JP2679305B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention provides an air-fuel ratio sensor (in this specification, an oxygen concentration sensor (O ₂ sensor)) on the upstream side and the downstream side of a catalytic converter, and O ₂ sensor air-fuel ratio control apparatus for an internal combustion engine in addition to the air-fuel ratio feedback control performing the air-fuel ratio feedback control by the O ₂ sensor downstream by, in particular, to an improvement of the catalyst deterioration determination.

[Conventional technology]

In simple air-fuel ratio feedback control (single O ₂ sensor system), an O ₂ sensor that detects oxygen concentration is provided at a point in the exhaust system as close as possible to the combustion chamber, that is, at the gathering portion of the exhaust manifold upstream of the catalytic converter. In addition, the variation in the output characteristics of the O ₂ sensor hinders the improvement of the air-fuel ratio control accuracy. Such O ₂ component variation variation and the fuel injection valve and the output characteristics of the sensor, to compensate for time or secular change, a second O ₂ sensor disposed downstream of the catalytic converter, by the upstream O ₂ sensor Downstream O ₂ in addition to air-fuel ratio feedback control
Double O ₂ with air-fuel ratio feedback control by sensor
Sensor systems have already been proposed (see:
-286550, JP-A-63-97852). In this double O ₂ sensor system, the O ₂ sensor provided on the downstream side of the catalytic converter has a lower response speed than the downstream O ₂ sensor, but the variation in the output characteristics is small for the following reasons. Have advantages.

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

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

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

Therefore, as described above, the air-fuel ratio feedback control based on the outputs of the two O ₂ sensors (double O ₂ sensor system), the variations in the output characteristic of the upstream O ₂ sensor can be absorbed by the downstream O ₂ sensor. Actually, as shown in FIG. 2, in the single O ₂ sensor system, when the O ₂ sensor output characteristic deteriorates, it directly affects the exhaust emission characteristic, while in the double O ₂ sensor system,
Even if the output characteristic of the upstream O ₂ sensor deteriorates, the exhaust emission characteristic does not deteriorate. That is, in the double O ₂ sensor system, good exhaust emission is guaranteed as long as the downstream O ₂ sensor maintains stable output characteristics.

The catalyst of the above-mentioned catalytic converter is designed so that its function is not significantly deteriorated as long as the catalyst is used within the range of the conditions where the vehicle is normally considered. However, if the wrong fuel (for example, leaded gasoline) is used, or if the high tension cord is pulled out and misfires for some reason during use, the function of the catalyst may be significantly reduced. In the former case, the user does not notice at all, and in the latter case, since the high tension cord may be reinserted, the catalyst is rarely replaced. As a result,
The catalytic converter may be driven without sufficiently purifying the exhaust gas.

In the above double O ₂ sensor system, as described above, the function of the catalyst is deteriorated, the number of reversals of the output V ₂ of the downstream O ₂ sensor increases, as a result, downstream O ₂
Disturbance is caused in the air-fuel ratio feedback control by the sensor, and a good air-fuel ratio cannot be obtained. As a result, fuel consumption is deteriorated, drivability is deteriorated, and HC, CO, NO _X emissions are deteriorated.

Therefore, the present applicant has downstream O ₂ as in the case where there is a catalyst deterioration shown in Figure 3B and if the output period of the downstream O ₂ sensor is long as in the case where there is no catalyst deterioration shown in Figure 3A in view of the difference in the output period of the sensor, already on, to determine the downstream O ₂ compared output inversion period of the sensor, or deterioration of the catalyst by output inversion cycle of the downstream O ₂ sensor (see JP 61 -286550), or the downstream side per a predetermined time
It has been proposed that the deterioration of the catalyst is determined by whether or not the output reversal number of the O ₂ sensor is a predetermined value or more (see Japanese Patent Laid-Open No. 63-97852).

[Problems to be solved by the invention]

However, in any of the catalyst deterioration determination methods described above, in order to detect a short output inversion cycle of the downstream O ₂ sensor due to catalyst deterioration, the sampling cycle of the output of the downstream O ₂ sensor is relatively small. , The same as the sampling cycle of the output of the upstream O ₂ sensor, for example, 4 ms.As a result, even if there is no catalyst deterioration in the normal condition, the downstream O ₂ sensor mounting position or the downstream O ₂ sensor ₂ If air leaks from near the upstream of the sensor, as shown in Fig. 3C, the intake of the air synchronized with the exhaust pulsation near the leak causes the downstream O ₂ sensor output to become lean and rich or lean. The number of inversions increases, and the output inversion cycle of the downstream O ₂ sensor becomes short. As a result, there is a problem that corrosion / cracking of the exhaust pipe, abnormality of the exhaust pipe at the distorted portion of the connecting portion of the catalytic converter and catalyst deterioration cannot be distinguished, and even if the catalyst is normal, it is erroneously determined as deterioration.

