JP2526587B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] 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 present invention relates to an air-fuel ratio control device for an internal combustion engine that performs air-fuel ratio feedback control using a downstream O ₂ sensor in addition to air-fuel ratio feedback control using an upstream O ₂ sensor.

[Conventional technology]

In simple air-fuel ratio feedback control (single O ₂ sensor system), an O ₂ sensor that detects oxygen concentration is provided in the exhaust system as close as possible to the combustion chamber, that is, at the exhaust manifold upstream of the catalytic converter. In addition, the variation in the output characteristics of the O ₂ sensor hinders the improvement of the air-fuel ratio control accuracy. Such O ₂ component variation variation and the fuel injection valve and the output characteristics of the sensor, to compensate for time or secular change, a second O ₂ sensor disposed downstream of the catalytic converter, by the upstream O ₂ sensor Downstream O ₂ in addition to air-fuel ratio feedback control
Double O ₂ with air-fuel ratio feedback control by sensor
Sensor systems have already been proposed (see:
-48756 publication). In this double O ₂ sensor system,
O ₂ sensor provided downstream of the catalytic converter, compared with the upstream O ₂ sensor, but has a low response speed,
There is an advantage that the variation in output characteristics is small for the following reasons.

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

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

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

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

[Problems to be solved by the invention]

However, the downstream O ₂ sensor in the double O ₂ sensor system for being positioned downstream of the catalytic converter, a rich delayed by a certain time, generating a lean output. That is, the output of the downstream O ₂ sensor is delayed due to the O ₂ storage effect of the catalytic converter (three-way catalyst). Therefore, when the output of the downstream O ₂ sensor changes from lean to rich, the air-fuel ratio upstream of the catalytic converter has already deviated to the rich side from the stoichiometric air-fuel ratio, and as a result, CO, HC
When the output of the downstream O ₂ sensor changes from rich to lean, which causes deterioration of emissions and fuel efficiency, on the other hand, the air-fuel ratio upstream of the catalytic converter has already deviated to the lean side by more than the theoretical air-fuel ratio. As a result, there is a problem that NO _x emission is deteriorated and drivability is deteriorated.

Explaining the O ₂ storage effect of the three-way catalyst, the three-way catalyst purifies NO _x , CO, and HC at the same time, and the purification rate η of the three-way catalyst is as shown in the dashed line in FIG. The purification rate of NO _x is larger on the rich side than the fuel ratio (λ = 1), and the purification rates of CO and HC are large on the lean side (HC is not shown, but has the same tendency as CO). As a result, if the required purification rate η is η ₀ , the controllable air-fuel ratio window w is very narrow (w = w ₁ ), and therefore the air-fuel ratio feedback control with respect to the theoretical air-fuel ratio is essentially in this range (w Must be done in ₁ ). However, three-way catalyst, O ₂ that reacted fuel ratio uptake of O ₂ when the lean, CO when the air-fuel ratio becomes rich, and O ₂ was incorporated at the time of lean crowded preparative HC Since the air-fuel ratio feedback control has a storage effect and positively utilizes such an O ₂ storage effect, the air-fuel ratio is controlled at an optimum frequency and amplitude. As a result, as shown by the solid line in FIG. 3, the purification rate η is improved during the air-fuel ratio feedback control, and the controllable air-fuel ratio window w is substantially widened (w = w ₂ ). In particular, when the control air-fuel ratio shifts to the lean side,
The purification rate η drops sharply, leading to an increase in NO _x emissions.

Therefore, an object of the present invention is to substantially increase the response speed of the downstream air-fuel ratio sensor (O ₂ sensor) to reduce CO, H
This will prevent deterioration of C, NO _x emissions, fuel consumption, drivability, etc.

In order to achieve the above-mentioned object, the applicant of the present application
Immediately after the change in rich or lean, the control constant is rapidly changed after that, and the update of the control constant after reversal from rich to lean and the update of the control constant after reversal from lean to rich are made symmetrical. I have already proposed (see:
Japanese Patent Application No. 61-241486).

