JP2002332904A - Catalyst deterioration detection device - Google Patents

Abstract

(57) [Summary] [Problem] HC contained in exhaust gas passing through a catalyst,
It is an object of the present invention to provide an apparatus for detecting catalyst deterioration directly from CO. A catalyst deterioration detection device according to the present invention is provided in an exhaust pipe of an internal combustion engine, and is provided in a catalyst for purifying HC and CO in exhaust gas, and is provided in an exhaust pipe downstream of the catalyst, respectively. A first air-fuel ratio sensor and a second air-fuel ratio sensor having different outputs in accordance with the concentration of CO, and the deterioration of the catalyst based on the output of each air-fuel ratio sensor due to the exhaust gas passing through the catalyst. It is configured to detect the degree. According to the present invention, when the purification ability of the catalyst is reduced, HC and CO in the exhaust gas pass through the catalyst without being sufficiently purified, so that the first air-fuel ratio sensor and the second air-fuel ratio sensor are large. An output difference occurs, and the degree of deterioration of the catalyst can be detected based on the output difference.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst deterioration detecting device provided in an exhaust system of an internal combustion engine such as an automobile for detecting the deterioration of a catalyst for purifying exhaust gas.

[0002]

2. Description of the Related Art A three-way catalyst for purifying exhaust gas of an internal combustion engine of an automobile reduces NOx by adsorbing and holding excess oxygen present in the exhaust gas when the air-fuel ratio of a mixture supplied to the engine becomes lean. On the other hand, when the air-fuel ratio becomes rich, the adsorbed and held oxygen is released to reduce HC and CO. For this reason, the purification ability of the catalyst decreases as the ability of the catalyst to accumulate oxygen (oxygen storage capacity; OSC) decreases. Traditionally, the degree of catalyst degradation has been detected by quantifying this oxygen storage capacity during engine operation. An apparatus for detecting catalyst deterioration utilizing this oxygen storage capacity is described in Japanese Patent Application Laid-Open No. 5-171922.

[0003]

In the method utilizing the oxygen storage capacity, the correlation between the purifying capacity of the catalyst and the oxygen storage capacity needs to be well matched. Accordingly, only the purification capacity of the catalyst may be reduced, or only the oxygen storage capacity may be reduced. In such a case, the correlation between the purifying ability of the catalyst and the oxygen storage ability cannot be obtained, and the deterioration of the catalyst cannot be accurately determined. Therefore, there is a demand for an apparatus that directly detects catalyst deterioration from the purification rate of exhaust gas that has passed through the catalyst, without depending on the oxygen storage capacity of the catalyst.

An object of the present invention is to provide an apparatus for directly detecting deterioration of a catalyst from HC and CO contained in exhaust gas passing through the catalyst.

[0005]

In order to solve the above-mentioned problems, a catalyst deterioration detecting device according to the present invention is provided in an exhaust pipe of an internal combustion engine and purifies HC and CO in exhaust gas. H is provided in each of the exhaust pipes on the downstream side.
A first air-fuel ratio sensor and a second air-fuel ratio sensor having different outputs according to the concentrations of C and CO, respectively, based on the output of each air-fuel ratio sensor by the exhaust gas passing through the catalyst. It is configured to detect the degree of deterioration of.

According to the present invention, when the purification ability of the catalyst is reduced, HC and CO in the exhaust gas pass through the catalyst without being sufficiently purified, so that the first air-fuel ratio sensor and the second air-fuel ratio sensor are used. And a large output difference is generated, and the degree of deterioration of the catalyst can be detected based on the output difference.

According to one embodiment of the present invention, the catalyst deterioration detecting device includes a means for obtaining an output difference between the first air-fuel ratio sensor and the second air-fuel ratio sensor, and an output of each air-fuel ratio sensor. Means for determining a reference value for deterioration determination according to the air-fuel ratio of the air-fuel mixture at the time of detection, and when the output difference of the detected air-fuel ratio sensor is larger than the reference value for deterioration determination, the catalyst deteriorates. And means for determining that there is.

