JP2006083746A - Exhaust emission control method and exhaust emission control system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control method and an exhaust emission control system for regenerating NOx storage reduction type catalyst without discharging NOx onto the downstream side of NOx storage reduction type catalyst even when an engine is in an exhaust gas low temperature operation region of low speed rotation and low load. <P>SOLUTION: In this exhaust emission control system 10 provided with NOx storage reduction type catalyst 50, exhaust temperature rise control is started when temperature of catalyst Tg is less than temperature of activity start Tc of precious metal catalyst and estimated amount Rnox of NOx storage reaches a second predetermined determination value R02 set to a value of NOx storage saturation amount R0 at the temperature of activity start Tc, and stoichiometric or rich control is performed when the temperature of catalyst Tg rises above the temperature of activity start Tc to perform regeneration control for restoring NOx storage capability of NOx storage reduction type catalyst 50. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification method and an exhaust gas purification system provided with a NOx occlusion reduction type catalyst that reduces and purifies NOx (nitrogen oxide) in exhaust gas of an internal combustion engine.

  Various studies and proposals have been made on NOx catalysts for reducing and removing NOx (nitrogen oxides) from exhaust gases of internal combustion engines such as diesel engines and some gasoline engines and various combustion devices. One of them is a NOx occlusion reduction type catalyst as a NOx reduction catalyst for diesel engines, which can effectively purify NOx in exhaust gas.

This NOx occlusion reduction type catalyst is formed of a monolith honeycomb 30M or the like having a structure as shown in FIG. 8, and this monolith honeycomb 30M is a structural material made of cordierite or stainless steel as shown in FIG. A plurality of polygonal cells 30S are formed on the carrier 31. As shown in FIGS. 9 and 10, a porous catalyst coat layer 34 serving as a catalyst support layer formed of alumina (Al ₂ O ₃ ) or zeolite is provided on the wall surface of the cell 30S. A supported noble metal (catalytically active metal) 32 and a NOx occlusion material (NOx occlusion material: NOx occlusion agent: NOx absorbent) 33 are carried on the surface of the catalyst coat layer 34 which has a surface area to be contacted, thereby generating a catalytic function. I am letting.

  11 and 12 show the arrangement of the catalyst materials 32 and 33 on the surface of the support layer of the NOx storage reduction catalyst and the NOx storage reduction mechanism. This NOx occlusion reduction type catalyst has a catalyst coat layer 34 with a supported noble metal 32 such as platinum (Pt) having an oxidation function and potassium (K), sodium (Na), lithium (Li), cesium (NOx occlusion function). NOx occlusion material 33 formed from some of alkali metals such as Cs), alkaline earth metals such as barium (Ba) and calcium (Ca), and rare earths such as lanthanum (La) and yttrium (Y). It is supported and has two functions, NOx occlusion and NOx release / purification, depending on the oxygen concentration in the exhaust gas.

As shown in FIG. 11, when the air-fuel ratio of the exhaust gas containing oxygen (O ₂ ) in the exhaust gas of a normal diesel engine, lean-burn gasoline engine or the like is in a lean air-fuel ratio state, Oxygen contained in the gas causes nitrogen monoxide (NO) discharged from the engine to be oxidized into nitrogen dioxide (NO ₂ ) by the oxidation catalyst function of the supported noble metal 32. Then, the nitrogen dioxide is occluded in the form of nitrate in the NOx occlusion material 33 such as barium having NOx occlusion function to purify NOx.

  However, in this state, the NOx occlusion material 33 having the NOx occlusion function is all changed to nitrate and loses the NOx occlusion function. Therefore, by changing the operating conditions of the engine or injecting fuel into the exhaust passage, there is no oxygen in the exhaust gas, the exhaust gas having a high carbon monoxide (CO) concentration and a high exhaust temperature, that is, , Create rich combustion exhaust gas and send it to the catalyst.

Then, as shown in FIG. 12, when the exhaust gas has no oxygen, the carbon monoxide concentration is high, and the exhaust gas temperature is raised to a rich air-fuel ratio state, the nitrate that occludes NOx releases nitrogen dioxide and releases the original nitrogen dioxide. Return to barium etc. Since the released nitrogen dioxide has no oxygen in the exhaust gas, carbon monoxide, hydrocarbons (HC), hydrogen (H ₂ ) in the exhaust gas are used as a reducing agent by the oxidation function of the supported noble metal 32. Reduce to water (H ₂ O), carbon dioxide (CO ₂ ), nitrogen (N ₂ ) and purify.

  Therefore, in an exhaust gas purification system equipped with a NOx occlusion reduction type catalyst, when the estimated NOx occlusion amount becomes the NOx occlusion saturation amount, the air-fuel ratio of the exhaust gas is made rich and the oxygen concentration of the inflowing exhaust gas is lowered. A regeneration operation is performed to release the absorbed NOx by performing rich control for recovering the NOx storage capacity and reduce the released NOx by a noble metal catalyst.

