JPH01113554A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To reduce catalytic exhaust smell by selecting an air-fuel ratio feedback control constant value according to duration of airfuel ratio feedback control conditions in a system having air-fuel ratio sensors disposed upstream and downstream from a catalytic converter. CONSTITUTION:Air-fuel ratio sensors 13, 15 are respectively disposed upstream and downstream from a catalytic converter 12 of an exhaust system. When it is discriminated that an engine is under air-fuel ratio feedback control conditions by downstream air-fuel ratio control discriminating means A, an air-fuel ratio feedback control constant is computed from output of the downstream sensor 15 by control constant arithmetic means B. The average value of the control constants is computed by arithmetic means C. Further, the duration of the air-fuel ratio feedback control condition is measured by timer means D. When the duration is less than a designated value, the control constant computed by the arithmetic means B is selected and when the duration is a designated value or more, the average value is selected by select means E. According to the control constant or the average value and output of the upstream sensor 13, an air-fuel ratio correction amount is computed by arithmetic means F.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides air-fuel ratio sensors (in this specification, oxygen flow rate sensors (0□ sensors)) on the upstream and downstream sides of a catalytic converter.
), and relates to an air-fuel ratio control device for an internal combustion engine that performs air-fuel ratio feedback control using a downstream 02 sensor in addition to air-fuel ratio feedback control using an upstream 02 sensor.

[Conventional technology]

Is it just an air-fuel ratio feedback system? In the ff1l (single 02 sensor system), the 0□ sensor that detects the oxygen concentration is installed in the exhaust system as close to the combustion chamber as possible, that is, in the gathering part of the exhaust manifold upstream of the catalytic converter, but the output of the 02 sensor Due to the variation in characteristics, it is difficult to improve the accuracy of air-fuel ratio control.

In order to compensate for variations in the output characteristics of the 02 sensor, variations in components such as fuel injection valves, and changes over time or aging, a second 02 sensor is provided downstream of the catalytic converter, and the air-fuel ratio feedback is provided by the upstream 02 sensor. A double 02 sensor system that performs air-fuel ratio feedback control using a downstream 02 sensor in addition to the control has already been proposed (see Japanese Patent Laid-Open No. 58-48756). In this double 0□ sensor system, although the 02 sensor installed on the downstream side of the catalytic converter has a lower response speed than the 02 sensor on the upstream side, it has the advantage of less variation in output characteristics for the following reasons. have.

(1) Downstream of the catalytic converter, the exhaust gas temperature is low, so 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 02 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 an equilibrium state.

Therefore, as described above, by performing air-fuel ratio feedback control (double 02 sensor system) based on the outputs of the two 02 sensors, variations in the output characteristics of the upstream 02 sensor can be absorbed by the downstream 0□ sensor. In fact, as shown in Figure 2, in a single 0□ sensor system, 0□
When the sensor output characteristics deteriorate, it directly affects the exhaust emission characteristics, whereas in the double 02 sensor system, even if the output characteristics of the upstream O sensor deteriorate, the exhaust emission characteristics do not deteriorate. In other words, double 02
In the sensor system, good exhaust emissions are guaranteed as long as the downstream 02 sensor maintains stable output characteristics.

[Problem that the invention seeks to solve]

However, in the double 02 sensor system described above, when catalyst deterioration is small or when the engine is operated in a low intake air range, the air-fuel ratio feedback control frequency by the downstream 02 sensor becomes small. For example, in a double 0□ sensor system that makes the skip amount variable (refer to Japanese Patent Application Laid-Open No. 61-234241), as shown in Figure 3, the output ■2 of the downstream 0□ sensor is reversed from lean to rich. However, due to the long lean duration, the IC,
C.

Catalyst exhaust odor (It2S) occurs for a certain period of time after emissions increase and skip 1iR3R reaches its peak value, and catalyst exhaust odor (irritating odor) occurs just before rich skip 1R3R reaches its peak value. do. Also,
Even when the output v2 of the downstream 0□ sensor is reversed from rich to lean, NOx emissions increase because the rich duration is long. As described above, when the lean duration or rich duration of the output of the downstream 02 sensor becomes longer due to the 0□ storage effect of the three-way catalyst, etc., there is a problem in that emissions deteriorate periodically.

Therefore, an object of the present invention is to reduce catalyst exhaust odor and prevent deterioration of emissions due to a decrease in the air-fuel ratio feedback control frequency of the downstream air-fuel ratio sensor.

[Means for solving problems]

A means for solving the above-mentioned problems is shown in FIG. That is, an upstream air-fuel ratio sensor for detecting the air-fuel ratio of the engine is provided in the exhaust passage upstream of the three-way catalyst CCR8 provided in the exhaust passage of the internal combustion engine, and the three-way catalyst CCR8 is provided in the exhaust passage of the internal combustion engine.
C,t. A downstream air-fuel ratio sensor for detecting the air-fuel ratio of the engine is provided in the exhaust passage on the downstream side of the engine. The downstream air-fuel ratio control determining means determines whether the air-fuel ratio control condition by the downstream air-fuel ratio sensor is an air-fuel ratio feedback control condition or an open-loop condition, and as a result, when the air-fuel ratio control condition is the air-fuel ratio feedback control condition, , the control constant calculation means calculates air-fuel ratio feedback control constants, for example, skip values R3R, R3L, according to the output V2 of the downstream side air-fuel ratio sensor, and furthermore, the control constant average value calculation means calculates the calculated air-fuel ratio feedback control constant R3R. , R3L average value (
(or annealed value), R2H, and R3L are calculated.
On the other hand, the timer means measures the duration during which the air-fuel ratio control condition is the air-fuel ratio feedback control condition, and as a result, the selection means selects the air-fuel ratio feedback control constant R3R from the control constant calculation means when the duration is less than a predetermined value. , R3
L is selected, and when the duration is longer than a predetermined value, the average value (or smoothed value) R2H, R3L of the air-fuel ratio feedback control constant from the control constant average value calculating means is selected. Therefore, the air-fuel ratio correction amount calculating means calculates the average value (or smoothed value) R of the selected air-fuel ratio foot bank control constants R3R, R3L or the air-fuel ratio feedback control constant.
Using 2H and R3L, an air-fuel ratio correction 11FAF is calculated according to the output (1) of the upstream air-fuel ratio sensor. The air-fuel ratio adjustment means adjusts the air-fuel ratio of the engine according to the air-fuel ratio correction amount FAF.

[For production]

According to the above-mentioned means, even after the air-fuel ratio feedback control condition by the downstream air-fuel ratio sensor is satisfied, when the duration thereof is equal to or greater than a predetermined value, the air-fuel ratio feedback control constants R3R and R3L are The previous average value (or smoothed value) R2H.

It is held at R3L, and therefore, the air-fuel ratio feedback control by the downstream air-fuel ratio sensor is stopped.

[Embodiment] 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 intake air amount, has a built-in potentiometer, and generates an analog voltage output signal proportional to the intake air amount. This output signal is quantified by the multiplexer built-in A/D converter 101 of the control circuit 10. The distributor 4 has a crank angle sensor 5 whose shaft generates a reference position detection pulse signal every 720° in terms of crank angle, and a crank angle sensor 5 which generates a reference position detection pulse signal every 30° in terms of crank angle. A crank angle sensor 6 is provided. The pulse signals of these crank angle sensors 5 and 6 are supplied to the input/output interface 102 of the control circuit 10, and the output of the crank angle sensor 6 is sent to the CPU 10.
3 interrupt terminal.

Further, the intake passage 2 is provided with a fuel injection valve 7 for supplying pressurized fuel from a fuel supply system to the intake boat for each cylinder.

Further, a water temperature sensor 9 for detecting the temperature of cooling water is provided in the water jacket 8 of the cylinder block of the engine body 1. Water temperature sensor 9 indicates cooling water temperature TH
Generates an analog voltage electrical signal according to W. This output is also supplied to the A/D converter 101.

A catalytic converter 12 that accommodates a three-way catalyst that simultaneously purifies three toxic components HC, CO, and NOx in exhaust gas is provided in the exhaust system downstream of the exhaust manifold 11.

The exhaust manifold 11 includes a catalytic converter 1.
A first Ot sensor 13 is provided upstream of the catalytic converter 12, and a second 02 sensor 15 is provided in the exhaust pipe 14 downstream of the catalytic converter 12.

The 0□ sensors 13 and 15 generate electrical signals corresponding to the concentration of oxygen components in the exhaust gas. That is, 02 sensor 13
, 15 of the control circuit 10 which outputs different output voltages depending on whether the air-fuel ratio is on the lean side or the lean side with respect to the stoichiometric air-fuel ratio.
This occurs in the D converter 101. The control circuit 10 is configured as a microcomputer, for example, and includes an A/D converter 1
01, input/output interface 102, CPU103
In addition to the above, a ROM 104, a RAM 105, a backup RAM 106, a clock generation circuit 107, etc. are provided.

Further, the throttle valve 16 of the intake passage 2 is provided with an idle switch 17 for detecting whether the throttle valve 16 is fully closed or not, and this output signal is supplied to the output horn interface 102 of the control circuit 10. be done.

18 is a vehicle speed sensor, the output of which is sent to the input/output ink face 10 via the vehicle speed forming circuit 111 of the control circuit 10.
2.

Further, in the control circuit 10, a down counter 108,
Flip-flop 109 and drive circuit 110 are for controlling fuel injection valve 7. That is, in the routine described later, when the fuel injection amount TAtJ is calculated, the fuel injection amount T8U 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 the clock signal (not shown) and its carrier terminal reaches the "1" level, the flip-flop 109
is set, and the drive circuit 110 stops energizing the fuel injection valve 7. That is, the fuel injection valve 7 is energized by the above-mentioned fuel injection amount 1TAtJ, and therefore, an amount of fuel corresponding to the fuel injection amount TAU is sent into the fuel chamber of the engine body 1.

Note that the interrupt generation of CPt1103 is caused by A/D converter 1.
When the A/D conversion of 01 is completed, the input/output interface 10
2 receives a pulse signal from the crank angle sensor 6, receives an interrupt signal from the clock generation circuit 107, etc.

