JP2000230421A - Exhaust gas purification device for internal combustion engine - Google Patents

Abstract

(57) Abstract: In an exhaust gas purifying apparatus for an internal combustion engine, a start timing of NOx release control is accurately determined to suppress deterioration of exhaust gas characteristics and fuel efficiency. A detects the oxygen concentration in the exhaust gas flowing into the upstream side of the occlusion-type NOx catalyst 24 # 1O ₂ sensor 26
While disposed, and disposed #. 2O ₂ sensor 27 for detecting oxygen concentration in exhaust gas discharged to the downstream side, # 1O ₂
From the output value D1 of the sensor 26 by subtracting the output value D2 of #. 2O ₂ sensor 27 obtains the output value [Delta] D, determines emission start timing of the NOx stored on the basis of the output difference [Delta] D.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine having a storage NOx catalyst in an exhaust passage.

[0002]

2. Description of the Related Art In recent years, a lean-burn internal combustion engine has been put to practical use in which the internal combustion engine is operated at a lean air-fuel ratio to improve fuel efficiency. This lean-burn internal combustion engine has a problem that, when operated at a lean air-fuel ratio, the three-way catalyst cannot sufficiently purify NOx (nitrogen oxide) in exhaust gas due to its purification characteristics. An NOx storage catalyst has been employed which stores NOx in exhaust gas and releases and reduces NOx stored during operation at a stoichiometric air-fuel ratio or a rich air-fuel ratio.

[0003] The occlusion-type NOx catalyst, the nitrate of the NOx in the exhaust gas in an excess oxygen state of the internal combustion engine (X-NO ₃₎
A catalyst having the property of releasing stored NOx in an excess state of carbon monoxide (CO) and reducing it to nitrogen (N ₂ ) (carbonate X-CO ₃ is generated at the same time) It is. Therefore, in practice, when the lean air-fuel ratio operation continues for a predetermined time, the air-fuel ratio is periodically switched to a rich air-fuel ratio operation in which the air-fuel ratio is controlled to a stoichiometric air-fuel ratio or a rich air-fuel ratio (this is referred to as a rich spike). C
By generating a reducing atmosphere rich in O and releasing the stored NOx and purifying and reducing (NOx purging), the storage-type NOx catalyst can be regenerated. Such a technique is disclosed in, for example, Japanese Patent No. 25876738.

When performing such a rich spike, it is necessary to set a rich spike start timing, an operation period, and a rich air-fuel ratio degree according to the amount of NOx stored in the storage NOx catalyst. For example, JP
According to the "method of diagnosing the state of the NOx trap" disclosed in Japanese Patent Publication No. When the duration time exceeds a predetermined time, rich spikes are started, and the estimated deposition NO
A predetermined time and a predetermined rich air-fuel ratio are set in advance in accordance with the x amount.

[0005]

However, in practice,
The amount of NOx deposited on the storage NOx catalyst depends on the storage NOx.
It affects the temperature of the catalyst, the flow rate of the exhaust gas, and the like, and changes depending on the operating conditions of the internal combustion engine or the state of deterioration of the catalyst.
Therefore, as described above, if the start time of the rich spike is set according to the duration during the lean air-fuel ratio operation,
There is a risk that a rich spike may be started at a time that does not correspond to the amount of NOx actually deposited on the storage NOx catalyst. That is, if the start time of the rich spike is early, an unnecessary rich air-fuel ratio operation is performed, which causes deterioration of fuel efficiency and the like. On the other hand, if the start time of the rich spike is delayed, NOx in the exhaust gas cannot be occluded sufficiently, which causes factors such as deterioration of the exhaust gas characteristics.

An object of the present invention is to solve such a problem and to provide an exhaust gas purifying apparatus for an internal combustion engine in which the start timing of NOx release control is accurately determined to suppress deterioration of exhaust gas characteristics and fuel efficiency. With the goal.

