JP2000080915A - NOx PURIFYING CATALYTIC EVALUATION METHOD AND DEVICE AS WELL AS NOx PURIFYING CATALYTIC EFFICIENCY CONTROL METHOD - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control an engine so as to make high efficiency maintainable by detecting a lowering of efficiency in order to prevent the efficiency lowering due to deterioration in the longtime operation of a catalyst in an internal combustion engine. SOLUTION: A catalyst 3 is set up in an exhaust pipe of an engine 1. Two sensors 7 and 8 detecting an exhaust gas state are installed in front and in the rear of the catalyst 3. As for these sensors 7 and 8, even such as an oxygen sensor wherein the output of power is varied stepwise in an equation of λ=1, by way of example, and another sensor wherein the output of power is varied in proportion to an air-fuel ratio may be used. Each detected value of both these sensors is taken in a control unit 9, and according to the compared results, catalytic conversion efficiency and the degree of deterioration are estimated, thereby controlling the engine so as to make the efficiency become optimized. In this constitution, the lowering of efficiency in the catalyst due to its longtime operation is accurately detected, whereby since the engine is controlled so as to avoid the lowering of efficiency, a high exhaust emission controlling characteristic is thus maintainable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating a NOx purification catalyst embedded in an exhaust system of an internal combustion engine, an apparatus for evaluating the same, and a method for controlling the efficiency of the catalyst.

[0002]

2. Description of the Related Art A conventionally proposed method for estimating a decrease in efficiency or deterioration of a NOx purifying catalyst is disclosed in Japanese Patent Laid-Open No. 4-26541.
As disclosed in Japanese Patent Application Publication No. 4 (1999), the mileage of an automobile is used as a parameter, and when the mileage exceeds a certain value, it is considered that the efficiency of the NOx purification catalyst has been sufficiently reduced.

A method for increasing the amount of HC when the amount of HC used for the NOx purification catalyst is insufficient is disclosed in Japanese Patent Laid-Open Publication No. HEI 9 (1999).
It is proposed in Japanese Patent Publication No. 3-229914.

[0004]

However, in the prior art, there is a problem that accurate evaluation cannot be performed because the catalyst is evaluated based on the traveling distance or the like. Also, no consideration is given to correctly evaluating the diagnosis of both catalysts having different characteristics.

In addition, since the amount of HC is controlled irrespective of the evaluation of the catalyst when the amount of HC is insufficient, there is a problem that the amount of HC cannot be controlled in an accurate state.

[0006]

An object of the present invention is to provide a composite catalyst which is provided in an exhaust system of an internal combustion engine and which comprises a combination of a NOx catalyst for purifying exhaust gas components and at least two of a three-way catalyst and an oxidation catalyst. And evaluating a catalyst constituting the composite catalyst based on a combination of any two of sensor outputs for detecting an exhaust gas specific component disposed upstream and downstream of the composite catalyst and between the composite catalyst. In the method for evaluating a catalyst, the problem is solved by a method for evaluating a catalyst, wherein an operating range for diagnosing the NOx catalyst and the three-way catalyst or the oxidation catalyst is different.

Another object is to evaluate an NOx purification catalyst provided in an exhaust system of an internal combustion engine and provided with a NOx purification catalyst for purifying NOx in exhaust gas. Is determined to be lower, the amount of HC supplied to the NOx catalyst is increased by controlling at least one of the assist air amount of the assist air injection valve and the swirl passage flow rate. The problem is solved by a catalyst efficiency control method characterized by the following.

[0008]

