JPH0788800B2 - Air-fuel ratio controller for internal combustion engine - Google Patents

Description

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

[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 upper O ₂ sensor provides 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 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) Since various poisons are trapped in the catalyst downstream of the catalytic converter, the amount of poisoning of the downstream O ₂ sensor is small.

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

Therefore, as described above, the air-fuel ratio feedback control based on the outputs of the two O ₂ sensors (double O ₂ sensor system), the variations in the output characteristic of the upstream O ₂ sensor can be absorbed by the downstream O ₂ sensor. Actually, as shown in FIG. 2, in the single O ₂ sensor system, when the output characteristic of the O ₂ sensor deteriorates, the exhaust emission characteristic is directly affected, whereas in the double O ₂ sensor system, the upstream Even if the output characteristic of the side O ₂ sensor deteriorates, the exhaust emission characteristic does not deteriorate. 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 therefore the upper limit value or the upper limit value may be close to the 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 air-fuel ratio feedback control, if the air-fuel ratio changes into 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, The fluctuation margin of the fuel ratio 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 air-fuel ratio deviation during open loop becomes large, Moreover, it takes time for the control air-fuel ratio to reach the required level when switching from the open loop to the air-fuel ratio feedback control, 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]

The learning control described above calculates the learning value FGHAC so that the average value FAFAV of the air-fuel ratio correction coefficient FAF converges to a predetermined value (1.0). Originally, it corresponds to the air density change (high altitude traveling). It is done. Therefore, in order to prevent the learning value FGHAC from being overcorrected and the air-fuel ratio becoming overrich or overlean due to the effect of evaporative control for some reason, the learning value FGHAC is also provided with upper and lower limits. ing. However, in the double O ₂ sensor system, the base air-fuel ratio is controlled by varying the amount of deviation from the predetermined value of the air-fuel ratio correction coefficient FAF, for example 1.0, so the average value FAFAV of the air-fuel ratio correction coefficient FAF is Even though there is no change in air density (running at high altitude), it changes due to the air-fuel ratio feedback control by the downstream O ₂ sensor, and as a result, the learning value FGHAC is learning-controlled and becomes a value close to its upper limit value or lower limit value. Sometimes. Therefore, in this case, the variation margin of the learning value FGHAC becomes small, and further learning control becomes impossible. In other words, in this state, even if there is a change in air density (traveling at high altitudes), it is impossible to compensate for the change in air density, resulting in deterioration of fuel efficiency.
There arises a problem that emission is deteriorated and drivability is deteriorated.

Furthermore, in the double O ₂ sensor system,
When the air-fuel ratio feedback control by both the downstream O ₂ sensor is performed, the air-fuel ratio feedback control involved constant example skip constant RSR, RSL are by the air-fuel ratio feedback control by the downstream O ₂ sensor, 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 non-correction coefficient FAF immediately before the skip becomes a predetermined value, for example, 1.0, the average value FAFAV becomes the air-fuel ratio correction coefficient FAF. There is also a problem that erroneous learning is performed because the average value is not accurately represented, that is, because the true air-fuel ratio deviation is not represented.

Therefore, an object of the present invention is to provide a double air-fuel ratio sensor system capable of performing sufficient and accurate learning control only when there is an original change in air density.

[Means for solving problems]

Means for solving the above problems are shown in FIGS. 1A and 1B.

