JPH06317204A - Air-fuel ratio controller of internal combustion engine - Google Patents

Abstract

PURPOSE:To prevent extension of a converging time to hatching of air-fuel ratio and to a theoretical air-fuel ratio when the storage item is applied, in an air-fuel ratio controller of an internal combustion engine. CONSTITUTION:An air-fuel ratio controller is provided with a first air-fuel ratio sensor 11 arranged downstream from a three-way catalyst 10, an output linear-type second air-fuel ratio sensor 12 arranged upstream from the three-way catalyst 10, a coarse adjust item calculating means for calculating the coarse adjust item by using a first skip value by output of the first air-fuel ratio sensor 11, a storage item calculating means for calculating the storage item by using a second skip value by output of the first air-fuel ratio sensor 11 and a second skip value correcting means for correcting the second skip value according to output of the second air-fuel ratio sensor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control system for an internal combustion engine.

[0002]

2. Description of the Related Art Generally, an exhaust system of an internal combustion engine is provided with a catalytic converter for purifying exhaust gas.
As this catalytic converter, a three-way catalytic converter is widely used which oxidizes harmful substances such as carbon monoxide and hydrocarbons in exhaust gas and reduces nitrogen oxides to convert them into harmless substances. The oxidation and reduction actions by this three-way catalytic converter are performed well in the exhaust gas of stoichiometric air-fuel ratio combustion, but when the air-fuel ratio becomes rich, the oxidation action becomes inactive due to lack of oxygen, and conversely lean. In this state, the reducing action becomes inactive due to excess oxygen.

Therefore, in an internal combustion engine having a three-way catalyst, it is necessary to feedback-control the air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio, and an air-fuel ratio control device therefor is described in Japanese Patent Laid-Open No. 3-160134. There is. This device has an air-fuel ratio sensor downstream of the three-way catalyst in the exhaust passage, and changes the air-fuel ratio correction amount based on the output of this air-fuel ratio sensor. To change this air-fuel ratio correction amount, perform proportional control when the output of the air-fuel ratio sensor reverses from the value indicating the exhaust gas rich state or lean state to the other state, and execute integral control when it does not reverse. Is calculated by performing the proportional control when the output of the air-fuel ratio sensor exceeds the predetermined range near the value indicating the stoichiometric air-fuel ratio combustion, and the integral control until it is within the predetermined range. Storage term is introduced. In this storage term, the second skip value used for the proportional control is set to be larger than the first skip value used in the rough adjustment term, and when the deviation of the air-fuel ratio is large, it is compared to the theoretical air-fuel ratio. It is intended to converge in a very short time.

Generally, a three-way catalyst has an ability to store oxygen, so that even if the air-fuel ratio becomes lean, excess oxygen is stored in the three-way catalyst to activate the reducing action. In addition, the stored oxygen is discharged even in the rich state to activate the oxidation action, and good exhaust gas purification performance can be maintained even if the air-fuel ratio deviates to some extent. This is called the O ₂ storage effect of the three-way catalyst. In the above-mentioned conventional technique, the air-fuel ratio sensor is arranged downstream of the three-way catalyst, and the actual air-fuel ratio fluctuates drastically, but since this fluctuation is mitigated by the O ₂ storage effect, a relatively stable output is obtained. Obtained and good air-fuel ratio control is realized. However, since the O ₂ storage effect decreases as the catalyst deteriorates, at this time, the second value set to a relatively large value when calculating the storage term.
If the skip value is used, it is overcorrected, and there is a problem that the skip is repeated on the opposite side in a short time and the air-fuel ratio is hatched. In order to solve this problem, Japanese Patent Laid-Open No. 4-8
Japanese Patent No. 6345 describes a device further having means for decreasing the second skip value each time the catalyst deteriorates.

