JP2780451B2 - Catalyst deterioration detection device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a catalyst deterioration detecting device provided in an exhaust system of an engine and detecting deterioration of a catalyst for purifying exhaust gas.

[Conventional technology]

FIGS. 7 (b) to 7 (d) show output signals of the downstream oxygen sensor according to the deterioration state of the catalyst corresponding to the output signals of the upstream oxygen sensor shown in FIG. 7 (a). FIG. 7 (b) shows an output signal when the catalyst is not deteriorated, and the output signal of the downstream oxygen sensor changes at a sufficiently long cycle with respect to the output signal of the upstream oxygen sensor due to the storage effect of the catalyst. .
FIG. 7 (c) shows the output signal in the initial stage of catalyst deterioration, which generally changes in a long cycle like the output signal in the case where the catalyst has not deteriorated, but with a decrease in the storage effect. The output waveform of the upstream oxygen sensor appears in the output waveform. FIG. 7 (d) shows an output signal when the catalyst is completely degraded. Since there is no storage effect, the output signal of the downstream oxygen sensor is almost the same as the output signal of the upstream oxygen sensor.

Therefore, when the deterioration of the catalyst is detected only by the frequency of the output signal of the downstream oxygen sensor, the deterioration is determined at the initial stage of the deterioration as described above. Therefore, conventionally, there has been disclosed a device for detecting deterioration of the catalyst by comparing the frequencies of output signals of oxygen sensors provided respectively on the upstream and downstream of the catalyst (for example, Japanese Utility Model Laid-Open No. 63-128221).

[Problems to be solved by the invention]

However, in the above-described apparatus, it is necessary to detect the frequencies of the output signals of the upstream and downstream oxygen sensors in order to detect the deterioration of the catalyst. Therefore, the storage capacity of the electronic control unit that controls the engine increases. In addition, there is a problem that the operation cycle becomes longer and the processing time becomes longer.

The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a device capable of accurately detecting catalyst deterioration by suppressing an increase in the load on an electronic control device. is there.

[Means for solving the problem]

As shown in FIG. 1, the present invention is provided in an exhaust system of an engine, a catalyst for purifying exhaust gas, and provided above and downstream of the catalyst, respectively. Upstream and downstream oxygen sensors for detecting rich or lean; engine control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine to near the stoichiometric air-fuel ratio in response to an output signal of the upstream oxygen sensor; An inversion cycle detection means for detecting an inversion cycle of the output signal of the downstream oxygen sensor within, a cycle deviation detection means for detecting a cycle deviation between a maximum value of the inversion cycle and a minimum value of the inversion cycle, and the predetermined period. A number of inversions detecting means for detecting the number of inversions of the output signal of the downstream oxygen sensor within, if the number of inversions is equal to or greater than a predetermined number and the period deviation is less than a predetermined time, the catalyst deteriorates It is summarized as deterioration detector of a catalyst and a catalyst deterioration detecting means for determining that.

[Action]

With the above configuration, the upstream oxygen is disposed upstream of the catalyst for purifying exhaust gas disposed in the exhaust system of the engine by the engine control means, and detects whether the air-fuel ratio is rich or lean with respect to the stoichiometric air-fuel ratio. The air-fuel ratio of the air-fuel mixture supplied to the engine is controlled near the stoichiometric air-fuel ratio in accordance with the output signal of the sensor.

On the other hand, according to the output signal of the downstream oxygen sensor which is disposed downstream of the catalyst and detects whether the air-fuel ratio is rich or lean with respect to the stoichiometric air-fuel ratio, the output signal of the downstream oxygen sensor within a predetermined period is determined by the inversion cycle detecting means. Is detected, and the deviation between the maximum value and the minimum value of the inversion period is detected by the period deviation detecting means. Further, the number of reversals of the output signal of the downstream oxygen sensor within a predetermined period is detected by the reversal number detecting means. When the number of reversals is equal to or greater than a predetermined number and the cycle deviation is less than a predetermined time, the catalyst deterioration detecting means detects the deterioration of the catalyst according to the cycle deviation and the number of reversals when the catalyst has deteriorated.

