JPH08338231A - Catalyst deterioration determining device for internal combustion engine - Google Patents

Abstract

PURPOSE: To eliminate influence due to fluctuation on an exhaust inlet condition so as to precisely determine whether a three way catalyst deteriorates or not. CONSTITUTION: O2 sensors 13, 15 are arranged individually on the upstream side and on the downstream side of an exhaust purifying catalyst 12 in an internal combustion engine 1, and during air-fuel ratio feedback control based on an output from the O2 sensor 13 on the upstream side, it is determined whether the catalyst 12 deteriorates or not on the basis of the orbital length of the output from the O2 sensor 15 on the downstream side. A control circuit 10 controls a time, in which an exhaust air-fuel ratio is shifted to a rich side, according to a rich degree of an air-fuel ratio for exhaust gas flowing into the catalyst 12. In this way, an O2 discharge quantity from the catalyst 12 is controlled so as to be brought to a fixed target value, and determination of deterioration in the catalyst 12 is carried out while this O2 discharge quantity converges to the target value. Therefore, an O2 discharge quantity is kept at the fixed target value even if an exhaust inlet condition fluctuates, so that erroneous determination in catalyst deterioration determination can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention feedback-controls an engine air-fuel ratio to a target air-fuel ratio based on at least an air-fuel ratio sensor output on the upstream side of a catalytic converter, and at least based on an air-fuel ratio sensor output on the downstream side of a catalytic converter. The present invention relates to a catalyst deterioration determination device for an internal combustion engine, which determines whether or not a catalytic converter has deteriorated.

[0002]

2. Description of the Related Art Air-fuel ratio sensors are arranged in the exhaust passages upstream and downstream of a catalytic converter in an exhaust system of an internal combustion engine, and the engine air-fuel ratio is determined based on at least the upstream air-fuel ratio sensor output. There is known a technique for controlling whether or not the catalyst is deteriorated based on the output of the downstream side air-fuel ratio sensor.

The three-way catalyst adsorbs oxygen in the exhaust gas when the passing exhaust air-fuel ratio is lean, and releases the adsorbed oxygen when the passing exhaust air-fuel ratio is rich.
₂ Performs storage function. Therefore, even if the exhaust air-fuel ratio on the upstream side of the catalytic converter repeatedly fluctuates between the lean air-fuel ratio and the rich air-fuel ratio in a relatively short cycle, if the three-way catalyst is normal, it passes through the catalytic converter. The subsequent change in the air-fuel ratio of the exhaust gas is moderated by the O ₂ storage action of the catalyst, and the exhaust air-fuel ratio on the downstream side of the catalyst is maintained near the stoichiometric air-fuel ratio. Therefore, if the three-way catalyst is normal, the amplitude of the downstream side air-fuel ratio sensor output is small and the fluctuation cycle is long.

On the other hand, the O ₂ storage action of the catalyst decreases with the deterioration of the catalyst, so that when the three-way catalyst deteriorates, the amount of oxygen absorbed and released by the catalytic converter decreases and the exhaust air-fuel ratio on the downstream side of the catalytic converter decreases. The fluctuation approaches the fluctuation of the exhaust air-fuel ratio on the upstream side of the catalytic converter. For this reason, when the three-way catalyst deteriorates, the amplitude of the downstream side air-fuel ratio sensor output becomes large, the fluctuation cycle becomes short, and the amplitude and cycle of the upstream side air-fuel ratio sensor output come close to.

Therefore, when the engine air-fuel ratio regularly fluctuates with a relatively short period between the lean air-fuel ratio side and the rich air-fuel ratio side centering on the theoretical air-fuel ratio, the downstream side air-fuel ratio Whether or not the three-way catalyst has deteriorated can be determined by monitoring the sensor output. However, when determining the presence or absence of catalyst deterioration based on the output of the downstream side air-fuel ratio sensor, an error may occur in the determination of presence or absence of deterioration if the air-fuel ratio fluctuation cycle becomes long.

As the cycle of the air-fuel ratio changes, the time during which the exhaust air-fuel ratio flowing into the catalyst swings toward the rich air-fuel ratio side and the time during which it leans toward the lean air-fuel ratio side become longer. In this case, for example, if the engine air-fuel ratio swings to the rich air-fuel ratio side for a long time, even if the three-way catalyst is normal, the catalyst releases all the adsorbed oxygen, and thereafter cannot release oxygen. Therefore, in such a state, even if the catalyst is normal, the exhaust air-fuel ratio after passing through the catalyst will be oscillated to the rich side similarly to the catalyst upstream-side exhaust air-fuel ratio. If the engine air-fuel ratio swings to the lean side for a long time, even if the three-way catalyst is normal, the catalyst adsorbs oxygen to the limit and can no longer adsorb oxygen. The fuel ratio also swings to the lean side. That is, when the fluctuation cycle of the air-fuel ratio flowing into the catalyst becomes long, the downstream side air-fuel ratio sensor output fluctuates with substantially the same amplitude and cycle as the upstream side air-fuel ratio sensor output even with a normal catalyst, and the downstream side When the deterioration of the catalyst is determined based on the output of the side air-fuel ratio sensor, it may be erroneously determined that the normal catalyst is deteriorated.

Japanese Patent Laid-Open No. 5-10182 discloses, for the purpose of solving the above-mentioned problem, catalyst deterioration in which the fluctuation cycle of the air-fuel ratio is made to coincide with a predetermined constant target cycle when executing catalyst deterioration determination. A detection device is disclosed. For example, when the air-fuel ratio swings to the rich side, if the catalyst releases all the adsorbed oxygen, the air-fuel ratio of the exhaust gas that has passed through the catalyst also swings to the rich side, and the downstream side air-fuel ratio sensor output The fluctuation amplitude and the cycle of are close to those of the upstream side air-fuel ratio sensor output. As the catalyst deteriorates, the O ₂ storage action of the catalyst decreases, and the amount of oxygen adsorbed by the catalyst also decreases. Therefore, when the exhaust air-fuel ratio flowing into the catalyst becomes rich, the deteriorated catalyst takes a shorter time than the normal catalyst. Oxygen will be exhausted at.
That is, as the degree of deterioration of the catalyst increases, the air-fuel ratio fluctuates only by swinging to the rich side for a short time (that is, even in a short air-fuel ratio fluctuation cycle).

The apparatus disclosed in the above publication has a cycle (target air-fuel ratio) in which the output of the downstream air-fuel ratio sensor does not fluctuate if the catalyst is normal, and fluctuates if the catalyst deteriorates to a certain extent. The fluctuation cycle) is set in advance, and the fluctuation cycle of the air-fuel ratio is forcibly matched with the target cycle when the catalyst deterioration is detected. In the device of the above-mentioned publication, the downstream air-fuel ratio sensor output does not fluctuate if the catalyst is normal, and the downstream air-fuel ratio sensor output fluctuates if the catalyst has deteriorated to a certain extent. By determining the presence or absence of catalyst deterioration based on the waveform of the downstream side air-fuel ratio sensor output in a state where it matches the constant target air-fuel ratio variation cycle such as We are trying to prevent this.

[0009]

However, it is not possible to accurately determine the presence or absence of catalyst deterioration simply by controlling the air-fuel ratio fluctuation cycle to a constant target cycle, as in the device disclosed in Japanese Unexamined Patent Publication No. 5-10812. There is a problem that sometimes occurs. The device of the above publication presupposes that when the exhaust air-fuel ratio flowing into the catalyst is a rich air-fuel ratio, the amount of O ₂ released from the catalyst is always proportional to time. In other words,
In the above publication, if the time during which the exhaust air-fuel ratio swings to the rich side is the same, the catalyst deterioration determination is performed on the assumption that the same amount of oxygen is always released from the catalyst.
However, in reality, the amount of O ₂ released from the catalyst largely changes depending on the exhaust conditions flowing into the catalyst.