  However, since this NOx occlusion reduction type catalyst has a very low reaction activity at a low catalyst temperature, a chemical occlusion reaction to NOx occurs in a low temperature operation region where the exhaust gas temperature of the engine is about 250 ° C. or lower. In addition, the NOx release / purification reaction due to the rich exhaust gas is also extremely reduced.

  Therefore, in order to quickly activate the exhaust purification catalyst disposed in the exhaust system of the internal combustion engine and maintain it in the activation start temperature range, a variable valve mechanism that changes the opening / closing timing and lift amount of the intake valve, An exhaust emission control device for a compression ignition type internal combustion engine has been proposed (see, for example, a patent) having an exhaust temperature raising means for controlling the throttle valve and controlling the fuel injection valve to perform post injection to raise the exhaust gas temperature. Reference 1).

  On the other hand, as shown in FIG. 13, the NOx occlusion estimated amount becomes the NOx occlusion saturation amount when the exhaust gas in which the engine is operating at a low load is in a very low temperature region where the exhaust gas is extremely low. When regeneration of the storage reduction catalyst is started, the temperature of the exhaust gas is raised by exhaust gas temperature rise control or the like that performs injection timing delay, post injection, etc. in the fuel injection control into the cylinder. As a result, the catalyst temperature is raised, and the catalytic activity during regeneration of the NOx storage reduction catalyst is improved.

  As one of such exhaust purification devices, the amount of NOx adsorbed on the NOx trap catalyst (NOx storage reduction catalyst) reaches a predetermined amount NOx1 set to a value smaller than the predetermined amount NOx2 corresponding to the regeneration timing. It is determined whether or not the regeneration timing is near, and when the regeneration preparation state is entered and it is determined that the catalyst bed temperature is lower than the activation start temperature of the catalyst, the excess air ratio λ By reducing the exhaust gas temperature and increasing the exhaust gas temperature, the NOx regeneration can be performed smoothly when the engine shifts to the rich spike mode thereafter, and the exhaust of the internal combustion engine that can avoid the deterioration of operability due to the long-term enrichment can be avoided. A purification device has been proposed (see, for example, Patent Document 2).

  However, in the extremely low temperature range below the catalyst activation temperature of the NOx occlusion reduction type catalyst, particularly 200 ° C. or less, it becomes a state of physical adsorption instead of chemical occlusion. Not only chemical occlusion in a state higher than the activation temperature but also this physical adsorption is expressed by “occlusion”.

  As shown in FIG. 7, when the catalyst temperature Tg is increased, the NOx occlusion saturation amount R0 of the NOx occlusion material is decreased, so that the regeneration time is close, and the relationship between the catalyst temperature and the NOx occlusion saturation amount is as follows. If the NOx occlusion reduction catalyst is at a low temperature and the temperature rise is started in preparation for regeneration, when a predetermined amount of NOx1 is set in the region A shown in FIG. 7, NOx occlusion reduction accompanies the temperature rise of the NOx occlusion reduction catalyst. Since the saturation amount R0 decreases, the NOx occluded in the NOx occlusion reduction type catalyst during the temperature rise is released and released.

  On the other hand, the relationship between the catalyst temperature of the noble metal catalyst and the purification rate is as shown in FIG. 14, and the catalyst temperature is the activation start temperature Tc of the noble metal catalyst of the NOx storage reduction type catalyst (depending on the catalyst). If the excess air ratio λ is reduced and the exhaust gas is made rich while the temperature does not reach about 250 ° C. to 300 ° C.), even if it is the B region shown in FIG. The stored NOx is released. Therefore, as in the exhaust gas purification apparatus of Patent Document 2, if the exhaust air temperature is raised with the excess air ratio λ reduced, NOx stored in the NOx occlusion reduction type catalyst is released during the temperature rise. Become.

  Before the catalyst temperature Tg reaches the activation start temperature Tc of the noble metal catalyst, the NOx released during the temperature rise is not reduced by the catalytic action and remains as NOx because the catalytic activity of the noble metal catalyst is low. It will be discharged downstream of the storage reduction catalyst. In addition, HC, CO, and the like to be a reducing agent during NOx reduction are also discharged downstream of the NOx storage reduction catalyst.

Therefore, it is important how to set the predetermined amount NOx1 when starting the exhaust gas temperature raising control as a preparation for regeneration, and until the catalyst temperature Tg reaches the activation start temperature Tc, the NOx occlusion reduction type. It is important to raise the temperature while preventing NOx stored in the catalyst from being released.
JP 2003-120368 A JP 2004-36552 A

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an exhaust gas purification system that uses a NOx occlusion reduction type catalyst for purifying NOx in exhaust gas. Exhaust gas purification method and exhaust gas capable of regenerating NOx occlusion reduction catalyst without releasing NOx downstream of NOx occlusion reduction catalyst even in a low load exhaust gas low temperature operation region It is to provide a purification system.

