JP3139328B2 - Catalyst deterioration determination device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to feedback control of an engine air-fuel ratio to a target air-fuel ratio at least based on an air-fuel ratio sensor output upstream of a catalytic converter, and at least based on an air-fuel ratio sensor output downstream of a catalytic converter. The present invention relates to a catalyst deterioration determination device for an internal combustion engine that determines whether a catalytic converter has deteriorated.

[0002]

2. Description of the Related Art An air-fuel ratio sensor is disposed in each of an exhaust passage on an upstream side and a downstream side of a catalytic converter of an exhaust system of an internal combustion engine, and an engine air-fuel ratio is determined based on at least an output of the air-fuel ratio sensor on the upstream side. And a technique for determining the presence or absence of catalyst deterioration based on the output of the downstream air-fuel ratio sensor.

The three-way catalyst adsorbs oxygen in exhaust gas when the passing air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and when the passing exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio. Performs an O ₂ storage function of releasing adsorbed oxygen. For this reason, even when 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, the catalytic converter passes The subsequent fluctuation in the air-fuel ratio of the exhaust gas is reduced by the O ₂ storage action of the catalyst, and the exhaust air-fuel ratio downstream of the catalyst is maintained near the stoichiometric air-fuel ratio. Therefore, if the three-way catalyst is normal, the amplitude of the output of the downstream air-fuel ratio sensor is small and the fluctuation period is long.

On the other hand, since the O ₂ storage action of the catalyst decreases in accordance with the deterioration of the catalyst, when the three-way catalyst deteriorates, the amount of oxygen that can be adsorbed by the catalytic converter decreases, and the fluctuation of the exhaust air-fuel ratio downstream of the catalytic converter is reduced. It becomes closer to the fluctuation of the exhaust air-fuel ratio on the upstream side of the catalytic converter. That is,
When the exhaust air-fuel ratio flowing into the catalyst fluctuates toward the rich air-fuel ratio, oxygen adsorbed from the catalyst is released, but O ₂
If the storage action is reduced, the amount of oxygen that can be adsorbed by the catalyst decreases, so if the inflow exhaust air-fuel ratio becomes rich, the deteriorated catalyst will release the adsorbed oxygen in a short time, and thereafter the inflow exhaust air-fuel ratio Is unable to release oxygen even if the fuel oil fluctuates toward the rich air-fuel ratio side. For this reason, after the catalyst has exhausted oxygen, the exhaust air-fuel ratio downstream of the catalyst also swings toward the rich air-fuel ratio. In addition, even when the inflow exhaust air-fuel ratio fluctuates to the lean air-fuel ratio side, the oxygen adsorbed amount of the catalyst is saturated in a short time because the maximum amount of oxygen that can be adsorbed by the deteriorated catalyst is reduced. When the inflowing exhaust air-fuel ratio fluctuates toward the lean air-fuel ratio, oxygen in the exhaust gas cannot be adsorbed, and the exhaust air-fuel ratio downstream of the catalyst also fluctuates toward the lean air-fuel ratio. That is, when the three-way catalyst is deteriorated, the amplitude of the output of the downstream air-fuel ratio sensor becomes large, and the fluctuation cycle becomes short, and approaches the amplitude and cycle of the output of the upstream air-fuel ratio sensor.

Therefore, if the engine air-fuel ratio fluctuates regularly between the lean air-fuel ratio side and the rich air-fuel ratio side with a relatively short cycle around the stoichiometric air-fuel ratio, the downstream air-fuel ratio By monitoring the sensor output, it is possible to determine whether the three-way catalyst has deteriorated. Tokuho 7-26578
This publication discloses a catalyst deterioration determining device that detects deterioration of the O ₂ storage function and determines deterioration of the catalyst.

[0006] the publication, the O ₂ sensor the air-fuel ratio of exhaust gas to generate different output voltages depending on whether rich or lean air-fuel ratio sensor disposed on an upstream side and a downstream side of the catalyst, the air-fuel ratio feedback control in in which the downstream O ₂ sensor output rich inversion period of the lean was made to determine the catalyst has deteriorated when it becomes shorter than a predetermined value. Also,
In this publication, in order to prevent an erroneous determination, the catalyst deterioration determination is performed only when the intake air amount per one rotation of the engine is within a predetermined deterioration determination execution region.

[0007] The amount of oxygen released from the catalyst changes according to the exhaust gas flow rate even if the level of the O ₂ storage action is the same. In other words, even if the level of the O ₂ storage action is the same, if the exhaust gas flow rate is large, the amount of oxygen absorbed and released by the catalyst per unit time increases. For this reason, even in the case of a normal catalyst in which the O ₂ storage effect is not reduced in a state where the exhaust gas flow rate is excessive, oxygen is released in a short time while the inflow exhaust air-fuel ratio swings to the rich air-fuel ratio side. After that, oxygen cannot be released. Similarly, the exhaust flow rate is <br/> excessive state, even normal catalyst O ₂ storage operation is not decreased, briefly while the inflow exhaust air-fuel ratio is oscillated at the lean air-fuel ratio side After that, the catalyst is saturated, so that oxygen in the exhaust gas cannot be adsorbed thereafter. For this reason, when the exhaust gas flow rate is excessive, even if the catalyst is normal, the output of the downstream air-fuel ratio sensor fluctuates in the same manner as the output of the upstream air-fuel ratio sensor, and the normal catalyst is degraded. An erroneous determination may occur.

[0008] In the under-state exhaust gas flow rate in the reverse, since the absorption and volume of oxygen from the catalyst per unit time decreases, O ₂ is reduced inflow exhaust air-fuel ratio even deteriorated catalyst of storage operation The entire amount of adsorbed oxygen cannot be released while swinging to the rich air-fuel ratio side, and oxygen cannot be absorbed to the saturation amount while the inflow exhaust air-fuel ratio swings to the lean air-fuel ratio side. For this reason, when the exhaust gas flow rate is too small, the output fluctuation of the downstream air-fuel ratio sensor becomes small even for a deteriorated catalyst, and a case where the deteriorated catalyst is erroneously determined to be normal may occur.

The apparatus disclosed in Japanese Patent Publication No. Hei 7-26578 is designed to prevent erroneous determination due to the fluctuation of the exhaust gas flow rate.
The catalyst deterioration determination is performed only when the exhaust flow rate is within a predetermined diagnostic range. On the other hand, the catalyst O ₂
There is a problem that the storage action varies greatly depending on the catalyst temperature as well as the degree of deterioration of the catalyst. That is, the O ₂ storage effect of the catalyst decreases as the catalyst deteriorates, but for a catalyst having the same degree of deterioration, the O ₂ storage effect increases as the catalyst temperature increases.
_{2 The} storage effect is high, and the lower the catalyst temperature, the lower the O ₂ storage effect. Therefore, if the catalyst deterioration is detected based on the output of the downstream air-fuel ratio sensor under different catalyst temperatures, the output of the downstream air-fuel ratio sensor changes differently even with the same catalyst. In some cases, the deterioration of the catalyst cannot be accurately determined.