〔Example〕

Hereinafter, an embodiment in which the present invention is applied to a vehicle engine will be described with reference to the drawings.

FIG. 2 is a schematic configuration diagram of the present embodiment. An air flow meter 3 is provided in an intake passage 2 of the engine 1. The air flow meter 3 directly measures the intake air amount Q guided through the air cleaner 4. Further, the intake passage 2 is provided with a throttle valve 6 that opens and closes in accordance with a driver's operation amount of an accelerator 5 and adjusts an intake air amount Q supplied to the engine 1. Further, each cylinder of the engine 1 is provided with a fuel injection valve 8 for supplying pressurized fuel from a fuel supply system 7 to an intake port for each cylinder.

The distributor 9 is provided with a reference position sensor 10 for generating a reference position detection signal every 720 crank angles (° C) and a crank angle sensor 11 for generating a crank angle detection signal every 30 ° C. I have.

Further, the water jacket 12 of the cylinder block of the engine 1 is provided with a water temperature sensor 13 for detecting a cooling water temperature Thw.

On the other hand, in the exhaust system, a three-way catalyst 1 that simultaneously purifies harmful components (HC, CO, NOx) in exhaust gas is provided downstream of the exhaust manifold 14.
5 are provided. An upstream oxygen sensor (O ₂ sensor) 16 is provided on the upstream side of the three-way catalyst 15, that is, on the exhaust manifold 14, and an exhaust pipe 17 on the downstream side of the three-way catalyst 15 is provided.
Is provided with a downstream O ₂ sensor 18. As we all know,
These upstream and downstream O ₂ sensors 16 and 18 generate different output voltages depending on whether the air-fuel ratio is lean or rich with respect to the stoichiometric air-fuel ratio.

Reference numeral 19 denotes an alarm for issuing a warning to a driver when an electronic control unit (ECU) 20 described later determines that the three-way catalyst 15 has deteriorated.

The ECU 20 is configured as, for example, a microcomputer, and as is well known, an A / D converter 101, an I / O port 102, a CPU 103, a ROM
104, RAM 105, backup RAM 106, clock generation circuit 107
Etc. are provided.

Hereinafter, a method for controlling the air-fuel ratio of the engine 1 and a method for detecting the deterioration of the three-way catalyst 15 will be described with reference to the flowcharts shown in FIGS.

FIG. 3 is a flowchart showing a fuel injection amount calculation routine for calculating the fuel injection amount TAU according to the detection signals from the various sensors described above. This routine is started and executed every predetermined period (for example, 360 ° C. in this embodiment).

In step 101, detection signals such as the intake air amount Q and the rotation speed NE are read. In step 102, the basic fuel injection amount Tp is calculated by the following equation.

 Tp ← K · Q / NE where K is a constant.

In the following step 103, the basic fuel injection amount Tp is subjected to various corrections such as air-fuel ratio feedback control described later, and the fuel injection amount TAU is calculated.

TAU ← Tp · FAF · F Here, FAF is an air-fuel ratio correction coefficient by air-fuel ratio feedback control, and F is various correction coefficients. Then, in step 104, the fuel injection amount calculated in step 103 described above.
A control signal corresponding to TAU is output to the fuel injection valve 8.

FIG. 4 shows a detection signal (upstream output value) V1 of the upstream O ₂ sensor 16.
Is an air-fuel ratio feedback control routine for setting an air-fuel ratio correction coefficient FAF. This routine is executed for a predetermined time (for example,
In this embodiment, it is started and executed every 4 msec).

