JPH0291440A - Catalyst deterioration determining device of internal combustion engine - Google Patents

Abstract

PURPOSE:To exactly judge the medium degree of deterioration of three-way catalyst by determining that the catalyst of a catalyst converter is deteriorated when the frequency of inversions of the output from a lower side air fuel ratio sensor per specified period is over the specified value. CONSTITUTION:A primary comparison means compares the output V1 of an upper side air fuel ratio sensor with a primary comparison voltage VR1. A load detecting means detects the quantity Q of intake air and according to the load Q, a comparison voltage computing means computes the secondary comparison voltage VR2 as such that when the load Q is high, the voltage is computed to the value of estimating the air fuel mixture rich and when the load Q is low, the voltage is to the value of estimating the mixture lean. A secondary comparison means compares the output V2 of a lower air fuel ratio sensor with the secondary comparison voltage VR2. According to both the results, an air fuel ratio adjusting means adjusts the air fuel ratio. Meanwhile an inversion number computing means counts the number CS of inversions of the output V2 from the lower air fuel ratio sensor per specified period and a catalyst deterioration determining means determines that the catalyst of the catalyst converter is deteriorated when the number CS of inversions is over the specified value CS0.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides a double air-fuel ratio sensor in which air-fuel ratio sensors (in this specification, oxygen concentration sensor (02 sensor)) are provided on the upstream and downstream sides of a catalytic converter. The present invention relates to a catalyst deterioration determination device in a system.

[Conventional technology]

In simple air-fuel ratio feedback control (single 02 sensor system), the 0□ 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 from the catalytic converter. Due to variations in the output characteristics of the 0□ sensor, it is difficult to improve the control accuracy of the air-fuel ratio. It takes 02
In order to compensate for variations in sensor output characteristics, variations in components such as fuel injection valves, and changes over time or aging, a second 0□ sensor is provided downstream of the catalytic converter, and air-fuel ratio feedback control is performed by the upstream 02 sensor. In addition to this, a double 02 sensor system has already been proposed that performs air-fuel ratio feedback control using a downstream 0□ sensor (
Reference: Japanese Unexamined Patent Publication No. 58-72647). This double 0
□In the sensor system, the 02 sensor installed on the downstream side of the catalytic converter has a lower response speed than the 02 sensor on the upstream side, but has the advantage of less variation in output characteristics for the following reasons. There is.

(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
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 O2 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.

The catalyst of a catalytic converter is designed so that its functionality does not deteriorate significantly as long as the vehicle is used within the range of normal usage conditions. However, if the user mistakenly adds leaded gasoline to the fuel, or if the high tension cord is disconnected or a misfire occurs for some reason during use, the catalyst's function may deteriorate significantly. In the former case, the user will not be aware of it, and in the latter case, the user will only need to reinsert the high tension cord, so it is unlikely that the catalyst will be replaced. It may be driven without being purified.

However, in the above-mentioned double 02 sensor system, as mentioned above, when the catalyst function deteriorates, HC,
The output characteristics of the downstream 02 sensor deteriorate due to the influence of unburned gas such as CD/H2. In other words, the number of reversals of the output of the downstream 02 sensor increases, and as a result, the downstream 02
This causes disturbance in the air-fuel ratio feedback control by the sensor, making it impossible to obtain a good air-fuel ratio, resulting in deterioration of fuel efficiency, deterioration of drivability, HC, C[l・N0M
This has the problem of causing deterioration of emissions, etc.

For this reason, the applicant has already calculated the downstream 0 per unit time.
It has been proposed to detect the deterioration of the catalyst based on the number of times the output of two sensors is reversed to a certain level (reference: Japanese Patent Application No. 61-241489).

[Problem to be solved by the invention]

Figure 3 shows the output V of the upstream side 02 sensor and the downstream side 02 sensor.
An example of sensor output v2 is shown. In other words, when the three-way catalyst is new or the degree of deterioration is small, as shown in Figure 3 (B), the 02 storage effect of the three-way catalyst is large, and the range of rich and lean air-fuel ratios of the gas entering the catalyst increases. Even if it is large, the air-fuel ratio of the emitted gas is averaged over time.

