JP4214923B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  A NOx storage reduction catalyst (hereinafter referred to as NOx catalyst) is disposed in the exhaust passage of the internal combustion engine, and nitrogen oxide (NOx) in the exhaust gas is stored in the NOx catalyst in an oxidizing atmosphere to form a reducing atmosphere. In this case, a technique is known in which NOx stored in the NOx catalyst is reduced to purify NOx in the exhaust gas.

This NOx catalyst is known to have a NOx occlusion capability that decreases with thermal deterioration and deterioration due to aging, and a technology for detecting this deterioration based on the outputs of oxygen sensors attached before and after the NOx catalyst is known. (For example, refer to Patent Document 1).
JP-A-11-93742 Japanese Patent No. 2692380 JP 2002-309928 A

  By the way, when the fuel is supplied to the NOx catalyst and NOx is reduced, it is required to accurately detect the air-fuel ratio in the exhaust gas by the air-fuel ratio sensor. However, if the cracking of the fuel contained in the exhaust gas is not sufficient, some fuel cannot pass through the diffusion resistance layer of the air-fuel ratio sensor. Therefore, the fuel is measured less than the actual amount, and the air-fuel ratio detected by the air-fuel ratio sensor is shifted to the lean side from the actual value. Such an air-fuel ratio shift is hereinafter referred to as “lean shift”. This lean shift makes it difficult to correct the fuel addition amount by feedback control.

  In addition, when the deterioration determination of the NOx catalyst is performed according to the conventional technique, it is required to accurately detect the oxygen concentration and the air-fuel ratio in the exhaust gas using an oxygen sensor or an air-fuel ratio sensor. However, there are cases where it is difficult to accurately determine the deterioration of the NOx catalyst for the above reasons.

  The present invention has been made in view of the above-described problems, and in an exhaust gas purification apparatus for an internal combustion engine, obtains an accurate value of the air-fuel ratio of exhaust flowing into the NOx catalyst when fuel is supplied to the NOx catalyst. Accordingly, an object of the present invention is to provide a technique capable of improving the accuracy of deterioration determination of the NOx catalyst.

In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention employs the following means. That is,
A first NOx storage catalyst provided in an exhaust passage of the internal combustion engine;
First air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the first NOx storage catalyst;
Fuel addition means for adding fuel to the first NOx storage catalyst from the upstream side of the first NOx storage catalyst;
NOx reduction means for reducing NOx stored in the first NOx storage catalyst by adding fuel by the fuel addition means;
Output stability determination means for determining whether or not the detected value of the first air-fuel ratio detection means when the fuel is added by the fuel addition means is richer than stoichiometric and stable within a predetermined change range. When,
Based on the air-fuel ratio detected by the first air-fuel ratio detecting means when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric. An inflow exhaust air / fuel ratio estimating means for estimating an air / fuel ratio of exhaust flowing into the first NOx storage catalyst during a period in which fuel is added by the fuel addition means;
When the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric, the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio detecting means and the Deterioration determining means for determining deterioration of the first NOx storage catalyst based on the air-fuel ratio estimated by the inflowing exhaust air-fuel ratio estimating means;
It is characterized by comprising.

  The greatest feature of the present invention is that the first NOx storage catalyst is determined from the air-fuel ratio when the air-fuel ratio of the exhaust gas detected downstream of the first NOx storage catalyst is a rich air-fuel ratio and is stable within a predetermined variation range. The air-fuel ratio of the exhaust gas flowing into the exhaust gas is estimated, and the deterioration determination of the first NOx storage catalyst is performed based on the estimated value.

  Here, when the fuel addition is performed by the fuel addition means, if the air-fuel ratio detected by the first air-fuel ratio detection means shifts to the rich side, the NOx stored in the first NOx storage catalyst. And the release of oxygen is complete. At this time, the air-fuel ratio detected by the first air-fuel ratio detection means is not affected by oxygen or NOx stored in the first NOx storage catalyst. Furthermore, since the fuel is sufficiently cracked by the first NOx storage catalyst, it is possible to suppress the occurrence of lean deviation of the detection value of the first air-fuel ratio detection means.