In step 201, it is determined whether or not a condition (first execution condition) of the main air-fuel ratio feedback control is satisfied.
Here, the first execution condition, for example, in the present embodiment is such that the upstream O ₂ sensor 16 and after an engine start is active. If it is determined in step 201 that the first execution condition is not satisfied, the process proceeds to step 202.
In step 202, the air-fuel ratio correction coefficient FAF is set to 1.0 (FAF ← 1.
0), and terminates this routine.

On the other hand, when it is determined in step 201 that the first execution condition is satisfied, a feedback process based on the upstream output value V1 in step 203 and thereafter is executed.

In step 203, the upstream output value V1 is read. In the following step 204, the upstream output value V1 is changed to the first comparison voltage VR1 (for example,
In this embodiment, it is determined whether or not the pressure is 0.45 V or less, that is, whether the air-fuel ratio is rich or lean. Here, when the upstream output value V1 is equal to or lower than the first comparison voltage VR1, that is, when the air-fuel ratio is lean, the process proceeds to step 205. In step 205, it is determined whether or not the first delay counter CDLY1 has a positive value, that is, whether or not the upstream output value V1 has inverted from rich to lean at the current control timing. Here, the first delay counter CDLY1 is a counter for measuring the elapsed time from when the upstream output value V1 crosses the first comparison voltage VR1, and the elapsed time in the rich state is a positive value and the elapsed time in the lean state is Elapsed time is defined as a negative value.

If the first delay counter CDLY1 has a negative value in step 205, the process proceeds to step 207. If the first delay counter CDLY1 has a positive value in step 205, the process proceeds to step 206. In step 206, the first delay counter CDLY1 is reset (CDLY1 ← 0), and the process proceeds to step 207. In step 207, the value of the first delay counter CDLY1 is decremented (CDLY1 ← CDLY1-1). Next steps
At 208, it is determined whether the value of the first delay counter CDLY1 is less than the first lean delay time TDL1. Here, the first lean delay time TDL1 is a count value corresponding to the delay time in the delay processing that holds the determination that the output signal of the upstream O ₂ sensor 16 is rich even when the output signal changes from rich to lean. And is defined by a negative value. If the first delay counter CDLY1 is equal to or longer than the first lean delay time TDL1 in step 208, the process proceeds to step 217.

Meanwhile, the first delay counter CDLY1 is less than a first lean delay time TDL1 at step 208, that is, when the output signal of the upstream O ₂ sensor 16 has passed over the delay time described above after changing from rich to lean step 209 Proceed to. Step 2
At 09, the first delay counter CDLY1 is set to the first lean delay time TDL1 (CDLY1 ← TDL1), and the routine proceeds to step 210. At step 210, the flag F1 indicating the state of the air-fuel ratio after the delay processing is reset (F1 ← 0), and the routine proceeds to step 217.
That is, when the flag F1 is in the reset state (F1 = 0), it indicates that the air-fuel ratio after the delay processing is lean.

Also, in step 204, the upstream output value V1 is changed to the first comparison voltage V1.
When it is larger than R1, that is, when the air-fuel ratio is rich, the process proceeds to step 211. In step 211, the first delay counter CD
LY1 is a negative value, that is, the upstream output value V
It is determined whether 1 has been inverted from lean to rich. Here, when the first delay counter CDLY1 is a positive value, the process proceeds to step 213.

On the other hand, if the first delay counter CDLY1 has a negative value in step 211, the process proceeds to step 212. In step 212, the first delay counter CDLY1 is reset (CDLY1 ← 0).
Then, the process proceeds to step 213.

In step 213, the value of the first delay counter CDLY1 is incremented (CDLY1 ← CDLY1 + 1). Next steps
At 214, it is determined whether the first delay counter CDLY1 is less than the first rich delay time TDR1. Here, the first rich delay time TDR1 corresponds to a delay time corresponding to the delay processing that holds the determination that the output signal of the upstream O ₂ sensor 16 is lean even when the output signal changes from lean to rich. Count value, defined as a positive value. If the first delay counter CDLY1 is equal to or longer than the first rich delay time TDR1 in step 214, the process proceeds to step 217.

On the other hand, if the first delay counter CDLY1 is larger than the first rich delay time TDR1 in step 214, that is, if the upstream O ₂
When the output signal of the sensor 16 has changed from lean to rich and the delay time has elapsed, the process proceeds to step 215. At step 215, the first delay counter CDLY1 is set to the first
(CDLY1 ← TDR1), and the process proceeds to step 216. At step 216, a flag F1 indicating the state of the air-fuel ratio after the delay processing is set (F1 ← 1), and at step 217
Proceed to. That is, when the flag F1 is in the set state (F1 = 1), it indicates that the air-fuel ratio after the delay processing is rich.

In step 217, it is determined whether or not the flag F1 has been inverted, that is, whether or not the state of the air-fuel ratio after the delay processing has been inverted. Here, when the state of the air-fuel ratio after the delay processing is reversed, skip processing of steps 218 to 220 is performed. First,
At step 218, it is determined whether or not the flag F1 is in a reset state. If the flag F1 is in the reset state, that is, if the flag F1 is inverted from rich to lean, the process proceeds to step 219. At step 219, the air-fuel ratio correction coefficient FAF is increased by the first rich skip amount RSR1 (FAF ← FAF + RSR1), and the routine proceeds to step 224. If the flag F1 is in the set state in step 218, that is, if the flag F1 is inverted from lean to rich, the process proceeds to step 220. In step 220, the air-fuel ratio correction coefficient FAF
Is reduced by the first lean skip amount RSL1 (FAF ← FAF
-RSL1), proceed to step 224;

On the other hand, if the state of the air-fuel ratio after the delay processing is not inverted at step 217, the integration processing of steps 221 to 223 is performed. First, at step 221, it is determined whether or not the flag F1 is in a reset state, that is, whether or not the flag is lean. If it is lean, the process proceeds to step 222. Step 2
At 22, the air-fuel ratio correction coefficient FAF is increased by the first rich integration constant KIR1 (FAF ← FAF + KIR1), and the routine proceeds to step 224.
If it is rich in step 221, the process proceeds to step 223. At step 223, the air-fuel ratio correction coefficient FAF is reduced by the first lean integration constant KIL1 (FAF ← FAF-KIL1), and the routine proceeds to step 224.

In step 224, guard processing is performed so that the air-fuel ratio coefficient FAF set as described above falls within a predetermined range (for example, 0.8 to 1.2 in the present embodiment), and the routine ends.

FIG. 5 is a sub air-fuel ratio feedback control for correcting the first rich skip amount RSR1 and the first lean skip amount RSL1 in the main air-fuel ratio feedback control based on the output value (downstream output value) V2 of the downstream O ₂ sensor 18. It is a flowchart which shows a routine. This routine is started and executed every predetermined time (for example, 1 second in this embodiment).

First, at step 301, it is determined whether or not the air-fuel ratio feedback condition (second execution condition) is satisfied, that is, whether or not to execute the sub-air-fuel ratio feedback control. Here, the second execution condition is, for example, in the present embodiment, the first execution condition is satisfied, that is, the main air-fuel ratio feedback control is being performed. Downstream O ₂ sensor 18 is equal in the active state.

If the second execution condition is not satisfied in step 301, the process proceeds to step 302. In step 302, the first rich skip amount RSR1 is set to a predetermined rich skip amount RSR0.
In the next step 303, the first lean skip amount RSL1 is set to a predetermined lean skip amount RSL0, and this routine ends.

