JPH06229291A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To rationalize sub-air-fuel ratio control at proper time, optimize air-fuel ratio control, and prevent deterioration of emission and drivability by stopping the sub air-fuel ratio control based on a downstream side concentration sensor of a catalyst converter until specific time when operation is transferred to feedback control from air-fuel ratio leaning control. CONSTITUTION:Both upstream and downstream side concentration sensors M4 and M5 to detect exhaust air concentration are arranged respectively on both upper and lower sides of a catalyst converter M3 installed in an exhaust air passage M2 of an internal combustion engine M1. An air-fuel ratio of the internal combustion engine M1 is controlled in a target air-fuel ratio by a control means M6 according to respective control results. In this case, the air-fuel ratio of the internal combustion engine M1 is leaned by a leaning means M7, and operations of the first and the second respective control parts M6a and M6b in the control means M6 are stopped at this leaning time by a stopping means M8. After it is leaned, a detecting means M9 detects time when a detecting value of the downstream side concentration sensor M5 exceeds a prescribed value. Operation of the second control part M6b is stopped by a stopping means M10 until rise time after it is leaned further.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention includes an air-fuel ratio sensor provided upstream and downstream of a catalytic converter, and an air-fuel ratio of an internal combustion engine for feedback control of the air-fuel ratio to a target air-fuel ratio based on output signals of both air-fuel ratio sensors. The present invention relates to a fuel ratio control device.

[0002]

2. Description of the Related Art Conventionally, an upstream O ₂ sensor provided upstream of a catalytic converter and a downstream O ₂ sensor provided downstream of a catalytic converter have been provided for the purpose of improving control accuracy of an air-fuel ratio. A double O ₂ sensor air-fuel ratio control system has been proposed which performs air-fuel ratio feedback control based on both output signals.

[0003] The double O ₂ sensor air-fuel ratio control system, specifically, and executes the air-fuel ratio feedback control based on the output signal of the upstream O ₂ sensor, during this execution, the output of the upstream O ₂ sensor The control constant of the air-fuel ratio correction coefficient FAF based on the signal, such as the skip amount RSR in the rich direction and the skip amount RSL in the lean direction, is variably controlled based on the output signal of the downstream O ₂ sensor.

By the way, such a double O ₂ sensor air-fuel ratio control system stops the air-fuel ratio feedback control based on the output signal of the upstream O ₂ sensor during fuel cut, deceleration or secondary air supply, Fuel ratio correction factor F
While the AF is fixed, the variable skip amount control based on the output signal of the downstream O ₂ sensor is stopped to fix the skip amount. Also, when the lean control such as the fuel cut and the secondary air supply described above is completed,
The air-fuel ratio feedback control based on the output signal of the upstream O ₂ sensor is immediately restarted, while the variable control of the skip amount based on the output of the downstream O ₂ sensor is restarted after a predetermined time delay. (JP-A-64-36
943).

The reason why the restart of the variable control of the skip amount based on the output signal of the downstream O ₂ sensor is delayed by a predetermined time is as follows. When the lean control is performed, a large amount of O ₂ is adsorbed in the catalytic converter due to the so-called O ₂ storage effect. When the variable control based on the output signal of the downstream O ₂ sensor is executed at the same time as the end of lean control, a large amount of O ₂ adsorbed in the catalytic converter is discharged into the exhaust passage, and the output signal of the downstream O ₂ sensor is The lean side is shown rather than the actual air-fuel ratio, and the air-fuel ratio is overcorrected to the rich side. Therefore, after the lean control end, by a predetermined time delay, waiting for the reduction of O ₂ storage amount of the catalytic converter, then the downstream O ₂
By restarting the variable control based on the output signal of the sensor, overcorrection of the air-fuel ratio to the rich side is eliminated.

Further, the delay time is set to the rich side when the output signal of the downstream side O ₂ sensor is set so that the decrease time of the O ₂ storage amount of the catalytic converter is more accurately determined and the air-fuel ratio control is performed with higher accuracy. There has been proposed a configuration up to the inversion (Japanese Patent Laid-Open No. 4-101038).

[0007]

By the way, the O ₂ sensor may output an output deviated to the lean side from the actual air-fuel ratio due to deterioration of the detection part, manufacturing variations, and the like.
When such lean deviation occurs in the upstream O ₂ sensor,
The above-mentioned conventional technique has the following problems.

[0008] occurs lean side to the upstream side O ₂ sensor, when the variable control based on the output signal of the downstream O ₂ sensor is insufficient, the lean control of the fuel cut or the like is performed, the downstream O _{2 The} sensor output signal sticks to the leanest side. After that, when the lean control is released, the upstream side O
By the air-fuel ratio feedback control based on the output signal of the ₂ sensor, the air-fuel ratio shifts to the stoichiometric side, and the output signal of the O ₂ sensor also gradually shifts to the stoichiometric side, but the upstream O
_{Since the two} sensors have a lean shift, the air-fuel ratio is only adjusted on the lean side, and the output signal of the downstream O ₂ sensor does not shift to the rich side. Therefore, the air-fuel ratio control based on the output signal of the downstream O ₂ sensor was not restarted, and the air-fuel ratio could not be properly adjusted. That is, the air-fuel ratio may not be properly adjusted after the end of lean control such as fuel cut, resulting in problems of emission deterioration and drivability deterioration.