This tendency increases as the purification performance of the three-way catalyst increases.

Furthermore, as the degree of deterioration of the three-way catalyst increases, that is, as the 02 storage effect of the three-way catalyst decreases, as shown in Figure 3 (
The amplitude and period of the output V2 of the downstream 0□ sensor shown in D) approach the amplitude and period of the output VI of the upstream 02 sensor shown in FIG. It is possible to determine the degree of deterioration of the three-way catalyst based on the number of times the output V2 of the side 02 sensor is reversed to a certain level.

However, if the degree of deterioration of the three-way catalyst is moderate,
As shown in Figure 3 (C), the output V of the downstream 02 sensor
Since the amplitude center value of 2 is shifted, it cannot be determined by the number of inversions with respect to a certain level. That is, as shown in FIG. 3(E), when the intake air amount Q becomes large, the gas flow rate in the three-way catalyst becomes faster, so the chemical reaction time in the three-way catalyst becomes shorter, and as a result, This is because the 0□ storage effect no longer works sufficiently, and therefore the level and frequency of the output V2 of the downstream 02 sensor become high. Furthermore, since various correction coefficients in the air-fuel ratio feedback control by the upstream 02 sensor are changed depending on the engine rotation speed and/or load, the output level of the downstream 02 sensor changes depending on the intake air amount Q. It is from.

Therefore, an object of the present invention is to provide a catalyst deterioration determination system that can reliably determine even a moderate degree of deterioration of a three-way catalyst.

[Means to solve the problem]

A means for solving the above problem is shown in FIG.

In Figure 1, the upstream and downstream sides of a catalytic converter for purifying exhaust gas installed in the exhaust system of an internal combustion engine are as follows:
Upstream and downstream air-fuel ratio sensors are respectively provided to detect the concentration of specific components in exhaust gas. The first comparison means converts the output Vl of the upstream air-fuel ratio sensor into a first comparison voltage V
Compare with RI.

The load detection means also detects the engine load, such as the intake air amount Q.
is detected, and the comparison voltage calculation means calculates the second comparison voltage VR2 according to the detected load Q. For example, when the detected load Q is large, the second comparison voltage VR2 is calculated to be determined to be rich, and when the detected load Q is small, the second comparison voltage V112 is calculated to be determined to be lean.

As a result, the second comparison means compares the output V2 of the downstream air-fuel ratio sensor with the second comparison voltage VR2. The air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine according to the comparison results of the first and second comparing means. On the other hand, the reversal number calculation means calculates the reversal number C8 of the output V2 of the downstream air-fuel ratio sensor per predetermined time, and the catalyst deterioration determination means determines that the reversal number C8 is a predetermined value C3O.
In the above cases, the catalyst of the catalytic converter is considered to have deteriorated.

[For production]

According to the above-mentioned means, catalyst deterioration is determined based on the number of times of reversal with respect to a level that matches the output amplitude center value of the downstream air-fuel ratio sensor.

〔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 the A/D converter 101 with a built-in multiplexer of the control circuit IO. 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.

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

The exhaust manifold 11 includes a catalytic converter 1.
A first O2 sensor 13 is provided on the upstream side of the catalytic converter 12, and a second O2 sensor 15 is provided on the exhaust pipe 14 on the downstream side of the catalytic converter 12.

The 02 sensors 13 and 15 generate electrical signals corresponding to the concentration of oxygen components in the exhaust gas. That is, 0□ sensor 13,
15, a control circuit 10 generates different output voltages to the A/D converter 101 depending on whether the air-fuel ratio is lean or rich with respect to the stoichiometric air-fuel ratio.

Further, the throttle valve 16 of the intake passage 2 is provided with an idle switch 17 for detecting whether the throttle valve 16 is fully closed, and this output signal is supplied to the input/output interface 102 of the control circuit 10. Ru.

18 is an alarm indicating deterioration of the three-way catalyst of the catalytic converter 12.

