JP2600277B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention provides an air-fuel ratio sensor (in this specification, an oxygen concentration sensor (O ₂ sensor)) on the upstream and downstream sides of a catalytic converter. O ₂ double air-fuel ratio sensor system performs air-fuel ratio feedback control by the O ₂ sensor downstream in addition to the air-fuel ratio feedback control by the sensor or to the catalytic converter downstream or in the catalytic converter
O ₂ and a sensor related to single air-fuel ratio sensor system for performing air-fuel ratio feedback control by the O ₂ sensor.

[Conventional technology]

In simple air-fuel ratio feedback control (single O ₂ sensor system), an O ₂ sensor that detects oxygen concentration is provided in the exhaust system as close as possible to the combustion chamber, that is, at the exhaust manifold upstream of the catalytic converter. In addition, the variation in the output characteristics of the O ₂ sensor hinders the improvement of the air-fuel ratio control accuracy. Such O ₂ component variation variation and the fuel injection valve and the output characteristics of the sensor, to compensate for time or secular change, a second O ₂ sensor disposed downstream of the catalytic converter, the upstream O ₂
In addition to air-fuel ratio feedback control by sensor, downstream side
Double with air-fuel ratio feedback control by O ₂ sensor
O ₂ sensor system has already been proposed. This double O ₂
The sensor system, O ₂ sensor provided downstream of the catalytic converter, compared with the upstream O ₂ sensor, but has a low response speed, has the advantage that variations in the output characteristics for the following reason is small ing.

(1) Since the exhaust gas temperature is low downstream of the catalytic converter, there is little thermal effect.

(2) Downstream of the catalytic converter, various poisons are trapped in the catalyst, so that the amount of poisoning of the downstream O ₂ sensor is small.

(3) The exhaust gas is sufficiently mixed downstream of the catalytic converter, and the oxygen concentration in the exhaust gas is close to the equilibrium state.

Therefore, as described above, the air-fuel ratio feedback control based on the outputs of the two O ₂ sensors (double O ₂ sensor system), the variations in the output characteristic of the upstream O ₂ sensor can be absorbed by the downstream O ₂ sensor. In fact, as shown in FIG. 2, when the output characteristics of the O ₂ sensor deteriorate in the single O ₂ sensor system, the exhaust emission characteristics are directly affected, whereas in the double O ₂ sensor system, the upstream side Even if the output characteristics of the O ₂ sensor deteriorate, the exhaust emission characteristics do not deteriorate. That is, in the double O ₂ sensor system, good exhaust emissions are guaranteed as long as the downstream O ₂ sensor maintains stable output characteristics.

On the other hand, as an input circuit for the output of the O ₂ sensor, there is a pull-down type input circuit shown in FIG. 3A. That is, the pull-down type input circuit (see Japanese Technical Disclosure 87-5098) is constituted by a pull-down resistor R ₁ and a noise absorbing capacitor C _1. When the element temperature is low, the internal resistance of the O ₂ sensor OX
Large R _0, therefore, as shown in FIG. 4A, the base air-fuel ratio O ₂ sensor output voltage V _OX even when the electromotive force of the O ₂ sensor OX rich becomes a low level, while when the element temperature becomes higher , O ₂ internal resistance R ₀ of the sensor OX is reduced, the base air-fuel ratio O ₂ sensor output voltage V _OX is the electromotive force of the O ₂ sensor OX in the case of rich electromotive force _{× R 1 / (R 0 +} R 1 ) It becomes a considerably high level. The activation determination of the O ₂ sensor OX when such a pull-down type input circuit is used is based on the O ₂ sensor output voltage V _OX
There is carried out by whether or reversed whether it has exceeded the predetermined value is usually based air-fuel ratio is even O ₂ sensor OX in the case of lean not determined that activity also be activated.

Therefore, regardless of whether the base air-fuel ratio is rich or lean, O ₂
As an input circuit that can determine the activity of the sensor OX, a pull-up type input circuit shown in FIG. 3B (refer to Published Technical Report No. 87-5098)
Has been proposed. That is, the pull-up type input circuit includes a pull-up resistor R ₂ and the noise absorbing capacitor C ₂
It consists of. O ₂ sensor when element temperature is low
Internal resistance R ₀ of OX is larger than the pull-up resistor R _2, the
As shown in FIG. 4B, the O ₂ sensor output voltage V _OX is almost equal to the power supply voltage (V _CC × R ₀ / (R ₀ +
R ₂ )), on the other hand, when the element temperature rises, the internal resistance R ₀ of the O ₂ sensor OX becomes smaller than the pull-up resistance R ₂ , and when the base air-fuel ratio is rich, the O ₂ sensor The output voltage V _OX becomes a high level equivalent to the electromotive force + V _CC × R ₀ / (R ₀ + R ₂ ), and when the base air-fuel ratio is lean, the O ₂ sensor output voltage V _OX becomes V _CC × R ₀ / The low level is equivalent to (R ₀ + R ₂ ). Therefore, when the pull-up type input circuit is used, the activation of the O ₂ sensor OX is determined by the O ₂ sensor output voltage.
This can be done by determining whether V _OX is a level slightly higher than the rich output level after warm-up, for example, lower than the activation level A shown in FIG. 4B.

[Problems to be solved by the invention]

However, in the above-described double O ₂ sensor system, when the pull-up type input circuit is used for the downstream O ₂ sensor, since the activation level A is not variable according to the base air-fuel ratio, the base air-fuel ratio Immediately after the activation is determined at the time of the lean operation, there is erroneous control of the air-fuel ratio due to the erroneous rich determination.
There is a problem that active / inactive hunting (range Y) occurs due to lean reversal, and the air-fuel ratio is deviated rich. That is, by setting as shown activity level A in FIG. 5, the base is air-fuel ratio is in the case of rich is determined that activity in element temperature T _3, a lower element temperature T ₁ when the base air-fuel ratio is lean It is quickly determined to be active, and in this case,
In the range X (T _{1 to} T ₂ ), it is erroneously determined to be rich, and as a result,
The air-fuel ratio is erroneously controlled. Further, when the element temperature is in the range of T _{1 to} T ₂ , active and inactive hunting occurs according to the base air-fuel ratio, and as a result, there is a problem that erroneous control of the air-fuel ratio occurs.

