JP5768607B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device including a NOx trap catalyst that adsorbs nitrogen oxides contained in exhaust gas of an internal combustion engine and purifies the exhaust gas.

  Conventionally, an exhaust gas purification apparatus provided with a NOx trap catalyst (also referred to as an occlusion type NOx catalyst or an occlusion reduction type NOx catalyst) as a catalyst for purifying nitrogen oxides (hereinafter referred to as NOx) contained in the exhaust gas of an internal combustion engine. It has been known. This NOx trap catalyst adsorbs NOx as nitrate on the catalyst support surface in an oxygen-excess atmosphere (also referred to as oxidizing atmosphere or lean), and adsorbs NOx adsorbed in an atmosphere having a low oxygen concentration (also referred to as reducing atmosphere or rich). It has the function of releasing and reducing.

When the amount of NOx adsorbed on the NOx trap catalyst (hereinafter referred to as NOx adsorption amount) approaches a saturated state, the exhaust purification device equipped with the NOx trap catalyst makes the ambient atmosphere of the NOx trap catalyst rich and the NOx trap catalyst. Is supplied with a reducing agent such as carbon monoxide (CO) generated by incomplete combustion of fuel or the like, and the nitrate adsorbed on the NOx trap catalyst reacts with a reducing agent such as CO in the exhaust. Thus, NOx is released from the NOx trap catalyst, and the released NOx is reduced to harmless nitrogen (N ₂ ) and discharged into the atmosphere.

  By the way, the NOx trap catalyst generally has a property of adsorbing a sulfur component (sulfur, hereinafter also simply referred to as S) contained in exhaust gas. Since the sulfur component adsorbed on the NOx trap catalyst has higher chemical stability than nitrate, only a small amount is released even if the ambient atmosphere of the NOx trap catalyst is made rich in order to release NOx from the NOx trap catalyst. . Therefore, the sulfur component remaining in the NOx trap catalyst increases with time, and the ability (performance) of adsorbing NOx, which is the original function of the NOx trap catalyst, is reduced.

  Therefore, a technique for performing a so-called S desorption process (hereinafter also referred to as S purge) in which the sulfur component adsorbed on the NOx trap catalyst is periodically released in order not to deteriorate the original function of the NOx trap catalyst is known. It has been. For example, in the exhaust emission control device of Patent Document 1, additional fuel is injected during the expansion stroke of the internal combustion engine to raise the temperature of the NOx catalyst to a high temperature and supply a large amount of CO as a reducing agent to the NOx catalyst. As a result, the sulfur component adsorbed on the NOx catalyst and CO are reacted to release the sulfur component from the NOx catalyst (S purge operation).

  By the way, in order to perform the S purge, the NOx trap catalyst needs to be heated to a higher temperature than during the NOx release process, and more fuel is injected than the NOx release process to cope with this (that is, the air-fuel ratio). Therefore, there is a concern about deterioration of fuel consumption. Therefore, it is preferable to quickly release the sulfur component adsorbed on the NOx trap catalyst to suppress deterioration of fuel consumption. In order to improve the release rate of the sulfur component (hereinafter referred to as S release rate), it is desired that the air-fuel ratio on the downstream side of at least the NOx trap catalyst is constantly made rich.

  In addition, it is known that the S release rate is faster at the start of the S purge when the amount of sulfur component adsorbed on the NOx trap catalyst is large, and becomes slower as the amount of sulfur component adsorbed as the S purge progresses. ing. As a technique for improving the S release rate, for example, Patent Document 2 estimates the degree of decrease in the release rate of SOx released from the NOx storage reduction catalyst during SOx poisoning recovery control (S purge operation). An exhaust purification system that raises the temperature of the NOx catalyst to a higher temperature target temperature as the degree of decrease increases is disclosed. According to this system, even if the degree of decrease in the SOx release rate increases, the catalyst bed temperature is increased to a higher target temperature to enhance the SOx reduction reaction in the NOx catalyst, and the SOx release rate (ie, Sx It is said that the SOx poisoning recovery control can be completed in a short period of time by suppressing a decrease in the release rate.

JP 11-229864 A JP 2009-68481 A

However, if the air-fuel ratio downstream of the NOx trap catalyst is constantly rich, most of the sulfur component released from the NOx trap catalyst is released into the atmosphere as hydrogen sulfide (H ₂ S). Even if this H ₂ S is a slight amount (level) that does not directly affect health, a human's sensitive olfaction may sense a odor peculiar to H ₂ S. Therefore, it is desired to suppress the amount of released H ₂ S (that is, H ₂ S concentration) as much as possible.

This case has been devised in view of such problems, and in an exhaust purification device equipped with a NOx trap catalyst, the H ₂ S concentration during the S desorption treatment can be reduced and odor can be suppressed. Another object of the present invention is to provide an exhaust purification device for an internal combustion engine.
The present invention is not limited to this purpose, and is a function and effect derived from each configuration shown in the embodiments for carrying out the invention described later, and other effects of the present invention are to obtain a function and effect that cannot be obtained by conventional techniques. Can be positioned.

(1) An exhaust gas purification apparatus for an internal combustion engine disclosed herein is provided in an exhaust passage of the internal combustion engine, and adsorbs nitrogen oxides and sulfur components in the exhaust gas in an oxidizing atmosphere where the air-fuel ratio of the exhaust gas is lean. A rich duration time in which the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst is richer than the stoichiometric air-fuel ratio and a lean duration time in which the air-fuel ratio is leaner than the stoichiometric air-fuel ratio are periodically repeated. An air-fuel ratio control means for controlling the air-fuel ratio, and an S desorption process for desorbing the sulfur component adsorbed on the NOx trap catalyst from the NOx trap catalyst in a reducing atmosphere where the air-fuel ratio is rich S a desorption process execution means, and S release speed estimating means for estimating a release speed at which the sulfur component from the NOx trap catalyst is desorbed, release the S If the release speed estimated by the speed estimating means is equal to or higher than the first predetermined speed, a determining means and limit condition is satisfied, Ru comprising a.
The S desorption process execution means, said When the restriction condition is satisfied S during the execution of the desorption process, by the air-fuel ratio control by the air-fuel ratio control means, the average air-fuel ratio at the outlet side of the NOx trap catalyst The rich degree with respect to the stoichiometric air-fuel ratio in the rich duration time is controlled so that the rich state is always in a rich state, and the front temperature on the exhaust upstream side and the rear temperature on the exhaust downstream side of the NOx trap catalyst are controlled by the NOx trap. By periodically changing the S desorption start temperature, which is the temperature at which the sulfur component begins to desorb from the catalyst, vertically, and swinging the front temperature and the rear temperature with a predetermined phase shift It is characterized by limiting the area of the NOx trap catalyst in which the sulfur component is desorbed. In addition, the area | region here means a spatial area | region. In addition, the release speed that the determination means compares with the first predetermined speed is a normal release speed during the S desorption process.

(2) It is preferable to provide an adsorption S amount estimation means for estimating the amount of the sulfur component adsorbed on the NOx trap catalyst . In this case, the S release speed estimating means, said a Turkey be estimated before Kiho out speed based on the amount of the sulfur component was estimated by adsorption S amount estimating means is preferred.