According to this embodiment, the catalyst deterioration detecting device
The deterioration of the catalyst can be determined by comparing with a deterioration determination reference value corresponding to the air-fuel ratio.

[0009]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 shows a schematic diagram of an engine 1 and its control device. The intake pipe 3 of each cylinder of the engine 1 includes a throttle valve 9 connected to a throttle valve opening sensor (θTH) 11. The throttle valve opening sensor 11 is
An electric signal is output according to the opening of the throttle valve 9, and the output is provided to an electronic control unit (hereinafter, referred to as “ECU”) 31. The intake pipe 3 is also provided with an absolute pressure sensor (PBA) 13 for detecting the pressure in the intake pipe, and the output thereof is provided by the ECU.
31.

The main body of the engine 1 has an engine speed sensor (NE) 21 for detecting the speed of the engine 1. The NE sensor 21 provides an output pulse to the ECU 31 every time the crankshaft of the engine 1 rotates 180 degrees.

The engine 1 includes various sensors in addition to the NE sensor 21. The outputs of these sensors are
The ECU 31 is used to detect the operating state of the engine. For example, as such a sensor, an engine coolant temperature (T
W) sensors. In this embodiment, such a plurality of sensors are collectively shown as a sensor 19.

An exhaust pipe 5 provided in the engine 1 is provided with a three-way catalyst 23 for purifying exhaust gas. An upstream air-fuel ratio sensor 17 is provided between the engine 1 and the catalyst 23 in the exhaust pipe 5. This sensor 17 detects the air-fuel ratio of the exhaust gas discharged from the engine 1. The sensor 17 may be a linear air-fuel ratio sensor that detects the air-fuel ratio of the entire region according to the model of the engine 1, or may be an O2 sensor that changes the level on and off at the stoichiometric air-fuel ratio. This detection signal is sent to the ECU 31 and used to control the air-fuel ratio of the engine 1.

The ECU 31 is constituted by a computer, and a ROM for storing programs and data, a RAM for extracting and storing necessary programs and data at the time of execution and providing an operation work area, a CPU for executing programs, and various types of CPUs. It has a circuit for processing an input signal from the sensor and an output circuit for sending a control signal to each part of the engine.

All outputs from the various sensors described above are input to the ECU 31 and processed according to a program incorporated in the ECU 31. In FIG. 1, the ECU 31 is shown by functional blocks based on such a hardware configuration.

The ECU 31 determines the operating state of various engines based on the above-described various sensor outputs, and performs air-fuel ratio feedback control and open-loop control (no air-fuel ratio feedback).

The ECU 31 includes functional blocks of a fuel injection control unit 61, an ignition timing control unit 63, an operation state detection unit 65, and an air-fuel ratio setting unit 67 for engine control. The operating state detector 65 determines the operating state of the engine based on the output of each sensor. The air-fuel ratio setting unit 67 receives the information on the operating state from the operating state detecting unit 65 and sets a target air-fuel ratio according to various processes. Ignition timing controller 63
Sends a signal for controlling the ignition timing based on the operating state of the engine, the target air-fuel ratio, and the like, and controls the ignition of the spark plug. The fuel injection control unit 61 calculates the fuel injection time according to the operating state of the engine, the target air-fuel ratio, and the like, and controls the fuel injection device 15.

The first air-fuel ratio sensor 25 and the second air-fuel ratio sensor 27 are provided for detecting the deterioration of the three-way catalyst 23.
Is provided in the exhaust pipe 5 on the downstream side. The two air-fuel ratio sensors respectively detect the oxygen concentration of the exhaust gas passing through the catalyst 23, and detect the HC and CO contained in the exhaust gas.
Output characteristics differ depending on the amount of. Therefore, an output difference is generated between the two air-fuel ratio sensors according to the amounts of HC and CO contained in the exhaust gas after passing through the catalyst.

If the purifying ability of the catalyst 23 is not reduced, the H contained in the exhaust gas after passing through the catalyst 23
C and CO are sufficiently reduced by the oxidation reaction of the catalyst 23. In that case, the output difference between the first air-fuel ratio sensor 25 and the second air-fuel ratio sensor 27 is relatively small. However, if the purification ability of the catalyst 23 is reduced, HC and CO contained in the exhaust gas are not sufficiently purified, and a large amount of HC and CO is contained in the exhaust gas even after passing through the catalyst. . In this case, the first air-fuel ratio sensor 25
And the second air-fuel ratio sensor 27 has a large output difference.