Moreover, Figure 6A in detail, the Figure 6B (second 6A enlarged view of portion B of Figure) using explaining, downstream O ₂ sensor is determined that the active base air-fuel ratio at while time t ₀ of lean And (V
_OX <A), the air-fuel ratio feedback control by the downstream O ₂ sensor, for example, the update of the rich skip amount RSR starts, but in this case, it is erroneously determined to be rich (V _OX >
V _R), the rich skip amount RSR is after being controlled to the lean side, the rich skip amount RSR at time t ₁ is controlled to the rich side as the original. Moreover, at the initial stage of activation of the downstream O ₂ sensor, the base air-fuel ratio is rich and lean fluctuates greatly due to fuel cut, increase, etc., and as shown in FIG. 6B,
At time _{t 2 ~t 3, t 4 ~t} 5, t 6 ~t 7, the inactive state of the downstream O ₂ sensor. During this time, the rich skip amount RSR is not updated, and the rich skip amount RSR is overcorrected toward the rich side. The dotted line RSR 'indicates the case where there is no rich side overcorrection.

Further, in order to prevent the latter active and inactive hunting, the activity determination value A can be set higher,
In this case, with time becomes longer rich misjudgment (t ₀ ~t _1), since the greater the air-fuel ratio feedback execution in half-activated state of the downstream O ₂ sensor can not be adopted.

The above-mentioned problem is the same in a single O ₂ sensor system in which an O ₂ sensor is provided only downstream of or in the catalyst.

The applicant has already started the air-fuel ratio feedback control by the downstream O ₂ sensor after a certain time delay after the output of the downstream O ₂ sensor, that is, the output of the pull-up type input circuit has become lower than the activation level A. By doing so, it has been proposed that the air-fuel ratio feedback control be performed in a time region where erroneous control is not performed (reference: Japanese Patent Application No. 62-262910).
issue). However, after this time delay,
There is no guarantee that the O ₂ sensor is activated.

Accordingly, an object of the present invention is to further advance the above-mentioned prior application and to provide a double air-fuel ratio in which deterioration of emission, deterioration of fuel efficiency, deterioration of drivability, etc. due to air-fuel ratio erroneous control after the determination of activation of the air-fuel ratio sensor is reliably prevented. It is to provide a sensor system and a single air-fuel ratio sensor system.

[Means for solving the problem]

The means for solving the above-mentioned problems are shown in FIGS. 1A and 1B.

FIG. 1A shows a double air-fuel ratio sensor system. That is, the upstream side of the exhaust passage of the three-way catalyst CC _RO provided in an exhaust passage of an internal combustion engine, it is provided upstream air-fuel ratio sensor for detecting an air-fuel ratio of the engine, also downstream of the three-way catalyst CC _RO The downstream exhaust passage is provided with a downstream air-fuel ratio sensor that detects the air-fuel ratio of the engine. When the downstream air-fuel ratio sensor is determined to be inactive, the pull-up type input circuit is higher than the rich output level of the downstream air-fuel ratio sensor after the downstream air-fuel ratio sensor is determined to be active. A voltage is applied and an output voltage of the downstream air-fuel ratio sensor is input. The first determination means determines whether or not the output of the pull-up type input circuit is lower than an activity determination level A slightly higher than the rich output level after warm-up. Then, from the time when the output of the pull-up type input circuit is determined to be lower than the activation determination level A, the second determining means determines whether the base air-fuel ratio is rich according to the rate of change of the output of the pull-up type input circuit with respect to time. Determine if it is lean. After the base air-fuel ratio is determined to be lean, the third determination means determines whether or not the output of the pull-up type input circuit is in a lean output level region after warm-up (less than a predetermined value C). As a result, after the base air-fuel ratio is determined to be rich, or after the output of the pull-up type input circuit is determined to be in the lean output level region after warm-up (less than the predetermined value C), the air-fuel ratio adjusting means is upstream. The output V ₂ of the downstream air-fuel ratio sensor in addition to the output V ₁ of the downstream air-fuel ratio sensor
The adjustment of the air-fuel ratio of the engine according to the above is started.

FIG. 1B shows a single air-fuel ratio sensor system. That is, in the three-way catalyst CC _RO downstream side of the exhaust passage or a three-way catalyst, the air-fuel ratio sensor for detecting an air-fuel ratio of the engine is provided. The pull-up type input circuit applies a voltage higher than the rich output level of the air-fuel ratio sensor after the air-fuel ratio sensor is determined to be in an active state when the air-fuel ratio sensor is determined to be in an inactive state, The output voltage of the air-fuel ratio sensor is input. The first determination means determines whether or not the output of the pull-up type input circuit is lower than an activity determination level A slightly higher than the rich output level after warm-up.
Then, from the time when the output of the pull-up type input circuit is determined to be lower than the activation determination level A, the second determining means determines whether the base air-fuel ratio is rich according to the rate of change of the output of the pull-up type input circuit with respect to time. Determine if it is lean. After the base air-fuel ratio is determined to be lean, the third determining means determines whether or not the output of the pull-up type input circuit is in a lean output level region after warm-up (less than a predetermined value C). As a result, after the base air-fuel ratio is determined to be rich or after it is determined that the output of the pull-up type input circuit is in the lean output level region after warming up (less than the predetermined value C), the air-fuel ratio adjusting means determines it is intended to initiate the adjustment of the air-fuel ratio of the engine corresponding to the output V ₂ of the downstream air-fuel ratio sensor.

(Operation)

The operation of the above means is shown in FIGS. 7A, 7B and 7C. That is, as shown in FIG. 7A, the output of the air-fuel ratio sensor in the fully activated state after warm-up, after crossing the activation determination level A, becomes the rich output level when the air-fuel ratio is rich. Region, that is, a relatively high first level region (A-B), and if the air-fuel ratio is lean, it should be in a lean output level region, that is, a relatively low second level region (less than C). is there. On the other hand, even if the output of the inactive air-fuel ratio sensor (the output of the pull-up type input circuit) crosses the activation determination level A, the first level region (A to B) and the second level region (less than C) Does not exist. However, determining whether the base air-fuel ratio is rich or lean before determining the first level region (A to B) and the second level region (less than C) increases reliability. The present invention determines such a base air-fuel ratio by the activation level A
Attention was paid to the rate of change of the output of the air-fuel ratio sensor with respect to time after crossing. That is, as shown by the arrow X ₁ in FIG. 7B, when the base air-fuel ratio is rich, the output V _OX of the pull-up type input circuit is changed to either slow, while, as shown in Figure 7C, the base air We focused on the fact that when the fuel ratio is lean, the output V _OX of the pull-up type input circuit changes rapidly.
That is, a change in the output with respect to time after the output of the air-fuel ratio sensor crosses the activity determination level A has a correlation with the rich / lean base air-fuel ratio. Accordingly, the air-fuel ratio base air-fuel ratio at the time of activation of the air-fuel ratio sensor according outputted from the time t ₁ that crosses the active discrimination level A to the rate of change of the output V ₂ of the pull-up type input circuit of the sensor is rich or lean Determine.
For example, the relationship between the time change rate of the output after crossing the activation determination level A and the base air-fuel ratio is obtained in advance by an experiment or the like, and the output change rate at which the base air-fuel ratio can always be determined to be rich is determined. (For example, the output change rate is equal to or less than a predetermined value D) and the output change rate range in which the base air-fuel ratio can always be determined to be lean (for example, the output change rate is equal to or more than a predetermined value E). deep. If the output change rate is smaller than the predetermined value D, the base air-fuel ratio is determined to be rich, and if the output change rate is larger than the predetermined value E, the base air-fuel ratio is determined to be lean. Rich base air-fuel ratio of the activity at the time, at a time point t ₂ when the lean determination result is output, based when the air-fuel ratio is rich, or the base air-fuel ratio in the case of a lean and a pull-up type input circuit output Is in the second level region (less than C), it is considered that the air-fuel ratio sensor has been completely activated, and the air-fuel ratio feedback control is started according to the output of the air-fuel ratio sensor. Note that F in FIG. 7 indicates the inactivity determination level.

