JP3424406B2 - Exhaust gas purification device for internal combustion engine - Google Patents

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, and more particularly to a first exhaust flow passage through which exhaust gas circulates a catalyst and a second exhaust flow passage through which a catalyst bypasses a catalyst. The present invention relates to a technique for diagnosing a failure of a flow path switching unit that selectively switches.

[0002]

2. Description of the Related Art In an exhaust gas purification apparatus for an internal combustion engine, in order to early activate an exhaust gas purification catalyst when it is cold, the catalyst is installed as far upstream as possible from the exhaust passage, and the catalyst is heated by exhaust gas of a relatively high temperature to raise it. A method of increasing the temperature is effective. However, in this case, there is a possibility that the catalyst is exposed to a very high temperature exhaust gas at the time of high rotation and high load after warming up, resulting in thermal deterioration.

Therefore, as a technique for solving such a problem, there is, for example, one shown in FIG. 10 (see Japanese Patent Laid-Open No. 62-99611). This is because the first catalyst 3 is provided in the exhaust passage 32 of the engine (hereinafter, engine) 31.
3 and an O ₂ sensor 34 located on the upstream side thereof are provided, and the O ₂ sensor 34 and the first catalyst 33 are provided in the exhaust passage 32.
A switching valve device 36 for switching the exhaust flow passage between a flow passage for flowing the exhaust gas to the first catalyst 33 and a flow passage for flowing the exhaust gas through the bypass passage 35. The exhaust passage 32 is provided upstream of the O ₂ sensor 34.

Therefore, in this type, when the exhaust gas is at a high temperature, the exhaust gas is caused to flow by bypassing the O ₂ sensor 34 and the first catalyst 33, thereby preventing the thermal deterioration of the exhaust gas and the O ₂ sensor 34 and the bypass passage. The exhaust gas is purified by using the second catalyst 37 provided in the exhaust passage 32 on the downstream side of the merging portion of 35.

[0005]

However, in such a conventional exhaust emission control device, the O ₂ sensor 34 is installed upstream of the first catalyst 33, and the O ₂ sensor 34 is also used when bypassing the air-fuel ratio. Since it is necessary to perform control, the exhaust passage 32 on the first catalyst 33 side is not completely blocked, and a part of the exhaust gas is guided to the first catalyst 33 side.

For this reason, the first catalyst 33 and the O ₂ sensor 34 are exposed to high temperature exhaust gas even if only a part of them, and thermal deterioration of these is inevitable. In order to solve such a problem, a second O ₂ sensor is provided upstream of the second catalyst so that the first catalyst and the first O ₂ can be provided at the time of bypass.
A method of completely shutting off the exhaust gas flow to the sensor by a switching valve device to protect the first catalyst and the first O ₂ sensor, and executing the air-fuel ratio control by using the second O ₂ sensor. You can use it.

However, even with such a construction, the first catalyst and the first O ₂ are lost due to a failure such as sticking of the switching valve device.
When the exhaust passage on the sensor side cannot be shut off, a large amount of high temperature exhaust gas flows into the exhaust passage on the first catalyst and first O ₂ sensor side at high speed and high load, and the first catalyst and the first O ₂ are exhausted. _{The 2} sensor suffers thermal deterioration as in the prior art, and the initial performance cannot be exhibited.

Therefore, it is necessary to detect the occurrence of a failure such as sticking of the switching valve device as described above because the leakage of the exhaust gas to the first catalyst side exceeds the allowable value. To detect such a failure, it is generally conceivable to provide an angle sensor in the switching valve device and determine whether or not the angle is at a target angle. However, it is necessary to newly provide an angle sensor, which causes an increase in cost. There are problems such as.

In view of the conventional problems as described above, the present invention provides a new sensor by improving the structure for determining the failure of the flow path switching means such as the switching valve device such as sticking. The problem is to eliminate the need and solve problems such as an increase in cost.