FIG. 1 shows an embodiment of the overall system of the present invention. A catalyst 3 is provided in an exhaust pipe 2 of the engine 1. The catalyst 3 includes a lean NOx catalyst 4 for purifying NOx at a lean air-fuel ratio and NOx, CO, HC at a stoichiometric air-fuel ratio.
A three-way catalyst or an oxidation five catalyst for purifying the catalyst is disposed.
The switching valve 6 is used so that the two can be used depending on the operating state. As the lean NOx catalyst, for example, a catalyst in which a metal is supported on a copper-zeolite system is considered. This catalyst generally has the property of deteriorating at high temperatures and rich air-fuel ratios. For this reason, it may be better to bypass the lean NOx catalyst in the output operation range, when the engine is warmed up, or the like.
For this purpose, a switching valve 6 is provided. In the lean operation range, the switching valve 6 is closed to supply the exhaust gas to the lean NOx catalyst. Further, the switching valve 6 is opened at the time of the engine output operation region in which the air-fuel mixture is rich, or at the time of warming-up, so that the exhaust gas is supplied to the downstream three-way catalyst or the oxidation catalyst.
In order to detect the NOx conversion efficiency of the lean NOx catalyst, for example, sensors 7 and 8 are provided before and after the catalyst 3 to detect an exhaust gas state. The sensors 7 and 8 may be, for example, an oxygen sensor whose output changes stepwise at an excess air ratio λ = 1, or an air-fuel ratio sensor whose output changes in proportion to the excess air ratio. The detection values of these two sensors are taken into the control unit 9 and the purification efficiency or the degree of deterioration of the catalyst is estimated based on the comparison result. Air cleaner 10
After the amount of air flowing in from the air is measured by the air amount sensor 11, the air flows into the collector 14 via the throttle 13 driven by the electric motor 12. Thereafter, it is sucked into the engine 1 through the independent intake pipe 15. The intake port portion 16 is provided with a bypass passage 17 for swirl generation and a flow dividing valve 18. In the lean operation region, it is necessary to improve the combustion by forming a swirling flow or swirl in the combustion chamber. Therefore, in such a case, the flow dividing valve 18 is closed to allow the air to pass through the bypass passage 17. The swirling of the air results in swirl being generated in the combustion chamber. Fuel is supplied from a fuel injection valve 19. The ignition of the air-fuel mixture is performed by a spark plug 20. A crank angle detector 21 for detecting the engine speed is provided on a crank shaft 22.

FIG. 2 shows the conversion efficiency of NOx to N2 in the lean NOx catalyst. FIG. 2A shows the relationship between the catalyst temperature and the conversion efficiency. This catalyst shows high efficiency in a certain temperature range. However, the temperature exhibiting this high efficiency has a characteristic that when the catalyst deteriorates, it shifts to a higher temperature side. This state is shown in FIG. It can be seen that the temperature indicating the highest efficiency increases as the traveling distance increases, that is, as the vehicle degrades. FIG. 3A shows the relationship between the amount of HC in the exhaust gas and the conversion efficiency. HC for a certain amount of NOx
There is an optimum value for the amount, and a higher purification rate can be obtained by controlling the HC amount so as to reach this point. However, the HC amount showing the highest conversion rate also changes when it deteriorates as shown in FIG. From this, it is necessary to detect deterioration and change the presence of HC accordingly.

Next, a method for detecting the conversion efficiency and the degree of deterioration of the catalyst will be described. As shown in FIG. 4A, the exhaust gas components on the engine side 30 of the lean NOx catalyst 4 installed in the exhaust pipe 3 are NOx, HC, and O ₂ , if only those relating to NOx reduction are mentioned. FIG. 4B schematically shows gas molecules. N for nitrogen, H for unburned hydrocarbons
C and oxygen are indicated by ○. On the catalyst, HC reacts with oxygen to form an intermediate product (indicated by HC enclosed in a square), which acts on NOx and is decomposed into N2.

For this reason, the exhaust gas downstream of the catalyst 31 is N
₂ , H ₂ O and CO ₂ , indicating that NOx has been reduced. In this case, as shown in FIG. 4B, the oxygen concentration changes before and after the catalyst. Therefore, as an example of a detection method for detecting a change in conversion efficiency, this lean NOx
A method of detecting the oxygen concentration before and after the catalyst can be considered.

FIG. 5 shows the principle of detection. As shown in FIG. 5A, sensors 7 and 8 for detecting the oxygen concentration are mounted before and after the catalyst. In the configuration of the sensors 7 and 8, for example, platinum electrodes 33a and 33b are provided on both sides of a zirconia solid electrolyte 32, for example. The electrode 3 on the exhaust gas side of these electrodes
A diffusion resistor 34 for controlling gas diffusion is formed on the exhaust side of 3a. The electrode 33a is configured to be connected to a housing such as the exhaust pipe 3 as a ground side. In this case, when a predetermined potential is applied to the other electrode, the current value at this time becomes proportional to the oxygen concentration on the exhaust gas side. That is, the oxygen concentration can be measured by detecting the current value. The configuration and operation principle of the sensor 8 are the same as those of the sensor 7. FIG. 5B shows a schematic diagram of the exhaust gas composition. Before the reaction, nitrogen, HC and oxygen coexist as shown in (a). However, the sensor 7
On the electrode 33a, HC is almost completely oxidized by the catalytic action of platinum. Therefore, as shown in (b), the detected oxygen concentration is surrounded by a dashed line where the amount of reacted oxygen is reduced. Amount. On the other hand, in the downstream side of the catalyst 4, as shown in (c), HC almost completely reacts and NOx
Since there is oxygen decomposed by the reduction of, the detected oxygen concentration becomes an amount surrounded by a dashed line, and is higher than the state of (b). Therefore, as can be seen by comparing the oxygen amounts enclosed by the dashed lines (b) and (c), the oxygen amounts detected before and after the catalyst 4 are different. As shown in FIG. 5C, the signal of the sensor for detecting the oxygen amount shows different values.