〔Example〕

FIG. 8 is an overall schematic diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention. In FIG. 8, an air flow meter 3 is provided in an intake passage 2 of an engine body 1. The air flow meter 3 directly measures the amount of intake air, and has a built-in potentiometer to generate an analog voltage output signal proportional to the amount of intake air. This output signal is output from the A / D converter 1 with built-in multiplexer in the control circuit 10.
Supplied to 01. The distributor 4 has a crank angle sensor 5 whose axis generates, for example, a reference position detection pulse signal every 720 ° in terms of crank angle, and a reference position detection pulse signal in every 30 degrees which is converted into crank angle. A generated crank angle sensor 6 is provided. The pulse signals of the crank angle sensors 5 and 6 are supplied to an input / output interface 102 of the control circuit 10, and the output of the crank angle sensor 6 is supplied to an interrupt terminal of the CPU 103.

Further, the intake passage 2 is provided with a fuel injection valve 7 for supplying pressurized fuel from a fuel supply system to an intake port for each cylinder.

The water jacket 8 of the cylinder block of the engine body 1 is provided with a water temperature sensor 9 for detecting the temperature of the cooling water. The water temperature sensor 9 detects the temperature TH of the cooling water.
Generates an analog voltage electric signal corresponding to W. This output is also supplied to the A / D converter 101.

The exhaust system downstream of the exhaust manifold 11 is provided with a catalytic converter 12 containing a three-way catalyst that simultaneously purifies three toxic components HC, CO, and NOx in the exhaust gas.

The exhaust manifold 11 includes a catalytic converter 12
A first O ₂ sensor 13 is provided on the upstream side, and a second O ₂ sensor 15 is provided on the exhaust pipe 14 downstream of the catalytic converter 12. The O ₂ sensors 13 and 15 generate an electric signal corresponding to the concentration of the oxygen component in the exhaust gas. That is, the O ₂ sensor 1
3 and 15 generate different output voltages to the A / D converter 101 via the pull-up type input circuits 111 and 112 of the control circuit 10 depending on whether the air-fuel ratio is lean or rich with respect to the stoichiometric air-fuel ratio.
The control circuit 10 is configured as, for example, a microcomputer, and includes an A / D converter 101, an input / output interface 102,
In addition to the PU 103, a ROM 104, a RAM 105, a backup RAM 106, a clock generation circuit 107, and the like are provided.

An idle switch 17 for detecting whether or not the throttle valve 16 is fully closed is provided at the throttle valve 16 of the intake passage 2.
The output signal is supplied to the input / output interface 102 of the control circuit 10.

In the control circuit 10, the down counter 108, the flip-flop 109, and the drive circuit 110
Is to control the That is, when the fuel injection amount TAU is calculated in a routine described later, the fuel injection amount TAU is preset in the down counter 108 and the flip-flop 109 is also set. As a result, the drive circuit 110 starts energizing the fuel injection valve 7. On the other hand, when the down counter 108 counts a clock signal (not shown) and its carry-out terminal finally becomes “1” level, the flip-flop 109 is reset and the drive circuit 110 is reset.
Stops the energization of the fuel injection valve 7. That is, the fuel injection valve 7 is energized by the above-described fuel injection amount TAU, so that an amount of fuel corresponding to the fuel injection amount TAU is sent to the combustion chamber of the engine body 1.

Note that the CPU 103 generates an interrupt when the A / D conversion of the A / D converter 101 ends, when the input / output interface 102 receives the pulse signal of the crank angle sensor 6,
When an interrupt signal from 7 is received, and so on.

The intake air amount data Q and the cooling water temperature data THW of the air flow meter 3 are fetched by an A / D conversion routine executed every predetermined time and stored in a predetermined area of the RAM 105.
That is, the data Q and THW in the RAM 105 are updated every predetermined time. Further, the rotation speed data Ne is calculated by an interruption of the crank angle sensor 6 at every 30 ° CA, and RA
It is stored in a predetermined area of M105.

Figure 9 is a first air-fuel ratio feedback control routine for calculating the air-fuel ratio correction coefficient FAF based on the output of the upstream O ₂ sensor 13 is executed at a predetermined time, for example, 4ms each.

In step 901, the air-fuel ratio of the closed loop by the upstream O ₂ sensor 13 (feedback) condition is determined whether or not satisfied. For example, when the cooling water temperature is equal to or lower than a predetermined value, during engine start, during increase after start, during warm-up, during power increase,
During OTP boost for catalytic heating cooling prevents the upstream O ₂ sensor 13
When the output signal has never been inverted, the closed loop condition is not satisfied during the fuel cut or the like, and the closed loop condition is satisfied in other cases. When the closed loop condition is not satisfied, the process proceeds directly to step 927, and the value is set to a value immediately before the end of the closed loop control. On the other hand, if the closed loop condition is satisfied, step
Proceed to 902.

