JPH0610738A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To maintain the excellent exhaust emission control performance for a long time in an air-fuel ratio control device for internal combustion engine, which controls the air-fuel ratio on the basis of air-fuel ratio sensors (Ox sensors) provided upstream and downstream of the catalyst, by switching plural Ox sensors before and after the deterioration of the catalyst. CONSTITUTION:Two catalysts 17, 18 and three Ox sensors 19-21 are alternately provided in an exhaust passage 16 of an internal combustion engine 11. Fluctuation frequency (FMAIN) of a first Ox sensor 19 upstream of the upstream catalyst is obtained (step 301). Fluctuation frequency (FSUBI) of a second Ox sensor 20 immediately downstream of the upstream catalyst is obtained (step 302). In the case where FMAIN/FSUBI is less than a predetermined value (step 303), deterioration of the catalyst is judged (step 306) to perform the sub feedback control with a third Ox sensor 21 positioned in the immediately downstream of the downstream catalyst instead of using the second Ox sensor 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly, to an air-fuel ratio sensor provided upstream and downstream of a catalyst, in addition to air-fuel ratio feedback control by the upstream air-fuel ratio sensor, a downstream oxygen sensor. The present invention relates to an air-fuel ratio control device for an internal combustion engine, which performs air-fuel ratio feedback control according to.

[0002]

2. Description of the Related Art In an internal combustion engine, exhaust gas contains three kinds of toxic components, namely CO, HC and NOx, and a three-way catalyst is used to purify them. The purification characteristics of this catalyst change greatly depending on the air-fuel ratio of the fuel gas,
The above 3 is most effective when the air-fuel ratio is near the stoichiometric air-fuel ratio.
Purifies toxic components of seeds. Therefore, in order to reduce the amount of toxic components emitted from the internal combustion engine, it is necessary to control the fuel gas supplied to the internal combustion engine within a narrow range near the stoichiometric air-fuel ratio.

For this reason, the applicant has provided oxygen sensors (Ox sensors) for measuring the oxygen concentration in the exhaust gas as air-fuel ratio sensors upstream and downstream of the catalyst, and the upstream Ox
In addition to the air-fuel ratio feedback control (main feedback control) by the sensor, the air-fuel ratio feedback control (sub-feedback control) by the downstream Ox sensor is performed to propose a device for controlling the air-fuel ratio of the fuel gas to near the stoichiometric air-fuel ratio. (Japanese Patent Laid-Open No. 61-286550).

As described above, in the apparatus having the Ox sensors upstream and downstream of the catalyst, when the catalyst deteriorates, CO, H
The downstream Ox sensor directly receives a change in the concentration of C, NOx, etc., and its output changes drastically. As a result, the downstream side O
Disturbance occurs in the sub-feedback control based on the x sensor output, a good air-fuel ratio cannot be obtained, and problems such as deterioration of exhaust gas purification performance, deterioration of fuel consumption, deterioration of drivability occur.

Therefore, in the apparatus described in the above publication, the state of the catalyst is monitored by monitoring the output of the downstream Ox sensor, and when the deterioration of the catalyst is detected, the feedback control by the downstream Ox sensor is stopped. Is taking place.

Therefore, according to this apparatus, a high purifying property is ensured until the catalyst is deteriorated, and deterioration of the purifying property after the catalyst deterioration is prevented.

9 (A) and 9 (B) are configuration diagrams showing an example in which the device described in the above publication is applied to an exhaust gas purifying device for an internal combustion engine having a plurality of catalysts.

In the figure, reference numeral 1 is an internal combustion engine, the intake port of which is connected to an air cleaner 4 upstream of a throttle valve 3 via an intake passage 2. Further, a fuel injection valve 5 is provided in a portion of the intake passage 2 that is connected to the internal combustion engine 1, and the air supplied from the intake passage 2 and the fuel supplied from the fuel injection valve 5 have a predetermined air-fuel ratio. Mixed internal combustion engine 1
Is supplied to.

The exhaust port of the internal combustion engine 1 has an exhaust passage 6
Are linked to. In the exhaust passage 6, a first catalyst 7 that acts as a sub catalyst and a second catalyst 8 that acts as a main catalyst are provided with a space therebetween. For this reason,
The exhaust gas discharged from the internal combustion engine 1 is first purified by the first catalyst 7 and sufficiently mixed, then purified by the second catalyst, and then released into the atmosphere.

