JP2000230420A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration of the exhaust gas characteristics and the fuel economy by accurately executing the NOx discharging control. SOLUTION: A #1O2 sensor 26 for detecting the oxygen concentration in an exhaust gas flowing into the upstream side with respect to an occlusion type NOx catalyst 24 is provided, while a #2O2 sensor 27 for detecting the oxygen concentration in an exhaust gas discharged to the downstream side is provided. Under a specific driving condition, such as where a state with an exhaust gas air-fuel ratio smaller than 14 continues for a predetermined time, a learned value Δd is calculated by subtracting an output value D2 of the #2O2 sensor 27 from an output value D1 of the #1O2 sensor 26. Based on a difference ΔD of the output value D1' corrected by the learned value Δd and the output value D2, the NOx purge control for the occlusion type NOx catalyst 24 by enrichment of the air-fuel ratio is finished.

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 such a rich spike is performed, the operation period and the rich air-fuel ratio degree of the rich air-fuel ratio operation required for the release of NOx are determined based on the duration of the lean air-fuel ratio operation. A predetermined time and a predetermined rich air-fuel ratio are set in advance in accordance with the accumulated NOx amount of the NOx catalyst, and the rich air-fuel ratio operation is performed under the predetermined rich air-fuel ratio for a predetermined time.

However, actually, the actual time required for NOx release varies depending on the operating conditions of the internal combustion engine which affect the NOx purification efficiency of the storage NOx catalyst and the flow rate of CO. Therefore, for example, if the operation period of the rich air-fuel ratio operation is short, the once released NOx may be directly discharged without being reduced due to a shortage of CO (reducing agent). On the other hand, if the operation period of the rich air-fuel ratio operation is too long, CO is continuously discharged in excess, which causes factors such as deterioration of fuel efficiency.

Therefore, a HEGO sensor (O ₂ sensor) is disposed upstream and downstream of the storage NOx catalyst, and the difference between the output voltages of the two sensors is compared with a predetermined value at the time of a rich spike to obtain a storage NOx. The change in the amount of NOx released from the catalyst is estimated to set the end time of purification, and the operating period of the rich air-fuel ratio operation is not set in advance, and this released NO
The rich air-fuel ratio operation is continued at a predetermined rich air-fuel ratio until the x amount is almost completely eliminated, thereby ensuring the storage NOx.
A technique for releasing NOx from a catalyst is disclosed in, for example,
No. 0-121944.

[0007]

The O ₂ sensor detects the amount of oxygen as a value correlated to the concentration of NOx in exhaust gas. The output of the O ₂ sensor includes a detection value based on individual differences. There is a possibility that the detection capability may be reduced due to variations in the data or deterioration over time. In the above-mentioned publication, the storage type N
And so as to exit the controlled release of NOx based on the upstream side and the output value of the O ₂ sensor provided downstream of the Ox catalyst. Therefore, if an error occurs due to a variation in the detection value of the O ₂ sensor or a decrease in detection ability due to deterioration over time, the NOx release control is terminated earlier than actual, and the NOx storage catalyst can be sufficiently regenerated. Otherwise, the characteristics of the exhaust gas may be deteriorated, or the control of the release of NOx may be continued for a longer time than actual, and the characteristics of the exhaust gas and the fuel efficiency may be deteriorated.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine in which NOx emission control is accurately performed to suppress deterioration of exhaust gas characteristics and fuel efficiency. I do.

[0009]

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 detection means and second detection means for detecting a value correlated with the concentration of NOx released from the storage NOx catalyst, and the storage and release of NOx by the storage NOx catalyst are substantially performed. Correction means for setting at least the output error of the detection value of the first detection means to a correction value in a specific operating state, and the exhaust air-fuel ratio is controlled by the NOx release control means to a rich air-fuel ratio or The air-fuel ratio is changed to release the NOx stored in the storage-type NOx catalyst, and the operation of the NOx release control means is stopped by the stop means based on the output difference of each detection means reflecting the correction value by the correction means. I have to.

Therefore, the correction means uses the output error of the detection value of the first detection means as a correction value, and even if the detection value varies due to individual differences of the first detection means or the detection performance decreases due to aging deterioration, The output error is corrected so that the output difference of the detection means becomes highly accurate, and the stop means can stop the NOx release control of the storage NOx catalyst at an appropriate time. Therefore, the NOx stored in the storage NOx catalyst
The operation period of the rich air-fuel ratio operation required for the emission of the exhaust gas becomes extremely appropriate, and it is possible to prevent the deterioration of the exhaust gas characteristics due to the early termination of the NOx emission control and the deterioration of the fuel efficiency due to the long-term NOx emission control.

