JP2008190461A - Exhaust emission control device and desulfurization method of exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve exhaust emission control performance while making exhaust gases cleaner even when a sulfur component absorbed in an NOx trap catalyst is eliminated. <P>SOLUTION: The exhaust emission control device is constituted by the NOx trap catalyst 15, an air-fuel ratio adjustment means 31, a means 34 of performing desulfurization processing, and a leanizing execution means 35. The NOx trap catalyst 15 provided to the exhaust gas passage 13 of an internal combustion engine 1 absorbs nitrogen oxides and sulfur components in the exhaust gas in a lean atmosphere, and the air-fuel ratio adjustment means 31 adjusts an air-fuel ratio. The means 34 of performing desulfurization processing performs enriching of the air-fuel ratio of the exhaust gas a little more than a theoretical air-fuel ratio, and performs control so that the desulfurization processing is performed which releases the sulfur components from the NOx trap catalyst when predetermined desulfurization processing execution conditions are met. Then the leanizing execution means 35 performs control so that the air-fuel ratio is made leaner than the theoretical air-fuel ratio only for a predetermined lean time period when the desulfurization processing is performed by the means 34 of performing desulfurization processing. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying apparatus having a NOx trap catalyst for purifying exhaust gas by storing nitrogen oxide (NOx) contained in exhaust gas of an internal combustion engine, and a desulfurization method for the exhaust gas purifying apparatus.

Conventionally, as a catalyst for purifying NOx contained in exhaust gas of an internal combustion engine, NOx is occluded in the catalyst in an oxygen-excess atmosphere (oxidizing atmosphere) and occluded in an atmosphere having a low oxygen concentration (reducing atmosphere). A NOx trap catalyst (storage type NOx catalyst) having a function of releasing NOx is used.
Such a NOx trap catalyst stores NOx contained in exhaust gas as nitrate in the oxidation atmosphere on the catalyst.

In such an exhaust gas purification apparatus using a NOx trap catalyst, when the NOx occlusion amount on the NOx trap catalyst approaches a saturation state, the ambient atmosphere of the NOx trap catalyst is made a reducing atmosphere with a low oxygen concentration, and incomplete combustion of fuel or the like CO or the like produced by the above is supplied as a reducing agent to the NOx trap catalyst. As a result, nitrate stored in the NOx trap catalyst reacts with a reducing agent such as CO in the exhaust to release NOx from the NOx trap catalyst and reduce the released NOx to harmless N ₂ , and then to the atmosphere. It is designed to discharge inside.

By the way, sulfa (sulfur) component is generally contained in exhaust gas, and the NOx trap catalyst has a property of storing a sulfur component called sulfur oxide (SOx) together with NOx.
However, the sulfur component on the NOx trap catalyst has higher chemical stability than nitrate, and only a small amount of sulfur component is released from the NOx trap catalyst even if the ambient atmosphere of the NOx trap catalyst is a reducing atmosphere. For this reason, the amount of the sulfur component remaining in the NOx trap catalyst increases with time, whereby the performance of the NOx trap catalyst as a catalyst is degraded. Thus, the sulfur component is occluded in the NOx trap catalyst, and the performance of the catalyst is lowered.

For this reason, techniques for eliminating S poisoning of NOx trap catalysts have been proposed.
For example, in the technique of Patent Document 1, the sulfur component stored in the NOx trap catalyst reacts well with the reducing agent at a high temperature and is released from the NOx trap catalyst. Sulfur components occluded in the NOx trap catalyst by increasing the temperature of the NOx trap catalyst by injecting additional fuel, etc., and supplying a large amount of carbon monoxide (CO) as a reducing agent to the NOx trap catalyst. Control is performed to release sulfur components from the NOx trap catalyst by reacting NO with CO (S purge operation).
JP 11-229864 A

By the way, when performing the S purge operation, it is necessary to raise the temperature of the NOx trap catalyst and supply CO, HC, H ₂ as the reducing agent to the NOx trap catalyst.
In the technique of Patent Document 1, the air-fuel ratio is set to be richer than the stoichiometric air-fuel ratio, and fuel is incompletely burned in the internal combustion engine to generate CO and supply it to the NOx trap catalyst.

FIG. 5 is a graph in which the horizontal axis represents the air-fuel ratio of the exhaust gas, and the vertical axis represents the CO concentration, NH ₃ concentration, and NOx concentration in the exhaust gas. As shown in FIG. 5, when the air-fuel ratio is set to be richer than the stoichiometric air-fuel ratio during the S purge operation, the NOx concentration in the exhaust gas decreases, but the CO concentration in the exhaust gas increases.

However, it has been found that when the air-fuel ratio is made richer than the stoichiometric air-fuel ratio, ammonia (NH ₃ ) is generated by the NOx trap catalyst, and the NH ₃ concentration in the exhaust gas increases.
That is, NH ₃ is considered to be generated by the following chemical reaction formulas (1) and (2) in the NOx trap catalyst.
NO + HC + H ₂ O → CO ₂ + NH ₃ (1)
2NO + 5H ₂ → 2H ₂ O + 2NH ₃ (2)
That is, when the NOx trap catalyst is in a reducing atmosphere, the above formulas (1), (2) can be obtained from NOx occluded in the NOx trap catalyst, hydrocarbons (HC) and water (H ₂ O) contained in the exhaust gas together with CO, ) Reaction may occur. Note that hydrogen (H ₂ ) in the above formula (2) is generated in the exhaust passage by the following chemical reaction formulas (3) and (4).
CO + H ₂ O → CO ₂ + H ₂ (3)
HC + H ₂ O → CO ₂ + CO + H ₂ (4)

Thus, when performing the S purge operation, NH ₃ produced in the exhaust gas may be discharged into the atmosphere. In particular, in order to sufficiently release the sulfur component from the NOx trap catalyst by the S purge operation, it is necessary to continue the S purge operation for a relatively long time (for example, about several minutes). However, by performing a relatively long time S purge operation, correspondingly, a large amount of NH ₃ is generated, the exhaust gas containing a large amount of NH ₃ is to be released to the atmosphere. In other words, since the problem that bad smell is generated in the exhaust gas due to NH ₃ occurs, it is desired to reduce NH ₃ generated during the S purge operation.

