JPH07116931B2 - Device for determining catalyst deterioration of internal combustion engine - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention controls the air-fuel ratio of an engine to a stoichiometric air-fuel ratio based on the output of an air-fuel ratio sensor provided at least in an exhaust passage upstream of a catalytic converter, and at least the downstream side of the catalytic converter. The present invention relates to a catalyst deterioration determination device for an internal combustion engine, which determines deterioration of a catalytic converter based on an output of an air-fuel ratio sensor provided in an exhaust passage.

[0002]

2. Description of the Related Art Conventionally, an air-fuel ratio sensor (O ₂ sensor) is provided in an exhaust system of an internal combustion engine, and the air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio by the output of this sensor, so that a catalytic converter provided in the exhaust system can be operated. Techniques for effectively utilizing the purifying ability to improve emission characteristics are well known. In recent years, the time change of the characteristics of the upstream O ₂ sensor to accurately compensate, even downstream of the catalytic converter provided with the O ₂ sensor is used for feedback control, so-called double O ₂ sensor A system has also been developed (see Japanese Patent Laid-Open No. 61-286550).

Even in such a technique, when the catalytic converter is deteriorated, the purifying ability of the components such as HC, CO, NOx in the exhaust gas is deteriorated, so that it is necessary to detect the deterioration of the catalytic converter and various catalysts are required. Deterioration determination methods and devices have been proposed. For example, when the catalyst deteriorates, the inversion cycle of the output of the downstream O ₂ sensor during air-fuel ratio feedback (the cycle of going up and down (or crossing) the theoretical air-fuel ratio equivalent value) becomes shorter, so the downstream O ₂ The ratio of the reversal cycle of the output of the sensor (or the number of reversals crossing the theoretical air-fuel ratio equivalent value) and the reversal cycle of the output of the upstream O ₂ sensor (or the number of reversals crossing the theoretical air-fuel ratio equivalent value) is calculated, and this ratio A method for discriminating the deterioration of the catalyst has been proposed (Japanese Patent Laid-Open Nos. 61-286550 and 63-98).
7852).

[0004]

In the above-mentioned double O ₂ sensor system, the control air-fuel ratio (average air-fuel ratio, central air-fuel ratio) resulting from the deviation of the characteristics of the upstream O ₂ sensor and the air-fuel ratio variation among the cylinders. center value for compensating using the air-fuel ratio correction value based on the output of the downstream O ₂ sensor deviation or the like from the stoichiometric air-fuel ratio, the air-fuel ratio characteristic of the upstream O ₂ sensor is feedback-controlled to change ( Even if the (average value) deviates from the stoichiometric air-fuel ratio, normally, the deviation of the air-fuel ratio from the stoichiometric air-fuel ratio is gradually compensated, and the center of the air-fuel ratio converges near the stoichiometric air-fuel ratio.

However, in order to stabilize the control system, or
It is possible to prevent incorrect overcorrection of the air-fuel ratio for any reason.
In order to achieve the above, the limit value (guard
Value) is often set. For this reason, the principle of air-fuel ratio
The deviation from the air-fuel ratio is downstream O ₂Based on sensor output
Increased beyond the range that can be compensated by the air-fuel ratio correction value
In case of downstream O₂Air-fuel ratio correction based on sensor output
When the value reaches the guard value above, the deviation from the theoretical air-fuel ratio
Is not compensated any more, and the engine is out of the stoichiometric air-fuel ratio
It will be driven at a ratio.

[0006] Although in normal operation does not occur such a situation, for example, cracking the upstream O ₂ sensor and or when abnormality such as disconnection has occurred, the upstream O ₂ sensor is susceptible specific cylinder arrangement on impact When an abnormality occurs in the fuel injection valve and the fuel injection amount becomes too large or too small, the output of the upstream O ₂ sensor does not reflect the actual air-fuel ratio, and the air-fuel ratio control is performed in a region that deviates greatly from the theoretical air-fuel ratio. As this happens, this can happen.

The air-fuel ratio feedback control center is theoretical air-fuel
If the ratio deviates significantly, the catalyst downstream O ₂At the position of the sensor
The exhaust air-fuel ratio should always remain on the rich side or the lean side.
Yes Downstream side O₂Sensor output is rich side output or lean side
The output is held as it is, and the output is not inverted. Above
Downstream of O₂Determining catalyst deterioration from the sensor output inversion cycle
In this case, the output does not reverse, so
Even if the catalyst is deteriorated,
It will be erroneously judged that it has not been made
There is a risk of delay in catalyst replacement at the time of inspection.

In order to prevent this, for example, the catalyst deterioration determination may be stopped if the next reversal does not occur even after a lapse of a predetermined time after the reversal of the downstream side air-fuel ratio sensor output. . However, the reversal cycle of the output of the downstream side air-fuel ratio sensor may fluctuate depending on the operating conditions and the like, so depending on the setting of the above-mentioned predetermined time, the deterioration determination is stopped even though the deterioration determination should be made. Cases can arise.

In view of the above problems, it is an object of the present invention to provide a catalyst deterioration determination device capable of accurately detecting a situation where catalyst deterioration determination should be stopped and preventing erroneous determination.