FIG. 6 shows the principle of detecting the degree of deterioration or conversion efficiency. FIG. 6A is an outline of the apparatus. Sensor 7,
A predetermined voltage V is applied to the electrode 33b of No. 8 and the voltage drop of the current flowing through the fixed resistors R1 and R2 at this time is represented by V1 and V2.
Are detected by the differential amplifiers 36 and 37. This V1, V
Reference numeral 2 denotes a current value flowing through the solid electrolyte 32 of each of the sensors 7 and 8, that is, a detected oxygen concentration. This V1,
The difference between V2 is detected again by the differential amplifier 39. This difference V2
-V1 is a value related to the degree of deterioration. As shown in FIG. 6B, the output of this sensor changes depending on the oxygen concentration, so that the difference between the oxygen concentration before and after the catalyst 4 can be detected. This is shown in FIGS. 6C and 6D. V2 has a higher output because of the higher oxygen concentration. Here, the output V1 of the sensor 7 before the catalyst can be used for air-fuel ratio control. Of course, the output V2 of the post-catalyst sensor 8 may also be used for air-fuel ratio control. Here, V1
The difference between V1 and V2 is detected by the differential amplifier 39.
V2 may be converted by an analog-to-digital converter, fetched into a microcomputer, and the difference may be obtained by arithmetic processing. A flowchart in that case will be described. First, FIG. 7 is a flowchart in the case of performing air-fuel ratio control. First, in step 100, V1 is measured, and in step 1
The target air-fuel ratio (A / F) as shown in FIG.
The target output Vref of the sensor with respect to
And come from the load map. V at step 120
1 and Vref, and if V1 is larger, it is determined that the current air-fuel ratio is leaner than the target, and step 1
At 30, the fuel injection amount Ti is increased to shift the air-fuel ratio to the rich side. If V1 is small, it is determined that the current air-fuel ratio is on the rich side, and Ti is decreased in step 140. T is determined as described above, and is output to the fuel injection valve 19 in step 150. In this way, air-fuel ratio control can be realized using a sensor that detects conversion efficiency or deterioration.

Next, FIG. 8 shows a flow for detecting the deterioration of the catalyst. In step 210, V1 and V2 are measured and the difference between them is calculated. Next, if the difference is equal to or smaller than the predetermined value in step 220, the process proceeds to step 230, where it is determined that the amount of increase in oxygen due to the reduction action of N ₂ by the catalyst is small, and it is presumed that the catalyst has deteriorated. That is, it is determined that the battery has deteriorated, and the degree of deterioration is displayed. Difference in oxygen concentration before and after the catalyst, that is, V1, V2
The larger the difference is, the stronger the reducing action of the catalyst is, which means that the catalyst has not deteriorated.

FIG. 9 shows a deterioration determination method with improved accuracy.
In step 300, the difference between V1 and V2 is calculated, and in step 3
At 10, it is determined whether the catalyst temperature Tc or the exhaust gas temperature is within a certain range. This is shown in FIG.
This is because the conversion efficiency of the catalyst depends on the temperature, and if the temperature changes, it may be determined that the efficiency is lowered. For this reason, deterioration determination is always realized only when the temperature is within a predetermined temperature range. This is also effective in the sense that the temperature characteristics of the sensor can be ignored. Deterioration determination after determining that the temperature is within a certain range is the same as in FIG. That is, if the difference between V2 and V1 is equal to or smaller than the reference value in step 320, it is determined that the deterioration has occurred in step 330,
Further, at step 340, the degree of deterioration corresponding to the difference is displayed.

FIG. 10 shows another detection method. The sensor used here is an oxygen sensor having a non-linear output characteristic with respect to the oxygen concentration (air-fuel ratio) as shown in FIG. In the case of such a sensor, a simple diffusion film 42 provided on the electrode 40a on the exhaust side of the sensors 7 and 8 is sufficient. Specifically, a layer thinner than the diffusion film 34 in FIG. 5 may be used. In the device shown in FIG. 10A, such an output uses a binary oxygen sensor. In such a sensor, the exhaust side electrode 40a is grounded, and the other electrode 40a is grounded.
Voltages (electromotive force) V1 and V2 generated in b are detected. The degree of deterioration is detected based on the difference between the voltages of the two sensors. More specifically, as shown in FIG. 10B, V1 and V2 when the exhaust gas is thinner than the stoichiometric air-fuel ratio are measured, and the difference between the output values is detected by the differential amplifier 44. However, as described above, the difference between the two may be obtained by calculation by taking it into the microcomputer. The degree of catalyst deterioration is estimated from this difference. FIGS. 10C and 10D show V
1 and 2 show output examples. The output after the catalyst is shown in FIG.
As shown in FIG. 10, since the gas contains a relatively large amount of oxygen, the output value is smaller than the output value shown in FIG. The difference between V1 and V2 is an index related to the degree of deterioration.