Therefore, an object of the present invention is to provide a double air-fuel ratio sensor system capable of distinguishing the case of true catalyst deterioration by distinguishing between the case of catalyst deterioration and the case of exhaust pipe abnormality even if the catalyst is normal.

[Means for solving the problem]

The means for solving the above-mentioned problem is shown in FIG. That is, the upstream side of the exhaust passage of the three-way catalyst CC _RO provided in an exhaust passage of an internal combustion engine, it is provided upstream air-fuel ratio sensor for detecting an air-fuel ratio of the engine, also the three-way catalyst CC _RO
A downstream air-fuel ratio sensor for detecting an air-fuel ratio of the engine is provided in an exhaust passage on the downstream side of the engine. The constant calculating means calculates a constant involved in the air-fuel ratio feedback control according to the output V ₂ of the downstream side air-fuel ratio sensor, for example, the skip amount RSR (RSL), and the limit value setting means calculates the constant involved in the air-fuel ratio feedback control. When the value of is out of the range, the constant value related to the air-fuel ratio feedback control is set as the limit value. Further, the air-fuel ratio correction amount calculation means calculates a change amount ΔRSR within a predetermined time of the constant RSR (RSL) involved in the corrected air-fuel ratio feedback control, and as a result, the value of the constant involved in the air-fuel ratio feedback control is When the change amount ΔRSR is not set to the limit value and the change amount ΔRSR is equal to or smaller than the predetermined value, the catalyst deterioration determining means determines that the three-way catalyst CC _RO has deteriorated.

(Operation)

The operation of the above means will be described again with reference to FIGS. 3A, 3B and 3C. That is, as shown in FIG. 3B, when the catalyst deteriorates, the output reversal cycle of the downstream side air-fuel ratio sensor becomes short, but the lean output and the rich output are alternately repeated with substantially the same length. Therefore, the update of the skip amount RSR calculated according to the output of the downstream side air-fuel ratio sensor in the increasing direction and the update of the skip amount RSR in the decreasing direction occur repeatedly. On the other hand, as shown in Figure 3A,
When there is no catalyst deterioration, the output inversion cycle of the downstream side air-fuel ratio sensor is long, and the skip amount RSR calculated according to the output of the downstream side air-fuel ratio sensor changes greatly. Further, as shown in FIG. 3C, when there is air leakage, the downstream air-fuel ratio sensor is reversed due to the influence of exhaust pulsation,
The lean output increases due to the air leakage, and the rich output decreases extremely. As a result, the skip amount RSR calculated according to the output of the downstream side air-fuel ratio sensor becomes large. Further, since the output of the downstream air-fuel ratio sensor changes relatively slowly due to the O ₂ storage effect of the catalyst, the skip amount RSR is updated for a relatively long time, for example, every 512 ms, so that the rich output Although the timing and the update timing of the skip amount RSR are made smaller, as described above, since the rich output is extremely small, there is no opportunity to update the skip amount RSR in the decreasing direction, and therefore the skip amount RSR is Is updated in the increasing direction, and eventually the upper limit guard MAX
I will stick to it.

Therefore, as can be seen from FIG. 3A, FIG. 3B, and FIG. 3C, in the inversion cycle of the downstream side air-fuel ratio sensor, it is not possible to distinguish between the abnormality of the exhaust pipe and the catalyst deterioration, but the change of the skip amount RSR within the predetermined time With the amount ΔRSR, when the exhaust pipe is abnormal, the skip amount RSR within a predetermined time sticks to the guard value, so that it is possible to distinguish between the exhaust pipe abnormality and the catalyst deterioration.

In FIGS. 3A, 3B, and 3C, Q indicates the intake air amount.