The control circuit 10 is configured as a microcomputer, for example, and includes an A/D converter 101, an input/output interface 102, a CPU 103, and a ROM 104°R.
An AM 105, a backup ROM 106, a clock generation circuit 107, and the like 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 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 reset 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 the CPU 103 is caused by the A/D converter 10.
1, the input/output interface 102
When the controller receives the crank angle sensor 6p pulse signal, when the interrupt signal from the clock generation circuit 107 is received, and so on.

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 Q and THW in RAM 105 are updated at predetermined time intervals. Further, the rotational speed data Ne is calculated by interruption every 30° CA of the crank angle sensor 6 and is stored in a predetermined area of the RAM 105.

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

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 satisfied, the process proceeds to step 527 and the FAF is set to the value immediately before the end of the closed loop control. Note that a certain value, for example 1.0
You can also use it as On the other hand, if the closed loop condition is satisfied, the process proceeds to step 502.

In step 502, the output V of the upstream 02 sensor 13,
is A/D converted and taken in, and in step 503 it is determined whether or not V is less than the comparison voltage V Rl, for example, 0.45V, 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+ ≦ V+u), it is determined in step 504 whether the delay counter CDLY is negative or not, and if -CDLY>Q, C is determined in step 505.
Set DLY to 0 and proceed to step 506. Step 50
In step 6, the delay counter CDLY is subtracted 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, set the first air-fuel ratio flag F1 to “0” (lean)
shall be. 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 (V+
>Vat), it is determined in step 510 whether the delay counter C port LY is positive or not, and if CDLY<Q, CDLY is set to 0 in step 511.
Proceed to step 2. In step 512, the delay counter CDL
Y is incremented by 1, and the delay counter CDLY is guarded at the maximum value TDR in steps 513 and 514.

In this case, when the delay counter CDLY reaches the maximum value TDR, the first air-fuel ratio flag F1 is set to "1" (rich) in step 515. In addition, 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 PAP -FAF+R3R
On the contrary, if it is a reversal from lean to rich, FAF -FAF- is increased in a skip manner in step 519.
Decrease R3L in a skip manner. In other words, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted in step 516, integration processing is performed in steps 520, 521, and 522. That is, in step 520, F1
If Fl=“0” (lean), FAF −FAF+KIR is set at step 521, and on the other hand, if F1=@l” (rich), FAF − is set at step 522. Let FAP- be IL.Here, integral constants KIR and KIL are skip amounts R3R and R3.
It is set sufficiently small compared to L, that is, KIR(
Kl<R2H(R5L). Therefore, step 521 gradually increases the fuel injection amount in a lean state (F1="0"), and step 522 gradually increases the fuel injection amount in a rich state (F1="0").
"1") gradually reduces the fuel injection amount.

The air-fuel ratio correction coefficient FAF calculated in steps 518, 519, 521, and 522 is guarded at a minimum value of, for example, 0.8 in steps 523 and 524, and is guarded at a maximum value of, for example, 1°2 in steps 525 and 526. Due to the fact that the air-fuel ratio correction coefficient FA is
When F becomes too large or too small, 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 in step 527.

In addition, in FIG. 5, the delay times TDR and TDL.

Although the skip amounts R5R and R3L and the integral constants KIR and KIL are set to constant values, they can also be made variable for each region of the intake air IQ.Furthermore, as will be described later, they can be used to finely adjust the deviation of the upstream 02 sensor, etc. For example, one of these skip amounts R3R, R3L is set to the output V2 of the downstream 0□ sensor 15.
It can also be made variable depending on the situation.

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

As a result, as shown in FIG. 6(C), a delayed air-fuel ratio signal A/F' (corresponding to flag F1) is formed. For example, time 1. Even if the air-fuel ratio signal A/F' changes from lean to rich at time t2, the delayed air-fuel ratio signal A/F' changes to rich at time t2 after being held lean for the rich delay time TDR. . Even if the air-fuel ratio signal A/F changes from rich to lean at time t, the air-fuel ratio signal A/F' subjected to the delay processing is delayed by the lean delay time (-
After being held rich by an amount equivalent to TDL), it changes to IJ+on at time t. However, the air-fuel ratio signal A/F' is at rich delay time TD as shown in time t5 i t6 , t.
When R is reversed in a short period, the delay counter CDLY
It takes time for TDR to reach the maximum value, and as a result,
At time t8, the air-fuel ratio signal A/F' after the delay process is inverted. In other words, the air-fuel ratio signal A/F' after the delay process is more stable than the air-fuel ratio signal A/F before the delay process. In this manner, the air-fuel ratio correction coefficient FAF shown in FIG. 6(D) is obtained based on the stable air-fuel ratio signal A/F' after the delay processing.

