas temperature sensor 35 are as follows. An example is shown in FIG.

  As shown in this figure, when regeneration control of the DPF 52 is started by the DPF regeneration control unit 60 (at time t0), the exhaust gas temperature is raised to around 600 ° C. When the exhaust gas temperature starts to exceed 300 ° C., the NOx concentration does not increase so much. This is because NOx adsorbed by the NOx adsorption catalyst device 54 begins to be desorbed, but the desorbed NOx is occluded by the NOx occlusion catalyst device 56, so that release of NOx to the outside is suppressed. This is probably because of this.

  On the other hand, if the exhaust gas temperature continues at a high temperature, the NOx concentration rises above the saturation determination value V1 (at time t1). This is because the NOx occlusion catalyst device 56 is saturated with respect to NOx occlusion, and NOx from the engine and NOx desorbed from the NOx adsorption catalyst device 54 are released without being occluded by the NOx occlusion catalyst device 56. It is thought to be done. When the NOx concentration becomes equal to or higher than the saturation determination value V1, the enrichment control is performed by the NOx release unit 78, and the air-fuel ratio of the exhaust gas is controlled to the NOx release air-fuel ratio (t1 to t2).

  When the enrichment control is performed by the NOx release unit 78, the NOx storage capability of the NOx storage catalyst device 56 is recovered, and NOx is stored again in the NOx storage catalyst device 56. In FIG. 5, the NOx concentration decreases after the enrichment control is started. In this way, NOx is again stored in the NOx storage catalyst device 56, and the release of NOx to the outside is suppressed. It is thought that it is because.

  When the regeneration control of the DPF 52 by the DPF regeneration control unit 60 ends (time t3), the air-fuel ratio of the exhaust gas is returned to a normal lean state (for example, 22). However, the NOx concentration gradually increases even after the regeneration control of the DPF 52 is completed. This is considered to be because immediately after the regeneration control is finished, the temperature of the exhaust gas is not sufficiently lowered, and NOx is desorbed from the NOx adsorption catalyst device 54.

  Here, in the present embodiment, as described above, the enrichment control is executed even after the regeneration control of the DPF 52 is completed. Therefore, when the concentration of the exhaust gas becomes equal to or higher than the saturation determination value V1, the enrichment control is performed by the NOx releasing unit 78, and the air-fuel ratio of the exhaust gas is controlled to the NOx releasing air-fuel ratio (t4 to t5). Then, the enrichment control restores the NOx storage capacity, and the release of NOx to the outside is again suppressed.

  In this way, NOx is stored by the NOx storage catalyst device 56 and enrichment control is performed by the NOx release part 78 to restore the NOx storage capability of the NOx storage catalyst device 56, so that the exhaust gas temperature is reduced. Even during the regeneration control period of the DPF 52 at a high temperature of 300 ° C. or higher, it is possible to suppress the release of NOx to the outside. In FIG. 5, the transition example of the NOx concentration when the NOx storage catalyst device 56 is not provided is shown by a broken line. As indicated by the broken line, when the NOx storage catalyst device 56 is not provided, the regeneration control of the DPF 52 is started and the exhaust gas temperature is raised, so that the NOx concentration detected by the NOx sensor 37 increases. That is, when the NOx storage catalyst device 56 is not provided, the exhaust gas temperature is raised, so that NOx discharged from the engine is released to the outside as it is and is adsorbed by the NOx adsorption catalyst device 54. NOx is desorbed and released, and a large amount of NOx is released to the outside as a whole.

  As described above, according to the exhaust gas purification apparatus 10, the NOx occlusion that occludes at a temperature higher than the temperature at which the NOx adsorbent adsorbs NOx is performed downstream of the NOx adsorption catalyst apparatus 54 containing the NOx adsorbent. Since the NOx occlusion catalyst device 56 containing the material is provided, even if NOx is desorbed from the NOx adsorbent due to the exhaust gas temperature becoming high, the desorbed NOx is used as the NOx occlusion material. Can be occluded.

  Further, if rich control is performed to make the air-fuel ratio of the exhaust gas richer than the stoichiometric air-fuel ratio in the NOx releasing section 78, the NOx stored in the NOx storage catalyst device 56 can be released. The NOx storage capacity of the storage catalyst device 56 can be recovered. The NOx occlusion catalyst device 56 is provided with an occlusion state determination unit 76 for determining the occlusion state of the NOx, and the enrichment control is performed based on the occlusion state of the NOx determined by the occlusion state determination unit 76. If done, the NOx occlusion capacity can be efficiently recovered.

  In particular, if the enrichment control is started when the NOx storage catalyst device 56 is saturated, the NOx storage capability of the NOx storage catalyst device 56 is recovered more efficiently.