In step 902, V ₁ is determined whether or not the comparison voltage V _R1 for example 0.45V or less uptake, at step 903 the output V ₁ of the upstream O ₂ sensor 13 converts A / D, that is, the air-fuel ratio It is determined whether the air-fuel ratio is lean (V ₁
If ≦ V _R1) a, delay counter CD at step 904
It is determined whether or not LY is positive. If CDLY> 0, step 905 is performed.
To set CDLY to 0, and proceed to step 906. In step 906, the count value of the delay counter CDLY is decremented by one.
At 8, guard the delay counter CDLY with the minimum value TDL.
In this case, when the delay counter CDLY reaches the minimum value TDL, the first air-fuel ratio flag F1 is set to "0" (lean) in step 909. Note that TDL is a lean delay state for holding the judgment that the rich state even if the change from rich to lean at the output of the upstream value O ₂ sensor 13 is defined by a negative value You. On the other hand, if rich (V ₁ > V _R1 ), it is determined in step 10 whether or not the delay counter CDLY is negative.
At 911, CDLY is set to 0, and the routine proceeds to step 912. Step 91
In step 2, the delay counter CDLY is incremented by 1, and steps 913 and 9
At 14, guard the delay counter CDLY with the maximum value TDR. In this case, when the delay counter CDLY reaches the maximum value TDR, in step 915, the first air-fuel ratio flag F1
To “1” (rich). Note that the maximum value TDR is the upstream O ₂
This is a rich delay time for maintaining the determination of the lean state even when the output of the sensor 13 changes from lean to rich, and is defined as a positive value.

In step 916, it is determined whether or not the sign of the first air-fuel ratio flag F1 has been inverted, that is, whether or not the air-fuel ratio after the delay processing has been inverted. If the air-fuel ratio is inverted, it is determined in step 917 whether the air-fuel ratio is inverted from rich to lean or from lean to rich, based on the value of the first air-fuel ratio flag F1. In the case of inversion from rich to lean, in step 918, FAF ← FAF + RSR is increased in a skipping manner. On the contrary, in the case of inversion from lean to rich, in step 919, FAF ← FAF−RSL is skipped. Decrease.
That is, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted at step 916, the integration processing is performed at steps 920, 921, and 922. That is, in step 920, it is determined whether or not F1 = "0". If F1 = "0" (lean), the FAF is determined in step 921.
← FAF + KIR. On the other hand, if A1 = “1” (rich), in step 922, FAF ← FAF + KIL. Where the integration constant
KIR, KIL is set to be sufficiently smaller than the skip amounts RSR, RSL, that is, KIR (KIL) <RSR (RSL). Accordingly, step 921 gradually increases the fuel injection amount in the lean state (F1 = "0") to shift the air-fuel ratio to the rich side, and step 922 reduces the fuel injection amount in the rich state (F1 = "1"). The air-fuel ratio is gradually decreased to shift to the lean side.

The air-fuel ratio correction coefficient FAF calculated in steps 918, 919, 921, 922 is guarded in steps 923, 924 with a minimum value, for example, 0.8, and is guarded in steps 925, 926 with a maximum value, for example, 1.2. Thus, if the air-fuel ratio correction coefficient FAF becomes too large or too small for some reason, the air-fuel ratio of the engine is controlled with that value to prevent over-rich or over-lean.

The FAF calculated as described above is stored in the RAM 105, and this routine ends in step 927.

FIG. 10 is a timing chart for supplementarily explaining the operation according to the flowchart of FIG. Rich As shown in FIG. 10 by the output of the upstream O ₂ sensor 13 (A), when the air-fuel ratio A / F is obtained, the delay counter CDLY is first
0 As shown in FIG. 7B, the count is incremented in the rich state and is counted down in the lean state. As a result,
As shown in FIG. 10 (C), the air-fuel ratio signal subjected to the delay processing
A / F '(corresponding to the flag F1) is formed. For example, the air-fuel ratio signal A / F at time t ₁ is changed from lean to rich, the air-fuel ratio signal A / F which is delayed processed 'is rich delay time T
It changes to the rich at time t ₂ after being held lean only DR. Even the air-fuel ratio signal A / F at time t ₃ is changed from rich to lean, the delayed air-fuel-fuel ratio signal A / F 'is the time after being held rich only equivalent lean delay time (-TDL) t It changes to lean at ₄ . However, when the air-fuel ratio signal A / F is reversed in a short period of rich delay time TDR as the time _{t 5, t 6, t 7} , takes time delay counter CDLY reaches the maximum value TDR, this result, the air-fuel ratio signal a / F after the delay is reversed at time t _8. That is, the air-fuel ratio signal A / F 'after the delay processing is more stable than the air-fuel ratio signal A / F before the delay processing. Thus, the air-fuel ratio correction coefficient FAF shown in FIG. 10 (D) is obtained based on the stable air-fuel ratio signal A / F 'after the delay processing.

Next, a description will be given of the second air-fuel ratio feedback control by the downstream O ₂ sensor 15. As the second air-fuel ratio feedback control, the skip amounts RSR, RSL, the integration constants KIR, KIL, the delay times TDR, TDL, or the output V ₁ of the upstream O ₂ sensor 13 as the first air-fuel ratio feedback control constant are determined. There are a system that makes the comparison voltage V _R1 variable, and a system that introduces the second air-fuel ratio correction coefficient FAF2.

For example, if the rich skip amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and if the lean skip amount RSL is reduced, the control air-fuel ratio can be shifted to the rich side, while if the lean skip amount RSL is increased, , The control air-fuel ratio can be shifted to the lean side, and the rich skip
Even if the RSR is reduced, the control air-fuel ratio can be shifted to the lean side. Accordingly, the air-fuel ratio can be controlled by correcting the rich skip amount RSR and the lean skip amount RSL in accordance with the output of the downstream O ₂ sensor 15. Also, the rich integration constant KIR
The control air-fuel ratio can be shifted to the rich side by increasing the control air-fuel ratio, and the control air-fuel ratio can be shifted to the rich side even if the lean integration constant KIL is reduced. The control air-fuel ratio can be shifted to the lean side even if the rich integration constant KIR is reduced. Accordingly, the air-fuel ratio can be controlled by correcting the rich integration constant KIR and the lean integration constant KIL in accordance with the output of the downstream O ₂ sensor 15. By increasing the rich delay time TDR, the control air-fuel ratio can be shifted to the rich side.
Even if the lean delay time (-TDL) is reduced, the control air-fuel ratio can shift to the rich side. Conversely, the lean delay time (−TD
By increasing L), the control air-fuel ratio can shift to the lean side,
Even if the rich delay time (TDR) is reduced, the control air-fuel ratio can be shifted to the lean side. That is, the air-fuel ratio can be controlled by correcting the delay time TDR, the TDL in accordance with the output of the downstream O ₂ sensor 15. Furthermore, if the comparison voltage V _R1 is increased, the control air-fuel ratio can be shifted to the rich side, and the comparison voltage V _R1 can be shifted.
By reducing _R1 , the control air-fuel ratio can be shifted to the lean side.
Therefore, according to the output of the downstream O ₂ sensor 15, the comparison voltage V _R1
Is corrected, the air-fuel ratio can be controlled.