[0010]

In order to solve the above problems, according to the present invention, there is provided a catalyst deterioration determination device having the configuration of FIG. That is, an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor that detect the air-fuel ratio of the engine exhaust are respectively provided in the upstream and downstream exhaust passages of the three-way catalyst CC _RO provided in the exhaust passage of the internal combustion engine. It is provided.

Further , based on the output of the downstream side air-fuel ratio sensor,
A constant that calculates the constant used for air-fuel ratio feedback control
Calculation means, the calculated constant and the upstream air-fuel ratio sensor
Feedback control of the air-fuel ratio of the engine based on
And an air-fuel ratio feedback control means.
Furthermore, the catalyst deterioration determination means is an air-fuel ratio feedback control.
Of the downstream side air-fuel ratio sensor within a predetermined period during execution.
The area surrounded by the output waveform and the predetermined value, and the downstream
At least one of the trace length of the output waveform of the side air-fuel ratio sensor
The deterioration of the catalyst is determined based on the above.

The prohibiting means is calculated by the constant calculating means.
Value of the constant used for the air-fuel ratio feedback control
Has reached a predetermined limit value (guard value), and if the guard value has been reached, the catalyst deterioration determination means prohibits the catalyst deterioration determination.

[0013]

The operation of the above means will be described with reference to FIG. FIG. 2A shows the output VOM of the upstream air-fuel ratio sensor. During the air-fuel ratio feedback control, the VOM alternates between the rich side and the lean side centering on the theoretical air-fuel ratio equivalent output. 2 (B) and 2 (C) show the characteristics of the upstream side air-fuel ratio sensor and the variation of the air-fuel ratio among the cylinders, and the downstream side air-fuel ratio sensor when the air-fuel ratio control center is near the theoretical air-fuel ratio. The output VOS is shown, FIG. 2 (B) shows the case where the catalyst is not deteriorated, and FIG. 2 (C) shows the case where the catalyst is deteriorated.

As can be seen from FIGS. 2B and 2C, when the catalyst is not deteriorated, the air-fuel ratio fluctuation on the downstream side of the catalyst is gentle due to the O ₂ storage effect of the catalyst. The inversion cycle between the rich side and the lean side of the output VOS is long (FIG. 2 (B)). However, when the catalyst deteriorates and the O ₂ storage effect decreases, the inversion cycle of VOS becomes shorter and gradually approaches the inversion cycle of the upstream side air-fuel ratio sensor output VOM (FIG. 2 (c)).

Therefore, the deterioration of the catalyst can be determined by monitoring the reversal period of the downstream side air-fuel ratio sensor and the locus length described later. Next, in FIGS. 2 (D) to (G), the characteristics of the upstream side air-fuel ratio are deviated, or the air-fuel ratio varies among the cylinders, so that the center side of the air-fuel ratio feedback control greatly deviates from the theoretical air-fuel ratio. Air-fuel ratio sensor output VO
The change in S is shown.

When the output characteristic of the upstream side air-fuel ratio sensor is greatly deviated as described above, for example, the upstream side air-fuel ratio sensor outputs the lean side output even though the actual engine air-fuel ratio is on the rich side. Occasionally, it will happen. In this case, the air-fuel ratio feedback control means controls the engine air-fuel ratio further to the rich side based on the output of the upstream side air-fuel ratio sensor, so that the control center of the air-fuel ratio shifts significantly to the rich side from the theoretical air-fuel ratio. .

Normally, such a deviation is compensated by the air-fuel ratio correction based on the output of the downstream side air-fuel ratio sensor, but the deviation of the characteristic is large and the downstream side air-fuel ratio correction amount reaches the limit value (guard value). After that, no further correction is performed, and the engine air-fuel ratio is maintained in a state of being significantly richer than the stoichiometric air-fuel ratio. Further, when the characteristics of the upstream side air-fuel ratio sensor largely deviate to the lean side, the engine air-fuel ratio is maintained in a state of being largely deviated to the lean side from the theoretical air-fuel ratio for the same reason.

FIGS. 2 (D) and 2 (E) show the downstream side air-fuel ratio sensor output VOS when the air-fuel ratio control center is largely shifted to the rich side.
2 shows the case where the catalyst is not deteriorated (FIG. 2 (D)) and the case where it is deteriorated (FIG. 2 (E)). 2 (F) and 2 (G) show that the catalyst is not deteriorated when the center of the air-fuel ratio control is largely shifted to the lean side (FIG. 2 (F)) and the catalyst is deteriorated (FIG. 2 (F)).
(G)) shows the downstream side air-fuel ratio sensor output VOS.

Since the center of the air-fuel ratio control is out of the stoichiometric air-fuel ratio, the downstream side air-fuel ratio sensor output VOS is fixed to the rich side or the lean side and does not reverse regardless of whether the catalyst is deteriorated. If the catalyst deterioration is detected by the cycle, it is erroneously determined that the catalyst is not deteriorated even if the catalyst is deteriorated. Further, particularly when an O ₂ sensor is used as the air-fuel ratio sensor, the sensor output changes greatly in the vicinity of the stoichiometric air-fuel ratio in accordance with the change in the air-fuel ratio, but the air-fuel ratio slightly fluctuates in the region greatly deviating from the stoichiometric air-fuel ratio. However, the sensor output does not show a large fluctuation.