〔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 an intake passage 2 of an engine main body 1. The air flow meter 3 directly measures the amount of intake air, and for example, incorporates a potentiometer and generates an output signal of an analog voltage proportional to the amount of intake air. This output signal is supplied to the A /
It is provided for the D converter 101. The distributor 4 has a crank angle sensor 5 whose axis generates, for example, a reference position detection pulse signal every 720 ° in terms of crank angle, and a reference position detection pulse signal in every 30 degrees which is converted into crank angle. A generated crank angle sensor 6 is provided. The pulse signals of the 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 supplied to the interrupt terminal of the CPU 103.

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

The water jacket 8 of the cylinder block of the engine body 1 is provided with a water temperature sensor 9 for detecting the temperature of the cooling water. The water temperature sensor 9 detects the temperature TH of the cooling water.
Generates an analog voltage electric signal corresponding to W. This output is also supplied to the A / D converter 101.

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

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

Further, the throttle valve 16 in the intake passage 2 is provided with an idle switch 17 for generating a signal LL indicating whether or not the throttle valve 16 is fully closed. This idle state output signal LL
Is supplied to the input / output interface 102 of the control circuit 10.

Reference numeral 18 denotes a secondary air introduction intake valve for supplying secondary air to the exhaust pipe 11 at the time of deceleration or idling to reduce HC and CO emissions.

Further, 19 is a catalyst deterioration alarm and 20 is an exhaust pipe abnormality alarm.

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

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

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

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

In step 501, the air-fuel ratio of the closed loop by the upstream O ₂ sensor 13 (feedback) condition is determined whether or not satisfied. For example, when the cooling water temperature is equal to or lower than a predetermined value, during engine start, during increase after start, during warm-up, during power increase,
When the output signal of the upstream O ₂ sensor 13 is never inverted during the increase of OTP for preventing catalyst overheating, the closed loop condition is not satisfied during the fuel cut, and the closed loop condition is satisfied in other cases. When the closed loop condition is not satisfied, the process directly proceeds to step 526. The air-fuel ratio correction factor FAF
May be 1.0. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 502.

In step 502, the output V ₁ of the upstream O ₂ sensor 13 is A / D converted and taken in, and in step 503 it is determined whether or not V ₁ is the comparison voltage V _R1 such as 0.45 V or less, that is, the air-fuel ratio is Discriminate between rich and lean, that is, lean (V ₁ ≤ V _R1 ).
If so, it is determined at step 504 whether or not the delay counter CDLY is positive. If CDLY> 0, at step 505 CDL
Set Y to 0, and proceed to step 506. In step 506, the delay counter CDLY is decremented by one, and in steps 507 and 508, the delay counter CDLY is guarded by the minimum value TDL. In this case, when the delay counter CDLY reaches the minimum value TDL, the air-fuel ratio flag F1 is set to "0" (lean) in step 509. Note that TDL is a lean delay state for holding the judgment that the rich state even if the change from rich to lean in the output of the upstream O ₂ sensor 13 is defined by a negative value You. On the other hand, rich (V ₁ >
V _R1 ), delay counter CDLY at step 510
Is determined to be negative, and if CDLY <0, CDLY is set to 0 in step 511, and the process proceeds to step 512. In step 512, the delay counter CDLY is incremented by 1, and in steps 513 and 514, the delay counter CDLY is guarded by the maximum value TDR. In this case, when the delay counter CDLY reaches the maximum value TDR, the air-fuel ratio flag F1 is set to "1" (rich) in step 515. It should be noted that the maximum value TDR is a rich delay state for holding the determination that it is in the lean state even if there is a change from lean to rich in the output of the upstream O ₂ sensor 13, and a positive value is defined. It

In step 516, it is determined whether or not the sign of the air-fuel ratio flag F1 is inverted, that is, it is determined whether or not the air-fuel ratio after the delay process is inverted. If the air-fuel ratio is reversed, it is determined in step 517 whether the air-fuel ratio is changed from rich to lean or lean to rich, depending on the value of the air-fuel ratio flag F1. If it is a reversal from rich to lean, step 51
In step 8, FAF ← FAF + RSR is increased in a skip manner, and conversely, if lean-to-rich inversion, FAF is executed in step 519.
← Skip to FAF-RSL. That is, skip processing is performed.