FIG. 11 shows still another embodiment. In this case, the front catalyst 53 and the downstream catalyst 54 are arranged in series, and three sensors 50, 51, and 52 are arranged respectively. The efficiency and deterioration of the pre-catalyst 53 are measured by the sensor 5
At 0,51, detection is performed by the method described above. In addition, the post catalyst 5
The efficiency and deterioration of 4 are detected by sensors 51 and 52 or by sensors 50 and 52. With such an arrangement, deterioration diagnosis of the composite catalyst system can be performed. As the type of catalyst, the pre-catalyst 53 is a NOx reduction catalyst, and the post-catalyst 54 is a three-way catalyst or an oxidation catalyst.
In this case, the method of determining the deterioration of the NOx reduction catalyst 53 is performed by comparing the detection outputs of the sensors 50 and 51 as described above. Further, the signal of the sensors 51 and 52 is used to judge the deterioration of the post-catalyst 54 or the sensors 50 and 52 are used.
The post-catalyst 54 may be diagnosed by using the 52. The signals of the sensors 50, 51, 52 are taken into the microcomputer 9 and are processed. FIG. 12 shows a control flowchart in this case. Since the NOx reduction catalyst 54 exhibits a catalytic action for NOx purification in the lean operation range,
Then, the diagnosis mode is started. That is, in step 410, the signals V1 and V2 of the sensors 50 and 51 before and after the catalyst 53 are measured, and in step 420 the degree of deterioration is diagnosed. This diagnosis uses the flowcharts of FIGS. Thereafter, the degree of deterioration of the corresponding catalyst is displayed in step 430. In the case of the post-catalyst, deterioration is determined in step 440 during operation at the excess air ratio λ = 1. In this operation state, at step 450, the sensor 5
The outputs V1 and V3 of 0 and 52 are measured, and deterioration is diagnosed in step 460 based on the signals. In this case, the outputs V2 and V3 of the sensors 51 and 52 may be measured to determine the deterioration in the same manner. As described above, in a composite catalyst system using a plurality of catalysts, a method of diagnosing the efficiency or deterioration of the catalyst in a region where each catalyst acts is preferable.

FIG. 13 shows still another embodiment. In this case, a plurality of catalysts are arranged in parallel. Catalyst 55,
Reference numerals 56 are arranged in parallel, and a switching valve 57 driven by a load or an actuator 58 operated by an electric motor selects a catalyst through which gas flows selectively according to the operation state. For example, when the switching valve 57A is opened so that the exhaust gas flows through the catalyst 56, the switching valve 57B is closed so that the exhaust gas does not flow through the catalyst 55. In this case, when the catalyst 56 is in an operating state in which it should operate, the efficiency and deterioration of the catalyst 56 are determined based on the signals of the sensors 58 and 59. Further, the switching valve 57A,
When the gas is caused to flow through the catalyst 55 by rotating 58B, the gas stops flowing through the catalyst 56.
In this case, the efficiency or deterioration of the catalyst 55 is determined based on signals from the sensors 58 and 59 when the catalyst 55 is in an operating state in which the catalyst 55 should operate. The operation of the actuator 58, the acquisition of signals from the sensors 58 and 59, and the calculation are performed by the microcomputer 9. FIG. 14 shows a flowchart in that case. FIG.
The case where one of the catalysts is a NOx reduction catalyst and the other is a three-way catalyst or an oxidation catalyst will be described. First, in step 500, it is determined whether or not the engine is in a lean operation range. If the engine is in a lean operation range, the switching valve 57A is operated in step 510 to supply exhaust gas to the NOx reduction catalyst. Thereafter, if the condition is satisfied, the signals of the sensors 58 and 59 are measured in step 520, the deterioration diagnosis for the NOx catalyst is performed in step 530, and the diagnosis result is displayed in step 540. In the case other than the lean operation, the switching valve 57A is closed at step 550, and the switching valve 57B is switched to open so that the exhaust gas flows through the three-way catalyst. Thereafter, in step 560, it is determined whether or not the engine is in the operating state with the stoichiometric air-fuel ratio λ = 1.
In the case of 1, the signals of the sensors 58 and 59 are measured in step 570, and a deterioration diagnosis for the three-way catalyst is performed in step 580, and then the diagnosis result is displayed in step 540. FIG.
FIG. 5 shows still another method. One of the sensors 61 has a catalyst 6
The configuration is such that a gas upstream of the catalyst is guided to zero and a gas downstream of the catalyst is guided to the other surface, and one sensor detects a difference in oxygen concentration in the gas before and after the catalyst. Here, most of the gas in the exhaust pipe 3 flows to the catalyst. However, a part thereof flows through the passage 62 and is guided to the chamber 63 provided on one side of the sensor 61. The gas flows through the passage 62, the chamber 63, and the passage 64 by the suction action of the venturi 65 provided upstream of the catalyst. On the other hand, a gas downstream of the catalyst 60 is guided to the exhaust pipe side of the sensor 61. This sensor 6
The structure of No. 1 is shown in FIG. The gas upstream of the catalyst is led to the left side of the sensor, and the gas downstream of the catalyst is led to the right side of the sensor. This sensor uses a zirconia solid electrolyte 6
6 are provided with platinum electrodes 67a, 67b on both sides thereof, and on the outside thereof, porous protective films 68a, 68b.
Is provided. Both sides of the solid electrolyte 66 have a catalytic action, and can oxidize HC. By doing so, the solid electrolyte 66 detects the residual oxygen after oxidation. In this case, a solid electrolyte which is an oxygen concentration cell is advantageous because a difference between oxygen concentrations on both sides may be detected. Further, one electrode 67b is grounded,
When the potential of the other electrode 67a is measured, the detected value indicates the oxygen concentration difference. The measured potential is taken into the microcomputer 9 and processed.