In order to reduce the production of NH ₃ during the S purge operation, for example, a method of changing the air-fuel ratio to the lean side can be considered. However, as shown in FIG. 5, when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, a sufficient amount of reducing agent (CO) is not supplied to the NOx trap catalyst, and the sulfur component can be sufficiently released from the storage-type NOx. Can not. That is, S poisoning of the NOx trap catalyst cannot be efficiently eliminated.

  The present invention was devised in view of such problems, and an exhaust gas purifying apparatus capable of improving exhaust gas purification performance while making the exhaust gas cleaner even when removing the sulfur component stored in the NOx trap catalyst, and It aims at providing the desulfurization method of an exhaust gas purification apparatus.

  In order to achieve the above object, an exhaust gas purifying apparatus according to the present invention (Claim 1) is provided in an exhaust gas passage of an internal combustion engine, and in a lean atmosphere where the air-fuel ratio of exhaust gas is lean, nitrogen oxides in the exhaust gas and When the NOx trap catalyst for storing the sulfur component, the air-fuel ratio adjusting means for adjusting the air-fuel ratio of the exhaust gas, and the predetermined desulfurization processing execution conditions are satisfied, the air-fuel ratio of the exhaust gas is slightly less than the stoichiometric air-fuel ratio. A desulfurization process executing means for controlling the air-fuel ratio adjusting means to perform a desulfurization process for releasing the sulfur component stored in the NOx trap catalyst after being enriched, and when the desulfurization process is performed by the desulfurization process execution means And a leaning execution means for controlling the air-fuel ratio adjusting means so as to make the air-fuel ratio of the exhaust gas leaner than the stoichiometric air-fuel ratio for a predetermined lean period. .

  Further, when the desulfurization processing is performed by the desulfurization processing execution unit, the leaning execution means periodically makes the air-fuel ratio leaner than the stoichiometric air-fuel ratio during the lean period periodically for each predetermined rich period. It is preferable to control the air-fuel ratio adjusting means (claim 2).

The leaning execution means preferably sets the rich period based on the operating state of the internal combustion engine.
For example, when performing the desulfurization process, if the rotational speed or load of the internal combustion engine is large, the flow rate of the exhaust gas is correspondingly increased, and it is considered that the amount of ammonia generated is increased in proportion thereto. By setting the enrichment period to be shorter, ammonia generation in the NOx trap catalyst may be effectively suppressed.

The leaning execution means preferably sets the lean period based on the operating state of the internal combustion engine.
For example, when performing the desulfurization process, if the rotational speed or load of the internal combustion engine is large, the flow rate of the exhaust gas is correspondingly increased, and it is considered that the amount of ammonia generated is increased in proportion thereto. By setting the lean period longer, ammonia generation in the NOx trap catalyst can be effectively suppressed.
The leaning execution means preferably sets the air-fuel ratio of the exhaust gas during the lean period based on the operating state of the internal combustion engine.

When performing the desulfurization process, if the rotational speed or load of the internal combustion engine is large, the exhaust gas flow rate will increase accordingly, and the amount of ammonia produced may increase proportionally. By setting the air-fuel ratio of the exhaust gas to be leaner, generation of ammonia in the NOx trap catalyst may be effectively suppressed.
In general, it is conceivable that the NOx trap catalyst is activated and the amount of ammonia generated increases as the temperature rises within a certain temperature range. For this reason, it is preferable that the leaning execution means is configured to set the lean period longer as the temperature becomes higher in the temperature region, or to set the air-fuel ratio of the exhaust gas in the lean period to the lean side.

The desulfurization treatment execution condition preferably includes that the temperature of the NOx trap catalyst is equal to or higher than a predetermined temperature (Claim 6).
In addition, a sulfur storage amount estimating means for estimating the amount of sulfur component stored in the NOx trap catalyst is provided, and the desulfurization execution condition includes the storage amount of the sulfur component estimated by the sulfur storage amount estimating means. Is preferably greater than or equal to a predetermined amount (Claim 7).

  Moreover, according to the desulfurization method of the exhaust gas purifying apparatus of the present invention according to claim 8, nitrogen oxides and sulfur components in the exhaust gas in a lean atmosphere provided in the exhaust gas passage of the internal combustion engine and in which the air-fuel ratio of the exhaust gas is lean Exhaust gas purification apparatus that performs a desulfurization process for releasing sulfur components stored in the NOx trap catalyst in an exhaust gas purification apparatus that includes a NOx trap catalyst that stores NOx and an air-fuel ratio adjustment unit that adjusts an air-fuel ratio of the exhaust gas The desulfurization method includes a enrichment step for enriching the air-fuel ratio of the exhaust gas with respect to the stoichiometric air-fuel ratio when a predetermined desulfurization treatment execution condition is satisfied, and the exhaust gas when the enrichment step is executed. And a partial leaning step for making the air / fuel ratio of the engine leaner than the stoichiometric air / fuel ratio for a predetermined lean period.