If the sign of the air-fuel ratio flag F1 is not inverted at step 512, integration processing is performed at steps 520, 521 and 522.
That is, in step 520, it is determined whether or not F1 = "0", and F1
If 1 = "0" (lean), then in step 521 FAF ← FAF +
Set to KIR, and if F1 = "1" (rich), step 5
At 22, set FAF ← FAF−KIL. Where the integration constants KIR, KIL
Is set to be sufficiently smaller than the skip amounts RSR and RSL, that is, KIR (KIL) <RSR (RSL). Therefore,
Step 521 gradually increases the fuel injection amount in the lean state (F1 = "0"), and step 522 is in the rich state (F1 =
In “1”), the fuel injection amount is gradually reduced.

Further, the air-fuel ratio correction coefficient FAF is learned in steps 523 and 524 for each skip processing. That is, in step 523, it is determined whether or not the learning condition is satisfied. For example, the intake air amount data Q is read from the RAM 105 and it is determined whether or not Q <constant value. If Q ≧ constant value, step 525
Go directly to and do not perform learning control. This is because if learning control is performed when the intake air amount Q is large, the learning value FGHAC may be erroneously learned (overcorrected) due to the effect of evaporation. On the other hand, if the learning condition is satisfied, the air-fuel ratio correction coefficient FAF is learned in step 524. The learning step 524 will be described later.

Next, in step 525, the air-fuel ratio correction coefficient FAF calculated in steps 518, 519, 521 and 522 is guarded at the minimum value, for example 0.8, and at the maximum value, for example 1.2. As a result, the air-fuel ratio correction coefficient FAF
Is too large or too small,
The value is used to control the air-fuel ratio of the engine to prevent over-rich or over-lean.

The FAF calculated as described above is stored in the RAM 105, and this routine ends at step 526.

FIG. 6 is a detailed flowchart of the learning step 524 of FIG. 5, and as described above, under the air-fuel ratio feedback control by the upstream O ₂ sensor 13, when the learning condition is satisfied, It is executed every time the fuel ratio correction coefficient FAF is skipped.

That is, in step 601, the average value FAFAV of the air-fuel ratio correction coefficient FAF is calculated by FAFAV ← (FAF + FAFO) / 2, where FAFO is the FAF value immediately after the previous skip, and in step 602, FAF is calculated next time. In preparation for this, FAFO ← FAF. Next, at step 603, ΔFAF ← FAFAV−1.0 is calculated.

Next, at step 604, it is determined whether or not ΔFAF> 0,
As a result, if ΔFAF> 0, the learning correction amount FGHAC is increased by FGHAC ← FGHAC + ΔFGHAC in step 605, and the maximum value is increased in steps 606 and 607, for example.
Guard at 1.05. On the other hand, if ΔFAF ≦ 0, in step 608 the learning correction amount FGHAC is decreased by FGHAC ← FGHAC−ΔFGHAC, and in step 609,610 the minimum value, for example,
Guard at 0.90. Note that FGHAC may be updated only when | ΔFAF |> K (a positive value). In this way,
According to the learning control, the learning correction amount FGHAC is increased or decreased so that the air-fuel ratio correction coefficient FAF converges to 1.0.

FIG. 7 is a timing chart for supplementarily explaining the operation according to the flowchart of FIG. Rich as shown in FIG. 7 by the output of the upstream O ₂ sensor 13 (A), when the air-fuel ratio A / F is obtained, the delay counter CDLY is
As shown in FIG. 7 (B), the count is performed in a rich state and the count is reduced in a lean state. As a result, as shown in FIG. 7 (C), a delayed air-fuel ratio signal A / F '(corresponding to the flag F1) is formed. For example, the air-fuel ratio signal A / F is changed from lean to rich at time t _1, the air-fuel ratio signal A / F which is delayed processed 'at time t ₂ after being held lean only the rich delay time TDR Change richly. Also the air-fuel ratio signal A / F from the rich at time t ₃ is changed to the lean air-fuel ratio signal A / F which is delayed processed '
Changes to lean at time t ₄ after being held rich only equivalent lean delay time (-TDL). But the air-fuel ratio signal
If A / F is reversed at time t _5, a short period of rich delay time TDR as the t _6, t _7, it takes time delay counter CDLY reaches the maximum value TDR, the result, at time t ₈ Thus, the air-fuel ratio signal A / F 'after the delay processing is inverted. That is, the air-fuel ratio signal A / F 'after the delay processing is more stable than the air-fuel ratio signal A / F before the delay processing. Thus, the air-fuel ratio correction coefficient FAF shown in FIG. 7D is obtained based on the stable air-fuel ratio signal A / F 'after the delay processing.