The exhaust gas purifying method for achieving the above-described purpose is to store NOx when the air-fuel ratio of the exhaust gas is lean, and to release and reduce NOx stored when it is rich. In order to restore the NOx occlusion capacity of the NOx occlusion reduction catalyst when the estimated NOx occlusion amount estimated to have been occluded by the NOx occlusion reduction catalyst reaches a predetermined judgment value. In the exhaust gas purification system that performs regeneration control of
Whether or not the catalyst temperature of the NOx occlusion reduction type catalyst is equal to or higher than the activation start temperature of the noble metal catalyst supported by the NOx occlusion reduction type catalyst based on the detection value of the temperature sensor that detects the temperature of the exhaust gas or the catalyst Determine whether
When the activation start temperature is equal to or higher than the activation start temperature, when the NOx occlusion estimated amount reaches a predetermined first determination value set by the NOx occlusion saturation amount in the catalyst, stoichiometric or rich control is performed to perform NOx occlusion reduction type catalyst. To restore NOx storage capacity of
When the NOx occlusion estimated amount reaches a predetermined second determination value set to the value of the NOx occlusion saturation at the activation start temperature when the activation start temperature is lower than the activation start temperature, the exhaust gas temperature raising control is started, After the catalyst temperature is raised to the activation start temperature or higher, stoichiometric or rich control is performed to perform regeneration control for recovering the NOx storage capacity of the NOx storage reduction catalyst.

  Regarding NOx purification of the NOx occlusion reduction type catalyst, it is in a state of physical adsorption rather than chemical occlusion at an extremely low temperature range below the catalyst activation temperature of the NOx occlusion reduction type catalyst, particularly 200 ° C. or less. However, here, the expression “occlusion” includes not only chemical occlusion in a state higher than the catalyst activation temperature but also this physical adsorption.

  In the exhaust gas purification method, the predetermined second determination value is set to a value at which the estimated NOx occlusion amount becomes the NOx occlusion saturation amount at the activation start temperature when the exhaust gas temperature raising control is finished. . That is, a value obtained by subtracting in advance the amount of NOx occluded during the exhaust gas temperature raising control, that is, the increment of the NOx occlusion estimated amount by the exhaust gas temperature raising control, is set as the predetermined second determination value.

  As described above, the predetermined second determination value is a value at which NOx is not released from the NOx occlusion reduction catalyst due to the temperature rise by the exhaust gas temperature raising control, so that the engine has a low rotation speed and low load exhaust gas low temperature operation region. Even in this case, the NOx storage reduction catalyst can be regenerated without releasing NOx to the downstream side of the NOx storage reduction catalyst.

  Further, since the stoichiometric or rich control is performed after the catalyst temperature reaches the activation start temperature of the noble metal catalyst, HC and CO generated in the exhaust gas by the stoichiometric or rich control are consumed as a reducing agent for NOx reduction. The NOx storage reduction catalyst is also prevented from being discharged to the downstream side.

Further, the exhaust gas purification system for achieving the above-described object occludes NOx when the air-fuel ratio of the exhaust gas is in a lean state and releases NOx that has been occluded when it is in a rich state. The NOx occlusion reduction type catalyst that is reduced together with the NOx occlusion reduction type catalyst is restored when the estimated NOx occlusion amount estimated to have been occluded by the NOx occlusion reduction type catalyst reaches a predetermined judgment value. In an exhaust gas purification system provided with a regeneration control means for performing regeneration control for
Catalyst temperature detecting means for detecting the catalyst temperature from the detection value of the temperature sensor for detecting the temperature of the exhaust gas or the catalyst;
Catalyst temperature determination means for determining whether the catalyst temperature detected by the catalyst temperature detection means is equal to or higher than the activation start temperature of the noble metal catalyst supported by the NOx storage reduction catalyst;
When the determination by the catalyst temperature determination means is equal to or higher than the activation start temperature, the stoichiometric or rich control is performed when the estimated NOx storage amount reaches a predetermined first determination value set by the NOx storage saturation amount in the catalyst. To restore the NOx occlusion capacity of the NOx occlusion reduction type catalyst, and when the NOx occlusion reduction catalyst temperature is lower than the activation start temperature, the estimated NOx occlusion amount is set to a predetermined second value set to the value of the NOx occlusion saturation at the activation start temperature. When the judgment value is reached, the exhaust gas temperature raising control is started and the catalyst temperature is raised to the activation start temperature or higher, and then the stoichiometric or rich control is performed to restore the NOx occlusion capacity of the NOx occlusion reduction type catalyst. And reproduction control means for performing reproduction control.

  In the exhaust gas purification system, the predetermined second determination value is set to a value at which the NOx occlusion estimated amount becomes the NOx occlusion saturation amount at the activation start temperature when the exhaust gas temperature raising control is finished. .

  Here, the air-fuel ratio state of the exhaust gas does not necessarily mean the state of the air-fuel ratio in the cylinder, but the amount of air and the amount of fuel supplied to the exhaust gas flowing into the NOx storage reduction catalyst (cylinder) (Including the amount burned inside).

  As described above, according to the exhaust gas purification method and the exhaust gas purification system of the present invention, NOx is stored in the NOx occlusion reduction type even when the engine is in the low-speed / low-load exhaust gas low-temperature operation region. Since the NOx occlusion reduction type catalyst can be regenerated without being released to the downstream side of the catalyst, it is possible to improve the NOx purification performance in the low temperature region and prevent deterioration of fuel consumption, HC, and CO exhaust gas.