In FIG. 1A, first and second air-fuel ratio sensors for detecting a specific component concentration in exhaust gas are 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. Each is provided. The air-fuel ratio correction amount calculation means calculates the air-fuel ratio correction amount FAF according to the constants involved in the air-fuel ratio feedback control, for example, the skip amounts RSR, RSL and the output V _{1 of the} upstream (first) air-fuel ratio sensor. The constant change amount calculation means calculates the change amount ΔRSR of the constant RSR involved in the air-fuel ratio feedback control, and the constant change amount determination means determines whether the change amount ΔRSR of the air-fuel ratio feedback control constant RSR is smaller than a predetermined amount α. To do. The constant calculation means determines whether the constant RSR, which is involved in the air-fuel ratio feedback control, has an empty value in accordance with the output V ₂ of the downstream (second) air-fuel ratio sensor when the change amount ΔRSR is not smaller than the predetermined amount α (ΔRSR ≧ α). The constants RSR and RSL related to the fuel ratio feedback control are calculated. Further, the learning means is configured such that the variation amount ΔRSR of the constant RSR involved in the air-fuel ratio feedback control is smaller than the predetermined amount α (ΔRSR <
α) only when the constants RSR and RSL related to the air-fuel ratio feedback control are set to a constant value so that the rich side change and lean side change of the air-fuel ratio correction amount FAF become constant and the average of the air-fuel ratio correction amount FAF The learning correction amount FGHAC is calculated so that the value FAFAV converges to the predetermined value 1.0. The air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine according to the air-fuel ratio correction amount FAF and the learning correction amount FGHAC.

Further, in FIG. 1B, compared with FIG. 1A, engine load detection means for detecting the engine load, for example, intake air amount Q, and engine load determination for determining whether the engine load Q is smaller than a predetermined value β or not. Means are added. The constant calculation means is the variation ΔR of the constant RSR involved in the air-fuel ratio feedback control.
When SR is not smaller than the predetermined amount α (ΔRSR ≧ α) or when the engine load Q is not smaller than the predetermined value β (Q ≧
In β), the constants RSR and RSL involved in the air-fuel ratio feedback control are calculated according to the output V ₂ of the downstream (second) air-fuel ratio sensor. Further, the learning means is provided only when the variation amount ΔRSR of the constant RSR involved in the air-fuel ratio feedback control is smaller than the predetermined amount α (ΔRSR <α) and the load Q of the engine is smaller than the predetermined value β (Q <β). , The constants RSR and RSL related to the air-fuel ratio feedback control are set to constant values so that the rich side change and the lean side change of the air-fuel ratio correction amount FAF are made constant, and the average value FAFAV of the air-fuel ratio correction amount FAF is a predetermined value. The learning correction amount FGHAC is calculated so that it converges to 1.0.

[Work]

According to the configuration of FIG. 1A described above, only when the change amount ΔRSR of the air-fuel ratio feedback control constant RSR is smaller than the predetermined amount α, and according to the configuration of FIG. 1B, when the load Q of the engine is smaller than the predetermined value β. Only, that is, the learning control is performed only when the air-fuel ratio feedback control by the downstream side air-fuel ratio sensor is sufficiently stable, at that time, the air-fuel ratio feedback control by the downstream side air-fuel ratio sensor is stopped, that is, the air-fuel ratio feedback Since the control constants RSR and RSL are constant and symmetrical (for example, RSR = RSL = 5-1%), there is no erroneous learning.

〔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 intake 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 built-in multiplexer of 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
To generate an analog voltage electric signal. 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 of the control circuit 10 that outputs different output voltages depending on whether the air-fuel ratio is lean or rich with respect to the theoretical air-fuel ratio.
Send to.

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
In addition to PU 103, ROM 104, RAM 105, backup RAM 10
6. A clock generation circuit 107 and the like are provided.

Further, in the control circuit 10, the down counter 18, 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 routine described later, the fuel injection amount TAU
Is preset in the down counter 108 and the flip-flop 109 is also set. As a result, the drive circuit 110
Starts energizing the fuel injection valve 7. On the other hand, when the down counter 108 counts a clock signal (not shown) and finally its carrier-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-mentioned fuel injection amount TA
Only U, 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 and the cooling water temperature data THW of the air flow meter 3 are fetched by an A / D conversion routine executed every predetermined time and stored in a predetermined area of the RAM 105.
That is, the data Q and THW in the RAM 105 are updated every predetermined time. In addition, the rotation speed data Ne is calculated by the interrupt of the crank angle sensor 6 every 30 ° C and RA
It is stored in a predetermined area of M 105.