[Means for solving problems]

The means for solving the above problem is shown in FIG. That is, the upstream side of the exhaust passage of the three-way catalyst CC _RO provided in an exhaust passage of an internal combustion engine, it is provided upstream air-fuel ratio sensor for detecting an air-fuel ratio of the engine, also the three-way catalyst CC _RO
A downstream air-fuel ratio sensor that detects the air-fuel ratio of the engine is provided in the exhaust passage downstream of the engine. Asymmetric control constant computing means for computing an air-fuel ratio feedback control constant example skip amounts RSR, the RSL in accordance with the output V ₂ of the downstream air-fuel ratio sensor, but the target air-fuel ratio of the output V ₂ of the downstream air-fuel ratio sensor After reversing from rich to lean or from lean to rich, the update rate of the air-fuel ratio feedback control constant per unit time is increased from before it was reversed and then gradually decreased,
At that time, the update speed after the reversal from rich to lean is set to be higher than the update speed after the reversal from lean to rich. As a result, the air-fuel ratio correction amount calculation means calculates the air-fuel ratio correction amount FAF according to the air-fuel ratio feedback control constants RSR, RSL and the output V ₁ of the upstream side air-fuel ratio sensor. The air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine according to the air-fuel ratio correction amount.

[Action]

According to the above means, the downstream air-fuel ratio sensor rich,
At the lean change point, the average air-fuel ratio upstream of the catalytic converter changes rapidly, but then gradually changes. In particular,
At that time, if the update speed after reversing from rich to lean is made higher than the update speed after reversing from lean to rich,
The average air-fuel ratio shifts to the rich side and NO _x emissions decrease.

〔Example〕

FIG. 4 is an overall schematic diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention. In FIG. 4, 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 amount of intake air, and has a built-in potentiometer to generate an analog voltage output signal proportional to the amount of intake air. This output signal is output from the A / D converter 1 with built-in multiplexer in the control circuit 10.
Supplied to 01. The distributor 4 has, for example, a crank angle sensor 5 that generates a reference position detection pulse signal every 720 ° converted into a crank angle, and a reference position detection 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 an input / output interface 102 of the control circuit 10, and the output of the crank angle sensor 6 is supplied to an interrupt terminal of the CPU 103.

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

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

The exhaust manifold 11 has a catalytic converter 12
A first O ₂ sensor 13 is provided on the upstream side, and a second O ₂ sensor 15 is provided on the exhaust pipe 14 downstream of the catalytic converter 12. The O ₂ sensors 13 and 15 generate an electric signal corresponding to the concentration of the oxygen component in the exhaust gas. That is, the O ₂ sensor 1
3 and 15 have different output voltages depending on whether the air-fuel ratio is leaner or richer than the theoretical air-fuel ratio.
Occurs at 01. The control circuit 10 is configured as, for example, a microcomputer, and in addition to the A / D converter 101, the input / output interface 102, and the CPU 103, the ROM 104, the RAM 105, the backup RAM 106, the clock generation circuit 107, and the like are provided.

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

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

The intake air amount data Q 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. Further, the rotation speed data Ne is calculated by the interrupt of the crank angle sensor 6 every 30 ° CA and RA
It is stored in a predetermined area of M105.

Figure 5 is a first air-fuel ratio feedback control routine for calculating the air-fuel ratio correction coefficient FAF based on the output of the upstream O ₂ sensor 13 is executed at a predetermined time, for example, 4ms each.

In step 501, the air-fuel ratio of the closed loop by the upstream O ₂ sensor 13 (feedback) condition is determined whether or not satisfied. For example, when the cooling water temperature is lower than or equal to a predetermined value, during engine start, during start increase, during warm up increase, during power increase,
When the output signal of the upstream O ₂ sensor 13 has never been inverted, the closed loop condition is not satisfied during the fuel cut or the like, and the closed loop condition is satisfied in other cases. When the closed loop condition is not satisfied, the routine proceeds to step 527, where the air-fuel ratio correction coefficient FAF is set to 1.0. Note that FAF may be set to a value immediately before the end of the closed loop control. In this case, step 528
Go directly to. On the other hand, if the closed loop condition is satisfied, step
Proceed to 502.