[0007]

According to the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the present invention, wherein NOx in exhaust gas is contained in an exhaust passage of the internal combustion engine when the exhaust air-fuel ratio is a lean air-fuel ratio. And a NOx catalyst that releases and reduces the stored NOx when the air-fuel ratio is the rich air-fuel ratio or the stoichiometric air-fuel ratio, and detects a value correlated to the NOx concentration flowing into the storage NOx catalyst. 1 detecting means, and second detecting means for detecting a value correlated with the NOx concentration stored in the storing NOx catalyst, wherein the release determining means stores the NOx based on the outputs of the first detecting means and the second detecting means. The timing of releasing the released NOx is determined.
The release control means changes the exhaust air-fuel ratio to a rich air-fuel ratio or a stoichiometric air-fuel ratio.

Accordingly, the release time of the NOx stored in the storage NOx catalyst is determined from the value correlated with the NOx concentration detected by the first and second detection means, so that the accumulated NOx of the storage NOx catalyst is determined. The amount can be estimated with high accuracy, and the NOx release control can be started at an appropriate time. for that reason,
The operating period of lean air-fuel ratio operation that stores NOx and NOx
The operation period of the rich air-fuel ratio operation required for the emission of NO becomes extremely appropriate, and deterioration of fuel efficiency due to early start of NOx release control and deterioration of exhaust gas characteristics due to delay in start of NOx release control can be prevented.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a schematic configuration of an exhaust gas purifying apparatus for an internal combustion engine according to an embodiment of the present invention, FIG. 2 is a graph showing an output characteristic of an O ₂ sensor with respect to an air-fuel ratio, and FIG. Flow chart of NOx purge control by
NOx purge control time chart in FIG. 4, O ₂ in FIG. 5
4 is a graph showing a target air-fuel ratio with respect to a difference between sensor output values.

The internal combustion engine (hereinafter referred to as an engine) of the present embodiment is, for example, in a fuel injection mode (operation mode).
The fuel injection in-cylinder ignition type in-line four-cylinder gasoline engine is capable of performing fuel injection during the intake stroke (intake stroke injection mode) or fuel injection during the compression stroke (compression stroke injection mode). The in-cylinder injection type engine 11 can be easily operated at a stoichiometric air-fuel ratio (stoichiometric ratio), at a rich air-fuel ratio (rich air-fuel ratio operation), or at a lean air-fuel ratio (lean air-fuel ratio). Operation) can be realized, and in particular, in the compression stroke injection mode, operation at an ultra lean air-fuel ratio is possible.

In this embodiment, as shown in FIG.
An electromagnetic fuel injection valve 14 is attached to a cylinder head 12 of the engine 11 together with an ignition plug 13 for each cylinder, and the fuel injection valve 14 enables direct injection of fuel into a combustion chamber 15. . This fuel injection valve 14
Is connected to a fuel supply device (fuel pump) having a fuel tank via a fuel pipe (not shown). The fuel in the fuel tank is supplied at a high fuel pressure, and the fuel is supplied from a fuel injection valve 14 to a combustion chamber. The fuel is injected at a desired fuel pressure into the fuel cell 15. At this time, the fuel injection amount is determined from the fuel discharge pressure of the fuel pump and the valve opening time (fuel injection time) of the fuel injection valve 14.

An intake port is formed in the cylinder head 12 in a substantially upright direction for each cylinder, and one end of an intake manifold 16 is connected to communicate with each intake port. A drive-by-wire (DBW) type electric throttle valve 17 is connected to the other end of the intake manifold 16.
Is provided with a throttle position sensor 18 for detecting the throttle opening θth. An exhaust port is formed in the cylinder head 12 in a substantially horizontal direction for each cylinder, and one end of an exhaust manifold 19 is connected to communicate with each exhaust port.

The engine 11 is provided with a crank angle sensor 20 for detecting a crank angle, and the crank angle sensor 20 can detect an engine rotation speed Ne. The above-described in-cylinder injection engine 1
1 is already known, and the details of its configuration will not be described here.