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 429, 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 and incorporated, and in step 403 V ₁ is the comparison voltage V _R1
If it is 0.45V or less, that is, if the air-fuel ratio is rich or lean, that is, if the air-fuel ratio is rich or lean (V ₁ ≦ V _R1 ), the first delay is performed in step 404. It is determined whether or not the counter CDLY1 is positive, and if CDLY1> 0, CDLY1 is set to 0 in step 405 and the process proceeds to step 406. In steps 407 and 408, the first delay counter CDL is set.
Y1 is guarded by 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. The minimum value TDL1 is the lean delay time for holding the determination that the output is the rich state even when there is a change from rich to lean in the output of the upstream O ₂ sensor 13,
Defined with a negative value. On the other hand, if rich (V ₁ > V _R1 ), it is determined in step 410 whether the first delay counter CDLY1 is negative, and if CDLY1 <0, in step 411 CD
Set LY1 to 0, and proceed to step 412. 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 TRD1, the first air-fuel ratio flag F1 is set to "1" in step 415. (Rich) The maximum value TDR1
Is a rich delay time for holding the judgment that the lean state is maintained even when the output of the upstream O ₂ sensor 13 changes from lean to rich, and is defined by a positive value.

In step 416, it is determined whether or not the sign of the first delay counter CDLY1 is inverted, that is, it is determined whether or not the air-fuel ratio after the delay process is inverted. If the air-fuel ratio is reversed, then in step 417, it is determined whether or not the learning condition is satisfied by the learning control execution flag F _G. As a result, when the learning condition is satisfied (F _G = “1”), the process proceeds to step 418 to perform learning control. The learning conditions and learning control will be described later.

Next, at step 419, it is judged from the value of the first air-fuel ratio flag F1 whether the reversal from rich to lean or the reversal from lean to rich. If it is inversion from rich to lean, in step 420, FAF ← FAF + RSR is increased in a skip manner, and conversely, if it is inversion from lean to rich,
In step 421, 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 422, 423 and 424. That is, in step 422, it is determined whether or not F1 = "0". If F1 = "0" (lean), in step 423 FAF ←
If FA1 + “1” (rich), FAF ← FAF−KI is set in step 424. Where the integration constant KI
Is set to be sufficiently smaller than the skip constants RSR and RSL, that is, KI <RSR (RSL). Therefore, the step
423 gradually increases the fuel injection amount in the lean state (F1 = "0"), and step 424 gradually decreases the fuel injection amount in the rich state (F1 = "1").

The air-fuel ratio correction coefficient FAF calculated in steps 420, 421, 423, 424 is guarded in steps 425, 426 at the minimum value, for example 0.8, and also in steps 427, 428, the maximum value, for example.
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 430.

FIG. 5 is a detailed flowchart of the learning control step 418 of FIG. 4, and as described above, when the learning condition is satisfied under the air-fuel ratio feedback control by the upstream O ₂ sensor 13, ( F _G = “1”), air-fuel ratio correction factor FAF
It is executed at every skip.

That is, in step 501, the average value FAFAV of the air-fuel ratio correction coefficient FAF is calculated by FAFAV ← (FAF + FAFO) / 2, where FAFO is the FAF value immediately before the previous skip, and in step 502, FAF is calculated next time. FAFO ← FAF in preparation for calculation. 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, the learning correction amount FGHAC is decreased by FGHAC → FGHAC−ΔFGHAC in step 508, and 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.

As will be described later, when the learning control routine of FIG. 5 is executed, the second air-fuel ratio feedback control of FIG. 7 described later is not performed.

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. 6C, the delayed air-fuel ratio control 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
F ′ changes to lean at time t ₄ after being held rich for a lean delay time (−TDL1). However, the air-fuel ratio signal A / F is the time t _5, t _6, t ₇ rich delay time as the TDR1
When inverted in a shorter period, the first delay counter CD
It takes time for LY1 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 constants involved in the first air-fuel ratio feedback control and the integration constants KIR, K
There are a system in which the IL delay time TDR1, TDL1 or the comparison voltage V _R1 of the output V ₁ of the upstream O ₂ sensor 13 is made variable, and a system in which the second air-fuel ratio correction coefficient FAF2 is introduced.