If the second execution condition is satisfied in step 301, the sub air-fuel ratio feedback processing based on the downstream output value V2 in step 304 and thereafter is executed. First, step 304
Reads the downstream output value V2. In step 305, the downstream output value V
It is determined whether or not 2 is equal to or lower than a second comparison voltage VR2 (for example, 0.45 V which is the same as the first comparison voltage VR1 in this embodiment), that is, whether the air-fuel ratio is rich or lean. Here, the downstream output value V2
Is equal to or lower than the second comparison voltage VR2, that is, when the air-fuel ratio is lean, the routine proceeds to step 306. In step 306, it is determined whether the second delay counter CDLY2 has a positive value, that is, whether or not the downstream output value V2 has been inverted from rich to lean at the current control timing. Here, the second delay counter CDLY2 has the same downstream output value V as the first delay counter CDLY1.
Reference numeral 2 denotes a counter for measuring an elapsed time after the second reference voltage VR2 is crossed, and a positive value is defined during the rich state and a negative value is defined during the lean state.

If the second delay counter CDLY2 has a negative value in step 306, the process proceeds to step 308. If the second delay counter CDLY2 has a positive value in step 306, the process proceeds to step 307. In step 307, the second delay counter CDLY2 is reset (CDLY2 ← 0), and the process proceeds to step 308.

In step 308, the value of the second delay counter CDLY2 is decremented (CDLY2 ← CDLY2-1). Next step 30
At 9, it is determined whether the value of the second delay counter CDLY2 is less than the second lean delay time TDL2. Here, the second lean delay time TDL2 is a count value corresponding to the delay time in the delay processing that holds the determination that the output signal of the downstream O ₂ sensor 18 is rich even when the output signal changes from rich to lean. And is defined by a negative value. If the second delay counter CDLY2 is equal to or longer than the second lean delay time TDL2 in step 309, the process proceeds to step 318.

On the other hand, the second delay counter CDLY2 is less than the second lean delay time TDL2 in step 309, i.e., when the output signal of the downstream O ₂ sensor 18 has passed over the delay time described above after changing from rich to lean step 310 Proceed to. Step 3
At 10, the second delay counter CDLY2 is set to the second lean delay time TDL2 (CDLY2 ← TDL2), and the routine proceeds to step 311. In step 311, the flag F2 indicating the state of the air-fuel ratio after the delay processing is reset (F2 ← 0), and the routine proceeds to step 318.
That is, when the flag F2 is in the reset state (F2 = 0), it indicates that the air-fuel ratio after the delay processing is lean.

Also, in step 305, the downstream output value V2 is changed to the second comparison voltage V2.
When it is larger than R2, that is, when the air-fuel ratio is rich, the process proceeds to step 312. In step 312, the second delay counter CD
LY2 is a negative value, that is, the downstream output value V
It is determined whether or not 2 is inverted from lean to rich. Here, if the second delay counter CDLY2 is a positive value, the process proceeds to step 314.

On the other hand, if the second delay counter CDLY2 has a negative value in step 312, the process proceeds to step 313. In step 313, the second delay counter CDLY2 is reset (CDLY2 ← 0).
Then, the process proceeds to step 314.

In step 314, the value of the second delay counter CDLY2 is incremented (CDLY2 ← CDLY2 + 1). Next steps
At 315, it is determined whether the value of the second delay counter CDLY2 is less than the second rich delay time TDR2. Here, the second rich delay time TDR2 corresponds to the delay time the output signal of the downstream O ₂ sensor 18 which corresponds to the delay processing for holding a judgment that even if there is a change from lean to rich is lean Count value, defined as a positive value. If the second delay counter CDLY2 is equal to or longer than the second rich delay time TDR2 in step 315, the process proceeds to step 318.

On the other hand, the step when the second delay counter CDLY2 in step 315 is greater than the second rich delay time TDR2, i.e. the output signal of the downstream O ₂ sensor 18 has passed the aforementioned delay time or more changes from the lean to rich Proceed to 316. In step 316, the second delay counter CDLY2 is set to the second rich delay time TDR2 (CDLY2 ← TDR2).
Proceed to. At step 317, the flag F2 indicating the state of the air-fuel ratio after the delay processing is set (F2 ← 1), and the routine proceeds to step 318. That is, when the flag F2 is set (F2 = 1), it indicates that the air-fuel ratio after the delay processing is rich.