  Therefore, when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable, the air-fuel ratio detected by the first air-fuel ratio detecting means is actually supplied to the first NOx storage catalyst. This indicates a value almost equal to the air-fuel ratio of the inflowing exhaust gas. That is, when the output stability determination means determines that the detection value of the first air-fuel ratio detection means is stable, the exhaust air-fuel ratio obtained from the first air-fuel ratio detection means flows into the first NOx storage catalyst. The air-fuel ratio of the exhaust gas being discharged can be set.

  The inflow exhaust air-fuel ratio estimating means is configured to detect the first air-fuel ratio detecting means based on the air-fuel ratio of the exhaust gas when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable. The air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst is estimated before the detected value is stabilized and the fuel addition means is adding fuel. This estimation is possible, for example, by experimentally determining the change in the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst while the fuel is being added from the fuel addition means. That is, the air-fuel ratio obtained by the first air-fuel ratio detection means when the output stability judgment means determines that the detection value of the first air-fuel ratio detection means is stable, and the exhaust air-fuel ratio obtained experimentally By correcting the experimentally obtained air-fuel ratio of the exhaust gas so that the air-fuel ratio that finally reaches is equalized, before the detected value of the first air-fuel ratio detecting means is stabilized during fuel addition In this period, the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be obtained.

By determining the deterioration of the first NOx storage catalyst using the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst thus obtained, the accuracy of the deterioration determination can be improved.
“Stable within a predetermined change width” can be exemplified by the fact that the change width of the air-fuel ratio is smaller than a predetermined value.

In the present invention, a second catalyst provided in an exhaust passage downstream of the first air-fuel ratio detection means and having an oxidation ability;
Second air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the second catalyst;
Further comprising
The output stabilizing means is detected by the first air-fuel ratio detecting means when the air-fuel ratio detected by the second air-fuel ratio detecting means changes richer than stoichiometric when fuel is added by the fuel adding means. It can be determined that the air-fuel ratio of the exhaust gas is stable at an air-fuel ratio richer than stoichiometric.

  Here, the second catalyst may have a NOx storage capacity. When the air-fuel ratio detected by the second air-fuel ratio detection means becomes a rich air-fuel ratio, it indicates that the rich air-fuel ratio exhaust gas has flowed into the second catalyst. In this case, the air-fuel ratio of the exhaust gas flowing out from the first NOx storage catalyst is a rich air-fuel ratio. This indicates that the reduction of NOx stored in the first NOx storage catalyst has been completed, and the air-fuel ratio of the exhaust detected by the first air-fuel ratio detection means is stabilized at a constant rich air-fuel ratio. It also shows that. As a result, the output stabilization means can determine that the air-fuel ratio of the exhaust detected by the first air-fuel ratio detection means is stable.

  In the present invention, there is further provided a third air-fuel ratio detecting means for detecting an air-fuel ratio of the exhaust upstream of the first NOx storage catalyst, and the inflowing exhaust air-fuel ratio estimating means is detected by the third air-fuel ratio detecting means. The air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be estimated from the air-fuel ratio and the air-fuel ratio detected by the first air-fuel ratio detection means.

  Since the third air-fuel ratio detection means is provided upstream of the first NOx storage catalyst, the third air-fuel ratio detection means detects the air-fuel ratio of the exhaust when the fuel is not sufficiently cracked. It is not affected by the release of NOx and oxygen stored in the water.

  Therefore, it is difficult to accurately obtain the air-fuel ratio of the exhaust gas flowing into the first storage NOx catalyst only with the air-fuel ratio obtained by the third air-fuel ratio detection means, but it is possible to detect a change in the air-fuel ratio. . Further, by correcting the air-fuel ratio of the exhaust gas obtained by the third air-fuel ratio detection means based on the air-fuel ratio detected by the first air-fuel ratio detection means, the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be more accurately determined. Obtainable. Therefore, it is possible to more accurately determine the deterioration of the first NOx storage catalyst.

  In the present invention, when the inflow exhaust air-fuel ratio estimating means determines that the air-fuel ratio detected by the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric by the output stability determining means. During the addition of fuel by the fuel addition means, the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio detection means is equal to the air-fuel ratio detected by the third air-fuel ratio detection means at this time. By correcting the air-fuel ratio detected by the third air-fuel ratio detection means, the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst can be estimated.