The catalyst deterioration detecting device of the present invention detects the degree of deterioration of the catalyst 23 using the output difference between the two air-fuel ratio sensors. Hereinafter, the first air-fuel ratio sensor 25 and the second air-fuel ratio sensor 27 in this embodiment will be described in detail. FIG. 2 is a diagram schematically showing the first and second air-fuel ratio sensors in the present invention. . The structure of the sensor shown in FIG. 2 is the same as a general air-fuel ratio sensor, and has the same structure as a so-called zirconia O2 sensor.

This sensor has a zirconia element 35
Is fired into a test tube, and platinum electrodes are provided on the inner wall 33 and the outer wall 43 thereof. The ceramic layer 45 protects the platinum electrode 43 on the outer wall and unburned components (H
C, CO) and oxygen can be diffused to the outer wall electrode 43. The inner wall side of the sensor is placed in an exhaust pipe so that the oxygen concentration is made constant by introducing the atmosphere, and the outer wall side of the sensor is exposed to exhaust gas. The air-fuel ratio sensor outputs a current corresponding to the component of the exhaust gas by applying a reverse voltage having the same magnitude as the electromotive force generated at the stoichiometric air-fuel ratio between the inner wall electrode 33 and the outer wall electrode 43.

Generally, the zirconia element 35 has a property of generating an electromotive force according to the difference in oxygen concentration between the inner and outer walls. This is caused when oxygen ions move from the high oxygen concentration side to the low oxygen concentration side when the oxygen concentration differs between the inner wall and the outer wall of the zirconia element 35. The electromotive force of the zirconia element 35 generated at this time increases according to the amount of moving oxygen ions (that is, according to the difference in oxygen concentration between the inner and outer walls).

Contrary to this property, the zirconia element 35
Has the property that oxygen ions move through the zirconia element 35 when a voltage is applied to both electrodes. Therefore, by applying a constant voltage to both electrodes of the zirconia element 35 and detecting oxygen ions flowing in the zirconia element as a current, it is possible to detect the oxygen concentration difference between the two electrodes. In the air-fuel ratio sensor, the oxygen concentration on the inner wall side of the zirconia element 35 is made constant by the atmosphere, and the outer wall side is exposed to exhaust gas. As a result, a change in the concentration of oxygen contained in the exhaust gas can be electrically detected.

The oxygen concentration in the exhaust gas before passing through the catalyst changes according to the air-fuel ratio of the mixture to be burned. The exhaust gas burned with the lean mixture contains a large amount of unreacted oxygen, and has a relatively small amount of HC and CO. On the other hand, the exhaust gas burned with the rich mixture contains almost no oxygen and contains a lot of unburned HC and CO.

Normally, the air-fuel ratio sensor is affected by not only oxygen contained in exhaust gas but also HC and CO, and changes its output. This is due to the platinum electrode provided on the zirconia element 35. The platinum electrode not only functions as an electrode for detecting an electromotive force, but also functions as a catalyst, and reacts HC and CO in exhaust gas with oxygen. When HC and CO react with oxygen, the electromotive force increases because the oxygen concentration near the platinum electrode on the outer wall decreases.

FIG. 3 schematically shows the operation of the air-fuel ratio sensor shown in FIG. 2, and the operation principle of the air-fuel ratio sensor will be described in more detail with reference to FIG. FIG. 3A shows the operation of the air-fuel ratio sensor in the exhaust gas burned at the stoichiometric air-fuel ratio.
In the case of exhaust gas burned at the stoichiometric air-fuel ratio, an electromotive force is generally generated between the inner wall electrode 33 and the outer wall electrode 43. However, in the air-fuel ratio sensor, since the constant voltage source 37 applies a reverse voltage having the same magnitude as the electromotive force between the inner wall electrode 33 and the outer wall electrode 43, as a result, in the case of exhaust gas having a stoichiometric air-fuel ratio. , The current flowing through the resistor 41 becomes I = 0.