In step 502, the output V ₁ of the upstream O ₂ sensor 13 is A / D converted and captured, and in step 503 V ₁ is the comparison voltage V _R1
Determine whether it is 0.45V or less. That is, it is determined whether the air-fuel ratio is rich or lean. If the air-fuel ratio is lean (V ₁ ≦ V _R1 ), the skip 504 determines whether the delay counter CDLY is positive, and if CDLY> 0, the step 505 calls CDLY.
Is set to 0 and the process proceeds to step 506. In step 506, the delay counter CDLY is decremented by 1, and in steps 507 and 508, the delay counter CDLY is guarded by the minimum value TDL. In this case, when the delay counter CDLY reaches the minimum value TDL, the first air-fuel ratio flag F1 is set to "0" (lean) in the skip 509. The minimum value TDL is a lean delay time for holding the determination that the output 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. It On the other hand, rich, (V ₁ >
V _R1 ), delay counter CDLY at step 510
Is determined to be negative, and if CDLY <0, CDLY is set to 0 in skip 511 and the process proceeds to step 512. In step 512, 1 is added to the delay counter CDLY, and in steps 513 and 514, the delay counter CDLY is guarded by the maximum value TDR. In this case, when the delay counter CDLY reaches the maximum value TDR, the first air-fuel ratio flag F1 is set to "1" in step 515.
(Rich). The maximum value TDR is the upstream O ₂ sensor 1
This is a rich delay time for maintaining the determination that the vehicle is in the lean state even when there is a change from lean to rich in the output of 3, and is defined as a positive value.

In step 516, it is determined whether or not the sign of the first air-fuel ratio flag F1 has been inverted, that is, whether or not the air-fuel ratio after the delay processing has been inverted. If the air-fuel ratio is inverted, it is determined in step 517 whether the air-fuel ratio is inverted from rich to lean or from lean to rich, based on the value of the first air-fuel ratio flag F1. If the inversion is from rich to lean, then in step 518, FAF ← FAF + RSR is skipped, and if the inversion is from lean to rich, in step 519, FAF ← FAF−RSL is skipped. Decrease.
That is, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted at step 516, integration processing is performed at steps 520, 521 and 522. That is, in step 520, it is determined whether or not F1 = "0". If F1 = "0" (lean), then in step 521 FAF
← FAF + KIR, while if F ₁ = “1” (rich), then in step 522 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 521 gradually increases the fuel injection amount in the lean state (F1 = "0"), and step 522 performs the rich state (F1 =
The fuel injection amount is gradually reduced by "1").

The air-fuel ratio correction coefficient FAF calculated in steps 518, 519, 521, 522 is guarded in steps 523, 524 with a minimum value, for example, 0.8, and is guarded in steps 525, 526 with a maximum value, for example, 1.2. Thus, if the air-fuel ratio correction coefficient FAF becomes too large or too small for some reason, the air-fuel ratio of the engine is controlled with that value to prevent over-rich or over-lean.

The FAF calculated as described above is stored in the RAM 105, and this routine ends at step 528.

FIG. 6 is a timing chart 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 delay counter CDLY shows
As shown in FIG. 6B, the rich state is counted up and the lean state is counted down. 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, 'also is changed from lean to rich, the air-fuel ratio signal A / F which is delayed processed' air-fuel ratio signal A / F at time t ₁ is the time t ₂ after being held lean only the rich delay time TDR It changes richly at. Also the air-fuel ratio signal A / F from the rich at time t ₃ is changed to the lean, the delayed air-fuel ratio signal A /
F 'is changed to the lean at time t ₄ after being held rich only equivalent lean delay time (-TDL). However, when the air-fuel ratio signal A / F ′ is inverted at a time shorter than the rich delay time TDR or the lean delay time (−TDL) at times t ₅ , t ₆ , and t ₇ , the delay counter CDLY reaches the maximum value TDR. Time is required, 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 is more stable than the air-fuel ratio signal A / F before the delay processing. In this way, the stable air-fuel ratio signal A /
Based on F ', the air-fuel ratio correction coefficient FAF shown in Fig. 6 (D)
Is obtained.

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 TDR, TDL, or the output V ₁ of the upstream O ₂ sensor 13 There is a system that makes the comparison voltage V _R1 variable.