Next, the second air-fuel ratio feedback control by the downstream 0□ sensor 15 will be explained. The second air-fuel ratio feedback control includes skip amounts R3R and R5L as first air-fuel ratio feedback control constants, and an integral constant K.
IR, KIL, delay time TDR.

There are two types of systems: one in which the comparison voltage V Rl of the output v1 of the upstream 02 sensor 13 is made variable; and the other is 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 02 sensor 15.

In addition, by increasing the rich integral constant KIR, the control air-fuel ratio can be shifted to the rich side, and the lean integral constant KIL
On the other hand, increasing the lean integral constant 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 (-TDL) 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 (-TDL) is set small
If the control air-fuel ratio is set to a small value (TDR), the control air-fuel ratio can be shifted to the lean side.In other words, the air-fuel ratio can be controlled by correcting the delay times TDR and TDL according to the output of the downstream 0□ sensor 15. , the control air-fuel ratio can be shifted to the rich side by increasing the comparison voltage Vll, and the control air-fuel ratio can be shifted to the lean side by decreasing the comparison voltage Vll. By correcting the air-fuel ratio, the air-fuel ratio can be controlled.

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

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

FIG. 7 shows a second air-fuel ratio feedback control routine that calculates skip amounts R3R and R3L based on the output of the downstream 0□ sensor 15, and is executed for a predetermined period of time, for example, every IS. In step 701, 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 upstream 02 sensor 13, when the output signal of the downstream 0
2) The closed loop condition does not hold when the throttle valve is fully open, and the closed loop condition holds in other cases. If the closed loop condition is not met, the process proceeds to step 716, where the rich skip amount R3R is calculated. For example, the intake air amount data Q is read out from the RAM 105 and a one-dimensional map stored in the ROM 104 is used to calculate rich skip ff1R3R by interpolation.

Alternatively, R3R may be set to 5%, or it may be held at the value immediately before the end of the closed loop, or it may be set as a learned value (value in the backup RAM 106).

If the closed loop condition is satisfied by the downstream 02 sensor 15, the process proceeds to step 702, reads the intake air amount data Q from the RAM 105, and calculates the change ΔQ by ΔQ-Q-QO, where Qo is the previous value of Q. However, in step 703, Q0←Q is set in preparation for the next execution. Next, in step 704, the intake air amount change ΔQl is set to a predetermined value Δ
Determine whether it is less than or equal to Q0.

If ΔQ1≦ΔQo, the process proceeds to steps 705 and 706, and if 1ΔQl>ΔQ0, the process proceeds to step 707. That is, only when a stable state in which the change 1ΔQ1 in the intake air amount Q is small, that is, a steady state, continues for CNTo X 1 s, the process proceeds to step 708, and in other cases, the process also proceeds to step 716.

In other words, the output of the downstream 02 sensor 15 is determined in steps 702 to 7 for steady state determination, taking into account the delay in exhaust gas transport and the reaction delay due to the 02 storage effect of the three-way catalyst.
07 is provided. In addition, to explain the 02 storage effect of a three-way catalyst, a three-way catalyst has NOX, CO
, HC at the same time, and the purification rate η is shown in Figure 8. On the rich side of the stoichiometric air-fuel ratio (λ = 1), the purification rate of NO is large, and on the lean side, the purification rate of CD/HC is large.
The purification rate is high. At this time, the three-way catalyst takes in 0□ when the air-fuel ratio is lean, takes in CD and HC when the air-fuel ratio becomes rich, and reacts with the 0□ taken in when the air-fuel ratio is lean. Provides a storage effect.