These skip amount, integration constant, the delay time, to a variable comparison voltage by the downstream O ₂ sensor has an advantage in, respectively. For example, the delay time allows very fine adjustment of the air-fuel ratio, and the skip amount can be controlled with good response without lengthening the air-fuel ratio feedback cycle unlike the delay time. Therefore, these variable amounts can be used in combination of two or more.

Next, explaining the skip amount of air-fuel ratio feedback control constant for double O ₂ sensor system variable, before that, the activity determination of the downstream O ₂ sensor 15 11
This will be described with reference to FIGS. This routine is executed every predetermined time, for example, every 512 ms.

In Figure 11, consider the case where the downstream O ₂ sensor 15 changes from an inactive state to an active state. Referring to the timing diagram of FIG. 13, the time t ₁ earlier, the downstream O ₂ sensor 15
Is inactive. That is, the temporary activation flag PF _AC , the activation flag F _AC , the base air-fuel ratio rich flag RF, and the base air-fuel ratio lean flag LF are all “0”. In this state, in step 1101, the output V _{2 of} the downstream O ₂ sensor 15 (here, the output of the pull-up type input circuit 112) is A / D-converted and taken in, and is passed through steps 1102, 1103, and 1104. Step 11
V ₂ is determined whether or not lower than the activity determination level A at 05. As a result, since V ₂ > A, the process proceeds to step 1115,
The flow ends.

Then, upon reaching the time t _1, the flow in step 1105 proceeds to step 1106, it sets a provisional activation flag _{PF AC (PF AC = "1} ").

When the temporary activation flag PF _AC is set, the base air-fuel ratio is judged whether rich or lean is at time t ₃ for example by routine Figure 12 below. As a result, when the base air-fuel ratio is rich, the base air-fuel ratio rich flag RF is set by the routine of FIG. 12, and therefore, when the routine of FIG. Proceeding to 1107, the output V _{2 of} the pull-up input circuit 112
Are within the warm-up rich output level regions A and B (B <
It is determined whether V ₂ <A). Since the base air-fuel ratio will be described later, determination is rich presupposes V ₂ <A, the determination in the step 1107 the V ₂ <A is not performed. As a result, the area A
If so, the activation flag _FAC is set in step 1109. On the other hand, when the base air-fuel ratio is lean, the base air-fuel ratio lean flag LF is set by the routine of FIG. 12, and therefore, when the routine of FIG. 11 is executed again,
Flow in step 1104 proceeds to step 1108, the output V ₂ of the pull-up type input circuit 112 determines whether it is in the lean output level region after warm-up (V ₂ <C). As a result, if it is less than the region C, the activation flag F
Set _AC .

Thus, after the activation flag F _AC is set, the flow in step 1102 proceeds to step 1110 to 1114. That is, in steps 1110 to 1112, the temporary activation flag PF _AC ,
The base air-fuel ratio rich flag RF, the base air-fuel ratio lean flag LF is reset, also, even in the active state, the output V ₂ is inactive discrimination level F of the pull-up input circuit 112
If (≧ A), the flow in step 1113 proceeds to step 1114, where the activation flag _FAC is reset.
As a result, the downstream O ₂ sensor 15 is inactive.

Thus, according to the routine of FIG. 11, after it is determined that the base air-fuel ratio is rich or lean,
Output of pull-up type input circuit 112 in steps 1107 and 1108
V ₂ is the first region, i.e. the warm-up after the rich output level region (A-B), or the second region, i.e. active until the lean output level region after warm-up (less than C) The operation of determination continues.

Was active flag F _AC obtained by as described above is used in the routine of FIG. 17.

Step 1107 in FIG. 11 can be omitted. That is,
As described later, the base air-fuel ratio rich (RF = "1") determination is a case where the change rate ΔV of the output V ₂ of the pull-up type input circuit 112 is small, usually, in this case, B <V ₂ <A Because they are satisfied.

As described above, when the base air-fuel ratio is rich, the output V ₂ of the pull-up type input circuit 112 in the active state is relatively high, therefore, in a transient state after passing through the active discrimination level A of the output V ₂ When the rate of change ΔV is small, while the base air-fuel ratio is lean, the output V ₂ of the pull-up type input circuit 112 is relatively low in the active state, and therefore, in the transient state after passing the activation determination level A, the rate of change ΔV of the output V ₂ is large. Such an output V of the pull-up input circuit 112
The rich / lean base air-fuel ratio is determined based on the change rate ΔV of ₂ .

Figure 12 is a base air-fuel ratio after passage through the output V ₂ of the pull-up type input circuit 112 active discrimination level A rich, a routine for determining a lean, it is executed at a predetermined time, for example, 4ms each. In Figure 12, to determine the base air-fuel ratio in accordance with the rate of change of the output V ₂ of the pull-up type input circuit 112 between time t ₂ between the two points for example FIG. 13, t ₃ (slope) [Delta] V. That is, in step 1201, the provisional activation flag PF _AC is determined not to have been set. Thus, the flow of steps 1202-1217 is executed starting at time t ₁ after the Figure 13.

In steps 1202 and 1203, a predetermined time (downstream O ₂ sensor 1
Shorter time than the inverting cycle of the output V ₂ of 5) is measured by the counter CNT to. Therefore, the flow of steps 1204 to 1217 is executed at regular intervals (CNT = CNT ₀ ). In FIG. 13, execution timing of the flow of time t ₂ is the first step 1204-1217, the time t ₃ is the second step 1204
1217 to the execution timing of the flow.

Steps 1204 to 1217 will be described. Step 1204
In takes the output V ₂ of the pull-up type input circuit 112 converts A / D, the time t ₂ of FIG. 13, i.e., the first (N = 0)
If so, the process proceeds to steps 1206-1208 via step 1205. In step 1206, 1207, and stores the V ₂ as V ₂₀ in RAM 106, N = 1 and counts up the N in step 1208
And then, to clear the counter CNT at step 1217 to start a re-count of a certain period of time CNT _0.

Next, the routine of FIG. 12 is executed, and the time of FIG.
If t ₃ , that is, the second time (N = 1), step 12
Proceed to steps 1209 to 1216 via 05. In step 1209, the rate of change of the output V ₂ of the pull-up type input circuit 112 [Delta] V
Is calculated by ΔV ← V ₂₀ −V ₂ . Then, with step 1210 for the next run, the V ₂ and V _20, and stores the V ₂₀ at step 1211 to the RAM 106.