  Hereinafter, an exhaust gas purification method and an exhaust gas purification system according to embodiments of the present invention will be described with reference to the drawings. Note that the exhaust gas stoichiometric and rich state here does not necessarily require stoichiometric combustion or rich combustion in the cylinder, but the amount of air and fuel supplied to the exhaust gas flowing into the NOx storage reduction catalyst (cylinder) (Including the amount burned inside) is in a state close to the stoichiometric air-fuel ratio, or in a stoichiometric or rich state in which the amount of fuel is larger than the stoichiometric air-fuel ratio.

  An exhaust gas purification system 10 shown in FIG. 1 is configured by disposing an NOx storage reduction catalyst 50 in an exhaust passage 20 of an engine (internal combustion engine) 1.

  The NOx occlusion reduction type catalyst 50 is formed of a monolithic catalyst, and a catalyst coat layer is provided on a carrier such as aluminum oxide or titanium oxide, and a noble metal catalyst (catalyst such as platinum (Pt) (Pd)) is provided on the catalyst coat layer. Metal) and NOx occlusion material (NOx occlusion material) such as barium (Ba) are supported.

  In this NOx occlusion reduction type catalyst 50, when the oxygen concentration is in the exhaust gas state (lean air-fuel ratio state), the NOx occlusion material occludes NOx in the exhaust gas, thereby purifying the NOx in the exhaust gas, In the exhaust gas state where the oxygen concentration is low or zero, the stored NOx is released and the released NOx is reduced by the catalytic action of the noble metal catalyst, thereby preventing NOx from flowing into the atmosphere.

  A catalyst inlet NOx sensor 51 and a catalyst inlet exhaust gas temperature sensor 52 for detecting the catalyst temperature Tg of the NOx storage reduction catalyst 50 are provided on the inlet side of the NOx storage reduction catalyst 50, and a catalyst outlet is provided on the outlet side. Each NOx sensor 53 is provided. Further, a turbine 21 a of the turbocharger 21 is disposed on the upstream side of the NOx occlusion reduction type catalytic converter 50 in the exhaust passage 20.

  On the other hand, the intake passage 30 is provided with a mass air flow sensor (MAF sensor) 31, a compressor (not shown) of the turbocharger 21, and an intake throttle valve (intake throttle valve) 32, and on the upstream side of the turbine 21a. An EGR passage 40 that connects the exhaust passage 20 and the intake passage 30 is provided, and an EGR cooler 41 and an EGR valve 42 are provided in the EGR passage 40.

  A control device (ECU: engine control unit) 60 that performs overall control of the operation of the engine 1 and also performs recovery control of the NOx purification ability of the NOx storage reduction catalyst 50 is provided. Detection values from the catalyst inlet NOx sensor 51, the catalyst inlet exhaust gas temperature sensor 52, the catalyst outlet NOx sensor 53, and the like are input to the controller 60, and the EGR valve 42 of the engine 1 and the common rail for fuel injection are input from the controller 60. A signal for controlling the fuel injection valve 61 and the intake throttle valve 32 of the electronically controlled fuel injection device is output.

  In the exhaust gas purification system 10, the air A passes through a mass air flow sensor (MAF sensor) 31 in the intake passage 30 and a compressor (not shown) in the turbocharger 21, and the amount of the air A is adjusted by the intake throttle valve 32. And enters the cylinder. The exhaust gas G generated in the cylinder drives the turbine 21a of the turbocharger 21 in the exhaust passage 20 and passes through the NOx storage reduction catalyst 50 to become purified exhaust gas Gc. It is discharged into the atmosphere through the vessel. A part of the exhaust gas G passes through the EGR cooler 41 of the EGR passage 40 as EGR gas Ge, and the amount thereof is adjusted by the EGR valve 42 and recirculated to the intake passage 30 side.

  The control device of the exhaust gas purification system 10 is incorporated in the control device 60 of the engine 1, and the exhaust gas purification system 10 is controlled in parallel with the operation control of the engine 1. The control device of the exhaust gas purification system 10 includes a NOx occlusion reduction catalyst control means C1 as shown in FIG.

  This NOx occlusion reduction type catalyst control means C1 is a means for controlling regeneration, desulfurization (sulfur purge), etc. of the NOx occlusion reduction type catalyst 50, NOx concentration detection means C11, catalyst temperature detection means C12, catalyst temperature determination. Means C13, a low temperature range regeneration control means C20, a high temperature range regeneration control means C30 and the like are configured.

  The NOx concentration detection means C11 is a means for detecting the NOx concentration in the exhaust gas, and has a catalyst inlet NOx sensor 51 and a catalyst outlet NOx sensor 53. When an exhaust component concentration sensor in which a NOx concentration sensor and an oxygen concentration sensor are integrated is used as the NOx sensor, the oxygen concentration (or excess air ratio) can be detected together with the NOx concentration.