Since the first catalyst 7 is provided in the vicinity of the exhaust port of the internal combustion engine 1, the first catalyst 7 is heated by the exhaust gas and reaches a temperature sufficient to exert the purification action relatively quickly even during cold start. However, the exhaust gas emitted by the internal combustion engine at the time of cold start can be quickly purified. Therefore, even when the second catalyst 8, which corresponds to the main catalyst of the exhaust gas purifying apparatus, does not reach a temperature sufficient to exert the purifying action at the cold start of the internal combustion engine, the exhaust gas Is thoroughly purified.

FIG. 1A shows a first Ox sensor 9 as the upstream Ox sensor in the apparatus described in the above publication.
The upstream side of the catalyst 7 of the
An example in which the Ox sensor 10 of 1 is arranged between the first and second catalysts 7 and 8 is shown.

Also, in FIG. 2B, the first Ox sensor, which is an upstream Ox sensor, is arranged upstream of the first catalyst 7 as in the example shown in FIG. An example in which the second Ox sensor 10 which is the above is arranged downstream of the second catalyst 8 is shown.

In these cases, the air-fuel ratio of the fuel gas can be controlled to a value close to the stoichiometric air-fuel ratio with high accuracy as in the device described in the above publication, and in addition, the purifying property at the time of starting the internal combustion engine, etc. In addition, due to the purifying action of a plurality of catalysts, the cleanliness of exhaust gas is further improved.

[0014]

However, in the above-mentioned conventional apparatus, when the structure shown in FIG. 9A is used, even if the second catalyst 8 is not deteriorated, if the first catalyst 7 is deteriorated. The sub feedback control based on the output of the second Ox sensor 10 cannot be performed.

Therefore, when the capacity of the second catalyst 8 which is the main catalyst begins to decline and it becomes necessary to control the second catalyst 8 closer to the stoichiometric air-fuel ratio with higher accuracy, the second Ox sensor 1
0 cannot be used, and the effect of providing a plurality of Ox sensors cannot be enjoyed.

On the other hand, in the catalyst at the early stage of use, in which the purifying ability is not deteriorated, oxygen is stored in the exhaust gas to form C
It has a high ability to reduce toxic components such as O, HC, and NOx, or to release oxygen into exhaust gas to oxidize the toxic components. Therefore, when the catalyst is in the initial stage of use, the fluctuation of the oxygen concentration in the exhaust gas can be suppressed to be smaller than that when the catalyst is deteriorated.

Therefore, in the case of the configuration shown in FIG. 9B, in the initial stage of use when the purification capacity of the catalyst is high, the fuel gas supplied during the internal combustion period 1 is kept rich or lean until the second The oxygen concentration in the exhaust gas around the Ox sensor 10 does not change. Therefore, in this configuration,
The sub-feedback control by the second Ox sensor 10 cannot be performed until the catalyst deteriorates to some extent.

As described above, in the above conventional device,
Due to the change in the purifying ability of the catalyst, the feedback control based on the outputs of the plurality of Ox sensors cannot be performed for a long period of time, and the good purifying property cannot be maintained for a long period of time.

The present invention has been made in view of the above points, and the air-fuel ratio control of an internal combustion engine which solves the above-mentioned problems by switching and using a plurality of Ox sensors for sub-feedback before and after the deterioration of the catalyst. The purpose is to provide a device.

[0020]

As shown in the principle diagram of FIG. 1, the above problem is solved by purifying the exhaust gas in the exhaust passage 32 of the internal combustion engine 31 at intervals in the exhaust gas flowing direction. A plurality of catalyst devices 33a, 3 provided in series for
3b, 33c ... and a plurality of air-fuel ratio sensors 34a, 34b, 34c ..
, Deterioration detecting means 35 for detecting deterioration of each of the plurality of catalyst devices 33a, 33b, 33c, and a catalyst located at the most upstream position of the catalyst devices estimated to be undeteriorated by the deterioration detecting means 35. This is solved by an air-fuel ratio control device for an internal combustion engine, which comprises a control means 36 for controlling the air-fuel ratio of the internal combustion engine 31 based on the outputs of the air-fuel ratio sensors arranged upstream and immediately downstream of the device.