In step 1212, 1213, or the rate of change ΔV is less than or of the output V ₂ of the pull-up type input circuit 112 (base air-fuel ratio is rich) large (base air-fuel ratio is lean) is determined. For example, as shown in FIG. 13, when the base air-fuel ratio is rich, the time t ₂ of the output V ₂ of the pull-up input circuit 112
Since the rate of change [Delta] V at ~t ₃ is small (0 <ΔV <D), the process proceeds to step 1214 to set the base air-fuel ratio rich flag RF. Incidentally, as shown in FIG. 13, when the rate of change in time t ₂ ~t ₃ is as large as [Delta] V> E, the base air-fuel ratio lean flag LF at step 1215 to determine the base air-fuel ratio lean with Is set. In other cases, ie, D ≦ Δ
If V ≦ E, the process directly proceeds to step 1216, where the counter N
Is cleared, and the counter CNT is cleared in step 1217 to prepare for the next execution.

Then, the routine in FIG. 12 ends in step 1218. The determination values D (step 1212) and E (step 1213) of the change rate ΔV at the time of determining whether the base air-fuel ratio is rich or lean are determined when the sensor is completely activated.
When ΔV <D, the base air-fuel ratio is a value that can always be determined to be rich, and when ΔV> E, the base air-fuel ratio is a value that can be determined always to be lean. Since the determination values D and E differ depending on the type of the O ₂ sensor, they are actually set in advance by experiments.

FIG. 14 is a modification of FIG.
Time t ₂ in Figure 15, t _3, to determine the base air-fuel ratio in accordance with the rate of change of the output V ₂ of the pull-up type input circuit 112 between t ₄ (slope) [Delta] V. Therefore, steps 1401 to 1411 are provided instead of steps 1209 to 1215 in FIG. The predetermined value CNT ₀ in step 1203 is smaller than in the case of Figure 12.

As in FIG. 12, in step 1201, the temporary activation flag PF
Determine whether _AC has been set. Thus, step 1202 and subsequent flow is performed beginning with the fifteenth time t ₁ after the Figure.

In steps 1202 and 1203, a predetermined time (downstream O ₂ sensor 1
Shorter time than the inverting cycle of the output V ₂ of 5) is measured by the counter CNT to. Therefore, the flow from step 1204 is executed at regular intervals (CNT = CNT ₀ ). In FIG. 15, execution timing of time t ₂ is later first step 1204 the flow, the time t ₃ is the second step 1204 the execution timing of the flow after the time t ₄ is the third step
This is the execution timing of the flow after 1204.

Step 1204 and subsequent steps will be described. In step 1204, takes in the output V ₂ of the pull-up type input circuit 112 converts A / D, the time t ₂ of Figure 15, i.e., the first (N = 0) is via a step 1205 if the step 1206 Proceed to ~ 1208. In step 1206, 1207, and stores the V ₂ as V ₂₀ in RAM 106, N = 1 and counts up the N in step 1208
And then, to clear the counter CNT at step 1217 to start a re-count of a certain period of time CNT _0.

Next, the routine of FIG. 14 is executed, and the time of FIG.
If t ₃ , that is, the second time (N = 1), step 12
The process proceeds to steps 1402 and 1403 via 05 and 1401. Step 14
In 01,1402 stores V ₂ as V ₂₁ in RAM 106, step
In 1208, N is counted up to N = 2, and step is performed.
At 1217, the counter CNT is cleared and re-counting of CNT ₀ is started for a certain period of time.

Next, the routine of FIG. 14 is executed, and the time of FIG.
If t ₄ , that is, the third time (N = 2), step 14
Go to 04-1419,1216. In step 1404 and 1405, two consecutive rate of change of the output V ₂ of the pull-up type input circuit 112 delta
V and ΔV ′ are calculated by ΔV ← V ₂₀ −V ₂₁ ΔV ′ ← V ₂₁ −V ₂ .

In step 1406, 1407, rate of change of the output V ₂ of the pull-up type input circuit 112 [Delta] V, or [Delta] V 'is less (base air-fuel ratio is rich) to determine whether. For example, as shown in Figure 15, the rate of change ΔV at time t ₂ ~t ₃ output V ₂ of the pull-up type input circuit 112 is smaller than F, the change rate at time t ₃ ~t ₄ Δ
Since V 'is smaller than G, the routine proceeds to step 1408, where the base air-fuel ratio rich flag RF is set. On the other hand, step 14
In 09,1410, the change rate of the output V ₂ of the pull-up type input circuit 112 [Delta] V, or [Delta] V 'is greater (base air-fuel ratio is lean) is determined or not. For example, as shown in FIG. 15, the change rate at time t ₂ ~t ₃ output V ₂ of the pull-up type input circuit 112 [Delta] V
Is greater than H, the change rate [Delta] V 'is at time t ₃ ~t ₄ greater than I, the process proceeds to step 1411 to set the base air-fuel ratio lean flag LF. In other cases, the process proceeds directly to step 1216, where the counter N is cleared. In step 1217, the counter CNT is cleared to prepare for the next execution.

Then, the routine in FIG. 14 ends in step 1218. The determination values F (step 1406) and H (step 1409) of ΔV, the determination values G (step 1407) of ΔV ′, I
The value of (step 1410) is set by an experiment similarly to the determination values D and E of V in FIG.

The output V _{2 of} four or more pull-up input circuits 112
The base air-fuel ratio may be determined in accordance with the rate of change of.

FIG. 16 also shows a modification of FIG. In Figure 16, the output V ₂ activity level A of the pull-up input circuit 112
After passing the, when the output V ₂ of the pull-up type input circuit 112 is in a predetermined time (CNT = J) a first region (A-B), the change rate of the output V ₂ of the pull-up type input circuit 112 Is assumed to be small, and therefore, the base air-fuel ratio is assumed to be rich and the downstream air-fuel ratio sensor 15 is assumed to be activated, while the output V ₂ of the pull-up type input circuit 112 is in the first region (A If the time required for exiting from .about.B) to a low level is short (CNT <K), the pull-up input circuit 112
Regarded as the rate of change in the output V ₂ of the large, therefore, the base air-fuel ratio is considered lean.

That is, in step 1601, the provisional activation flag PF _AC is determined not to have been set. Therefore, time t _{1 in} FIG.
For the first time thereafter, the flow of steps 1602 to 1609 is executed.

In step 1602, the output V of the pull-up input circuit 112 is
Incorporation ₂ converts A / D, the output V ₂ of the pull-up type input circuit 112 at step 1603 determines whether it is in the first region (A-B).

For example, consider the case where the base air-fuel ratio is rich.
In this case, since B ≦ V ₂ <A, the flow in step 1603 proceeds to step 1604, counts up the counter CNT, and proceeds to step 1610 via step 1605. This state is maintained until CNT> J is satisfied, that is,
It continues until time t ₄ of FIG. 17. As a result, if CNT> J, the flow in step 1605 proceeds to step 1606, in which the base air-fuel ratio rich flag RF is set, and
At 1609, the counter CNT is cleared, and the process proceeds to Step 1610. As a result, the activation flag F _{AC is}
Is set.