At step 318, it is detected whether the flag F2 is in a reset state, that is, whether the air-fuel ratio after the delay processing is lean or rich. Here, when the flag F2 is in the reset state, that is, when the air-fuel ratio after the delay processing is lean, the process proceeds to step 319. In step 319, the first rich skip amount RSR1 is increased by a predetermined value RS (RSR1 ← RSR1 + RS), and the process proceeds to step 320. In step 320, the first lean skip amount RSL1 is reduced by a predetermined value RS (RSL1 ← RSL1-RS), and step 323 is performed.
Proceed to.

On the other hand, if the flag F2 is set in step 318, that is, if the air-fuel ratio after the delay processing is rich, the process proceeds to step 321. In step 321, the first rich skip amount RSR1 is reduced by a predetermined value RS (RSR1 ← RSR1-RS), and in step 322.
Proceed to. In step 322, the first lean skip amount RSL1 is increased by a predetermined value RS (RSL1 ← RSL1 + RS), and step 32
Proceed to 3.

In step 323, guard processing is performed so that the first rich skip amount RSR1 and the first lean skip amount RSL1 set as described above fall within the predetermined ranges, and this routine ends.

FIG. 6 is a catalyst deterioration detection control routine for detecting deterioration of the three-way catalyst 16 based on the rich / lean inversion cycle and the number of inversions of the downstream output value V2 within a predetermined period (for example, 20 seconds in this embodiment). . This routine is started and executed every predetermined period (for example, 16 msec in this embodiment).

In step 401, whether the deterioration detection condition is satisfied,
That is, it is determined whether or not to execute the deterioration detection processing. Here, as the deterioration detection condition, for example, in the present embodiment, the main / sub air-fuel ratio feedback control is being performed. The air-fuel ratio correction coefficient FAF, the first rich skip amount RSR1, and the first lean skip amount RSL1 are not guard values. For example, the engine 1 is in a steady state. If the deterioration detection condition is not satisfied in step 401, the process proceeds to step 422.

On the other hand, if the deterioration detection condition is satisfied in step 401, deterioration detection processing in step 402 and subsequent steps is executed. First, at step 402, it is detected whether or not the counter T1 is equal to or greater than a predetermined value α (a count value corresponding to a predetermined period). here,
The counter T1 is a counter for measuring the inversion cycle detection time of the downstream output value V2. When the counter T1 is smaller than the predetermined value α in step 402, the process proceeds to step 403. Step 403
Then, the counter T1 is incremented (T1 ← T1 + 1), and the inversion cycle detection processing of steps 404 to 414 is performed.

In step 404, it is detected whether the downstream output value V2 is equal to or lower than the second comparison voltage V2, that is, whether the air-fuel ratio is lean or rich. If the downstream output value V2 is equal to or lower than the second comparison voltage V2, that is, if the air-fuel ratio is lean, the process proceeds to step 405. In step 405, the flag F3 indicating the state of the air-fuel ratio is reset (F
3 ← 0), and proceed to step 407. If the downstream output value V2 is larger than the second comparison voltage V2, that is, if the air-fuel ratio is rich, the process proceeds to step 406. In step 406, a flag F3 indicating the state of the air-fuel ratio is set (F3 ← 1), and step 407 is performed.
Proceed to.

At step 407, it is detected whether or not the flag F3 has been inverted, that is, whether or not the air-fuel ratio has changed. If the flag F3 has not been inverted, the process proceeds to step 408. In step 408, the counter CR for measuring the inversion cycle is incremented (CR ← CR + 1), and this routine ends.