For example, if the rich step amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and the lean
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. Also, 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 decreased, 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 the control air-fuel ratio can be shifted to the lean side even if the rich integration constant KIR is reduced. 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 a constant involved in the air-fuel ratio feedback control 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 closed loop condition by the downstream O ₂ sensor 15 is satisfied. 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 excessive operation, 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 736 and 737, and the skip amounts RSR and RSL are set to constant values, for example, 5%.
That is, symmetrical skip control is performed. In this case, if the other air-fuel ratio feedback control constants KIR, KIL; TDR, TDL are also symmetrical, the air-fuel ratio correction coefficient FAF will be controlled symmetrically by the routine of FIG.

If it is a closed loop, the process proceeds to step 702, where the output V ₂ of the downstream O ₂ sensor 15 is A / D converted and fetched, and in step 703 V ₂
Is a comparison voltage V _R2, for example, 0.55 V or less, that is, it is determined whether the air-fuel ratio is rich or lean. It should be noted that the comparison voltage V _R2 is the comparison voltage of the output of the upstream O ₂ sensor 13 considering that the output characteristics due to the influence of raw gas are different upstream and downstream of the catalytic converter 14 and the deterioration speed is different.
Set higher than V _R1 . Note that steps 703 to 715 correspond to steps 403 to 415 in FIG. Therefore, the step
The comparison result in 703 is delayed by the delay times TDR2 and TDL2, and the second air-fuel ratio flag F2 is set.

Steps 716 to 720 are a routine for determining the learning condition. That is, in step 716, the intake air amount data Q is read from the RAM 105 and it is determined whether or not Q <β (constant value). If Q ≧ β, the process proceeds to step 717, the learning execution crag F _{G is set} to 0, and the process proceeds to steps 721-732. That is, learning control is not performed. This is because if the learning control is performed when the intake air amount Q is large, that is, when the engine load is large, the learning value FGHAC may be learned (overcorrected) due to the effect of evaporation. On the other hand, Q <
If β, steps 718 and 719 determine learning conditions according to changes in the rich skip amount (RSR in this case).
That is, in step 718, it is determined whether or not the second air-fuel ratio flag F2 is inverted. As a result, the learning condition is determined in step 719 only when it is inverted, and if it is not inverted, the learning is executed. Flag F _G is unchanged. The learning condition determination in step 719 will be described later. Then, in step 720, it is determined whether or not the learning execution crag F _G is “1”: If this result F _G = “1”, the process proceeds to steps 736 and 737, the skip amounts RSR and RSL are set to a constant value of 5%, and F _{If G} = "0", proceed to steps 721-736.

In step 721, it is determined whether or not the second air-fuel ratio flag F2 is "0". As a result, if F2 = "0" (lean), the process proceeds to steps 722 to 727, while F2 = "1". If it is (rich), proceed to steps 728-732.

In step 722, RSR _i ← RSR _i + ΔRS (a constant value, for example,
0.08%), that is, the rich skip amount RSR during air-fuel ratio feedback is increased to shift the air-fuel ratio to the rich side. In steps 724 and 725, RSR _i is guarded at the maximum value MAX, for example 6.2%. In addition, RSL at step 725
← RSL-ΔRS, that is, the rich skip amount RSL _i during feedback of the air-fuel ratio is decreased to shift the air-fuel ratio to the rich side. In steps 726 and 727, RSL is guarded with the minimum value MIN, for example, 2.5%.

On the other hand, when rich (F2 = “1”), RSR _i ← RSR _i −ΔRS is set in step 728, that is, the rich skip amount RSR _i during air-fuel ratio feedback is decreased to shift the air-fuel ratio to the lean side. . In steps 730 and 731, RSR _i is guarded with the minimum value MIN. Furthermore, in step 731, RSL _i ←
RSL _i + ΔRS (constant value), that is, the lean skip amount RSL _i during the air-fuel ratio feedback is increased to shift the air-fuel ratio to the lean side. In steps 732 and 733, RSL _i is guarded with the maximum value MAX.