  Further, a NOx release air-fuel ratio calculating unit 80 for calculating the NOx release air-fuel ratio is provided, and this NOx release air-fuel ratio is determined according to the NOx storage state in the NOx storage catalyst device 56 determined by the storage state determination unit 76. Thus, the air-fuel ratio of the exhaust gas is controlled according to the NOx occlusion state in the NOx occlusion catalyst device 56, and the recovery of the NOx occlusion capability of the NOx occlusion catalyst device 56 is more efficiently performed. Done.

  The storage state determination unit 76 determines the NOx storage state of the NOx storage catalyst device 56 based on the NOx concentration detected by the NOx sensor 37 provided on the downstream side of the NOx storage catalyst device 56. Thus, this occlusion state can be determined more accurately.

  Further, if the exhaust gas temperature detected by the exhaust gas temperature sensor 35 is equal to or higher than the normal operation permission temperature, if it is configured to permit the execution of the enrichment control, excess fuel consumption associated with the enrichment is reduced. It can be suppressed, leading to improved fuel economy.

  When the NOx adsorption catalyst device 54 and the NOx occlusion catalyst device 56 are provided on the downstream side of the DPF 52, the NOx is occluded even when the exhaust gas becomes hot during the combustion of PM in the DPF 52, Since this NOx emission to the outside can be suppressed, it is more effective.

  Further, a DPF regeneration control unit 60 for controlling the regeneration of the DPF 52 is provided, and during the regeneration control of the DPF 52 by the DPF regeneration control unit 60 and after the regeneration control of the DPF 52 is finished, the temperature of the exhaust gas is allowed to be the normal operation permission. If the enrichment control is permitted to be performed when the temperature is higher than the temperature, the NOx storage capacity of the NOx storage catalyst device 56 can be recovered more efficiently.

  Here, in the above-described embodiment, the case where the enrichment control by the NOx releasing unit 78 is performed during the DPF regeneration control is shown, but the detection is performed by the exhaust gas temperature sensor 35 regardless of whether the DPF regeneration control is being performed. The enrichment control may be started when the temperature of the exhaust gas is equal to or higher than a predetermined value (for example, 300 ° C.) set in advance.

  In this case, the enrichment control is performed in accordance with the value of the exhaust gas temperature that determines the NOx release timing of the NOx adsorption catalyst device 54, and the NOx occlusion capability of the NOx occlusion catalyst device is efficiently improved. Can be recovered. Further, NOx emission can be suppressed even when the exhaust gas temperature is 300 ° C. or higher during high-speed operation or the like.

  The NOx release air-fuel ratio may be a constant value. In the occlusion state determination unit 76, the NOx release air-fuel ratio is set to a constant value, while the air-fuel ratio of the exhaust gas is maintained at the NOx release air-fuel ratio in accordance with the NOx concentration detected by the NOx sensor 37. And the enrichment control may be performed in the NOx releasing unit 78 only during the calculated period. Furthermore, the air-fuel ratio of the exhaust gas may be held at a predetermined constant value for a predetermined period. In the NOx releasing unit 78, for example, the air-fuel ratio of the exhaust gas may be changed in small increments between the NOx releasing air-fuel ratio and the lean air-fuel ratio.

  In addition, specific values such as the saturation determination value, the enrichment end determination value, and the normal operation permission temperature are not limited to the above. These values may be set for each operating condition. Further, in the above embodiment, the enrichment control is performed when the exhaust gas temperature is equal to or higher than the same normal allowable temperature both during the regeneration control of the DPF 52 by the DPF regeneration control unit 60 and after the regeneration control ends. However, the temperature determination value for permitting the enrichment control during the regeneration control may be different from the normal operation permission temperature for permitting the enrichment control after the regeneration control. For example, the determination value during regeneration control may be 300 ° C., and the normal operation permission temperature after regeneration control may be 400 ° C. And if it does in this way, the period enriched after reproduction | regeneration control can be shortened.

  The specific configurations of the NOx adsorption catalyst device 54 and the NOx storage catalyst device 56 and the arrangement of the NOx adsorption catalyst device 54, the NOx storage catalyst device 56 and the DPF 52 are not limited to the above. The DPF 52 can be omitted.

  Further, the method for enriching the air-fuel ratio of the exhaust gas in the NOx releasing section 78 and the like is not limited to the above.

  Moreover, this exhaust gas purification apparatus 10 is applicable not only to a diesel engine but to other lean burn engines.

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 adsorption catalyst apparatus in the exhaust gas purification apparatus shown in FIG. It is a schematic sectional drawing which expands and shows the internal structure of the NOx storage catalyst apparatus in the exhaust gas purification apparatus shown in FIG. 2 is a schematic flowchart of enrichment control by a NOx release unit in the exhaust gas purification apparatus shown in FIG. 1. It is explanatory drawing for demonstrating the change of the air fuel ratio of exhaust gas, NOx density | concentration, and the temperature of exhaust gas at the time of using the exhaust gas purification apparatus shown in FIG.