As described above, when it is determined by the output stability determination means that the detection value of the first air-fuel ratio detection means is stable, the release of NOx and oxygen stored in the first NOx storage catalyst is completed, The air-fuel ratio detected by the first air-fuel ratio detection means and the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst are substantially equal. Therefore, the third air-fuel ratio detection means is set so that the detection value of the third air-fuel ratio detection means when the detection value of the first air-fuel ratio detection means is stable becomes equal to the detection value of the first air-fuel ratio detection means. Correct the detection value. At this time, a coefficient for correcting the third air-fuel ratio detecting means is obtained. By applying this coefficient to all detection values of the third air-fuel ratio detection means at the time of fuel addition, the detection value of the third air-fuel ratio detection means before the detection value of the first air-fuel ratio detection means stabilizes is corrected. Is possible. That is, by storing the detected value of the third air-fuel ratio detecting means and correcting this stored value, it becomes possible to obtain the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst at the time of fuel addition.

  The present invention further includes fuel addition ending means for ending fuel addition by the fuel addition means when the air-fuel ratio of the exhaust detected by the second air-fuel ratio detection means becomes richer than a predetermined air-fuel ratio. be able to.

  Here, when the detection value of the second air-fuel ratio detection means becomes the rich air-fuel ratio, the release of NOx stored in the first NOx storage catalyst is completed. At this time, the estimation of the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst by the inflowing exhaust air-fuel ratio estimation means is also completed. Therefore, it is possible to determine the deterioration of the first NOx storage catalyst even when the fuel addition is terminated. Moreover, by setting it as such an end time, it can suppress that excess fuel passes a 1st and 2nd catalyst, and flows out downstream.

  In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, by obtaining an accurate value of the air-fuel ratio of the exhaust gas flowing into the NOx catalyst when the fuel is supplied to the NOx catalyst, the accuracy of determining the deterioration of the NOx catalyst is improved. Can do.

  Hereinafter, specific embodiments of an exhaust emission control device for an internal combustion engine according to the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 to which an exhaust gas purification apparatus for an internal combustion engine according to this embodiment is applied and an exhaust system thereof.
The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine.

An exhaust passage 2 leading to the combustion chamber is connected to the internal combustion engine 1. This exhaust passage 2 communicates with the atmosphere downstream.
In the middle of the exhaust passage 2, an oxidation catalyst 3, a first NOx catalyst 4, and a second NOx catalyst 5 are provided in order from the internal combustion engine 1 side.

  The first NOx catalyst 4 and the second NOx catalyst 5 occluded NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and occluded when the oxygen concentration of the inflowing exhaust gas decreased and a reducing agent was present. This is a NOx storage reduction catalyst having a function of reducing NOx. Note that the NOx storage reduction type NOx catalyst may be supported on the first NOx catalyst 4 and / or the particulate filter. When only the deterioration determination of the first NOx catalyst is performed, the second NOx catalyst 5 may be a catalyst having a simple oxidation ability. Furthermore, the oxidation catalyst 3 may be another type of catalyst such as a three-way catalyst or a NOx catalyst as long as it has an oxidation ability.

  A third air / fuel ratio sensor 6 for detecting the air / fuel ratio of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 downstream of the oxidation catalyst 3 and upstream of the first NOx catalyst 4. A first air-fuel ratio sensor 7 for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 downstream of the first NOx catalyst 4 and upstream of the second NOx catalyst 5. . Further, a second air-fuel ratio sensor 8 for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 downstream of the second NOx catalyst 5.

  By the way, when the internal combustion engine 1 is operated in lean combustion, the NOx occluded in the first NOx catalyst 4 and the second NOx catalyst 5 is reduced before the NOx occlusion capacity of the first NOx catalyst 4 and the second NOx catalyst 5 is saturated. There is a need.

  Therefore, in this embodiment, a fuel addition valve 9 for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage 2 upstream from the oxidation catalyst 3 is provided. Here, the fuel addition valve 9 is opened by a signal from the ECU 10 described later to inject fuel. The fuel injected into the exhaust passage 2 from the fuel addition valve 9 makes the air-fuel ratio of the exhaust flowing from the upstream of the exhaust passage 2 rich, and the NOx stored in the first NOx catalyst 4 and the second NOx catalyst 5. Reduce. During NOx reduction, so-called rich spike control is performed in which the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 and the second NOx catalyst 5 is made rich in a spike (short time) in a relatively short cycle.

  The internal combustion engine 1 configured as described above is provided with an ECU 10 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 10 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver.