FIG. 16 shows a more specific configuration of the sensor 61. The sensor is arranged in the protection tube 70. The electrode 67b on the exhaust gas side downstream of the catalyst is grounded by a protective tube 70 via a conductive wire printed on the insulating material 71. Also,
The other electrode 67a on the exhaust gas side upstream of the catalyst is guided to the outside from the connector section 69. The sensor body is an exhaust pipe 72
Is fixedly screwed in. With such a configuration, the exhaust gas before and after the catalyst can be guided to each surface of the sensor. Since this sensor is an oxygen concentration cell, it exhibits characteristics as shown in FIG. 16B depending on the difference in oxygen concentration on both sides of the sensor. When the sensor does not deteriorate, the oxygen of NOx is reduced as shown in FIG. 5, so that the oxygen concentration in the exhaust gas downstream of the catalyst increases. When the difference between the oxygen concentrations on both sides of the sensor is large, the output of the sensor becomes low as shown in FIG. On the other hand, when the difference between the oxygen concentrations on both sides is small, an electromotive force is generated in the solid electrolyte 66, and the output increases as shown in (b). Therefore, as shown in FIG. 16C, the output of the sensor increases as the elapsed time increases. If this is detected, the deterioration of the catalyst can be detected. As described above, when detection is performed by one sensor, the detection surfaces are the same in temperature, so that the temperature dependency of the detection is reduced, and the detection accuracy is improved.

FIG. 17 shows a flowchart of the detection. First, at step 600, it is determined whether or not the engine is in a lean operation range.
It is determined whether g is within a predetermined value. If the temperature is within a certain range, the sensor is also activated, and the temperature dependence of the catalyst can be avoided. By selecting only the temperature at which the sensor is activated, it is not necessary to provide a heater in the sensor. Thereafter, the output of the sensor is measured in step 620, and the degree of deterioration is determined in step 630. This determination method can be performed by determining whether the output value of the detected sensor is equal to or greater than a certain reference value. Then, in step 640, the degree of deterioration is determined and displayed.

FIG. 18 shows still another embodiment. During the lean operation, the exhaust gas is reduced to NO by closing the switching valve 6.
It flows to the x reduction catalyst 4 and then flows to the three-way catalyst 5 located downstream. At this time, the switching valve 6 is made so as to leak some gas so that the gas upstream of the catalyst 4 flows through the sensor 73 as shown in FIG. Further, the gas passing through the catalyst 4 flows on the other surface of the sensor 73. The signal of the sensor is taken into the microcomputer 9 and subjected to arithmetic processing. When the switching valve 6 is opened, the exhaust gas does not flow to the NOx reduction catalyst 4 but flows only to the three-way catalyst.

FIG. 19 shows another embodiment. Here, a sensor 73 as shown in FIG. 16A is arranged upstream of the switching valve 6. The switching valve 6 is driven by a command from the microcomputer 9 via a driving device 74. Sensor 7
The gas before and after the catalyst is led to 3. Switching valve 6
Is brought downstream of the sensor 73 because the sensor 7
This is to prevent the gas downstream of the catalyst 4 from flowing into the surface upstream of the catalyst 3.