  According to the exhaust gas purification apparatus of the present invention (Claim 1) and the desulfurization method of the exhaust gas purification apparatus of the present invention (Claim 8), the generation of ammonia is reduced when the sulfur component stored in the NOx trap catalyst is removed. This makes it possible to clean the exhaust gas and suppress the malodor of the exhaust gas. Further, carbon monoxide as a reducing agent can be sufficiently supplied to the NOx trap catalyst, and the sulfur component can be removed from the NOx trap catalyst. Therefore, exhaust gas purification performance can be improved.

According to the exhaust gas purification apparatus of the present invention (Claim 2), it is possible to continuously perform the release of the sulfur component remaining in the NOx trap catalyst and the suppression of the generation of ammonia.
According to the exhaust gas purification apparatus of the present invention (Claim 3), by setting the lean period according to the operating state of the internal combustion engine, it is possible to effectively suppress the release of the sulfur component remaining in the NOx trap catalyst and the generation of ammonia. And can be executed.

According to the exhaust gas purification apparatus of the present invention (Claim 4), by setting the rich period according to the operating state of the internal combustion engine, it is possible to effectively suppress the release of sulfur components remaining in the NOx trap catalyst and the generation of ammonia. And can be executed.
According to the exhaust gas purification apparatus of the present invention (Claim 5), by setting the air-fuel ratio of the exhaust gas in the lean period according to the operating state of the internal combustion engine, it is possible to effectively release the sulfur component remaining in the NOx trap catalyst. Ammonia production can be suppressed.

According to the exhaust gas purification apparatus of the present invention (Claim 6), the sulfur component can be effectively removed from the NOx trap catalyst by performing the desulfurization treatment in a state where the temperature of the NOx trap catalyst is equal to or higher than the predetermined temperature. .
According to the exhaust gas purifying apparatus of the present invention (Claim 7), the NOx trap catalyst is obtained by including in the desulfurization treatment execution condition that the storage amount of the sulfur component estimated by the sulfur storage amount estimation means is a predetermined amount or more. The catalyst performance can be recovered by performing desulfurization treatment in a state where the catalyst performance of the catalyst is reduced to a predetermined level by S poisoning, and unnecessary desulfurization treatment is prevented when the degree of S poisoning is small. be able to.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 to 4 are all for explaining an exhaust gas purifying apparatus and a desulfurization method for an exhaust gas purifying apparatus according to an embodiment of the present invention, and FIG. 1 is a configuration diagram schematically showing the entire configuration, FIG. 2 is a map used for fuel injection control, FIG. 3 is a flowchart showing the procedure for desulfurization processing, and FIG. 4 shows the operation and effect. The horizontal axis indicates time, and the vertical axis indicates air-fuel ratio, CO concentration, is a graph plotting the NH ₃ concentration.

  As shown in FIG. 1, a spark plug 3 is attached to a cylinder head 2 of the engine 1, and an electromagnetic fuel injection valve 4 is attached. Fuel is directly injected into the combustion chamber 5 from the fuel injection valve 4. A piston 7 is supported on the cylinder 6 of the engine 1 so as to be slidable in the vertical direction, and a cavity 8 recessed in a hemispherical shape is formed on the top surface of the piston 7. The cylinder head 2 is provided with an intake valve 11 and an exhaust valve 12.

The intake port 9 communicates with the combustion chamber 5 via the intake valve 11, but is opened and closed by driving the intake valve 11. The intake port 9 is connected to an intake pipe, a surge tank, an air cleaner, a throttle valve, etc. (not shown).
On the other hand, the exhaust port 10 communicates with the combustion chamber 5 via the exhaust valve 12, but is opened and closed by driving the exhaust valve 12.

  That is, in the engine 1, the intake flow that flows into the combustion chamber 5 from the intake port 9 forms a reverse tumble flow by the cavity 8 on the upper surface of the piston 7, and fuel is injected in the latter half of the compression stroke to use the reverse tumble flow. A small amount of fuel is collected only in the vicinity of the spark plug 3 disposed in the center of the top of the combustion chamber 5, and an extremely lean air-fuel ratio is obtained at a portion separated from the spark plug 3, and only in the vicinity of the spark plug 3 is the stoichiometric air-fuel ratio. Alternatively, by making the air-fuel ratio rich, it is possible to suppress fuel consumption while realizing stable stratified combustion (stratified super lean combustion).

  Of course, when it is necessary to obtain a high output from the engine 1 in addition to the stratified combustion, the fuel is mixed into the entire combustion chamber 5 by injecting fuel from the fuel injection valve 4 into the intake stroke, and premixed combustion is performed. It can be done. Of course, the higher the stoichiometric air-fuel ratio or the rich air-fuel ratio, the higher the output than when the lean air-fuel ratio is obtained. In this case, the fuel chamber is sufficiently atomized and vaporized. Fuel injection is performed at the timing, and high output is efficiently obtained.