(3) Preferably, the S desorption processing execution means executes the S desorption processing by releasing the restriction of the area when a predetermined restriction release condition is satisfied when the area is restricted.
(4) Preferably, the determination unit determines that the restriction release condition is satisfied when the release rate estimated by the S release rate estimation unit is less than a second predetermined rate. The release speed that the determination means compares with the second predetermined speed is the release speed during the S desorption process in which the region is limited.

( 5 ) The S desorption execution means limits the region from which the sulfur component is desorbed by setting the predetermined phase as an opposite phase and swinging so that the phase is reversed at the S desorption start temperature. It is preferable to do.
(6) the air-fuel ratio control means, said controls the richness of the rich duration, it is preferred to substantially the same and the lean duration time and the rich time duration.
( 7 ) It is preferable that two NOx trap catalysts are arranged in series in the exhaust passage.

According to the disclosed exhaust purification device for an internal combustion engine, when a predetermined restriction condition is satisfied during execution of the S desorption process for desorbing the sulfur component from the NOx trap catalyst, the region in which the sulfur component is desorbed from the NOx trap catalyst ( Therefore, the sulfur component adsorbed on the NOx trap catalyst can be prevented from being released from the entire NOx trap catalyst. That is, since the amount of sulfur component released from the NOx trap catalyst per unit time can be reduced, the amount of H ₂ S released per unit time released into the atmosphere can be reduced. In other words, the maximum value of the H ₂ S concentration released into the atmosphere (that is, the peak value of the H ₂ S concentration) can be lowered. Thus, it is possible to suppress the odor of H ₂ S.

1 is a schematic overall configuration diagram showing an exhaust gas purification apparatus for an internal combustion engine according to an embodiment. It is a graph which shows the relationship between the quantity (S adsorption amount) of the sulfur component adsorbed by the NOx trap catalyst and the rate (S release rate) at which the sulfur component is released during the S desorption process. FIG. 6 is a graph for explaining an S desorption process, in which (a) is an air-fuel ratio state of exhaust gas that is changed by air-fuel ratio control, (b) is a temperature state of a NOx trap catalyst that is changed by air-fuel ratio control; ) Shows the change in the S release rate during the S desorption process. It is a flowchart which shows the control content of S desorption process.

  Hereinafter, embodiments will be described with reference to the drawings. Note that the embodiment described below is merely an example, and there is no intention to exclude various modifications and technical applications that are not explicitly described in the following embodiment.

[1. Device configuration]
The exhaust purification device of this embodiment is applied to a diesel engine (internal combustion engine) 1 mounted on a vehicle. FIG. 1 shows one of a plurality of cylinders 2 provided in the engine 1, but the other cylinders 2 have the same configuration. A piston 3 that reciprocates vertically is provided in the cylinder 2 of the engine 1. The piston 3 is connected to the crankshaft 5 via the connecting rod 4. The piston 3 is formed with a cavity 3a serving as a combustion chamber on the top surface.

  The cylinder head 6 above the cylinder 2 is provided with an injector 7 for fuel injection. The injector 7 has a tip projecting from the in-cylinder space of the cylinder 2 and directly injects fuel into the cylinder 2. The injection direction of the fuel injected from the injector 7 is set to a direction toward the cavity 3 a of the piston 3. A fuel pipe 7 a is connected to the base end portion of the injector 7, and pressurized fuel is supplied from the fuel pipe 7 a to the injector 7.

  The cylinder head 6 is provided with an intake port 8 and an exhaust port 9 communicating with the in-cylinder space of the cylinder 2, and an intake valve 10 and an exhaust valve 11 for opening and closing these ports 8 and 9 are provided. An intake passage 12 having an air cleaner and a throttle valve (not shown) is connected to the intake port 8, and an exhaust passage 13 is connected to the exhaust port 9. The exhaust passage 13 is provided with a fuel addition device 14 for adding fuel into the exhaust passage 13, and an exhaust purification device 20 for purifying the exhaust is disposed downstream of the fuel addition device 14. The exhaust passage 13 is provided with a temperature sensor 15 for detecting the temperature of the exhaust gas flowing through the exhaust passage 13, an air-fuel ratio sensor 16 for detecting the air-fuel ratio of the exhaust, and a flow rate sensor 17 for detecting the flow rate of the exhaust. It is done.

The exhaust purification device 20 includes a front-stage oxidation catalyst 21, a filter 22, a NOx trap catalyst 23, and a rear-stage oxidation catalyst 24, which are arranged in order from the exhaust upstream side. Here, the front-stage oxidation catalyst 21, the filter 22, the NOx trap catalyst 23, and the rear-stage oxidation catalyst 24 are accommodated in the three cases interposed in the exhaust passage 13, respectively.
The pre-stage oxidation catalyst 21 is an oxidation catalyst having an oxidizing ability with respect to components in exhaust gas, and is a catalyst in which a catalyst material is supported on a honeycomb-shaped carrier made of metal, ceramics or the like.

  The filter 22 is a porous filter (for example, a ceramic filter) that collects particulate matter (hereinafter abbreviated as PM) contained in the exhaust gas. In addition, PM is a particulate material in which fuel, lubricant components, sulfur compounds, and the like that remain unburned around carbon black smoke (soot) are attached. The inside of the filter 22 is divided into a plurality along the flow direction of the exhaust by a porous wall. A large number of pores having a size commensurate with the particulates of PM are formed in the wall, and PM is collected on the wall and on the surface of the wall when exhaust passes near or inside the wall. The filter 22 performs regeneration control for purifying exhaust gas by continuously oxidizing and removing the collected PM.

  The NOx trap catalyst 23 adsorbs nitrogen oxides (hereinafter referred to as NOx) as nitrates on the catalyst support surface in an oxygen-excess oxidizing atmosphere (a state where the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio). Has a function of releasing and reducing adsorbed NOx in a reducing atmosphere where the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio. The amount of NOx that can be adsorbed by the NOx trap catalyst 23 (ie, the maximum amount) is determined, and when the amount of NOx adsorbed by the NOx trap catalyst 23 approaches this maximum amount (ie, saturated state), the adsorbed NOx Is controlled to reduce and purify (hereinafter, this control is referred to as NOx purge).

The NOx purge is performed by making the ambient atmosphere of the NOx trap catalyst 23 a reducing atmosphere (rich) with a low oxygen concentration and supplying a reducing agent such as carbon monoxide (CO) or hydrocarbon (HC) to the NOx trap catalyst 23. Is done. Here, the fuel is added to the exhaust passage 13 from the fuel addition device 14 provided on the upstream side of the NOx trap catalyst 23 to enrich the air-fuel ratio and supply the reducing agent. By performing the NOx purge, NOx is released from the NOx trap catalyst 23, and the released NOx is reduced and converted into harmless nitrogen (N ₂ ) to purify the exhaust gas.

  In the exhaust purification apparatus 20 shown in FIG. 1, two identically shaped NOx trap catalysts 23 having the same performance are arranged in series in the exhaust passage 13. Hereinafter, the NOx trap catalyst 23 disposed on the exhaust upstream side is referred to as a front-stage NOx trap catalyst 23F, and the NOx trap catalyst 23 disposed on the exhaust downstream side is referred to as a rear-stage NOx trap catalyst 23R. Further, when the two are not particularly distinguished, they are simply referred to as NOx trap catalyst 23.