For this reason, when the air-fuel ratio control center deviates greatly from the stoichiometric air-fuel ratio, the fluctuation range of the output VOS becomes very small even if the catalyst deteriorates, and the catalyst deterioration due to the locus length of the downstream side air-fuel ratio sensor output VOS described later. Even if the detection is performed, it may be difficult to accurately detect the deterioration, and an erroneous determination may be made. Further, even if it is determined that the catalyst has deteriorated when the ratio of the locus length of the downstream side air-fuel ratio sensor output to the locus length of the upstream side air-fuel ratio sensor output is a predetermined value or more, the downstream side air-fuel ratio correction amount is guarded. When the value is limited, the value of this ratio comes to be close to the case where the catalyst is deteriorated and the case where the catalyst is not deteriorated, and therefore the same erroneous determination may be made.

As described above, when the catalyst downstream side air-fuel ratio sensor output does not show the behavior according to the degree of deterioration of the catalyst, the catalyst deterioration determination method using the catalyst downstream side air-fuel ratio sensor output makes an erroneous determination. May occur. The present invention provides that the air-fuel ratio correction amount calculated based on the downstream side air-fuel ratio sensor output VOS has reached the guard value, that is,
By detecting that the output VOS of the downstream side air-fuel ratio sensor is in the state shown in FIG. 2 ((D) to (G)) and stopping the catalyst deterioration determination, erroneous determination is prevented.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is an overall schematic view showing an embodiment of an air-fuel ratio control system for an internal combustion engine according to the present invention. In FIG.
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, and has, for example, a potentiometer built therein and generates an output signal of an analog voltage proportional to the intake air amount. This output signal is provided to the A / D converter 101 with a built-in multiplexer of the control circuit 10. The distributor 4 includes a crank angle sensor 5 whose axis generates a reference position detection pulse signal at every 720 ° converted to a crank angle, and a reference position detection pulse signal at every 30 ° converted to a crank angle. A crank angle sensor 6 for generating is provided. These crank angle sensors 5, 6
Is 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 the fuel supply system to the 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 generates an electric signal of analog voltage according to the temperature THW of the cooling water. This output is also supplied to the A / D converter 101.

The exhaust system downstream of the exhaust manifold 11 has an O ₂ storage effect for purifying three toxic components HC, CO and NOx in the exhaust gas at the same time. A catalytic converter 12 that accommodates a three-way catalyst having a function of performing) is provided. The exhaust manifold 11 includes a catalytic converter 12
A first O ₂ sensor 13 is provided on the upstream side of, and a second O ₂ sensor 15 is provided on the exhaust pipe 14 on the downstream side of the catalytic converter 12. The O ₂ sensors 13 and 15 generate electric signals according to the concentration of oxygen components in the exhaust gas. That is, the O ₂ sensors 13 and 15 output different output voltages to the A / D converter 101 of the control circuit 10 depending on whether the air-fuel ratio is leaner or richer than the stoichiometric air-fuel ratio. The control circuit 10 is configured as, for example, a microcomputer, includes an A / D converter 101, an input / output interface 1
02, CPU104, ROM104, RAM10
5, backup RAM 106, clock generation circuit 10
7 etc. are provided.

Further, the throttle valve 16 of 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. The 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 during deceleration or idling to reduce HC and CO emissions.

Reference numeral 19 is an alarm activated when the three-way catalyst of the catalytic converter 12 is deteriorated, and reference numeral 20 is an alarm activated when the catalyst deterioration determination is stopped. 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 the 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 finally its output terminal becomes the "1" level, the flip-flop 109 is set and the drive circuit 110 is provided with the fuel injection valve 7. Stop the momentum. That is, the fuel injection valve 7 is the same as the fuel injection amount TAU described above.
Is energized, and therefore, an amount of fuel corresponding to the fuel injection amount TAU is sent to the combustion chamber of the engine body 1.

It should be noted that the interrupt generation of the CPU 103 is A /
For example, when the input / output interface 102 receives the pulse signal of the crank angle sensor 6 after the A / D conversion of the D converter 101 is completed. The intake air amount data Q and the cooling water temperature data THW of the air flow meter 3 are fetched by an A / D conversion routine executed at a predetermined time or at a predetermined crank angle and stored in a predetermined area of the RAM 105.
That is, the data Q and THW in the RAM 105 are updated every predetermined time. Also, the rotation speed data N
e is calculated by interruption of the crank angle sensor 6 every 30 ° CA and is stored in a predetermined area of the RAM 105.

The operation of the control circuit shown in FIG. 3 will be described below.
4 and 5 show an 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, which is executed every predetermined time, for example, 4 ms. At step 401, it is judged if the closed loop (feedback) condition of the air-fuel ratio by the upstream O ₂ sensor 13 is satisfied. For example, when the cooling water temperature is equal to or lower than a predetermined value, the output signal of the upstream O ₂ sensor 13 is provided during engine startup, after startup increase, during warmup increase, power increase, fuel injection amount increase to prevent catalyst overheating. Is not reversed at all, 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 routine proceeds to step 425, the air-fuel ratio feedback flag XMFB is set to "0", and the routine proceeds to step 426. The air-fuel ratio correction coefficient FAF is 1.
It may be 0. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 402.