[0021]

According to the above structure, the plurality of catalyst devices 33 are provided.
If a, 33b, 33c, ... Are new, the deterioration detecting means 35 does not detect deterioration of the catalyst device. Therefore, the catalyst device 33a becomes a catalyst device which is not deteriorated and is located in the uppermost stream, and the control means 36 controls the internal combustion engine 31 based on the outputs of the air-fuel ratio sensors 34a and 34b.
The air-fuel ratio control is performed. Therefore, in this case, the fuel gas supplied to the internal combustion engine 31 is maintained in the vicinity of the stoichiometric air-fuel ratio with high accuracy, as in the conventional device at the initial stage of use.

When the internal combustion engine 31 discharges exhaust gas, deterioration proceeds from the catalyst located upstream due to waste heat and oil scattering. Therefore, the catalyst device 33a is the fastest to deteriorate among the plurality of catalyst devices. Therefore, when the deterioration detecting means 35 detects the deterioration of the catalyst device 34a, the catalyst device 33b becomes the catalyst device which is not deteriorated and is located in the uppermost stream, and the control means 36 causes the air-fuel ratio sensor 34a or 34b and The air-fuel ratio control of the internal combustion engine 31 is performed based on the output of the fuel ratio sensor 34c.

In this case, since the purifying ability of the catalyst device 33a is deteriorated, the exhaust gas discharged from the internal combustion engine 31 is blown to the downstream side thereof without being purified.
Therefore, when any of the air-fuel ratio sensors located upstream of the catalyst device 33b and the air-fuel ratio sensor 33c located immediately downstream of the catalyst device 33b is used, highly accurate air-fuel ratio control is performed. .

Thereafter, as the deterioration of the catalyst progresses, air-fuel ratio control is performed based on the output of a pair of air-fuel ratio sensors similarly arranged in the space before and after the undeteriorated catalyst, and the most downstream catalyst deteriorates. Until then, the control using the two air-fuel ratio sensors is continued.

[0025]

FIG. 2 is a block diagram of an embodiment of an air-fuel ratio control system for an internal combustion engine according to the present invention. In the figure, reference numeral 11 denotes an internal combustion engine equipped with the device of this embodiment, and like the conventional internal combustion engine 1 shown in FIG. 9, its intake port has an intake passage 12 connected to an air cleaner 14 upstream of a throttle valve 13 and a fuel injection. A valve 15 is provided.

An exhaust passage 16 is provided at the exhaust port of the internal combustion engine 11.
Are connected. In the exhaust passage 16, a first catalyst 17 that serves as a sub catalyst device and a second catalyst 18 that serves as a main catalyst device are provided with a space therebetween.

Further, in the apparatus of this embodiment, the first Ox sensor 19 is provided upstream of the first catalyst 17 and the first catalyst 1
A second Ox sensor 20 is provided between the second catalyst 7 and the second catalyst 18.
Further, a third Ox sensor 21 is provided downstream of the second catalyst 18.

These Ox sensors 19 to 21 correspond to air-fuel ratio sensors, and are sensors that output signals corresponding to the oxygen concentration in the exhaust gas to which they are exposed. Also,
It has the characteristic of suddenly changing the output depending on whether the fuel gas is leaner or richer than the stoichiometric air-fuel ratio.

The first catalyst 17 is provided in the vicinity of the exhaust port of the internal combustion engine 11, is heated by the exhaust gas, and reaches a temperature sufficient to promptly perform the purifying action. Therefore, even in the cold state, the activation temperature required for purifying the exhaust gas is reached immediately after the internal combustion engine 11 is started, and the incomplete combustion gas discharged by the internal combustion engine 11 can be burned during the cold start.

The first to third Ox arranged at the above positions
The sensors 19 to 21 are control devices 22 corresponding to control means.
Is connected to a predetermined terminal of
The output signal based on the oxygen concentration in the exhaust gas in each region is transmitted.

In addition to the output signals of the Ox sensors 19-22,
Signals for calculating the fuel injection time are transmitted to the control device 22 from an air flow meter, an intake air temperature sensor, a water temperature sensor, a throttle sensor, etc., which are not shown.

The control device 22 calculates the basic fuel injection time based on these signals, and feeds back the fuel gas from the output signals of the Ox sensors 19 to 21 to maintain the fuel gas near the stoichiometric air-fuel ratio. The drive signal is transmitted to 15.