When the change ΔQ in the intake air amount Q becomes stable as described above, the process proceeds to step 708, where the comparison voltage VR2 is calculated. That is, the intake air amount data Q is read out from the RAM 105 and a one-dimensional map stored in the ROM 104 is used to interpolate and calculate the comparison voltage V12. Here, the intake air amount Q
If the intake air amount Q is large, the comparison voltage VR2 is made large, and conversely, if the intake air amount Q is small, the comparison voltage Va□ is made small. Therefore, the output characteristics of the 02 sensor are shown as shown in Figure 9, so if the intake air amount Q is large, the comparison voltage VR2 is calculated to be rich, and conversely, if the intake air amount Q is small, the comparison voltage VR2 is calculated to be rich. The voltage VR2 will be calculated on the lean side. In addition,
When Q<Q, the element temperature of the downstream 0□ sensor 15 decreases, and when Q<Q2, the OTP increase range occurs, so in step 601, the air-fuel ratio feedback control condition is set as Q1≦Q≦Q2.

Also, at Q ” Q +, V112 is 0.2 to 0.4V
, and in Q ” Q 2, VIL2 is 0.6 to 0.8
It is considered to be V.

In addition to the intake air amount Q, the throttle valve opening and the intake pipe pressure can be used as the load, and the comparison voltage VR2 may be determined from a map or calculation of these loads and the engine speed.

In step 709, the output V2 of the downstream side 02 sensor 15 is
is A/D converted and taken in, and in step 710 it is determined whether V2 is less than or equal to the comparison voltage Vlk2, that is, it is determined whether the air-fuel ratio is rich or lean.

If V2≦V12 (lean) in step 710, proceed to steps 711 and 712, and on the other hand, Va>Vi+□
(rich), the process advances to steps 713 and 714.

In step 711, the second air-fuel ratio flag F2 is set to "0".
Then, in step 712, R2H-R3R+ΔR3 (constant value) is set, that is, the rich skip amount R3R is increased to shift the air-fuel ratio to the rich side.

On the other hand, when V2 > Vl12 (rich), the second air-fuel ratio flag F2 is set to "0" in step 713,
In step 714, R2H-R2H-ΔR3 is set, that is, the rich skip amount R8R is decreased to shift the air-fuel ratio to the lean side.

In step 715, the rich skip amount R3R calculated as described above is guarded, for example, with the minimum value MIN=2.5% and the maximum value MAX=7.5%. Note that the minimum value MIN is a value at a level that does not impair transient followability, and the maximum value MAX is a level at which drivability does not deteriorate due to air-fuel ratio fluctuations.

In step 717, the lean skip fiR3L is
Calculate as 3L -10%-R2H. That is, R3R+R3L=10%.

After the R2H calculated as described above is stored in the RAM 105, this routine ends in step 718.

FIG. 10 shows a catalyst deterioration determination routine, which is performed every predetermined period of time, for example, 4 ms. In step 1001, similarly to step 701 in FIG.
It is determined whether the closed loop condition for the downstream 0□ sensor 15 is satisfied, and the process proceeds to step 1002 only if the closed loop condition for the downstream 0□ sensor 15 is satisfied. In step 1002, the rotational speed data Ne is read from the RAM 105, and N≦Ne≦N, for example, 11.
000rl1≦Ne≦300Orpm (Determine whether it is in the D range or not, and in step 1003, read the intake air amount data Q from the RAM 105 and set Ql≦Q≦Q, for example, 0.511 /rev≦Q/Ne≦1.OR /rev
Determine whether it is within the range of . In other words, step 1 is only performed in steady state, excluding idle state, acceleration/deceleration state, fuel increase region, etc.
It is set to proceed to 004. Otherwise, proceed directly to step 1014.

In step 1004, the counter CT is incremented by +1,
In step 1005, it is determined whether a predetermined time period CTo X 4 ms has elapsed based on whether CT≦CTo.

11000rp≦Ne≦300Orpm and 0.51
/rev≦Q/Ne≦L If the state of O1/rev continues before the elapse of a predetermined time (CT≦CTo), step 1
006.