The exhaust manifold 1 of the engine 11
An exhaust pipe (exhaust passage) 21 is connected to 9, and a muffler (not shown) is connected to the exhaust pipe 21 via an exhaust purification catalyst device 22. And this exhaust pipe 21
A high-temperature sensor 23 for detecting the exhaust gas temperature is provided on the upstream side of the exhaust gas purifying catalyst device 22 in FIG.

This exhaust purifying catalyst device 22 is a storage type NO
x catalyst 24 and a three-way catalyst 25, and the three-way catalyst 25 is a storage-type NOx catalyst 24.
It is arranged further downstream than. The storage type NOx catalyst 24 has a function of temporarily storing NOx in an oxidizing atmosphere and releasing NOx mainly in a reducing atmosphere where CO is present to reduce it to N ₂ (nitrogen) or the like. Specifically, the storage NOx catalyst 24 is configured as a catalyst having platinum (Pt), rhodium (Rh) or the like as a noble metal, and an alkali metal such as barium (Ba) or an alkaline earth metal as a storage material. Has been adopted.

Further, a position upstream of the storage type NOx catalyst 24 is provided.
And # 1O_TwoProvided with a sensor (first detection means) 26
And furthermore, located downstream of the storage NOx catalyst 24 #
2O _TwoA sensor (second detection means) 27 is provided.
These # 10_TwoSensor 26 and # 2O_TwoThe sensor 27
The oxygen concentration as a value correlated to the NOx concentration in the exhaust
It is to detect. These O_TwoThe sensors 26 and 27
It takes a large value when the amount of oxygen is small, and conversely
It is configured to take a small value when it is large. One
Mari, O_TwoThe output characteristics of the sensors 26 and 27 are shown in FIG.
As such, rich air fuel exists in which oxygen is present as compound CO.
Large in specific atmosphere, steep near stoichiometric atmosphere
And in a lean air-fuel ratio atmosphere with excess oxygen
It has been made smaller. Therefore, these # 10_Two
Sensor 26 and # 2O_TwoOxidation type NOx by sensor 27
By detecting the amount of oxygen upstream and downstream of the catalyst 24,
The air-fuel ratio upstream and downstream of the NOx storage catalyst 24
Each can be detected well.

Further, an input / output device and a storage device (ROM, R
AM, nonvolatile RAM, etc.), central processing unit (CPU),
An ECU (Electronic Control Unit) 28 having a timer counter and the like is provided, and the ECU 28 controls the exhaust gas purifying apparatus of the present embodiment including the engine 11 comprehensively. That is, on the input side of the ECU 28,
Various sensors such as the high-temperature sensor 23 and the NOx sensors 26 and 27 described above are connected, and detection information from these sensors is input. On the other hand, on the output side of the ECU 28,
The above-described ignition plug 13, fuel injection valve 14, and the like are connected via an ignition coil. The ignition coil, the fuel injection valve 14, and the like are connected to a fuel injection amount calculated based on detection information from various sensors. The optimum values such as the ignition timing are output. As a result, an appropriate amount of fuel is injected from the fuel injection valve 14 at an appropriate timing, and ignition is performed by the spark plug 13 at an appropriate timing.

Actually, the ECU 28 determines the target in-cylinder pressure corresponding to the engine load, that is, the target average effective pressure based on the throttle opening information θth from the throttle position sensor 18 and the engine rotation speed information Ne from the crank angle sensor 20. Pe is determined, and the fuel injection mode is set from a map (not shown) according to the target average effective pressure Pe and the engine rotation speed information Ne. For example, when the target average effective pressure Pe and the engine rotation speed Ne are both low, the fuel injection mode is set to the compression stroke injection mode, and fuel is injected in the compression stroke, while the target average effective pressure Pe increases, or When the rotation speed Ne increases, the fuel injection mode is set to the intake stroke injection mode, and fuel is injected during the intake stroke.

