JPH11182296A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To rapidly converge an air-fuel ratio on a stoichiometric air-fuel ratio after switching the operating state of an internal combustion engine from transient operation to steady operation. SOLUTION: An air-fuel ratio control device for an internal combustion engine is provided with a catalytic converter 12, a catalyst upper reaches side O2 sensor 13, a lower reaches side O2 sensor 15, a main air-fuel ratio control means for increasing/decreasing the value of an air-fuel ratio correction factor FAF on the basis of a detection value of the upper reaches side O2 sensor 13, and an auxiliary air-fuel ratio control means for increasing/decreasing the value of skip quantities RSR, RSL concerned in an air-fuel ratio correction factor FAF, on the basis of the detection value of the lower reaches side O2 sensor 15. At the time of switching from steady operation where main and auxiliary air-fuel ratio control are executed to transient operation where only main air-fuel ratio control is executed, a value of the skip quantities RSR, RSL is increased/decreased to the specified value, and succeedingly at the time of switching from transient operation to steady operation, the value of the skip quantities RSR, RSL is made the value of the skip quantities RSR, RSL immediately before switching from steady operation to transient operation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an air-fuel ratio control device for an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, there has been known an air-fuel ratio control device for an internal combustion engine provided with an O ₂ sensor upstream of a catalytic converter for purifying exhaust gas in order to perform feedback control of the air-fuel ratio of the internal combustion engine. In order to compensate for variations in output characteristics of the O ₂ sensor and components such as fuel injection valves and their aging, as an improved form, an O ₂ sensor is provided not only on the upstream side but also on the downstream side of the catalytic converter. An air-fuel ratio control device is known. An example of this type of air-fuel ratio control device for an internal combustion engine is disclosed in Japanese Patent Publication No. Hei 6-13857. The air-fuel ratio control device for an internal combustion engine described in the above publication performs the air-fuel ratio control by controlling the fuel injection amount based on the following equation. TAU = TAUP · FAF · ((FWL + p) + q) Here, TAU is the final injection amount of fuel finally injected from the fuel injection valve, and TUP is the basic fuel amount calculated from the engine speed, the accelerator opening, and the like. The injection amount and FAF are air-fuel ratio correction coefficients for correcting the basic injection amount TAUP according to the air-fuel ratio of the internal combustion engine and the like, and FWL is warm-up for correcting the basic injection amount TAUP according to the warm-up state of the internal combustion engine. The increase coefficients p and q indicate correction coefficients for correcting the basic injection amount TAUP according to other operating state parameters.

At the time of operation such as immediately after the start of an internal combustion engine,
The air-fuel ratio control device for an internal combustion engine described in the above publication does not perform the air-fuel ratio feedback control based on the output of the upstream O ₂ sensor or the downstream O ₂ sensor, and the air-fuel ratio correction coefficient FAF
Is set to a constant value (= 1.0), and the air-fuel ratio open control is performed based on the above equation.

[0004] During steady operation of the internal combustion engine, air-fuel ratio control apparatus for an internal combustion engine according to the above publication, a main air-fuel ratio feedback control based on the output value of the upstream O ₂ sensor, the output of the downstream O ₂ sensor The sub air-fuel ratio feedback control based on the value is simultaneously executed. In detail, the main air-fuel ratio feedback control is executed to increase or decrease the air-fuel ratio correction coefficient FAF. O ₂ based on the output value of the sensor, air-fuel ratio of the internal combustion engine to detect whether a rich or a lean. If it is determined that the air-fuel ratio of the internal combustion engine has transitioned from rich to lean, the air-fuel ratio of the internal combustion engine converges from lean to the stoichiometric air-fuel ratio. Add RSR (FAF ← FAF + RSR),
The final injection amount TAU is increased. When it is determined that the air-fuel ratio of the internal combustion engine has transitioned from lean to rich, the lean skip amount RSL is calculated from the previous air-fuel ratio correction coefficient FAF to converge the air-fuel ratio of the internal combustion engine from rich to stoichiometric air-fuel ratio.
(FAF ← FAF-RSL), and the final injection amount TA
Decrease U. If it is determined that the air-fuel ratio of the internal combustion engine is still lean, the air-fuel ratio of the internal combustion engine is converged from lean to the stoichiometric air-fuel ratio.
An integration constant KI (KI is sufficiently smaller than RSR and RSL) is added to F (FAF ← FAF + KI) to slightly increase the final injection amount TAU. When it is determined that the air-fuel ratio of the internal combustion engine remains rich, the integration constant KI is subtracted from the previous air-fuel ratio correction coefficient FAF (FAF ← F
AF-KI), the final injection amount TAU is slightly reduced.
On the other hand, the sub air-fuel ratio feedback control uses the skip amount RS used for the main air-fuel ratio feedback control described above.
This is executed to increase or decrease R and RSL. Downstream O ₂
In the sub-air-fuel ratio feedback control based on the output value of the sensor, the air-fuel ratio control device of the internal combustion engine described in the above publication uses, for example, every 1 sec, the exhaust gas on the downstream side of the catalytic converter based on the output value of the downstream O ₂ sensor. Is detected whether the air-fuel ratio is lean or rich. If it is determined that the air-fuel ratio of the exhaust gas downstream of the catalytic converter is lean,
In order to make the air-fuel ratio of the exhaust gas downstream of the catalytic converter converge from lean to the stoichiometric air-fuel ratio, the previous time (that is, one second before)
The skip amount change coefficient ΔRS to the rich skip amount RSR of
(For example, a fixed value of 0.0008) is added (RSR ← R
SR + ΔRS), the skip amount change coefficient ΔRS is subtracted from the previous lean skip amount RSL (RSL ← RSL−).
ΔRS). If it is determined that the air-fuel ratio of the exhaust gas downstream of the catalytic converter is rich, in order to converge the air-fuel ratio of the exhaust gas downstream of the catalytic converter from rich to the stoichiometric air-fuel ratio,
The skip amount change coefficient ΔRS is subtracted from the previous rich skip amount RSR (RSR ← RSR−ΔRS), and the skip amount change coefficient ΔRS is added to the previous lean skip amount RSR (RSL ← RSL + ΔRS).