FIG. 3B shows the operation of the air-fuel ratio sensor in the exhaust gas burned with the lean mixture. In this case, since the lean exhaust gas contains a large amount of unreacted oxygen, oxygen ions (O ²⁻ ) move from the outer wall electrode side of the zirconia element 35 to the inner wall electrode side. A current (I> 0) flows. The magnitude of the current flowing at this time is determined by the amount of oxygen diffusing in the ceramic layer 45.

FIG. 3c shows the operation of the air-fuel ratio sensor in the exhaust gas burned with the rich mixture. In this case, oxygen contained in the rich exhaust gas reacts with HC and CO by the catalytic action of the platinum electrode 43, and a large oxygen concentration difference occurs between the outer wall and the inner wall. In response to this, an electromotive force is generated on the outer wall and the inner wall, so that oxygen ions (O ²⁻ ) move from the inner wall to the outer wall. The moved oxygen ions further react with HC and CO on the outer wall side, and a negative current (I> 0) flows through the resistor 41. The magnitude of the current flowing at this time is determined by the amounts of HC and CO diffusing in the ceramic layer 45.

FIG. 4 schematically shows the current output of the air-fuel ratio sensor. The solid line indicated by reference numeral 53 indicates the current output of a typical air-fuel ratio sensor. In a typical air-fuel ratio sensor, the current value of the air-fuel ratio sensor changes almost proportionally over a wide range from rich to lean, but the slope of the straight line changes at the boundary of the stoichiometric air-fuel ratio. This is because, as described with reference to FIG.
This is because it changes in accordance with the amount of oxygen and the amount of unburned components in the exhaust gas diffusing 5. That is, on the lean side, the current value is determined mainly by diffusion of the ceramic layer 45 by oxygen, whereas on the rich side, HC,
The current value is affected by diffusion of the ceramic layer 45 due to unburned components such as CO.

In the present invention, the diffusion rates of the unburned components (HC, CO) of the respective ceramic layers 45 in the first and second air-fuel ratio sensors are intentionally changed.
That is, the sensor currents on the rich side of the first and second air-fuel ratio sensors have different slopes. For example, by slowing the diffusion of the unburned components of the air-fuel ratio sensor, the gradient of the sensor current on the rich side can be reduced as indicated by reference numeral 57 in FIG. Conversely, if the diffusion of the unburned components in the ceramic layer 45 of the air-fuel ratio sensor is accelerated, the gradient of the sensor current on the rich side increases as indicated by reference numeral 55 in FIG.

As described above, by changing the gradient of the sensor current on the rich side of the first air-fuel ratio sensor and the gradient of the sensor current on the rich side of the second air-fuel ratio sensor, the respective air-fuel ratio sensors HC contained in exhaust gas,
Even if the amount of CO is the same, different current values are output.

Next, how the output of the air-fuel ratio sensor is output for each air-fuel ratio will be described with reference to FIG. FIG. 5 shows sensor currents of the first and second air-fuel ratio sensors 25 when feedback control is performed to a predetermined lean air-fuel ratio, a stoichiometric air-fuel ratio, and a predetermined rich air-fuel ratio, respectively. FIG. 5B is a diagram when the purification capability of the catalyst 23 is normal, and FIG. 5B is a diagram when the purification capability of the catalyst 23 is reduced.

In the example of FIG. 5, the first air-fuel ratio sensor 25
The sensor has a relatively small gradient of sensor current on the rich side, and its output is indicated by a solid line in the figure. The second air-fuel ratio sensor 27 is a sensor having a relatively large sensor current gradient on the rich side, and its output is indicated by a broken line in the figure.

First, the output of the first air-fuel ratio sensor and the output of the second air-fuel ratio sensor when the purifying ability of the catalyst is normal will be described with reference to FIG. When the air-fuel ratio in the engine is feedback-controlled to the lean side, both the air-fuel ratio sensors output a positive sensor current because the oxygen concentration in the exhaust gas after passing through the catalyst is relatively high. At this time, since the unburned components in the exhaust gas have been sufficiently purified by the catalyst, no output difference occurs between the first air-fuel ratio sensor and the second air-fuel ratio sensor.