For example, if the rich skip amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and even if the lean skip amount RSL is decreased, the control air-fuel ratio can be shifted to the rich side, while the lean skip amount RSL is increased. , The control air-fuel ratio can be shifted to the lean side, and the rich skip amount
Even if the RSR is reduced, the control air-fuel ratio can be shifted to the lean side. Therefore, the air-fuel ratio can be controlled by correcting the rich skip amount RSR and the lean skip amount RSL according to the output of the downstream O ₂ sensor 15. Also, the rich integration constant KIR
The control air-fuel ratio can be shifted to the rich side by increasing the control air-fuel ratio, and the control air-fuel ratio can be shifted to the rich side even if the lean integration constant KIL is reduced. The control air-fuel ratio can be shifted to the lean side even if the rich integration constant KIR is reduced. Accordingly, the air-fuel ratio can be controlled by correcting the rich integration constant KIR and the lean integration constant KIL in accordance with the output of the downstream O ₂ sensor 15. Rich delay time TDR>
If the lean delay time (-TDL) is set, the control air-fuel ratio can shift to the rich side, and conversely, the lean delay time (-TDL)
By setting> rich delay time (TDR), the control air-fuel ratio can shift to the lean side. That is, the air-fuel ratio can be controlled by correcting the delay times TDR, (-TDL) according to the output of the downstream O ₂ sensor 15. Furthermore, it shifts the control air-fuel ratio by increasing the comparison voltage V _R1 to the rich side, also, possible shifts the control air-fuel ratio by decreasing the reference voltage V _R1 to the lean side. Accordingly, the air-fuel ratio can be controlled by correcting the reference voltage V _R1 in accordance with the output of the downstream O ₂ sensor 15.

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

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.

Figure 7 is skip amount based on the output of the downstream O ₂ sensor 15 RSR, a second air-fuel ratio feedback control routine for calculating the RSL, is executed at predetermined time, for example 512ms. In step 701, it is determined whether or not the downstream O ₂ sensor 15 is in a closed loop condition. For example, a closed loop condition is not satisfied by the upstream O ₂ sensor 13, when the output signal of the downstream O ₂ sensor 15 is also not reversed once, when the downstream O ₂ sensor 15 is faulty, neither etc. closed loop condition is not satisfied In other cases, the closed loop condition is satisfied. If it is not a closed loop condition, proceed to steps 725 and 726 and skip amount RS
Let R and RSL be constant values RSR ₀ and RSL ₀ . For example, RSR ₀ = 5% RSL ₀ = 5%. The skip amounts RSR and RSL may be values immediately before the end of the closed loop control. In this case, go directly to step 727.

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 captured. Next, at step 703, it is determined whether or not V ₂ is the 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. Incidentally, than the comparison voltage V _R1 of the output of the comparison voltage V _R2 upstream O ₂ sensor 13 upstream of the catalytic converter 12, it and the degradation rate output characteristics due to the influence of the raw gas is different downstream in consideration of different like Although set high, this setting may be arbitrary.

If V ₂ ≤ V _R2 (lean) in step 703, step
Proceeding to 704, the second air-fuel ratio flag F2 is set to "0", and conversely, V
_{If 2} > _VR2 (rich), the routine proceeds to step 705, where the second air-fuel ratio flag F2 is set to "1".

In step 706, it is determined whether or not the second air-fuel ratio flag F2 has been inverted, that is, whether or not the output V _{2 of the} downstream O ₂ sensor 15 has been inverted. If it is reversed, the process proceeds to step 707, and the correction amount ΔRS as the update speed of the skip amounts RSR, RSL.
Set the initial value A ₁ to, that is, ΔRS ← A ₁ . Note that A ₁ can be a constant value or variable according to the load parameters Q, Ne and the like. On the other hand, if not reversed, the correction amount ΔRS is multiplied by a predetermined ratio k ₁ (<1) in step 708. That is, ΔRS ← ΔRS · k ₁ .

Therefore, when the routine of FIG. 7 is executed n times after the change from rich to lean, ΔRS becomes ΔRS = A ₁ · ▲ k ⁿ ₁ ▼. That is, the correction amount ΔRS as the update speed sharply increases immediately after the output V ₂ of the downstream O ₂ sensor 15 is reversed, and then gradually decreases.