[0010]

Therefore, according to the invention of claim 1, as shown in FIG. 1, a catalyst is provided in an exhaust passage of an engine, and a bypass passage for bypassing the catalyst is provided in the exhaust passage. And a flow passage switching means for selectively switching between a first exhaust flow passage through which the exhaust gas flows through the catalyst and a second exhaust flow passage through which the exhaust gas bypasses the catalyst, and the flow path switching means. A first air-fuel ratio detecting means provided between the flow path switching means and the catalyst for detecting the air-fuel ratio of the exhaust gas, and a first air-fuel ratio detecting means provided in the exhaust passage downstream of the confluence portion of the bypass passage to detect the air-fuel ratio of the exhaust gas. The second air-fuel ratio detecting means for detecting, the air-fuel ratio detected by each of the air-fuel ratio detecting means when the exhaust gas is passed through the catalyst and when the exhaust gas is passed through the bypass passage so as to approach the target air-fuel ratio. Air-fuel ratio feedback of the basic control value of Air-fuel ratio feedback control means for performing an increase / decrease correction with a positive value to feedback-control the air-fuel ratio, and first air-fuel ratio detection for detecting a signal of the first air-fuel ratio detection means when exhaust gas is passed through the bypass passage. Means signal detection means, and the flow path switching means based on the signal of the first air-fuel ratio detection means and the signal of the second air-fuel ratio detection means detected by the first air-fuel ratio detection means signal detection means. An exhaust emission control device for an internal combustion engine, comprising: a failure diagnosis means for diagnosing a failure.

According to a second aspect of the present invention, the failure diagnosing means has a cycle in which the air-fuel ratios detected by the first and second air-fuel ratio detecting means are reversed from rich to lean or from lean to rich. And a second air-fuel ratio detecting means for detecting the air-fuel ratio detected by the first air-fuel ratio detecting means when the exhaust gas is passed through the bypass passage. Inversion cycle ratio calculation means for calculating the ratio of the detected air-fuel ratio to the inversion cycle, and comparison means for comparing the inversion cycle ratio calculated by the inversion cycle ratio calculation means with a predetermined value, Based on the comparison result by the comparison means, when the calculated inversion period ratio is larger than a predetermined value, the flow path switching means is diagnosed as a failure.

According to a third aspect of the present invention, in the failure diagnosing means, the time difference at which the air-fuel ratios detected by the first and second air-fuel ratio detecting means are reversed from rich to lean or from lean to rich. And an inversion time difference detection means for detecting the air-fuel ratio by the first air-fuel ratio detection means detected by the inversion time difference detection means when the exhaust gas is passed through the bypass passage. The inversion time difference calculating means for calculating the difference between the detected air-fuel ratio and the inversion time, and the comparing means for comparing the inversion time difference calculated by the inversion time difference calculating means with a predetermined value, and the comparing means. On the basis of the result of the comparison according to, when the calculated reversal time difference is smaller than a predetermined value, the flow path switching means is diagnosed as a failure.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 2 shows a system diagram common to the embodiments of the invention described in claims 1 to 3. In this figure, the engine 1 opens the intake valve 2 and introduces the air-fuel mixture from the intake port 3 into the cylinder 4, compresses it with the piston 5 and ignites and burns it with the spark plug 6, and after the combustion, the exhaust valve 7
And exhausts the exhaust gas to the exhaust passage 9 through the exhaust port 8.

The intake air is introduced from the upstream side of the air cleaner 10 and the flow rate is detected by an air flow meter 11 provided downstream thereof. Further, a throttle valve 12 is provided downstream of the throttle valve 12, and the opening of the throttle valve 13 is detected by a throttle sensor 13 provided side by side. These detection signals together with other information relating to the operating state, that is, the engine rotation speed detected by the engine rotation speed sensor 23, the cooling water temperature detected by the water temperature sensor 14, and the control unit 15
And the fuel injection amount according to the operating state of the engine 1 is calculated.

The control unit 15 also has a clock 15a and a counter 15b for measuring time. On the other hand, two catalysts, that is, a first catalyst 18 and a second catalyst 2 are provided on the upstream side and the downstream side of the exhaust passage 9 of the engine 1.
0 and 0 are inserted in series. In the exhaust passage 9, after branching from the exhaust passage 9 and joining the exhaust passage 9 again,
A bypass passage 9b for bypassing the catalyst 18 is connected in communication with the first exhaust passage through which exhaust flows through the two catalysts 18 and 20 at a branch portion of the bypass passage 9b from the exhaust passage 9. , Bypassing the first catalyst 18 to the second
The switching valve device 16 as a flow path switching means for selectively switching to the second exhaust flow path through which only the catalyst 20 of FIG. This switching valve device 16 includes a valve 1
Actuator 1 for switching and driving 6A and the valve 16A
7 and 7.

