JPH01121538A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

PURPOSE:To enhance the control accuracy by calculating the air-fuel ratio correction amount from the control constant in accordance with the output from an O2 sensor downstream catalyst and the output of O2 sensor downstream catalyst, and by computing the air-fuel ratio correction amount in rectangular waveform with this air-fuel ratio correction amount in center when the frequency is below a certain value. CONSTITUTION:An upstream O2 sensor 13 is installed upstream a catalyst converter 12 while a downstream O2 sensor 15 provided downstream the same, and a control circuit 10 calculates No.1 air-fuel ratio correction amount on the basis of a control constant set upon the sensed value by the downstream O2 sensor 15 and the output of the upstream O2 sensor 13. When the frequency of this air-fuel ratio correction amount is over a certain value, control is made with No.1 air fuel ratio correction amount. When the frequency is below specified value, control is made with No.2 air-fuel ratio correction amount in rectangular waveform having specific frequency and amplitude with the central value of No.1 air-fuel ratio correction amount in the center.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides air-fuel ratio sensors (herein, oxygen concentration sensors (0□ sensors)) on the upstream and downstream sides 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 0□ sensor in addition to air-fuel ratio feedback control using an upstream 02 sensor.

[Conventional technology]

In simple air-fuel ratio feedback control (single 0□ sensor system), the 0□ sensor that detects oxygen concentration is installed in the exhaust system as close as possible to the combustion chamber, that is, in the gathering part of the exhaust manifold upstream of the catalytic converter. ,0. Improving the accuracy of air-fuel ratio control is hindered by variations in sensor output characteristics. Ox
In order to compensate for variations in sensor output characteristics, variations in components such as fuel injection valves, and changes over time or aging, a second 0□ sensor is provided downstream of the catalytic converter, and air-fuel ratio feedback control is performed 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 above has already been proposed (
Reference: Japanese Patent Application Laid-Open No. 58-48756). This double 0
In a two-sensor system, the O2 sensor installed downstream of the catalytic converter has a lower response speed than the upstream O2 sensor, but has the advantage of less variation in output characteristics for the following reasons. ing.

(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 Ox 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 air-fuel ratio feedback control (double 0□ sensor system) based on the outputs of the two O2 sensors, variations in the output characteristics of the upstream 02 sensor can be absorbed by the downstream 02 sensor. In fact, as shown in Figure 2, in a single 02 sensor system, 02
If the sensor output characteristics deteriorate, it will directly affect the exhaust emission characteristics, whereas in the double 0□ sensor system, even if the output characteristics of the upstream 02 sensor deteriorate, the exhaust emission characteristics will not deteriorate. Double 0□
In the sensor system, good exhaust emissions are guaranteed as long as the downstream 0□ sensor maintains stable output characteristics.

[Problem that the invention seeks to solve]

In the double 02 sensor system mentioned above, for example, in the double 02 sensor system in which the skip amount is variable (see Japanese Patent Laid-Open No. 61-234241), the downstream Air-fuel ratio correction amount by side 0□ sensor For example, rich skip amount R3R (lean skip amount R3L = 10% - R
3R) becomes too large or too small, and as a result, the downstream o
The control center air-fuel ratio corrected by the output of the two sensors is 0 on the upstream side.
When the air-fuel ratio becomes richer than the control center air-fuel ratio of the two sensors themselves, the output of the upstream O2 sensor increases the rich duty ratio DR as shown in Fig. 3 (A), and conversely, when the air-fuel ratio becomes lean, the output of the upstream Ox sensor increases. As shown in FIG. 3(B), the rich duty ratio DR becomes small. In this way, as the rich duty ratio DR increases, the air-fuel ratio correction coefficient FAF becomes asymmetrical (for example, R
3R=8%.

R3L=2%), and the rich duty ratio DR
As shown in Fig. 4(B), when FAF becomes small, the air-fuel ratio correction coefficient FAF becomes asymmetric (for example, R5R=2%, R
3L=8%). As a result, the air-fuel ratio feedback frequency decreases (for example, about 0.511z), the purification performance based on the 0□ storage effect decreases, and the purification window W of the three-way catalyst shown in Fig. 5 becomes smaller (W = W
+ ), its purification performance decreases. Therefore, IIC,
There is a problem in that it causes deterioration of Co, No, and emissions. In particular, the deterioration of emissions becomes remarkable with a deteriorated catalyst with a small 02 storage effect, and furthermore, during acceleration such as a shift change (transient time), as shown in Figure 4 (C
), the fuel increase skips occur continuously and the emissions worsen further.