[0011]

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 learning value setting control by
FIG. 4 is a flowchart of the NOx purge control by the exhaust gas purification apparatus of the present embodiment, FIG. 5 is a time chart of the NOx purge control, and FIG. 6 is a graph showing the target air-fuel ratio with respect to the difference between the output values of the O ₂ sensor.

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 the present 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.
Has a throttle sensor 1 for detecting the throttle opening θth.
8 are provided. Also, the cylinder head 12 has
An exhaust port is formed in a substantially horizontal direction for each cylinder,
Exhaust manifold 1 so that it communicates with each exhaust port
9 are connected to each other.

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 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 Pe based on the throttle opening information θth from the throttle sensor 18 and the engine rotation speed information Ne from the crank angle sensor 20. 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
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.

Hereinafter, the operation of the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment configured as described above will be described.

In the storage type NOx catalyst 24 of the exhaust purification catalyst device 22, nitrate is generated from NOx in the exhaust gas in an atmosphere having an excessively high oxygen concentration such 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 end (stop means) of the period of the purge control (NOx release control means) necessary for releasing the NOx stored in the storage type NOx catalyst 24 is determined. the output value of the # 1O ₂ sensor 26 upstream of the mold NOx catalyst 24, occlusion-type NOx catalyst 2
Downstream of 4 is #. 2O ₂ are determined based on the difference between the output value of the sensor 27. Then, the beginning of the NOx purge control, and # 1O ₂ sensor 26 #. 2O ₂ sensor 27
(Correction means) for correcting an error generated due to a variation in output values due to individual differences and a decrease in detection capability due to aging.

[0026] First, the setting control of the learning value for correcting the error between the # 1O ₂ sensor 26 #. 2O ₂ sensor 27, will be described with reference to the flowchart of FIG.

As shown in FIG. 3, in step S1, for example, it is determined whether the exhaust air-fuel ratio detected by the # 1 O ₂ sensor 26 is smaller than 14. This is to determine whether the exhaust air-fuel ratio is rich atmosphere, a stable air-fuel ratio of exhaust gas to the upper limit sensor output if there is a rich atmosphere, and # 1O ₂ sensor 26 #. 2O ₂ sensor 2
7, the difference between the output value and the output value becomes clear. In step S1, if the exhaust air-fuel ratio is smaller than 14, the process proceeds to step S2, and it is determined whether this state has continued for a predetermined time (for example, 10 seconds), that is, whether the sensor output has been stable for a predetermined time. judge. And this step S2
If it is determined that the state in which the exhaust air-fuel ratio is smaller than 14 has continued for a predetermined time, the process proceeds to step S3 to calculate a learning value.

That is, in step S3, the storage NO
output value D of # 10 ₂ sensor 26 upstream of x catalyst 24
1 obtains the output difference between the two by subtracting the output value D2 of #. 2O ₂ sensor 27 downstream, set this output difference as a learning value [Delta] d. In this case, the # 1O ₂ sensor 26
Of Without output difference of the output values D1 and #. 2O ₂ sensor 27 is intended no error or the like occurring in reduction in detection ability of the output value of the variation or aged deterioration due to individual differences in both sensors 26 and 27, the learning The value Δd becomes 0.

If it is determined in steps S1 and S2 that the state in which the exhaust air-fuel ratio is smaller than 14 has continued for a predetermined period of time, the process proceeds to step S3 to calculate a learning value. It may be changed according to the degree of deterioration of the storage NOx catalyst 24. For example, a predetermined air-fuel ratio and a predetermined time may be obtained from a deterioration degree map corresponding to a traveling distance set in advance according to a learning value.

Next, the NOx of the storage type NOx catalyst 24 in the exhaust gas purifying apparatus of this embodiment using the learning value Δd
The purge control will be described based on the flowchart in FIG. 4 and the time chart in FIG.

As shown in the flowchart of FIG. 4, first, in step S11, the NO of the storage NOx catalyst 24
x Determine whether the occlusion amount exceeds the allowable value. On this occasion,
The NOx occlusion amount is calculated by, for example, the following equation (1). Q (n) = Q (n-1) + q1-q2 (1) where Q (n) is the current integrated amount of NOx, that is, the NOx storage amount, and Q (n-1) is the routine of this routine. N at the previous execution of
This is the integrated amount of Ox. And q1 is a storage type NOx catalyst 2
4 shows the newly absorbed NOx amount, and q2
Indicates the amount of desorbed NOx removed from the storage NOx catalyst 24.