[0005] Transient such as during high load acceleration operation of an internal combustion engine
During operation, the air-fuel ratio control of the internal combustion engine described in the above publication
The control device is used to increase or decrease the air-fuel ratio correction coefficient FAF.
Outflow side O_TwoMain air-fuel ratio fee based on sensor output
The feedback control is executed in almost the same way as during steady-state operation.
Downstream O for increasing / decreasing kipping amounts RSR, RSL _Two
Sub air-fuel ratio feedback system based on sensor output value
Do not perform control. Instead, the rich skip amount RSR and
And the lean skip amount RSL are, for example, 0.0
A fixed value such as 5 is used. It should be noted that high load acceleration operation of the internal combustion engine
At the time of turning, by the main air-fuel ratio feedback control,
The air-fuel ratio of the internal combustion engine is NO_XIn order to control the emission of
Slightly richer than the stoichiometric air-fuel ratio
It is controlled to be bundled.

[0006]

When the operation state of the internal combustion engine is switched from the high load acceleration operation state to the steady operation state, the air-fuel ratio control device for the internal combustion engine described in the above publication discloses a sub air-fuel ratio feedback control. When the execution is resumed and the air-fuel ratio of the exhaust gas downstream of the catalytic converter is determined to be rich, the rich skip amount RSR of the previous time, that is, one second before, is determined.
Is subtracted from the skip amount change coefficient ΔRS (= 0.0008).

However, immediately after the restart of the sub air-fuel ratio feedback control, the sub air-fuel ratio feedback control has not been executed yet, that is, one second before, because the sub air-fuel ratio feedback control has not yet been restarted. Therefore, the rich skip amount RS 1 sec before used for the sub air-fuel ratio feedback control
R is a relatively large fixed value (= 0.05) as described above. Therefore, immediately after switching the operation state of the internal combustion engine from the high load acceleration operation state to the steady operation state,
The air-fuel ratio control apparatus for an internal combustion engine described in the above-mentioned publication, despite wishing to quickly converge the slightly rich air-fuel ratio to the stoichiometric air-fuel ratio, achieves a relatively large value of the rich skip amount RSR. To perform the main air-fuel ratio feedback control. As a result, the air-fuel ratio of the internal combustion engine cannot be quickly converged to the stoichiometric air-fuel ratio.

In addition, even if the sub air-fuel ratio feedback control is restarted, as described above, the relatively large value of the rich skip amount RSR is 0.0008 per second.
It is reduced only at a rather slow rate of change (= ΔRS). Therefore, the air-fuel ratio control device for an internal combustion engine described in the above publication requires a considerable amount of time to reduce the rich skip amount RSR to a value appropriate for the air-fuel ratio of the internal combustion engine to converge on the stoichiometric air-fuel ratio. Would. As a result, the air-fuel ratio of the internal combustion engine cannot be quickly converged to the stoichiometric air-fuel ratio.

In view of the above-mentioned problems, the present invention provides an internal combustion engine having an air-fuel ratio that is slightly rich, and after the operation state of the internal combustion engine is switched from a high load acceleration operation state to a steady operation state. An object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine that can improve emission and prevent deterioration of fuel efficiency due to useless fuel injection by rapidly converging the fuel ratio to a stoichiometric air-fuel ratio.

[0010]

According to the present invention, an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine and an air-fuel ratio upstream of the exhaust gas purifying catalyst are detected. An upstream air-fuel ratio sensor, a downstream air-fuel ratio sensor for detecting an air-fuel ratio downstream of the exhaust purification catalyst, and a fuel injection amount to be injected into the internal combustion engine. A fuel injection amount calculating means for calculating the fuel injection amount based on an air-fuel ratio correction coefficient calculated from a detection value of the upstream air-fuel ratio sensor. The main air-fuel ratio control means for bringing the air-fuel ratio to be approached to a predetermined target air-fuel ratio, and the value of a control constant related to the air-fuel ratio correction coefficient based on the detection value of the downstream air-fuel ratio sensor are determined in advance. The sub air-fuel ratio control means for increasing / decreasing the ratio and the main air-fuel ratio control means and the sub air-fuel ratio control means are operated to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to the target air-fuel ratio. Switching from the first state to the second state in which only the main air-fuel ratio control means is operated to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to a predetermined air-fuel ratio slightly richer than the target air-fuel ratio. A control constant increasing / decreasing means for increasing / decreasing a value of the control constant to a predetermined value, and a value of the control constant at the time of switching from the second state to the first state, from the first state to the first state. An air-fuel ratio control device for an internal combustion engine, comprising: control constant setting means for setting a control constant value immediately before switching to the second state.

In the air-fuel ratio control device for an internal combustion engine according to the first aspect, the value of the control constant increased by the control constant increasing / decreasing means as the value of the control constant when switching from the second state to the first state. Is used instead of a relatively large value, i.e., a relatively small value that is the value of the control constant immediately before switching from the first state to the second state. Therefore, from the high load acceleration operation state in which only the main air-fuel ratio control means in which the air-fuel ratio detected by the upstream air-fuel ratio sensor is slightly rich is operated.
After the operation state of the internal combustion engine is switched to a steady operation state in which the main air-fuel ratio control means and the sub air-fuel ratio control means are operated to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to the stoichiometric air-fuel ratio, The air-fuel ratio detected by the side air-fuel ratio sensor can quickly converge from a slightly rich state to the stoichiometric air-fuel ratio. Therefore, emission can be improved, and deterioration of fuel efficiency due to useless fuel injection can be prevented.