In step 402, the upstream O ₂ sensor 1
A / D conversion of the output VOM of 3 is taken in, and step 40
At 3, it is determined whether the air-fuel ratio is rich or lean depending on whether VOM is equal to or lower than the comparison voltage V _R1 . The comparison voltage V _R1 is normally O
₂ The voltage at the center of the amplitude of the sensor output, which is V in this embodiment.
_R1 = 0.45V. If the air-fuel ratio is lean in step 403, that is, if VOM ≦ V _R1, it is determined in step 404 whether or not the delay counter CDLY is positive, and CDL is set.
If Y> 0, CDLY is set to 0 in step 405,
Go to step 406. In step 406, the delay counter CDLY is decremented by 1, and steps 407 and 408 are executed.
The delay counter CDLY is guarded by the minimum value TDL. In this case, the delay counter CDLY has the minimum value T
When DL is reached, the air-fuel ratio flag F1 is set to "0" (lean) in step 409. The minimum value TDL
Is a lean delay state for holding the determination that it is in the rich state even if the output of the upstream O ₂ sensor 13 changes from rich to lean, and is defined by a negative value. On the other hand, if rich (VOM> V _R1 ), it is determined in step 410 whether the delay counter CDLY is negative, and if CDLY <0, in step 411 CDLY.
Is set to 0 and the process proceeds to step 412. In step 412, the delay counter CDLY is incremented by 1, and step 41
In 3,414, the delay counter CDLY is set to the maximum value TD.
Guard with R. In this case, the delay counter CDLY
When reaches the maximum value TDR, the air-fuel ratio flag F1 is set to "1" (rich) in step 415. 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 when the output of the upstream O ₂ sensor 13 changes from lean to rich, and is defined by a positive value. It

Next, at step 416, it is judged if the sign of the air-fuel ratio flag F1 is reversed, that is, if the air-fuel ratio after the delay processing is reversed. If the air-fuel ratio is reversed, at step 417, the air-fuel ratio flag F
The value of 1 determines whether the reversal from rich to lean or the reversal from lean to rich. If it is the reverse from rich to lean, in step 418 FAF ← FAF +
RSR is skipped and increased, and conversely, if lean to rich inversion, in step 419, FAF ← F
AF-RSL and skip-like reduction. That is, skip processing is performed.

If the sign of the air-fuel ratio flag F1 is not reversed in step 416, steps 420, 421, 4
At 22, the integration process is performed. That is, in step 420, it is determined whether or not F1 = "0". If F1 = "0" (lean), in step 421 FAF ← FAF + KI.
If R1 and F1 = "1" (rich), FAF ← FAF-KIL is set in step 422. here,
The integration constants KIR and KIL are set sufficiently smaller than the skip amounts RSR and RSL, that is, KIR (KI
L) <RSR (RSL). Therefore, step 42
1 is the lean state (F1 = “0”) and gradually increases the fuel injection amount, and step 422 is the rich state (F1 =
In "1"), the fuel injection amount is gradually reduced.

Next, in step 423, step 42
The air-fuel ratio correction coefficient FAF calculated at 3,424,426,427 is guarded at the minimum value, for example 0.8, and is also guarded at the maximum value, for example 1.2. Thus, when 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 by the value to prevent overrich or over lean.

In step 424, the air-fuel ratio feedback flag XMFB is set to "1", the FAF calculated as described above is stored in the RAM 105, and this loop ends in step 426. Next, the present invention is applied to the output V ₁ of the upstream O ₂ sensor 13 and the output V ₁ of the downstream O ₂ sensor 15.
A case where the invention is applied to a double O ₂ sensor system that performs air-fuel ratio feedback control using both of the _two will be described.

FIG. 6 is based on the flowcharts of FIGS. 4 and 5.
FIG. 8 is a timing chart for supplementarily explaining the operation. Upstream O₂SE
As shown in FIG. 6 (A), the output VOM of the sensor 13
If the air-fuel ratio signal A / F for the determination of
The ray counter CDLY has a threshold value as shown in FIG. 6 (B).
The count is incremented in the lean state and the count is counted in the lean state.
Will be released. As a result, as shown in FIG.
The calculated air-fuel ratio signal A / F '(corresponding to flag F1)
Is made. For example, time t₁At air-fuel ratio signal A / F '
Even if changes from lean to rich, delayed air-fuel
The ratio signal A / F 'is kept lean for the rich delay time TDR.
Time t after being held₂Changes to rich. Time t₃
The air-fuel ratio signal A / F changes from rich to lean
Also, the delayed air-fuel ratio signal A / F 'is
At the time t after being held rich for a period (-TDL)
_FourChanges to lean. However, when the air-fuel ratio signal A / F is
Tick t _Five, T₆, T₇From rich delay time TDR
If it is reversed in a short period, the delay counter CDLY will
It takes time to reach the maximum TDR, and as a result, the time
t₈At A, the air-fuel ratio signal A / F 'after the delay processing is inverted.
It That is, the air-fuel ratio signal A / F ′ after the delay processing is delayed by the delay processing.
It becomes more stable than the air-fuel ratio signal A / F before the fact. like this
Based on the stable air-fuel ratio signal A / F 'after delay processing
As a result, the air-fuel ratio correction coefficient FAF shown in FIG. 6 (D) is obtained.

Next, the second air-fuel ratio feedback control by the downstream O ₂ sensor 15 will be described. The second air-fuel ratio feedback control includes skip amounts RSR and RSL as first air-fuel ratio feedback control constants.
Integration constants KIR, KIL, delay times TDR, TDL, or a comparison voltage V of the output VOM of the upstream O ₂ sensor 13
System that makes _R1 variable and the second air-fuel ratio correction factor FA
There is a system that introduces F2.