The air-fuel ratio control system for an internal combustion engine according to the present invention has been made in view of these problems. It appropriately adjusts the air-fuel ratio at the time of shifting from lean control such as fuel cut to air-fuel ratio feedback control. Therefore, it is intended to prevent deterioration of emission, deterioration of drivability, and the like.

[0010]

In order to achieve such an object, the following constitution was adopted as a means for solving the above problems.

That is, the air-fuel ratio control system for an internal combustion engine of the present invention, as illustrated in FIG. 1, has an exhaust passage M of an internal combustion engine M1.
2, a catalytic converter M3 provided on the upstream side of the catalytic converter M3, an upstream side concentration sensor M4 provided on the upstream side of the catalytic converter M3, which outputs a signal according to the concentration of the specific component that changes according to the air-fuel ratio reflected in the exhaust gas, and the catalyst. The downstream side concentration sensor M5, which is provided on the downstream side of the converter M3 and outputs a signal according to the concentration of the specific component that changes according to the air-fuel ratio reflected in the exhaust gas, and the signal output from the upstream side concentration sensor M4. A second controller M6a for adjusting the air-fuel ratio of the internal combustion engine M1 and a second controller for adjusting the air-fuel ratio of the internal combustion engine M1 based on signals output from the downstream concentration sensor M5.
An air-fuel ratio control apparatus for an internal combustion engine, comprising: a control unit M6b; and a control unit M6 that controls the air-fuel ratio of the internal combustion engine M1 to a stoichiometric air-fuel ratio by the operation of both control units M6a and M6b. Is in a predetermined operating state, the leaning means M7 for forcibly adjusting the mixture ratio of the fuel and air supplied to the internal combustion engine M1 to control the air-fuel ratio of the internal combustion engine M1 to the lean state. While the air-fuel ratio control is being performed by the leaning means M7, the first control unit M
6a and the leaning stop means M8 for stopping the operations of the second control section M6b, and after the end of the air-fuel ratio control by the leaning means M7, the signal output from the downstream side concentration sensor M5 becomes the theoretical air-fuel ratio. Signal rising detection means M9 for detecting a time when a predetermined level lower than the corresponding level is exceeded.
And a post-lean stop means M10 for stopping the operation of the second control unit after the end of the air-fuel ratio control by the lean means M7 until the timing is detected by the signal rise detection means M9. That is the summary.

[0012]

The air-fuel ratio control system for an internal combustion engine according to the present invention constructed as described above operates as a first controller M6a based on the upstream concentration sensor M4 and a second controller M6b based on the downstream concentration sensor M5. In this way, the control means M6 controls the air-fuel ratio of the internal combustion engine M1 to the stoichiometric air-fuel ratio.
When the internal combustion engine M1 is in a predetermined operating state, the leaning means M7 forcibly adjusts the mixing ratio of the fuel and air supplied to the internal combustion engine M1 to set the air-fuel ratio of the internal combustion engine M1 to the lean state. During the control, the lean stop means M8 stops the operation of the first control unit M6a and the second control unit M6b during the control. After the end of the air-fuel ratio control by the leaning means M7, the signal rising detection means M9 detects the time when the signal output from the downstream side concentration sensor M5 exceeds a predetermined level lower than the level corresponding to the theoretical air-fuel ratio. Then, after the end of the air-fuel ratio control by the leaning means M7, until the time is detected, the operation after leaning stop means M10 stops the operation of the second controller M6b.

In this way, the second control unit M6b continues to stop even after the end of the air-fuel ratio control by the leaning means M7, and the signal output from the downstream side concentration sensor M5 is lower than the level corresponding to the theoretical air-fuel ratio. When it exceeds the level, the stop is released and the air-fuel ratio adjustment by the second controller M6b is restarted. Therefore, as compared with the conventional configuration in which the restart timing of the second control unit M6b is determined when the output signal of the downstream side concentration sensor M5 is determined to be on the rich side, the second control unit M6b has a lower predetermined level on the lean side. Can be restarted.
Therefore, even if the upstream side concentration sensor M4 has a lean shift and the output signal of the downstream side concentration sensor M5 becomes low, the possibility of restarting the second control unit M6b is increased.

By the way, the output signal of the downstream side concentration sensor M5 sticks to the lean side at the time of leaning, but after leaning, it gradually shifts to the stoichiometric air-fuel ratio side by the main air-fuel ratio feedback control. At the time of this transition, there is a time point when the O ₂ adsorbed during lean conversion is discharged from the catalytic converter M3 and the O ₂ storage amount becomes almost zero. At this time point, the output signal of the downstream side concentration sensor M5 is detected. Appears as an inflection point. Therefore, by presetting a predetermined level that is a comparison value for the determination by the signal rise detection means M9 as the level of the inflection point, the time when the O ₂ storage amount of the catalytic converter M3 after leaning becomes almost zero. Can be accurately determined. As a result, the air-fuel ratio is not overcorrected to the rich side when the second control unit M6b is restarted.