In addition, in the double 0□ sensor system that performs double skip (reference: JP-A-61-197737), the air-fuel ratio feedback control frequency does not decrease, but the skip amounts RSR and RSL become too large, and the air-fuel ratio changes during the skip. The sudden change increases the torque fluctuation, and therefore,
Drivability deteriorates.

Therefore, an object of the present invention is to prevent deterioration of emissions due to a decrease in the air-fuel ratio feedback control frequency of the upstream 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 that detects the air-fuel ratio of the engine is provided in the exhaust passage upstream of the three-way catalyst CC*o provided in the exhaust passage of the internal combustion engine, and the three-way catalyst C
A downstream air-fuel ratio sensor that detects the air-fuel ratio of the engine is provided in the exhaust passage downstream of Cao. The control constant calculation means calculates an air-fuel ratio feedback control constant, for example SKIP!, according to the output v2 of the downstream air-fuel ratio sensor. ! kRSR,
Calculate RSL. As a result, the first air-fuel ratio correction amount calculating means calculates the air-fuel ratio correction amount FAF according to the air-fuel ratio feedback control constants RSR, RSL and the output (2) of the upstream air-fuel ratio sensor. On the other hand, the center value calculation means calculates the center value FAFM of the first air-fuel ratio correction amount FAF, and the second air-fuel ratio correction amount calculation means calculates a predetermined value about the center value FAFM of the first air-fuel ratio correction amount. Rectangular wave-like second air-fuel ratio correction amount (automatic control waveform FAF') having a period and amplitude
Calculate. The frequency detection means is the first air-fuel ratio correction ff1
The frequency of the FAF is detected directly or indirectly, for example, by the above-mentioned air-fuel ratio feedback control constant R3RO value. As a result, when the frequency of the first air-fuel ratio correction amount FAF is equal to or higher than the predetermined value, the air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine according to the first air-fuel ratio correction amount FAF, and performs the first air-fuel ratio correction. When the frequency of the amount FAF is less than a predetermined value, the air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine in accordance with the second air-fuel ratio correction amount FAF'.

[For production]

According to the above means, when the frequency of the first air-fuel ratio correction 11FAF is high, the air-fuel ratio of the engine is adjusted according to the first air-fuel ratio correction amount FAF, that is, the air-fuel ratio control amount by the double air-fuel ratio sensor control, On the other hand, the first air-fuel ratio correction amount F
When the AF frequency is low, the first air-fuel ratio correction IFA
Instead of F, the air-fuel ratio of the engine is adjusted in accordance with the second air-fuel ratio correction amount FAF', that is, the control amount of the self-excitation control waveform.

〔Example〕

FIG. 6 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. 6, an air flow meter 3 is provided in the intake passage 2 of the engine body l. 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 supplied to an A/D converter 101 with a built-in multiplexer 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 degrees in terms of crank angle, and a crank angle sensor 5 which generates a reference position detection pulse signal every 30 degrees in terms of crank angle. A crank angle sensor 6 is provided. The pulse signals of these crank angle sensors 5.6 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 port 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.

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 (IC, Co, and NO) in the exhaust gas.

The exhaust manifold 11 includes a catalytic converter 1.
A first O2 sensor 13 is provided on the upstream side of the catalytic converter 12, and a second O2 sensor 15 is provided on the exhaust pipe 14 on the downstream side 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, 0□ sensor 13
, 15, the control circuit 10 outputs different output voltages depending on whether the air-fuel ratio is lean or rich with respect to the stoichiometric air-fuel ratio.
This occurs in the D converter 101. The control circuit IO is configured as a microcomputer, for example, and is connected to an A/D converter 1.
01, input/output interface 102, CPU10
3, 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, and this output signal is supplied to the input/output interface 102 of the control circuit 10. Ru.

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, the fuel injection amount TAU
is calculated, the fuel injection ITAU becomes the down counter 1.
08 and flip-flop 109
is also set. As a result, the drive circuit 110 starts energizing the fuel injection valve 7. On the other hand, the down counter 108
counts the clock signal (not shown) and finally when its carrier terminal becomes "L" level, the flip-flop 109 is reset and the drive circuit 110 activates the fuel injection valve 7. Stop. That is, the fuel injection valve 7 is energized by the amount of fuel injection -ITAU described above, and therefore, an amount of fuel corresponding to the fuel injection amount TAU is sent into the combustion chamber of the engine body 1.

Note that the interrupt generation of the CPU 103 is caused by the A/D converter 10.
1, the input/output interface 102
When the controller receives a pulse signal from the crank angle sensor 6, when it receives an interrupt signal from the clock generation circuit 107, etc.

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

FIG. 7 shows a first air-fuel ratio feedback control routine for calculating the air-fuel ratio correction coefficient FAF, which is executed every predetermined period of time, for example, every 4 ms.

In step 701, 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, when the engine is starting, when the star is increasing after starting, when the warm-up quantity is increasing, when the power is increasing, when the quantity is increasing for catalyst overheat protection (OTP quantity increasing), upstream side 0□ sensor 13 When the output signal of is never inverted, the closed loop condition is not satisfied in any case such as fuel cut, and the closed loop condition is satisfied in other cases. If the closed loop condition is not satisfied, the process proceeds to step 708 and 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 closed loop control.

In this case, proceed directly to step 709. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 702.

In step 702, the self-help control execution flag XJIRIE
Determine whether I is "0" or not. Note that the automatic control execution flag XJIREI is set in the routine shown in FIG. 14, which will be described later.

If XJIREI="0" at step 702, the process proceeds to steps 703 to 705, where double 02 sensor control is performed, and when XJIREI="1", self-help control is performed at steps 706 and 707.

That is, in steps 703 to 705, the self-excitation control execution flag XJIRt! Only when I starts to reverse from "1" to 0, the air-fuel ratio feedback control 11 by the upstream Ot sensor 13 (in this case, the variable skip amount RSI?, R
Since SL is used, double 0□ sensor control is performed. Note that the air-fuel ratio feedback control step 705 will be described later.

In steps 706 and 707, automatic control processing is performed in step 512.
It is performed every 111 seconds. Note that automatic control step 707
This will be discussed later.

This routine then ends in step 709.

Figure 8 shows air-fuel ratio feedback control step 7 in Figure 7.
This is the detailed routine of 05. That is, step 80
1, the output vI of the upstream Ox sensor 13 is A/D converted and taken in, and in step 802 it is determined whether V is less than the comparison voltage V□, for example, 0.45V, that is, is the air-fuel ratio rich? It is determined whether the air-fuel ratio is lean, that is, if the air-fuel ratio is lean (Vl≦Vat), it is determined in step 803 whether the delay counter CDLY is positive or not, and CDL is determined.
If Y>0, set CDLY to O in step 804,
Proceed to step 805.

In step 805, the delay counter CDLY is decremented by 1, and in steps 806 and 807, the delay counter CDLY is guarded at 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" in step 808.
(lean). Note that the minimum value TDL is 0 on the upstream side!
This is a lean delay state for maintaining the determination that the state is rich even if the output of the sensor 13 changes from rich to lean, and is defined as a negative value. On the other hand, if it is rich (Vl > VRI), it is determined in step 809 whether the delay counter CDLY is negative or not, and CDL
If Y<O, CDLY is set to 0 in step 810 and the process proceeds to step 811.

In step 811, the delay counter CDLY is incremented by 1, and in steps 812 and 813, the delay counter C is incremented by 1.
Guard DI and Y 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 814.
(Rich). Note that the maximum value TDR is on the upstream side Ot
This is a rich delay time for maintaining the determination that the lean state is present even if the output of the sensor 13 changes from lean to rich, and is defined as a positive value.

In step 815, it is determined whether the sign of the first air-fuel ratio flag F1 has been inverted, that is, it is determined whether the air-fuel ratio after the delay processing has been inverted. If the air-fuel ratio is inverted, the average value (center value) of the air-fuel ratio correction coefficient FAF is calculated in step 816, which will be described later.

Next, in step 817, the first air-fuel ratio flag Fl
The value of determines whether the reversal is from rich to lean or from lean to rich. If it is a reversal from rich to lean, in step 818 FAF - PAF + l
? If the SR is increased in a skip manner, and conversely, if it is a reversal from lean to rich, FAF-FA is increased in step 819.
F-RSL and skipping decrease. In other words, skip processing is performed.

If the sign of the first air-fuel ratio flag Fl is not inverted in step 815, the process proceeds to steps 820 to 822 and an integral process is performed. That is, in step 820, F1="
0", and if F1="0" (lean), at step 821, set PAP←FAF+)KIR,
On the other hand, if F1="1" (rich), step 8
At step 22, PAP←PAP-KIL. Here, the integral constant KIR(KIL) is set sufficiently small compared to the skip constants RSR and RSL, that is, KIR(KI
U) < R2H(RSL). Therefore, step 1321 gradually increases the fuel injection amount in lean state a (F1=“0”), and step 822 gradually increases the fuel injection amount in lean state a (F1=“0”), and step 822 gradually increases the fuel injection amount in lean state a (F1=“0”).
"1") gradually reduces the fuel injection amount.

Step 817. 818. The air-fuel ratio correction coefficient FAF calculated in steps 821 and 822 is
4 is guarded at a minimum value, for example 0.8, and at steps 825 and 826 it is guarded at a maximum value, for example 1.2. 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 over-rich or over-lean conditions.

F A F -t-RAM10 calculated as described above
5, and the routine ends at step 827.

Figure 9 shows step 8 of calculating the central value FAFM in Figure 8.
19 detailed routines. That is, step 90
1, it is determined whether the counter CNTl exceeds 7 or not. As a result, if CNTl≦7, in step 902, the cumulative value FAFI of the air-fuel ratio correction coefficient FAF is set to FAFI
-FAFI +FAF, step 903
At step 908, the counter CNTl is counted up by +1. On the other hand, in step 901, CNTl
> 7, in step 904 the central value FAFM of the air-fuel ratio correction coefficient FAF is calculated as follows: FAFM←FAPI/(CNTi 1 ) and stored in the backup RAM 106 in step 905. Note that in this case, CNT1=7.

Then, in step 906, the counter CNTl is cleared, and in step 907, the accumulated value FAFI is cleared, and the process proceeds to step 908.

That is, according to the routine shown in FIG. 9, as shown in FIG. 10, the center value FAFM is calculated from the average value of six FAF values immediately before skipping. Note that the number of FAF values may be another number (however, an even number), and FApM may be obtained by successively integrating FAF instead of the value FAF for each skip.

FIG. 11 shows a detailed routine of the self-excitation control step 707 in FIG. That is, in step 1101, the self-excitation control frequency FREQ is interpolated and calculated using the one-dimensional map stored in the RO old 04 according to the load, for example, Q/Ne. In other words, when the load is large, the automatic control frequency FR
Increase the EQ to increase the purification performance of the catalyst. In addition,
As loads, other operating parameters Q, PM, Ne, T
A etc. may also be used.

In step 1102, the frequency FRIEQ is converted to the period T of the automatic control waveform shown in FIG. 12, and in step 1103
Now, the center (direct F) calculated by double 0□ sensor control
The AFM is moved from the backup RAM 106 to the RAM 105, and its value is set to tRAM. In step 1104, the reference value FAFB of the automatic control waveform shown in FIG.
AFB_tRAM_A/ However, A is the amplitude and is a kind of skip amount.

This skip iA is set to the maximum value without adversely affecting drivability. For example, A=10%.

In step 1105, it is determined whether 1/2 of the period T has elapsed, and steps 1106 to 1108 are performed every T/2 elapsed.
The FAF is inverted from FAFB to FAFB + A or from FAFB + A to FAFB. It is assumed that the counter CNT2 is incremented by a timer routine shown in FIG. 13 for a predetermined period of time, for example, every 4n+s. Next, in step 1109, the counter CNT2
is cleared and the process proceeds to step 1110.

In this way, according to the routine shown in FIG. 11, the automatic control waveform shown in FIG. 12 is generated.

FIG. 14 is a timing diagram supplementary explanation of the operation according to the flowchart of FIG. 8. 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. 14(A), the delay counter CDLY is set to indicate the rich/lean ratio as shown in FIG. It is counted up in the state and counted down in the lean state. As a result, as shown in FIG. 14(C), a delayed air-fuel ratio signal A/F' (corresponding to flag F1) is formed. For example, time 1. Air-fuel ratio signal A/F'
Even if the air-fuel ratio signal A/F' changes from lean to rich, the delayed air-fuel ratio signal A/F' changes to rich at time t2 after being held lean for the rich delay time TDR. Time t
Even if the air-fuel ratio signal A/F changes from rich to lean at It changes in a straight line. However, the air-fuel ratio signal A/F
' is inverted in a short period of the rich delay time TDR as at time tS+t6+t7, the delay counter C
It takes time for DLY to reach the maximum value TDR, and as a result, the air-fuel ratio signal A/F' after delay processing at time (8)
is reversed. In other words, 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, stable air-fuel ratio signal A/F after delay processing
Based on ', the air-fuel ratio correction coefficient FAF shown in FIG. 14(D) is obtained.

Next, the second air-fuel ratio feedback control by the downstream 02 sensor 15 will be explained. The second air-fuel ratio feedback control includes skip amounts RSR and RSL as the first air-fuel ratio feedback control constants, and an integral constant K.
IR, IL, delay time TDR.

There is a system in which TDL or the comparison voltage □ of the output v1 of the upstream O2 sensor 13 is made variable, and a system in which a second air-fuel ratio correction coefficient FAF2 is introduced.

For example, by increasing rich skip IR3R, the control air-fuel ratio can be shifted to the rich side, and even by decreasing lean skip 1iR3L, the control air-fuel ratio can be shifted to the rich side; on the other hand, by increasing lean skip IR3L,
The controlled air-fuel ratio can be shifted to the lean side, and even if the rich skip amount R3R is made small, 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 IR5R and the lean skip amount R3L according to the output of the downstream 02 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
On the other hand, increasing the lean integral constant RIL allows the controlled air-fuel ratio to shift to the lean side, and even if the rich integral constant KIR is decreased, the controlled air-fuel ratio 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 Ot sensor 15.Rich delay time TDR>lean delay time (-TDL ), the control air-fuel ratio can be shifted to the rich side, and conversely, if the lean delay time (-TDL)>
By setting the rich delay time (TDR), the controlled air-fuel ratio can be shifted to the lean side. In other words, the downstream side 0□ sensor 15
The air-fuel ratio can be controlled by correcting the delay times TDR and TDL according to the output. Furthermore, the comparison voltage Vl
By increasing ll, the control air-fuel ratio can be shifted to the rich side,
Further, by decreasing the comparison voltage V□, the control air-fuel ratio can be shifted to the lean side. Therefore, by correcting the comparison voltage V□ according to the output of the downstream 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.

A double 02 sensor system in which the skip amount as an air-fuel ratio feedback control constant is made variable will be described with reference to FIGS. 15 and 16.

FIG. 15 shows a second air-fuel ratio feedback control routine that calculates skip lR2H and RSL based on the output v2 of the downstream 02 sensor 15, and is executed every predetermined period of time, for example, every 512 m5. Steps 1501-15
In step 05, it is determined whether the downstream side 01 sensor 15 is in a closed loop condition. For example, in addition to the failure of the closed loop condition determined by the upstream 0□ sensor 13 (step 1501), when the cooling water temperature THW is below a predetermined value (for example, 70° C.) (step 1502), the throttle valve 16 is fully opened (
When LL="1" (step 1503), the downstream side is 0! When the output v2 of the sensor 15 has never crossed the reference voltage (that is, when the downstream Oz sensor 15 is not activated) (step 1504), when the load is light (Q/Ne <X+) (step 1505) , etc., the closed-loop condition is not satisfied, and in other cases, the closed-loop condition is satisfied. If the condition is not a closed loop condition, the process directly advances to step 1516.

If the loop is closed, the process proceeds to step 1506 and the downstream 0
Output V of 2 sensors 15! Then, in step 1507, it is determined whether v2 is less than the comparison voltage (2), for example 0.55V, that is, it is determined whether the air-fuel ratio is rich or lean. Note that the comparison voltage Vl11 is set to 0 on the upstream side and the downstream side of the catalytic converter 12, taking into account that the output characteristics are different due to the influence of raw gas and the deterioration rate is different between the upstream and downstream sides of the catalytic converter 12. Comparison voltage VRI of the output of sensor 13
Although it is set higher, this setting may be arbitrary.