For example, if the rich skip amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and even if the lean skip amount RSL is decreased, the control air-fuel ratio can be shifted to the rich side, while the lean skip amount RSL is increased. Is increased, the control air-fuel ratio can be shifted to the lean side, and even if the rich skip amount RSR is decreased, the control air-fuel ratio can be shifted to the lean side. Therefore, the downstream O ₂ sensor 15
The air-fuel ratio can be controlled by correcting the rich skip amount RSR according to the output of In addition, the rich integration constant KI
When R is increased, the control air-fuel ratio can be shifted to the rich side,
Further, even if the lean integration constant KIL is reduced, the control air-fuel ratio can be shifted to the rich side, while if the lean integration constant KIL is increased, the control air-fuel ratio can be shifted to the lean side and the rich integration constant KIR is reduced. However, the control air-fuel ratio can be shifted to the lean side. Therefore, the downstream O ₂ sensor 15
The air-fuel ratio can be controlled by correcting the rich integration constant KIR and the lean integration constant KIL according to the output of
If the rich delay time TDR is set large or the lean delay time (-TDL) is set small, the control air-fuel ratio can shift to the rich side, and conversely, the lean delay time (-TDL) is set large or the rich delay time (TDR) is set. If it is set small, the control air-fuel ratio can shift to the lean side. That is, the delay time TD depends on the output VOS of the downstream O ₂ sensor 15.
The air-fuel ratio can be controlled by correcting R and TDL.
Furthermore, if the comparison voltage V _R1 is increased, the control air-fuel ratio can be shifted to the rich side, and if the comparison voltage V _R1 is decreased, the control air-fuel ratio can be shifted to the lean side. Therefore, the air-fuel ratio can be controlled by correcting the comparison voltage V _R1 according to the output VOS of the downstream O ₂ sensor 15.

These skip amount, integration constant, delay time,
It is advantageous in that the comparison voltage is variable by the downstream O ₂ sensor. For example, if the delay time is variable, it is possible to adjust the air-fuel ratio very delicately, and if the skip amount is variable, a control with good response without increasing the air-fuel ratio feedback cycle like the delay time is possible. Is possible. Therefore, these variable amounts can naturally be used in combination of two or more.

Next, a double O ₂ sensor system in which the skip amount as the air-fuel ratio feedback control constant is variable will be described. 7 and 8 show the downstream O ₂ sensor 15
Is a second air-fuel ratio feedback control routine based on the output VOS of the above, and is executed every predetermined time, for example, 512 ms. In steps 701 to 706, the downstream O ₂ sensor 15 determines whether or not the closed loop condition is satisfied. For example, when the closed-loop condition is not satisfied by the upstream O ₂ sensor 13 (step 701) and the cooling water temperature THW is a predetermined value (for example, 70 ° C.) or less (step 702),
When the throttle valve 16 is fully closed (LL = "1") (step 703), the rotation speed Ne, the vehicle speed, the idle switch 1
When the secondary air is introduced based on the signal LL of 7 and the cooling water temperature THW (step 704) and when the load is light (Q / Ne <X ₁ ) (step 705), the downstream O ₂ sensor 15 If not activated (step 706),
Etc., the closed loop condition is not satisfied, and in other cases, the closed loop condition is satisfied. If the closed loop condition is not satisfied, the routine proceeds to step 719, where the air-fuel ratio feedback flag XS
FB is reset ("0"), and if the closed loop condition is satisfied, the routine proceeds to step 708, where the air-fuel ratio feedback flag XS
Set FB ("1").

The flow of steps 709 to 718 will be described. In step 709, the output VOS of the downstream O ₂ sensor 15 is A / D converted and fetched, and in step 710.
At VOS, the comparison voltage V _R2 (for example, V _R2 = 0.55
V) or less, that is, it is determined whether the air-fuel ratio is rich or lean. The comparison voltage V _R2 is more than the comparison voltage V _R1 of the output of the upstream O ₂ sensor 13 in consideration of the difference in output characteristics due to the influence of raw gas and the deterioration speed on the upstream and downstream sides of the catalytic converter 12. Although it is set high, this setting may be arbitrary. (For example, V
(0.45V same as _R1 ) As a result, if VOS ≦ V _R2 (lean), proceed to steps 711, 712 and 713,
If VOS> V _R2 (rich), steps 714 and 71
Proceed to 5,716. That is, in step 711, R
SR ← 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 712 and 713, the RSR is set to the maximum value M.
AX (9%) is guarded, while RSR ← RSR-ΔRS is set in step 714, that is, the rich skip amount RSR is decreased to shift the air-fuel ratio to the lean side, and RSR is set in steps 715 and 716. Minimum value MIN (1
%) To guard. It should be noted that the minimum value MIN is a value at which the transient followability is not impaired, and the maximum value MA
X is a value at a level at which drivability deterioration does not occur due to air-fuel ratio fluctuations.

At step 717, the lean skip amount R
Let SL be RSL ← 10% −RSR. That is, RSR + RSL = 10%. In step 718, the skip amounts RSR and RSL are stored in the RAM1.
It stores in 05. Then, the process proceeds to step 720.