Further, they are provided in the exhaust passage 9 immediately upstream of the first catalyst 18 and upstream of the second catalyst 20 and downstream of the confluence of the bypass passage 9b, respectively, to detect the air-fuel ratio of the exhaust gas. Oxygen sensors (hereinafter referred to as O ₂ sensors) 19 and 21 are provided as first and second air-fuel ratio detecting means. The detection signals of both O ₂ sensors 19 and 21 are sent to the control unit 15, the injection amount is corrected accordingly, and the fuel is injected from the fuel injection valve (injector) 22 based on this. ing.

Here, when the exhaust gas is passed through the first catalyst 18 and the bypass passage 9b, the air-fuel ratios detected by the respective O ₂ sensors 19 and 20 are exhausted so as to approach the target air-fuel ratio. An air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio by increasing or decreasing the basic control value of the fuel ratio with the air-fuel ratio feedback correction value.
When the exhaust gas is passed through the bypass passage 9b, it is detected by the first air-fuel ratio detecting means signal detecting means for detecting the signal of the first O ₂ sensor 19 and the first O ₂ sensor signal detecting means. The switching valve device 16 is based on the signal of the first O ₂ sensor 19 and the signal of the second O ₂ sensor.
Failure diagnosis means for diagnosing the failure of
Each of these means is installed in the control unit 15 by software.

Next, claims 1 and 2 based on the above configuration
The exhaust passage switching control of the embodiment of the described invention will be described in detail based on the flowchart of FIG. First, step 1
Then, engine cooling water temperature Tw and engine rotation speed N
e, the throttle valve opening TVO, and the intake air amount Qa are read, and the basic fuel injection amount Tp is calculated in step 2 based on these information.

In step 3, the engine cooling water temperature Tw is equal to or higher than the predetermined value TwH and the engine rotation speed Ne is
Is a predetermined value NeH or more, and the throttle valve opening TVO is a predetermined value TVOH or more. If this is determined, it is determined that the engine is in the high-speed / high-load operation state after warming up, and the routine proceeds to step 4, where the exhaust gas is exhausted in order to prevent thermal deterioration of the first catalyst and the first O ₂ sensor. The switching valve device is switched so that the gas flows to the bypass passage side.

In the next step 5, the basic fuel injection amount Tp
For use in feedback correction and switching valve device failure diagnosis 2
The two O ₂ sensor outputs VO ₂ 1 and VO ₂ 2 are read. In the next step 6, carry out the Tp feedback correction by the second O ₂ sensor output VO ₂ 2. The details and next step 7
Will be described later. On the other hand, if it is determined in step 3 that the high speed / high load is not determined, it is determined that there is no risk of thermal deterioration of the first catalyst and the first O ₂ sensor, and the process proceeds to step 8 where the exhaust gas is discharged to the first exhaust gas. The switching valve device is switched so as to pass through the catalyst, and the first O ₂ sensor output VO ₂ 1 is read in step 9, and based on this, the next step 10
Tp feedback correction is performed.

FIG. 4 shows a flowchart of the Tp feedback correction. The figure corresponds to step 6 and step 10 in FIG. First, in step A1, it is determined whether or not the operating state is in a region where Tp correction control is performed. The engine cooling water temperature Tw is equal to or higher than a predetermined value TwL and the throttle valve opening TV
When O is within the predetermined range, that is, in the Tp correction control region, the process proceeds to step A2, and the Tp correction control coefficients Pr and P are set.
l and I are retrieved and read from the maps assigned to the Tp and Ne lattices, respectively. In the next step 3, the second O ₂
Sensor output VO 2 ₂ is equal to or greater than a predetermined value VSL.

Since the output voltage of the O ₂ sensor output is higher when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, the fact that the output is VSL or higher means that , Shows that the air-fuel ratio is rich.
In the case of step 6 in FIG. 3, the O ₂ sensor output to be referred to is the second O ₂ sensor output VO ₂ 2, and step 10
, The referenced O ₂ sensor output is the first O ₂ sensor output VO ₂ 1.