The above-described sensor or the like is connected to the ECU 10 via an electrical wiring, and an output signal from the sensor or the like is input thereto.
On the other hand, the fuel addition valve 9 is connected to the ECU 10 via an electric wiring, and can be controlled by the ECU 10.

  By the way, the first NOx catalyst 4 and the second NOx catalyst 5 may be deteriorated by aging or heat. This deterioration appears significantly in the NOx storage capacity. Then, the NOx occlusion capacity decreases, and a part of the NOx in the exhaust gas may flow out downstream of the first NOx catalyst 4 and the second NOx catalyst 5. In contrast, for example, a decrease in the NOx storage capacity of the first NOx catalyst 4 can be detected using the air-fuel ratio sensors 6 and 7 before and after the first NOx catalyst 4. Thereby, it becomes possible to add fuel according to the degree of deterioration. It is also possible to prompt the driver or the like to replace the first NOx catalyst 4.

  Here, when NOx is occluded in the first NOx catalyst 4, if rich air-fuel ratio exhaust gas is supplied to the first NOx catalyst 4, NOx and oxygen occluded in the first NOx catalyst 4 are released. The rich air-fuel ratio exhaust gas flows into the first NOx catalyst 4 due to the rich spike, and while the NOx and oxygen are released from the first NOx catalyst 4, the air-fuel ratio downstream of the first NOx catalyst 4, that is, the first air-fuel ratio sensor 7. It is known that the air-fuel ratio detected by the above is stoichiometric. Then, after the release of NOx and oxygen from the first NOx catalyst 4 is completed, the air-fuel ratio detected by the first air-fuel ratio sensor 7 shifts to the rich air-fuel ratio. In this way, the time from when the first air-fuel ratio sensor 7 detects the stoichiometric gas and shifts to the rich air-fuel ratio becomes longer as the amount of NOx and oxygen stored in the first NOx catalyst 4 increases.

  As the degree of deterioration of the first NOx catalyst 4 increases, the amount of NOx and oxygen that can be stored by the first NOx catalyst 4 decreases. Therefore, as the degree of deterioration of the first NOx catalyst 4 increases, the amount of fuel necessary for reducing NOx stored in the first NOx catalyst 4 and releasing oxygen decreases. Further, the time until the shift to the rich air-fuel ratio after the stoichiometric detection is detected by the first air-fuel ratio sensor 7 during the rich spike, that is, the stoichiometric duration time is shortened. From the above, based on the amount of fuel required to reduce or release NOx and oxygen stored in the first NOx catalyst 4, or the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 and the duration of the stoichiometry, the first NOx. The degree of deterioration of the catalyst 4 can be determined.

Incidentally, in order to determine the degree of deterioration of the first NOx catalyst 4, it is necessary to accurately know the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4. However, at the time of rich spike, cracking of the fuel added to the exhaust gas is not sufficiently performed, and the third air-fuel ratio sensor 6 may cause a lean shift. This lean shift is improved to some extent by providing the oxidation catalyst 3,
It is difficult to eliminate the lean deviation of the third air-fuel ratio sensor 6 at all. Therefore, it becomes difficult to detect the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 during the rich spike control only by the third air-fuel ratio sensor 6.

  In that respect, according to the present embodiment, the exhaust gas detected by the second air-fuel ratio sensor 8 when the output signal of the second air-fuel ratio sensor 8 becomes stable within a predetermined change width after the rich air-fuel ratio becomes stable. The detected value of the third air-fuel ratio sensor 8 can be corrected based on the air-fuel ratio.

  Here, FIG. 2 shows the time transition of the air-fuel ratio obtained from the third air-fuel ratio sensor 6, the first air-fuel ratio sensor 7, and the second air-fuel ratio sensor 8 when the fuel addition amount correction control according to this embodiment is performed. It is the time chart figure which showed. The solid line indicates the output value of the third air-fuel ratio sensor 6, the broken line indicates the output value of the first air-fuel ratio sensor 7, the alternate long and short dash line indicates the output value of the second air-fuel ratio sensor 8, and the dotted line indicates the target air-fuel ratio.