In steps 734 and 735, RSR ← RSR _i , RSL ← RSL _i are set, and RSR _i , R
After storing RSR and RSL together with SL _i in RAM 105,
At 735, this routine ends.

FAF, RSR _i , R calculated during air-fuel ratio feedback
SL _i can be once converted into other values FAF ′, RSR ′, RSR ′ and stored in the backup RAM 106, which is also useful for improving drivability at the time of restart or the like. The minimum value MIN in FIG. 7 is a level value at which transient followability is not impaired, and the maximum value MIN is a value at a level at which drivability deterioration due to air-fuel ratio fluctuations does not occur.

As described above, the learning execution flag F _G is set to “1” and the learning control is executed only when the intake air amount Q is small (Q <β) and the change in the air-fuel ratio feedback control constant, for example, RSR, is small. At that time, the skip amounts RSR and RSL are set to a constant value, for example, 5% (RSR = RSL) in steps 736 and 737. Therefore, in this case, the first air-fuel ratio feedback control by the upstream O ₂ sensor 13 is the air-fuel ratio correction. The coefficient FAF will be controlled symmetrically.

FIG. 8 is a detailed flowchart of the learning condition determining step 719 of FIG. 7, which is executed every time the delayed output signal of the downstream O ₂ sensor 15 is inverted, as described above. In step 801, the average value RSRAV of the rich skip amount RSR is calculated by RSRAV ← (RSR + RSRO) / 2, where RSRO is the value of RSR immediately before the inversion of the second delay counter CDLY2, and in step 802, The average value RSRAVX of the average value RSRAV Calculate with. In step 803, the change amount calculation counter CRSRAV of the smoothed value RSRAVX is incremented by 1, and step 80
At 4, it is determined whether the counter CRSRAV has reached a predetermined value C ₀ , that is, whether the number of times the output signal of the downstream O ₂ sensor 15 has been inverted has reached C ₀ . If CRSRAV> C ₀ , step 80
Proceed to step 5, and if CRSAFAV ≦ C ₀ , jump to step 812.

In step 805, the amount of change in smoothed value RSRAVX, i.e. the air-fuel ratio change amount ΔRSRAVX, ΔRSRAVX ← | RSRAVX-RSRAVXO | However, RSRAVXO computes number of reversals C ₀ values of the previous RSRAVX, by, empty at step 806 Fuel ratio change amount ΔRSRAVX
Is determined to be greater than the predetermined value α. As a result, Δ
If RSRAVX> α, the learning control execution flag F _{G is set} to “0” in step 807. On the other hand, if ΔRSRAVX ≦ α, the process proceeds to step 808, and it is determined whether other learning conditions are satisfied. To do. Other learning conditions include, for example, air-fuel ratio feedback control by the upstream O ₂ sensor 13 (step 401).
In addition, the cooling water temperature THE is 70 ° C <THW <90 ° C, and so on. If even one of these other learning conditions is not satisfied, the learning control execution flag F _{G is set} to “0” in step 807, and the learning control execution flag is set in step 809 only when all the learning conditions are satisfied. Set F _G to “1”. In step 810, the counter CRSRAV is cleared and step 8
In step 11, RSRAVX is set to RSRAVXO, and RSR is set to RSRO in step 812 in preparation for the next execution. In step 813, this routine ends.

In the routine of FIG. 8, the change amount of the lean skip amount RSL may be used instead of the change amount of the rich skip amount RSR.

That is, the routine of FIG. 8 prohibits the learning control when the air-fuel ratio feedback control by the downstream O ₂ sensor 15 is unstable and the skip amount change amount is large, and the air-fuel ratio by the downstream O ₂ sensor 15 is prohibited. When the feedback amount is stable and the amount of change in the skip amount is small, the learning control is performed together with the air-fuel ratio feedback control by the downstream O ₂ sensor 15.