For example, when the rich degree of the exhaust air-fuel ratio flowing into the catalyst is large (the air-fuel ratio is small), it is released from the catalyst even at the same time as compared with the case where the rich degree is small (the air-fuel ratio is large). The amount of O _{2 that is} discharged increases. Further, even when the exhaust air-fuel ratio flowing into the catalyst has the same rich degree, when the exhaust flow rate is large, the amount of O ₂ released at the same time is larger than when the exhaust flow rate is small.

Therefore, as in the device of the above publication, the air-fuel ratio fluctuation cycle (that is, the time during which the exhaust air-fuel ratio flowing into the catalyst swings to the rich side) is simply controlled to be a constant target cycle. Then, the amount of released O ₂ may fluctuate depending on the exhaust conditions, resulting in an erroneous determination. For example, even when the air-fuel ratio fluctuation cycle is controlled to the target air-fuel ratio, when the exhaust gas flowing into the catalyst is rich and the exhaust gas flow rate is large, the amount of O ₂ released from the catalyst also becomes extremely large. However, not only the deteriorated catalyst but also the normal catalyst will release the whole amount of oxygen adsorbed. In this case, the output of the downstream side air-fuel ratio sensor fluctuates even if the catalyst is normal, and it is erroneously determined that the normal catalyst is deteriorated.

On the contrary, when the richness of the exhaust gas flowing into the catalyst is low and the flow rate of the exhaust gas is small, the amount of O ₂ released from the catalyst is also extremely small, and the exhaust air-fuel ratio is on the rich side in the target cycle. It becomes impossible to release the total amount of oxygen from the deteriorated catalyst within the period of time when the temperature is fluctuating. Therefore, in this case, the output of the downstream side air-fuel ratio sensor does not fluctuate even if the catalyst has deteriorated, and it is erroneously determined that the deteriorated catalyst is normal.

In view of the above problems, the present invention, when determining the presence or absence of catalyst deterioration based on the output of the air-fuel ratio sensor on the downstream side of the catalyst, does not cause erroneous determination due to the difference in exhaust conditions flowing into the catalyst. It is intended to provide a discriminating device.

[0014]

According to the present invention, a three-way catalyst having an O ₂ storage function, which is arranged in an exhaust passage of an internal combustion engine, and an exhaust passage upstream of the three-way catalyst, An upstream air-fuel ratio sensor for detecting the exhaust air-fuel ratio on the upstream side of the three-way catalyst, and a downstream air for detecting the exhaust air-fuel ratio on the downstream side of the three-way catalyst arranged in the exhaust passage on the downstream side of the three-way catalyst. Based on the output of the fuel ratio sensor and at least the upstream side air-fuel ratio sensor, the exhaust air-fuel ratio flowing into the three-way catalyst alternates between the rich air-fuel ratio side and the lean air-fuel ratio side cyclically around the theoretical air-fuel ratio. and air-fuel ratio feedback control means for changing, based on the exhaust gas condition flows into the three-way catalyst, and O ₂ emission calculation means for calculating the O ₂ release from the three-way catalyst, by the O ₂ emission calculation unit If the calculated amount of released O ₂ is A release amount control means for setting a time period during which the air-fuel ratio feedback control controls the exhaust air-fuel ratio to the rich side so that the target value is set, and the release amount control means controls the O ₂ release amount to a target value. When, the deterioration determination means for determining the presence or absence of deterioration of the three-way catalyst based on at least the downstream air-fuel ratio sensor output,
A catalyst deterioration determination device for an internal combustion engine is provided.

[0015]

[Function] O₂The discharge amount calculating means is an exhaust gas flowing into the catalyst.
O from three-way catalyst ₂Calculate the amount released and release
The quantity control means is O₂O calculated by the discharge amount calculation means
₂The air-fuel ratio is adjusted so that the emission amount reaches the predetermined target value.
The feedback control means controls the exhaust air-fuel ratio to the rich air-fuel ratio side.
Set the time to do. Therefore, the air-fuel ratio is rich
While swinging to the side (controlled to the rich air-fuel ratio side)
O released from the three-way catalyst₂The amount will always be the target value
Sea urchin rich time (when exhaust is swinging to the rich air-fuel ratio side
Interval) is adjusted.

For example, the rich time is set short when the rich degree of the exhaust gas flowing into the three-way catalyst and the exhaust flow rate are large, and conversely, the rich time is set long when the exhaust rich degree and the exhaust flow rate are small. It Therefore, the adsorption amount of the three-way catalyst has deteriorated O ₂ is a three-way catalyst is adsorbed while the exhaust air-fuel ratio is oscillated at the rich side if is smaller than the target value of the O ₂ emission The total amount of oxygen is released, and the downstream air-fuel ratio sensor output fluctuates similarly to the upstream air-fuel ratio sensor output. Further, when the three-way catalyst is normal and the O ₂ adsorption amount is larger than the target value of the O ₂ release amount, the fluctuation of the downstream side air-fuel ratio sensor output does not occur while the exhaust air-fuel ratio is swinging to the rich side.

[0017]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing an overall schematic configuration of an embodiment when the present invention is applied to an internal combustion engine for automobiles. In FIG. 1, 1 is an internal combustion engine body, 2a is an intake manifold connected to an intake port of each cylinder of the engine 1, and 11 is an exhaust manifold connected to an exhaust port of each cylinder.

The intake manifold 2a is connected to the intake passage 2 via a common surge tank 2b. Reference numeral 3 in FIG. 1 is an air flow meter for detecting the intake air amount of the engine 1. As the air flow meter 3, for example, a movable vane type with a built-in potentiometer is used, and generates a voltage signal proportional to the amount of intake air. The intake passage 2 is provided with a throttle valve 16 that opens according to the operation amount of the accelerator pedal by the driver.
An idle switch 17 is provided near 6 to generate an idle state signal (LL signal) when the throttle valve 16 is fully closed.

Reference numeral 7 in FIG. 1 indicates an intake manifold 2a.
Is a fuel injection valve disposed near the intake port of each cylinder. The fuel injection valve 7 opens in response to a signal from a control circuit 10 described later, and injects pressurized fuel into each intake port of each cylinder. The fuel injection control from the fuel injection valve 7 will be described later. The exhaust manifold 11 is connected to the catalytic converter 12 via a common exhaust pipe. Catalytic converter 1
2 has a built-in three-way catalyst and can purify three components of HC, CO, and NO _x in the exhaust gas at the same time. Further, an upstream side air-fuel ratio sensor 13 is provided on the upstream side of the catalytic converter 12, that is, on the exhaust collecting portion of the exhaust manifold 11, and a downstream side air-fuel ratio sensor 15 is provided on the downstream side exhaust pipe 14 of the catalytic converter 12.
Are provided respectively. In this embodiment, as the air-fuel ratio sensors 13 and 15, O ₂ sensors that generate a voltage signal according to the oxygen component concentration in the exhaust gas are used. That is,
The O ₂ sensors 13 and 15 generate different output voltages depending on whether the exhaust air-fuel ratio is leaner or richer than the stoichiometric air-fuel ratio.

Further, the ignition distributor 4 of the engine 1
Is provided with two crank angle sensors 5 and 6 which generate a pulse signal each time the engine crankshaft rotates at a constant speed. In the present embodiment, the crank angle sensor 5 outputs a reference position detection pulse signal, for example, every time a specific cylinder reaches the compression top dead center (that is, every 720 ° of crank rotation angle), and the crank angle sensor 6 outputs, for example, the crank. Rotation angle 30 °
A pulse signal for crank rotation angle detection is output every time.

The water jacket 8 of the cylinder block of the engine 1 is provided with a cooling water temperature sensor 9 which outputs an analog voltage according to the engine cooling water temperature. The control circuit 10 includes, for example, an input / output interface 102,
A microcomputer having a known structure in which a CPU 103, a ROM 104, and a RAM 105 are mutually connected by a bidirectional bus, and further includes a multiplexer built-in AD converter 10.
1. Backup RA that can be stored directly even if the engine ignition switch is off by being directly connected to the power supply
An M106, a clock generation circuit 107, etc. are provided.

The control circuit 10 performs basic control such as fuel injection control and ignition timing control of the engine, and in this embodiment, as will be described later, the upstream O ₂ sensor 13 and the downstream O ₂ sensor 1 are controlled.
Air-fuel ratio feedback control and catalyst 12 based on the output of 5
Deterioration judgment is performed. In order to execute these controls, the control circuit 10 sends the engine intake air amount signal from the air flow meter 3, the cooling water temperature signal from the cooling water temperature sensor 9, and the O ₂ sensors 13 and 15 via the AD converter 101. In addition to the respective air-fuel ratio signals of 1, the pulse signals from the crank rotation angle sensors 5 and 6 and the idle signal from the idle switch 17 are input through the input / output interface 102.

The engine intake air amount signal and the cooling water temperature signal are fetched by an AD conversion routine executed every fixed crank time, and the engine intake air amount data Q and the cooling water temperature data THW are respectively stored in predetermined areas of the RAM 105.
Is stored as Every time the pulse signal of the crank rotation angle sensor 6 is input, the engine rotation speed is calculated from the pulse interval by a routine (not shown).
5 is stored as engine speed data Ne in a predetermined area.

On the other hand, the control circuit 10 is connected to the fuel injection valve 7 via the input / output interface 102 and controls the fuel injection from the fuel injection valve 7. 1 in FIG.
Denoted at 09 and 110 are a down counter, a flip-flop, and a drive circuit for controlling the fuel injection amount from the fuel injection valve 7, respectively. That is, when the fuel injection amount (time) TAU is calculated in the routine described later, the fuel injection time TAU is preset in the down counter 108, the flip-flop 109 is set, and the drive circuit 110 causes the drive signal of the fuel injection valve 7 to be set. Is output. As a result, the fuel injection valve 7 is opened and fuel injection is started. The down counter 108 counts the clock signal of the clock 107 and outputs a set signal to the flip-flop 109 when the preset time TAU elapses. As a result, the flip-flop 109 is set, so that the drive circuit 110 stops the drive signal of the fuel injection valve 7, and the fuel injection valve 7 is closed. Therefore, the fuel injection valve 7 is opened for the time corresponding to the calculated fuel injection time TAU, and the fuel of the amount corresponding to TAU is injected from the fuel injection valve 7 to the engine 1.

Further, the control circuit 10 uses the input / output interface 102 to alarm 1 activated when the catalyst deteriorates.
9 is connected. In this embodiment, O ₂ release from the air-fuel ratio feedback control the three-way catalyst 12 in as described later and controls the engine air-fuel ratio to be the target value, and O ₂ emission is equal to the target value In this state, the presence / absence of deterioration of the three-way catalyst is determined based on the output of the downstream side air-fuel ratio sensor 15. Therefore, before describing the deterioration detection, the air-fuel ratio feedback control of this embodiment, which is the premise thereof, will be briefly described first.

FIG. 2 is a flow chart showing a fuel injection amount calculation routine of this embodiment. This routine is executed by the control circuit 10 at every constant crank rotation angle (for example, every 360 °).
Is executed. In the routine of FIG. 2, the fuel injection amount, that is, the fuel injection time TAU of the fuel injection valve 7 is calculated based on the intake air amount Q / Ne per engine revolution and the air-fuel ratio correction coefficient FAF described later.

That is, in the routine shown in FIG. 2, the intake air amount data Q and the rotational speed data Ne are read from a predetermined area of the RAM 105, and the intake air amount Q / N per engine revolution is read.
In addition to calculating e (step 201), the basic fuel injection time TAUP is calculated as TAUP = α × Q / Ne (step 202). Here, the basic fuel injection time TAUP is a fuel injection time required to make the air-fuel mixture supplied to the combustion chamber have a stoichiometric air-fuel ratio, and α
Is a constant.

Further, the actual fuel injection time TAU is calculated as a value obtained by correcting the above TAUP with the air-fuel ratio correction coefficient FAF, TAU = TAUP × FAF × β + γ (step 203). Where β, γ
Are constants determined according to the engine operating state. When the fuel injection time TAU is calculated as described above, the time TAU is reduced by the down counter 10 in step 204.
8 is set, and the amount of fuel corresponding to the time TAU is injected from the fuel injection valve 7.

Next, at step 203, the air-fuel ratio correction coefficient F
The calculation of AF will be described. The air-fuel ratio correction coefficient FAF is the first air-fuel ratio feedback control based on the output of the upstream O ₂ sensor 13 and the _second air-fuel ratio correction control based on the output of the downstream O ₂ sensor 15.
Of the air-fuel ratio feedback control. 3 and 4 are flowcharts showing the first air-fuel ratio feedback control based on the output of the upstream O ₂ sensor 13.
This routine is executed by the control circuit 10 at regular time intervals (for example, every 4 ms).

In this routine, the output VOM of the upstream O ₂ sensor 13 is compared with the comparison voltage V _R1 (theoretical air-fuel ratio equivalent voltage), and the exhaust air-fuel ratio on the upstream side of the catalytic converter is richer than the theoretical air-fuel ratio (VOM> When V _R1 ), the air-fuel ratio correction amount FAF is decreased, and when lean (VOM ≦ V _R1 ), FAF is increased. The O ₂ sensor outputs a voltage signal of, for example, 0.9 V when the exhaust air-fuel ratio is richer than the theoretical air-fuel ratio, and outputs a voltage signal of, for example, about 0.1 V when the exhaust air-fuel ratio is leaner than the theoretical air-fuel ratio. Output voltage signal. In this embodiment, the comparison voltage V _R1 is 0.45.
It is set to about bolts. As described above, the air-fuel ratio correction amount F
By increasing / decreasing AF according to the exhaust air-fuel ratio, the engine air-fuel ratio can be accurately brought close to the stoichiometric air-fuel ratio even if there are some errors in the devices of the fuel supply system such as the air flow meter 3 and the fuel injection valve 7. Will be fixed.

The flow charts of FIGS. 3 and 4 will be briefly described below. Step 301 shows whether or not the feedback control execution condition is satisfied. The feedback control execution condition is, for example, that the O ₂ sensor is activated, that the engine warming up is completed, that a predetermined time has elapsed after returning from the fuel cut, and the like.
Only when the execution condition is satisfied, the FAF calculation in and after step 302 is performed. If the feedback control execution condition is not satisfied, the routine proceeds to step 3 in FIG.
In step 25, the value of the flag XMFB is set to 0 and the routine ends. The flag XMFB is a flag indicating whether or not the first air-fuel ratio feedback control is being executed.
FB = 0 means that the first air-fuel ratio feedback control is stopped.

Steps 302 to 315 indicate the determination of the air-fuel ratio. Flag F1 shown in steps 309 and 315
Indicates that the engine air-fuel ratio is rich (F1 = 1) or lean (F1 =
0) is an air-fuel ratio flag indicating that F1 = 0 to F1 =
When switching from 1 (lean to rich), the upstream O ₂ sensor 13 continues for a predetermined time (TDR) or longer and the rich signal (V
OM > V _R1 ) is output (steps 303, 3
10 to 315), and switching from F1 = 1 to F1 = 0 (rich to lean) is continued by the upstream O ₂ sensor 13 for a predetermined time (−TDL) or longer and the lean signal (VOM). ≤
V _R1 ) is output (steps 303 to 309). CDLY is a counter for determining the air-fuel ratio flag switching timing.

In steps 316 to 323 of FIG. 4, the FAF is increased or decreased according to the value of the flag F1 set as described above. That is, the value of F1 at the time of executing this routine is compared with the value of F1 at the time of executing the previous routine, and it is determined whether the value of F1 has changed, that is, whether the air-fuel ratio is reversed from rich to lean or lean to rich. Yes (Step 31
6). When the current value of F1 is F1 = 0 (lean), the FAF is skipped by a relatively large value RSR immediately after changing (reversing) from F1 = 1 to F1 = 0 (rich to lean). (Steps 317, 3
18) After that, while F1 = 0, FAF is gradually increased by a relatively small value KIR each time the routine is executed (steps 320 and 321). Similarly, when the current value of F1 is F1 = 1 (rich), first, F1 = 0 to F
Immediately after reversing from 1 to 1 (from lean to rich), FAF is reduced in a skip manner by RSL (steps 317, 3).
19) thereafter, while F1 = 1, FAF is gradually decreased by KIL for each routine execution (step 32).
0, 322). Further, after guarding the value of FAF calculated as described above so as not to exceed the range defined by the maximum value (FAF = 1.2 in this embodiment) and the minimum value (FAF = 0.8 in this embodiment) (step 323), the value of the flag XMFB is set to 1 (step 324), and this routine ends.

Next, the second air-fuel ratio feedback control based on the output of the downstream O ₂ sensor 15 will be described. 5 and 6 show a second air-fuel ratio feedback control routine. In this routine, the control circuit 10 causes the control circuit 10 to perform a predetermined interval longer than the first air-fuel ratio feedback control (for example, 5).
Every 00 ms). In this routine, the downstream side O
₂ The output VOS of the sensor 15 is compared with the comparison voltage V _R2 (theoretical air-fuel ratio equivalent voltage, for example, 0.45 V), and the exhaust air-fuel ratio on the downstream side of the catalytic converter is richer than the theoretical air-fuel ratio (VOS> V _R2 ). Sometimes, the correction amount RSR (step 318 in FIG. 4) used in the first air-fuel ratio feedback control is decreased and RSL (step 319 in FIG. 4) is increased. Further, it performs an operation of reducing the RSL with increasing correction amount RSR is when the exhaust gas air-fuel ratio of the catalytic converter downstream leaner than the stoichiometric air-fuel ratio (VOS ≦ V _R2). As a result, when the exhaust air-fuel ratio is rich on the downstream side of the catalytic converter, the FAF value is generally set to be small in the first air-fuel ratio feedback control, and conversely, the exhaust air-fuel ratio on the downstream side is set to be small. FA in case of rich
The value of F is generally set to be large. Therefore, even if the output of the upstream O ₂ sensor 13 deviates from the actual exhaust air-fuel ratio due to the deterioration of the upstream O ₂ sensor 13 or the strong influence of the exhaust gas of a specific cylinder, the value of FAF is set to the downstream. Since it is corrected based on the output of the side O ₂ sensor 15, the engine air-fuel ratio is accurately maintained at the stoichiometric air-fuel ratio.

The flow charts of FIGS. 5 and 6 will be briefly described below. Steps 501 and 502 of FIG. 5 show the determination as to whether or not the feedback control execution condition is satisfied. The determination condition of step 501 is as shown in step 301 of FIG.
In addition to the above, the condition is that the engine is not in idle operation (that is, the LL signal from the idle switch 17 is not input). Further, in step 502, it is determined whether or not the first air-fuel ratio feedback control is being executed, and the control of step 504 and subsequent steps is executed only when the control is being executed (flag XMFB = 1). If the control is not executed (XMFB ≠ 1), the value of the flag XSFB is set to 0 in step 503, and the routine ends. The flag XSFB is a flag indicating whether or not the second air-fuel ratio feedback control is being executed, and XS
FB = 0 means that the second air-fuel ratio feedback control is stopped.

When the first air-fuel ratio feedback control is being executed in step 502, the value of the flag XSFB is set to 1 in step 504, and then the exhaust air-fuel ratio detected by the downstream O ₂ sensor 15 is An operation of increasing or decreasing the values of the correction amounts RSR and RSL is performed depending on whether or not it is rich. That is, in step 505 of FIG. 6, the output VOS of the downstream O ₂ sensor 15 is AD-converted and read, and in step 506 it is determined whether or not VOS is the lean air-fuel ratio equivalent value (VOS ≦ V _R2 ), and the value of VOS is If it is the lean air-fuel ratio equivalent value, the value of RSR is increased by a fixed amount ΔRS in step 507, and the increased RSR does not exceed a predetermined maximum value MAX (MAX = 0.09 in this embodiment). (Steps 508 and 509). If the VOS value is the rich air-fuel ratio equivalent value (VOS> V _R2 ) in step 506, the RSR value is changed by a constant amount Δ in step 510.
Only the RS is reduced, and the reduced RSR is a predetermined minimum value MI.
Guarding is performed so as not to become smaller than N (MIN = 0.01 in this embodiment) (steps 511 and 512).

Further, using the RSR value calculated above, in step 513, the value of RSL (step 319 in FIG. 4) used in the first air-fuel ratio feedback control routine is calculated as RSL = 0.1-RSR. . That is, the sum of RSR and RSL is always held at a constant value (0.1) in this embodiment, and when RSR increases, RS
When L decreases and RSR decreases, RSL increases.

By executing the second air-fuel ratio feedback control routine, when the exhaust air-fuel ratio detected by the downstream O ₂ sensor 15 is rich, RSR decreases and RSL increases, and the exhaust air-fuel ratio becomes lean. In this case, RSR is increased and RSL is decreased at the same time. FIG. 7 shows a case where the first air-fuel ratio feedback control of FIGS. 3 and 4 is performed.
The counter CDLY (same (B)), the flag F1 (same (C)), the air-fuel ratio correction coefficient FAF (same) for the change in the air-fuel ratio (A / F) detected by the upstream O ₂ sensor 13 (FIG. 7A). (D))
Shows the change. As shown in FIG. 7 (A), even if the A / F changes from lean to rich, the air-fuel ratio flag F1
The value of (Fig. 7 (C)) does not change from 0 to 1 immediately, and is maintained at 0 for the time (Fig. 7 (C) T ₁ ) until the value of the counter CDLY increases from 0 to TDR. Then, after the lapse of T ₁ , it changes from 0 to 1. Also, when the A / F changes from rich to lean, the value of F1 is the time during which the value of the counter CDLY decreases from 0 to TDL (TDL is a negative value) (T _{2 in} FIG. 7 (C)). Is maintained as 1 and changes from 1 to 0 after T ₂ . Therefore, as indicated by N in FIG. 7 (A), even if the output of the upstream O ₂ sensor 13 changes in a short cycle due to disturbance or the like, the value of the flag F1 does not change following the change of the air-fuel ratio. Control is stable.

As a result of the first air-fuel ratio feedback control,
The value of the air-fuel ratio correction coefficient FAF repeatedly increases and decreases as shown in FIG. 7 (D), and the engine air-fuel ratio alternates between the rich air-fuel ratio and the lean air-fuel ratio. Further, as described with reference to FIG. 2, when the value of FAF increases, the fuel injection time TAU increases, and when the value of FAF decreases, the fuel injection time TAU also decreases.

As can be seen from FIG. 7 (D), when RSR is increased and RSL is decreased by the second air-fuel ratio feedback control (FIGS. 5 and 6), the swing range to the rich air-fuel ratio side is increased. The air-fuel ratio shifts to the rich air-fuel ratio side as a whole). Conversely, when RSR decreases and RSL increases,
The swing range of the engine air-fuel ratio to the lean air-fuel ratio side increases and the air-fuel ratio shifts to the lean air-fuel ratio side as a whole.

Therefore, when the values of RSR and RSL are increased or decreased by the second air-fuel ratio feedback control, the engine air-fuel ratio changes to the rich side or the lean side. It should be noted that in this embodiment, RSR and RSL are controlled by the second air-fuel ratio feedback control.
However, the engine air-fuel ratio can also be changed by setting another correction amount in the first air-fuel ratio control in the second air-fuel ratio feedback control.

For example, KIR, KIL (step 4 in FIG. 5)
21 and 422) or the values of TDR and TDL (steps 407 and 413 in FIG. 4) based on the second air-fuel ratio feedback control, the engine air-fuel ratio can be similarly changed. Or upstream O
It is also possible to change the engine air-fuel ratio in the same manner by setting the value of the comparison voltage V _R1 (step 303 in FIG. 3) of the _two- sensor 13 based on the second air-fuel ratio feedback control.

Next, the catalyst deterioration determination of this embodiment will be described. In this embodiment, the presence or absence of catalyst deterioration is determined by detecting a decrease in the O ₂ storage action of the three-way catalyst. As described above, the three-way catalyst adsorbs oxygen in the exhaust gas when the inflowing exhaust air-fuel ratio is lean (that is, when the air-fuel mixture leaner than the stoichiometric air-fuel ratio is burning in the engine combustion chamber). However, when the inflowing exhaust air-fuel ratio is rich (when the air-fuel mixture richer than the stoichiometric air-fuel ratio is being burned in the combustion chamber), the adsorbed oxygen is released and absorbed. Therefore, even when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst fluctuates alternately between the rich side and the lean side around the stoichiometric air-fuel ratio as shown in FIG. 7 (A), O ₂ Due to the storage action, the fluctuation of the air-fuel ratio of the exhaust gas flowing out from the catalyst is reduced.

However, as the catalyst deteriorates, the O ₂ storage function of the catalyst decreases, and the amount of oxygen that the catalyst can adsorb and retain decreases. Therefore, when the catalyst deteriorates, the adsorbed oxygen is exhausted in a short time when the exhaust air-fuel ratio swings to the rich side, and after the oxygen is completely released, the air-fuel ratio of the exhaust gas flowing out from the catalyst is rich. You can swing to the side.

FIG. 8 is a diagram for explaining changes in the air-fuel ratio fluctuation on the downstream side of the catalyst due to catalyst deterioration. FIG. 8 (A) shows the fluctuation of the exhaust air-fuel ratio flowing into the catalyst, and FIG. 8 (B) shows the case where the catalyst is normal (that is, when the O ₂ storage action of the catalyst is not lowered). ) Indicates the fluctuation of the exhaust air-fuel ratio on the downstream side of the catalyst when the catalyst is deteriorated (when the O ₂ storage function of the catalyst is reduced).

As shown in FIG. 8A, the exhaust gas flowing into the catalyst is
The air-fuel ratio is regularly on the rich air-fuel ratio side and the lean air-fuel ratio side.
Exhaust that flows in with a normal catalyst when it seems to fluctuate
Is a lean air-fuel ratio, the catalyst is
It absorbs excess oxygen, and when it has a rich air-fuel ratio, it does not
Because the released oxygen is released, the exhaust air-fuel ratio on the downstream side of the catalyst is
It is always close to the stoichiometric air-fuel ratio, and the second air-fuel ratio
Due to the effect of the feedback control, the lean air-fuel ratio side and the
H Change to the air-fuel ratio side (see Fig. 8 (B)). On the other hand, the catalyst
Deteriorated and O₂When the storage effect decreases, O₂Straight
The exhaust gas flow into the catalyst,
O that can be released when the fuel ratio becomes rich air-fuel ratio₂Amount of
Is reduced and the inflowing exhaust air-fuel ratio becomes rich,
Guni O₂Has been exhausted, then O₂Emit
Can not do. In addition, the inflowing exhaust air-fuel ratio
When the lean air-fuel ratio is reached, O can be adsorbed by the deteriorated catalyst.
₂Because the amount of exhaust gas is small, the inflowing exhaust air-fuel ratio is lean
As soon as the ratio is reached, the catalyst O ₂The adsorption amount is saturated,
Then O₂Cannot be adsorbed.

Therefore, when the catalyst deteriorates, the exhaust air-fuel ratio on the downstream side of the catalyst also fluctuates alternately between the rich air-fuel ratio side and the lean air-fuel ratio side as shown in FIG. 8 (C). FIG. 9 shows the output of the O ₂ sensor when the air-fuel ratio changes as shown in FIG. 8. In FIG. 9 (A), the exhaust air-fuel ratio flowing into the catalyst is rich / lean as shown in FIG. 8 (A). The output VOM of the upstream O ₂ sensor 13 when the fluctuation of the above is repeated, and FIG. 9B shows the downstream side corresponding to the fluctuation of the exhaust air-fuel ratio on the downstream side of the catalyst when the catalyst is normal (FIG. 8B). O ₂ sensor 15 output VOS,
FIG. 9C shows the output VOS of the downstream O ₂ sensor 15 corresponding to FIG. 8C when the catalyst is deteriorated.

As shown in FIGS. 8 (B) and 8 (C), the catalyst is normal.
In such a case (Fig. 9 (B)), the downstream side O₂Change of sensor 15 output
The movement has a very long cycle, and the output trajectory length becomes short.
When the catalyst deteriorates, as shown in Fig. 8 (C), the downstream side
O₂The fluctuation cycle of the sensor 15 output becomes short, and the sensor output
The locus length of becomes large. Therefore, for example, the downstream side O ₂
Same as the trajectory length LVOS of the sensor 15 output within a certain period,
Upstream O within the same period₂Sensor 13 output trajectory length LVO
By monitoring the value of the ratio LVOS / LVOM with M
It is possible to determine the presence or absence of catalyst deterioration. That is, L
VOS grows with catalyst deterioration, so the trajectory
The long ratio LVOS / LVOM is greater than or equal to a predetermined judgment value.
If so, it can be determined that the catalyst has deteriorated.

However, as can be seen from the above description, in order to determine the presence or absence of catalyst deterioration based on the waveform (trajectory length) of the output of the downstream O ₂ sensor 15, the condition of the exhaust gas flowing into the catalyst has deteriorated. the catalyst downstream O ₂ sensor 15 outputs varies greatly, it is necessary to have become conditions such as the downstream O ₂ sensor 15 outputs varies slowly in normal catalyst. That is, as will be described later, the amount of O ₂ released from the catalyst varies depending on the exhaust conditions flowing into the catalyst, but in order to detect the presence or absence of deterioration of the catalyst, this O ₂ release amount is
It is necessary to set the exhaust conditions that flow into the catalyst so that the amount of oxygen that can be adsorbed by the deteriorated catalyst is greater than the amount of oxygen that can be adsorbed by the normal catalyst. For example, if the amount of O ₂ released from the catalyst is set to be larger than the amount of oxygen that can be adsorbed by the normal catalyst, even if the normal catalyst is adsorbed while the inflow exhaust swings to the rich air-fuel ratio side. Since the entire amount of oxygen is exhausted, the output of the downstream O ₂ sensor starts to fluctuate as in the case of the deteriorated catalyst, and it may be erroneously determined that the normal catalyst has deteriorated. Occurs. Conversely, if the amount of O ₂ released from the catalyst is set to be smaller than the amount of oxygen that the deteriorated catalyst can adsorb, even if the deteriorated catalyst is adsorbed while the inflowing exhaust air-fuel ratio is swinging to the rich side. Since the total amount of oxygen cannot be released, the output of the downstream O ₂ sensor does not fluctuate, and the deteriorated catalyst may be erroneously determined to be normal.

By the way, the inflow exhaust conditions that determine the amount of O ₂ released from the catalyst are roughly classified into the rich degree (air-fuel ratio) of the inflow exhaust, the exhaust flow rate, and the time during which the air-fuel ratio is swinging to the rich side (rich time. ). Therefore, the amount of O ₂ emission cannot be made constant only by making the air-fuel ratio feedback cycle (that is, the rich time) constant. For example, even if the rich times are the same, if the rich degree of the inflowing exhaust gas is large (the air-fuel ratio is small), the amount of O ₂ released from the catalyst increases, and if the rich degree is small (the air-fuel ratio is large).
And the amount of O ₂ released from the catalyst is reduced. Similarly, if the exhaust flow rate is large even if the rich time is the same, the O
_{2 The} emission amount increases, and if the exhaust flow rate is small, the O
₂ Emissions decrease.

In this embodiment, the O ₂ emission amount of the catalyst is calculated from the exhaust gas inlet condition as described below, and the exhaust gas condition that flows in is feedback-controlled so that this calculated amount matches a certain target value. Thus, the presence / absence of catalyst deterioration is accurately determined by making the catalyst deterioration determination after setting the O ₂ emission amount from the catalyst to a constant target value suitable for catalyst deterioration determination.

Next, the control of the amount of O ₂ released from the catalyst in this embodiment will be described. As described above, the amount of O ₂ released from the catalyst depends on the rich degree of the inflow exhaust, the exhaust flow rate, the time during which the air-fuel ratio of the inflow exhaust swings to the rich side,
It is determined by the following three. Therefore, if the above three conditions are controlled at the same time, the amount of released O ₂ can be made to converge to a constant target value. In the present embodiment, in order to simplify the control, the catalyst deterioration is detected during the idle operation when the exhaust gas flow rate is substantially constant, and the rich time is adjusted according to the rich degree of the inflowing exhaust gas, so that the O ₂ from the catalyst is reduced. The operation to make the release amount converge to a constant target value is performed.

When the other inlet exhaust conditions are constant, the amount of O ₂ released from the catalyst is considered to increase substantially in proportion to the rich degree of the inflow exhaust and also in proportion to the rich time. Further, the rich degree of the inflow exhaust is the amount of excess fuel supplied to the engine, that is, the difference between the value of the air-fuel ratio correction coefficient FAF and the value of the air-fuel ratio correction coefficient FAF ₀ that gives the theoretical air-fuel ratio (FAF-FAF ₀ ). Proportional to. Therefore, the amount of O ₂ released from the catalyst is the product of the value of (FAF-FAF ₀ ) and the rich time, that is, the time integration of the value of (FAF-FAF ₀ ) during the period when the exhaust air-fuel ratio is swinging to the rich side. It can be considered to be proportional to the value.

FIG. 10 shows the first air-fuel ratio feedback system.
Upstream O during execution (Figs. 3 and 4) ₂Sensor 13 output V
Change in OM (Fig. 10 (A)) and the air-fuel ratio correction coefficient FAF
It is a figure which shows change (FIG.10 (B)). Like above-mentioned,
The value of the correction factor FAF is O on the upstream side.₂Sensor 13 output VOM
Is changed from lean air-fuel ratio output to rich air-fuel ratio output,
After the elapse of the time TDR, only RSL decreases in a skip manner,
After that, KIL is gradually decreased by each routine execution.
Now VOM is lean to rich or rich to rich
The exhaust air-fuel ratio at the moment when the
Because the VOM is rich,
The value of the air-fuel ratio correction coefficient FAF at the moment of reversing from lean to lean
FAF value equivalent to stoichiometric air-fuel ratio (FAF₀)
And O from the catalyst₂Release amount ((FAF-FAF₀)time
(Integral value) is proportional to the area shaded in Fig. 10 (B)
Will be. On the other hand, the exhaust air-fuel ratio is swinging to the rich side.
Time (VOM swings to the rich side)
H can be adjusted by increasing or decreasing the delay time TDR.
it can.

Therefore, in this embodiment, the value of FAF calculated by the first air-fuel ratio feedback control and the upstream side O
₂ Based on the output of the sensor 13 and the VOM, each time the VOM reverses from rich to lean, the area shaded in FIG. 10 (B) is calculated, and the first area is adjusted so that this area becomes a constant target value.
Rich delay time T in the air-fuel ratio feedback control of
It controls DR.

The target value of the above area (that is, O ₂
The target value of the release amount) is set to a value that is smaller than the maximum oxygen amount that can be adsorbed by a normal catalyst and that is larger than the maximum oxygen amount that can be adsorbed by the catalyst of the deterioration level that is desired to be detected. By setting the target value in this way, it becomes possible to accurately determine the catalyst having an arbitrary deterioration level.

11 and 12 are flow charts showing the above-mentioned catalyst deterioration determining operation of this embodiment. This routine is executed by the control circuit 10 at regular time intervals. When the routine starts in FIG. 11, step 11
In 01, it is determined whether or not the catalyst deterioration determination execution condition is satisfied. In the present embodiment, the execution conditions of step 1101 are that the first air-fuel ratio feedback control (FIGS. 3 and 4) is being executed, the engine is in idle operation, and the second air-fuel ratio feedback control ( 5 and 6) are stopped, and the engine operating condition is stable.

The above condition is because it is necessary to determine the deterioration of the present embodiment while the first air-fuel ratio feedback control is being executed, as described above. Whether or not the first air-fuel ratio feedback control is being executed. Is determined from whether or not the value of the flag XMFB (steps 324 and 325 in FIG. 4) is set to 1. Further, the condition is that the catalyst deterioration determination is performed under the condition that the exhaust flow rate is substantially constant in the present embodiment. Whether or not the idle operation is currently being performed is determined by the L of the idle switch 17.
It is determined by whether or not the L signal is on (LL = 1). Further, the above conditions are the same as the catalyst O _{2 in} this embodiment.
Although it is necessary to maintain the release amount at a constant target value, if catalyst deterioration determination is performed during execution of the second air-fuel ratio feedback control, the rich time is affected by the second air-fuel ratio feedback control, and catalyst O ₂ release This is because there are cases where the amount cannot be maintained constant. Further, the above conditions are required to make catalyst deterioration determination in a state where the engine exhaust flow rate is substantially constant. Whether the engine operating condition is stable depends on, for example, whether the difference between the intake air amount during the execution of the previous routine and the intake air amount during the execution of the current routine is less than or equal to a predetermined value. And whether the difference in engine speed during execution of this routine is less than or equal to a predetermined value, or the like.

In step 1101, one of the above conditions is satisfied.
If none of the above holds, this routine clears the value of the counter CT described later (step 1133),
12, the process immediately ends without executing the catalyst deterioration determination. If all of the above conditions are satisfied in step 1101, then in step 1103, a counter C that measures the time after the condition of step 1101 is satisfied.
The value of T is incremented by 1, and in step 1105, it is determined whether or not the value of the counter CT after the increment is equal to or larger than a predetermined value CT ₃ . CT ≧ CT in step 1105
If it is ₃ , the routine proceeds to step 1135, where KIL, KIR used for the first air-fuel ratio feedback control,
The routine is terminated after the TDL control parameter is returned to the normal value. CT ₃ is a value larger than CT ₁ (step 1115) and CT ₂ (step 1129) described later, and is a time sufficient for completing the catalyst deterioration determination after the condition of step 1101 is satisfied. In the present embodiment, as will be described later, KIL, KIR, and TD are set while the catalyst deterioration determination is being performed.
Although the value of each parameter of L is fixed to a constant value, the catalyst deterioration determination operation is stopped when the time sufficient for completing the catalyst deterioration determination is elapsed as described above, and the value of each parameter is set to the normal value. After the completion of the catalyst deterioration determination, the normal first air-fuel ratio feedback control is performed.

If CT <CT ₃ in step 1105, the values of the control parameters of KIL, KIR and TDL used for the first air-fuel ratio feedback control in step 1107 are constant according to the intake air amount. Fixed to the value. As will be described later, in this embodiment, the rich time of the air-fuel ratio feedback control is adjusted by changing the value of the control parameter TDR used for the first air-fuel ratio feedback control when determining the catalyst deterioration. Therefore, KI
The values of L and KIR are set to smaller constant values as the intake air amount is larger in order to minimize the time change of FAF and increase the sensitivity of the rich time with respect to TDR. Further, in the present embodiment, the lean time is set so that the catalyst adsorbs maximum O ₂ while the exhaust air-fuel ratio is swinging to the lean side, that is, the O ₂ adsorption amount of the catalyst is saturated at the lean air-fuel ratio. Since it is necessary to make it sufficiently long, the value of TDL is set to a larger value as the intake air amount is smaller.

At step 1109, the output VOM of the upstream O ₂ sensor 13 is AD-converted and read, and VO
It is determined whether M is a rich air-fuel ratio output (VOM ≧ V _R1 ). When VOM ≧ V _R1 , that is, when the output OOM of the upstream O ₂ sensor 13 is the rich air-fuel ratio output, at step 1111 the current air-fuel ratio correction coefficient FAF
The integrated value W ₁ of the values of
At 3, the value of the counter N is incremented by 1. F as described below
The integrated value W _{1 of the} AF and the counter N are always cleared in step 1127 of FIG. 12 when the VOM is the lean air-fuel ratio output, so the integrated value W ₁ calculated in step 1111.
Represents the integrated value of FAF after VOM becomes rich output, and the value of the counter N represents the integrated number of FAF after rich air-fuel ratio. Also, step 1
After executing 111 and 1113, the routine proceeds to step 1129 in FIG. Operations after step 1129 will be described later. Next, when the output VOM of the upstream O ₂ sensor 13 is the lean air-fuel ratio output in step 1109, it is determined in step 1115 whether or not the value of the clock counter CT is equal to or more than the predetermined value CT ₁ , that is, the condition of step 1101 is It is determined whether or not a predetermined time has elapsed after all the conditions are satisfied. If CT <CT ₁ , the process proceeds to step 1127 in FIG. 12 and the integrated value W ₁ and the value of the counter N are cleared. That is, in this embodiment, the TDR setting operation in step 1119 and thereafter is not performed until the time (CT ₁ ) when it is determined that the engine operating state has been completely stabilized after the conditions are satisfied.

If CT ≧ CT ₁ in step 1115, the flow advances to step 1117 to determine whether or not the value of the counter N is N ≠ 0. If N = 0, similarly, FIG. Proceeding to step 1127, W ₁ and N are cleared. If N ≠ 0 at step 1117,
Steps 1119 and below are executed. If the VOM is rich, the value of the counter N is counted up in step 1113, and if it is lean, step 1127.
Therefore, N = 0 is always satisfied in step 1117 except when the routine is executed immediately after the VOM value is inverted from rich to lean. Therefore, the TDR setting operation from step 1119 to step 1125 is executed only once every time the VOM is reversed from rich to lean.

At step 1119, the current value of the air-fuel ratio correction coefficient FAF is stored as FAF ₀ (see FIG. 10B). Further, in step 1121, the area WS of the shaded portion in FIG. 10B is calculated as WS = W ₁ −FAF ₀ × N. That is, the area W of the shaded area in FIG.
S Should be calculated as an integrated value of (FAF-FAF ₀ ), but in the present embodiment, the value of FAF when the VOM is inverted from the rich output to the lean output is used as FAF _0. FAF value is added to
₁ is obtained, and after the value of FAF ₀ is confirmed, FAF ₀
Is multiplied by the number of times of integration N and subtracted from W ₁ to calculate the shaded area WS.

After obtaining the area WS from the above,
In 1123, the value of WS and the preset target value WS₀When
The value of the TDR correction amount ΔTDR is set according to the deviation of
It In step 1125, the value of TDR is supplemented by the above.
The positive amount ΔTDR is added. In this embodiment, WS-WS
₀The larger the positive value of ΔTDR, the larger the negative value
Value, and then the first air-fuel ratio feedback control
Value of TDR when the routine (Figs. 3 and 4) is executed is small
It gets worse. Therefore, the VOM becomes a rich air-fuel ratio output.
It takes a short time to decrease FAF in a skip manner
Therefore, the area of the shaded area in FIG. 10 (B) becomes smaller. sand
O from the catalyst₂The amount released is reduced. In addition, this implementation
In the example, WS-WS₀The larger the negative value of
TDR is set to a large positive value, then the first air-fuel ratio
Value of TDR when the feedback control routine is executed
Grows. Therefore, conversely, the shaded area in Fig. 10 (B)
The area becomes large. Therefore, to perform the above operation
Therefore, the shaded area in Fig. 10 (B) is the target value WS₀Controlled by
Is done. That is, O from the catalyst₂Release amount is WS ₀Against
The target release amount will be controlled accordingly.

After controlling the amount of O ₂ released from the catalyst to the target value as described above, the routine proceeds to step 1127, where
Step 112 is performed after clearing the integrated value W ₁ and the counter N.
Proceed to 9. Steps 1129 to 1131 show the catalyst deterioration determination operation. In step 1129, it is determined whether or not the value of the above-mentioned clock counter CT is greater than or equal to the predetermined value CT ₂ , and when CT <CT ₂ , the routine is ended without performing catalyst deterioration determination. CT ₂ is set to a constant value larger than the above-mentioned CT ₁ (step 1115), and the O ₂ emission amount control started after the lapse of time CT ₁ after the condition of step 1101 is satisfied (steps 1119 to 1
125) allows sufficient time for the O ₂ emission amount to converge to the target value.

If the time (CT ₂ ) sufficient for the amount of O ₂ released from the catalyst to converge to the target value has elapsed in step 1129, then the deterioration determination subroutine is executed in step 1131. FIG. 13 shows step 11 in FIG.
31 is a flowchart showing details of a deterioration determination subroutine executed in step 31.

In FIG. 13, in step 1301,
The time counter KT for the catalyst deterioration determination operation time is incremented by one. In this subroutine, as will be described later, the trajectory lengths of the upstream O ₂ sensor 13 output VOM and the downstream O ₂ sensor 15 output VOS for a certain period of time are calculated, and the presence or absence of catalyst deterioration is determined based on this trajectory length. The counter KT is a counter for timing the trajectory length calculation period. Since this subroutine is executed every time the routines of FIGS. 11 and 12 are executed, the value of the counter KT becomes a value corresponding to the cumulative time during which the catalyst deterioration determining operation is executed.

Next, at step 1303, it is judged if the value of the counter KT has become a predetermined value KT ₀ or more. In the present embodiment, the value of KT ₀ is set to the number of times of routine execution in FIGS. 11 and 12 corresponding to about 20 seconds. That is, in this embodiment, the locus length of the O ₂ sensor output is calculated for a period of about 20 seconds, and the presence or absence of catalyst deterioration is determined based on the result.

If KT <KT ₀ in step 1303, upstream O ₂ in steps 1305 to 1309.
Path length LVOM of sensor 13 output and downstream O ₂ sensor 1
The 5-output trajectory length LVOS is integrated. In this embodiment, the locus lengths LVOM and LV of the outputs of the O ₂ sensors 13 and 15 are set.
The OSs are approximately | VOM-VOM _i-1 | and |
The integrated value of VOS-VOS _i-1 | is calculated by the following formula (see FIG. 14).

LVOM = LVOM + | VOM-VOM _i-1 | LVOS = LVOS + | VOS-VOS _i-1 | where VOM _i-1 and VOS _i-1 are the values of VOM and VOS at the time of execution of the previous routine, respectively. is there. That is, in step 1305, the output VOM of the upstream O ₂ sensor 13 and the output VOS of the downstream O ₂ sensor 15 are AD-converted and read, and in step 1307, the integrated values LVOM and LV are obtained.
It is calculated by the OS, and in step 1309, the values of VOM _i-1 and VOS _i-1 are updated in preparation for the next subroutine execution, and the routine ends. The above trajectory lengths LVOM and LV
The calculation of OS is repeated every time the subroutine is executed until the value of the counter KT reaches KT ₀ in step 1303.

On the other hand, if the value of the counter KT has reached the predetermined value KT ₀ in step 1303, the routine proceeds to step 1311 and the ratio of LVOS and LVOM calculated so far, that is, within a certain period. Track length ratio LVO
S / LVOM is calculated. In step 1313, it is determined whether or not the calculated value of the locus length ratio LVOS / LVOM is equal to or greater than a predetermined value R ₀ .

As described above, when the amount of O ₂ released from the catalyst is controlled to an appropriate constant value, the variation of the output VOS of the downstream O ₂ sensor 15 becomes large in the deteriorated catalyst, and the upstream O 2 _{2 The} output of the sensor 13 approaches the fluctuation of VOM. Therefore, the value of the locus length LVOS of the output of the downstream O ₂ sensor 15 also becomes large, and the value of LVOS / LVOM comes close to 1. Therefore, step 1313
And the value of LVOS / LVOM is a predetermined value R ₀ (R ₀ is 1.0
If it is equal to or more than the following constant), it is determined that the catalyst has deteriorated, and the value of the deterioration flag ALM is set to 1 in step 1315. When LVOS / LVOM <R ₀ , it is determined that the catalyst is normal, and the value of the deterioration flag ALM is set to 0 in step 1317.

When the value of the flag ALM is set to 1, the deterioration alarm 19 (FIG. 1) is lit by a separately executed routine (not shown) to notify the driver of the deterioration of the catalyst. Further, the value of the flag ALM set in steps 1315 and 1317 is stored in the backup RAM 106 of the control circuit 10 in step 1319 so as to be prepared for the next repair or inspection.

When the catalyst deterioration determining operation is completed, the integrated values LVOM and LVOS and the value of the counter KT are cleared in step 1321, and this subroutine is completed. As described above, according to the present embodiment, the time during which the exhaust air-fuel ratio swings to the rich side is adjusted according to the conditions of the exhaust gas flowing into the catalyst, and the O ₂ emission amount from the catalyst is set to a constant target value. After converging, the presence or absence of catalyst deterioration is determined. Therefore, even if the exhaust conditions flowing into the catalyst fluctuate, the amount of O ₂ released from the catalyst is maintained at a constant target value, and accurate catalyst deterioration determination can be performed.

In the embodiment of FIGS. 11 and 12, the catalyst deterioration determination is always executed when the condition of step 1101 (FIG. 11) is satisfied.
Instead of executing the catalyst deterioration determination of FIG. 11 and FIG. 12 every time the condition of 01 is satisfied, for example, the catalyst deterioration determination (FIG. 13) based on the trajectory length of the O ₂ sensor output during normal traveling is executed.
As a result, only when it is determined that the catalyst has deteriorated, the catalyst deterioration determination of FIGS. 11 and 12 may be performed to verify the determination result during normal traveling.

Further, in the embodiment shown in FIGS. 11 and 12, since the air-fuel ratio sensor is the O ₂ sensor which produces different output voltage depending on whether the exhaust air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio. , The degree of exhaust richness is estimated using the air-fuel ratio correction coefficient FAF. However, if an air-fuel ratio sensor that generates a continuous output corresponding to the exhaust air-fuel ratio is used as the air-fuel ratio sensor, It is possible to directly calculate the rich degree of exhaust from the output.

Furthermore, in the embodiments of FIGS. 11 and 12, the rich time of the air-fuel ratio control is adjusted by increasing or decreasing the rich delay time, but other control parameters, such as RSR, in the air-fuel ratio feedback control are changed. It is also possible to adjust the rich time.

[0078]

According to the present invention, the time during which the exhaust air-fuel ratio swings toward the rich side is adjusted according to the conditions of the exhaust gas flowing into the catalyst, and the O ₂ emission amount from the catalyst is converged to a constant target value. After that, the catalyst deterioration determination is performed based on the output of the downstream side air-fuel ratio sensor.Therefore, it is possible to accurately determine the catalyst deterioration regardless of the change in the exhaust conditions flowing into the catalyst. Play.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment in which the present invention is applied to an internal combustion engine for automobiles.

FIG. 2 is a flowchart showing a fuel injection amount control routine of the embodiment of FIG.

FIG. 3 is a part of a flowchart showing a first air-fuel ratio feedback control routine.

FIG. 4 is a part of a flowchart showing a first air-fuel ratio feedback control routine.

FIG. 5 is a part of a flowchart showing a second air-fuel ratio feedback control routine.

FIG. 6 is a part of a flowchart showing a second air-fuel ratio feedback control routine.

FIG. 7 is a timing diagram for supplementarily explaining the routines of FIGS. 3 and 4.

FIG. 8 is a diagram for explaining changes in air-fuel ratio fluctuations on the downstream side of the catalyst due to catalyst deterioration.

9 is a diagram showing changes in the downstream O ₂ sensor output corresponding to the air-fuel ratio fluctuations in FIG. 8.

FIG. 10 is a diagram illustrating the principle of controlling the amount of O ₂ released from the catalyst.

FIG. 11 is a part of a flowchart showing a catalyst deterioration determination routine.

FIG. 12 is a part of a flowchart showing a catalyst deterioration determination routine.

FIG. 13 is a flowchart showing a catalyst deterioration determination subroutine.

FIG. 14 is a diagram illustrating a trajectory length of an O ₂ sensor output.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine body 10 ... Control circuit 12 ... Catalytic converter 13 ... Upstream O ₂ sensor 15 ... Downstream O ₂ sensor

Claims (1)

[Claims] 1. O ₂ disposed in an exhaust passage of an internal combustion engine
A three-way catalyst having a storage function, an upstream air-fuel ratio sensor which is arranged in an exhaust passage on the upstream side of the three-way catalyst and detects an exhaust air-fuel ratio on the upstream side of the three-way catalyst, and a downstream side of the three-way catalyst. A downstream side air-fuel ratio sensor for detecting an exhaust air-fuel ratio on the downstream side of the three-way catalyst, based on at least the output of the upstream side air-fuel ratio sensor,
Air-fuel ratio feedback control means for periodically changing the exhaust air-fuel ratio flowing into the three-way catalyst alternately between the rich air-fuel ratio side and the lean air-fuel ratio side centering on the theoretical air-fuel ratio, and the exhaust gas flowing into the three-way catalyst based on the conditions, and O ₂ emission calculation means for calculating the O ₂ release from the three-way catalyst, as O ₂ release amount calculated by the O ₂ emission calculation unit becomes a predetermined target value , An emission amount control means for setting a time period during which the air-fuel ratio feedback control controls the exhaust air-fuel ratio to a rich side, and at least the downstream side when the O ₂ emission amount is controlled to a target value by the emission amount control means A catalyst deterioration determination device for an internal combustion engine, comprising: deterioration determination means for determining the presence or absence of deterioration of the three-way catalyst based on the output of the side air-fuel ratio sensor.