  The catalyst temperature detection means C12 is a means for detecting the catalyst temperature based on the exhaust gas temperature Tg detected by the catalyst inlet exhaust gas temperature sensor 52. Strictly speaking, the exhaust gas temperature Tg at the catalyst inlet is different from the catalyst temperature and needs to be corrected. However, considering the simplicity of control, the exhaust gas temperature Tg is often used as the catalyst temperature. The exhaust gas temperature (catalyst inlet gas temperature) Tg is used as the temperature. When the catalyst temperature sensor is provided and the catalyst temperature is directly measured, the measured temperature becomes the detected catalyst temperature as it is.

  The catalyst temperature determination means C13 is configured so that the catalyst temperature (in this case, the exhaust gas temperature is substituted) Tg is in a low temperature region lower than the activation start temperature Tc of the noble metal catalyst of the NOx storage reduction catalyst, or the activation start temperature Tc. It is a means to determine whether it exists in the above high temperature range.

  The low temperature region regeneration control means C20 is means for performing regeneration control of the NOx occlusion reduction type catalyst when the catalyst temperature Tg is lower than the activation start temperature Tc. The NOx catalyst regeneration start determining means C21, the exhaust gas temperature raising control means. C22, NOx catalyst regeneration control means C23 and the like.

  The NOx catalyst regeneration start judging means C21 is a judgment value (predetermined second judgment) for starting the exhaust gas temperature raising control from the map data set beforehand by experiments or the like from the engine speed Ne and the load Q indicating the operating state of the engine. Value) R02 is calculated. This determination value R02 for starting the exhaust gas temperature raising control is a predetermined determination value (predetermined second determination value) for determining the timing for starting the regeneration control. In this low temperature region regeneration control, the NOx occlusion reduction type It is set to the same value as the NOx occlusion saturation amount R0 at the activation start temperature Tc of the catalyst.

  Although the map data needs to be set, this predetermined second determination value is used as the NOx occlusion estimated amount when the exhaust gas temperature raising control is finished, and the NOx occlusion reduction amount becomes the NOx occlusion saturation at the activation start temperature Tc of the NOx occlusion reduction type catalyst. If the amount R02 is set to a value, it is possible to further reduce the release of NOx to the downstream side of the NOx storage reduction catalyst. In this case, the NOx occlusion amount during the exhaust gas temperature raising control is taken into consideration, and the value obtained by subtracting the NOx occlusion amount during the exhaust gas temperature raising control from the NOx occlusion saturation amount R02 is the predetermined second determination value. Is done.

  That is, the NOx occlusion saturation is read, the exhaust gas temperature increase control, the exhaust gas temperature increase control, the threshold value at which NOx is not released even if the catalyst temperature rises is obtained in advance by experiments, and this threshold value is input as map data. Then, the exhaust gas temperature raising control is started based on this threshold value.

  On the other hand, from the catalyst inlet NOx concentration Cnoxin, the catalyst outlet NOx concentration Cnoxex, the fuel injection amount (fuel weight) Qg, and the intake air amount (intake air weight) Ag indicating the state of the exhaust gas, Rnox1 = (Qg + Ag ) × (Cnoxin−Cnoxex), the first NOx occlusion estimated amount Rnox1 that is the NOx emission amount per unit time is calculated. Since the NOx occlusion amount slightly affects the temperature, a value obtained by correcting the first NOx occlusion estimation amount Rnox1 as a function is cumulatively calculated to obtain the NOx occlusion estimation amount Rnox.

  Then, the NOx occlusion estimated amount Rnox is compared with the determination value R02 for starting the exhaust gas temperature raising control. When the NOx occlusion estimated amount Rnox becomes equal to or greater than the predetermined determination value R02, it is determined that the regeneration control start time is reached. to decide.

  Further, the exhaust gas temperature raising control means C22 is a means for raising the exhaust temperature by fuel injection control such as after injection, delayed injection, multistage injection, post injection, etc., intake / exhaust throttle control, etc. in fuel injection into the cylinder. is there. 5 and 6 show examples of after injection and delayed injection. In the after injection control of FIG. 5, pilot injection Pf is performed before the main injection Mf, and after injection Af is performed after the main injection Mf. In the delayed injection control of FIG. 6, the pilot injection Pf is performed before the main injection Mf, and the main injection Mf is delayed from the normal main injection Mf. In these exhaust gas temperature raising controls, the NOx occlusion reduction catalyst noble metal catalyst is not activated during the temperature rise, and NOx cannot be reduced. Therefore, during the temperature rise, the NOx occlusion reduction catalyst uses the NOx occlusion material. The excess air ratio λ is maintained lean (for example, about 5.0 to 2.0) so that the stored NOx is not released.

  In this exhaust temperature increase control, the catalyst inlet gas temperature Tg becomes equal to or higher than the temperature increase determination temperature Ts calculated from the map data set by experiments or the like in advance from the engine rotational speed Ne indicating the engine operating state and the load Q. If finished. This temperature rise determination temperature Ts is normally set to a temperature equal to or higher than the activation start temperature Tc of the noble metal catalyst of the NOx storage reduction catalyst.