On the other hand, an exhaust pipe (exhaust gas passage) 13 is connected to the exhaust port 10, and a three-way catalyst 14 and a NOx trap catalyst 15 are provided in series as an exhaust purification device in series from the engine 1 side.
The three-way catalyst 14 is a catalyst having a three-way function that can purify CO, HC, and NOx in the exhaust gas when the air-fuel ratio is the stoichiometric air-fuel ratio. On the other hand, the NOx trap catalyst 15 occludes NOx contained in the exhaust gas in an atmosphere where the oxygen in the exhaust gas is excessive (oxidizing atmosphere), has a low oxygen concentration in the exhaust gas, and contains a large amount of reducing agent in the exhaust gas. It is a catalyst having a function of making harmless N ₂ by releasing NOx stored in the atmosphere (reducing atmosphere) and reducing and purifying it.

That is, these three-way catalyst 14 and NOx trap catalyst 15 purify harmful components such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) in the exhaust gas, and then release them to the atmosphere. It has become so.
That is, when the air-fuel ratio is the stoichiometric air-fuel ratio, CO, HC, NOx in the exhaust gas is purified by the three-way catalyst 14, and in an oxidizing atmosphere where the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, CO, Although incomplete combustion products such as HC are hardly contained, the NOx concentration in the exhaust gas increases, but the NOx that has passed through the three-way catalyst 14 is occluded by the downstream NOx trap catalyst 15 and removed from the exhaust gas. It is like that.

The exhaust gas contains sulfur oxides (hereinafter referred to as sulfur components) such as sulfur dioxide (SO ₂ ) in addition to CO, HC, and NOx. It also has the property of occluding components.
A catalyst temperature sensor 16 is installed between the three-way catalyst 14 and the NOx trap catalyst 15. A NOx sensor 17 is installed on the downstream side of the NOx trap catalyst 15. Further, the further downstream side of the NOx sensor 17 in the exhaust pipe 13 is communicated with the atmosphere via a silencer (not shown).

The vehicle of the present embodiment is provided with an electronic control unit (ECU) 30 that performs comprehensive control of the engine 1.
The ECU 30 includes an input / output device (not shown), a storage device that stores a control program, a control map, and the like, an arithmetic device, a timer, a counter, and the like.
Further, the ECU 30 includes, as functional elements, an air-fuel ratio adjustment unit (air-fuel ratio adjustment unit) 31, a catalyst temperature raising unit 32, a NOx purge execution unit 33, a desulfurization process execution unit (desulfurization process execution unit, sulfur storage amount estimation unit) 34. And a leaning execution unit (leaning execution means) 35.

  The ECU 30 is connected to a catalyst temperature sensor 16, a NOx sensor 17, an air flow sensor 20, an engine speed sensor 21, an accelerator opening sensor (APS) 22, and other sensors (not shown). The catalyst temperature sensor 16 provides the catalyst temperature T, the NOx sensor 17 provides the NOx sensing signal C, the airflow sensor 20 provides the intake air amount A, the engine speed sensor 21 provides the engine speed Ne, and the accelerator opening sensor 22 provides The accelerator opening θ is input to the ECU 30.

Further, the ignition plug 3 and the fuel injection valve 4 are connected to the ECU 30. The air-fuel ratio adjustment unit 31 of the ECU 30 determines the fuel injection amount, the fuel injection timing, and the ignition timing based on various input information and an operation mode described later, and drives and controls the fuel injection valve 4 and the ignition plug 3. is there.
The air-fuel ratio adjusting unit 31 controls the engine 1 in accordance with four operation modes of a stratified super lean mode, an intake lean mode, a stoichiometric mode, and an enriched mode as fuel injection modes during normal running. .

The stratified super lean mode is an operation mode in which fuel injection is performed during the compression stroke in order to realize the lean operation by the above-described stratified super lean combustion and improve fuel efficiency.
The intake lean mode is an operation mode in which fuel injection is performed during the intake stroke in order to achieve operation at an air / fuel ratio leaner than the stoichiometric air / fuel ratio by premixed combustion and to obtain an output by slow acceleration.
The stoichiometric mode is an operation mode in which fuel injection is performed during the intake stroke in order to realize stoichiometric feedback operation at an air-fuel ratio near the stoichiometric air-fuel ratio by premixed combustion and to improve output compared to the intake lean mode.

The enrich mode is an operation mode in which operation at an air / fuel ratio richer than the stoichiometric air / fuel ratio is performed by premixed combustion.
Further, the air-fuel ratio adjusting unit 31 controls the fuel injection amount, the fuel injection timing, and the ignition timing according to these four operation modes.
More specifically, the air-fuel ratio adjusting unit 31 performs one operation mode from the above operation modes according to the engine speed Ne and the average effective pressure Pe according to a map as shown in FIG. Select. Then, a fuel injection control map corresponding to the selected operation mode is selected, and normal combustion is performed according to the engine speed Ne and the average effective pressure Pe using the selected fuel injection control map (not shown). A fuel injection amount and an injection timing (that is, a fuel injection end timing and a fuel injection start timing) to be performed are set.

Note that the average effective pressure Pe is determined by the air-fuel ratio adjusting unit 31 in place of the engine speed Ne and the accelerator opening θ (or the throttle opening θ instead of the accelerator opening θ when the throttle opening always corresponds to the accelerator opening θ). The degree of opening may be used).
The catalyst temperature raising unit 32 is for raising the temperature of the three-way catalyst 14 and the NOx trap catalyst 15. The catalyst temperature raising unit 32 affects the output of the engine 1 separately from the drive of the fuel injection valve 4 in the main fuel injection (fuel injection in the compression stroke or the intake stroke) for normal combustion in the combustion chamber 5. Additional fuel injection (fuel injection at the end of the expansion stroke) that drives the fuel injection valve 4 at a timing that is difficult to perform is performed, and this additional fuel is combusted on the exhaust pipe 13 side so that high-temperature exhaust gas is vented to the exhaust pipe 13. As a result, the temperature of the three-way catalyst 14 and the NOx trap catalyst 15 is increased.