[0015]

Preferred embodiments of the present invention will be described below in order to further clarify the structure and operation of the present invention described above. FIG. 2 is a schematic configuration diagram showing an automobile engine equipped with an air-fuel ratio control device according to an embodiment of the present invention and peripheral devices thereof.

As shown in the figure, in the intake passage 2 of the engine 1, from the intake air intake port to the air cleaner 3,
Throttle valve 5, surge tank 6 that suppresses pulsation of intake air, and fuel injection valve 7 that supplies fuel to engine 1.
Is provided. The intake air taken in through the intake passage 2 is mixed with the fuel injected from the fuel injection valve 7 and taken into the combustion chamber 11 of the engine 1. This fuel-air mixture is spark-ignited by the spark plug 12 in the combustion chamber 11 to drive the engine 1. The gas (exhaust gas) burned in the combustion chamber 11 is guided to the catalytic converter 16 through the exhaust passage 15, is purified, and is then discharged to the atmosphere side.

A high voltage from an igniter 22 is applied to the spark plug 12 via a distributor 21,
The ignition timing is determined by this application timing. The distributor 21 is for distributing the high voltage generated by the igniter 22 to the ignition plugs 12 of the respective cylinders.
A rotation speed sensor 23 that outputs four pulse signals is provided.

Further, the engine 1 has a rotation speed sensor 23 as a sensor for detecting its operating state, and an idle switch for detecting the opening degree of the throttle valve 5 and the fully closed state of the throttle valve 5. 5
0 (Fig. 3) built-in throttle position sensor 5
1, an intake air temperature sensor 52 arranged in the intake passage 2 for detecting the temperature of intake air (intake air), an air flow meter 53 for detecting the amount of intake air, and a water temperature sensor 54 for detecting the cooling water temperature arranged in the cylinder block. An upstream O ₂ sensor 55 arranged in the exhaust passage 15 upstream of the catalytic converter 16 to detect the oxygen concentration in the exhaust; and an oxygen O _{2 in} the exhaust passage 15 arranged downstream of the catalytic converter 16 in the exhaust passage 15. A downstream O ₂ sensor 56 that detects the concentration, a vehicle speed sensor 57 that detects the vehicle speed V, and the like are provided.

The detection signals of the above-mentioned sensors are input to an electronic control unit (hereinafter referred to as ECU) 70. Figure 3
As shown in FIG. 2, the ECU 70 is configured as a logical operation circuit centered on a microcomputer, and more specifically, CPUs 70a and CPs that execute various arithmetic processes for controlling the engine 1 according to a preset control program.
ROM 70b in which control programs and control data necessary for executing various arithmetic processes in U70a are stored in advance,
Similarly, a RAM 70c in which various data necessary for executing various arithmetic processes in the CPU 70a is temporarily read and written,
Backup RAM 70d capable of holding data even when power is off, A / D converter 70e for inputting detection signals from the above-mentioned sensors, and input processing circuits 70f, C
An output processing circuit 70 that outputs a drive signal to the fuel injection valve 7, the igniter 22 and the like according to the calculation result of the PU 70a.
g and so on. Further, the ECU 70 uses the battery 71
A power supply circuit 70h connected to the output processing circuit 70
It is also possible to apply a high voltage from g.

With the ECU 70 thus constructed,
The fuel injection valve 7 and the igniter 22 are drive-controlled according to the operating state of the engine 1, and fuel injection control, ignition timing control, air-fuel ratio control, etc. are performed.

Next, the fuel injection control processing routine executed by the CPU 70a of the ECU 70 will be described with reference to FIG. It should be noted that this control processing routine is executed every predetermined crank angle, for example, every 360 ° CA.

When the processing is started, the CPU 70a first detects the A / D converter 7 by the air flow meter 53.
The intake air amount Q A / D converted in 0e is stored in the RAM 70c.
The process of reading from is executed (step S100). Then, the input processing circuit 70 is detected by the rotation speed sensor 23.
A process of reading the rotation speed Ne input via f from the RAM 70c is executed (step S110).

Then, using the intake air amount Q and the rotation speed Ne read in steps S100 and 110,
The basic fuel injection amount TP is calculated according to the following equation (1) (step S120). TP ← k · Q / Ne (where k is a constant) (1)

Then, the actual fuel injection amount TAU is calculated by multiplying the basic fuel injection amount TP by various correction coefficients so as to comply with the following equation (2) (step S130). TAU ← TP / FAF / FWL / FFC / a / b ... (2)

FAF is an air-fuel ratio correction coefficient,
It is calculated by a main air-fuel ratio feedback control processing routine described later. FWL is a warm-up increase correction coefficient,
While the cooling water temperature THW is 60 ° C. or less, it takes a value of 1.0 or more. FFC is a correction coefficient at the time of fuel cut, and normally has a value of 1 and has a value of 0 at the time of fuel cut. a and b are other correction coefficients, for example, correction coefficients relating to intake air temperature correction, transient correction, power supply voltage correction, and the like.

