JPS62223433A - Air-fuel ratio controller for internal combustion engine - Google Patents

Abstract

PURPOSE:To prevent drivability and exhaust characteristics from deteriorating, by stopping the operation of a control constant by an oxygen sensor at the downstrem side when this oxygen sensor at the downstream side is deteriorated and its output amplitude comes smaller, in a device provided with each oxygen sensor at both upstream and downstream sides of a catalyzer. CONSTITUTION:A first oxygen sensor 13 is installed at the upstream of a catalytic converter 12 and a second oxygen sensor 15 at the downstream, respectively, and inputted into a control circuit 10 together with an air flow meter 3 and crank angle sensors 5 and 6. The control circuit 10 secures an air-fuel ratio signal for rich-lean discrimination on the basis of detection value of the oxygen sensor at the upstream side at time of the specified driving state, and sets a control constant including a skip quantity, an integration constant or the like on the basis of full force of the oxygen sensor 15 at the downstream side, setting it down to an air-fuel ratio feedback control signal. The control circuit 10 stops the operation of the control constant on the basis of the oxygen sensor at the downstream side when amplitude of the control constant based on the oxygen sensor 15 at the downstream side is less than the specified value.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides air-fuel ratio sensors (herein, oxygen concentration sensors (02 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]

In simple air-fuel ratio feedback control (single 02 sensor system), the 02 sensor that detects oxygen concentration is installed at a location in the exhaust system as close to the combustion chamber as possible, that is, at the gathering part of the exhaust manifold upstream of the catalytic converter. Improving the accuracy of air-fuel ratio control is hindered by variations in sensor output characteristics. It takes 02
In order to compensate for variations in sensor output characteristics, variations in parts such as fuel injection valves, and changes over time or aging, a second 02 sensor is provided downstream of the catalytic converter, and air-fuel ratio feedback control by the upstream 02 sensor is performed. In addition, a double 02 sensor system that performs air-fuel ratio feedback control using a downstream 02 sensor has already been proposed (
Reference: JP-A-55-37562, JP-A-55-3
Publication No. 7647, etc.). In this double 02 sensor system, although the 02 sensor installed downstream of the catalytic converter has a lower response speed than the upstream O2 sensor, it has the advantage of less variation in output characteristics for the following reasons. are doing.

fl) Downstream of the catalytic converter, the temperature of the exhaust gas 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 02 sensor. In fact, as shown in Figure 2, in a single 02 sensor system, 02
If the output characteristics of the sensor deteriorate, it will directly affect the exhaust emission characteristics, but with the double 02 sensor system, even if the output characteristics of the upstream 02 sensor deteriorate,
Exhaust emission characteristics are not deteriorated. That is, double 0
In a two-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, when the downstream 02 sensor described above deteriorates, for example due to thermal deterioration, its output shifts to the lean side (when using the outflow type processing circuit described below) or to the rich side (when using the inflow type processing circuit described below). Lean. As a result, downstream side 0
Even if air-fuel ratio feedback control is performed using two sensors, the engine air-fuel ratio cannot be tracked sufficiently, and therefore lean over-correction or lean over-correction is performed, leading to deterioration of drivability, deterioration of emissions, etc. There is.

[Means for solving problems]

An object of the present invention is to provide a double 02 sensor system that prevents deterioration of drivability and emissions even when the downstream 02 sensor deteriorates, and the configuration thereof is shown in FIG.

In FIG. 1, 1. The second air-fuel ratio sensor means detects the specific component concentration in the exhaust gas on the upstream side and the downstream side of the catalytic converter for exhaust gas purification provided in the exhaust system of the engine, and the control constant calculation means detects the concentration of a specific component in the exhaust gas on the downstream side. The air-fuel ratio feedback control constant is calculated according to the output of the (second) air-fuel ratio sensor means, and the air-fuel ratio adjustment means is operated according to the output of the upstream (first) air-fuel ratio sensor means and the air-fuel ratio feedbank control constant. Adjust the engine's air-fuel ratio. At this time, the amplitude determining means determines whether the amplitude of the air-fuel ratio feedback control constant is within a predetermined value, and as a result, when the amplitude of the air-fuel ratio feedback control constant is within the predetermined value, the stopping means operates the air-fuel ratio feedbank. This stops adjustment of the engine's air-fuel ratio using control constants.