FIG. 20 shows still another embodiment. During the lean operation, the switching valve 75 is opened to flow the exhaust gas to the NOx reduction catalyst 4. In this case, the gas downstream of the catalyst 4 flows on the exhaust gas surface downstream of the catalyst 4 of the sensor 78. However, since a small amount of gas flows from the switching valve 75 toward the exhaust pipe 76, the gas upstream of the catalyst flows on the exhaust gas surface of the sensor 78 upstream of the catalyst 4. In an operation state other than the lean operation, the switching valve 75 is switched as shown by a dotted line to flow exhaust gas toward the exhaust pipe 76. The three-way catalyst 5 is disposed in the exhaust pipe 76. Even with such a configuration, the deterioration of the catalyst can be detected by one sensor.

Next, an engine control method for improving the efficiency after detecting the deterioration and the efficiency of the catalyst will be described. FIG. 21 is an overall view of the system. Sensors 7 and 8 are attached before and after the catalyst 4. Further, a sensor 80 for detecting the temperature of the gas is attached to the exhaust pipe. After determining the deterioration of the catalyst from the outputs of the sensors 7 and 8, as shown in FIG.
It is necessary to control the concentration. A method for this will be described. One method of controlling the temperature is to control the circulation of the cooling water 86 of the engine 1. When the circulation amount of the cooling water is reduced by the control valve 87, the engine 1
And the exhaust temperature rises. That is, when it is determined that the catalyst 4 has deteriorated, the circulation of the cooling water is controlled to increase the temperature of the exhaust gas, thereby preventing a decrease in efficiency. Also,
Another method of controlling the temperature is to control the ignition timing of the ignition device 84 and the ignition plug 20. If the ignition timing is delayed, the exhaust gas temperature rises. That is, when the catalyst deteriorates, the ignition timing is slightly delayed to raise the exhaust gas temperature and prevent the efficiency from lowering. Further, since the required HC amount changes due to the deterioration, the state of deterioration needs to increase the HC amount. The method will be described below. When the fuel injection timing of the injection valve 19 is changed, the amount of HC emission changes. Therefore, when the fuel injection timing is deteriorated, the controller 9 changes the set fuel injection timing. Further, in the case where the injection valve 19 is an air-assisted injection valve that atomizes fuel by airflow, if the amount of assist air is reduced or stopped, atomization of fuel becomes worse, and the amount of HC emission increases. Further, when the flow dividing valve 18 is closed to allow the intake air to flow through the swirl passage 17, a swirling flow is generated in the combustion chamber, and the fuel is improved. For this reason,
By opening the flow dividing valve 18 halfway, the swirl passage 1
Increasing air passing through other than 7 exacerbates combustion. This increases the amount of HC emissions. Furthermore, when the circulation amount of the cooling water is increased and the engine is cooled, HC
Emissions increase. Further, since the amount of HC emission increases even if the ignition timing is advanced, if the catalyst deteriorates, the ignition timing may be advanced according to the degree of the deterioration. Also, the catalyst temperature and the amount of HC emission can be increased on the exhaust side. The fuel vaporized in the upper part of the fuel tank 82 is removed by the pump 8
Supply to the exhaust pipe upstream of the catalyst at 3 or the like. The vaporized fuel is HC
Therefore, the efficiency of the catalyst can be improved by supplying it to the catalyst. Furthermore, since the vaporized fuel is light HC, it is easy to burn, and the burning raises the catalyst temperature.
Another way to raise the temperature is to use a catalyst 8
8 there is a method of heating. The driving circuit 81 supplies electricity to the heater according to a signal from the controller 9. The heater is heated, the catalyst temperature rises, and the efficiency improves. In this heater, the catalyst carrier may be made of a conductive material so that electricity flows.

FIG. 22 shows another method of changing the temperature of the catalyst. This adopts a configuration in which a combustor is provided in the exhaust pipe 3, and a spark is formed by a spark plug 91 by a voltage from a drive circuit 90 for an ignition device. Fuel injection valve 92
From the fuel is injected into the combustor 93. Also, the combustor 9
3 is supplied with air by a pump 94. The fuel injected into the chamber 93 is ignited by the ignition plug 91 and a flame is formed in the exhaust pipe before the catalyst. The flame raises the temperature of the catalyst. Further, the configuration of the electric heater 88 is also shown in the drawing. By configuring the electric heater 88 so as to wrap the outside of the catalyst 4, the catalyst can be heated. Incidentally, the respective methods may be combined or independently. In other words, any one of the methods is valid.