The target air-fuel ratio (target A / A), which is the control target, is determined from the target average effective pressure Pe and the engine speed Ne.
F) is set, and the appropriate amount of fuel injection is set to the target A / F
Is determined based on The catalyst temperature Tcat is estimated from the exhaust gas temperature information detected by the high temperature sensor 23. Specifically, the high-temperature sensor 23 and the storage NOx catalyst 24
In order to correct an error caused by being arranged at least a little apart from each other, a temperature difference map is set in advance by an experiment or the like in accordance with the target average effective pressure Pe and the engine rotation speed information Ne. The catalyst temperature Tcat is uniquely estimated when the target average effective pressure Pe and the engine speed information Ne are determined.

The operation of the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment having the above-described configuration will be described below.

In the storage type NOx catalyst 24 of the exhaust purification catalyst device 22, nitrate is generated from NOx in exhaust gas in an atmosphere having an excessively high oxygen concentration as in the super-lean combustion operation in the lean mode, whereby NOx is stored. Exhaust gas purification is performed. On the other hand, in the three-way catalyst 25, the nitrate stored in the storage NOx catalyst 24 reacts with the CO in the exhaust gas in an atmosphere having a reduced oxygen concentration to generate carbonate and release NOx. Therefore, the storage NOx catalyst 24
As the NOx storage proceeds, the oxygen concentration is reduced by enriching the air-fuel ratio or performing additional fuel injection, and the NOx is released from the storage NOx catalyst 24 to maintain the function.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment, the output value of the # 1O ₂ sensor 26 upstream of the storage NOx catalyst 24 and the output value # of the storage NOx catalyst 24 downstream
From the difference with the output value of the 2O ₂ sensor 27, this storage NOx
The timing of releasing the NOx stored in the catalyst 24 is determined (release determining means), and based on the determination result, the storage type NOx is determined.
The start of the period of the purge control (NOx release control means) necessary for releasing the NOx stored in the catalyst 24 is determined.
In the exhaust gas purifying apparatus, the end of the purge control period is determined based on the difference between the output values of the sensors 26 and 27.

Here, the NOx purge control of the storage type NOx catalyst 24 in the exhaust gas purification apparatus of the present embodiment will be described.
A description will be given based on the flowchart of FIG. 3 and the time chart of FIG.

As shown in the flowchart of FIG. 3, when the lean operation is started in step S11, first, in step S12, the output value D1 of the # 10 ₂ sensor 26 upstream of the storage NOx catalyst 24 is compared with the output value D1. , based on the output value D2 of #. 2O ₂ sensor 27 downstream, obtaining the output difference ΔD from the following equation. ΔD = D1-D2

Next, in step S13, the output difference Δ
If D is larger than the predetermined value ΔD0 (minimum value or value 0), the current lean operation is continued in step S14.
On the other hand, if the output difference ΔD is equal to or smaller than the predetermined value ΔD0, the NOx purge control or the S purge control is executed from step S15.

That is, as described above, when the exhaust gas in the oxygen-rich atmosphere flows into the storage NOx catalyst 24 in the lean operation, the storage NOx catalyst 24 generates nitrate from NOx in the exhaust gas. NOx is stored, but at this time, CO corresponding to the stored NOx is discharged, so that a downstream side of the storage NOx catalyst 24 has a rich atmosphere. Therefore, when the lean operation is being performed, the #
The output value D1 of the 1O ₂ sensor 26 and the downstream # 2O ₂
If the output difference ΔD from the output value D2 of the sensor 27 is larger than ΔD0, the downstream side of the storage type NOx catalyst 24 is rich, and the storage type NOx catalyst 24 can still sufficiently store NOx in the exhaust gas. It can be estimated that there is, and the current lean operation is continued in step S14.

On the other hand, the output value D of each O ₂ sensor 26, 27
If the output difference ΔD between the first and the second D2 is equal to or less than ΔD0, the storage type N
The downstream side of the Ox catalyst 24 has a lean atmosphere, and the storage type N
Since it can be estimated that the Ox catalyst 24 cannot occlude NOx in the exhaust gas any more, the process proceeds to step S15. In this step S15, if the lean operation continuation time T exceeds the predetermined time T _S , it is NO in step S16.
Start x purge control.