This adsorbed NOx amount q1 means the amount of NOx discharged from the engine 11 and adsorbed by flowing into the storage type NOx catalyst 24. The target average effective pressure Pe and the engine rotational speed information described above are used. Ne is easily obtained based on Ne. The desorbed NOx amount q2 is obtained from the following equations (2) and (2) '. q2 = q2'.Za (2) Za = Qa. (A / F coefficient) (2) 'where q2' is the desorbed NOx amount per unit intake air amount (unit desorbed NOx amount). Where Za is the flow rate of the reducing agent (for example, CO) flowing into the storage NOx catalyst 24, and Qa is the intake air amount. At this time, Za may be obtained in advance from the target average effective pressure Pe and the engine rotation speed information Ne. It should be noted that the NOx storage amount can be estimated to some extent by the operating time at a lean air-fuel ratio.

Then, in step S11, NOx
If the occlusion amount has not reached the allowable value, the routine exits without performing any operation. On the other hand, if it is determined that the NOx storage amount has exceeded the allowable value, the process proceeds to step S12. In this step S12, # 1 is determined based on the learning value Δd described above.
The output value D1 of the O ₂ sensor 26 is corrected by the following equation (3). D1 error between the output value D2 of the #. 2O ₂ sensor 27 '= D1-Δd ...... ( 3) the output value D1 of # 1O ₂ sensor 26 by the correction' is corrected,
Proceeding to step S13, NOx purging is started.

In step S13, 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, a value of 10). As a result, as shown in FIG. 5A, the NOx stored in the storage NOx catalyst 24 is rapidly released and begins to be reduced and removed. Next, in step S14, # 1O ₂ corrected output after D1 of the 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, the output of the # 1O ₂ sensor 26 D1 'is returned to step S12 if not exceed the predetermined value Ds target A
/ F is kept at a predetermined initial value, that is, an initial rich air-fuel ratio (for example, a value of 10). 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.

[0035] In this way, then in step S15, # 1O ₂ output of the sensor 26 D1 'and #. 2O ₂ but of performing a calculation using the output D2 of the sensors 27,
# 1O ₂ elements of a response delay in the initial stage of NOx purge of the sensor 26 and #. 2O ₂ sensor 27 are eliminated (masking), it is can be accurately calculated. On the other hand, if in step S14, # 1O output D1 of the _sensor 26 'is equal to or greater than a predetermined value Ds, the elements of a response delay of # 1O ₂ sensor 26 and #. 2O ₂ sensor 27 is considered to have been eliminated, step Move to S15.

[0036] In step S15, the following equation (4) and the output D1 'of # 1O ₂ sensor 26 #. 2O ₂ sensor 2
7 is calculated from the output D2. ΔD = D1'-D2 ...... (4 ) that is, in FIG. 5 (b), the output D of the # 1O ₂ sensor 26
1 'with one-dot chain line, #. 2O ₂ but the output D2 of the sensor 27 is shown in solid lines, these output D1' calculates the difference between the outputs D2 and the amount of oxygen in the upstream side of the exhaust of the occlusion-type NOx catalyst 24 And the amount of oxygen in the exhaust gas on the downstream side, that is, the difference in the air-fuel ratio in the exhaust gas.

[0037] Thus, NOx stored in the occlusion-type NOx catalyst 24 is released by the NOx purging 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. For this reason, 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 NO
In x 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 O ₂ has a large amount of generated, thus occlusion-type NOx catalyst 24 output D2 of #. 2O ₂ sensor 22 located downstream of the are held at small. That is, the output D1 '
Is greater than or equal to the predetermined value Ds, the difference ΔD, that is, the amount of NOx released, is relatively large, as shown in FIGS.

Then, in the next step S16, the optimum target A / F is determined by calculation based on the difference ΔD thus obtained. In practice, for example, as shown in FIG.
A map indicating the relationship between the difference ΔD and the target A / F is set in advance by experiments or the like, and the target A / F is obtained from this map according to the difference ΔD. According to the figure, when the output D1 'becomes equal to or more than the predetermined value Ds, the difference Δ
Since D is relatively large, the target A / F is set to the same air-fuel ratio (for example, a value of 10) as the initial rich air-fuel ratio.