  In addition, when the NOx purge execution condition is satisfied, that is, when it is determined that the NOx stored in the NOx trap catalyst 15 is saturated or approaching saturation, the NOx purge execution unit 33 Instead of the operation mode, the NOx purge mode for releasing / reducing (purifying) NOx stored in the NOx trap catalyst 15 is selected.

When the NOx purge execution unit 33 determines that the NOx purge execution condition is satisfied and selects the NOx purge mode, the air-fuel ratio adjustment unit 31 performs additional fuel injection (rich spike) in addition to normal fuel injection (main injection). The fuel injection valve 4 is driven so that the air-fuel ratio of the exhaust gas becomes richer than the stoichiometric air-fuel ratio.
That is, in consideration of the effects of securing HC and CO as reducing agents for promoting the release of NOx stored in the NOx trap catalyst 15 and the influence on the output torque of the engine 1 (preferably at the end of the expansion stroke). When the additional fuel injection is performed at a timing close to), the total air-fuel ratio (total A) that combines the normal fuel injection (main injection) and the additional fuel injection while suppressing the change in the output torque of the engine 1 is suppressed. / F) becomes rich.

  When the NOx purge mode is selected, the air-fuel ratio may be made rich only in normal fuel injection (main injection) instead of performing additional fuel injection as described above. However, in this case, the engine output may fluctuate due to enrichment, and therefore processing that suppresses fluctuations in the engine output by ignition timing control, intake air amount control, and the like is necessary.

When the NOx purge mode is selected in this way, the air-fuel ratio adjustment unit 31 sets the air-fuel ratio to be richer than the stoichiometric air-fuel ratio, makes the ambient atmosphere of the NOx trap catalyst 15 a reducing atmosphere, and reduces the amount of fuel such as CO. By supplying the reducing agent resulting from complete combustion to the NOx trap catalyst 15, NOx occluded on the NOx trap catalyst 15 is released from the NOx trap catalyst 15, and NOx occluded in the NOx trap catalyst 15 is removed. At the same time, NOx is reduced to harmless N ₂ by reacting with a reducing agent (CO) and released to the atmosphere.

Here, the air-fuel ratio adjustment unit 31 determines that both of the following NOx purge execution conditions (A) and (B) are satisfied, and selects the NOx purge mode.
· NOx purge execution condition (A): that the catalyst temperature T is higher than the predetermined temperature T ₁ of the temperature of the activation temperature near the NOx trap catalyst 15 · NOx purge execution condition (B): than the air-fuel ratio is the stoichiometric air-fuel ratio In the lean state (stratified super lean mode or intake lean mode), the NOx sensing signal C input from the NOx sensor 17 is not less than the predetermined value C ₀ , that is, in the state where the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. When the NOx sensing signal C is equal to or greater than the predetermined value C ₀ , it can be determined that NOx in the exhaust gas is not sufficiently occluded / removed by the NOx trap catalyst 15.

As the NOx purge execution condition is not limited thereto, the catalyst temperature T may be selected the NOx purge mode when lean operation simply continues for a predetermined time, higher than the predetermined temperature T ₁ of .
Further, when the catalyst temperature T is lower than a predetermined temperature T ₁ that is a temperature near the activation temperature of the NOx trap catalyst 15, the catalyst temperature may be raised by the catalyst temperature raising unit 32.

Hereinafter, the desulfurization process which is the most characteristic part of the present invention will be described.
The sulfur component stored in the NOx trap catalyst 15 has a higher chemical stability than the NOx stored in the NOx trap catalyst 15 and is only partially released even in the NOx purge mode. The amount of residual sulfur component increases with time. As a result, the so-called S poisoning in which the NOx occlusion performance of the NOx trap catalyst 15 gradually decreases occurs.

Therefore, when it is determined that the following preset S purge execution conditions (desulfurization process execution conditions) (a) and (b) are satisfied, the desulfurization process execution unit 34 performs each operation during the normal travel described above. Instead of the mode, the S purge mode is selected as the operation mode.
S purge execution condition (a): the catalyst temperature T is a predetermined temperature T ₂ (ie, T ₁ <T ₂ ) that is higher than a predetermined temperature T ₁ that is near the activation temperature of the NOx trap catalyst 15. S purge execution condition (b): The vehicle travels a predetermined distance D from the previous S purge execution. The S purge execution condition (a) is set when the catalyst temperature T is too low. This is because the present inventors paid attention to the point that the sulfur component remaining in the NOx trap catalyst 15 cannot be sufficiently released.

  The S purge execution condition (b) is based on the condition that the estimated storage amount of the sulfur component is equal to or greater than a predetermined amount. In other words, the predetermined distance D may be obtained in advance by experiment or the like so that the sulfur component can be accumulated to such an extent that the NOx purification function of the NOx trap catalyst 15 is lowered. That is, in this case, the desulfurization processing execution unit 34 functions as a sulfur storage amount estimation unit that estimates the amount of sulfur component stored in the NOx trap catalyst 15.