The rear-stage oxidation catalyst 24 arranged on the downstream side of the NOx trap catalyst 23 is an oxidation catalyst having the ability to oxidize components in the exhaust, like the front-stage oxidation catalyst 21, and is formed on a honeycomb-shaped carrier made of metal, ceramics, or the like. A catalyst material is supported.
The temperature sensor 15, air-fuel ratio sensor 16, and flow rate sensor 17 are all provided downstream of the filter 22 and upstream of the NOx trap catalyst 23, and the temperature, air-fuel ratio, and flow rate of the exhaust gas flowing into the NOx trap catalyst 23. Are detected respectively. Information detected by these sensors 15, 16, and 17 is transmitted to the ECU 30.

  The ECU (electronic control unit) 30 includes a CPU that executes various arithmetic processes, a ROM that stores programs and data necessary for the control, a RAM that temporarily stores arithmetic results in the CPU, and an external interface. It has an input / output port for inputting and outputting signals. A temperature sensor 15, an air-fuel ratio sensor 16, and a flow rate sensor 17 are connected to the input side of the ECU 30, and a fuel addition device 14 is connected to the output side. The ECU 30 performs various controls such as the above-described regeneration control and NOx purge. In particular, the ECU 30 performs the following S purge.

  The S purge is a so-called S desorption process in which the sulfur component adsorbed on the NOx trap catalyst 23 is periodically released. The NOx trap catalyst 23 adsorbs not only NOx in the exhaust gas but also a sulfur component (sulfur, hereinafter simply referred to as S) generally contained in the exhaust gas. Since the amount of NOx that can be adsorbed on the surface of the catalyst carrier is determined as described above, the amount of NOx that can be adsorbed decreases when the NOx trap catalyst 23 adsorbs the sulfur component. Furthermore, since only a small amount of the sulfur component is released by the above-described NOx purge, the sulfur component increases with time, and this reduces the ability of the NOx trap catalyst 23 to adsorb NOx, which is the original function. In order to prevent this, S purge is performed.

[2. Control configuration]
Next, a control configuration of the exhaust gas purification apparatus for an internal combustion engine according to the present embodiment will be described with reference to FIGS. 1, 2, and 3 (a) to 3 (c).
In order to perform the above-described S purge, the ECU 30 functions as an adsorption S amount estimation unit 31, a function element as an S release rate estimation unit 32, a function element as a determination unit 33, and an air-fuel ratio control unit. 34 and a functional element as the S purge execution unit 35.

The adsorption S amount estimation unit (adsorption S amount estimation means) 31 estimates the amount of sulfur component adsorbed by the NOx trap catalyst 23 (hereinafter referred to as adsorption S amount) _SAD . The adsorption S amount S _AD depends on the amount of sulfur component discharged from the engine 1, and the amount of sulfur component discharged from the engine 1 depends on the amount of fuel used in the engine 1 and the type of fuel. . Therefore, the adsorption S amount estimation unit 31 integrates, for example, the fuel amount F that has been used since the end of the previous S purge, and, as shown in the following equation (1), the type of fuel is added to this accumulated used fuel amount ΣF. The adsorption S amount S _AD is estimated by multiplying by a coefficient k corresponding to. The coefficient k is a value that varies depending on the type of fuel (for example, fuel used in Japan or fuel used in the United States). This is because the amount of sulfur component discharged varies depending on the type of fuel used.

Further, during the execution of S purge, the adsorption S amount estimation unit 31 is estimated by the later-described S release rate estimation unit 32 from the adsorption S amount S _AD estimated by multiplying the above-described accumulated fuel consumption ΣF by the coefficient k. The release S amount S _EX (= V × t _SP ) obtained by multiplying the S release speed V by the S purge time t _SP is subtracted. Thereby, the amount of the sulfur component adsorbed by the NOx trap catalyst 23 during the execution of the S purge is estimated. Hereinafter, the adsorption S amount during execution of the S purge is referred to as a residual adsorption S amount S _RE .

The estimation methods for the adsorption S amount S _AD and the residual adsorption S amount S _RE are expressed by the following equations (1) and (2).
S = S _AD = ΣF × k (1)
S = S _RE = S _AD −V × t _SP (2)
Information on the estimated adsorption S amount S _AD and residual adsorption S amount S _RE is transmitted to the S release speed estimation unit 32 and the determination unit 33. Hereinafter, the adsorption S amount S _AD and the residual adsorption S amount S _RE are simply referred to as the adsorption S amount unless otherwise distinguished.

  The S release rate estimation unit (S release rate estimation means) 32 calculates a release rate (hereinafter referred to as S release rate) V when the sulfur component is released (desorbed) from the NOx trap catalyst 23 based on the adsorbed S amount. To be estimated. The S release speed V depends on the amount of adsorbed S, the air-fuel ratio (A / F) of exhaust gas, the temperature, and the flow rate. Here, the S release speed V is estimated by paying attention to the amount of adsorbed S. If the conditions of the exhaust gas flowing into the NOx trap catalyst 23 (that is, the air-fuel ratio, temperature, and flow rate of the exhaust gas) are constant, the S release rate V increases as the adsorption S amount increases as shown in FIG. Therefore, a plurality of maps as shown in FIG. 2 corresponding to various exhaust conditions are stored in advance, and values detected by the temperature sensor 15, the air-fuel ratio sensor 16 and the flow rate sensor 17 from the plurality of maps are stored. Select the corresponding map. The S release rate V is estimated by applying the adsorption S amount estimated by the adsorption S amount estimation unit 31 to this map.

In FIG. 2, the solid line indicates the S release speed _VSL with respect to the adsorption S amount when the below-described limited S purge is performed, and the alternate long and short dash line indicates the S release speed V with respect to the adsorption S amount when the normal S purge is performed. _S is shown. As shown in FIG. 2, the S release rate V is smaller when the limited S purge is performed than the normal S purge when the adsorption S amount is the same. Hereinafter, the release rate during normal S purge is referred to as S release rate V _S , and the release rate during limited S purge is referred to as limited S release rate V _SL, and these are referred to as S release rate V unless otherwise distinguished. The estimated S release rate V is transmitted to the determination unit 33.

  The determination unit (determination unit) 33 determines whether or not S purge is necessary, determines whether or not the S purge start condition is satisfied, and whether restriction control is necessary when executing the S purge. The determination of whether or not and the determination of whether or not to release the restriction control are performed. Hereinafter, these determinations will be described.