Next, a description will be given of the second air-fuel ratio feedback control by the downstream O ₂ sensor 15. As the second air-fuel ratio feedback control, the skip amounts RSR, RSL as the constants involved in the first air-fuel ratio feedback control, the integration constant KI
There are a system that makes R, KIL, delay times TDR, TDL, or a comparison voltage V _R1 of the output V ₁ of the upstream O ₂ sensor 13 variable, and a system that introduces a second air-fuel ratio correction coefficient FAF2.

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

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

Next, a double O ₂ sensor system in which the skip amount as a constant involved in the air-fuel ratio feedback control is variable will be described.

FIG. 8 shows a second air-fuel ratio feedback control routine based on the output of the downstream O ₂ sensor 15, which is executed every predetermined time, for example, 512 ms. In steps 801-806,
It is determined whether or not the closed loop condition by the downstream O ₂ sensor 15 is satisfied. For example, when the closed loop condition is not satisfied by the upstream O ₂ sensor 13 (step 801) and the cooling water temperature THW is below a predetermined value (for example, 70 ° C.) (step 802), the throttle valve 16 is fully closed (LL = “ 1 ") (step 803),
When the secondary air is not introduced based on the rotation speed Ne, the vehicle speed, the signal LL of the idle switch 17, the cooling water temperature THW, etc. (step 804), when the load is light (Q / Ne <X ₁ ) (step 805) The closed loop condition is not satisfied when the downstream O ₂ sensor 15 is not activated (step 806), and the closed loop condition is satisfied in other cases. If it is not the closed loop condition, the process proceeds to step 817, and if it is the closed loop condition, the process proceeds to step 807.

In step 807, the output V ₂ of the downstream O ₂ sensor 15 is A / D converted and taken in, and in step 809 it is determined whether or not V ₂ is the comparison voltage V _R2, for example, 0.55 V or less, that is, the air-fuel ratio is Determine whether it is rich or lean. Incidentally, than the comparison voltage V _R1 of the output of the comparison voltage V _R2 upstream O ₂ sensor 13 upstream of the catalytic converter 12, it and the degradation rate output characteristics due to the influence of the raw gas is different downstream in consideration of different like Although set high, this setting may be arbitrary. As a result, V ₂
If ≦ V _R2 (lean), proceed to steps 809,810,811
If V ₂ > V _R2 (rich), proceed to steps 812, 813, 814. That is, in step 809, RSR ← RSR + ΔRS (constant value), that is, the rich skip amount RSR is increased to shift the air-fuel ratio to the rich side, and in steps 810 and 811, RSR is set to the maximum value MAX (= 7.5%). Guard, on the other hand,
In step 812, RSR ← RSR−ΔRS is set, that is, the rich skip amount RSR is decreased to shift the air-fuel ratio to the lean side, and in steps 813 and 814, RSR is guarded to the minimum value MIN (= 2.5%). It should be noted that the minimum value MIN is a value at which the transient followability is not impaired, and the maximum value MAX is a value at which driveability does not deteriorate due to air-fuel ratio fluctuations.

 In step 815, the lean skip amount RSL is set to RSL ← 10% −RSR. In other words, RSR + RSL = 10%.

In step 816, it is determined whether the catalyst is deteriorated or the exhaust pipe is abnormal. Step 816 will be described later.

 Then, in step 817, this routine ends.

FIG. 9 is a step for determining catalyst deterioration / exhaust pipe abnormality in FIG.
8 is a detailed flowchart of 816. That is, in steps 901 and 902, the learning value FGHAC calculated in FIG.
It is determined whether the fuel injection valve 7 is abnormal by determining whether the fuel injection valve 7 is stuck at the lower limit value 0.90 or the upper limit value 1.05.
Further, in step 903, it is determined whether or not the steady state is present, for example, by whether or not the intake air change amount is a fixed value or less or the load Q / Ne change amount is a fixed value or less. Fuel injector malfunction,
Alternatively, in the unsteady state, the skip amount RSR does not become a normal value, so the routine proceeds to step 921, the counters C1 and C2 are cleared, and the routine proceeds to step 922. Substantial determination of catalyst deterioration / exhaust pipe abnormality is not performed. Otherwise, go to step 904.

At step 904, the rich skip amount RSR is the maximum value MAX.
Whether or not RSR = MAX, and if RSR ≠ MAX as a result, steps 905 to 913 are determined.
, And if RSR = MAX, step 914
Continue to ~ 920.