[0010] For example, the catalyst may be the degree of degradation is still sufficiently used low, but is determined to be normal by performing the deterioration determination in a high temperature state, O ₂ storage effect of low temperatures the deterioration determination in a low temperature state May be erroneously determined to be deteriorated due to a decrease in On the other hand, even if the catalyst has deteriorated and needs to be replaced, the O ₂ storage effect increases at high temperatures. Therefore, when the deterioration is determined in a high temperature state, it is erroneously determined that the catalyst is normal despite being deteriorated. Cases arise.

In order to solve this problem, Japanese Patent Laid-Open Publication No.
Japanese Patent No. 48227 discloses a catalyst deterioration determination device that changes a determination value used for catalyst deterioration determination depending on a catalyst temperature. The catalyst deterioration determination device disclosed in the publication is disclosed in
In the same manner as in the device disclosed in Japanese Patent No. 26578, the area surrounded by the locus of the downstream O ₂ sensor output during the air-fuel ratio feedback control, the lean reversal cycle, or the locus of the upstream O ₂ sensor output and the locus of the downstream O ₂ sensor output. In order to determine that the catalyst has deteriorated when the difference from the enclosed area is equal to or less than a predetermined determination value, and to prevent erroneous determination due to a change in the O ₂ storage action due to the catalyst temperature. The determination value used for the above-mentioned deterioration determination is changed according to the catalyst temperature.

[0012]

However, as in the apparatus disclosed in Japanese Patent Application Laid-Open No. Hei 5-248227, there is a case where the deterioration of the catalyst cannot be accurately determined only by changing the determination value for the deterioration determination in accordance with the catalyst temperature. . For example, as described above, the amount of oxygen absorbed and released from the catalyst fluctuates according to the exhaust flow rate. Therefore, when the exhaust flow rate fluctuates during the deterioration determination, not only does the determination value change depending on the catalyst temperature, but also depends on the exhaust flow rate. Also, accurate deterioration determination cannot be performed unless the determination value is changed. Also, in order to determine catalyst deterioration, it is necessary to monitor the output of the downstream air-fuel ratio sensor for a certain period of time,
When the catalyst temperature fluctuates during monitoring, there is a problem that even if the determination value is determined based on the temperature at the time of determining the deterioration after the end of monitoring, the deterioration cannot be accurately determined.

The output of the downstream air-fuel ratio sensor is rich,
The area surrounded by the lean inversion cycle and the sensor output trajectory is
Even if the degree of deterioration of the catalyst is the same, since it greatly varies depending on the exhaust flow rate and the response of the O ₂ sensor, etc., erroneous determination is prevented even if the discrimination value is changed according to the catalyst temperature in an area where the variation is originally large. It is not possible.

On the other hand, the catalyst deterioration determination is performed in an operating state in which the above-described problem does not occur (for example, a state in which the catalyst temperature and the exhaust flow rate are kept constant for a certain period of time in a region where the determination result does not vary). If it is executed only at the time, it is possible to prevent the erroneous determination and perform the accurate catalyst deterioration determination. However, since such conditions rarely continue for a long time in actual operation, if the catalyst deterioration determination is performed only when such a condition is satisfied, the frequency of performing the catalyst deterioration determination is large. And deterioration of the catalyst cannot be detected early.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a catalyst deterioration determining apparatus for an internal combustion engine capable of accurately determining the presence or absence of catalyst deterioration while maintaining a high frequency of catalyst deterioration determination while preventing erroneous determination. It is intended to provide.

[0016]

According to the present invention, there is provided a three-way catalyst having an O ₂ storage function disposed in an exhaust passage of an internal combustion engine, and disposed in 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-fuel ratio sensor disposed in the exhaust passage on the downstream side of the three-way catalyst for detecting the exhaust air-fuel ratio on the downstream side of the three-way catalyst. A fuel ratio sensor, and air-fuel ratio feedback control means for performing feedback control of an engine air-fuel ratio to a target air-fuel ratio based on at least an output of the upstream air-fuel ratio sensor;
Catalyst temperature detecting means for detecting the temperature of the three-way catalyst, intake air amount detecting means for detecting the amount of engine intake air, and during execution of the air-fuel ratio feedback control, the engine intake air amount falls within a predetermined determination execution area. When the three-way catalyst has deteriorated based on at least the output of the downstream air-fuel ratio sensor, and a temperature of the three-way catalyst detected by the catalyst temperature detecting means, An apparatus for determining catalyst deterioration of an internal combustion engine, comprising: an area setting means for setting the determination execution area of the engine intake air amount.

[0017]

The area setting means sets an engine intake air amount area in which the deterioration determining means determines the presence or absence of catalyst deterioration according to the catalyst temperature. In the engine intake air amount area, an area is set in which there is little variation in the determination result of the presence or absence of catalyst deterioration based on the output of the downstream air-fuel ratio sensor for each catalyst temperature and the presence or absence of deterioration can be clearly determined.

[0018]

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 a vehicle. In FIG. 1, 1 is an internal combustion engine main 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. In FIG. 1, reference numeral 3 denotes an air flow meter for detecting an intake air amount of the engine 1. As the air flow meter 3, for example, a movable vane type having 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 having an opening corresponding to the operation amount of the accelerator pedal by the driver.
In the vicinity of 6, an idle switch 17 for generating an idle state signal (LL signal) when the throttle valve 16 is fully closed is provided.

In FIG. 1, reference numeral 7 denotes an intake manifold 2a.
Are the fuel injection valves arranged near the intake ports of the respective cylinders. The fuel injection valve 7 opens in response to a signal from a control circuit 10 described later, and injects pressurized fuel for each intake port of each cylinder. The control of the fuel injection amount 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 12, HC in the exhaust gas built-in three-way catalyst, CO, NO _X
Can be purified simultaneously. An upstream air-fuel ratio sensor 13 is provided on the upstream side of the catalytic converter 12, that is, an exhaust gas collecting portion of the exhaust manifold 11, and a downstream air-fuel ratio sensor 1 is provided on a downstream exhaust pipe 14 of the catalytic converter 12.
5 are provided. In this embodiment, an O ₂ sensor that generates a voltage signal according to the concentration of the oxygen component in the exhaust gas is used as the air-fuel ratio sensors 13 and 15. That, O ₂ sensor 13 and 15 respectively exhaust air-fuel ratio to generate different output voltages in accordance with the lean air-fuel ratio side or a rich air-fuel ratio side or the theoretical air-fuel ratio.