First, in step S17, when starting the NOx purge, the target A / F is set to a predetermined initial value, that is, an initial rich air-fuel ratio (for example, value 10). As a result, as shown in FIG. 4A, the NOx stored in the storage NOx catalyst 24 is rapidly released and begins to be reduced and removed. Next, in step S18, the output D1 of # 1O ₂ sensor 26 is a predetermined value Ds (e.g., output value corresponding to the stoichiometric (see FIG. 2)) to determine whether it is above.
Here, # 1O ₂ output D1 is a predetermined initial value the target A / F returns to step S17 if not exceed the predetermined value Ds of the sensor 26, i.e., the initial rich air-fuel ratio (e.g., a value 1
0). Note that the initial rich air-fuel ratio (for example, value 10) is preferably set to a value having the largest degree of richness used in the NOx purge control.

In this way, the next step is step S19.
# 1O_TwoOutput D1 of sensor 26 and # 2O_TwoSen
The operation is performed using the output D2 of the sensor 27.
10 _TwoSensor 26 and # 2O_TwoNOx par of sensor 27
The element of the response delay at the beginning of the
), And accurate calculation is made possible. Meanwhile, step
In S18, # 1O_TwoThe output D1 of the sensor 26 is a predetermined value D
# 10_TwoSensor 26 or # 2O_TwoSensor
If the 27 response delay elements are deemed to have been eliminated,
The process moves to step S19.

In step S19, the next step is performed as described above.
By the formula, # 1O_TwoOutput D1 of sensor 26 and # 2O_TwoC
The output difference ΔD from the output D2 of the sensor 27 is calculated. ΔD = D1−D2 That is, in FIG._TwoOutput D1 of sensor 26
By a dashed line, # 2O _TwoThe output D2 of the sensor 27 is represented by a solid line.
As shown, the difference between these outputs D1 and D2 is calculated.
And the amount of oxygen in the exhaust gas on the upstream side of the storage NOx catalyst 24.
The difference from the amount of oxygen in the downstream exhaust, that is, the air-fuel ratio in the exhaust
Try to find the difference between

[0032] Thus, released NOx stored in occlusion-type NOx catalyst 24 by NOx purge When reduced with N ₂ is O ₂ (oxygen) is being generated, the amount of O ₂ generated by this reduction Is appropriately detected, and thus the amount of released NOx can be appropriately detected. When the air-fuel ratio is changed from the lean air-fuel ratio to the rich air-fuel ratio, the amount of oxygen is abrupt. Therefore, as shown in FIG. The output D1 of the _second sensor 26 is kept constant after changing to a large side with a steep gradient. On the other hand, at this time, the storage NOx
In the catalyst 24, NOx stored initial rich air-fuel ratio (e.g., the value 10) emitted by a large amount of CO which occurs under, is reduced and O ₂ is a large amount of generation, therefore the occlusion-type NOx catalyst 24 the output D2 of the downstream side is #. 2O ₂ sensor 27 is held at a small. That is, when the output D1 becomes equal to or more than the predetermined value Ds, the difference ΔD, that is, the NOx release amount is relatively large, as shown in FIGS.

Then, in the next step S20, an optimum target A / F is determined by calculation based on the output difference ΔD thus obtained. Actually, for example, as shown in FIG. 5, a map indicating the relationship between the output difference ΔD and the target A / F is set in advance by experiments or the like, and the target A / F is determined from this map in accordance with the difference ΔD. Desired. According to the figure, when the output D1 becomes equal to or more than the predetermined value Ds, the difference AD is relatively large as described above, and thus the target A / F is the same as the initial rich air-fuel ratio (for example, the value 10). It is said.

In the next step S21, it is determined whether or not the output difference ΔD has become equal to or smaller than a predetermined value ΔD0. That is, the amount of O ₂ generated by the reduction becomes extremely small, and the stored NOx
It is determined whether the amount has almost been reduced and it is no longer necessary to perform the NOx purge. At this point, the difference Δ
Since D is relatively large, steps S19 and S
Step 20 is repeated and continued.