FIG. 9 shows an injection amount calculation routine, which is executed every predetermined crank angle, for example, every 360 ° C. RA in step 901
Intake air amount data Q and rotation speed data Ne from M 105
Is read and the basic injection amount TAUP is calculated. For example TAUP ←
KQ / Ne (K is a constant). In step 902, the cooling water temperature data THW is read from the RAM 105 and stored in the ROM 104 1
The warm-up increase value FWL is interpolated by the dimensional map. In step 903, the final injection amount TAU is calculated by TAU ← TAUP · (FAF + FGHAC) · (FWL + α) + β. Note that α and β are correction amounts determined by other operating state parameters. Next, at step 904, the injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection. Then, in step 905, this routine ends.

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

FIG. 10 is a timing diagram for supplementary explanation of the flowcharts of FIGS. 4, 5, 7, 8 and 9. When the intake air amount Q changes as shown in FIG. 10 (A) and the rich skip amount RSR changes as shown in FIG. 10, at times t _{1 to} t ₂ and times t _{5 to} t ₆ , As shown in FIG. 10 (C), the learning value FGHAC is updated by the learning control. At that time, the air-fuel ratio feedback control by the downstream O ₂ sensor 15 is stopped, and the rich skip amount RSR and the lean skip amount RSL are As shown in FIG. 10 (B), the same constant value is set. On the other hand, at time t ₃ ~t _4, but the intake air amount Q is small (Q <beta), learning control the change ΔRSR the skip amount RSR is large is prohibited, air by the downstream O ₂ sensor 15 Fuel ratio feedback control is performed.

In addition, in the learning control of FIG. 5, the arithmetic mean of the air-fuel ratio correction coefficient FAF immediately before the skip is calculated as the average value FAFAV, but the integrated amount of the air-fuel ratio correction coefficient FAF may be used as the average value FAFAV. it can. That is, in this case, the eleventh
As shown in the figure, the learning value FGHAC is updated so that the positive integral amount S _P (or its smoothed value) and the negative integral amount S _N (or its smoothed value) become equal. This allows
Even if the air-fuel ratio correction coefficient FAF changes asymmetrically, the correct learning correction amount FGHAC can be obtained.

Further, as the load detection in step 717 of FIG. 7, other operating state parameters such as intake air pressure, throttle valve opening, and engine speed can be used.

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,
Either one of the skip amounts RSR, RSL can be fixed and only the other can be made variable, or one of the delay times TDR1, TDL1 can be fixed and only the other 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.

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. Alternatively, 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.

Further, although the O ₂ sensor is used as the air-fuel ratio sensor in the above-described embodiment, a CO sensor, a lean mixture sensor, or the like may 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. Also, the present invention can be applied to those that control the air-fuel ratio by introducing air into the slow passage, those that adjust the amount of secondary air sent to the exhaust system of the engine, and the like. In this case, the basic fuel injection amount corresponding to the basic injection amount TAUO in step 901 is determined by the carburetor itself, that is, it is determined in accordance with the intake pipe negative pressure according to the intake air amount and the engine speed, and step 90
At 3, 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, the learning control is prohibited when the load is large or when the air-fuel ratio feedback control is unstable from the downstream side air-fuel ratio sensor.
The air-fuel ratio feedback control can be preferentially performed by the downstream side air-fuel ratio sensor, and the constants involved in the air-fuel ratio feedback control are fixed symmetrically when the learning value is updated. As a result, it is possible to prevent deterioration of fuel consumption, deterioration of emission, deterioration of drivability, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are overall block diagrams for explaining the configuration of the present invention, and FIG. 2 is an exhaust emission characteristic diagram for explaining a single O ₂ sensor system and a double O ₂ sensor system. FIG. 3 is an overall schematic view showing an embodiment of an air-fuel ratio control system for an internal combustion engine according to the present invention, FIG. 4, FIG. 5, FIG. 7, FIG. 8 and FIG. 6 is a flow chart for explaining the operation of the control circuit, FIG. 6 is a timing chart for supplementary explanation of the flow chart of FIG. 4, and FIG. 10 is FIG. 4, FIG. 5, FIG. 7, FIG. FIG. 11 is a timing chart for supplementary explanation of the flowchart of FIG. 9, and FIG. 11 is a timing chart for explaining a modification 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 side (first) O ₂ Sensor, 15 ... Downstream (second) O ₂ sensor.