  The NOx catalyst regeneration control means C23 is a control for reducing the NOx occluded from the NOx occlusion material of the NOx occlusion reduction catalyst by reducing the air-fuel ratio of the exhaust gas to a stoichiometric or rich state, and reducing it by a noble metal catalyst, Inlet throttle control such as valve throttle, exhaust valve throttle, EGR control, etc., and post injection or exhaust pipe fuel injection control are used in combination, and the excess air ratio λ is stoichiometric or rich (for example, 1.0 to 0.90). To do.

  In this regeneration process, the excess air ratio is always measured together with the NOx concentration by the catalyst inlet NOx sensor 51 or the like formed by the exhaust component concentration sensor, fed back, and controlled below the stoichiometry. At this time, the control may be performed so that the deep side is gradually increased and the stoichiometric side is gradually increased. In this way, it is held for a predetermined time below the stoichiometric condition, and the process ends.

  The high temperature region regeneration control means C30 is means for performing regeneration control of the NOx storage reduction catalyst when the catalyst temperature Tg is equal to or higher than the activation start temperature Tc, and is the same as the regeneration control of the prior art. The high temperature region regeneration control means C30 includes a NOx catalyst regeneration start determining means C31, a NOx catalyst regeneration control means C32, a desulfurization control start determining means C33, a desulfurization control means C34, and the like.

  The NOx catalyst regeneration start judging means C31 is set in advance by experiments or the like from the engine rotation speed Ne and the load Q indicating the operating state of the engine, similarly to the NOx catalyst regeneration start judging means C21 of the low temperature region regeneration control means C20. The determination value R01 for starting the reproduction control is calculated from the map data. This determination value R01 is a predetermined determination value (predetermined first determination value) for determining the timing for starting the regeneration control. In this high temperature region regeneration control, the activation start temperature Tc of the NOx storage reduction catalyst is determined. It is set to the same value as the NOx storage saturation amount R0.

  Further, from the catalyst inlet NOx concentration Cnoxin, the catalyst outlet NOx concentration Cnoxex, the fuel injection amount (fuel weight) Qg, and the intake air amount (intake air weight) Ag, Rnox1 = (Qg + Ag) × (Cnoxin−Cnoxex) The first NOx occlusion estimated amount Rnox1 that is the NOx emission amount per unit time is calculated. Since the NOx occlusion amount slightly affects the temperature, a value obtained by correcting the first NOx occlusion estimation amount Rnox1 as a function is cumulatively calculated to obtain the NOx occlusion estimation amount Rnox.

  Then, the NOx occlusion estimated amount Rnox is compared with a predetermined first determination value R01, and when the NOx occlusion estimated amount Rnox becomes equal to or greater than the predetermined first determination value R01, it is determined that it is the time to start regeneration control. To do.

  The NOx catalyst regeneration control means C32 is a means for controlling the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio (theoretical air-fuel ratio) or a rich state, and controls the EGR valve 42 to increase the EGR amount, The throttle valve 32 is controlled to reduce a new intake amount, or fuel is added to the exhaust gas by the fuel injection control in the cylinder injection, so that the air-fuel ratio of the exhaust gas is lowered and the exhaust gas is reduced. The air fuel ratio is made lower than the stoichiometric air fuel ratio.

  By these controls, the air-fuel ratio of the exhaust gas is set to 1.00 to 0.90 in terms of excess air ratio (λ) before the catalyst, and in a predetermined temperature range (almost 200 ° C. to 600 ° C. depending on the catalyst). And the NOx storage capacity, that is, the NOx purification capacity, is recovered, and the NOx catalyst is regenerated.

  Then, the desulfurization control start determination means C33 determines whether or not to start sulfur purge control depending on whether or not sulfur has accumulated until the NOx occlusion capacity decreases by a method such as integrating the sulfur (sulfur) accumulation amount. When the sulfur accumulation amount exceeds a predetermined judgment value, desulfurization is started. The desulfurization control means C34 is a means for efficiently performing desulfurization while suppressing discharge of carbon monoxide (CO) into the atmosphere, and controls the air-fuel ratio of exhaust gas by in-pipe injection or post-injection. Then, EGR control and intake throttle control are performed to raise the temperature of the NOx storage reduction catalyst to a temperature at which desulfurization is possible.

  In the exhaust gas purification system 10, the control means C1 of the exhaust gas purification system of the control device of the exhaust gas purification system 10 incorporated in the control device 60 of the engine 1 follows the control flow as illustrated in FIG. Regeneration control of the NOx occlusion reduction type catalytic converter 10 is performed. FIG. 4 schematically shows an example of a time series of the NOx occlusion estimated amount Rnox, the catalyst temperature Tg, the exhaust gas temperature raising control (after-injection control), and the NOx regeneration control (air-fuel ratio rich control) according to the control flow of FIG. Indicate.

  The control flow in FIG. 3 is shown as being executed in parallel with other control flows of the engine when the engine 1 is operated. Further, the present invention relates to a low temperature region regeneration control of the NOx catalyst. Since the conventional technology can be used for the high temperature region regeneration control, the description of the high temperature region regeneration control is omitted.