When the actual fuel injection amount TAU is calculated in step S130, subsequently, the fuel injection time corresponding to the actual fuel injection amount TAU is set in a counter (not shown) that determines the valve opening time of the fuel injection valve 7 ( Step S14
0). As a result, the fuel injection valve 7 is driven to open for the valve opening time set in the counter. After that, the process returns to "return" and the process is terminated.

Next, a fuel cut processing routine executed by the CPU 70a of the ECU 70 will be described with reference to FIG. This fuel cut processing routine calculates the fuel cut flag XFC, and is executed at predetermined time intervals by interruption. When the processing is started, the CPU 70a first outputs the output signal LL of the idle switch 50 incorporated in the throttle position sensor 51 to the RAM 70.
The process of reading from c is executed (step S200).
Then, whether or not the output signal LL has the value 1, that is,
It is determined whether or not it is in an idle state (step S21
0).

If it is determined in step S210 that the vehicle is in the idle state, first, step S11 is performed every 360 ° CA.
It is determined whether the latest rotation speed Ne read at 0 is equal to or higher than the fuel cut rotation speed NC (step S22).
0). Here, if it is determined that Ne is NC or more,
Fuel cut flag XF, indicating that fuel cut is necessary
The value 1 is set in C (step S230). afterwards,
Exit to "Return" to end the process.

On the other hand, if it is determined in step S220 that Ne is smaller than NC, then it is determined whether the rotational speed Ne is equal to or lower than the return rotational speed NR (<NC) (step S240). Here, if it is determined that Ne is equal to or less than NR, then it is determined whether the fuel cut flag XFC has already become 1 (step S25).
0). If it is determined that the flag XFC has a value of 1, the flag XFC is set to a value of 0 in order to stop the fuel cut (step S260), and then the sub air-fuel ratio feedback control described later is prohibited. A value 1 is set to the prohibition flag XSB shown (step S270). After that, the process ends.

On the other hand, if it is determined in step S240 that the rotation speed Ne is higher than the return rotation speed NR, or if it is determined in step S250 that the fuel cut flag XFC is not the value 1, the processing is immediately terminated. .
If it is determined in step S210 that the output signal LL of the idle switch 80 is not the value 1, the process proceeds to step S250.

According to the fuel cut processing routine having such a structure, when the rotation speed Ne exceeds the fuel cut rotation speed NC in the idle state, the value 1 is set in the fuel cut flag XFC, as shown in FIG. In response to the setting of this value 1, the value 0 is set in the fuel cut correction coefficient in step S130 of the fuel injection control processing routine of FIG. 4, the actual fuel injection amount TAU becomes 0, and fuel cut is performed. Then, the rotation speed Ne decreases and becomes lower than the return rotation speed NR, which is lower than the fuel cut rotation speed NC. Here, the value 0 is set to the fuel cut flag XFC to stop the fuel cut. In response to the setting of the value 0, the value 1 is set in the fuel cut correction coefficient in step S130, and the fuel injection is restarted. When the fuel injection is restarted, the prohibition flag XSB related to the sub air-fuel ratio feedback control is set to the value 1.

Next, a main air-fuel ratio feedback (hereinafter, feedback will be referred to as F / B) control processing routine executed by the CPU 70a of the ECU 70 will be described with reference to FIG. This main air-fuel ratio F / B control processing routine is performed by the output voltage MO _{2 of the} upstream O ₂ sensor 55.
The air-fuel ratio is feedback-controlled based on the above, and is executed by a predetermined time, for example, every 4 msec by interruption.

When the processing is started, the CPU 70a first determines whether or not the F / B condition of the air-fuel ratio is satisfied (steps S300 to S304). Specifically, it is determined in step S30 whether or not the engine is starting at step S300.
In step S304, it is determined whether or not the engine has been warmed up, and in step S304, whether or not the fuel is being cut. The determination at the time of starting is determined based on whether or not the starter is rotating, and the determination after warm-up is determined based on whether or not the cooling water temperature THW detected by the water temperature sensor 54 exceeds 30 to 60 [° C]. The determination during fuel cut is made based on whether the fuel cut flag XFC set in the fuel cut processing routine of FIG. In addition, as the F / B condition of the air-fuel ratio, other than OTP
The configuration may be such that conditions such as that the amount is not being increased and that the upstream O ₂ sensor 55 is not broken down are added.

By steps S300 to S304,
The F / B condition is satisfied when it is determined that the fuel is not being cut after the warm-up, not at the time of starting, and in other cases, the F / B condition is not satisfied. When it is determined that the F / B condition is not satisfied, the processing of this routine is once ended without executing the air-fuel ratio F / B control. On the other hand, if it is determined that the F / B condition is satisfied, the following process is performed.