When the air-fuel ratio in the engine is feedback-controlled to the stoichiometric air-fuel ratio, the catalyst 23 sufficiently purifies the HC and CO contained in the exhaust gas. Value, and no output difference occurs between the first air-fuel ratio sensor and the second air-fuel ratio sensor.

When the air-fuel ratio in the engine is feedback-controlled to the rich side, since the air-fuel ratio is out of the purification window region of the catalyst 23, the H
C and CO are not completely purified. Therefore, both the air-fuel ratio sensors output a negative sensor current according to the amounts of HC and CO that have not been purified by the catalyst. At this time, the first air-fuel ratio sensor 25 and the second air-fuel ratio sensor 27 generate an output difference because the gradients of the sensor currents on the rich side are different from each other.

Next, the outputs of the first air-fuel ratio sensor and the second air-fuel ratio sensor when the purifying ability of the catalyst is reduced will be described with reference to FIG. When the feedback control is performed on the lean side, the air-fuel ratio sensor outputs a positive sensor current in accordance with the oxygen concentration of the exhaust gas after passing through the catalyst. An output difference of the fuel ratio sensor occurs.

Similarly, when the air-fuel ratio in the engine is feedback-controlled to the stoichiometric air-fuel ratio, the output of the air-fuel ratio sensor slightly decreases from 0 because HC and CO contained in the exhaust gas are not sufficiently purified. Then, an output difference of the air-fuel ratio sensor occurs.

When the air-fuel ratio in the engine is feedback-controlled to the rich side, a large amount of HC and CO contained in the exhaust gas are discharged as they are, and both the air-fuel ratio sensors output a negative sensor current. This large amount of HC and C
O causes a large air-fuel ratio sensor output difference.

Therefore, in the present invention, the deterioration of the catalyst is determined by detecting the output difference ΔI between the two air-fuel ratio sensors with respect to a certain air-fuel ratio and comparing it with a predetermined reference value. For example, when the air-fuel ratio at the time of detecting the output difference ΔI of the air-fuel ratio sensor is AF _1, the detected output difference ΔI
Is compared with a predetermined reference value corresponding to the AF _1.
If the detected output difference ΔI is larger than the reference value, it is determined that the catalyst has deteriorated because HC and CO to be purified have passed without being purified. If the detected output difference ΔI is larger than the reference value, it is determined that the catalyst is normal because HC and CO have been sufficiently purified by the catalyst.

FIG. 6 shows one embodiment of a flow chart for determining catalyst deterioration. The processing of this catalyst deterioration determination is performed according to EC
The process is executed every predetermined time (for example, 10 msec) in U15. First, in step 103, it is determined whether or not a precondition (hereinafter, referred to as "precondition") for accurately executing the deterioration determination process is satisfied. For example, when the process for another component is being executed, the deterioration determination process should not be performed, and thus it is determined in step 103 that the precondition for the deterioration determination process is not satisfied. The processing for such other components includes a sensor deterioration monitor, an evaporative fuel emission suppression system failure monitor, a fuel system abnormality monitor, and the like.

Further, the operating state of the engine is determined based on the output from each sensor. If the operating state is inappropriate for determining the deterioration of the catalyst, it is determined that the precondition is not satisfied. For example, in an operating state in which the purge of evaporative fuel from the canister to the intake pipe should be cut, and when the air-fuel ratio correction coefficient is stuck to a predetermined upper limit or lower limit for a certain period of time or the like, the precondition is not satisfied. It is determined that it is not established.
The values of the intake air temperature, the engine water temperature, the engine speed, the vehicle speed, etc. are obtained from the outputs of the sensors, and even if these values are not within the predetermined range for determining the catalyst deterioration, the precondition is not satisfied. Is determined. The fluctuation amount of the vehicle speed V and the fluctuation amount of the absolute pressure PBA in the intake pipe during a certain time are also determined in step 103.