Proceeding to step 1007, the number of times the output V2 of the downstream O2 sensor 15 is reversed is counted by the number counter C8. That is,
In step 1006, it is determined whether or not the second air-fuel ratio flag F2 has been inverted, and in step 1007, the number counter CS is incremented by +1 each time the second air-fuel ratio flag F2 is inverted.

Next, 11000rpm≦Ne≦300Orpm and 0.
51! /rev≦Q/Ne≦1.01 When the state of /rev has passed for a predetermined period of time (CT > CT o), the flow of step 1005 proceeds to step 1008. As a result, in step 1008, it is determined whether the number of reversals C8 of the downstream 0□ sensor 15 is equal to or greater than a predetermined value C3o. C3<
If it is C3°, it is determined that there is no catalyst deterioration and step 1 is performed.
The alarm is stopped (or canceled) at step 009, and the alarm flag FAL14 is set to "0" at step 1010.
shall be. On the other hand, if C8≧C3o, it is determined that the catalyst has deteriorated, and the alarm 18 is activated in step 1011, and the alarm flag FALM is set to " in step 1012.
1". Then, in step 1013, the counter C
Clear both T and C3 and proceed to step 1014Q. Note that the alarm flag FALX is the backup RAM1
Therefore, by reading out the alarm flag FALM with a special reading device, the catalyst deterioration can be detected.
Thereby, catalyst exchange can be performed.

Note that the predetermined value C8o in step 1008 can also be made variable depending on the operating state parameter, for example, the load.

FIG. 11 also shows the catalyst deterioration determination routine, and the 10th
Unlike the case shown in the figure, catalyst deterioration is determined based on the ratio of the number of reversals of the output Vl of the upstream 02 sensor 13 and the number of reversals of the output V2 of the downstream 02 sensor 15. Step 1101
Steps 1001 to 1103 are the same as steps 1001 to 1003 in FIG. In steps 1104 and 1005, the counter C8 counts the number of inversions of the output V2 of the downstream 02 sensor 15, that is, the number of inversions of the second air-fuel ratio flag F2, and in steps 1006 and 1107, the output V1 of the upstream 0□ sensor 13 is counted. The counter CM counts the number of inversions of the first air-fuel ratio flag F1, that is, the number of inversions of the first air-fuel ratio flag F1. As a result, if CM≧200, step 110
The flow at step 8 proceeds to step 1109.

In step 1109, it is determined whether C8≧CM/2.

If C3<CM/2, it is determined that there is no catalyst deterioration and the alarm is stopped (or canceled) in step 1110,
Further, in step 1111, the alarm flag FALM is set to "0". On the other hand, if C8≧CM/2, it is determined that the catalyst has deteriorated, and the alarm 18 is activated in step 1112, and the alarm flag FA is activated in step 1113.
Set LM to "1". and steps 1114, 11
Proceed to step 15. In addition, in this case as well, the alarm flag FAL
M is stored in the backup RAM 106, so by reading out the alarm flag FAL14 with a special reading device, catalyst deterioration can be detected and the catalyst can be replaced.

In other words, normally, the output V1 of the upstream 02 sensor 13 is
Although it has a high frequency as shown in Figure (A), the output V2 of the downstream side 02 sensor 15 has a low frequency as shown in Figure 3 (B), but when the catalyst of the catalytic converter 12 deteriorates, the 02 storage effect decreases, downstream 0□
The frequency of the output v2 of the sensor 15 is as shown in Fig. 3 (C), (D
), the frequency approaches the output Vl of the upstream 0□ sensor 13. Therefore, catalyst deterioration is possible with the relative values of these two frequencies.

FIG. 12 shows an injection amount calculation routine, which is executed at every predetermined crank angle, for example, every 360° CA.