In this way, the guard value (MAX, M
As a result of providing (IN), after RSR reaches the guard value,
Further air-fuel ratio correction by the downstream O ₂ sensor output is not performed. FIG. 9 is an injection amount calculation routine, which is executed at a predetermined crank angle, for example, 360 ° CA. In step 901, the intake air amount data Q and the rotational speed data Ne are read from the RAM 105 and the basic injection amount TAUP (injection time for obtaining the theoretical air-fuel ratio) is calculated. For example TAU
Let P ← α · Q / Ne (α is a constant). Step 902
Then, the final injection amount TAU, TAU ← TAUP ・ FAF
・ Calculate by β + γ. Note that β and γ are correction amounts that are determined by other operating state parameters. Next, at step 903, the injection amount TAU is reduced by the down counter 108.
And the flip-flop 109 is set to start fuel injection. Then, in step 904, 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 output signal of the down counter 108 and the fuel injection ends. 10 and 11 show a catalyst deterioration determination routine, which is executed every predetermined time, for example, 4 ms.
In step 1001, the air-fuel ratio feedback flag X
It is determined whether or not the air-fuel ratio feedback control by the output VOM of the upstream O ₂ sensor 13 is being performed (XMFB = "1") by MFB, and in step 1002, the output VOM of the upstream O ₂ sensor 13 is lean by the lean monitor. It is determined whether or not it has been stuck for a predetermined period of time or more, and step 100
At 3, the rich monitor determines whether or not the output VOM of the upstream O ₂ sensor 13 is stuck to the rich side for a predetermined period or longer. At step 1004, the output of the downstream O ₂ sensor 15 is determined by the air-fuel ratio feedback flag XSFB. During air-fuel ratio feedback control by VOS (XSFB =
It is determined whether it is "1").

As a result, the output V of the upstream O ₂ sensor 13
During air-fuel ratio feedback control by OM (XMFB =
“1”), the output VOM is not stuck to the lean side or the rich side, and the air-fuel ratio feedback control is being performed by the output VOS of the downstream O ₂ sensor 15 (XSFB.
= “1”) only, step 1005 and subsequent steps in FIG. 11 are executed.

Steps 1005 and 1006 determine whether the skip amount RSR calculated based on the output of the downstream O ₂ sensor has reached the guard value MAX or MIN. That is, in step 1005, it is determined whether or not RSR has reached MAX (9%), and in step 1006 whether or not RSR has reached MIN (1%). Only when MIN <RSR <MAX. In step 1007, the catalyst deterioration determination described later is executed.

When RSR reaches MAX or MIN, steps 1009 to 1015 are executed. In step 1009, a counter CT ₁ used for catalyst deterioration determination and a deterioration determination parameter (CS,
VOM _i , VOS _i, etc.) are reset and step 10
At 10, the counter CT ₂ is incremented by +1.

Next, at step 1011 the determination period T ₂
If CT ₂ > T ₂ , then the judgment stop alarm flag ALMS is set (“1”) in step 1012, and the judgment stop alarm 20 is set in step 1013.
To notify that the catalyst deterioration determination has been stopped. Next, at step 1014, the alarm flag ALMS is stored in the backup RAM RAM 106 for repair and inspection, the counter CT ₂ is reset at step 1015, and the routine ends. Even when the catalyst deterioration determination is executed, CT ₂ is reset in step 1008.

As a result, RSR becomes the guard value MAX or M.
When it reaches IN, the catalyst deterioration determination is stopped, an erroneous determination is prevented, a deterioration determination stop alarm is issued, and the abnormality is notified to the driver. Next, step 100
The catalyst deterioration determination subroutine shown in 7 will be described.

FIG. 12 shows the catalyst deterioration determination when the number of reversals of the downstream O ₂ sensor output VOS is used. When the subroutine starts in FIG. 12, the counter CT ₁ is incremented by +1 in step 1201, it is determined in step 1202 whether or not a predetermined determination period T ₁ has elapsed, and if CT ₁ <T ₁ , the process proceeds to step 1203. Whether or not the output VOS of the advanced downstream side air-fuel ratio sensor 15 is reversed from the rich side (VOS ≧ _VR2 ) to the lean side (VOS < _VR2 ) or from the lean side to the rich side as compared with the time when the previous routine is executed. If it is determined that VOS is inverted, the counter CS is incremented by +1 in step 1204. The counter CS is a counter for counting the number of VOS inversions within a predetermined time.

When the determination period T ₁ has elapsed in step 1202, the process proceeds to step 1205, and it is determined from the value of the counter CS whether or not the catalyst has deteriorated. That is, when the number of VOS inversions within the determination period T ₁ is equal to or greater than the predetermined value CS ₀ , it is considered that the state is as shown in FIG. 2C, so it is determined that the catalyst has deteriorated, and the alarm flag ALM is determined in step 1206. Is set ("1"), and the catalyst deterioration alarm 19 is activated in step 1207 to notify that the catalyst has deteriorated. In step 1205, CS <
If it is CS ₀ , it is considered that the state is as shown in FIG. 2 (B), so it is determined that there is no catalyst deterioration, the alarm flag ALM is reset (“0”) in step 1208, and step 12
The alarm is deactivated at 09. Step 121 after the above execution
At 0, it indicates that the catalyst deterioration determination is executed, so the determination stop alarm flag ALMS is reset (“0”) and the determination stop alarm 20 is deactivated, and at step 1211, the flags ALM and ALMS are set for repair and inspection. The counter is stored in the backup RAM RAM 106, and the counters CT ₁ and CS are cleared in step 1212, and then the subroutine ends. The determination period T of the upstream air-fuel ratio sensor output VOM
_The number of VOM inversions within ₁ is also counted by another counter CM, and when (CS / CM) is a predetermined value or more, it can be determined that the catalyst has deteriorated.