FIG. 23 shows a flowchart of the control. If the degree of catalyst deterioration is detected in step 710, step 7
At 20, it is determined whether the amount of HC needs to be increased or decreased according to the degree of deterioration. If it is determined that it is necessary, the amount of increase or decrease of HC is determined in step 730 according to the degree of deterioration. Then, step 74
At 0, the HC supply amount to the catalyst is increased or decreased by combining one or some of the HC increasing / decreasing means shown in FIGS. Generally, when the NOx reduction catalyst has deteriorated, it is necessary to increase the amount of HC supplied to the catalyst in order to ensure its efficiency. When the degree of deterioration of the catalyst is detected in step 710, the degree of deterioration is displayed in step 750.

FIG. 24 shows a control flow for changing the temperature of the catalyst. When the degree of deterioration of the catalyst is detected in step 800, the degree of deterioration is displayed, and in step 810, it is determined whether or not the catalyst operating temperature needs to be changed. If it is determined that it is necessary, the temperature change amount is determined in step 820, and in step 830, one or some of the temperature changing means shown in FIGS. 21 and 22 are combined to change the temperature of the catalyst. Thereafter, it is determined in step 840 whether or not the temperature has reached a predetermined temperature, and when the temperature has reached the predetermined temperature, the flow ends.

FIG. 25 shows the flow of a method for controlling the conversion efficiency of the catalyst to always be the highest. First, it is determined in step 900 whether or not the engine is in a lean operation range, and in the case of lean operation, it is next determined in step 910 whether or not the engine is in a steady operation. In the case of steady operation during lean operation, step 92
At 0, the signal of the sensor for measuring the efficiency of the catalyst is read, and at step 930, the conversion efficiency of the catalyst is estimated. Next, if the efficiency is low in step 940, step 95
Each parameter is controlled by an efficiency changing means for changing the catalyst temperature and the HC supply amount at zero. Thereafter, it is determined in step 960 whether or not the efficiency has been improved. If the efficiency has been improved, the process ends with the parameter changed. If the efficiency has not been improved, the process returns to the state before changing the parameter in step 970. Ends. By doing so, the engine can always be operated with high conversion efficiency.

FIG. 26 shows another method for detecting the efficiency and the degree of deterioration. In FIG. 26A, the variation width ΔV of the output signals of the sensors 7 and 8 before and after the catalyst 4 is compared. FIG.
26 (B), when the oxygen concentration upstream of the catalyst 4 fluctuates as shown in FIG. 26 (C), the state of the fluctuation can be recognized by the magnitude of the fluctuation width ΔV1 of the sensor 7. If the catalyst is new,
Due to the active NOx reduction reaction, the oxygen concentration downstream of the catalyst 4 also fluctuates greatly. However, when the catalyst deteriorates, NO
Since the reducing action of x becomes slow, the change in the oxygen concentration is less. Therefore, the fluctuation width ΔV2 of the output signal of the sensor 8 downstream of the catalyst 4 becomes smaller. By detecting the change in the fluctuation range, the degree of deterioration of the catalyst is detected. The flowchart is shown in FIG. In step 1000, it is determined whether or not the catalyst is in the lean operation range. When it is determined in step 1010 that the catalyst is in the predetermined temperature range in the lean operation range, ΔV is determined in step 1020.
1, ΔV2 is detected and calculated. Then step 103
At 0, ΔV2 / ΔV1 is calculated. Next, step 1040
If this value is equal to or smaller than the predetermined value, it is determined that the efficiency has decreased, and the degree of deterioration is displayed in step 1050 and the efficiency improving means is operated in step 1060. When the efficiency is determined, control of the efficiency improving means can be omitted.

FIG. 28 shows another means for detecting the degree of deterioration and efficiency. In FIG. 28A, the air-fuel ratio supplied to the engine 1 is changed by intentionally changing the injection amount of the fuel injection valve 19 and the throttle opening, and the signals of the sensors 7 and 8 before and after the catalyst 4 at that time are changed. The degree of catalyst deterioration is detected from the difference in the behavior of the catalyst. As shown in FIG. 28B, when the air-fuel ratio is changed stepwise, the outputs of the sensors 7 and 8 also change stepwise. However, when the catalyst has deteriorated, the difference in response between the sensors 7 and 8 before and after the catalyst changes compared to when the catalyst has not deteriorated. For example, the solid line in the figure shows the behavior of the sensor that shows a linear output with respect to the air-fuel ratio, and becomes larger when the difference in the response time constant τ deteriorates. The dotted line shows the behavior of the output of the normal oxygen sensor, but also becomes large when the difference in the response time constant τ deteriorates. FIG. 28 (C) changes the air-fuel ratio on the supply side randomly or according to a certain rule. The degree of deterioration of the catalyst is detected from the correlation between the changes in the signals of the sensors 7 and 8 at this time. If the signal from the sensor 8 is greatly distorted (dulled) compared to the signal from the sensor 7, it can be determined that the catalyst has deteriorated. FIG. 29 shows a flow chart at that time. If it is determined in step 1100 that the operation is in the lean operation range or the steady operation, in step 1110, the catalyst is determined to be in the predetermined temperature range. Next, when this is satisfied, the air-fuel ratio supplied to the engine is changed in step 1120. Next, step 1130
Then, the degree of deterioration of the catalyst is detected by the method shown in FIG. 28 from the difference in the behavior of the change time τ between the signals V1 and V2 of the sensors before and after the catalyst at this time. If it has deteriorated in step 1140, the degree of deterioration is displayed in step 1150, and in step 1160, the means for improving the efficiency shown in FIGS. 21 and 22 is operated.