[For production]

According to the above configuration, deterioration of the downstream 02 sensor is detected by the magnitude of the amplitude of the air-fuel ratio feedback control constant that is feedback-controlled by the downstream 02 sensor, and when deterioration of the downstream 02 sensor is detected, , the air-fuel ratio feedback control by the downstream 02 sensor is stopped.

〔Example〕

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 3 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. 3, an air flow meter 3 is provided in the intake passage 2 of the engine body 1. As shown in FIG. 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 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 1.
03 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 TI
Generates an analog voltage electrical signal according to IW. This output is also supplied to the A/D converter 101.

The exhaust system downstream of the exhaust manifold 11 is provided with a catalytic converter 12 that accommodates a three-way catalyst that simultaneously purifies three harmful components 11c, Co, and NOx in the exhaust gas.

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

The 02 sensors 13 and 15 generate electrical signals according to the concentration of oxygen components in the exhaust gas. That is, 02 sensor 13
, 15 output different output voltages to the A/D converter 101 via signal processing circuits 111 and 112 of the control circuit 10 depending on whether the air-fuel ratio is lean or rich with respect to the stoichiometric air-fuel ratio.
occurs in

The signal processing circuit 111 (112") has a wide type (separately, an outflow type and an inflow type. The outflow type has a grounded resistor R1 and a buffer OP, as shown in FIG. 4A). t-(Iiff), therefore, if the 02 sensor 13 (15) is inactive, its output voltage will disappear, and as a result, the sink current flowing through resistor R1 will cause signal processing regardless of whether it is active or inactive. circuit 1
The input of 11 (112) becomes low level, therefore,
The output V1 (V2) becomes low level. In other words, 0
As shown in Figure 5A, the temperature characteristics of the 02 sensor are as follows: When the air-fuel ratio /F is rich, the output (rich signal) of the 02 sensor rises as the element temperature rises and remains stable at a high level. On the other hand, when the air-fuel ratio A/F is lean, it stabilizes at a low level as the element temperature rises.

Therefore, if the 02 sensor deteriorates, the output of the signal processing circuit 111 (112) will generate a lean signal, and as a result, if air-fuel ratio feedback control is performed by the downstream 02 sensor 15, lean overcorrection will occur. It will be done.

On the other hand, the flow-in type is equipped with a resistor R2 and a buffer OP connected to the power supply Vcc, as shown in FIG. The voltage disappears, and as a result, the source current flowing from the power supply Vcc to the resistor R2 activates the
Regardless of the inactive state, the signal processing circuit 111 (112
) input becomes high level, therefore, the output V1 (
V2) becomes high level. In other words, the temperature characteristics of the 02 sensor are as shown in Figure 5B. When the air-fuel ratio A/F is lean, the output of the 02 sensor (lean signal) decreases to a low level as the element temperature rises. stabilized at
On the other hand, when the air-fuel ratio A/F is rich, it stabilizes at a high level as the element temperature rises.

Therefore, when the 02 sensor deteriorates, the output of the signal processing circuit 111 (112>) generates a rich signal, and as a result, if air-fuel ratio feedback control is performed by the downstream 02 sensor 15, rich overcorrection occurs. It will be done.

In addition, in the following explanation, the signal processing circuit 111 (112)
Use the leaked format as .

The control circuit 10 is configured as a microcomputer, for example, and includes an A/D converter 101, an input/output interface 102, a CP port 103, and signal processing circuits 111, 11.
In addition to 2, a 120M 104, a RAM 105, a backup RAM 106, a clock generation circuit 107, etc. are provided.

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 T8υ
is calculated, fuel injection 1 TAU becomes 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, when the down counter 108 counts the clock signal (not shown) and finally its carrier terminal reaches the "1" level, the flip-flop 109 is turned on and the drive circuit 110 controls the fuel injection valve 7. Stop biasing. In other words, the above fuel injection ITA
The fuel injection valve 7 is energized by U, and therefore the fuel injection amount T
An amount of fuel corresponding to the AU is sent to the combustion chamber of the engine body 1.

Note that the interrupt generation of the CPU 103 is caused by the 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.

The intake air amount data Q and cooling water temperature data THW 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. In other words, I? Data Q and Tl+- in AM 105 are updated at predetermined intervals. Moreover, the rotational speed data Ne is 30°C of the crank angle sensor 6.
It is calculated by the interrupt for each A and stored in a predetermined area of the 12AM 105.

The operation of the control circuit shown in FIG. 3 will be explained below.

FIG. 6 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 ms.

In step 601, 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 fuel is being increased after starting, when the fuel is being warmed up, when the power is increasing, when the output signal of the upstream side 02 sensor 13 has never been inverted, when the fuel is being cut. The closed-loop condition is not satisfied in all cases of 2nd grade, and the closed-loop condition is satisfied in the other cases. When the closed loop condition is not satisfied, the air-fuel ratio feedback correction amount FAF is set to 1.0 in step 628, that is, open loop control is performed. In this case,
The FAF may be a learned value, an average value during a closed loop, or a value immediately before the end of the closed loop. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 602.

In step 602, the output v1 of the upstream 02 sensor 13 is
is A/D converted and taken in, and in step 603 it is determined whether (1) is less than the comparison voltage VRI, for example 0.45V, that is, it is determined whether the air-fuel ratio is rich or lean. If it is lean (■1≦VRI), the first
Determine whether the delay counter CDLY 1 is positive or not,
If CDLY1>O, the first delay counter CDLY1 is set to O in step 605. Step 60
In step 6, the first delay counter CDLY 1 is decremented by 1, and in step 607 it is determined whether CDLYI<TDL 1. In addition, TDL 1 is upstream side 02 sensor 1
It is a lean delay time for maintaining the determination that the state is rich even if there is a change from rich to lean in the output of No. 3, and is defined as a negative value. Therefore, only when CDLY<TDL 1 in step 607, CDLY-TDL is set to 1 in step 608, and step 60
At step 9, the air-fuel ratio flag F1 is set to "0" (lean state). On the other hand, if it is rich (Vl>Vat) in step 603, the first delay counter C is determined in step 610.
It is determined whether DLY 1 is negative or not, and if CDLY 1>O, the first delay counter CDL is set in step 611.
Let Y1 be O. In step 612, the first delay counter CDLY 1 is increased by 1, and in step 613
It is determined whether CDLY 1<TDR1. In addition,
TDR1 is a rich delay time for maintaining the determination that the engine is in a lean state even if the output of the upstream 02 sensor 13 changes from lean to rich, and is defined as a positive value.

Therefore, only when CDLY<TDR1 in step 613, CDLY-TDR1 is set in step 614, and the air-fuel ratio flag F1 is set to "1" (rich state) in step 615.

In step 616, it is determined whether the sign of the 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 reversed, the process advances to step 617, where it is determined whether the air-fuel ratio is reversed from rich to lean (F
l = “0”), or a reversal from lean to rich (F1 =
“1”).

If the reversal is from rich to lean, step 618 skip-increases FAF-FAF+RSR; conversely, if the reversal is from lean to rich, step 61
9, it is decreased in a skip manner as FAF-FAF-RSR. In other words, skip processing is performed.

If the sign of the air-fuel ratio flag F1 is not inverted in step 616, steps 620, 621. Integration processing is performed at 622. That is, in step 620, Fl =
It is determined whether or not it is "O", and if Fl = "0" (lean), FAF-FAF + KL is set in step 621, and on the other hand, if Fl = "1" (rich), FAF is set in step 622. -FAF- is set to L. Here, the integral constant Kl is set sufficiently small compared to the skip constants R5R and RSL, that is, K I <RSR(RS
L). Therefore, step 621 is in the lean state (
At step 622, the fuel injection amount is gradually increased at Fl-“0”), and at step 622, the fuel injection amount is gradually decreased at Lynch-like a (Fl=“1”).

Steps 618, 619, 621. The air-fuel ratio correction coefficient FAF calculated in step 622 is applied to steps 623 and 624.
It is guarded at a minimum value, for example, 0.8, and at steps 625 and 626, 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.

The FAF calculated as described above is stored in the RAM 105, and the routine ends at step 628. FIG. 7 is a timing diagram supplementary explanation of the operation according to the flowchart of FIG. 6. When the air-fuel ratio signal A/F for rich/lean discrimination is obtained from the output of the upstream 02 sensor 13 as shown in FIG. 7(A), the first delay counter CDLY 1 is activated as shown in FIG. 7(B). As shown, the counter returns to O at the time of change from rich to empty or vice versa, counts up in the rich state, and counts down in the lean state. As a result, as shown in Figure 7(C),
A delayed air-fuel ratio signal A/F" is formed.

For example, even if the air-fuel ratio signal A/F changes from -7 to rich at time L1, the delayed air-fuel ratio signal A/F
' is maintained lean for a rich delay time (TDR1) and then changes to rich at time t2. 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 (TD
After being held rich by an amount equivalent to R1'), it changes to lean at time t4. However, the air-fuel ratio signal A/F at time t
5.

When reversed in a period shorter than the rich or lean delay time like t6 and t7, the first delayer '7:
/ Evening CDLY1 is maximum value TDR1 or minimum value TDL
It takes time to reach L, and as a result, the air-fuel ratio signal A/I''' after the delay process is inverted at time t8.

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 way, the air-fuel ratio correction coefficient FAF shown in FIG. 7(D) is obtained based on the stable air-fuel ratio signal A/l'' after the delay processing.

Next, the second air-fuel ratio feedback control by the downstream 02 sensor 15 will be explained. As the second air-fuel ratio feedback control, the skip amount R3R, I? as the first air-fuel ratio feedback control constant is used. SL, delay time TDR1, TDL1, integral constant + (in this case, rich integral constant KIIR and lean integral constant KILL are set separately), or make the comparison voltage VRI of the output ■1 of the upstream 02 sensor 13 variable. system and the second
There is a system that uses the air-fuel ratio correction coefficient FAF 2 for four people.

For example, Rich Skip Il? By increasing sR, the control air-fuel ratio can be shifted to the rich side, and lean skip iI? Even if sL is decreased, the control air-fuel ratio can be shifted to the rich side.On the other hand, if lean skip IRsL is increased, the control air-fuel ratio can be shifted to the lean side, and even if rich skip 1iR5R is decreased, the control air-fuel ratio can be shifted to the lean side. You can move to Therefore, depending on the output of the downstream side 02 sensor 15, the rich skip amount 1? SR and Lean Skip 1Rs
By correcting L, the air-fuel ratio can be controlled. Also, rich delay time (TDI? 1) > lean delay time (
If the control air-fuel ratio is set to TDL 1 ), the control air-fuel ratio can shift to the rich side, and conversely, the lean delay time (TDL 1 ) >
By setting the IJ Sochi delay time (TDR1), the controlled air-fuel ratio can be shifted to the lean side. That is, the air-fuel ratio can be controlled by correcting the delay times TDR1 and TDL1 according to the output of the downstream side 02 sensor 15. Furthermore,
Increasing the rich integral constant KIIR can shift the controlled air-fuel ratio to the rich side, and decreasing the lean integral constant KILL can also shift the controlled air-fuel ratio to the rich side. On the other hand, increasing the lean integral constant KIIL can shift the controlled air-fuel ratio to the rich side. can be shifted to the lean side, and the control air-fuel ratio can also be shifted to the lean side even if the rich integral constant KTIR is made small. Therefore,
The Ricci integral constant K is determined according to the output of the downstream 02 sensor 15.
The air-fuel ratio can be controlled by correcting IIR and lean integral constant KILL.

Furthermore, when the comparison voltage VRI is increased, the controlled air-fuel ratio can be shifted to the rich side, and when the comparison voltage VRI is decreased, the controlled air-fuel ratio can be shifted to the lean side. Therefore, by correcting the comparison voltage VRI according to the output of the downstream 02 sensor 15, the air-fuel ratio can be controlled.

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. 8, 9, and 10.

FIG. 8 shows a routine for determining deterioration of the downstream side 02 sensor 15, which is executed every predetermined period of time, for example, every 1 second. In step 801, it is determined whether a closed loop (feedback) condition for the air-fuel ratio by the downstream side 02 sensor 15 is satisfied. For example, when the cooling water temperature is below a predetermined value,
The closed-loop condition is not satisfied during engine startup, during fuel increase after startup, during warm-up, during power increase, when the output signal of the downstream O2 sensor 15 has never reversed, and during fuel cut. In other cases, the closed loop condition is satisfied.

If the closed loop condition is not met, the process proceeds to step 819, where the air-fuel ratio feed bank execution flag FB2 indicating execution of air-fuel ratio feedback by the downstream side 02 sensor 15 is cleared, and the sample period counter C is cleared at step 820. Furthermore, in step 821, the maximum value of rich skip 1R5R is 1? SR1 and minimum value RSR
RSR with an initial value of 2.

(for example, 5%), and in step 822, the maximum value R5L 1 and the minimum value R3L of the lean skip amount R5L are determined.
The initial value is R5Lo (for example, 5%), and the routine ends at step 824. In this way, if the closed loop condition is not satisfied, air-fuel ratio feedbank control by the downstream 02 sensor 15, which will be described later, is stopped.

If the closed loop condition is satisfied, the process advances to step 802. In step 802, the current amount R5
Compare R with its maximum value R5R1, ll5R>RSR1
At step 803, Near I? SR1-RSR
The maximum value RSR1 is updated as follows. Also, step 80
4, the current Rich Skip i+? sR is compared with its minimum value RSR2, and when RSR<ll5R2, the minimum value 1 is set as near RSR2-RSR in step 805.
? Update SR2. Similarly, in step 806, the current lean skip amount RsL is set to its maximum value R3L 1
When compared with RSL > RSL 1 (7), step 807 shows the maximum value 1 as RSL 1-RSL?
Update SL 1. Also, in step 808, the current lean ski knob amount RSL is compared with its minimum value R5L2, and I? When SL < RSL 2, the minimum value 11SL is set as RSL 2←R3L in step 809.
Update 2.

In step 810, a sample period counter C is counted up, and in step 811, the counter C is set to a predetermined value c.
It is determined whether h (for example, 10) has been reached.

If the sample period has not been reached (C<Ch)
In step 823, the air-fuel ratio feedback execution flag FB2 is set to "1" and the process proceeds to step 824. In other words,
The execution of the air-fuel ratio feedback control by the downstream side 02 sensor 15 is continued.

On the other hand, when the sample period is reached (C<Ch),
Proceed to step 812.

In step 812, the amplitude A of the rich skip amount RSR is
R is calculated by A R-R5R1-R3R2, and in step 813, the amplitude AL of the lean skip amount R5L is calculated by A L-RSL1-RSL2. Next, in step 814, rich skip ll? It is determined whether the amplitude AP of sR is greater than the allowable value ARh, and in step 815 it is determined whether the amplitude AL of the lean skip 3iR5L is greater than the allowable value ALh. As a result, AR>ARh and AL>
When ALh, it is assumed that the downstream side 02 sensor 15 has not deteriorated, and the process proceeds to step 816, where the air-fuel ratio feedback control execution flag FB2 is set. On the other hand, A
I? When ≦＾Rh or AL≦ALh, the downstream side is 0.
2 sensor 15 is considered to be deteriorated, and the process proceeds to step 817.
, 818, the duration A is measured. As a result,
The state of AR≦ARh or AL≦ALh remains for a predetermined time (A
h) When it exceeds (A>Ah), the air-fuel ratio feedback control execution flag FB2 is cleared in step 816, and the air-fuel ratio feedback control by the downstream side 02 sensor 15 is stopped.

In this way, when the downstream 02 sensor 15 deteriorates, its output will retain a lean signal (in the case of a flow-in type signal processing circuit) or a rich signal (in the case of a flow-in type signal processing circuit) for a long time. Therefore, the air-fuel ratio feedback control constant (in this case, rich skip amount RSR, lean skip full?sL'') is overcorrected by air-fuel ratio feedbank control by the downstream side 02 sensor 15, which will be described later, and its amplitude becomes small. In addition, when the amplitudes of the air-fuel ratio feedback control constants R3R and RSL become small, the air-fuel ratio feedbank control by the downstream side 02 sensor 15 is stopped (FB2="0").

FIG. 9 shows a second air-fuel ratio feedback control routine that calculates rich skip IR3R and lean skip IR5L based on the output of the downstream 02 sensor 15,
It is executed every predetermined time, for example, 1 second. Step 901
Then, it is determined whether the air-fuel ratio feedback control execution flag FB2 calculated in the routine of FIG. 8 is "1". If FB2="0", steps 929, 930
Then, the rich skip 1iRsIl and the lean skip amount RsL are set to constant values. For example, R3R"R3Ro (5%) R5L+R3Lo (5%). In this case, RSI?, RSL can also be a learned value, an average value during a closed loop, or a value immediately before the end of a closed loop. On the other hand, When FB2="1", the process advances to step 902.

In step 902, the output ■2 of the downstream sensor 15
is A/D converted and taken in, and the step knob 903 determines whether or not (2) is equal to or lower than the comparison voltage VR, for example, 0.55V. In other words, it determines whether the air-fuel ratio is lynch or lean. Note that the comparison voltage VR2 is higher than the comparison voltages VP and I of the output of the upstream side 02 sensor 13, taking into consideration that the output characteristics differ due to the influence of raw gas and the deterioration rate differs between the upstream and downstream of the catalytic converter 12. Set.

Steps 904-915 are steps 604-6 in FIG.
Similar to No. 15, this is for delay processing the air-fuel ratio determination result. That is, the air-fuel ratio flag 1?2 is set based on the lean delay time TDR2 and the lean delay time TDL2.

In step 916, the air-fuel ratio after the delay process is determined based on the air-fuel ratio flag F2. As a result, if F2 = “0” (lean), the process proceeds to steps 917 to 922;
If F2 = “1” (rich), step 923~
Proceed to 928.

In step 917, RSR−R3R+ΔI? S (constant value, for example 0.08%), that is, the rich skip amount R5I? is increased to shift the air-fuel ratio to the rich side.

In steps 918 and 919, the RSR is guarded at a maximum value MAX, for example, 6.2%. Furthermore, step 92
I at 0? 5R-RSL-ΔI? S, that is, the rich skip IR5R is decreased to shift the air-fuel ratio to the rich side. In steps 921 and 922, the RSL is guarded at a minimum value MIN, for example 2.5%.

On the other hand, when F2 = "l" (rich), in step 923, l? 5R-1? SI? -Δr+s, that is, the rich skip -7JR5R is decreased to shift the air-fuel ratio to the lean side. In steps 924 and 925, ll5R is guarded at the minimum value MIN. moreover,
In step 926, RSL-RSL+ΔRS (constant value) is set, that is, lean skip 1R5L is increased to shift the air-fuel ratio to the lower side. Step 927, 9
In step 28, RSL is guarded at the maximum value MAX.

RSR and RSL calculated as above are l? AM 1
05, the routine ends at step 931.

In addition, FAF calculated during air-fuel ratio feedback
, RSR, RSL are temporarily changed to other values FAF', R5R'
, RSL” and backup RAM 106
This also helps improve drivability during restarts, etc. The minimum value MIN in FIG. 9 is a value at which transient followability is not impaired, and the maximum value MAX is a value at a level at which drivability does not deteriorate due to air-fuel ratio fluctuations.

As described above, according to the routine shown in FIG. 9, if the output of the downstream side 02 sensor 15 is lean, the rich skip fi
RsR is gradually increased and lean skip amount R5L
is gradually decreased, thereby shifting the air-fuel ratio to the rich side. Further, if the output of the downstream side 02 sensor 15 is rich, the rich skip 1RsR is gradually reduced,
In addition, the lean skip 11R5L is gradually increased, thereby shifting the air-fuel ratio to the lean side.

FIG. 10 shows an injection amount calculation routine, which is executed at every predetermined crank angle, for example, every 360° CA.

In step 1001, the intake air amount data Q and rotational speed data Ne are read from the RAM 105, and the basic injection I
Calculate TAUP'. For example, TAUP ←K Q/
Ne (K is a constant). RA at step 1002
Continuously output cooling water temperature data T11- from M105 and RO
The warm-up increase value FWL is calculated by interpolation using the one-dimensional map stored in M104. In step 1003, the final injection ITAU is calculated as TAU=TA[IP-FAF・(1+FWL
+α) + β. Note that α and β are correction amounts determined by other operating state parameters. Next, in step 1004, 1 TAU of injection is set in the down counter 108 and the flip-flop 10 is
Set 9 to start fuel injection. This routine then ends in step 1005.

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

FIG. 11 is a timing diagram of skip i RSR and RSL obtained by the flowcharts of FIGS. 8 and 9, and shows a case where the downstream side 02 sensor 15 is normal. 1st
As shown in FIG. 1(A), when the output voltage V2 of the downstream side 02 sensor 15 changes, as shown in FIG. 11(B),
Rich skip 1 if lean state (v2≦VR2)
R5l'il increases, but lean skip IJRsL
decreases. On the other hand, as shown in FIG. 11(C), if the engine is in a rich state, the rich skip amount RSR decreases and the lean skip 1R3L increases. Kono Toki, RSR, R
SL changes from MAX to MIN, but its amplitude is relatively large.

Further, FIG. 12 is also a timing diagram of the skip 131?5Ill and RSL obtained by the flowcharts of FIGS. 8 and 9, and shows a case where the downstream side 02 sensor 15 is in a deteriorated state. Here, a case is shown in which a spill type (see FIG. 4A) is used as the signal processing circuit of the 02 sensor. At this time, the output ■2 of the downstream side 02 sensor 15 is
As shown in FIG. 12(A), the lean signal is held for a long time, so as shown in FIGS. 12(B) and (C), the rich skip amount is 1? sR and lean-skip IRsL are overcorrected and therefore their amplitudes are small. As a result, the air-fuel ratio feedback execution flag FB2 changes from "1" to "O", and the rich skip amount RSR and lean skip amount R5L are kept at constant values RSRo and RSL.

Fixed.

Further, FIG. 13 is also a timing diagram of skip il R2H and RSL obtained by the flowcharts of FIGS. 8 and 9, and shows a case where the downstream side 02 sensor 15 is in a deteriorated state. Here, a case is shown in which a flow-in type (see FIG. 4B) is used as the signal processing circuit of the 02 sensor. At this time, the output 2 of the downstream 02 sensor 15 retains the rich signal for a long time, as shown in FIG. 13(A), so
As shown in FIGS. 13(B) and (C), the rich skip amount R5R and lean skip 1R5L are over-corrected,
Therefore, their amplitudes become smaller. As a result, the air-fuel ratio feedback execution flag FB2 changes from "1" to "O", and the rich skip amount R5R and lean skip 3]RSL are also fixed at constant values R5Ro and RSLo.

Note that the first air-fuel ratio feedback control is performed every 4 ms and the second air-fuel ratio feedback control is performed every 1 s because the air-fuel ratio feedback control is mainly controlled by the upstream 02 sensor with good response. This is because the control by the downstream 02 sensor, which has poor response, is performed as a secondary control.

In addition, other control constants in the air-fuel ratio feedback control by the upstream 02 sensor, such as delay time and integral constant 2, are corrected by the output of the downstream 02 sensor.
The present invention can also be applied to a two-sensor system.

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, 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 amount of air bleed from the carburetor to control the air-fuel ratio. The present invention can also be applied to systems that control the air-fuel ratio by introducing atmospheric air into the slow system passage, systems that adjust the amount of secondary air sent to the exhaust system of an engine, and the like. In this case, the basic injection amount TAUP in step 1001
A corresponding basic fuel injection amount is determined by the carburetor itself, that is, depending on the intake pipe negative pressure depending on the intake air amount and the engine rotational speed, and in step 1003, the supply air amount corresponding to the final fuel injection ITAU is determined. is calculated.

Furthermore, in the above embodiment, the 02 sensor was used as the air-fuel ratio sensor, but a CO sensor, 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, when the downstream side 02 sensor deteriorates and its output amplitude becomes small, the air-fuel ratio feedback control by the downstream side 02 sensor is stopped. This can prevent deterioration of emissions, etc.

[Brief explanation of drawings]

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 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. 4A and 4B are the signal processing circuit shown in FIG. 3. Figures 5A and 5B are graphs showing the output characteristics of the signal processing circuit of the 02 sensor in Figures 4A and 4B, Figures 6, 8, 9, and 10 are A flowchart for explaining the operation of the control circuit shown in FIG. 4, and FIG.
11, FIG. 12, and FIG. 13 are timing diagrams for supplementary explanation of the flowcharts of FIGS. 8 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 side (1st)゛・02“Se・bury, 1
5...Downstream (second) 02 sensor.

Claims (1)

[Scope of Claims] 1. First and second air vents for detecting the concentration of specific components in exhaust gas on the upstream and downstream sides of a catalytic converter for purifying exhaust gas provided in the exhaust system of an internal combustion engine, respectively. fuel ratio sensor means; control constant calculation means for calculating an air-fuel ratio feedback control constant according to the output of the second air-fuel ratio sensor means; an air-fuel ratio adjusting means for adjusting the air-fuel ratio of the engine accordingly; an amplitude determining means for determining whether the amplitude of the air-fuel ratio feedback control constant is within a predetermined value; and an amplitude determining means for determining whether the amplitude of the air-fuel ratio feedback control constant is within a predetermined value an air-fuel ratio control device for an internal combustion engine, comprising: stopping means for stopping adjustment of the air-fuel ratio of the engine using the air-fuel ratio feedback control constant when the air-fuel ratio feedback control constant is within the range of the air-fuel ratio; 2. The internal combustion engine according to claim 1, wherein the stopping means maintains the air-fuel ratio feedback control constant at an average value calculated when the amplitude of the air-fuel ratio feedback control constant exceeds the predetermined value. Engine air-fuel ratio control device. 3. The internal combustion engine according to claim 1, wherein the stopping means holds the air-fuel ratio feedback control constant at a fixed value when the amplitude of the air-fuel ratio feedback control constant exceeds the predetermined value. Air-fuel ratio control device.