On the other hand, if the flag F3 is inverted at step 407, the process proceeds to step 409. In step 409, the counter CR is the maximum value TM of the counter CR set up to the previous control timing.
Detects if it is greater than AX. Here, when the counter CR is equal to or smaller than the maximum value TMAX, the process proceeds to step 411. If the counter CR is larger than the maximum value TMAX in step 409, the process proceeds to step 410. In step 410, the maximum value TMAX is reset to the count value of the counter CR, and the process proceeds to step 411.

At step 411, it is detected whether or not the counter CR is less than the minimum value TMIN of the counter CR set up to the previous control timing. If the value of the counter CR is equal to or greater than the minimum value TMIN, the process proceeds to step 413. If the counter CR is smaller than the minimum value TMIN in step 411, the process proceeds to step 412. Step 4
In step 12, the count value of the counter CR is reset as the minimum value TMIN, and the process proceeds to step 413.

In step 413, the counter CT for counting the number of inversions is incremented (CT ← CT + 1), and the process proceeds to step 414. Reset the counter CR in step 414 (CR ← 0)
Then, this routine ends.

On the other hand, if the counter T1 is equal to or more than the predetermined value α in step 402, the process proceeds to a deterioration determination routine in steps 415 to 421. In step 415, it is detected whether or not the counter CT, that is, the number of inversions is equal to or more than a predetermined value β (for example, 10 in this embodiment).
If the value of the counter CT is less than the predetermined value β, step 42
Go to 0.

If the value of the counter CT is equal to or larger than the predetermined value β in step 415, the process proceeds to step 416. In step 416, a period deviation T between the maximum value TMAX and the minimum value TMIN is calculated (T ← TMAX−TMIN),
Proceed to step 417. In step 417, the period deviation T is a predetermined value γ (for example, a count value corresponding to 1 second in the present embodiment).
It is detected whether it is less than. Here, the period deviation T is a predetermined value γ
If the value is less than the predetermined value β and the period deviation is less than the predetermined time, it is determined that the three-way catalyst 15 has deteriorated, and the process proceeds to step 418. Three-way catalyst for the driver in step 418
The alarm 19 is turned on to warn of the deterioration of 15, and the process proceeds to step 419. At step 419, a flag XCAT indicating the deterioration of the three-way catalyst 15 is set (XCAT ← 1), and the routine proceeds to step 422.

On the other hand, if the period deviation T is equal to or larger than the predetermined value γ in step 417, the process proceeds to step 420. If the number of reversals is not equal to or more than the predetermined value β or the period deviation is less than the predetermined time, it is determined that the three-way catalyst 15 has not deteriorated, and the alarm 19 is not turned on in Step 420, and Step 421 Proceed to.
At step 421, the flag XCAT is reset (XCAT ← 0),
Proceed to step 422.

Steps 422 to 425 are an initial value setting routine. Setting a lower limit value MIN of the inversion cycle of the downstream O ₂ sensor 18 which is set in advance the maximum value TMAX at step 422 (TMAX ← MI
N) and proceed to step 423. Minimum value TMIN in step 423
Is set to the preset upper limit value MAX of the reversal cycle of the downstream O ₂ sensor 18 (TMIN ← MAX), and the routine proceeds to step 424. In step 424, the counter CT is reset (CT ← 0), and the process proceeds to step 425. In step 425, the counter T1 is reset (T1 ← 0), and this routine ends.

FIGS. 7A to 7D show the waveform of the upstream output value V1 and the waveform of the downstream output value V2 according to the state of deterioration of the three-way catalyst 15. FIG. As is apparent from FIG. 7, the waveform of the upstream output value V1 by the main air-fuel ratio feedback control has a substantially constant rich-lean inversion cycle in a steady state as shown in FIG. 7 (a).

When the three-way catalyst 15 is not deteriorated, the waveform of the downstream output value V2 has a longer cycle than the upstream output value V1 due to the storage effect, as shown in FIG. 7B. Therefore, since the reversal period CR1 is large and the number of reversals is small, it is determined that the three-way catalyst 15 has not deteriorated.