These preconditions are prerequisites that are determined for accurately determining the deterioration of the catalyst. Unless all of these conditions are satisfied, the processing of the deterioration determination does not proceed to the next step. If it is determined in step 103 that all the preconditions are satisfied, the process of catalyst deterioration determination proceeds to step 105.

Next, at step 105, the output I1 of the first air-fuel ratio sensor and the output I2 of the second air-fuel ratio sensor are detected. In this embodiment, the first air-fuel ratio sensor 25 is a sensor having a small gradient of the sensor current on the rich side, and the second air-fuel ratio sensor 27 is a sensor having a large gradient of the sensor current on the rich side. Although the outputs from these sensors are shown directly connected to the ECU 31 in the block diagram of FIG. 1, they are actually input to the ECU 31 via current-voltage conversion, A / D conversion, and the like.

In step 107, the output difference ΔI between I1 and I2 is calculated. In step 109, ΔIREF serving as a deterioration determination reference is determined according to the air-fuel ratio in the engine at the time when I1 and I2 are detected. For example, ΔIREF corresponding to the air-fuel ratio may be stored in the ROM in the ECU 13 as a table with a predetermined value based on an experimental value, or may be determined as a function for the air-fuel ratio.

In step 111, the output difference ΔI is compared with the criterion value ΔIREF determined in step 109. If the catalyst is normal, the value of the output difference ΔI becomes smaller than the reference value ΔIREF. However, if the purification ability of the catalyst is reduced, HC and CO to be purified pass through the catalyst without being purified. Therefore, the value of the output difference ΔI becomes larger than the reference value ΔIREF. Therefore, step 11
As a result of the comparison of 1, if ΔI> ΔIREF, the catalyst is determined to be normal (step 115), and if ΔI <ΔIREF, the catalyst is determined to be deteriorated.

Although the present invention has been described with reference to a specific embodiment, the present invention is not limited to such an embodiment, and various modifications which can be easily made by those skilled in the art fall within the scope of the present invention. included.

[0047]

According to the present invention, it is possible to detect the degree of deterioration of the catalyst based on the output difference between the two air-fuel ratio sensors provided downstream of the catalyst.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an engine 1 and a control device thereof.

FIG. 2 is a diagram schematically showing an air-fuel ratio sensor according to the present invention.

FIG. 3 is a diagram schematically showing the operation of the air-fuel ratio sensor shown in FIG. 2;

FIG. 4 is a diagram showing a current output of an air-fuel ratio sensor.

FIG. 5 is a diagram showing sensor currents of the first and second air-fuel ratio sensors 25 when feedback control is performed at each of a lean air-fuel ratio, a stoichiometric air-fuel ratio, and a rich air-fuel ratio.
5A is a diagram when the purifying ability of the catalyst is normal, and FIG. 5B is a diagram when the purifying capability of the catalyst is reduced.

FIG. 6 is a flowchart of catalyst deterioration determination.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 engine 5 exhaust pipe 31 ECU 23 three-way catalyst 17 upstream air-fuel ratio sensor 25 first air-fuel ratio sensor 27 second air-fuel ratio sensor 69 output difference detection unit 71 deterioration determination unit

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference)

Claims (2)

[Claims] 1. A catalyst provided in an exhaust pipe of an internal combustion engine for purifying HC and CO in exhaust gas, and a catalyst provided in an exhaust pipe on the downstream side of the catalyst, and different depending on the concentrations of HC and CO. A first air-fuel ratio sensor and a second air-fuel ratio sensor having outputs; and a catalyst deterioration detection device for detecting a degree of deterioration of the catalyst based on an output of each air-fuel ratio sensor due to exhaust gas passing through the catalyst. . 2. A means for determining an output difference between the first air-fuel ratio sensor and the second air-fuel ratio sensor, and a deterioration determination in accordance with the air-fuel ratio of the air-fuel mixture when the output of each air-fuel ratio sensor is detected. 2. The means according to claim 1, further comprising: means for determining a reference value of: and means for determining that the catalyst has deteriorated when the detected output difference of the air-fuel ratio sensor is larger than the deterioration determination reference value. Catalyst deterioration detection device.