First, the output voltage MO ₂ of the upstream O ₂ sensor 55 input through the input processing circuit 70f is transferred to the RAM 7.
0c is read (step S310),
It is determined from the output voltage MO ₂ whether or not the air-fuel ratio is in the rich state (step S320). In this embodiment, when the output voltage MO ₂ is larger than the threshold level of 0.45 [V], it is determined that the air-fuel ratio is in the rich state.

When it is determined in step S320 that the air-fuel ratio is in the rich state, it is then determined whether or not the rich state is the first rich state after the lean state has transitioned, that is, whether or not the lean-to-rich reversal has occurred. It is determined whether or not (step S330). When it is determined in step S330 that the time of reversal to rich is determined, the skip amount RSL in the lean direction (RSL> 0) is subtracted from the air-fuel ratio correction coefficient FAF (step S340). If it is determined that the integration amount KIL (K
IL> 0) is subtracted (step S350). The skip amount RSL is set sufficiently larger than the integration amount KIL.

If it is determined in step S320 that the air-fuel ratio is not in the rich state but in the lean state, then it is determined whether or not the lean state is the first lean state after the transition from the rich state, that is, inversion from rich to lean. It is determined whether it is time (step S360). If it is determined in step S360 that the engine is leaning, the air-fuel ratio correction factor FA
The skip amount RSR (RSR> 0) in the rich direction is added to F (step S370). On the other hand, if it is determined that it is not the time of reversal to lean, the integration amount KIR (KIR> 0) is added to the air-fuel ratio correction coefficient FAF. Is added (step S38
0). The skip amount RSR is set sufficiently larger than the integration amount KIR.

Air-fuel ratio correction coefficient FAF calculated in step S340, S350, S370 or S380
Is stored in the RAM 70c (step S390), and then the processing of this routine is once ended.

The control shown in steps S350 and S380 is called integral control, and is executed in step S340.
The control shown in S370 and S370 is called skip control. By both controls, the air-fuel ratio will be balanced before and after the stoichiometric air-fuel ratio. Specifically, as shown in FIG. 8, at time t1, the output voltage MO of the upstream O ₂ sensor 55 is increased.
_{When 2} becomes 0.45 [V] or more, that is, in the rich state,
The CPU 70a receiving this signal drops the air-fuel ratio correction coefficient FAF by RSL in a stepwise manner, and thereafter gradually lowers it by the amount indicated by the integrated amount KIL. As a result, since the fuel injection amount TAU is reduced, the air-fuel ratio becomes thinner than the theoretical air-fuel ratio, the output voltage MO _{2 of the} upstream O ₂ sensor 55 drops, and the output voltage MO ₂ becomes 0.45 [V].
It becomes smaller (time t2).

Output voltage MO ₂ smaller than 0.45 [V]
The CPU 70a that has received the value jumps up the air-fuel ratio correction coefficient FAF stepwise by RSR, and thereafter, the integrated amount KI
Gradually increase by the size indicated by R. as a result,
The fuel injection amount TAU increases, the air-fuel ratio eventually becomes richer than the stoichiometric air-fuel ratio, and the output voltage MO _{2 of the} upstream O ₂ sensor 55 jumps up (time t3). By repeating such processing, the negative feedback control is constantly applied to the air-fuel ratio, and the air-fuel ratio is balanced before and after the stoichiometric air-fuel ratio.

Next, the sub air-fuel ratio feedback control processing routine executed by the CPU 70a of the ECU 70 will be described with reference to FIG. This sub air-fuel ratio F /
The B control processing routine feedback-controls the air-fuel ratio based on the output voltage SO ₂ of the downstream O ₂ sensor 56. Specifically, the skip amounts RSR and RSL calculated in the main air-fuel ratio F / B control processing routine are used. By correcting based on the output voltage SO ₂ of the downstream O ₂ sensor 56, feedback control of the air-fuel ratio is indirectly performed using the main air-fuel ratio F / B control. This control processing routine is executed by interruption every predetermined time, for example, every 512 msec, which is much longer than the execution interval of the main air-fuel ratio F / B control processing routine.

When the processing is started, the CPU 70a determines whether or not the main air-fuel ratio F / B control processing by the main air-fuel ratio F / B control processing routine described above is being executed (step S400). If it is determined that the fuel cut is being executed, then it is determined whether the fuel cut flag XFC is set to the value 1, that is, whether the fuel cut is being performed (step S410). Step S410
If it is determined that the fuel cut is not in progress, then step S42.
Go to 0. In step S400, the main air-fuel ratio F /
When it is determined that the B control is not in progress, or step S4
When it is determined in 10 that the fuel is being cut, the process returns to "return" and ends the process once. In addition to the determinations in steps S400 and S410, the cooling water temperature TH
A configuration may be added in which determination that W has exceeded 60 to 80 [° C.] and has been completely warmed up, that the downstream O ₂ sensor 56 is not defective, and the like is added.