On the other hand, consider the case where the base air-fuel ratio is lean. Also in this case, while B ≦ V ₂ <A, the flow in step 1603 proceeds to step 1604, counts up the counter CNT, and proceeds to step 1610 via step 1605. However, in this case, the output V of the pull-up
₂ reaches level B relatively quickly. That is, at time t ₂ of FIG. 17 reaches a level B, Setsu換Ru result, the step 1607 flow in step 1603. This results in a step
If the condition of CNT <K in 1607 is satisfied, step 1607
The flow proceeds to step 1606, sets the base air-fuel ratio lean flag LF, clears the counter CNT in step 1609, and proceeds to step 1610. As a result, at time t ₃
Activity flag F _AC is to be set by V ₂ <Figure 11 routine step in C the time t ₃ when being satisfied 1108 and 1109.

Figure 18 is the skip amount based on the output of the downstream O ₂ sensor 15 RSR, a second air-fuel ratio feedback control routine for calculating the RSL, is executed at predetermined time, for example 512ms. At step 1801 to 1,805, and determines whether the closed loop condition by the downstream O ₂ sensor 15. For example, upstream
Failure of the closed loop condition by the O ₂ sensor 13 (step 140
In addition to 1), when the cooling water temperature THW is equal to or lower than a predetermined value (for example, 70 ° C.) (step 1802), the throttle valve 16 is fully closed (LL
= “1”) (step 1803), and when the load is light (Q / Ne)
<X ₁ ) (step 1804), when the downstream O ₂ sensor 15 is not activated (step 1805), the closed loop condition is not satisfied, and in other cases, the closed loop condition is satisfied. If it is not a closed loop condition, the process directly proceeds to step 1813.

Activity determination of the downstream O ₂ sensor 15 in step 1805 is performed by the active flag F _AC obtained by routine Figure 11.

If the condition is satisfied in the closed loop condition, the process proceeds to step 1806, where the output V ₂ of the downstream O ₂ sensor 15 is A / D converted and captured,
V ₂ is determined whether or not the comparison voltage V _R2 eg 0.55V or less at step 1807, i.e., the air-fuel ratio is determined whether rich or lean. Note that the upstream O ₂ sensor concerning the comparison voltage V _R2 is upstream of the catalytic converter 12, it output characteristics due to the influence of the raw gas is different downstream and that the degradation rate is different, etc.
13 is set higher than the comparison voltage V _R1 of the output of FIG. 13, but this setting is provided if V ₂ is within the region between the first level region and the second level region (ie, C <V ₂ <If B)

If V ₂ ≦ VR ₂ (lean) in step 1807, the process proceeds to steps 1808 and 1809, while if V ₂ > VR ₂ (rich), the process proceeds to steps 1810 and 1811. In step 1808, RSR ← RS
R + ΔRS, that is, while increasing the rich skip amount RSR to shift the air-fuel ratio to the rich side,
RSL ← RSL−ΔRS, that is, the lean skip amount RS
L is reduced to further shift the air-fuel ratio to the rich side.
On the other hand, at step 1810, RSR ← RSR−ΔRS, that is,
The air-fuel ratio is shifted to the lean side by decreasing the rich skip amount RSR, and RSL ← RSL + ΔRS is set in step 1811, that is, the lean-skip amount RSL is increased to further shift the air-fuel ratio to the lean side.

Step 1812 carries out guard processing of the RSR and RSL calculated as described above. For example, the maximum value MAX = 7.5
Guard at%, minimum value MIN = 2.5%. Note that the minimum value MI
N is a value at a level at which transient followability is not impaired, and a maximum value MAX is a value at which drivability does not deteriorate due to air-fuel ratio fluctuation.

Then, the routine in FIG. 18 ends in step 1813.

In this way, when the output of the downstream O ₂ sensor 15, that is, the output V _{2 of the} pull-up type input circuit 112 changes, V ₂
<After A, the base air-fuel ratio is determined to be rich or lean,
Then the output of the pull-up input circuit 112 is the downstream O ₂ sensor 15 determines whether or not the active state (F _AC = "1") depending on whether the level region in accordance with the base air-fuel ratio. Therefore, the rich skip amount RSR and the lean skip amount RSL are not overcorrected toward the rich side. In addition, there is no erroneous rich determination immediately after the activation determination when the base air-fuel ratio is determined to be lean.

FIG. 19 shows an injection amount calculation routine which is executed at every predetermined crank angle, for example, every 360 ° CA. In step 1501, the intake air amount data Q and the rotation speed data are read from the RAM 105.
Ne is read to calculate the basic injection amount TAUP. For example, TAUP
← α · Q / Ne (α is a constant). RAM1 in step 1902
The cooling water temperature data THW is read out from 05, and the warm-up increase value FWL is calculated by interpolation using a one-dimensional map stored in the ROM 104.
In step 1903, the final injection amount TAU is set as TAU ← TAUP / FAF
・ Calculate by (FWL + β) + γ. Here, β and γ are correction amounts determined by other operation state parameters. Next, at step 1904, the injection amount TAU is
Set to 08 and set the flip-flop 109 to start fuel injection. Then, in step 1905, this routine ends.

As described above, when the time corresponding to the injection amount TAU has elapsed, the flip-flop 109 is reset by the carry-out signal of the down counter 108, and the fuel injection ends.

In a single O ₂ sensor system in which an O ₂ sensor is provided downstream of or only in the catalyst to perform air-fuel ratio feedback control, the RSR of the second air-fuel ratio feedback routine is replaced with the first air-fuel ratio feedback routine. , RSL may be calculated as FAF. In addition, although the output value of the pull-up type input circuit is used as the output of the sub O ₂ sensor to be reflected in the air-fuel ratio feedback, the output of the downstream O ₂ sensor can be directly used without passing through the pull-up type input circuit. .

Also, the first air-fuel ratio feedback control is performed every 4 ms,
Also, the reason why the second air-fuel ratio feedback control is performed every 512 ms is that the air-fuel ratio feedback control mainly performs the control by the upstream O ₂ sensor having a high response, and the control by the downstream O ₂ sensor having a low response. It is to do so.