In step 1507, if ■2≦■8□ (lean), it is inferior to steps 1508 and 1509, and on the other hand, vz>
If v*z (rich), steps 1510, 151
Go to 1.

In step 1508, RSR-R3R+ΔRS,
In other words, the rich skip 1R3R is increased to shift the air-fuel ratio to the rich side, and at step 15o9
? sL←I? Sl, -ΔR5, that is, the lean skip flR3L is decreased to further shift the air-fuel ratio to the rich side. On the other hand, in step 1510 v2>Vat
(Rich), in step 1510 RSR4
−RSR−ΔIts, that is, rich skip amount R
SR is decreased to shift the air-fuel ratio to the lean side, and at step 1511, RSL -RSL+ΔR5 is set, that is, the lean skip 1JR3L is increased to further shift the air-fuel ratio to the lean side.

, in step 1512, the skip IRsR is 3% to 7.
Determine whether it is within the % range. As a result, 3%〈RSR〉
If it is 7%, clear the self-excitation control execution flag XJIREI in step 1513, and if R3R≧7% or R3R
If it is 53%, the automatic control execution flag XJIREI is set in step 1514. In this way, skip
When lR3R is too large or too small, the asymmetry of the air-fuel ratio correction coefficient FAF obtained using this skip amount increases, and as a result, the frequency of FAF decreases. Execution flag XJIRE
Set I. That is, the FAF frequency obtained in the routine of FIG. 8 is detected by skip@R8R.

Step 15I5 is the RSR calculated as described above.

This is to perform RSL guard processing, and guard at a maximum value MAX=9% and a minimum value MIN=1%, for example. Note that the minimum value MEN 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.

The routine of FIG. 15 then ends at step 1516.

In addition, the skip @ RS obtained by the routine in Figure 15
R and RSL are not reflected in the FAF calculation when the routine in Figure 8 (double 08 sensor control) is not executed, but the routine in Figure 15 is similar to the routine in Figure 8 and the routine in Figure 11 (automatic control). In either execution, the
Therefore, under the conditions of steps 1501-1505,
RSR and RSL are constantly updated. As a result, double 0. Switching from sensor control to self-excitation control and vice versa is also possible.

FIG. 16 shows an injection amount calculation routine, which is executed at every predetermined crank angle, for example, every 360° CA.

In step 1601, the intake air amount data Q and rotational speed data Ne are read from the RAM 105, and the basic injection amount T is read out.
Calculate AUP. For example, TAIJP"-cx ・Q
/ Ne (α is a constant), R in step 1602
Read the cooling water temperature data THW from AM105 and store it in ROM.
The warm-up increase value FW is determined by the one-dimensional map stored in 104.
Calculate L by interpolation. In step 1603, the final injection amount TAU is set as TAU −TAUP −FAF・(FWF+
Calculate by β>+r. Note that β and γ are correction amounts determined by other operating state parameters. Next, in step l604, the injection ff1TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection. And step 16
This routine ends at 05.

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

As described above, switching between double 0□ sensor control and self-excitation control is performed according to the skip IR3RO value, as shown in FIG. 17.

Although the air-fuel ratio feedback control constant was used as the frequency detection means in the above embodiment, the frequency may be determined by detecting the length of the rich or lean time indicated by the upstream 02 sensor, or the frequency may be determined by detecting the length of the rich or lean time indicated by the upstream 02 sensor. It may be directly detected by the number of inversions of the first air-fuel ratio flag (step 815, etc.), or the FAFM determined in step 816 and FIG.
The amount of deviation from 1.0 may be used.

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 every 512 m5 because the air-fuel ratio feedback control is mainly performed by the upstream O2 sensor, which has good responsiveness, and is secondary to the control by the downstream O2 sensor, which has poor responsiveness. This is for the purpose of doing so.

In addition, a double 0□ sensor system that corrects other control constants in air-fuel ratio feedback control by the upstream 02 sensor, such as delay time, integral constant, comparison voltage, etc., by the output of the downstream 02 sensor also has a second The present invention can also be applied to a double 02 sensor system that introduces an 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 also possible to fix one of skip fi R5R, R5L and make only the other variable.
It is also possible to fix one of i 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 capretor 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, and an electric bleed air control valve adjusts the air bleed of the carburetor to control the main system passage and The present invention can be applied to devices that control the air-fuel ratio by introducing atmospheric air into the slow system passage, devices 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 amount TAUP in step 1601 is determined by the carburetor itself, that is, it is determined according to the intake pipe negative pressure depending on the intake air amount and the rotational speed of the engine, and in step 1603 The amount of supplied air corresponding to the final fuel injection ITAU is calculated.