The flow of steps 905 to 913 is for determining whether there is no catalyst deterioration shown in FIG. 3A or there is catalyst deterioration shown in FIG. 3B. That is, in step 905, the third
A diagram, the counter C1 for counting the determination period C1 ₀ shown in Figure 3B is increased +1, while clearing the counter C2 for measuring the exhaust pipe abnormality determining period C2 ₀ shown in Figure 3C.
In step 907, the change amount (amplitude) ΔRSR of the rich skip amount RSR is calculated. Step 907 will be described later. Then, determining period C1 _0, it is determined whether or not the elapsed at step 908, the result, until after determining period proceeds directly to step 922, after determining period step 909-91
Proceed to 3.

That is, in step 909, the change amount DerutaRSR the rich skip amount RSR is determined whether or not less than the predetermined value X _2, determines a result, the first 3B catalyst deterioration shown in Figure If DerutaRSR <X _2, step a catalyst Illuminates the deterioration alarm 19, while
If ΔRSR ≧ X ₂ , it is determined that there is no catalyst deterioration shown in FIG. 3A, and the catalyst deterioration alarm 19 is turned off in step 911.
Then, in step 912, the counter C1 is cleared and initialized, and in step 913, the upper limit value RSRU, the lower limit value RSRL, and the change amount ΔRSR used in step 907 are respectively set as RSRU ← RSR RSRL ← RSR ΔRSR ← Initialize with O.

The flow of steps 914 to 920 is to determine whether or not there is an exhaust pipe abnormality shown in FIG. 3C. That is, step 914
In the counter C1 is cleared, +1 increases the counter C2 for the exhaust pipe abnormality determining period C2 ₀ measured in step 915. At step 916, it is determined whether or not determination period C2 ₀ elapse of time, as a result, C2> proceeds to step 917 as has advanced enrichment of the rich skip amount RSR by C2 ₀ a long if the exhaust pipe abnormal, The exhaust pipe abnormality alarm 20 is turned on, and the catalyst deterioration alarm 19 is turned off in step 918. That is, when the exhaust pipe is abnormal, the catalyst deterioration determination is substantially not performed. Then, in step 919, the counter C2 is cleared and initialized, and in step 920, the rich skip amount RSR is initialized to, for example, 5%.

Then, in step 922, the routine of FIG. 9 ends.

FIG. 10 is a detailed flowchart of the variation ΔRSR calculation step 907 in FIG. 9, in which the upper limit value RSRU and the lower limit value RSRL are initialized to RSR in advance (see step 913 in FIG. 9). In steps 1001 and 1002, the current rich skip amount RSR is compared with the upper limit value RSRU, and only when RSR> RSRU,
Update the upper limit value RSRU. In steps 1003 and 1004, the current rich skip amount RSL is compared with the lower limit value RSRL, and the lower limit value RSRL is updated only when RSR <RSRL. Then, in step 1005, the change amount ΔRSR is calculated by ΔRSR ← RSRU−RSRL, and in step 1006, this routine ends.

FIG. 11 shows an injection amount calculation routine, which is executed every predetermined crank angle, for example, at 360 ° CA. R in step 1101
The intake air amount data Q and the rotation speed data Ne are read from the AM 105 to calculate the basic injection amount TAUP. For example TAUP ← α
・ Q / Ne (α is a constant). In step 1102, the final injection amount TAU is calculated by TAU ← TAUP · (FAF + FGHAC) · β + γ. Here, β and γ are correction amounts determined by other operation state parameters. Then, in step 1103,
The injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection.
Then, in step 1104, this routine ends.

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

In the above-described embodiment, the control of the rich skip amount RSR is continued even when the catalyst deterioration is determined, but the RSR may be fixed to a constant value in the routine of FIG.

Also, the first air-fuel ratio feedback control is performed every 4 ms,
Also, the reason why the second air-fuel ratio feedback control is performed every 512 ms is that the air-fuel ratio feedback control mainly performs the control by the upstream O ₂ sensor having a high response, and the control by the downstream O ₂ sensor having a low response. It is to do so.