Then, as the release and reduction of NOx stored in the storage NOx catalyst 24 progresses, O generated by the reduction proceeds.
₂ gradually increases and then decreases, and the storage NOx catalyst 2
4 also transitions to the rich air-fuel ratio side to # 2O
The output D2 of the _second sensor 27 also starts to change to the large side. As described above, when the output D2 changes to the large side, FIG.
As shown in (b), the difference ΔD gradually decreases with respect to the output D1. The difference ΔD is, for example, a predetermined value ΔD1
When it becomes smaller, the target A / F based on the map of FIG.
Is the air-fuel ratio with a slightly smaller degree of enrichment (for example, value 12)
(See FIG. 4C). That is, NOx emission,
When the reduction has progressed to some extent, the reducing agent, CO, is no longer required so much. Therefore, in this case, the target A / F is changed to a side with a lower degree of enrichment by reducing the fuel supply amount or the like. As a result, surplus CO is not discharged, and deterioration of fuel efficiency and the like are suitably prevented.

[0036] By the way, the output of # 2O ₂ sensor 27 D2
Actually, as shown in FIG. 4 (b), the difference ΔD continues for a while while being kept at the predetermined value ΔD2. This is because the NOx stored in the storage NOx catalyst 24 is
When a certain amount of NOx is released, only a portion of the NOx that is difficult to release remains, and this difficult-to-release NOx is released little by little and takes time to release (FIG. 4).
(See (a).) During this time, the target A / F is kept at the air-fuel ratio (for example, the value 12) having a slightly smaller degree of enrichment in order to completely release the hardly released NOx.

When the hardly released NOx is almost completely released and reduced, the difference ΔD starts decreasing again. Accordingly, the target A / F is set based on the map shown in FIG.
Is gradually reduced.
As a result, the CO in the exhaust gas is gradually reduced, that is, the N which remains as a reducing agent is not released.
The amount required for the reduction of Ox is suppressed to the minimum necessary amount, so that the excess CO is not discharged and the deterioration of the fuel efficiency can be prevented very suitably.

Thereafter, in step S21, the output difference ΔD
Is smaller than the predetermined value ΔD0, it can be determined that the NOx stored in the storage type NOx catalyst 24 has been almost completely removed. In this case, CO as the reducing agent is no longer necessary, and this NOx purge control is terminated. . Thereby, the storage NO
The NOx stored in the x catalyst 24 is reliably removed without excess or shortage, and the lean operation is started again.

As shown in FIG. 4, when the difference ΔD becomes a predetermined value ΔD0 (minimum value or value 0), the target A
/ F is a slightly rich air-fuel ratio (for example, a value of 14.5) near stoichiometric (a value of 14.7).
x Storage-type NO while maintaining rich air-fuel ratio until the end of purge
This is to prevent NOx from being newly stored in the x-catalyst 24. Furthermore, if the air-fuel ratio is slightly rich (for example, a value of 14.5), the generation of CO and HC is ensured because the air-fuel ratio is close to stoichiometric. It is because it can be suppressed to.

In this case, as shown in FIG. 4C, the target A / F is set to the lean air-fuel ratio again after the end of the NOx purge. This is because the injection mode is set to a mode corresponding to the lean air-fuel ratio. In the stoichiometric feedback mode, the target A / F is set to stoichiometric, and in the open loop mode, the target A / F is set to the rich air-fuel ratio.