Furthermore, in the above embodiment, the 02 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, the optimum air-fuel ratio central value (FAFM) is used to obtain the optimum zero! Since a storage effect can be obtained, deterioration of emissions can be prevented. That is, in the double 02 sensor system □, air-fuel ratio feedback control based on the output of the upstream air-fuel ratio sensor is possible, the transient response is good, and the optimum control center can be found. The center value of the FAF waveform is shifted toward the rich side or the lean side by the output of the downstream air-fuel ratio sensor, and as a result, the air-fuel ratio control frequency decreases, the O2 storage effect decreases, and emissions deteriorate. The present invention can eliminate such control limits. In addition, even if the output characteristics of the downstream air-fuel ratio sensor from lean to rich or rich to lean are slow due to the O2 storage effect, etc., over-correction of rich or lean over-correction of control constants etc. can be prevented, reducing exhaust emissions. This is useful for reducing fuel consumption, improving fuel efficiency, and preventing deterioration of drivability.

[Brief explanation of the drawing]

Figure 1 is an overall block diagram for explaining the present invention in detail, Figure 2 is a single 0□ sensor system and a double 02 sensor system.
Figures 3 and 4 are exhaust emission characteristic diagrams that explain the sensor system; Figures 3 and 4 are timing diagrams that explain the point of view that the present invention aims to solve; Figure 5 is a graph that explains the purification performance of the three-way catalyst; FIG. 6 is an overall schematic diagram showing an embodiment of the air-fuel ratio control device for an internal combustion engine according to the present invention; FIGS. 7, 8, 9, 11, 13, and 15.
Figure 16 is a flowchart to explain the operation of the control circuit in Figure 6, Figure 10 is a timing diagram to supplement the flowchart in Figure 9, and Figure 12 is a supplementary flowchart in Figure 11. FIG. 14 is a timing diagram to supplement the flowchart of FIG. 8. FIG. 17 is a timing diagram to explain the present invention in detail. 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 side Oz sensor, 15...Downstream 02 sensor, 17...Idle switch.

Claims (1)

[Claims] 1. A three-way catalyst (12) provided in the exhaust passage of an internal combustion engine and having an O_2 storage effect; and a three-way catalyst (12) provided in the exhaust passage upstream of the three-way catalyst, controlling the air-fuel ratio of the engine. an upstream air-fuel ratio sensor (13) that detects; a downstream air-fuel ratio sensor (15) that is provided in the exhaust passage downstream of the three-way catalyst and detects the air-fuel ratio of the engine; and the downstream air-fuel ratio sensor control constant calculation means for calculating an air-fuel ratio feedback control constant according to the output of the air-fuel ratio feedback control constant; and a first air-fuel ratio correction amount for calculating a first air-fuel ratio correction amount according to the air-fuel ratio feedback control constant and the output of the upstream air-fuel ratio sensor. an air-fuel ratio correction amount calculation means; a center value calculation means for calculating a center value of the first air-fuel ratio correction amount; and a rectangle having a predetermined period and amplitude centered on the center value of the first air-fuel ratio correction amount. a second air-fuel ratio correction amount calculation means for calculating a wave-like second air-fuel ratio correction amount; a frequency detection means for directly or indirectly detecting the frequency of the first air-fuel ratio correction amount; When the frequency of the air-fuel ratio correction amount is equal to or higher than a predetermined value, the air-fuel ratio of the engine is adjusted according to the first air-fuel ratio correction amount, and when the frequency of the first air-fuel ratio correction amount is less than the predetermined value, the air-fuel ratio of the engine is adjusted according to the first air-fuel ratio correction amount. 2. 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 air-fuel ratio correction amount of No. 2. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the frequency detection means detects the frequency of the first air-fuel ratio correction amount based on the value of the air-fuel ratio feedback control constant. 3. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the period of the second air-fuel ratio correction amount changes depending on the load of the engine.