  The air-fuel ratio detected by the first air-fuel ratio sensor 7 decreases with fuel addition. When stoichiometric, NOx and oxygen are released from the first NOx catalyst 4. In other words, the detected value of the first air-fuel ratio sensor 7 becomes stoichiometric until the release of NOx and oxygen from the first NOx catalyst 4 is completed. Thereafter, when the release of NOx and oxygen from the first NOx catalyst 4 is completed, the air-fuel ratio detected by the first air-fuel ratio sensor 7 decreases to a rich air-fuel ratio and changes at a substantially constant rich air-fuel ratio. The detected value by the first air-fuel ratio sensor 7 while changing at this constant rich air-fuel ratio becomes a value substantially equal to the air-fuel ratio of the actual exhaust gas flowing into the first NOx catalyst 4.

  Next, the air-fuel ratio detected by the second air-fuel ratio sensor 8 becomes stoichiometric until the release of NOx and oxygen from the second NOx catalyst 5 is completed. When the release of NOx and oxygen from the second NOx catalyst 5 is completed, the air / fuel ratio becomes rich. At that time, the addition of fuel is stopped, so the air / fuel ratio immediately increases.

Further, the air-fuel ratio detected by the third air-fuel ratio sensor 6 has a lean shift due to fuel addition.
On the other hand, FIG. 3 is a time chart showing the time transition of the air-fuel ratio when the fuel addition amount correction control according to this embodiment is performed and when the output of the third air-fuel ratio sensor is corrected.

  Here, in the period indicated by A in FIG. 3 (hereinafter referred to as period A), the output signal from the first air-fuel ratio sensor 7 shows a substantially constant value. In this case, it can be said that the detected value of the first air-fuel ratio sensor 7 is a rich air-fuel ratio and is stable within a predetermined change width. In the present embodiment, in the period A, the detection value of the third air-fuel ratio sensor 6 is corrected so that the detection value of the first air-fuel ratio sensor 7 and the detection value of the third air-fuel ratio sensor 6 become equal. . The correction at this time is not limited to the period A, and the detection value of the third air-fuel ratio sensor 6 detected during the rich spike before that is also corrected.

  That is, in FIG. 3, the lean air-fuel ratio before starting fuel addition (hereinafter referred to as the base air-fuel ratio) so that the air-fuel ratio before “third air-fuel ratio sensor correction” becomes equal to the air-fuel ratio of “first air-fuel ratio sensor” in period A. The air-fuel ratio “before the third air-fuel ratio sensor correction” is moved at the same ratio in the direction of becoming richer on the basis of the air-fuel ratio), and the air-fuel ratio “after the third air-fuel ratio sensor correction” is obtained.

  In this case, first, a difference X between the base air-fuel ratio and the air-fuel ratio obtained from the first air-fuel ratio sensor 7 in the period A is obtained. Next, a difference Y between the base air-fuel ratio and the air-fuel ratio obtained from the third air-fuel ratio sensor 6 in the period A is obtained. Then, a value obtained by dividing the difference X by the difference Y is obtained as a correction coefficient Z. This correction coefficient Z is applied to other than the period A. That is, a new difference T is obtained by multiplying the difference S between the base air-fuel ratio and the air-fuel ratio obtained from the third air-fuel ratio sensor 6 during fuel addition by a correction coefficient Z, and a value obtained by subtracting the difference T from the base air-fuel ratio is obtained. It can be obtained as a corrected air-fuel ratio.

As described above, the output value of the third air-fuel ratio sensor 6 can be corrected by applying the correction coefficient Z calculated in the period A to all the periods at the time of fuel addition.
In this case, the detection value of the third air-fuel ratio sensor 6 is stored in the ECU 10, and after the correction coefficient Z is calculated in the period A, the stored detection value of the third air-fuel ratio sensor 6 is corrected. Then, the amount of fuel required for the release or reduction of NOx and oxygen stored in the first NOx catalyst 4 is calculated, and the deterioration of the first NOx catalyst 4 is determined based on the calculated fuel amount.

Here, FIG. 4 is a diagram showing the amount of fuel required for releasing or reducing NOx and oxygen stored in the first NOx catalyst 4.
The amount of fuel added to release NOx and oxygen stored in the first NOx catalyst 4 is obtained from the area hatched as “first NOx catalyst component”, and the hatched area as “second NOx catalyst component”. Thus, the fuel addition amount required for releasing the NOx and oxygen stored in the second NOx catalyst 5 is obtained.

  It can be seen that the greater the amount of fuel added required to release the NOx and oxygen stored in the first NOx catalyst 4, the more NOx and oxygen were stored in the first NOx catalyst 4. Based on this, the degree of deterioration of the first NOx catalyst 4 can be determined.