  In addition, the sulfur storage amount estimating means is configured to estimate the storage amount of the sulfur component stored in the NOx trap catalyst 15 based on the integrated value of the fuel injection amount from the previous execution of the S purge. Also good. In this case, instead of the S purge execution condition (b), it may be set as the S purge execution condition that the integrated value of the fuel injection amount from the previous S purge execution reaches the predetermined fuel injection amount E. That is, a value that is considered to accumulate the sulfur component to the extent that the NOx purification function of the NOx trap catalyst 15 is reduced based on the relationship between the content of the sulfur component contained in the fuel and the sulfur component stored in the NOx trap catalyst 15 is tested in advance. You may ask for it.

As the S purge execution condition, in addition to the S purge execution condition (a), a condition that the sulfur component is considered to remain to such an extent that the NOx purification function of the NOx trap catalyst 15 is reduced may be set as appropriate. .
If the catalyst temperature T is lower than the predetermined temperature T ₂ , the catalyst temperature raising unit 32 may raise the catalyst temperature. At this time, a predetermined temperature T ₃ that is lower than the predetermined temperature T ₂ is set in advance, and when the catalyst temperature T is higher than the predetermined temperature T ₃ , the catalyst temperature riser 32 sets the catalyst temperature T. You may comprise so that it may heat up.

That is, if the temperature does not reach the predetermined temperature T ₂ but can be easily raised by the catalyst temperature raising unit 32, the temperature of the NOx trap catalyst 15 is raised by the catalyst temperature raising unit 32 and then the S purge is performed. May be configured to be executed. In this way, it is possible to reduce deterioration in fuel consumption due to a significant increase in the catalyst temperature T.
When the desulfurization processing execution unit 34 determines that the above-described S purge execution condition is satisfied and selects the S purge mode, the air / fuel ratio adjustment unit 31 adjusts the air / fuel ratio to adjust a predetermined S purge execution time (normally). The target air-fuel ratio is set slightly richer than the stoichiometric air-fuel ratio for a few minutes) to perform feedback control (rich shifted SF / B operation). As a result, a desulfurization process (also referred to as S purge) for removing sulfur components accumulated in the NOx trap catalyst 15 is performed.

Further, when the desulfurization processing execution unit 34 selects the S purge mode, at the same time, the leaning execution unit 35 determines the lean period based on the detected operating state of the engine 1 such as the engine speed Ne and the catalyst temperature T. T _L and rich period T _R are set.
Incidentally, the rich period T _R is rich shifted Ted S-F / B performs operation time. Lean period T _L is the air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter, intermittent referred lean air-fuel ratio R _{A / F)} is the time to perform the operation, in a sufficiently shorter time than the rich period T _R It is set up.

Further, the lean period T _L and the rich period T _R is the intake air amount A in each case, and is periodically set according to the engine rotational speed Ne and catalyst temperature T.
At this time, lean execution unit 35, the intake air amount A, the larger the value of the engine rotational speed Ne and catalyst temperature T, set a short or rich period T _R is set longer lean period T _L, the intake air the amount a, so that the greater the value of the engine rotational speed Ne and catalyst temperature T is smaller, setting a longer or rich period T _R is set short lean period T _L.

In addition, the air-fuel ratio (referred to as intermittent lean air-fuel ratio) R _{A / F} in the lean period _TL is also set by the leaning execution unit 35 based on the intake air amount A, the engine speed Ne, and the catalyst temperature T. ing.
The intermittent lean air-fuel ratio R _{A / F} is set to a leaner side as the intake air amount A, the engine speed Ne and the catalyst temperature T are larger, and the values of the intake air amount A, the engine speed Ne and the catalyst temperature T are set. The smaller the is, the more it is set to the rich side.

Then, the air-fuel ratio adjusting unit 31, an interval of rich period T _R set by the leaning execution unit 35, only the lean period T _L which is set, an empty from a rich shift Ted S-F / B operation described above The fuel ratio is made leaner than the stoichiometric air fuel ratio (lean spike). The air-fuel ratio set at this time (hereinafter referred to as intermittent lean air-fuel ratio _{RA / F} ) is set based on the operating state of the engine 1 such as the engine speed Ne and the catalyst temperature T. ing.

Since the exhaust gas purifying apparatus and the desulfurization method of the exhaust gas purifying apparatus according to an embodiment of the present invention are configured as described above, the following operations and effects are achieved when the S purge is performed.
The execution mode of the S purge mode will be described with reference to FIG.
First, as step S10, the desulfurization processing execution unit 34 detects the operating state of the engine 1 by reading the intake air amount A and the engine speed Ne.

Next, as step S <b> 20, the desulfurization processing execution unit 34 reads the catalyst temperature T to detect or estimate the temperature of the NOx trap catalyst 15.
As step S30, the desulfurization process executing unit 34 compares the a predetermined temperature T ₂ the catalyst temperature T as part of the S purge execution condition. When the catalyst temperature T is equal to or higher than the predetermined temperature T ₂ (that is, when T ₂ ≦ T), the process proceeds to step S40.

As described above, the predetermined temperature T ₂ is set to a temperature higher than the activation temperature when NOx is stored and released in the NOx trap catalyst 15. Thereby, the sulfur component occluded in the NOx trap catalyst 15 is moved to the periphery of the NOx trap catalyst 15 under a temperature higher than the activation temperature (that is, the predetermined temperature T ₁ ) when NOx is occluded and released. It effectively reacts with CO, which is a reducing agent, and can effectively release the sulfur component from the NOx trap catalyst 15.