Also in step 709, it is determined whether or not the second air-fuel ratio flag F2 is inverted, that is, whether or not the output V _{2 of the} downstream O ₂ sensor 15 is inverted. If it is reversed, the process proceeds to step 710, and the correction amount ΔRS as the update speed of the skip amounts RSR, RSL.
Set the initial value A ₂ to, that is, ΔRS ← A ₂ . A ₂ can be a constant value or variable according to the load parameters Q, Ne, etc. In this case, set A ₁ > A ₂ to reduce NO _x emissions. Plan. On the other hand, if not reversed, the correction amount ΔRS is multiplied by a predetermined ratio k ₂ (> 1) in step 711. That is, ΔRS ← ΔRS · k ₂ .

Therefore, after inversion from lean to rich, the routine of Figure 7 is executed n times, .DELTA.Rs becomes ^{_{ΔRS = A 2 · ▲ k n}} 2 ▼. That is, the correction amount ΔRS as the update speed sharply increases immediately after the output V ₂ of the downstream O ₂ sensor 15 is reversed, and then gradually decreases.

The above-mentioned ΔRS is set, for example, as shown in FIG. The ΔRS after reversal from rich to lean changes according to A ₁ = 0.4, k ₁ = 0.9 or A ₁ = 0.4, k ₁ = 0.6, and ΔRS after reversal from lean to rich.
Changes according to A ₂ = 0.2, k ₂ = 0.6. In this case, an upper limit guard value may be set to reduce the air-fuel ratio fluctuation.

Next, at step 712, the second air-fuel ratio flag F2 is "0".
If F2 = "0" (lean), as a result, the process proceeds to steps 713 to 718. If F2 = "1" (rich), the process proceeds to steps 719 to 724.

In step 713, RSR ← RSR + ΔRS is set, that is, the rich skip amount RSR is increased to shift the air-fuel ratio to the rich side. In steps 714 and 715, RSR is guarded at the maximum value MAX, for example 6.2%. Further, at step 716, RS
L ← RSL−ΔRS, that is, the lean skip amount RSL is decreased to shift the air-fuel ratio to the rich side. Step 717,
In 718, RSL is guarded with a minimum value MIN, for example 2.5%.

On the other hand, when F2 = "1" (rich) in step 712, RSR ← RSR-ΔRS is set in step 719, that is, the rich skip amount RSR is decreased and the air-fuel ratio is shifted to the lean side. In steps 720 and 721, RSR is guarded with the minimum value MIN. Further, in step 722, RSL ← RSL + ΔRS is set, that is, the lean skip amount RSL is increased to shift the air-fuel ratio to the lean side. In steps 723 and 724, RSL is guarded with the minimum value MAX.

The RSR, RSL, calculated as described above are stored in the RAM 105, and then the routine ends at step 727.

In addition, FAF, RSR, calculated during the air-fuel ratio feedback,
The RSL can be once converted into another value and stored in the backup RAM 106, which is also useful for improving drivability at the time of restart or the like. Minimum value MI in Fig. 7
N is a value at which the transient followability is not impaired, and the maximum value MAX is a value at which driveability deterioration due to air-fuel ratio fluctuation does not occur.

FIG. 9 shows an injection amount calculation routine, which is executed every predetermined crank angle, for example, every 360 ° CA. In step 901
Intake air amount data Q and rotation speed data Ne from RAM 105
To calculate the basic injection amount TAUP. For example TAUP ←
Let α · Q / Ne (α is a constant). RAM 105 in step 902
The cooling water temperature data THW is read out and the warmer increase value FWL is interpolated by the one-dimensional map stored in the ROM 104. In step 903, the final injection amount TAU is TAU ← TAUP ・ FAF ・
It is calculated by (FWL + β) + γ. Here, β and γ are correction amounts determined by other operation state parameters. Next, at step 904, the injection amount TAU is
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 amount TAU has elapsed, the flip-flop 109 is reset by the carry-out signal of the down counter 108, and the fuel injection ends.