Intake air amount data Q and cooling water temperature data T of the air flow meter 3) (W is taken in by an A/D conversion routine executed at predetermined time intervals and stored in a predetermined area of the RAM 105. In other words, data Q and cooling water temperature data in the RAM 105 THW is updated at predetermined time intervals.Rotational speed data Ne is calculated by the crank angle sensor 6's interruption every 30° CA and stored in a predetermined area of the RAM 105.

FIG. 5 shows a first air-fuel ratio feedback control routine for calculating an air-fuel ratio correction coefficient FAF based on the output of the upstream 02 sensor 13, and is executed every predetermined period of time, for example, every 4 ma.

In step 501, it is determined whether a closed loop (feedback) condition for the air-fuel ratio by the upstream 02 sensor 13 is satisfied. For example, when the cooling water temperature is below a predetermined value, while the engine is starting, during the initial increase after starting, during warm-up increase, during increase in power, during OTP increase to prevent catalyst overheating, upstream side 02
When the output signal of the sensor 13 has never been inverted, the closed loop condition is not satisfied in any case such as a fuel cut, and the closed loop condition is not satisfied in any other case. If the closed loop condition is not satisfied, the process proceeds to step 527 and the air-fuel ratio correction coefficient F A F is set to 1.0. Note that FAF may be set to a value immediately before the end of closed loop control. In this case, proceed directly to step 528. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 802.

In step 502, the output vl of the upstream 02 sensor 13 is
is A/D converted and taken in, and in step 503 it is determined whether vI is lower than the comparison voltage Vll, for example 0.45V, that is, it is determined whether the air-fuel ratio is rich or lean, that is, lean (V+ ≦ V Rl ), it is determined whether the delay counter CDLY is positive or not in skip 504, and if CDLY>O, CDL is determined in step 505.
Set Y to 0 and proceed to step 806. In step 506, the delay counter CDLY is decremented by 1, and in step 5
07, the delay counter CDLY is guarded at the minimum value TDL at 508. In this case, the delay counter CD
When LY reaches the minimum value TDL, step 509
The first air-fuel ratio flag F1 is set to "0" (lean). Note that the minimum value TDL is a lean delay state to maintain the determination that the rich state is present even if the output of the upstream 0□ sensor 13 changes from rich to lean, and is defined as a negative value. Ru. On the other hand, rich (V+ >V
R+), it is determined whether the delay counter CDLY is negative or not in step 510, and if CDLY<0, CDLY is set to O in skip 511, and step 812
Proceed to.

In step 512, the delay counter CDLY is incremented by 1, and in steps 513 and 514, the delay counter C
Guard DLY with 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). Note that the maximum value TDR is a delay time for maintaining the determination that the engine is in a lean state even if there is a change from lean to rich in the output of the upstream 02 sensor 13, and is defined as a positive value. .

In step 516, it is determined whether the sign of the first air-fuel ratio flag Fl has been inverted, that is, it is determined whether the air-fuel ratio after the delay processing has been inverted. If the air-fuel ratio is reversed, in step 517, it is determined whether the reversal is from rich to lean or from lean to rich, based on the value of the first air-fuel ratio flag F1. If it is a reversal from rich to lean, in step 518 FAF - FAF + R3R
On the contrary, if it is a reversal from lean to rich, FAF←FAF − is increased in step 519.
Decrease R5L in a skip manner. In other words, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted in step 512, integration processing is performed in steps 520, 521, and 522. That is, in step 520, F1
="0" or not, and if F1="0" (lean), in step 521 FAF 4-FAF+KIR
On the other hand, if F1="1" (rich), then in step 522 FAF←FAF-NIL is set. here,
Integral constants KIR and KIL are skip amounts R3R.

It is set sufficiently small compared to R3L, that is, the old R
(K IL) < RSR (R5L). Therefore,
Step 521 gradually increases the fuel injection amount in a lean state 1 (Fl-“0”), and step 522 gradually increases the fuel injection amount in a rich state (Fl-“0”).
F1="1") gradually decreases fuel injection ■.

The air-fuel ratio correction coefficient FAF calculated in steps 518, 519, 521, and 522 is guarded at a minimum value of, for example, 0.8 in steps 523 and 524, and is guarded at a maximum value of, for example, 1.2 in steps 525 and 526. will be guarded. As a result, 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 using that value to prevent overrich.
Prevent over lean.

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

FIG. 6 is a timing diagram supplementary explanation of the operation according to the flowchart of FIG. When the air-fuel ratio signal A/F for rich/lean discrimination is obtained from the output of the upstream 0□ sensor 13 as shown in FIG. 6(A), the delay counter CD
As shown in FIG. 6(B), LY is counted up in a rich state and counted down in a lean state.

As a result, as shown in FIG. 6(C), a delayed air-fuel ratio signal A/F' (corresponding to flag F1) is formed. For example, time 1. Even if the air-fuel ratio signal A/F' changes from lean to rich at time t2, the delayed air-fuel ratio signal A/F' changes to rich at time t2 after being held lean for the rich delay time TDR. . Even if the air-fuel ratio signal A/F changes from rich to lean at time t3, the air-fuel ratio signal A/F' subjected to the delay processing is delayed by the lean delay time (
-TDL) at time 1. Changes to lean. However, if the air-fuel ratio signal A/F' is inverted in a short period of the rich delay time TDR, such as at time tS+t&+L, it takes time for the delay counter CDLY to reach the maximum value TDR, and as a result, at time 1
At s, the air-fuel ratio signal A/F' after the delay processing is inverted.

In other words, the air-fuel ratio signal A/F' after the delay process is more stable than the air-fuel ratio signal A/F before the delay process. In this manner, the air-fuel ratio correction coefficient FAF shown in FIG. 6(D) 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 0□ sensor 15 will be explained. The second air-fuel ratio feedback control includes skip amounts R3R and R3L as first air-fuel ratio feedback control constants, and an integral constant KI.
There is a system in which R, KIL, delay time TDR, TDL, or comparison voltage VRI of the output 1' of the upstream 02 sensor 13 is made variable, and a system in which a second air-fuel ratio correction coefficient FAF2 is introduced.

For example, if the rich skip amount R3R is increased, the controlled air-fuel ratio can be shifted to the rich side, and even if the lean skip IR3L is decreased, the controlled air-fuel ratio can be shifted to the rich side.On the other hand, if the lean skip fiR3L is increased,
The controlled air-fuel ratio can be shifted to the lean side, and even if the rich skip 1R3R is reduced, the controlled air-fuel ratio can be shifted to the lean side.

Therefore, the air-fuel ratio can be controlled by correcting the rich skip IR3R and the lean skip R3L according to the output of the downstream 0□ sensor 15.

In addition, by increasing the rich integral constant KIR, the control air-fuel ratio can be shifted to the rich side, and the lean integral constant KIL
The control air-fuel ratio can be shifted to the rich side even if KIR is decreased, and on the other hand, the control air-fuel ratio can be shifted to the lean side by increasing the lean integral constant KIL. can be shifted to the lean side. Therefore, the air-fuel ratio can be controlled by correcting the rich integral constant KIR and the lean integral constant KIL according to the output of the downstream 02 sensor 15. If rich delay time TDR>lean delay time (-TDL) is set, the control air-fuel ratio can be shifted to the rich side, and conversely, lean delay time (-TDL)
> IJ Sochi delay time (TDR), the control air-fuel ratio can be shifted to the lean side. That is, the air-fuel ratio can be controlled by correcting the delay times TDR and TDL according to the output of the downstream 02 sensor 15. Furthermore, if the comparison voltage Vl11 is increased, the controlled air-fuel ratio can be shifted to the rich side, and if the comparison voltage □ is decreased, the controlled air-fuel ratio can be shifted to the lean side. Therefore, by correcting the comparison voltage ■□ according to the output of the downstream side 02 sensor 15, the air-fuel ratio can be controlled.

Making these skip amount, integral constant, delay time, and comparison voltage variable by the downstream 02 sensor has its own advantages. For example, the delay time allows for very delicate air-fuel ratio adjustment, and the skip amount can be controlled with good response without lengthening the air-fuel ratio feedback period unlike the delay time. Therefore, naturally, two or more of these variable amounts can be used in combination.

Next, a double 02 sensor system in which the skip amount as an air-fuel ratio feedback control constant is made variable will be described.

Figure 17 shows downstream O! Skip 'W! based on the output of sensor 15! This is a second air-fuel ratio feedback control routine that calculates tR3R and R3L, and is executed every predetermined period of time, for example, every 512 m5. Steps 701-705
Now, it is determined whether the downstream side 02 sensor 15 is in a closed loop condition. For example, in addition to the failure of the closed loop condition determined by the upstream 02 sensor 13 (step 701), when the cooling water temperature THW is below a predetermined value (for example, 70°C) (step 702), the throttle valve 16 is fully closed (LL = "1").
”) (step 703), and when the load is light (Q/
Ne < If it is not a closed loop condition, the process proceeds to steps 706 to 709 for open loop initialization.

In step 706, the cumulative value MRSR of rich skip 1iR3R is reset, and in step 707, the cumulative value MRSL of lean skip 1iRsL is reset,
In step 708, the reversal counter CR3 for measuring the air-fuel ratio feedback control duration by the downstream side 02 sensor 15 is reset, and in step 709,
An execution flag FB is set in preparation for execution of air-fuel ratio feedback control by the downstream side 02 sensor 15.

When the closed loop condition is satisfied and the oven loop control is shifted to the closed loop control, the flow of steps 701 to 705 proceeds to step 710. In this case, since FB="1" is set during open loop control, step 71
Flows 2 to 725 are executed. That is, in step 711, the output ■2 of the downstream sensor 15 is A/D converted and taken in, and in step 712, v2 is set to the comparison voltage ■9.
□For example, determine whether it is 0.55V or less, that is,
Determine whether the air-fuel ratio is rich or lean. Note that the comparison voltage 1 is set higher than the comparison voltage V1 of the output of the upstream O2 sensor 13, taking into account that the output characteristics differ due to the influence of raw gas and the deterioration rate differs between upstream and downstream of the catalytic converter 12. However, this setting may be optional. Step 712 V! If =≦Vllll (lean), the process advances to step 713 and the second air-fuel ratio flag F2 is set to "0", while if V,>Vo (rich), the process advances to step 714 and the second air-fuel ratio flag F, 2 is set. is "1". In step 715, it is determined whether the second air-fuel ratio flag F2 has been inverted, that is, whether the air-fuel ratio A/F detected by the downstream O2 sensor 15 has been inverted. As a result, steps 716 to 718 are executed only when the second air-fuel ratio flag F2 is inverted. That is,
At step 716, the cumulative rich skip IMRsR is
MRSR−MRSR−R3R, and in step 714, the cumulative lean skip amount MR5L is set as MR5L 4−MR.
5L+RSL, and in step 715, the reversal counter CR3 is incremented by +1. In this case, since the reversal count counter CR3 is less than the predetermined value CR3o (10 to 20 minutes in terms of time), the process advances to steps 720 to 725 via step 719. Note that the predetermined value CR
8° is an even value in order to calculate the average value of the peak values of each skip 1iRsR, R3L.

In steps 720 to 725, air-fuel ratio feedback control is substantially performed using the downstream 02 sensor 15. That is, in step 720, the second air-fuel ratio flag F2 is set to “
0” (lean) or not, and as a result, F2="
0” (lean), proceed to steps 721 and 722, and if F2="1” (rich), proceed to step 72
3, proceed to 724. That is, in step 721,
RSR←R5R+ΔR5, that is, the rich skip amount R8R is increased to shift the air-fuel ratio to the rich side, and at step 722, RSL 4−RSL+ΔRS
That is, lean skip (iR3L is decreased to further shift the air-fuel ratio to the rich side. On the other hand, in step 723, RSR-RSR-ΔR5 is set, that is, the rich skip 'MR8R is decreased to shift the air-fuel ratio to the lean side. At the same time, in step 724, RSL-RS
L-ΔR5, that is, the lean skip amount R3L is increased to further shift the air-fuel ratio to the lean side. Step 725 performs guard processing on the RSR and RSL calculated as described above. For example, the maximum value MA
Guard at X = 7.5% and minimum value MIN = 2.5%. Note that the minimum value MIN is a value at a level that does not impair transient followability, and the maximum value MAX is a value at a level at which drivability does not deteriorate due to air-fuel ratio fluctuations.

Note that the calculated skip amounts R8R and R8L are stored in the back amplifier RAM 106.

Under closed loop conditions (steps 701-705),
The flow of steps 711 to 725 is executed until the reversal counter CRS reaches the predetermined value CR3o. In this state, the reversal counter CR8 reaches the predetermined value CR.
3o, the flow of steps 720-725 is changed to step 726 by switching in step 719.
The flow switches to 728.

In step 726, the rich skip amount R5R is calculated as the average value. That is, RSR=MRSR
/CR5° Also, in step 727, the lean skip amount R3L is calculated as the average value. That is, RSL=MR
5L/CR3° Then, in step 728, the execution flag FB is reset.

The routine of FIG. 7 then ends at step 729.

In this way, when the routine of FIG. 7 is executed again while the closed loop condition remains satisfied, the flow at step 710 is directly changed to step 72 because the execution flag FB=“0”.
9, and therefore, under the closed loop condition, the skip amounts R5R and R8L are held at their average values. In other words, it is open loop control.

In addition, in FIG. 7, the closed loop condition is satisfied (step 70
1 to 705), but the predetermined time period may be directly measured. Also steps 726 and 727
Although the average value of the skip amounts R3R and R3 is calculated in , a smoothed value may be used, and further, successive integration may be performed instead of calculation for each inversion.

FIG. 8 shows an injection amount calculation routine, which is executed at every predetermined crank angle, for example, 360° CA. Step 150
1, the intake air amount data Q and rotational speed data Ne are read out from the RAM 105 to calculate the basic injection amount RAUP. For example, assume that TAUP←α·Q/Ne (α is a constant). At step 802, the cooling water temperature data THW is read from the RAM 105, and a warm-up increase value FWL is calculated by interpolation using the one-dimensional map stored in the ROM 104. In step 803, the final injection amount TAU is set as TAU←TAUP.
Calculate by -FAF ・(PWL+β)+γ.

Note that β and T are correction amounts determined by other operating state parameters. Next, in step 804, the injection amount T
AU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection.

This routine then ends in step 1505.

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

FIG. 9 is a timing diagram for supplementary explanation of the routines shown in FIGS. 5, 7, and 8. That is, time 1. Previously, it is assumed that the closed loop condition (step 501) by the upstream O2 sensor 13 and the closed loop condition (steps 701 to 705) by the downstream O2 sensor 15 have been satisfied, and furthermore, the closed loop condition by the upstream O2 sensor 13 (Step 501) has been satisfied. Assume that the output ■1 changes as shown in FIG. 9(A), and the output ■2 of the downstream 02 sensor 15 changes as shown in FIG. 9(B). In this case, the reversal counter CR3
As shown in FIG. 9(C), the downstream 0□ sensor 15
■Add 1 count for each reversal of 2.

Furthermore, as shown in FIG. 9(D), the rich skip 1R3R gradually increases when the output ■2 of the downstream side 02 sensor 15 is lean, and on the other hand, the downstream side O! When the output V2 of the sensor 15 is rich, it gradually decreases. Similarly, as shown in FIG. 9(E), the lean skip fiR3L gradually decreases when the output v2 of the downstream 02 sensor 15 is lean, and on the other hand, when the output v2 of the downstream 0□ sensor 15 is rich If so, increase gradually. At this time, downstream O! When the rich duration or lean duration of the output (2) of the sensor 15 becomes longer, the air-fuel ratio A/F (periodically) becomes rich (IC, C) as shown in FIG. 9(F).

(increase in emissions) or lean (increase in NOx emissions).

When the reversal counter CR3 reaches the predetermined value CR3o, the execution flag FB changes from "1" to "O", and as a result, as shown in FIGS. 9(D) and (E), the rich skip ilRS R, lean The skip amount R3L is held above its average value R3R, R3, and as a result, as shown in FIG.
As shown in B), the amplitude of the output v2 of the downstream 02 sensor 15 becomes smaller and the frequency becomes larger, so that
As shown in FIG. 9(F), the periodic rich/lean deviation of the air-fuel ratio A/F disappears. In this way, even after the closed-loop conditions (steps 701 to 705) are satisfied, open-loop control is performed after a predetermined period of time has elapsed.

FIG. 10 shows a modification of FIG. 7, in which the skip amount R3R and the hold value on R3 are determined for each region of the vehicle speed SPD. Skip R3R and hold above R3 to the value that is optimal for each area of SPD. In this routine, the vehicle speed SPD has the following four regions, and a counter, a skip amount, and an execution flag are defined for each region.

Nod area Counter Skip amount Execution flag SP
D < a CNTl RSRI
FBI5LI a≦5pIl <b CNT2 R5R2F
B25L2 b≦SPD <c CNT3 R5R3FB3
5L3 SPD≧c CNT4 RSR4R3L4
FB4 Steps 1001' to 1005 are step 70 in FIG.
1 to 705, and is a closed loop condition by the downstream 02 sensor 15. If it is not a closed loop condition, the process proceeds to initialization steps 1006 to 1008.In step 1006, the time measurement counters CNT1 to CNT4 for each region are
In step 1007, the skip amount of each area, for example, the maximum value RSR1max + minimum value R5R1m1n of RSRI is set to 5%, and in step 100B
In order to prepare for execution of air-fuel ratio feedback control by the downstream O2 sensor 15, execution flags FBI to FB4 are set.

When the closed loop condition is satisfied and the open loop control shifts to the closed loop control, the flow of steps 1001 to 1005 proceeds to step 1009. In step 1009, the vehicle speed SPD is taken in, region 1: SPD<a, region ■: a≦spo<b, 6B region m: b≦SPD<C region I
V: Determine whether SPD>c. If it is area I, the process advances to step 1010 and processes area I, and area ■
If so, the process advances to step 1011 to process the area (2); if it is the area (2), the process proceeds to step 1012 to process the area (2); if it is the area (2), the process proceeds to step 1013 to process the area (2).

The routine then ends at step 1014.

Next, for each region I, n, I[[, TV, the first
This will be explained with reference to FIGS. 1 to 14.

FIG. 11 shows a detailed routine of processing step 1010 in area M in FIG. 10. When this routine is executed for the first time, in step 1101, FBI-1'' is set during open loop or when processing other areas, so
The flow of steps 1102-1118 is executed. In step 1102, counters CNT2 . C
0NT3, reset CNT3, step 110
3 skips other areas 1iR3R2.

RSL2, R5R3, RSL3, R3R4, R
SL4 is initialized (5%), and in step 1104, execution flags FB2 and FB3 are set to prepare for processing other areas.
, set FB4.

In step 1105, a counter CNTl is incremented by one in order to measure the duration of region I.

Next, in step 1106, the output v2 of the downstream side 02 sensor 15 is A/D converted and taken in, and in step 1107, V2 is the comparison voltage ■. For example, it is determined whether the voltage is 0.55V or less, that is, it is determined whether the air-fuel ratio is rich or lean.

If ■2≦vlI□ (lean) in step 1107, the process proceeds to step 1108 and sets the second air-fuel ratio flag F2 to “
On the other hand, if Vz>VRl (rich), the process proceeds to step 1109 and sets the second air-fuel ratio flag F2 to "1".In step 1110, the second air-fuel ratio flag F2 is set to "1".
It is determined whether or not the air-fuel ratio A/F detected by the downstream 0□ sensor 15 has been reversed. As a result, step 1111 is executed only when the second air-fuel ratio flag F2 is inverted. That is, in step 1111, the maximum and minimum values of rich skip IRsRI and lean skip amount RSLI are updated.

That is, the rich skip amount RSRI and its maximum value R5R1max.

Read the minimum value R5R1m1n from the back amplifier RAM 106, and if RSRI > RSRlmax, RSRl
max”R5R1, RSRI < R5R1n1n
If so, it is set as R5R1n1rr-RSRI.

Similarly, the lean skip amount RSLI, its maximum value R5L1max, and minimum value RSL1min are read from the bank up RAM106, and RSLI>RSLlm
ax, set RSLlmax-R3[,1, and RSL
If I < R3L1min, RSL1min=RS
Let it be LI. Similarly, lean skip 1RsL1, its maximum value RSL1max, minimum (I!R3Lmin
is released from the backup RAM 106, and RSLI>
If RSLmax, set RSLlmax = RSL1, and if RSLI < R5L4min, set RSLlm
in "RSLI. In step 1112, the counter CNTl is set to a predetermined value CNTo (10 to 20 in terms of time).
minute), the process advances to steps 1113 to 1118. Steps 1113 to 1118 are substantially downstream 0
2 sensor 15 performs air-fuel ratio feedback control.

That is, in step 1113, it is determined whether or not the second air-fuel ratio flag F2 is "D" (lean), and as a result, F
If 2="0" (lean), step 1114.

The process advances to step 1115, and if F2="bi (rich), the process advances to steps 1116 and 1117. That is, step 11
In 14, RSRI←RSRI+ΔRS, that is,
Rich skip 1JRsR1 is increased to shift the air-fuel ratio to the rich side, and at step 1115 RSL
I←RSLI +ΔR5, that is, the lean skip ffiR3LfcM is decreased to further shift the air-fuel ratio to the rich side.

On the other hand, in step 1116, R5RI←R3RI−Δl
In other words, the rich skip amount R3R is decreased to shift the air-fuel ratio to the lean side, and step 1
117, R3LI←R3LI−ΔRS, that is,
The lean skip amount R3LI is increased to further shift the air-fuel ratio to the lean side. Step 1118 performs guard processing on the RSRI and RSLI calculated as described above. Note that the calculated skip filR3R1, RS
LI is stored in bank up RAM 106.

In steps 1122 and 1123, the area I
7P RSRI, RSLI execution ski knob amount ER5
Let R, ERSL.

Note that the execution skip amounts ER5R and ERSL are RAM10.
It is stored at 5.

Under the closed loop condition (steps 1001-1005), the flow of steps 1113-1118 is executed until the counter CNTl reaches the predetermined value CNTo. In this state, the counter CNTl is set to a predetermined value (UCNTo
When reaching , the flow of steps 1113 to 1118 is changed to step 11 by switching in step 1112.
The flow switches to 19 to 1121.

In step 1119, rich skip 1JR3R1 is calculated as the average value. That is, RSRI =
(RSRlmax + R5R1m1n) / 2 Also, in step 1120, the lean skip amount RSLI
is calculated as the average value. That is, RSLI”
(R5L1max+R5L1min)/2 Then, in step 1121, the execution flag FBI is reset.

At steps 1122 and 1123, RSRI and RS
The routine of FIG. 11 ends at step 1124 with LI set to the execution values ERSR and ERSL.

In this way, when the routine of FIG. 11 is executed again while the closed loop condition remains satisfied, the execution flag FBI="0"
”, the flow at step 1101 goes directly to step 1124, thus skipping ERSR(RSRI), ERSL(RSL) under the closed loop condition.
I) is held at its average value. In other words, it is open loop control.

In addition, in FIG. 11, the predetermined time is directly counted by the counter C.
Closed loop condition established by NTl (steps 1001 to 1
005) The subsequent connection time is measured, but it may also be measured by the number of output reversals of the downstream side 02 sensor 15. Also,
Skip 1iR3R at steps 1119 and 1120
, R3L is calculated, but a smoothed value may be used, and further, successive integration may be performed instead of calculation for each inversion.

FIG. 12 shows a detailed routine of processing step 1011 in area ■ in FIG. 10, FIG. 13 shows a detailed routine in processing step 1012 in area ■ in FIG. The detailed routine of step 1013 is the same as that in FIG.

That is, in each region, when the closed loop condition is established and a predetermined period of time has elapsed, the skip amount ERSR(R
SR2~RSR4), ERSL (RSL2~R5
L4) is held at its average value, resulting in open loop control.

In addition, when FIGS. 10 to 14 are used instead of FIG. 7, the values R at steps 518 and 519 in FIG.
3R and R3L are assumed to be execution values ER5R and ERSL.

Furthermore, although the regions in FIG. 10 are divided based on vehicle speed, other driving state parameters such as intake air amount Q, engine rotational speed Ne, etc. may also be used (also, the number of regions may be other than the number of regions).

In addition, the first air-fuel ratio feedback control is performed every 4 ms.
In addition, the second air-fuel ratio feedback control is performed for 512 ms.
The reason why the air-fuel ratio feedback control is performed every time is that the air-fuel ratio feedback control is mainly performed by the upstream 02 sensor, which has good responsiveness, and the downstream 02 sensor, which has poor responsiveness, is used as a secondary control.

In addition, other control constants in air-fuel ratio feedback control by the upstream 0□ sensor, such as delay time, integral constant, etc., are corrected by the output of the downstream 02 sensor.
The invention can be applied to two-sensor systems as well as double-02 sensor systems that introduce a second air-fuel ratio correction factor. Furthermore, controllability can be improved by simultaneously controlling two of the skip amount, delay time, and integral constant.

Furthermore, it is possible to fix one of the skip amounts R3R and R3L and make only the other variable.
It is also possible to fix one of L and make only the other variable.
Or Rich integral constant KIR, Lean integral constant KIL
It is also possible to have one of them fixed and the other variable.

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

Furthermore, in the above embodiment, the fuel injection amount is calculated according to the intake air amount and the engine rotation speed, but the fuel injection amount is calculated according to the intake air pressure and the engine rotation speed, or the throttle valve opening and the engine rotation speed. The fuel injection amount may also be calculated.

Further, in the above embodiment, an internal combustion engine is shown in which the amount of fuel injected into the intake system is controlled by a fuel injection valve, but the present invention can also be applied to a carburetor type internal combustion engine. For example, the electric link air control valve (EACV) adjusts the intake air amount of the engine to control the air-fuel ratio, and the electric link bleed air control valve adjusts the air bleed pattern of the carburetor. The present invention can be applied to devices that control the air-fuel ratio by introducing atmospheric air into system passages and slow system passages, those that adjust the amount of secondary air sent to the exhaust system of an engine, and the like. In this case, the basic fuel injection amount corresponding to the basic injection pattern TAUP in step 801 is determined by the carburetor itself, that is, it is determined according to the intake pipe negative pressure according to the intake air amount and the engine rotation speed, and in step 803 The supplied air amount corresponding to the final fuel injection amount TAU is calculated.

Further, in the above embodiment, a 0□ sensor was used as the air-fuel ratio sensor, but a CO sensor, a lean mixture sensor, etc. may also be used.

Furthermore, although the embodiments described above are constructed using a microcomputer, that is, a digital circuit, they may also be constructed using an analog circuit.

〔Effect of the invention〕

As explained above, according to the present invention, even after the closed-loop condition is established by the downstream air-fuel ratio sensor, open-loop control is performed after a predetermined period of time has elapsed, and at that time, the air-fuel ratio control amount by the downstream air-fuel ratio sensor is e.g. Since the skip amount is an average value during closed loop control, periodic deterioration of emissions can be prevented.

[Brief explanation of the drawing]

Figure 1 is an overall block diagram for explaining the present invention in detail, Figure 2 is a single 02 sensor system and a double 02 sensor system.
Fig. 3 is a timing chart illustrating the problem to be solved by the present invention; Fig. 4 shows an embodiment of the air-fuel ratio control device for an internal combustion engine according to the present invention; Overall schematic diagram; Figures 5, 7, 8, and 10 to 14 are flowcharts for explaining the operation of the control circuit in Figure 4; Figure 6 supplements the flowchart in Figure 5. Timing diagram for explanation FIG. 9 is a timing diagram supplementary to the flowcharts of FIGS. 5, 7, and 8. 1... Engine body, 3... Air flow meter,
4... Distributor, 5.6... Crank angle sensor, lO... Control circuit, 12... Catalytic converter,
13... Upstream 0□ sensor, 15... Downstream 02 sensor, 17... Idle switch.

Claims (1)

[Claims] 1. Three-way catalyst (12) provided in the exhaust passage of an internal combustion engine
an upstream air-fuel ratio sensor (13) provided in the exhaust passage upstream of the three-way catalyst to detect the air-fuel ratio of the engine; and an upstream air-fuel ratio sensor (13) provided in the exhaust passage downstream of the three-way catalyst to detect the air-fuel ratio of the engine. a downstream air-fuel ratio sensor (15) that detects the air-fuel ratio of the downstream air-fuel ratio, and downstream air-fuel ratio control determining means that determines whether the air-fuel ratio control condition by the downstream air-fuel ratio sensor is an air-fuel ratio feedback control condition or an open-loop condition; control constant calculation means for calculating an air-fuel ratio feedback control constant according to the output of the downstream air-fuel ratio sensor when the air-fuel ratio control condition is an air-fuel ratio feedback control condition; and an average of the calculated air-fuel ratio feedback control constants. value(
control constant average value calculation means for calculating a control constant average value calculation means (or a smoothed value), a timer means for measuring a duration during which the air-fuel ratio control condition is an air-fuel ratio feedback control condition, and when the duration is less than a predetermined value, the control constant Selecting an air-fuel ratio feedback control constant from the calculation means, and selecting an average value (or smoothed value) of the air-fuel ratio feedback control constant from the control constant average value calculation means when the duration is equal to or greater than the predetermined value. and an air-fuel ratio that calculates an air-fuel ratio correction amount according to the output of the upstream air-fuel ratio sensor using the selected air-fuel ratio feedback control constant or the average value (or rounded value) of the air-fuel ratio feedback control constant. An air-fuel ratio control device for an internal combustion engine, comprising: a correction amount calculation means; and an air-fuel ratio adjustment means for adjusting the air-fuel ratio of the engine according to the air-fuel ratio correction amount. 2. The control constant average value calculation means calculates an average value (or smoothed value) of the air-fuel ratio feedback control constant for each region of the operating state parameters of the engine, and the selection means is configured to set the duration to the predetermined value. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, which selects an average value (or smoothed value) of the air-fuel ratio feedback control constant according to the range of the operating state parameter in the above cases. 3. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the timer means measures the duration based on the number of inversions of the output of the downstream air-fuel ratio sensor.