The determination unit 33 first determines whether or not S purge is necessary based on the adsorption S amount S _AD estimated by the adsorption S amount estimation unit 31. The determination unit 33 compares the adsorption S amount S _AD estimated by the adsorption S amount estimation unit 31 with a predetermined value S _TH set in advance, and when the adsorption S amount S _AD is equal to or larger than the predetermined value S _TH , the S purge is performed. If the adsorption S amount S _AD is less than the predetermined value S _TH , it is determined that the S purge is not necessary. That is, when the following (3) is established, it is determined that S purge is necessary. Note that the predetermined value S _TH used for this determination is the amount of adsorption S that is thought to have accumulated sulfur components to such an extent that the NOx purification performance of the NOx trap catalyst 23 is reduced by, for example, experiments in advance.
(3) Adsorption S amount S _AD ≧ predetermined value S _TH

  The determination unit 33 determines whether or not the S purge can be started when the above (3) is satisfied, in other words, whether or not the S purge can be performed (that is, whether or not the S purge start condition is satisfied). ). The S purge start condition is, for example, that the exhaust gas is stable at a high temperature while the vehicle is traveling on a road such as an expressway at a high speed or traveling stably in a suburb. Even if the above (3) is satisfied, if the S purge start condition is not satisfied, the S purge is not executed.

  If it is determined that the S purge start condition (3) is satisfied, the determination unit 33 determines whether or not restriction control is necessary when performing the S purge. The restriction control is control for restricting the amount of sulfur component released from the NOx trap catalyst 23 per unit time. Hereinafter, this control is referred to as a limited S purge. That is, the limited S purge is a control that limits the region of the NOx trap catalyst 23 from which the sulfur component is released. The area here means a spatial area (that is, a place). In other words, the limited S purge means that the location where the sulfur component is released from the NOx trap catalyst 23 is limited and the sulfur component is partially desorbed from the NOx trap catalyst 23. Specific control for executing the limited S purge will be described later.

The determination unit 33 compares the S release rate V _S when the normal S purge estimated by the S release rate estimation unit 32 is performed with a preset first predetermined rate V _TH1, and the S release rate V _S. in There determines that it is necessary to limit the control in the case of the above first predetermined speed V _TH1, S release rate V _S is not required restriction control in the case of less than the first predetermined speed V _TH1 (i.e., normal S purge Is good). That is, the restriction S purge start condition (restriction condition) is the following condition (4). Note that the first predetermined speed V _TH1 used for this determination is, for example, an H ₂ S concentration at which humans do not feel the H ₂ S odor obtained by experiments in advance, and the S release speed calculated from this H ₂ S concentration. It is.
(4) S release speed V _S ≧ first predetermined speed V _TH1

The determination unit 33 determines whether or not to release the restriction S purge (restriction control) when the restriction condition (4) is satisfied and the restriction S purge is performed. The release of the restriction control means that the restriction S purge is terminated and switched to the normal S purge. Based on the residual adsorption S amount S _RE during execution of the limited S purge estimated by the adsorption S amount estimation unit 31, the determination unit 33 performs the S release rate during execution of the limited S purge estimated by the S release rate estimation unit 32. V _SL is compared with a predetermined second predetermined speed V _TH2, and it is determined that the restriction control is released when the limit S release speed V _SL is less than the second predetermined speed V _TH2 , and the limit S release speed V _SL is It is determined that the restriction control is not released when the second predetermined speed V _TH2 or more. That is, the restriction S purge release condition (restriction release condition) is the following condition (5). Note that the second predetermined speed V _TH2 used for this determination is, for example, an S release speed smaller than the first predetermined speed V _TH1 .
(5) Restricted S release speed V _SL <second predetermined speed V _TH2

  The air-fuel ratio control unit (air-fuel ratio control means) 34 controls the air-fuel ratio (A / F) of the exhaust gas flowing into the NOx trap catalyst 23. In general, the air amount and the fuel amount may be controlled in order to control the air-fuel ratio, but here, in order to control only the air-fuel ratio of the exhaust gas without affecting the output, the fuel addition device 14 enters the exhaust passage 13. Control the amount of fuel added. The air-fuel ratio control unit 34 increases the amount of fuel added from the fuel addition device 14 when the exhaust air-fuel ratio is made richer than the stoichiometric air-fuel ratio, and when the air-fuel ratio is made leaner than the stoichiometric air-fuel ratio. The amount of fuel added from the fuel addition device 14 is decreased (or the amount of fuel added is reduced to zero).

  The air-fuel ratio control unit 34 controls the rich degree (rich A / F) with respect to the stoichiometric air-fuel ratio, the rich duration time when the air-fuel ratio is rich, and the lean duration time when the air-fuel ratio is lean. In other words, the air-fuel ratio control unit 34 controls the amount of fuel added from the fuel addition device 14 and the fuel addition time.

  The air-fuel ratio control unit 34 controls the amount of fuel added from the fuel addition device 14 and the fuel addition time, thereby controlling the air-fuel ratio of the exhaust and controlling the temperature of the exhaust. In other words, the air-fuel ratio control unit 34 increases the amount of fuel added from the fuel addition device 14 when increasing the temperature of the exhaust gas, causes the pre-oxidation catalyst 21 to oxidize (combust), and decreases the temperature of the exhaust gas. Causes the amount of fuel added from the fuel addition device 14 to be zero.

  The S purge execution unit (S desorption process execution means) 35 executes a limited S purge or a normal S purge according to the determination result by the determination unit 33 when the S purge start condition is satisfied. When the determination unit 33 determines that the S purge can be performed and determines that the restriction control is necessary (when the restriction condition (4) is satisfied), the S purge execution unit 35 performs the restriction control. The limited S purge is executed until it is determined to cancel. The S purge execution unit 35 determines that the S purge can be performed by the determination unit 33 but determines that the restriction control is not necessary (when the restriction condition (4) is not satisfied), and When it is determined that the restriction control is released during the restriction control (when the restriction release condition (5) is satisfied), the normal S purge is executed.

In order to execute the S purge, it is necessary to establish both (6) and (7) below.
(6) The ambient atmosphere around the NOx trap catalyst 23 is rich. (7) Temperature of the NOx trap catalyst 23 ≧ S desorption start temperature T _SP
In other words, in the NOx trap catalyst 23, the sulfur component is released from the portion where both (6) and (7) are satisfied, and the sulfur component is released from the portion where only at least one of the conditions is satisfied. Not. The S desorption start temperature T _SP here is a temperature at which sulfur components start to be desorbed from the NOx trap catalyst 23 when the ambient atmosphere of the NOx trap catalyst 23 is rich, and differs depending on the catalyst.

The S purge execution unit 35 executes the limited S purge using the above (7). In other words, limiting S purge while maintaining the atmosphere around the NOx trapping catalytic converter 23 always rich state, the NOx trap catalyst 23, S desorption start point temperature T _SP or more portions and S desorption start point temperature T _SP of less than Make the part deliberately. Thus, the region where the sulfur component is released from the NOx trap catalyst 23 is limited, and the amount of the sulfur component released per unit time when viewed from the entire NOx trap catalyst 23 is limited (ie, reduced). . The limited S purge is executed when the S purge execution unit 35 issues a command to the air-fuel ratio control unit 34.

Air-fuel ratio controller 34 from the S purge execution portion 35 receives an instruction of limiting the S purge execution, the NOx trap catalyst 23, and S desorption start point temperature T _SP or more portions and S desorption start point temperature T _SP below parts Therefore, the air-fuel ratio is controlled so as to periodically repeat the rich continuation time and the lean continuation time. Further, the air-fuel ratio control unit 34 controls the degree of rich when the air-fuel ratio is rich in order to keep the atmosphere around the NOx trap catalyst 23 always rich. In other words, the air-fuel ratio control unit 34 periodically and alternately repeats the time during which fuel is added by the fuel addition device 14 (rich continuation time) and the time when fuel is not added (lean continuation time) (that is, the air-fuel ratio is changed). Periodically swing between rich and lean). Then, while the air-fuel ratio is swung in this way, the amount of added fuel is controlled so that the atmosphere around the NOx trap catalyst 23 is constantly rich, and the rich degree in the rich duration time is controlled.