In the next step S17, it is determined whether or not the difference ΔD has become equal to or smaller than a predetermined value ΔD0 (minimum value or value 0). In other words, O ₂ generated by the reduction becomes extremely small, and the stored NOx amount is almost completely reduced to NO 2
x Determine if you no longer need to purge. At this point, since the difference ΔD is relatively large, step S1
5 and step S16 are repeated and continued.

When the release and reduction of the NOx stored in the storage NOx catalyst 24 progresses, O generated by the reduction is reduced.
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 ΔD
When it is reduced to 1, the target A /
F is an air-fuel ratio with a slightly smaller degree of enrichment (for example, a value of 1
2) (see FIG. 5C). In other words, when NOx emission and reduction have progressed to some extent, CO as a reducing agent is no longer necessary, and in this case, the target A / F is shifted to a lower side of the degree of enrichment by reducing the fuel supply amount or the like. change. As a result, surplus CO is not discharged, and deterioration of fuel efficiency and the like are suitably prevented.

[0041] By the way, the output of # 2O ₂ sensor 27 D2
Actually, as shown in FIG. 5 (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 it takes time to release (FIG. 5).
(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 S17, if the difference ΔD is smaller than a predetermined value ΔD0 (minimum value or value 0), it can be determined that the NOx stored in the storage type NOx catalyst 24 has been almost completely removed. Is no longer a reducing agent, CO
Is unnecessary, and this NOx purge control is ended. As a result, the NOx stored in the storage NOx catalyst 24 is reliably removed without excess or deficiency.

As shown in FIG. 5, 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.

Also, in this case, as shown in FIG. 5C, after the NOx purge is completed, the target A / F is again set to the lean air-fuel ratio. 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.

[0046] 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 particular operating conditions, for example, when the exhaust air-fuel ratio is 14 less than state continues for a predetermined time, # 1O ₂ the output value D2 of #. 2O ₂ sensor 27 from the output value D1 of the sensor 26 is subtracted obtains the learned value Δd, the output value D which is corrected by this learned value Δd
Based on the difference ΔD between 1 ′ and the output value D2, the NOx purge control of the storage NOx catalyst 24 by enriching the exhaust air-fuel ratio is terminated.

Therefore, the # 1O ₂ sensor 26 and the # 2O ₂
Variations in output values due to individual differences of the sensor 27 and a decrease in detection ability due to deterioration over time can be reliably corrected, high accuracy can be achieved, NOx purge control can be terminated at an appropriate time, and NOx purge control can be terminated early. This can prevent the exhaust gas characteristics from deteriorating and the fuel efficiency from deteriorating due to long-term NOx purge control.

In the above embodiment, # 1O ₂
Obtains the learning value Δd from the output difference of the output value D2 of the output value D1 of the sensor 26 #. 2O ₂ sensor 27, # 1O ₂ sensor 2
The output D1 of 6 by subtracting the learned value Δd has been sought # output correction value D1 of 1O ₂ sensor 26 ', #. 2O ₂ sensor 2
7 may be corrected. Further, at this rich air-fuel ratio, hydrogen H ₂ increases in the exhaust gas, and due to the structure of the O ₂ sensors 26 and 27, if H ₂ coexists in the exhaust gas, an error occurs in the detected value of O _2. . The amount of generated H ₂ increases or decreases in accordance with the operation state of the engine 11, but the amount is about 80% in proportion to the increase or decrease of CO. Therefore, in this case, since the generation amount of H ₂ it can be estimated according to the operating state of the engine 11, depending on the generation amount of the estimated H ₂ #
The output value D1 of the 10 ₂ sensor 26 may be corrected.

As the deterioration of the O ₂ sensors 26 and 27 progresses, the response at the time of rising of the output also decreases. Therefore, by utilizing the fact that the output and the response when the O ₂ sensors 26 and 27 are deteriorated have a correlation, the sensor output error may be obtained from the response time and corrected. In this case, the sensor output error is, for example, the target A /
By changing the F obtains the response time of the O ₂ sensor may be obtained from the difference between the two on the basis of a map which sets the response time of the O ₂ sensor is not deteriorated in advance.