Further, the double O ₂ for correcting other control constant in the air-fuel ratio feedback control by the upstream O ₂ sensor, for example the delay time, the integration constant, or the like by the output of the downstream O ₂ sensor
The present invention can be applied to a sensor system and also to a double O ₂ sensor system that introduces a second air-fuel ratio correction coefficient. In addition, two of the skip amount, the delay time, and the integration constant
The controllability can be improved by controlling the two at the same time. Further, the fixed amount of the skip amounts RSR and RSL may be fixed and only the other variable, or one of the delay times TDR and TDL may be fixed and only the other variable, or the rich integration constant KIR or lean It is also possible to fix one of the integration constants KIL and make the other variable.

Further, instead of an air flow meter, a Karman vortex sensor, a heat wire sensor, or the like can be used as the intake air amount sensor.

Further, in the above-described embodiment, the fuel injection amount is calculated according to the intake air amount and the engine speed, but the fuel injection amount is calculated according to the intake air pressure and the engine speed, or the throttle valve opening and the engine speed. The fuel injection amount may be calculated.

Further, in the above-described embodiment, the internal combustion engine in which the fuel injection valve controls the fuel injection amount to the intake system is described. However, the present invention can be applied to a carburetor-type internal combustion engine. For example, the air-fuel ratio is controlled by adjusting the intake air amount of the engine using an electric air control valve (EACV).
Electric bleed air control valve adjusts the air bleed amount of the carburetor to control the air-fuel ratio by introducing air into the main passage and the slow passage, and controls the amount of secondary air sent to the exhaust system of the engine. The present invention can be applied to a device to be adjusted. In this case, the basic fuel injection amount corresponding to the basic injection amount TAUP in step 1901 is determined by the carburetor itself, that is, determined in accordance with the suction pipe negative pressure corresponding to the intake air amount and the engine speed, and step 1903 At the final fuel injection amount TAU
Is calculated.

Further, although the above-described embodiment is constituted by a microcomputer, that is, a digital circuit, it may be constituted by an analog circuit.

〔The invention's effect〕

As described above, according to the present invention, the activity determination of the downstream air-fuel ratio sensor is performed in the level region corresponding to the base air-fuel ratio after crossing the activity determination level, so that the active / inactive determination can be reliably performed. As a result, there is no hunting of the activation / inactivation discrimination, and there is no rich misjudgment immediately after the activation discrimination when the base air-fuel ratio is determined to be lean, thus eliminating the erroneous control of the air-fuel ratio. ,
As a result, overcorrection of the control constant, the air-fuel ratio control amount, and the like can be prevented, which is useful for reducing exhaust emissions, improving fuel efficiency, preventing deterioration of drivability, and the like.

[Brief description of the drawings]

1A and 1B are overall block diagrams for explaining the configuration of the present invention, FIG. 2 is an exhaust emission characteristic diagram for explaining a single O ₂ sensor system and a double O ₂ sensor system, FIGS. 3A and 3B Figure is a circuit diagram showing an example of the input circuit of the output of the O ₂ sensor, figures 4A, Figure 4B is Figure 3A, the output characteristic diagram of the circuit of Figure 3B, the Fig. 5 activity determination of the O ₂ sensor FIGS. 6A and 6B are timing diagrams for explaining the problem to be solved by the present invention; FIGS. 7A, 7B and 7C are timing diagrams for explaining the operation of the present invention; FIG. 1 is an overall schematic diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention, FIG. 9, FIG. 11, FIG. 12, FIG. 14, FIG.
FIG. 19 is a flowchart for explaining the operation of the control circuit of FIG. 8, and FIGS. 10, 13, 15, and 17 are respectively ninth and nineteenth embodiments.
FIG. 17 is a timing chart for supplementarily explaining the flowcharts of FIGS. 12, 12, 14 and 16. 1 ...... engine body, 3 ...... air flow meter, 4 ...... distributor, 5,6 ...... crank angle sensor, 10 ...... control circuit, 12 ...... catalytic converter, 13 ...... upstream O ₂ sensor, 15 ...... downstream O ₂ sensor, 17 ...... idle switch.

Claims (2)

(57) [Claims] A three-way catalyst provided in an exhaust passage of an internal combustion engine; and an upstream air-fuel ratio sensor provided in an exhaust passage upstream of the three-way catalyst for detecting an air-fuel ratio of the engine. 13); a downstream air-fuel ratio sensor (15) provided in an exhaust passage downstream of the three-way catalyst and detecting an air-fuel ratio of the engine; and determining that the downstream air-fuel ratio sensor is in an inactive state. A voltage higher than the rich output level of the downstream air-fuel ratio sensor after the downstream air-fuel ratio sensor is determined to be in the active state, and a pull-down input for inputting the output voltage of the downstream air-fuel ratio sensor. An up-type input circuit (112); first determination means for determining whether an output of the pull-up type input circuit is lower than an activation determination level slightly higher than a rich output level after warm-up; The output of the mold input circuit is Second determination means for determining whether the base air-fuel ratio is rich or lean in accordance with the rate of change of the output of the pull-up type input circuit with respect to time from the time when the base air-fuel ratio is determined to be lean. Third determining means for determining whether or not the output of the pull-up type input circuit is in a lean output level region after warm-up after the determination is made, and after determining that the base air-fuel ratio is rich or After the output of the pull-up type input circuit is determined to be in the lean output level region after warm-up, the air-fuel ratio of the engine according to the output of the downstream air-fuel ratio sensor in addition to the output of the upstream air-fuel ratio sensor An air-fuel ratio control device for an internal combustion engine, comprising: air-fuel ratio adjusting means for starting the adjustment of the air-fuel ratio. 2. A three-way catalyst (12) provided in an exhaust passage of an internal combustion engine, and an air-fuel ratio of the engine provided in an exhaust passage downstream of the three-way catalyst or in the three-way catalyst. When the air-fuel ratio sensor (15) and the air-fuel ratio sensor are determined to be in an inactive state,
A pull-up type input circuit (112) for applying a voltage higher than the rich output level voltage of the air-fuel ratio sensor after the air-fuel ratio sensor is determined to be in an active state and for inputting the output voltage of the air-fuel ratio sensor; First determining means for determining whether an output of the pull-up input circuit is lower than an activity determination level slightly higher than a rich output level after warm-up; and determining whether an output of the pull-up input circuit is the activity determination. Second determining means for determining whether the base air-fuel ratio is rich or lean according to the rate of change of the output of the pull-up type input circuit with respect to time from the time when the base air-fuel ratio is determined to be lower than the level. A third determining means for determining whether or not the output of the pull-up type input circuit is in a lean output level region after warm-up, and determining that the base air-fuel ratio is rich. Air-fuel ratio adjustment that starts adjusting the air-fuel ratio of the engine according to the output of the air-fuel ratio sensor after the output of the pull-up type input circuit is determined to be in the lean output level region after warm-up. Means for controlling an air-fuel ratio of an internal combustion engine, comprising: