JPH01262339A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To surely prevent erroneous control by stopping feed-back control of air-fuel ratio in accordance with a signal from a downstream side air-fuel ratio sensor when the compensating value of air-fuel ratio reaches its upper or lower limit value, and by preventing this feed-back control from being restarted until the compensating value of air-fuel ratio comes away from the upper or lower limit. CONSTITUTION:In an exhaust passage (a) of an internal combustion engine, there are provided a ternary catalyst (b) and air-fuel ratio sensors (c, d) upstream and downstream of the catalyst (b). In this arrangement, a means (e) controls the compensating value of air-fuel ratio in accordance with output signals from the air-fuel ratio sensors (c, d). Further, a means (f) guards the compensating value of air-fuel ratio, and a means (g) adjusts the air-fuel ratio of the engine in accordance with the compensating value of air-fuel ratio. Further, a means (h) discriminates whether the compensating value of air-fuel ratio reaches its upper or lower limit value or not, and stops the computation in accordance with an output signal from the downstream side sensor (d) if the compensating value of air-fuel ratio comes to the value. A means (e) discriminates whether the compensating value of air-fuel ratio returns into such a condition that it can be controlled in a predetermined allowable range or not, and restarts the computation in accordance with an output signal from the downstream side sensor (d).

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides air-fuel ratio sensors (oxygen concentration sensor (02 sensor) in this specification) 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 a 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 0□
In order to compensate for variations in sensor output characteristics, variations in parts such as fuel injection valves, and changes over time, a second 02 sensor is provided downstream of the catalytic converter, and air-fuel ratio feedback control is performed by the upstream 0□ 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. 61-234241). In this double 02 sensor system, the 0□ sensor installed downstream of the catalytic converter has a
Although it has a low response speed, it has the advantage of small variations in output characteristics for the following reason.

(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 air-fuel ratio feedback control (double 0□ 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
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 02 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.

In the double 02 sensor system described above, the downstream 0
During execution of the air-fuel ratio feedback control using the two sensors, the control constant of the air-fuel ratio correction amount FAF based on the output of the upstream 02 sensor, for example, the rich skip amount R3R,
There is a system that variably controls the IJ-n skip amount R3L based on the output of the downstream 02 sensor.
When stopping the variable control of the control constant by the output of the downstream 02 sensor due to sensor inactivity, etc., the value stored in the backup RAM when the control constant was being variably controlled is used to change the output of the upstream 02 sensor. Only air-fuel ratio feedback control was performed (see Gokai Sho 61-192828). Furthermore, in systems that variably control control constants, upper and lower limit values are set so as not to impair transient followability and to prevent deterioration of drivability due to air-fuel ratio fluctuations.
As a result, the control constants are subjected to guard processing (see Japanese Unexamined Patent Publication No. 61-234241).

[Problem to be solved by the invention]

However, for example, in an internal combustion engine equipped with a fuel vapor exhaust prevention device, if the temperature in the fuel tank is high and a large amount of vapor is generated from the fuel in the fuel tank, the vapor in the tank will be removed by the purge system. It passes through the canister and is drawn into the combustion chamber, resulting in a very rich air-fuel ratio. If this state continues, the air-fuel ratio correction amount FAF based on the output of the upstream 02 sensor will be corrected to the lean side, and will stick to the lower limit value provided to prevent overleaning. Moreover, in this case, the air-fuel ratio cannot be completely corrected by the air-fuel ratio correction amount FAF alone, and
Control constant R3R based on sensor.

R3L will also be overcorrected to the lean side. After the engine is stopped in such a state, when the engine is brought back to a near-steady running state, the air-fuel ratio correction amount FAF is adjusted using the control constants RSR and R3L, which are stored in the backup RAM and are overcorrected toward the lean side. is calculated, the air-fuel ratio correction amount F
The rate of increase in AF is small, and since the engine stops, the fuel temperature in the tank decreases and there is less steam, so the downstream 0□ sensor maintains a lean output for a while, and the NO
, there were issues such as increased emissions and deterioration of drivability.

In addition, for example, in an internal combustion engine that performs high altitude learning correction, if it moves from high altitude to low altitude without learning, the amount of intake air will decrease due to the drop in atmospheric pressure, and the air-fuel ratio will therefore become very rich, causing skip The amounts RSR and R3L are over-corrected to the lean side. When moving to a lowland in such a state, the amount of intake air increases due to the rise in atmospheric pressure, and the downstream 02 sensor maintains a lean output for a while, causing the same problem.

In this way, if the air-fuel ratio correction amount FAF based on the upstream 02 sensor output is guarded to the lower limit value due to some factor, the air-fuel ratio of the upstream 02 sensor output that is original to the double 02 sensor system There is a problem that the function of detecting the deviation in the average air-fuel ratio due to the feedback result and controlling it to the desired air-fuel ratio cannot be achieved, and as mentioned above, if the air-fuel ratio leans toward the lean or rich side,
Furthermore, in the case where the downstream side 02 sensor output is determined to have been activated upon output reversal from lean to rich or rich to lean, and air-fuel ratio feedback based on the downstream side 02 sensor output is started, the start of the feedback is delayed. There is also the issue of being left behind.

For this reason, the applicant of this application has already proposed that the air-fuel ratio feedback control by the downstream 0□ sensor should be stopped when the air-fuel ratio correction amount FAF reaches its permissible range (see: Japanese Patent Application No. 62 -35820). In this case, for example, as shown in FIG. 3, if you move from a lowland to a highland without learning, the control air-fuel ratio is rich, so the time t.

At , the air-fuel ratio correction amount FAF sticks to the limit value, and as a result, updating of the air-fuel ratio correction amount FAF stops, but until the air-fuel ratio correction amount FAF completely sticks to the lower limit value,
z), the air-fuel ratio correction amount FAF leaves the lower limit value and reaches 0 on the downstream side.
The air-fuel ratio feedback condition based on the outputs of the two sensors is satisfied. As a result, the control constant is still updated by the output of the downstream 0□ sensor, and for example, the rich skip amount R3R is decreased and the lean skip amount R3L is increased. Similarly, even in the period (ts to t) when the air-fuel ratio correction IFAF starts to deviate from the lower limit value, the air-fuel ratio feedback control condition based on the output of the downstream 02 sensor is satisfied, and the downstream 0□ sensor outputs The control constants are updated by In this way, during the period when the air-fuel ratio correction amount FAF begins to reach the upper limit value, lower limit value, and begins to deviate from the upper limit value, the control constants are erroneously controlled, resulting in deterioration of emissions, deterioration of drivability, deterioration of fuel efficiency, and catalyst exhaust. This may lead to the generation of strange odors.

Therefore, an object of the present invention is to provide a downstream air-fuel ratio when the air-fuel ratio correction amount FAF sticks to the upper limit value or lower limit value due to some factor such as moving from a lowland to a highland or from a highland to a lowland without learning. The object of the present invention is to prevent erroneous control of air-fuel ratio feedback control based on the output of a sensor, thereby preventing deterioration of emissions, deterioration of drivability, deterioration of fuel efficiency, generation of abnormal odor in catalyst exhaust, etc.

[Means and operations for solving the problems] Means for solving the above problems are shown in FIG. That is, upstream and downstream air-fuel ratio sensors that detect the air-fuel ratio of the engine are provided in the exhaust passages upstream and downstream of the three-way catalyst CC1o provided in the exhaust passage of the internal combustion engine. The air-fuel ratio correction amount calculation means uses the output Vl of the upstream and downstream air-fuel ratio sensors.
.

V2, the air-fuel ratio correction amount FAF is calculated, and the guard means adjusts the calculated air-fuel ratio correction amount to a predetermined allowable range, for example, 0.
．． 8 to 1.2, and the air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine according to the air-fuel ratio correction amount FAF. On the other hand, the permissible width reaching determination means determines whether the air-fuel ratio correction amount FAF has reached the upper limit or lower limit of the predetermined permissible width, and determines whether the air-fuel ratio correction amount FAF has reached the upper or lower limit of the predetermined permissible width. When the air-fuel ratio correction amount FAF reaches the output V2 of the downstream side air-fuel ratio sensor, the calculation of the air-fuel ratio correction amount FAF is stopped, and the return-within-permissible-range determination means sets the air-fuel ratio correction amount FAF to a state where it is controlled within a predetermined allowable range. It is determined whether or not the condition has been restored, and when the condition has been restored, the calculation of the air-fuel ratio correction amount FAF based on the output V2 of the downstream side air-fuel ratio sensor is restarted.

[For production]

According to the above means, when the air-fuel ratio correction amount FAF reaches the upper limit value or the lower limit value of its allowable range, the air-fuel ratio feedback control based on the output of the downstream air-fuel ratio sensor is stopped, but the air-fuel ratio correction amount FAF It will not be restarted until it completely deviates from the upper and lower limits of the allowable range. Therefore, during periods 1 and -14 in FIG. 3, the air-fuel ratio feedback control based on the output of the downstream air-fuel ratio sensor is stopped.

〔Example〕

FIG. 4 is an overall schematic diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention. In FIG. 4, an air flow meter 3 is provided in the intake passage 2 of the engine body 1. The air flow meter 3 directly measures the 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 includes 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 which generates a signal. 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 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 HC, Co, and No in 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.

02 sensor 13.15 generates an electrical signal according to the concentration of oxygen components in the exhaust gas. That is, 02 sensor 13,
15 generates different output voltages in the A/D converter 101 of the control circuit 10 depending on whether the air-fuel ratio is lean or rich with respect to the stoichiometric air-fuel ratio. The control circuit 10 is configured as a micro combi coater, for example, and includes an A/D converter 101.
, input/output interface 102 , ROM 104 , RAM 105 , backup RAM in addition to CPt 1103
M106, 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 open or not, and this output signal is supplied to the input/output interface 102 of the control circuit 10. .

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 amount TAU is counted down by the down counter 1.
08 and flip-flop 10
9 is also set. As a result, the drive circuit 110 starts energizing the fuel injection valve 7. On the other hand, down counter 10
8 counts a clock signal (not shown) and finally when its carrier terminal reaches the "1" level, the flip-flop 109 is set and the drive circuit 110 stops energizing the fuel injector 7. do. That is, the fuel injection valve 7 is energized by the above-mentioned fuel injection amount TAU, so that 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 CPt1103 is caused by A/D converter 1.
After completing the A/D conversion of 01, 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. That is, data a and THW in the RAM 105 are updated at predetermined intervals. Further, the rotational speed data Ne is calculated by an interrupt every 306 CA of the crank angle sensor 6 and stored in a predetermined area of the RAM 105.

The operation of the control circuit shown in FIG. 4 will be explained below.

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

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, the output signal of the upstream 02 sensor 13 is When not inverted, the closed loop condition does not hold true when there is a fuel cut, etc., and the closed loop condition holds true in other cases. If the closed loop condition is not met, the process proceeds directly to step 535. That is,
The air-fuel ratio correction coefficient FAF is set immediately before the end of closed loop control.

On the other hand, if the closed loop condition is satisfied, the process proceeds to step 502.

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 below the comparison voltage V□, for example 0.45 V, that is, it is determined whether the air-fuel ratio is rich or lean, that is, whether the air-fuel ratio is If lean (V, ≦VILl), it is determined whether the delay counter CDLY is positive or not in step 504, and if CDLY>[), C is determined in step 505.
DLY is set to 0 and the process proceeds to step 506. Step 5
In step 06, the delay counter CDLY is decremented by 1, and in steps 507 and 508, the delay counter CDLY is guarded at the minimum value TDL. In this case, when the delay counter CDLY reaches the minimum value TDL, step 5
At 09, the first air-fuel ratio flag F1 is set to "0" (lean). Note that the minimum value TDL is a lean delay state for maintaining the determination that the rich state is present even if there is a change from rich to lean in the output of the upstream 02 sensor 13, and is defined as a negative value. . On the other hand, Rich (VI
> VIu), it is determined in step 510 whether the delay counter C[lLY is negative or not. If CDLY<0, CDLY is set to 0 in step 511, and the process proceeds to step 512. In step 512, delay counter C
DLY is incremented by 1, and in steps 513 and 514, the delay counter CDLY is guarded at the maximum value TDR.

In this case, when the delay counter CDLY reaches the maximum value TDR, the first air-fuel ratio flag F1 is set to "1"('J petit) in step 515. Note that the maximum value TD
R 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.

In step 516, 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 reversed, then 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, the air-fuel ratio feedback stop flag 5FB is set based on the output V2 of the downstream side 02 sensor 15.
Determine whether L is "1" or not, and proceed to steps 519 and 520 only when 5FBL="12".Step 519
In this case, the air-fuel ratio correction coefficient FAF is the lower limit value of guard processing 0.
If FAF>0.8, the flag 5PBL is reset in step 520. In step 521, the air-fuel ratio feedback stop flag 5FB is increased by skipping FAF -FAF+R3R, and on the other hand, if it is reversed from lean to rich, in step 522, the air-fuel ratio feedback stop flag 5FB is set based on the output V2 of the downstream side 02 sensor 15.
It is determined whether R is "1" or not, and only when 5FBR="1", the process proceeds to steps 523 and 524. Step 523
In this case, the air-fuel ratio correction coefficient FAF is the upper limit value of guard processing 1.
If FAF<1.2, the flag 5FBR is reset in step 524. In step 525, FAF 4-PAF-R3L is decreased in a skip manner. That is, step 521,
At 525, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted in step 516, steps 526 and 527 are performed.

Integration processing is performed at 528. That is, in step 526, it is determined whether F1="0" or not, and if F1="0" (lean), then in step 527, IR is set to FAF←PAP+, and on the other hand, F1="1" (rich). ), in step 528 it is set to FAF -FAF-.

Here, IR and KIL are the integral constants and the skip amount R5R.

It is set sufficiently small compared to R3L, that is, the I
R(KIL) < RR3(R3L). Therefore,
Step 527 gradually increases the fuel injection amount in a lean state (F1="0") to shift to the rich side, and step 528 gradually decreases the fuel injection amount in a rich state (F1="1"). Shift to lean side.

The air-fuel ratio correction coefficient FAF calculated in steps 521, 525, 527, and 528 is
At step 30, the lower limit value is set to 0.8, and at steps 532 and 533, the upper limit value is set to 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. In this case, when the air-fuel ratio correction coefficient FAF reaches the lower limit value 0.8, the air-fuel ratio stop flag 5FBL is set in step 531, and on the other hand, when the air-fuel ratio correction coefficient FAF reaches the upper limit value 1.2, At step 534, the air-fuel ratio stop flag 5FBR is set.

The FAF calculated as described above is backed up in RAM1.
06, and the routine ends at step 535.

That is, as shown in FIG. 6A, the air-fuel ratio correction coefficient FA
Flag 5FBL is set when F reaches the lower limit value of guard processing 0.8, and as a result, downstream side 02 described below
The air-fuel ratio feedback control based on the output v2 of the sensor 15 is stopped, and the air-fuel ratio correction coefficient F immediately before rich skip processing is
When the AF leaves the lower limit value of 0.8, the flag 5FBL is reset and the air-fuel ratio feedback control based on the output V2 of the downstream 02 sensor 15 is restarted. Similarly, the 6th B
As shown in the figure, the flag 5FBR is set when the air-fuel ratio correction coefficient FAF reaches the upper limit value 1.2 of the guard process, thereby stopping the air-fuel ratio feedback control based on the output V2 of the downstream 02 sensor 15, which will be described later. However, the air-fuel ratio correction coefficient FAF immediately before the lean skip process reaches the upper limit value 1.
2, the flag 5FBR is reset and the air-fuel ratio feedback control based on the output V2 of the downstream side 02 sensor 15 is restarted. Note that the air-fuel ratio feedback control based on the output V2 of the downstream 0□ sensor 15 is stopped when either one of the stop flags 5FBL and 5FBH is set, as will be described later. In addition, the 6th A
As shown by the dotted line in Fig. 6B, the stop flag 5FBL
, 5FBR is reset in steps 519 and 52
It is also possible to stabilize by delaying by a predetermined time after the condition 3 is satisfied.

FIG. 7 is a timing diagram supplementary explanation of the operation according to the flowchart of FIG. 5. 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 delay counter CD
As shown in FIG. 7(B), LY is counted up in a rich state and counted down in a lean state.

As a result, as shown in FIG. 7(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 for the lean delay time (-
TDL) After being held rich by a corresponding amount, at time t, it changes to green. However, the air-fuel ratio signal A/F' is at time 1.
5.16.1. When the rich delay time TDR is reversed as shown in the figure, the delay counter CDLY reaches the maximum value T.
It takes time to reach DR, and as a result, the air-fuel ratio signal A/F' after the delay process is inverted at time ts. 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. 7(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 02 sensor 15 will be explained. The second air-fuel ratio feedback control includes skip amounts R3R, R3L as the first air-fuel ratio feedback control constants, and an integral constant K.
IR, KIL, delay time TDR.

There is a system in which TDL or the comparison voltage V11 of the output V1 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 amount R3L is decreased, the controlled air-fuel ratio can be shifted to the rich side.On the other hand, if the lean skip amount R3L is increased, the controlled air-fuel ratio can be shifted to the rich side. , 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 amount R3R and the lean skip amount 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
On the other hand, increasing the lean integral constant KIL allows the controlled air-fuel ratio to be shifted to the lean side, and even if the rich integral constant KIR is decreased, the controlled air-fuel ratio cannot be shifted to the rich side. You can move 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 0□ sensor 15. If the rich delay time TDR is set large or the lean delay time (-TDR) is set small, the control air-fuel ratio can be shifted to the rich side; conversely, if the lean delay time (-TDL) is set large or the lean delay time (-TDR) is set small. By setting (TDR) small, 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 TDR and TDL according to the output of the downstream O2 sensor 15. Furthermore, if the comparison voltage Vll is increased, the controlled air-fuel ratio can be shifted to the rich side, and if the comparison voltage VRl 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.

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 very delicate air-fuel ratio adjustments, and the skip amount allows for responsive control without requiring a long air-fuel ratio feedback cycle like the delay time. Of course, two or more of these variable amounts can be used in combination.

Next, a double 0□ sensor system in which the skip amount as an air-fuel ratio feedback control constant is made variable will be described.

FIG. 8 shows a second air-fuel ratio feedback control routine that calculates the skip amounts R3R and R3L based on the output of the downstream side 02 sensor 15.
Executed every m5. In steps 801 to 806, 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 by the downstream 02 sensor 13 (step 801), the cooling water temperature TH
Step 8 when W is below a predetermined value (for example, 70°C)
02), when the downstream side 0□sensor 15 is not activated, the stelabe 803) is in the light load region (
Q/Ne≦X, ) (step 804), etc., the closed loop condition is not satisfied.

Furthermore, in steps 805 and 806, it is determined whether the stop flags 5FBL and 5FBR calculated in the routine of FIG. In other cases, the closed loop condition is satisfied.If the closed loop condition is satisfied, the process proceeds to step 807, and if the closed loop condition is not satisfied, the process proceeds to step 813.

In step 807, the output V2 of the downstream 0□ sensor 15 is A/D converted and incorporated, and in step 708, it is determined whether or not V2 is less than the comparison voltage Vi, for example, 0.55 V. Note that the comparison voltage VR□ is upstream of the catalytic converter 12,
The upstream side 0□sensor 1
Although the comparison voltage V I I of the output of No. 3 is set higher than the comparison voltage V I I, this setting may be arbitrary. In other words, it determines whether the air-fuel ratio is rich or lean. As a result, in step 808, if V2≦Vi2(J-n), the process proceeds to step 809, and on the other hand, if V2>V12 (!J small), the process proceeds to step 810.

In step 809, the rich skip amount R3R is read from the RAM 105, and R2H-R3R+ΔR3 (constant value)
In other words, the rich skip amount R3R is increased to shift the air-fuel ratio to the rich side, and on the other hand, V,>Vll(!J
When it is small, the rich skip amount R3R is read from the RAM 105 in step 810, and R2H-R2H-
ΔR3, that is, the rich skip amount R3R is decreased to shift the air-fuel ratio to the lean side. Step 811 is to perform guard processing of R2H calculated as described above, for example, maximum value MAX = 7.5%, minimum value M
Guard at IN=2.5%. Note that the minimum value MIN is a value at a level where transient followability is not impaired, and
The maximum value MAX is a level at which drivability does not deteriorate due to air-fuel ratio fluctuations.

In step 812, the lean skip amount R3L is set to R3
Calculate L←10%-R3R. That is, RSR+R3L=10%.

The routine then ends at step 813.

FIG. 9 shows an injection amount calculation routine, which is executed at every predetermined crank angle, for example, every 360 degrees CA.

In step 901, the intake air amount data Q and rotational speed data Ne are read from the RAM 105, and the basic injection amount TA is read out.
Calculate UP. For example, TAUP←α・Q/Ne(α
is a constant). In step 902, 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 903, the final injection amount TAU is set to TAt
Calculate by l -TAUP ・FAF -(FWL+β)+T. Note that β and γ are correction amounts determined by other operating state parameters. Then step 904
At , the injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection. This routine then ends in step 905.

As described above, when the time corresponding to the injection amount TAU has elapsed, the flip-flop 109 is set by the carrier signal of the down counter 10g, and the fuel injection ends.

FIG. 10 shows a modification of FIG. 5, and FIG. 11 shows a modification of FIG. 8.

In FIG. 10, steps 1001-1015 are the same as steps 501-515 in FIG.

In step 1016, 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 process has been reversed. If the air-fuel ratio is reversed, in step 1017, the skip counter C3K
Increase P by 1 count. In addition, the skip counter C
3KP is for counting the number of skips of the air-fuel ratio correction coefficient FAF after the air-fuel ratio correction coefficient FAF reaches the lower limit value 0.8 or the upper limit value 1.2. Next, in step 1018, 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 1019, FAF 4-FAF+R3R
On the contrary, if it is a reversal from lean to rich, FAF 4-PA is increased in step 1020.
Decrease in skips with F-R3L. In other words, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted in step 1016, steps 1021 and 1022
.

Integration processing is performed at 1023. That is, step 102
0, it is determined whether F1="0" or not, and F1="0" (
lean), in step 1022 FAF←FAF
+KIR, and on the other hand, if F1="1" (rich), then in step 1023 FAF + -FAF-KIL is set.

Steps 1019, 1020, 1022, 102
The air-fuel ratio correction coefficient FAF calculated in step 3 is calculated in step 10.
24 and 1025 with a lower limit value of 0.8, and in steps 1027 and 1028 with an upper limit value of 1.2. In this case, when the air-fuel ratio correction coefficient FAF reaches the lower limit value 0.8, the skip counter C3KP is reset in step 1026, and on the other hand, when the air-fuel ratio correction coefficient FAF reaches the upper limit value 1.2, In step 1029, the skip counter C3KP is reset.

The FAF calculated as described above is backed up in RAM1.
06, and the routine ends at step 1030.

That is, when the air-fuel ratio correction coefficient FAF reaches the lower limit value 0.8 or the upper limit value 1.2 of the guard process, the skip counter C3KP is cleared, and the downstream side 02 sensor 1 described later is
The air-fuel ratio feedback control using the output V2 of No. 5 is stopped. This stop continues until the value of the skip counter C3KP reaches a predetermined value a. This is accomplished by replacing steps 805 and 806 in the routine of FIG. 8 with step 1101 of FIG. That is,
It is determined by the number of FAF skips that the air-fuel ratio correction coefficient FAF is controlled within the range of the lower limit value 0.8 and the upper limit value 1.2, and the air-fuel ratio feedback is performed using the output V2 of the downstream side 02 sensor 15. Resume control.

Note that the lower limit value of the guard process for the air-fuel ratio correction coefficient FAF is 0.
8. The average value FAFAV of the air-fuel ratio correction coefficient FAF is determined to be controlled within the range of the upper limit value 1.2. For example, the average value AFAV of the values immediately before skip shown in FIG. 12 = (FAF immediately before rich skip + IJ - immediately before rich skip) FAF)/2, and FAFAV is within a predetermined range, for example 0.9~
1.1.

In this case, in step 519 of FIG.
It is determined whether FAV > 0.9, and in step 523, FAV
<1.1 Determine whether or not.

In addition, the lower limit value of the air-fuel ratio correction coefficient FAF for guard processing is 0.
8. It can also be determined that the duty ratio between the rich output time and lean output time of the upstream 0□ sensor 13 is within a predetermined range that the control is within the upper limit value of 1.2.

In addition, the first air-fuel ratio feedback control is performed every 4 ms.
Also, the second air-fuel ratio feedback control is performed every 512 m5, because the air-fuel ratio feedback control mainly controls the upstream 02 sensor, which has a good response, and subordinates the control by the downstream 02 sensor, which has a poor response. This is for the purpose of doing so.

In addition, other control constants in the air-fuel ratio feedback control by the upstream 02 sensor, such as delay time, integral constant, etc., are corrected by the output of the downstream 02 sensor.
The present invention can be applied to a □ sensor system as well as a double 0□ sensor system that introduces 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, fixing one of the skip amounts R3R and R3L and making only the other variable can also reduce the delay time TDR.

It is also possible to fix one of TDL and make only the other variable, or to fix one of Ricci integral constants KIR and IJ-N and make 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, Electric Air Control Valve (IEAcV)
The electric bleed air control valve adjusts the air bleed amount of the carburetor and introduces atmospheric air into the main system passage and slow system passage to control the air fuel ratio. The present invention can be applied to things such as those that control the exhaust system of an engine, and those that adjust the amount of secondary air sent into the exhaust system of an engine. In this case, the basic twist injection amount corresponding to the basic injection amount TAUP in step 901 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, At step 903, the amount of supplied air corresponding to the final fuel injection amount TAU 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, the air-fuel ratio correction amount FAF on the downstream side is fixed at the upper and lower limits of the guard processing due to some factor such as moving from a lowland to a highland or vice versa without learning. Erroneous control of the air-fuel ratio feedback control due to the output of the sensor can be more reliably prevented, and therefore deterioration of emissions, deterioration of drivability, deterioration of fuel efficiency, generation of catalyst exhaust odor, etc. can be prevented.

[Brief explanation of the drawing]

Figure 1 is an overall block diagram for explaining the invention in detail, Figure 2 is a single 02 sensor system and a double 02 sensor system.
FIG. 3 is an exhaust emission characteristic diagram explaining the sensor system; FIG. 3 is a timing diagram explaining the problem to be solved by the present invention; FIG. 4 is an overall diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention. 5, 8, 9, 10, and 11 are flowcharts for explaining the operation of the control circuit in FIG. 4, and FIGS. 6A, 6B, and 7 are schematic diagrams. FIG. 12 is a timing diagram for supplementary explanation of the flowchart in FIG. 5, and FIG. 12 is a timing diagram showing another embodiment of the present invention. 1... Engine water L 3... Air flow meter, 4... Distributor, 5, 6... Crank angle sensor, 10... Control circuit, 12... Catalytic converter, 13... Upstream side 02 sensor, 15... Downstream 02 sensor, 17... Idle switch. NOx Figure 6A Figure 7 Figure 9 Figure 11 Figure 12

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 air-fuel ratio; an air-fuel ratio correction amount calculation means that calculates an air-fuel ratio correction amount according to the outputs of the upstream and downstream air-fuel ratio sensors; guard means for guarding the fuel ratio correction amount within a predetermined allowable range; air-fuel ratio adjusting means for adjusting the air-fuel ratio of the engine according to the air-fuel ratio correction amount; and the air-fuel ratio correction amount is within the upper limit of the predetermined allowable range. The air-fuel ratio correction amount is determined by the output of the downstream air-fuel ratio sensor when the air-fuel ratio correction amount reaches the upper limit value or the lower limit value of the predetermined allowable range. and determining means for determining whether or not the air-fuel ratio correction amount has returned to a state in which the air-fuel ratio correction amount is controlled within the predetermined permissible range, and when returning to the state, the downstream air An air-fuel ratio control device for an internal combustion engine, comprising: return-to-within-permissible-range determining means for restarting calculation of the air-fuel ratio correction amount based on the output of a fuel ratio sensor.