Reference numeral 18 in FIG. 1 denotes a secondary air introduction valve (ASV) for introducing secondary air into the exhaust system. The secondary air introduction valve 18 performs an operation to reduce HC and CO emissions by introducing secondary air into the exhaust manifold 11 at the time of engine deceleration, idling operation, and the like. Further, the ignition distributor 4 of the engine 1 is provided with two crank angle sensors 5 and 6 for generating a pulse signal at every constant rotation of the engine crankshaft. In this embodiment, the crank angle sensor 5 outputs a pulse signal for detecting the reference position, for example, every time the specific cylinder reaches the compression top dead center (ie, every 720 ° of the crank rotation angle). A pulse signal for detecting the crank rotation angle is output every 30 rotation angles.

The water jacket 8 of the cylinder block of the engine 1 is provided with a cooling water temperature sensor 9 for outputting an analog voltage corresponding to the engine cooling water temperature. The control circuit 10 includes, for example, an input / output interface 102,
A microcomputer having a well-known configuration in which a CPU 103, a ROM 104, and a RAM 105 are connected to each other by a bidirectional bus.
1. a backup RAM 106 which is directly connected to a power supply and can retain the stored contents even when the engine switch is off;
A clock generation circuit 107 and the like are provided.

The control circuit 10 performs basic control such as fuel injection control and ignition timing control of the engine. In the present embodiment, the upstream O ₂ sensor 13 and the downstream O ₂
-Fuel ratio feedback control based on the output of
Is performed. In order to execute these controls, the control circuit 10 supplies the engine intake air amount signal from the air flow meter 3 via the AD converter 101, the cooling water temperature signal from the cooling water temperature sensor 9, and the O ₂ sensors 13 and 15 to the control circuit 10. , A pulse signal from the crank rotation angle sensors 5 and 6, an idle signal from the idle switch 17, and the like are input via 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 at regular intervals, and are stored in predetermined areas of the RAM 105 as engine intake air amount data Q and cooling water temperature data THW, respectively. Is stored. Further, every time a pulse signal from the crank rotation angle sensor 6 is input, an engine rotation speed is calculated from a pulse interval by a routine (not shown) and stored in a predetermined area of the RAM 105 as engine rotation speed data Ne.

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. 108, 1 in FIG.
Reference numerals 09 and 110 denote 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 a 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 drives the fuel injection valve 7 Output a signal. As a result, the fuel injection valve 7 opens to start fuel injection. The down counter 108 counts the clock signal of the clock 107 and outputs a set signal to the flip-flop 109 when a 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 closes. Accordingly, the fuel injection valve 7 is opened for a time corresponding to the calculated fuel injection time TAU, and an amount of fuel corresponding to TAU is injected from the fuel injection valve 7 to the engine 1.

The control circuit 10 is connected via an input / output interface 102 to the aforementioned secondary air introduction valve 18 and an alarm 19 that is activated when the catalyst is deteriorated.
In the present embodiment, as described later, the temperature of the three-way catalyst 12 is indirectly detected from the engine operating state, and the engine is operated in an exhaust flow rate (intake air amount) region set according to the catalyst temperature. The upstream O ₂ sensor output trajectory length LVOM
LVOS between the output locus length LVOS and the downstream O ₂ sensor output locus length LVOS
It is determined whether the catalyst 12 has deteriorated based on the value of / LVOM. FIG. 2 shows the upstream O ₂ during the air-fuel ratio feedback control.
Sensor 13 output VOM and downstream O ₂ sensor 15 output VO
It is a figure which shows the change of S by catalyst deterioration typically. FIG. 2A shows changes in VOM and VOS when the catalyst is normal. As described above, when the catalyst is normal, even when the air-fuel ratio of the exhaust gas flowing into the catalyst had changed into a cyclically lean and rich by the air-fuel ratio feedback control, the catalyst downstream the O ₂ storage function of the catalyst The fluctuation of the exhaust air-fuel ratio on the catalyst side is reduced, and the exhaust air-fuel ratio on the downstream side of the catalyst fluctuates between lean and rich in a long cycle due to the influence of a second air-fuel ratio feedback control described later. Accordingly, the upstream O ₂ sensor output VOM and the downstream O ₂
The change from the sensor output VOS is as shown in FIG. In this case, the length LVO of the locus between VOM and VOS
Comparing M and LVOS (see FIG. 2 (A)), VOM has a locus length LVOM because the fluctuation in a relatively short cycle is repeated.
Becomes larger, while the fluctuation period of the VOM is extremely long, so that the locus length LVOS becomes smaller. Therefore, if the catalyst is normal, the ratio LVOS / LVOM between LVOS and LVOM becomes an extremely small value.

On the other hand, when the catalyst is deteriorated and the O ₂ storage function is reduced, the amount of oxygen that can be adsorbed by the catalyst decreases, so that the catalyst adsorbs while the inflow exhaust air-fuel ratio fluctuates toward the rich air-fuel ratio. Since the entire amount of oxygen is released, and the amount of adsorbed oxygen is saturated while swinging to the lean air-fuel ratio side, the exhaust air-fuel ratio on the downstream side also fluctuates in a relatively short cycle like the upstream side. Therefore, the catalyst becomes short fluctuation period of the downstream O ₂ sensor output VOS as deteriorated, when the catalyst has deteriorated significantly as shown in FIG. 2 (B), the downstream O ₂ sensor output VOS even upstream It changes like the O ₂ sensor output VOS. Therefore, the trajectory length ratio LV between the downstream O ₂ sensor output and the upstream O ₂ sensor output
OS / LVOM increases as the catalyst degrades and increases to 1.0
Get closer to.

In this embodiment, the control circuit 10 determines the trajectory length LVOS of the downstream O ₂ sensor output and the upstream O 2
₂₎ The locus length LVOM of the sensor output is calculated, and when the ratio LVOS / LVOM of these locus lengths becomes equal to or greater than a predetermined value, it is determined that the catalyst has deteriorated, and the LVOS / LV is determined.
When OM is equal to or less than the predetermined value, it is determined that the catalyst is normal.

As described above, in the catalyst deterioration determination of this embodiment, the air-fuel ratio flowing into the catalyst is periodically feedback-controlled to the rich air-fuel ratio side and the lean air-fuel ratio side around the stoichiometric air-fuel ratio. Is required. Therefore, before describing the detection of deterioration of the catalyst, the air-fuel ratio feedback control of the present embodiment, which is a premise thereof, will be briefly described first.

FIG. 3 is a flowchart showing a fuel injection amount calculation routine according to 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. 3, 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 one rotation of the engine and an air-fuel ratio correction coefficient FAF described later.

That is, in the routine of FIG. 3, the intake air amount data Q and the rotation speed data Ne are read from predetermined areas of the RAM 105, and the intake air amount Q / N per one rotation of the engine is read.
e is calculated (step 301), and the basic fuel injection time TAUP is calculated as TAUP = α × Q / Ne (step 302). Here, the basic fuel injection time TAUP is a fuel injection time required to make the air-fuel mixture supplied to the combustion chamber a stoichiometric air-fuel ratio.
Is a constant.