  In this embodiment, the third air-fuel ratio sensor 6 is obtained by the air-fuel ratio obtained by the first air-fuel ratio sensor detected immediately before the air-fuel ratio obtained by the second air-fuel ratio sensor 8 becomes rich from stoichiometric. The detected value may be corrected.

  Here, the detected value of the second air-fuel ratio sensor 8 shifting from stoichiometric to rich means that the release of NOx and oxygen stored in the first NOx catalyst 4 and the second NOx catalyst 5 has been completed. . That is, immediately before this, the release of NOx and oxygen stored in the first NOx catalyst 4 has been completed, and the air-fuel ratio of the exhaust detected by the first air-fuel ratio sensor 7 and the exhaust flowing into the first NOx catalyst 4 The air-fuel ratio is substantially equal. Further, it can be said that the detected value of the first air-fuel ratio sensor 7 is a rich air-fuel ratio and is stable within a predetermined change width. Further, the detection value of the second air-fuel ratio sensor 8 is not easily affected by temperature changes or the like, and it is accurate that the detection value of the first air-fuel ratio sensor 7 is a rich air-fuel ratio and is stable within a predetermined change range. Can be detected well.

  Therefore, the detection value of the third air-fuel ratio sensor 6 can be corrected based on the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio sensor 7 at this time. This correction can be performed in the same manner as the correction by the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio sensor detected during the period A.

  Further, in this embodiment, the detected value of the first air-fuel ratio sensor 7 is the rich air-fuel ratio after a predetermined time has elapsed after the air-fuel ratio of the exhaust gas detected by the first air-fuel ratio sensor 7 has shifted from rich to rich. Therefore, the detected value of the third air-fuel ratio sensor 6 may be corrected on the basis of the air-fuel ratio of the exhaust detected by the first air-fuel ratio sensor 7 at this time.

Further, the detection value of the first air-fuel ratio sensor 7 for correcting the detection value of the third air-fuel ratio sensor 8 may be a value at an arbitrary time in the period A, and is an average value of the period A. Also good.
In this embodiment, the detection value of the third air-fuel ratio sensor 6 is corrected by the detection value of the first air-fuel ratio sensor 7, but instead, fuel is added from the fuel addition valve 9. In the meantime, the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 may be experimentally obtained, and the experimentally obtained value may be corrected.

  That is, the exhaust gas experimentally determined so that the air-fuel ratio obtained by the first air-fuel ratio sensor 7 in the period A is equal to the air-fuel ratio finally reached by the experimentally determined air-fuel ratio. By correcting the air-fuel ratio, it is possible to obtain the air-fuel ratio of the exhaust gas flowing into the first NOx catalyst 4 during the fuel addition and before the period A.

  As explained above, according to the present embodiment, the detection value of the third air-fuel ratio sensor 6 is corrected by the detection value of the period when the detection value from the first air-fuel ratio sensor 7 is rich and stable. can do. Thereby, a more accurate deterioration determination of the first NOx catalyst 4 can be performed.

In this embodiment, the end time of fuel addition is determined by the output signal of the second air-fuel ratio sensor 8.
Here, the second air-fuel ratio sensor 8 can most accurately detect the air-fuel ratio of the exhaust gas flowing through the exhaust system. This is because the fuel is sufficiently cracked by the oxidation catalyst 3, the first NOx catalyst 4, and the second NOx catalyst 5. However, the second air-fuel ratio sensor 8 is located away from the fuel addition valve 9, and it takes time for the fuel added from the fuel addition valve 9 to reach the second air-fuel ratio sensor 8. Therefore, if the air-fuel ratio control during rich spike is performed by the output signal of the second air-fuel ratio sensor 8, the air-fuel ratio may vary.

In that respect, fluctuations in the air-fuel ratio can be suppressed by using the output signal of the second air-fuel ratio sensor 8 only for determining the end timing of fuel addition.
Here, in this embodiment, when the output signal of the second air-fuel ratio sensor 8 shifts from stoichiometric to rich, the fuel addition from the fuel addition valve 9 is terminated. When the output signal of the second air-fuel ratio sensor 8 shifts from stoichiometric to rich, it means that the release of NOx and oxygen stored in the first NOx catalyst 4 and the second NOx catalyst 5 has been completed, and catalyst deterioration determination It is possible to calculate the amount of fuel added necessary for the operation. Therefore, even if fuel addition is terminated at this time, the catalyst deterioration determination described in the first embodiment can be performed, and excessive fuel addition can be suppressed.