  When the control flow of FIG. 3 starts, in step S11, the catalyst temperature detecting means C12 detects the catalyst temperature (here, substituted by the catalyst inlet gas temperature) Tg and inputs it. In the next step S12, the catalyst temperature determination means C13 determines whether the catalyst temperature Tg is in a low temperature region below the activation start temperature Tc of the NOx storage reduction catalyst or in a high temperature region above the activation start temperature Tc.

  If it is determined in step S12 that the temperature is in the high temperature range, the high temperature range regeneration control unit C30 performs high temperature range regeneration control in step S20. After passing through the high temperature region regeneration control routine, the process returns to step S11. In this high temperature range regeneration control, regeneration control and desulfurization control are performed if various conditions are established, but if the conditions are not met, the control returns without performing regeneration control and desulfurization control.

  If it is determined in step S12 that the temperature is in the low temperature range, the process goes to step S13, and the NOx catalyst regeneration start determination means C21 of the low temperature range regeneration control means C20 determines the engine speed indicating the engine operating state. A judgment value (predetermined second judgment value) R02 for starting the exhaust gas temperature raising control is calculated from map data preset by experiments or the like from the speed Ne and the load Q. Further, from the catalyst inlet NOx concentration Cnoxin, the catalyst outlet NOx concentration Cnoxex, the fuel injection amount (fuel weight) Qg, and the intake air amount (intake air weight) Ag, Rnox1 = (Qg + Ag) × (Cnoxin−Cnoxex) The first NOx occlusion estimated amount Rnox1 that is the NOx emission amount per unit time is calculated. Since the NOx occlusion amount slightly affects the temperature, a value obtained by correcting the first NOx occlusion estimation amount Rnox1 as a function is cumulatively calculated to obtain the NOx occlusion estimation amount Rnox.

  In the next step S14, the NOx occlusion estimated amount Rnox is compared with the judgment value R02 for starting the exhaust gas temperature raising control. If the NOx occlusion estimated amount Rnox is less than the predetermined second judgment value R02, the regeneration control start is started. It is determined that it is not time, and the process returns to step S11. If the NOx occlusion estimation amount Rnox is equal to or greater than the predetermined determination value R02, it is determined that it is time to start regeneration control, and the process proceeds to step S15.

  In step S15, the exhaust gas temperature raising control means C22 performs exhaust gas temperature raising control. In this exhaust temperature raising control, the exhaust temperature is raised by fuel injection control such as after injection, delayed injection, multistage injection, post injection, etc., intake / exhaust throttle control, etc. in fuel injection into the cylinder. In this exhaust temperature increase, the excess air ratio λ is about 5.0 to 2.0. After this exhaust temperature raising control is performed for a predetermined time, the process goes to step S16. The predetermined time is a time related to the check interval of the catalyst inlet gas temperature.

  In step S16, a temperature increase determination temperature Ts is calculated from map data set beforehand through experiments or the like from the engine speed Ne indicating the operating state of the engine and the load Q, and the catalyst inlet gas temperature Tg is input.

  In the next step S17, it is checked whether or not the catalyst inlet gas temperature Tg is equal to or higher than the temperature rise determination temperature Ts. If it is lower than the temperature rise determination temperature Ts, the process returns to step S15 and the exhaust temperature rise control is repeated. When the catalyst inlet gas temperature Tg becomes equal to or higher than the temperature rise determination temperature Ts, the exhaust gas temperature rise control is terminated, and the process proceeds to the next step S18.

  In step S18, NOx regeneration control is performed by the NOx catalyst regeneration control means C23, the exhaust gas air-fuel ratio is stoichiometric or rich, and the NOx occluded from the NOx occlusion material of the NOx occlusion reduction type catalyst is released. Reduction with a noble metal catalyst. In this NOx regeneration control, intake throttle control such as intake valve throttle, exhaust valve throttle, EGR control, etc., and fuel injection control are used in combination, and the excess air ratio λ is set to 1.00-0.90. This NOx regeneration control is performed until regeneration of the NOx occlusion reduction catalyst is completed, and when completed, the process returns to step S11.

  The control flow of FIG. 3 is repeatedly executed until the engine is stopped. If the engine key is turned off during the control, an interrupt in step S19 is generated, and after performing necessary termination processing (not shown) at each step where the interrupt occurred, the process returns. The control flow ends with the end of the main control.

  According to the exhaust gas purification system 10 having the above configuration, in the regeneration control of the NOx storage capacity of the NOx storage reduction catalyst 50, even when the engine is in the low-speed exhaust gas low temperature operation region of low rotation and low load. NOx storage reduction catalyst can be regenerated without releasing NOx to the downstream side of the NOx storage reduction catalyst, improving NOx purification performance in the low temperature range and preventing deterioration of fuel consumption, HC and CO exhaust gas can do.