When the air-fuel ratio is swung by the air-fuel ratio controller 34, the temperature of the exhaust gas flowing into the NOx trap catalyst 23 also periodically rises and falls following the air-fuel ratio swing. As a result, the temperature of the NOx trap catalyst 23 also periodically rises and falls. That is, by controlling the air-fuel ratio of the exhaust gas, the temperature of the NOx trap catalyst 23 changes, so that the temperature of the NOx trap catalyst 23 would change periodically in the vertical direction by sandwiching the S-desorption start point temperature T _SP In addition, air-fuel ratio control (the cycle of rich duration and lean duration and the degree of richness) by the air-fuel ratio controller 34 is set in advance. Thus, if the air-fuel ratio control by the air-fuel ratio control unit 34 is performed, the limited S purge is automatically executed.

  Here, since the two NOx trap catalysts 23 are arranged in series in the exhaust passage 13, the upstream upstream NOx trap catalyst 23F has an exhaust temperature before the downstream downstream NOx trap catalyst 23R. Affected by change. That is, when the exhaust gas temperature rises, the temperature of the front NOx trap catalyst 23F rises first, and then the temperature of the rear NOx trap catalyst 23R rises. Similarly, when the temperature of the exhaust gas decreases, the temperature of the front NOx trap catalyst 23F decreases first, and then the temperature of the rear NOx trap catalyst 23R decreases.

The S purge execution unit 35 controls the temperature of the front NOx trap catalyst 23F (hereinafter also referred to as the front temperature) _TF and the temperature of the rear NOx trap catalyst 23R (hereinafter referred to as the rear temperature) by the air / fuel ratio swing control by the air / fuel ratio control unit 34. A temperature shift is caused between T _R and T _R. That is, the S purge execution unit 35 performs a vertical cycle of the front temperature T _F and the rear temperature T _{R of} the NOx trap catalyst 23 with the S desorption process start temperature T _SP interposed therebetween by the air / fuel ratio control by the air / fuel ratio control unit 34. to change, further it has a front temperature T _F and the rear temperature T _R is controlled so as to swing with a shift of the predetermined phase.

Thus, in the portion exceeding the S desorption start point temperature T _SP runs the S purge, will not be executed S purge part of less than S desorption start point temperature T _SP, when viewed in the entire NOx trap catalyst 23, The amount of sulfur component released is limited (decreased). At this time, the phase shift between the front stage NOx trap catalyst 23F and the rear stage NOx trap catalyst 23R is reversed phase, the air-fuel ratio by the air-fuel ratio control unit 34 as phase reversal becomes S desorption start point temperature T _SP If the control is set, when the sulfur component is released from the front NOx trap catalyst 23F, the sulfur component is not released from the rear NOx trap catalyst 23R, and when the sulfur component is released from the rear NOx trap catalyst 23R. No sulfur component is released from the trap catalyst 23F. That is, a certain amount of sulfur component is always released from the NOx trap catalyst 23.

The above is the control content for the limited S purge. On the other hand, the normal S purge is an S purge that is not a limited S purge, and is an S purge that is performed with the entire NOx trap catalyst 23 being in a state in which the S purge can be performed. When executing the normal S purge, the S purge execution unit 35 establishes both the above (6) and (7) in the entire NOx trap catalyst 23 and releases the sulfur component from the entire NOx trap catalyst 23. When the sulfur component has been released from the NOx trap catalyst 23 (that is, when the residual adsorption S amount S _RE becomes zero), the S purge execution unit 35 ends the S purge. That is, this is the S purge end condition.

  Next, the contents of control by the above-described S purge execution unit 35 will be described with reference to FIGS. FIG. 3A is a graph showing the state of the air-fuel ratio (A / F) that is changed by the air-fuel ratio control performed in the air-fuel ratio control unit 34 based on the command from the S purge execution unit 35, and the theoretical air-fuel ratio. In contrast to (stoichiometric), the lean degree increases as the air-fuel ratio increases, and the rich degree increases as the air-fuel ratio decreases. In FIG. 3A, the solid line represents the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst 23, and the alternate long and short dash line represents the air-fuel ratio on the outlet side of the NOx trap catalyst 23. The air-fuel ratio on the outlet side of the NOx trap catalyst 23 corresponds to the air-fuel ratio (ambient atmosphere) around the NOx trap catalyst 23.

FIG. 3B is a graph showing the temperature state of the NOx trap catalyst 23 that changes with the air-fuel ratio control shown in FIG. 3A. The solid line indicates the temperature T _F of the preceding NOx trap catalyst 23F, and the alternate long and short dash line indicates the temperature T _R of the rear stage NOx trap catalyst 23R. FIG. 3C is a graph showing the change in the S release rate during the S purge, where the solid line is the S release rate by the present exhaust purification device, and the alternate long and short dash line is the S release rate by the conventional exhaust purification device. Moreover, all the time of the horizontal axis of Fig.3 (a)-(c) respond | corresponds.

As shown in FIG. 3A, when the determination unit 33 determines that the limited S purge is necessary at time t ₀ , the S purge execution unit 35 causes the limited S purge to the air-fuel ratio control unit 34. A command is issued to execute the air-fuel ratio control. Receiving this command, the air-fuel ratio control unit 34 first makes the air-fuel ratio richer than stoichiometric. As a result, the temperature of the NOx trap catalyst 23 rises as shown in FIG. Then, the temperature of the pre-stage NOx trap catalyst 23F (front temperature) T _F is earlier reached S desorption start temperature T _SP, the temperature higher than the S desorption start temperature T _SP. The time at this time (the time when the forward temperature T _F reaches the S desorption start temperature T _SP ) is defined as t ₁ .

As shown in FIG. 3A, the air-fuel ratio control unit 34 gradually increases the air-fuel ratio when time t ₁ is reached (that is, when the forward temperature _TF reaches the S desorption start temperature T _SP ). I will make it leaner than stoichiki (start of lean continuation). Thus, it rises and have forward temperature T _F was gradually decreased, to a temperature lower than the S desorption start temperature T _SP. On the other hand, the temperature of the rear NOx trap catalyst 23R rises behind the front NOx trap catalyst 23F. Here, as shown in FIG. 3 (b), upon reaching the front temperature T _F is lowered S desorption start temperature T _SP, the temperature of the rear stage NOx trap catalyst 23R (rear temperature) T _R increases Thus, the S desorption start temperature T _SP is reached. That is, the front temperature T _F and the rear temperature T _R have an opposite phase shift with the S desorption start temperature T _SP as a boundary.