Claims (14)

[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, an air-fuel ratio correction amount calculation means for calculating an air-fuel ratio correction amount according to an output of the first air-fuel ratio sensor and a constant involved in air-fuel ratio feedback control, and a constant involved in the air-fuel ratio feedback control A constant change amount calculating means for calculating a change amount, a constant change amount determining means for determining whether or not a constant change amount involved in the air-fuel ratio feedback control is smaller than a predetermined amount, and a constant change amount determining means involved in the air-fuel ratio feedback control. A constant operation for calculating a constant involved in the air-fuel ratio feedback control according to the output of the second air-fuel ratio sensor when the amount of change in the constant is not smaller than the predetermined value. Calculation means, only when the amount of change in the constant involved in the air-fuel ratio feedback control is smaller than the predetermined amount, the constant involved in the air-fuel ratio feedback control is the rich side change and the lean side change of the air-fuel ratio correction amount. And a constant value so as to be symmetric, and a learning unit that calculates a learning correction amount so that the average value of the air-fuel ratio correction amount converges to a predetermined target value, and the air-fuel ratio correction amount and the learning correction amount. An air-fuel ratio adjusting means for adjusting the air-fuel ratio of the engine in accordance with the above. 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 control device for an internal combustion engine according to claim 1, wherein an average value of the air-fuel ratio correction amount is an integrated amount of the air-fuel ratio correction amount. 4. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the constant involved in the air-fuel ratio feedback control is a skip constant. 5. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the constant involved in the air-fuel ratio feedback control is an integration constant. 6. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the constant involved in the air-fuel ratio feedback control is a delay time. 7. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the constant involved in the air-fuel ratio feedback control is a comparison voltage of the output of the first air-fuel ratio sensor. 8. 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, an air-fuel ratio correction amount calculation means for calculating an air-fuel ratio correction amount according to an output of the first air-fuel ratio sensor and a constant involved in air-fuel ratio feedback control, and a constant involved in the air-fuel ratio feedback control Constant change amount calculating means for calculating the change amount of, the constant change amount determining means for determining whether the change amount of the constant involved in the air-fuel ratio feedback control is smaller than a predetermined amount, and the load of the engine is detected. The detected load detection means, the engine load determination means for determining whether or not the load of the engine is smaller than a predetermined value, and the change amount of a constant involved in the air-fuel ratio feedback control is greater than the predetermined amount. When not small, or when the load of the engine is not smaller than the predetermined value, constant calculating means for calculating a constant involved in the air-fuel ratio feedback control according to the output of the second air-fuel ratio sensor, and the air-fuel ratio Only when the amount of change in the constant involved in the feedback control is smaller than the predetermined amount and the load of the engine is smaller than the predetermined value, the constant involved in the air-fuel ratio feedback control is changed to the rich side change of the 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 target value while keeping a constant value so that the lean side change is symmetrical, and the air-fuel ratio correction amount and An air-fuel ratio control device for an internal combustion engine, comprising: an air-fuel ratio adjusting means for adjusting an air-fuel ratio of the engine according to the learning correction amount. 9. The air-fuel ratio control device for an internal combustion engine according to claim 8, 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. 10. The air-fuel ratio control device for an internal combustion engine according to claim 8, wherein an average value of the air-fuel ratio correction amount is an integrated amount of the air-fuel ratio correction amount. 11. The air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the constant involved in the air-fuel ratio feedback control is a skip constant. 12. The air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the constant involved in the air-fuel ratio feedback control is an integration constant. 13. The air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the constant involved in the air-fuel ratio feedback control is a delay time. 14. The air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the constant relating to the air-fuel ratio feedback control is a comparison voltage of the output of the first air-fuel ratio sensor.