With respect to other points, both steps are exactly the same and conform to FIG. When it is determined in step A3 that the air-fuel ratio is rich, it is determined in the next step A4 whether or not the previous air-fuel ratio was lean. Flag F
Is 1 when the previous air-fuel ratio is rich and 0 when it is lean
Since F has a value of, if F = 0, the previous lean, that is,
This time, the air-fuel ratio has changed from lean to rich. In that case, the process proceeds to step A5, Pr is subtracted from the air-fuel ratio correction value α, and the flag F is set to 1 in step A6.
Is substituted. Further, if F ≠ 0 in step A4, that is, if the previous time is also determined to be rich, the process proceeds to step A8, and I is subtracted from the air-fuel ratio correction value α.

On the other hand, in step A3, when VO ₂ 2 is smaller than VSL, that is, when the air-fuel ratio is judged to be lean, in steps A9 to A12, 0 of the flag F is set to 1 and 1 is set to 0. , Pr for Pl and α are replaced by “add” and “add” to replace step A4 to
The description is omitted because it is similar to A6 and A8. If it is determined in step A1 that the operating state is not within the Tp correction control range, the process proceeds to step A13, where α
Is 100%. After calculating these α, finally, in step A7, Tp is multiplied by α to obtain the final injection both Ti.

The relationship between α and the air-fuel ratio in the above control is shown in FIG. FIG. 6 shows a flowchart of the exhaust passage switching valve device failure diagnosis. This is a failure diagnosis means, is to use a ratio of the first O ₂ sensor output and the inversion period of the second O ₂ sensor output. This figure corresponds to step 7 in FIG. First, in step B1, the present and the previous VO ₂ 2 determination flags F and FB are checked and the second O
₂ lean sensor output VO 2 ₂ detects the reversal to the rich. When it is detected, the process proceeds to step B2 and time t
Then, the cycle T2 is calculated from the difference between the previous inversion time t2 and t is t2.
To.

Next, in step B3, it is judged whether or not the first O ₂ sensor output VO ₂ 1 is equal to or more than a predetermined value VSL, that is, the air-fuel ratio is rich at that location. If it is rich, step B4 is executed. Then, the VO ₂ 1 determination flag F1 is set to 1, and if lean, the process proceeds to step B10 to set F1 to 0. In the next step B5, the present and the previous VO ₂ 1 determination flags F1 and F1B are checked to detect the inversion of the first O ₂ sensor output VO ₂ 1 from lean to rich. When it is detected, the process proceeds to step B6, the period T1 is obtained from the difference between the time t and the previous inversion time t1, and t is substituted for t1.

By the way, when the bypass passage is used, the amount of exhaust gas flowing through the exhaust passage on the side of the first catalyst is very small due to the leakage of the switching valve device. Therefore, the fluctuation range of the air-fuel ratio is small, and the reversal cycle is large. T1 is also longer than the inversion period T2 of the second O ₂ sensor output. However, if the leakage flow rate increases due to a failure of the switching valve device, the reversal cycle T1
Will be shorter than normal. From this, in this embodiment, it is determined in step B7 whether the ratio of T2 and T1 is larger than the predetermined value K, that is, whether T2 is larger than the product of T1 and the predetermined value K. It is determined that there is a large leak flow rate, that is, a failure, the process proceeds to step B8, the ALARM is turned on, and the driver is informed of the failure. Finally, in step B9, the flags F and F1 are substituted into FB and F1B, respectively, for reference in the next control.

Next, another embodiment will be described with reference to FIG. In this embodiment, as the failure diagnosis means, the first O
₂ sensor output and an embodiment in which the difference in their inversion time of the second O ₂ sensor output. The configuration other than the failure diagnosis corresponding to FIG. 6 and FIG. 7 is the same as that of the first embodiment described above, and thus the description thereof is omitted. It should be noted that this figure also corresponds to step 7 in FIG. 3, as in FIG.

First, at step BB1, the present and previous VO ₂ 2 determination flags F and FB are checked to detect the inversion of the second O ₂ sensor output VO ₂ 2 from lean to rich. When it is detected, the process proceeds to step BB2 and the time t is read into tF2. Next, in step BB3, the first O ₂
It is determined whether or not the sensor output VO ₂ 1 is equal to or higher than a predetermined value VSL, that is, whether the air-fuel ratio is rich at that location. If it is rich, the process proceeds to step BB4 and the VO ₂ 1 determination flag F
If 1 is set to 1, and if lean, proceed to step BB16,
F1 is set to 0.

At the next step BB5, the current and previous V
The O ₂ 1 determination flags F1 and F1B are checked to detect the lean-to-rich inversion of the first O ₂ sensor output VO ₂ 1. When it is detected, the process proceeds to step BB6 and the time t is read into tF1. By the way, in FIG.
_{The 2} sensor 21 requires more time than the first O ₂ sensor 19. That is, when calculating the time delay of the inversion of the first O ₂ sensor output with respect to the inversion of the _second O ₂ sensor output, this arrival required time difference tFD is subtracted from the second O ₂ sensor output determination time tF2 in advance. There is a need. Since this arrival required time difference depends on the flow rate of exhaust gas, in this embodiment, tFD is read from the map assigned to the Tp and Ne lattices in step BB7, and tF is read in next step BB8.
Subtract D and substitute the value into tF. Next step BB
At 9, it is determined whether tF is tF1 or less.

When tF1 is smaller, as shown in FIG.
Since it is thought that it is due to the inversion of one cycle before,
Do not perform failure diagnosis. If tF is tF1 or less,
The process proceeds to step BB10 and the difference between tF1 and tF is the predetermined value tF.
_NGDetermine if less than. As shown in Figure 9
If the switching valve device has a large amount of leakage, the first O₂
The time delay of sensor output inversion is small. That is, tF1
And tF is a predetermined value tF _NGSwitching valve when smaller
There is a possibility that the device is out of order, in this case, step BB
11 is a calculation for tF1 and tF2 different from the previous time.
If it is determined that the failure occurs, the process proceeds to step BB12 and the failure occurs.
Flag F_NGIs increased by 1. However, in this case, for example
As shown in 9, the first O₂Inversion period of sensor output and
Second O₂When the sensor output reversal period is significantly different
May be judged to be defective even if it is normal. this
In order to prevent this, in the present embodiment, two inversions are repeated.
Subsequently, the difference between tF1 and tF is the predetermined value tF._NGIf less than
It is diagnosed as a failure.

That is, in step BB10, tF1
Difference tF between proceeds to step BB17 when it is determined that the predetermined value tF _NG above, the failure flag F _NG to 0, steps if these results, step BB13 Oite, F _NG is 2 or more BB14 judged it to be a failure and ALA
Turn on the RM to notify the driver of the failure. Finally, in step BB15, since F, F1, tF1, and tF2 are referred to in the next control, FB, F1B, tF1B, and t, respectively.
Substitute in F2B.

As described above, when the exhaust gas is passed through the bypass passage 9b, the exhaust passage 9 on the first catalyst side 18 is shut off, and the air-fuel ratio control is performed by the second O ₂ sensor 21. Means is provided for detecting a cycle or a time difference at which the air-fuel ratios detected by the second O ₂ sensors 19 and 20 are respectively changed from rich to lean or from lean to rich, and based on the inversion cycle or the inversion time difference. The first catalyst 18 at the time of exhaust bypass, which directly causes thermal deterioration of the first catalyst 18 and the first O ₂ sensor 19.
Since the excessive leakage of the exhaust gas to the exhaust passage on the side is determined to determine the failure of the switching valve device 16, the failure of the switching valve device 16 in the exhaust passage is prevented without causing an increase in cost due to the addition of a sensor or the like. The diagnosis can be easily performed and the driver can be notified, and the first catalyst 18 and the first catalyst 18
There is an effect that the thermal deterioration of the O ₂ sensor 19 can be prevented at an early stage.

[0034]

As described above, according to the first aspect of the present invention, the failure diagnosis of the passage switching means of the exhaust passage can be easily performed and the operation can be performed without increasing the cost due to the addition of the sensor. It is possible to notify the person concerned, and it is possible to prevent the catalyst and the first air-fuel ratio detecting means from being deteriorated by heat at an early stage.

According to the invention described in claims 2 and 3, the first
And a means for detecting a cycle or a time difference at which the air-fuel ratio detected by the second air-fuel ratio detecting means is changed from excessive rich to lean or lean to excessive rich, or a time difference at which the air-fuel ratio is inverted, based on this inversion cycle or inversion time difference. It is possible to judge the failure of the flow path switching means by judging the excessive exhaust leakage state to the exhaust passage on the catalyst side at the time of exhaust bypass, which directly causes the thermal deterioration of the catalyst and the first air-fuel ratio detecting means.

[Brief description of drawings]

FIG. 1 is a configuration diagram of the invention according to claim 1.

FIG. 2 is a system diagram common to the embodiments of the invention described in claims 1 to 3.

FIG. 3 is a flow chart of the switching valve device switching control showing the control content of the embodiment of the invention according to claims 1 and 2.

FIG. 4 is a flowchart of air-fuel ratio feedback correction control showing the same control contents as above.

[Figure 5] Characteristic diagram

FIG. 6 is a flowchart of a switching valve device failure diagnosis showing the same control contents as above.

[Figure 7] Characteristic diagram

FIG. 8 is a flow chart of a switching valve device failure diagnosis showing the control content of the embodiment of the invention as claimed in claims 1 and 3.

[Figure 9] Characteristic diagram

FIG. 10 is a diagram showing a conventional technique.

[Explanation of symbols]

1 engine 9 exhaust passage 9b bypass passage 15 control unit 16 switching valve device 18 first catalyst 20 second catalyst 19 first O ₂ sensor 21 second O ₂ sensor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI F02D 41/14 ZAB F02D 41/14 ZAB 41/22 305 41/22 305M ZAB ZAB (72) Inventor Keiji Okada Kanagawa Yokohama, Kanagawa No. 2 Takaracho, Ward Nissan Motor Co., Ltd. (56) Reference JP-A-8-86215 (JP, A) JP-A-5-340238 (JP, A) JP-A-5-231137 (JP, A) JP Hei 5-231138 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) F01N 3/08-3/24 F02D 41/02-41/22

Claims (3)

(57) [Claims] 1. A first exhaust flow passage in which a catalyst is interposed in an exhaust passage of an engine, a bypass passage for bypassing the catalyst is connected to the exhaust passage, and exhaust gas flows through the catalyst; A flow passage switching means for selectively switching to a second exhaust flow passage that bypasses and flows; and a flow passage switching means provided in the exhaust passage between the flow passage switching means and the catalyst for detecting the air-fuel ratio of the exhaust gas. No. 1 air-fuel ratio detection means, second air-fuel ratio detection means provided downstream of the bypass passage confluence portion of the exhaust passage for detecting the air-fuel ratio of the exhaust gas, and when the exhaust gas is passed through the catalyst When flowing through the passage, the air-fuel ratio feedback control is performed by increasing / decreasing the basic control value of the air-fuel ratio by the air-fuel ratio feedback correction value in order to bring the air-fuel ratio detected by each air-fuel ratio detecting means close to the target air-fuel ratio. Fuel ratio A readback control means, when the circulating exhaust in the bypass passage, said first
First air-fuel ratio detecting means signal detecting means for detecting a signal of the air-fuel ratio detecting means, and a signal of the first air-fuel ratio detecting means detected by the first air-fuel ratio detecting means signal detecting means and the second 2. An exhaust gas purification device for an internal combustion engine, comprising: a failure diagnosis means for diagnosing a failure of the flow path switching means based on a signal from the air-fuel ratio detection means. 2. The inversion cycle detection means for detecting a cycle in which the air-fuel ratios detected by the first and second air-fuel ratio detection means are inverted from rich to lean or from lean to rich, respectively. And a reversal cycle of the air-fuel ratio detected by the first air-fuel ratio detection means detected by the reversal cycle detection means and a reversal of the air-fuel ratio detected by the second air-fuel ratio detection means when the exhaust gas is passed through the bypass passage. The inversion period ratio calculating means for calculating the ratio with the period, and the comparing means for comparing the inversion period ratio calculated by the inversion period ratio calculating means with a predetermined value, and the comparison result by the comparing means. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas purifying apparatus according to claim 1 is configured to diagnose a failure of the flow path switching means when the calculated reversal cycle ratio is larger than a predetermined value. 3. The reversal time difference detection means for detecting a time difference at which the air-fuel ratios detected by the first and second air-fuel ratio detecting means respectively change from excessive rich to lean or from lean to excessive rich. And an inversion time of the air-fuel ratio detected by the first air-fuel ratio detection means detected by the inversion time difference detection means and an inversion of the air-fuel ratio detected by the second air-fuel ratio detection means when the exhaust gas is passed through the bypass passage. A reversing time difference calculating means for calculating a difference with time, and a comparing means for comparing the reversing time difference calculated by the reversing time difference calculating means with a predetermined value, and based on a comparison result by the comparing means. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purifying apparatus according to claim 1 is configured to diagnose a failure of the flow path switching unit when the calculated reversal time difference is smaller than a predetermined value.