FIG. 10 is a timing chart for explaining the air-fuel ratio A / F on the upstream side of the catalyst obtained by the flow charts of FIGS. 5, 7, and 9. The output V ₂ of the downstream O ₂ sensor 15 is shown in Fig. 10 (A).
The second air-fuel ratio flag F2 changes to the 10th
It changes as shown in FIG. As a result, as shown in FIG. 10 (C), the update speed ΔRS is large when the flag F2 is reversed and then gradually decreased, and changes lean and rich asymmetrically. If the output V _{2 of} the downstream O ₂ sensor 15 is lean (F2 = “0”), as shown in FIGS. 10 (D) and (E), the rich skip amount RSR suddenly increases immediately after reversal. After that, the lean-skip amount RSL is gradually increased immediately after reversal, and then gradually decreased thereafter, and if the output V _{2 of} the downstream O ₂ sensor 15 is rich (F2 = “1”), The rich skip amount RSR sharply decreases immediately after the reversal and then gradually decreases, and the lean skip amount RSL sharply increases immediately after the reversal and gradually increases thereafter. In this case, since A ₁ > A ₂ and k ₁ > k ₂ , the skip amounts RSR and RSL are shown in FIGS. 10 (D) and (E).
It changes asymmetrically as shown in. Since the air-fuel ratio correction coefficient FAF is calculated by the routine shown in FIG. 5, which has a high calculation speed, the skip amounts RSR, RSL are reflected in the air-fuel ratio correction coefficient FAF. Therefore, as shown in FIG. The upstream air-fuel ratio A / F (which shows the opposite tendency to the air-fuel ratio correction coefficient FAF) is
The output V ₂ (F2) of the downstream O ₂ sensor 15 changes greatly immediately after reversal, and then changes slightly. As a result, immediately after the output V ₂ (F2) of the downstream O ₂ sensor 15 is reversed, the air-fuel ratio A / F can be brought close to the stoichiometric air-fuel ratio early, and after that, the air-fuel ratio A / F is reversed and the downstream side Overcorrection of the air-fuel ratio A / F that occurs until the output V ₂ (F2) of the O ₂ sensor 15 is re-inverted can be suppressed, and the amplitude of the air-fuel ratio A / F becomes smaller.

When symmetric control (A ₁ = A ₂ , k ₁ = k ₂ ) is performed in steps 706 to 711 of FIG. 7, the air-fuel ratio A / F ′ is as shown by the alternate long and short dash line in FIG. It is easy to bias toward the lean side,
In particular, it causes an increase in NO _x emissions. Further, as in the conventional case, the air-fuel ratio correction coefficient FAF is set to the output V ₂ of the downstream O ₂ sensor 15.
When skip control is performed with a constant value ΔRS according to (F2),
As shown by the dotted line in FIG. 10 (F), the air-fuel ratio A / A upstream of the catalyst
The amplitude of F becomes large.

Although the correction amount ΔRS is reduced by the predetermined ratios k ₁ and k ₂ (<1) in steps 708 and 711 of FIG. 7, it may be reduced by subtracting the predetermined amount.

Also, the first air-fuel ratio feedback control is performed every 4 ms,
The second air-fuel ratio feedback control is performed every 512 ms. The air-fuel ratio feedback control is mainly performed by the upstream O ₂ sensor with good response, and the downstream O ₂ sensor with poor response is used. This is because the control is performed subordinately.

Further, the double O ₂ for correcting other control constant in the air-fuel ratio feedback control by the upstream O ₂ sensor, for example the delay time, the integration constant, or the like by the output of the downstream O ₂ sensor
The present invention can be applied to a sensor system. Controllability can be improved by simultaneously controlling two of the skip amount, the delay time, and the integration constant. Further skip amount RS
Either one of R and RSL can be fixed and only the other can be made variable, or one of delay time TDR, (-TDL) can be fixed and only the other can be made variable, or the rich integration constant KI
It is also possible to fix one of R and the lean integration constant 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 speed, but the fuel injection amount is calculated according to the intake air pressure and the engine speed, or the throttle valve opening and the engine speed. The fuel injection amount may be calculated.