13 and 14 are flow charts showing another embodiment of the catalyst deterioration determination. In this embodiment, the upstream side O
₂ The locus length LVOM of the output VOM of the sensor 13 and the downstream side O
_{2 The} catalyst deterioration determination is performed using the locus length LVOS of the output VOS of the sensor 15. As shown in FIGS. 2 (B) and 2 (C), when the air-fuel ratio control center is not largely deviated from the stoichiometric air-fuel ratio, the downstream side O ₂ sensor output VOS has a large fluctuation range as the catalyst deteriorates, and The fluctuation cycle becomes shorter. Therefore, the trace length of the VOS waveform after catalyst deterioration (Fig. 2 (C)) is
It becomes larger than when the catalyst is not deteriorated (FIG. 2 (B)).
Therefore, the deterioration of the catalyst can be determined by monitoring the VOS trajectory length.

In this embodiment, the downstream side O₂Sensor output VOS
Trajectory length LVOS and upstream O₂Sensor output VOM trajectory
The catalyst is inferior when the ratio with the long LVOM exceeds a specified value.
It is determined that it has become. In FIG. 13, the subroutine is
Then, in step 1301, the upstream side O ₂Sensor 13
The output trajectory of the output VOM is approximately LVOM ← L
VOM + | VOM-VOM_i-1 | Is calculated. This
This is VOM_iIs the upstream O when the previous routine was executed₂Sensor
This is the output (see FIG. 18). Then in step 1302
Is VOM for the next routine execution_i-1 ← Update VOM
Is done.

At steps 1303 and 1304, the locus length LVOS of the downstream O ₂ sensor output VOS is calculated and VOS _i-1 is updated in the same manner as described above. Next, in FIG. 14 and step 1305, the counter CT ₁ is incremented by +1 and in step 1306 it is determined whether or not CT ₁ has exceeded the predetermined determination period T ₁ . When CT ₁ > T _1, it is determined in step 1307 whether the ratio of LVOS to LVOM is a predetermined value K or more, and LVOS / LVOM is determined.
If ≧ K, it is determined that the catalyst has deteriorated, and step 130 is performed.
At 8, 1309, the alarm flag ALM is set and the catalyst deterioration alarm is activated. Also LVOS / LVOM
If <K, the alarm flag ALM is reset (step 1310) and the catalyst deterioration alarm is deactivated (step 1311).

After completion of the above operation, the alarm flag ALM
As in the embodiment of FIG. 12, resetting S, deactivating the determination stop alarm 20 (step 1312), storing in the backup RAM RAM 106 (step 1313), and clearing parameters (step 1314) are performed. In the present embodiment, the downstream O ₂ sensor output VOS
The catalyst deterioration is determined using the ratio of the locus length LVOS of the above and the locus length LVOM of the upstream O ₂ sensor output VOM, but the LVOS is determined using only the locus length LVOS of the downstream O ₂ sensor output VOS. It may be determined that the catalyst has deteriorated when it becomes equal to or more than a predetermined value.

Next, another example of catalyst deterioration determination is shown in FIGS.
An example of is shown. In this embodiment, the above-mentioned trajectory length LVOS,
In addition to LVOM, VOS and VOM are compared with each other.
Pressure V _R2 , V_R1Area surrounded by and AVOS, AVO
By using the M
It enables more accurate determination. Shown in Figure 2 (B)
If the catalyst is not deteriorated, the downstream side O₂Sensor out
The trajectory length LVOS of the force VOS is relatively small, but
The area AVOS surrounded by the comparison voltage is relatively large.
It

On the other hand, when the catalyst is deteriorated as shown in FIG. 2C, the locus length LVOS becomes relatively large as described above, while the area AVOS becomes relatively small. Therefore, the presence or absence of catalyst deterioration can be more accurately determined by monitoring the area AVOS together with the track length LVOS. Details of these are described in Japanese Patent Application No. 3-331810 filed by the applicant of the present application.

In this embodiment, the upstream side O₂Sensor output VOM
Trajectory length LVOM and area AVOM and downstream O₂Sensor
Using the trajectory length LVOS and the area AVOS of the output VOS
Calculate LVOS / LVOM, AVOS / AVOM
The map shown in FIG. 17 (A) or FIG. 17 (B)
The presence or absence of catalyst deterioration is determined from the above. In Figure 15, the routine
Once started, steps 1501 and 1502 are upstream
O ₂Trace length LVOM of output VOM of sensor 13, area A
VOM and downstream O₂Output VOS trajectory length of sensor 15
LVOS and area AVOS are calculated. Here, the locus length L
VOM and AVOS are calculated by the same formula as in FIG. 13, and the area A
VOM and AVOS are approximately calculated using the following equations.
(See FIG. 18).