The actual fuel injection time TAU is calculated as the value obtained by correcting the above TUP with the air-fuel ratio correction coefficient FAF, TAU = TAUP × FAF × β + γ (step 303). Where β, γ
Are constants determined according to the engine operating state. When the fuel injection time TAU is calculated as described above, in step 304, the time TAU is counted down by the down counter 10.
8 is set, and the amount of fuel corresponding to the time TAU is injected from the fuel injection valve 7.

Next, at step 303, the air-fuel ratio correction coefficient F
The calculation of AF will be described. The air-fuel ratio correction coefficient FAF is determined based on the first air-fuel ratio feedback control based on the output of the upstream O ₂ sensor 13 and the _second air-fuel ratio feedback control based on the output of the downstream O ₂ sensor 15.
Of the air-fuel ratio feedback control. FIGS. 4 and 5 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 fixed time intervals (for example, every 4 ms).

In this routine, the output VOM of the upstream O ₂ sensor 13 is compared with a comparison voltage V _R1 (voltage corresponding to the stoichiometric air-fuel ratio), and the exhaust air-fuel ratio upstream of the catalytic converter is richer than the stoichiometric air-fuel ratio (VOM> reducing the air-fuel ratio correction amount FAF when the V _R1), performs control to increase the FAF when the lean (VOM ≦ V _R1). The O ₂ sensor outputs a voltage signal of, for example, 0.9 volts when the exhaust air-fuel ratio is on the rich air-fuel ratio side from the stoichiometric air-fuel ratio. A voltage signal of about 1 volt is output. In this embodiment, the comparison voltage _VR1 is set to about 0.45 volt. By increasing / decreasing the air-fuel ratio correction amount FAF according to the exhaust air-fuel ratio as described above, even when some errors occur in the fuel supply system such as the air flow meter 3 and the fuel injection valve 7, the engine air-fuel ratio is increased. Is corrected exactly to near the stoichiometric air-fuel ratio.

Hereinafter, the flowcharts of FIGS. 4 and 5 will be briefly described. Step 401 shows a determination as to whether or not a feedback control execution condition is satisfied. The feedback control execution conditions include, for example, that the O ₂ sensor is activated, that the engine warm-up has been completed, that a predetermined time has elapsed after returning from fuel cut, that the secondary air introduction valve 18 That is, the next air is not supplied, and the FAF calculation in step 402 and subsequent steps is performed only when the execution condition is satisfied. If the feedback control execution condition is not satisfied, the routine proceeds to step 425 in FIG. 5, sets the value of the flag XMFB to 0, and ends the routine. The flag XMFB is a flag indicating whether or not the first air-fuel ratio feedback control is being executed.
XMFB = 0 means that the first air-fuel ratio feedback control is stopped.

Steps 402 to 415 show the determination of the air-fuel ratio. Flag F1 shown in steps 409 and 415
Indicates that the engine air-fuel ratio is rich (F1 = 1) or lean (F1 =
0) is an air-fuel ratio flag indicating whether F1 = 0 to F1 =
1 (lean to rich), the upstream O ₂ sensor 13 continues the rich signal (V
OM > V _R1) when the output (step 403,4
10 from 415), also F1 = 1 from the F1 = 0 (lean switched from rich lean) to the upstream side O ₂ sensor 13 for the predetermined period (-TDL) or signal (VOM ≤
(V _R1 ) is output (steps 403 to 409). CDLY is a counter for determining the air-fuel ratio flag switching timing.

In steps 416 to 423 in FIG. 5, the FAF is increased or decreased according to the value of the flag F1 set as described above. That is, by comparing the value of F1 at the time of executing the current routine with the value of F1 at the time of executing the previous routine, it is determined whether the value of F1 has changed, that is, whether the air-fuel ratio has been inverted from rich to lean or from lean to rich. (Step 41
6). If the current value of F1 is F1 = 0 (lean), the FAF is skipped by a relatively large value RSR immediately after the change (inversion) from F1 = 1 to F1 = 0 (rich to lean). (Steps 417, 4
18) Thereafter, as long as F1 = 0, the FAF is gradually increased by a relatively small value KIR every time the routine is executed (steps 420 and 421). Similarly, when the current value of F1 is F1 = 1 (rich), first, F1 = 0 to F1
Immediately after inverting 1 = 1 (lean to rich), the FAF is skipped and the FAF is reduced by RSL (steps 417 and 417).
19) Thereafter, as long as F1 = 1, the FAF is gradually reduced by KIL every time the routine is executed (step 42).
0, 422). After guarding the value of the FAF calculated as described above so as not to exceed the range defined by the maximum value (FAF = 1.2 in the present embodiment) and the minimum value (FAF = 0.8 in the present embodiment) (step). 423), the value of the flag XMFB is set to 1 (step 424), 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. 6 and 7 show a second air-fuel ratio feedback control routine. This routine is performed by the control circuit 10 at a predetermined interval longer than the first air-fuel ratio feedback control (for example, 5
(Every 00 ms). In this routine, the downstream O
₂ The output VOS of the sensor 15 is compared with a comparison voltage V _R2 (the voltage corresponding to the stoichiometric air-fuel ratio, for example, 0.45 volt), and the exhaust air-fuel ratio downstream of the catalytic converter is richer than the stoichiometric air-fuel ratio (VOS> V _R2 ). Sometimes, the correction amount RSR (step 418 in FIG. 5) used in the first air-fuel ratio feedback control is decreased, and the RSL (step 419 in FIG. 5) is increased. When the exhaust air-fuel ratio on the downstream side of the catalytic converter is leaner than the stoichiometric air-fuel ratio (VOS ≦ _VR2 ), an operation of increasing the correction amount RSR and decreasing the RSL is performed. As a result, when the exhaust air-fuel ratio is rich on the downstream side of the catalytic converter, the value of FAF is generally set small in the first air-fuel ratio feedback control, and conversely, the exhaust air-fuel ratio on the downstream side is reduced. FA for rich
The value of F is generally set to be large. Therefore, even when the upstream O ₂ sensor 13 is deteriorated or strongly affected by the exhaust of a specific cylinder, the output of the upstream O ₂ sensor 13 deviates from the actual exhaust air-fuel ratio. Since the correction is made 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.