Further, in this embodiment, the air-fuel ratio feedback control by the Ox sensor is performed by the main feedback control and the sub-feedback control as in the conventional air-fuel ratio control device. Therefore, two Ox sensors are selected from the first to third Ox sensors as needed, and the upstream side is used for main feedback control and the downstream side is used for sub feedback control.

3 and 4 show a flow chart of an example of the air-fuel ratio feedback control executed by the control device 22 of the present embodiment device. The operation of the apparatus of this embodiment will be described below with reference to the drawings.

The routine shown in FIG. 3 is a main feedback control routine, which is started every predetermined time, for example, 4 ms. When this routine is started, it is first determined whether or not it is in the air-fuel ratio feedback (F / B) control region (step 101).

The condition for performing the air-fuel ratio feedback control is satisfied when the internal combustion engine 11 is in a state in which it should be operated with the fuel gas having the theoretical air-fuel ratio. Therefore, for example, the conditions are not satisfied during cranking of the internal combustion engine 11, during fuel amount increase correction for warm-up or power increase, during lean control, during fuel cut processing, and the like. In this case, since the air-fuel ratio feedback is not performed, the process is finished as it is and waits for the next activation.

When it is determined that the air-fuel ratio feedback control is performed, the first to third Ox sensors 19 to 19 are operated.
It is determined from the output signal of the Ox sensor (main Ox sensor) to be used for the main feedback control based on the below-mentioned reference out of 21 whether or not the air-fuel ratio is reversed from the previous processing (step 102). .

When the air-fuel ratio is inverted, it is determined whether the inversion is from rich to lean or from lean to rich in order to perform feedback control so as to cancel the inversion. ).

In the case of the reversal from rich to lean, it is necessary to increase the fuel injection amount and return the air-fuel ratio to the rich side. Therefore, by adding the reversal feedback constant RS1 to the air-fuel ratio correction coefficient FAF, skipping is performed. The FAF is increased (step 104), and the process ends.

In the case of lean to rich inversion, the air-fuel ratio needs to be turned back to the lean side, and the feedback constant RS2 at inversion is subtracted from FAF to reduce it in a skip manner (step 105). finish.

On the other hand, when it is determined in step 102 that the air-fuel ratio is not reversed, it is determined whether or not the main OX signal is rich in order to know the current air-fuel ratio state (step 106), and then that state is determined. Feedback control is performed to gradually increase or decrease the fuel injection amount in the direction of canceling

That is, when the air-fuel ratio is rich, the feedback constant KI1 during non-reversal is subtracted from FAF to reduce the fuel injection amount supplied to the internal combustion engine 11 (step 107), and the air-fuel ratio is lean. If it is, the feedback constant KI2 at the non-reversal time is added to FAF to increase the fuel injection amount (step 108), and the process is ended.

In the above main feedback control, the inversion feedback constants RS1 and RS2 are set to values sufficiently larger than the non-inversion feedback constants KI1 and KI2. Therefore, according to the above control, for example, when the air-fuel ratio is reversed from rich to lean,
First, the fuel injection amount is corrected to the rich side (the stoichiometric air-fuel ratio side) in a relatively large step, and then gradually comes to the stoichiometric air-fuel ratio side until the air-fuel ratio is reversed again. Therefore, the fuel gas supplied to the internal combustion engine is accurately maintained near the stoichiometric air-fuel ratio.

The feedback constant RS for each inversion
1, RS2 and feedback constant KI1 at each non-inverting time,
KI2 is set to the same value when the device is started.

The processing shown in FIG. 4 is a sub-feedback control routine, which is the above-mentioned RS1, RS2 and KI.
This is a routine for changing the values of 1 and KI2. Further, this processing is performed based on the output signal of the Ox sensor (sub-Ox sensor) which measures the oxygen concentration in the exhaust gas after passing through the catalyst, and is activated at intervals sufficiently longer than the main feedback control, for example, every 1 second.

When this routine is started, it is judged whether or not it is in the sub feedback control region as in the case of the main feedback control (step 201). In this case, for example, when the catalyst located upstream of the sub-Ox sensor is not activated, the condition for starting the sub-feedback control becomes unsatisfied, and the processing of this time is ended as it is.