A V _R1, V _R2 comparison voltage of the upstream respectively downstream O ₂ sensor output | [0057] AVOM ← AVOM + | VOM-V R1 | AVOS ← AVOS + | VOS-V R2. Next, at step 1503, VOM _i-1 ← VOM, VOS _i-1 ← in preparation for the next routine execution.
The VOS is updated, and the process proceeds to step 1504 in FIG.

In step 1504, the counter CT₁+
1 count-up is performed, and in step 1505 CT
₁Is the predetermined judgment time T₁It is determined whether or not
CT ₁> T₁If it is, the path length is determined in step 1506.
Ratio LVOS / LVOM and area ratio AVOS / AVOM
Calculate and calculate the path length ratio and area ratio in step 1507.
Based on FIG. 17 (A) or FIG. 17 (B) based on
The presence or absence of catalyst deterioration is determined from the above.

If it is determined in step 1507 that the catalyst has deteriorated, the alarm flag ALM is set (step 15).
08), catalyst deterioration alarm activation (step 1509)
However, if it is determined that the catalyst has not deteriorated, the alarm flag ALM is reset (step 1510),
The catalyst deterioration alarm is deactivated (step 1511). After completion of these operations, the alarm flag ALMS is reset and the judgment cancellation alarm 20 is deenergized (step 151).
2), storing in backup RAM RAM 106 (step 1513), clearing parameters (step 151)
4) is performed in the same manner as in the embodiment of FIGS.

Further, in the above embodiment, only the case of the air-fuel ratio control in which the skip amount RSR is changed based on the output of the downstream side O ₂ sensor has been described, but the present invention is based on the output of the downstream side O ₂ sensor. The same can be applied to the case of the air-fuel ratio control in which the control constants such as the integration constant, the delay time and the comparison voltage are changed. In this case, it is necessary to stop the deterioration determination (see FIG.
1, steps 1005 and 1006) may be determined by whether or not these control constants reach the guard value instead of whether or not the RSR reaches the guard value.

[0061]

According to the present invention, the catalyst deterioration determination operation is stopped when the air-fuel ratio correction amount based on the output of the downstream side air-fuel ratio sensor reaches the guard value. Since it is possible to prevent an erroneous determination of catalyst deterioration from occurring when the deviation of the catalyst occurs, it is possible to prevent a delay in the catalyst replacement timing due to the erroneous determination.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of the present invention.

FIG. 2 is a diagram illustrating a change in a downstream air-fuel ratio sensor output due to catalyst deterioration.

FIG. 3 is an overall schematic diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention.

4 is a part of a flowchart for explaining the operation of the control circuit of FIG.

5 is a part of a flowchart for explaining the operation of the control circuit of FIG.

FIG. 6 is a timing diagram for supplementarily explaining the control operation of FIGS. 4 and 5.

7 is a part of a flowchart for explaining the operation of the control circuit of FIG.

8 is a part of a flow chart for explaining the operation of the control circuit of FIG.

9 is a part of a flowchart for explaining the operation of the control circuit in FIG.

10 is a part of a flowchart for explaining a catalyst deterioration determination operation of the control circuit of FIG.

11 is a flow chart for explaining a catalyst deterioration determination operation of the control circuit of FIG.

12 is a part of a flowchart showing an embodiment of a catalyst deterioration determination subroutine of FIG.

FIG. 13 is a part of a flowchart showing another embodiment of the catalyst deterioration determination subroutine of FIG.

14 is a part of a flowchart showing another embodiment of the catalyst deterioration determination subroutine of FIG.

FIG. 15 is a flowchart of a catalyst deterioration determination subroutine of FIG.
15 is a flowchart showing an embodiment different from those of FIGS.

16 is a flowchart of the catalyst deterioration determination subroutine of FIG.
15 is a flowchart showing an embodiment different from those of FIGS.

FIG. 17 is a diagram showing a map used for catalyst deterioration determination in FIGS. 15 and 16;

FIG. 18 is a diagram showing the definition of the locus length and area of the air-fuel ratio sensor output.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine body 2 ... Air flow 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 19 ... Catalyst deterioration alarm 20 … Judgment stop alarm

Claims (1)

[Claims] 1. O ₂ provided in an exhaust passage of an internal combustion engine
A three-way catalyst having a storage effect, an upstream air-fuel ratio sensor provided in an exhaust passage upstream of the three-way catalyst for detecting an air-fuel ratio of the engine, and provided in an exhaust passage downstream of the three-way catalyst. And a downstream air-fuel ratio sensor for detecting the air-fuel ratio of the engine, and an air-fuel ratio sensor based on the output of the downstream air-fuel ratio sensor.
Constant calculating means for calculating a constant used for feedback control; and a machine based on the constant and the upstream air-fuel ratio sensor output.
Air-fuel ratio feedback bar for feedback control of air-fuel ratio of Seki
And click control means, within a predetermined time period in the air-fuel ratio feedback control execution
Of the output waveform of the downstream side air-fuel ratio sensor and a predetermined value.
Area of the encircled part and the output wave of the downstream side air-fuel ratio sensor
Based on at least one of the trajectory length of the shape
A catalyst deterioration judging means for judging deterioration of the medium and an air-fuel ratio feedback calculated by the constant calculating means.
When the value of the constant used for control reaches the specified guard value
Catalyst deterioration determination device for an internal combustion engine, characterized in that a prohibition means for prohibiting the catalyst deterioration determination by said catalyst deterioration determining means.