[0031]

According to the present invention, accurate diagnosis suitable for each operating environment can be performed for each catalyst. Further, engine control can be performed so as to avoid a decrease in efficiency, and high exhaust purification characteristics can be maintained.

[Brief description of the drawings]

FIG. 1 is an overall system diagram.

FIG. 2 is a characteristic diagram of conversion efficiency of a catalyst.

FIG. 3 is a characteristic diagram of conversion efficiency of a catalyst.

FIG. 4 is a diagram illustrating the principle of catalyst purification.

FIG. 5 is a diagram illustrating the principle of deterioration detection according to the present invention.

FIG. 6 is a schematic diagram of a detection method of the present invention.

FIG. 7 is a flowchart of the present invention.

FIG. 8 is a flowchart of the present invention.

FIG. 9 is a flowchart of the present invention.

FIG. 10 is a schematic diagram of another detection method of the present invention.

FIG. 11 is a schematic diagram of another detection device of the present invention.

FIG. 12 is a flowchart of the control in FIG. 11;

FIG. 13 is a schematic diagram of another detection device of the present invention.

FIG. 14 is a flowchart of the control in FIG. 13;

FIG. 15 is a schematic diagram of another detection method of the present invention.

16 is a configuration diagram of the detection sensor of FIG.

FIG. 17 is a flowchart of the control in FIG. 15;

FIG. 18 is a schematic diagram of another detection method of the present invention.

FIG. 19 is a schematic diagram of another detection method of the present invention.

FIG. 20 is a schematic diagram of another detection method of the present invention.

FIG. 21 is an overall syssim diagram.

FIG. 22 is an overall system diagram.

FIG. 23 is a flowchart of the control in FIGS. 21 and 22;

FIG. 24 is a flowchart of the control in FIGS. 21 and 22.

FIG. 25 is a flowchart of the control in FIGS. 21 and 22;

FIG. 26 is a schematic diagram of another detection method of the present invention.

FIG. 27 is a flowchart of the control in FIG. 26;

FIG. 28 is a schematic diagram of another detection method of the present invention.

FIG. 29 is a flowchart of the control in FIG. 28;

[Explanation of symbols]

4 NOx reduction catalyst or three-way catalyst, 5 three-way or oxidation catalyst, 7, 8 sensor, 9 control unit,
17: swirl generation passage, 32: solid electrolyte, 34: diffusion resistor, 50, 51, 52: sensor, 57: exhaust switching valve, 82: fuel tank, 88: heater, 91: spark plug, 92: fuel injection valve .

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02B 31/00 321 F02M 69/00 310E F02M 69/00 310 B01D 53/36 101Z

Claims (3)

[Claims] 1. An exhaust system for an internal combustion engine, comprising: a NOx catalyst for purifying exhaust gas components; and a composite catalyst comprising a combination of at least two of a three-way catalyst and an oxidation catalyst. A catalyst evaluation method for evaluating a catalyst constituting the composite catalyst based on a combination of any two of sensor outputs for detecting an exhaust gas specific component disposed between the downstream and the composite catalyst. An evaluation method for a catalyst, wherein an operation range for diagnosing the catalyst and the three-way catalyst or the oxidation catalyst is different. 2. The method for evaluating a catalyst according to claim 1, wherein the operating range for diagnosing the NOx catalyst is a lean operating range, and the operating range for diagnosing the three-way catalyst or the oxidation catalyst is air. A method for evaluating a catalyst, wherein the operating range is an excess ratio λ = 1. 3. A system provided with an NOx purification catalyst provided in an exhaust system of an internal combustion engine for purifying NOx in exhaust gas, wherein the NOx purification catalyst is evaluated, and as a result, purification efficiency is reduced. When it is determined, the amount of HC supplied to the NOx catalyst is increased by controlling at least one of the assist air amount of the assist air injection valve and the swirl passage flow rate. Catalyst efficiency control method.