It is a figure which shows the structure of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which shows the structure of the control means of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which shows an example of the control flow for regeneration of a NOx occlusion reduction type catalyst. FIG. 4 is a diagram schematically showing temporal changes in an estimated NOx storage amount, catalyst temperature, exhaust gas temperature raising control, and NOx regeneration control according to the control flow of FIG. 3. It is a figure which shows after injection typically. It is a figure which shows delayed injection typically. It is a figure which shows the relationship between catalyst temperature and NOx absorption saturation amount. It is a figure which shows a monolith honeycomb. It is the elements on larger scale of a monolith honeycomb. It is an enlarged view of the cell wall part of a monolith honeycomb. It is a figure which shows typically the structure of a NOx occlusion reduction type catalyst, and the purification mechanism of the state at the time of lean control. It is a figure which shows typically the structure of a NOx occlusion reduction-type catalyst, and the purification mechanism of the state at the time of rich control. It is a figure which shows a cryogenic region typically. It is a figure which shows the relationship between a catalyst temperature and a purification rate.

Explanation of symbols

1 Engine 10 Exhaust Gas Purification System 20 Exhaust Passage 30 Intake Passage 32 Intake Throttle Valve (Intake Throttle Valve)
40 EGR passage 42 EGR valve 50 NOx occlusion reduction type catalyst 51 Catalyst inlet NOx sensor 52 Catalyst inlet exhaust gas temperature sensor 53 Catalyst outlet NOx sensor C1 NOx occlusion reduction type catalyst control means C1 1 NOx concentration detection means C12 Catalyst temperature detection means C13 Catalyst temperature determination means C20 Low temperature range regeneration control means C21 NOx catalyst regeneration start judgment means C22 Exhaust temperature rise control means C23 NOx catalyst regeneration control means C30 High temperature range regeneration control means C31 NOx catalyst regeneration start judgment means C32 NOx catalyst regeneration control means C33 Desulfurization control start determination means C34 Desulfurization control means

Claims (4)

A NOx occlusion reduction type catalyst that occludes NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces NOx occluded when it is rich is provided in the NOx occlusion reduction type catalyst. In an exhaust gas purification system that performs regeneration control for recovering the NOx occlusion capacity of the NOx occlusion reduction catalyst when the estimated NOx occlusion amount estimated to be occluded reaches a predetermined determination value,
Whether or not the catalyst temperature of the NOx occlusion reduction type catalyst is equal to or higher than the activation start temperature of the noble metal catalyst supported by the NOx occlusion reduction type catalyst based on the detection value of the temperature sensor that detects the temperature of the exhaust gas or the catalyst Determine whether
When the activation start temperature is equal to or higher than the activation start temperature, when the NOx occlusion estimated amount reaches a predetermined first determination value set by the NOx occlusion saturation amount in the catalyst, stoichiometric or rich control is performed to perform NOx occlusion reduction type catalyst. To restore NOx storage capacity of
When the NOx occlusion estimated amount reaches a predetermined second determination value set to the value of the NOx occlusion saturation at the activation start temperature when the activation start temperature is lower than the activation start temperature, the exhaust gas temperature raising control is started, An exhaust gas purification method comprising performing regeneration control for recovering the NOx occlusion capacity of a NOx occlusion reduction catalyst by performing stoichiometric or rich control after raising the catalyst temperature to an activation start temperature or higher.   The predetermined second determination value is set to a value at which the estimated NOx occlusion amount becomes the NOx occlusion saturation amount at the activation start temperature when the exhaust gas temperature raising control is finished. Exhaust gas purification method. A NOx occlusion reduction type catalyst that occludes NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces NOx occluded when it is rich is provided in the NOx occlusion reduction type catalyst. In an exhaust gas purification system provided with a regeneration control means for performing regeneration control for recovering the NOx storage capacity of the NOx storage reduction catalyst when the estimated NOx storage amount estimated to be stored reaches a predetermined determination value ,
Catalyst temperature detecting means for detecting the catalyst temperature from the detection value of the temperature sensor for detecting the temperature of the exhaust gas or the catalyst;
Catalyst temperature determination means for determining whether the catalyst temperature detected by the catalyst temperature detection means is equal to or higher than the activation start temperature of the noble metal catalyst supported by the NOx storage reduction catalyst;
When the determination by the catalyst temperature determination means is equal to or higher than the activation start temperature, the stoichiometric or rich control is performed when the estimated NOx storage amount reaches a predetermined first determination value set by the NOx storage saturation amount in the catalyst. To restore the NOx occlusion capacity of the NOx occlusion reduction type catalyst, and when the NOx occlusion reduction catalyst temperature is lower than the activation start temperature, the estimated NOx occlusion amount is set to a predetermined second value set to the value of the NOx occlusion saturation at the activation start temperature. When the determination value is reached, exhaust temperature raising control is started, and after raising the catalyst temperature to be higher than the activation start temperature, stoichiometric or rich control is performed to restore the NOx occlusion capacity of the NOx occlusion reduction type catalyst. An exhaust gas purification system comprising a regeneration control means for performing regeneration control. The predetermined second determination value is set to a value at which the estimated NOx occlusion amount becomes the NOx occlusion saturation amount at the activation start temperature when the exhaust gas temperature raising control is finished. Exhaust gas purification system.

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