According to the second aspect of the present invention, there is provided an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine,
An upstream air-fuel ratio sensor for detecting an air-fuel ratio on the upstream side of the exhaust purification catalyst, a downstream air-fuel ratio sensor for detecting an air-fuel ratio on the downstream side of the exhaust purification catalyst, and A fuel injection amount calculating means for calculating a fuel injection amount to be injected based on an operation state of the internal combustion engine, and the fuel injection based on an air-fuel ratio correction coefficient calculated from a detection value of the upstream air-fuel ratio sensor. Main air-fuel ratio control means for increasing or decreasing the air-fuel ratio detected by the upstream air-fuel ratio sensor to approach a predetermined target air-fuel ratio, and correcting the air-fuel ratio based on the detection value of the downstream air-fuel ratio sensor. Sub air-fuel ratio control means for increasing or decreasing the value of a control constant relating to the coefficient at a predetermined rate, and the above-mentioned air-fuel ratio for making the air-fuel ratio detected by the upstream air-fuel ratio sensor close to the target air-fuel ratio. In order to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to a predetermined air-fuel ratio slightly richer than the target air-fuel ratio from the first state in which the in-air-fuel ratio control means and the sub air-fuel ratio control means are operated. Control constant increasing / decreasing means for increasing / decreasing a value of a control constant to a predetermined value when switching to the second state in which only the main air-fuel ratio control means is operated; and from the second state to the first state. Control constant correction means for increasing or decreasing the value of the control constant relating to the air-fuel ratio correction coefficient at a rate larger than the predetermined rate after the switching of the air-fuel ratio. An apparatus is provided.

[0013] In the air-fuel ratio control apparatus for an internal combustion engine according to the second aspect, after switching from the second state to the first state,
The value of the control constant relating to the air-fuel ratio correction coefficient is not reduced at a predetermined ratio used in the sub-air-fuel ratio control, but is reduced at a larger ratio than the predetermined ratio. Therefore, the air-fuel ratio detected by the upstream air-fuel ratio sensor is changed from the stoichiometric air-fuel ratio to the air-fuel ratio detected by the upstream air-fuel ratio sensor from a high-load acceleration operation state in which only the main air-fuel ratio control means is slightly rich. After the operation state of the internal combustion engine is switched to a steady operation state in which the main air-fuel ratio control means and the sub air-fuel ratio control means are operated in order to approach the upstream air-fuel ratio sensor, the upstream air-fuel ratio sensor is reduced. Can quickly converge from a slightly rich state to a stoichiometric air-fuel ratio. Therefore, emission can be improved, and deterioration of fuel efficiency due to useless fuel injection can be prevented.

According to the third aspect of the present invention, the predetermined value is a value of the control constant at the end of the first state and an average value of the control constant during the first state. 3. The method according to claim 1, wherein the difference is determined based on a difference between
The invention provides an air-fuel ratio control device for an internal combustion engine.

In the air-fuel ratio control device for an internal combustion engine according to the third aspect, the predetermined value used by the control constant increasing / decreasing means for increasing / decreasing the value of the control constant is determined when the first state ends. The determination is made based not only on the value of the control constant but also on the difference between the value of the control constant at the end of the first state and the average value of the control constants in the first state. Therefore, highly accurate air-fuel ratio control can be performed.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is an overall schematic diagram showing a first embodiment of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention. In FIG. 1, 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 supplied to the A / D converter 101 with a built-in multiplexer of the control circuit 10. The distributor 4 has its shaft converted to, for example, a crank angle of 72.
A crank angle sensor 5 for generating a reference position detection pulse signal every 0 ° and a crank angle sensor 6 for generating a reference position detection pulse signal every 30 ° in terms of crank angle are provided. The pulse signals of these crank angle sensors 5 and 6 are supplied to the input / output interface 102 of the control circuit 10.
And the output of the crank angle sensor 6 is C
It is supplied to the interrupt terminal of the PU 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 generates an analog voltage electric signal corresponding to the cooling water temperature THW. This output is also supplied to the A / D converter 101.

An exhaust system downstream of the exhaust manifold 11 has a catalytic converter 12 containing a three-way catalyst for simultaneously purifying three harmful components HC, CO and NOx in the exhaust gas.
Is provided. A first O ₂ sensor 13 is provided in the exhaust manifold 11, that is, upstream of the catalytic converter 12.
And an exhaust pipe 14 downstream of the catalytic converter 12.
Is provided with a second O ₂ sensor 15. 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 sensors 13 and 15 output different output voltages to the A / D converter 10 by the control circuit 10 depending on whether the air-fuel ratio is lean or rich with respect to the stoichiometric air-fuel ratio.
Occurs at 1.

The control circuit 10 is configured as, for example, a microcomputer, and includes an A / D converter 101, an input / output interface 102, a CPU 103, and a ROM 10
4, a RAM 105, a backup RAM 106, a clock generation circuit 107, and the like.

In the control circuit 10, the down counter 108, the flip-flop 109, and the driving circuit 1
Reference numeral 10 is for controlling the fuel injection valve 7. That is, when the control circuit 10 calculates the fuel injection amount TAU, 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 1
09 is set, and the drive circuit 110 stops energizing 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.

The CPU 103 generates an interrupt at A
For example, when the A / D conversion of the / D converter 101 is completed, when the input / output interface 102 receives a pulse signal of the crank angle sensor 6, or when an interrupt signal from the clock generation circuit 107 is received. The intake air amount data Q and the cooling water temperature data THW of the air flow meter 3 are taken in by an A / D conversion routine executed at predetermined time intervals, and are taken into RA.
It is stored in a predetermined area of M105. That is, the RAM 10
5, the data Q and THW are updated every predetermined time. The rotation speed data Ne is calculated by an interruption of the crank angle sensor 6 at every 30 ° CA, and is stored in the RAM.
105 is stored in a predetermined area.

Hereinafter, an air-fuel ratio control method by the air-fuel ratio control device for an internal combustion engine according to the present embodiment will be described. FIG. 2 is a flowchart illustrating an air-fuel ratio control method of the air-fuel ratio control device for an internal combustion engine according to the present embodiment. As shown in FIG. 2, when starting the air-fuel ratio control, the air-fuel ratio control device for the internal combustion engine determines in step 201 whether or not to execute the main air-fuel ratio feedback control. If so, the process proceeds to step 202, where the main air-fuel ratio feedback control is executed. On the other hand, if not executed, the process proceeds to step 205. The air-fuel ratio control device of the internal combustion engine is, for example, 4 ms
The main air-fuel ratio feedback control is executed to calculate the air-fuel ratio correction coefficient FAF based on the output of the upstream O ₂ sensor 13 every predetermined time such as ec.