It is a figure which shows schematic structure of the internal combustion engine which applies the exhaust gas purification apparatus of the internal combustion engine which concerns on an Example, and its exhaust system. FIG. 6 is a time chart showing the time transition of the air-fuel ratio obtained from the third air-fuel ratio sensor, the first air-fuel ratio sensor, and the second air-fuel ratio sensor when the fuel addition amount correction control according to the embodiment is performed. FIG. 6 is a time chart showing the time transition of the air-fuel ratio when the fuel addition amount correction control according to the embodiment is performed and the output of the third air-fuel ratio sensor is corrected. It is the figure which showed the amount of fuel required for discharge | release or reduction | restoration of NOx and oxygen which were occluded by the 1st NOx catalyst.

Explanation of symbols

1 internal combustion engine 2 exhaust passage 3 oxidation catalyst 4 first NOx catalyst 5 second NOx catalyst 6 third air-fuel ratio sensor 7 first air-fuel ratio sensor 8 second air-fuel ratio sensor 9 fuel addition valve 10 ECU

Claims (5)

A first NOx storage catalyst provided in an exhaust passage of the internal combustion engine;
First air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the first NOx storage catalyst;
Fuel addition means for adding fuel to the first NOx storage catalyst from the upstream side of the first NOx storage catalyst;
NOx reduction means for reducing NOx stored in the first NOx storage catalyst by adding fuel by the fuel addition means;
Output stability determination means for determining whether or not the detected value of the first air-fuel ratio detection means when the fuel is added by the fuel addition means is richer than stoichiometric and stable within a predetermined change range. When,
Based on the air-fuel ratio detected by the first air-fuel ratio detecting means when the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric. An inflow exhaust air / fuel ratio estimating means for estimating an air / fuel ratio of exhaust flowing into the first NOx storage catalyst during a period in which fuel is added by the fuel addition means;
When the output stability determining means determines that the detected value of the first air-fuel ratio detecting means is stable at an air-fuel ratio richer than stoichiometric, the air-fuel ratio of the exhaust gas obtained by the first air-fuel ratio detecting means and the Deterioration determining means for determining deterioration of the first NOx storage catalyst based on the air-fuel ratio estimated by the inflowing exhaust air-fuel ratio estimating means;
An exhaust emission control device for an internal combustion engine, comprising: A second catalyst provided in an exhaust passage downstream of the first air-fuel ratio detection means and having an oxidation ability;
Second air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust downstream of the second catalyst;
Further comprising
The output stabilizing means is detected by the first air-fuel ratio detecting means when the air-fuel ratio detected by the second air-fuel ratio detecting means changes richer than stoichiometric when fuel is added by the fuel adding means. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas is determined to be stable at an air-fuel ratio richer than stoichiometric.   Third air-fuel ratio detecting means for detecting the air-fuel ratio of the exhaust upstream of the first NOx storage catalyst is further provided, wherein the inflowing exhaust air-fuel ratio estimating means is configured to detect the air-fuel ratio detected by the third air-fuel ratio detecting means and the first air-fuel ratio. 3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst is estimated from the air-fuel ratio detected by one air-fuel ratio detection means.   The inflow exhaust air / fuel ratio estimation means is configured to detect the first air / fuel ratio when the output stability determination means determines that the air / fuel ratio detected by the first air / fuel ratio detection means is stable at an air / fuel ratio richer than stoichiometry. The third air-fuel ratio during fuel addition by the fuel addition means so that the air-fuel ratio of the exhaust gas obtained by the fuel-fuel ratio detection means is equal to the air-fuel ratio detected by the third air-fuel ratio detection means at this time. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the air-fuel ratio of the exhaust gas flowing into the first NOx storage catalyst is estimated by correcting the air-fuel ratio detected by the detection means.   The fuel addition means further comprises a fuel addition end means for ending the fuel addition by the fuel addition means when the air-fuel ratio of the exhaust detected by the second air-fuel ratio detection means becomes richer than a predetermined air-fuel ratio. The exhaust emission control device for an internal combustion engine according to any one of claims 2 to 4.

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