In the following, the flowcharts of FIGS. 6 and 7 will be briefly described. In FIG. 6, steps 601 to 606 show the determination as to whether or not the conditions for executing the feedback control are satisfied. The execution condition of the second air-fuel ratio feedback control is that the first air-fuel ratio feedback control is being executed (XMFB =
1) (step 601), that the engine warm-up is completed (the cooling water temperature THW is a predetermined value (for example, 70
° C) or higher) (step 602), the engine is not idling (L from the idle switch 17).
L signal is not input) (step 603), secondary air is not introduced (step 604), and engine load (intake air amount Q / N per one engine revolution)
e) is equal to or greater than a predetermined value (step 605), and the downstream O ₂ sensor is activated (step 60).
6) is a condition. If any one of the above conditions is not satisfied, the second air-fuel ratio feedback control is not executed, and the flag XSFB is determined in step 619.
Is set to 0, and the routine ends. Flag X
SFB is a flag indicating whether or not the second air-fuel ratio feedback control is being executed, and XSFB = 0 means that the second air-fuel ratio feedback control is stopped.

If all the conditions of Steps 601 to 606 are satisfied, the routine proceeds to Step 608 where the flag XSFB is set.
Is set to 1 and the correction amounts RSR and R are determined based on whether the exhaust air-fuel ratio detected by the downstream O ₂ sensor 15 is rich or not.
An operation of increasing or decreasing the value of SL is performed. That is, the output VOS of the downstream O ₂ sensor 15 in FIG. 7 step 609 AD
In step 610, it is determined whether or not VOS is a lean air-fuel ratio equivalent value (VOS ≦ _VR2 ).
Is a lean air-fuel ratio equivalent value, the value of RSR is increased by a fixed amount ΔRS in step 611, and the RSR after the increase is increased to a predetermined maximum value MAX (MAX =
0.09) (step 61).
2,613). If the value of VOS is a rich air-fuel ratio equivalent value (VOS> _VR2 ) in step 606,
In step 614, the value of RSR is reduced by a fixed amount ΔRS, and guard is performed so that the reduced RSR does not become smaller than a predetermined minimum value MIN (in this embodiment, MIN = 0.01) (steps 615 and 616).

At step 617, the value of RSL (step 419 in FIG. 5) used in the first air-fuel ratio feedback control routine is calculated as RSL = 0.1-RSR using the RSR value calculated as described above. . That is, in this embodiment, the sum of RSR and RSL is always kept at a constant value (0.1).
As 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, the RSR decreases and the RSL increases, and when the exhaust air-fuel ratio becomes lean, In such a case, the increase of the RSR and the decrease of the RSL are performed simultaneously. FIG. 8 shows a case where the first air-fuel ratio feedback control of FIGS. 4 and 5 is performed.
The counter CDLY (same (B)), the flag F1 (same (C)), and the air-fuel ratio correction coefficient FAF (same as above) with respect to the change in the air-fuel ratio (A / F) detected by the upstream O ₂ sensor 13 (FIG. 8A). (D))
Shows the change. As shown in FIG. 8A, even when the A / F changes from lean to rich, the air-fuel ratio flag F1
The value of (FIG. 8 (C)) does not change immediately from 0 to 1, during the time until the value of the counter CDLY increases from 0 to TDR (Fig. 8 (C) T ₁₎ is kept at the 0 It is changed from 0 after T ₁ elapses 1. Further, during the A / F is time to reduce the values of the counter CDLY of F1 may have changed from rich to lean is 0 to TDL (TDL is a negative value) (FIG. 8 (C) T ₂₎ is held at 1, changes from 1 after T ₂ elapses 0. Therefore, even when the output of the upstream O ₂ sensor 13 changes in a short cycle due to disturbance or the like as indicated by N in FIG. 8A, the value of the flag F1 does not change and the air-fuel ratio Control becomes stable.

As a result of the first air-fuel ratio feedback control,
The value of the air-fuel ratio correction coefficient FAF periodically increases and decreases as shown in FIG. 8D, and the engine air-fuel ratio alternately fluctuates between a rich air-fuel ratio and a lean air-fuel ratio. As described in FIG. 3, the fuel injection time TAU increases as the value of FAF increases, and the fuel injection time TAU decreases as the value of FAF decreases.

As can be seen from FIG. 8D, when RSR increases and RSL decreases by the second air-fuel ratio feedback control (FIGS. 6 and 7), the swing width toward the rich air-fuel ratio increases. The overall air-fuel ratio shifts to the rich air-fuel ratio side). Conversely, when RSR decreases and RSL increases,
The swing width of the engine air-fuel ratio toward the lean air-fuel ratio increases, and the air-fuel ratio shifts to the lean air-fuel ratio 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 air-fuel ratio side or the lean air-fuel ratio side. Note that in the present embodiment, R
The case where SR and RSL are set has been described.
The engine air-fuel ratio can also be changed by setting another correction amount in the air-fuel ratio control in the second air-fuel ratio feedback control. For example, KIR, KIL (FIG. 5)
Step 421, 422) value, or TDR, TDL
The engine air-fuel ratio can be similarly changed by setting the value of (steps 408, 413 in FIG. 5) based on the second air-fuel ratio feedback control, or the comparison of the upstream O ₂ sensor 13 can be made. The engine air-fuel ratio can be similarly changed by setting the value of the voltage V _R1 (step 403 in FIG. 4) based on the second air-fuel ratio feedback control. Next, determination of catalyst deterioration according to this embodiment will be described. As described above, in the present embodiment, the presence or absence of catalyst deterioration is determined based on the trajectory length ratio between the output of the downstream O ₂ sensor and the output of the upstream O ₂ sensor (FIG. 2A,
(B)). However, the fluctuation of the output of the downstream O ₂ sensor varies depending on various conditions even if the degree of deterioration of the catalyst is the same.

For example, the main factors affecting the fluctuation of the output of the downstream O ₂ sensor are the catalyst temperature and the exhaust gas flow rate. That is, the higher the catalyst temperature, the higher the O ₂ storage capacity of the catalyst. Therefore, if the other conditions are the same, the downstream O ₂ sensor output becomes less likely to fluctuate as the catalyst temperature increases (ie, the locus length ratio LVOS / LVOM). Is smaller), and the lower the catalyst temperature is, the more likely it is to change (that is, the locus length ratio LVOS / LVOM increases).

Also, the larger the flow rate of exhaust gas flowing into the catalyst, the more the catalyst releases the entire amount of adsorbed oxygen in a shorter time,
In addition, it adsorbs oxygen to a saturated amount in a short time. For this reason, if the other conditions are the same, the output of the downstream O ₂ sensor tends to fluctuate as the exhaust gas flow rate increases (ie, the trajectory length ratio L
VOS / LVOM increases ), and the smaller the exhaust flow rate, the more difficult it is to change (that is, the locus length ratio LVOS / LV).
OM is smaller).

Further, while maintaining the catalyst temperature and the exhaust gas flow rate constant using the same catalyst, the trajectory length ratio LVOS / LV
Even when OM is measured, the locus length ratio LVOS / LVOM
It has been found that the measurement results vary relatively greatly due to the response of the O ₂ sensor and the like. 9 (A) and 9 (B) show the actual downstream of the normal catalyst (FIG. 9 (A)) and the deteriorated catalyst (FIG. 9 (B)) by changing the engine intake air amount Q and the catalyst temperature T _CAT. The figure shows the results of actual measurement of the trajectory length ratio LVOS / LVOM between the output of the side O ₂ sensor and the output of the upstream O ₂ sensor. Since the engine intake air amount Q is substantially equal to the exhaust flow rate, Q in FIGS. 9A and 9B can be considered as the exhaust flow rate.

Referring to FIG. 9 (A) (normal catalyst), even for the same catalyst, the trajectory length ratio LVOS / LVOM shows the exhaust flow rate Q (Q ₁ <Q ₂ <Q ₃ ), the catalyst temperature T _CAT and Changes, the value of LVOS / LVOM varies greatly even at the same exhaust flow rate and catalyst temperature. For example, the exhaust flow rate Q ₁ in FIG. 9 (A), the the catalyst temperature is seen for the case of T _CAT1, LVOS / LVOM
Values vary between the maximum value R ₁ and the minimum value R _2, and the magnitude of the variation is ± 0.3 with respect to the center value at the maximum.
It is about. The size of the variation becomes smaller when the catalyst temperature is high to some extent, for example, T _{CA T} 20
In the region above 0 ° C., the variation is relatively small. This tendency is observed when the flow rate is larger (Q ₂ , Q ₃ )
The same applies to. That is, the variation in the trajectory length ratio changes according to the combination of the catalyst temperature and the exhaust flow rate.

In general, when the measurement is performed in an ideal state with no variation, if the LVOS / LVOM is 0.5 or less, the catalyst has not deteriorated and it can be determined that the catalyst is normal. In the state where the trajectory length ratio varies by about ± 0.3 at the maximum even at the same exhaust flow rate and temperature as in (A), if the trajectory length ratio determination value for normality determination is set to 0.5,
A catalyst having a trajectory length ratio of about 0.5 when measured in an ideal state has a trajectory length ratio of 0.5 when measured during actual operation.
It will vary from 2 to 0.8. For this reason, even if the determination value (0.5) is changed in accordance with the temperature and the exhaust gas flow rate, it is difficult to make an accurate deterioration determination.

That is, when the determination value is set to 0.5, the normal catalyst (a catalyst having a locus length ratio smaller than 0.5 measured in an ideal state) is always determined to be normal. Considering the variation of the ratio, the deterioration judgment is performed under the conditions of the catalyst temperature and the exhaust gas flow rate at which the trajectory length ratio of the normal catalyst always becomes 0.2 or less (a value obtained by subtracting the magnitude of the variation 0.3 from 0.5). Need to do. That is, if the catalyst deterioration is determined under the other conditions of the catalyst temperature and the exhaust gas flow rate, the normal catalyst may be determined to be deteriorated due to the influence of the variation. (For example, in FIG. 9A, the catalyst temperature T
_CAT1, when performing the deterioration determination in the conditions of the exhaust gas flow rate Q _1, either value also becomes possible to obtain take, trajectory length even normal catalyst between the measured trajectory length ratio R ₁ and R ₂ In some cases, the ratio exceeds the determination value of 0.5. Looking at the combination of exhaust flow rate and temperature at which a normal catalyst can always be determined to be normal from FIG.
° locus length ratio always normal catalyst if the exhaust flow rate Q ₃ or less in the case of it can be seen that of 0.2 or less. Similarly, the exhaust flow when the catalyst temperature is 300 ° C. is Q ₂ or less, the exhaust flow rate in the case of 200 ° C. It can be seen that it is possible to determine that always normal normal catalyst if Q ₁ or less.

Next, the determination of the deteriorated catalyst will be described with reference to FIG. FIG. 9 (B) shows a deteriorated catalyst (a catalyst having a locus length ratio of 0.5 or more in an ideal state without variation).
About the exhaust flow rate Q and the catalyst temperature T _CAT as in FIG.
The results of the actual measurement of the trajectory length ratio while changing are shown. FIG.
In FIG. 9B, the mutual relationship between the exhaust flow rates Q ₁ ′, Q ₂ ′, and Q ₃ ′ is Q ₁ ′ <Q ₂ ′ <Q ₃ ′, and Q ₁ , Q ₂ , and Q ₂ in FIG. the relationship between the Q _{3 is, Q 1 '<Q 2'} <Q 1 <
And it has a _{Q 3 '<Q 2 <Q} 3.

That is, in the deteriorated catalyst, the region where the trajectory length ratio is small generally shifts to the side where the exhaust gas flow rate is large and the catalyst temperature is high. Contrary to the case of FIG. 9 (A), in FIG. 9 (B), the condition that the deteriorated catalyst is always equal to or more than the determination value (0.5 + 0.3 = 0.8 in this case) in consideration of the variation, that is, Consider a combination of the exhaust gas flow rate and the catalyst temperature at which the deteriorated catalyst is not erroneously determined to be normal. FIG.
As can be seen from (B), this condition is when the catalyst temperature is 400 ° C.
In the case of (1), the exhaust flow rate is not less than Q ₃ ′, at 300 ° C. is not less than Q ₂ ′, and at 200 ° C., is not less than Q ₁ ′.

From the above, it is also possible to determine that a normal catalyst is normal without erroneous determination and to fix the deteriorated catalyst to a combination of exhaust gas flow rate and catalyst temperature. It is possible. For example, as can be seen from FIG. 9, when the exhaust flow rate Q is Q ₂ ′ ≦ Q ≦ Q ₁ and the catalyst temperature T
If the determination is performed only in the region where _CAT is 200 ° C. ≦ T _CAT ≦ 300 ° C., it is possible to always perform accurate deterioration determination. However, if the determination execution condition is limited to only this range, the area where the determination can be performed becomes extremely narrow, and the frequency of the actual execution of the determination decreases, so that early detection of catalyst deterioration may not be possible. Thus, in the present embodiment, the determination execution condition is set for each catalyst temperature, thereby expanding the determination execution area during operation and increasing the frequency of execution of the determination.

FIG. 10 is a diagram showing conditions for executing the catalyst deterioration determination of this embodiment. 10, the line indicated by Q _H maximum exhaust flow rate to perform the catalyst deterioration determination performed at each catalyst temperature, Q _L denotes the minimum exhaust flow rate. For example, when the catalyst temperature T _CAT is 300 ° C., the maximum exhaust flow rate at which deterioration determination can be performed is Q ₂ , and the minimum exhaust flow rate is Q ₂ ′. That is, the line Q _H the maximum exhaust flow rate can be determined to always normal normal catalyst in the catalyst temperature, the line Q _L represents the minimum exhaust flow rate can be determined that the deteriorated catalyst was always degraded I have. As described above, by setting the exhaust flow rate region in which the deterioration determination is performed according to each catalyst temperature, the deterioration determination condition is fixed to one combination of the exhaust flow rate and the catalyst temperature as described above (for example, Q ₂ '≦ Q ≦
Q ₁ and _{200 ℃ ≦ T CAT ≦ 300 ℃} , ( 10 hatched area) in the case of fixing) in comparison with the deterioration determining condition is greatly increased.

By the way, when the exhaust flow rate region for performing the deterioration determination is set for each catalyst temperature T _CAT as described above,
In order to determine whether or not deterioration can be determined, it is necessary to detect the current exhaust gas flow rate Q and the catalyst temperature T _CAT to determine whether or not Q and T _CAT are within the deterioration determination execution region. Here, since the exhaust flow rate Q is substantially equal to the engine engine intake air amount, it is possible to determine the area of the exhaust flow rate using the engine intake air amount detected by the air flow meter 3.

As for the catalyst temperature T _CAT , it is possible to directly detect the catalyst temperature by arranging a temperature sensor on the catalyst bed of the catalytic converter 12, but in the present embodiment, a method described below is used. The catalyst temperature is indirectly detected from the operating state of the engine to prevent the cost increase due to the installation of the temperature sensor. During actual operation, the catalyst temperature fluctuates according to the engine operating state.However, for example, when the engine air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio, the combustion temperature in the engine combustion chamber does not change significantly. The rising speed and the falling speed of the catalyst temperature per unit time are determined by the exhaust gas flow rate (engine intake air amount) and the catalyst temperature at that time.

FIGS. 11 and 12 are graphs showing the change over time of the catalyst temperature T _{CAT with} the engine intake air amount Q.
FIG. 12 shows the case of temperature rise (heating), and FIG. 12 shows the case of temperature decrease (cooling). FIGS. 11 and 12 show an execution interval t of a catalyst temperature detection routine described later as a unit time on the horizontal axis. As can be seen from FIG. 11, the catalyst temperature increases rapidly as the current catalyst temperature is low and the engine intake air amount is large. It converges to a certain temperature determined by the amount of intake air. For example, in FIG. 11, when looking at the case of the engine intake air amount Q ₂ , when the initial temperature is T ₀ , the catalyst temperature T _CAT initially increases by T ₂ −T ₀ per unit time t, Initial temperature is T
_In the case of _4, only T ₅ −T ₄ increases per unit time,
The temperature rise 0 and more when the initial temperature is T ₅ (final temperature reached in the case of the engine intake air quantity Q _2).

In the case of cooling, as shown in FIG. 12, the catalyst temperature rapidly decreases as the current catalyst temperature increases and the engine intake air amount decreases, and the decreasing speed decreases as the catalyst temperature decreases. After a certain time, the temperature converges to a constant temperature determined by the engine intake air amount. For example, in FIG. 12, looking for the case of engine intake air quantity Q _2, the catalyst temperature T _CAT when the initial temperature was T ₉ is initially lowered only T ₉ -T ₇ per unit time, the initial T ₇ per unit time in the case of temperature T ₇ -T
_When only ₆ drops and the initial temperature is T ₅ (the final temperature in the case of the engine intake air amount Q ₂ ), the temperature drop becomes zero.

In this embodiment, the relationship between FIG. 11 and FIG. 12 for the combination of the change amount ΔT _CAT per unit time of the catalyst temperature T _CAT per unit time and the engine intake air amount Q is obtained in advance by experiments or the like, as shown in FIG. It is stored in the ROM 104 of the control circuit 10 as a numerical table (map) in a simple format. FIGS. 11 to 13 show the change in the catalyst temperature during the execution of the air-fuel ratio feedback control. However, for example, the same relationship as that in FIG. 11 or FIG. In this embodiment, a change in the catalyst temperature per unit time during the fuel increase and the fuel cut during the fuel increase is also prepared in advance by a numerical table having a format similar to that shown in FIG. The circuit 10 constantly calculates the current catalyst temperature by integrating the change ΔT _CAT per unit time of the catalyst temperature T _CAT using these maps during engine operation.

FIG. 14 is a flowchart showing a catalyst temperature detection routine according to this embodiment. This routine is executed by the control circuit 1
It is executed at regular intervals (unit time t in FIGS. 11 and 12) by 0. When the routine starts in FIG. 14, in step 1401, the current engine intake air amount Q becomes
In step 1403, the current catalyst temperature T _CAT is read from a predetermined area of the RAM 105. In addition,
During engine starting, or high temperature of the intake air temperature or the cooling water temperature is used as the initial value of T _CAT.
In step 1405, it is determined whether the fuel increase is currently being performed. In step 1407, it is determined whether the fuel cut is currently being performed. If neither the fuel increase nor the fuel cut is currently performed (that is, the air-fuel ratio feedback control is being performed). ), The process proceeds to step 1409, and FIG.
From the map shown in FIG. 3, the catalyst temperature change amount ΔT _CAT per unit time is determined using the current engine intake air amount Q and the catalyst temperature T _CAT .

Next, in step 1411, ΔT _CAT
, The catalyst temperature at the end of the current routine is T _CAT +
It is calculated as ΔT _CAT . In step 1413, the calculated value of T _CAT is stored in a predetermined area of the RAM 105, and the execution of the current routine ends. In step 1405, the fuel is currently being increased, or in step 1407.
In the case where fuel cut is currently being performed, ΔT _CAT is calculated using the map of ΔT _CAT at the time of fuel increase or fuel cut separately in step 1415 or step 1417, and the value of ΔT _CAT is calculated using the value of ΔT _CAT. Steps 1411 and subsequent steps are executed. By executing this routine, the current catalyst temperature _TCAT is always stored in the predetermined area of the RAM 105.

FIGS. 15 and 16 are flowcharts showing a catalyst deterioration determination routine of the present embodiment using the catalyst temperature T _CAT . This routine is executed by the control circuit 10 at regular intervals. When the routine is started in FIG. 15, it is determined in steps 1501 and 1503 whether or not a precondition for catalyst deterioration determination is satisfied. Since it is assumed that the catalyst deterioration determination of this embodiment is performed during the execution of the air-fuel ratio feedback control, step 1501 is executed.
In step 1503, the first air-fuel ratio feedback control is being executed (the value of the flag XMFB is set to 1), and in step 1503, the second air-fuel ratio feedback control is being executed (the flag Value is set to 1).
If any one of 01 and 1503 does not hold, the routine proceeds to FIG. 16 and ends without executing catalyst deterioration determination.

If both the conditions of steps 1501 and 1503 are satisfied, at step 1505 the RAM 105
The engine intake air amount Q and the catalyst temperature T _CAT calculated in the routine of FIG.
Maximum intake air amount Q _H and the minimum intake air amount catalyst deterioration determining viable in the current catalyst temperature map of FIG. 10 on the basis of the catalyst temperature T _CAT in 7 Q _L and is determined.