FIG. 3 is a flowchart showing a main air-fuel ratio feedback control routine of FIG. As shown in FIG. 3, the air-fuel ratio control device for the internal combustion engine first determines in step 301 whether a closed-loop (feedback) condition for the air-fuel ratio by the upstream O ₂ sensor 13 is satisfied. For example, when the coolant temperature is below a predetermined value, during engine start, when in after startup increase in warming increase in power boosting, the output signal of the upstream O ₂ sensor 13 is not also inverted once, the fuel cut moderate In any case, the closed loop condition is not satisfied, and in other cases, the closed loop condition is satisfied. When the closed loop condition is not satisfied, the process proceeds to step 321 to set the air-fuel ratio correction coefficient FAF to 1.0. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 302.

In step 302, the output V ₁ of the upstream O ₂ sensor 13 is A / D converted and taken in. In step 303, the comparison voltage V _R1 where V ₁ is, for example, 0.45V
It is determined whether or not it is below, that is, whether the air-fuel ratio is rich or lean. If lean (V ₁ ≦ V _R1), the first delay counter CDL at step 304
Y1 is decremented by one, and in steps 305 and 306, the first delay counter CDLY1 is guarded by the minimum value TDR1. Note that the minimum value TDR1 is a rich delay time for maintaining the determination that the output is the lean state even when the output of the upstream O ₂ sensor 13 changes from lean to rich, and is defined as a negative value. You. On the other hand, rich (V ₁ >
If V _R1 ), the first delay counter CDLY1 is incremented by 1 in step 307, and steps 308 and 30 are added.
9, the first delay counter CDLY1 is set to the maximum value TD.
Guard at L1. It should be noted that the maximum value TDL1 is a lean delay time for maintaining the judgment that the state is a rich state even when the output of the upstream O ₂ sensor 13 changes from rich to lean, and is defined as a positive value. You. Here, the reference of the first delay counter CDLY1 is set to 0, and CD
When LY1> 0, the air-fuel ratio after the delay processing is regarded as rich, and when CDLY1 ≦ 0, the air-fuel ratio after the delay processing is regarded as lean.

In step 310, it is determined whether or not the sign of the first delay counter CDLY1 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 311 whether the air-fuel ratio is inverted from rich to lean or from lean to rich. If the inversion is from rich to lean, in step 312, the air-fuel ratio correction coefficient FAF is set to FAF ← F
On the other hand, if the inversion is from lean to rich, the air-fuel ratio correction coefficient FAF is skipped to FAF ← FAF−RSL in step 313. That is, skip processing is performed.

In step 310, if the sign of the first delay counter CDLY1 is not inverted, the integration processing is performed in steps 314, 315, 316. That is,
At step 314, it is determined whether or not CDLY1 ≦ 0,
If CDLY1 ≦ 0 (lean), the air-fuel ratio correction coefficient FAF is set to FAF ← FAF + KI in step 315, while if CDLY1> 0 (rich), step 316 is performed.
, The air-fuel ratio correction coefficient FAF is set to FAF ← FAF-KI. Here, the integration constant KI is the skip constant RSR, RS
L is set sufficiently smaller than L, that is, KI <R
SR (RSL). Therefore, step 315 gradually increases the fuel injection amount in the lean state (CDLY1 ≦ 0), and step 316 gradually decreases the fuel injection amount in the rich state (CDLY> 0).

Steps 312, 313, 315, 316
The air-fuel ratio correction coefficient FAF calculated in
Guarded at a minimum value such as 0.8 at 7, 318, and at 1.2 at steps 319 and 320, for example.
Is guarded by the maximum value such as 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 by the value to prevent over-rich or over-lean.
The air-fuel ratio correction coefficient FAF calculated as described above is
The program is stored in M105, and this routine ends in step 322. Step 321 in FIG. 3 can be omitted. In this case, the value immediately before the end of the air-fuel ratio feedback control is used as the FAF.

FIG. 4 is a timing chart for supplementarily explaining the operation according to the flowchart of FIG. Upstream O ₂ sensor 1
When the air-fuel ratio signal A / F1 for rich / lean discrimination as shown in FIG. 4A is obtained from the output of FIG. 3, the first delay counter CDLY1 counts up in a rich state as shown in FIG. And the counter is decremented in the lean state. As a result, the air-fuel ratio signal A / F1 'subjected to the delay processing is formed as shown in FIG. For example, even if the air-fuel ratio signal A / F1 is changed from lean to rich at time t _1, the air-fuel ratio signal A / F1 delayed processed '
To change to the rich at time t ₂ after being held to lean only rich delay time (-TDR1). Even if the air-fuel ratio signal A / F1 changes from rich to lean at time t _{3, the} air-fuel ratio signal A / F1 ′ subjected to the delay processing has the lean delay time T
It changes to lean at time t ₄ after being held to the rich only DL1 equivalent. However, the air-fuel ratio signal A / F1 is at time t
_5, t _6, t ₇ rich delay time as the (-TDR1)
Inverting in a shorter time, the first delay counter C
DLY1 takes time to cross a reference value of 0, a result, the air-fuel ratio signal A / F1 after the delay is reversed at time t _8. That is, the air-fuel ratio signal A / F after the delay processing
1 'is more stable than the air-fuel ratio signal A / F1 before the delay processing. Thus, the stable air-fuel ratio signal A after the delay processing
/ F1 ′, the air-fuel ratio correction coefficient F shown in FIG.
AF1 is obtained.

Returning to FIG. 2, the air-fuel ratio control device for the internal combustion engine subsequently determines in step 203 whether or not to execute the sub-air-fuel ratio feedback control. If it is to be executed, the routine proceeds to step 204, where sub air-fuel ratio feedback control is executed. On the other hand, if not executed, the process proceeds to step 205. The air-fuel ratio control device of the internal combustion engine performs the downstream O ₂ sensor 1 every predetermined time, for example, 1 _second.
5, the skip amounts RSR, R as constants related to the main air-fuel ratio feedback control described above.
The sub air-fuel ratio feedback control is executed to set SL or the like.

The sub air-fuel ratio feedback control includes skip amounts RSR, RSL, delay time TDR1,
TDL1, integration constant KI (in this case, rich integration constant K
Set separately I1R and lean integration constant KI1L), or a system for the comparison voltage V _R1 of the output V ₁ of the upstream O ₂ sensor 13 to the variable, and a system for introducing a second air-fuel ratio correction coefficient FAF2 is there.