The air-fuel ratio control unit 34 reduces the air-fuel ratio again when half of the preset lean continuation time (a time leaner than the stoichiometric state) is finished, and when the stoichiometric condition is reached, the lean continuation time ends. Further, the air-fuel ratio is made richer than stoichiometric (start of rich continuation). When the air-fuel ratio becomes richer than the stoichiometric again, forward temperature T _F is increased to reach the S desorption start point temperature T _SP again, a higher temperature than S desorption start point temperature T _SP. Subsequent NOx trap catalyst 23R, since temperature decreases with a delay upstream NOx trap catalyst 23F, the rear temperature T _R when the front temperature T _F began to rise begins to decrease, the front temperature T _F is increased S When the desorption start temperature T _SP is reached, the S desorption start temperature T _SP is reached and becomes lower than the S desorption start temperature T _SP .

The air-fuel ratio control unit 34 increases the air-fuel ratio again when half of the preset rich continuation time (the time in a state richer than the stoichiometric state) is finished, and when the stoichiometric condition is reached, the rich continuation time ends. Further, the above-described lean continuation is started by making the air-fuel ratio leaner than stoichiometric. As described above, the S purge execution unit 35 causes the air-fuel ratio control unit 34 to swing the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst 23 so as to repeat rich and lean. As a result, the front temperature of the NOx trap catalyst 23 (the temperature of the front NOx trap catalyst 23F) T _F and the rear temperature (the temperature of the rear NOx trap catalyst 23R) T _R are in opposite phases across the S desorption start temperature T _SP . Swing up and down with a gap.

  The S purge execution unit 35 swings the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst 23 by the air-fuel ratio control unit 34 so as to repeat rich and lean, but the ambient atmosphere of the NOx trap catalyst 23 (that is, NOx The air / fuel ratio on the outlet side of the trap catalyst 23) controls the degree of richness in the rich duration so that it is always in a rich state on average as shown by a one-dot chain line in FIG. That is, control is performed so that the rich degree with respect to stoichiometry is larger than the lean degree (so that the amplitude toward the rich side becomes large). Here, the rich duration and the lean duration are set to be substantially the same by controlling the rich degree.

The sulfur component released from the NOx trap catalyst 23 by such limited S purge is released at a speed as shown by a solid line in FIG. As indicated by a solid line in FIG. 3C, from time t ₁ when the front temperature T _F reaches the S desorption start temperature T _SP , sulfur components start to be released from the NOx trap catalyst 23, and the front temperature T _F In a state where the temperature is equal to or higher than the S desorption start temperature T _{SP, the} sulfur component is released from the front NOx trap catalyst 23F.

Conventionally, since the sulfur component starts to be released from the entire NOx trap catalyst 23 at a stretch, the S release rate (that is, the amount of the sulfur component released per unit time) rapidly increases as shown by the alternate long and short dash line, and the time is short. A large amount of sulfur component is released. For this reason, a large amount of H ₂ S is generated, the H ₂ S concentration is increased, and a odor peculiar to H ₂ S is felt.

On the other hand, when the limited S purge is performed, the atmosphere around the NOx trap catalyst 23 is always rich, but the forward temperature T _F and the rear temperature T _R alternate between the execution of the limited S purge. Since the S desorption start temperature T _SP is exceeded, sulfur components are alternately released from the front NOx trap catalyst 23F and the rear NOx trap catalyst 23R. In other words, since the temperature of the NOx trap catalyst 23 is divided into a portion that exceeds the S desorption start temperature T _SP and a portion that does not exceed the S desorption start temperature T _SP , the sulfur component is only from the portion that exceeds the S desorption start temperature T _SP. Is not released.

That is, although the above (6) is always established, there are a portion where (7) is established and a portion where (7) is not established. The amount is greatly reduced compared to the conventional one. In FIG. 3C, V _TH1 is a first predetermined speed used in the start condition (limit condition) of the limited S purge, and V _TH2 is a second predetermined speed used in the limit S purge release condition (limit release condition). Speed.

If the sulfur component is continuously released from the NOx trap catalyst 23 by the limited S purge, the residual adsorption S amount _SRE adsorbed on the NOx trap catalyst 23 gradually decreases. Thus, S release rate (limit S release rate) V _SL during limit S purge gradually decreases, it becomes smaller speed than the second predetermined speed V _TH2. The time when the limit S release speed V _SL decreases and reaches the second predetermined speed V _TH2 is defined as t ₂ . That is, the restriction control is released at time t ₂ , and the restriction S purge shifts to the normal S purge.

At time t ₂ , the S purge execution unit 35 issues a command to the air / fuel ratio control unit 34 to perform normal S purge air / fuel ratio control. Upon receiving this command, the air-fuel ratio control unit 34 stops the air-fuel ratio swing and always makes it rich as shown in FIG. As a result, as shown in FIG. 3B, the front temperature T _F and the rear temperature T _R are always maintained at a temperature _{equal to} or higher than the S desorption start temperature T _SP . That is, since the ambient atmosphere of the NOx trap catalyst 23 is always in a rich state and the temperature of the NOx trap catalyst 23 is higher than the S desorption start temperature T _{SP, the} entire NOx trap catalyst 23 (that is, the preceding NOx trap catalyst 23F). And the remaining sulfur component is released from the post-stage NOx trap catalyst 23R).

Then, as shown in FIG. 3 (c), the residual adsorption S amount _SRE gradually decreases, so that the S release rate also gradually decreases, and when the residual adsorption S amount _SRE disappears, the S release rate becomes zero. . Time t ₃ at this time is the S purge end time.

  In FIG. 3 (c), the time for ending the S purge by the exhaust purification apparatus and the S purge by the conventional method is substantially the same. This is because when the limited S purge is used, even if the amount of sulfur released during the limited S purge (S release rate) is limited, all sulfur components are released quickly after the transition from the limited S purge to the normal S purge. Because it is done. That is, while the peak value of the S release rate is lowered by the limited S purge, the time for executing the S purge is substantially the same as the conventional time.

[3. flowchart]
Next, an example of the procedure of the S purge control (S desorption process control) executed by the ECU 30 will be described with reference to FIG. This flowchart operates at a predetermined cycle T (for example, a number [s] cycle). Each of the following steps is performed by each function (means) assigned to the hardware of the computer being operated by software (computer program).

  In this exhaust purification apparatus, when an ignition switch (not shown) is turned on by a driver and the engine 1 is started, the following control flow is started. Note that a flag F in the flowchart indicates a control mode (control state) by the S purge execution unit 35, a flag F = 0 indicates a state in which S purge is not performed, a flag F = 1 indicates that a limited S purge is being performed, F = 2 indicates that the normal S purge is being executed. At the start of the control, the flag F is set to F = 0.

As shown in FIG. 4, in step S10, it is determined whether or not the flag F is F = 0. Since the flag F is F = 0 at the start of control, the process proceeds from the YES route to step S20. In step S20, the detected values (sensor values) of the temperature sensor 15, the air-fuel ratio sensor 16, and the flow rate sensor 17, and the integrated value ΣF of the used fuel amount F after the end of the previous S purge are acquired. In step S30, the amount of sulfur component adsorbed on the NOx trap catalyst 23 (adsorption S amount) S _AD is estimated from the integrated value ΣF acquired in step S20.