Further, in the above-described embodiment, the internal combustion engine in which the fuel injection valve controls the fuel injection amount to the intake system is described. However, the present invention can be applied to a carburetor-type internal combustion engine. For example, the air-fuel ratio is controlled by adjusting the intake air amount of the engine using an electric air control valve (EACV).
The air bleed amount of the carburetor is adjusted by the electric blade air control valve to control the air-fuel ratio by introducing the atmosphere into the main passage and the slow passage, and the amount of secondary air sent to the exhaust system of the engine is controlled. The present invention can be applied to things that are adjusted. In this case, the basic fuel injection amount corresponding to the basic injection amount TAUP in step 901 is determined by the carburetor itself, that is, determined in accordance with the intake pipe negative pressure corresponding to the intake air amount and the engine speed, and step 903. Then, the supply air amount corresponding to the final fuel injection amount TAU is calculated.

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

Further, 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 response speed of the downstream side air-fuel ratio sensor is substantially increased by increasing the amount of change in the air-fuel ratio feedback control constant immediately after the inversion of the output of the downstream side air-fuel ratio sensor, After that, it is possible to prevent a large deviation of the air-fuel ratio upstream of the catalyst, and thus to prevent emission deterioration, fuel consumption deterioration, drivability deterioration, and the like, and asymmetrical control reduces NO _x emissions in particular. It is useful.

[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 O _{2 of a} three-way catalyst. FIG. 4 is a graph for explaining the storage effect, FIG. 4 is an overall schematic view showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention, and FIGS. 5, 7, and 9 are control circuits of FIG. 6 is a timing chart for supplementary explanation of the flow chart of FIG. 5, FIG. 8 is a timing chart for explaining ΔRS of FIG. 7, FIG. 10 is FIG. FIG. 10 is a timing chart for explaining the air-fuel ratio upstream of the catalyst obtained by the flowcharts of FIGS. 7 and 9. DESCRIPTION OF SYMBOLS 1 ... Engine body, 3 ... Air flow meter, 4 ... Distributor, 5, 6 ... Crank angle sensor, 10 ... Control circuit, 12
… Catalytic converter, 13… Upstream O ₂ sensor, 15… Downstream O ₂
Sensor.

Claims (7)

(57) [Claims] 1. A three-way catalyst provided in an exhaust passage of an internal combustion engine; an upstream air-fuel ratio sensor provided in an exhaust passage upstream of the three-way catalyst for detecting an air-fuel ratio of the engine; A downstream side air-fuel ratio sensor that is provided in the exhaust passage on the downstream side of the original catalyst and detects the air-fuel ratio of the engine, and an air-fuel ratio feedback control constant is calculated according to the output of the downstream side air-fuel ratio sensor, and the downstream side After the inversion from rich to lean or lean to rich with respect to the target air-fuel ratio of the output of the air-fuel ratio sensor, the update speed per unit time of the air-fuel ratio feedback control constant is increased from before the inversion and then gradually decreased, At that time, an asymmetric control constant calculating means for setting the update speed after reversal from rich to lean to be greater than the update speed after reversal from lean to rich, the air-fuel ratio feedback control constant and the above-mentioned Air-fuel ratio correction amount calculation means for calculating an air-fuel ratio correction amount for calculating an air-fuel ratio correction amount according to the output of the upstream side air-fuel ratio sensor, and an air-fuel ratio for adjusting the air-fuel ratio of the engine according to the air-fuel ratio correction amount An air-fuel ratio control device for an internal combustion engine, comprising: an adjusting unit. 2. The internal combustion engine according to claim 1, wherein the control constant calculation means performs the gradual calculation speed decrease of the air-fuel ratio feedback control constant at a predetermined ratio at predetermined time intervals. Air-fuel ratio controller. 3. The internal combustion engine according to claim 1, wherein the control constant calculation means performs the gradual calculation speed decrease of the air-fuel ratio feedback control constant by a predetermined amount at predetermined time intervals. Air-fuel ratio controller. 4. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio feedback control constant is a skip control constant. 5. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio feedback control constant is an integral control constant. 6. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio feedback control constant is a delay time. 7. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio feedback control constant is a comparison voltage of the output of the upstream side air-fuel ratio sensor.