In step 1201, the intake air amount data Q, rotational speed, and data Ne are read out from the RAM 105 to calculate the basic injection amount TAUP. For example, TAUP←α・Q
/Ne (α is a constant). RA at step 1202
Read the cooling water temperature data THW from M105 and store it in ROM1.
The warm-up increase value FWL is determined by the one-dimensional map stored in 04.
Calculate by interpolation. As shown in the figure, this warm-up increase value FWL is set to decrease as the current cooling water temperature THW increases. In step 1203, the final injection amount TAU is calculated by TAU-TAUP.FAF.(FWL+β)+γ. Note that β and γ are correction amounts determined by other operating state parameters, such as a signal from a throttle position sensor (not shown), a signal from an intake air temperature sensor, battery voltage, etc. These are also correction amounts determined by the RAM 105. is stored in. Then,
At step 1204, the injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection. And step 1
The routine ends at 205. As described above, when the time corresponding to the injection amount TAU has elapsed, the flip-flop 109 is reset by the carrier signal of the down counter 108, and the fuel injection ends.

As mentioned above, if catalyst deterioration is determined,
Closed loop control by the downstream 02 sensor 15 can also be stopped, thereby preventing deterioration of emissions.

Furthermore, in the above embodiment, when making the comparison voltage Vi2 variable according to the intake air amount Q, the control constants, for example, the skip amounts R3R, R5L, can be divided into blocks for each intake air amount Q. This makes it possible to quickly approach the required air-fuel ratio even when the engine load changes to a different range, which helps prevent deterioration of fuel efficiency, drivability, and emissions due to control delays.

Furthermore, the first air-fuel ratio feedback control is performed every 4 ms, and the second air-fuel ratio feedback control is performed every IS. This is because the control by the downstream 0□ sensor, which has poor responsiveness, is performed as a secondary control.

In addition, other control constants in the air-fuel ratio feedback control by the upstream 0□ sensor, such as delay time, integral constant, and comparison voltage of the upstream 02 sensor (reference: JP-A-55-3
The present invention can also be applied to a double 0□ sensor system that corrects the air-fuel ratio (No. 7562) etc. using the output of the downstream 02 sensor, or a double 0□ sensor system that introduces a second air-fuel ratio correction coefficient.

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 the amount of fuel injected, but the present invention can also be applied to a carburetor type internal combustion engine. For example, an electric air control valve (EACV) is used to control the air-fuel ratio by adjusting the intake air amount of the engine; The present invention can also be applied to devices that control the air-fuel ratio by introducing atmospheric air into the slow system passage, and devices that adjust the amount of secondary air sent to the exhaust system of an engine. In this case, the basic injection amount TAU in step 1201
The basic fuel injection amount corresponding to P is determined by the carburetor itself, that is, it is determined according to the intake pipe negative pressure corresponding to the intake air amount and the rotational speed of the engine, and in step 1203, the supply corresponding to the final fuel injection amount TAU is determined. The amount of air 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, even when the degree of deterioration of the three-way catalyst is relatively small, the degree of deterioration can be accurately determined.

[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 exhaust emission characteristic diagram explaining the sensor system; FIG. 3 is a timing diagram explaining the problems to be solved by the present invention; FIG. 4 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 Figure 5, Figure 7, Figure 1O, Figure 11, Figure 12 are
Flowchart for explaining the operation of the control circuit in Figure 6, Figure 6 is a timing diagram for supplementary explanation of the flowchart in Figure 10, Figure 8 is a diagram for explaining the 02 storage effect, Figure 9 is a diagram for explaining the 02 storage effect.
□This is a graph showing the output characteristics of the sensor. 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 0□ sensor, 15...downstream 0□ sensor, 17...idle switch. Ox Figure 2 Diagram explaining the task A/F Figure 8 A/F Figure 9 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) for detecting the air-fuel ratio of the engine; a first comparing means for comparing the output of the upstream air-fuel ratio sensor with a first comparison voltage; and a load detecting means for detecting the load of the engine. and a comparison voltage calculation means for calculating a second comparison voltage according to the detected load; and a second comparison means for comparing the output of the downstream air-fuel ratio sensor with the second comparison voltage; air-fuel ratio adjusting means for adjusting the air-fuel ratio of the engine according to the comparison results of the first and second comparing means; and reversal number calculating means for calculating the number of times the output of the downstream air-fuel ratio sensor is reversed per predetermined time. and a catalyst deterioration determining means for determining that the catalyst of the catalytic converter has deteriorated when the number of reversals is equal to or greater than the predetermined value.