Next, in step S40, it is determined whether or not the adsorption S amount S _AD estimated in step S30 is greater than or equal to a predetermined value S _TH set in advance. That is, the determination in step S40 is a determination as to whether or not the above-described (3) S purge is necessary, and is determined by the determination unit 33. When the adsorption S amount S _AD is equal to or greater than the predetermined value S _TH , it is determined that S purge is necessary, and the process proceeds to step S50 to determine whether or not the S purge can be performed by the determination unit 33 (the S purge start condition is Whether or not established) is determined. If it is determined in step S40 that the adsorption S amount S _AD is less than the predetermined value S _TH and if it is determined in step S50 that the S purge cannot be performed, the process returns.

If the S purge start condition is satisfied in step S50, the process proceeds to step S60, and the S release speed V _S is estimated. In step S70, it is determined whether or not the S release speed V _S is equal to or higher than a preset first predetermined speed V _TH1 . That is, the determination in step S70 is the above-described restriction S purge start condition (4), which is determined by the determination unit 33. If the S release speed V _S is equal to or higher than the first predetermined speed V _{TH1, the} process proceeds to step S80, the flag F is set to F = 1 in step S80, and the limited S purge is executed by the S purge execution unit 35 in step S90. . On the other hand, if it is determined in step S70 that the S release speed V _S is less than the first predetermined speed V _TH1 , the process proceeds to step S140, the flag F is set to F = 2, and the process proceeds to step S150.

When the limited S purge is executed in step S90, the sulfur component is gradually released from the NOx trap catalyst 23. In step S100, the amount of sulfur component (residual adsorption S amount) _SRE remaining in the NOx trap catalyst 23 is estimated. The residual adsorbed S amount S _RE of this time, the limit S purge start immediately (i.e., first when it proceeds to step S100), the adsorption amount of S S _AD is directly estimated as residual adsorption amount of S S _RE, following from control cycle it is estimated using equation (2) described above using the restriction S release rate V _SL estimated in step S110 in the previous cycle.

Next, in step S110, the limit S release speed V _SL is estimated from the residual adsorption S amount S _RE . Then, in step S120, whether the second or lower than the predetermined speed V _TH2 restriction S release rate V _SL is set in advance. That is, the determination in step S120 is the restriction S purge release condition (5) described above, and is determined by the determination unit 33. When the limit S release speed V _SL is less than the second predetermined speed V _TH2 , the process proceeds to step S130, the flag F is set to F = 2, and the limit control is released. In step S150, a normal S purge is executed.

On the other hand, if it is determined in step S120 that the limited S release speed _VSL is equal to or higher than the second predetermined speed _VTH2 , the process returns from the NO route. In this case, since the flag F is set to F = 1, the determination in step S10 proceeds from the NO route to step S180, where it is determined whether or not the flag F is F = 1. Since F = 1, the process proceeds from the YES route to step S90, and the limited S purge is executed. Then, the limited S purge is executed until the normal S purge is shifted according to the determination in step S120.

In step S150, normal S purge is executed. Thus, since the sulfur component is gradually released from the NOx trap catalyst 23, the residual adsorption S amount _SRE is estimated in step S160, and whether or not the sulfur component has been released from the NOx trap catalyst 23 in step S170 (that is, , Whether or not the S purge end condition is satisfied) is determined. If the S purge end condition is satisfied, the control flow is ended. If the S purge end condition is not satisfied, the process returns and proceeds to the determination in step S10. In this case, both the determinations at step S10 and step S180 are NO, so the routine proceeds to step S150 where the normal S purge is executed and repeated until the S purge end condition is satisfied.

[4. effect]
Therefore, according to the present exhaust purification apparatus, if the above-described restriction condition (4) is satisfied during the execution of the S desorption process (S purge) for desorbing the sulfur component from the NOx trap catalyst 23, the NOx trap catalyst 23 detects sulfur. Since the region where the components are desorbed (that is, the portion where the S purge is performed) is limited, it is possible to prevent the sulfur component adsorbed by the NOx trap catalyst 23 from being released from the entire NOx trap catalyst 23. That is, since the amount of the sulfur component released from the NOx trap catalyst 23 per unit time can be reduced, the amount of H ₂ S released per unit time released into the atmosphere can be reduced. In other words, the maximum value of the H ₂ S concentration released into the atmosphere (that is, the peak value of the H ₂ S concentration) can be lowered. Thus, it is possible to suppress the odor of H ₂ S.

Moreover, for determining that S release rate V _S released from the NOx trap catalyst 23 is limited conditions when a predetermined speed of the (first predetermined speed) V _TH1 or more is satisfied, to limit the area where the sulfur component is desorbed Thus, it is possible to appropriately determine whether or not to perform S purge (that is, whether or not to perform limited S purge). When the S release rate V _S is equal to or higher than the first predetermined rate V _TH1 (that is, when it is estimated that a large amount of sulfur component is released at once and the peak value of the S release rate needs to be lowered), the sulfur component is removed. By limiting the separation, the H ₂ S odor can be reliably suppressed.

  Further, when the above-described restriction release condition (5) is satisfied during the restriction S purge execution (that is, during the area restriction), the restriction S purge is released (the area restriction is released), and the normal S purge is executed. Therefore, the sulfur component can be quickly released from the NOx trap catalyst 23. That is, while suppressing the peak value of the S release rate, the time for executing the S purge can be made substantially the same as the conventional time, and deterioration of fuel consumption can be suppressed.

In addition, when the estimated S release rate (restricted S release rate) V _SL is less than the predetermined rate (second predetermined rate) V _TH2 , it is determined that the restriction release condition is satisfied. It is possible to appropriately determine whether or not to cancel (whether or not to shift from the limited S purge to the normal S purge). When the limited S release speed V _SL becomes less than the second predetermined speed V _TH2 , the routine shifts to normal S purge, so that the S purge can be finished early.

Further, by repeated a rich duration and lean duration periodically by the air-fuel ratio control unit 34, a front temperature T _F and the rear temperature T _R of the NOx trap catalyst 23 across the S desorption start point temperature T _SP In order to limit the S desorption region by swinging the front temperature T _F and the rear temperature T _R with a predetermined phase shift, thereby changing the region between the front and rear of the NOx trap catalyst 23. And the peak value of the H ₂ S concentration can be reduced.

The amount of the sulfur component and NOx trap catalyst 23 forward temperature T _F and the rear temperature T _R is to be swing in opposite phase to the boundary of the S desorption start temperature T _SP, is released view the entire NOx trap catalyst 23 It is possible to prevent variation.
In addition, the air-fuel ratio control unit 34 maintains the air-fuel ratio around the NOx trap catalyst 23 to be always rich by controlling the rich degree in the rich continuation time, and makes the rich continuation time and the lean continuation time substantially the same. , Control can be simplified.

In addition, here, the two NOx trap catalysts 23 (the front-stage NOx trap catalyst 23F and the rear-stage NOx trap catalyst 23R) are arranged in series (tandem arrangement) in the exhaust passage 13, whereby the temperature of the upstream NOx trap catalyst 23F ( The phase shift between the front temperature (T _F ) and the temperature (rear temperature) T _R of the downstream NOx trap catalyst 23R can be clearly set. That is, the area to be purged with S can be more clearly defined, and the limited S purge can be performed more accurately.