Further, other constants involved in the air-fuel ratio feedback control by the upstream O ₂ sensor, such as delay time, integration constant,
The present invention can be applied to a double O ₂ sensor system that corrects the above by the output of the downstream O ₂ sensor, and also to a double O ₂ sensor system that introduces a second air-fuel ratio correction coefficient. Controllability can be improved by simultaneously controlling two of the skip amount, the delay time, and the integration constant. Furthermore, one of the skip amounts RSR and RSL can be fixed and only the other can be made variable, or one of the delay times TDR and TDL can be fixed and only the other can be made variable, or the rich integration constant
It is also possible to fix one of KIR and lean integration constant KIL and make the other variable.

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

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

Further, in the above-described embodiment, the internal combustion engine in which the fuel injection valve controls the fuel injection amount to the intake system is described. However, the present invention can be applied to a carburetor-type internal combustion engine. For example, the air-fuel ratio is controlled by adjusting the intake air amount of the engine using an electric air control valve (EACV).
An electric bleed air control valve is used to adjust the air bleed amount of the carburetor to control the air-fuel ratio by introducing air into the main passage and the slow passage, and the 2 o'clock air amount sent to the exhaust system of the engine. The present invention can be applied to things that are adjusted. In this case, the basic fuel injection amount corresponding to the basic injection amount TAUP in step 1101 is determined by the carburetor itself, that is, the intake pipe negative pressure according to the intake air amount and the engine speed, and step 1102 At the final fuel injection amount TAU
Is calculated.

Furthermore, in the above-described embodiment, the O ₂ sensor is used as the air-fuel ratio sensor, but a CO sensor, a lean mixture sensor, or the like may be used. In particular, when a TiO ₂ sensor is used as the upstream air-fuel ratio sensor, the control response is improved and overcorrection due to the output of the downstream air-fuel ratio sensor can be prevented.

Further, although the above-described embodiment is constituted by a microcomputer, that is, a digital circuit, it may be constituted by an analog circuit.

〔The invention's effect〕

As described above, according to the present invention, the deterioration of the catalyst can be surely discriminated from the case of the exhaust pipe abnormality, and as a result, it is possible to reduce the running of the vehicle in the deteriorated state of the catalyst as much as possible.

[Brief description of the drawings]

FIG. 1 is an overall block diagram for explaining the configuration of the present invention, FIG. 2 is an exhaust emission characteristic diagram for explaining a single O ₂ sensor system and a double O ₂ sensor system, and FIGS. 3A to 3C are the present invention. FIG. 4 is a diagram for explaining problems and actions to be solved by the present invention, FIG. 4 is an overall schematic diagram showing one embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention, FIG. 5, FIG. 6, FIG. 9, 10 and 11 are flow charts for explaining the operation of the control circuit of FIG. 4, and FIG. 7 is a timing chart for supplementary explanation of the flow chart of FIG. 1 ...... engine body, 2 ...... airflow meter, 4 ...... distributor, 5,6 ...... crank angle sensor, 10 ...... control circuit, 12 ...... catalytic converter, 13 ...... upstream O ₂ sensor, 15 ...... Downstream O ₂ sensor, 17 …… Idle switch.

Claims (1)

(57) [Claims] 1. A three-way catalyst (12) provided in an exhaust passage of an internal combustion engine, and an upstream air-fuel ratio sensor for detecting an air-fuel ratio of the engine provided in an exhaust passage upstream of the three-way catalyst ( 13), a downstream side air-fuel ratio sensor (15) provided in the exhaust passage downstream of the three-way catalyst for detecting the air-fuel ratio of the engine, and an air-fuel ratio feedback according to the output of the downstream side air-fuel ratio sensor. A constant calculating means for calculating a constant involved in the control, and a limit value setting for setting a value of the constant involved in the air-fuel ratio feedback control to a limit value when the value of the constant involved in the air-fuel ratio feedback control is out of the range. Means, an air-fuel ratio correction amount calculation means for calculating an air-fuel ratio correction amount according to a constant involved in the air-fuel ratio feedback control and the output of the upstream side air-fuel ratio sensor, and the engine according to the air-fuel ratio correction amount. Air-fuel ratio Air-fuel ratio adjusting means for adjusting, a change amount calculating means for calculating a change amount of a constant involved in the air-fuel ratio feedback control within a predetermined time, and a value of a constant involved in the value of the air-fuel ratio feedback control is the limit value. And a catalyst deterioration determining unit that determines that the three-way catalyst has deteriorated when the change amount is equal to or less than a predetermined value, and an air-fuel ratio control device for an internal combustion engine.