When it is determined that the sub feedback control region is set, it is determined whether or not the output signal of the sub Ox sensor is rich (step 202). As described above, the sub-Ox sensor is provided downstream of the catalyst,
The oxygen concentration in the exhaust gas after purification is monitored.

As described above, the purification of the exhaust gas is performed by the oxidation or reduction of the toxic components, and at this time, supply and demand of oxygen is performed between the exhaust gas and the catalyst. Further, the exhaust gas discharged from the internal combustion engine is mixed when passing through the catalyst and reaches the sub Ox sensor.

Therefore, the oxygen concentration in the atmosphere to which the sub-Ox sensor is exposed has a characteristic in which the oxygen concentration in the exhaust gas discharged from the internal combustion engine is smoothed. Therefore, by monitoring the oxygen concentration, it is possible to determine whether the fuel gas corrected by the main feedback control every 4 ms is richer or leaner in the long term.

That is, when it is determined in step 202 that the output signal of the sub Ox sensor is rich, it is determined that the air-fuel ratio control by the main feedback control is shifted to the rich side. Then, the feedback constant used in the main feedback control is shifted to the lean side (RS1, KI2 is decreased, RS2, KI1 is increased) (step 203), and the process is ended.

When it is determined in step 202 that the output signal of the sub-Ox sensor is lean, the reverse process of step 203, that is, the feedback constant is shifted to the rich side (RS1, KI2 is increased, RS2, KI1 is increased.
Is reduced (step 204) and the process is terminated.

As a result, the control center of the main feedback control fluctuates gently around the stoichiometric air-fuel ratio, and the main feedback control is not biased to the rich side or the lean side. Therefore, the air-fuel ratio of the fuel gas is further maintained near the stoichiometric air-fuel ratio.

In the apparatus of this embodiment, as described above, two Ox sensors are selected from the first to third Ox sensors as necessary, and the upstream side of them is the main Ox sensor and the downstream side is the sub Ox sensor. It is used as an Ox sensor. The selection process of the Ox sensor, which is the main part of this embodiment, will be described below.

FIG. 5 is an example of a subroutine relating to the Ox sensor selection processing executed by the apparatus of this embodiment, and is a flow chart of a routine for detecting deterioration of the first catalyst and switching the Ox sensor.

In the apparatus of this embodiment, the first Ox sensor 19 shown in FIG. 2 is initially set as the main Ox sensor, and the second Ox sensor 20 is initially set as the sub Ox sensor.

In FIG. 5, when the catalyst deterioration detection routine is started, the output of the first Ox sensor 19 is first monitored for a predetermined time, and the fluctuating frequency FMAIN is calculated and stored (step 301).

Since the first Ox sensor 19 monitors the exhaust gas immediately after being discharged from the internal combustion engine 11, the FM
AIN has the same value as the fluctuating frequency of the oxygen concentration in the fuel gas supplied to the internal combustion engine 11. Also, this frequency is
Determined by the correction speed of the main feedback control,
This value does not change depending on the deterioration of the catalyst.

After the calculation of the theoretical air-fuel ratio FMAIN is completed, the second O2 sensor which is initially set to the sub-Ox sensor is set.
From the output of the Ox sensor 20 of the
1 is calculated and stored (step 302).

As described above, since the second Ox sensor 20 is provided downstream of the first catalyst 17, the oxygen concentration to be detected is the oxygen concentration in the exhaust gas immediately after being discharged from the internal combustion engine 11. It becomes a smoothed value. Therefore, FSUB
1 becomes a smaller value than FMAIN, and the value approaches FMAIN as the purification capacity of the first catalyst 17 decreases.

FIG. 6 shows the first to third Ox sensors 19-2.
The output waveform of 1 is shown. FIG. 1A shows the first Ox sensor 1
When the output is 9, the output fluctuation of the second Ox sensor 20 is smoothed as shown in FIG. 7B before the deterioration in which the purification capacity of the first catalyst 17 is high.

When the purification performance deteriorates due to the use of the first catalyst 17, the amount of oxygen exchanged between the exhaust gas and the catalyst decreases, and the second Ox sensor as shown in FIG. The output fluctuation frequency of 20 becomes high and approaches the waveform shown in FIG.

As described above, the sub feedback control is performed to correct the bias due to the main feedback control. That is, the sub-Ox sensor detects a deviation of the output of the main Ox sensor that fluctuates little by little, and the output fluctuation must be moderate to some extent as compared with the output fluctuation of the main Ox sensor.