In step S40, the desulfurization processing execution unit 34 determines whether or not the S purge execution condition is satisfied. When it is determined that the S purge execution condition is satisfied, the process proceeds to step S50, and the desulfurization process execution unit 34 executes the desulfurization process.
That is, as step S50, the air-fuel ratio adjusting unit 31 performs the S purge operation. That is, as shown by the solid line L ₁ in FIG. 4, the rich shifted SF / B operation is performed only during the rich period T _R (T _R1 , T _R2 ,...) Set each time by the leaning execution unit 35. At this time, the operation is performed with the air-fuel ratio set to the intermittent lean air-fuel ratio R _{A / F} for the set lean period T _L (T _L1 , T _L2 ,...) (Partial leaning step). ) Is periodically repeated, and when the S purge execution time is reached, the S purge operation is terminated.

Thus, according to the exhaust gas purification device and the desulfurization method of the exhaust gas purification device of the present invention, as will be described later with reference to FIG. 4, CO as a reducing agent can be sufficiently supplied to the NOx trap catalyst 15, The sulfur component accumulated in the NOx trap catalyst 15 can be released and the production of NH ₃ can be suppressed.
4, a thick broken line L ₂ corresponds to the conventional S purge operation, only the S purge execution time, shows a case of performing continuous rich shifted Ted S-F / B operation. As described above, the solid line L ₁ indicates the case where the S purge operation in the present embodiment is performed.

That is, in the conventional S purge operation, although the rich shift SF / B operation is continuously performed, the CO concentration in the exhaust gas can be increased and the reducing agent can be supplied to the NOx trap catalyst. A large amount of NH ₃ is generated in the exhaust gas, and the generated NH ₃ is released into the atmosphere as it is, so that the exhaust gas contains a strong odor.
On the other hand, in the present embodiment, compared to the case where the rich shifted SF / B operation is continuously performed, an increase in the concentration of NH _{3 in} the exhaust gas can be suppressed and the NOx trap catalyst. The NOx trap catalyst 15 can be supplied with CO as a reducing agent sufficient to release the sulfur component accumulated in the NOx 15.

This makes it possible to release the accumulated sulfur components in the NOx trap catalyst 15, to suppress the generation of NH _3, it is possible to prevent a situation that contains odor in the exhaust gas.
That is, the reaction of generating NH ₃ contained in the exhaust gas occurs when the air-fuel ratio of the exhaust gas is a rich air-fuel ratio continuously for a certain period of time with a catalyst. For this reason, the amount of NH ₃ produced in the catalyst can be suppressed by periodically making the air-fuel ratio lean.

Further, in the present embodiment, each rich period T _R (T _R1 , T _R2 ,...), Lean period T _L (T _L1 , T _L2 ,...), And intermittent lean air-fuel ratio R _{A / F} Is set each time by the leaning execution unit 35 based on the detected engine speed Ne and the catalyst temperature T.
Therefore, a sufficient amount of CO is accurately supplied to the NOx trap catalyst 15 according to the operating state of the engine 1 and the degree of activity of the NOx trap catalyst 15, and the sulfur component accumulated in the NOx trap catalyst 15 is released to thereby release the NOx trap. The NOx purification function of the catalyst 15 can be recovered, the production of NH ₃ can be suppressed, and the malodor of the exhaust gas can be reduced.

Further, when executing the desulfurization process, if the engine speed Ne or the load is large, the amount of exhaust gas flowing through the exhaust pipe 13 is increased accordingly. It is conceivable that the amount of NH ₃ produced in the exhaust pipe 13 increases in proportion to the exhaust gas flow rate. Therefore, by setting longer the lean period, it is possible to effectively suppress the generation of NH ₃ in the NOx trap catalyst.

Further, it can be considered that as the exhaust gas flow rate flowing into the NOx trap catalyst 15 increases, the amount of NH ₃ generated near the NOx trap catalyst increases while a large amount of CO flows. Further, it is considered that the higher the catalyst temperature T is, the higher the degree of activity of the catalyst is, so that the amount of NH ₃ produced is increased. Therefore, by setting the rich period T _R , the lean period T _L and the intermittent lean air-fuel ratio R _{A / F based} on the intake air amount A, the engine speed Ne, and the catalyst temperature T, the operating state of the engine 1 and NOx According to the degree of activity of the trap catalyst 15, it is possible to suppress the generation of NH ₃ with higher accuracy and supply as much CO as possible to the NOx trap catalyst to more effectively remove the sulfur component from the NOx trap catalyst.

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.
For example, in the above-described embodiment, an in-cylinder injection type gasoline engine has been described as an example. However, the internal combustion engine to which the exhaust gas purifying apparatus of the present invention is applied is not limited to this and may be applied to a diesel engine. Further, the present invention is not limited to the in-cylinder injection type gasoline engine as long as it has a NOx trap catalyst and can change the air-fuel ratio. Further, an engine having a plurality of cylinders may be used.

In the embodiment, the rich period T _R , the lean period _TL, and the intermittent lean air-fuel ratio R _{A / F} are set each time according to the engine operating state and the catalyst temperature. _R , the lean period _TL, and the intermittent lean air-fuel ratio _{RA / F} may be set as fixed values, respectively.
In the embodiment, the rich period T _R and the lean period T _L are alternately and periodically repeated during the execution of the S purge. However, it is not always necessary to periodically perform the generation, and the generation of NH ₃ can be suppressed. If so, the lean period T _L may be configured to be only once during the execution of the S purge.