Explanation of symbols

1 Engine body 2 ECU
22 Exhaust passage 35 Exhaust gas temperature sensor (exhaust gas temperature detection means)
37 NOx sensor (NOx concentration detection means)
40 Intake throttle 41 EGR valve 52 DPF (particulate filter)
54 NOx adsorption catalyst device 54a Cordierite carrier substrate 54b NOx adsorption catalyst layer 54c NH ₃ adsorption catalyst layer 56 NOx occlusion catalyst device 56a Cordierite carrier substrate 56b NOx occlusion catalyst layer 60 DPF regeneration control unit (regeneration control means)
76 Occlusion State Judgment Unit (Occlusion State Judgment Unit)
78 NOx release part (NOx release means)
80 NOx emission air-fuel ratio calculation unit (NOx emission air-fuel ratio calculation means)
A / F Air-fuel ratio PM Particulate matter V1 Saturation judgment value V2 Enrichment completion judgment value T1 Normal operation permission temperature

Claims (8)

An exhaust gas purification device for a lean burn engine,
The NOx adsorbent that is provided in the exhaust passage of the engine and adsorbs NOx contained in the exhaust gas discharged from the combustion chamber of the engine and the NOx adsorbed on the NOx adsorbent are converted in an oxygen-deficient atmosphere. a NOx trap catalyst device containing a NH ₃ adsorbing material for adsorbing NH ₃ comprising,
A NOx occlusion catalyst device that is provided on the downstream side of the NOx adsorption catalyst device and contains a NOx occlusion material that occludes NOx in the exhaust passage at a temperature higher than the temperature at which the NOx adsorbent adsorbs NOx ;
Occlusion state determination means for determining the NOx occlusion state in the NOx occlusion catalyst device;
Based on the NOx occlusion state in the NOx occlusion catalyst device determined by the occlusion state determination means, the enrichment control is performed to make the air-fuel ratio of the exhaust gas richer than the stoichiometric air-fuel ratio to the NOx release air-fuel ratio, An exhaust gas purification device comprising NOx storage means for releasing NOx stored in the NOx storage material included in the NOx storage catalyst device and reducing and purifying the released NOx . The exhaust gas purification apparatus according to claim 1,
The NOx release means starts the enrichment control when the storage state determination means determines that the NOx storage catalyst device is saturated with respect to storage of NOx. apparatus. The exhaust gas purification apparatus according to claim 1 or 2,
An exhaust gas purification apparatus comprising NOx release air-fuel ratio calculation means for calculating the NOx release air-fuel ratio in accordance with the NOx storage state in the NOx storage catalyst device determined by the storage state determination means . In the exhaust gas purification apparatus according to any one of claims 1 to 3,
Provided on the downstream side of the NOx occlusion catalyst device, comprising NOx concentration detection means for detecting the concentration of NOx in the exhaust passage;
The exhaust gas purification device, wherein the storage state determination means determines the NOx storage state in the NOx storage catalyst device based on the NOx concentration detected by the NOx concentration detection means . In the exhaust gas purification apparatus according to any one of claims 1 to 4,
An exhaust gas temperature detecting means provided on the upstream side of the NOx adsorption catalyst device for detecting the temperature of the exhaust gas in the exhaust passage;
The NOx releasing means permits the enrichment control to be performed when the temperature of the exhaust gas detected by the exhaust gas temperature detecting means is equal to or higher than a predetermined value set in advance . In the exhaust gas purifying apparatus according to any one of claims 1 to 5,
A particulate filter that collects particulate matter discharged from the engine and burns and removes the collected particulate matter;
The exhaust gas purification device, wherein the NOx adsorption catalyst device and the NOx storage catalyst device are provided on the downstream side of the particulate filter . The exhaust gas purification apparatus according to claim 6,
Regeneration control is performed so that the air-fuel ratio of the exhaust gas is richer than the air-fuel ratio during normal operation and leaner than the stoichiometric air-fuel ratio, and the particulate matter collected by the particulate filter is burned Having a regeneration control means for removing,
The exhaust gas purification apparatus , wherein the NOx releasing means permits the enrichment control to be performed during the regeneration control by the regeneration control means . The exhaust gas purification apparatus according to claim 7,
An exhaust gas temperature detecting means provided on the upstream side of the NOx adsorption catalyst device for detecting the temperature of the exhaust gas in the exhaust passage;
The NOx releasing means is immediately after the regeneration control by the regeneration control means is completed and during a period in which the temperature of the exhaust gas detected by the exhaust gas temperature detecting means is equal to or higher than a preset normal operable temperature. The exhaust gas purifying apparatus , wherein the enrichment control is performed when the storage state determination means determines that the NOx storage catalyst device is saturated with respect to storage of NOx .

Images