[5. Modified example]
Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.
In the above embodiment, the determination is performed using different predetermined speeds for the start condition (restriction condition) and the release condition (restriction release condition) of the limited S purge, but the predetermined speed may be the same. That is, the first predetermined speed V _TH1 and the second predetermined speed V _TH2 may be the same value. In this case, the control configuration is simplified.

In the above embodiment, the S release speed V _S is estimated and it is determined whether or not the limited S purge is executed based on the S release speed V _S. This determination may be made based on, for example, the adsorption S amount S _AD . As the amount of sulfur component adsorbed on the NOx trap catalyst 23 increases, the rate at which the NOx trap catalyst 23 is released increases. Therefore, when the adsorption S amount S _AD exceeds a certain determination threshold value, the limited S purge may be executed. . In this case, since it is not necessary to estimate the S release rate, the control configuration is simplified.

Further, in addition to the above, the estimation of the adsorption S amount S _AD is obtained by, for example, obtaining the relationship between the adsorption S amount and the operating time of the engine 1 in advance by experiments and mapping it, and integrating the operating time of the engine 1. The amount of adsorption S may be estimated by applying the accumulated operation time to this map. Further, for example, the relationship between the amount of adsorption S and the travel distance may be mapped in advance, and the travel distance from when the previous S purge was performed may be acquired and applied to this map. Moreover, you may estimate using the numerical formula previously calculated | required by experiment etc. instead of a map.

In addition, when a sensor capable of detecting the amount of adsorbed S and the S release rate is provided, it may be detected directly by these. In this case, it is not necessary to provide the adsorption S amount estimation unit 31 and the S release rate estimation unit 32.
Further, when performing the above-described limited S purge or normal S purge, the S purge may be executed after the NOx trap catalyst 23 has been heated to some extent in advance. In this case, it is possible to NOx trap catalyst 23 is able to reduce the time required to reach the S desorption start temperature T _SP, starts S purge early.

Further, only the limited S purge may be performed without shifting from the limited S purge to the normal S purge. In this case, it is not necessary to determine the restriction release condition.
In the above embodiment, the region where the sulfur component is desorbed from the NOx trap catalyst 23 is controlled by controlling the air-fuel ratio of the exhaust gas. However, the temperature condition of the NOx trap catalyst 23 varies depending on the location (that is, As long as the NOx trap catalyst 23 can be partially changed, the region limitation is not limited to that by air-fuel ratio control.

Further, when executing the limited S purge, the rich continuation time and the lean continuation time may be different. In this case, for example, the ambient atmosphere of the NOx trap catalyst 23 may be always made rich by controlling the rich duration according to the rich degree.
Further, in the above embodiment, two NOx trap catalysts 23 are arranged in series, but one NOx trap catalyst 23 may be provided. Even in this case, a temperature difference occurs between the front temperature on the exhaust upstream side of the NOx trap catalyst 23 and the rear temperature on the exhaust downstream side, and the same operation and effect as the above-described limited S purge can be obtained. . Three or more NOx trap catalysts 23 may be arranged.

Further, instead of the fuel addition device 14 provided in the exhaust passage 13, an injector 7 for fuel injection may be used.
Further, the exhaust purification device 20 (the front-stage oxidation catalyst 21, the filter 22, the NOx trap catalyst 23, and the rear-stage oxidation catalyst 24) may be arranged in the exhaust passage 13 in the order described above, and has the shape and storage state as shown in FIG. Not limited to.
Moreover, if it has a NOx trap catalyst, it will not be restricted to a diesel engine.

1 Diesel engine (internal combustion engine)
DESCRIPTION OF SYMBOLS 13 Exhaust passage 14 Fuel addition apparatus 15 Temperature sensor 16 Air-fuel ratio sensor 17 Flow sensor 20 Exhaust purification apparatus 21 Pre-stage oxidation catalyst 22 Filter 23 NOx trap catalyst 24 Post-stage oxidation catalyst 30 ECU (electronic control unit)
31 Adsorption S amount estimation unit (adsorption S amount estimation means)
32 S release rate estimation unit (S release rate estimation means)
33 determination unit (determination means)
34 Air-fuel ratio control unit (air-fuel ratio control means)
35 S purge execution part (S desorption process execution means)

Claims (7)

A NOx trap catalyst that is provided in an exhaust passage of the internal combustion engine and that adsorbs nitrogen oxides and sulfur components in the exhaust in an oxidizing atmosphere in which the air-fuel ratio of the exhaust is lean;
The air / fuel ratio so as to periodically repeat a rich duration in which the air / fuel ratio of the exhaust gas flowing into the NOx trap catalyst is richer than the stoichiometric air / fuel ratio and a lean duration in which the air / fuel ratio is leaner than the stoichiometric air / fuel ratio. An air-fuel ratio control means for controlling
S desorption processing execution means for performing S desorption processing for desorbing the sulfur component adsorbed on the NOx trap catalyst from the NOx trap catalyst in a reducing atmosphere in which the air-fuel ratio is rich ;
S release rate estimating means for estimating a release rate when the sulfur component is desorbed from the NOx trap catalyst;
Determination means for determining that a limiting condition is satisfied when the release speed estimated by the S release speed estimation means is equal to or higher than a first predetermined speed , and
The S desorption process execution means, said When the restriction condition is satisfied S during the execution of the desorption process, by the air-fuel ratio control by the air-fuel ratio control means, the average air-fuel ratio at the outlet side of the NOx trap catalyst The rich degree with respect to the stoichiometric air-fuel ratio in the rich duration time is controlled so that the rich state is always in a rich state, and the front temperature on the exhaust upstream side and the rear temperature on the exhaust downstream side of the NOx trap catalyst are controlled by the NOx trap. By periodically changing the S desorption start temperature, which is the temperature at which the sulfur component begins to desorb from the catalyst, vertically, and swinging the front temperature and the rear temperature with a predetermined phase shift , and limits the area of the NOx trap catalyst, wherein the sulfur components are desorbed, exhaust purification device of an internal combustion engine. An adsorption S amount estimating means for estimating the amount of the sulfur component adsorbed on the NOx trap catalyst;
The S release speed estimating means, estimate the pre SL release rate based on the amount of the sulfur component where the estimated by adsorption S amount estimating means
Characterized the this, the exhaust gas purifying apparatus for an internal combustion engine according to claim 1. The S desorption process executing means executes the S desorption process by canceling the restriction of the area when a predetermined restriction release condition is satisfied when the area is restricted. An exhaust gas purification apparatus for an internal combustion engine as described. 4. The internal combustion engine according to claim 3, wherein the determination unit determines that the restriction release condition is satisfied when the release rate estimated by the S release rate estimation unit is less than a second predetermined rate. 5. Exhaust purification device. The S desorption execution means limits the region from which the sulfur component is desorbed by setting the predetermined phase as an opposite phase and swinging so that the phase is reversed at the S desorption start temperature. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 4, wherein the exhaust gas purification device is an internal combustion engine. The air-fuel ratio control means, said controls the richness of the rich duration, characterized by substantially the same and the rich continuation time and the lean duration claim 1 2. An exhaust gas purification apparatus for an internal combustion engine according to 1. Wherein the NOx trap catalyst is disposed two in series in the exhaust passage, an exhaust purification system of an internal combustion engine according to any one of claims 1-6.

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