Furthermore, in the above-described embodiment, a sensor or the like for detecting the NH ₃ concentration in the exhaust gas is further provided, and when the NH ₃ concentration in the exhaust gas is larger than a predetermined concentration when performing the desulfurization process, the exhaust gas is only in the lean period. The air-fuel ratio may be set to be leaner than the stoichiometric air-fuel ratio.
A general NOx sensor has a sensitivity to NH ₃ when the air-fuel ratio of exhaust gas is the stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio. it may be used as the NH ₃ sensor during the desulfurization process execution.

BRIEF DESCRIPTION OF THE DRAWINGS It is for demonstrating the exhaust gas purification apparatus which concerns on one Embodiment of this invention, and the desulfurization method of an exhaust gas purification apparatus, Comprising: It is a block diagram which shows typically the whole structure. BRIEF DESCRIPTION OF THE DRAWINGS It is a map for demonstrating the exhaust gas purification apparatus which concerns on one Embodiment of this invention, and the desulfurization method of an exhaust gas purification apparatus, Comprising: It is a map used for fuel injection control. It is a flowchart for demonstrating the desulfurization method of the exhaust gas purification apparatus and exhaust gas purification apparatus which concerns on one Embodiment of this invention, Comprising: It is a flowchart which shows the procedure concerning a desulfurization process. There is shown the operation and effect of the method for desulfurizing exhaust gas purifying apparatus and an exhaust gas purifying apparatus according to an embodiment of the present invention, with respect to time, is a graph showing respectively the air-fuel ratio, CO concentration, the characteristics of the NH ₃ concentration. Provided for the purpose of illustrating the prior art, it is a graph showing relative air-fuel ratio of the exhaust gas, CO concentration, respectively, NH ₃ concentration, the characteristics of the NOx concentration.

Explanation of symbols

1 Engine 2 Cylinder Head 3 Spark Plug 4 Fuel Injection Valve 5 Combustion Chamber 6 Cylinder 7 Piston 8 Cavity 9 Intake Port 10 Exhaust Port 11 Intake Valve 12 Exhaust Valve 13 Exhaust Pipe (Exhaust Gas Path)
14 Three-way catalyst 15 NOx trap catalyst 16 Catalyst temperature sensor 17 NOx sensor 20 Air flow sensor 21 Engine speed sensor 22 Accelerator position sensor 30 Electronic control unit (ECU)
31 Air-fuel ratio adjustment unit (air-fuel ratio adjustment means)
32 catalyst temperature raising unit 33 NOx purge execution unit 34 desulfurization processing execution unit (desulfurization processing execution means, sulfur storage amount estimation means)
35 Leanification execution unit (leanization execution means)

Claims (8)

A NOx trap catalyst which is provided in an exhaust gas passage of an internal combustion engine and occludes nitrogen oxides and sulfur components in the exhaust gas in a lean atmosphere where the air-fuel ratio of the exhaust gas is lean;
Air-fuel ratio adjusting means for adjusting the air-fuel ratio of the exhaust gas;
When a predetermined desulfurization treatment execution condition is satisfied, the air-fuel ratio of the exhaust gas is made slightly richer than the stoichiometric air-fuel ratio and the desulfurization treatment is performed to release the sulfur component stored in the NOx trap catalyst. Desulfurization processing execution means for controlling the fuel ratio adjusting means;
Leaning execution means for controlling the air-fuel ratio adjusting means so that the air-fuel ratio of the exhaust gas is made leaner than the stoichiometric air-fuel ratio for a predetermined lean period when the desulfurization processing means is performed. An exhaust gas purifying device characterized by comprising: When the desulfurization processing is performed by the desulfurization processing execution means, the leaning execution means periodically makes the air-fuel ratio leaner than the stoichiometric air-fuel ratio during the lean period periodically during the lean period. 2. The exhaust gas purifying apparatus according to claim 1, wherein the fuel ratio adjusting means is controlled. The exhaust gas purifying apparatus according to claim 2, wherein the leaning execution means sets the rich period based on an operating state of the internal combustion engine. The exhaust gas purification apparatus according to any one of claims 1 to 3, wherein the leaning execution means sets the lean period based on an operating state of the internal combustion engine. The exhaust gas purification unit according to any one of claims 1 to 4, wherein the leaning execution means sets an air-fuel ratio of the exhaust gas during the lean period based on an operating state of the internal combustion engine. apparatus. The exhaust gas purifying apparatus according to any one of claims 1 to 5, wherein the desulfurization treatment execution condition includes that the temperature of the NOx trap catalyst is equal to or higher than a predetermined temperature. A sulfur storage amount estimating means for estimating the amount of sulfur component stored in the NOx trap catalyst;
7. The desulfurization treatment execution condition includes that the storage amount of the sulfur component estimated by the sulfur storage amount estimation means is equal to or greater than a predetermined amount. The exhaust gas purification apparatus according to 1. NOx trap catalyst that is provided in the exhaust gas passage of the internal combustion engine and occludes nitrogen oxides and sulfur components in the exhaust gas in a lean atmosphere where the air-fuel ratio of the exhaust gas is lean, and air-fuel ratio adjusting means that adjusts the air-fuel ratio of the exhaust gas And a desulfurization method for an exhaust gas purification apparatus for performing a desulfurization process for releasing the sulfur component stored in the NOx trap catalyst,
A enrichment step of enriching the air-fuel ratio of the exhaust gas more than the stoichiometric air-fuel ratio when a predetermined desulfurization treatment execution condition is satisfied;
A desulfurization method for an exhaust gas purification apparatus, comprising: a partial leaning step for leaning the air-fuel ratio of the exhaust gas from the stoichiometric air-fuel ratio for a predetermined lean period when the enrichment step is executed.

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