Next, in the initial stage of the deterioration of the three-way catalyst 15, the waveform generally changes in a long cycle, but as the storage effect decreases, the waveform is disturbed as shown in FIG. 7 (c). . Therefore, the number of inversions increases, but the maximum value of the inversion cycle
Since the cycle deviation between CR20 and the minimum value CR21 is large, a three-way catalyst
15 is judged not to have deteriorated.

Finally, if the three-way catalyst 15 is deteriorated, the storage effect is lost, so that the waveform becomes similar to the upstream output value V1 as shown in FIG. 7 (d). Therefore, the number of reversals increases and the reversal periods CR30 and CR31 become substantially constant, so that the period deviation also decreases, and it is determined that the three-way catalyst 15 has deteriorated.

Therefore, erroneous determination of the initial stage of deterioration can be prevented by the inversion cycle and the number of inversions within the predetermined period α of the downstream output value V2 as in the above embodiment. Further, since deterioration is detected only by the downstream output value V2, an increase in the load on the ECU can be prevented.

In the above embodiment, a preset lower limit and upper limit are set as initial values of the maximum value TMAX and the minimum value TMIN, respectively. However, the minimum value TM detected during this deterioration detection
IN may be set as the initial value of the maximum value TMAX, and the maximum value TMAX detected at the time of this deterioration detection may be set as the initial value of the minimum value TMIN.

〔The invention's effect〕

As described above in detail, in the present invention, the deterioration of the catalyst is detected based on the cycle deviation between the maximum value and the minimum value of the reversal cycle and the number of reversals within a predetermined period of the downstream oxygen sensor. That is, since the deterioration of the catalyst is detected only by the output signal of the downstream oxygen sensor, there is an excellent effect that the deterioration of the catalyst can be accurately detected without increasing the load on the electronic control unit.

[Brief description of the drawings]

FIG. 1 is a diagram corresponding to claims of the present invention, FIG. 2 is a schematic configuration diagram of an embodiment to which the present invention is applied, FIGS. 3 to 6 are flowcharts for explaining the operation of the embodiment, FIG. 7A, 7B, 7C, and 7D are output waveform diagrams of the upstream and downstream oxygen sensors according to the state of deterioration of the three-way catalyst 15. FIG. 1… Engine, 8… Injector, 15… Three-way catalyst, 16
…… Upstream O ₂ sensor, 18 …… Downstream O ₂ sensor, 19 …… Alarm, 20… ECU.

Continued on the front page (51) Int.Cl. ⁶ Identification code FI F02D 45/00 345 F02D 45/00 345Z 368 368H

Claims (2)

(57) [Claims] A catalyst disposed in an exhaust system of an engine for purifying exhaust gas, and disposed upstream and downstream of the catalyst, respectively, for detecting whether an air-fuel ratio is rich or lean with respect to a stoichiometric air-fuel ratio. Upstream and downstream oxygen sensors, engine control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine to near the stoichiometric air-fuel ratio in accordance with the output signal of the upstream oxygen sensor, and the downstream oxygen sensor within a predetermined period An inversion cycle detection means for detecting an inversion cycle of the output signal of the above, a cycle deviation detection means for detecting a cycle deviation between a maximum value of the inversion cycle and a minimum value of the inversion cycle, and the downstream oxygen sensor within the predetermined period. A reversal number detecting means for detecting the number of reversals of the output signal of the above; Deterioration detecting apparatus of a catalyst, characterized in that it comprises a reduction detection means. 2. An engine control unit comprising: a control amount setting unit configured to set a control amount of the engine according to an output signal of the upstream oxygen sensor; and a control amount setting unit according to an output signal of the downstream oxygen sensor. The catalyst deterioration detecting device according to claim 1, further comprising: control constant correction means for correcting the control constant of (1).