[0005]

[Patent Document 1] Japanese Patent Application Laid-Open No. 4-86345
Also in the prior art of the publication, the second skip value is a constant during a predetermined period in which the degree of deterioration of the catalyst is equal. When the output of the air-fuel ratio sensor downstream of the three-way catalyst exceeds a predetermined range near the value indicating stoichiometric air-fuel ratio combustion, the storage term is applied, but at this time, if the actual air-fuel ratio deviation is not so large, The set second skip value is too large and may be overcorrected, and the air-fuel ratio may still be hatched. If the second skip value is set to a relatively small value in order to prevent this, the convergence time to the stoichiometric air-fuel ratio becomes longer when the actual deviation of the air-fuel ratio is large.

Therefore, it is an object of the present invention to perform hatching of the air-fuel ratio and convergence time to the stoichiometric air-fuel ratio in the air-fuel ratio control using the rough adjustment term and the storage term as described above, especially when the storage term is applied. It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine, which can prevent the extension of the engine.

[0007]

An air-fuel ratio control system for an internal combustion engine according to the present invention is a three-way catalyst installed in an exhaust system of an internal combustion engine, and a first three-way catalyst downstream of the three-way catalyst in the exhaust system.
The air-fuel ratio sensor, the output linear type second air-fuel ratio sensor arranged on the upstream side of the three-way catalyst of the exhaust system, and the output of the first air-fuel ratio sensor are on the rich side with a value indicating stoichiometric air-fuel ratio combustion as a boundary. Alternatively, when the lean side is reversed to the other side, proportional control is executed using the first skip value, and when the output is not reversed, integral control is executed to calculate the coarse adjustment term of the air-fuel ratio correction amount. And a second skip value is used to perform proportional control when the output of the first air-fuel ratio sensor exceeds a predetermined range in the vicinity of a value indicating stoichiometric air-fuel ratio combustion and becomes rich or lean, and the output is Storage term calculation means for performing integration control until the value falls within the range to calculate the storage term of the air-fuel ratio correction amount, and the output of the first air-fuel ratio sensor exceeds a predetermined range near the value indicating stoichiometric air-fuel ratio combustion. Rich side or lean And a second skip value correction means for correcting the second skip value according to the output of the second air-fuel ratio sensor, and an air-fuel ratio for adjusting the air-fuel ratio of the internal combustion engine based on the rough adjustment term and the storage term. Fuel ratio adjusting means,
And is provided.

[0008]

In the above-described air-fuel ratio control device for an internal combustion engine, the coarse adjustment term calculation means is arranged such that the output of the first air-fuel ratio sensor arranged on the downstream side of the three-way catalyst in the exhaust system has a value indicating the stoichiometric air-fuel ratio combustion. When performing the reverse from the rich side or the lean side to the other side, the proportional control is executed using the first skip value, and when the output does not reverse, the integral control is executed to calculate the coarse adjustment term of the air-fuel ratio correction amount, and the storage is performed. The term calculation means executes proportional control using the second skip value when the output of the first air-fuel ratio sensor exceeds the predetermined range near the value indicating the stoichiometric air-fuel ratio combustion and becomes the rich side or the lean side, and the output is The integral control is executed until it is within the range, the storage term of the air-fuel ratio correction amount is calculated, and the second skip value correction means is a predetermined range near the value at which the output of the first air-fuel ratio sensor indicates the stoichiometric air-fuel ratio combustion. Over rich side or lean At this time, the second skip value is corrected in accordance with the output of the output linear type second air-fuel ratio sensor arranged on the upstream side of the three-way catalyst of the exhaust system, and the air-fuel ratio adjusting means sets the coarse adjustment term and the storage term. Based on this, the air-fuel ratio of the internal combustion engine is adjusted.

[0009]

1 is a schematic sectional view of an air-fuel ratio control system for an internal combustion engine according to the present invention. In the figure, 1 is an internal combustion engine, 2 is an intake passage, and 3 is an exhaust passage. A throttle valve 5 interlocking with an accelerator pedal 4 is provided upstream of the surge tank 2a in the intake passage 2, and an air flow meter 6 for detecting the intake air amount is provided further upstream thereof. A fuel injection valve 7 is installed for each cylinder in the downstream passage of the surge tank 2a. Each fuel injection valve 7 is connected to a fuel tank 9 via a pump 8.

On the other hand, a three-way catalytic converter 10 for purifying the exhaust gas is arranged in the exhaust passage 3, and the exhaust gas at this position is rich or lean based on the oxygen concentration on the downstream side thereof. The first air-fuel ratio sensor 11 capable of detecting the
An output linear type second air-fuel ratio sensor 12 capable of detecting how rich or lean the exhaust gas at this position is based on the oxygen concentration is installed.

A control device 20 for controlling the amount of fuel injected by the fuel injection valve 7 is provided.
For example, it is composed of a microcomputer system,
D converter 20a, input / output interface 20b,
It has a CPU 20c, a ROM 20d, a RAM 20e, a backup RAM 20f, a clock generation circuit 20g and the like. The A / D converter 20a includes a first and a second
The outputs of the air-fuel ratio sensors 11 and 12, the output of the water temperature sensor 13 for detecting the engine cooling water temperature, and the output of the air flow meter 6 are supplied, and the input / output interface 2 is also supplied.
At 0b, the output of the throttle valve opening sensor 14 for detecting the opening of the throttle valve 5 and the distributor 15 are provided.
First crank angle sensor 16 that generates a pulse signal every °
And the output of the second crank angle sensor 17, which generates a pulse signal every 30 ° in terms of crank angle.

Further, in the control device 20, the down counter 20h, the flip-flop 20i and the drive circuit 20.
j is for controlling the fuel injection valve 7. When the fuel injection amount TAU is calculated in a fuel injection amount calculation routine which will be described later, the calculation result is set in the down counter 20h and the flip-flop 20i is simultaneously set. As a result, the drive circuit 20j energizes the fuel injection valve 7. The down counter 20h starts counting clock pulses (not shown), resets the flip-flop 20i when the value of the down counter 20h becomes zero, and the drive circuit 20j stops energizing the fuel injection valve 7. That is,
The fuel injection valve 7 is energized only during the period calculated by the fuel injection amount calculation routine, and the fuel corresponding to the calculation result TAU is supplied to each cylinder of the internal combustion engine 1.

FIG. 2 is a first flowchart for calculating a rough adjustment term that functions as an air-fuel ratio correction amount for determining the fuel injection amount TAU. This will be explained below.
This flowchart is executed, for example, every 16 ms, and first, at step 101, it is judged if the flag XFB indicating that the condition for performing the air-fuel ratio feedback control is satisfied is 1. For example, the flag XFB is 0 when the fuel is being cut or the fuel is being increased, and when the condition is not satisfied, the processing ends.

When the determination in step 101 is affirmative, the routine proceeds to step 102, where the output V _{ox of} the first air-fuel ratio sensor 11 is a value V _R (eg 0.45) indicating stoichiometric air-fuel ratio combustion.
V) is less than or equal to. When this determination is affirmed, in step 103, the flag XOX indicating the state of the exhaust gas this time is set to the value 0 indicating the lean state, and the routine proceeds to step 104.

In step 104, it is judged whether or not the flag XOXO showing the state of the previous exhaust gas is the value 1 showing the rich state. When this determination is denied, the routine proceeds to step 105, where the output V of the first air-fuel ratio sensor 11
_It is determined whether or not the counter CNT representing the cycle in which _OX is inverted from the lean side and the rich side to the other side is smaller than the predetermined value KCNT. When this judgment is affirmed, step 10
At 6, the counter CNT is incremented by 1 and ends.

When the determination in step 105 is negative, that is, when the counter CNT reaches the predetermined value KCNT, the counter CNT is set to 0 in step 107.
To the coarse adjustment term A in step 108.
Fc is increased by ΔAFc2. That is, if the lean state of the exhaust gas is maintained, the coarse adjustment term AFc indicates the KCNT of the execution interval (for example, 16 ms) of this flowchart.
Each time the doubled time elapses, ΔAFc2 is increased by an integral amount.

When the determination in step 104 is affirmative, that is, when the exhaust gas reverses from the rich state to the lean state, the flag XOXO is set in step 109.
Is set to 0, and in step 110, the coarse adjustment term A
Fc is increased by ΔAFc1 in a skip manner, the counter CNT is reset to 0 in step 111, and the process ends.

On the other hand, when the determination in step 102 is negative, that is, when the exhaust gas is rich, the flag XOX is set to 1 in step 113. Hereinafter, the same processing as in the exhaust gas lean state is executed. However, step 117 corresponding to step 108
, The coarse adjustment term AFc is reduced by ΔAFc2 in an integrated manner, and step 119 corresponding to step 110 is performed.
In, the coarse adjustment term AFc is skipped by ΔAFc1.

When the deviation of the air-fuel ratio from the theoretical air-fuel ratio is relatively small, good air-fuel ratio control can be realized by controlling the fuel injection amount based on this rough adjustment term AFc, but if this deviation becomes large. Since the first skip value ΔAFc1 and the first integration value ΔAFc2 in the rough adjustment term AFc are set to relatively small values, it takes a long time to return the air-fuel ratio to the stoichiometric air-fuel ratio with only the rough adjustment term AFc. Need. Therefore, the storage term is introduced in the air-fuel ratio control in this embodiment. A second flow chart for the calculation of this storage term is shown in FIG. This will be explained below.

This flow chart is executed, for example, every 16 ms. First, in step 201,
As with the first flowchart, the flag XFB indicating that the condition for performing the air-fuel ratio feedback control is satisfied is 1
Is determined. If this determination is denied, the processing ends, but if affirmed, step 202
Then, it is determined whether the output V _{OX of} the first air-fuel ratio sensor 11 is smaller than the first threshold value V1 which is smaller than the value V _R indicating stoichiometric air-fuel ratio combustion. When this judgment is affirmed,
That is, when it is estimated by the first air-fuel ratio sensor 11 that the air-fuel ratio has largely deviated to the lean side, the _routine proceeds to step 203, where the integrated value AF _CCROi of the storage term is increased by ΔAF _ccRO . Next, in step 204, the flag F
Is determined to be zero. When this judgment is affirmed, the _routine proceeds to step 205, where the correction value AF _CCROP2 of the second skip value AF _CCROP1 is determined from the first map shown in FIG. 4 based on the output of the second air-fuel ratio sensor 12 representing the actual air-fuel ratio. Then, in step 206, the above-mentioned flag F is set to 1, and in step 207, the storage item AF _{CC RO}
Is calculated as the sum of the second skip value AF _CCROP1 , its correction value AF _CCROP2, and the integral value AF _CCROi . If the correction value AF _CCROP2 is already set and the flag F is 1 in step 204, steps 205 and 20 are _executed.
The process of 6 is omitted and the process proceeds to step 207. The second skip value AF _CCROP1 used here is set to a considerably smaller value _than in the conventional case.

On the other hand, when the determination in step 202 is negative, the routine proceeds to step 208, where the first air-fuel ratio sensor 11
The output V _OX of whether greater than the value V _R is greater than the second threshold value V2 indicating the stoichiometric combustion is determined. When this determination is affirmed, that is, the first air-fuel ratio sensor 11
When it is estimated that the air-fuel ratio has largely deviated to the rich side, the routine proceeds to step 209, where the integrated value A of the storage term
F _CCROi is reduced by ΔAF _ccRO . Next step 2
At 10, it is determined whether the flag F is 0. When this determination is positive, the _routine proceeds to step 211, where the correction value AF _CCROP2 of the second skip value AF _CCROP1 is determined from the first map shown in FIG. 4 based on the output of the second air-fuel ratio sensor 12, and at step 212, The aforementioned flag F is set to 1, and in step 213, the storage item AF
_CCRO is the second skip value-AF _CCROP1 and its correction value AF
It is calculated as the sum of _CCROP2 and the integrated value AF _CCROi . If the correction value AF _CCROP2 has already been set and the flag F is 1 in step 210, step 211
The processing of steps 212 and 212 is omitted and the process proceeds to step 213.
When the determination in step 208 is negative, that is, when the first air-fuel ratio sensor 11 estimates that the deviation of the air-fuel ratio is relatively small, a good air-fuel ratio control is realized only by the rough adjustment term AFc described above. Step 2 to be done
At 10 the integral AF _CCROi is set to 0, at step 211 the storage term AF _CCRO is set to 0 and the flag F is reset to 0.

In the first map shown in FIG. 4, Δ on the horizontal axis
The A / F is a value obtained by subtracting the theoretical air-fuel ratio 14.6 from the actual air-fuel mixture air-fuel ratio calculated based on the output of the second air-fuel ratio sensor 12. Therefore, if ΔA / F is a positive value, the air-fuel ratio is lean, and if it is a negative value, it is rich, and the greater the absolute value, the greater the lean or rich degree. The correction value AF _CCROP2 is 0 when ΔA / F is near 0, and has the same sign as that of other ΔA / F, and is set to increase stepwise as the value increases. ing.

Storage term A calculated in this way
F _CCRO is calculated when the first air-fuel ratio sensor 11 which can obtain a relatively stable output due to the O ₂ storage effect of the three-way catalyst estimates that the air-fuel ratio has largely deviated, and its skip value is a relatively small value. Since the sum of the second skip value AF _CCROP1 set to and the correction value AF _CCROP2 set based on the actual air-fuel ratio detected by the second air-fuel ratio sensor 12 is used, It becomes an appropriate skip value, and the hatching of the air-fuel ratio and the extension of the convergence time to the stoichiometric air-fuel ratio are prevented.

When the O ₂ storage effect is reduced due to deterioration of the three-way catalyst, the skip value becomes large if the actual deviation of the air-fuel ratio is large, and it is necessary to skip to the opposite side in a relatively short time. However, unlike the past, the skip value is not a constant, and the skip value at this time is determined to be smaller than the previous time based on the actual air-fuel ratio, so the air-fuel ratio does not hatch, and the stoichiometric air-fuel ratio is ensured. Can be converged.

Further, in this embodiment, the first map used for determining the correction value AF _{CCROP2 in} order to simplify the control is such that the correction value AF _CCROP2 changes stepwise according to the air-fuel ratio. , Correction value AF _CCROP2 as shown in FIG.
By using a second map where
The skip value of the storage term becomes a more appropriate value. In addition, the correction value AF determined by the first and second maps
_Instead of using _CCROP2 as it is, the time differential value d (A of the air-fuel ratio at this time obtained from the second air-fuel ratio sensor 12 is obtained.
/ F) / dt from the third map shown in FIG.
It is also possible to obtain _α and use _α AF _{CC ROP2} as a correction value. As shown in this map, the coefficient α becomes 1 when the absolute value of the time differential value d (A / F) / dt is less than or equal to a predetermined value, that is, when the gradient of the change in the air-fuel ratio is gentle, the correction value AF _CCROP2 is used as it is, but as this slope becomes _steeper , the absolute value of the correction value AF _CCROP2 is set to be larger, so that the correction value calculated as αAF _CCROP2 is the future of the air-fuel ratio. The value also takes into account the change in

FIG. 7 shows the fuel injection amount T based on the rough adjustment term AFc and the storage term AF _CCRO calculated in this way.
It is a flow chart for calculating AU, and is executed for every predetermined crank angle. First, in step 301, the intake air amount Q detected by the air flow meter 6
Based on the engine speed N detected by the crank angle sensors 16 and 17, the basic fuel injection amount TAUP is calculated by the following equation (1). TAUP = kQ / N (1) where k = constant

Next, in step 302, the following equation (3)
Then, the actual fuel injection amount TAU is calculated. TAU = TAUP (AFc + AF _CCRO + δ) + γ (2) where δ, γ = constant This calculation result is calculated in step 303 in the control device 20.
By setting it in the counter 20h, the predetermined amount of fuel is injected from the fuel injection valve 7, and good air-fuel ratio control is realized. FIG. 8 shows a first air-fuel ratio sensor 1 according to this embodiment.
3 is a time chart of the output of 1 and the air-fuel ratio correction amount (AFc + AF _CCRO ).

By the way, the conventional air-fuel ratio control device has only the air-fuel ratio sensor provided downstream of the three-way catalyst for detecting the rich state and the lean state of the exhaust gas, so that the combustion is performed during engine warm-up. Although it was impossible to execute the air-fuel ratio control for making the air-fuel ratio a predetermined rich state for the purpose of stabilizing the engine and early warming up, the air-fuel ratio control device of the present embodiment outputs to the three-way catalyst upstream. Since the linear second air-fuel ratio sensor is provided, this sensor can be used to perform such air-fuel ratio control.

FIG. 9 shows a fourth flowchart for this air-fuel ratio control. This flow chart is, for example, 4 ms
It is executed every time. First, at step 401, it is judged if the second air-fuel ratio sensor 12 is activated. For this determination, for example, when the second air-fuel ratio sensor 12 is not activated, its output shows an abnormal value. When the determination in step 401 is negative, the air-fuel ratio control based on the output is impossible, so the process ends.

When the determination at step 401 is affirmative, the routine proceeds to step 402, where the increase / decrease value DLAF of the correction coefficient FDLAF is calculated by the following equation (3). DLAF = (AFt−AF) / AFt (3) In this equation, AFt is the target air-fuel ratio, and AF is the air-fuel ratio calculated based on the output of the second air-fuel ratio sensor 12. Next, at step 403, it is judged if the absolute value of the increase / decrease value DLAF is larger than the predetermined value β1. When this judgment is affirmed, the routine proceeds to step 404, where the count value CDLAF is reset to zero. Then step 405
Correction coefficient FDLAF originally set to 1
Is subtracted from the increase / decrease value DLAF calculated in step 402. In this process, the air-fuel ratio AF calculated based on the output of the second air-fuel ratio sensor 12 is the target air-fuel ratio A.
If it is lean with respect to Ft, the increase / decrease value DLAF becomes a negative value, and the correction coefficient FDLAF is substantially increased. On the contrary, if it is rich, the correction coefficient FD is substantially
LAF is reduced.

On the other hand, when the determination at step 403 is negative, the routine proceeds to step 406, where the count value CDL
AF is increased by one. Next, at step 407, it is judged whether or not the count value CDLAF is larger than the predetermined value KCDLAF, and when this judgment is negative, the processing ends. That is, when the air-fuel ratio AF calculated based on the output of the second air-fuel ratio sensor 12 is close to the target air-fuel ratio AFt, the current correction coefficient FDLAF ends without changing. However, when the determination in step 407 is affirmative, that is, when the air-fuel ratio AF is maintained near the target air-fuel ratio AFt for a predetermined time t (4KCDLAFms) or more, the routine proceeds to step 408, where the current correction coefficient FDLAF
It is determined whether or not the absolute value of the value obtained by subtracting 1 from the predetermined value β1 is equal to or smaller than the predetermined value β1. When the determination is negative, the process ends as it is, but when the determination is affirmative, the correction coefficient FDLA
F is forcibly set to 1, and the correction of the substantial fuel injection amount is stopped.

The output of the second air-fuel ratio sensor 12 has a certain degree of error. Thus, when the air-fuel ratio AF calculated based on this output is close to the target air-fuel ratio AFt, it is set within this error range. Without changing the correction coefficient FDLAF,
Further, when the correction coefficient FDLAF is maintained in the vicinity of 1 for a predetermined time, it is determined that the target air-fuel ratio has already converged, and the correction coefficient FDLAF is set to 1 to prevent meaningless change in the air-fuel ratio. be able to. FIG. 10 shows the second
6 is a time chart of the air-fuel ratio AF and the correction coefficient FDLAF when this air-fuel ratio control is executed by the air-fuel ratio sensor 12. In the same figure, the alternate long and short dash line shows the change in the air-fuel ratio due to disturbance when the air-fuel ratio control is not executed.

If it is detected from the output of the water temperature sensor 13 that the engine warm-up is completed, it is confirmed that the first air-fuel ratio sensor 11 is activated, and then the feedback control to the stoichiometric air-fuel ratio is executed. To be done.

Since the second air-fuel ratio sensor 12 is exposed to unpurified exhaust gas, carbon particles and the like are likely to adhere to the second air-fuel ratio sensor 12, and the reliability of its output gradually deteriorates. Therefore, good air-fuel ratio control is executed by periodically calibrating the output. FIG. 11 is a fifth flow chart for performing this calibration.

First, at step 501, it is judged if the output V _{OX of} the second air-fuel ratio sensor 11 is equal to or more than the first threshold value V1 and equal to or less than the second threshold value V2.
When this judgment is affirmed, that is, when the air-fuel ratio is substantially converged to the stoichiometric air-fuel ratio, the routine proceeds to step 502, where the count value CNTSY is incremented by one.

Next, in step 503, the current second
The output AF _VALUEi of the air-fuel ratio sensor 12 is integrated and AF
_VALUE is set, and the process proceeds to step 504, where the count value CN
It is determined whether TSDY is greater than or equal to a predetermined integer value KCNTSDY. If this determination is denied, the processing ends, but if affirmed, the processing proceeds to step 505, during which the average value AF _VALUE 'of the output of the second air-fuel ratio sensor 12 is reached.
Calculated by dividing the KCNTSDY the integrated value AF _{VALU E} of outputs.

Next, in step 506, the calibration value AF
_OFFSET is a value A indicating the theoretical air-fuel ratio of the second air-fuel ratio sensor 12.
It is calculated by subtracting the average value AF _VALUE 'from FV. On the other hand, when the determination in step 501 is denied, that is, when the output of the first air-fuel ratio sensor 11 deviates greatly from the value indicating stoichiometric air-fuel ratio combustion, the routine proceeds to step 507,
The count value CNTSDY is reset to 0, and the integrated value AF _VALUE is reset to 0 in step 508.

The average value AF _VALUE 'used for calculating the calibration value AF _OFFSET is the output of the first air-fuel ratio sensor 11 for a predetermined time (KCNTSDY times the execution interval of the flowchart).
It is an average of the outputs of the second air-fuel ratio sensor 12 when it is converged to the stoichiometric air-fuel ratio, and is a value indicating the present theoretical air-fuel ratio combustion of the second air-fuel ratio sensor. Therefore, this calibration value AF
Good air-fuel ratio control is realized by calibrating the output of the second air-fuel ratio sensor 12 based on _OFFSET .

The calibration of the second air-fuel ratio sensor using such average output is performed not only by the air-fuel ratio control using the coarse adjustment term and the storage term as in the present embodiment, but also by O ₂
It can also be applied to the air-fuel ratio control in which the forced oscillation term for effectively utilizing the storage effect is used.

By the way, in the conventional feedback control to the stoichiometric air-fuel ratio, the response of the first air-fuel ratio sensor 11 is deteriorated when the exhaust gas amount is small and the load is light.
Only at this time, the first skip value of the rough adjustment term was increased to increase the variation range of the air-fuel ratio to improve the convergence to the stoichiometric air-fuel ratio. However, since the increase of the first skip value is unequivocally executed by ignoring the actual air-fuel ratio, it may simply deteriorate the air-fuel ratio.

FIG. 12 shows a time chart of the air-fuel ratio and the correction coefficient at a light load by the air-fuel ratio control apparatus of this embodiment.
The correction coefficient is calculated as the sum of the coarse adjustment term AFc that is normally used and the correction value FDLAF used for air-fuel ratio control during engine warm-up. However, this correction value FDLAF is a value for the stoichiometric air-fuel ratio. By such air-fuel ratio control, good air-fuel ratio control considering the actual air-fuel ratio is realized, and vibration at this time is reduced. In this air-fuel ratio control, when the light load operation is performed after the fuel cut is executed, the air-fuel ratio correction amount during the fuel cut is held in the rough adjustment term AFc immediately before that, and then the correction value FDLAF becomes 0 for a while. To be done. This ensures that the fuel cut effect is maintained.

[0042]

As described above, according to the air-fuel ratio control device of the present invention, when the storage term is calculated on the basis of the output of the first air-fuel ratio sensor arranged on the downstream side of the three-way catalyst of the exhaust system, it is used for it. The second skip value to be corrected is corrected according to the output of the output linear type second air-fuel ratio sensor arranged on the upstream side of the three-way catalyst in the exhaust system, so that a good air-fuel ratio considering the actual air-fuel ratio is obtained. The control is realized, and the hatching of the air-fuel ratio and the extension of the convergence time to the stoichiometric air-fuel ratio can be prevented.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an air-fuel ratio control system for an internal combustion engine according to the present invention.

FIG. 2 is a first flowchart for calculating a rough adjustment term.

FIG. 3 is a second flowchart for calculating a storage term.

FIG. 4 is a first map for determining a correction value for a second skip value used in the second flowchart.

FIG. 5 is a second map used instead of the first map.

FIG. 6 is a third map for determining a coefficient of a correction value.

FIG. 7 is a third flowchart for determining a fuel injection amount.

FIG. 8 is a time chart showing air-fuel ratio control according to the present invention.

FIG. 9 is a fourth flowchart for air-fuel ratio control during engine warm-up.

FIG. 10 is a time chart showing air-fuel ratio control during engine warm-up according to the present invention.

FIG. 11 is a fifth flowchart for calibrating the output of the second air-fuel ratio sensor.

FIG. 12 is a time chart showing air-fuel ratio control during light engine load according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2 ... Intake passage 3 ... Exhaust passage 6 ... Air flow meter 7 ... Fuel injection valve 10 ... Three-way catalytic converter 11 ... First air-fuel ratio sensor 12 ... Second air-fuel ratio sensor 20 ... Control device

Claims (1)

[Claims] 1. A three-way catalyst installed in an exhaust system of an internal combustion engine, a first air-fuel ratio sensor located downstream of the three-way catalyst in the exhaust system, and an upstream side of the three-way catalyst in the exhaust system. The output of the output linear type second air-fuel ratio sensor and the first skip value is used when the output of the first air-fuel ratio sensor is reversed from the rich side or the lean side to the other side with a value indicating stoichiometric air-fuel ratio combustion as a boundary. Is performed to perform proportional control, and when the output does not reverse, integral control is performed to calculate a rough adjustment term of the air-fuel ratio correction amount, and the output of the first air-fuel ratio sensor is the theoretical air-fuel ratio combustion. When a rich side or a lean side is exceeded beyond a predetermined range near the value indicating, the proportional control is executed by using the second skip value, and the integral control is executed until the output falls within the range to perform the air-fuel ratio correction amount. Term computing means for computing the storage term of And when the output of the first air-fuel ratio sensor goes to a rich side or a lean side beyond a predetermined range near the value indicating stoichiometric air-fuel ratio combustion, the second skip value is corrected according to the output of the second air-fuel ratio sensor. An air-fuel ratio controller for adjusting the air-fuel ratio of the internal combustion engine based on the rough adjustment term and the storage term. .