In step S420, the input processing circuit 70
The output voltage SO ₂ of the downstream O ₂ sensor 56 input via f is read from the RAM 70c. Subsequently, it is determined whether the prohibition flag XSB set in the fuel cut processing routine of FIG. 5 is 1 (step S
430), if it is determined that the value is 1, it is determined whether the output voltage SO ₂ read in at step S420 exceeds a predetermined voltage level α that is set in advance (step S440). This determination is based on the prohibition flag XSB.
After the end of fuel cut when the value is set to the value 1, the output voltage SO ₂ stuck to the lean side is the predetermined voltage level α.
It is to determine the time when the value exceeds. Predetermined voltage level α
Is smaller than the threshold level corresponding to the theoretical air-fuel ratio, and corresponds to the level of the inflection point at which the output voltage SO ₂ rising from the lean side after fuel cut changes most rapidly.

The inflection point of the output voltage SO ₂ is a point at which the O ₂ adsorbed when the fuel is cut off in the catalytic converter M3 is discharged and the O ₂ storage amount becomes almost zero. The value of the predetermined voltage level α is determined so that the inflection point or its vicinity becomes the predetermined voltage level α. The predetermined voltage level α is 0.1 to 0.4 [V] when the threshold level of the downstream O ₂ sensor 56 is 0.45 [V].
Can be set to 0.25 [V] in this embodiment.

When it is determined in step S440 that the output voltage SO ₂ does not exceed the predetermined voltage level α, the output voltage SO ₂ stuck to the lean side has not reached the predetermined voltage level α after the fuel cut. As a result, the processing of this routine is once ended without executing the sub air-fuel ratio F / B control by the processing of the subsequent steps. On the other hand, step S440
When it is determined that the output voltage SO ₂ is higher than the predetermined voltage level α, the prohibition flag XSB is cleared to the value 0 (step S450), and the sub air-fuel ratio F / B control by the processing of the subsequent steps is performed. Run.

First, it is determined from the output voltage SO ₂ read in step S420 whether the air-fuel ratio is in a rich state (step S460). In this embodiment, the output voltage SO
_{When 2} is larger than the threshold level of 0.45 [V], it is determined that the air-fuel ratio is in the rich state. Here, if it is determined that the air-fuel ratio is in the rich state, then it is determined whether or not the rich state is the first rich state that has transitioned from the lean state, that is, whether or not it is the time of reversal from lean to rich. (Step S470). If it is determined in step S470 that the time is reversal to rich, a predetermined skip amount RSrsrL (RSrs is calculated from the rich skip amount RSR obtained in the main air-fuel ratio F / B control processing routine of FIG.
rL> 0) is subtracted (step S480), and when it is determined that it is not the time of reversal to rich, a predetermined century amount KIrsrL (KIrs) is determined from the skip amount RSR in the rich direction.
rL> 0) is subtracted (step S490). The skip amount RSrsrL is set sufficiently larger than the integration amount KIrsrL.

On the other hand, if it is determined in step S460 that the air-fuel ratio is not rich but lean.
It is determined whether or not the lean state is the first lean state after the transition from the rich state, that is, whether or not at the time of reversal from rich to lean (step S500). When it is determined in step S500 that the engine is leaning to the lean side, a predetermined skip amount RSrsrR (RSrsrR) is added to the skip amount RSR in the rich direction obtained in the main air-fuel ratio F / B control processing routine.
> 0) is added (step S510). On the other hand, if it is determined that it is not the time of lean reversal, the predetermined amount of integration KIrsrR (KIrsrR> is added to the skip amount RSR in the rich direction.
0) is added (step S520). The skip amount RSrsrR is set sufficiently larger than the integration amount KIrsrR.

FIG. 10 shows how the skip amount RSR in the rich direction changes by the skip control in steps S480 and S510 and the integration control in steps S490 and S520. As shown in FIG. 10, the output voltage S of the downstream O ₂ sensor 56 at time t4
When O ₂ is 0.45 [V] or more, that is, in the rich state, the CPU 70a receiving this signal drops the skip amount RSR in the rich direction by RSrsrL in a stepwise manner, and then the magnitude indicated by the integral amount KIrsrL. Gradually decrease gradually. The air-fuel ratio eventually becomes thinner than the theoretical air-fuel ratio, and the output voltage SO _{2 of the} downstream O ₂ sensor 56 becomes 0.4.
When it becomes smaller than 5 [V] (time t5), the CPU 70a, which receives this signal, skips R in the rich direction.
Raise SR in steps by RSrsrR, then
Gradually increase by the amount indicated by the integrated amount KIrsrR (time t6). By repeating such processing, the skip amount in the rich direction is balanced so that the output signal SO ₂ of the downstream O ₂ sensor 56 is at the threshold level.

When the skip amount RSR in the rich direction is updated in step S480, S490, S510 or S520, subsequently, the skip amount RS in the rich direction is updated.
The sum of R and the skip amount RSL in the lean direction is a predetermined amount β
Therefore, RSL ← β-RSR is calculated to obtain the lean skip amount RSL (step S530). After that, the process returns to "return" and the process is terminated.

By repeating the sub air-fuel ratio F / B control processing routine thus constructed, the skip amount RSL in the rich direction is updated. Hereinafter, how the skip amount RSR in the rich direction and other variables change with time will be described based on the timing chart of FIG. 11.

As shown in FIG. 11, when the value 1 is set to the fuel cut flag XFC by the fuel cut processing routine (time t11), the fuel cut is started, and thereafter,
When the fuel cut flag XFC is cleared to 0 (time t12), the fuel cut ends. When the fuel cut is released, a value 1 is set in the prohibition flag XSB indicating that the sub air-fuel ratio feedback control is prohibited. During the fuel cut from time t11 to t12, neither the main air-fuel ratio feedback control nor the sub air-fuel ratio feedback control is performed, and the output signal of the downstream O ₂ sensor 56 sticks to the lean side most.

After the fuel cut is released, the air-fuel ratio shifts to the stoichiometric side by the main air-fuel ratio feedback control, and the output voltage SO _{2 of the} downstream O ₂ sensor 56 also gradually shifts to the stoichiometric side. At this time, the prohibition flag XSB has a value of 1, and the sub air-fuel ratio feedback control is not being performed. After that, when the output voltage SO ₂ exceeds the predetermined voltage level α (<0.45 [V]) set in advance (time t13), the prohibition flag XSB is cleared to the value 0, and the downstream O ₂ sensor 56 is detected. The sub air-fuel ratio feedback control based on the output voltage SO ₂ is restarted.

As described above, in this embodiment, both the main air-fuel ratio feedback control and the sub air-fuel ratio feedback control are stopped while the fuel cut is being executed, and only the main air-fuel ratio feedback control is executed after the fuel cut is released. The sub air-fuel ratio feedback control is restarted after waiting for the output voltage SO ₂ of the downstream O ₂ sensor 56 to exceed the predetermined voltage level α. Therefore, the upstream side O ₂
In the case where the sensor 55 has a lean shift and the output signal SO ₂ of the downstream side O ₂ sensor 56 does not reach the threshold level only by the main air-fuel ratio feedback control (indicated by a chain line in the figure), Conventional example (Japanese Patent Laid-Open No. 4-101)
No. 038)), the sub air-fuel ratio feedback control could not be restarted. On the other hand, if the output signal SO ₂ reaches a predetermined voltage level α lower than the threshold level, The air-fuel ratio feedback control can be restarted. Therefore, during the transition to air-fuel ratio feedback control after fuel cut,
The air-fuel ratio can be adjusted appropriately, emission can be reduced, and drivability can be improved.

Further, in the present embodiment, the predetermined voltage level α used for the above determination is set to the level of the inflection point at which the output voltage SO ₂ rising from the lean side changes sharply after the fuel cut. It is possible to accurately determine the later time when the O ₂ storage amount of the catalytic converter 16 becomes almost zero. As a result, in the conventional example in which the delay time after leaning is set to a constant value (as represented by JP-A-64-36943), the amount of O ₂ storage in the catalytic converter 16 becomes zero (constant from time t12). After time T0)
Then, the sub air-fuel ratio feedback control is restarted, and as shown by the two-dot chain line in the figure, the skip amount RSR in the rich direction sharply increases and the air-fuel ratio is overcorrected to the rich side. In the embodiment, there is no such rapid rise in RSR, and the air-fuel ratio is not overcorrected to the rich side. Therefore, the emission can be further reduced and the fuel consumption can be improved.

In the above embodiment, the predetermined voltage level α used for the determination of the restart timing of the sub air-fuel ratio feedback control is set in advance, but instead of this, the downstream O ₂ sensor 56 is replaced. The minimum value (or maximum value) of the output voltage SO ₂ may be obtained by learning control, and the voltage level used for the above determination may be set in the form of a deviation from this minimum value (or maximum value). With this configuration, even when the output voltage SO ₂ of the downstream O ₂ sensor 56 deviates with time, the predetermined voltage level can be made highly accurate, and the emission is further reduced and the drivability is improved. And so on.

In the above embodiment, the leaning means M7 is used.
However, the configuration is not limited to this, and the fuel supply amount during deceleration or the like is reduced, or the intake air amount such as secondary air supply is increased. You may.

Although one embodiment of the present invention has been described in detail above, the present invention is not limited to such an embodiment. For example, the upstream and downstream concentration sensors M4, M4,
It is needless to say that the present invention can be implemented in various modes such as a configuration using a CO sensor, a lean mixed sensor, etc. in place of the O ₂ sensors 55, 56 as M5 without departing from the scope of the present invention. That is.

[0058]

As described above, in the air-fuel ratio control system for an internal combustion engine of the present invention, at the time of transition from lean control such as fuel cut to air-fuel ratio feedback control, the main signal based on the output signal of the upstream side concentration sensor is used. Only the air-fuel ratio control is restarted, and the sub-air-fuel ratio control based on the output signal of the downstream side concentration sensor is restarted after waiting for the output signal of the downstream side concentration sensor to exceed a predetermined level lower than the level corresponding to the theoretical air-fuel ratio. is doing. Therefore, even if the upstream side concentration sensor has a lean deviation and the output signal of the downstream side concentration sensor does not reach the level corresponding to the theoretical air-fuel ratio by only the main air-fuel ratio control, the predetermined level is reached. Then, the sub air-fuel ratio control can be restarted. Therefore, the air-fuel ratio can be properly adjusted when shifting to the air-fuel ratio feedback control after the lean control.
Emissions can be reduced and drivability can be improved.

Further, in the present invention, the predetermined level which is the judgment value of the output signal of the downstream side concentration sensor is set to the level of the inflection point at which the output signal rising from the lean side after fuel cut changes most rapidly. It is possible to adjust the restart timing of the sub air-fuel ratio control after the lean control to an appropriate timing when the O ₂ storage amount in the catalytic converter becomes almost zero. Therefore, it is possible to prevent overcompensation of the air-fuel ratio due to the restart timing being too early, and to reduce emissions and improve fuel efficiency, and to prevent deterioration of the air-fuel ratio due to the restart timing being too late. It is also possible to reduce the emission.

[0060]

[Brief description of drawings]

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

FIG. 2 is a schematic configuration diagram showing an automobile engine equipped with an air-fuel ratio control device according to an embodiment of the present invention and peripheral devices thereof.

FIG. 3 is a block diagram showing an electrical configuration of a control system centered on an ECU.

FIG. 4 is a flowchart showing a fuel injection control processing routine executed by a CPU of the ECU.

FIG. 5 is a flowchart showing a fuel cut processing routine which is also executed by the CPU.

FIG. 6 is a graph showing a fuel cut state according to a rotation speed Ne.

FIG. 7 is a flowchart showing a main air-fuel ratio feedback control processing routine executed by a CPU.

FIG. 8 is a timing chart showing the contents of the main air-fuel ratio feedback control process.

FIG. 9 is a flowchart showing a sub air-fuel ratio feedback control processing routine executed by a CPU.

FIG. 10 is a timing chart showing the contents of the sub air-fuel ratio feedback control process.

FIG. 11 is a timing chart showing an operation based on various control processes executed by the CPU.

[Explanation of symbols]

M1 ... Internal combustion engine M2 ... Exhaust passage M3 ... Catalytic converter M4 ... Upstream concentration sensor M5 ... Downstream concentration sensor M6 ... Control means M6a ... First control portion M6b ... Second control portion M7 ... Leaning means M8 ... During leaning Stopping means M9 ... Signal rising detection means M10 ... Stopping means after leaning 1 ... Engine 2 ... Intake passage 3 ... Air cleaner 5 ... Throttle valve 6 ... Surge tank 7 ... Fuel injection valve 11 ... Combustion chamber 12 ... Spark plug 15 ... Exhaust Passage 16 ... Catalytic converter 21 ... Distributor 22 ... Igniter 23 ... Rotation speed sensor 50 ... Idle switch 51 ... Throttle position sensor 52 ... Intake temperature sensor 53 ... Air flow meter 54 ... Water temperature sensor 55 ... Upstream O ₂ sensor 56 ... Downstream O ₂ sensor 57 ... vehicle speed sensor 70 ... ECU 70a ... CPU 70 ... ROM 70c ... RAM FAF ... air-fuel ratio correction coefficient KIL ... integration amount KIR ... integration amount KIrsrL ... integration amount KIrsrR ... integration amount RSL ... skipping amount RSR ... skipping amount RSrsrL ... skipping amount RSrsrR ... skipping amount XFC ... fuel cut flag XSB ... Prohibition flag α ... predetermined voltage level

Claims (1)

[Claims] 1. A catalytic converter provided in an exhaust passage of an internal combustion engine, and an upstream side provided on the upstream side of the catalytic converter and outputting a signal according to a concentration of a specific component that changes according to an air-fuel ratio reflected in exhaust gas. A concentration sensor, a downstream concentration sensor that is provided on the downstream side of the catalytic converter and that outputs a signal according to the concentration of a specific component that changes depending on the air-fuel ratio reflected in the exhaust gas, and a signal that is output from the upstream concentration sensor And a second control unit for adjusting the air-fuel ratio of the internal combustion engine based on a signal output from the downstream side concentration sensor. An air-fuel ratio control device for an internal combustion engine, comprising: a control means for controlling the air-fuel ratio of the internal combustion engine to a stoichiometric air-fuel ratio by an operation, wherein the internal Forcibly adjusting the mixing ratio of the fuel and air supplied to the engine, and leaning means for controlling the air-fuel ratio of the internal combustion engine to a lean state, and during execution of the air-fuel ratio control by the leaning means, The first
A lean stop means for stopping the operation of the control section and the second control section, and a signal output from the downstream side concentration sensor at a level corresponding to the stoichiometric air-fuel ratio after the end of the air-fuel ratio control by the leaning section. A signal rising time detecting means for detecting a time exceeding a lower predetermined level, and the second control section of the second control unit after the end of the air-fuel ratio control by the leaning means until the signal rising time detecting means detects the time. An air-fuel ratio control device for an internal combustion engine, comprising: a post-lean stop means for stopping the operation.