For example, if the rich skip amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and even if the rich skip amount RSR is reduced, the control air-fuel ratio can be shifted to the lean side. Therefore, the air-fuel ratio can be controlled by correcting the rich skip amount RSR and the lean skip amount RSL according to the output of the downstream O ₂ sensor 15. If rich delay time (-TDR1)> lean delay time (TDL1) is set, the control air-fuel ratio can shift to the rich side, and conversely, lean delay time (TDL1)> rich delay time (-TDR1) Then, the control air-fuel ratio can shift to the lean side. That is, the air-fuel ratio can be controlled by correcting the delay time TDR1, TDL1 in accordance with the output of the downstream O ₂ sensor 15. Furthermore, if the rich integration constant KI1R is increased, the control air-fuel ratio can be shifted to the rich side, and if the lean integration constant KI1L is reduced, the control air-fuel ratio can be shifted to the rich side, while the lean integration constant KI1L is increased. Then, the control air-fuel ratio can be shifted to the lean side, and even if the rich integration constant KI1R is reduced, the control air-fuel ratio can be shifted to the lean side. Therefore, the downstream O ₂
The air-fuel ratio can be controlled by correcting the rich integration constant KI1R and the lean integration constant KI1L according to the output of the 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
Is reduced, 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
_The air-fuel ratio can be controlled by correcting _R1 .

Returning to the drawings, FIGS. 5 and 6 are flowcharts showing the sub air-fuel ratio feedback control routine of FIG. As shown in FIGS. 5 and 6, the air-fuel ratio control device for the internal combustion engine firstly performs
_{2 It} is determined whether or not the condition is a closed loop condition by the sensor 15. For example, when the air-fuel ratio detected by the upstream O ₂ sensor 15 is set to be slightly rich (λ <1.0) during the high load acceleration operation of the internal combustion engine, when the cooling water temperature is equal to or lower than a predetermined value, the downstream O ₂ sensor When the output signal at 15 is never inverted, when the downstream O ₂ sensor 15 is out of order, or when the internal combustion engine is in a transient operation, the closed loop condition is not satisfied, and in other cases, the closed loop condition is satisfied. . If the condition is not the closed loop condition, the process proceeds to step 531. If the condition is the closed loop condition, the process proceeds to step 531.
Move to 02.

In step 502, it is determined whether or not sub-feedback has been performed during the previous execution of the sub-air-fuel ratio feedback control routine 204.
At 01, it is determined whether the determination is Yes or No. Previous sub air-fuel ratio feedback control routine 20
If the sub-feedback is executed during the execution of step 4, that is, if Yes is determined in step 501, the process proceeds to step 504. On the other hand, when the sub feedback is not executed at the time of executing the previous sub air-fuel ratio feedback control routine 204,
If No is determined in step 1, the process proceeds to step 503 to set the rich skip amount RSR to RSR ← RSRi (RSR
i is described in step 530), and the lean skip amount RSL is set to RSL ← RSLi (RSL
i will be described in step 530), and then the process proceeds to step 504.

In step 504, the downstream O ₂ sensor 15
A / D-converts and takes in the output V ₂ of
Comparison voltage as the output V _2, for example, 0.55V at V _R2
The downstream O ₂ sensor 15
It is determined whether the detected air-fuel ratio is rich or lean. still,
The comparison voltage V _R2 is set to be higher than the comparison voltage V _R1 of the output of the upstream O ₂ sensor 13 in consideration of, for example, the difference in output characteristics due to the influence of raw gas and the difference in the degradation speed between the upstream and downstream of the catalytic converter. Is done. Lean (V ₂ ≤VR ₂ )
If so, in step 506, the second delay counter CDLY2 is decremented by 1, and in steps 507 and 508, the second delay counter CDLY2 is guarded by the minimum value TDR2. Note that the minimum value TDR2 is a rich delay time for maintaining a lean state even when there is a change from lean to rich, and is defined as a negative value. On the other hand, rich (V
If _2> V _R2), the second delay counter CDLY2 is incremented by one at step 509, step 510,
At 511, the second delay counter CDLY2 is guarded by the maximum value TDL2. Note that the maximum value TDL2 is a lean delay time for maintaining a rich state even when there is a change from rich to lean, and is defined as a positive value. Here, the reference of the second delay counter CDLY2 is set to 0, the air-fuel ratio after the delay processing is regarded as rich when CDLY2> 0, and the air-fuel ratio after the delay processing is regarded as lean when CDLY2 ≦ 0. I do.

[0036] At step 512, reads the intake air amount data Q from the RAM 105, to determine whether Q> Q _1, determines whether or not step 513 the Q> Q _2.
Note that Q ₁ <Q _2. For example, Q ₁ = 20 m ³ / h, Q ₂
= 80 m ³ / h. As a result, if Q ≦ Q ₁ , in step 514, the upper limits L of the skip amounts RSR and RSL are set.
If U is 4% and Q ₁ <Q ≦ Q ₂ , step 515
At the upper limit value LU is 6% to 8% the upper limit LU at step 516 if Q> Q _2. In this way,
As the parameter Q increases, the skip amounts RSR, RS
The upper limit LU of L is set large. Note that the upper limit LU may be obtained as a value that continuously changes by interpolation calculation.
Also, instead of the intake air amount Q, the calculation may be performed in accordance with the vehicle speed SPD, the engine speed Ne, the load Q / Ne, and the throttle valve opening degree TA. May be used for interpolation calculation.

At the throttle 517, it is determined whether or not the second delay counter CDLY2 satisfies CDLY2 ≦ 0. As a result, if CDLY2 ≦ 0, the air-fuel ratio is determined to be lean, and steps 518 to 523 are performed. On the other hand, if CDLY2> 0, it is determined that the air-fuel ratio is rich, and the routine proceeds to steps 524 to 529. At step 518, RSR ← RSR + ΔRS (for example, a constant value of 0.08%), that is, the rich skip amount RSR is increased to shift the air-fuel ratio to the rich side. In steps 519 and 520, the rich skip amount RSR is guarded by the upper limit LU. Further, in step 521,
RSL ← RSL−ΔRS, that is, the lean skip amount RSL is reduced to shift the air-fuel ratio to the rich side.
In steps 522 and 523, the lean skip amount RSL
Is guarded at a lower limit of, for example, 2.5%. On the other hand, when rich (V ₂ > V _R2 ), at step 524, R
SR ← RSR−ΔRS, that is, the rich skip amount RSR is reduced to shift the air-fuel ratio to the lean side. In steps 525 and 526, guarding is performed with the rich skip amount RSR at the lower limit of 2.5%. Further, step 527
Is set to RSL ← RSL + ΔRS, that is, the lean skip amount RSL is increased to shift the air-fuel ratio to the lean side. In steps 528 and 529, the lean skip amount R
Guard SL at upper limit LU. The skip amounts RSR and RSL calculated as described above are stored in the RAM 105 as RSRi and RSLi, respectively, in step 530, and then the process proceeds to step 533 to execute this routine 20
4 is ended.

On the other hand, in step 531, as in step 502, whether sub-feedback has been performed during the previous execution of the sub-air-fuel ratio feedback control routine 204, that is, whether Yes was determined in step 501 or not is determined. It is determined whether it has been determined. If the sub feedback is not executed during the previous execution of the sub air-fuel ratio feedback control routine 204, that is, if No is determined in step 501, the process proceeds to step 533, that is, the skip amounts RSR and RSL are increased or decreased. This routine 204 ends without being performed. On the other hand, if the sub feedback is executed during the previous execution of the sub air-fuel ratio feedback control routine 204, that is, if it is determined Yes in step 501, step 53 is executed.
2 and the rich skip amount RSR is changed to RSR ← RSR.
+ Α and the lean skip amount RSL is RSL ← R
Then, the routine proceeds to step 533, where the routine 204 is terminated.

The predetermined value α is determined by the skip amount RSR (RSR in FIG. 7) at the end of the previous sub air-fuel ratio feedback control routine 204 (time t _{1 in} FIG. 7 described later).
i) and RSL, engine speed Ne, intake air amount Q, load Q / Ne, throttle valve opening TA, and the like.

FIG. 7 is based on the flowcharts of FIGS.
FIG. 9 is a timing chart for supplementarily explaining the operation of FIG. Figure 7
The solid line indicates the change in the rich skip amount RSR of the present embodiment.
And the broken line indicates the rich skip amount RS of the prior art.
R indicates the change in R, and the two-dot chain line
9 shows a change in the rich skip amount RSR of the mode. Figure
As shown in FIG.₁Previous internal combustion engine steady state
During operation, the upstream O _TwoThe air-fuel ratio detected by the sensor 13 is
Main air-fuel ratio feedback to approximate stoichiometric air-fuel ratio
Control and sub air-fuel ratio feedback control are executed
In both the embodiment and the prior art, the rich skip amount RSR is simply
The value is reduced by a constant value ΔRS for each time period. Then at time t
₁At the time of switching to high load acceleration operation of the internal combustion engine in
In both the present embodiment and the prior art, the rich skip amount RSR is
The predetermined value α is added. Then at time t₁~ T_TwoIn
During high load acceleration operation of the internal combustion engine, the upstream O_TwoSensor 1
3 approaches the slightly richer air-fuel ratio.
Only the main air-fuel ratio feedback control is
In both the present embodiment and the prior art, the rich skip amount RS
R is maintained as it is. Then at time t_TwoLater
At the time of steady operation of the internal combustion engine, at the time t₁
As before, the rich skip amount RSR is one per unit time.
It is reduced only by the constant value ΔRS. Therefore, the air-fuel ratio
It is possible to quickly converge from a little rich to the stoichiometric air-fuel ratio.
I can't. On the other hand, in the case of the present embodiment, the time t_TwoIn
When the operating state of the fuel engine is switched, the rich skip amount RSR
Is time t₁Set to the immediately preceding rich skip amount RSRi
You. As a result, the air-fuel ratio can be quickly changed from slightly rich to the stoichiometric air-fuel ratio.
It can be converged quickly.

Returning to FIG. 2, the air-fuel ratio control device for the internal combustion engine subsequently determines in step 205 whether or not to end this air-fuel ratio control. For example, when stopping the internal combustion engine, the air-fuel ratio control ends, and when the operation of the internal combustion engine is continued, step 201 is executed again.

FIG. 8 is a flowchart showing a fuel injection amount calculation routine for calculating the fuel injection amount using the air-fuel ratio correction coefficient FAF calculated by the air-fuel ratio control shown in FIGS. . This fuel injection amount calculation routine is executed at every predetermined crank angle such as every 360 ° CA. In step 801, the intake air amount data Q and the rotation speed data Ne are read from the RAM 105 to calculate the basic injection amount TAUP. For example, TAUP ← KQ
/ Ne (K is a constant). RAM at step 802
105 reads out cooling water temperature data THW from ROM 105
04, the warm-up increase value FWL by the one-dimensional map stored in
Is calculated by interpolation. In step 803, the final injection amount TA
U is expressed as TAU ← TAUP · FAF · ((FWL + p) +
q). Here, p and q are correction amounts determined by other operation state parameters. Then, step 8
At 04, the fuel injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection. Then, in step 805, this routine ends.

The main air-fuel ratio feedback control is 4
The sub-air-fuel ratio feedback control is performed every msec, and the sub-air-fuel ratio feedback control is performed every 1 second. The air-fuel ratio feedback control is performed mainly by the upstream O ₂ sensor 13 having good responsiveness, and the downstream O ₂ sensor having poor responsiveness. _This is because the control by the _two sensors 15 is performed in compliance.

Hereinafter, a second embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention will be described. The overall configuration of the present embodiment is substantially the same as the overall configuration of the first embodiment shown in FIG. FIG. 9 is a flowchart illustrating an air-fuel ratio control method of the air-fuel ratio control device for an internal combustion engine according to the present embodiment. The flowchart shown in FIG. 9 is different from the flowchart shown in FIG. 2 in the sub air-fuel ratio feedback control 204 ′. FIGS. 10 and 11 are flowcharts showing the sub air-fuel ratio feedback control routine of FIG. As shown in FIGS. 10 and 11, the difference between the sub air-fuel ratio feedback control 204 (FIGS. 5 and 6) and the sub air-fuel ratio feedback control 204 ′ (FIGS. 10 and 11) is that 'In that steps 502 and 503 do not exist and steps 551 to 551
The point 56 is provided and the step 530 'is modified.

The sub air-fuel ratio feedback control 204 'will be described with reference to the timing chart of FIG. Time t ₂
When the operation state of the internal combustion engine is switched from the high-load acceleration operation to the steady operation in the above, the air-fuel ratio control device of the internal combustion engine skips steps 552 and 553 when the air-fuel ratio is lean and RSR is smaller than RSRi. Quantity RS
R and RSL are RSR ← RSR + ΔKLRSR, RSL ←
RSL-ΔKLRSR. On the other hand, if the air-fuel ratio is rich and RSR is greater than RSR, step 55
In steps 5 and 556, the skip amounts RSR and RSL are set to RS
R ← RSR−ΔKLRSR, RSL ← RSL + ΔKLR
SR. Here, ΔKLRSR is a value larger than ΔRS.

That is, during the steady operation of the internal combustion engine after time t ₂ , in the case of the conventional technique (dotted line in the figure), at time t 2
_As before ₁ , the rich skip amount RSR is reduced only by a constant value ΔRS per unit time. Therefore, it is not possible to quickly converge the air-fuel ratio from slightly rich to the stoichiometric air-fuel ratio. On the other hand, in the case of the present embodiment, the rich skip amount RSR is a value ΔKL larger than ΔRS for each unit time.
It is reduced by RSR until it becomes RSRi. for that reason,
The air-fuel ratio can be quickly converged from slightly rich to the stoichiometric air-fuel ratio.

The value ΔKLRSR may be a constant value as long as it is larger than the value ΔRS, or may be a variable value according to, for example, the engine load. Also, the value ΔKLR
The SR, can be variable in accordance with the length of the main air-fuel ratio feedback control execution time (time t ₁ ~ time t ₂ in FIG. 7). If the execution time of the main air-fuel ratio feedback control is long, the amount of oxygen stored in the catalytic converter 12 is reduced, so that the air-fuel ratio is quickly returned to the stoichiometric air-fuel ratio, that is, the amount of oxygen in the catalytic converter 12 is increased. The value ΔKLRSR is set to a large value for the purpose of. On the other hand, when the execution time of the main air-fuel ratio feedback control is short, since the amount of oxygen stored in the catalytic converter 12 is relatively large, the air-fuel ratio is stabilized, that is,
The value ΔKLRSR is set to a small value for the purpose of stabilizing combustion.

Alternatively, the value ΔKLRSR is set to the execution interval of the main air-fuel ratio feedback control (4 m in this embodiment).
It is also possible to make it variable according to (sec). When the execution interval of the main air-fuel ratio feedback control is large, the value ΔKLRSR is set large, and when the execution interval of the main air-fuel ratio feedback control is small, the value ΔKLRSR is set small. Further, the value ΔKLRSR is set to the rich integration time of the main air-fuel ratio feedback control (FIG. 4D).
Of time t ₄ ~ time t ₈₎ and the lean integration time (Fig. 4
It is also possible to variably depending on the time t ₂ ~ time t ₄₎ of (D). When the value of (rich integration time / lean integration time) is large, the value ΔKLRSR is set to be large, and when the value is small, the value ΔKLRSR is set to be small.

For the above-described reason, the operating state of the internal combustion engine is switched from the high load acceleration operating state to the fuel cut state (both main and sub air-fuel ratio feedback controls are open) and subsequently switched to the steady operating state. In this case, the value ΔKLRSR is set to a small value, while the value ΔKLRSR is set to a large value when the OT is increased from the high-load acceleration operation state (both the main and sub air-fuel ratio feedback controls are opened), and subsequently the state is switched to the steady operation state. .

Incidentally, the main air-fuel ratio feedback control routine and the fuel injection amount calculation routine of this embodiment are almost the same as those of the first embodiment.

As a modification of the present embodiment and the first embodiment, step 53 is replaced with step 53.
RSR ← RSR + α + α ′ and RSL ← RS as 2 ′
It is also possible to perform L-α-α ′. Where α '
This means that the skip amount RSRi at the end of the period in which the main air-fuel ratio feedback control and the sub-air-fuel ratio feedback control are executed (time t _{1 in} FIG. 7), the main air-fuel ratio feedback control, and the sub-air-fuel ratio feedback control it is a value calculated based on the difference between the skip amount average value RSRG during the period that runs (time t ₁ earlier). In step 532 ', the skip amount RSR of time t ₁ in FIG. 7
When i and the average value RSRG of the skip amounts before time t ₁ are significantly different, that is, even when the skip amount RSRi includes a large error, the skip amount RSR from time t ₁ to time ₂ can be accurately calculated. Can be set,
Therefore, highly accurate air-fuel ratio control can be performed.

A double O ₂ sensor system for correcting other constants, such as a delay time and an integration constant, related to the air-fuel ratio feedback control by the upstream O ₂ sensor 13 by the output of the downstream O ₂ sensor 15. The present invention can also be applied to Further, controllability can be improved by simultaneously controlling two of the skip amount, the delay time, and the integration constant. Further, it is also possible to fix one of the skip amounts RSR and RSL and to make only the other variable, instead of the delay time TDR.
It is also possible to fix one of TDL1 and TDL1 and make only the other variable, or to fix one of the rich integration constant KIR and lean integration constant KIL and make the other variable. Further, instead of the air flow meter, a Karman vortex sensor, a heat wire sensor, or the like may be used as the intake air amount sensor.

In the above embodiment, the fuel injection amount is calculated according to the intake air amount and the engine speed.
The fuel injection amount may be calculated according to the intake air pressure and the engine speed, or the throttle valve opening and the engine speed. 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,
Electric air control valve (EAC
V) to control the air-fuel ratio by adjusting the intake air amount of the engine, and to adjust the air bleed amount of the carburetor by the electric freed air control valve and to introduce air into the main system passage and the slow system passage. The present invention can be applied to a device for controlling a fuel ratio, a device for adjusting an amount of secondary air sent to an exhaust system of an engine, and the like. In this case, the basic fuel injection amount corresponding to the basic injection amount TAUP in step 801 is determined by the carburetor itself,
That is, it is determined according to the intake pipe negative pressure according to the intake air amount and the engine speed, and at step 803, the supply air amount corresponding to the final fuel injection amount TAU is calculated. Furthermore,
In the above-described embodiment, the O ₂ sensor is used as the air-fuel ratio sensor, but a CO sensor, a lean mixture sensor, or the like may be used. Further, the above-described embodiment is configured by a microcomputer, that is, a digital circuit, but may be configured by an analog circuit.

[0054]

According to the invention described in claim 1 or 2,
The air-fuel ratio detected by the upstream air-fuel ratio sensor is slightly rich The air-fuel ratio detected by the upstream air-fuel ratio sensor approaches the stoichiometric air-fuel ratio from the high load acceleration operation state in which only the main air-fuel ratio control means is operated. After the operating state of the internal combustion engine is switched to a steady operating state in which the main air-fuel ratio control means and the sub air-fuel ratio control means are operated, the air-fuel ratio detected by the upstream air-fuel ratio sensor is theoretically changed from a slightly rich state. It is possible to quickly converge to the air-fuel ratio. Therefore, emission can be improved, and deterioration of fuel efficiency due to useless fuel injection can be prevented.

According to the third aspect of the present invention, highly accurate air-fuel ratio control can be performed.

[Brief description of the drawings]

FIG. 1 is an overall schematic diagram of a first embodiment of an air-fuel ratio control device for an internal combustion engine of the present invention.

FIG. 2 is a flowchart illustrating an air-fuel ratio control method of the air-fuel ratio control device for an internal combustion engine according to the first embodiment.

FIG. 3 is a flowchart illustrating a main air-fuel ratio feedback control routine according to the first embodiment.

FIG. 4 is a timing chart for supplementarily explaining the operation according to the flowchart of FIG. 3;

FIG. 5 is a flowchart illustrating a sub air-fuel ratio feedback control routine according to the first embodiment.

FIG. 6 is a flowchart illustrating a sub air-fuel ratio feedback control routine according to the first embodiment.

FIG. 7 is a timing chart for supplementarily explaining the operation according to the flowcharts of FIGS. 5 and 6;

FIG. 8 is a flowchart illustrating a fuel injection amount calculation routine according to the first embodiment.

FIG. 9 is a flowchart illustrating an air-fuel ratio control method of the air-fuel ratio control device for an internal combustion engine according to the second embodiment.

FIG. 10 is a flowchart illustrating a sub air-fuel ratio feedback control routine according to a second embodiment.

FIG. 11 is a flowchart illustrating a sub air-fuel ratio feedback control routine according to a second embodiment.

[Explanation of symbols]

1 ... engine 12 ... catalytic converter 13 ... upstream O ₂ sensor 15 ... downstream O ₂ sensor 10 ... control circuit

Claims (3)

[Claims] 1. An exhaust purification catalyst for purifying exhaust gas discharged from an internal combustion engine, an upstream air-fuel ratio sensor for detecting an air-fuel ratio on an upstream side of the exhaust purification catalyst, and a downstream of the exhaust purification catalyst. A downstream air-fuel ratio sensor for detecting an air-fuel ratio on the side, a fuel injection amount calculating means for calculating a fuel injection amount to be injected to the internal combustion engine based on an operating state of the internal combustion engine, For increasing or decreasing the fuel injection amount based on an air-fuel ratio correction coefficient calculated from a detection value of an upstream-side air-fuel ratio sensor to bring the air-fuel ratio detected by the upstream-side air-fuel ratio sensor closer to a predetermined target air-fuel ratio. Main air-fuel ratio control means, and sub-air-fuel ratio control means for increasing or decreasing a value of a control constant related to an air-fuel ratio correction coefficient at a predetermined rate based on a detection value of the downstream air-fuel ratio sensor, The first air-fuel ratio control means and the sub air-fuel ratio control means are operated to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to the target air-fuel ratio.
When switching from the state to the second state in which only the main air-fuel ratio control means is operated to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to a predetermined air-fuel ratio slightly richer than the target air-fuel ratio, A control constant increasing / decreasing means for increasing / decreasing a value of the control constant to a predetermined value, and a value of the control constant at the time of switching from the second state to the first state.
An air-fuel ratio control device for an internal combustion engine, comprising: a control constant setting unit that sets a value of a control constant immediately before switching from the state to the second state. 2. An exhaust purification catalyst for purifying exhaust gas discharged from an internal combustion engine, an upstream air-fuel ratio sensor for detecting an air-fuel ratio on an upstream side of the exhaust purification catalyst, and a downstream of the exhaust purification catalyst. A downstream air-fuel ratio sensor for detecting an air-fuel ratio on the side, a fuel injection amount calculating means for calculating a fuel injection amount to be injected to the internal combustion engine based on an operating state of the internal combustion engine, For increasing or decreasing the fuel injection amount based on an air-fuel ratio correction coefficient calculated from a detection value of an upstream-side air-fuel ratio sensor to bring the air-fuel ratio detected by the upstream-side air-fuel ratio sensor closer to a predetermined target air-fuel ratio. Main air-fuel ratio control means, and sub-air-fuel ratio control means for increasing or decreasing a value of a control constant related to an air-fuel ratio correction coefficient at a predetermined rate based on a detection value of the downstream air-fuel ratio sensor, The first air-fuel ratio control means and the sub air-fuel ratio control means are operated to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to the target air-fuel ratio.
When switching from the state to the second state in which only the main air-fuel ratio control means is operated to bring the air-fuel ratio detected by the upstream air-fuel ratio sensor closer to a predetermined air-fuel ratio slightly richer than the target air-fuel ratio, Control constant increasing / decreasing means for increasing / decreasing the value of the control constant to a predetermined value; and changing the value of the control constant relating to the air-fuel ratio correction coefficient after switching from the second state to the first state. An air-fuel ratio control device for an internal combustion engine, comprising: control constant correction means for increasing / decreasing at a rate larger than a predetermined rate. 3. The control device according to claim 2, wherein the predetermined value is determined based on a difference between a value of the control constant at the end of the first state and an average value of the control constant during the first state. The air-fuel ratio control device for an internal combustion engine according to claim 1 or 2, wherein: