JPH0788801B2 - Air-fuel ratio controller for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to an air-fuel ratio sensor (in this specification, an oxygen concentration sensor (O ₂ sensor)) upstream and downstream of a catalytic converter.
The provided, relates to an air-fuel ratio control apparatus for an internal combustion engine which performs air-fuel ratio feedback control by the O ₂ sensor downstream in addition to the air-fuel ratio feedback control by the O ₂ sensor in the upstream side.

[Conventional technology]

In simple air-fuel ratio feedback control (single O ₂ sensor system), an O ₂ sensor that detects the oxygen concentration is provided at a location in the exhaust system that is as close to the combustion chamber as possible, that is, at the collective portion of the exhaust manifold that is upstream from the catalytic converter. , O ₂ sensor's output characteristic variation hinders improvement of air-fuel ratio control accuracy. In order to compensate for such variations in the output characteristics of the O ₂ sensor, variations in parts such as the fuel injection valve, and changes over time or over time, a second O ₂ sensor is provided downstream of the catalytic converter and the upstream O ₂ sensor is used. A double O ₂ sensor system has already been proposed that performs air-fuel ratio feedback control by a downstream O ₂ sensor in addition to air-fuel ratio feedback control (see JP-A-58-58).
48756 publication). In this double O ₂ sensor system, the O ₂ sensor provided downstream of the catalytic converter is
Although it has a low response speed as compared with the O ₂ sensor, it has an advantage that variation in output characteristics is small for the following reason.

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

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

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

Therefore, as described above, the air-fuel ratio feedback control based on the outputs of the _two O ₂ sensors (the variation in the output characteristics of the double O ₂ can be absorbed by the downstream O ₂ sensor. Actually, as shown in FIG. In the O ₂ sensor system, O ₂
When the output characteristic of the sensor deteriorates, it directly affects the exhaust emission characteristic, whereas in the double O ₂ sensor system, the exhaust emission characteristic does not deteriorate even if the output characteristic of the upstream O ₂ sensor deteriorates. That is, in the double O ₂ sensor system, good exhaust emission is guaranteed as long as the downstream O ₂ sensor maintains stable output characteristics.

Even in the above-mentioned double O ₂ sensor system, the air-fuel ratio is corrected by variations in manufacturing of parts such as the air flow meter (or pressure sensor) and fuel injection valve, changes over time or over time, changes in air density (changes in atmospheric pressure), etc. The coefficient FAF is largely deviated, and thus may be close to its upper limit value or its lower limit value. The upper limit value and the lower limit value are provided to prevent the air-fuel ratio correction coefficient FAF from being excessively corrected and becoming too large or too small due to a failure of the upstream O ₂ sensor for some reason. . For example, during the air-fuel ratio feedback control, if the air-fuel ratio changes to a large transient state such as sudden acceleration or deceleration, the air-fuel ratio correction factor FAF will stick to its upper limit or lower limit and the air-fuel ratio The fluctuation margin of the correction coefficient FAF becomes small, and further correction becomes impossible. Therefore, it may be impossible to compensate for the transient air-fuel ratio change. Further, when the difference between the air-fuel ratio correction coefficient during air-fuel ratio feedback control and the air-fuel ratio correction coefficient (constant value) during non-air-fuel ratio feedback control (open loop) becomes large, the open-loop time air-fuel ratio deviation becomes large, and At the time of switching from the open loop to the air-fuel ratio feedback control, it takes time for the control air-fuel ratio to reach the required level, resulting in insufficient correction. As a result, fuel consumption is deteriorated due to overrich, HC and CO emissions are deteriorated, and
Deterioration of drivability and deterioration of NO _x emission due to over lane will occur.

For this reason, learning control is introduced in the double O ₂ sensor system, so that the average value of the air-fuel ratio correction coefficient FAF, that is, the average value FAFAV of the air-fuel ratio correction coefficient FAF immediately before skipping, changes around a predetermined value, for example, 1.0. The applicant of the present invention has already proposed (see Japanese Patent Application No. 60-16742).
issue). Therefore, the air-fuel ratio correction factor FAF is always the specified value (1.0)
Since the value is close to, the fluctuation margin is large, and therefore, the change in the air-fuel ratio during the transition during the air-fuel ratio feedback control can be compensated, and the difference in the air-fuel ratio correction coefficient between the air-fuel ratio feedback and the open loop becomes small.
Therefore, the deviation of the air-fuel ratio during the open loop becomes smaller, and the control air-fuel ratio immediately approaches the required level when switching from the open loop to the air-fuel ratio feedback control.

[Problems to be solved by the invention]

However, in the double O ₂ sensor system, when air-fuel ratio feedback control is performed by both the upstream and downstream O ₂ sensors, the air-fuel ratio feedback control constants such as the skip control constants RSR and RSL are emptied by the downstream O ₂ sensor. Due to the fuel ratio feedback control, it is usually asymmetric (RSR ≠ RSL). Therefore, during this period, as described above, when the learning correction amount FGHAC is calculated so that the average value FAFAV of the air-fuel ratio correction coefficient FAF immediately before the skip becomes a predetermined value, for example, 1.0, the average value FAFAV is the average of the air-fuel ratio correction coefficient FAF. Since the value is not accurately represented, that is, the true air-fuel ratio deviation is not represented, erroneous learning is performed, and as a result, the learning correction amount FGHAC deviates from the original value. Therefore,
When the air-fuel ratio feedback control by both upstream and downstream O ₂ sensors is switched to open loop, the base air-fuel ratio is displaced by the amount of the deviation of the learning correction amount value FGHAC, fuel efficiency is deteriorated, drivability is deteriorated, HC, CO, NO _X Emissions may be deteriorated.

On the other hand, the air-fuel ratio feedback control by the downstream O ₂ sensor is stopped and the skip control constants RSR and RSL are symmetric (RSR = RS
However, the air-fuel ratio feedback control is performed by the upstream O ₂ sensor,
Moreover, the average value FAFAV of the air-fuel ratio correction coefficient FAF immediately before the skip is
When the learning correction amount FGHAC is calculated so as to be 1.0, in this case, the average value FAFAV almost accurately represents the average value of the air-fuel ratio correction coefficient FAF. Therefore, the upstream side, the air-fuel ratio feedback control by both the downstream O ₂ sensor to the upstream O ₂ sensor only air-fuel ratio feedback control by, or when a reverse switching is performed, for example, from the off idle state to the on idle , Or vice versa, the learning correction amount FGHAC is corrected by the air-fuel ratio feedback control by the amount of deviation, and therefore the base air-fuel ratio is deviated during such a transition, which also deteriorates fuel consumption and drivability. , HC, CO, NO _X emission will be worsened.

Therefore, an object of the present invention is to provide a double air-fuel ratio sensor system that prevents erroneous learning.

[Means for solving problems]

A means for solving the above problems is shown in FIG.

That is, first and second air-fuel ratio sensors, which are respectively provided on the upstream side and the downstream side of a catalytic converter for purifying exhaust gas provided in an exhaust system of an internal combustion engine, for detecting the concentrations of specific components in the exhaust gas, Control constant calculation means for calculating an air-fuel ratio feedback control constant according to the output of the second air-fuel ratio sensor, and an air-fuel ratio for calculating an air-fuel ratio correction amount according to the output of the second air-fuel ratio sensor and the air-fuel ratio feedback control constant A correction amount calculation means, a learning means for calculating a learning correction amount so that the average value of the air-fuel ratio correction amount converges to a predetermined value, and an asymmetry for determining whether or not the asymmetry of the air-fuel ratio correction amount is within a predetermined range. Determination means, prohibition means for prohibiting the operation of the learning control means when the asymmetry of the air-fuel ratio correction amount is outside a predetermined range, and the air-fuel ratio of the engine is adjusted according to the air-fuel ratio correction amount and the learning correction amount. Air fuel Comprising an adjustment means.

[Work]

According to the above-mentioned means, when the air-fuel ratio feedback control constant, for example RSR, is out of the predetermined range, that is, when the asymmetry (RSR ≠ RSL) of the air-fuel ratio correction amount FAF becomes large, the learning is stopped, and therefore the asymmetry. False learning is minimized.

〔Example〕

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

FIG. 3 is an overall schematic diagram showing an embodiment of an air-fuel ratio control system for an internal combustion engine according to the present invention. In FIG. 3, an air flow meter 3 is provided in the intake passage 2 of the engine body 1. The air flow meter 3 directly measures the intake air amount, and has a built-in potentiometer to generate an output signal of an analog voltage proportional to the intake air amount. This output signal is the A / D converter 101 with a built-in multiplexer in the control circuit 10.
Is being supplied to. The distributor 4 includes a crank angle sensor 5 whose axis generates a reference position detecting pulse signal every 720 ° converted into a crank angle, and a reference position detecting pulse signal every 30 ° converted into a crank angle. A crank angle sensor 6 for generating is provided. The pulse signals of the 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 supplied to the interrupt terminal of the CPU 103.

Further, the intake passage 2 is provided with a fuel injection valve 7 for supplying pressurized fuel from the fuel supply system to the 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 is the temperature of the cooling water THW
Generates an electric signal of analog voltage according to. This output is also supplied to the A / D converter 101.

The exhaust system downstream of the exhaust manifold 11 is provided with a catalytic converter 12 that accommodates a three-way catalyst that simultaneously purifies three harmful components HC, CO, and NO _x in the exhaust gas.

A first O ₂ sensor 13 is provided on the exhaust manifold 11, that is, on the upstream side of the catalytic converter 12, and a second O ₂ sensor 15 is provided on the exhaust pipe 14 on the downstream side of the catalytic converter 12. . The O ₂ sensors 13 and 15 generate electric signals according to the concentration of oxygen components in the exhaust gas. That is, the O ₂ sensor 13,1
5 is an A / D converter 101 that outputs different output voltage by the control circuit 10 depending on whether the air-fuel ratio is lean or rich with respect to the theoretical air-fuel ratio.
Occurs in.

The control circuit 10 is configured as, for example, a microcomputer, and includes an A / D converter 101, an input / output interface 102, a C
A ROM 104, a RAM 105, a backup RAM 106, a clock generation circuit 107, and the like are provided outside the PU 103.

Further, in the control circuit 10, the down counter 108, the flip-flop 109, and the drive circuit 110 are for controlling the fuel injection valve 7. That is, when the fuel injection amount TAU is calculated in the postoperative routine, the fuel injection amount T
The AU is preset in the down counter 108 and the flip-flop 109 is also set. As a result, drive circuit 1
10 Start energizing the fuel injection valve 7. On the other hand, when the down counter 108 counts a clock signal (not shown) and finally its carry-out terminal becomes the "1" level, the flip-flop 109 is set and the drive circuit 110 causes the fuel injection valve 7 to operate. Stop energizing. That is, the above fuel injection amount TAU
Only the fuel injection valve 7 is energized, and therefore the fuel injection amount TAU
The amount of fuel corresponding to is sent to the combustion chamber of the engine body 1.

The CPU 103 generates an interrupt when the A / D conversion of the A / D converter 101 is completed, when the input / output interface 102 receives the pulse signal of the crank angle sensor 6, the clock generation circuit 107.
When an interrupt signal from is received.

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

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

In step 401, it is determined whether or not the closed loop (feedback) condition of the air-fuel ratio by the upstream O ₂ sensor 13 is satisfied. For example, when the cooling water temperature is equal to or lower than a predetermined value, during engine start, during starting increase, during warm-up increase, during power increase, when the output signal of the upstream O ₂ sensor 13 has never been reversed,
The closed loop condition is not satisfied during the fuel cut, and the closed loop condition is satisfied in other cases. When the closed loop condition is not satisfied, the routine proceeds to step 430, where the air-fuel ratio correction coefficient FAF is set to 1.0. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 402.

In step 402, the output V ₁ of the upstream O ₂ sensor 13 is A / D converted to work, and in step 403 V ₁ is compared voltage V _R1
It is determined whether 0.45V or less, that is, whether the air-fuel ratio is rich or lean. If the air-fuel ratio is lean (V ₁ ≦ V _R1 ), it is determined in step 404 whether the first delay counter CDLY1 is positive, and if CDLY1> 0, in step 405.
Set CDLY1 to 0, and proceed to step 406. Step 407,408
Then, the first delay counter CDLY1 is guarded with the minimum value TDL1, and in this case, when the first delay counter CDLY1 reaches the minimum value TDL1, the first air-fuel ratio flag F1 is set to “0” (lean) at step 409. And The minimum value TDL1
Is a lean delay time for holding the judgment that the state is rich even if there is a change from rich to lean in the output of the upstream O ₂ sensor 13, and is defined as a negative value.
On the other hand, if rich (V ₁ > VR ₁₎ , in step 410, the first
It is determined whether the delay canceler CDLY1 of is negative or not, and CDLY1 <
If it is 0, CDLY1 is set to 0 in step 411, and step 41
Go to 2. In steps 413 and 414, the first delay counter CDLY1 is guarded with the maximum value TDR1. In this case, when the first delay counter CDLY1 reaches the maximum value TDR1, the first air-fuel ratio flag F1 is set to "1" in step 415. (Rich) The maximum value TDR1 is the rich delay time for holding the judgment that the output is the lean state even if there is a change from the lean to the rich in the output of the upstream O ₂ sensor 13, and is defined as a positive value. It

In step 416, it is determined whether or not the sign of the first air-fuel ratio flag F1 has been inverted, that is, it is determined whether the air-fuel ratio after the delay process has been inverted. If the air-fuel ratio is reversed,
In step 417, it is determined whether or not the learning condition is satisfied. The learning conditions are, for example, i) the cooling water temperature THW is 70 ° C. <THW <90 ° C., ii) the intake air amount change ΔQ is smaller than a certain value, and is stable for a certain period of time. When the learning conditions are satisfied, step 418
Otherwise go to step 420.

In step 418, it is determined whether or not the air-fuel ratio feedback control constant calculated by the routine of FIG. 7, which will be described later, in this case, the rich skip amount RSR is within a predetermined range, X ₁ <RSR <X ₂ . X ₁ is, for example, 2%, and X ₂ is, for example, 6%. In other words, if it is within this range, the asymmetry of the air-fuel ratio correction coefficient FAF, which will be described later, is tolerable, and is regarded as almost symmetrical, and the learning step of step 419 is proceeded to. Conversely, if it is outside this range, the air-fuel ratio correction coefficient
The FAF has a large asymmetry, and as a result, in order to prevent erroneous learning, that is, erroneous correction of the learning correction amount FGHAC, step 420
Proceed to and prohibit learning control.

In the above example, the system in which the skip amount RSR, RSL is variable has been described, but only one of the skip amount RSR, RSL, only one of the integration amounts KIR, KIL, or the delay time TDR1,
Even in a system in which only one of TDL1 is variable, learning permission / prohibition can be determined depending on whether the variable control amount is within a predetermined range.

At step 420, according to the value of the first air-fuel ratio flag F1,
Determine whether the reversal from rich to lean or from lean to rich. If it is a reversal from rich to lean,
In step 421, FAF ← FAF + RSR is increased in a skip manner, and conversely, when lean is reversed to rich, in step 422, FAF ← FAF−RSL is reduced in a skip manner. That is, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted in step 416, integration processing is performed in steps 423, 424, 425. That is, in step 423, it is determined whether or not F1 = "0". If F1 = "0" (lean), in step 424 FAF ←
If FAF + KIR, on the other hand, if F1 = "1" (rich), then in step 425, FAF ← FAF-KIL. Where the integration constant KI
R and KIL are set sufficiently smaller than the skip constants RSR and RSL, that is, KIR (KIL) <RSR (RSL). Therefore, step 424 gradually increases the fuel injection amount in the lean state (F1 = "0"), and step 425 in the rich state (F1
= "1") gradually decreases the fuel injection amount.

The air-fuel ratio correction coefficient FAF calculated in steps 421, 422, 424, 425 is guarded in steps 426, 427 at the minimum value, for example 0.8, and at steps 428, 429, for example, the maximum value.
Guarded at 1.2. Thus, when 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 that value to prevent overrich or over lean.

The FAF calculated as described above is stored in the RAM 105, and this routine ends at step 431.

FIG. 5 is a detailed flowchart of the learning control step 419 of FIG. 4, and as described above, the learning condition is satisfied and rich under the air-fuel ratio feedback control by the upstream O ₂ sensor 13. When the skip amount RSR is within the predetermined range (F _G = “1”), it is executed every time the air-fuel ratio correction coefficient FAF is skipped.

That is, in step 501, the average value FAFAV of the air-fuel ratio correction coefficient FAF is calculated by FAFAV ← (FAF + FAF0) / 2, where FAF0 is the FAF value immediately before the previous skip, and in step 502, FAF is calculated next time. In preparation for calculation, set FAF0 ← FAF. Next, in step 503, ΔFAF ← FAFAV−1.0 is calculated.

Next, in step 504, it is determined whether or not ΔFAF> 0, and if ΔFAF> 0 as a result, in step 505 the learning correction amount is determined.
Increase FGHAC by FGHAC ← FGHAC + ΔFGHAC, and set the maximum value in steps 506 and 507.
Guard at 1.05. On the other hand, if ΔFAF ≦ 0, in step 508 the learning correction amount FGHAC is decreased by FGHAC ← FGHAC−ΔFGHAC, and in steps 509 and 510 the minimum value, for example,
Guard at 0.90. Note that FGHAC may be updated only when | ΔFAF |> K (a positive value). In this way
According to the learning control, the learning correction amount FGHAC is increased or decreased so that the air-fuel ratio correction coefficient FAF converges to 1.0.

FIG. 6 is a timing diagram for supplementarily explaining the operation according to the flowchart of FIG. When the rich / lean air-fuel ratio signal A / F is obtained from the output of the upstream O ₂ sensor 13 as shown in FIG. 6 (A), the first delay counter CDLY1
6 is counted up in the rich state and counted down in the lean state as shown in FIG. 6 (B). As a result, as shown in FIG. 6 (C), the delayed air-fuel ratio signal A / F '(corresponding to the flag F1) is formed. For example, the air-fuel ratio signal A / F is changed from lean to rich at time t _1, the air-fuel ratio signal A / F1 delayed processed 'at time t ₂ after being held lean only rich delay time TDR1 Change to rich. Even if the air-fuel ratio signal A / F changes from rich to lean at time t ₃ , the delayed air-fuel ratio signal A / F '
Changes to lean at time t ₄ after being held rich only equivalent lean delay time (-TDL1). However, when the air-fuel ratio signal A / F is inverted in a period shorter than the rich delay time TDR1 as at times t ₅ , t ₆ , and t ₇ , the first delay counter CDLY1
Takes a long time to reach the maximum value TDR1, and as a result, the delayed air-fuel ratio signal A / F ′ is inverted at time t ₈ .
That is, the air-fuel ratio signal A / F ′ after the delay processing becomes more stable than the air-fuel ratio signal A / F before the delay processing. In this way, the air-fuel ratio correction coefficient FAF shown in FIG. 6D is obtained based on the stable air-fuel ratio signal A / F 'after the delay processing.

Next, the second air-fuel ratio feedback control by the downstream O ₂ sensor 15 will be described. As the second air-fuel ratio feedback control, the skip amounts RSR, RSL as the first air-fuel ratio feedback control constants, the integration constants KIR, KIL, the delay times TDR1, TDL1, or the output V ₁ of the upstream O ₂ sensor 13 There are a system that makes the comparison voltage V _R1 variable and a system that introduces the second air-fuel ratio correction coefficient FAF2.

For example, if the rich skip amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and the lean skip amount
Even if RSL is reduced, the control air-fuel ratio can be shifted to the rich side,
On the other hand, if the lean skip amount RSL is increased, the control air-fuel ratio can be shifted to the lean side, and the rich skip amount RSR
The control air-fuel ratio can be shifted to the lean side even if is decreased.

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 the rich integration constant KIR is increased, the control air-fuel ratio can be shifted to the rich side, and
Even if the lean integration constant KIL is reduced, the control air-fuel ratio can be shifted to the rich side, while if the lean integration constant KIL is increased, the control air-fuel ratio can be shifted to the lean side, and even if the rich integration constant KIL 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 integration constant KIR and the lean integration constant KIL 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. Conversely, if lean delay time (-TDL1)> rich delay time (TDR1) is set, control can be performed. The air-fuel ratio can shift to the lean side. That is, the air-fuel ratio can be controlled by correcting the delay times TDR1 and TDL1 according to the output of the downstream O ₂ sensor 15. Furthermore, if the comparison voltage V _R1 is increased, the control air-fuel ratio can be shifted to the rich side.
_The control air-fuel ratio can be shifted to the lean side by reducing _R1 .
Therefore, according to the output of the downstream O ₂ sensor 15, the comparison voltage V _R1
The air-fuel ratio can be controlled by correcting

A double O ₂ sensor system in which the skip amount as the air-fuel ratio feedback control constant is variable will be described with reference to FIG. 7.

FIG. 7 is a second air-fuel ratio feedback control routine for calculating the skip amounts RSR, RSL based on the output of the downstream O ₂ sensor 15, and is executed every predetermined time, for example, every 1 _s . In step 701, it is determined whether or not the downstream O ₂ sensor 15 is in a closed loop condition. For example, when the cooling water temperature is below a predetermined value, the output signal of the downstream O ₂ sensor 15 never reverses, when the downstream O ₂ sensor 15 has failed, during transient motion, on idle (LL = The closed loop condition is not satisfied in all cases such as "1"), and the closed loop condition is satisfied in other cases. If it is not the closed loop condition, the process proceeds to steps 730 and 731, and the skip amounts RSR and RSL are set to constant values RSR _o and RSL _o . For example, RSR _o = 5% RSL _o = 5%. That is, symmetrical skip control is performed. In this case, other air-fuel ratio feedback control constants KIR, KIL; TDR, TD
If L is also symmetrical, the air-fuel ratio correction coefficient FAF will be controlled symmetrically by the routine of FIG.

In step 703, the output V ₂ of the downstream O ₂ sensor 15 is A / D converted and captured, and in step 704, V ₂ is the comparison voltage V _R2, for example.
It is determined whether it is 0.55V or less, that is, whether the air-fuel ratio is rich or lean. It should be noted that the comparison voltage V _R2 is higher than the comparison voltage V _R1 of the output of the upstream value O ₂ sensor 13 in consideration of the difference in output characteristics due to the influence of raw gas and the deterioration rate on the upstream and downstream _sides of the catalytic converter 12. Set high. Note that steps 704 to 716 are steps 403 to 4 in FIG.
Equivalent to 15. Therefore, the comparison result in step 704 is delayed by the delay times TDR2 and TDL2, and the second air-fuel ratio flag F2 is set. In step 717, it is determined whether or not the second air-fuel ratio flag F2 is "0", and as a result, F2
If = "0" (lean), the process proceeds to steps 718 to 723. If F2 = "1" (rich), the process proceeds to steps 724 to 729.

In step 718, RSR ← RSR + ΔRS (constant value, for example, 0.0
8%), that is, the rich skip amount RSR is increased to shift the air-fuel ratio to the rich side. In steps 719 and 720, RSR is guarded at the maximum value MAX, for example 6.2%. Further, in step 721, RSL ← RSL−ΔRS, that is,
The lilli skip amount RSL is reduced to shift the air-fuel ratio to the rich side. In steps 722 and 723, RSL is guarded with the minimum value MIN, for example, 2.5%. On the other hand, rich (F2 =
If "1"), RSR ← RSR-ΔRS is set in step 724, that is, the rich skip amount RSR is decreased and the air-fuel ratio is shifted to the lean side. In steps 725 and 726, RSR is guarded with the minimum value MIN. Further, in step 727, RS
L ← RSL + ΔRS (constant value), that is, the lean skip amount RSL is increased to shift the air-fuel ratio to the lean side. In steps 728 and 729, RSL is guarded with the maximum value MAX.

After the RSR and RSL calculated as described above are stored in the RAM 105, this routine ends in step 732.

FAF, RSR, RS calculated during air-fuel ratio feedback
L can be once converted into other values FAF ′, RSR ′, RSR ′ and stored in the backup RAM 106, which also helps improve drivability at the time of restart. The minimum value MIN in FIG. 7 is a value at which the transient followability is not impaired, and the maximum value MIN is a value at which the deterioration of drability due to the air-fuel ratio fluctuation does not occur. In addition,
It goes without saying that the values X ₁ and X ₂ defining the predetermined range in FIG. 4 are set between the minimum value MIN and the maximum value MAX.

As described above, according to the routine of FIG. 7, if the output of the downstream O ₂ sensor 15 is lean, the rich skip amount RSR is gradually increased, and the lean skip amount RSL is gradually decreased. , The air-fuel ratio is shifted to the rich side.
If the output of the downstream O ₂ sensor 15 is rich, the rich skip amount RSR is gradually reduced and the lean skip amount RSL is gradually increased, whereby the air-fuel ratio is shifted to the lean side.

FIG. 8 shows an injection amount calculation routine, which is executed every predetermined crank angle, for example, every 360 ° CA. RA in step 801
The intake air amount data Q and the rotation speed data Ne are read from M105 to calculate the basic injection amount TAUP. For example TAUP ← KQ
/ Ne (K is a constant). In step 802, the cooling water temperature data THW is read from the RAM 105, and the warm-up increase value FWL is interpolated by the one-dimensional map stored in the ROM 104. Step 8
In 03, the final injection amount TAU is calculated by TAU ← TAUP · (FAF + FGHAC) · (FWL + α) + β. Note that α and β are correction amounts that are determined by other operating state parameters. Then step 804
At the same time, the injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start the fuel injection. Then, in step 805, this routine ends.

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

In addition, the first air-fuel ratio feedback control is performed every 4 ms, and the second air-fuel ratio feedback control is performed every 1 S.
This is because the control by the O ₂ sensor is mainly performed, and the control by the downstream O ₂ sensor, which has poor response, is performed as the _secondary control.

Further, other control constants in the air-fuel ratio feedback control by the upstream O ₂ sensor, such as delay time, integration constant,
The present invention can be applied to a double O ₂ sensor system that corrects the above by the output of the downstream O ₂ sensor, and also to a double O ₂ sensor system that introduces a second air-fuel ratio correction coefficient. In addition, controllability can be improved by simultaneously controlling two of the skip amount, delay time, and integration constant. further,
One of the skip amounts RSR and RSL can be fixed and only the other can be made variable, or one of the delay times TDR1 and TDL1 can be fixed and only the local region can be made variable, or the rich integration constant KIR, lean It is also possible to fix one of the integration constants KIL and make the other variable. Therefore, the steps of FIG.
The determination of 418 can also be applied to the air-fuel ratio feedback control constant by various upstream O ₂ sensors. For example, it may be X ₁ <RSL <X ₂ or may be a value of | RSR−RSL |.