[0065] In step 1509, whether the current intake air quantity Q is in the intake air region defined by the above maximum value Q _H and the minimum value Q _L is determined, figure when not in region Proceeding to 16, the routine ends without executing catalyst deterioration determination. If the current intake air quantity Q was in the deterioration determining carried region at step 1509 (Q _L ≦ Q
≦ Q _H), then the locus length LVOM of the upstream O ₂ sensor output VOM in steps 1511 1515 and the locus length LVOS of the downstream O ₂ sensor output VOS is calculated. That is, in step 1511, 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 1513, the VOM and the VOS
And the trajectory lengths LVOM and LVOS are calculated.

In this embodiment, as shown in FIG. 17, the trajectory lengths LVOM and LVOS are respectively set to | VOM-VOM.
_i-1 | and | VOS-VOS _i-1 |. Here, VOM _i-1 and VOS _i-1 are the values of VOM and VOS at the time of the previous execution of the routine, respectively. After calculating the integrated value in step 1503, VOM _i-1 and VOS _i-1
Is updated in preparation for the next routine execution.

In FIG. 16, steps 1517 to 1531 are a catalyst deterioration judging operation. In this embodiment, step 15
Until 01,1503,150 of 9 times the deterioration determination condition is satisfied in the accumulation reaches a predetermined time, performs multiplication of the locus length LVOM and LVOS, these locus length every time the accumulated time reaches a predetermined time It is used to determine the presence or absence of catalyst deterioration. Thus, steps 1501, 1503, 150
Even if the condition of No. 9 is not continuously satisfied for a predetermined time, the catalyst deterioration determination can be performed if the accumulation of the time in which the condition is satisfied reaches the predetermined time. Increase.

[0068] In Figure 16 step 1517, the value of the timing counter CT for counting the above conditions satisfied cumulative time is plus 1 increases, whether the value of step 1519 the counter CT exceeds a predetermined value CT _0, i.e. conditions It is determined whether or not the accumulated cumulative time has reached a predetermined time. Here, the value of CT ₀ is, for example, the number of executions of this routine corresponding to about 20 seconds (for example, if the execution interval of this routine is 50 ms, CT ₀ = 20 / 0.05 = 400). If the was the CT ≦ CT ₀ at step 1519, for integration of the locus length is insufficient, the locus length integrated value of the up to now the execution of the routine is terminated in intact. That is, the integration of the trajectory length is performed, but the deterioration determination is not performed.

If CT> CT ₀ at step 1519, catalyst deterioration determination at step 1521 and thereafter is performed. That is, in step 1521, the ratio R = LVO between the integrated value LVOS and LVOM of the trajectory length obtained so far.
S / LVOM is calculated, and it is determined in step 1523 whether or not the trajectory length ratio R is equal to or smaller than a predetermined value K. In this embodiment, as shown in FIGS. 9A and 9B, the trajectory length ratio determination value K for determining the presence or absence of catalyst deterioration is set to 0.5.

If R ≦ K in step 1523, that is, since the catalyst is normal, the value of the alarm flag ALM is set to 0 in step 1525, and step 152
If R> K in 3, the value of the alarm flag ALM is set to 1 in step 1527. When the value of the flag ALM is set to 1, the deterioration alarm 19 (FIG. 1) is turned on by a routine (not shown) separately executed by the control circuit 10 to notify the driver that the catalyst has deteriorated. Also, the value of the flag ALM is stored in the backup RAM 106 of the control circuit 10 in step 1529, and is prepared for the next repair and inspection. And step 1531
Then, the counter CT, the integrated value of the track length LVOM, LVO
S is cleared and this routine ends.

In this embodiment, the discrimination value K for determining the catalyst deterioration of the locus length ratio is a constant value (for example, K = 0.5) regardless of the catalyst temperature. The determination of presence / absence is performed in the operating region where the normal catalyst is always determined to be normal and the deteriorated catalyst is always determined to be deteriorated, taking into account the variation in the trajectory ratio. Thus, it is possible to accurately determine the presence or absence of catalyst deterioration.

[0072]

According to the present invention, since the catalyst temperature is detected and the engine intake air amount region for performing the catalyst deterioration determination is set in accordance with the detected catalyst temperature, the frequency of execution of the catalyst deterioration determination is improved. It is possible to determine the presence / absence of catalyst deterioration extremely accurately without reducing the temperature.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating the principle of catalyst deterioration determination based on an O ₂ sensor output locus length ratio.

FIG. 3 is a flowchart illustrating a fuel injection amount calculation 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 first 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 part of a flowchart showing a second air-fuel ratio feedback control routine.

FIG. 8 is a timing diagram illustrating the routine of FIGS. 4 and 5;

FIG. 9 is a diagram illustrating a change in an O ₂ sensor output locus ratio depending on a catalyst temperature and an exhaust gas flow rate.

FIG. 10 is a diagram illustrating a relationship between a catalyst temperature and an intake air amount region in which catalyst deterioration determination is performed.

FIG. 11 is a diagram showing a relationship between a catalyst temperature change and an exhaust gas flow rate.

FIG. 12 is a diagram showing a relationship between a catalyst temperature change and an exhaust gas flow rate.

FIG. 13 is a diagram showing a format of a numerical map used for catalyst temperature calculation.

FIG. 14 is a flowchart illustrating a catalyst temperature detection routine.

FIG. 15 is a part of a flowchart showing a catalyst deterioration determination routine.

FIG. 16 is a part of a flowchart showing a catalyst deterioration determination routine.

FIG. 17 is a diagram illustrating a method for calculating the trajectory length of the O ₂ sensor output according to the present embodiment.

[Explanation of symbols]

1 ... engine body 3 ... air flow meter 10 ... control circuit 12 ... catalytic converter 13 ... upstream O ₂ sensor 15 ... downstream O ₂ sensor

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) F01N 3/20 ZAB F02D 41/14 310

Claims (1)

(57) [Claims] 1. A disposed in an exhaust passage of an internal combustion engine, O ₂
A three-way catalyst having a storage function, an upstream air-fuel ratio sensor disposed in an exhaust passage on the upstream side of the three-way catalyst and detecting 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 air-fuel ratio sensor that is disposed in the exhaust passage of the three-way catalyst and detects an exhaust air-fuel ratio on the downstream side of the three-way catalyst, based on at least an output of the upstream air-fuel ratio sensor.
Air-fuel ratio feedback control means for feedback-controlling the engine air-fuel ratio to a target air-fuel ratio; catalyst temperature detection means for detecting the temperature of the three-way catalyst; intake air amount detection means for detecting an engine intake air amount; During the feedback control, when the engine intake air amount falls within a predetermined determination execution region, a deterioration determination unit that determines whether or not the three-way catalyst has deteriorated based on at least the output of the downstream air-fuel ratio sensor; A catalyst deterioration determining device for an internal combustion engine, comprising: a region setting unit that sets the determination execution region of the engine intake air amount according to the temperature of the three-way catalyst detected by the catalyst temperature detecting unit.