By making the storage NOx catalyst 24 and the three-way catalyst 25 contain cerium Ce, the storage NO
x It is common practice to increase the efficiency of regeneration of the catalyst 24 and to improve the durability. This cerium
The engine 1 has a function of storing oxygen by adsorbing O ₂ and forming ceria (cerium oxide CeO ₂ ).
In lean operation 1, O ₂ in the exhaust gas is adsorbed and stored, and in rich operation, O ₂ is released to oxidize and remove H ₂ and CO in the exhaust gas. In this case, since O ₂ is released during the rich operation of the engine 11, the # 2O ₂ sensor 27 located downstream of the storage NOx catalyst 24 is used.
Is to detect the oxygen concentration including O ₂ released from ceria by the storage type NOx catalyst 24,
₂ , an error occurs in the output value D2.

Therefore, in the exhaust gas purifying apparatus of this embodiment,
For example, the cerium Ce content of the storage NOx catalyst 24 is reduced or eliminated to such an extent that the exhaust gas purifying function is not deteriorated, and O ₂ sensors 26 and 27 are disposed immediately upstream and downstream thereof, respectively.

Further, the amount of O ₂ released from cerium Ce contained in the storage NOx catalyst 24 is measured or estimated, and the # 2O ₂ sensor 27 downstream of the storage NOx catalyst 24 is measured.
Is corrected. For example, the actual O ₂ emission amount map obtained in advance according to the operating state of the engine 11
₂ emissions estimates, obtain a correction coefficient from a previously determined correction map according to the O ₂ emission, #. 2O ₂ output value D of the correction coefficient obtained on the output value D2 corrected by multiplying the sensor 27
2 'may be used.

Further, in the above-described embodiment, the storage type NOx catalyst 24 capable of performing both 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. In this case, #. 2O ₂ sensor 27 is preferably provided downstream of the three-way catalyst 25.

Further, without using the three-way catalyst 25, the storage type NOx catalyst 24 having both the function of releasing NOx and the function of reducing NOx.
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.

Further, in this embodiment, is performed a termination judgment of the NOx release control using two O ₂ sensors 26 and 27, capable of detecting NOx sensor the NOx concentration in place of the O ₂ sensors 26 and 27 May be used. In this case, it is disposed upstream and downstream of the storage NOx catalyst 24, or disposed upstream or downstream.

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.

[0057]

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 output error of the detection value of the first detection means in a specific operating state is used as a correction value. , The first detecting means reflecting the correction value and the second detecting means.
Since the operation of the NOx emission control means is stopped based on the output difference from the detection means, variations in the detection value due to individual differences of the first detection means and a decrease in detection ability due to deterioration with time are surely corrected, and The output difference of the detection means is made highly accurate, the operation period of the rich air-fuel ratio operation required for the release of the NOx stored in the storage NOx catalyst becomes extremely appropriate, and the NOx release control is executed with high accuracy. Deterioration of gas characteristics and fuel efficiency can be suppressed.

[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 setting control of a learning value by the exhaust gas purification apparatus of the present embodiment.

FIG. 4 is a flowchart of NOx purge control by the exhaust gas purification device of the present embodiment.

FIG. 5 is a time chart of NOx purge control.

FIG. 6 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 ( (Second detection means) 28 electronic control unit, ECU (correction means,
NOx release control means, stop means)

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 41/04 305 F02D 41/04 305A (72) Inventor Kojiro Okada 5-33-8 Shiba 5-chome, Minato-ku, Tokyo No. Mitsubishi Motors Corporation F-term (reference) 3G091 AA12 AA17 AA23 AA24 AB03 AB06 BA14 BA15 BA19 BA27 BA33 BA34 CB02 CB03 CB05 CB07 DB06 DB07 DB10 DC02 EA01 EA07 EA17 EA30 EA31 EA33 EA34 FB10 GB04 GB03 HA36 HA38 HA42 HA47 3G301 HA01 HA04 HA06 HA15 JA15 JA16 JA25 JA26 JB01 JB09 LA01 LB03 LB04 MA01 MA11 MA18 NA08 ND21 NE13 NE14 NE15 PA11A PA11B PD02A PD02B PD11A PD11B PE01A PE01B

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 released from the catalyst; and at least the second detection means in a specific operating state in which the storage and release of NOx by the storage-type NOx catalyst are not substantially performed. (1) correction means for setting an output error of a detection value of the detection means to a correction value; and NOx release control for changing an exhaust air-fuel ratio to a rich air-fuel ratio or a stoichiometric air-fuel ratio when releasing NOx stored in the storage-type NOx catalyst. Means, and stop means for stopping the operation of the NOx release control means based on an output difference between the first detection means and the second detection means in which the correction value reflected by the correction means is reflected. An exhaust gas purification device for an internal combustion engine.