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

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

Further, although the internal combustion engine in which the fuel injection amount is controlled by the fuel injection valve is shown in the above-mentioned embodiment, the present invention can be applied to a carburetor type internal combustion engine. For example, an electric air control valve (EACV) adjusts the intake air amount of the engine to control the air-fuel ratio, an electric bleed air control valve adjusts the air bleed amount of the carburetor, and the main system passage and The present invention can be applied to those that control the air-fuel ratio by introducing the atmosphere into the slow passage and those that adjust the amount of secondary air sent into the exhaust system of the engine. 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, in accordance with the intake pipe negative pressure according to the intake air amount and the rotation speed of the engine. At, the supply air amount corresponding to the final fuel injection amount TAU is calculated.

Furthermore, in the above-mentioned embodiment, the O ₂ sensor is used as the air-fuel ratio sensor, but a CO sensor, a lean mixture sensor or the like can be used.

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

〔The invention's effect〕

As described above, according to the present invention, when the air-fuel ratio feedback control constant is within the predetermined range, the air-fuel ratio correction coefficient FAF
Since the learning control is performed when the asymmetry is small, the erroneous correction of the learning correction amount FGHAC due to the asymmetry of the air-fuel ratio correction coefficient FAF can be reduced, and therefore, the fuel consumption is deteriorated during open loop, during transients, and drivability is reduced. It is possible to prevent deterioration and emission deterioration.

[Brief description of drawings]

FIG. 1 is an overall block diagram for explaining the configuration of the present invention, FIG. 2 is an exhaust emission characteristic diagram for explaining a single O ₂ sensor system and a double O ₂ sensor system, and FIG. 3 is an internal combustion engine according to the present invention. 6 is an overall schematic view showing an embodiment of the air-fuel ratio control device of FIG. 4, FIG. 5, FIG. 7, FIG. 8 and FIG. 8. FIG. 6 is a flow chart for explaining the operation of the control circuit of FIG. 6 is a timing diagram for supplementary explanation of the flowchart of FIG. 1 ... Engine main body, 3 ... Air flow meter, 4 ... Distributor, 5,6 ... Crank angle sensor 10 ... Control circuit, 12 ... Catalytic converter, 13 ... Upstream (first) O ₂ sensor , 15 ...... Downstream (second) O ₂ sensor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 55-134731 (JP, A) JP 62-31178 (JP, B2) JP 6-10445 (JP, B2)

Claims (5)

[Claims] 1. A first and a second detector, which are respectively provided on the upstream side and the downstream side of a catalytic converter for purifying exhaust gas provided in an exhaust system of an internal combustion engine, for detecting a concentration of a specific component in the exhaust gas. An air-fuel ratio sensor, a control constant calculating means for calculating the air-fuel ratio feedback control constant according to the output of the second air-fuel ratio sensor, and an output of the second air-fuel ratio sensor and the air-fuel ratio feedback control constant Air-fuel ratio correction amount calculating means for calculating an air-fuel ratio correction amount, learning means for calculating a learning correction amount so that the average value of the air-fuel ratio correction amount converges to a predetermined value, and asymmetry of the air-fuel ratio correction amount Is within a predetermined range, an asymmetry determining means, a prohibiting means for inhibiting the operation of the learning control means when the asymmetry of the air-fuel ratio correction amount is outside a predetermined range, the air-fuel ratio correction amount and The above Air-fuel ratio control system for an internal combustion engine having a, and the air-fuel ratio adjusting means for adjusting the air-fuel ratio of the engine in accordance with the learning correction amount. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the average value of the air-fuel ratio correction amount is an arithmetic average value immediately before the inversion of the air-fuel ratio correction amount. 3. The air-fuel ratio of the internal combustion engine according to claim 1, wherein the asymmetry determining means determines asymmetry based on a difference between the rich-side skip constant and the lean-side skip constant. Control device. 4. The empty space of the internal combustion engine according to claim 1, wherein the asymmetry determining means determines asymmetry based on a difference between a rich side integration constant and a lean side integration constant. Fuel ratio control device. 5. The air-fuel ratio of the internal combustion engine according to claim 1, wherein the asymmetry determining means determines asymmetry based on a difference between the rich-side delay time and the lean-side delay time. Control device.