On the other hand, FIGS. 6D and 6E show the third O
The output waveforms of the x sensor 21 show waveforms when the catalyst is new and when it is deteriorated. The third Ox sensor 21 is
The oxygen concentration in the exhaust gas purified by the first and second Ox sensors 20 and 21 is detected. Therefore, as shown in (D) of the same figure, the output of the third Ox sensor 21 when the catalyst is new and the purification capacity is high compared to the fluctuation of the oxygen concentration in the exhaust gas before purification shown in (A) of the figure. Shows extremely gradual fluctuations.

If the sub-feedback control is performed with the output signal exhibiting such a gentle fluctuation, the sub-Ox
The output fluctuation of the sensor is too gentle, and the feedback constants diverge alternately to the rich side or the lean side. That is, by performing the sub-feedback control, the air-fuel ratio will be biased to one side.

By the way, as shown in FIG. 6E, when the purifying ability of the catalyst is deteriorated, the fluctuation frequency of the third Ox sensor 21 is high as in the case of the second Ox sensor 20. Become. Therefore, the fluctuating frequency approaches the output fluctuating frequency of the first Ox sensor 19, and the sub-feedback control can be performed by the output of the third Ox sensor 21.

Therefore, in the apparatus of this embodiment, the second Ox sensor 20 is used as a sub-Ox sensor until the catalyst deteriorates, and the third Ox sensor 21 is used as a sub-Ox sensor when the catalyst deteriorates. Further, in the apparatus of this embodiment, the method of monitoring the change in the output fluctuation frequency of the Ox sensor is used as the catalyst deterioration detecting means.

That is, in FIG. 5, step 30
The fluctuation frequency FMA of the first Ox sensor 19 is 1, 302.
IN and the fluctuation frequency FSUB1 of the second Ox sensor 20
After storing, the ratio of these values (FMAIN / FS
UB1) is calculated, and it is determined whether the ratio is smaller than a predetermined threshold value A (step 303). The threshold value A is a value that is obtained as an inflection point where good control accuracy is obtained when the third Ox sensor 21 is used as the sub Ox sensor rather than the second Ox sensor 20.

In step 303, the FSUB1 is still
Is determined to be sufficiently gentle and FMAIN / FSUB1 ≧ A is satisfied, it is determined that the first catalyst 17 has not deteriorated yet (step 304), and the second Ox sensor 20 performs the sub feedback control. Continuing (step 305), the process ends.

In step 303, FMAIN / F
When it is determined that SUB1 <A, it is determined that the first catalyst has deteriorated (step 306). in this case,
The use of the second Ox sensor 20 as the sub-Ox sensor is stopped, the sub-feedback control is performed by the third Ox sensor 21 thereafter (step 307), and the process ends.

As described above, according to the apparatus of this embodiment,
The deterioration of the first catalyst 17 can be detected, and the second Ox sensor 20 and the third Ox sensor 21 can be switched and used as the sub-Ox sensor. Therefore, the sub feedback control can be performed in addition to the main feedback control regardless of the deterioration of the first and second catalysts 17 and 18. Therefore, it is possible to maintain a good purifying property for a long term as compared with the conventional device.

FIG. 7 is a flowchart of another example of the subroutine relating to the Ox sensor selection processing executed by the apparatus of this embodiment.

In the figure, steps 401 to 407 have the same processing contents as steps 301 to 307 of FIG. That is, the routine of this embodiment is performed in steps 408 and 409.
The routine is the same as that of the above-mentioned embodiment except that is inserted.

Therefore, when this routine is started, first, the output from the first and second Ox sensors 19 and 20 is changed to F.
Calculate MAIN and FSUB1 (steps 401, 4
02), and then the deterioration state of the first catalyst 17 is judged from the ratio (step 403), and better control accuracy of the first and second Ox sensors will be obtained.
The x sensor is a sub Ox sensor (steps 404 to 4).
07).

Further, in the routine of this embodiment, when FMAIN / FSUB <A in step 403, in order to judge how much the deterioration is, the value B smaller than A, the first and Second Ox sensor 19, 20
A comparison is made with the fluctuating frequency ratio FMAIN / FSUB1 (step 408).