On the other hand, if the lean operation continuation time T is equal to or less than the predetermined time T _S in step S15, the S purge control is started in step S22. That is, the sulfur component contained in the fuel and the lubricating oil also exists in the exhaust gas, and the storage NOx
The catalyst 24 is provided with SOx and NOx in an atmosphere having an excessive oxygen concentration.
Also occludes. That is, the sulfur component is oxidized into SOx, and a part of the SOx further reacts with the NOx storage agent on the storage NOx catalyst 24 to become a sulfate and is stored in the storage NOx catalyst 24. . Further, the storage NOx catalyst 24 has a function of releasing the stored SOx when the oxygen concentration decreases. That is, in an atmosphere in which the oxygen concentration is reduced, a part of the sulfate stored in the storage type NOx catalyst 24 reacts with CO in the exhaust to generate carbonate and release SOx. However, sulfate has a higher stability as a salt than nitrate, and only a part of it is decomposed even in an atmosphere having a reduced oxygen concentration.
The amount of sulfate remaining on x catalyst 24 increases with time. As a result, the storage capacity of the storage NOx catalyst 24 decreases with time, and the performance of the storage NOx catalyst 24 deteriorates (S poisoning). For this reason, when it is determined that the lean operation continuation time T is equal to or less than the predetermined time T _S and the S poisoning of the storage NOx catalyst 24 has progressed, in the present embodiment, the catalyst is set to a predetermined temperature (for example, 650 ° C.) or higher. And the air-fuel ratio is reduced to a reducing atmosphere to release the stored SOx (S purge operation).

[0042] In this manner, in the exhaust purification system of an internal combustion engine of the present embodiment, disposing the # 1O ₂ sensor 26 for detecting oxygen concentration in exhaust gas flowing into the upstream side of the occlusion-type NOx catalyst 24 on the other hand, arranged to #. 2O ₂ sensor 27 for detecting oxygen concentration in exhaust gas discharged to the downstream side, the output value D1 of # 1O ₂ sensor 26 during operation of a lean air-fuel ratio
#. 2O ₂ subtracts the output value D2 of the sensors 27 seeking output difference [Delta] D, and so as to start the NOx purge control of occlusion-type NOx catalyst 24 by enriching the exhaust air-fuel ratio based on the output difference [Delta] D from .

Therefore, the amount of NOx stored in the storage type NOx catalyst 24 can be estimated with high accuracy, the NOx purge control can be started at an appropriate time, and the operating period of the lean air-fuel ratio operation for storing NOx and the NOx amount can be reduced. The operation period of the rich air-fuel ratio operation required for release becomes extremely appropriate, and NOx
Deterioration of fuel efficiency due to early start of purge control and deterioration of exhaust gas characteristics due to delay in start of NOx purge control can be prevented.

In the above-described embodiment, # is provided as the first detecting means upstream of the storage NOx catalyst 24.
Although the 1O ₂ sensor 26 is used, the invention is not limited to this.
A map value that is set and stored in advance based on the operating state of No. 1 may be used. In addition, the O ₂ sensors 26 and 27 are used as the first and second detecting means, and the release determining means uses the storage type N.
Based on the oxygen concentration difference between the upstream and the bottom sulfur of the Ox catalyst 24, N
Although the Ox purge start timing is determined, the NOx release determination may be performed from the difference between the upstream and downstream air-fuel ratios (A / F) using an air-fuel ratio sensor or the like.

Further, the output difference Δ between the O ₂ sensors 26 and 27
The predetermined value ΔD0 for D is a fixed value.
It may be a map value that is set and stored in advance based on the operating state of No. 1.

In the present embodiment, the O ₂ sensors 26 and 27 are used as the first and second detecting means. However, the N ₂ sensors 26 and 27 are used to detect NOx concentration in the exhaust gas.
Based on the output value of the Ox sensor, the occlusion state of the occlusion type NOx catalyst 24 is determined, for example, by the amount of NOx to be discharged (=
x NOx amount flowing into x catalyst 24-NOx stored
Amount) or the storage rate of the storage NOx catalyst 24 (= the amount of NOx flowing into the storage NOx catalyst 24 / N stored
It is also possible to determine the NOx purge start timing that has calculated the (Ox amount) and the like. The NOx sensor is a storage type NOx
It may be disposed only on the upstream or downstream side of the catalyst 24.

Further, in the above-described embodiment, the storage type NOx catalyst 24 capable of performing both the release and reduction of NOx at the time of purging NOx is used.
Oxidation type NOx that allows the Ox catalyst 24 to only release NOx
The catalyst may be configured such that the catalyst is reduced by the three-way catalyst 25 on the downstream side.