The current threshold value B is FM in the normal state.
AIN / FSUB1 <B is set to a value that cannot occur. Therefore, if in step 408 FMAIN / FSU
When it is determined that B1 <B is established, it can be estimated that the first catalyst 17 has an abnormality such as oil adhesion. In this case, it is conceivable that an abnormality such as oil adhesion has also occurred in the first Ox sensor 19 and an abnormality signal is being output.

Therefore, in the routine of the present embodiment, in such a case, the main feedback control is thereafter performed by the second Ox sensor 20 (step 409), and the process ends.

That is, in the routine of this embodiment, as shown in the table of FIG. 8, FMAIN / FSUB1
When ≧ A, it is determined that the first catalyst 17 is in a new state, the first Ox sensor 19 is the main Ox sensor, the second Ox sensor 20 is the sub Ox sensor, and B <FMAIN.
When / FSUB1 <A, it is determined that the first catalyst 17 is in a deteriorated state, and the first Ox sensor 19 is set to the main O
The x sensor and the third Ox sensor 21 are sub-Ox sensors, and when FMAIN / FSUB1 <B, the first Ox sensor is used.
It is estimated that the x sensor 19 may be abnormal, and air-fuel ratio feedback control is performed using the second Ox sensor 20 as the main Ox sensor and the third Ox sensor 21 as the sub Ox sensor.

Therefore, according to the routine of this embodiment,
Even if an abnormality occurs in the first Ox sensor 19 which is the most harsh environment, the second and third Ox sensors 20 and 21 can continue the complete air-fuel ratio feedback control. Therefore, it becomes possible to maintain the purifying property for a remarkably long period as compared with the conventional device.

[0079]

As described above, according to the present invention, when a plurality of catalyst devices provided in the exhaust passage of an internal combustion engine are sequentially deteriorated from the upstream side as they are used, the catalyst device is always undeteriorated. Among them, the catalyst device located in the uppermost stream can be detected, and the air-fuel ratio control can be performed based on a set of air-fuel ratio sensors that monitor the exhaust gas in the space before and after the catalyst device.

Therefore, highly accurate air-fuel ratio control based on the two air-fuel ratio sensors can be performed from the time when all the catalyst devices are new to the time when the most downstream catalyst device deteriorates. For this reason, it is possible to maintain good purifying properties for a significantly long period as compared with the conventional device.

[Brief description of drawings]

FIG. 1 is a principle diagram of an air-fuel ratio control device for an internal combustion engine according to the present invention.

FIG. 2 is a configuration diagram of an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention.

FIG. 3 is a flowchart of an example of a main feedback control routine used in the apparatus of this embodiment.

FIG. 4 is a flowchart of an example of a sub-feedback control routine used in the device of this embodiment.

FIG. 5 is a flowchart of an example of a catalyst deterioration detection routine used in the apparatus of this embodiment.

FIG. 6 is a diagram showing an output waveform of an Ox sensor of the apparatus of this embodiment.

FIG. 7 is a flowchart of another example of a catalyst deterioration detection routine used in the apparatus of this embodiment.

FIG. 8 is a diagram showing an operating state of each Ox sensor when another example of the catalyst deterioration detection routine used in the apparatus of this embodiment is used.

FIG. 9 is a configuration diagram of a conventional air-fuel ratio control device for an internal combustion engine.

[Explanation of symbols]

 11, 31 Internal combustion engine 16, 32 Exhaust passage 17 First catalyst 18 Second catalyst 19 First Ox sensor 20 Second Ox sensor 21 Third Ox sensor 22 Control device 33a, 33b, 33c ... Catalyst device 34a, 34b, 34c ... Air-fuel ratio sensor 35 Deterioration detection means 36 Control means

Claims (1)

[Claims] 1. A plurality of catalytic devices arranged in series in an exhaust passage of an internal combustion engine at intervals in a flow direction of the exhaust gas to purify the exhaust gas, and upstream and downstream of each catalytic device. A plurality of air-fuel ratio sensors arranged downstream, a deterioration detection unit that detects deterioration of each of the plurality of catalyst devices, and a catalyst device that is estimated to be undegraded by the deterioration detection unit, located at the uppermost position. A control device for controlling the air-fuel ratio of the internal combustion engine based on the outputs of the air-fuel ratio sensors arranged upstream and immediately downstream of the catalyst device.