The storage type NOx catalyst 24 having both the function of releasing NOx and the function of reducing NOx without using the three-way catalyst 25.
It is good also as a structure using only. Further, after the NOx purge is started, # 1O ₂ output until the output D1 of the sensor 26 is equal to or more than a predetermined value Ds D1, without calculation of the difference ΔD output D2, # 1O ₂ sensor 26 and #. 2O ₂ sensor 27
Although the element of the response delay is excluded (masking), the initial rich air-fuel ratio may be simply continued until a predetermined time elapses. In this case, the predetermined time may be obtained from a map set in advance according to the operating conditions of the engine 11, or may be a fixed value set in advance.
Further, the output D1 and the output D2 may be corrected by first-order lag correction or the like without performing such masking.

In the above embodiment, the engine 1
Although 1 is an in-cylinder injection spark ignition type in-line 4-cylinder gasoline engine, the engine 11 may be an intake pipe lean burn engine having a storage NOx catalyst.

[0050]

As described above in detail in the embodiment, according to the exhaust gas purifying apparatus for an internal combustion engine of the present invention, the amount of NOx deposited on the storage NOx catalyst is estimated with high accuracy, and the NOx release is performed at an appropriate time. The control can be started, and as a result, the operating period of the lean air-fuel ratio operation for storing NOx and N
The operation period of the rich air-fuel ratio operation required for the release of Ox becomes extremely appropriate, and it is possible to prevent deterioration of fuel efficiency due to early start of NOx release control and deterioration of exhaust gas characteristics due to delay of start of NOx release control.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an exhaust gas purification device for an internal combustion engine according to an embodiment of the present invention.

FIG. 2 is a graph showing an output characteristic of an O ₂ sensor with respect to an air-fuel ratio.

FIG. 3 is a flowchart of NOx purge control by the exhaust gas purification device of the present embodiment.

FIG. 4 is a time chart of NOx purge control.

FIG. 5 is a graph showing a target air-fuel ratio with respect to a difference between output values of an O ₂ sensor.

[Explanation of symbols]

11 engine (internal combustion engine) 15 combustion chamber 17 throttle valve 21 exhaust pipe (exhaust passage) 22 exhaust purification catalyst device 24 occlusion-type NOx catalyst 25 three-way catalyst 26 # 1O ₂ sensor (first detecting means) 27 #. 2O ₂ sensor ( 2nd detection means) 28 electronic control unit, ECU (release determination means, NOx release control means)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 41/04 305 F02D 41/04 305A F-term (Reference) 3G091 AA12 AA17 AA24 AA28 AB03 AB06 BA01 BA11 BA14 CB02 DA02 DB07 DC01 EA01 EA17 EA18 EA33 FC02 GB02Y GB03Y GB05W GB06W HA08 HA36 HA37 HA38 HA42 HA47 3G301 HA01 HA04 HA06 HA15 JA02 JA25 JA33 MA01 NA06 NA08 NC02 ND01 NE13 PD04Z PD09Z

Claims (1)

[Claims] 1. An exhaust passage provided in an internal combustion engine for storing NOx in exhaust gas when the exhaust air-fuel ratio is a lean air-fuel ratio and for storing NOx when the exhaust air-fuel ratio is a rich air-fuel ratio or a stoichiometric air-fuel ratio. Storage NOx that releases and reduces
A catalyst, first detection means for detecting a value correlated with the NOx concentration flowing into the storage NOx catalyst, and the storage NOx
Second detection means for detecting a value correlated with the concentration of NOx stored in the catalyst, and occlusion in the storage-type NOx catalyst based on the output of the detection value of the first detection means and the detection value of the second detection means. Emission determination means for determining the timing of releasing the exhausted NOx, and NOx emission control means for changing the exhaust air-fuel ratio to a rich air-fuel ratio or a stoichiometric air-fuel ratio when the release determination means determines the release timing of NOx. An exhaust gas purification device for an internal combustion engine, comprising: